Topics in Self-Powered
Controllers
Special Emphasis on Push Buttons and Switches for
Industrial Applications
Jason E. Harmon
A Seminar submitted to the Faculty of Rensselaer at Hartford in partial fulfillment
of the requirements for the Degree of MASTER of Science
Major Subject: Engineering Science
The original of the seminar is on file at the Rensselaer at Hartford Library.
Approved by Seminar Advisor Ernesto Gutierrez-Miravete
Rensselaer at Hartford
Hartford, CT
April, 2003
Contents
1
2
3
4
5
6
7
Abstract………………………………………………..3
Historical Background………………………………...3
Theory of Self-Powered Control………………………4
3.1
Solar Self-Power………………4
3.2
Mechanical Self-Power………..6
3.3
Piezoelectric Self-Power………7
3.4
Critical Characteristics………..7
Applications of Self-Powered Control………………..9
Developments in Self-Powered Control……………...12
5.1
Recent Developments………...12
5.2
Possible Future Applications…14
Conclusion……………………………………………16
References……………………………………………17
List of Figures
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Figure 9
Figure 10
Figure 11
Semiconductors in a photovoltaic cell………….………5
Self-winding watch…………………………………….6
Piezoelectric effect…………………………………….7
User interface of a control panel……………….………10
View of push button controls………………………….10
Doorway of control panel………………….………….11
Hardwired 3-button control panel………….………….11
Power harvesting shoe inserts from MIT Media Lab…13
Power output from piezoelectric shoe generators…….13
Prototype of self-powered pushbutton control………..13
Various pushbuttons………………………………….16
List of Tables
Tables 1 & 2 Summary information on current self-powered technologies…..8, 9
2
1
Abstract
Traditional control signals travel from a controller to a device using energy
derived from an outside electrical energy source that employs conductive material to
carry the current, or from a battery integrated within the electronic circuitry of the
controller. Both of these energy sources have disadvantages. The major disadvantage of
an outside energy source is that it requires wires to transfer the energy, therefore is not
easily mobile. The battery is mobile, but has the disadvantage of a finite life, thereby
requiring replacement.
Certain devices have been able to overcome both of these disadvantages by using
other means to generate the energy required to operate. Devices such as self-winding
watches and acoustic TV remote controls are able to generate the energy necessary to
operate or send signals without outside electric sources or batteries. The movement
necessary to actuate the device generates their operating energy. Researchers and
engineers have started to apply this concept to switch and push-button devices. The
energy the operator uses to push the button or flick the switch is transformed to create the
energy necessary to send the signal to the down stream device.
This paper elaborates on the history, theory, applications, and recent
developments of such devices. A historical background is given to describe the
progression and this technology, as well as the development of the need for this
technology. Theory is discussed concentrating on the three main types of self-powering
techniques: solar, mechanical, and piezoelectric. Several possible industrial applications
are discussed. Recent developments centered on power harvesting, power scavenging,
and piezoelectric applications are explored before wrapping up with final conclusions and
a look to the future.
2
Historical Background
Since the Industrial Revolution, the industrialized world has gone through several
stages of technological evolution. As technologies advanced, the dominate technical
trends touching every household have progressed from mechanical machines, to electrical
machines, to electronic machines, and finally to computerized (microprocessor
controlled) machines. Since the late nineteenth century when the first electrical
distribution systems were developed, electric power developed into the primary driving
force behind all devices in the factory, home, and office. Thomas Alva Edison was an
early pioneer of electrical distribution. He developed the first DC power system at the
Pearl Street Station in New York in 1882. A few years later, George Westinghouse and
William Stanley pioneered an early single phase AC distribution system in Great
Barrington, Massachusetts. Eventually AC distribution was determined to be superior to
DC distribution. The first three-phase AC transmission line in the US was developed in
1893 in Southern California. This was the start of electric energy distribution as we
know it today. On the residential level, little has changed in the fundamental concepts of
energy distribution since that time. In the home, energy is still transferred using wires.
The main user interface for most hard-wired electrically controlled devices in the home is
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the hard-wired switch. Protection and safety features such as fuses and circuit breakers
were developed in the early and mid-twentieth century to protect the wires in the
residence from causing fires.
For non-hard wired applications in the home, the battery is king. Typically a
lithium-based battery is used to power household devices. Rechargeable types may come
in Nickel or other material bases. Probably one of the most commonly used batterypowered, non-hard wired devices in a typical US home is the television remote control.
