Self-Powered Wireless Sensor System using MEMS Piezoelectric Micro Power
Generator (PMPG)
by
YuXin Xia
S. B. Electrical Science and Engineering
Massachusetts Institute of Technology, 2005
Submitted to the Department of Electrical Engineering and Computer Science
in Partial Fulfillment of the Requirements for the Degrees of
Master of Engineering in Electrical Engineering and Computer Science
at the Massachusetts Institute of Technology
MASS
ACUsETs
I i UT1
OF TECHNOLOGY
gay 26,
2006
Copyright 2006 YuXin Xia. All rights reserved.
LIBRARIES
The author hereby grants to M.I.T. permission to reproduce and
distribute publicly paper and electronic copies of this thesis
and to grant others the right to do so.
Author
Department of Electrical Engineering and Computer Science
May 26, 2006
Certified by_
Sang-Gook Kim
Thesis Supervisor
Accepted by
Arthur C. Smith
Chairman, Department Committee on Graduate Theses
BARKER
Self-Powered Wireless Sensor System using MEMS Piezoelectric Micro Power
Generator (PMPG)
by
YuXin Xia
Submitted to the
Department of Electrical Engineering and Computer Science
May 26, 2006
In Partial Fulfillment of the Requirements for the Degree of
Master of Engineering in Electrical Engineering and Computer Science
ABSTRACT
A thin-film lead zirconate titanate, Pb(Zr,Ti)0 3 , MEMS Piezoelectric Micro Power
Generator (PMPG) has been integrated with a commercial wireless sensor node (Telos),
to demonstrate a self-powered RF temperature sensor module. PMPG and a power
management module are designed to satisfy sensor node's power requirement. An
electro-mechanical model of PMPG has been developed to maximize power output. The
2 nd generation PMPG is designed to provide 0.173 mW power at 3 V DC with a natural
frequency of 155.5 Hz. The power management module is developed to provide AC-DC
rectification, energy storage, and active switching between PMPG and application circuit.
To minimize power consumption, sensor data is taken at a discontinuous interval. A test
bed is developed, which mimics that of a liquid gas pipeline used in the Alaska, where
the self-powered sensor be used to monitor pipeline temperature.
Thesis Supervisor: Sang-Gook Kim
Title: Esther & Harold Edgerton Associate Professor of Mechanical Engineering
2
Acknowledgments
I would first like to thank my advisor Professor Sang-Gook Kim for his support
and guidance during the past two years. He has constantly reminded me the importance
of looking at the big picture in technical research, and kept me motivated in this project.
His technical insight was invaluable for the completion of this project. Besides research,
there are many things I have learned from him that I will treasure for the rest of my life.
Thank you very much and best of luck in your future research!
Second, I would like to thank my former office mate Wonjae Choi for helping me
with PMPG modeling. He was always there tireless answering my questions, and
provided support both as a friend and a mentor. I wish him and his family the very best.
I would like to thank all the MTL staffs as well as fellow researchers at MTL for
your advices and supports. You have all made the countless hours at MTL more bearable.
I especially like to thank Dave Terry, Dennis Ward and Vicky Diadiuk for help with
processing, and be extremely patient in helping me with machines.
I like to thank my fellow office mates Zach Traina, Sunil Doddabasanagouda,
Soohyung Kim, Dr. Hyungwoo Lee and Ray Hardin for technical advices and help.
I also thank Professor Dave Perreault for help with power electronics, and
Professor Brian Wardle and Noel Eduard du Toit for help with PMPG modeling. Joe
Polastre of U.C. Berkeley was also extremely helpful in nesC programming.
I like to thank my friends for providing all the entertainment and support I needed
during those stressful times. I would not be able to finish my thesis without them.
Lastly, I would like to thank my dad and mom for all the sacrifices they made to
let me do what I enjoy doing. I would not be able to become who I am today without the
unselfish love and guidance they have provided. I also thank my brother Matthew for
always being there for me, and I wish him the very best!
3
Contents
1 Introduction
6
2 Self-Powered Sensor Node Architecture
8
2.1 Introduction .........................................................................
8
2.2 Ultra-Low Power Components .....................................................
9
2.3 Wireless Sensor Nodes ...........................................................
2.4 nesC ...............................................................................
11
. 15
2.4.1 nesC Language ..........................................................
15
2.4.2 nesC Application ......................................................
17
2.4.3 Telos Operation and Power Consumption .........................
19
2.5 Power Management Module ....................................................
21
2.5.1 PMPG Modeling ......................................................
22
2.5.2 Wireless Sensor Node Modeling ...................................
22
2.5.3 System Architecture Design ........................................
23
2.5.4 Implementation: Voltage Monitoring Module .....................
27
2.5.5 Implementation: AC-DC Rectifier ...................................
29
2.5.6 Energy Storage Capacitor Sizing ...................................
33
2.5.6.1 Calculating Lower Bound ................................
33
2.5.6.2 Capacitor Charging Time Constant .......................
34
2.5.6.3 ESR and DCL .............................................
35
2.5.6.4 Effects of ESR and DCL on Cstorage Sizing ...............
36
2.5.6.5 Double Layer Capacitor (DLC) ..........................
38
3 PMPG Design
41
3 .1 Introduction .........................................................................
41
3.2 Piezoelectric Effect ...............................................................
43
3.3 PMPG Structure ..................................................................
45
3.4 Electro-Mechanical Model of PMPG ..........................................
46
4
3.4.1 Introduction .............................................................
46
3.4.2 Parameter Considerations for Maximizing Power Output .........
49
3.4.2.1 Effect of Mass ...............................................
50
3.4.2.2 Effect of Damping ..........................................
51
3.4.3 PMPG Design .........................................................
54
3.5 Previous Piezoelectric Devices .................................................
57
3.6 Type 1 PMPG Electrical and Mechanical Characteristics ...................
58
3.7 Type 2 PMPG Design ...........................................................
61
4 Self-Powered Sensor Node Tests and Results
63
4.1 Telos Characterization ...........................................................
63
4.2 Telos Current Consumption Measurement ...................................
65
4.3 Power Management Module Testing ..........................................
68
4.4 Self-Powered Sensor Natural Gas Pipeline Test Bed .......................
76
5 PMPG Fabrication
78
5.1 Type 1 D evice ....................................................................
5.2 Recip e 1 ........................................................................
78
..
5.3 Type 2 D evice ....................................................................
5.4 Recip e 2 ..........................................................................
80
84
.. 87
5.5 Fabrication Result: PZT on ZrO2 /SiO 2 (PECVD)/SiNx/Si ..................
90
5.6 Fabrication Result: PZT on ZrO2/SiO 2 (thermal)/Si .........................
93
6 Conclusions
96
6.1 C ontribution .......................................................................
96
6.2 Recommendations for Future Works ..........................................
97
Bibliography
99
Appendix A nesC Codes
103
Appendix B Matlab Code for PMPG Sizing
108
5
Chapter 1
Introduction
There has recently been a surge of interest in the study of wireless sensor network
to monitor social infrastructure at remote locations. Current technological advances allow
a larger wireless network at a cheaper cost. Many research groups, such as UC Berkeley's
Smart Dust and MIT's p-AMPS are researching on low power sensor modules that would
make a large scale wireless sensor network feasible. However, we have now encountered
the bottle neck of the problem: limits of power supplies to sensors and transmitters.
Recently, embedded and remote Micro-Electro-Mechanical Systems (MEMS)
devices have begun to play active roles in medical implants and micro-scale wireless
sensors. At the same time, supplying power to these devices has remained a serious
problem. Traditionally, remote devices have used batteries as energy supply. However,
batteries only have a limited life span and finite amount of energy. The need to replace
them could contribute to considerable problems and costs, especially in medical implants
and applications in hazardous environment. As the size of the network grows, it is almost
impossible to replace them manually.
An alternative to conventional battery as power supply is to make use of the
parasitic energy available from the environment. Specifically, one could use piezoelectric
energy harvesting to generate electric power from the environmental vibration. The topic
of this research, a piezoelectric micro power generator (PMPG), converts mechanical
vibrations into electrical energy.
By using energy harvesting, one could create autonomous wireless sensors that
are posed to have very important roles in the future. The self-powering mechanism allows
6
the sensors to be more cost effective and easier to manage. A wireless sensor network
will link millions of sensor nodes to one control base station, allowing a large monitoring
coverage with limited numbers of operators. The cost of replacement batteries will be
eliminated as a result of the self-powered characteristics. Power lines and human
resources that are needed for power replacement will also be greatly reduced.
There are many immediate applications for autonomous sensors. Engineers can
embed these sensors to buildings to monitor structural health under vibration caused by
strong wind or earthquake. They will also be effective in monitoring rotary mechanical
systems such as helicopter blades. Surgical cost will be greatly minimized if such device
is used in artificial heart implants. Surgeries related to battery replacement can be
eliminated, which will also reduce tissue damages. This monitoring system can report
real-time information, even in dangerous or inaccessible environment. It will become an
invaluable asset in homeland security applications, in which countless numbers of low
cost sensors could be used for border control. Autonomous sensors will also cut the cost
of airport inspections, therefore allowing more inspection points and reducing airport
traffics.
The goal of this project is to integrate a sensor circuit and PMPG to create a selfpowered wireless monitoring system. The first step of the research is to fabricate a PMPG
device that satisfies the sensor circuit's power consumption requirement. In recent years,
advances in low power DSP and VLSI system-design have reduced electronics power
requirements to hundreds of pW. This lowered power requirement has made selfpowered sensor possible. Specifically, the PMPG only requires to generate 100 iW to
sustain our sensor system.
A RF temperature monitoring system will be developed as a part of this study. It
will be used for temperature monitoring for natural gas pipelines. Different electronics
components of the system will be studied and optimized in order to achieve minimum
power consumption. Lastly, other sensing applications will also be explored and
incorporated into this self-powered monitoring system.
7
Chapter 2
Self-Powered Sensor Node Architecture
2.1 Introduction
The goal of this project is to create a self-powered wireless sensor node, the
standard unit of a wireless sensor system. Such sensor node collects data, and transmits
them wirelessly to base station (Figure 2-1). The basic components of a sensor node that
can fulfill the requirements of general sensor monitoring applications are illustrated in
Figure 2-2. The sensor node requires a source of energy such as battery or power supply.
In this design, the sensor node is self-powered, by extracting energy from ambient
vibrational source using piezoelectric micro power generator (PMPG). The power
management module serves three functions:
.
Rectification of PMPG AC output to DC voltage.
.
Energy storage.
.
Active switching between the application circuit and power source.
The application circuit consists of three main power-consuming components:
microcontroller, sensor, and data transceiver. This design focuses on temperature sensing
application and uses digital temperature sensor. However, it can be expanded to
incorporate other sensing applications. The microcontroller manages system's peripherals
and communication between them. It instructs the temperature sensor to convert the
8
ambient temperature to digital format and reads the converted temperature. The data is
then sent to base station or other sensor nodes through an output channel. For basic
sensor application, one-way communication between the sensor nodes and base station
using RF transmitter is sufficient. In this design, the temperature data is outputted
through either a transmitter or the transmit mode of a RF transceiver, which modulates
the digital temperature data into FM signal. Two-way communication using infrared
transceiver is also possible for complex wireless system, but will demand more power
from PMPG.
Self-powered
A
Sensor Node
Base Station
Figure 2-1: Wireless sensor system schematics. The self-powered sensor node, focus of
this project, transmits data to a receiver at the base station.
PMPG
-
Power
Mamgerniert
*1
Power Supply
Module
TMP
Seriior
4-0 Microcordroller
RF Tx
Figure 2-2: Schematics of self-powered sensor node.
2.2 Ultra-Low Power Components
Current researches in the field of ultra-low power electronics have produced
devices that only require milliwatts of power (Table 2-1). Atmel and Texas Instruments
are two of the dominant producers of ultra-low power microcontrollers (Figure 2-3).
9
Devices such as ATmega128L [1], ATmega163 and MSP430 [2] require active current of
less than 10 mA and sleep current in the order of pAs. In particular, TI MSP430 has five
power modes and interval timers. The interval timer sourced by a 32 kHz crystal can
wake up the system from sleep mode. In power mode 3, the device draws as low as 2 pA
with 3 V power supply. Its sleeping current less than 1 uA has made it particularly
attractive to low power circuit design engineers.
Table 2-1: Self-powered monitoring system components.
ISleep (pA)
Twmp. Sensor
V
I (mA)
Power
RF Transmttr
Frequency (MHz)
V
I Active (mA)
DS1620
2.7-5.5
1
2.7 mW
Tx3
902-928
2.2-12
7.5
-
DS1626
2.7-55
1-1.25
2.7 mW
CC1000
315/433/868/915
2.1-3.6
7.4
2
TMP100
2.7-5.5
0.07
0.21 mW
CC2420
2400
2.1-3.6
17
20
AD7814
2.7-5.5
0.25
<80 L/
TR1000
916.5
2.7-3.5
12
0.75
AD7301
2.7-5.B
1.6
0.631 mW
Tx6000
916.5
2.7-3.5
12
0.75
LM75
3-5.5
1
3mW
XE1201A
300-500
2.4
5.0-6.0
0.2
MAX6625
3-5
1
3 mW
Microcontroller
V
I Active (mA)
SX28
2.7-5.5
17
Panasonic ERT-
Sleep (pA)
8
J1VR103J
-
-
100 mW
ATmegal28L
2.7-5.5
6
SHT11
2.4-5.5
0.028
30 pW
ATmegal63L
2.7-5.5
5
1.9
MSP430
1.6-3.6
0.46
0.7
LL 11111, I'l.
4F fI I
f7J7J'
b)
(a)
Figure 2-3: (a) TI MSP430. (b) Atmel ATMEGA128L.
Low power temperature sensors are also available for ultra-low power
applications. There are many ways to measure temperature. The simplest way is to
measure the voltage across a thermistor. However, this may require an ADC to translate
analog voltage level into a digital form for the microcontroller. Furthermore, a thermistor
draws considerable amount of power, thus is not suitable for low power applications.
Digital sensors such as TI TMP100 [3], which draws 70 pA at 3 V, are viable choices as
temperature sensors. Sensirion SHT 11 Humidity and temperature sensor [4] includes a
10
capacitive polymer sensing element for relative humidity and a bandgap temperature
sensor (Figure 2-4). This 12/14 bit sensor only consumes 28 pA at 3 V, and thus is the
best candidate for our application.
Figure 2-4: Sensirion SHT 11 Humidity and Temperature Sensor.
RF Monolithics [5] and Chipcon [6] produce some of the best ultra-low power RF
transceivers (Figure 2-5). To minimize power consumption, the sensor circuit will only
transmit and not receive data. RFM TR1000 has an active current of 12 mA, and Chipcon
CCl1000, CC2420 have active currents of 7 mA and 17 mA. However, output power and
transmission rate should also be considered in deciding a suitable RF transmitter.
(b)
(a)
Figure 2-5: (a) RFM TR1000. (b) Chipcon CC2420.
2.3 Wireless Sensor Nodes
Advances
in networking
and ultra-low power electronics have allowed
researchers to develop generations of systems that integrate low power CPU, radio
communication and MEMS-based sensors. These wireless sensor nodes are colloquially
11
referred in sensor research community as "motes" [7]. Each mote consists of RF
transceiver, CPU, and sensors. The task of each mote is to collect sensor data and
transmit them to outside world via radio link. This task, as well as other applications, is
programmed in nesC language (see later section for a detail discussion). Target costs are
less than 10 cents per unit, which enable networks with potentially tens of thousands of
motes. Motes are typically powered by batteries, which can have significant replacement
costs, as well as the limitation of their size due to batteries.
Motes designs are based on the principle of minimizing power consumption.
Since continuous data is not required for most applications, sensor data are taken and
processed at discontinuous time intervals. To reduce power, motes are powered down, or
put to sleep during the majority of the duty cycle. Some components are turned off to
reduce sleeping current.
As shown before, the major power consumption blocks are the computing unit
(microcontroller), a sensing unit, and a communication unit. The computing unit, or the
microcontroller, generally has 3 modes (active, sleep, and idle), and power consumptions
differ in each mode. Microcontroller is put into sleep mode during most of the cycle,
hence minimizing its sleeping current, which is crucial in reducing power consumption.
Microcontroller that has a fast wakeup time from sleep to active mode is also required to
reduce energy loss. Lastly, the microcontroller should have low active power and fast
processing rate.
