A LOW-COST LINEAR-RESPONSE TEMPERATURE SENSOR FOR
EXTREME ENVIRONMENTS
A Thesis Presented
by
Michael Fortney
to
The Faculty of the Graduate College
of
The University of Vermont
In Partial Fulfillment of the Requirements
For the Degree of Master of Science
Specializing in Electrical Engineering
March, 2007
Accepted by the Faculty of the Graduate College, The University of Vermont, in partial
fulfillment of the requirements for the degree of Master of Science, specializing in
Electrical Engineering.
Thesis Examination Committee:
______________________________
Jeff Frolik, Ph. D.
Advisor
______________________________
Steve Titcomb, Ph. D.
______________________________
Jun Yu, Ph. D.
Chairperson
______________________________
Frances Carr, Ph. D.
Vice President of Research
Date: December 4, 2006
1
Abstract
This Thesis contributes to the area of Wireless Sensor Networks (WSN) by
presenting a system level evaluation of design strategies and routing algorithms.
Specifically, power consumption (and consequently, network lifetime) and cost of
implementation are considered. To date, these two facets of WSN design have inhibited
the deployment of sensor networks on a large scale envisioned by many researchers.
Thus, this research strives to minimize these parameters whenever the opportunity
presents itself.
Existing hardware implementations were deemed too complex and too costly to
implement a sensor network on the desired scale of around 50 nodes. As such, custom
hardware (the UVM-WSN) was designed and utilized in this work. The hardware design
includes an ALOHA node which provides a transmit only sensor platform, a Hopper node
which acts as both a sensor node and a repeater node for other sensors, and a Gateway
which provides an embedded web server with power over Ethernet capability. The
MOTE units, developed by the University of California at Berkeley, were used in a
comparison study to evaluate improvements in network lifetime gained through an
implemented hierarchical design and through the use of a power aware routing algorithm.
Specifically, the objectives of this research include:
1) The design and development of a hierarchical sensor network that takes advantage of
the transmit-only functionality of the ALOHA nodes to conserve power and reduce cost.
2) The development and simulation of an energy-aware routing algorithm in this sensor
network that uses a new form of route determination known as VAARP, Voluntary
Assisted Energy Aware Routing Protocol, which increases network lifetime under
specific constraints.
3) A comparison study of existing WSN routing protocols with respect to the VAARP
protocol.
The platform developed for this research is inherently open ended. Through the
use of a simple I2C communications protocol, communications with many different
sensors may be rapidly achieved. However, as presented, the sensor board will track five
parameters - battery voltage, humidity, temperature, acceleration, and barometric
pressure. The hardware interface used in the design of the sensor board allows other
sensors to be easily deployed and utilized for a specific application. It is expected that the
UVM-WSN capable of tracking temperature with 48 nodes would have a lifetime of
approximately 1.8 years for ALOHA nodes and 1 week for Hopper nodes and a cost
around $750.
Acknowledgements
This project would not have been successful without the strong support of the
Electrical and Computer Engineering Department at the University of Vermont. I am
particularly indebted to my advisor, Dr. Jeff Frolik, for his patience and persistence in
helping me see this project through to conclusion. His suggestions have kept me on track
and focused on completing relevant research without getting sidetracked on one of the
many other avenues that we have discovered along the way. I would also like to thank
Benji Capsuto for the great deal of time and effort the he invested in developing a superb
web interface for the sensor network. Lastly I would like to thank Jake Galbreath,
Christian Bird and Mike Fortney for providing many great technical suggestions that
would result in the best final product.
Finally, I would like to dedicate this thesis to my father Jay Maciejowski, my
mother Ginger Maciejowski and my sisters Karli and Katie Maciejowski for all the love
and support they have given me and to Allaire Diamond for supporting me through all
my endeavors.
Table of Contents
Acknowledgements
3
List of Tables
6
CHAPTER 1: INTRODUCTION....................................................................................................................9
1.1.
Thesis Motivation .........................................................................................................................9
1.2.
Thesis Objective ......................................................................................................................... 11
1.3.
Examples of Existing Sensor Networks ..................................................................................... 12
1.4.
Contributions .............................................................................................................................. 14
1.5.
Thesis Organization.................................................................................................................... 16
CHAPTER 2: ADAPTATION OF A LOW-COST WIRELESS SENSOR FOR FRESHMAN AND
OUTREACH PROGRAMS ....................................................................................................... 17
2.1.
Intro to Paper .............................................................................................................................. 17
2.2.
Abstract
2.3.
Introduction ................................................................................................................................ 17
2.4.
The CricketSat System ............................................................................................................... 18
2.5.
UVM CricketSat Development and Testing ............................................................................... 20
2.6.
High School Outreach ................................................................................................................ 21
2.61.
2003 – 2004 HELiX Team ......................................................................................................... 23
2.62.
2004 – 2005 HELiX Teams........................................................................................................ 24
2.7.
College Freshman Engineering Course ...................................................................................... 29
2.8.
Conclusions ................................................................................................................................ 30
2.9.
Acknowledgements .................................................................................................................... 30
17
CHAPTER 3: CRICKETSAT SENSOR AND SYSTEM DEVELOPMENT .............................................. 32
3.1.
Introduction ................................................................................................................................ 32
3.2.
The Original Stanford CricketSat Design (1999) ....................................................................... 34
3.3.
UVM CricketSat Workshop Planning (Spring 2003) ................................................................. 35
3.4.
UVM CricketSat Revision A (Spring 2003) .............................................................................. 36
4
3.5.
2003 CricketSat Workshop and Test Flights (June 16 – June 20) .............................................. 40
3.6.
UVM CricketSat Test Flight (08/08/2003) ................................................................................. 42
3.7.
GIV CricketSat Workshop and Test Flight (08/09/2003) ........................................................... 44
3.8.
HELiX CricketSat Weather Stations (12/2003 – 02/2004) ........................................................ 45
3.9.
UVM CricketSat Revision C (January 2004) ............................................................................. 48
3.10
UVM CricketSat Revision D (May 2004) .................................................................................. 49
3.11.
2004 CricketSat Workshop and Test Flights (June 14 – June 18) .............................................. 51
3.12.
First Collaborative BalloonSat Flight (July 17, 2004) ............................................................... 53
3.13.
Second Collaborative BalloonSat Flight (July 30, 2004) ........................................................... 55
3.14.
UVM CricketSat Revision E (December 2004) ......................................................................... 59
3.15.
UVM CricketSat Revision F (May 2005) .................................................................................. 62
3.16.
Conclusions ................................................................................................................................ 63
CHAPTER 4: A LOW-COST LINEAR-RESPONSE TEMPERATURE SENSOR FOR EXTREME
ENVIRONMENTS .................................................................................................................... 66
4.1.
Intro to Paper .............................................................................................................................. 66
4.2.
Abstract
4.3.
Introduction ............................................................................................................................... 66
4.4.
555-Timer Astable Oscillator .................................................................................................... 70
4.5.
Linear Frequency Control Methods........................................................................................... 74
4.5.1.
Threshold Control Voltage Method............................................................................................ 74
4.5.2.
Ladder Voltage Control Voltage Method ................................................................................... 79
4.5.3.
Current Source Method .............................................................................................................. 82
4.5.4
Comparison of alternative methods ............................................................................................ 85
4.6
Implementation and Test ............................................................................................................ 86
4.61
Implementation of a linear sensor .............................................................................................. 86
4.62
Test Scenarios ............................................................................................................................ 87
4.63
Test Results ................................................................................................................................ 89
66
5
4.7
Conclusions ................................................................................................................................ 93
4.8
Acknowledgement ...................................................................................................................... 93
CHAPTER 5: SUMMARY AND CONCLUSIONS ..................................................................................... 95
5.1.
Conclusions ................................................................................................................................ 95
5.2.
Future Research .......................................................................................................................... 97
5.3.
Final Thoughts ......................................................................................................................... 100
Bibliography
101
List of Tables
Table 2.1: HELiX CricketSat Workshop Schedule ...................................................................................... 22
Table 3.1: UVM CricketSat sensor Revsions A-E and proposed Revision G. ............................................. 64
Table 3.2: UVM CricketSat significant milestones. ..................................................................................... 64
Table 4.1: Comparison of simulation results for various the oscillator control methods. ............................ 85
Table 4.2: Calibration methods and sources ................................................................................................. 90
Table 4.3: Mean error/standard deviation (ºC) produced using various calibration methods. “Z” calibration
includes absolute zero data point, “NZ” does not. ................................................................................. 91
List of Figures
Figure 1.1: Great Duck Island Network Topology ........................................................................................ 10
Figure 1.2: Huntington Gardens Monitoring Project .................................................................................... 13
Figure 1.3: Commonly available sensor nodes .............................................................................................. 14
Figure 2.1: CricketSat Wireless Temperature System .................................................................................. 19
Figure 2.2: UVM CricketSat Wireless Temperature Sensor ........................................................................ 21
Figure 2.3: The John D. O'Bryant School CricketSat presentation of the MHS-1 flights. The team received
a 2nd-place finish at the 2005 Boston Regional Science Fair ................................................................. 25
Figure 2.4: Medgar Evers College (MEC) team preparing for a BalloonSat launch at the Milton High
School. The CUNY school provided flight support for the MHS-2 payload. ...................................... 26
Figure 2.5: The Milton High School CricketSat Array System flown on the MHS-3 flight. This project
allows the measurement of several CricketSat sensors over 50 miles away. ........................................ 27
Figure 2.6: Properly segregated data collected from the Milton High School developed CricketSat Array
System. Raw frequency results are shown. .......................................................................................... 27
Figure 2.7: Example student design projects: Wireless Wind-Chill Instrument (left) and Wireless Door
Alarm (right) ......................................................................................................................................... 30
Figure 3.1: The original Stanford CricketSat schematic and completed circuit board. ................................ 34
Figure 3.2: UVM initial redesign (RevA) of the Stanford University CricketSat. ....................................... 37
Figure 3.3: UVM RevA CricketSat temperature, pressure and humidity sensors. Pressure and humidity
sensors are inserted in the prototype area. ............................................................................................. 39
Figure 3.4: CricketSat flights conducted from the bridge near the Sandbar State Park. Students are shown
preparing, launching, and tracking CricketSat sensors. ......................................................................... 41
6
Figure 3.5: Results from a pulse-mode CricketSat launched from the UVM campus. The graph to the left
shows the flight temperature profile from launch to landing. The right graph represents an estimated
altitude profile based on an adiabatic cooling rate. ............................................................................... 43
Figure 3.6: CricketSat weather station located at the Walforf High School fitted with a solar panel for
long-term use. ........................................................................................................................................ 45
Figure 3.7: The Waldorf CricketSat weather station. Plastic-plate and threaded-rod construction (left),
stacked CricketSat sensors (center), sensor selection switch and battery (right). .................................. 46
Figure 3.8: CricketSat receiver designed to work with the CricketSat weather station and was later used for
all CricketSat applications. .................................................................................................................... 47
Figure 3.9: Schematic diagram of the UVM RevC CricketSat. ................................................................... 48
Figure 3.10: CricketSat RevC temperature sensor assembled circuit board. Green solder mask, white
labeling and strain-relief holes were added to this design. .................................................................... 49
Figure 3.11: CricketSat RevD schematic diagram. The voltage regulator provides circuit stability and is
required for active sensors. .................................................................................................................... 50
Figure 3.12: CricketSat RevD temperature and pressure sensors. ................................................................ 50
Figure 3.13: Students using the Spectra RTA software to perform CricketSat calibration measurements. . 52
Figure 3.14: CricketSat daytime temperature sensor flight. The sensor experienced a 114 ºF temperature
change during the flight. ........................................................................................................................ 52
Figure 3.15: Night-flight data collection from a CricketSat pressure sensor used as an altimeter. The
graph shows the linear ascent rate of the balloon during the flight. ...................................................... 53
Figure 3.16: Students prepare for a BalloonSat flight from the Milton High School. Temperature data (red
trace) in the figure to the right shows the communications flight bag temperature during flight. ......... 54
Figure 3.17: CricketSat sensor array timing diagram. .................................................................................. 55
Figure 3.18: CricketSat power sequencing circuit using a BASIC Stamp II controller. .............................. 56
Figure 3.19: CricketSat Sensor Array System ............................................................................................. 57
Figure 3.20: BalloonSat flight path (left) and raw segregated CricketSat temperature and pressure data
(right) received during the flight. .......................................................................................................... 57
Figure 3.21: Converted CricketSat temperature data (left) and altimeter data (right). CricketSat data is
compared to known data for sensor validation. ..................................................................................... 59
