Development of a wireless sensor unit for tunnel
monitoring
by
Sivaram M.S.L. Cheekiralla
Bachelor of Technology, Civil Engineering (2001)
Indian Institute of Technology, Bombay
Submitted to the Department of Civil and Environmental Engineering
in partial fulfillment of the requirements for the degree of
Master of Science in Civil and Environmental Engineering
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
February 2004
© Massachusetts Institute of Technology 2004. All rights reserved.
A uth o r .........................
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Department of Civil and Environmental Engineering
January 16, 2004
Certified by...
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Ruaidhri M. O'Connor
Assistant Professor
The is Supervisor
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Accepted by ......................
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Heidi M. Aepf
Chairman, Department Committee on Graduate Students
MASSACHUSETTS INSTITE
OF TECHNOLOGY
FEB 19 2004
LIBRARIES
BARKER
2
Development of a wireless sensor unit for tunnel monitoring
by
Sivaram M.S.L. Cheekiralla
Submitted to the Department of Civil and Environmental Engineering
on January 16, 2004, in partial fulfillment of the
requirements for the degree of
Master of Science in Civil and Environmental Engineering
Abstract
In this thesis we describe the development of a wireless sensor module for tunnel
monitoring. The tunnel in question is a part of the London Underground system.
Construction of a new tunnel beneath the existing tunnel is anticipated to cause
quantifiable vertical displacement. To ensure safe operation of the tunnel during the
construction activity, a real-time monitoring system has been created to measure
vertical displacements along the critical zone near Highbury & Islington station. A
geomechanical analysis, provided by a third party, is used to establish the allowable
maximum displacement.
A custom wireless sensor module was developed from off-the-shelf components.
This module consists of a sensor device, microcontroller, ADC and RF transmitter.
The integration of these components is described in detail. Deployment details and
some preliminary results are presented.
Thesis Supervisor: Ruaidhri M. O'Connor
Title: Assistant Professor
3
4
Acknowledgments
First of all, I would like to thank Prof. Rory O'Connor for his support. His words of
encouragement during tough times were really motivating. I am grateful to him for
being my advisor.
James Brooks has been a great person to work with. Most of this work has been
done with him. His keen insights for solving problems are really inspiring. I appreciate
his patience during the tough times we had during this project. I also thank him for
listening to some of my fancy suggestions and actually implementing some of them!
I would also like to thank all the members of the CMI team. I am thankful to
Prof. Whittle and Dr. Germaine for their constructive criticism and useful suggestions
during the course of this project. Special thanks to Prof. Amaratunga and George
Kokossalakis for their help and support for my qualifying exams.
I am thankful to my friends Hari, Phani, Deepak, Dhanush, Sreekar, Bhanu,
Dinesh and Srini for all their support and encouragement. Special thanks to Kunal
Kunde and Murthy for all the help over the years.
I am thankful to my relatives for providing me a homely environment whenever I
felt home-sick. Special thanks to Manasa and Jyotsna.
I would like to thank my parents, brother and grandmother for all their love,
support and encouragement.
I would like to dedicate this thesis to my late friend Hari Narayan Sugavanam.
This research was sponsored by the The Cambridge MIT Institute's New Technologies for Condition Assessment and Monitoring of Ageing Infrastructure project.
5
6
Contents
1
Introduction
1.1
M otivation . . . . . . . . . . . . . . . . . . . . . . . . .
15
1.2
Wireless monitoring . . . . . . . . .
. . . . . . . . . . .
16
1.2.1
Great Duck Island project
. . . . . . . . . . .
18
1.2.2
Wireless Modular Monitoring Systems (WiMMS)
18
1.3
1.4
2
15
LUL tunnel monitoring project
.
. . . . . . . . . . . ..
21
1.3.1
Other Methods
. . . . . . . ...........
23
1.3.2
Proposed Method . . . . . . . . . . . . . . . . .
23
Present work
. . . . . . . . . . . . ...........
25
Sensors
27
2.1
Sensors..
.........
2.2
Accelerometers
. . . . . . . .
27
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
28
2.2.1
Sensing Method . . . . . . . . . .
28
2.2.2
Capacitive accelerometers
. . . .
29
2.2.3
Piezoelectric accelerometers
. . .
29
2.2.4
Potentiometric accelerometers
. .
29
2.3
Strain gages . . . . . . . . . . . . . . . .
30
2.4
Linear Variable Differential Transformer
30
2.5
Sensors used in the tunnel monitoring proj ect
31
2.5.1
Pressure transducers . . . . . . .
31
2.5.2
Ultrasonic transducers
32
2.6
. . . . . .
MEMS
36
7
2.6.1
3
3.2
3.3
3.4
41
41
. . . . . . . . . . . . . . . . . . . . . . . . .
43
3.1.1
Clocking scheme
3.1.2
Memory Organization
. . . . . . . . . . . . . . . . . . . . . .
44
3.1.3
Pin Description . . . . . . . . . . . . . . . . . . . . . . . . . .
46
Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
47
Universal Synchronous Asynchronous Receiver Transmitter (US-
ART) module . . . . . . . . . . . . . . . . . . . . . . . . . . .
48
3.2.2
Analog to Digital Converter . . . . . . . . . . . . . . . . . . .
48
3.2.3
Master Synchronous Serial Port (MSSP) module . . . . . . . .
49
3.2.4
Timer and interrupt features . . . . . . . . . . . . . . . . . . .
49
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
50
3.3.1
Programming in assembly language . . . . . . . . . . . . . . .
50
3.3.2
Programming in C
. . . . . . . . . . . . . . . . . . . . . . . .
51
Operation of the microcontroller . . . . . . . . . . . . . . . . . . . . .
51
Instruction Set
53
Communications
4.1
4.2
38
A rchitecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.1
5
. . . . . . . . . . . . . . . . . . . . . . . .
Microcontroller
3.1
4
Fiber optic sensors
Wire-based systems . . . . . . . . . . . . . .
53
4.1.1
Synchronous serial communications
53
4.1.2
Asynchronous serial communications
55
4.1.3
Bit-banging . . . . . . . . . . . . . .
58
Wireless Technologies . . . . . . . . . . . . .
59
4.2.1
MICA motes
. . . . . . . . . . . . .
61
4.2.2
i-Beans . . . . . . . . . . . . . . . . .
62
4.2.3
Linx Technologies RF ICs
. . . . . .
64
67
Integration of components
5.1
Interfacing the ADC
. . . . . . . . . . . . . . . . . . . . . . . . . . .
67
5.2
Interfacing the i-Bean . . . . . . . . . . . . . . . . . . . . . . . . . . .
74
8
6
5.3
Interfacing the ultrasonics
5.4
Others . . . . . . . . . . . . . . . . ..
82
5.5
Sensor Board
. . . . . . . . .
84
5.6
Protocols
. . . . . . . . . . . . . . . .
86
5.7
Protocol for i-Bean . . . . . . . . . . .
87
5.8
Protocol for the Ultrasonic transducer
88
. . . ..
. . . . . . .
78
Summary
91
6.1
Sum m ary
6.2
Directions for future research
6.3
................
91
. . . . .
93
6.2.1
Wireless technology . . . . . . .
93
6.2.2
Hardware
. . . . . . . . . . . .
95
6.2.3
Operating system . . . . . . . .
96
Conclusions
. . . . . . . . . . . . . . .
96
A Configuration details for ultrasonic transducer using HE860
97
B Sample microcontroller program in assembly language and in C
99
B.1
Program in assembly language . . . . . . . . . . . . . . . . . . . . .
B.2
Program in C .....
.............
.....
....
...
..
C Latest version of the microcontroller code
C.1
Code to interface the ultrasonics . . . . . . . . . . . . . . . . . . . .
D ASCII version of CMI Communication Protocol
9
99
103
105
121
125
10
List of Figures
1-1
Architecture diagram for the wireless sensor network deployed on the Great
D uck Island [53].
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
19
1-2
Figure showing the MICA mote in an enclosure [53].
. . . . . . . . . . .
20
1-3
Figure showing the prototype sensing unit for WiMMS [37]. . . . . . . . .
21
1-4
Figure showing a tunnel boring machine used in the CTRL project [13].
22
1-5
Planned sensor installation in the tunnel.
. . . . . . . . . . . . . . . . .
25
1-6
Figure showing the layout of the wireless sensors to measure pressure . . .
26
2-1
Simplified model of an accelerometer [23].
. . . . . . . . . . . . . . . . .
28
2-2
Figure showing the parts of an LVDT [36]. . . . . . . . . . . . . . . . . .
31
2-3
The MPX2200AP pressure transducer [49].
. . . . . . . . . . . . . . . .
33
2-4
HE860 positioning device and HE 240 transducer head [25]. . . . . . . . .
34
2-5
E-152, ultrasonic transducer from Massa corporation [40]. . . . . . . . . .
35
2-6
Figure showing a MEMS accelerometer [3].
. . . . . . . . . . . . . . . .
37
2-7
Figure showing a fiber optic strain gage [59]..
. . . . . . . . . . . . . . .
40
3-1
Block diagram of Harvard and Neumann architectures.
3-2
Pin diagram of PIC16F877 [45].
4-1
Figure showing the data pulses for sending the character 'U' at a baud rate
of 9600...........
. . . . . . . . . .
42
. . . . . . . . . . . . . . . . . . . . . .
46
....................................
60
4-2
a) figure showing the MICA processor board and b) MICA sensor board [12]. 61
4-3
Figure showing i-Beans and the base station.
4-4
. . . . . . . . . . . . . . .
Linx RM series a)transmitter [34] b) receiver [33].
11
. . . . . . . . . . . .
63
64
5-1
Pin diagram of AD7713 [4].
. . . . . . . . . . . . . . . . . . . . . . . .
68
5-2
Interfacing the ADC with the microcontroller. . . . . . . . . . . . . . . .
70
5-3
Timing diagram for writing data to the ADC [4].
. . . . . . . . . . . . .
72
5-4
Timing diagram for reading data from the ADC [4]. . . . . . . . . . . . .
74
5-5
Interfacing the i-Bean with the microcontroller.
. . . . . . . . . . . . . .
76
5-6
Timing diagram for the i-Bean provided by Millennial Net, Inc. . . . . . .
77
5-7
Figure showing the interface between microcontroller and the ultrasonic
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
80
5-8
Figure showing the ultrasonic transducers in boxes. . . . . . . . . . . . .
82
5-9
Graph of the calibration experiment. . . . . . . . . . . . . . . . . . . . .
83
5-10 Schematic of the sensor board. . . . . . . . . . . . . . . . . . . . . . . .
85
5-11 Figure showing the sensor board and the wireless sensor unit. . . . . . . .
86
6-1
Figure showing the installed sensors in the tunnel. . . . . . . . . . . . . .
92
6-2
Figure showing the receiving unit and the laptop placed in the access shaft
transducers.
6-3
of the tunnel. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
93
Summary of the wireless sensor operation. . . . . . . . . . . . . . . . . .
94
12
List of Tables
5.1
Table showing the timing constraints provided by Millennial Net, Inc.
A. 1
Table showing the Configuration String
.
75
. . . . . . . . . . . . . . . . . .
97
A.2
Table showing the bits of the PCB for a Beacon . . . . . . . . . . . . . .
98
A.3
Table showing the bits of the PCB for a Pilot . . . . . . . . . . . . . . .
98
13
.
14
Chapter 1
Introduction
1.1
Motivation
Infrastructure 1 monitoring can be defined as the use of sensor technologies to characterize a system's condition, performance or response.
Civil infrastructure undergo a lot of changes in their structural and serviceability characteristics over time.
Some of the common reasons for these changes are
degradation of material properties, adverse loading and climatic conditions, improper
maintenance, etc. These changes may be gradual over time or can be sudden due
to events like earthquakes.
In order to ensure safety to the public; rehabilitation,
retrofit and repair of the infrastructure becomes necessary. To establish the need for
maintenance, the existing state of the structure needs to be known for applying the
techniques mentioned above.
Apart from the safety issue, economic factors are also important.
"Civil infras-
tructure systems are generally the most expensive investments/assets in any country
(an estimated $ 20 trillion in the United States) and these systems are deteriorating
at an alarming rate" [11].
For example, in the United States itself, nearly 50% of
the bridges were built before the 1940's. A survey in 1996 showed that 42% of the
bridges are functionally deficient or obsolete and the cost of correcting all of these
'The word infrastructure here refers to Civil infrastructure such as buildings, bridges, tunnels,
etc.
15
deficient bridges exceeds 90 billion dollars [41]. The nationwide maintenance cost for
civil infrastructure is estimated at 1.4 trillion dollars [24].
These figures show that
there is a strong necessity to evaluate the state of the existing infrastructure.
The state of the infrastructure can be predicted or estimated by monitoring and
other Non Destructive Evaluation (NDE) methods. A review of different methods
for structural health monitoring can be found here [14].
Monitoring also helps in
understanding the structure's performance and in studying the structural response
during hazardous or other events which might affect the performance of the structure.
Another scenario for infrastructure monitoring is the concept of I-city.
"The
concept of I-city, envisages an entire metropolis linked to a web-based monitoring
system" [58]. With the availability of cheap computing power and advances in sensor
technology, the concept of I-city is much more feasible.
Again, the chief task in
envisaging such a concept is monitoring.
These are just some of the reasons for structural monitoring. The goal of this
thesis is to explain the different components of a wireless sensor unit, which can be
used for wireless infrastructure monitoring. A specific example of a wireless sensor
unit is described in this thesis. This sensor unit is currently being used to monitor
the behavior of an aging underground train tunnel system in London, UK.
1.2
Wireless monitoring
Wireless infrastructure monitoring makes use of wireless sensor technologies. Wireless
sensors use wireless communications as a means to transmit data to and from a base
station or for inter-sensor communication. Most (if not all) of the monitoring systems
existing in Civil Engineering are wire-based systems. The primary purpose of the
wires is for data communications, and may also be used for powering the sensors.
Some of the important limitations of wire-based systems are:
1. It can be difficult to instrument an existing structure with a wire-based monitoring system. This is because of the practical difficulties associated with laying
out wires in an existing structure. Accessibility for laying out wires, and shield16
ing for the wires are major issues. For outdoor installations, the wires need
additional shielding from the harsh weather conditions.
2. Wire-based systems are expensive because of the initial cost associated with
laying out the wires and the maintenance costs associated with the system. To
install a wire-based system, installing the wires alone be approximately 25% of
the total system cost and 75 % of the installation time focussed solely on the
installation of system wires [37].
3. Wire-based systems require additional architectural and engineering design to
incorporate wires unobtrusively.
Wireless systems, to a great extent, are devoid of the above mentioned problems.
Wireless monitoring systems also have the added advantage of being modular. They
can be moved to new locations as needed. The availability of low-powered and cheap
computing power (microcontrollers, Digital Signal Processing (DSP) chips, etc.), Radio Frequency (RF) Integrated Circuits and the development of new wireless standards has fueled a lot of interest in wireless sensor systems. Wireless sensor technology
is listed as one of the promising ten technologies of the future [60].
A wide variety of research areas related to wireless sensors have emanated. These
include development of new energy-efficient wireless networking algorithms, real-time
distributed sensing and control, routing protocols for sensor networks, etc. On the
application side, a lot of research is being done in the use of wireless sensor systems
for structural and environment monitoring. In structural monitoring, wireless sensors
are used for sensing structural integrity during and after an earthquake [20], developing modular systems for monitoring bridges and buildings [38]. In environmental
monitoring, wireless sensors are used for monitoring rare and endangered species of
plants [54], wild life habitats [22], vineyards [63], etc. In the next couple of sections,
a few of these applications are described.
17
1.2.1
Great Duck Island project
The Great Duck Island habitat monitoring project is a testbed for wireless sensor
network technologies. The wireless sensors used are a custom version of the MICA
motes2 (which are discussed in chapter 4 of this thesis). Great Duck Island lies near
Desert Island, Maine. It approximately contains 5000 pairs of petrels '. The goal of
this project is to study the petrel usage pattern of nesting burrows and to monitor the
environmental changes during the breeding season. One of the primary requirement
of the sensor system was that it should be non-invasive [53]. Presently, around 50
weather motes (for monitoring weather) and around 70 burrow motes (to monitor
temperature, humidity, etc. of the burrows) are deployed.
A specially manufactured weather board was used for this project. This sensor
board contains photo-resistor (for light), temperature, barometric pressure, humidity
and passive infrared sensors (for occupancy). Figure 1-2 shows the MICA mote with
its enclosure. The enclosure is designed for robustness and minimum disruption in
the functionality of the mote.
Figure 1-1 shows the architecture diagram for the
monitoring project. More information about this project and live sensor data can be
found at the following site [22].
1.2.2
Wireless Modular Monitoring Systems (WiMMS)
The WiMMS group at Stanford University is concentrating on the development of
low-cost modular structure monitoring systems.
The focus is also on using a de-
centralized approach for monitoring. In a centralized approach, all the sensors report
to a base station 4. The base station is the unit which is responsible solely for taking
decisions, collecting data, etc. Sensors in this modality usually don't communicate
among themselves. In a de-centralized approach, the sensors have some intelligence
and they can communicate with other sensors and make autonomous decisions if
2
MICA motes are a compact wireless sensor module developed by University of California, Berkeley and now sold commercially by Crossbow Technology, Inc.
3
A petrel is a kind of seabird.
4
The base station need not be a sensor. It is usually a receiving unit connected to a PC or a
laptop.
18
Figure 1-1: Architecture diagram for the wireless sensor network deployed on the Great
Duck Island [53].
necessary [37].