The first commercially available television remote control was introduced by the Zenith
in the early 1950’s. In 1957 Zenith released a television remote control that worked
using ultrasonic waves. Although this remote control was self-powered, it eventually
gave way to the battery operated infrared remote control. Since then, the battery has
remained the dominant energy source for mobile control applications in the home.
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Theory of Self-Powered Control
Self-powered devices derive the power they need to operate on their own.
Generally, they use the environment or operating conditions around them to generate
power to operate. A solar calculator is an example of such a device. It uses a
photovoltaic cell attached to the calculator to generate electric energy. However, its
operation is limited to environments with light. This would not be acceptable for many
control applications.
A self-winding watch is another example of a self-powered machine. A selfwinding watch is an interesting mechanical device that generates energy from the
movements of the arm of the person wearing the watch. This device relies on an
assumption regarding the manner in which the watch will be used.
Both of these devices operate without power being generated from outside
sources such as batteries or “plug-in” electricity. Both make assumptions regarding the
manner and environment in which the devices are to operate. The ability to make
assumptions regarding how the device will be used seems to be a common theme in selfpowered devices.
There are typically three general technologies that are used in self-powered
devices. Each has pros and cons.
3.1
Solar Self-Power
The solar powered calculator may be the most common self-powered device used
today. Most rely solely on photovoltaic cells to transform photon light energy to electric
current without the use of battery storage. Photovoltaic cells use semiconductor
materials. The most common material used is silicon. Light actually contains waves of
several different wavelengths. When light of the correct wavelength hits the cell some of
the energy from the photons is absorbed within the semiconductor. This energy knocks
electrons loose within the semiconductor. The electrons then move from one side of the
semiconductor to the other in search of a hole, or free space, in which to reside. The
electric field created by the junction of the two sides of the semiconductor creates a
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voltage to move the electrons in a specific direction. This movement creates an electric
current. This current and voltage from the electric fields creates the power needed to run
the calculator.
Figure 1: Semiconductors in a photovoltaic cell
Wireless self-powered systems using photovoltaic cells have been developed in
the past. Most of the solar self-powered designs have been made to power small
electronic devices of low power consumption, such as infrared or RF remote control
systems.
On a sunny day a photovoltaic cell of only 2 cm^2 in diameter can generate 1
mWh of energy. This number drops to about .2 mWh for cloudy or inside conditions (E
= 1000 lx). To operate a TV remote control requires approximately 60 mW. Assuming
that the remote control is operated 60 times each day for a period of 1 second (remote
would be operated a daily total of 1 minute), this means that the total daily energy to
operate the TV remote control would be 1 mWh (60 mW for 1 operation * 60 operations
of 1 second each = 60 mWminutes. 60 mWminutes = 1 mWh). Since this amount of
power could only be obtained on sunny days, this would limit the applications for a selfpowered solar system. Typical push buttons have a button surface of approximately 5
cm^2. Therefore, a typical push button with a photovoltaic cell mounted on the face of
the button could produce about .4 mWh in cloudy conditions. This would still not be
enough to power the TV remote control under most conditions. Generally, 6 photovoltaic
cells of 2 cm^2 connected in series would be recommended for an application such as a
solar powered TV remote control. Such a circuit configuration using batteries and
accumulators offers efficiencies of approximately 60 to 65%. This space may be
available in some switching applications (perhaps on the face mounting plate of a
standard residential light switch), but this space requirement would certainly limit
applications at this time.
Space constraints will continue to be a limiting factor to photovoltaic selfpowered systems until the efficiencies of photovoltaic cells can be increased. The trend
is positive. Typical photovoltaic cell efficiencies have risen from about 4% in the 1950’s
to efficiencies approaching 20% today. Efficiencies around 15% are most typical. The
world record holder for photovoltaic cell efficiency is presently 24.5%.
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Solar PV cells provide inexpensive power to operate the device, but their obvious
disadvantage is that they can not be used in the absence of light. Battery storage is
possible, but it is typically not used in this application because battery storage adds
considerable cost and size to the system. Because of this rather significant disadvantage,
photovoltaic cells can not be used to power the majority of household wireless devices.
3.2
Mechanical Self-Power
Mechanical self-power typically relies on the storage of kinetic energy in the form
of potential energy, then releasing it at the appropriate time. Taking advantage of
mechanical springs, gears, and levers is a common way to achieve this. The self-winding
watch is a great example of this at work.