Figure 3.22: CricketSat RevE circuit board. ................................................................................................ 60
Figure 3.23: CricketSat RevE schematic. .................................................................................................... 61
Figure 3.24: CricketSat RevF schematic. .................................................................................................... 62
Figure 3.25: CricketSat RevF circuit board. ................................................................................................ 63
Figure 4.1: CricketSat non-linear frequency response to temperature ......................................................... 68
Figure 4.2: 555-timer astable oscillator CricketSat design ........................................................................... 70
Figure 4.3: 555-timer internal circuitry composed of the threshold voltage ladder, comparators, RD latch
and discharge transistor. Image for Texas Instruments NE555, SA555, SE555 Precision Timers data
sheet....................................................................................................................................................... 72
Figure 4.4: PSpice astable timing waveforms of the 555-timer circuit. The top trace shws the voltage on
the timing capacitor. The bottom trace represents the digital output of the timer. ............................... 73
Figure 4.5: PSpice simulation circuit for using a voltage source (V2) to override the timer’s internal
threshold compare voltages. The 5-Volt source is used to represent the regulated voltage, which is
now necessary for this mode of operation. ............................................................................................ 75
Figure 4.6: Simulation waveforms showing the effect of control voltage on the charging cycle of the
timing capacitor. The top and bottom waveform shows the capacitor voltage using a control voltage
of 4.0 Volts and 2.0 Volts, respectively. Note that the lower limit is always one-half the control
threshold value. ..................................................................................................................................... 76
Figure 4.7: Simulation results demonstrating 555-timer frequency response to varying threshold control
voltage. .................................................................................................................................................. 79
Figure 4.8: Simulation circuit used to investigate the effect of RC ladder voltage (V2) on timer frequency.
In comparison to Fig. 4.5, the value of R2 has been reduced to increase duty cycle and minimize the
non-linearity. ......................................................................................................................................... 80
Figure 4.9: Simulated response of timer frequency to control of RC ladder voltage. .................................. 82
7
Figure 4.10: PSpice simulation circuit used to investigate the use of a current source method for 555-timer
frequency control. .................................................................................................................................. 83
Figure 4.11: Schematic showing implementation of the LM234 current source device as a temperature
sensor. The value of R1 (i.e. RSET) is selected to provide a suitable sensitivity and frequency range. .. 87
Figure 4.12: CricketSat module modified using the LM234 current source for use as a temperature sensor.
............................................................................................................................................................... 88
Figure 4.13: Test results for the thermistor-based and current-source-base CricketSat temperature sensors.
............................................................................................................................................................... 90
Figure 4.14: Figure to the right demonstrates the linear response of the CricketSat oscillator circuit over a
wide current range. The highlighted region near the origin is shown in the left figure, showing the
trend line passing near the origin. .......................................................................................................... 92
Figure 5.1: Hopper node and ALOHA node photo ........................................................................................ 96
8
CHAPTER 1: INTRODUCTION
1.1.
Thesis Motivation
Wireless Sensor Networks (WSN) are rapidly becoming prevalent in many facets
of everyday life. Sensor networks facilitate easy retrieval of useful data collected from
sources that may be spread over a wide area without the intensive infrastructure
requirements of a wireline network. In fact, a recent study found that 90% of wireline
sensor system cost for industrial settings was due to the cabling required to connect a
sensor and the data acquisition system [1].
There are myriad possibilities for the
deployment of wireless sensors, from measuring environmental parameters to monitoring
border security. One specific deployment of a wireless sensor network is the Great Duck
Island Project.
This project utilizes MOTES units, developed by UC-Berkeley, to
monitor the environmental conditions of snow petral nests on Great Duck Island off the
coast of Maine [2]. This project provides specific insight into the behavior of snow petral
with respect to the environmental conditions in which they reside. The data extracted
from this network can, in addition, be used by wildlife management personnel to make
decisions regarding habitat conservation.
Figure 1.1 shows a typical multi-hop network topology adopted by many WSN
implementations. Data is collected from a number of sensors deployed in the field and
relayed back to a Base-station or Gateway through the use of packet forwarding. This
data is then presented to a client or user via the Internet in some meaningful form. The
data may then be utilized by the client for research purposes or general monitoring.
9
Figure 1.1: Great Duck Island Network Topology
Given their capability to gather a large variety of data, WSN have the potential to
significantly affect private decision making as well as public policy. Hopefully, WSN
will provide policymakers with a powerful tool for educated decision making. As such,
MIT Technology Review has named WSN one of ten technologies that will change the
world [3].
Numerous problems must be addressed before large-scale deployments of WSN
may be realized. First, no universal standard has been established for WSN design, thus
unlike wireless standards such as 802.11b, WSN has unique problems specific to each
platform that restrict portability. Generally, the goal of a WSN is to extract and present a
large amount of environmental data on a near continuous time basis from a group of lowcost, widely distributed sensors. Since maximum network life is desired and the amount
of energy available for each node is finite (nodes are typically battery powered), low
power implementations are a requisite to meet this objective. This objective differs from
typical wireless networks because a product implementing the 802.11 standard typically
has no restriction on the amount of power it may use to move packets around a network
(they are typically connected to the grid). The lack of a WSN standard is an issue that
needs to be addressed and is being looked at by IEEE 802.15 Task group 4 [4]. This
working group developed the 802.15.4 specification that Motorola Corporation has
adopted in the development of a new RF modem (available early 2004). Additionally
Motorola has adopted the open architecture networking protocol for the MAC (Media
10
Access Control) layer offered by the Zigbee Alliance [46]. This platform offers a great
deal of functionality over existing platforms including support for frequency hopping,
high data rates, spread spectrum encoding and support for star, mesh and peer to peer
network topologies. This will provide a good platform for future sensor node firmware
development once it becomes available. The research presented herein precedes this latest
development and takes advantage of the lack of restrictions placed on the ISM band to
facilitate custom designs for hardware and communication protocols.
Another desirable attribute of a WSN is to have it be as non-intrusive and low
cost as possible. This objective implies that a node will be sized to minimize impact on
the environment in which they are deployed and that the node's design will also seek to
minimize costs in order to facilitate large scale deployments.
1.2.
Thesis Objective
The work presented herein seeks to address two areas of interest in the WSN
arena – (1) power consumption and (2) cost. These two areas are addressed through the
development and implementation of a hierarchical WSN with a power aware routing
algorithm. Further, the network developed for this thesis work utilizes a custom built,
low-cost platform that achieves a large volume (i.e.; sensed temperature values every 14
seconds from each of 40 nodes) of data collection at a relatively low cost in comparison
with commercially available WSN implementations.
The designed network was
evaluated for overall effectiveness in terms of network lifetime. Though an few specific
sensing applications are investigated, the open ended design allows for future
enhancements to adapt the WSN for unique sensing tasks such as thermal monitoring or
11
traffic monitoring on a bicycle path.
Specifically, the main objectives of this thesis are as follows:
1
Develop low cost hardware that can implement a hierarchical WSN architecture.
This objective included the design and development of a hierarchical WSN with nodes
consisting of three different node classes - ALOHA, Hopper, and Gateway. ALOHA
nodes are transmit-only sensor platforms while the Hopper and Gateway nodes have both
receive and transmit capabilities.
The nodes are developed using a low cost
microcontroller, transmitter and receiver chips.
2
Develop a power aware routing algorithm for moving data within a WSN.
This objective encompassed the development of the VAARP protocol which allows the
routing tree within the UVM-WSN (University of Vermont Wireless Sensor Network) to
adjust itself in an attempt to achieve a longer overall network lifetime.
3
Evaluate the performance of the routing algorithm and hierarchical design.
The UVM-WSN is evaluated with and without a power aware algorithm installed in the
firmware of the Hopper nodes. Additionally, the UVM-WSN is evaluated with respect to
existing WSN implementations with regard to power consumption and cost.
1.3.
Examples of Existing Sensor Networks
WSN is an emerging field in which many companies and organizations are
developing their own implementations to meet specific sensing criteria. For example,
Microstrain [5] corporation of Williston, Vermont utilizes a single hop, single channel
strategy to collect acceleration data in its G-Link modules. These units operate in the 433
MHz ISM band and allow the base station to poll a single sensor module many times a
12
second to receive acceleration data.
These units are very application specific, are
essentially wireless accelerometer units, and provide little flexibility in signal forwarding
in a large-scale network. They do, however, effectively collect the requisite acceleration
data and present it in a timely fashion with limited complexity. Specific disadvantages of
this platform is the limited range (less than 50 feet line of sight), a 9 volt battery power
supply which increases the size of the G-Link package, and the cost of the package as a
whole (approximately $400).
UC-Berkeley [6] has pursued WSN through the development of the MOTES
package and recently the miniaturized version of the MOTE unit aptly named SPEC. A
clear disadvantage of the MOTES units are cost [7]. These units, now manufactured by
Crossbow, cost $215 for the nodes themselves and an additional $100-$200 for the sensor
boards required to collect data. The MOTES offer more flexibility than a G-Link module
in that they can perform multi-hop packet forwarding from remote sensors and may be
configured to poll at different intervals.
UCLA [8] and NASA's Jet Propulsion Labs
[8] have invested time and effort in developing their own WSN platforms.
The
Huntington Gardens monitoring project monitors environmental parameters in botanical
gardens using JPL sensors [9]. Figure 1.2 shows the overall topology of the sensor
network used in this application.
Figure 1.2: Huntington Gardens Monitoring Project
In the Huntington Gardens Monitoring Project, data is multi-hopped from a set of
JPL nodes distributed throughout the gardens as shown in Figure 1.2. The data is moved
13
through the network through the use of packet forwarding to the "Mother" node which
acts as the data aggregation point and a link to the client. For example, data from node
14 may be relayed through nodes 12, 18, 5 and 6 before reaching the mother node. The
advantage of multihop topologies is that RF transmission power levels for each sensor
may be reduced since its signal does not directly need to reach the gateway (i.e. mother
node). Reducing transmission power significantly reduces a node’s energy requirement,
thus enabling the use of smaller batteries, and or extending node life.
These latter implementations (i.e. JPL and WINS) attempt to address data
collection through the development of a single class of nodes that can place themselves in
different functional modes that attempt to reduce power consumption based upon specific
operating parameters. Figure 1.3 illustrates the nodes discussed.
Figure 1.3: Commonly available sensor nodes
The open architecture of some of these platforms, such as the MOTES platform,
facilitates rapid development of code to test energy aware routines such as the LEACH
protocol [10] and hardware power control routines such as those described in [11]. The
major distinction between these WSN architectures and that of the UVM-WSN presented
herein is the aggressive use of hierarchy through the development of ALOHA protocol,
transmit-only nodes that save power, cost and complexity by removing receivers from the
design of one class of nodes.
1.4.
Contributions
To meet the objectives outlined above, the UVM-WSN was developed with a
14
tiered structure of three types of nodes. Each class of nodes is based upon the commonly
available PIC microcontroller from Microchip [12]. An ALOHA class of sensor nodes
utilizes a modified, pure transmit ALOHA multiple access protocol to inject their data
into the sensor network. This reduces the cost and complexity of that class of nodes
while also significantly reducing power consumption by removing the receiver
altogether. A higher level node, the Hopper node, has a complete transceiver section that
allows ALOHA data to be forwarded and allows coordination with other nodes to deal
with routing issues. The third, highest level node was developed that consists of an
embedded Ethernet interface that easily plugs into the Hopper node making a Gateway
node. This node also implements the power-over-Ethernet standard to steal power from
the network port further reducing the complexity of introducing a fixed power or the need
for a large battery.
Within the Hopper nodes, an energy aware routing algorithm (VAARP) was
simulated and tested with limited success in improving Quality of Service.
The
simulation used a base case of 40 ALOHA nodes and 8 Hopper nodes using static routing
and VAARP.
The key contributions of this work is a new hierarchical energy aware WSN
implemented in low cost hardware and a demonstration system to prove the effectiveness
of the aforementioned techniques.
To evaluate the UVM-WSN design, both simulation and laboratory experiments
we used to determine the power requirements of the network as a whole and to test the
effectiveness of the power aware routing algorithm.
15
1.5.
Thesis Organization
This chapter presented an introduction to the field of wireless sensor networks
along with some applications and implementations. The differences between the existing
designs and the proposed UVM-WSN were highlighted. The remainder of this thesis is
organized as follows. Chapter 2 reviews existing algorithms and design methods that
attempt to improve the power requirements of WSN. Chapter 3 describes the protocols
used in the ALOHA and Hopper nodes for energy aware routing and the specific
implementation details of the UVM-WSN. Chapter 4 describes the results yielded from
the test setup used to evaluate the UVM-WSN. This chapter also presents a performance
comparison to a Crossbow MOTES network of approximately similar size. Finally,
Chapter 5 summarizes the key results of this research and proposes further improvements
and avenues for future research.
16
CHAPTER 2: ADAPTATION OF A LOW-COST WIRELESS SENSOR FOR
FRESHMAN AND OUTREACH PROGRAMS
Mike Fortney and Jeff Frolik
University of Vermont
Underrepresented Groups in Engineering
2.1.
Intro to Paper
This paper was published in the ASEE Journal, Vol #, date…... and was presented at the
proceedings conference, Connecticut, date.
2.2.
Abstract
This paper details the development of new CricketSat designs and education programs at
the University of Vermont (UVM). UVM first explored the use of this wireless sensor in
Summer 2002 after attending a NASA Starting Student Space Programs workshop. Work
at the university has since involved improvements to the design to expand functionality
and facilitate successful student circuit assembly. High school, undergraduate and
graduate level students are involved with CricketSat sensors and systems, design and
testing. Collaborative and outreach programs involve other institutions.
2.3.
Introduction
The CricketSat wireless temperature sensor was originally designed in 1999 at
Stanford University's Space System Development Laboratory as part of the NASA Space
Grant "Crawl, Walk, Run, Fly" student satellite program i. The purpose of this NASA
program is to instruct students into methods of space hardware development. Student
satellites range from the simple balloon-borne CricketSat to the more complex earthorbiting CubeSat. To assist colleges and universities in developing their own programs,
"Starting Student Space Hardware Programs" workshops are held frequently at the
University of Colorado campus in Boulderii. The workshop covers the range of student
satellite designs, with emphasis on the BalloonSat program.
17
Representatives from UVM attended workshops and a CricketSat program was
implemented at the University in 2002.
UVM CricketSat objectives involve