The WiMMS group have developed a wireless sensing unit which has a MEMS 5based accelerometer, a wireless modem, a 16-bit Analog to Digital Converter (ADC)
and a computational core consisting of a 8-bit microcontroller. Figure 1-3 shows the
prototype sensing units. The group also has developed another sensing unit that has
the additional computational power of a 32-bit microcontroller. This increased power
of the microcontroller allows computation to calculate estimates for damage detection using statistical time-series algorithms. Damage detection is predicted from the
frequency response of the structure. The validation of the sensing unit was performed
on the Almosa Canyon Bridge [39]
5
Micro Electro-Mechanical Systems.
19
... ....
....
.
Figure 1-2: Figure showing the MICA mote in an enclosure [53].
The positive thing about these monitoring applications is that the research community is actively interested in using the latest technology to solve some of the problems in structural and environmental monitoring. Also, these applications serve as
good educational tools. Students can learn how complex systems behave in the real
world. A good example of such a project is the flagpole project at MIT. A virtual
laboratory was developed as a part of this project. The virtual laboratory was created
to " enhance the comprehension of concepts in structural dynamics, sensor technology and signal processing using a real-world structural system as a laboratory model"
[58].
The complex nature of civil engineering systems makes it difficult to develop monitoring systems which are more practical and useful. Also, the heterogeneous nature
of the infrastructure means each application is different from the other. Therefore,
20
Figure 1-3: Figure showing the prototype sensing unit for WiMMS [37].
wireless systems should be designed based on the application rather than designing
for a generic application. Thus, for the tunnel monitoring project, a new wireless sensor system had to be developed. Some of the reasons for this design are discussed in
the subsequent chapters of this thesis. In the next section, the London Underground
Limited tunnel monitoring project is described.
1.3
LUL tunnel monitoring project
The core of this thesis describes the design and development of a wireless sensor unit,
to be used in a tunnel monitoring application. The wireless sensor unit is intended for
deployment in a section of the London underground tunnel system during a disruptive
construction activity. The construction activity being the construction of a new tunnel
below the existing and operational London underground tunnel. The new tunnel is
being bored using a Tunnel Boring Machine (TBM, see figure 1-4).
Due to the
removal of material to accommodate the TBM movement, some displacement in the
existing tunnel is expected. These displacements should not exceed the critical value
that forces the tunnel to be shutdown.
The goal of this project is to use wireless
21
technology to measure these displacements in real-time. An additional objective is to
evaluate the potential of wireless technology for monitoring applications.
The new tunnel that is being constructed is a part of the Channel Tunnel Rail
Link (CTRL) Project. The CTRL is a high speed line running for 109 km between St.
Pancras station in London and the Channel Tunnel. This project is being constructed
in two sections named Section I and Section II. Section I was opened and became
operational in September 2003, while Section II is expected to be completed by the
end of 2006. The new tunnel is a part of the work comprising of construction of twin
7.5 km bored tunnels from Stratford station to King's Cross station [13]. The existing
tunnel's diameter is around 12 ft and is a section of the Victoria Line (Northbound).
The nearest station to the tunnel section of interest is Highbury & Islington. The new
tunnel will be perpendicular to the existing tunnel. See figure 1-5
Figure 1-4: Figure showing a tunnel boring machine used in the CTRL project [13].
22
1.3.1
Other Methods
A recently established method to monitor displacements due to excavation makes use
of electrolevels. This method is also used for monitoring the section of the tunnel
where the wireless sensor system will be deployed [28]. An electrolevel is a " robust,
stable, gravity referenced tilt transducer" [15]. Tilt can be thought of as being synonymous with slope. Electrolevels are used for measuring the distortion and alignment of
various parts of a structure [15]. Using electrolevels is one of the ways of to measure
displacements vertically in the direction of the tunnel. The tilts are integrated over
length to get vertical displacements in the direction of the tunnel. This method has
the following disadvantages.
" Errors get accumulated over the distance and are also exacerbated through the
integration process.
" System is not redundant in the sense that if one of the electrolevels fail, all
subsequent electrolevels become disconnected from the absolute reference frame.
Another potential way of measuring these displacements is to use fiber optics. The
research group at the Department of Civil Engineering, The University of Birmingham, UK is studying the use of fiber optic sensors for tunnel monitoring. The goal is
to develop a remote system for measuring tunnel settlement, distortion and rotation
of tunnels. Their current research is focused on developing a test rig for simulating
tunnel movements and measuring them using fiber optic sensors and strain gages [17].
Some of the limitations aforementioned are reduced by the wireless tunnel monitoring scheme proposed below.
1.3.2
Proposed Method
The two critical values that need to be measured continuously in this project are the
vertical displacements along the length of the tunnel and the transverse deformation
at the section where the tunnel is directly undercut by the TBM. This section is the
critical section. These displacements are calculated in the following manner.
23
1. Vertical displacements along the length of the tunnel: Measurements of the
relative change in the absolute pressure in a fixed hydraulic line are used for
calculating these displacements. The hydraulic line is laid out along the length
of the section of a tunnel (around 50 m of length) and is connected to a fixed
reservoir at a known reference location (in this case, this is the access shaft of
the tunnel).
The relative (with respect to a fixed point) pressure variation in
the tube is then proportional to the vertical displacement. Absolute pressure at
a few points along the length of the tunnel is measured using pressure transducers. The absolute pressure is measured with respect to the fixed reservoir. This
is depicted in figure 1-6. At these points, the boxes containing the sensor boards
are fixed. These points are approximately placed every 5 meters. The sensor
farthest from the construction activity is assumed 6 to be the fixed point and is
used as a reference for calculating the displacements. After taking the pressure
measurements, these data are sent wirelessly over to the base station. Temperature is also measured on the sensor board to correct the pressure transducer
measurements for temperature.
These adjustments are based on calibration
data previously determined in a controlled laboratory setting.
2. Transverse deformation at the critical section: Two ultrasonic transducers facing each other are to be installed in the tunnel for calculating the transverse
displacements at the critical section. The transducers measure the time of flight
for an acoustic signal between the pair and use this measurement to calculate
the distance between them. Any change in the transducer measurements indicate movement of the transducers or deformation at the critical section '. After
the distance between the two transducers is calculated, the distance readings
are sent wirelessly to the base station. Figure 1-5 shows how the sensors are to
be installed in the tunnel.
6
This point is assumed to be fixed. If this point moves, the displacements calculated will be
relative to the displacement of this fixed point.
7
This change in transducer measurements could be also due to a change in temperature, pressure
relative humidity etc. Calibration for these factors is also necessary. Only preliminary calibration has
been performed to date, and additional benchmarking remains necessary for accurate measurement.
24
Critical
Section
50 m (monitoring distance)
12 ft
Highbury &Islington
Nearest Station (Circle Line)
loin
D)
10
Hydraulic Line & Wireless
Sensor Node
Ultrasonic Transducer
Channel Tunnel
Rail Link (CTRL)
Figure 1-5: Planned sensor installation in the tunnel.
The proposed scheme is considered better than that of the electrolevels for the
following reasons:
" The system is redundant in the sense if one of the pressure transducers fail, it is
still possible to calculate displacements. Although, if the hydraulic line breaks,
the system also fails.
" It is easier to install this system.
* It is relatively cheaper and reusable for other similar applications.
1.4
Present work
In this thesis, a detailed discussion of the different components of a wireless sensor
unit for infrastructure monitoring and integration of these components to construct
the wireless sensor unit are presented.
Chapter 2 discusses the various sensors that are commonly used in civil infrastructure monitoring and then talks about the sensors used in this project.
25
Reservoir
()
Pressure sensor
with RF module
Hydraulic
Line
Figure 1-6: Figure showing the layout of the wireless sensors to measure pressure
Chapter 3 discusses microcontroller architectures and specifically PIC16F877,
which has been used in this project. The architecture, memory details and other
relevant features of this microcontroller are explained.
Chapter 4 discusses the various forms of communication arising in this project.
In particular this chapter addresses synchronous and asynchronous (wire-based communications) communication standards and some relevant wireless technologies that
have been tested during the course of this project.
Chapter 5 describes how the various components were integrated so as to construct
the wireless sensor unit.
Chapter 6 summarizes the thesis and states the directions for future research.
26
Chapter 2
Sensors
This chapter gives a brief overview of some sensor types used for various monitoring applications.
These include the accelerometer, strain gage and Linear Variable
Differential Transformer (LVDT). Following this, a brief overview of the sensors used
for the tunnel monitoring project is given. In the later sections a brief discussion on
MEMS and fiber optic sensors follows.
2.1
Sensors
A sensor is a device which measures a physical quantity. Sensors are typically used
for measuring the parameters which affect a system's state. Common examples include temperature sensors, pressure sensors, etc. For monitoring applications, sensors
provide the data by measuring various physical parameters over time. A transducer
is a subset of a sensor; a transducer converts the physical quantity measured into an
electrical signal proportional to the value of the measured physical parameter [50].
The electrical output of the transducer (which is an analog signal) can be digitized
and can be processed using a microcontroller or a Digital Signal Processing (DSP)
chip. To convert an analog signal to a digital signal, an Analog to Digital Converter
(ADC) is typically used.
In the next couple of sections, a brief description of some of the more commonly
used sensors in monitoring applications for Civil Engineering systems is given. These
27
are accelerometers, strain gages and LVDT.
2.2
Accelerometers
As the name suggests, accelerometers or acceleration transducers measure acceleration. Measures of acceleration can also be interpreted to detect tilt, vibration and
shock [50]. Linear acceleration is measured in m/s 2 , or often expressed in terms of
g's, where g is the gravitational constant. The standard value of g is 9.81m/s 2 . There
are many types of accelerometers and a few of which are discussed below.
2.2.1
Sensing Method
Accelerometers use a "sensing method in which the acceleration acts upon a seismic
mass (proof mass) that is restrained by a spring and whose motion is usually damped
in a spring-mass system" (figure 2-1). Acceleration is proportional to the distance
moved by the proof mass from its reference position [23].
Spring
Damper
Mass
Figure 2-1: Simplified model of an accelerometer [23].
28
2.2.2
Capacitive accelerometers
In this type of accelerometers, the proof mass is a freely suspended electrode, which
moves with respect to a fixed electrode or stator plate. When acceleration is applied,
the distance between the electrodes change and this changes the capacitance. This
change in capacitance measured is proportional to the value of the acceleration [50].
2.2.3
Piezoelectric accelerometers
The basic principle of these accelerometers is piezoelectric transduction. The proof
mass is connected to a piezoelectric crystal. The piezoelectric crystal is pre-loaded and
this loading changes whenever acceleration is experienced. This changes the electrical
output of the piezo-material. This gives a relative measure of the acceleration. Typical
piezoelectric materials include quartz crystals, Barium Titanate, etc. Piezoelectric
transducers usually have a low output signal and very high output impedance. This
requires the use of amplifiers which not only magnify the signal, but also act as
impedance converters [50].
2.2.4
Potentiometric accelerometers
These accelerometers can be produced at a relatively low cost, have moderate, but
acceptable accuracy and a limited frequency range. The displacement of the springmass system causes a displacement of a potentiometric element '.
This change in
resistance is measured to get the value of the acceleration.
Details about other types of accelerometers can be found at [50] and [23]. In the
next section, strain gages are discussed
1
Potentiometric element has a potentiometer and the resistance of this element changes because
of the displacement.
29
2.3
Strain gages
Strain is defined as "the ratio of dimensional change to the total value of the dimension
in which the change occurs" [50]. Since strain is dimensionless (as it is a ratio of two
physical quantities having the same units), there is no specific unit for strain. It is
usually measured in percent or microstrain.
Strain is usually measured using piezoresistive materials. Piezoresistive substances
undergo a change in their resistance when a stress is applied. Stress is related to
strain by the modulus of elasticity, and is defined as the ratio of stress to strain. The
sensitivity of a strain gage is measured by the gage factor which is defined as the ratio
of unit change in resistance to the ratio of unit change in length.
GageFactor,GF =AR/R = AR/R
AL/L
C
The resistance change of a strain gage is converted into voltage using a Wheatstone
bridge [50]. Details about different kinds of strain gages can be found at [50] and [23].
2.4
Linear Variable Differential Transformer
Linear Variable Differential Transformer (LVDT) is used for measuring displacement.
The absence of physical contact of the sensing element gives the LVDT a high degree
of robustness [35]. The LVDT consists of one primary and two secondary magnetic
coils and a magnetic core. In its null position, the magnetic field in the two secondary
coils gets canceled. Hence, at its null position, the output of a LVDT is zero volts.
When the magnetic core is displaced, there is be an imbalance in the electromagnetic
field in the two secondary coils. This imbalance in electromagnetic field causes a
differential output voltage that is proportional to the displacement and the direction
of displacement. Apart from being a very rugged instrument, an LVDT has a high
resolution [36]. Figure 2-2 shows the different parts of an LVDT.
In the next section the sensors used in the London underground tunnel project
are discussed.
30
COIL WINDING ASSEMBLY
MAGNETIC CORE
ELECTROMAGNETIC SHIELDING
CYLINDRICAL CASE
Figure 2-2: Figure showing the parts of an LVDT [36].
2.5
Sensors used in the tunnel monitoring project
The sensors used in the tunnel monitoring project are the pressure transducer and the
ultrasonic transducers. Though ultrasonic transducers haven't yet been installed in
the tunnel, they are described for completeness. It is anticipated that the ultrasonics
will be deployed in future monitoring projects.
2.5.1
Pressure transducers
Pressure transducer measurements could be absolute (i.e. with reference to a vacuum or zero pressure) or relative (with respect to atmosphere). Pressure transducers
are widely used in pressure control systems, flow measurement systems, vehicle air
bag deployment, etc [50].
There are different types of pressure transducers. Com-
mon types are piezoelectric pressure transducers, strain-gage pressure transducers,
31
capacitive pressure transducer, etc.
Information about different types of pressure
transducers can be found here [50].
The pressure transducer used in the project is
the MPX2200AP from Motorola Corporation (shown in figure 2-3).
This pressure
transducer is a silicon piezoresistive sensor.
A piezoresistive substance changes its resistance when it is stressed. In a piezoresistive transducer, when there is a change in pressure at the input of the transducer,
the stress on the piezoresistive material also changes and hence its resistance. This
change in resistance is usually detected as a differential voltage across a bridge circuit.
Some of the advantages of using a mono-crystalline semiconductor as the piezoresistive element are:
* "High sensitivity > 10 mV/V" [52].
0 "Good linearity at a constant temperature" and the "ability to track pressure
changes without hysteresis" [52].
Some of the disadvantages are large initial offset, strong non-linear dependence
on temperature and strong drift with temperature. These can be compensated with
proper electronic circuitry. More details can be found here [52].
The MPX2200AP measures absolute pressure (i.e.
or zero pressure).
with reference to a vacuum
It has a good linearity (±0.25 % linearity) and is temperature
compensated over the range of 00 C to 85 C. Other details of this transducer could
be found here [49].
The differential outputs of the transducer are connected to an
ADC. The ADC converts the analog output of the transducer to digital format. The
digital output from the ADC is collected by the microcontroller.
These details are
discussed in Chapter 5.
In the next section, the ultrasonic transducers are discussed.
2.5.2
Ultrasonic transducers
Ultrasonic waves are those sound waves whose frequency is greater than 20 kHz.
Ultrasonic transducers can be classified into two kinds. They are:
32
Figure 2-3: The MPX2200AP pressure transducer [49].
" Magnetostrictive transducers, which are mainly used in ultrasonic cleaning
baths (used by watchmakers and in the electronics industry) [57].
" Piezoelectric transducers, they have low power consumption than the magnetostrictive transducers and are used in security devices etc. [57].
There are two important parameters related to ultrasonic transducers. They are
beam spread and target angle. Beam spread defines the maximum spreading of the
ultrasonic sound beam from the transducer. The target angle measures the maximum
amount by which the target can be tilted and still be detected by the ultrasonic
transducer [57].
Ultrasonic transducers are used to calculate distances by using the time of flight
principle. The precision of ultrasonic transducer system allows it to be used to measure small movements (< 1 mm) [25]. The idea of using ultrasonic transducers is to
measure deformations at the critical section. That is, the cross-section of the tunnel
below which the tunnel boring machine will pass. This is where the largest vertical
33
displacement and related deformation of the tunnel liner occur. The deformations
might be in the order of 2 mm or less [56]. The original idea was to place ultrasonic
transducers along the circumference of the tunnel and calculate the distance between
any two pair of transducers and observe any changes in these distance measurements
over time. The change in distance is an indication of relative deformation at the
critical cross-section 2
The transducers that were chosen in order to achieve the goal were the HE240
series of transducers from Hexamite Devices (shown in figure 2-4). In addition to
the transducers, a positioning device is needed (for each transducer) which has the
necessary electronic and control circuitry for operating the ultrasonic transducers.
The positioning device measures the time of flight and calculates the distance from
this measurement and sends this measurement over a RS-485 network to a controlling
device. The positioning device that was chosen was HE860 (from Hexamite Devices)
[25].
Figure 2-4: HE860 positioning device and HE 240 transducer head [25].
One of the main obstacles in using the transducers was the problem of unstable
readings. The transducers gave very unstable readings when they were not facing each
other and moderately stable readings when they were directly facing each other. The
2
This is not strictly true as temperature and other factors might affect the distance measurements
made by the ultrasonic transducers
34
reason for this might be a narrow beam angle of the Hexamite ultrasonic transducer.