The communication unit, in this case the RF transceiver, often consumes the most
power among the three blocks. Its power consumption is determined by the modulation
type, data transmission rate and transmission power. If a RF transceiver is used instead of
RF transmitter, power is also consumed when switching between transmit and receive
mode. In order to minimize power, The RF transceiver is designed only for data
transmission. It is also put to sleep by microcontroller during majority of operating cycle.
On the other hand, the power consumption of sensing unit is difficult to assess, as
it varies among different application. Most motes provide multiple output ports or sensor
board that allows users to attach the sensor of their choices. Temperature sensing is
chosen in this project, but it can be expanded to other applications as well.
12
Many generations of motes have been developed over the years, and have been
summarized in hardware survey [8].
Some of the most notable research groups are
SmartDust at UC Berkeley [9] and R-AMPS at MIT [10]. In addition, these motes are
commercialized by companies such as Ember Corp. [11], Dust. Inc. [12], Crossbow
Technology [13], and Millennial Net [14]. Sensor nets developed using these motes, are
intended to serve as easy and inexpensive methods of controlling building environments,
safeguarding sites and optimizing all kinds of industrial processes and business
operations. Recently, members of Intel Berkeley Research Lab founded Moteiv [15],
which provides latest generation motes to sensor community for research purposes.
Table 2-2 illustrates an enormous improvement in mote performance over the last
decade. Microcontroller's active power, sleep power and wakeup time are drastically
reduced as new chips become available to the wireless sensor community. Data
transmission rate also increases multifold while transmit power remains approximately
constant.
Table 2-2: Generations of motes [16].
Microcontroller
Nonvolatile storage
Size ((B)
Communication
MVd*aton teni
Concation p
Power Consumption
I iiiimi
Operation(V
lTte Apti Power (W)
Programming and Sensor
Interface
For proof of concept, commercial available motes are used to develop the
autonomous wireless system. However, future generation autonomous sensor node can be
13
made by integrating ultra-low power components together with a portable power
generator, to create a self-powered sensor node.
In this project, two motes, Moteiv's Telos and Crossbow Technology's Mica2Dot,
are considered (Figure 2-6, 2-7). At a first glance, the latter may be more favorable
because of its physical size. Mica2Dot, with diameter of only 0.25 cm, is much smaller
than Telos (6.5 cm x cm xl.5 cm). However, an external sensor board is required for
Mica2Dot, which increases the overall size of the sensor node. On the other hand, sensors
are integrated on Telos. Furthermore, Telos' physical dimension will be greatly reduced
once its two AA batteries cartilage is replaces by a MEMS piezoelectric micro power
generator (PMPG).
Telos also consumes less power than Mica2Dot. Telos is integrated with SHT 11,
a digital temperature sensor which consumes significantly less power than the thermistor
used by Mica2Dot. Telos also uses TI MSP430, a microcontroller with an average 2.4 RA
sleeping current and a maximum wakeup time of 6 ps. Mica2Dot, on the other hand, uses
ATmegal28 with an average 30 pA sleeping current and a maximum wakeup time of 60
ps. Lastly, Telos CC2420 RF transceiver consumes active current twice that of Mica2Dot
TR1000. Nevertheless, CC2420 has a data rate of 240 kbps, while TR1000 has data rate
of only 40 kbps. Ultimately, Telos can transmit data faster with less power per
transmission.
Telos is chosen for this project because it consumes less power. As stated
previously, a board level design will be made in the future, that integrates ultra-lower
power components with PMPG to create a self-powered sensor node. Since Telos is
already using some of the most energy efficient components (e.g. MSP430), using it to
implement the prototype will make the development of future autonomous sensor's
architecture easier. A detail description of Telos operation will be discussed in a later
section.
Figure 2-6: Left: Mica2Dot. Right: Telos.
14
Tunerst
Sensor
Phobpeinthelicaly
Active Raiation
User
Reset
g
roW Sollr
Rain
/
6-pin expansion
conneeI)/
1-i
xaso
conneco
UPs
Connector
irdernat
USS
Receive LED
LEDs
CC2420
M11crowolbor
Radio
Switch
i
JTA
bowing USO iun
conneco
rnicroconlrller
TOxM Instruments
USP430 F1611
SMA
Antenn
Connecior
(pf)f
sei Kn
/8btN~
U
Flash (2kB)
32L
osWator
ST
Flash (1MB)
Figure 2-7: Top: Telos Front View. Bottom: Telos Back View.
2.4 nesC
2.4.1 nesC Language
Sensor nodes have vastly different system requirements from those of traditional
computer. nesC programming language is specifically designed for networked embedded
systems [17, 18]. An example of such a system is a sensor network, which consists of
thousands of tiny, low power nodes, each of which execute concurrent, reactive programs
that must operate with severe memory and power constraints.
15
There are a number of sensor network programming challenges that nesC must
addresses. First, sensors are driven by interaction with environment. They are used for
data collection and control, rather than traditional computer that are used for general
purpose computation. Concurrency issues must be addressed, since event arrivals and
data processing occur simultaneously.
Second, sensor node has extremely limited resources. It must be produced in low
cost, have minimum power consumption with a small package size. These parameters are
mostly set by the availabilities of low power components and new packaging
technologies. Nevertheless, an operating system with small footprint and memory is
desired.
Reliability for long-lived applications is another concern for environmental
sensing applications. Since autonomous sensor can have unlimited energy supply, its life
time is limited by the reliability of applications. An important goal is to reduce run-time
errors, since there is no real recovery mechanism in the field except for automatic reboot.
Lastly, sensor network has soft real-time requirements, since most tasks are not
time-critical. A few time-critical tasks are sensor acquisition and radio timing.
nesC supports a programming model that integrates reactivity to the environment,
concurrency, and communication. A nesC application is built out of components which
provides and requires interfaces. Application is wired together using components and
configurations.
nesC has many advantages that support sensor network requirements. It is an
extension of C, therefore provides all the low-level features necessary to produce
efficient codes. nesC programs are subjected to whole program analysis, which allows
aggressive cross-component inlining, and static race condition detection. There is no
separate compilation, further reducing the limited program size. nesC is a static language,
which makes program analysis and optimization simpler and more accurate. It has no
dynamic memory allocation, or dynamic component instantiation/destruction. However,
its lack of functional pointer can make whole-program analysis difficult.
TinyOS, written in nesC, is an operating system that is designed for sensor
embedded system. This OS has been used in most of existing sensor nodes. It has a
number of important features that are influenced by nesC design. Its component-based
16
architecture allows different OS services to be decomposed into separate components. As
a result, unused services can be excluded from applications, greatly reducing the
application size. It solves concurrency issues by separating task, a deferred computation
mechanism, from event that runs in completion.
2.4.2 nesC Application
A nesC application, Tpmpg, has been written to support Telos temperature
measurement using SHT 11 sensor. The application is written such that the sensor will
take and transmit 14 bit temperature readings at 1 second interval (see Appendix for
scripts). This application is composed of two components: a module TpmpgM.nc, and a
configuration Tpmpg.nc. TpmpgM.nc provides the implementation of the application.
Tpmpg.nc is the source file that the nesC compiler uses to generate an executable file. It
wires TpmpgM.nc to other TinyOS components that the application requires.
Tpmpg.nc links Tpmpg application to existing TinyOS components TimerC,
GenericComm and HumidityC. As a result, Tpmpg application can use commands
provided by other components. For example, by creating a wire
TpmpgM.Temperature -> HumidityC.Temperature
in Tpmpg.nc, calling Temperature.getData() is equivalent to calling the same function in
HumidityC.
HumidityC is the SHT1 1 sensor driver that controls humidity and temperature
measurements.
The two measurements
can be obtained separately by calling
HumidityM.Temperature and HumidityM.Humidity. To reduce power consumption, only
temperature
data
is
retained
and
later
transmitted
by
RF
transceiver.
HumidityC.TemperatureError is used for error detection, such that it would return TRUE
when errors occur during measurement. The other component GenericComm, is the RF
transceiver driver that controls data transmission. TinyOS package provides a high level
application Oscope that uses GenericComm, and is used in many communication related
applications. However, calling GenericComm directly allows more flexibility in
controlling transmission time and bit rate, thus reduces power consumption. TimerC
provides clock for task scheduling.
17
Furthermore, TinyOS's power management support is used by invoking
HPLPowerManagement.
Such function is activated and deactivated by calling
GenericComm.start( ) and GenericComm.stop( ). By doing so, the RF transceiver is
turned off and microcontroller returns to sleep mode at the end of data transmission.
A set of commands and events are defined in TpmpgM.nc. Commands are
functions that can be called by other functions within the applications. It can invoke other
commands and events. Events define functions that are called when commands end in
completions. For example, the application calls HumidityControl.start( ). When it runs to
completion, it returns TRUE and invokes event HumidityControl.startDone( ). This event
will then call commands such as Timer.start and CommControl.stop().
Components such as StdControl and HumidityControl require initiation, start and
stop. These functions are defined by init ( ), start () and stop ( ). As shown by previous
example, when the function runs to completion, it will invoke an event that triggers
another command.
The commands timing is controlled by the timer defined in the application. If
timer is fired while Telos is active, it will take temperature measurement by calling
Temperature.getData( ). When data is ready and the timer fires again, it calls
SendMsg.send to transmit message. When message is sent, SendMsg.sendDone is
triggered, and calls CommControl.stop( ) to put Telos to sleep. The timer will fire 1000
ms later to reactivate Telos and runs the application for another cycle.
An external TpmpgMsg.h is implemented. It defines the length of data transmitted
by CommControl.
The Telos functions as a sensor node is installed with Tpmpg application. As
shown in Figure 2-1, a receiver at the base station is required to collect the transmitted
data. Another Telos is installed with TOSBase application provided by TinyOS. This
mote is connected to a PC and functions as a receiver for the base station. The TinyOS
application, Oscope is modified and used to display data using Java applet. The script has
been modified to include data length change and temperature conversion. The scripts are
recompiled, and two jar files are created. To run SerialForwarder, the Java application
which allows user to manage data pockets and displays many useful statistics, use the
command,
18
java -jar sf.jar
To run a java applet that shows a graphical display of temperature measurements, run
java -jar oscope.jar
2.4.3 Telos Operation and Power Consumption
Telos' MSP430 is programmed in nesC with application described by previous
section. For this research, sensors other than SHT 1 are ignored. Figure 2-8, 2-9 and 2-10
illustrate the minimum current, energy and power consumptions, assuming that most
efficient code is used. They are constructed according to Telos [19], MSP430, SHT1 1,
and CC2420's data sheets (Figure 2-8, 2-9, 2-10). MSP430 wakes up at the beginning of
each duty cycle with wakeup time of -6 is. After wakeup, SHTl1 temperature sensor is
activated and takes 14 bit temperature sample for -210 ms. CC2420 is next activated by
MSP430. It then processes the data for duration of 500 jts, switches to transmission mode,
and transmits the 14 bit data for 10 ins. Duty cycle ends as CC2420 and MSP430 return
to sleep. Each duty cycle has duration of 221 ms and consumes 2270 [J energy.
Figures indicate that sensor operates for the longest duration and consumes the
most energy. SHT1 1 can also be programmed to read 12 bit data at a rate of 55 ms per
measurement. 14 bit mode is chosen instead, to insure data robustness. Such result also
demonstrates the importance of choosing a low power temperature sensor. On the other
hand, CC2420 consumes the most power. Its energy consumption is greatly reduced as
transmission time is shortened. Lastly, 15 pW sleeping power would be -15% of the
power supplied, if Telos were to be powered by PMPG (provides -100 pW). Therefore,
eliminating sleeping power is important in increasing power efficiency.
19
25
19.4
20
19.4
19.4
17
15
10
2
2.55
0.005
6 us
MSP430
Wakeup
210 ms
600 us
S HT1l1 Temp. CC2420
Measurement Startup
580 us
CC2420
Crystal
Startup
500 us
192 us
10 ms
CC2420
CC2420
Programming Switch
to Tx
CC2420
Tx
Is
Time
Standby
Figure 2-8: Current drawn by Telos at 3 V per data point (221 ms).
70
58.2
60 -
58.2
58.2
51
50-
40 -
30 -
20
-
12
10 -
6
0-
7.65
U
0.015
6 us
210 ms
600 us
580 us
MSP430
Wakeup
S HT1 1 Temp.
NMeasurement
CC2420
Startup
CC2420
Crystal
Startup
192 us
10 ms
Is
CC2420
CC2420
Programming Switch
to Tx
CC2420
Tx
Standby
500 us
Figure 2-9: Telos power consumption at 3 V per data point (221 ms).
20
Time
1800
Total Energy without Sleeping = 2270 uJ
1606.50
1600
1400
1200
.
1000
582.00
600
400
200
0.04
30.60
6.96
29.10
11.17
6 us
210 ms
600 us
580 us
500 us
192 us
10 Ms
1s
MSP430
Wakeup
SHT11 Temp.
Measurement
CC2420
Startup
CC2420
Crystal
Startup
CC2420
Programming
CC2420
Switch
to Tx
CC2420
Tx
Standby
Time
Figure 2-10: Telos energy consumption at 3 V per data point (221 ms).
2.5 Power Management Module
The power management module is used to integrate commercial wireless sensor
node Telos with piezoelectric micro power generator (PMPG), to create the self-powered
wireless sensor node. It defines the system architecture and sequence of procedures
performed by different components. A system level model that incorporates PMPG and
Telos is first developed. Such model provides a better understanding of energy flow in
between the devices. Power management module is then designed to satisfy the
architectural requirements indicated by the model.
21
2.5.1 PMPG Modeling
For simplicity, PMPG and Telos are modeled as simple circuit elements (Figure
2-11, 2-12). Previous researches has modeled piezoelectric element as an ac current
source i (t)
=
I, sin(wot) with an internal capacitance C, [20]. C, is determined by
intrinsic parameters of the piezoelectric element and co is the input vibration frequency.
The magnitude of the current I, varies with mechanical excitation level, and is also
dependent on external loading. However, the voltage across external load varies at a slow
rate. Using first order approximation, ip(t) can be modeled as an independent AC current
source. C, is assumed to be much smaller than the capacitors in rectification circuit and
energy storage. A more elaborate electro-mechanical model with be used in Chapter 3 for
PMPG design. PMPG will be designed to provide 100 pW of power at 3 V. The input
vibration frequency will be natural frequency of liquid gas pipeline, which is in the range
of 100-200 Hz.
Cp
ip =Ipsin(wt)
Figure 2-11: PMPG is modeled as an AC current source with internal capacitance Cp.
2.5.2 Wireless Sensor Node Modeling
The wireless sensor node, Telos, is modeled as a variable resistor that has
resistance of greater than 20 k9 during off state and 125 Q during operation. It has turnon voltage of 2.094 V and draws a maximum current of 20 mA. It has a maximum
voltage rating of 3.6 V. However, it will operate at a supply voltage less than 3 V, in
order to avoid damages to the microcontroller, radio, and other components. It is
programmed to take 221 ms of data in I second interval.
22
RTelos
Figure 2-12: Telos is modeled as a variable resistor with R,=125 Q and Rof > 20 kn.
2.5.3 System Architecture Design
A system level model is shown in Figure 2-13a. Power management module must
be designed to satisfy the three major requirements of this system architecture:
.
Provides DC voltage to sensor node (Telos).
.
Manages power flow from source (PMPG) to application (Telos).
.
Energy storage.
AC-DC converter is used to rectify AC PMPG output to DC. A detail analysis of
rectifiers will be presented in a later section.
Power management module also addresses the issue of power flow from PMPG to
Telos. If PMPG were to power Telos continuously at 3 V, such that the sensor takes
temperature reading at 1 second interval, it will need to provide an average power of 10
mW. In order to support this power requirement, 100 0.1 mW PMPGs will be required.
On the other hand, most sensor applications do not require a continuous data stream. The
power management module
will manage
power such that Telos
is powered at
discontinuous time intervals. Energy is first transferred to an energy storage element
(Cstorage) during charging mode (Figure 2-13b). Once the capacitor has stored enough
energy to support greater than one data point measurement, the system will transfer
power from the capacitor to Telos during active mode (Figure 2-13c). Such process
repeats every cycle.
23
Power Management Module
AC-DC
Rectifier
PMPG
RTeos
Cstorage
---..............
(a)
AC-DC
Rectifier
PMPG %
Cstorage
(b)
Cstorage
RTeIos
(c)
Figure 2-13: (a) Self-powered sensor system level model. Power management module
consists of AC-DC rectifier that rectifies PMPG's AC output to DC, and a switching
mechanism that manages power flow from PMPG to Telos. (b) Charging mode: energy
transfer from PMPG to
Cstorage.