improvements to the original design, its use as an educational tool, and outreach
activities. This paper discusses the following activities which take place within the
program:
1. CricketSat development and testing
2. Collaborative work with other colleges and universities
3. High school outreach
4. Freshman introduction to engineering course
2.4.
The CricketSat System
The CricketSat system (Fig. 2.1) is composed of a single wireless sensor and a
receiving station. The CricketSat transmitter contains a simple, 555 timer-based circuit
that produces an audio tone that changes frequency in response to changing temperature.
This tone amplitude modulates a 434 MHz carrier. Calibration of the sensor is performed
by measuring the tone frequency taken at various temperatures. From the calibration,
graphs are produced for converting frequency to temperature during use.
18
Figure 2.1: CricketSat Wireless Temperature System
For flight, the CricketSat device can be attached to a helium balloon as small as 2
feet in diameter. During flights, the 434 MHz signal is received by the ground station.
The ground station consists of a Yagi antenna, a UHF radio receiver and an audio
frequency measurement device. The frequency of the tone is measured with a frequency
counter or audio-spectrum software. Flights have been tracked for 90 minutes before the
signal becomes too weak to measure reliably. During this time, the balloon may travel a
distance over 150 km, reach an altitude of 10 km and experience temperatures less than –
70 C. The sensor and balloon are seldom recovered.
BalloonSat is a much larger system, toting a payload of several pounds, and a
price in excess of ~$500. This system contains a GPS device and a radio transmitter used
to broadcast the position coordinates for tracking during flight. Sensor data is usually
collected and stored during the flight. Recovery of the payload is necessary for the
expensive equipment and data. Complexity, cost, and logistics for flight preparation and
tracking may make this system undesirable. Flights may achieve altitudes of 30 km in
100 minutes before the balloon bursts and the payload parachutes back to earth.
19
In comparison to BalloonSat, the CricketSat system has benefits of low cost
($10), low weight, and live data telemetry. It is also simple to understand, easy to
assemble, and simple to use.
Drawbacks include single-sensor operation, and
requirements for calibration and frequency conversion. Work at the UVM is involved
with improving the performance and flexibility of this device.
2.5.
UVM CricketSat Development and Testing
Development work at UVM includes improvements to the original design,
adaptations for new sensing capabilities, and the design of multi-sensor systems.
Improvements have also been made to better the electrical performance, system
reliability, and the likelihood of successful assembly. Concerning the latter, component
outlines and designations have been added to the board, and a protective layer to
minimize electrical shorts.
A prototype area has been expanded to support student
adaptations to the CricketSat design. The most recent UVM CricketSat design is shown
in Fig. 2.2.
20
Figure 2.2: UVM CricketSat Wireless Temperature Sensor
One objective of the UVM CricketSat work is to extend the circuit’s capability
beyond simply measuring temperature.
Common sensors for air pressure, humidity,
light level, and acceleration can be now accommodated in the design. In addition, easily
built sensor circuitsiii will also interface with this design. With this added flexibility, the
CricketSat may used as a platform for a wide variety of wireless sensor applications. To
date, multi-sensor CricketSat designs have also been developed by both university and
high school students. Eventually, the improved circuitry will lead to the development of
a low-cost student radiosonde (CricketSonde) containing meteorological and other
scientific instruments.
Such a design may enable community-based, meteorological
measurements at a much finer spatial resolution than those currently available using the
current network of National Weather Service radiosonde stations. This work toward this
end is detailed in the following sections.
2.6.
High School Outreach
The HELiX (Hughes Endeavor for Life Science Excellence) Program at UVM
HELiX/EPSCoRiv is a NSF funded outreach program supporting area college and
high school students. Among the HELiX activities is a summer workshop for high
school students that is designed to provide students insight into the "real world" of
science. Teams, consisting of a teacher and a few students, conduct a research project,
assisted by scientists at the university. At least one of the students must be female. A
21
HELiX-sponsored CricketSat workshop titled "Building and Launching Cricket Satellites
to Measure Various Atmospheric Conditions" was conducted during the summers of
2003 and 2004 (one is also planned for June 2005). The one-week session, outlined in
Table 2.1, involves lectures and hands-on activities for the students and teachers.
Classroom instruction includes an introduction to the earth's atmosphere and operation of
the CricketSat sensors. Hands-on activities involved the assembly, soldering, calibration,
and flight of these sensors. Students fly balloons, collect data and analyze the results.
School teams must then conduct a related research project to be conducted over the
following year.
Table 2.1: HELiX CricketSat Workshop Schedule
Day
Monday
Tuesday
Wednesday
Thursday
Friday
Activity
Temperature profile of the atmosphere
Introduction to the CricketSat sensors
Practice soldering
CricketSat assembly
CricketSat testing
CricketSat calibration
Balloon flights and data collection
Spreadsheet data entry
Analysis and results
The high school teams have provided an important role of testing and evaluating the
CricketSat sensor designs.
These teams have also developed and evaluated more
complex multi-sensor CricketSat designs. Each balloon flight strives to surpass previous
results, contributing towards advancing the system. Parameters analyzed for each flight
are duration, distance, minimum temperature and maximum altitude.
22
2.61.
2003 – 2004 HELiX Team
This initial workshop (2003) involved a team from the Waldorf High School in
Charlotte, Vermont, consisting of a female science teacher and three female students.
The June flights involved the initial testing of the newly developed pressure and humidity
sensors and ground station receiving system. Balloons were released over the water from
a causeway on Lake Champlain. The results were not very encouraging. Frequency
measurements using a meter became unstable less than 10 minutes into the flight. The
meter did a poor job of measuring the signal in the presence of background radio noise.
Expected qualitative variations were observed for the temperature, pressure and humidity
sensors.
In short, the system worked satisfactory for close-range work, but not for balloon
flights. As such, for their long term research project, the team decided to build a wireless
weather station consisting of temperature, pressure and humidity sensors. The goal was
to make automatic measurements at periodic intervals. A frequency measurement meter
was connected to a computer for data collection. Since all of the CricketSat sensors share
the same radio frequency, a rotary switch was added to the station to provide power to the
CricketSat sensor to be measured. The system worked for three-hour intervals before the
meter would turn itself off. This appeared to be caused by the meter’s inability to
properly track the changing signal.
A new method was devised for performing the frequency measurements during
balloon flights. Several audio spectrum analyzer programs were investigated. These
programs allow for individual frequencies in the audio signal to be “seen” on the
23
computer and measured. The CricketSat signal was easily identifiable, even when far
away, allowing it to be measured reliably. The SpectraRTA v software was selected at the
time due to its data logging capability. Spectrogramvi is now recommended due to its low
cost.
2.62.
2004 – 2005 HELiX Teams
This improved platform was utilized for the second HELiX workshop (2004) in
which two high schools participated. The first team was from the Milton High School
located in Milton, Vermont. The team was composed of a female science teacher and
two female students. The second team was from the John D. O'Bryant (JDOB) School of
Mathematics and Science (Boston Public Schools) located in Roxbury, Massachusetts.
This team consisted of a female science teacher and two students: one female and one
male.
Day and evening CricketSat flights (MHS-1) to monitor atmospheric temperature
profiles were conducted. A CricketSat humidity sensor was also flown along with an
experimental audio alarm device attached. Collectively, the flights were a remarkable
success. The SpectraRTA software performed well, allowing measurement of the tone
signals for a much longer period of time than the previous method using a frequency
meter. The shortest flight was tracked for 45 minutes and the longest for 91 minutes.
This allowed for measurements much higher in the atmosphere. Accordingly, the lowest
measured temperature was -41 F (-40 C), and the highest recorded altitude was 26,732
feet (8.1 km). Factoring in the velocities of the upper-air winds obtained from the
National Weather Service (NWS), the longest CricketSat altimeter flight was 144 km.
24
For their follow-on project, the JDOB team prepared a detailed presentation of the
MHS-1 flights, placing second at the 2005 Boston Regional Science Fair in March 2005
(Fig. 2.3). They will go on to compete at the Massachusetts State Science Fair to be held
in May at MIT.
Figure 2.3: The John D. O'Bryant School CricketSat presentation of the MHS-1 flights. The team received
a 2nd-place finish at the 2005 Boston Regional Science Fair
For the Milton High School team, their work was just beginning. The school
hosted two BalloonSat flights in July 2005 for students from Medgar Evers College
(MEC) of the City University of New York (CUNY). CricketSat sensors were flown as
payload to provide real-time flight support data for the MEC team. These flights also
allowed CricketSat sensors to achieve altitudes and conditions not normally experienced
with the smaller balloon flights.
25
Figure 2.4: Medgar Evers College (MEC) team preparing for a BalloonSat launch at the Milton High
School. The CUNY school provided flight support for the MHS-2 payload.
For the first BalloonSat flight (MHS-2), the MHS team monitored the temperature
inside the BalloonSat instrument flight bag (Fig. 2.4). The temperature was measured for
125 minutes and never dropped below 63 F (17 C). For the MEC team, this validated
the use of the insulated lunch bag for holding instruments during BalloonSat flights. This
experiment also demonstrated the compatibility between CricketSat and BalloonSat
payloads concerning radio co-interference.
With the successful results of the single CricketSat sensor on the MHS-2 flight,
the team was now presented a challenge of measuring data using several CricketSat
sensors. Unlike the earlier weather station, this system would need to sequence through
the sensors automatically. The problem was presented to the Milton team, and with a
little guidance, they devised a sequential timing algorithm for segregating and identifying
sensors during flight. In addition, the design needed to be light weight for the balloon
application. A circuit was designed for the students using a BASIC Stamp controller.
One ambitious student assembled the circuit, wrote a PBASIC program employing the
26
timing algorithm, and tested the controller.
Figure 2.5: The Milton High School CricketSat Array System flown on the MHS-3 flight. This project
allows the measurement of several CricketSat sensors over 50 miles away.
The CricketSat Array System (CAS) assembled for flight is shown in Fig. 2.5.
During the BalloonSat flight (MHS-3), the CricketSat flight bag and external
temperatures were measured, as well as altitude (air pressure). The system worked very
well, properly segregating the data from the various CricketSat sensors, as seen in Fig.
2.6.
Figure 2.6: Properly segregated data collected from the Milton High School developed CricketSat Array
System. Raw frequency results are shown.
27
New levels of performance were achieved.
The flight was tracked for 134
minutes, to an altitude of 85,781 feet (26 km), and with a bone-chilling external
temperature of –92 F (-69 C). The CricketSat altimeter worked properly below 32,000
feet (10 km), meeting expectations. External temperature versus altitude data correlated
with NWS sounding balloon data. The CricketSat altimeter data agreed well with altitude
data provided by the onboard GPS. The results were presented at the Northeast Regional
Space Grant Conference held in October 2004 in South Burlington, Vermont by the
author and the two Milton High School students.
Design changes to the CricketSat are necessary for improvement to the
temperature and pressure measurements. As can be seen in Fig. 2.6 (Ext Temp 1 & 2),
for very low temperatures, the CricketSat frequency is very low and difficult to measure.
In addition, the CricketSat pressure sensor only works properly up to altitudes of 10 km.
To completely characterize the environment experience during these large balloon
launches, the sensors need to perform measurements to altitudes of 30 km with
temperatures as low as –90 C. As such, thesis work is in progress by the author towards
the development of a linear frequency response CricketSat temperature sensor for use in
extreme cold environments. This may lead to the development of a family of linear
response CricketSat sensors for radiosonde experiments (i.e., CricketSonde). The linear
response provides benefits of uniform sensitivity, and simplified calibration and
conversion methods. The sensors will be evaluated on future HELiX and BalloonSat
flights.
28
2.7.
College Freshman Engineering Course
As a result of the above successful programs, the CricketSat was chosen in Spring
2004 as a project platform for UVM’s freshman design course for electrical and
mechanical students (instructed by the co-author)vii.
In this course, students first
fabricate, test and calibrate the basic wireless temperature sensor. Then, working in
teams, the 60 students adapted over a six week period this sensor for an application of
their own choosing.
The project requires electrical modification of the circuit and
mechanical design requisite of the application. Student projects from the first offering
included a wireless wind-chill instrument (Fig. 2.7-left), a wireless synthesizer and a
wireless alarm system (Fig. 2.7-right). We view the breadth of these designs as being
indicative of the flexibility and simplicity of the CricketSat platform to accommodate a
variety of introductory-level student projects. The course is currently in its second
offering to 70 students and utilizing the improved CricketSat design illustrated in Fig.
2.2.
29
Figure 2.7: Example student design projects: Wireless Wind-Chill Instrument (left) and Wireless Door
Alarm (right)
2.8.
UVM’s
CricketSat
activities
Conclusions
have
in
addition
enabled
collaborative
meteorological studies with the University of Alaska along with the aforementioned work
with Medgar Evers College. Like UVM, the University of Alaska is involved with
CricketSat development and testing. Designs and methods are being shared between the
two schools towards a common goal of improving this design. With this wide range of
“customer” input, we view the UVM CricketSat design as rapidly migrating towards a
simple, flexible and yet a powerful platform upon which meaningful projects can be
developed for a wide range of wireless monitoring applications. We hope that with this
paper along with additional material (schematics, project ideas and kit information)
available onlineviii will provide a resource that other institutions may utilize to develop their
own entry level sensor programs. The author encourages interested institutions to contact himix should they
have any questions.
2.9.
Acknowledgements
The authors would like to acknowledge the Vermont and Colorado Space Grant
Consortiums, and the UVM HELiX outreach program for their support in the
development of the CricketSat program at UVM.
The authors would also like to
acknowledge Dr. Shermane Austin of Medgar Evers College and Dr. Neal Brown of the
University of Alaska for their flight and technical contributions, respectively.
30
31
CHAPTER 3: CRICKETSAT SENSOR AND SYSTEM DEVELOPMENT
3.1.
Introduction
In this chapter we discuss the development of the UVM CricketSat sensor and
systems beginning with the original Stanford CricketSat design and concluding with the
current UVM Revision F (RevF) design. System development is also discussed relating
to CricketSat sensor arrays and a reliable ground-based measurement system.
Educational applications, discussed in Chapter 2, provided the motivating factors driving
the development work. These factors were primarily related to performance, simplicity,
and reliability. In addition, we considered the likelihood of successful assembly, testing
and adaptability. Limitations of the RevF design discussed at the end of this chapter, lead
to the development of a linear CricketSat sensor, discussed in Chapter 4.
CricketSat performance relates to sensor accuracy and resolution. For example,
of primary concern is that the oscillator change frequency based on sensed parameter.
Resolution may suffer due to a non-linear frequency response to the measured parameter
and this is addressed in detail in Chapter 4.
 Performance
 Simplicity
 Reliability
A simple design is important to allow students to understand the operation with
little or no electronics experience. The 555-timer based design was maintained due to its
32
simplicity and wide-spread use. Operation of the 555-timer oscillator is discussed in
Chapter 4. A circuit with fewer components could have been developed using a small
microcontroller, but the “black box” circuit would not demonstrated simple electrical
principles and would have required a custom pre-programmed microcontroller which
may not be available in the future.
Likelihood of successful assembly is important for students untrained in soldering
skills and circuit board assembly. A quality assembly manual, in addition to a clearly
labeled and coated circuit board, helps guarantee proper insertion of components and
minimization of soldering shorts.
Reliability is mostly concerned with mechanical issues relating to wires and
components breaking from exposure and repetitious use. The use of strain-relief holes