This problem was partially solved when a transducer head from another company
(Massa Corporation, figure 2-5) was used [40]. The probable reason for this could
be a wider beam angle of the Massa transducer head than the Hexamite transducer
head. Because of the narrow beam angle, it was decided that only two transducers
will be used,and these transducers will be installed such that they are facing each
other directly (see figure 1-5). Some of the results of the tests that were done using
the transducers from these two different companies are discussed in Chapter 5.
Figure 2-5: E-152, ultrasonic transducer from Massa corporation [40].
Each ultrasonic transducer is configured either as a pilot or a beacon. One of the
pilot nodes (called the master node), initiates a timing distance acquisition signal.
The master node transmits a synchronization signal (electrical) and measures the
time to receive an ultrasonic wave from the beacons. This measured time gives the
distance between the pilot and the beacon nodes [25].
Chapter 5 gives the details
of the interface between the microcontroller and the ultrasonic transducer. Details
of configuring the ultrasonic transducers using the positioning device are given in
Appendix A.
In the next two sections, two (relatively) new sensor technologies are briefly discussed. These are Micro Electro-Mechanical Systems (MEMS) and Fiber Optic sen-
35
sors. These new technologies will play an important role in the future monitoring
applications.
2.6
MEMS
Micro Electro-Mechanical Systems or MEMS "is the integration of mechanical elements, actuators, and electronics on a common silicon substrate through the use of
microfabrication technology" [23] . As evident from the name, the size of MEMS devices are on the order of the microns (10-6). Since bulk fabrication techniques from
the micro-electronic devices can be applied (lithography, etching, micro-machining,
etc. ), mass fabrication of these devices make them potentially cheap and more reliable than their macro-mechanical counterparts. Three of the main application areas
of MEMS are:
* Biotechnology, applications of MEMS in biotechnology include developing biochips
for detecting hazardous chemicals and biological agents and developing microsystems for drug delivery [42].
" Communications, the focus in this area is to develop MEMS-based electronic
components that can be used for making low-power and low-cost, high-frequency
RF circuits. An important use for this is in making light-weight cellular phones
[42]. Other important application is developing micro-mirrors for telecommunications.
" Accelerometers, low-cost MEMS accelerometers are being used for crash air-bag
systems in automobiles [42]. MEMS (figure 2-6) accelerometers are also being
used in wireless sensors. A particular example of this is the MICA mote developed by University of California, Berkeley and Crossbow Corporation. Details
of the MICA mote are provided in Chapter 4.
MEMS have great potential in monitoring applications providing low-cost and
low-power sensors. Unfortunately, there are some challenges in using this technology
widely. They are:
36
Figure 2-6: Figure showing a MEMS accelerometer [3].
1. For manufacturing or prototyping MEMS devices, micro-fabrication technology
is required and this fabrication technology is not easily accessible.
Building
such a fabrication unit is very expensive [42].
2. Packaging MEMS is more difficult than packaging ICs. One reason is protection
against the harsh environments in which MEMS devices may be used. This is
because of the harsh environments in which MEMS devices may be used. Also,
the packaging has to be customized for different applications. Packaging MEMS
devices is one of the major obstacle the MEMS community is facing [42].
3. Numerical and simulating tools for MEMS are very limited [42].
4. In order to design a MEMS device, the designer would need an extensive knowledge of fabrication techniques. The process of fabricating even a simple MEMS
37
device requires a lot of effort and research [42].
More details on MEMS can be found here [42].
2.6.1
Fiber optic sensors
This section focuses on fiber optic sensors. Fiber optic sensors have existed for quite
some time (past 10 years approximately) and hence is not a new technology. "Fiber
optics (optical fibers) are long, thin strands of very pure glass about the diameter of a
human hair. They are arranged in bundles called optical cables and used to transmit
light signals over long distances" [27]. Fiber optic sensors can be broadly classified as
extrinsic and intrinsic sensors. Extrinsic sensors use an optical fiber to convey light to
a sensing element and use the same or another optical fiber to convey the processed
light to a photo detector. On the other hand, intrinsic sensors are those sensors, which
use a sensing mechanism that is a part of the optical fiber [41]. Another classification
of fiber optics sensors is to classify them on the property of light that is affected by
the sensing mechanism. They are:
" Intensiometric sensors: these sensors use the variation of light power transmitted
through optical fiber for sensing mechanism [41].
* Interferometric sensors: these sensors use the changes in phase of the light as
the sensing mechanism [41].
" Polarimetric sensors: these sensors measure temperature or strain by detecting
the changes in polarization eigenmodes of an optical fiber [41].
" Modalmetric sensors: these sensors use the changes in "transverse spatial mode
distribution of light" as the sensing mechanism [41]
" Spectrometric sensors: these sensors use Raman and Brillouin scattering mechanisms for determining temperature profiles [41].
38
Details of these mechanisms is beyond the scope of the thesis and more information
could be found here [41]. Some of the important applications (related to monitoring)
of fiber optic sensors are:
1. For bridge monitoring fiber optic sensors are potentially useful in performing
the following tasks [41].
(a) Fiber optic sensors could be used in evaluating the long term performance of new construction materials like Carbon Fiber Reinforced Polymers (CFRP).
(b) For studying the wind and seismic response of bridges by integrating Fiber
Reinforced Polymer (FRP) strengthening elements with fiber optic sensing
technology. This can also be used for studying bridge vibration due to
traffic loading and other environmental conditions.
2. Fiber optic sensors can be used to measure other mechanical and thermal information. For example, a fiber optic strain gage is shown in figure 2-7. They can
also be used as both the sensor and the data channel which has such uses such
as measuring strain and temperature information from any point of its length
[41].
(a) They could be used for monitoring the degree of cure and temperature
of concrete structures and hence indicate when it is appropriate to posttension or moving the concrete structure [41].
Important considerations that should be kept in mind while using this technology
is the integrity of fiber optic sensors such that installation should not affect the light
properties adversely. More details on fiber optic sensors can be found here [41]
Smart sensors are those sensors which integrate some kind of intelligence from a
microcontroller, DSP chip, or an application-specific integrated circuit (ASIC) [18].
The wireless sensor units developed for the tunnel monitoring project are smart sensors as they have integrated microcontrollers. In the next chapter, the brain of the
wireless sensor unit, the microcontroller is discussed.
39
Figure 2-7: Figure showing a fiber optic strain gage [59].
40
Chapter 3
Microcontroller
A Microcontroller is a miniature computer. It is an Integrated Chip (IC) that has a
Central Processing Unit (CPU), Random Access Memory (RAM), Read Only Memory
(ROM) and other components that are also present in a computer. The present chapter describes the PIC 'family of microcontroller in detail. Specifically, the PIC16F877
microcontroller is examined, which is used in this project. The PIC microcontroller
is manufactured by Microchip Technology, Inc.
In this chapter, the architecture and instruction set for the PIC mid-range devices
are described. In the architecture section, the clocking scheme, the memory organization and pin description of PIC16F877 microcontroller are discussed.
Specific
features of the PIC16F877 microcontroller are described and finally the operation of
the microcontroller on the sensor board is described.
3.1
Architecture
This section explains the architecture of PIC mid-range microcontrollers
2.
The clock-
ing scheme, memory organization and pin description of the PIC16F877 microcontroller are described.
iPIC stands for Peripheral Interface Controller. PICgis a registered trademark of Microchip
Technology, Inc.
2PIC microcontrollers which have 14 bit length instructions are classified as mid-range
microcontrollers
41
Many of the present day microcontrollers (including the PIC16F877) have a Harvard Architecture. In Harvard Architecture, the program memory and data memory
reside separately in two different blocks of memory. While in a Neumann Architecture, the program and data memory reside in the same block of memory. This is
depicted in the figure 3-1, which shows the block diagrams of both the architectures.
As seen in the figure below, in a Neumann Architecture a single or a shared bus is
used to access data and program memory, as they reside in the same block of the
memory. In Harvard Architecture separate/different buses are used for accessing the
program and data memory. This allows execution of an instruction using data from
the data memory and fetching of the next instruction from the program memory[31]
in the same instruction cycle.
DATA
MEMORY
CPU
M
PROGRAM
MEMORY
MMR
PROGRAM
MEMORY
CPU
DATA
MEMORY
a) Harvard Architecture
b) Neumann Architecture
Figure 3-1: Block diagram of Harvard and Neumann architectures.
Apart from having a Harvard Architecture, the PIC16F877 has some common features that are found in other RISC (Reduced Instruction Set Computers) microcon-
trollers3 [47]. They are discussed below:
" Long word instructions, each instruction is 14 bits long for a PIC16F877 microcontroller.
" Single word instructions: the microcontroller has a 14 bit program memory bus
which can access the program memory. Since the instructions are single word
3
RISC microcontrollers have an instruction set comprising of very few instructions.
PIC16F877 microcontroller has only 35 instructions.
42
The
instructions, all the locations in the program memory are all valid instructions.
This leads to a better utilization of the program memory. This also implies that
the microcontroller has to read just a single memory location in the program
memory for executing an instruction.
3.1.1
Clocking scheme
In this section, the clocking scheme and instruction cycle of the PIC microcontroller
are described. A microcontroller needs a clock input to synchronize and execute the
instructions in its program memory. The external clock input to the microcontroller is
through pins OSC1 and OSC2. The PIC16F877 microcontroller communicates with
the external world using the pins. The pin description of the PIC16F877 is given
later. The four valid clock modes are4 described below:
1. RC mode. In this mode, an external resistor and a capacitor circuit provides
a clock input to the microcontroller on the OSCI pin. The frequency of the
clock input is a function of the supply voltage, and the values of the resistor
and capacitor. The device data sheet has more details regarding the range of
resistors and capacitors that can be used [45]. In this mode, the microcontroller
outputs a clock pulse on the OSC2 pin, which can be used for clocking other
devices. The frequency of this clock pulse is one-fourth of the frequency of the
RC clock signal on the OSC1 pin.
2. LP mode.
In this mode, the microcontroller uses an external crystal of low
speed (usually less than 2 MHz, for exact values, see [45]). Of all the 4 modes,
this mode consumes the least power.
3. XT mode. In this mode, the microcontroller uses an external crystal. Typical
clock speeds include 2-4 MHz [45].
4. HS mode. In this mode, the microcontroller uses an external crystal. The clock
4
All the modes that are discussed here are the external clocking modes. Some microcontrollers
have internal oscillators/clocks. The PIC16F877 does not have an internal oscillator.
43
speeds are usually greater than 4MHz. Of all the 4 modes, this mode consumes
the highest power.
The clock input from the OSCI pin of the microcontroller is formed into four
non-overlapping quadrature clock cycles namely Q1, Q2,
Q3 and Q4. These four
clock cycles together form an instruction cycle for the microcontroller.
During an
instruction cycle, an instruction is executed and the next instruction is fetched simultaneously. Hence, each instruction of the microcontroller takes a time of 4/F..c
seconds, where F,,, is the frequency of the clock input to the microcontroller. Complex instructions like GOTO, which branch the program take more than one instruction
cycle to execute [45]. Understanding clocking scheme and the instruction cycle helps
in writing better programs to program the microcontroller.
3.1.2
Memory Organization
In this section, the memory organization of the PIC16F877 is discussed. The memory of a PIC microcontroller is divided into two main components.
They are the
program memory and the data memory. The PIC16F877 is an Electrically Erasable
Programmable Read Only Memory (EEPROM) device (also known as a flash microcontroller) [45].
Program Memory
The program memory of the PIC16F877 microcontroller is an EEPROM. The size
of this memory is 8K X14 bits (as each instruction is 14 bits long). This program
memory is divided into four equally sized blocks called pages. The two important
locations in the program memory are:
* Reset vector. This is the address at which the microcontroller looks for instructions whenever there is a device reset or whenever the microcontroller starts.
The address of the reset vector is Oh 5.
5
0h is 0 in hexadecimal notation.
44
* Interrupt vector. This is the address to which the program branches whenever there is an enabled interrupt. Interrupts are special events which might
have to be serviced depending upon the application. Common examples are
timer interrupts, serial port interrupts, etc. In order to service an interrupt,
the interrupt enable flag for that particular interrupt and the global interrupt
flag should be set. If the aforementioned flags are set, whenever the interrupt
occurs, the microcontroller services the interrupt by making the program jump
to the interrupt vector. Enabling and disabling of the interrupts is done in the
program.
Data Memory
Data Memory can be divided into three categories. They are:
1. General Purpose Registers. General Purpose Registers (GPR) and the Special
Function registers (SFR) form the RAM (Random Access Memory)6 of the microcontroller. The size of this RAM is 368 bytes for PIC16F877 [45]. GPR are
used for storing the variables and other temporary values that are used in the
program.
2. Special Function Registers.
The SFR controls the various core and periph-
eral features of the microcontroller. The core features are the features necessary for the microcontroller to operate. These include the STATUS, PCL, etc.
The peripheral features include features like the Analog to Digital Converter
(ADC), USART (Universal Synchronous Asynchronous Receiver Transmitter),
etc. These features are explained later in this chapter.
3. Data EEPROM. The Data EEPROM is an EEPROM. This can be used for
storing permanent data or some other valuable information.
The microcon-
troller uses a protocol for writing and reading data from the data EEPROM
6
Similar to the computer's RAM, the microcontroller's RAM is volatile. After a device reset all
the values in the RAM registers are reset to their reset values.
45
so that the existing data is not written over accidentally. PIC16F877's data
EEPROM has 256 locations and the size of each location is 1 byte [45].
3.1.3
Pin Description
As stated earlier in this chapter, the microcontroller communicates with the external world using its pins. In this section, the pin layout of PIC16F877 is described.
Figure 3-2 shows the pin diagram of PIC16F877. These can broadly be classified as
Input/Output (I/O) pins and power pins. Each class is now described.
TIONPP/THw 1
RAW/ANO
2
m(
RA1/AN1
3
rs-a[
RA24AN2NREF-..( 4
R A3ANNRIF +
6
RAC/TOCKI
e[
6
RAFWAN4/b"7I
r--[7
REBM"EAN5 S8
REE1/WRAN6
m[
9
RE2CIAN7
L 10
VDD -B -[
11
12
OSC1UCLKIN
13
-[
OSC2/CLKOUrT
r---[14
RCT10SO/T1CKI ,m-E [R is
RC1ITISVCCP2 "--[
16
RC2VCCP1
L-[ 17
RC3SCKISCL
18
[
RDCVPSP0 e--[
19
RD1/PSP1
mC 20
K740
39 J -38
37 ]]
'4--b
---
36 ) S- --.
35 j
34
_
r
CD
CD
33
32
R
D
R87NPGD
RBGJPGC
RBS
RB4
RB3jPGM~
RB2
RB:1
RB0.1NT
VD
-3
31B ]
30
29
28 )3-a
27 :]1] 4----- ,
26 3 s-a
-a
25V]3
24
N/SS
23
RC4/SDIX-RDA
RD30PSP3
RD)+PSP2
22
21
3]--.
RD7,rPSP7
RDGPSP6
RDWPS
RD4VPSP4
RC7/RXQDT
RC&TX(CK
RMsSDO
Figure 3-2: Pin diagram of PIC16F877 [45].
I/O pins
I/O ports are the ports by which the microcontroller communicates with external
devices for I/O operations. The PIC16F877 has 40 pins, of which 33 are I/O pins.
Most of the I/O pins are multiplexed to perform other functions. The 33 I/O pins are
divided into 5 ports named A,B,C,D and E. The registers corresponding to these ports
46
are PORTA, PORTB, PORTC, PORTD and PORTE respectively [45]. An I/O pin
can be either configured as input or output by setting or clearing7 the corresponding
bit in a corresponding register.
These registers are referred by the prefix TRIS.
(TRISA corresponding to PORTA and so on). TRIS stands for tri-state. As the name
suggests, tri-state devices have three states. These are logical 1, logical 0 (when the
device is in its output mode) and off state (when the device is in its input mode). For
example, to configure PIN6 of PORTB as an input pin, bit 6 of the TRISB register
has to be set high or logical 1.
Power pins and other pins
These pins correspond to the pins which are either connected to the power or ground
lines.
The other pins apart from the power pins are the pins for the clock input,
which are OSCI and OSC2.
3.2
Features
In this section, some of the important features of the microcontroller are described.
These features are not the complete set of features. Details of other features can be
found here [45]. The features that are now described are:
1. Universal Synchronous Asynchronous Receiver Transmitter (USART) module
2. Analog to Digital Converter (ADC)
3. Master Synchronous Serial Port (MSSP) module
4. Timer and interrupt features
7
By set we mean writing a "1" and by clearing we mean writing a "0" at the appropriate memory
location.
47
3.2.1
Universal Synchronous Asynchronous Receiver Transmitter (USART) module
PIC16F877 has an inbuilt module which can be used for synchronous and asynchronous serial communications. Serial communications are explained in detail in the
next chapter. USART module can be used for the commonly used asynchronous serial communication standard, RS-232. This standard is used to connect with a PC or
other RS-232 compatible devices. The USART module is also used for synchronous
serial communications for talking to devices like Analog to Digital Converters (ADCs)
and other microcontrollers. Some of the standards for synchronous serial communication are also described in the next chapter.
Serial communication can be done either in hardware by using the USART module
or in software by writing the appropriate code which would generate the required data
pulses. Using the hardware USART has the advantage of using the interrupt features
of the microcontroller. An example of such an interrupt is the interrupt generated
when the microcontroller receives a byte of data in the asynchronous mode.