(c) Active mode: energy transfer from Cstorage to Telos.
A switching scheme has been implemented such that Telos is disconnected from
Cstorage
during charging mode, and PMPG is disconnected during active mode.
24
An in-depth analysis of this circuit will bring insights to such conclusion. If Telos
were connected during charging mode, PMPG would provide power to a parallel
RTeiosCstorage circuit (Figure 2-14). It is important to remember that, the goal is to
maximize power transfer to the capacitor during the transient response of the circuit, and
not power transfer to the load resistor during steady state. Most published papers in the
past have analyzed the steady state response [20, 22, 24]. A matching resistor is used to
maximize power transfer. Such approach is not suitable for this application.
PMPG AC current source is rectified to DC, Irect by AC-DC converter. The
voltage across Cstorage is:
-t
VCstorage
(2.1)
I rectR Telos (1 - e RrectCstorage
PSpice simulation (Figure 2-15) indicates that, as Vcstorage reaches steady state
voltage, more power is transferred to the load resistor and less to the energy storage
capacitor. When voltage across capacitor reaches steady state, all power is transfer to the
load resistor. Since the goal is to maximize power transfer to
Cstorage,
any additional
resistive loading that acts as energy sink should be eliminated. Therefore, Telos is
disconnected during charging cycle.
Irect
Cstorage
RTeIos
Figure 2-14: PMPG connected to Cstorage and RTelos.
25
Power Delivers to RC Circuit
osiUw-
- - - - - - - - - - --- - - - - - - - - - - -PRTelos+PCstorage
I
PRTelos
PCstorage
-------------r1
r-------- r--------- r-
es
U1(01)*I(01)
*
6Ks
KSs
2Ms
Is
V1(C1)*-CtR1) 71(1)*(I(C1)-I(R1))
IGKs
Tine
Figure 2-15: PSpice simulation of current source in parallel with RC circuit.
Furthermore, the sensor node Telos should be disconnected from the circuit
during charging process, in order to charge capacitor above the sensor node's minimum
operating voltage (Vmin). Since Telos shuts down when Vcstorage decays below the
minimum operating voltage, only the energy stored by the capacitor above Vmin can be
used. If Telos were connected to Cstorage during charging mode, it will prevent Vcstorage
from rising above Vmin. Therefore, Telos must be disconnected from Cstorage during
charging mode. A comparator will be implemented along with the switch to monitor the
voltage across the capacitor.
Lastly, PMPG should be disconnected from energy storage capacitor during Telos
operation (active mode), in order to prevent current from flowing back to the PMPG
device. This will minimize power loss due to leakage and maximize energy efficiency.
Figure 2-16 is an operation flow chart of the self-powered sensor node. Cstorage is
connected to PMPG and charged up to Vhigh during the charging mode. Once Vcstorage
reaches Vhigh, a single pole/double throw switch disconnects Cstorage from PMPG and
connects it to Telos. Telos operation starts, and begins to take temperature measurement
at 1 s time interval. As Telos consumes energy, Vcstorage drops to V10 , Vi., is set at a
threshold higher than the Telo's minimum operating voltage Vmin. When Vcstorage reaches
Vi 0w, active mode ends and charging mode begins for the next cycle.
26
Charging Mode
Vcstorage: Vi0o -
Vhigh
Active Devices: PMPG, Cstorage
Active Mode Starts
Vcsorage: Vhigh
Active Device: Cstorage, Telos
Telos Operation
VCstorage: Vhigh-Vow
Active Device: Cstorage, Telos
Sensor Reading: 221 ms/measurement, every 1 s
Active Mode Ends
Vcstorage: Vio,
Active Device: Cstorage
Figure 2-16: Operation flow chart.
2.5.4 Implementation: Voltage Monitoring Module
The switching scheme has been implemented using single pole/double throw
(SPDT) switch Intersil ISL84714 and Maxim MAX9118 comparator. ISL84714 is an
analog switch with an ultra-low on resistance of 0.38 2 at 3V supply voltage. A small Rn
can minimize conduction loss and increase power efficiency from PMPG to Cstorage.
MAX9118 comparator is an ultra low power comparator with an internal reference
voltage Vref=1.252 V. It also has a 4 mV internal hysteresis band, which allows two
different threshold voltages to be set. Specifically, the output generates low when Vin <
27
VI., (or VTHF in Figure 2-17), and high when Vi > Vhigh (or VTHR). A larger hysteresis
band can be achieved by using external biasing resistors.
THRESHOLDS
IN+
THR
IN-
Figure 2-17: MAX9118 internal hysteresis band [21].
Figure 2-18 is the schematics of voltage monitoring module. Hysteresis is
generated from the positive feedback constructed using R1, R2, R3 and R4 . These resistors
are sized according to equations listed on the datasheet. The resistors have been sized to
R1=1 M9, R2=1.1 Mn, R3=1.8 M9, and R4=1 M2. Vhigh is set to 3 V, a voltage that is
well below the Telos rated voltage. Vi., is set at 2.2 V (greater than Telos' Vmin), so that
Telos remains in operation during entire period of active mode. Both comparator and
switch are powered by Cstorage.
R3
R,
Cstorage
R4
+SL84714
S2MAX9118
no-
Telos
Figure 2-18: Voltage monitoring module schematics.
28
PMPG
+
AC-DC Rectifier
2.5.5 Implementation: AC-DC Rectifier
PMPG is excited using a vibrational mechanical input, and generates an AC
current source. The AC power must be converted to DC before it can be used by other
electrical components in the system. First generation PMPG was previously tested using
a full wave rectifier [22]. However, four diode drops will be lost as a result of the circuit
topology, resulting in a lower voltage level at the output. In this design, a voltage doubler
rectifier will be used instead. Since only two diodes are used in this topology, the voltage
loss will be half of that of full-wave rectifier.
STMicroelectonics 1N5711 diodes (Di and D2) are used to construct the voltage
doubler rectifier (Figure 2-19). They are Schottky diodes with very low forward voltage
drop VF. Maximum forward voltage VF is rated at 0.41 V, and it has a maximum forward
current IF of 1 mA. It also has a very low reverse current of 200 nA at 50 V. Since the
system only operates up to 3 V, the reverse current will peak out at approximately 5 nA.
As a result, very little current will leak away during reverse biasing of the diodes.
Z\D1
PMPG
+
Cstorage
C1
ip=jpsin(wt)
CP
D2
Figure 2-19: Voltage doubler rectifier.
To determine power flow characteristic of the rectifier, PMPG is again assumed
to be a sinusoidal current source Ipsin(ot), in parallel with PMPG intrinsic capacitance Cp.
During each charging cycle, C1 serves as intermediary energy storage element between
29
PMPG and Cstorage. The polarities of the two capacitors C, and Cstorage are defined as
shown in Figure 2-19. In the following analysis, Cstorage is assumed to be relatively large
compare to C1 and Cp.
During the negative cycle of sinusoidal input, D2 is forward biased, connecting
positive terminal of C1 to ground. D, is reverse biased, and no current flows to Cstorage.
The parallel capacitors C, and C1 charges up to a maximum negative voltage, (or positive
according to polarities indicated in Figure 2-20a). This voltage is dependent on frequency
o and current amplitude Ip.
During the positive cycle of sinusoidal input, D2 is reversed biased. The positive
input charges C,, and reverses the polarity of the capacitor. When Vc, + Vc > Vcs,,, D1
becomes forward biased, connecting C, to output capacitor Cstorage. The sum of the
voltages across C1 and C, are used to charge up Cstorage (Figure 2-20b). This power flow
scheme repeats every cycle. However, it is important to remember that the topology
operates in transient regime instead of steady state. Specifically, Vcstorage can increase
indefinitely until Cstorage is cut off from PMPG by the comparator. Therefore, steady state
analysis that is used in references [20] and [22] is inappropriate for this application.
PMPG
i=sin(wt) %
C1
C
(a)
C1-
Cstorage
+C
PMGip=Ipsin(wt)
,
CP
(b)
Figure 2-20: Circuit topology when (a) i, <0, (b) i, >0.
30
The voltage doubler rectifier is simulated using PSpice (Figure 2-21 a). Current
source is driven at amplitude of 1 mA at 150 Hz. Cp and C1 are sized as 0.1 pF, and
Cstorage 0.1 mF. Vci is defined as the node voltage between D, and D2 . Vcp exhibits a
sinusoidal behavior as predicted by the model. However, deviation from pure sinusoidal
response is shown in Figure 2-21b, due to the non-idealistic behavior of diodes, switch,
and capacitors. When input is negative, both Vci and Vcp reach the same minimum
voltage. When input is positive, Vci reaches
Vcstorage
of the previous cycle.
Vcstorage
increases indefinitely, and it follows the transient response profile of Vci.
.Cstorage
VC1I
Vcp.
1
13
C pCpI\
D 1s
-D2
Open=.
FeUw.....
(a)
31
torage
.....---rwnfl.t Aes**m,.
Cp
H
(0
1*
15
15r
15
H
asa
Vs
H
H
IH
(b)
1w1
~~~~Ti~~m
$
I~
VCP
...
{ Sf1iz)
mS8tQ~apa1)
am ...
...
..........as .. .
......
iia
(c)
Figure 2-21: (a) PSpice simulation of voltage doubler rectifier. (b) 10 s simulation result.
(c) Simulation result from 8.1 s to 8.2 s.
32
2.5.6 Energy Storage Capacitor Sizing
Energy storage capacitor must be sized such that it can store enough energy to
provide voltage between 2.2 V to 3 V to allow Telos operation. On the other hand, it must
be small, so the charging time constant remains acceptable. Capacitor characteristics
Equivalent Series Resistance (ESR) and DC Leakage Current (DCL) will also affect the
capacitor choice.
Steps in choosing capacitor:
1.
Set largest acceptable time constant.
2. Choose the capacitance that corresponds to this time constant.
3. Consider capacitances with the smallest DCL and ESR.
4. If ESR and DCL does not satisfy power requirement, repeat step 1 with a larger
time constant.
The following section explores these parameters, as well as energy storage
elements other than conventional capacitors.
2.5.6.1 Calculating Lower Bound
One can calculate the minimum capacitance by considering Telos' energy
consumption. According the energy distribution taken from pervious section, 2270 pJ is
consumed in each cycle. Capacitor need to be sized such that 2270 [1J is stored in the
capacitor between Vi 0,=2.2 V and Vhigh= 3 V.
E=Cog
1
2270 x 10- 6 =-C
2
Cstorage
(Vhigh
(32
2
_
V
2
)
(2.2)
-2.22)
1 mF.
Therefore, a minimum size of 1 mF is required. A larger size might be required if
loss from leakage current is included in the modeling.
33
2.5.6.2 Capacitor Charging Time Constant
We have assumed that the PMPG could provide 0.1 mW of power. If we were to
further assume that the power remains constant at the voltage levels of our interest, then
the charging time r from 2.2 V to 3 V will be,
C,
,(V
,storage(
2
,,2
_V ,2
high
-
V
2
PPMPG
X10 4 Cstorage
-2.08
(2.3)
The first cycle will require the capacitor to charge from 0 to 3 V. In such case, the
charging time r1 will be,
1
-Ctorage(3
2
-02)
PPMPG
=4.x104Cstorage
4.
If Cstorage= mF, r=20.8 s, and
Ti=
(2.4)
45 s. Charging time constants increase linearly
with capacitance. For example, 100 mF will require 75 minutes charging time in the first
cycle, and 30 minutes in between successive cycles. Nevertheless, such time constants
remain acceptable in most applications. The advantage of using a larger capacitor is that
it allows Telos to operate for a longer time interval, therefore taking more data
continuously. On the other hand, it would require a longer charging time.
34
2.5.6.3 ESR and DCL
ESR
C
Rleak
Figure 2-22: Capacitor model.
A more realistic model of capacitor includes two loss elements ESR and DCL.
Leakage current DCL is represented in the model by a resistive element R1eak (Figure 222). Capacitor size is limited by ESR and leakage current. This section provides a brief
overview of these two concepts.
ESR is the internal resistance that leads to heat generation in the capacitor. In
order to have a low ESR, it is necessary to control the characteristics of the electrolyte,
the ohmic contact resistance, and other intrinsic capacitor parameters. ESR is inversely
proportional to frequency. For our application, Cstorage operates at DC. Therefore, it will
be associated with the maximum ESR rated.
Another source of power loss is capacitor's leakage current DCL. The dielectric
of a capacitor has a very high resistance which prevents the flow of DC current. However,
there are some areas in the dielectric that allow a small amount of leakage current to pass.
The areas allowing current flow are due to very small impurity sites which are not
homogeneous, and the dielectric formed over these impurities does not create a strong
bond. When high voltage or temperature is applied to the capacitor, these bonds break
down and the leakage current increases.
Capacitor also exhibits "dielectric absorption" charge storage. When the capacitor
is initially charged to full voltage, a considerable amount of inflow current occurs as a
result of ions getting soaked up by the dielectric material. It is not a true leakage because
it can be recovered when the capacitor is discharged. Such current decays with a long
time constant, and makes actual leakage current measurement extremely difficult.
However, dielectric absorption could dramatically increase capacitor charging time. The
effect is more pronounced in double layer capacitors (DLC). For example, PowerStor
35
Aerogel capacitor's inflow current (sum of dielectric absorption and leakage currents),
changes from 10 mA to leakage current of 5 ptA over the course of 1000 hours. CAP-XX
DLC requires three to five days in order to reach true leakage current. Therefore,
dielectric absorption must be considered in the choice of capacitor.
2.5.6.4 Effects of ESR and DCL on Csto-age Sizing
Capacitor size has complex relationships with ESR and leakage current. Within
each type of capacitor (tantalum, electrolytic, DLC), capacitance is inversely proportional
to ESR and proportional to leakage current. Such relationship is harder to qualify across
different types of capacitors. Double layer capacitor is generally designed for ultra-low
leakage applications. It also has higher energy density than conventional capacitors.
Therefore, some DLCs have a leakage rate much lower than other type of capacitors of
smaller size. For example, a CAP-XX GW2 90 mF capacitor has 2 pA leakage current,
while an AVX tantalum capacitor has 40 pA of leakage current. Similarly, some
capacitor type is designed for ultra-low ESR application, and has a lower ESR than DLC.
Furthermore, dielectric absorption may present additional current loss during early cycles
of operations, and it varies across capacitor types.
The maximum power loss through ESR and DCL were arbitrarily set as 50% of
0.1 mW. Current of 20 mA flows across ESR during sensor node's active mode. The
power loss is described by:
pESR
=
I 2ESR
= 4 x 10-4ESR
(2.5)
Furthermore, maximum power loss due to leakage occurs when capacitor is fully charged
to 3 V. Therefore, the power loss due to DCL is:
PDCL =DCL - V= 3 -DCL
PESR
DCL
50x 10-6 W
36
(2.6)
Equations (2.5) and (2.6) indicate a maximum ESR of 125 mK2 and DCL of 16 pA.
Capacitor generally has ESR on the hundreds of mQ range, while DCL on mA range,
which causes
PESR
to be generally smaller than
PDCL.
DCL should be minimized prior to
ESR.
The amount of commercially available capacitors for commercial use also limits
the choices of sizes. Tantalum and electrolytic capacitor generally ranges from jFs to 1.5
mF. On the other side of the spectrum, double layer capacitor ranges from tens of mF to
hundreds of F. In between the two ranges are electrolytic capacitors that have large ESR
and DCL, and are not suitable for our application. With a lower bound of 1 mF, the
system would require either multiple tantalum capacitors or a relatively small size DLC.
Despite the fact that each tantalum capacitor could have a smaller ESR and DCL than
DLC, the sum of tens of them could have considerable power loss. In addition, DLC is
designed to have very low leakage current. Therefore, DLC will be used in this project.
Nevertheless, there are disadvantages such as a longer charging time constant and
problems associated with dielectric absorption.
The process of choosing a correct capacitor is a tradeoff between charging time
constant and power loss. One should first determine the charging time constant that can
be tolerated, and set the maximum allowed capacitance. A larger capacitance implies a
longer charging time but less ESR power loss. For capacitors with relatively same
leakage current, the one with the smaller ESR should be chosen.
Figure 2-23 compares the power loss due to ESR and DCL from different types
of capacitors. Result indicates that CAP-XX 250 mF, CAP-XX 90 mF, and AVX
BestCap 60 mF satisfy the power limitation. Depends on the charging time that is
allowed, the largest among the three should be chosen to minimize power loss.