helps alleviate wires from breaking. Components should be mounted tight to the circuit
board to avoid flexing. Vertically mounted components may be placed in proximity of
other components for physical protection or mounted horizontally if room permits.
Testability is important for debugging newly assembled circuit boards. Providing
clearly labeled test points on the electrical schematic and the printed circuit board allows
for confirmation of proper signals in debugging a non-functional circuit. The test points
also aid in instructional use for demonstrating 555-timer operation with the use of test
instruments.
Adaptability allows the CricketSat circuit to interface with additional types of
sensors primarily for student-based designs. Test points provide access to power and
timing signals to interface with custom circuitry.
For small circuits, the on-board
33
prototype area can be used. Larger circuits may be constructed on external prototype
circuit boards and wired to the CricketSat circuit board at the test points.
3.2.
The Original Stanford CricketSat Design (1999)
The original CricketSat was developed in 1999 for the Space and Systems
Development Laboratory (SSDL) located at Stanford University.
David Joseph
(W7AMX), a student mentor, designed the CricketSat at the suggestion of Professor Bob
Twiggs, director of the laboratory. The circuit (Fig. 3.1a) is composed of a simple 555timer based oscillator, a thermistor, an LED and an RF transmitter module. The circuit
produced tones or clicks in a receiver (or flashes on the LED), dependent on the size of
the timing capacitor, C1. A prototype area (Fig. 3.1b) is provided on the circuit board
supporting adaptations.
(a)
(b)
Figure 3.1: The original Stanford CricketSat schematic and completed circuit board.
The printed circuit board was designed using Express PCB software. This was a
good choice, as the software is free to download and is widely used. The circuit board
design files were provided at the first “Starting Student Satellite Hardware Programs”
34
workshop held by the Colorado Space Grant Consortium at the University of Colorado in
2002.
3.3.
UVM CricketSat Workshop Planning (Spring 2003)
Dr. Mark Miller (UVM), attended the 2002 Colorado workshop and decided to
use the CricketSat for student outreach, sponsored through the HELiX program at the
University. A workshop entitled “Building and Launching Cricket Satellites to Measure
Various Atmospheric Conditions” was planned for Summer 2003. The goal was to
perform balloon-borne atmospheric profile measurements similar to those made by the
U.S. National Weather Service. These data would be compared to NOAA radiosonde
sounding data and used to validate know atmospheric relationships taught in the
classroom.
A UHF ham radio transceiver (Kenwood THD-7A), tuned to 433.92 MHz, was
designated to receive the remote signal. Methods use to measure the received data vary
dependent on the CricketSat mode of operation. For a CricketSat operating in pulse
mode, a stopwatch is used to count clicks heard in the speaker over a specified time
interval (i.e. 15 seconds) or used to measure individual click intervals. A CricketSat
operating in the tone mode requires a frequency measurement device to measure the
audio tone produced.
For allowing students to duplicate measurements made by NOAA, the CricketSat
sensor would be required to support the use of pressure, humidity and various other
sensors. Fortunately, due to the versatility of the 555 timer circuit, interfacing with a
35
variety of sensors types is simple. Passive (resistive and capacitive) and active (voltage
and current output) sensor types are easily interfaced with the timer oscillator circuit.
Various methods of interfacing these sensor types for control of the timer are analyzed
and discussed in Chapter 4.
The immediate concerned required changes to the printed in support of a
replacement for the TWS-434 radio transmitter module. The original six-pin module was
replaced by the manufacturer with a four-pin version, with no equivalent substitute
available. Since the circuit board required modification, the opportunity was taken to
make a few additional changes. Unknown was which CricketSat operational mode (pulse
or tone) would prove superior relating to long-distance reception of the signal. Features
allowing the selection between these CricketSat modes of operation would be useful for
initial evaluation.
3.4.
UVM CricketSat Revision A (Spring 2003)
For this initial release of the UVM CricketSat, changes were made to support the
new transmitter module, along with those to provide safeguards and flexibility. To
protect the CricketSat circuitry (Fig. 3.2) in the case of a reverse-battery connection,
diode (D2), in series with the power, allows current to flow if the battery is properly
connected. Three shorting-block jumpers (JP1-JP3) provide for flexibility of circuit
operation and provide connectivity for other sensor types.
36
Figure 3.2: UVM initial redesign (RevA) of the Stanford University CricketSat.
In support of the two modes (pulse and tone) of CricketSat operation, JP2 allows
mode selection without replacement of the timing capacitor. Pads were provided on the
RevA circuit board supporting two timing capacitors, C1 and C3. C1 is intended to be
the smaller capacitor (0.1uF) and C3 the larger (100uF) electrolytic capacitor. JP2
connects the larger, C3, to the circuit, placing it in parallel with C1, which is always
enabled. Since the electrolytic capacitor is typically hundreds of times larger than the
disc capacitor, its value dominates the oscillator timing intervals while connected.
Therefore, tone mode is provided with JP2 disconnected and pulse mode with it installed.
Two additional jumpers were added to support two other features. Jumper JP1 is
used to disable the LED, conserving power for extended use. Jumper JP3 allows for the
connection of active sensors to the threshold control (Pin5) of the 555 timer. Varying the
threshold voltage may be used to control the oscillator frequency as described in Chapter
37
4.
One goal of the redesign was to simplify the circuit by eliminating the need for
the two inductors L1 and L2 (Fig. 3.1a). These inductors appear to serve the primary
purpose of noise decoupling, typically accomplished using capacitors. Two decoupling
capacitors were added to the design as inductor replacements, while the inductors were
retained until their purpose was understood. Inductor L1, associated with the 555 timer
circuit, appears to serve a decoupling purpose and its replacement more straightforward.
The use of L2 is more complicated, appearing to serve a dual purpose of noise decoupling
and aiding modulation.
Pins 1, 2, and 6 of the TWS-434 transmitter module (Fig. 3.1a) are tied together,
driven by the logic output of the 555 timer IC and coupled through L2 to the battery
power. The transmitter power pins (1 and 2) are connected to the data pin (6) on the
circuit board using a pinched metal trace, intending it to be cut. Indications are that this
connected arrangement may provide a stronger modulation of the transmitter. For this
RevA design, the data and power signals to the new transmitter module were separated.
As a precaution, an unnamed jumper was added to the circuit board to provide
reconnection if needed. The L2 inductor was configured to couple the transmitter power
pins to the supply voltage.
To make the prototype area more usable, the bottom row was freed from the
power rail, providing four rows of unconnected pads (Fig. 3.3a). This change allows for
inclusion of an 8-pin DIP IC, such as another timer, or an op amp to be used in the space.
38
Power and ground connections were added to additional prototype pads to provide access
to those signals.
(a)
(b)
(c)
Figure 3.3: UVM RevA CricketSat temperature, pressure and humidity sensors. Pressure and humidity
sensors are inserted in the prototype area.
Active pressure and passive humidity versions of the CricketSat sensor were
created. A Motorola MPX4115AP pressure sensor (Fig. 3.3b) was used as an altitude
sensor for balloon flights and as barometric pressure sensor for ground-based
measurements.
The active device produces a voltage linearly proportional to
temperature. This voltage was used to vary the threshold control voltage on the 555
timer, affecting the frequency of oscillation.
For the CricketSat humidity sensor (Fig. 3.3c), a passive humidity sensor
(Humirel HS1101) replaced the timing capacitor (C1) in the oscillator circuit. The
HS1101 data sheet includes a 555-timer based circuit that works well with the layout of
CricketSat circuit board. Unfortunately, the center frequency of the circuit is greater than
6000 Hz, exceeding the bandwidth of the RF transmitter. A doubling of the timing
resistors reduced the frequency in half, allowing adequate operation.
Two CricketSat sensors of each type were assembled and calibrated (tone mode)
in preparation for the first HELiX CricketSat workshop. The tone mode was thought to
39
provide a higher resolution and allow automated data logging to a laptop computer. A
Radio Shack multi-meter with and RS-232 interface and accompanying software was
selected to perform the sensor frequency measurements.
3.5.
2003 CricketSat Workshop and Test Flights (June 16 – June 20)
Three female students and their science teacher representing the Waldorf High
School (Charlotte, Vermont), participated in this first workshop, whose format is outlined
in Chapter 2. Students assembled and calibrated four CricketSat temperature sensors.
Due to the wiring complexity of the pressure and humidity sensors, sensors assembled
and calibrated in the prior section were used.
Test flights were conducted Wednesday (18th) from the Sandbar Bridge (Fig.
3.4a) in Colchester, Vermont.
Since no flight procedures were available from the
Colorado workshop, this was a flight of firsts. The flight objectives were four-fold.
 Develop flight procedures relating to preparation, launch and data collection
 Observe qualitative and quantitative atmospheric effects on sensor types
 Evaluate the data collection system
 Develop benchmarks to evaluate CricketSat and system performance
o
Tracking time
o
Estimated range
o
Coldest temperature
o
Highest altitude
o
Lowest humidity
40
(a)
(b)
(c)
Figure 3.4: CricketSat flights conducted from the bridge near the Sandbar State Park. Students are shown
preparing, launching, and tracking CricketSat sensors.
Four sensors were flown (Fig. 3.4b) and tracked (Fig. 3.4c) in sequence and
students recorded measurements at 20-second intervals. It was obvious from the first
balloon flight that the data collection system using the frequency meter had a problem.
After only a couple minutes of flight, the meter could not discriminate the tone signal
from the background noise. Little temperature or humidity variation was observed over
the water surface and therefore no quality quantitative measurements were obtained in the
short span.
Qualitative temperature results were better since the audio signal was audible for
several minutes and a reduction in the tone frequencies were noticeable in
correspondence with decreasing temperature and altitude.
impressive for the pressure sensor.
Results were even more
This sensor produced a noticeable decrease in
frequency after a couple minutes of ascent.
Unfortunately, the antenna of wildly
swinging sensor popped the balloon. The descent to the lake surface produced a tone
variation sound similar to that of incoming artillery; that is up until impact with the water.
41
Several things were learned from this flight.
 Qualitative results were observed for the temperature and pressure sensors,
demonstrating sensor response to the stimulus
 The frequency meter was inadequate for making distant measurements
 Future balloon flights would use CricketSat sensors configured for pulse-mode
operation, using a stopwatch for measurement until a better frequency measurement
method is adopted
 The CricketSat sensors require a longer balloon attachment string to stabilize
the flight system and avoid popping balloons
 Measurements over water do not provide adequate temperature or humidity
variation versus altitude
3.6.
UVM CricketSat Test Flight (08/08/2003)
This test flight was conducted to evaluate feasibility of using stopwatch
measurements relating to pulse-mode CricketSat balloon flights. This was in preparation
of a CricketSat workshop and flight to occur the following day. A CricketSat sensor was
prepared and released on the green behind the UVM Cook Science physics building.
Instead of counting CricketSat “clicks” in a 15-second period, a more accurate method
was adopted which measured the time interval of five consecutive clicks. The resolution
of the stopwatch provided a much improved resolution for the measurements. A second
watch was used to provide elapsed time since launch for each measurement.
42
This flight and results and were extremely encouraging. The temperature dropped
from 83 ºF to 40 ºF in 25 minutes (Fig. 3.5a), at which point it quickly began warming.
We realized that the balloon had burst and was rapidly descending. The signal was
tracked to landing, only six minutes later, indicated by a return to the original
temperature. Using a directional antenna and a RF spectrum analyzer, the balloon was
located in the top of a maple tree located two miles away in Winooski.
(a)
(b)
Figure 3.5: Results from a pulse-mode CricketSat launched from the UVM campus. The graph to the left
shows the flight temperature profile from launch to landing. The right graph represents an estimated
altitude profile based on an adiabatic cooling rate.
Using an adiabatic cooling rate of 4.5 ºF per 1000 feet of altitude, the second plot
(Fig. 3.5b) provides an altitude profile for the flight, indicating a burst altitude of 11,000
feet. This graph can be used to estimate ascent and descent rates due to the nearly linear
characteristics. From the data, the balloon ascended at a rate of 440 fpm (5.00 mph) and
struck the maple tree at 1833 fpm (20.8 mph).
This flight clearly demonstrated the successful operation of the CricketSat
temperature sensor and the pulse-mode stopwatch measurement system. This flight also
demonstrated use of the CricketSat to demonstrate atmospheric characteristics in a short
time interval. The linear temperature profile, similar to those produced by BalloonSat
43
flights indicates a linear temperature change over time. Since these balloons (and
BalloonSat weather balloons) rise at a nearly linear rate, a constant adiabatic rate in the
lower atmosphere is demonstrated.
3.7.
GIV CricketSat Workshop and Test Flight (08/09/2003)
This workshop was conducted at 20th anniversary celebration for the Governor’s
Institute of Vermont (GIV) held at Shelburne Farms in Shelburne, Vermont. A small
group of six students assembled pulse-mode CricketSat temperature sensors which they
were allowed to take home.
Temperature measurements could be made using the
flashing LED and a stopwatch to perform measurements.
A calibrated pulse-mode CricketSat sensor was used for the demonstration flight
on a cold day and occurring during a thunderstorm. The CricketSat sensor was placed in
a zip-lock bag to protect it from the rain. In an attempt to achieve a higher flight than the
previous day (11,000 feet), less helium was placed in today’s balloon. The underinflation of the balloon and the additional weight of the rain-laden bag and balloon
resulted in a slow ascent rate.
The flight was successful in relation to benchmarks relating to flight duration,
minimal temperature and estimated terrestrial range. After 90 minutes of tracking the
flight, the minimum temperature experienced was +20 ºF. After factoring in an adiabatic
cooling rate, an estimate for an altitude was determined to be approximately 8000 feet.
Factoring in flight time, strength of the winds and the low angle of the Yagi antenna, the
terrestrial flight range was estimated to be in excess of 15 miles. As in the previous
44
flight, the pulse-mode CricketSat using a stopwatch proved to be a very successful
system for data collection and measurement.
3.8.
HELiX CricketSat Weather Stations (12/2003 – 02/2004)
HELiX teams and their UVM sponsors have a one-year commitment to develop a
project in response to knowledge and skills obtained during the summer workshop. The
Waldorf team decided to use CricketSat sensors to develop two weather stations to
investigate lake-effect on temperature. One station was located at the Waldorf High
School (Fig. 3.6), 1.5 miles inland from Lake Champlain, at an altitude of approximately
150 feet above sea level. The second station was located in Williston, Vermont, six miles
from the lake at an altitude of approximately 400 feet. The weather stations were fitted
with solar panels and rechargeable batteries for long-term operation.
Figure 3.6: CricketSat weather station located at the Walforf High School fitted with a solar panel for
long-term use.
The mechanical construction of the weather stations consisted of plastic plates
(Fig. 3.7a), sheet aluminum, shelf brackets, threaded rods and nuts.
Along with
45
temperature, the pressure and humidity sensors developed during the workshop were
stacked (Fig. 3.7b) and mounted center in the weather station. Autonomous data logging
capability was required, requiring the CricketSat sensors to operate in tone-mode. Again,
the Radio Shack meter was used, but since the receiver was in close proximity to the
weather station, the signal noise issues, experienced in the first flight, were not replicated.
(a)
(b)
(c)
Figure 3.7: The Waldorf CricketSat weather station. Plastic-plate and threaded-rod construction (left),
stacked CricketSat sensors (center), sensor selection switch and battery (right).
Since CricketSat sensors are always transmitting data, only one is allowed to be
powered on at a time to avoid interference. Consequently, the weather stations were
fitted with a switch box (Fig. 3.7c), allowing the selection of one of the three sensors.
Since temperature was the measurement of interest in this application, this restriction did
not cause a serious problem. The station would remain in the temperature mode for
autonomous data logging and only briefly switched to pressure or humidity to make those
measurements manually.
To eliminate the dependency of the Kenwood receiver, costing hundreds of
dollars, a CricketSat receiver (Fig. 3.8a) was designed to work with the weather station.
The circuit (Fig. 3.8b) uses the accompanying receiver module to the transmitter module
46
used on the CricketSat. The signal weak produced by the receiver module is amplified by
a LM386 amplifier, driving the speaker and the Radio Shack frequency meter.
(a)
(b)