The
software mode does not have these features, but this mode gives the flexibility of
choosing any 2 I/O pins for serial communication. The hardware USART restricts
this to pins RC6 and RC7 on the PIC16F877 [45]
3.2.2
Analog to Digital Converter
An Analog to Digital Converter (ADC) converts an analog input signal into a digital
output. The PIC16F877 has an inbuilt ADC, which has a 10-bit resolution. It has
8 input analog channels. These channels are multiplexed with PORTA and PORTE
pins of the microcontroller. The 10-bit resolution is sufficient for many applications
(10-bit resolution provides a resolution of 0.1% over the range of the ADC). The input
signal to the analog channel is a voltage which is usually in the range of 0 to 5 volts.
Other possible ranges and configurations in which the ADC can be used are found in
the device data sheet [45].
48
3.2.3
Master Synchronous Serial Port (MSSP) module
This module is used for communicating in synchronous serial communication standards such as Serial Peripheral Interface (SPI) and IIC (12C). Protocols, based on
these standards are also commonly used for interfacing devices. These standards are
explained in Chapter 4. The use of MSSP module makes the application of these
protocols quite easy.
3.2.4
Timer and interrupt features
Other important features of the microcontroller are the timers and interrupts. The
PIC16F877 has 3 inbuilt timers. These are TimerO, Timeri and Timer2. These
timers can be used for measuring the amount of time elapsed between any two events
(need not be necessarily related to microcontroller internal events) and other timekeeping functions.
Interrupts allow the microcontroller to be notified of important events. These
include data reception on the USART receiving pin, completion of an analog to digital
conversion, etc. As mentioned earlier, the microcontroller should enable the relevant
interrupt flag (for a particular interrupt) and the global interrupt flag so that the
microcontroller can be interrupted when the particular interrupt occurs. Using the
timer and interrupt features one can measure an interval of time. An example to
measure an interval of 5 seconds is provided.
For example, if a 4 MHz oscillator is used, each instruction takes a time of 1
microsecond to execute. Using the Timer2 scaled down by a factor of 16 (so that
time interval between 2 ticks of Timer2 is 1x16 = 16 microseconds), a reset value
of 250 (i.e. timer resets itself to zero after it reaches 250 and starts from 0 again)
and a post-scaling factor of 10 (i.e. after every 10 resets, an interrupt is generated),
therefore, the time interval between any two Timer2 interrupts is 16 x 250 x 10 = 40
milliseconds. 125 such interrupts make 5 seconds. So a counter is used that keeps
track of the number of interrupts that have occurred. The counter can be reset once
5 seconds have elapsed. In this way, one can use the timer and interrupt features to
49
measure 5 seconds.
3.3
Instruction Set
The microcontroller needs to be programmed for it to perform any task.
The in-
structions to the microcontroller are written in hexadecimal format which is not a
convenient way (for humans) to code the microcontroller. So one has to use either
an assembly language or a high-level language like C or BASIC to write the program.
This program is then compiled by a special compiler which compiles it into the hexadecimal format, which the microcontroller understands. A programmer then burns
(electrically) this program onto the microcontroller's program memory. The following
sub section discuss assembly language programming and programming in C.
3.3.1
Programming in assembly language
The assembly language for the PIC microcontroller is known as MPASM 8. It was developed by Microchip Technology, Inc., which manufactures the PIC microcontrollers.
The PIC16F877 is a RISC microcontroller and has just 35 instructions [45].
These
instructions can be broadly classified as:
* Control Operations, which control the program flow and other basic features of
the microcontroller.
" Byte-oriented instructions, dealing with addition, subtraction or other arithmetic or logical operations on the whole register (as a register is 1 byte long).
" Bit-oriented instruction, dealing with setting or clearing of a particular bit in a
register.
" Literal operations, dealing with the operations which involve constants/literals.
Appendix B gives an example of a sample program in assembly language.
8
MPASM @is a registered trademark of Microchip Technology, Inc.
50
3.3.2
Programming in C
Programming in C is simpler and easier compared to programming in assembly language. But an understanding of the assembly language is definitely an advantage in
programming the microcontroller. There are commercial C compilers for programming
the PIC microcontrollers. Some commonly used C compilers are CCS [8], Hi-Tech [26],
etc. Programming in C for small programs may not lead to a proper utilization of the
program memory [19]. This depends on the compiler and most of the compilers have
various optimization levels to achieve a better efficient usage of the program memory.
The main advantage of programming in C is the ease of programming. Appendix B
also gives an example of a sample program in C programming language.
Operation of the microcontroller
3.4
This section discusses an example operation of a microcontroller in a wireless sensor
unit. The microcontroller can perform the following functions.
1. Periodically collect data from an external ADC, which is usually connected to
a sensor. The microcontroller can also use its in-built ADC to collect data from
sensors.
2. Packetize this data. The microcontroller after collecting the above readings/data
composes a string which has other relevant information like the ID of the sensor board, the length of payload and checksum bytes in addition to the readings/data.
3. Transfer the message string to a RF transceiver. The microcontroller after packetizing the data sends the data to the RF transceiver, which then communicates
the readings to the base station.
This chapter described some of the basic features of the PIC16F877 microcontroller.
Chapter 5, gives the details of the interface between the PIC16F877 and
51
other external devices. In Chapter 4, communication aspects arising in the development of the wireless sensor unit are described.
52
Chapter 4
Communications
This chapter discusses the communications component of the wireless sensor unit.
At a broad level, this chapter discusses wire-based and wireless communications and
some of the standards and technologies related to these. In wire-based communications, synchronous and asynchronous serial communications are described. In wireless
communications, different RF technologies considered for the developing the wireless
sensor unit are described.
4.1
Wire-based systems
Wire-based systems use a wire or a cable as a medium for transferring the data. For
example, the telephone system at MIT. Serial communication, by definition sends data
bit by bit. Telephone modem is an example of a device that uses serial communication.
Synchronous and Asynchronous serial communications are described now.
4.1.1
Synchronous serial communications
In this section, synchronous serial communications at the microcontroller level are
discussed. At this level, for the synchronization of devices, one of the devices, provides
the clock signal. This clock signal is used for timing the data pulses. The device which
provides the clock signal is the master device and the rest of them are slaves. The
53
master device coordinates the communication between other devices and itself.
Two important standards for synchronous serial communications, which are used
for inter-device communications are Serial Peripheral Interface (SPI) and Inter IC
Communication (12C). Some microcontrollers have inbuilt modules which facilitate
inter-device communications using these standards. If the microcontroller doesn't
have such a module, a simple approach called bit-banging can be used as an alternative
form of communication. Bit-banging is a term used for software written to generate
data pulses for serial (synchronous as well as asynchronous) communications. Now,
SPI and 12C standards are described.
SPI
SPI standard was specified by Motorola Corporation for inter-device communication.
SPI is a data exchange format. Data is exchanged, rather than being transmitted
or received. Devices using can only exchange data. A device is either a master or a
slave and there is only one master device in a network at a given time. Master device
can select the slave device to which it wishes to communicate with. SPI specifies four
signals. These are SCLK (clock signal), MISO (input for the master or output for
slave). MOSI (output for the master and slave input) and SS (slave select signal for
selecting a slave) [30].
The PIC16F877 contains a MSSP (Master Synchronous Serial Port) module which
simplifies the process of configuring the devices for SPI communications. There is a
good tutorial on using the PIC microcontroller for SPI communications [46]. Many
of the external devices like EEPROMS and ADCs have SPI features which enables
the PIC microcontroller to communicate with these devices.
12C
12C was specified by Philips Corporation. 12C is a half-duplex communication scheme
(devices cannot transmit and receive at the same time). Devices are addressed by their
address and before initiating any communication with the slave(s), the master sends
54
the address of the slave to which it desires to communicate. Address is of 7 bit length,
and the 8th bit indicates whether the slave should transmit or receive. Recipient slave
then sends an acknowledgment bit confirming the receipt of the previous message. The
master and slave communicate in this fashion with the receiver device (can be either
the master or the slave) acknowledging the message after each byte of transmission.
After communicating with the slave, the master issues a stop bit indicating the end
of the transmission. Master device, if necessary might initiate the communication
scheme again to communicate with a particular slave (or a broadcast) [29].
12C,
because of the acknowledgment bit and the need to send the address of the slave
for communication cannot achieve a high data rate. The maximum data rate is on
the order of 100 Kbps. On the other hand, SPI, which is a data exchange format
has a higher data rate (reaching Mbps). There is a good tutorial on using the PIC
microcontroller for 12C [44]. The specifications for 12C can be found here [51].
4.1.2
Asynchronous serial communications
In this section, the commonly used asynchronous communication standards, RS-232
and RS-485 1 are discussed. Asynchronous communication assumes that the devices
are not synchronized and don't know in advance when they are going to receive data.
RS-232
RS-232
2 is
a commonly used standard for serial communication between a PC (the
COM port is used) and another device (ex:modem). There are different ways of communicating using RS-232 standard. The simplest way is to use a 8-bit transmission,
with no parity bit and a single stop bit and start bit. This simple scheme doesn't
use any handshaking schemes. RS-232 uses three lines for communication.
are TX (transmission), RX (receiving) and GND (ground reference).
These
While trans-
mitting a logical 1 (also called MARK), the voltage on the transmitting line can be
1
2
RS stands for Recommended Standard.
The present version of this standard is known as EIA232 (Electronic Industries Association).
55
anywhere between -5 to -15 Volts 3, and while receiving the voltage on the receiving
line should be between -3 to -15 Volts. Similarly when transmitting a logical 0 (also
called SPACE), the voltage on the transmitting line can be anywhere between 5 to
15 Volts and while receiving, the voltage on the receiving line can be anywhere between 3 to 15 Volts. The difference in voltages while transmitting and receiving is
to compensate transmission losses. Typically, RS-232 is used when the transmitting
distances are not greater than 100 ft. For transmission distances greater than 100 ft,
RS-485 is preferred [21].
RS-232, being an asynchronous communication protocol doesn't use any clock line
(for synchronization) for communication. The beginning and end of a byte are indicated by the stop and start bits. The stop bit is a logical 1 (MARK) and the start
bit is a logical 0 (SPACE). RS-232 is a full-duplex communication scheme allowing
simultaneous reception and transmission. This is possible because of the two separate and independent lines for transmission and reception. Between any 2 bytes of
transmission/reception, the transmitting and receiving lines use continuous stop bits
and wait for a start bit of the next byte . Therefore, in the idle state, the receiving
and transmitting lines are in the MARK state.
Baud rate is equal to the bits transmitted per second. For transmitting a byte,
8
+ 2 (one for start and one for stop) = 10 bits are transmitted. So in theory, 10
bits are transmitted for conveying 8 bits of information. Transmission and reception
using RS-232 is described now (using the simple scheme aforementioned).
Before transmitting, the transmission line is in the idle state (MARK or logical
1). The start bit is transmitted first (which is a logical 0 or SPACE). The duration
of this bit is equal to 1/(baud rate) seconds. After the start bit the individual data
bits are transmitted with the LSB (Least Significant Bit) being transmitted first and
the MSB (Most Significant Bit) being transmitted last.
The duration of each subsequent bit is also 1/(baud rate) seconds. After the MSB
is transmitted, the stop bit is transmitted and the line is held in that state until the
start bit of the next byte/character has to be transmitted. See figure 4-1, which shows
3
These voltages are with respect to the ground line.
56
how the character 'U' ' is transmitted at 9600 baud (the voltage levels correspond
to non RS-232 voltage levels which, need to be converted to RS-232 levels using a
RS-232 driver).
The receiving line receives the start bit first which pulls the receiving line from
a MARK (idle state) to a SPACE (start bit). When the line goes to SPACE, the
receiver waits for a time equal to half the bit width (bit width is 1/baud rate) and
proceeds further only if the line is still in the SPACE state. ' After this the line is
sampled after every bit width of time until all the 8 bits of information are retrieved
[61]. The line again goes to the idle state waiting for the next byte.
Some of the limitations of RS-232 are stated below.
1. RS-232 is a peer-to-peer communication scheme. It cannot be used in a network
mode.
2. Range of RS-232 is limited. Typical range is less than 100 ft.
The advantages of RS-232 are its full duplex communication capabilities and its
simplicity. Since the voltage outputs from the PIC microcontroller are usually 0 V
(for logical 0) and 5 V 6 (for logical 1), these voltages need to be converted to RS-232
compatible voltages. This is achieved by using a RS-232 driver, which converts the
output voltage from the microcontroller to RS-232 compatible voltage. Details of
some commonly used drivers can be found here [5].
RS-485
RS-485
'
uses a balanced differential line driver for communication. RS-485 is primar-
ily meant for creating a network of devices where all the devices use a shared medium
to communicate. The shared medium is a twisted pair of cables and a ground line.
A logical 0 or 1 depends upon the voltage difference between the two cables of the
twisted pair of cables. These cables are labeled as A and B and both of them carry
4
ASCII value of 'U' is 85, which in binary is 01010101.
1f the line is not in the SPACE state, the receiver assumes the transition to be noise.
6assuming the supply voltage to be 5 V.
7
Also known as EIA/TIA-485 (Telecommunications Industry Association).
5
57
the same signal. The voltage difference between these 2 lines, Vb dictates the logic.
If Vab is between -0.2 V to -6 V, the logic state is MARK or logical 1 and if Vab is
between 0.2 to 6 V, the logic state is SPACE or logical 0. These ranges allow for
attenuation and transmission losses in the cables 8. The mean voltage in the cables
is called the common mode voltage, Vm, and this voltage should lie between -7 to 12
Volts [6]. The cable labeled A is also known as non-inverting (+) input and the cable
labeled as B is known as inverting (-) input. Devices which are RS-485 compatible
usually have control lines, which control when the device can receive or transmit.
Since, RS-485 uses a shared medium, only one device can transmit at a time. RS-485
compatible devices have control lines which enable them to transmit or to receive.
Some of the advantages of RS-485 are:
1. Networking is possible with RS-485.
2. Up to 32 devices can be connected in a single network.
3. Transmission distances up to 4000 ft can be achieved.
Once again, the voltage output from the PIC microcontroller is either 0V or 5V.
These need to be converted to RS-485 compatible voltages. This is achieved by using a
RS-485 driver. One such example of RS-485 driver is ADM1485 from Analog Devices
Inc. Details of this device can be found here [2].
Now the bit-banging approach is described, which can be used for both synchronous
and asynchronous serial communications.
4.1.3
Bit-banging
Bit-banging is the term for software written (instead of using the hardware capabilities
to produce data pulses) for producing data pulses. The data pulse durations should be
exact or nearly exact durations that are obtained by using hardware USART module.
For synchronous serial communications, two lines are used, one for the clock signal
and the other for the data.
8
RS-485 uses twisted pair of cables for communication. Using a twisted pair reduces a lot of
electromagnetic interference [6].
58
Fundamentally, serial communication is transfer of information using data pulses.
In asynchronous communications, the bit-width is governed by the baud rate, while
in synchronous communications bit-width is directly dependent on the width of clock
pulses (the width of clock pulses depends on the specific requirements of the devices).
Most of the present-day microcontrollers have modules for generating the data pulses
(of appropriate width). Bit-banging is a very crude (but efficient) way of mimicking
the data pulses. Bit-banging is best explained by an example.
Consider again transmitting the ASCII character 'U' at a baud rate of 9600. The
waveform appears in the following fashion (see figure 4-1) at the microcontroller if the
in-built module is used. The same data pulses can be generated using appropriate
microcontroller instructions (i.e. bit-banging). For example, if an oscillator of 11.0592
MHz is used, length of each instruction cycle is 0.362 microseconds approximately.
Figure 4-1 gives the bit-width or duration of a bit is 0.104 milliseconds. To generate
a duration of this interval, the microcontroller has to output a line high or low for 288
(0.104 ms/0.362 us) instruction cycles. In this way a single bit is mimicked. Similarly
the other bits are mimicked to transfer information. This is a very crude way of doing
things but this approach is useful for microcontrollers which don't have an in-built
USART module.
Also, if C language is used for programming the microcontroller
the compiler generates the appropriate code for bit-banging (in asynchronous mode
only). For synchronous mode, the clock line is also used. For interfacing the external
ADC with the PIC16F877 (for developing the sensor unit), the bit-banging approach
is used.
4.2
Wireless Technologies
In this section some of the relevant wireless technologies that were considered for this
project are discussed.
RF based systems do not require a line of sight for transferring data (some of
them like GPS receivers need a line of sight for communication).
One of the major
technologies in using RF systems is the Bluetooth Standard. The Bluetooth standard
59
stop
idle state
stp
5V
0V
0
0
0
start 0.1042 ms
bit
Figure 4-1: Figure showing the data pulses for sending the character 'U' at a baud rate of
9600.
has been primarily developed for wireless communications between devices in a personal area network. Bluetooth compatible devices operate in the 2.4 GHz frequency
band and have a working range of 10 m. Bluetooth compatible devices have an attractive feature of operating at a low power, but the short range of these devices is a
major limitation [7]. Other RF systems that can be used for monitoring applications
are usually devices which work in the frequency ranges that fall in the Industrial,
Scientific and Medical (ISM) band of the RF spectrum or other license free bands 9.
In the following sections, the various technologies (RF) considered for developing
the wireless sensor units for the project are described. They are:
1. MICA motes from Crossbow Technology, Inc.
2. i-Bean from Millennial Net,Inc.
3. Linx Technology RF ICs
These are discussed in detail in the following sub-sections.
9
Primary license free bands in US are 900-928 MHz and 2.39-2.5 G Hz bands. Since the final
deployment would be in UK, we used devices which have a frequency of 433 MHz [16].