Swing arm
Figure 2: Self-winding watch
The semi-circular swing arm shown in Figure 2 is the key to the self-winding
watch. This arm moves from gravity as the watch wearers arm moves. This swing arm is
attached to a gear train. The gear train is like a mechanical transformer. It transforms the
movement of the swing arm into the slow gear movement that is needed to actuate the
hands on the watch. Each movement of the swing arm winds a spring. This spring stores
the energy needed to keep the watch operating even when the swing arm is stationary.
Typical self-winding watches can operate for about a day and a half without any
movement.
The self-winding watch is a clever device that accounts for the environment in
which the watch is used to scavenge some energy to operate. The obvious disadvantage
is that if the watch were to be left stationary for a period of time it would no longer have
the stored energy necessary to operate.
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3.3
Piezoelectric Self-Power
One of the more recognizable piezoelectric self-powered devices in the home is
the lighter in a gas BBQ. The gas grill lighter uses a spring-loaded button to hammer a
piece of piezoelectric material to create a high voltage, low current spark. This spark is
similar to the static electric spark after rubbing your feet on a rug. Quartz, Rochelle salt,
and PZT ceramic (lead-zircononate-titanate) are common piezoelectric materials.
Pierre and Jacques Curie discovered the piezoelectric effect in 1880.
Piezoelectric materials become electrically charged when subjected to a mechanical
stress. Conversely, when a voltage is applied to the material, the piezoelectric material
can deflect. If an electric oscillation is applied to a piezoelectric material the material can
respond with a vibration. The piezoelectric effect in quartz is very stable. This is the
reason quartz is typically chosen as the time signal element in watches.
Figure 3: Piezoelectric effect. One the left, applied strain creates a voltage. On
the right, applied voltage creates a displacement.
Piezoelectric materials are used in applications such as clocks, timers, speakers,
microphones, sensors, oscillators. Because of the high voltage, low current nature of
piezoelectricity, harvesting this energy to power electric devices can be difficult.
Piezoelectric material-based devices mounted into the sole of shoes have been able to
scavenge up to 1 W of energy.
3.4
Developing Critical Characteristics of Self-Powered Systems, An Example
Knowing the pros and cons of the three most common general technologies for
self-powered systems; how would one chose a technology for a particular application?
This would depend very much on the application. Recent efforts in self-powered control
seem to have two goals that represent the pinnacle of self-powered utopia. The first
primary goal is to be able to power a stripped down lap top computer solely from selfpower, power harvesting, and/or energy scavenging devices. The second goal is to be
able to create a battery-less/wireless television remote control. The energy needed to
power a lap top computer is significantly more than the energy needed to power a TV
remote control. Therefore, the quest for self-powered lap top computing involves a wide
range of different devices still in the infancy stages from a technical standpoint.
However, achieving the goal of a self-powered TV remote control is within the grasp of
existing technology.
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The TV remote control is an exciting pursuit for self-powered technology
researchers because it is a device nearly everyone uses. Not only does nearly everyone
use it, but also nearly everyone enjoys using it! In concept, the TV remote control is a
multi-function switch. It sends a signal to a powered device to complete a particular
command. Although the technologies are significantly different, this is essentially the
same concept as a light switch sending a signal to a light to turn ON or OFF. The
difference is that the light switch is simply “releasing the dam” on energy that is already
there to for the purpose of powering the final device, while the TV remote control must
generate energy from some outside source (a battery) to send a signal. The self-powered
TV remote control would ideally send a signal as effortlessly as a light switch.
In order to make a self-powered TV remote control one must consider what
characteristics would make such a device successful, and what technologies would be
best suited to achieve those characteristics. Table 1 shows some of the critical to quality
characteristics for self-powered systems. The characteristics in this table apply very well
to the TV remote control. An engineer would wish the device to have low power
consumption requirements and enough space to fit all necessary components. However,
the key factor for self-powered systems is not necessarily the device, but how the device
is used. The TV remote control is not generally used in a high inertia environment like a
wristwatch might be. Therefore, mechanical systems would probably not be suitable.
Solar systems can quickly be eliminated for the obvious reason that light is required at all
times. This leaves the engineer left with piezoelectric systems to consider. Most of the
recent developments in self-powered switching control devices have taken advantage of
piezoelectric technology. This technology is a natural fit to switch and push button
applications because the force used to depress the button or flip the switch can be used to
create the piezoelectric signal. Couple that with a spring loaded “booster” and a switch
can fully take advantage of piezoelectric technology. This is especially true considering
many industrial switches contain very large springs with over-centering mechanism
features.