Figure 3.8: CricketSat receiver designed to work with the CricketSat weather station and was later used for
all CricketSat applications.
The station functioned with frequent problems mostly relating to loss of data
acquisition after a few hours. Apparently the meter was again the culprit, now failing to
take frequency measurements, even in the presence of a strong signal. Turning the meter
off briefly returned it to proper operation. Apparently, the meter assesses the initial
signal and makes a determination of a threshold voltage level to used to convert the
analog input signal into a digital one for counting. Changes in the CricketSat signal
strength resulting form battery level or duty cycle variations would render the initial
threshold level in error, resulting in the loss of data.
Results from this work demonstrate the need to find or develop a reliable method
for measurement of CricketSat data in the presence of noise. It also demonstrated the
need for an automatic multi-sensor array system sequencing through the sensors. Such a
system could be used for ground based or balloon-borne applications. Progress is this
area is achieved later during the year, resuming in Section J. Meanwhile, during the
47
course of this weather station project, a new CricketSat sensor was developed for use in
the newly created freshman design class at UVM.
3.9.
UVM CricketSat Revision C (January 2004)
The CricketSat sensor development now jumps from RevA to RevC. CricketSat
RevB was not manufactured and only exists in prototypes. Revision C was designed to
address previous problems with RevA and to meet the needs of the new freshman design
class. The most significant change for this version was the addition of strain-relief holes
for the battery wires. Electrical FIG 3.9. These connections were frequently breaking at
the board surface due to flexing, and were difficult for students to repair.
Figure 3.9: Schematic diagram of the UVM RevC CricketSat.
This is the first CricketSat version to be used for the freshman design course
(Chapter 2), requiring a quality PCB (Fig. 3.10) that would be easy to assemble and
tolerant of soldering errors. Production quality boards were ordered which provided
solder-mask and silkscreen layers. The solder mask, covering all metal traces, except for
the solder pads, minimized short circuits caused by newly developed soldering skills.
48
The silkscreen layer provided reference labeling to help guide students relating to
component location and orientation on the circuit board.
Figure 3.10: CricketSat RevC temperature sensor assembled circuit board. Green solder mask, white
labeling and strain-relief holes were added to this design.
To reduce complexity and assembly for the students, only one timing capacitor
location was provided on the board. However, to accommodate multiple size capacitors,
three pads provided a choice of 0.1” or 0.2” pin spacing. This feature supported disc,
electrolytic and a capacitive humidity sensor as well. A similar three-pad feature was
provided for R1 in support resistor, thermistor or photocell devices.
3.10
UVM CricketSat Revision D (May 2004)
CricketSat Revision D (RevD) was designed to directly support pressure and
humidity sensors and address problems with the earlier designs. Prior testing revealed a
sensitivity of the oscillator circuit to battery supply voltage, later evident due to selfheating of the thermistor. The addition of a 5-Volt regulator (Fig. 3.11) resolved the
problem and provided the specified voltage for the active pressure and humidity
(Honeywell HIH3610) sensors.
49
Figure 3.11: CricketSat RevD schematic diagram. The voltage regulator provides circuit stability and is
required for active sensors.
The unregulated voltage remained connected to the RF transmitter module
(through D2) to provide maximum power and reception range. A self-protection, lowdropout regulator (Texas Instrument TL750L05C) allowed stable circuit operation down
to 5.3 Volts, extending effective battery life in comparison to a traditional 2-Volt dropout
regulator (LM7805).
The addition of the regulator provided isolation from the RF
module, allowing the removal of the L2 (Fig. 3.9) inductor associated with the timer.
A mechanical improvement was the enlargement of the four mounting holes,
allowing the use of common #4-40 mounting hardware. This is necessary for stacking
the CricketSat sensors in the weather station and for use in student projects.
(a)
(b)
Figure 3.12: CricketSat RevD temperature and pressure sensors.
50
Pre-wired pads and component outlines were provided to directly support the
Motorola pressure and Honeywell humidity sensors. This would simplify the assembly
and allow the students to install the sensors themselves, unlike the RevA version. The
prototype area was sacrificed to retain the original dimensions of the CricketSat. Ground
(G) and data (D) test points were added to allow testing of the oscillator and allow offboard interconnections.
3.11.
2004 CricketSat Workshop and Test Flights (June 14 – June 18)
A series of CricketSat balloon flights during this workshop produced results
exceeding all benchmarks established on previous flights. More importantly, a reliable
tone-measurement system was successfully demonstrated. During the search for viable
audio spectrum analyzer software, a team of students from the UVM freshman design
class discovered the Spectra RTA program.
This software, shown for use during
CricketSat calibration (Fig. 3.13), allowed CricketSat tones to be visualized and
measured even in the presence of noise. This allowed for flight measurements to be
collected over a longer duration, higher altitudes and terrestrial distance than by using a
frequency counter.
51
Figure 3.13: Students using the Spectra RTA software to perform CricketSat calibration measurements.
Three flights were conducted during the daytime and three in the evening
involving temperature, pressure and humidity sensors. One of the daytime flights (Fig.
3.14a) involved a CricketSat temperature sensor. Data (Fig. 3.14b) was collected nearly
continuously for 73 minutes resulting in a minimum temperature of -41 ºF (114 ºF drop)
in during the flight. Eventually, measurements could not continue due to the complete
loss of the signal. Prior to that finality, even after loss of the signal discernable to the ear,
for a period of time, the Spectra RTA software continued to display a tone peak and
allow measurements to continue.
(a)
(b)
Figure 3.14: CricketSat daytime temperature sensor flight. The sensor experienced a 114 ºF temperature
change during the flight.
52
The highlight of the evening was the CricketSat altimeter flight (Fig. 3.15a). This
flight was tracked for 91 minutes, reaching an altitude of 26,732 feet (Fig. 3.15b) after 90
minutes. The flattening of the data after 80 minutes is due to the limited pressure range
of the MPX4115 sensor resulting in an artificially low result. Wind velocities from
Albany sounding balloon data along with the CricketSat rate-of-ascent altitude slope (Fig
3.15b) was used to generate a flight path and estimate the terrestrial distance (51 miles).
(a)
(b)
Figure 3.15: Night-flight data collection from a CricketSat pressure sensor used as an altimeter. The
graph shows the linear ascent rate of the balloon during the flight.
The collective sets of flights throughout the day resulted in all previous
measurement benchmarks to be surpassed due to the success of the measurement system
based on the Spectra RTA software. This system now proved viable for performing longrange atmospheric measurements for single-sensor flights, meeting the first of two goals
outlined at the end of the Section G.
3.12.
First Collaborative BalloonSat Flight (July 17, 2004)
As described in Chapter 2, collaboration was established between Vermont and
New York Space Grant Consortia (Medgar Evers College), the UVM HELiX high school
53
outreach program and the Milton High School. The Medgar Evers College had launched
their first BalloonSat (Condor-1) in June, but had lost communication with it in midflight. One of several possibilities related to temperature of the flight bag, housing the
GPS and communications equipment.
Too cold of a temperature might cause the
equipment to fail.
(a)
(b)
Figure 3.16: Students prepare for a BalloonSat flight from the Milton High School. Temperature data (red
trace) in the figure to the right shows the communications flight bag temperature during flight.
Since the June CricketSat flights demonstrated that measurements could be
collected as far as 51 miles away, a CricketSat sensor could be used to provide real-time
BalloonSat flight-bag temperature measurements.
This experiment would also
demonstrate CricketSat compatibility with the BalloonSat system, allowing the
measurement of various sensor data.
The flight was conducted at the Milton High School (Fig. 3.16a) and the
temperature data was collected by three HELiX students. The flight bag temperature is
shown as the red trace in the graph (Fig. 3.16b). The temperature was collected 125
minutes and only dropped to +66 ºF, indicating that low temperature was not likely to
have been the cause for communication loss in the Condor-1flight. Additional results of
54
the flight include a GPS validation of altitude exceeding 45,000 feet and a terrestrial
range of 18 miles. Results were probably greater than these, but cannot be confirmed due
to loss of GPS data on this flight as well (green trace).
This flight demonstrated the CricketSat could provide real-time data flown on
board BalloonSat flights. Since BalloonSat flights are typically recovered, they provide a
good test platform for CricketSat projects, especially those that are more costly. The next
goal was to allow for an array of CricketSat sensors to be used on a single flight.
3.13.
Second Collaborative BalloonSat Flight (July 30, 2004)
Two weeks following the previous BalloonSat flight, Medgar Evers College again
returned to the Milton High School for a second flight. The HELiX high school students
were busy designing and building an elaborate CricketSat system to make multiple
measurements on this flight. Of course, the problem with CricketSat is that each sensor
is always transmitting and “jams” the others. A system needed to be designed that would
allow the operation of multiple sensors, all sharing the same domain of tonal frequencies,
and be able to identify one from the other.
Figure 3.17: CricketSat sensor array timing diagram.
55
A deterministic timing method was adopted (Fig. 3.17) to meet the requirements
by supplying battery power to each sensor in succession. Gaps in the timing are provided
to segregate sensor data and to indicate the first sensor, providing synchronization. The
sequence cycles every 60 seconds allowing sensor measurements to be made at oneminute intervals. A BASIC Stamp circuit board was developed (Fig. 3.18) to provide the
power sequencing and additional operations.
Figure 3.18: CricketSat power sequencing circuit using a BASIC Stamp II controller.
The complete sensor system (Fig. 3.19a) was comprised of CricketSat
temperature, altimeter and accelerometer sensors as well as a camera, strobe light and
audible alarm. All of the materials (Fig. 3.19b) were enclosed in an insulated lunch bag
and attached inline with other payloads to the weather balloon.
56
(a)
(b)
Figure 3.19: CricketSat Sensor Array System
The
BalloonSat
flight
(CONDOR-3)
containing
the
CricketSat
and
communication payloads was launched from the Milton High School once again. The
Milton students tracked the flight (Fig. 3.20a) from the high school and collected
CricketSat data using a Yagi antenna, Kenwood receiver and the Spectra RTA software.
The flight was tracked for 134 minutes, bursting at an altitude of 85,781 feet, and traveled
51 terrestrial miles. CricketSat data was collected for nearly the entire flight to within
6,000 feet of landing. Below this altitude, the radio signals were blocked by the Green
Mountains, located about halfway between the launch and the landing sites.
Burst
(a)
(b)
Figure 3.20: BalloonSat flight path (left) and raw segregated CricketSat temperature and pressure data
(right) received during the flight.
The CricketSat array system performed successfully, indicated by the segregation
57
of tonal data (Fig. 3.20b) received from the various sensors.
Two CricketSat sensors
were used to measure the external atmospheric temperature (dark blue and light blue
traces), another sensor measured the CricketSat flight bag temperature (red trace) and a
third measured atmospheric pressure (green trace).
Data was collected nearly continuously for the entire flight. Collection of data for
the external temperature sensors was complicated due to the extremely low tone
frequencies experienced at the coldest temperatures. One of the CricketSat temperature
sensors (light blue trace) was modified to double its frequency to attempt to resolve
problem.
Unfortunately, the change was not significant enough for the lower
frequencies.
The converted temperature and altitude sensor data (Fig. 3.21a) was overlaid with
known data for correlation. For the external temperature data (green trace), the source
was NOAA sounding balloon data from Albany, New York. The external temperature
data tracks very well except at higher altitudes. The difference is very noticeable in the
stratosphere (>60,000 feet), where radiation heating is more intense and the thermal
conductivity of the air is reduced. Unfortunately, the black tips of the thermistors were
not shielded from the radiation, resulting in the erroneous heating. Painting the tips with
white paint or covering them with tin foil minimizes the radiation heating effect.
58
Flight Bag
Burst
(a)
(b)
Figure 3.21: Converted CricketSat temperature data (left) and altimeter data (right). CricketSat data is
compared to known data for sensor validation.
CricketSat altimeter data (Fig. 3.21b) was correlated with altitude data collected
from the GPS flown in the communications flight bag. The CricketSat data (blue trace)
tracked closely with the GPS data (green trace) during the ascent until the limits of the
pressure sensor were exceeded near 30,000 feet and remaining steady until the descent.
Later, during the descent, the CricketSat altimeter responds, but presents a shallower
slope than the GPS data. In the lower atmosphere, the pressure increased rapidly, likely
causing the collapse of the moderately sealed flight bag and restricting the flow of air into
it. This would produce a delayed pressure response, resulting in an effective lagging of
altitude.
The minimum temperature of -92 F was experienced in the upper tropopause near
60,000 feet.
3.14.
UVM CricketSat Revision E (December 2004)
Several changes were made for this revision to make it the most versatile and
reliable. For prior versions of the CricketSat the area of the circuit board remained
59
unchanged; that of a 9-Volt battery. The Revision E CricketSat (Fig. 3.22) was grown
larger allow more features and to simplify assembly.
Figure 3.22: CricketSat RevE circuit board.
An enlarged prototype area was reintroduced, pre-wired like a breadboard. This
allowed for the use of single 14 or 16-pin DIP ICs, or dual 8-pin ICs. Pads on the center
two columns were wired vertically to provide two power bus strips. The outer two pair
of columns were wired to provide connectivity to the ICs, ach horizontal pad pair
interconnected. Pre-wiring the prototype area reduced the amount or wiring required for
students using it for custom applications.
Labeled test points (Fig 3.22) were added to serve instructional, debugging and
connectivity uses. Test points were connected to all power and timing circuit nodes (Fig.
3.23). For circuit debugging and instruction, the larger pads provide convenient probing
locations for test instruments such as multimeters and oscilloscopes for verifying correct
voltage levels and viewing oscillating signals. With large through-holes, they also serve
as a convenient way to interface power, and timer control and output signals to the
prototype area and external circuitry.
60
Figure 3.23: CricketSat RevE schematic.
Power and ground planes were added to improve the stability of the oscillator
circuit. Placing one’s hand near the prior CricketSat revisions would affect the oscillator
frequency. Inclusion of ground and power planes eliminated the sensitivity.
Resistors were orientated horizontally to simplify assembly and improve
mechanical reliability. In earlier CricketSat versions, there was a problem with the
antenna wires breaking due to necessary bending and re-positioning.
Also, while
straightening the antennae, the insulation would frequently slide up, exposing the bare
wire. This problem was not easily reversible, causing concern for short circuits. Antenna
strain relief holes were added, solving both problems.
The last inductor was finally removed, after testing indicated that there appeared
to be no benefit to keeping it in place. The CricketSat signal was received over the
farthest terrestrial distance with the power and data lines separate on the transmitter
module. A decoupling capacitor was placed on the power trace near the module. A
61
decoupling capacitor was also added to the battery side of the 5-Volt regulator, in case
long leads or an unclean power source was used.
3.15.
UVM CricketSat Revision F (May 2005)
The most important change for RevF CricketSat was the addition of the power
switch (Fig. 3.24, Fig. 3.25). Besides the benefit of conveniently switching power, it also
eliminated failures with the 9-Volt snap connectors due to the constant stress of removal
of the connector.
Figure 3.24: CricketSat RevF schematic.
This revision also included the use of thermal pads on the PCB (Fig. 3.25) for
those pads and vias connecting to the power and ground plane. The thermal pads form
and “X” connection to those metal planes, minimizing the heat-sink effect on the solder
pads. The soldering of these pads had been troublesome for students who were issued
lower-wattage soldering irons, resulting in many cold solder connections.
Using a
higher-wattage soldering iron would likely damage the circuit board, or components.
62
Figure 3.25: CricketSat RevF circuit board.
Three changes are anticipated relating to a future CricketSat design (RevG).
Switching CricketSat operation between pulse and tone modes has been determined to be
useful and the dual-capacitor selection will be reintegrated in the circuit board. All
component solder pads will be made larger, making the process of soldering easier for the
students. The soldering of connections using thermal pads connected to power and
ground planes still proves somewhat difficult for students, requiring new thermal pads
with a higher thermal resistance to be created.
3.16.

