Download Piezoelectric Transducers to Harvest Energy from Human Motion

Residential Piezoelectric Energy Sources
Andrew Katz
July 21, 2004
Abstract
The DELTA Smart House is going
to have a large number of sensors and
microelectronic devices located throughout
the house. These devices will need a clean,
reliable source of energy that won’t need
constant maintenance. The goal of this
project is to utilize piezoelectric energy
sources to provide power to certain
applications in the house. Initially the plan
was to create an energy scavenging floor
that used piezoelectric transducers to
harvest wasted energy in the foot strike of a
human being. In consideration of the high
cost and minimal power output of these
piezo-sources, it seems more feasible to
create small, localized energy sources rather
than one large unified system. This idea has
lead to several potential applications. The
first is to combine a piezoelectric power
source with sensors such that there would
be no need to ever change the batteries in
these sensors. The next application is to use
piezoelectric cable throughout the floors of
the house as a means of tracking. The final
application is to combine piezoelectrics with
a device to eliminate vibrations in
household appliances.
Technical Overview
Piezoelectric materials exhibit the
unique property known as the piezoelectric
effect. When these materials are subjected to
a compressive or tensile stress, an electric
field is generated across the material,
creating a voltage gradient and a subsequent
current flow. This effect stems from the
asymmetric nature of their unit cell when a
stress is applied. As seen in Figure 1, the
unit cell contains a small positively charges
particle in the center. When a stress is
applied this particle becomes shifted in one
direction which creates a charge
distribution, and subsequent electric field.
These materials come in several different
forms. The most common is crystals, but
Fig.1: Lead Zirconate Titanate unit cell
they are also found as plastics and ceramics.
Existing Technologies
There are several companies and
research institutes throughout the world
who are focusing on finding useful
applications for piezoelectric energy sources.
Several years ago a project was done at MIT
entitled, “Energy Scavenging with ShoeMounted Piezoelectrics.” 1 In this project
the researchers lined the bottom of a shoe
with piezoelectric transducers and saw what
kind of power they got out of it. They
eventually attached an RF-transmitter to the
shoe that was powered by the piezoelectrics.
The two materials they used were
polyvinylidene fluoride (PVDF) and lead
zirconate titanate (PZT). Their initial results
were that the PVDF material produced 1.3
mW per foot strike and the PZT produced
around 8.4 mW. They went back and tried
numerous other approaches but they were
confined to working with the limitation of a
shoe. In this report they hinted at the fact
that much great power output could be
achieved if they were not confined to
working with a shoe.
Another company that is looking
into using piezoelectric sources to power
networks of wireless sensors is MicroStrain
Shenck, Nathan S. and Joseph A. Paradiso. Energy
Scavenging with Shoe-Mounted Piezoelectrics.
http://www.computer.org/micro/homepage/may_jun
e/shenck/?SMIDENTITY=NO
1
Inc.2 This company setup an experiment
where piezoelectric transducers were
attached to the support beams in a structure.
As the structure was constantly under strain,
the voltage created by the piezoelectrics was
stored up in a capacitor. Once the capacitor
voltage reached a certain level, the power
was than transferred to a transmitter which
sent a wireless signal to some receiver. It
was reported that the cycle time was about
20 to 80 seconds to store up a charge of 9.5
V on the capacitor given the size of the
piezoelectric was 17 cm2. This report was
done on January 5, 2004 and at that time
research was still being conducted. This
seems to be a very promising project and
MicroStrain is looking to have a
commercially available product by mid 2005.
A German based company called
EnOcean3 already has a commercially
available product. The slogan for this
company is “no batteries and no wires.”
They create products that use piezoelectric
transducers to power RF transmitters. One
particular product is a light switch that
requires no wiring at all. Behind the actual
switch is a piezoelectric transducer. When
the light switch is flipped, this motion is
used by the transducer to power the RF
transmitter, which signals the receiver on
the actual light to turn on. This would be an
interesting technology to demonstrate in the
house because it enables you to have light
switches on places you never could before
such as on a window or outside in the
middle of the yard.
Two other companies, Ferro
Solutions Inc. and Continuum Control
Corp., make small ambient energy
harvesting sources. Ferro Solutions makes a
product called the Energy Harvester.4 This
little device about the size of two AA
batteries contains an electromagnetic
generator inside. There are two magnets and
in between them is a coil of wire. When
vibrations cause the coil of wire to move
around in the magnetic field, current is
generated in the wire. This small energy
source could be used in place of batteries or
as a means to recharge batteries. The device
supplies about 1 to 10 mW of power and
the company is looking to license this
technology to wireless sensor companies.
