Piezoelectric Transducers to Har

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					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                  Fig.1: Lead Zircon ate Titanate unit cell
applications in the house. Initially the plan
was to create an energy scavenging floor        they are also found as plastics and ceramics.
that used piezoelectric transducers to
harvest wasted energy in the foot strike of a   Existing Technologies
human being. In consideration of the high               There are several companies and
cost and minimal power output of these          research institutes throughout the world
piezo-sources, it seems more feasible to        who are focusing on finding useful
create small, localized energy sources rather   applications for piezoelectric energy sources.
than one large unified system. This idea has    Several years ago a project was done at MIT
lead to several potential applications. The     entitled, “Energy Scavenging with Shoe-
first is to combine a piezoelectric power       Mounted Piezoelectrics.” 1 In this project
source with sensors such that there would       the researchers lined the bottom of a shoe
be no need to ever change the batteries in      with piezoelectric transducers and saw what
these sensors. The next application is to use   kind of power they got out of it. They
piezoelectric cable throughout the floors of    eventually attached an RF-transmitter to the
the house as a means of tracking. The final     shoe that was powered by the piezoelectrics.
application is to combine piezoelectrics with   The two materials they used were
a device to eliminate vibrations in             polyvinylidene fluoride (PVDF) and lead
household appliances.                           zirconate titanate (PZT). Their initial results
                                                were that the PVDF material produced 1.3
Technical Overview                              mW per foot strike and the PZT produced
        Piezoelectric materials exhibit the     around 8.4 mW. They went back and tried
unique property known as the piezoelectric      numerous other approaches but they were
effect. When these materials are subjected to   confined to working with the limitation of a
a compressive or tensile stress, an electric    shoe. In this report they hinted at the fact
field is generated across the material,         that much great power output could be
creating a voltage gradient and a subsequent    achieved if they were not confined to
current flow. This effect stems from the        working with a shoe.
asymmetric nature of their unit cell when a             Another company that is looking
stress is applied. As seen in Figure 1, the     into using piezoelectric sources to power
unit cell contains a small positively charges   networks of wireless sensors is MicroStrain
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.    1Shen ck, Nathan S. and Joseph A. Paradiso. Energy
                                                Scavenging with Sho e-Mounted Piezoelectrics.
These materials come in several different       http://www.computer.org/micro/homepage/may_jun
forms. The most common is crystals, but         e/shen ck/?SMIDENTITY=NO
Inc. 2 This company setup an experiment         product called the Energy Harvester.4 This
where piezoelectric transducers were            little device about the size of two AA
attached to the support beams in a structure.   batteries contains an electromagnetic
As the structure was constantly under strain,   generator inside. There are two magnets and
the voltage created by the piezoelectrics was   in between them is a coil of wire. When
stored up in a capacitor. Once the capacitor    vibrations cause the coil of wire to move
voltage reached a certain level, the power      around in the magnetic field, current is
was than transferred to a transmitter which     generated in the wire. This small energy
sent a wireless signal to some receiver. It     source could be used in place of batteries or
was reported that the cycle time was about      as a means to recharge batteries. The device
20 to 80 seconds to store up a charge of 9.5    supplies about 1 to 10 mW of power and
V on the capacitor given the size of the        the company is looking to license this
piezoelectric was 17 cm2. This report was       technology to wireless sensor companies.
done on January 5, 2004 and at that time        Continuum Control Corp. makes two
research was still being conducted. This        different products called the PiezoFlex5 and
seems to be a very promising project and        the iPower Generator. The PiezoFlex is a
MicroStrain is looking to have a                new type of piezoelectric material that is
commercially available product by mid 2005.     both flexible and robust. At the same time it
        A German based company called           also cheaper to manufacture than most
EnOcean3 already has a commercially             other piezoelectric materials. The second
available product. The slogan for this          product, the iPower Generator contains a
company is “no batteries and no wires.”         piezoelectric transducer that converts
They create products that use piezoelectric     mechanical input into electrical output. This
transducers to power RF transmitters. One       product was used was used as a backup
particular product is a light switch that       energy source in an Antarctic Expedition
requires no wiring at all. Behind the actual    several years ago. The device has a crank
switch is a piezoelectric transducer. When      and as you turn it, the mechanical energy is
the light switch is flipped, this motion is     converted into electrical energy.
used by the transducer to power the RF                  At the MIT Media Lab, researchers
transmitter, which signals the receiver on      in the Responsive Environments Group
the actual light to turn on. This would be an   created a piezoelectric floor called the
interesting technology to demonstrate in the    “Magic Carpet.” 6 This floor contained a
house because it enables you to have light      grid of piezoelectric cables spaced 4” apart.
switches on places you never could before       The goal of this project was to create a floor
such as on a window or outside in the           that could track the movements of the
middle of the yard.                             person walking across it. This technology
        Two other companies, Ferro              was then combined with lights and sound
Solutions Inc. and Continuum Control            such that depending on where you walked a
Corp., make small ambient energy                different sound would play. Today you can
harvesting sources. Ferro Solutions makes a     find examples of the magic carpet in some
                                                4
                                                  O’Handley, K evin. “Energy Harvester: Converts
                                                Low-Level Vibrations into Usable Energy.” Ferro
                                                Solutions Corp.