Researchers estimate that they are less than 2 years away from producing a
commercially viable self-powered TV remote control utilizing piezoelectricity.
The key point is that self-power researchers must not only concentrate on the
operational characteristics of a particular device, but also the way the user uses the
device. This knowledge of the way in which the device is used is generally what alloys
the developer to take advantage of self-power opportunities.
Critical to Quality Characteristics for Self-Powered System Materials
General CTQs
Customer CTQs
Engineering CTQs
Reliability
Ease of Use
Size to power ratio
Size
Installation flexibility
Life span, in time and operations
Cost
Backwards compatible
Able to operate in variety of environments
Safety
Susceptibility to interference signals
Low volume, not loud
Efficiency
Tables 1 & 2: Summary information on current self-powered technologies
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Technology
Solar System
Individual Solar Cell
Piezoelectric
Mechanical
4
Efficiencies of Self-Powered Systems
Approximate Percent Efficiency
Notes
60-65
Several cells circuited together
15
.5-15
Mechanical to electrical energy
70
Friction is key player
Applications of Self-Powered Control
The progression of electronic devices into the homes of the future continues.
Devices like cell phones, electronic organizers, and DVD-players are just a few of the
devices that have become standard equipment in many lives. People continue to add an
array of home electronics and smart devices to the already extensive list of electronic
devices for the home. The advancement of electronic devices is not limited to the
residential environment. Just like in the home, industrial and commercial entities are
continually finding ways to use electronics to the advantage of their bottom lines. As
electronic devices continue to grow and evolve, the control of these devices has generally
remained constant; provided by either hardwired user interfaces or battery powered
remote controls. Mobility and flexibility are limited with hardwired devices, while
batteries require periodic replacement. Self-powered controllers have the potential to
address both of these drawbacks, as well as address the expense associated with wiring or
periodic battery replacement.
Self-powered control could save significant labor and costs associated with
hardwired devices. Figure 4 shows a hardwired user interface. The user
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Figure 4: User interface of a control panel for industrial generator motor
interface shown is a control panel for an industrial generator motor. This interface uses
individually wired push buttons to transmit control signals. Figure 5 shows a close up of
the push button controls. Since each pushbutton is wired individually, a substantial
Figure 5: View of push button controls
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amount of wiring is required to operate this user interface. Figure 6 shows a view of just
one doorway of the generator control panel shown in figures 4 and 5. One could well
Figure 6: One doorway showing wiring of a industrial generator control panel.
imagine the expense and effort required to wire this type of control interface. Ignoring
the many application details and concentrating on the concept, a significant amount of
this wiring could possibly be eliminated if a self-powered wireless controller could send a
control signal through air (RF, infrared, acoustic, etc.) to a central receiver. That is not to
say that self-powered wireless controllers would only be beneficial in such advanced
industrial applications. Even relatively simple hardwired interfaces could benefit from
development of self-powered wireless controls. Figure 7 shows a simple 3-button control
panel. Each push button used in this simple control panel uses a minimum of 3 wires.
Figure 7: Hardwired 3 button control panel
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Self-powered wireless control could possibly eliminate these wires. Additionally, a
signal receiver could take on much of the work that the contact relay is presently
performing in this control panel.
So far, industrial and commercial/industrial applications have been explored.
However, perhaps the most exciting applications for self-powered wireless controllers are
in residential applications. Imagine a TV remote control operating without batteries, or a
keyless entry remote for an automobile that never required battery replacement. Imagine
a house in which the light switch were wireless. Central receivers could be placed at
specified locations throughout the home. Since the switch would be wireless only the
load device would need to be hardwired. This means that the switch wires no longer
need to be protected. This would eliminate not only the need for wiring, but also the
need for circuit breakers or fuses for that portion of the home electrical system. Light
switches would no longer need to be stationary. Since they are not held immobile by the
hardwiring requirement, switches could be moved from wall to wall of the home at will.
The possibilities are limited only by the imagination.