Continuum Control Corp. makes two
different products called the PiezoFlex5 and
the iPower Generator. The PiezoFlex is a
new type of piezoelectric material that is
both flexible and robust. At the same time it
also cheaper to manufacture than most
other piezoelectric materials. The second
product, the iPower Generator contains a
piezoelectric transducer that converts
mechanical input into electrical output. This
product was used was used as a backup
energy source in an Antarctic Expedition
several years ago. The device has a crank
and as you turn it, the mechanical energy is
converted into electrical energy.
At the MIT Media Lab, researchers
in the Responsive Environments Group
created a piezoelectric floor called the
“Magic Carpet.” 6 This floor contained a
grid of piezoelectric cables spaced 4” apart.
The goal of this project was to create a floor
that could track the movements of the
person walking across it. This technology
was then combined with lights and sound
such that depending on where you walked a
different sound would play. Today you can
find examples of the magic carpet in some
O’Handley, Kevin. “Energy Harvester: Converts
Low-Level Vibrations into Usable Energy.” Ferro
Solutions Corp.
5 “Piezoflex” Continuum Control Corp.
6
J. Paradiso, C. Abler, “The Magic Carpet: Physical
Sensing for Immersive Environments.”
4
D.L. Churchill, M.J. Hamel, “Strain Energy
Harvesting for Wireless Networks.”
3 http://www.enocean.com/indexe.html
2
of the museums at MIT. A similar project
was done at Georgia Tech. At Georgia Tech
they built a “Smart Floor”7 that used
piezoelectrics as sensing devices to monitor
and predict when people were walking
across it. They could predict with about
90% accuracy what person was walking
across the floor just given the way the
piezoelectrics reacted to their foot strike.
Application to Residential
There are several ways to
incorporate piezoelectric technology into a
residential setting. The first would be to use
small piezoelectric sources in the place of
batteries. These piezoelectric sources could
be used to power sensors throughout the
house so that the batteries would never need
to be changed. Another application was one
proposed by Dr. Rob Clark and Dr. Henri
Gavin. Their idea was to use piezoelectric
materials to cancel out vibrations in certain
household appliances. They envisioned a
device that could autonomously adapt the
amount of dampening based on the
magnitude of the vibrations. They proposed
that first the device be built with a battery
power source and eventually switch to an
energy source that converted the vibrations
into energy using either piezo or
electromagnetic generators. Their second
idea was the opposite of the first: rather
than cancel the vibrations, you would
enhance them. This idea originated from the
fact that subwoofers only go down to about
20 Hz, yet the electrical signasl coming from
the actual media contains frequencies below
20 Hz. These signals are too low for us to
hear, but they can be converted into
vibrations. For example when there is an
explosion in a movie, there are frequencies
below 20 Hz, but the subwoofer can’t make
a sound with that long of a wavelength. If
you were to install proof mass actuators into
7
Orr, Robert J. “FCE Smart Floor.” Georgia Tech
the floor of the media room, whenever
there was a signal below 20 Hz, these
actuators would respond accordingly and
literally shake the room.
The next set of applications would
involve piezoelectric cable. Piezoelectric
cable behaves much the same way as flat
piezoelectric transducers only the cable is
much cheaper. The cable resembles the
standard coaxial cable that plugs into the
TV, but the piezo cable has a layer of
piezoelectric polymer wrapped into it. This
cable could be used to create a grid across
the floor that could generate power from
people walking across it, or the more viable
purpose would be for tracking. These cables
come with varying degrees of sensitivity.
The cable could be used to track both the
location and the orientation of a person in
the house. Based on the way they walk, the
floor might also be able to identify who the
person is.
Design
The major obstacle in designing your
own piezoelectric circuit is finding a way to
maximize the power output. The major
components involved in this circuit would
be an AC/DC rectifier, a filter capacitor,
and a DC-DC converter. The AC/DC
rectifier converts the AC signal from the
piezo-source into DC current. The filter
capacitor smoothes electrical flow and the
DC-DC converter is what allows the battery
to store the energy. Most of the power
conversion comes into play in the DC-DC
converter.
Fig. 2: Adaptive Energy Harvesting Circuit
Recently in a paper published in Power
Electronics entitled “Adaptive Piezoelectric
Energy Harvesting Circuit for Wireless
Remote Power Supply,”8 researchers found
a way to increase power output by roughly
400% compared to when a converter is not
used. They used an adaptive control
technique in the converter that adjusts itself
to find the optimal power transfer options
for every moment.
Last year, Dr. Gavin created a setup
to demonstrate the power output you could
achieve using the vibrations in a plane wing.