60
4.2.1
MICA motes
At the time, this project started (September 2002) MICA motes (see figure 4-2) were
the primary choice for the RF technology. The mote technology has been developed
by University of California, Berkeley and the motes are commercially manufactured
by Crossbow Technology, Inc. The MICA motes come with a processing board, which
has the microprocessor, the wireless transceiver and a sensor board which can be connected to this processing board with a 51 pin connector. The particular sensor board
which was used for testing the MICA motes had a dual-axis MEMS accelerometer,
magnetometer, photo diode, thermistor, microphone and a sounder. MICA motes
use TinyOS as the operating system for the microprocessor [12].
"
TinyOS is a
component-based runtime environment designed to provide support for deeply embedded systems which require concurrency intensive operations while constrained by
minimal hardware resources" [62].
Prof.David Culler's research group at University of California, Berkeley is primarily concentrating on developing and improving TinyOS. More details of TinyOS can
be found at this place [62].
a)
b)
Figure 4-2: a) figure showing the MICA processor board and b) MICA sensor board [12].
The utility of the MICA motes for monitoring applications was done as a part
of a class project at MIT. The primary goal of the class project was to use the
61
MICA motes for monitoring a MIT building (building 48), which was experiencing
noticeable (human noticeable) vibration due to the adjacent construction work. Some
of the important observations that were made from the project are:
1. The MICA mote uses the inbuilt ADC (Analog to Digital Converter) of the
microcontroller for performing analog to digital conversions. This inbuilt A/D
converter has a 10 bit resolution [9]. This resolution is insufficient for the tunnel
monitoring application.
2. Some of the features of the MICA motes are not accessible for modification
.
This greatly reduces the flexibility of the MICA motes [9].
3. With the generation of MICA mote that were used in the class project, it is
not possible to use a generic sensor board. One is forced to use the sensors
like photo, temperature sensors, accelerometer, magnetometer, etc [9]. These
sensors are not required (except for the temperature sensor) for the tunnel
monitoring project.
The MICA motes are being used for extensive research in developing wireless
sensor networking algorithms and other research activities [22].
Different monitoring applications require different sensors and hence a greater flexibility is required for using application specific sensors, which the MICA motes lacked.
This was the major limitation in using the MICA motes for the tunnel monitoring
project.
4.2.2
i-Beans
i-Bean (see figure 4-3) is a wireless transceiver with analog and digital inputs and a
microcontroller to control its operation. i-Beans have the capability of forming self
configuring networks and they are optimized for power consumption [48]. For the
present project, Millennial Net Inc. has custom manufactured i-Beans with a serial
interface. Any generic sensor/device which can output the data in serial format can
ioThe MICA motes are pre-manufactured and it is not an easy task to modify the existing circuitry
62
use this interface to transfer data to the i-Bean and utilize the existing i-Beans' self
configuring networking feature to develop a wireless sensor networking system. Some
of the important features that were absent in these devices are:
1. With the serial interface, the i-Beans are just transmitters and the i-Bean base
station, a receiver.
2. The original payload requested was 40 bytes, but the actual payload capability
is 20 bytes.
Figure 4-3: Figure showing i-Beans and the base station.
Other important limitations with the i-Beans are:
1. The serial interface was sensitive to the oscillator's frequency. The data received
at the base station was incorrect at a clock speed of 2 MHz.
63
2. The i-Bean base station does not work on all PCs.
3. The i-Bean uses a proprietary protocol for transmitting the data over the radio.
This forces to use the manufacturer's software. This places some limitations in
developing the software for storing the data in a database.
These limitations and the strict time line meant an alternative technology had to
be used for building the wireless sensor unit. The subsequent RF transmitter that
was chosen is the Linx RF IC (RM series), manufactured by Linx Technologies, Inc.
[32].
4.2.3
Linx Technologies RF ICs
Linx Technologies manufactures RF ICs which can be easily interfaced with a microcontroller. These ICs are simple transmitters and receivers with no special features
(which i-Bean and MICA motes have, ex: self configuring network capability). The
specific series that is currently used is the RM series (see figure 4-4). The details for
the transmitter and receiver can be found at [33], [34] respectively.
0.?9 in.
0,83 in,
POB level
Oin spacing: D1 in
1.20 In.
X In,
a)
Figure 4-4:
b)
Linx RM series a)transmitter [34] b) receiver [33].
Some of the limitations of this technology are discussed below.
1. It has been found that the transmitter has to transmit 3 times so that the
receiver can receive properly.
64
2. There will be a loss of data due to collisions, when two or more sensors transmit
at the same time.
3. These devices are not really power efficient.
Typical power consumption is
greater than 50 mW.
From the discussion on wireless technologies, one finds that there is no perfect
technology which can be easily used for any monitoring application.
The i-Bean
seems to be a promising technology with the serial interface. One would hope that
the latest version of i-Beans will have a well developed and tested serial interface
which gives the i-Beans some kind of flexibility, so that they can be used in different
applications.
In this chapter, the various aspects of communications in developing the wireless
sensor unit were discussed. In the next chapter the details of the integration of various
components to form the wireless sensor unit are described.
65
66
Chapter 5
Integration of components
The present chapter first discusses the integration of the components that form the
wireless sensor unit. Following this, the protocols for composing the messages are
discussed.
5.1
Interfacing the ADC
In this section, the details of the interface between the microcontroller and the external ADC are discussed. The reason for using an external ADC is to acquire sufficient
resolution in the pressure measurements. In order to have a resolution of 1 mm displacement at a pressure of 20 m (10 m atmospheric pressure plus 10 m water head)
that corresponds to a resolution of 1 in 20,000. This requires an ADC that has a minimum resolution of logj0000 bits or 15 bits. The on-board ADC of the microcontroller
has a resolution of 10 bits which is in-sufficient for acquiring the desired resolution.
Hence an external ADC is used.
The external ADC used in this project is the AD7713 model from Analog Devices,
Inc. The resolution of the AD7713 is 24 bits. The microcontroller talks to the ADC
using synchronous serial communication protocols. Synchronous serial communications have been discussed in the previous chapter. A brief review of synchronous
serial communications is below.
Synchronous Communications (at the microcontroller level) requires a clock to
67
time the data pulses.
Serial communication, by definition sends data bit by bit.
The device which provides the clock signal for timing the data pulses is the master
device. The master device coordinates all the communication between the master
device and all other devices, which are called slave devices. In the present case, the
microcontroller is the master device and the ADC is the slave device.
The microcontroller has to be interfaced with other components (like the RF
transmitter) other than the ADC. Hence, it should have control over all the communication between the ADC and itself (rather than the ADC controlling). Therefore,
the microcontroller is in the master mode and the ADC is in the slave mode. Some
ADC readings are lost when the ADC is in the slave mode. This is because the microcontroller collects data from the ADC at a different rate (0.2 Hz) from the conversion
rate (0.5 Hz)of the ADC. But this loss does not affect the monitoring requirements
in any drastic way as the readings from the ADC are only required to be collected
once in every 5 seconds.
SCLK [
24 DGND
0
MCLK IN 2
23
SYNC _
MODE 6
DVOD
22 SDATA
21 ORDY
MCLK OUT 3
AOT
4
AD7713
20 RFS
TOP VEW
19 VS
AGNO
AINI(+) f (Not to Scale)
AIN1(-)8
17 AIN3
9
16 RTD2
AIN2(+)
AIN2(-) 10
15 REF IN(+)
STANDBY 1
AVD12
AVD
14 REF IN(-)
13 R~
RTD1
Figure 5-1: Pin diagram of AD7713 [4].
Figure 5-1 shows the pin configuration of the ADC. Some of the important pins
and their functions are explained below. The description for others can be found at
68
[4].
* SCLK, this pin carries the clock signal for timing the data pulses. If the ADC
is in slave mode, the microcontroller provides this clock signal (as in the present
case), else the ADC provides the clock signal.
* MCLK IN & MCLK OUT, these pins are used for providing the clock/oscillator
input to the ADC. This clock is needed for the ADC to function and is completely different from the clock signal the SCLK pin carries. The frequency of
this clock can be from 400 kHz to 2 MHz. Presently a 2 MHz crystal is used.
* AIN2(+) & AIN2(-), these pins are the differential input pins to the ADC.
The pressure transducer that is used has a differential output and hence the
differential input pins are used on the ADC.
* SDATA, a bidirectional communication pin on which all the data to/from the
ADC is transmitted/received.
* DRDY, this pin goes low whenever the ADC has finished a conversion and the
data is ready to be collected. This is a hardware indicator for the microcontroller
to collect data.
* RFS and TFS, Receive Frame Synchronization and Transmit Frame Synchronization pins. RFS is made low when the microcontroller receives data. TFS is
made low when the microcontroller transmits data to the ADC.
" REF IN(+) & REF IN(-), these pins are the reference voltage pins. For the
present case, these values are 2.5V and OV.
.
Ao,
this pin controls which register (data or control) of the ADC is accessed.
If this pin is low, the control register is accessed, else (i.e. this pin is high) the
data register is accessed.
Figure 5-2 shows the various connections between the microcontroller and the
ADC. The ADC on power reset is in its default mode. In the default mode, the ADC
69
outputs only 2 bytes of the digital output (instead of 3 bytes). Therefore, the ADC
needs to be configured so that it can output 24 bits or 3 bytes. This is done by writing
a 24 bit code to the ADC's control register. This 24 bit code defines how the ADC
should operate. One important consideration when writing to the control register is
that all the 24 bits should be written completely when writing to the control register.
Else, the control register fails to load the code in the control register. This causes the
ADC to operate in the default mode or the mode in which the ADC was previously
operating. The 24-bit code contains single bit flags and integer values [4].
B7
B6
SCLK
A2
DGnd
nvai'
o2
SDAT
DRD
M4z
SYNC A
Mo
A5
EQ
RFS
TFq
El
E2
A N3
S |AITN](+)
-AINI(-)
AIN2(+)
AIN2(-)
o 5V
AD
10K
7713
Ref In(
5AV
AV
PFC
16F877
10K
PRESSURE
TRANSDUCE1
-
5V
Figure 5-2: Interfacing the ADC with the microcontroller.
Some of the features which the control register controls are described below and the
particular settings that are used in this project are also explained.
70
"
calibration mode, the ADC can operate in any of these four calibration modes.
They are normal mode (i.e. no calibration mode and this is the default state),
system-calibration, self-calibration and active background calibration mode. In
the active background calibration mode, the ADC operates in a continuous
calibration mode. This mode compensates for the re-calibration of the device
when there is a change in the ambient temperature. In this mode, only one
input channel of the ADC is used and the other channels are shorted (zeroed)
for the background calibration. For details of other modes, see [4].
" Programmable Gain Amplification (PGA). This is used for amplifying the differential input from the pressure transducer. Presently, a value of 64 is used.
This implies that both the input signals from the transducer are amplified by a
factor of 64 before the conversion takes place.
* Channel, selecting which channel on the ADC to use. Presently, channel 2 is
used for converting the pressure transducer readings. There are three channels
on the ADC of which 2 are differential input channels and the third one is a
single input channel [4].
* Word length, to select if the digital output from the ADC should be 2 or 3
bytes. ADC is configured to output 3 bytes.
" Bipolar or unipolar output selection. If bipolar output is selected, data is output
in offset binary. Presently, bipolar output is used.
After selecting the desired configuration settings (i.e. the 24 bit code), the microcontroller transmits this code to the ADC. Then the ADC loads this code into the
control register. The steps to write the code are as follows.
1. Initially TFS, RFS and
2.
AO
AO
pins are made high.
pin is pulled low. Bringing
Ao
register instead of the data register.
71
pin low designates access to the control
3. TFS pin is pulled low to indicate the ADC that the microcontroller is transmitting.
4. The 24 bit configuration code is sent, starting with the most significant bit
(MSB).
5. TFS and
A0
pins are pulled back high.
Step 3 of the above procedure is implemented using a bit-banging approach, which
is described now.
The microcontroller has two pins allocated for sending data to the ADC, SDATA
(for data) and SCLK (for the clock signal).
See figure 5-2.
Both these pins are
configured as outputs and initially they are low. Once the microcontroller is ready to
send the data, it does the following:
SCLK I1
SEDATA (1
M5
Figure 5-3: Timing diagram for writing data to the ADC [4].
1. Modulate the SDATA pin high or low (1 or 0) '. The state of SDATA is held
high/low until it is changed explicitly.
2. After a delay of 1 cycle, the SCLK line is made high and is held high for 2
cycles.
'These values are when the microcontroller is using a 4 MHz clock and the ADC is using a 2
MHz clock. 1 cycle is 1 microsecond.
72
3. SCLK line is then made low and after 1 cycle, step 1 is repeated. This operation
is repeated until all the 24 code bits are transmitted.
Figure 5-3 shows the timing pulses (SCLK), settings of
A0,
TFS,DRDY and a
sequence of data bits on the SDATA line in the write mode. The constraints on the
different timings (shown as labels on the figure) can be found here [4].
Now the steps for receiving data from the ADC are explained.
1. The ADC pulls the DRDY pin low whenever the conversion data is ready to be
taken. The microcontroller keeps polling to see if it is low. If the data in the
control register of the ADC has to be accessed
2,
the microcontroller need not
wait for this pin to go low. It can access the ADC's control register even if the
DRDY pin is high.
2. Once the DRDY pin goes low, then the RFS pin is made low. However,if the
ADC's control register has to be accessed, the microcontroller need not wait for
the DRDY to go low, it can directly make RFS pin low.
3. Microcontroller starts receiving the data with the MSB coming first. Since, the
microcontroller is in the master mode it has to provide the clock signal so that
the ADC can send the data.
4. After the data has been received, the RFS pin and if necessary the
AO
are
pulled high.
Again a bit-banging procedure is used to read the data from the ADC. This is
explained in more detailed terms to elucidate the operation. Once again, the SDATA
and SCLK lines are used.
See figure 5-2.
SDATA is made an input pin as the
microcontroller has to read data synchronously from the ADC.
1. Once the RFS pin is made low, the microcontroller waits for 2 cycles and then
reads the SDATA line.
2
This might be done to examine the contents of the control register and see if the settings are
satisfactory.
73
2. The SCLK pin is made high and is held high for 2 cycles.
3. The SCLK pin is made low and the step 1 is repeated. This operation is repeated
until all the bits have been received.
Figure 5-4 shows the timing pulses (SCLK), settings of
Ao,
RFS and a sequence
of data bits on the SDATA line in the read mode (from the data register).
The
constraints on the different timings (shown as labels on the figure) can be found here
[4].
t~~
-- t 1 5T
SA
Figure 5-4: Timing diagram for reading data from the ADC [4].
5.2
Interfacing the i-Bean
The previous chapter discussed some of the various RF technologies including the iBean. This section focuses on the details of the interface between the microcontroller
and the i-Bean. This task is described as it formed a prototype communication interface for the project. As mentioned in the previous chapter, the i-Beans were specially
configured for the project with a serial interface by the manufacturer (Millennial Net
74
Inc. ). There are three lines/pins through which the microcontroller interacts with
the i-Bean. They are:
1. RTS (Ready To Send), the microcontroller pulls this line low whenever it wants
to transmit data to the i-Bean. This is a signal to the i-Bean that the microcontroller is ready to send data.
2. CTS (Clear To Send), this line signals (line goes low) an indication from the
i-Bean to the microcontroller that it is ready to receive data from the microcontroller. This happens after the microcontroller makes the RTS line low.
3. DATA or RX, When the CTS goes low, the microcontroller knows that the
i-Bean is waiting to get data from it. The microcontroller sends data (in serial
format L9600 baud) using this line.
Figure 5-5 shows the connections from the microcontroller to the i-Bean.
The
i-Bean operates at 3V and hence a special voltage regulator (LP2950) was used. This
converts the 9V supply (power supply to the sensor module) to a 3V output, which
powers the i-Bean. For the same reason, a voltage bridge is used, which converts
the 5V output from the PIC microcontroller to meet the i-Bean requirements. These
voltage bridges appear on the RTS and DATA/RX lines.
Fig 5-6 shows the timing diagram for the different pins. The constraints on the
timings that are shown in figure 5-6 and are also listed in table 5.1.
Parameter
ti
t2
t3
Description
CTS response time
CTS timeout
CTS release after end of
message or 20 bytes received
Minimum
1
0
0
Maximum Unit
40
millisecond
20
millisecond
20
millisecond
Table 5.1: Table showing the timing constraints provided by Millennial Net, Inc.
The code to interface the i-Bean with the microcontroller is explained now.
75
PIC
16F877
CTS
RTS
C5
D3
GND
-'
1
7-W
RX
V cc (3V)
LP2950-3.C
9V
I
+
22
uF
Figure 5-5: Interfacing the i-Bean with the microcontroller.
1. The microcontroller collects the digital pressure transducer readings from the
external ADC and the thermistor and board voltage readings from the in-built
ADC of the microcontroller. The data is formed into a message string which is
passed to the i-Bean. When the microcontroller is ready to send the message
string, it pulls the RTS line low. The message composition and the protocol
used for composing the message are discussed in a later section of this chapter.
2. After the microcontroller pulls the RTS line low, it keeps polling for the CTS
line to see if it is low. The time interval for the CTS line to go low (after RFS
becomes low) is between 1 to 40 ms. The microcontroller keeps polling the CTS
line for 40 ms. Now two situations arise. They are:
76
RTS
ti
CTS
t2
t3
DATA
DATA
Figure 5-6: Timing diagram for the i-Bean provided by Millennial Net, Inc.
(a) CTS line goes low in the 40 ms time.
sends the data to the i-Bean.
In this case, the microcontroller
The data is sent at a baud rate of 9600
and as suggested by the manufacturers, two additional stop bits are used.
This is approximately 200 us at 9600 baud. As mentioned in the previous
chapter, the maximum payload is 20 bytes.
(b) In the second case, the CTS line doesn't go low in the 40 ms time.
this case, the microcontroller pulls the RTS line high.