5
Developments In Self-Powered Control
5.1
Recent Developments
Piezoelectric energy generation has been the focus of much of the recent
developments in self-powered control. The continued growth of portable electronic
“gadgets” (lap top computers, electronic organizers, cell phones, beepers, digital cameras
etc.) has driven the need for supplemental energy generation. The ground work for
piezoelectric control was laid by researchers taking advantage of piezoelectric materials
for energy “scavenging” or “parasitic power harvesting”. Energy scavenging is the
concept of using normal body movements like walking, breathing, arm motion, and
typing to generate energy to run personal electronic devices. Piezoelectric shoe inserts
have been invented that generate energy while walking. This energy is used to
compliment battery energy to run personal electronic devices. This work in piezoelectric
energy harvesting laid the groundwork for developments in self-powered control.
Some of the more exciting developments in self-powered control have taken place
at the Massachusetts Institute of Technology Media Lab in Cambridge, Massachusetts.
The MIT Media Lab is devoted to “the study, invention, and creative use of digital
technologies to enhance the ways that people think, express, and communicate ideas, and
explore new scientific frontiers.” This lab produced most of the recent technology
advances in both parasitic power and self-powered control. Parasitic power research has
been done using piezoelectric inserts in shoes to try to harvest the power generated from
a foot strike during walking. The final goal of this research is to be able to scavenge
enough power from normal human movement to power a lab top computer. Other
applications that have been explored would be a self-powered RF tag system. In this
system, the power-generating shoe would activate a RF transmitter to send signals while
a person walked. This signal could be used to alert an area of the wearer’s presence.
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This would allow the movement of the wearer to be tracked throughout a building. It
could also give a particular area of a building advanced notice of the arrival of the
wearer. This would allow lights to be turned on, doors opened, etc. in preparation for the
arrival of the wearer. Figure 9 shows the power output capabilities of piezoelectric shoe
inserts. A typical RF device requires about 60 mW of power to operate. Figure 9 shows
that piezoelectric shoe inserts are certainly capable of achieving such power outputs.
Figure 8: Power harvesting shoe inserts from MIT Media Lab
Figure 9: Power output from piezoelectric shoe generators
In April 2002 researchers Joseph Paradiso and Mark Feldmeier developed a
prototype battery-less controller. The researchers modified a fireplace/cigarette lighter
and an automobile keyless entry remote to create a self-powered device that can transmit
digital codes at a distance of more than 20 meters. A picture of the prototype device is
shown below in figure 8. The researches have visions
Figure 10: Prototype of self-powered automobile keyless entry remote from MIT
Media Lab
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that such a device could eliminate the need for batteries in devices like TV remote
controls. Although only in the prototype stage, the researchers are attempting to have
commercial products using this technology on the market within 2 years. If successful,
this technology has the potential to have significant impact on control of electronic
devices.
The self-powered device uses a spring-loaded pushbutton to strike the
piezoelectric material. To get maximum energy output from the piezoelectric material,
the material is allowed to vibrate at the resonate frequency. This produces a high voltage,
low current electric pulse. A step-down electronic circuit is used to transform this
electric energy to a usable form. The RF automobile keyless entry device requires
approximately 7.5 mW at 3 Volts for 20 ms to create a 12-bit signal. This totals to an
energy requirement of about 150 microJoules. The prototype device produces about .5
mJ of energy per button push. This is more than enough energy to power the RF device.
The prototype was created for less than $5 dollars.
The researchers at the MIT Media Lab proved that the concept of self-powered
control could be achieved. Their concept used “recycled” parts just for the purpose of
proving the concept. Refinement of the components could yield even better power
generation, allowing for more features and functions to be added to the device.
5.2
Possible Future Applications
The MIT Media Lab appears to be leading the way on expanding the concept of
self-powered control to the many commercial battery-powered devices already
commonly used. However, could this same technical concept be expanded to some of the
industrial applications discussed in section 4? There are several issues that researchers
would have to address before the concept of self-powered control could expand to
industrial applications.
Technical Issues:
Most of the industrial devices shown in section 4 use a push-button interface to
send control signals. The MIT Media Lab pushbutton produces 15 N of force. The
typical industrial pushbutton requires a minimum of about 11 N of force to operate.
Therefore, as they presently are designed the standard industrial pushbutton would not be
able to deliver enough force to operate the MIT Media Lab piezoelectric circuit.
However, the MIT Media Lab took advantage of a spring-loaded pushbutton to deliver an
accelerated and very quick force. The traditional industrial pushbutton would most likely
have to be re-designed to incorporate a spring-loaded mechanism for self-powered
applications.
RF seems to be the logical choice for most self-power wireless pushbutton control
at this time. RF does not require a line-of-sight like infrared, and is capable of sending
signals on the range of 50-100 feet in this application. Noise interference, receipt of data
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(rather than transmit-only), array of signals, and security would be just some of the issues
to address with RF technology.