37
1400
1200-
1000-
800 -
E ESR
Power Loss(^W
0DCL Power Loss(pW)
o 600-
400-
200-
0
AVX TPS
Series II
Tantalum
(lmF)
Kemet T530
Tantalum
(ImF)
AVX
BestCap
(33mF)
AVX
BestCap
(5OmF)
AVX
BestCap
(6OmF)
CAP-XX
GW2 DLC
(9OmF)
CAP-XX
GS2 DIC
(250 mF)
PowerStor Maxwell PC5 NESSCAP
DLC (4F) PSHLR DLC
Aerogel A
(20F)
Series DLC
(47OmF)
Capacitors
Figure 2-23: Power Loss Comparison of various capacitors. AVX BestCap (60 mF) has
T=20 min, CAP-XX GW2 DLC (90 mF) has T=31.2 min, and CAP-XX GS2 DLC (250
mF) has T=87 min.
2.5.6.5 Double Layer Capacitor (DLC)
Double layer capacitor technology can be viewed as a hybrid technology that lies
between lead-acid battery technology and conventional electrolytic capacitor technology.
It posses some of the better qualities of battery device and combines those qualities with
some of the desirable attributes of electrolytic capacitors.
Energy storage devices are characterized by their energy density (energy stored
per unit volume) and by their power (how fast the energy can be delivered from the
device). Conventional capacitors have enormous power but only tiny amounts of energy.
They are useful in supporting high current peaks, but their low energy density requires
additional energy storage element in actual applications. Batteries can store lots of energy,
but have low power because of their long charge and discharge time. Their lifetimes are
greatly reduced when they are fully discharged. DLC bridges the gap between batteries
38
and capacitors by having both high power and high energy properties. It has high specific
power densities, between 10-20 times greater than leading battery technology. Its energy
densities can exceed those of conventional capacitors by approximately 10-100 times.
Other advantages include charge/discharge efficiencies superior to that of batteries, near
infinite shelf life and rechargeability, minimal maintenance, and can be fully discharged
without reducing the lifetime of the devices.
Different types of DLC have been designed to support various applications. AVX
BestCap (ranging from 10 mF to 560 mF) is designed to support pulse current that
reduces the lifespan of conventional batteries. It uses an electrolyte that yields positive
hydrogen ions, one of the most mobile species, which leads to a very low ESR of tens to
hundreds of m~s.
Cooper Bussmann's PowerStor capacitors (ranging from 220 mF to 50 F) use
carbon electrodes but in a foamed aerogel form. Their electrodes property results in very
low ESR capacitors. They are best known for a leakage current of only few gAs, which
allows capacitors to hold charges for few weeks. On the other hand, 1000 hours are
required to diminish dielectric absorption current. Its low voltage rating of 2.5 V means
multiple capacitors are required in high voltage applications.
NEC-Tokin DLC (ranging from 10 mF to 100 F) concentrates on backup
applications for SRAM, microsystem support, and low-impedance applications such as
actuators and valves. Cap-XX capacitors (ranging from 90 mF to 2.3 F) offer supports to
compact flash and PC card and other portable devices.
Other double layer capacitors are designed based on the high-power end of the
DLC spectrum. Epcos's UltraCaps (ranging from 3.3 to 5000 F) are generally used in
automotives and other high voltage, high power applications. Maxwell Technologies'
Boostcaps (ranging from 2 to 2600 F) are also designed for automotive uses in cars,
railway, tramway, telecommunications-backup,
and energy-generation applications.
NessCaps (ranging from 1.5 to 5000 F) have been evaluated in electric car applications to
improve the performance of conventional batteries.
Considering the long charging time constant associated with large DLCs, only mF
capacitors are considered. NEC-Tokin has capacitor size closest to the minimum size
39
requirement, but has >300 mQ ESR. PowerStor Aerogel capacitors, Cap-XX capacitors
and AVX BestCap capacitors are the prime candidates for this project.
40
Chapter 3
PMPG Design
3.1 Introduction
The major component of any large scale wireless network is its power supply.
Previous discussion indicates enormous technological advances in network theory, as
well as creating very small scale energy efficient wireless sensors. Comparatively
speaking, very little progress has been made on the energy supply side of the system. In
many systems, power supplies remain the bulkiest of the entire architecture (e.g. AA
batteries slot on Telos, MICA mode).
The issue of replenishing energy is especially important in wireless applications.
If conventional batteries were to be used, one would also need to include the cost of
battery replacement. Replacing batteries at hostile or inaccessible environment can be
extremely expensive. Wireless sensors that are embedded in un-disassemblable structures
such as buildings, bridges or rotary systems will not allow foreign access to replenish
their energies. Replacing batteries for artificial heart can be expensive and harmful to the
patient.
An alternative to conventional battery is to make use of the parasitic energy
available from the environment. Energy harvester, a device that can perpetually provide
energy to the system, will eliminate the needs of battery replacement.
Energy harvesting from the environment has been actively explored using solar
power (most popular and powerful ambient energy source), thermal gradients and fluid
flow, among many [22, 23].
41
Table 3-1: Comparison of Energy Source [22, 24].
Power density (10 year lifetime)
Energy Source
(v/cnt)
Solar (outdoor)
15,000
Solar (indoor)
6
Vibration (piezoelectric)
Vibration (electrostatic)
Temperature gradient
Batteries (non-rechargeable)
Batteries (rechargeable)
Hydrocarbon fuel (micro heat engine)
Fuel cells (methanol)
250
50
15(at 10 0C gradient)
3.5
7*
33
28
Comparison of potential ambient sources for energy harvesting has been done
previously. Vibrational energy harvesting via piezoelectric conversion has a high power
density (250 W/cm 3), only second to solar power (15,000 W/cm3) [22, 24]. If one were to
take into account the cost of manufacturing and applications such as powering low cost
devices, then solar, hydrocarbon fuel and fuel cells are too extravagant and expensive.
Solar power is also impractical, especially in imbedded applications. As a result,
piezoelectric energy harvesting becomes a more attractive option.
Piezoelectric power generators have been used previously to power sensor in
shoes [25] as well as other RF applications [26, 27]. However, large scale piezoelectric
materials are used. These materials are not feasible in making small scale integrated
devices. In this research, MEMS scale piezoelectric power generator is used as energy
supply of the wireless sensors. By integrating the piezoelectric power generator, PMPG,
with the wireless sensor, one could create an autonomous wireless system with very small
sensor nodes. In this project, this system is used to monitor liquid gas pipeline that
provides energy through its fixed natural frequency.
42
3.2 Piezoelectric Effect
The piezoelectric effect was discovered by Jacques and Pierre Curie in 1880.
They found that if a certain crystals were subjected to mechanical strain, they became
electrically polarized and the degree of polarization was proportional to the applied strain
[28]. Conversely, the same material will be physically deformed when subjected to an
electric field. Such phenomenon is known as the piezoelectric effect.
The piezoelectric materials used in this research are piezoceramics known as PZT
(lead zirconium titanate, (Pb(Zr,Ti)0 3)). These are polycrystalline ferroelectric materials
with perovskite crystal structure, a tetragonal/rhombahedral structure very close to a
cubic. PZT consists of mass of crystallites. Above a critical temperature, or Curie point,
these crystallites align themselves to exhibit a simply cubic symmetry (Figure 3-1a).
Such structure has positive and negative charge sites coinciding, so there are no dipoles
present in the materials. Below the Curie point, however, the crystallites take on
tetragonal symmetry, in which the positive and negative charge sites no longer coincide,
resulting in dipole formations (Figure 3-1b). These built-in dipoles are crucial to the
piezoelectric effect.
(b)
(a)
Figure 3-1: (a) Cubic lattice (above Curie point). (b) Tetragonal lattice (below Curie point)
[28].
The dipoles are not randomly oriented throughout the tetragonal PZT materials.
Neighboring dipoles align with each other to form regions of local alignment known as
43
Weiss domains. Each Weiss domain has a net polarization. On the other hand, different
Weiss domains are aligned in random directions; hence no overall polarization or
piezoelectricity is exhibited by the material. The ceramic materials become piezoelectric
through a poling process at a temperature below Curie point. When the material is placed
in an electric field, domains that are aligned to the direction of the field will grow at the
expense of other domains. When the field is removed, the dipoles remain locked, giving
the ceramic material a permanent deformation (Figure 3-2).
+I
ILI
Figure 3-2: (a) Before poling. (b) During poling. (c)After poling [28].
The piezoelectricity of a material is described by its hysteretic behavior. When a
sample of PZT is subjected to an increasing electric field, the dipoles become aligned
with the field and polarization increases as shown by Figure 3-3. When the field has
increased to a certain value in which all the dipoles are aligned with the field, polarization
will cease to increase. The material then reaches the saturation point, with saturation
polarization P.
If the field is now reduced to zero, dipoles will become less aligned. However,
they do not return to their original alignment, since they are bounded to a preferred
direction defined by the applied field during the first part of the hysteresis. Since there is
still a high degree of alignment, the polarization does not fall back to zero but to a value
somewhat lower than the saturation polarization known as the remanent polarization Pr.
PZT layer are generally made using solgel spin coating method. Crystal structure,
or perovskite phase of PZT is formed after the final annealing step. However, depending
on processing parameter, some regions of PZT will remain in amorphous, or pyrochlore
44
phase. A large percentage of PZT in perovskite phase is required in order for it to be
piezoelectric. This material property is proportional the ratio between P, and PS. A large
Pr/Ps often indicates PZT with high piezoelectricity.
*
It~W~
...............
.
........
...
. ..
U
..........
............
4*
4*
US
S
----------
..........
..
...
..................
....
*5
Figure 3-3: Hysteresis behavior of PZT [28].
3.3 PMPG Structure
Piezoelectric Micro Power Generator (PMPG), is a composite MEMS energy
harvesting device. Type 1 PMPG device was fabricated according to the layout shown in
Figure 3-4 [29]. The MEMS device is built on a Si substrate, which is later etched away
to release the cantilever beam. The basic design of the multilayer cantilever consists of
five layers as follows: membrane layer (SiO 2 and/or SiNx) for controlling bow of the
cantilever structure, diffusion barrier/buffer layer (ZrO2) for preventing electrical charge
diffusion from the piezoelectric layer, piezoelectric layer (PZT), top IDT electrode (Pt/Ti),
and optional proof mass layer (SU-8). Pt is the dominant material of the top electrodes,
while Ti acts as an adhesion layer between Pt and PZT.
45
Interdigitated Electrode
_ r of Mass
membrane
Figure 3-4: Cross section and top views of Type 1 PMPG [29].
There are two piezoelectric modes (d31, d33 ), which are commonly used in piezoelectric
devices. While the d3 1 mode has separate top and bottom electrodes, the d33 mode
eliminates the need for a bottom electrode by employing an interdigitated top electrode.
The d33 mode design gives much higher open-circuit voltage than the d31 type transducer
at similar beam dimensions. The electrodes in Type 1 PMPG are designed to take
advantage of d33 mode of PZT material [30].
Figure 3-5: SEM photograph of Type-I PMPG [29].
3.4 Electro-Mechanical Model of PMPG
3.4.1 Introduction
Previous evaluation of wireless sensors indicates power requirements of 100 1IW
at 3 V from the power generator. In addition, the energy harvesting device needs to have
a low resonant frequency in order to convert parasitic mechanical vibrations available
from the environment. Specifically, the natural vibration of liquid gas pipeline is on the
order of 100-200 Hz. PMPG will be sized according to its electro-mechanical model in
order to meet these power requirements.
46
There are two piezoelectric modes (d31 and d33 ) that are commonly used in
piezoelectric devices (Figure 3-6). They are distinguished by whether the electric field
direction is perpendicular to the input strain direction (d31 ) or parallel to it (d3 3 ).
Operating piezoelectric elements in d33 mode is more advantageous. While the d3 1 type
must have separate top and bottom electrodes, the d33 type eliminates the need for a
bottom electrode by employing an interdigitated (IDT) top electrode. It reduces the
number of photo mask, as well as complications in fabrications due to multiple layers.
Device in d33 mode also has high strain and larger open circuit voltage. Equations
(3.1) and (3.2) show the relation between stress axx (or strain x 3) and electric field Ei (or
voltage V3 ) [29].
x3
V~, = og
where
x3
(3.1)
= dA
3
(3.2)
L,
is the strain, V 31 the open circuit voltage, d 3i (V/m) and g3i (Vm/N) the
piezoelectric constants, and Li is the distance between electrodes. In d31 mode, electrode
spacing L is dictated by the thickness of the PZT layer. On the other hand, L can be
controlled arbitrarily in d33 mode by changing the interdigitated top electrodes spacing.
Furthermore, the piezoelectric constants in d33 are generally 2 to 2.5 times larger than d31
mode. According to equations (3.1) and (3.2) these imply a larger open circuit voltage,
larger strain and power output. As a result, d33 mode was employed in our PMPG design.
d33 mode
d3 mode
L
Et
V, aCtYg
V =a, Lg 33
Figure 3-6: Two modes of piezoelectric conversion from input mechanical stress [40].
Many previous researches have developed mathematical model describing
behaviors of piezoelectric structures. Wardle et al. have derived a model to estimate
47
power output for a 1-dimentional system in d33 mode. The effect of proof mass, beam
shape and damping on the harvesting performance have been modeled to provide a
design guideline for maximizing power output [31].
The 1-D model consists of a piezoelectric element excited by a base input,
WB.
The piezoelectric element has a mass m, with proof mass, M, and is connected to a power
harvesting circuit, modeled simply as resistor R, (Figure 3-7). The piezoelectric leakage
resistance is modeled as Rp. The resonance structure is damped out by both mechanical
damping and electrical damping, with electrical damping dominant during power transfer
from PMPG to output circuit. This model is strictly valid only for harvesters where
electrical damping is linear and proportional to velocity. However, most output circuitry
consists of AC-DC converter with nonlinear behavior. Nevertheless, this model is still
useful in evaluating first order responses, relative importance of generator's parameters,
and providing design insights.
WD
x2
Figure 3-7: General 1-D structure of piezoelectric energy harvester.
The estimated displacement, open circuit voltage, and power are determined by
the following equations:
48
B=
K
[-(1 + 2,,R,)
2
+-2
(3.3)
24, + 1
]2 +
,ReQ -
ReQ'
KC,
B
v
KCP e
(3.4)
..
[I-(1+ 24,mRe,)
P
(WVB
~
2
]2
+
~
2
j
,m+
j2
Re J - R ef2'
KCP
1+
1 R
P
2
2
[1 - (1+ 2mRe )Q2 ] +
2,;m +
-2(35
2
{1+
}Re}
- ReQ3
IKCP
Parameters intrinsic to MEMS device are capacitance of piezoelectric element CP, the
mechanical damping ratio ;m, total structural mass M, electromechanical coupling factor
0, and stiffness of the cantilever K. Dimensionless factors Re=oiR1Cp and Q=o/oj are
introduced, where
o is the base input frequency, and co, = [K/M . Bf is the forcing
function that accounts for the inertial loading due to the base excitation. Electrical
damping coefficient qe is introduced when power is drawn from the system by electrical
loading.
3.4.2 Parameter Considerations for Maximizing Power Output
A design rule to maximize the power output is developed. We have developed our
design rule to maximize the power output rather than the conversion efficiency, since the
environmental vibrational energy is abundant and the magnitude of the continuous power
from the harvesting device is the most important measure to many wireless sensing
applications.
49
Energy conversion from mechanical to electrical via a PZT beam follows steps
below:
" Input vibration - applies acceleration to the beam structure.
" Effective mass - converts input acceleration into force.
" Beam shape - bends according to the force to result in strain along the PZT
layer. Mechanical damping dissipates energy during oscillation.
" Piezoelectric layer of the beam - converts mechanical strain into electrical
charge, which results in electrical damping of the vibrating beam.
" Electrodes - collect generated charge and electrical damping results.
According to equation (3.5), generated power is modeled with 6 parameters:
amplitude of vibration o, mass of cantilever beam M, its natural frequency ol,
mechanical and electrical damping coefficients ((m and
ce),
and electromechanical
coupling coefficients. Amplitude of vibration is fixed by the input source, and the natural
frequency of piezoelectric element is adjusted to that of input frequency. Therefore,
power can only be maximized by adjusting M and damping coefficient through varying
cantilever beam shape and mass.