In
This data is lost
because the i-Bean could not receive the data from the microcontroller.
The microcontroller just proceeds with collecting data again, composing
the message string and try sending data again to the i-Bean.
The above procedure is used for sending data to the i-Bean. Once the data is sent
to the i-Bean, this data is relayed by the i-Bean to the base station. The base station
has a receiver which receives messages from i-Beans and is connected to a PC serial
port. The PC runs the software provided by the manufacturers of the i-Bean. This
software displays the incoming data at the base station and stores this data in a text
77
file. The software run on the base station had to be modified so that it is possible
to check for the integrity of the messages. The i-Beans are not being used in the
project because of operational issues arose around the stability of RF communications.
Given the strict time line of the project, alternative technologies were considered.
Subsequently, the RF transmitter manufactured by Linx Technologies was chosen.
The technical details of Linx transmitter were discussed in Chapter 4. The Linx
RF IC is not integrated with a formal operating system to manage networking and
other hardware issues. This factor forced to resort to raw transmission of data with
out collision detection. In future versions, for the better management of networking
issues, a system with an operating system would be constructed.
This operating
system can be a variant of the TinyOS or some similar operating system.
5.3
Interfacing the ultrasonics
In this section, details of the interface between the microcontroller and the ultrasonic
transducers are discussed. In order to interface the ultrasonics, some changes had to
be made to the sensor board. Ultrasonic transducers were discussed in Chapter 2. As
mentioned, the positioning devices consist of the basic circuitry for the ultrasonic to
operate and calculate the distance. The ultrasonic receiver is a Hexamite HE240 series
receiver [25] and the transmitter head, a transceiver (E-152) from Massa Corporation
[40].
The ultrasonic transducers have to be configured before they can output data.
The PIC microcontroller sends the configuration details over the RS-485 network
3.
Therefore,the configuration details have to be known in advance to program the microcontroller. An alternative way is to configure the ultrasonic transducers dynamically.
This possibly can be done in two ways.
1. By using a RF transceiver on the sensor board, it is possible for the sensor unit
to listen for any messages from the base station. In such case, it is possible to
3
This network consists of the two ultrasonic transducers and the sensor unit. All these devices
use a shared medium of wires to communicate.
78
send the configuration data and instructions for the microcontroller to configure
the ultrasonics.
2. Using a serial interface to the sensor board and programming the microcontroller
so that it can listen for any incoming data. In order to do this, the microcontroller has to listen periodically or an interrupt can be generated whenever it
receives data. In order to use the interrupt feature, the microcontroller should
use hardware USART module for receiving data. Since the ultrasonic transducers also use the hardware USART, one should disconnect the ultrasonics before
giving instructions to the microcontroller to configure the ultrasonic transducers. Listening periodically is not a very efficient way, as it is difficult to get the
microcontroller's attention. Another limitation is that this has to be done in
the field/tunnel.
For this project, the microcontroller is programmed so that it can configure the
ultrasonic transducer system comprising a single transmitter and a single receiver.
This was the proposed configuration of the ultrasonic transducers. Since, the hexamite positioning devices come with pre-assigned addresses, the configuration data has
to be programmed correctly in the microcontroller. The configuration data that the
microcontroller sends to the ultrasonic transducer system is shown below.
#W
84 OD 01 01 00 00 93\r#S 1C OD 01 00 00 00 2A\r\r 4
Configuration data is explained in Appendix A. As mentioned, the ultrasonic
transducers use a shared medium (twisted pair of cables connecting them) to communicate with each other and the microcontroller. They use the RS-485 standard
for communication, which was been explained in the previous chapter. The microcontroller outputs data in serial format, which is converted to RS-485 compatible
differential outputs using a standard RS-485 driver. ADM1485 was used for this purpose [2]. The microcontroller needs two pins to communicate on the RS-485 network
and two other pins to enable its reception and transmission on the RS-485 network.
4
\r is carriage return, since microcontroller is programmed in C.
79
Figure 5-7 shows the various connections between the microcontroller, ADM1485 and
the ultrasonic transducers.
ADM 1485
00
C7
5 V
I
RS-485
Network
C6
Twisted pair of cables
D3
D2
Figure 5-7: Figure showing the interface between microcontroller and the ultrasonic transducers.
Before sending the configuration data to the ultrasonics, the microcontroller should
enable the transmission line (by pulling the data transmission enable line high) and
disabling the reception line (by pulling the data reception enable line high). If the
data reception enable line is not disabled, the microcontroller would hear an echo of
the data it sends. However, this can be used as a test to check if the data sent by the
microcontroller is right or wrong.
The microcontroller before transmitting the configuration data, captures the attention of the ultrasonic transducers by sending a control code, a carriage return
followed by the escape character.
When the microcontroller sends these characters, the ultrasonic transducers wait
to hear the configuration data from the controlling device. In this case, it happens to
80
be the microcontroller. The microcontroller then sends the configuration data to the
ultrasonic transducers. Following this, the microcontroller disables its transmission
to the ultrasonic transducer network5 and enables its reception. The ultrasonics send
data every 0.25 seconds 6.
Specifically, the format of the data collected from the ultrasonics appears in this
fashion.
-1234
-1233
There is a new line character at the end of each line of data. The microcontroller
is programmed in such a a way that it only considers characters between "-" and the
new line character. As the data is in a string format, it needs to be converted to
a long ' before it can be stored. After converting the data from string format into
long format, the microcontroller collects the next reading and after 20 readings have
been collected, the microcontroller averages those 20 readings, composes the message
string and sends it to the RF transmitter.
Figure 5-8 shows the sensors that were intended to be used in the tunnel.
A simple experiment was done to determine the resolution of the ultrasonic transducers. The two transducer boxes (shown in figure 5-8) were placed facing each other
on two tables. They were separated by a distance of around 12 ft (the same size
as the tunnel diameter). One of the transducers (the Hexamite device) was moved
towards the other transducer by a distance of 10 cm in 10 equal steps. At each step,
the distance measurements were made, and the average of 200 such measurements
was taken as a representative value. The transducer was moved backwards in 10
equal steps to its starting position. During these movements, the transducers were
still facing each other. This process of moving one of the transducers forwards and
5
Only one device can transmit in a RS-485 network. Disabling the microcontroller's transmission
avoids
any accidental data transmission from the microcontroller.
6
This rate is fixed.
7
For the CCS compiler, long is a 16 bit integer [19].
81
=7
7-
MORM
Figure 5-8: Figure showing the ultrasonic transducers in boxes.
backwards was repeated once, and at each step the average of 200 readings was taken.
All these values were plotted as shown in figure 5-9. From the graph, one can see
that the response of the ultrasonic transducers is linear, which is satisfactory. But
the, standard deviation (or the resolution) of the 200 readings taken at each step was
typically around 2 mm (not shown in the graph). As mentioned in Chapter 1, the
deformations are expected to be in the order of 2 mm. By a simple rule of thumb,
to measure 2 mm deformations, the least resolution required is 0.2 mm. Clearly,
the ultrasonic transducers will not be able to achieve the task of measuring 2 mm
deformation. Hence they haven't been deployed in the tunnel.
5.4
Others
Some other minor, but important details of the sensor board are discussed in this
section. They are listed below.
1. The microcontroller has two pins which are connected to the ADM233LJN (RS232 driver). It is possible to use a female end of a serial port connector and use
it for debugging purposes or talking to the microcontroller.
2. There is a thermistor on the sensor board which is in series with a 10 K ohm
82
0
88
~
fowad-21
I
*
1;a:.
~4
4~~
a
.
back iart:. 2
f8A0
.
d-2
b. - kThr
800
1
distanc. f tom reof pd nt Inmm
Figure 5-9: Graph of the calibration experiment.
resistor.
The thermistor's resistance changes with temperature changes and
this can be used for measuring the temperature at the sensor board.
The
node which joins these two components is connected to an ADC channel of the
microcontroller.
3. There are two 1 M ohm resistors which are directly connected to the power supply of the board. These two resistors form a voltage divider and the voltage drop
across one of the resistor is an input to an ADC channel of the microcontroller.
This voltage gives an indication of the power supply to the board.
4. Finally, there are two voltage regulators on the board for supplying 5 V (power
for most of the components) and 3 V (for the i-Bean).
83
5.5
Sensor Board
Figure 5-10 shows the schematic of the sensor board.
The sensor board was de-
signed after testing the interface of the ADC with the microcontroller, considering
the requirements from the Millennial Net Inc. (for i-Beans) and the future intention
of interfacing the ultrasonics with the same board. The software that was used for
generating the required circuit schematic, and the necessary files for manufacturing
the PCB (Printed Circuit Board) is Protel DXP [1]. Some changes have been made
from the original design. They are:
1. The original sensor boards had 2 MHz ceramic oscillators and it was found that
some of the boards were not working properly. The ceramic oscillators were
found to be the cause of the malfunction. Hence the oscillators were changed
to 2 MHz crystals and then consistent operation was possible.
2. When testing the i-Beans on the sensor board, it was found that the i-Beans
were also not working properly. As mentioned in the previous chapter, 4 MHz
oscillators had to be used to fix this problem. Hence, the 2 MHz crystals ' were
subsequently replaced with 4 MHz crystals.
3. In the original schematic, the ADM1485's enable pins were wrongly connected to
the power line. This had to be changed for interfacing the ultrasonics. A small
bread board is used, which houses the ADM1485 and for other connections.
Also, the thermistor and the pressure transducer were removed from the board
to access some more pins of the microcontroller, so that the ADM1485 can be
interfaced to the microcontroller. This was done for interfacing the ultrasonic
transducers only. All changes were performed manually and should be amended
on the original design for any future work.
4. Two additional wires were soldered at the back of the board so that the hardware
USART lines of the microcontroller can be used for interfacing the ultrasonics.
The reason for using the hardware USART lines is that the microcontroller is
8
The 2 MHz crystals were replaced only for the microcontroller and not for the external ADC.
84
k
4~
AE
14:
[C ]
-T-
AL
E
E
]
D-
-
C
bd[
r
I
E
30-E 3
16
---4- L2
E:
]L3
2]
E
:
E
]
I
FCI
/f 1 X2200A
cJJz
L
LD
rnsd uc er--
0
I
---- - -
LP2950-!IJ3
q
uF
fI
I~
~tter'y
Figure 5-10: Schematic of the sensor board.
85
um
W
Figure 5-11: Figure showing the sensor board and the wireless sensor unit.
more efficient in receiving data from the ultrasonics. Even with the last two
modifications, the sensor board could not be used to interface the ultrasonics
because of a defective power supply to the sensor board. Hence, two printed
prototyping boards were required to interface the ultrasonics with the microcontroller. This is shown in figure 5-8
Apart from these changes it was found that one of the board also had defective voltage
regulators.
The sensor boards are enclosed in a metallic box, so that they can be used in the
tunnel (see figure 5-11). In the next section the protocols for composing the messages
are discussed.
5.6
Protocols
This section briefly describes the protocols for composing the message strings, that
are transmitted to the base station by the RF transmitter. The base station software
in turn checks the validity of the data and incorporates it into the database if the data
is valid. This protocol was written by James Brooks, but some minor changes and
suggestions were made by the author and described further in this section. Appendix
C lists the details of the latest version of the protocol. In the next couple of sections,
86
the protocol that is used for sending data when the i-Bean is used as the transmitter
and the protocol for the ultrasonics when a Linx transmitter is used.
5.7
Protocol for i-Bean
This section explains the protocol that is used for sending data from the i-Bean to the
base station. Some of the changes that have been made to the base station software
(provided by Millennial Net Inc.) are also mentioned. The software changes had to
be made so that the data could be checked for its integrity at the base station.
The message format is as follows.
1. The first two bytes of the message are the start bytes. These are 123 and 43 in
decimal. All messages require these start bytes.
2. The next two bytes are for the length of the message. The LSB (Least Significant
Byte) is followed by the MSB (Most Significant Byte).
3. The next two bytes are for the sensor id. Again, the LSB is followed by the
MSB.
4. The next two bytes are for the reading number. The reading number starts
from 1 and continues until 65536 and starts from 0 again. The reading number
helps determine some readings are missing. This is indicated if there is a skip
in the reading number instead of being consecutive.
5. The next three bytes are for the pressure transducer reading. The LSB comes
first and the MSB last
6. The next two bytes are from the channel 0 of the in-built ADC of the microcontroller. MSB follows LSB. This gives the thermistor reading
7. The next two bytes are from the channel 1 of the in-built ADC of the microcontroller. MSB follows LSB. This gives the supply voltage to the board.
87
8. The next two bytes are the checksum bytes. The check sum bytes are the sum
of all the bytes starting from the start bytes to the bytes before the checksum
bytes.
An example of a (correct) message string is below (in decimal format).
43 , 17, 0,
123,
1,
0, 10,
0,
100, 230,
11,
55, 3,
189, 8,22, 3
Since the i-Bean uses proprietary protocols for transmitting data,one has to use
the code provided by i-Bean manufacturers at the base station for interpreting the
data. This code had to be modified so that this could also check the integrity of
the data that the base station received using the above mentioned protocol. The
modified code at the base station first checks if the start bytes are right and see if the
message length bytes indicate the true message length and finally the code checks if
the checksum bytes are right. If either the start bytes are wrong or if the checksum
bytes are wrong, the code would mention so.
5.8
Protocol for the Ultrasonic transducer
In this section, the protocol for transmitting the ultrasonic data from the sensor unit
to the base station is discussed. This protocol is a part of the complete protocol
described in Appendix B. The protocol is described below.
1. The first three characters of the message string are CMI followed by the character
","
2. The next two characters of the message string are UR followed by the character
",".
UR for Ultrasonic report.
3. The next character is the sensor id followed by the character "
4. The next two characters mentions the transmitter and the receiver followed by
the character ",
5. The next four characters indicate the output from the ultrasonics in decimal
followed by an asterisk.
88
6. Finally, the last two characters are the LSB (in hexadecimal) of the checksum.
The checksum is the sum of all the ASCII values of the characters from the
starting character to the character (including ",") before the asterisk.
An example of a correct message string is shown below.
CMI,UR,Q,SU,1234*F3
In this chapter, the details of the interfacing of various components have been
discussed. These components form the sensor unit of the wireless sensor network. The
data protocols and some relevant details of the sensor board also have been discussed.
The next chapter summarizes the thesis and states the directions for future research.
89
90
Chapter 6
Summary
6.1
Summary
The goal of the London Underground monitoring project is to develop a real-time
system to measure displacements and deformations. This thesis discusses the various components of the wireless sensor unit that was developed.. In particular, the
discussion is focused more on the hardware details, where the different electronic components are interfaced with the microcontroller. The wireless sensors were installed
in the tunnel on the 17th October, 2003. Presently, the sensors are collecting data
and reporting to a base station receiver in the access shaft of the tunnel.
The receiver collects the data and transmits the data to a laptop. The program
running on the laptop checks for the integrity of the data using the protocol described
in Appendix B. These data are then stored in a text file and a wireless modem
periodically sends them to an MIT server. Figure 6-1 shows a sensor installed in the
tunnel. The figure also shows the location of the various wireless sensors installed in
the tunnel. Figure 6-2 shows the receiving unit and the laptop in the access shaft of
the tunnel. Figure 6-3 summarizes the whole operation of the wireless sensors in the
tunnel.
There are still areas for improvement in the project and are addressed in the next
section. Even though data is being collected, a lot of processing has to be done to
remove noise from the readings. The processing also accounts for data that was lost
91
1H
Figure
6-1: Figure showing the installed sensors in the tunnel.
perhaps due to faulty communication or extreme interference due to the passing of a
train or to correct for various environmental effects. These effects might be because
of the temperature variation in the tunnel or again transient effects such as a change
in the pressure, when a train passes through the tunnel.
The present processing
scheme employed averages the readings over 5 minute intervals. For intervals that
do not have any readings (because of lost readings), a linearly interpolated value is
used between the previous and next intervals which have a reading. After this, a low
pass filter is used to remove the sensor noise. The filtered values are also stored in
the database and are used to calculate displacements. This installation highlighted
some of the important issues in designing a wireless sensor system. However, because
of the limited time-frame of the project, many other considerations remain unsolved.
For example, on-chip processing for energy conservation, appropriate communication
protocols for collision detection, etc.
92
Figure 6-2: Figure showing the receiving unit and the laptop placed in the access shaft of
the tunnel.
6.2
Directions for future research
In this section, some of the improvements that can be made to the existing system
are discussed. This discussion focuses on the wireless technology and the hardware
that is being currently used and how they might be enhanced.
6.2.1
Wireless technology
The RF IC that was used for this project is the Linx RM series ICs. These are simple
devices with no ability to apply a communications protocol.
This fact is obvious,
especially if these ICs are compared with the MICA motes or the i-Beans.
The
network topology of the wireless sensors installed in the tunnel is similar to a limited
hub and spoke topology. Since inter-sensor communication and contention resolution
are not possible, there will be often collisions when two or more transmitters try to
transmit at the same time and this causes loss of data. This might not be such a
critical issue if only few sensors
1
are used and data is not extremely valuable. Such a
system is inherently unscalable. Additionally integrity and continuity is critical such
'In the present project, 16 of them are being used.