Conclusions
Accomplishments
o Sensors improvements

Comparison of versions of the UVM CricketSat is summarized in
Table 3.1.
o Receiver data collection system

Table 3.2 highlights achievements with various test flights
63

Shortcomings
o Low resolution at very cold temperatures
o Difficult calibration for upper atmospheric temperatures
o Lead-in for Chapter 4 linear sensor
Table 3.1: UVM CricketSat sensor Revsions A-E and proposed Revision G.
Revision
Date
Major Changes
A
2003
4-pin transmitter, protection diode, pulse-tone mode, larger prototype area
B
Jan 2004
Dead end
C
Jan 2004
Battery strain relief holes, solder mask coating, component labeling
D
May 2004
5-Volt regulator, larger mounting holes, dedicated pressure and humidity
sensors
E
Dec 2004
Enlarged circuit board, improved prototype area, test points, antenna strain
relief
F
May 2005
Power switch, thermal pads
G
Dec 2006
Proposed: Selectable pulse-tone mode, increased size of solder pads
Table 3.2: UVM CricketSat significant milestones.
CricketSa
t
Mode
Measurement
Tool
Temperature
pressure
humidity
Tone
Frequency
meter
Temperature
Pulse
Stopwatch
CricketSat
Event
Date
Sensor(s)
Sandbar
Flights
(RevA)
06/18
2003
UVM
Flight
(RevA)
08/08
2003
Key Results
Qualitative results observed.
Noisy signal hampered
frequency measurements less
than two minutes into flights.
First successful measurements
Flight time: 31 minutes
Range: 2 miles
Temperature: +83 F > +40 F
Estimated altitude: 11,000 feet
Burst
64
Governor’s
Institute
Flight
(RevA)
08/09
2003
Temperature
Pulse
Stopwatch
Weather
Station
(RevA)
12/2003
02/2004
Temperature
pressure
humidity
Tone
Frequency
meter
MHS
Flights
(RevD)
06/16
2004
Temperature
pressure
humidity
Tone
Spectra RTA
software
First
BalloonSat
Flight
(RevD)
07/17
2004
Temperature
Tone
Spectra RTA
software
Second
BalloonSat
Flight
(RevD)
07/30
2004
Temperature
pressure
humidity
Tone
Spectra RTA
software
Longest measured flight
Flight time: 90 minutes
Estimated range: >15 miles
Minimum temperature: +20 F
Estimated altitude: 8,000 feet
First multi-sensor system
Frequency meter never worked
more than three hours. Better
frequency measurement system
needed.
Longest duration, furthest,
coldest and highest flights
Successful frequency
measurements
Flight time: 91 minutes
Estimated range: 51 miles
Minimum temperature: -41 F
Estimated altitude: 26,732 feet
First CricketSat application
Flight time: 125 minutes
Flight bag temperature: +66 F
Range: >18 miles
Altitude: >45,000 feet
First multi-sensor flight
Flight time: 134 minutes
External temperature: -92 F
Range: 51 miles
Altitude: 85,781 feet
65
CHAPTER 4: A LOW-COST LINEAR-RESPONSE TEMPERATURE SENSOR
FOR EXTREME ENVIRONMENTS
Mike Fortney and Jeff Frolik
University of Vermont
4.1.
Intro to Paper
This paper was submitted to the IEEE Sensors Journal, date…..
4.2.
Abstract
This work details the development of a low-cost (<$15) wireless temperature sensor to
measure the wide-range temperatures found in extreme environments. The work is
motivated by educational and outreach programs which utilize a simple timer-based
circuit for a variety of environmental monitoring applications. This circuit, commonly
referred to as the CricketSat, is often configured as a temperature to frequency converter
for the purpose of atmospheric profiling. Unfortunately, the nonlinear response of the
present circuit significantly reduces resolution and accuracy for temperatures below -20
ºC. Herein we provide analyses of three possible linear strategies in order to extend this
range over extreme, naturally occurring temperatures. A suitable method is identified
and implemented in hardware. This design and test results are presented.
4.3.
Introduction
Wireless sensors are touted as being a means by which to improve our understanding
of natural, industrial and military environments through their ability to enable increased
spatial and temporal resolution of data. While most research in recent years has focused
on wireless sensor networks for applications such as target tracking, wildlife monitoring
[x] and microclimate assessment [xi], there are also applications that may take advantage
of low-cost, non-networked wireless sensors. An example application, which motivates
the work herein, is the use of expendable, balloon-borne instrumentation packages for
meteorological data collection.
66
Atmospheric data is crucial in weather forecasting so much so that, for example, each
day 150 weather balloons are launched by the National Weather Service (NWS) from 93
different locations in the continental United States, Alaska and Puerto Rico [xii]. The data
collected forms the basis of computer models. However due to their cost ($250 each
profile), the spatial resolution provided by current weather balloons is typically 250km in
populated regions, 1000km in sparsely populated regions, with a U.S. average separation
of 315km [xiii]. As a result, conditions that are localized (e.g., valley fog) may not be
captured appear from current atmospheric data. To monitor such local microclimates, the
use of low-cost balloon-borne instrumentation has been proposed [xiv] along the lines of
the hardware developed to support the NASA National Space Grant Fellowship "Crawl,
Walk, Run, Fly" Student Satellite Program [xv]. This program has been in place since
2001 to teach students the fundamentals of space hardware development through project
ranging in complexity from the very simple CricketSat to advanced earth-orbiting
satellites. The low-cost (<$15) CricketSat, developed at Stanford University (circa 1999),
forms the foundation of the work herein. This design is similar to an earlier "Electronic
Cricket" circuit, published (prior to 1990) by the popular amateur scientist and author
Forrest Mims III [xvi]. This circuit differs from the CricketSat in using a speaker as an
output device instead of a radio transmitter.
For outreach activities, the primary application of the CricketSat is its use as an
balloon-borne telemetry sensor that students build, test, calibrate and launch. During the
flight,
students
remotely
record
measurements
of
atmospheric
temperatures.
Measurement ranges of over 100 km have been reported using off the shelf radio
67
equipment and antennas [xvii]. In this capacity, the CricketSat has been utilized by several
NASA Space Grant K-12 outreach programs, including those in Alaska, Colorado [xviii],
Louisiana [xix], Vermont [xx], Washington [xxi], and Vermont [8]. Furthermore, at the
University of Vermont it has been used to form the basis of interdisciplinary wireless
sensor projects conceived and developed by students in a first-year engineering design
course [xxii].
In short, the use of the CricketSat for educational activities is well
established and thus motivating improvements to its design while still maintaining its
many positive qualities including low-cost, long-range wireless capability, circuit
simplicity and adaptability.
Figure 4.1: CricketSat non-linear frequency response to temperature
As illustrated in Fig. 4.1, one shortcoming of the current CricketSat is the limited range
of temperatures that the sensor can accurately measure. The current design (discussed in
detail in § II), is effectively limited to measurements between -20 ºC and +60 ºC. One
68
also notes the non-linear response below 0 ºC necessitating multipoint calibration which
must include temperatures not necessarily easily obtained by the K-12 programs utilizing
this design. As such, in this work, we present and compare three possible approaches to
improve the measurement range and linearity of the CricketSat.
The development of linear temperature to frequency circuits has been previously
addressed. Earliest designs [xxiii] were centered on transistorized multivibrator bridges
producing an error less than 0.17 ºC across a range of 30 ºC to 60 ºC. Op-amp based
designs [xxiv,
xxv
], minimized the thermistor exponential response through logarithmic
means, yielding a 1 % error across a 64 ºC range. Sengupta [xxvi] clearly summarized
these prior methods and extended linearity over a wider temperature range. His design
predicted a linear frequency response resulting in a deviation of 1.4 % across a
temperature range of -100 ºC to 225 ºC. These approaches, while effective, were specific
to linearizing thermistor response and thus are not appropriate for a platform that is
intended to support a variety of sensors.
Due to the CricketSat’s popularity, its
educational value and versatility, our goal is to maintain a linear response for this 555timer based design.
While we focus on temperature linearization herein, our
investigation for controlling oscillation is applicable to active and passive sensors of
many types (e.g., pressure, humidity and light).
In particular, our objective is to enable this platform to provide data for a variety of
environmental monitoring applications that may range from atmospheric monitoring
(with temperatures as low as -89 ºC [xxvii]) to on-ground temperatures observed in deserts
(with temperatures as high as +58 ºC [xxviii]). The remainder of this paper is structured as
69
follows. In §II, analysis of the basic 555-timer which forms the basis of the CricketSat is
reviewed. In §III, we present analyses and simulation results for three alternative designs
which are shown to extend the range and improve the linear response of the CricketSat.
§IV compares the designs and in §V a prototype design is presented along with test
results. The paper concludes in §VI with discussion and extensions to other sensing
modalities.
4.4.
555-Timer Astable Oscillator
The heart of the CricketSat circuit is the popular 555-Timer [xxix] IC, configured as an
oscillator, as shown in Fig. 4.2.
Figure 4.2: 555-timer astable oscillator CricketSat design
70
The frequency of oscillation, f, is determined by resistive and capacitive timing
components R1, R2 and C1. All component values are fixed except for the thermistor, R1,
whose resistance decreases with temperature. The capacitor charges from the power
supply through the series combination of R1 and R2. Thus, as the temperature increases,
the thermistor resistance decreases, thereby shortening the charging interval, tCHG. The
discharge interval, tDIS, remains constant since it occurs only through the fixed value
resistor, R2. Charging discharging curves can be seen in the top panel of Fig. 4.4. In
short, this circuit is a temperature to frequency converter. The oscillation is presented to
an LED for visual cue and RF transmitter for wireless use. The modulation method is onoff keying (OOK) of a 434 MHz carrier enabled by the TWS-434 AM transmitter chip
(10 mW output power).
In existing CricketSat designs, R1 is 10kΩ thermistor and R2 is a 3.3 kΩ. C1 is
chosen to produce either an audible tone (C1 = 0.1 μF) or a pulse (C1 = 47.0 μF) visually
observable using the LED. The pulse mode is useful when radio receiver and frequency
measurement equipment is not available.
This situation is likely to occur when
CricketSat sensors are assembled for student use at home. In this low frequency mode,
LED flashes, which are produced once every ½ to 10 seconds, are counted over a timed
interval to determine the temperature. The tone mode of operation will typically produce
an oscillation in the range of 500-2400 Hz.
This mode is preferred for wireless
measurements, since it provides better temperature resolution.
71
Figure 4.3: 555-timer internal circuitry composed of the threshold voltage ladder, comparators, RD latch
and discharge transistor. Image for Texas Instruments NE555, SA555, SE555 Precision Timers data sheet.
The timer works to monitor and control the charging and discharging of timing
capacitor C1. The internal circuitry is seen in Fig. 4.3. The circuit has a pair of voltage
comparators with thresholds set for 1/3 and 2/3 of the supply voltage. These values
determine the voltage limits of the signal on the timing capacitor and state transition.
The 555 timer contains a latch, which is used to control the charge/discharge using
thresholds of 1/3 and 2/3 the supply voltage, respectively. The resulting digital output is
illustrated in the bottom trace of Fig. 4.4.
72
Figure 4.4: PSpice astable timing waveforms of the 555-timer circuit. The top trace shws the voltage on
the timing capacitor. The bottom trace represents the digital output of the timer.
The 555-timer astable oscillator design is well known [xxx] as is the resulting expression
for the frequency of oscillation (1)
f 
1
1.44