3.4.2.1 Effect of Mass
Although the actual charge generation occurs at the piezoelectric layer, the
maximum strain on the PZT layer is set by the effective mass of the beam. For the same
resonant frequency, the heavier the mass and the stiffer the beam, the more energy can be
stored into the structure at resonance. Assuming the input vibration is fixed, o and oih are
constant. Also assuming the beams have constant 0, K, C, and the same harvesting circuit,
the most dominant factor for the power output shown in equation (3.5) becomes Bf,
which is a function of inertial mass of the structure. The maximum energy harvested by a
piezoelectric transducer is
e * Estored,
in which
Estored
is mainly determined by the
effective mass of the beam structure [30]. The more the electrical power is drawn from
the vibrating beam, the higher the electrical damping. Then the high electrical damping
would result in a significant reduction of the beam deflection at the resonance. If the
beam has more stored energy, however, more electrical energy can be drawn from the
50
beam while maintaining the electrical damping low, which is the ratio of Egenerated/
Estored-
Therefore the key idea for more power generation is to maximize the stored energy. The
maximum power harvesting, therefore, can be achieved by having an effective mass as
heavy as possible unless it results other adverse effects such as excessive stress or
damping.
There are two approaches to increase the total mass M of the cantilever. One can
add a proof mass at the end of the beam, as illustrated in Figure 3-8. By adding a proof
mass, one can obtain a larger total mass in a smaller volume. It should be noted that beam
b not only increases M, but also decreases the mechanical air damping significantly. Less
mechanical damping will result in a larger power output.
Ml.
M1M
(b)
(a)
Figure 3-8: The effect of the proof mass on deflection.
The second approach is to enlarge the total structure. Since Si (beam substrate)
has a higher density than SU8 (proof mass), it is sometimes more feasible to increase the
cantilever beam thickness instead of adding a proof mass. On the other hand, a large
beam structure will have a larger mechanical damping that will in turn reduce power.
Furthermore, reducing the size of power generator is another requirement of PMPG
design. Therefore, the most effective way to increase total mass is to use a proof mass
much denser than the beam.
3.4.2.2 Effect of Damping
Damping coefficients
also affect power output [33].
Electrical damping
coefficient ge is set by the electrical load. On the other hand, mechanical damping
coefficient ;m can be varied by adjusting beam dimensions. The prior discussion of mass
51
has already illustrated the advantage of a smaller beam in reducing air damping. As a
matter of fact, the difference between the two approaches in adjusting mass is the
mechanical damping coefficient. Basically, harvested power is proportional to the
maximum deflection, so consequently proportional to the inverse of damping coefficient
as shown in equation (3.5). Resonant deflection is determined by the following equations:
Static case: ,=
(3.6)
M/K
Resonant case: 6r= t M/K / 2 T=tM/K *
is the total damping coefficient of PMPG,
where
Q
(3.7)
Q is the quality factor, or the ratio of
resonant tip deflection to the static tip deflection. So minimizing
m
is essential for
maximum power.
As
m
is the ratio of damping force to the effective mass, the exact expression
becomes:
(m
=
(3.8)
b/2Mcoi
where b is damping force. We can reduce
mby
minimizing b which is proportional to
the surface area. Consider the vibrating cantilever at the MEMS scale. The damping of
the cantilever structure can come from the flapping air, internal friction, beam support
losses, and squeezed air film underneath as shown in Figure 3-9. The dominant
contributors at the MEMS scale are the air damping and internal structure damping [34].
Air damping
Support Loss
Vibration
Internal friction
Squeeze Effect
Figure 3-9: Damping mechanisms.
52
Damping coefficients from the air damping and internal structure damping is determined
by the governing equations below:
(3.9)
4m = 4air +4intemal
air
1 2
beam = (L/W)(3pt7b+0.75nb 2(2PaOP) / )/(2M(oi)
airroof =
(3pnd+0.757td 2(2PaOmP)
4intemal = (mbeam/(mproof + mbeam))*i1/
(3.10)
)/(2Moi)
(3.11)
2
(3.12)
where L is beam length, W the beam width, d width of a proof mass, P the viscosity of air,
Pa
the density of air, and lithe viscosity of beam material.
From the model of these damping mechanisms, the damping energy loss can be
reduced considerably by changing the beam and proof mass geometry. We found that
minimizing the projected area significantly reduces the air damping. The shapes of
structure and proof mass should be optimized to minimize the damping while to maintain
the stiffness of the beam for low resonance frequency. A simple comparison of the two
beams at the same resonant frequency and static deflection has been made as shown in
Figure 3-10.
63um
20um
63um
u
3umt7
90um
lurn
lum $tm
26 um
70um
(b)
(a)
Figure 3-10: Comparison of two PMPGs: (a) non-optimized, (b) damping optimized.
Using the dimensions shown in Figure 3-10, a 77% reduction of the damping coefficient
of a new device is anticipated. This results in a significant increase of the power density
as shown in Table 3-2. The reduction in damping coefficient enables 4.3 times larger
resonance amplitude of the cantilever structure and 10.2 times larger maximum strain of
53
the PZT layer. As a result, power density increases up to 1300% of the non-optimized
device.
Table 3-2: Comparison of PMPGs estimated performance: (a) non-optimized, (b)
damping optimized.
(a)
(b)
Dimension
261 x 190 x 51 pm 3 70 x 172 x 64pm
4.
0.0044
0.00102
Max. deflection
1.8pm
7.8ptm
Max. strain
0.0142%
0.145%
Power output
1.01 pW
4.37pW
Power density
0.74mW/cm 2
13.70mW/cm2
3
3.4.3 PMPG Design
Wireless sensor application requires a high power output (-0. 1 mW) at very low
resonance frequency (-150 Hz). A large effective mass is required in order to achieve this
power level. However, a fabrication process that includes heavy proof mass on PZT, such
as metal, has yet to be developed. SU8 is well understood and has been frequently used as
proof mass in different MEMS structures. On the other hand, there is little knowledge of
alternative proof mass material. As a result, we use the second approach to increase the
total mass of the cantilever beam. Specifically, Si wafer will be etched using DRIE to
create a cantilever using the entire wafer thickness. Since Si is much denser than SU8,
adding proof mass will not have a significant impact on effective mass. Therefore proof
mass is not used in the design. Using the second approach can increase mechanical
damping, and severely reduces power output. This problem will be reconsidered for
future PMPG generation designs.
The cantilever dimensions W, L, H are adjusted to obtain the desired power
output at resonance frequency. Cantilever beam thickness H is fixed by the thickness of
54
Si wafer, tsi, since thicknesses of other layers, such as PZT, are negligible compare to tsi.
Beam length determines stiffness, which directly couples with natural frequency. Width,
on the other hand, is independent of natural frequency. Increasing width of the cantilever
is analogous to having multiple beams in parallel. It increases power output while
retaining the same natural frequency. Therefore, L is adjusted to obtain the desired
natural frequency, and then vary width to reach the power specification. The natural
frequency is set to be the same as input frequency to maximize power output.
It should be noted that strain also plays an important role in power output. It
should range from 0.1% to 0.2% in order for PZT to achieve the desired power output.
However, strain is directly coupled with length. In this design, length is used to vary
natural frequency, while leaving strain fixed. If we were to use proof mass, it would
introduce another degree of freedom, allowing us to vary the strain.
The following discussion present a detail calculation of parameters used in
equations (3.3) to (3.5). Please refer to references [31] and [32] for an in-depth discussion
of the source of these equations.
The neutral plane, NP of the cantilever is first solved using equation (3.13),
t
Si
si -x
c33, zdz
-X+ti0
'Si
2
Jc 33 ,Szo 2 zdz +
+
t
-X+tSij2
+1Z02
fc
33, Zro2
_si-x
-x
where tsi,
tSi 0
2
, tZrO2
Si
-x+1Si0
2
+1Zr0
zdz +
2
tSi -X+tS02
+IPZR
+tZr,
-X+tSiO,
+tZr 0
tSi -X+tSi02
2
+tPZT
+1p,
c 33 ,Pzdz =
c 33 ,PZTzdz +
t
Si
2
0
(3.13)
1
+tZr02 + PZT
and tpt are thickness of different layers of the cantilever beam, and c33
is Young's modulus.
El, where E is the axial modulus of the beam, and I the second moment of area of
the beam is next calculated using equation (3.14). Since Pt is patterned as interdigitated
electrodes, only 1/10 of total beam area is considered in calculating contribution by the
electrode layer.
2
El =-W ((tsi
3
- NP)3 +
Np3 )C33, +
(tPZr + t.i - NP)' c 3 3 ,PZT
(ta,
+
+
tP
+t
- NP)3 c3
10
Potential distribution V(r) for a uniformly distributed beam can be modeled as:
55
,
)
(3.14)
_-3L)
r 2(r
1(r) = r(
L
(3.15)
Using potential distribution, the total mass is calculated by including the thickest and
heaviest three materials, PZT, Si and Pt electrodes,
t
L p, / 2
L tpzr
L tsi
M = W Jpsiw 2(r)dzdr + W
00
PPZT T 2(r)dzdr +
W f fp,T 2 (r)dzdr
0 0
0
t
(3.16)
PZT
where p is material density.
Stiffness K is calculated as:
(L
tsi-NP
K=WJf c 33
0 -NP
L tsi-NP+tPz7+tp
L tSi-NP+tPZr
,zN(r)
2dzdr +
0
fc3 3 ,PZTzN(r) 2dzdr +
tsi -NP
Jt
3
I
zN(r)2dzdr
0 tsi-NP+pT
(3.17)
1z
r-L
where N(r) =
El
Forcing function and electromechanical coupling factor are calculated as:
B
=
- psi ts + PPZTtPZT +
L tsi--NP+IPZT
z(r - L) c ,Pzr
33
O=W f
El
tsi-NP
0
p,t,,)W fJ
33
Lspacing
(r)dr
(3.18)
Zdr
(3.19)
where Lspacing is the spacing between interdigitated electrodes. Lastly, the PMPG intrinsic
capacitance is defined in terms of constrained permittivity efs as,
L
'Pzr
C, =W J
S
C33
2
dzdr
0 0 Lspacing
56
(3.20)
With these parameters, equations (3.3) to (3.5) are used to calculate the displacement,
voltage, and output power per acceleration of the device. To check if strain lies inside the
acceptable range of values, it is calculated by,
R_
Strain
..
= N(r)(tsi - NP +tPZr )-R-
3.1
(3.21)
3.5 Previous Piezoelectric Devices
Previous section has explored the feasibility of using piezoelectric material PZT
(Pb(Zr,Ti)0 3) to extract power from ambient source. In fact, several bulk piezoelectric
generators have been developed.
Mid6 Technology Corporation has developed a chain of Quick Pack, or bulk
piezoelectric devices [35]. They are two layer bimorph devices that are designed for
bending operation, and generate voltage when strained using d31 mode. They are
constructed from four piezoceramic wafers embedding in a Kapton and epoxy matrix
(Figure 3-11). Their nominal dimensions range from 0.685 x 0.5 x 0.015 in. to 4.00 x
1.00 x 0.02 in. These devices have been extensively used in research relating to
piezoelectric generator modeling and wireless sensor applications [26, 36].
Figure 3-11: Mide Technology Corporation Quick Pack 20W.
57
Other bulk size piezoelectric generators have also been developed. A study at
MIT Media Lab constructed a shoe generator, by placing a sandwiched structure of metal
midplate and two PZT unimorph layers on the sole of a shoe [37]. Such shoe insert is
capable of generating 8.4 mW of power in normal walking condition. Paul Wright of UC
Berkeley has constructed a power generator for RF transmitter using two layer
piezoelectric bender mounted as a cantilever beam [27]. Such device has a total length of
3 cm and maximum power output of 375 gW at 60 Hz with driving vibration of 2.25 m/s2 .
MEMS scale piezoelectric generator has been attempted using thick-film piezoelectric
technology [38]. Such device operates in d31 mode, with power output of 3 pW.
However, most of these devices are too bulky for system integration. Power
generator remains to be the largest part in a sensor node. Furthermore, these devices uses
d31 mode, therefore which require a larger size in order to obtain the same power output
as in d33 mode. In conclusion, no d33 thin film piezoelectric generator has been fabricated
at the MEMS scale, until the fabrication of first generation PMPG device.
3.6 Type 1 PMPG Electrical and Mechanical Characteristics
Type 1 piezoelectric micro power generator has been developed [22, 29]. The
device structure is described in the previous section. The bow of the composite cantilever
beam was controlled by changing the membrane layer from thermal SiO 2 to PECVD SiO 2,
to PECVD SiO 2/SiNx. Since PZT is compressive, a tensile layer is required to create a
flat cantilever beam. Device with thermal Si0
2
of 300 MPa as a membrane layer has a
severely curled cantilever. The bow is greatly diminished when PECVD Si0 2 was used
instead, as residual stress is reduced to only tens of MPa. On the other hand, it has a small
elastic modulus regime. Therefore, a small perturbation in stress will cause a large
change in curvature. SiNg of high elastic modulus (315 GPa) and tensile stress (190 MPa)
was chosen as the membrane layer. Its large elastic modulus increases the robustness of
the device, and resulting in a nearly flat cantilever beam (Figure 3-12). PECVD Si0 2 was
still retained as an adhesion layer.
58
(a)
(b)
(c)
Figure 3-12: SEM images of the stress-controlled cantilevers consisting of:
(a) PZT/ZrO2/SiO2(thermal), (b) PZT/ZrO2/SiO2(PECVD) and (c)
PZT/ZrO2/SiO2(PECVD)/SiNx.
PZT film's microstructure and crystallization were determined using a fieldemission scanning electron microscope and an X-ray diffractometer (XRD). The
microstructure (Figure 3-13a) of the PZT film on ZrO2/SiO 2/SiNx/Si has an approximate
grain size of 100 nm in diameter. Figure 3-13b illustrates the relative peaking of PZT
according to XRD. The polarization properties of the PZT thin films were measured
using the Radiant Technologies@ RT-66A as shown in Figure 3-13c. The spontaneous
polarization (Ps), remnant polarization (Pr), coercive field, and dielectric constant are 50
piC/cm 2, 20 pC/cm 2, 38 kV/cm, and 1200Eo, measured respectively.
UO -
I-N
d33mode type
Inter-digited electrode dista: ce: 4pm
40. PZT thickness 0.5
P,:20pC/cm'
:
20
-4
~E
-
20
30
40
2o
(a)
(b)
50
60
#.Id
-100
-50
0
Applied voltage (V)
5
100
(c)
Figure 3-13: Experimental results of characterization of PZT thin film deposited on
ZrO 2 /SiO 2 /SiNx/Si: (a) SEM image, (b) XRD pattern, (c) P-V hysteresis curve.
PMPG device, a 170 pmx260 ptm size cantilever, was directly excited by a PZT
exciter at ±3V AC voltage source, sweeping from 0 to 200 kHz. The devices showed
59
resonant modes at frequencies 13.9, 21.9 and 48.5 kHz. Since the first resonance mode
shows the largest amplitude of displacement, it was used for power generation. A
piezoelectric shaker was used to apply vibration directly to the PMPG at the frequency
the 1 't mode resonance. The magnitude of the base displacement was varied by changing
the shaker drive voltage.
For base shaking experiments, the PMPG device was connected to a rectifying
bridge circuit with a resistive load across the storage capacitor (Figure 3-14). Figure 3-15
shows the load voltage and power delivered to the load. 1.01 pW was delivered for the
5.2 M9 load, with 2.4V output. Given the device area (including room for on-chip
rectification and energy storage circuitry), this translates to an energy density of 0.74
mW-h/cm 2 . The storage capacitor charging and discharging times were approximately 0.2
and 0.3 seconds, respectively.
dq
d,
PMPG
Rectifier
Storage Load
Figure 3-14: Equivalent electrical model of the PMPG power system.
1.2
5.0
4
---
Power delivered to Load
1.0
4.
4.0
3.5
0
-j
3.0
--
2.5
c
2.0
CU
1.5
0.6
-
0.4
0)
00
>
1.0
01
.
0.2
0.5
9a-
-- 0-- Voltage across Load
0.0
0
2
4
6
8
1
10
1 0.0
Load Resistance [MQ]
Figure 3-15: Load voltage and output power at 13.9 kHz resonance.
60
3.7 Type 2 PMPG Design
Type 2 PMPG is designed to meet the power requirements of wireless sensor
application. Specifically, it is designed to provide greater than 100 gW of power at
natural frequency of 100-200 Hz. In order to provide sufficient power output at low
frequency, a large effective mass is required. The total mass is increased from Type 1
PMPG by including Si substrate in the beam structure (Figure 3-16). The area around the
device will be etched away using Deep Reactive Ion Etching (DRIE) to release the
cantilever beam. Since the beam structure is predominantly silicon, it will counteract with
compressive PZT to create a flat beam.