93
Int.mrir,
lr
ful
n
Acces
Expectedtettlentnt lorf L L
ExAtting
s
Are reWitstrnW
*
rervo
itAchted I turnnel wall
ied
n ht
New CTRL
tun-e1
6rn dinjo
C~ItL 0,06TunnO Oosring Michine
Figure 6-3: Summary of the wireless sensor operation.
an architecture is wholly inadequate. Another limitation of the Linx ICs used is the
lack of bi-direction communication capabilities. This limitation further impacts the
scalability and loss of data factors mentioned. Bidirectional communication capability
would also provide the option of sending data in hops, i.e.
we could send data
through intermediate sensor nodes. Bidirectional capability also increases redundancy
in the routes through which data is routed in the system. This capability makes the
system more sophisticated and provides an ideal platform to study the performance
of different ad-hoc networking and multi-hop routing protocols.
In order to achieve bidirectional capability, some of the possible options are:
. Bluetooth devices, which are low-cost, low-power devices and have bidirectional
94
capabilities. The limitation of these devices is the communication range (around
10 m) of these devices.
" Other RF ICs which have bidirectional communication capability. These include
RF ICs from RFM [55] and Chipcon [10]. This then increases the functionality
that the microcontroller has to provide for proper handling of bidirectional
communications.
In addition they have a microcontroller integrated on chip,
which allows specific customization of communication protocols suitable for each
specific monitoring task.
" Future versions of the MICA motes or the i-Bean might be useful for a similar
kind of application. The MICA motes do have bidirection communication capability, while the present version of i-Bean (i.e. the version that was tested for
this project) do not have this capability.
6.2.2
Hardware
The microcontroller that was used in this project is a PIC16F877. The typical power
consumption of this microcontroller is 0.6 mA 03V and a 4 MHz. There are newer microcontrollers from Microchip Technology, Inc. such as the nanowatt range that have
lower power consumption than this microcontroller. A list of such microcontrollers
can be found here [43].
Lower power microcontrollers are important for making the wireless sensor unit
autonomous, with minimum human intervention. Lower power increase the life of
the sensing unit, compared to the present one, and also allows more devices to be
interfaced with the microcontroller, again compared to the present one.
Another important component to consider is a DSP chip. This would allow efficient local processing of the data. With local processing of data, it is possible to
minimize the number of messages sent to the base station. This in turn saves a lot
of power as RF communications are power expensive.
95
6.2.3
Operating system
Another important area for future work is to use a low-level operating system on the
microcontroller.
This provides the microcontroller with multi-tasking and schedul-
ing features. One such example is TinyOS [621. For future applications, a TinyOS
compatible microcontroller may be used or a basic operating system (like TinyOS)
may be developed for the microcontroller(s) that is(are) used. This allows a lot of
flexibility in developing the applications.
6.3
Conclusions
This thesis described the development of a wireless sensor module for tunnel monitoring. A real-time monitoring system was developed and deployed to measure vertical
displacements along the critical zone near Hihbury & Islington station. Real-time
measurements were not possible as the data had to be processed before the displacements could be calculated.
To conclude, there is a lot of exciting opportunity to
develop more sophisticated sensor devices in the future and to enhance the society
through distributed monitoring of infrastructure.
96
Appendix A
Configuration details for ultrasonic
transducer using HE860
The composition of the configuration string for configuring the ultrasonic transducers
using HE860(as the positioning device) is shown in table A.1.
Byte
1
2
3
4
5
Parameter
Program Control Byte(PCB)
Termination Byte(TB)
Number of Beacons Byte(NBB)
Beacon Designation Byte(BDB)
Noise Suppression Byte(NSB)
Table A.1: Table showing the Configuration String
A beacon is a device which can tarnsmit ultrasonic waves and a pilot is a device
which can receive ultrasonic waves. The PCB(for a beacon) settings are shown in A.2.
For the configuration described in Chapter 5 (i.e. one beacon and one pilot connected
over serial network), Bits 2 and 7 should be set (making it 84 in hexadecimal) for
the beacon. The terminal byte is ODh (in hexadecimal) for the beacon. NBB = 01h,
BDB = 01h and NSB = 00h for the beacon.
For the PCB of the pilot (table A.3),Bit 2 (though it is unnecessary) , Bit 3 and
97
Bits
Bit 0
Bit 1
Bit 2
Bit 3
Bit 4
Bit 5
Bit 6
Bit 7
Bit Function(Beacon)
Synchronization using ultrasound
Synchronization override
Beacon is linked on a serial network
Ignored
Ignored
Ignored
Ignored
Should be set for a Beacon
Table A.2: Table showing the bits of the PCB for a Beacon
Bit 4 are set, which makes it ICh. TB = ODh, NBB = 01h, BDB =00h and NSB
= 00h for the pilot. The configuration bytes for the beacon(s) should be sent before
sending the configuration bytes for the pilot.
Bits
Bit
Bit
Bit
Bit
Bit
Bit
0
1
2
3
4
5
Bit Function(Pilot)
Synchronization using ultrasound
Reserved
Reserved
Pilot acquires data continuously
Device is the master (initiator) on the network
Ignored
Bit 6
Ignored
Bit 7
Should be cleared for a Pilot
Table A.3: Table showing the bits of the PCB for a Pilot
98
Appendix B
Sample microcontroller program in
assembly language and in C
B.1
Program in assembly language
A simple program which counts in binary from 0 to 255 and starts again from 0.
list p=16f877
include "pl6f 877. inc"
; Start at the reset vector
;(this is where the chip boots from by
;default)
org
Ox000
goto
start
start
; the SFRs we want to use are in bank 1 so we set the STATUS bits
99
; to make bank 1 active (i.e. set context)
; SFRs TRISA,B,C,D,E correspond to the respective I/O ports/pins
; on chip
; Each bank defines an offset address applied/prepended to the
; value of the mnemonic used to refe to a register
; e.g. if banki is at some address say 1000, and TRISA has a
; reference value/token of 77, then the absolute address of
; TRISA is 1000 + 77 = 1077 (check spec for actual address values
bsf
STATUS,RPO
; set
bcf
STATUS,RP1
; clear bit RP1 in STATUS
bit RPO in STATUS
; we want to assign the prescalar multiplier for timer TMRO
; by through the SFR OPTIONREG bits.
- place required settings in W and mov to OPTIONREG
- here we use prescalar of 1:256 for TMRO, use internal clock
cycle
movlw b'00000111'
movwf
OPTIONREG
; load Os into working reg w
; clear all TRISs bits (0) i.e. these ports are entirely O/P
; We are setting ALL of the I/0 pins as O/P to ensure we know
; their designation
movlw H'00'
movwf TRISA
movwf TRISB
movwf TRISC
100
movwf TRISD
movwf TRISE
; need to move to bank 0 to access PORTA,B,C,D,E
; since we know bit RP1 is 0, we only need to change (clear)
; RPO to specify bank 0
bcf STATUS, RPO
; bank 0
; now setup value in portB to be zero, i.e. we are
; clearing all bits (0). Simplest way is to clear register
; in one instruction using _clrf_. We could clear each bit
; individually but it is unecessary.
clrf PORTB
; now loop through binary values by incrementing PORTB by
1 (i.e. b'00000001')
each loop increment corresponds to 256 timer ticks,
each timer tick
is 256 usec, so each update of the vlaue in PORTB takes
256^2 usec ~= 0.06 sec.
We'll see this barely as the frequency is 16 Hz
the logic of the loop is to poll the INTCON reg to see
if the TOIF bit
is set using the instruction btfss (bit test file skip-if-set)
if not set it continues to instruction "goto $ - 1"
which tells it to
go to the previous line (i.e. $ means current line, $ - 1,
101
means the
previous line).
so it keeps testing INTCON,TOIF until TOIF is set
when it determines TOIF bit is set this indicates overflow
of the timer
and tells program to "skip" the next instruction.
So the program reaches the "bcf INTCON,TOIF" instruction
which clears the
TOIF bit and at the same time since TIMERO has overflowed it
effectively
wraps around to restart at 0, so we are back at the same
initial starting
conditions for the loop i.e. TIMERO is 0 and the TOIF bit
is clear (0).
count
incf PORTB,1
btfss
goto
INTCON, TOIF
$ -1
bcf INTCON, TOIF
goto
end
count
; et fini
102
B.2
Program in C
Similar program in C, but uses interrupts instead of polling.
#include <16F877.h> //
header file for address definitions
#fuses XTNOWDT,NOPROTECT,PUT,BROWNOUT,NOLVP
#use delay (clock=4000000) //
int value = 0; //
clock speed
4MHz
8 bit int, global variable
#INTTIMERO // Interrupt service routine
void isr
0
{
value ++; // increment value
outputb(value); // and show the new ''value'' portb on
}
main()
{
settris-a(OxOO); //
make all the ports output
settris.b(OxOO);
settris-c(OxOO);
settris-d(OxOO);
settris-e(OxOO);
output-b(value);// output 0 on portb
enableinterrupts(GLOBAL); //
enables interruptst
//at the global level
enableinterrupts(INTTIMERO)// enables TimerO
103
//
interrupt
settimerO(O) ; sets the timer to zero;
do
}while(true); //infinite loop so that interrupts are
//
always generated
}
104
Appendix C
Latest version of the
microcontroller code
The present verison of the microcontroller code and the code to interface the ultrasonics is listed below.
//Code for CMI LUL pressure module microcontroller
//Version 5 31July2003
//ADC Reset after negative or low values
//4MHz clock
//sending thru Linx at 9600 baud
//config comm always thru RS232, 2400 baud
//delays function of sensorid
#include <16F877.h>
#device *=16 ADC=10
#fuses XT,NOWDT,NOPROTECT,PUT,BROWNOUT,NOLVP
#include <stdlib.h>
#use delay(clock=4000000)
#define SYNC PINB6
#define ADRINP PINB7
#define SCLK PINA2
#define SDATA PINA5
#define DRDY PINEO
#define RFS PIN_El
#define TFS PINE2
#define RADIOPWR PINC5
105
#define RCVPIN PIN_B1
#define MAXMSGLEN 32
#define NUMFIELDS 5
#separate
void init_7713_adc();
void listenmodeo;
void sendserial-messageo;
int data[31 ={ObOOOOOOOO, Ob0000000,
unsigned int presstmr = 0;
unsigned int temp-tmr = 0;
char gstr[421;
int chk-ptr;
int field-ptr[NUMFIELDS];
char sum, sensorid;
int gi,gj;
int data-to-port=1;
unsigned int press-freq = 5;
unsigned int temp-freq = 30;
#INTTIMER2
void istr2()
{
static int mscount = 25;
static short lastval=1;
if (input(RCVPIN) != lastval)
{
listenmode(;
if(last-val==0) lastval=1;
else lastval=0;
}
mscount--;
if(mscount==0)
{
presstmr++;
temp-tmr++;
mscount = 25;
}
}
void add_checksum(char *str)
106
Ob000000};
{
gi = 0;
sum = Ox00;
while(
str[gil
!= OxCO)
{
sum += strEgil;
gi++;
}
str[gil =
sprintf(str + gi + 1, "XX", sum);
str[gi+31 = OxOO;
}
void chop-checksum(char *str)
gi = 0;
while(str[gil
OxOO)
{
if (strEgil ==
{
str[gi] = OxOO;
return;
}
gi++;
}
}
void send radiomessage()
#use rs232(baud=9600, xmit=PINC4, rcv=PINB1)
if (data-to-port==1)
{
sendserialmessageo;
chop-checksum(gstr);
}
addchecksum(gstr);
output-high(RADIOPWR);
for(gi=0; gi<20; gi++)
putc(Ox00);
putc(OxOD);
107
putc(OxOA);
printf (gstr);
putc(OxOD);
putc(OxOA);
printf(gstr);
putc(OxOD);
putc(OxOA);
printf (gstr);
putc(OxOD);
putc(OxOA);
delay-ms(10);
outputjlow(RADIOPWR);
}
void sendserial-message()
#use rs232(baud=2400, xmit=PINBO, rcv=PINB1)
addchecksum(gstr);
printf(gstr);
putc(OxOD);
putc(OxOA);
}
void send-ack(char *msg-type, int result)
sprintf(gstr, "CMI,AR,%c,%s,%d", sensorid, msg-type,
result);
sendserialmessageo;
}
void sendversion()
{
sprintf(gstr, "CMI,VR,%c,P,Ver 5,31JulO3",sensorjid);
sendserialmessageO;
}
void sendid()
{
sprintf(gstr, "CMI,IR,%c",sensorjid);
108
sendserial-messageo;
}
void setid(char id)
writeeeprom(Ox02, id);
sensorid = read-eeprom(Ox02);
sendidO;
}
void sendschedule()
{
sprintf(gstr, "CMI,SR,Xc,/u,%u", sensor id, pressjfreq,
temp-freq);
sendserialmessageO;
}
void setschedule(char *pres, char *tp)
{
int pi, ti;
char temp[51 ,type [31;
sprintf (type, "SS");
if ((presEO]
< '0')
11
(presEO
> '9'))
{
sendack(type,2);
return;
}
if ((tp[0]
< '0')
11
(tp[0] > '9'))
{
sendack(type,2);
return;
}
for (gi=0; gi<5; gi++)
{
tempEgi] = pres[gi];
if (pres[gil==',')
{
temp[gi]=0x00;
break;
}
}
pi = atoi(temp);
109
if ((pi<3) 11
(pi>250))
{
send-ack(type,2);
return;
}
for (gi=0; gi<5; gi++)
{
temp[gil = tp[gil;
if (tp[gil=='*')
{
temp[gi]=OxOO;
break;
}
}
ti = atoi(temp);
if ((ti<3) 11 (ti>250))
{
send-ack(type,2);
return;
}
writeeeprom(Ox04, pi);
writeeeprom(Ox05, ti);
pressjfreq = readeeprom(0x04);
temp-freq = read-eeprom(Ox05);
if ((press-freq != pi) 11
(temp-freq != ti))
{
send-ack(type, 3);
return;
}
sendack(type,O);
sendscheduleo;
}
void senddataout()
sprintf(gstr,"CMI,DR,%c,%d", sensorid, datato-port);
send-serialmessageO;
}
void setdataout(char setting)
char temp[31;
sprintf(temp,"DS");
110
if (setting == '0')
writeeeprom(Ox03, 'n');
else if (setting == '1')
writeeeprom(Ox03, 'y');
else
sendack(temp,2);
datatoport = 1;
if (read-eeprom(0x03)
==
'n') data-to-port
=
0;
senddata-out(;
}
#separate
void sendmisc()
char stri[321;
strcpy(strl,gstr);
strl[311 = Ox00;
sprintf(gstr, "CMI,MR,%c,%s", sensorid, stri);
sendserial-messageO;
chop-checksum(gstr);
sendradiomessageO;
}
short my-get-char(char *the-byte, long timeout)
{
static long i;
for (i=0; i<timeout; i++)
if (kbhit())
*the-byte = getcho;
return 1;
}
delay-us(10);
}
return 0;
}
short my-get-message(char *string, int length,
long timeout)
111
static int byte-count;
bytecount = 0;
while (byte-count < length)
{
if (my-get-char((string + byte-count),
timeout) == 0)
string[byte-count] = Ox00;
if (byte-count==0) return 0;
else return 1;
}
byte-count++;
}
return 1;
}
short checkmessage(char *msg)
chk-ptr = 0;
for (gi=0; gi<NUMFIELDS; gi++)
field-ptr[gil = 0;
for (gi=0; gi<MAXMSGLEN; gi++)
{
if (msg[gi] == '*')
chk-ptr
break;
=
gi;
}
}
if (chk-ptr
==
0) return 0;
gj=100;
for (gi=0; gi<MAXMSGLEN - 2; gi++)
{
if ((msg[gil=='C') && (msg[gi+1]=='M') &&
(msg[gi+21=='I'))
gj = 0;
field-ptr[0] = gi;
112
break;
}
}
if (gj == 100) return 0;
sum = Ox00;
for (gi=field-ptr[0]; gi<chk.ptr; gi++)
sum = sum + msg[gil;
sprintf(gstr, "X", sum);
if ((gstr[0] != msg[chk-ptr + 11)
11 (gstr[1] != msg[chk-ptr + 21))
return 0;
for (gi=field-ptr[01; gi<chkptr; gi++)
if (msg[gi]==',')
{
gj++;
if (gj >= NUMFIELDS) return 0;
field-ptr[gjl = gi+1;
}
}
return 1;
}
void listen-mode()
static char rcvmsg[MAXMSGLEN];
sprintf(gstr,"?");
sendserialmessageO;
rcv-msg[0] = OxOO;
rcv-msg[MAXMSGLEN-1] = OxOO;
if (my-get-message(rcv-msg,
MAXMSGLEN, 65000) == 1)
{
if (check-message(rcv-msg)==0)
continue;
113
if (field-ptr[1] == 0) continue;
if ((rcv-msg[field-ptr[11]
&& (rcv-msg[field-ptr[1]
== 'V')
+ 11 == 'Q'))
{
if ((rcvmsg[field-ptr[211 == sensor-id)
11 (rcv-msg[field-ptr[2]1 == '#'))
{
sendversion();
}
}
else if ((rcv-msg[field-ptr[1]] == 'I')
&& (rcv-msg[field-ptr[1] + 1] == 'Q'))
send-ido;
else if ((rcv-msg[field.ptr[1]] == 'I')
&& (rcv-msg[field-ptr[1
+ 11 == 'S'))
set-id(rcv-msg[field-ptr[211);
else if ((rcv-msg[field-ptr[1]] == 'S')
&& (rcv-msg[field-ptr[1] + 11 == 'Q'))
{
if ((rcv-msg[field-ptr[2]1 == sensor-id)
11 (rcv-msg[field-ptr[2]1 == '#'))
{
sendscheduleo;
}
}
else if ((rcv-msg[field-ptr[1]] == 'S')
&& (rcv-msg[field-ptr[1] + 11 == IS'))
{
if ((rcv-msg[field-ptr[2]1 == sensor-id)
11 (rcv-msg[field-ptr[211 == 1#1))
{
setschedule(&rcv-msg[field-ptr[311,
&rcvmsg[field-ptr[411);
}
}
else if ((rcvmsg[field-ptr[1]] == 'D')
&& (rcv-msg[field.ptr[1] + 11 == 'Q'))
{
if ((rcv-msg[field-ptr[2]1 == sensor-id)
11 (rcv-msg[field-ptr[2]1 == '#))
114
{
senddataout();
}
}
else if ((rcv.msg[field-ptr[1]
==
'D')
(rcv-msg[field-ptr[1
==
'S'))
+ 11
&&
{
if ((rcv-msg[field-ptr[2]1 == sensor-id)
11 (rcv-msg[field-ptr[2]1 ==
{
setdataout(rcv-msg[field-ptr[3]1);
}
}
else {
sprintf(gstr,"Not know <<s>>",rcvmsg);
sendserialimessageO;
}
}
else
{
sendserial-messageo;
}
delay-ms(50);
}
void write_7713_adc()
{
int x[31 = {OblO100001, OblOO00111, OblO111001};
//
LSB
MSB
//mode = act. background calib.