tCHG  t DIS R1  2 R2 C1
(4.1)
Inspection of the equation leads to a couple of observations.
frequency has no dependency on the supply voltage, VCC.
First, the oscillator
Second, the non-linear
response of the thermistor, R1, results in the non-linear temperature/frequency
characteristics illustrated in Fig. 1. An approach to linearize the thermistor is to place it
in series or parallel with a fixed resistor [xxxi]. However, in the CricketSat design,
linearizing the thermistor response does not help since R1 is in the denominator of (4.1).
As noted, we also have the objective to ensure sensitivity is sufficient to discern
changes in temperature across a wide range.
In the current design, sensitivity is
approximately 10 Hz/ºC in the linear region (0 ºC to 40 ºC) which is adequate for a
resolution of 0.3 ºC. This performance comes from the ~3 Hz frequency resolution
73
obtained using Spectrogram software [xxxii] for analyzing the audible tones.
In
comparison to (4.1), PSpice simulation of the circuit illustrated in Fig. 4.2 results in at
most a 0.3% error in resonant frequency. As such, we use PSpice as a means of assessing
alternative designs presented in §III.
4.5.
Linear Frequency Control Methods
For our educational purposes, we are motivated to linearize the temperature to
frequency response of this circuit to enable calibration of student projects using fewer
data points. Furthermore, a linear design will enable students to extrapolate data beyond
their calibration region with good confidence. Three methods for linearized frequency
control were investigated and are now detailed. These methods were evaluated on the
basis of linearity, oscillator frequency span and sensitivity to power supply variations.
4.5.1. Threshold Control Voltage Method
This method takes advantage of the control voltage (pin 5) on the 555 timer, by
way of forcing an external voltage, V2 as demonstrated in Fig. 4.5. This upper threshold
control voltage is normally produced by an internal resistor ladder network composed of
three equal-value resistors. The resistors are connected between VCC and ground (GND),
producing node voltages of 1/3 VCC and 2/3 VCC. The control pin is attached to the weak
2/3 VCC node, allowing the upper threshold voltage to be overridden from an external
source.
74
Figure 4.5: PSpice simulation circuit for using a voltage source (V2) to override the timer’s internal
threshold compare voltages. The 5-Volt source is used to represent the regulated voltage, which is now
necessary for this mode of operation.
The lower threshold voltage is always one-half of the upper threshold level (V2).
Introducing a larger forced voltage creates a wider operational window, resulting in
longer timing intervals and a lower oscillation frequency.
Example oscillations are
shown in Fig. 4.6. The upper waveform represents the timing capacitor voltage for a
forced control voltage of 4.0 Volts, and the lower waveform for a value of 2.0 Volts.
75
Figure 4.6: Simulation waveforms showing the effect of control voltage on the charging cycle of the
timing capacitor. The top and bottom waveform shows the capacitor voltage using a control voltage of 4.0
Volts and 2.0 Volts, respectively. Note that the lower limit is always one-half the control threshold value.
We now present the analysis for this design to ascertain the parameters which
determine the oscillation rate; that is, an equation analogous to (4.1) for the threshold
control voltage method.
In particular we will determine the charge and discharge
intervals (tCHG and tDIS, respectively).
For the charging interval, the capacitor voltage, VC (t ) has the following time response
(4.2).
VC (t )  V0  A(1  e t /  C )
Where V0 is the initial capacitor voltage which is


voltage potential, VCC 
(4.2)
1
V2 in this case; A is the applied
2
V2 
 ; and  C is the charging time constant, R1  R2 C1 .
2
The capacitor is noted to be charged when VC (t )  V2 . Substituting these values into the
general equation yields:
76
V2 
V2 
V 
 VCC  2 (1  e  t CHG /  C )
2 
2
(4.3)
Solving for the charge time results in (4).
tCHG




1

R  R C
  ln 1 
2
1
2VCC

 1
1

V2


(4.4)
Note the dependency on the supply voltage (VCC), unlike the common astable
configuration, which has no dependency on the supply. This circuit will therefore require
a voltage regulator to minimize the effects of supply variation. For example, a typical 9Volt battery may experience a range of 6 Volts to 9.5 Volts over its useful life thereby
demonstrating the need for the regulator.
The discharge interval follows the familiar form:
VC (t )  Ae t /  D
(4.5)
Where A  V0  0  VTHR and  D is the discharge time constant, R2C1 .
The discharge is noted to be complete when VC (t ) 
V2
. Substituting these values into the
2
general equation yields:
V2
 V2 * e  t DIS /  D
2
(4.6)
Which yield the discharge time (4.7)
77
t DIS  0.6931R2C1
(4.7)
This discharge expression is the same as for the astable oscillator and as such has no
dependency on the supply voltage. From (4.4) and (4.7) the oscillating frequency can be
found to be (4.8)
f 
Note that when V2 
1




1

R  R C  0.6931R C
 ln 1 
2
1
2 1
2VCC

 1

1


V2


(4.8)
VCC
the scenario is equivalent to the astable oscillator and (4.8)
3
reduces to (4.1). Results of the PSpice simulation for this design are shown in Fig. 4.7
below. An input stimulus across the range of 2.0V to 4.0V produces an oscillator
frequency span of 1030 Hz with a linear correlation factor of 0.991
78
Figure 4.7: Simulation results demonstrating 555-timer frequency response to varying threshold control
voltage.
Note that over a limited range shown, the effect is linear. Error bars represent the
error introduced by the 5-Volt regulator (TL75L05) tolerance of +/- 0.25 Volts (+/- 5%).
The circuit suffers a worst case 13% frequency deviation in response these variations in
the regulated supply when V2 = 5V; clearly not a desirable result even for a low-cost
design.
4.5.2. Ladder Voltage Control Voltage Method
A second method considered attempts to control the oscillator frequency by
varying the voltage, VLAD , at the top of the R1-R2-C1 ladder. Note that in the common
astable configuration, this voltage is tied to the VCC supply voltage. We will show, as
with the previous method, that varying this voltage has an effect on the charging timing
interval but none on the discharge interval. In contrast to the previous method, the
threshold levels in this approach are fixed at 1/3 and 2/3 of VCC. The simulation circuit
diagram is shown below.
79
Figure 4.8: Simulation circuit used to investigate the effect of RC ladder voltage (V2) on timer frequency.
In comparison to Fig. 4.5, the value of R2 has been reduced to increase duty cycle and minimize the nonlinearity.
As with the previous design, we first consider the charge time for this
configuration.
Beginning with (4.2), we now have that the initial voltage on the
discharged capacitor, V0 , is
1
VCC . The applied voltage potential, A , is VLAD  V0  .
3
The charging time constant, as before is  C  R1  R2 C1 .
2
3
As with the astable configuration, the capacitor is fully charged when VC (t )  VCC
resulting in (4.9) from which the charge time, tCHG , is found as given in (4.10).
2VCC VCC 
V 

 VLAD  CC (1  e  t CHG /  C )
3
3 
3 

VCC
tCHG   ln 1 
 3VLAD  VCC

R1  R2 C1

(4.9)
(4.10)
80
Now, the discharge interval is dependent only on the initial voltage on C1 and the
component values of C1 and R2, having no dependence on the applied ladder voltage.
Therefore, t DIS remains the same as in the astable case (4.7).
The resulting oscillator
frequency is given by (4.11). Note that when VLAD  VCC then the scenario is equivalent
to the astable case and as expected (4.11) simplifies to (4.1).
f 
1