Interdigitated Electrode
PZT layer
ZrO 2 layer
SiO 2 (Thermal) layer
Si substrate
Figure 3-16: Type 2 PMPG cross section and top view.
The device structure is designed according to equations described by PMPG
electro-mechanical model (please see previous section for detail) (Table 3-3). An input
vibration of 5 m/s 2 is assumed. Since the beam is predominantly silicon substrate, its
thickness is assumed to be the Si substrate thickness (475 pm - 575 [tm). Device
dimension has been over-designed, in order to compensate for thickness variations.
Wider devices are designed in order to increase W/L ratio and minimize bending.
Maximum strain < 0.2%, and power > 100 ptW are achieved.
61
Table 3-3: Type 2 PMPG device sizing using electro-mechanical model.
Natural
Length
Width
Thickness
Maximum
Tip Displacement
Frequency
Power
Devices:
(mm)
(mm)
(mm)
Strain
(mm)
(Hz)
(uW)
1
50
3
525
0.11%
3.5
155.5
173
2
51
3
525
0.12%
3.8
149.5
184
3
52
3
525
0.12%
4.1
144
194
4
51
5.1
525
0.12%
3.8
149.5
312
5
51
10.2
525
0.12%
3.8
149.5
618
6
50
3
475
0.12%
4.2
141
185
7
50
3
575
0.10%
3
170
163
8
52
3
575
0.11%
3.5
157.5
183
62
Chapter 4
Self-Powered Sensor Node Tests and
Results
4.1 Telos Characterization
Telos energy consumption and operation time had been experimentally measured.
Two Telos nodes were used to form a network (Figure 4-1). The sensor node Telos was
programmed with Tpmpg application, and base station Telos is programmed with
TOSBase application. One device was placed in a controlled environment as sensor node.
The other Telos was connected to computer, serving as base station. A testing box had
been constructed, in which a light source was used as heating element for Telos (Figure
4-2). Alternatively, cooling source was applied using ice. Experimental result showed
temperature data displayed at the base station varied accordingly (Figure 4-3).
63
Rseries
3V Power Supply:
Battery, Capacitor or
Signal Generator
Telos Sensor Mote
Telos Base Station
Heat Source
Figure 4-1: Telos experiment setup.
Figure 4-2: Temperature measurement testing box.
27
Applied
Ice
25-
23 -
21 -
CL
19-
Removed
Ice
17 -
150
200
400
600
1000
800
1200
1400
Time(sec)
Figure 4-3: Temperature measurements.
64
1600
1800
4.2 Telos Current Consumption Measurement
Telos energy consumption and operation sequence were experimentally measured
and characterized. Sensor node Telos was programmed to sleep 1 second in between
measurement with Tpmpg application. Telos was connected to a 3 V power supply and a
3.5 K2 resistor. The instantaneous current consumption was acquired by measuring the
voltage across the series resistor (Figure 4-4). Telos current consumption is shown in the
following Figures.
I measurement
Rwe=3.5 0
3V V
Telos
Figure 4-4: Telos current consumption measurement.
Figure 4-5 illustrated current measurement over a period of 5 s, and Figure 4-6, 47 are zoom-in views of startup and radio active period.
According to Figure 4-5, there existed a delay period of 340 ms (labeled as power
delay) between voltage applied to Telos, and the time when Telos started to respond. This
period seemed to be intrinsic to the hardware, and cannot be reduced. It was followed by
a startup period of approximately 6 ins, with two current spikes separated by a gap of 3
ms (Figure 4-6). The current spikes would not inflict damages to the system, since they
are applied to a large energy storage capacitor that acted as a filter. The first data
measurement cycle began at the end of the startup period.
Each data measurement began by waking up the microcontroller MSP430 (6 pis,
negligible and omitted in graph), and SHT 11 taking temperature measurement for 210 ms
(Figure 4-5). The first cycle was completed after radio initialized and transmitted data for
approximately 10 ms. Telos slept for one second as programmed, before the start of the
65
next cycle. In summary, each successive cycle consisted of MSP430 waking up, SHT1 1
taking data measurement, radio transmitting data and returning to sleeping mode.
Figure 4-7 is a detailed view of the transmission period. It indicated less than 1 ms
radio initialization, followed by 3 ms of radio crystal startup and 6 ms of radio
transmission. Periods of 210 ms temperature measurement and 10 ms radio transmission
agreed with approximation made according to Telos datasheet (Figure 2-8). 20 mA of
current drawn during radio transmission also matched the expected level of current
consumption. Power delay and startup were not expected, but could not be eliminated.
Furthermore less than 1 mA was consumed during sleeping mode. The exact value could
not be measured due to instrumental constraints and background noises. Nevertheless, it
illustrated the effectiveness of reducing power consumption by putting Telos to sleep in
between measurements. On the other hand, sleeping current could be completely
eliminated with the switching scheme of the power management module.
3.50)
> -0.5
(a) (b) (c) (d)
(e)
(e)
(c) (d)
(e)
(c) (d)
(c) (d) (e)
40-
35 -
(a)
(b)
(c)
(d)
30
(e) Sleep (is)
Radio
Transmission
25
Startup
E
Power Delay (-340 ms)
Startup (-6 ms)
SHT1 1 Temp. (-210 ms)
Radio (-10 ms)
20
10
5
0
)
-5
0.
1.5
1
------------------
2.5
2
-
3
3.5
----
4
4.5
f-----
Time (s)
Figure 4-5: Telos current consumption over 5 s interval, with 1 s sleep. There was a 340
ms power delay between voltage applied to Telos (top) and Telos startup. After startup
period, Telos operated periodically through 210 ms of temperature measurement, 10 ms
of radio transmission, and 1 s of sleeping.
66
45
First Cycle SHT11 Temp.
Startup
Power Delay
40
35
30
25
E
&
20
10
5
PA,IRA
Ia 101"
.a
POMPI-P
-014
10-5
1
2
3
4
5
6
7
8
9
13
Time (ms)
Figure 4-6: Telos startup period, consisted of two current spikes separated by 3 ms.
35SHT1 1 Temp.
Radio
Init
I
Radio Crystal Startup
Sleep
Radio Transmission
III
30 -
25 -
20
~E15
U
10
5
K11
I-I,...
I1
i
-I--- I -- I 4 M
1111111111
IIIIIIIIIII
11
11111
.iL~.i.
I
n
2
4
6
8
10
12
14
16
18
2
-5L
Time (ms)
Figure 4-7: Telos radio active period. less than 1 ms radio initialization, followed by 4 ms
of radio crystal startup and 6 ms of radio transmission.
67
4.3 Power Management Module Testing
A test model of the self-powered sensor node was constructed, by integrating
Telos, power management module and a power source (Figure 4-8, 4-9). RI, R2 , R 3 and
R4 were sized to 1 ME, 1.1 M9, 1.8 Mn, and 1 M9, as determined by previous section.
Viow=2.18 V and Vhigh= 2 .7 1 V were achieved. Loading effect from Telos and other circuit
components caused a deviation from desired threshold voltage of 2.2 V and 3 V.
Nevertheless, V,10 was greater than Vmin and Vhigh less than 3 V, ensuring that the system
operates correctly. A sinusoidal current source provided by PMPG was simulated using
function generator output in series with a 1 kE resistor. o was set to 150 Hz while V, was
varied to change the power input to
Cstorage.
C1 was set to 0.1 pF, about 10000 times
smaller than the energy storage capacitor. For testing purpose, electrolytic capacitors of
size 470 [tF to 2 mF were used. More ideal capacitors as determined by previous section
should be used in the final system. As illustrated by the switching scheme described in
the previous section, Telos would continue taking and transmitting temperature at 1
second interval using energy from Cstorage, until Vcstorage dropped below Viw.
R3
R1
R4
Cstorage
lpsin(wt)
4D1I
R2R
C1
-
Telos
D2
r
vP=VPsin(wt)
Figure 4-8: Schematics of self-powered wireless sensor node test setup. R1=1 M9,
R2=1.1
MQ, R3 =1.8 M, and R4=1 MQ. Threshold voltage Vi 0w=2.18 V and Vhigh
2 .71
V were achieved. PMPG sinusoidal current source was modeled by a function generator
output and series resistor R=1 kQ. o was set to 150 Hz, the input frequency to PMPG.
68
Figure 4-9: Self-powered wireless sensor node test setup.
69
Function generator input voltage VP was adjusted to set I, to 10 mA. The size of
Cstorage
was varied from 470 1iF to 2 mF. The voltages across the energy storage
capacitors were measured. 470 pF capacitor provided insufficient energy to generate data
using Telos. On the other hand, 670 RF could provide sufficient energy for sensor node
Telos to measure and transmit one data point to base station. During charging mode,
capacitor drew energy from function generator, and Vcstorage ramped up from 2.2 V to 2.7
V for duration of 3.5 s (Figure 4-10a). When
pole/double throw switch connected Telos to
Vcstorage
Cstorage. Vcstorage
reached Vhigh, the single
decayed for 0.5 s as Telos
consumed energy during data measurement and transmission, until it reached V 0w. The
data were received at the base station and displayed by a Java GUI applet. A variation in
temperature around the sensor node was clearly shown in the output.
On the other hand, 1 mF capacitor was able to provide enough energy for three
data measurement (Figure 4-11). The capacitor had a charging time of 4.7 s, and
discharged through Telos for duration of 3 s. Lastly, 2 mF capacitor supported nine data
measurements, and has a charging and discharging time of 10.3 s and 11.3 s (Figure 4-12).
Results indicated that a 670 pF capacitor could support one data measurement, a
capacitance smaller than the 1 mF predicted previously. One reason was that, the
previous
calculation
assumed
to
Vcstorage
remain
relatively
constant
during
charging/discharging cycle. In order to achieve a relatively constant voltage, a larger
capacitance would be required. In reality, Telos could continue to operate as VCstorage
decayed to Vmin. Therefore, a smaller capacitor could be used instead.
Furthermore, I, was set at a value larger than that could be provided by PMPG. In
order to achieve power output of 0.1 mW, Ip would need to be set to 0.1 mA. Such low
level of amplitude could not be achieved due to instrumental constraints. On the other
hand, varying Ip should not affect the minimum Cstorage size. It would, however, increase
the capacitor charging time constant. The effect of Ip was measured experimentally.
As expected, the minimum capacitance required remained constant, as I, varied
from 4 to 5 mA (Figure 4-13, 4-14). It was also comparable to the 670 [.F required by
previous experiment. Nevertheless, charging time increased drastically from 16.8 s to
46.8 s, as Ip changed from 5 to 4 mA.
70
2.9
2.7 -
2.5 -
2.3 -
0
> 2.1 -
1.9-
1.7
-
1.5
0
2
4
6
10
8
12
14
16
18
20
Time (s)
(a)
(b)
Figure 4-10: (a) Voltage profile of 670 pF Cstorage capacitor, with power source Ip=10 mA.
The capacitor was charged for 3.5 s and discharged for 0.5 s. (b) One temperature data
point was measured at base station during each discharging period.
71
2.9
2.7
2.5
2.3
0
> 2.1
1.9
1.7
1.5
0
2
4
6
10
8
12
14
16
18
20
Time (s)
(a)
(b)
Figure 4-11: (a) Voltage profile of 1 mF Cstorage capacitor, with power source Ip=10 mA.
The capacitor was charged for 4.7 s and discharged for 3 s. (b) Three temperature data
points were measured at base station during each discharging period.
72
.-
liii
-
,.-
2.9
2.7 -
2.5 -
2.3 -
0
> 2.1 -
1.9 -
1.7 -
1.50
5
10
15
25
20
30
35
40
45
50
Time (s)
(a)
(b)
Figure 4-12: (a) Voltage profile of 2 mF Cstorage capacitor, with power source Ip=10 mA.
The capacitor was charged for 10.3 s and discharged for 11.3 s. (b) Nine temperature data
point were measured at base station during each discharging period.
73
2.9 -T -
-
-----------------------------------
----
--
2.7-
2.5-
2.30
> 2.1 -
1.9-
1.7-
1.5
0
5
10
15
25
20
30
35
40
45
50
Time (s)
Figure 4-13: Voltage profile of 1 mF Cstorage capacitor, with power source Ip=5 mA. The
capacitor was charged for 16.8 s and discharged for 2.1 s. One temperature data point was
measured at base station during each discharging period.
74
2.9
2.7-
2.5-
2.3
0
> 2.1-
1.9
1.7-
1.5
0
20
40
60
80
100
120
140
160
180
200
Time (s)
Figure 4-14: Voltage profile of 1 mF Cstorage capacitor, with power source Ip=4 mA. The
capacitor was charged for 46.8 s and discharged for 2 s. One temperature data point was
measured at base station during each discharging period.
75
4.4 Self-Powered Sensor Natural Gas Pipeline Test Bed
A test bed that modeled a 900 mile Alaskan pipeline was constructed to test the
designed autonomous sensor system [39]. As mentioned previously, one application of
this system is liquid gas pipeline temperature monitoring. By placing the autonomous
sensor node on a pipeline, PMPG resonates with pipeline's natural frequency, and
generates electricity for sensor node. A survey of liquid gas pipeline indicated range of
frequencies of 120 Hz - 250 Hz. For proof of concept, a miniaturized model of Alaskan
pipeline was created, as a source of input vibration to PMPG.
The pipeline system design was outlined in Figure 4-15. Water was used instead
of liquid gas in this setup. Pipeline was sized such that the water turbulent flow matched
that of liquid gas. In this setup, all the water started and ended with the reservoir. This
allowed for a circular path where the water flowing through the pipe could be recycled.
The reservoir connected to a pump of specified 4000 gph that would pump into the
designed system, gauged by a manual control valve. This will flow into a meter for flow
rate measurement and documentation and finally the test bed system. Frequency would
be measured via an accelerometer. After flowing through the test bed, the water would
return back to the reservoir. Since the pump vibrated, it could generate simultaneous
vibrations nearby and affected the test bed. A wooden table with rubber feet was used to
damp out the pump vibration. Such setup isolated the pump from the rest of the system.
Filter
Re servoir
SSy Stem
Pump
____
A ccele rometerf
M icro pho neL_
|LabPro
Valve
F Foufrier Transform
Figure 4-15: Schematics of test bed.
76
The test bed was constructed as shown in Figure 4-16. Experimental results
showed a vibrational frequency of 251.01 Hz ± 0.447 with a 95% confidence interval,
illustrating the repeatability of the test bed system. The result was on the high end of the
spectrum described by liquid gas pipeline survey. It was also higher than PMPG's
designed frequency. The test bed will be further modified to include frequency tunability.
(b)
(a)
(d)
(c)
Figure 4-16: Test bed setup for measuring pipeline vibration frequency: (a) The pipeline
was isolated from the rest of the pump system so there were no compounding effects. (b)
The pump was also isolated from the rest of the system so there were no compounding
effects. (c) The 3-way valve provided control for flow and drainage. (d) Logger Pro
software and data collection material. The accelerometer was later taped securely to the
copper tubing system for data collection.
77
Chapter 5
PMPG Fabrication
5.1 Type 1 Device
PMPG cantilever structure similar to Type 1 device was fabricated, using recipe 1
(Figure 5-la). This recipe was modified from the one used in reference [22]. SU8 proof
mass was used to increase power by increasing effective mass.
The device was
fabricated on a 4" silicon substrate. Si substrate acted as a sacrificial layer in Type 1
device. Therefore, its thickness, quality and crystal orientation were not important. 4000
A SiNx was used to reduce bowing in the cantilever structure, and PECVD oxide was
used as adhesive layer. Oxide was annealed at 950 'C for 4 hours.
500
A
ZrO 2 was coated, and pyrolyzed for 1 minute at 80 'C, followed by 8
minutes at 350 'C. It was then annealed at 750 0 C for 3 hours. A long annealing time is
required in order to obtain high quality PZT without cracks. After annealing, the wafer
was cooled down gradually. Furnace door was first opened for two minutes while the
wafer was still in furnace. The wafer was then brought out to hot plate, to cool down to
350 'C. Lastly, the wafer was cooled at a crystal boat to room temperature. Pyrolysis,
annealing time and temperature were varied to adjust PZT quality.