Reliability Issues:
Industrial pushbuttons are typically rated for operations in the millions. 5 million
operations are not out of the ordinary. This number of operations would most certainly
tax the life expectancy of any spring-loaded mechanism.
The reliability of piezoelectric materials also would have to be considered.
Piezoelectric materials can often develop micro cracks that diminish the effectiveness of
the material after time. This issue would have to be addressed in order to move the selfpowered push button technology into industrial applications.
Cost Issues:
Although cost is always an issue, is seems that it would not be a significant
limiting factor for self-powered control. The prototype self-powered controller
developed at the MIT Media Lab was constructed from components costing less than $5.
When considering cost, the actual component cost is not the only factor. A
system cost would have to be considered. Increased component cost could easily be
made up by the reduced wire cost and labor cost of traditional hard-wired pushbutton
interfaces. It is estimated that up to 1/3 of the cost of some industrial switchgear and
motor control centers is for wire and wiring labor. Although this cost would not be
eliminated, it could be considerably reduced with wireless control.
Size Issues:
Size is not usually the limiting factor in industrial applications. Industrial
installations tend to be rather large in size, so there is typically ample space for the wiring
involved in the hard-wired interface. With the small size of modern electronic
components it is reasonable to assume that self-powered controls could be applied to
industrial control applications without much impact to available space. Figure 9 shows
the installation side of several types of pushbutton controls. It shows that some of these
pushbuttons take up a relatively large volume of space on the installed side. Selfpowered controllers would have to consider making their package take up this same
volume, or less.
15
Figure 11: Interface side of several styles of industrial pushbuttons (left).
Installation side of several styles of industrial pushbuttons (right).
Customer Acceptance:
Customer acceptance of the new technology is more of a business issue rather
than a technical or engineering issue, but still an issue that must be addressed. The
industrial market can be adverse to radical technology change. This is true of all markets,
but may be more of an issue in the industrial market where reliability and performance
are often paramount. Industrial control often is responsible for complex machines and
systems. If they go down because of a technical issue, it could cause the loss of
significant amounts of productivity. This is not generally the case in residential
applications. Although the TV remote control breaking is an inconvenience, it is not the
same as having to close an entire factory floor because of non-functioning equipment.
Self-powered control would have to clear this hurdle to get into the industrial market.
6
Conclusion
Self-powered wireless control has some exciting potential. Researchers have
made the first steps towards making self-powered wireless applications a reality.
Piezoelectricity is the preferred energy source for the new self-power wireless control
technologies. Devices such as self-powered TV remote controls may be on the market as
soon as 2 years from now.
Several obstacles are yet to be overcome to utilize self-powered wireless control
technology in industrial applications. Customer acceptance, reliability, and technical
design remain as the largest obstacles to overcome.
Technologies such as MEMS (micro-electrical-mechanical systems) have great
potential to open new doors for self-powered wireless controllers. As MEMS advances
continue, hopes for breakthrough technologies continue in the area of self-powered
control.
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Taking this technology out to the furthest extreme yields the potential for some
high impact concepts. The wireless house or control panel is perhaps the most exciting.
Wireless automobiles are also an exciting concept to consider. Technical universities and
corporate research centers are continuing to progress on self-powered wireless control
technology. It is an emerging field that will certainly be heard from in the coming
decades.
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References
1) Kymisis, J., Kendall, C., Paradiso, J., Gershenfeld, N., “Parasitic Power Harvesting in
Shoes,” Proc. Of the Second IEEE International Conference on Wearable
Computing, (ISWC), IEEE Computer Society Press, October 1998.
2) Marshall, B., “Remote Car Entry,” website, Howstuffworks.com
3) Paradiso, J., Feldmeier, M., “A Compact, Wireless, Self-Powered Pushbutton
Controller,” Responsive Environments Group, MIT Media Laboratory, 2002.
4) “Piezoelectric Materials,” Materials by Design, Article, Cornell University, 1996.
5) Shenck, N., Paradiso, J., “Energy-Scavenging with Shoe-Mounted Piezoelectrics,”
IEEE Micro, Vol. 21, No. 3, May-June 2001, pp.30-42
6) Starner, T., “Piezoelectric Shoe Inserts,” website, http://web.media.mit.edu,
November 9, 1996.
7) “The Piezoelectric Effect,” PZT Corporation Application Manual, Technical manual,
2001.
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