//PGA gain = 64
//channel AIN2
////
//24 bit word
//RTD off
//burn-out off
//bi-polar
//filter so 2Hz
output-low(ADRINP);
delay-cycles(1);
115
output low(TFS);
for (gi=0; gi<24; gi++)
{
output-bit(SDATA, shiftleft(x,sizeof(x),0));
delay-cycles(1);
outputhigh(SCLK);
delay-cycles(2);
outputlow(SCLK);
delay-cycles(1);
}
output-low(SDATA);
delay-cycles(1);
output._high(TFS);
delay-cycles(1);
output-high(ADRINP);
}
#separate
void read_7713_adc()
{
gj =0;
while (true)
if (input (DRDY)==0)
{
outputlow(RFS);
for(gi=0; gi<24; gi++)
{
delay-cycles(2);
shiftleft(data,3,input(SDATA));
outputhigh(SCLK);
delay-cycles(2);
outputjlow(SCLK);
}
output-high(RFS);
break;
}
delay-ms(100);
if (gj>100)
{
116
sprintf(gstr,"7713ResetADC");
send-misco;
init_7713_adco;
gj=0;
}
}
}
void read_7713_adcreg()
{
outputjlow(ADRINP);
output_low(RFS);
for(gi=O; gi<24; gi++)
{
delay-cycles(2);
shift_left(data,3,input(SDATA));
output-high(SCLK);
delay-cycles(2);
outputilow(SCLK);
}
output-high(ADRINP);
output-high(RFS);
}
void report-reg()
{
int bit;
char inbin[251;
for(bit=0; bit<24; bit++)
{
if(shift§left(data,3,0)==1)
inbin[bit] = '1';
else
inbinEbit] = '0';
}
in-bin[24] = Ox00;
sprintf(gstr,"ADC: %s", in-bin);
sendmisco;
}
117
#separate
void send-press-msg()
{
float step = 0.5000;
float value = 0.0000;
//int offsetbinary = true;
for (gi=0; gi<24; gi++)
{
if (shiftjleft(data,3,0)==1)
value += step;
step = step/2.000;
}
value
-=
0.5000;
sprintf(gstr,"CMI,PR,%c,%01.9f", sensorid, value);
sendradiomessageO;
if (value < 0.01)
{
sprintf(gstr,"SPMResetADC");
sendmisco;
init_7713_adco; //reset ADC
}
}
long onboardadc(int num)
{
long reading;
setadcchannel(num);
delay-ms(100);
reading = read-adco;
return reading;
}
#separate
void send-temp-msg()
{
long reading;
float ftemp, fvolt;
reading = onboardadc(0);
118
fvolt = (reading/1023.00)*10.00;
reading = onboard-adc(1);
ftemp = (reading/1023.00);
sprintf(gstr, "CMI,TR,Xc,1.lf,%01.3f", sensorid,
fvolt, ftemp);
sendradiomessageO;
}
#separate
void init_7713_adc()
settris-a(ObOO001011);
output-high(RFS);
delay-ms(1);
output-high(TFS);
delay.ms(1);
output-high(ADRJINP);
delay-ms(1);
output-low(SYNC);
delay-ms(1000);
output-high(SYNC);
delay-ms(1);
write_7713_adc0;
settris-a(ObOO101011);
delay-ms(1);
read_7713_adcreg();
report-rego;
}
main()
{
int badreads=O;
char seq-char;
settris-a(ObOO001011);
settris-b(ObOO001100);
settris-c(ObOO010000);
settrisAd(OxOO);
settris-e(ObOO000001);
setup-adc(adc-clock-div_32);
119
setup-adc-ports(RAORA1_ANALOGRA3_REF);
output-low(RADIOPWR);
sensorid = read-eeprom(Ox02);
if (read-eeprom(0x03) ==
'n') datato-port = 0;
pressjfreq = read.eeprom(Ox04);
temp-freq = read-eeprom(Ox05);
if ((press-freq > 250) 11 (pressjfreq < 3))
pressjfreq = 5;
if ((temp-freq > 250) 11 (temp-freq < 3))
tempjfreq = 30;
if ((sensorid >= 'A') && (sensorid <= 'Z'))
{
for (seq-char = 'A'; seq-char < sensorid;
seq-char++)
delay-ms(222);
}
sprintf(gstr,"Hello");
sendmisco;
init_7713_adcO;
enableinterrupts(INTTIMER2);
enable-interrupts(GLOBAL);
setup-timer_2(T2_DIVBY_16, 250, 10);
settimer2(0);
while(true)
{
read_7713_adc();
if(data[0]==0x00 && data[1]==OxOO &&
data [21 ==0x00)
badreads++;
else if (data[01==0xFF && data[1]==OxFF
&& data[21==0xFF)
badreads++;
120
else
badreads=O;
if (badreads>2)
{
sprintf(gstr,"MainResetADC.");
send-misco;
init_7713_adcO;
badreads=O;
}
while (press-tmr < pressjfreq)
delay-ms(10);
if ((sensorid >= 'A') && (sensorid <= 'Z'))
{
for (seq-char = 'A'; seq-char < sensorjid; seq-char++)
delay-ms(73);
}
sendpress-msgo; //pressure
press-tmr = 0; //reset timer
if (temp-tmr >= tempjfreq) //time to send temperature
{
send-temp-msgo;
temp-tmr = 0;
}
}
}
C.1
Code to interface the ultrasonics
#include <16F877.h>
#device *=16 ADC=10
#fuses XT,NOWDT,NOPROTECT,PUT,BROWNOUT,NOLVP
#use delay (clock=4000000)
#include<stdlib.h>
#define DE PIND2 // TP9 driver enable pin for transmitting,
I/ high while transmitting
#define RE PIND3 II TP8 receive enable pin for receiving,
121
#define
#define
#define
#define
#define
//
low while receiving
TX485 PINC6
RX485 PINC7
TXRF PINC4
RXRF PIN_B1
RADIOPWR PINC5
char v;
void config()
{
int i;
int data[7];
output-high(DE); //
delay-cycles(1);
b6 is driver enable for transmitting
#use rs232(baud=19200, xmit=TX485, rcv=RX485)
putc(OxOD);
putc(Ox1B);
printf("#W840D0101000093\r# S1COD010000002A\r\r");
putc('-');
output-low(DE);
output-low(RE);
}
main()
{
int data[7];
char chars[301;
int i, j,k,l,m, sum;
int32 value;
long values[201;
write-eeprom(2,'U');
output-high(RE);
output_low(DE);
config();
do
122
{
#use rs232(baud=19200, xmit=TX485, rcv=RX485)
gets(data);
if(data[0] =
{
values [kl= atol(data);
value += values[k];
k ++;
}
if(k==20)
{
value = value/20;
sprintf(chars,"CMI,UR,U,SW,%lu",value);
for(l=0; 1<30; 1++)
{
if (chars [1
!=0x00)
sum += chars[1];
else break;
}
#use rs232(baud=9600, xmit=TXRF,
output-high(RADIPWR);
delay.cycles(1);
printf("CMI,UR,U,SW,Xlu*XX\r\n",
printf("CMI,UR,U,SW,Xlu*%X\r\n",
printf("CMI,UR,U,SW,Xlu*%X\r\n",
delay-ms(10);
outputjlow(RADIOPWR);
k =0;
value =0;
sum =0;
}
} while(true);
}
123
rcv=RXRF)
value,sum);
value,sum);
value,sum);
124
Appendix D
ASCII version of CMI
Communication Protocol
This is the actual draft of the protocol used for composing the messages. This has
been written by James Brooks, who also worked on this project.
This is a protocol for communication between field sensors and the controlling
base station. It is not as data-efficient as a binary protocol, but it is easier to parse
and debug as the messages can be read by users. New messages can be added at any
time within this framework. The specifics outlined here pertain to Version 4 of the
microcontroller code currently loaded on the pressure sensors' microcontrollers.
For the current installation, the radio link is through a Linx-brand transmitter at
9600 baud. Data messages (Pressure and Temperature Reports) and Miscellaneous
Reports are sent out over the radio link to the base station.
The microcontroller boards also have the option to communicate directly to a
computer through a serial port at 2400 baud. (Faster baud rates were found problematic with some computers.) The computer can configure the microcontroller over
this link. Messages to query or to set settings on the microcontroller come from the
controlling computer over this link. Messages reporting the status of various settings
pass from the microcontroller to the controlling computer over this link. Optionally,
the data messages containing the readings the sensor is taking also come over this
link.
125
A software utility written for this project can be used to send messages to the microcontroller. Any application (such as HyperTerminal) set to 2400 baud can listen to
the microcontroller output. Sending messages is more difficult as the microcontroller
is not always listening to the serial port. Our application sends out the byte OxFF
continuously until it hears a response. When the microcontroller hears the bytes coming in on the serial port, it sends back the character '?' meaning it is ready to listen.
The application on the computer side then sends the message to the microcontroller.
Data Fields: The different fields of the messages will be separated by commas
for easy parsing.
Start bytes: Every message will start with three bytes, the letters "CMI" these
indicate that a message is starting. These letters will never appear elsewhere in any
message.
Message Type: The next field after the start bytes will be the message type.
This will be a two letter code identifying what the message is about. The types fall
into three categories: Queries, Reports, and Settings, the codes for these messages
end in 'Q', 'R', and 'S' respectively.
ID: Each sensor has a unique ID. This is one character, stored in non-volatile
memory on the microcontroller. Most messages contain the ID of either the sensor
reporting in, or of the sensor that the message is intended for. This unique ID is in
some cases duplicated by the fact that some wireless systems have a feature that the
source of messages is tagged. However, having our own identities frees us to use any
method for transmitting the messages, even a simple wired connection. The character
'#' as the ID in a message means that this message is intended for all sensors who
hear it.
Checksum: Every message ends with a checksum. The purpose of the checksum
is to identify when the bytes of a message have been corrupted. The receiver of the
message should check that the checksum is correct and discard the message if it is
not. The checksum is two characters long and appears after an asterisk appended to
the message. To calculate the checksum, start with a byte set to zero. Starting at
the first character of the message (the 'C' of "CMI") and ending before the asterisk,
126
add the binary value of each byte into the sum. The resulting one-byte value can
be represented by a two-digit hexadecimal number. These two digits serve as the
checksum.
Messages
Identity Query(IQ): This is sent by the base to a certain sensor to ask what that
sensor's ID is. The sensor should answer with Identity Response.
Fields: Start, Type
Example: CMI,IQ*9F
Meaning: "What is your name?"
Identity Report (IR): The sensor uses this to report what its ID is. For the moment
the identity is one-byte long, but future applications can use longer IDs.
Fields: Start, Type, ID
Example: CMI,IR,B*OE
Meaning: "I am Sensor B"
Identity Set (IS): This is sent by the base to a sensor to set its ID. It is anticipated
that it will only ever need to be set once for each sensor. It is important that only
this sensor is listening when this message is sent out, otherwise multiple sensors will
have the same ID. The sensor replies with the Acknowledgement Report to say if the
ID switch was successful.
Fields: Start, Type, ID
Example: CMI,IS,B*OF
Meaning: "You will be called B from now on."
Version Query (VQ): The base uses this to find out from the sensor what version
of firmware is running on the sensor's microcontroller. It is answered with Version
Report.
Fields: Start, Type, ID
Example: CMI,VQ,E*1D
Meaning: "Sensor E,what is your firmware version?"
127
Version Response (VR): The sensor responds to a Version Query with this message
containing its firmware.
Fields: Start, Type, ID, Type of Sensor (P=pressure, U=ultrasonic, more as needed),
Version, Date
Example: CMI,VR,E,P,Ver2,17JunO3*69
Meaning: "I am Sensor E, a Pressure sensor, running version 2 of the firmware, from
June 17th."
Pressure Report (PR): The pressure sensor sends this one out automatically based
on the schedule it's been programmed with.
Fields: Start, Type, ID, Pressure Reading
Example: CMI,PR,E,0.12654336*40
Meaning: "Sensor E reporting a pressure of 0.12654336."
Temperature Report (TR): The pressure sensor also sends out (with less frequency)
this message containing the supply voltage to its board and the thermistor reading.
The conversion of thermistor reading to temperature will take place at the base.
Fields: Start, Type, ID, Supply Voltage, Thermistor Voltage
Example: CMI,TR,E,8.6,0.878*15
Meaning: "Sensor E reporting a supply voltage of 8.6 volts, and a thermistor reading
of 0.878."
Schedule Query (SQ): This is to ask the microcontroller the frequency of its automatically outputted messages. Fields: Start, Type, ID
Example: CMI,SQ,G*1C
Meaning: "Sensor G, how are your messages scheduled?"
Schedule Report (SR): The microcontroller on a pressure module uses this message
to report how its automatic reporting schedule is set. It sends it in response to a SQ
message and also after it sets itself in response to an SS message. Frequencies are in
seconds.
Fields: Start, Type, ID, Pressure Frequency, Temperature Frequency
Example: CMI,SR,G,5,30*OD
128
Meaning: "This is Sensor G. I send pressure measurements every 5 seconds and
temperature readings every 30 seconds."
Schedule Set (SS): This is to schedule the frequency of the automatically outputted
messages on a pressure sensor. The values are in seconds. Acceptable values are from
3 seconds to 250 seconds. The temperature messages cannot be more frequent than
the pressure messages.
Fields: Start, Type, ID, Pressure Frequency, Temp. Frequency
Example: CMI,SS,G,10,40*3B
Meaning: "Sensor G, you are to report pressure every 10 seconds, temperature every
40 seconds."
Data Messages Query (DQ): This is to ask the microcontroller whether the data
messages (PR and TR) are sent only to the radio, or also to the serial connection.
A user attached with a serial cable can view the data messages if they are on, but if
the module is going to be wireless for a long period, with no one listening through a
serial cable, it is wasteful to send the messages out both ways. A value of 1 means
the data messages also go to the serial port; a value of zero means the data messages
do not go to the serial port. Default is 1.
Fields: Start, Type, ID
Example: CMI,DQ,C*09
Meaning: "Sensor C, do you send data messages to the serial port?"
Data Messages Report (DR): The microcontroller on a pressure module uses this
message to report whether or not it is outputting data messages (PR and TR) on the
serial port as well as the radio port. A value of 1 means the messages go out on both,
a value of 0 means they go only out the radio port.
Fields: Start, Type, ID, Value
Example: CMI,DR,C,1*67
Meaning: "This is Sensor C. I send my data messages out both the radio port and
the serial port."
129
Data Messages Set (DS): This is to set whether or not the data messages (PR and
TR) are sent out over the serial port as well as the radio port. The only acceptable
value is 0 (messages only over radio port) or 1 (messages over both radio and serial
ports).
Fields: Start, Type, ID, Value
Example: CMI,DS,C,0*67
Meaning: "Sensor C, stop sending data reports over the serial port."
Miscellaneous Report (MR): Whenever the sensor has anything else to relate to
the outside world, it can use this message. What action the base takes is undefined
at the moment, but this should be useful for debugging.
Fields: Start, Type, ID, Message
Example: CMI,MR,B,Resetting ADC in MainO*C8
Meaning: "This is sensor B and I felt the need to tell you that."
Acknowledgement Report (AR): Certain messages sent by the base are answered
by this message to communicate whether the action was taken or not.
Fields: Start, Type, ID, Type of Message Being Acknowledged, Error Code Error Codes: 0 =Incoming message accepted and action undertaken successfully 1 =
Checksum of incoming message was wrong 2 = Message accepted, but contained unacceptable values 3 = Message accepted, but action not taken Others to be added as
necessary
Example: CMI,AR,G,SS,0*39
Meaning: "Sensor G accepts the Schedule Setting message just sent to it."
Ultrasonic Distance Report (UR): The microcontroller running the Hexamite ultrasonic distancing system reports measured distances back to the base using this
message. It specifies the ID of the microcontroller reporting in as well as the identities of the two transducers acting as endpoints for the measurement. The identity
of the ultrasonic transmitter comes first and the identity of the ultrasonic receiver
comes last. The distance reported is in the same units as output by the Hexamite
system; one unit is approximately 0.7mm. The distance reading is in decimal form
130
as opposed to the hexadecimal output by the Hexamite.
Fields: Start, Type, ID, Path of measurement (Unit "S" to unit "U"), Distance
Example: CMI,UR,Q,SU,1234*F3
Meaning:
"Sensor
Q
reports that the distance measured from transceiver S to
transceiver U is 1234 units."
131
132
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