To create a thick layer of PZT of 0.48 pm, four separate coatings of thinner layers
were used. After coating the first layer, PZT was pyrolyzed for 1 minute at 80 "C,
followed by 5 minutes at 350 0C. After it was cooled to room temperature, wafer was
coated with second layer of PZT, following by the same pyrolysis step. It was then
annealed for 15 minutes at 650 'C. PZT annealing temperature should be set lower than
78
ZrO 2 annealing temperature, in order to separate the annealing processes. It followed the
same cooling procedure as used after ZrO2 annealing. After the wafer cooled to room
temperature, another two layers were coated using the same steps for coating the first two
layers. Pyrolysis, annealing time and temperature were varied to adjust PZT quality.
After a photolithography step, Pt/Ti interdigitated electrodes were deposited using
e-beam. SU-8 proof mass was patterned using a second mask. The cantilever beam
structure was then isotropically etched and released using XeF 2 (Figure 3-4).
50 pm SU8
2000 A Pt/200 A Ti
0.48 pm PZT
500 A ZrO 2
1000 A SiO 2
(PECVD)
4000 A SiNx
2000 A Pt/200 A Ti
0.48 im PZT
500 A ZrO 2
1000 A SiO 2
(thermal)
Si
Si
(a)
(b)
Figure 5-1: (a) PMPG structure using recipe 1, (b) PMPG structure using recipe 2.
79
5.2 Recipe 1
Beginning substrate:
P-type <100> 4" silicon wafer, 1-side polished, 475-575 jim thickness, TTV < 3 gm,
Bow&Wrap < 10 pm, 1-50 Q-cm.
Step 1: Cleaning of the wafer
0
RCA cleaning.
Step 2: Create SiNk nitride membrane
*
Target thickness: 4000 A.
"
Equipment: SVG/Thermco 7000 Series Vertical Thermal Reactor.
Step 3: Create PECVD SiO 2 oxide layer
*
Target thickness: 1000 A.
" Equipment: STS PECVD.
" Si0 2 Anneal.
o Anneal at 950 *C for 240 min.
o Equipment: Thermolyne Furnace 6000.
Step 4: Create diffusion barrier (ZrO2)
*
*
Spin-coat ZrO2 sol-gel.
o
Target thickness: 500 A.
o
15 s at 3000 rpm.
o
Equipment: Specialty Coating Systems, Spin Coater 6700 Series.
Pyrolysis: 80 *C for 1 min, followed by 350 0C for 8 min.
" ZrO 2 Anneal.
o
Anneal at 750 *C for 180 min. Long anneal time insures PZT
without crack.
o
Equipment: Thermolyne Furnace 6000.
80
*
Open furnace door to cool for 2 min. Bring to hot plate to cool to 350 'C.
Cool to room temperature.
Step 5: PZT deposition and anneal
"
Spin-coat four layers of Mitsubishi PZT sol-gel: Pb(Zr,Ti)0 3 , with total
thickness of 0.48 ptm.
" Spin-coat first PZT layer.
o
3 s at 500 rpm, 15 s at 3000 rpm.
o
Target thickness: 0.12 [tm.
o Equipment: Specialty Coating Systems, Spin Coater 6700 Series.
" Pyrolysis: 80 'C for 1 min, followed by 350 'C for 5 min.
*
Cool to room temperature.
"
Spin-coat second PZT layer with the same target thickness. Repeat the
same pyrolysis step.
"
PZT 1st Anneal.
o
Anneal at 650 'C for 15 min.
o
Equipment: Thermolyne Furnace 6000.
" Open furnace door to cool for 2 min. Bring to hot plate to cool to 350 *C.
Cool to room temperature.
" Spin-coat and pyrolyze two additional layers using the same steps as the
first two layers.
*
PZT 2 nd Anneal.
o
Anneal at 650 "C for 15 min.
o
Equipment: Thermolyne Furnace 6000.
o
Follow same cooling procedure as the first annealing step.
Step 6: Pt/Ti top electrode deposition and lift-off
*
HMDS.
" Spin-coat image reverse photoresist AZ 5214E.
o
Dispense speed to 500 rpm for 6 s, spread speed to 750 rpm for 6 s,
spin speed to 5000 rpm for 30 s.
81
o
Target thickness: 1.8 tm.
o
Equipment: Manual Photoresist Coater Solitec Inc. Model 5110.
" Prebake at 90 'C for 30 min using oven.
" Exposure with electrode mask.
o
1.6 s with 365-450 nm wavelength at 10 mW/cm2
o
Equipment: Electronic Visions EV620.
" Post exposure bake at 90 *C for 30 min using oven.
" Flood exposure.
o
60 s with 365-450 nm wavelength at 10 mW/cm 2.
o
Equipment: Electronic Visions EV620.
" Develop for 2-3 min.
*
Pt/Ti deposition.
o
Target deposition: 200 A Ti (deposition rate: 1 A Is) and < 500 OC
substrate temperature.
o Target deposition: 2000 A Pt (deposition rate: 2 A/s).
o Equipment: Temescal Semiconductor Electron-Beam Evaporator.
*
Acetone lift-off on electrode overnight; followed by methanol and 2propanol for cleaning.
" Ultrasonic -2-3 min for quicker lift-off and clearing lift-off residues.
Check for possible peel-off.
" Cleaning of solvents and contaminants with: acetone / methanol / 2propanol and multiple rinse dumps.
Step 7: Create SU8 proof mass
" Spin-coat MicroChem Corp. SU-8.
o
Target thickness: 50 ptm.
o
Equipment: SU8 spinner.
" Prebake at hot plate. 65 'C for 2 min, 95 'C for 4 min.
" Exposure with SU8 mask.
o
25 s with 365-450 nm wavelength at 10 mW/cm 2.
o Equipment: Electronic Visions EV620.
82
*
Post exposure at hot plate. 65 *C for 2 min, 95 'C for 4.5 min.
" Develop with PM Acetate for 4 min.
" Hard bake 1 hour in SU8 oven.
Step 8: PZT Etch Pattern
" Pattern resist profile with 5 ptm thick resist using third mask. 1 tm resist
gives severe undercut.
" Dry etching of PZT:
o
C12/BC13=10/30 sccm ratio; 150W RF power; etch rate expected:
0.15um/min; resist etch: 0.27um/min.
o
Equipment: Plasmaquest.
Step 9: XeF 2 release etch to release cantilever beam
0
Remove silicon layer using XeF 2 etch.
83
5.3 Type 2 Device
Type 2 device was design to operate at 150 Hz and generate 150 pW of power (5lb). Please refer to Chapter 2 for a detail discussion. The fabrication recipe was also
modified to accommodate change in device design.
First, Si substrate was incorporated in the cantilever structure. The PMPG
effective mass was increased using Si instead of SU8 proof mass. As a result, one less
mask and photolithography step was needed. The final PMPG structure would be
patterned and etched using DRIE. On the other hand, the wafer thickness became
important, since the substrate became the dominant layer in the cantilever beam. PMPG
were designed around the lower bound thickness, in order to guarantee sufficient power
output. (Table 3-3).
Second, high quality PZT fabricated on ZrO 2 /SiO 2 (PECVD)/SiNx/Si could not be
achieved, using Type 1 device recipe (see next chapter for detail discussion). Thermal
oxide was required for good quality PZT. In order to grow thermal oxide directly on Si
substrate, nitride layer was eliminated. Since a thick Si layer was used, bowing would not
occur even after nitride layer was removed.
Two masks were designed for Type 2 structure (Figure 5-2, 5-3). Electrode mask
(digitized dark) defined the interdigitated electrodes, and device mask (digitalized clear)
defined the device area. The exposed area after patterning with device mask would be
etched using DRIE, and releasing the cantilever beam.
The devices were sized according to Table 3-2. 3 mm by 50 mm could achieve
187 pW power output, assuming wafer thickness was ptm. Devices with 1/10 and 1/5
W/L ratio were constructed, in order to prevent structural bending. Specifically, devices
sizes W (mm)/L (mm) from left to right were: 3/50, 3/51, 3/52, 5.1/51, 10.2/51, and six
3/50 on the right side of the wafer. Electrode gap sizes were also varied to insure
successful PZT pooling. Specifically, nine devices on the left had gap size of 5 [.m, while
the two on the right had gap size of 3 m and 8 ptm. Recipe 2 was used to fabricate the
devices.
84
(a)
(b)
Figure 5-2: (a) Electrode mask. Devices sizes W (mm)/L (mm) from left to right were:
3/50, 3/5 1, 3/52, 5.1/5 1, 10.2/5 1, and six 3/50 on the right side of the wafer. Electrode
gap size was 5 pm for nine devices on the left and 3 jim and 8 pm for two devices on the
right. (b) Zoom-in view of the top of portion of interdigitated electrodes.
85
Figure 5-3: Device mask.
86
5.4 Recipe 2
Beginning substrate:
P-type <100> 4" silicon wafer, 1-side polished, 475-575 ptm thickness, TTV < 3 pim,
Bow&Wrap < 10 jim, 1-50 Q-cm.
Step 1: Cleaning of the wafer
0
RCA cleaning.
Step 2: Create PECVD SiO 2 oxide layer
*
Target thickness: 1000 A.
" Equipment: MRL Industries Model 718 A2 Tube.
Step 3: Create diffusion barrier (ZrO2)
" Spin-coat ZrO2 sol-gel.
o Target thickness: 500 A.
o 15 s at 3000 rpm.
o Equipment: Specialty Coating Systems, Spin Coater 6700 Series.
*
Pyrolysis: 80 *C for 1 min, followed by 350 C for 8 min.
" ZrO2 Anneal.
o Anneal at 750 *C for 180 min Long anneal time insures PZT
without crack.
o
*
Equipment: Thermolyne Furnace 6000.
Open furnace door to cool for 2 min. Bring to hot plate to cool to 350 'C.
Cool to room temperature.
Step 4: PZ T deposition and anneal
"
Spin-coat four layers of Mitsubishi PZT sol-gel: Pb(Zr,Ti)0 3, with total
thickness of 0.48 jim.
"
Spin-coat first PZT layer.
87
o
3 s at 500 rpm, 15 s at 3000 rpm.
o
Target thickness: 0.12 ptm.
o
Equipment: Specialty Coating Systems, Spin Coater 6700 Series.
" Pyrolysis: 80 'C for 1 min, followed by 350 'C for 5 min.
" Cool to room temperature.
" Spin-coat second PZT layer with the same target thickness. Repeat the
same pyrolysis step.
" PZT 1st Anneal.
o Anneal at 650 'C for 15 min.
o
Equipment: Thermolyne Furnace 6000.
" Open furnace door to cool for 2 min. Bring to hot plate to cool to 350 *C.
Cool to room temperature.
"
Spin-coat and pyrolyze two additional layers using the same steps as the
first two layer.
" PZT 2 Anneal.
o
Anneal at 650 'C for 15 min.
o
Equipment: Thermolyne Furnace 6000.
o
Follow same cooling procedure as the first annealing step.
Step 5: Pt/Ti top electrode deposition and lift-off
*
HMDS.
*
Spin-coat image reverse photoresist AZ 5214E.
o Dispense speed to 500 rpm for 6 s, spread speed to 750 rpm for 6 s,
spin speed to 5000 rpm for 30 s.
o Target thickness: 1.8 [tm.
o Equipment: Manual Photoresist Coater Solitec Inc. Model 5110.
" Prebake at 90 'C for 30 min using oven.
" Exposure with electrode mask.
o
1.6 s with 365-450 nm wavelength at 10 mW/cm 2.
o
Equipment: Electronic Visions EV620.
* Post exposure bake at 90 'C for 30 min using oven.
88
"
Flood exposure.
o
60 s with 365-450 nm wavelength at 10 mW/cm 2 .
o
Equipment: Electronic Visions EV620.
" Develop for 2-3 min.
" Pt/Ti deposition.
o
Target deposition: 200 A Ti (deposition rate: 1 A /s) and < 500 *C
substrate temperature.
o
Target deposition: 2000 A Pt (deposition rate: 2 A /s).
o
Equipment: Temescal Semiconductor Electron-Beam Evaporator.
" Acetone lift-off on electrode overnight; followed by methanol and 2propanol for cleaning.
" Ultrasonic ~2-3 min for quicker lift-off and clearing lift-off residues.
Check for possible peel-off.
" Cleaning of solvents and contaminants with: acetone / methanol / 2propanol and multiple rinse dumps.
Step 6: DRIE etch to release cantilever beam
" Pattern resist profile with device mask.
" Etch entire wafer (including silicon substrate) using DRIE.
o
Equipment: STS/Multiplex ICP (non-MESC) STS 1.
89
5.5 Fabrication Result: PZT on ZrO2 /SiO 2 (PECVD)/SiN,/Si
PZT was fabricated on ZrO 2/SiO 2 (PECVD)/SiNx/Si, but the quality was
unsatisfactory. Various processing parameters were modified to improve the PZT quality,
and their effects were illustrated in the following section.
The original recipe annealed PECVD oxide at 950 "C overnight. ZrO 2 annealed
for 20 minutes at 750 *C, and spin-coated four layers of PZT with two 20 min anneals at
650 'C. The fabricated PZT displayed crack and hillocks (Figure 5-4). Minor cracks were
formed after the first PZT anneal, and additional denser cracks were formed after the
second PZT anneal. Hillocks, on the other hand, were formed after either the first or the
second PZT annealing. Hillocks were formed when PZT was oxidized by diffused oxide
through ZrO 2.
Figure 5-4: PZT with crack and hillock.
Three key parameters that affected PZT qualities were:
.
SiO 2 annealing time and temperature.
.
ZrO 2 annealing time and temperature.
.
PZT annealing time and temperature.
SiO 2 need to be annealed at 950 'C for minimum of 2 hours. Oxidizing the wafer
overnight would form dust particles in the oven and caused PZT contamination. The
annealing time was reduced to four hours.
To remove PZT crack, ZrO 2 must anneal for a minimum of three hours. ZrO2
annealing temperature was set higher than that of PZT to separate the annealing steps.
90
750 'C was used. Varying this temperature did not improve PZT quality. Using fresh PZT
(less than three months old) also could help to avoid crack.
PZT annealing temperature needed to be set above 650 'C in order to form good
perovskite structures. Unlike rapid thermal anneal (RTA), increasing annealing time in
furnace anneal would not improve crystal formation. However, hillocks were formed at
temperature equal to or greater than 650 'C. PZT without cracks or hillocks were formed
at temperature less than or equal to 625 C (Figure 5-5). However, a large amount of
perovskite phase PZT was not formed at this temperature.
Other parameters were also adjusted, but showing very little impact in perovskite
PZT formation. To increase the number of perovskite seed sites, PZT pyrolysis time and
temperature were increased to 5 min at 150 'C and 5 min at 400 'C, before the 625 'C
anneal. PZT crystallization was not improved, and hillock formations occurred in some of
the samples. Spin-coating PT solgel as seed layer prior to PZT layer also did not improve
perovskite formation.
Figure 5-6 illustrated an XRD pattern of PZT annealed at 625 'C. Perovskite
PZTs were formed, but a high density of pyrochlore PZT still remained, as shown by
peaking at 290. PZT was poled, and a P-V hysteresis curve was constructed (Figure 5-7).
The spontaneous polarization (Ps) and remanent polarization (Pr) were 40 pC/cm2 and
9.55 pC/cm 2 . Pr/PS of 24% was achieved, and it was half of that obtained in previous
fabrications [22]. A better quality PZT would need to be fabricated in order to produce
PMPG with sufficient power output.
Other fabrication parameters could also be investigated to improve PZT quality.
The ramp up rate of furnace should be characterized. Temperature overshoot could cause
PZT to be oxidized at the required 650 'C. The exact furnace temperature should be
measured. Lastly, the hot plate pyrolysis temperature should be checked. During
pyrolysis, wafer was placed on a metal plate on top of the hot plate. As a result, the metal
plate created a temperature difference between the hot plate and the wafer. However, the
most effective way was to removed the PECVD oxide which caused hillock formation.
To do so, SiNx was removed in the second recipe, and thermal oxide was grown instead.
91
Figure 5-5: PZT annealed at 625 'C.
PZT <011>
Pyrochlore
0
PZT <111>
PZT <1 12>
PZT <102>
PZT <001>
PZT <002>
20
25
30
35
45
40
50
55
60
65
Two-Theta (Degree)
Figure 5-6: XRD pattern of PZT on ZrO2 /SiO 2 (PECVD)/SiNx/Si with PZT anneal at 625
Oc.
92
E
0
0
S-2 )0
-150
-100
50
,R)
100
150
2 10
0U
I.