He didn’t use piezoelectrics, but rather
electromagnetic generators. Regardless, the
circuit for capturing and storing the energy
would still be the same. He still has the
circuit and is willing to let us use it in
designing a piezoelectric energy source.
Simulation Results
In my physics 171L class for my
final project I modeled a circuit I found in a
book9 in the simulation program PSpice.
The circuit used a piezoelectric transducer
as the voltage source which than fed into an
op amp. The purpose of the circuit was to
provide a voltage gain of 100. Our
simulation results proved that the op amp
did in fact multiply the voltage from the
piezo-transducer by a factor of 100.
Unfortunately, in the program we could not
find an actual piezo-transducer so we had to
model it with an ideal voltage source with an
estimated equivalent Thevenin resistance. I
considered this project a success on the
grounds that it proved that piezotransducers can in fact act as voltage
sources. It also showed that the voltage
provided by the transducer can be
multiplied by several orders of magnitude
8
Ottman, Geoffrey K. and Lesieutre, George A.
“Optimized Piezoelectric Energy Harvesting Circuit.”
9 Horowitz, Paul and Winfield Hill, The Art of
Electronics. p.1039
using certain circuits. This circuit essentially
failed to answer the real question of what
kind of maximum power one can get out of
the piezo-transducer.
Cost Analysis
Buying a piezoelectric transducer can
be quite expensive. Two different suppliers
I looked at were Face International Corp.
and Active Control eXperts. Their piezotransducers sold for about $100 and $200
respectively. Buying piezoelectric cable is
much cheaper. One supplier in the UK,
Ormal Electronics Ltd., gave a quote of
£2.75 per meter for purchases of more than
2000 meters. Another supplier in the US,
Measurement Specialties Inc., sells piezocable for $8.00/m for more than 1000 m.
Even if we were to use piezo-cable to line
the house at 4” apart, the cost would still
exceed $30,000.
Contacts Established
I have written letters to Dr. Aaron
Bent of Continuum Control Corp. and
Kevin O’Handley of Ferro Solutions Inc. I
am still waiting to hear back from them. I
have also contacted MicroStrain in order to
elicit their support of the project. They
informed me that they are a small company
with limited resources, but could support
the project by supplying us with materials at
a discounted price. I also contacted Mike
Redman of Ormal Electronics Ltd. He
quoted me with a price on the piezoelectric
cable and said he could supply us with some
free samples if necessary.
Future Direction
The next step would be to team up
with Dr. Gavin and work with the energy
harvesting setup he has. Once you
understood the fundamentals of it, you
could model some of your own circuits on a
simulation program. Right now the most
likely simulation to use would be Ansys.
This simulation program is installed on the
acpub machines as well as several civil
engineering computers. Prof. Dolbow
would be the person to see about learning to
use this program. With this simulation
program, you could customize the circuit to
maximize the power you get out of the
piezo-source. The next step would be to
start building the actual prototype. You
would need to continue contacting
companies in order to get their support and
perhaps some donations of products.
References
Shenck, Nathan S. and Joseph A. Paradiso. Energy Scavenging with Shoe-Mounted
Piezoelectrics. MIT Media Laboratory, Responsive Environments Group.
http://www.computer.org/micro/homepage/may_june/shenck/?SMIDENTITY=NO
D.L. Churchill, M.J. Hamel, “Strain Energy Harvesting for Wireless Networks.” Microstrain Inc.
http://www.microstrain.com/white_strain_energy_harvesting.htm
Enocean http://www.enocean.com/indexe.html
O’Handley, Kevin. “Energy Harvester: Converts Low-Level Vibrations into Usable Energy.” Ferro
Solutions Corp. http://www.ferrosi.com/files/FS_product_sheet_wint04.pdf
Bent, Aaron. “Piezoflex” Continuum Control Corp.
http://www.powerofmotion.com/technology/ipower/characteristics.html
Orr, Robert J. “FCE Smart Floor.” Georgia Tech. http://www.cc.gatech.edu/fce/smartfloor/
Horowitz, Paul and Winfield Hill, The Art of Electronics. 2nd Edition, Cambridge: Cambridge
University Press. 1989. p.1039
Ottman, Geoffrey K. and Lesieutre, George A. “Optimized Piezoelectric Energy Harvesting
Circuit.” Power Electronics. Vol.18, No.2, March 2003.
The Magic Carpet: Physical Sensing for Immersive Environments J. Paradiso, C. Abler, KY. Hsiao,
M. Reynolds, in Proc. of the CHI '97 Conference on Human Factors in Computing Systems,
Extended Abstracts, ACM Press, NY, pp. 277-278(1997)