VCC
 ln 1 
 3VLAD  VCC

R1  R2 C1  0.6931R2C1

(4.11)
Fig. 9 shows the simulated frequency response to the RC ladder voltage. An input
stimulus across the range of 3.5V to 5.0V produces an oscillator frequency span of 823
Hz with a linear correlation factor of 0.9963. Note that usage in the region beyond 5.0V
is possible as well, dependent on available battery voltage, however use below 3.5 V
results in a non-linear response.
81
Oscillator Frequency vs RC Ladder Voltage
2000
1800
Frequency (Hz)
1600
1400
1200
1000
y = 549x - 1527
R2 = 0.9963
800
600
400
200
0
3
3.5
4
4.5
5
RC Ladder Voltage (V)
5.5
6
Figure 4.9: Simulated response of timer frequency to control of RC ladder voltage.
Since (11) is also affected by the VCC supply voltage, a sensitivity analysis such
as discussed in the previous method was performed using PSpice. This circuit suffers a
worst case 36% frequency deviation in response to variations in the regulated supply; this
occurs when VLAD = 3.5 V.
4.5.3. Current Source Method
The previous two methods, while producing linear responses are highly
susceptible to fluctuations in supply voltage even in cases where a regulator is utilized.
As such a third design in which a high-side current source is used to replace the resistor
R1 is considered (Fig. 4.10).
82
Figure 4.10: PSpice simulation circuit used to investigate the use of a current source method for 555-timer
frequency control.
In this design, the capacitor charging period is inversely proportional to the
strength of the current source, I SRC .
Beginning with the fundamental expression
Q  CV an equation can be derived relating the charging current and charging interval
(4.12). Since current is the rate of charge (electron) flow, the fundamental equation may
be rewritten as,
I SRC 
Q
V
 C1
t
t
(4.12)
83
The charging is complete when the voltage rises from
1
2
VCC to VCC (i.e.,
3
3
1
V  VCC ) which occurs in t  tCHG ; or using (4.12) and solving for tCHG yields
3
(4.13).
t CHG 
C1VCC
3I SRC
(4.13)
Solving for f yields,
f 
t DIS
1
C *V
 1 CC
3 * I SRC
(4.14)
If the duty cycle is maintained above ~95%, then f is approximately linearly dependent
on I SRC (15). This condition is achieved by having tDIS is kept proportionally small; in
our case 23 μsec using R1=330 Ω and C1= 0.1 μF.
f 
3I SRC
C1VCC
(4.15)
Component values for (4.15) were selected that produced a sensitivity of 4.81
Hz/μA in simulation. The linear result, with a correlation factor of 0.9995, was produced
using an input range of 175 μA to 375 μA.
From (4.15) we see that there is a
dependence on the VCC supply voltage to be evaluated. The circuit suffers a worst case
4.5% frequency deviation in response to variations in the regulated supply independent of
source current.
84
4.5.4
Comparison of alternative methods
Table 4.1 below summarizes the range of operations, linearity and power supply
sensitivity for each of the three methods considered. All three methods produced similar
oscillator frequency spans. The current source method demonstrates the best linearity
and least supply sensitivity of the three methods. This method also requires the least
amount of support circuitry, thereby reducing the complexity and cost of the final circuit.
Therefore, the current source method was selected for the temperature sensor design
detailed in the following section.
Table 4.1: Comparison of simulation results for various the oscillator control methods.
Control
Range
Threshold
Voltage
Ladder
Voltage
Current
Source
2.0 –
4.0
Volts
3.5 –
5.0
Volts
175 –
375 uA
Frequency
Range
(Hz)
Frequency
Span (Hz)
Oscillator
Linearity
VCC
Supply
Sensitivity
Sensor
Support
Circuitry
595 - 1630
1035
0.9910
13 %
Signal
Conditioning
375 - 1200
825
0.9963
36 %
Signal
Conditioning
975 - 1937
962
0.9995
4.5 %
Single
Resistor
85
4.6
Implementation and Test
4.61
Implementation of a linear sensor
To implement the current source design, a sensor is needed that produces a
temperature dependent current. The LM334 current source is such a device, commonly
used by hobbyists and for commercial temperature measurement and control applications.
Product examples include a temperature/humidity sensing kit sold by Fascinating
Electronics and temperature probes sold by Greystone Energy Systems.
The LM334 produces a current that is related to the value of a set resistor (RSET)
and with sensitivity to the absolute temperature. Specifically,
I SRC 
227 V /  K
RSET
(4.16)
The above equation represents a straight line with an intercept at the ordinate.
The manufacturer claims that there are no offset errors, only a gain factor, providing for a
single-point calibration. The LM134 and LM234 devices, of the same family, guarantee
higher initial accuracy and less slope error across a wider temperature range [ xxxiii] than
the common LM334 (0 ºC to +70 ºC). In our work, we utilized the LM234 due to this
extended temperature range. All three part numbers conform to equation (4.16).
The LM334 in conjunction with a 555-timer circuit appears in a patent from
General Electric Company [xxxiv]. However in that proposed application, only room
temperatures were considered and no analysis was provided. In Fig. 4.12, a LM234 is
86
shown incorporated in to the CricketSat timer circuit, with R1 representing the RSET
resistor. With R1 assigned a value of 200 Ohms, the current source produces a current
range of 208 μA to 344 μA over a respective temperature range of -90 ºC to +30 ºC.
This aligns the desired operational temperature range within the desired current range
shown in Table 4.1. Combining the resulting sensor sensitivity of 1.13 uA/ ºC with the
oscillator sensitivity of 4.81 Hz/μA yields an overall sensitivity of 5.44 Hz/ ºC.
Figure 4.11: Schematic showing implementation of the LM234 current source device as a temperature
sensor. The value of R1 (i.e. RSET) is selected to provide a suitable sensitivity and frequency range.
4.62
Test Scenarios
To further validate our analyses and simulations of the proposed design, six
CricketSat modules were modified to test the design (Fig. 4.14). Four of the modules
87
utilized the LM234 (from different lots to accentuate process variation) discussed thus far
and two modules utilized the LM334 (a lower cost alternative). Our tests were also
compared to results of three, unmodified (thermistor-based) CricketSats.
Oscillator
frequency data was collected for each module in four environments: 25 ºC (room
temperature), +5 ºC (household refrigerator compartment), -15 ºC (household freezer
compartment), -58 ºC (laboratory freezer) and at 40 ºC and 61.1 ºC (in an electrically
heated compartment).
Temperatures were measured using multi-meters (ExTech
MiniTec 26) with a temperature adapter (ExTech 381277) with a K-type thermocouple
probe. The multi-meters were also used to measure oscillation frequency.
Figure 4.12: CricketSat module modified using the LM234 current source for use as a temperature sensor.
In the tests, only the temperature sensors (LM234, LM334 or thermistor) were
placed in the environment and the remaining circuitry stayed at room temperature. This
setup for cold temperatures mimics balloon-borne tests where instrumentation is placed
in an insulated and heated flight bag. Once the sensor was placed in the environment,
88
temperature and frequency readings were allowed to stabilize. Three measurements were
made over a three minute window for each test.
4.63
Test Results
Frequency response measurements of three thermistor-based and six currentsource-based CricketSat sensors can be seen in the left graph in Fig. 4.13 below. Data
was averaged for each type of sensor and standard deviation is shown. The thermistorbased CricketSat data aligned nearly perfectly to the predicted response, indicated by the
short-dashed line.
For comparison, the longer-dashed line represents the predicted
response of a linearized-thermistor [22] based CricketSat sensor. This demonstrates that
even though the oscillator frequency has not approached zero Hertz at very cold
temperatures, the slope of the response is nearly zero, rendering the circuit ineffective
over the extreme ranges we are considering.
The averaged results from the six current source based CricketSat designs is
shown in the right graph of the Fig. 4.13. The trend line is fitted to data values of 25 ºC
and colder, since this is the primary range of interest for weather balloon applications.
The results of this four-point calibration using all six linearized circuits indicate a
linearity of 0.9997 (0.317 %) and a standard deviation error of 14.4 Hertz (1.15%). There
appears to be no significant statistical difference between the LM234 and LM334
sensors, with both types functioning across the tested temperature range. A tight 95%
confidence interval is shown with variance of ± 5.2 Hertz to ± 9.3 Hertz across the tested
temperature range.
Not shown to the left of the y-axis is an x-intercept of -273.47 ºC,
89
only 0.32 ºC from absolute zero. This result reinforces the argument for the use of an
absolute zero data point to simplify calibration as to be discussed shortly.
(a)
(b)
Figure 4.13: Test results for the thermistor-based and current-source-base CricketSat temperature sensors.
Concerning calibration, multiple options were explored, driven by the need to
simplify calibration in the classrooms. Two aspects of calibration were investigated; the
number of calibration points used and the effect of including absolute zero. Two, three
and four-point calibrations were investigated while not including absolute zero as part of
the set.
Using absolute zero as part of the set allowed the inclusion of a single-point
calibration.
Table 4.2: Calibration methods and sources
Room
Temperature
(25.0 ºC)
One-Point
(plus 0ºK)
Two-Point
Three-Point
Four-Point
Refrigerator
(4.44 ºC)
Refrigerator
Freezer
(-18.1 ºC)
Laboratory
Freezer
(-67.2 ºC)
X
X
X
X
X
X
X
X
X
X
90
Using only temperature points in Table 4.2 for curve fitting we find (Table 4.3)
that by using our frequency measurements we can ascertain temperature over our test
range with very good accuracy (better than ± 0.5 ºC or 0.32%) and precision (within ±
0.9 ºC).
Two cases are worth noting. The first is the four-point calibration without using
absolute zero (noted in bold). Over the range of measurements we achieve the best fit to
the actual data of the non-zero methods (as expected). We also note an offsets at the
above room temperatures of ~ -2.2 ºC at +40 ºC and ~ -3.5 ºC at +61.1 ºC. This offset is
systematic for all methods and expected. At these higher temperatures, reduction of the
tCHG
interval increases the significance of
t DIS
, resulting in non-linearity and requiring
the use of (4.14). For our ballooning efforts, this range is not of interest, but we
demonstrate applicability of the circuit in these upper extremes (with slight degradation
of precision).
Table 4.3: Mean error/standard deviation (ºC) produced using various calibration methods. “Z” calibration
includes absolute zero data point, “NZ” does not.
Temperature
Calibration Type
-67.2 ºC
-18.1 ºC
4.44 ºC
25.0 ºC
40.0 ºC
61.1 ºC
NZ
0.192/0.718
-0.646/1.57
0.872/0.614
-0.346/0.753
-2.12/2.01
-3.55/1.59
Z
0.200/0.999
-0.648/1.55
0.852/0.524
-0.369/0.828
-2.16/1.78
-3.59/1.56
NZ
0.674/4.60
-0.394/0.257
0.862/0.640
-0.522/0.402
-2.46/0.909
-3.99/1.99
Z
0.243/1.26
-0.605/1.33
0.896/0.653
-0.326/0.935
-2.12/2.02
-3.55/1.67
NZ
-5.52/5.09
-3.11/2.51
0.000672/0.0304
0.000694/0.0327
-0.826/1.85
-0.998/2.83
4-Point
3-Point
2-Point
91
1-Point
Z
0.0578/0.947
-0.812/1.82
0.674/0.584
-0.558/0.567
-2.36/1.53
-3.80/1.33
Z
0.435/1.04
-0.355/2.07
1.17/1.12
-0.0315/0.0448
-1.81/1.40
-3.22/1.39
Of special interest to our outreach activities is the results obtained using absolute
zero in the calibrations in particular the 1-point calibration italicized in Table 4.3. The 1point calibration error/precision differs little from the results of the four-point calibration
not using the absolute zero point. For outreach activities the advantage is clear: good
calibration using only room temperature data. To validate the use of a 1-point calibration
of our design, we replaced the LM234 with a controlled current source to test the
remaining CricketSat circuit.
This current source enabled us to emulate current
developed by the LM234 per (4.16) over temperatures from absolute zero (-273.15 ºC) to
+79 ºC. The oscillator frequency approaches zero Hertz as the source current approaches
zero (as illustrated in the magnified region of Fig. 4.14). Thus not only do we justify the
use of absolute zero in our calibration but also we can, with good confidence, extrapolate
outside our tested ranges (e.g., down to – 90 ºC).
(a)
(b)
Figure 4.14: Figure to the right demonstrates the linear response of the CricketSat oscillator circuit over a
wide current range. The highlighted region near the origin is shown in the left figure, showing the trend
line passing near the origin.
92
4.7
Conclusions
Herein we have presented an analysis and a design to linearize the temperature to
frequency response of a popular circuit used in engineering education and outreach
settings. The design maintains the use of a simple and low-cost wireless device and
provides linearity better than 0.317 % and was validated over the range of -60ºC to room
temperature. Circuit response above this range, while non-linear, is predictable and
temperatures below the tested range will also produce linear response with a high degree
of confidence. Furthermore, the design lends itself to simple calibration which is also of
advantage to its intended use in educational programs.
While this work has focused solely on the measurement of temperature, there is
opportunity to utilize the developed methods to investigate other modalities. Any sensor,
but in particular, pressure, humidity and light sensors, which produce linear voltage or
current responses may used.
The LX1972 light sensor from Microsemi is directly
compatible with the CricketSat design, producing a proportional output current up to 200
uA. A simple three-transistor voltage-controlled current source circuit may be used to
interface the voltage-output type sensors. Pressure (Motorola MPX4115) and humidity
(Honeywell HIH-3610) CricketSat prototypes have been developed by the authors, and
initial testing has demonstrated linear frequency response.
4.8
Acknowledgement
The work presented herein originated with support from Vermont NASA Space
Grant.
Additional support has been received through UVM HELiX program.
The
93
authors would also like to thank Neil Brown of the University of Alaska, Chris Koehler
of the University of Colorado and the first-year engineering students at UVM for their
utilizing the CricketSat platform thereby by motivating the work herein. Special thanks
to Gary Visco with UVM’s Mathematics and Statistics Department for assistance with
our data analysis.
94
CHAPTER 5: SUMMARY AND CONCLUSIONS
5.1.
Conclusions
This thesis presented an alternate approach for the design of a wireless sensor
network that achieves low-power consumption, extended network lifetime and low cost.
The UVM-WSN uses three different classes of nodes which employ different methods of
communication to move sensed data to a base station. The base class of nodes (ALOHA)
possess a much lower level of complexity and functionality than the higher level nodes
(Hopper and Gateway). The developed UVM-WSN was lab tested to determine overall
power consumption when utilizing an energy aware routing protocol (VAARP) and
compared with an existing WSN developed by UC-Berkeley. The results of the power
test indicate that the UVM-WSN achieves reduced overall power consumption and cost
when compared with a single hop UC-Berkeley WSN and shows a favorable network
lifetime curve under some severe network constraints when using the VAARP over a
static routing protocol.
The specific contributions that this work makes to improve upon current WSN
design methodologies are:

Reduced Power Consumption and Cost Through Hierarchical Architectural Design –
All WSN topologies attempt to reduce energy use through hardware and software
design. The UVM-WSN in addition utilizes major changes in the WSN architecture
to achieve reduced power consumption. The design of the ALOHA node reduces
functionality, but significantly reduces energy requirements. These lower class nodes,
which are more prevalent in the UVM-WSN, also have significantly reduced cost.
95
The ALOHA node has no receiver on board and roughly halves the cost of the design
as a result.

Reduced Power Consumption Through Close Control of the Hardware Layer – Many
of the components utilized in the design of the sensor nodes in the UVM-WSN
offered power down functionality. These parts, such as the sensor board A/D, offer
multiple operating modes that raise or lower the power requirements of the device.
Additionally, microcontroller sleep modes are taken advantage of to reduce power
consumption between transmissions in the ALOHA node.

Smaller Form Factor – The ALOHA node requires less power to operate and
therefore requires a smaller battery for its power source. This has the added benefit of
reducing the size of the node. The node itself is designed to be as compact as
possible and since it lacks a receiver, the form factor is further reduced in size. The
assembled nodes are shown in Figure 5.1
Figure 5.1: Hopper node and ALOHA node photo

Investigation of a Protocol That Attempts to Achieve Extended Network Lifetime – An
analysis of energy aware routing protocols resulted in VAARP. This protocol was
simulated and analyzed in the context of the UVM-WSN. The result is a uniform dieoff of nodes instead of patch failures of groups of nodes that lose their packet
forwarding node.
Performance wise, the VAARP did not significantly improve
network lifetimes in the UVM-WSN. Development of VAARP brought to light
serious issues that need to be addressed during design and development of a WSN
96
with an energy aware routing protocol.
These issues specifically include the
difference between normal and packet forwarding power, the rate of packet
forwarding and the length of the data packet. These issues dictate how much benefit
may be gained by using an energy aware routing protocol and whether it is
worthwhile to implement one in a WSN from the perspective of a cost benefit
analysis.

Remotely Accessible – The web enabled Ethernet interface in development will allow
the UVM-WSN to directly update an online database to continuously update sensed
parameters. This will allow data to be extracted from the network to be easily utilized
and viewed.
5.2.
Future Research
This work is still, in many ways, in progress. The software on all the nodes could
be improved for greater power efficiency and the hardware design could also be modified
to further reduce power consumption. Some specific improvements that could be made
are listed below.

The Linx receiver chip utilized on the Hopper board design has a power down feature
which is not used in the presented design. This feature could be used to intentionally
power down the receiver in between packets from its handled nodes. The Hopper
node can keep track of the frequency of transmissions from its handled ALOHA nodes
and power down the receiver when no packet is expected. This technique is basically
a form of slotting.
97

Improvement of the VAARP to permit more then two hops and non-fixed size
networks. This could include dynamic creation of tables of available nodes as they
get added to the network or die-off. This improvement would also improve the
effectiveness of VAARP as the packet load on a given node would probably increase
with additional hops.

A different receiver and transmitter combination could be used in the design. An FSK
modulation scheme would make for a more reliable wireless link. Microchip is
marketing a PIC microcontroller that has a built in FSK/ASK transmitter [36]. If a
suitable low cost and low power receiver could be found to use with this transmitter,
the size, cost, and power requirements of the ALOHA and Hopper nodes could be
reduced. Additionally, the data rate could be increased over the limited 300-5000
bits/second of the Linx transmitter and receiver pair. The Motorola RF modem [37]
could be used in future designs as it offers QPSK (Quadrature Phase Shift Keying)
modulation.

The onboard EEPROM on the Hopper node is not currently utilized. This hardware
could be used to implement a logging feature so that data sets could be sent as a
whole or filtered in some fashion on the node.

Some form of data aggregation technique [38] could be used to reduce the number of
ALOHA and Hopper packets on the channel and thus free up the channel for more
users. This would reduce the restrictions imposed on the number of users as shown in
the protocol curves of Chapter 2.
98

An error checking a correction methodology could be implemented to repair mangled
packets. The byte needed for ECC is already present in the communications packet,
thus ECC only requires implementation.

The Gateway node needs to be completed and optimized for use in the UVM-WSN.
Much future research can be done using the UVM-WSN platform. Some examples of
this are listed below

Investigate the user of power scavenging technologies to remove the requirement for
a battery on the sensor nodes. The ALOHA node uses so little power that power
scavenging is a viable option for this class of sensor nodes. NASA is utilizing small
solar panels to power planetary rovers [39]. These more efficient panels could be
used in sensor networks in trigger scenario in which sensor nodes would only
function when it was light out. The thermoelectric effect is another possibility for
powering sensors. In many instances in a sensing environment, temperature gradients
are present and could be used to power a very low power sensor through exploitation
of the thermoelectric effect. [40] describes the basics of thermoelectric cooling and
power generation and some approaches that are being taken to improve
thermoelectric effects in different materials. Even more exotic is the use of organic
material to power a sensor node. This would make a sensor effectively able to
consume organic material and produce energy from this source. The “SlugBot” [41]
attempts to achieve power autonomy by capturing and decomposing slugs. This
approach could be adapted for use in decomposing plant material for energy on a
small scale for a sensor node.
99

Determine node location through the use of echo location or GPS. GPS chips are
progressively become smaller and more power miserly and will, at some future date,
become practical for use in a WSN. This would enable specific determinations of
node to node distances and allow the nodes to adjust their transmission strength to
reflect the actual distances between nodes. Specific specifications for GPS chips
currently being manufactured may be found at [42].

Frequency hopping could be utilized [43] to increase the maximum node load for the
channel by placing different users on different channels. This would use multiple
bands, but collisions could be reduced. Like GPS chips, frequency agile transmitters
are rapidly becoming smaller and more cost effective.
Atmel Corporation is
producing a transceiver that has software selectable carrier frequencies that could be
used to implement a frequency agile networking scheme [44].

Investigations of the interactions between the CSMA and ALOHA protocols when
the same band is utilized.
This thesis assumes no conflict between Hopper
transmissions and ALOHA transmissions. This would only be the case if the two
nodes transmitted on different frequencies. However, this is not the case. Equations
for throughput of WSN using both CSMA and ALOHA protocols need to be created
to quantify the interplay between nodes in the network.
5.3.
Final Thoughts
This work has personally introduced me to a number of issues associated with digital
communications protocols, namely wireless channel sharing protocols and problems
associated with various modulation and encoding schemes. Additionally, this work has
100
enhanced my understanding of embedded system design from the bottom up. This work
required the design and development of three separate boards from scratch and borrowed
from all aspects of embedded system design. After creating the UVM-WSN, I hope that
it may be utilized in a research project at some time in the future for data collection and
analysis.
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