*
+
..
+*
-20 -
-30
-40
Applied Voltage (Volt)
Figure 5-7: P-V hysteresis curve of PZT. The spontaneous polarization (Ps) and remanent
polarization (Pr) were 40 ptC/cm 2 and 9.55 pC/cm2
5.6 Fabrication Result: PZT on ZrO 2 /SiO 2 (thermal)/Si
To obtain higher quality PZT, it was fabricated on ZrO 2 /SiO 2 (thermal)/Si
substrate. Type 2 device was designed to use Si substrate to increase effective mass.
Since bowing was no longer an issue, nitride layer was omitted. Thermal oxide was used
instead of PECVD oxide, and hillock problem was completely eliminated from PZT
(Figure 5-8). PZT was fabricated using 650 'C annealing temperature. Minor cracks
occurred near the edge of the wafer in some samples. Since the cracks were away from
device region, they would not affect the PMPG structures. On the other hand, using
fresher ZrO 2 might help to solve the problem.
Figure 5-9 is an SEM image of PZT grain structures. Grain size of 4 to 10 tm was
achieved. XRD pattern showed perovskite formations with peakings at corresponding
angles. The intensity of pyrochlore at 290 was greatly reduced.
93
Figure 5-8: PZT annealed at 650 'C.
Figure 5-9: SEM image of PZT on ZrO 2 /SiO 2 (thermal)/Si.
94
PZT <011>
Pyrochlore
PZT <111>
PZT <1 12>
PZT <001>
PZT <002>
20
25
30
35
40
45
PZT <1 02>
50
55
60
65
Two-Theta (degree)
Figure 5-10: XRD pattern of PZT on ZrO 2/SiO 2 (thermal)/SiNx/Si with PZT anneal at
650 "C.
95
Chapter 6
Conclusions
6.1 Contributions
The system architecture of a self-powered wireless sensor node has been
developed, and was tested for temperature sensing application. PMPG and a power
management module were designed to satisfy sensor node's power requirement. An
electro-mechanical model of PMPG was developed to maximize power output.
2 nd
generation PMPG was designed to provide 0.173 mW power at 3 V DC with a natural
frequency of 155.5 Hz. The power management module was developed to provide ACDC rectification, energy storage, and active switching between PMPG and application
circuit. Lastly, a test bed was developed, which mimicked that of a liquid gas pipe used in
the Alaska where the PMPG device will be used to generate power for temperature
sensors.
The first part of this work was dedicated to an evaluation of available ultra-low
power electronics for constructing low power wireless sensor node. A commercial
product Telos provided the necessary sensor components, and was used in this project for
testing purposes. The mote was programmed with nesC temperature sensor application
with minimum power consumption and operation time. An evaluation of the sensor mote
illustrated an operation time of 221 ms and maximum current consumption of 20 mA.
Telos power specification was evaluated and used to design a power management module.
96
A power management module was implemented to manage power flow from the
power source PMPG to sensor node. Since PMPG could not provide sufficient
instantaneous power, a switching scheme was developed to first store energy in an energy
storage reservoir, before transferring it to the sensor node. Different energy storage
reservoirs
were investigated,
and recommendations
were provided. The power
management module was tested with Telos and power source, and the temperature
sensing application functioned as expected.
An electro-mechanical model of PMPG was developed to maximize power output.
In order to provide sufficient power of ~0.1 mW at 150 Hz, a large effective mass was
required. Type 2 PMPG was designed to satisfy such power requirement. Effective mass
was increased by including Si substrate in the cantilever beam.
PMPG
fabrication
Zr0 2/SiO 2 (PECVD)/SiNx/Si
recipe
and
was
developed.
ZrO2/SiO 2(thermal)/Si
PZT
were
fabrications
studied
in
on
detail.
Parameters such as annealing and pyrolysis conditions were thoroughly investigated. PZT
cracks were eliminated by increasing ZrO2 annealing time, and hillock problem were
removed by using thermal oxide. PZT with low density of pyrochlore structure was
achieved.
6.2 Recommendations for Future Works
There are many improvements that can be made in designing future generation
self-powered wireless sensor systems. For testing purposes, Telos was used for sensor
applications. For next generation autonomous sensor node, microcontrollers as well as
other low power sensor components can be integrated with PMPG to create a board level
system design. By integrating it with additional sensors, this system can also be applied
to other sensing applications such as pressure and acceleration. In addition, one can also
incorporate the receiver function of the RF transceiver on the autonomous sensor node, to
develop a more complex wireless sensor network.
Type 2 PMPG was designed with millimeter scale length to achieve low
resonance frequency. To reduce the size of PMPG, other designs should also be
97
considered. One suggested design folds the cantilever into a spiral structure [33].
However, twisting mode can occur and reduce PMPG power output. To solve such
problem, a double beam sandwich can be used instead. By having two beams supporting
the proof mass at both ends, the primary mode resonance occurs vertically and the
horizontal and twisting modes of vibration can be effectively suppressed.
(a)
(b)
(c)
Figure 6-1: Serpentine structures: (a) Basic concept, (b) double beam sandwich, (c)
tethered structure.
Type 2 PMPG used silicon substrate to increase effective mass. To reduce
thickness, a heavy proof mass should be used instead. Patterning metal as proof mass on
PZT can be an alternative to increase PMPG effective mass. However, techniques for
fabricating tm thick metal on PZT must be developed first.
Power management module can also be improved. Using MEMS switch instead
of CMOS switch can reduce power consumption and provide an ease of integration with
PMPG. An array of PMPG can be used if a higher power output is required. Using
multiple PMPG can reduce charging time and increase measurement rate. An auxiliary
energy source mechanism should also be implemented. In an emergency situation such as
pipeline overheating, a large amount of sensor data will be required, and additional power
to support data measurement will be needed. A control circuitry that monitors deviation
from the average in sensor data, will activate a backup power source and provide energy
in this scenario.
98
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Energy Scavenger, MIT Master Thesis, 2003.
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Transmit Beacon using Environmentally Scavenged Energy," ISLPED, 2003.
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102
Appendix A
nesC Codes
Tpmpg.nc
/* *
*
Author: YuXin Xia
*/
includes TpmpgMsg;
configuration Tpmpg { }
implementation
{
components Main
,TpmpgM
, TimerC
GenericComm as Comm
HumidityC
#ifdef
AVR
, HPLPowerManagementM as PM
#endif
#ifdef
AVR
CountSleepRadioM.PowerManagement -> PM;
CountSleepRadioM.Enable -> PM.Enable;
#endif
Main.StdControl
Main.StdControl
Main.StdControl
TpmpgM.Timer
TimerC;
Comm;
-> TpmpgM;
->
->
->
TimerC.Timer[unique("Timer")];
103
TpmpgM.HumidityControl -> HumidityC;
//HumidityC controls both temperature and humidity measurement.
TpmpgM.Temperature -> HumidityC.Temperature;
//Call only the temperature component of humidity.
TpmpgM.TemperatureError -> HumidityC.TemperatureError;
TpmpgM.CommControl -> Comm;
TpmpgM.SendMsg -> Comm.SendMsg[AMTPMPGMSG];
}
TpmpgM.nc
/* *
*
Author: YuXin Xia
*/
//Successfully put sensor node to sleep using CommControl.stop.
//Successfully allowed 220 ms operation time and 1000 seconds in between
measurements.
//CHANGE IN HUMIDITYM: when Humidity.start, it draw 20mA of current for 80 ms.
HumidityM is changed so it only draws for 2ms.
//The data length is fixed to only leave with 2 remaining digit. This works with the new
oscope (sf.jar and oscope.jar).
includes Timer;
module TpmpgM
{
provides interface StdControl;
uses {
interface Timer;
interface SplitControl as HumidityControl;
//This calls the HumidityC though Tpmpg.
interface StdControl as CommControl;
interface ADC as Temperature;
interface SendMsg;
104
interface ADCError as TemperatureError;
}
#ifdef AVR
uses interface PowerManagement;
uses command resultt Enableo;
#endif
}
implementation
{
TOSMsg mmsg;
bool start;
command resultt StdControl.initO {
start=FALSE;
#ifdef AVR
call EnableO;
call PowerManagement.adjustPowero;
#endif
call CommControl.inito; //doesnt really matter.
return SUCCESS;
}
command resultt StdControl.starto
{
call HumidityControl.starto;
return SUCCESS;
}
command resultt StdControl.stopo {
return SUCCESS;
}
event resultt HumidityControl.initDoneo {
return SUCCESS;
}
event resultt HumidityControl.startDoneo {
call CommControl.stopo;
call Timer.start(TIMERONESHOT, 1);
105
//triggertimer fired and starts measuring data.
return SUCCESS;
}
event resultt HumidityControl.stopDoneo {
call TemperatureError.disableo;
}
event resultt Timer.firedo {
if (start==FALSE)
{
call Temperature.getDatao;
}
else if (start==TRUE)
{
call SendMsg.send(TOSBCASTADDR, 2, &m msg);
//2 is the number of significant digit, after 10 fixed digits.
}
}
async event resultt Temperature.dataReady(uintl 6t data) {
* ((uint 16t*)m msg.data)=data;
start=TRUE;
call CommControl.starto;
call Timer.start(TIMERONESHOT, 1);
//I ms is required so the data is send after CommControl starts.
return SUCCESS;
I
event result t TemperatureError.error(uint8_t token) {
return SUCCESS;
I
event resultt SendMsg.sendDone( TOSMsgPtr msg, result t success)
{
start=FALSE;
call Timer.start(TIMERONESHOT, 1000);
//sleeping time.
call CommControl.stopo;
//Crucial! so the radio actually turns off.
return SUCCESS;
}
}
106
TpmpgMsg.h
enum
{
OSCOPEBUFFER_SIZE=1,
AMTPMPGMSG=10,
};
typedef struct TpmpgMsg
{
uintI6_t n;
uintI6_t src;
uintl 6_t data[OSCOPEBUFFER SIZE];
} TpmpgMsgt;
107
Appendix B
Matlab Code for PMPG Sizing
% Calculation sequence for PMPG.
% Equations come from Noel's Design Considerations for MEMs-Scale.
% Mechanical damping coefficient
deltam = 0.001;
% Amplitude of vibration (m/s^2%)
acceleration = 5;
% Physical size - unit: meter
Length = 52000E-6; %set length to fix natural frequency
Width = 3000E-6;
% Different layers thicknesses
PiezoT = 0.5E-6;
Si T = 525E-6;
SiO2_T = 0.00001E-6;
ZrO2_T = 0.00005E-6;
Pt_T = 0.2E-6;
% Electrode spacing
E_Pitch = 8E-6;
E_Width = 4E-6;
% Material property
% Densities... silicon, pzt, pt, etc
Rho_s= 2300;
Rhop =7750;
Rhopt = 21440;
RhoSiO2 =2300;
RhoZrO2
=
6000;
108
%Young's Modulus
C33s = 47E9;
C33p = 64E9;
C33pt = 168E9;
C33SiO2
=
69E9;
C33ZrO2 = 244E9;
d33 = 200E-12;
e33
=
C33p*d33;
eps33s = 1200*8.85E-12;
x = sym('x');
z = sym('z');
%NeutralPlane
temp=int(z*C33s,z,-x,SiT-x)+int(z*C33SiO2,z,SiT-xSi_Tx+SiO2_T)+int(z*C33ZrO2,z,SiT-x+SiO2_TSiT-
x+SiO2 _T+ZrO2 T)+int(z*C33p zSiT-x+SiO2 T+ZrO2_TSiTx+SiO2_T+ZrO2_T+PiezoT)+int(z*C33pt/O,z,SiTx+SiO2_T+ZrO2_T+Piezo_TSiT-x+Si02_T+ZrO2_T+PiezoT+PtT);
NeutralPlane=eval(solve(temp, x));
%El number for the structure (stiffness)
ElBeam = 2/3*Width*(((SiTNeutralPlane)^3+NeutralPlaneA3)*C33s+(PiezoT+SiTNeutralPlane)^3*C33p+(PtT+Piezo_T+SiT-NeutralPlane)A3*C33pt/10);
% Proof mass, set to zero since it is not used
M_Proof= Oe-5;
R_load = sym('R load');
w_input = sym('w_input');
% Defines mode shapes for energy method
Phi r = x^2*(x-3*Length)/(6*EIBeam);
N_r = (x-Length)/(EIBeam);
% Input force equation
B_f = eval((Rhos*SiT+Rho-p*Piezo_T+Rhopt*PtT) * Width * int(Phi_r,x,0,Length)
+ LengthA2*(-2*Length)*MProof/(6*EIBeam));
% Mass equation
M_Si = Width*int(int(Phi_r^2*Rho_s,z,-
SiT,O),x,0,Length)+MProof*(4*LengthA6/36/ElBeam^2);
MPZT = Width*int(int(Phi rA2*Rhop,z,OPiezo_T),x,0,Length);
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M_Pt = Width*int(int(Phi_rA2*Rhopt,z,Piezo_TPiezoT+PtT/2),x,O,Length);
M_s=M_Si + M_Pt;
M_p = MPZT;
M_total = eval(M_s + M_p);
% Stiffness equation
K_s = Width*int(int((z*Nr)2*C33s,z,-Neutral_Plane,SiT-Neutral_Plane),x,O,Length);
K-p = Width*int(int((z*N r)A2*C33p,z,Si_T-Neutral_PlaneSiTNeutralPlane+PiezoT),x,O,Length);
K_pt = Width*int(int((z*Nr)A2*C33pt/10,z,Si_T-NeutralPlane+PiezoT,SiTNeutralPlane+PiezoT+PtT),x,O,Length);
K_total_Ori = eval(K s + K-p + Kpt);
% Electrical & Piezoelectric equation
Theta_p_Ori = eval(Width*int(int(-(z*(x-Length)/EIBeam)*e33/(EPitchEWidth),z,SiT-NeutralPlane,SiT-NeutralPlane+PiezoT),x,O,Length));
Capp = eval(Width*int(int((1/(EPitch-EWidth)A2*eps33s),z,0,PiezoT),x,O,Length));
Coupling2_Ori = (Theta_p_Ori)A2/(K-totalOri)/Capp;
w_n = ((K total_Ori)/M total)AO.5;
%w_n_n = 839.5888;
w_anti = w_n*(1+(Coupling2_Ori))O.5;
% Assuming that the cantilever will match the frequency (input freq=resonance freq)
Omega = 1;
% RESONANCE FREQUENCY
f_n = (w-n)/2/pi;
% Define optimal resistance
R_optimal = ((OmegaA4+(4*delta mA2-2)*OmegaA2+1)/(OmegaA6+(4*delta_mA2-
2*(1 +(ThetapOri^2/K_totalOri/Cap_p)))*Omega^4+(1 +(Theta-pOriA2/K-totalOri/
Capp))2*OmegaA2))A0.5;
R_sys = sym('R.sys');
% Maximum power & displacement using optimal resistance
Poutperacc2 =
((Bf)*(Theta_p_Ori)*Omega/(Ktotal_Ori))2*(w n)*R-optimal/Capp/(( 1(1+2*deltam*R optimal)*OmegaA2)A2+((2*delta m+(1+(Theta_p_OriA2/(K totalOri
*Cap_p)))*R-optimal)*Omega-R-optimal*OegaA3)A2);
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% POWER
POWER=Poutper-acc2*accelerationA2;
x=Length, Dispper acc =
eval(Phi r*(Bf)/(K-totalOri)*(1 +((Roptimal)*Omega)A2)AO.5/((1(1+2*deltam*(R optimal))*OmegaA2)A2+((2*delta m+(1+(Theta_p_Ori)A2/((K total_
Ori)*Capp))*(Roptimal))*Omega-(Roptimal)*Omega^3)2)AO.5);
%should stay constant as we change width. To get final result, set x=length.
% DISPLACEMENT
Disp=Dispperacc* acceleration;
Disp_per-accl = (Bf)/(K totalOri)*(1 +((Roptimal)*Omega)A2)AO.5/((1(1+2*deltam*(R optimal))*OmegaA2)A2+((2*delta±m+(1+(Theta-pOri)A2/((K
Ori)*Capp))*(Roptimal))*Omega-(R-optimal)*OmegaA3)A2)A0.5;
total_
%tip displacement without N r. needed for Strain calculation.
x=O, Strainper ace = eval(Nr * (SiT-NeutralPlane+PiezoT) * Disp-per-ace_1);
%set x in n r and phi-r =0.
% STRAIN
STRAIN=Strainperacc * acceleration;
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