2
  D.L. Churchill, M.J. Hamel, “Strain Energy    5 “Piezoflex” Continuum Control Corp.
                                                6
Harvesting for Wireless Networks.”                J. Paradiso, C. Abler, “The Mag ic Carpet: Physical
3 http://www.enocean.com/indexe.html            Sensing for Immersive Environ ments .”
of the museums at MIT. A similar project             the floor of the media room, whenever
was done at Georgia Tech. At Georgia Tech            there was a signal below 20 Hz, these
they built a “Smart Floor”7 that used                actuators would respond accordingly and
piezoelectrics as sensing devices to monitor         literally shake the room.
and predict when people were walking                          The next set of applications would
across it. They could predict with about             involve piezoelectric cable. Piezoelectric
90% accuracy what person was walking                 cable behaves much the same way as flat
across the floor just given the way the              piezoelectric transducers only the cable is
piezoelectrics reacted to their foot strike.         much cheaper. The cable resembles the
                                                     standard coaxial cable that plugs into the
Application to Residential                           TV, but the piezo cable has a layer of
        There are several ways to                    piezoelectric polymer wrapped into it. This
incorporate piezoelectric technology into a          cable could be used to create a grid across
residential setting. The first would be to use       the floor that could generate power from
small piezoelectric sources in the place of          people walking across it, or the more viable
batteries. These piezoelectric sources could         purpose would be for tracking. These cables
be used to power sensors throughout the              come with varying degrees of sensitivity.
house so that the batteries would never need         The cable could be used to track both the
to be changed. Another application was one           location and the orientation of a person in
proposed by Dr. Rob Clark and Dr. Henri              the house. Based on the way they walk, the
Gavin. Their idea was to use piezoelectric           floor might also be able to identify who the
materials to cancel out vibrations in certain        person is.
household appliances. They envisioned a
device that could autonomously adapt the             Design
amount of dampening based on the                              The major obstacle in designing your
magnitude of the vibrations. They proposed           own piezoelectric circuit is finding a way to
that first the device be built with a battery        maximize the power output. The major
power source and eventually switch to an             components involved in this circuit would
energy source that converted the vibrations          be an AC/DC rectifier, a filter capacitor,
into energy using either piezo or                    and a DC-DC converter. The AC/DC
electromagnetic generators. Their second             rectifier converts the AC signal from the
idea was the opposite of the first: rather           piezo-source into DC current. The filter
than cancel the vibrations, you would                capacitor smoothes electrical flow and the
enhance them. This idea originated from the          DC-DC converter is what allows the battery
fact that subwoofers only go down to about           to store the energy. Most of the power
20 Hz, yet the electrical signasl coming from        conversion comes into play in the DC-DC
the actual media contains frequencies below          converter.
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             Fig. 2: Adaptive Energy Harvesting Circuit


7
    Orr, Robert J. “FCE Smart Floor.” Georgia Tech
Recently in a paper published in Power                 using certain circuits. This circuit essentially
Electronics entitled “Adaptive Piezoelectric           failed to answer the real question of what
Energy Harvesting Circuit for Wireless                 kind of maximum power one can get out of
Remote Power Supply,”8 researchers found               the piezo-transducer.
a way to increase power output by roughly
400% compared to when a converter is not               Cost Analysis
used. They used an adaptive control                           Buying a piezoelectric transducer can
technique in the converter that adjusts itself         be quite expensive. Two different suppliers
to find the optimal power transfer options             I looked at were Face International Corp.
for every moment.                                      and Active Control eXperts. Their piezo-
        Last year, Dr. Gavin created a setup           transducers sold for about $100 and $200
to demonstrate the power output you could              respectively. Buying piezoelectric cable is
achieve using the vibrations in a plane wing.          much cheaper. One supplier in the UK,
He didn’t use piezoelectrics, but rather               Ormal Electronics Ltd., gave a quote of
electromagnetic generators. Regardless, the            £2.75 per meter for purchases of more than
circuit for capturing and storing the energy           2000 meters. Another supplier in the US,
would still be the same. He still has the              Measurement Specialties Inc., sells piezo-
circuit and is willing to let us use it in             cable for $8.00/m for more than 1000 m.
designing a piezoelectric energy source.               Even if we were to use piezo-cable to line
                                                       the house at 4” apart, the cost would still
Simulation Results                                     exceed $30,000.
        In my physics 171L class for my
final project I modeled a circuit I found in a         Contacts Established
book9 in the simulation program PSpice.                        I have written letters to Dr. Aaron
The circuit used a piezoelectric transducer            Bent of Continuum Control Corp. and
as the voltage source which than fed into an           Kevin O’Handley of Ferro Solutions Inc. I
op amp. The purpose of the circuit was to              am still waiting to hear back from them. I
provide a voltage gain of 100. Our                     have also contacted MicroStrain in order to
simulation results proved that the op amp              elicit their support of the project. They
did in fact multiply the voltage from the              informed me that they are a small company
piezo-transducer by a factor of 100.                   with limited resources, but could support
Unfortunately, in the program we could not             the project by supplying us with materials at
find an actual piezo-transducer so we had to           a discounted price. I also contacted Mike
model it with an ideal voltage source with an          Redman of Ormal Electronics Ltd. He
estimated equivalent Thevenin resistance. I            quoted me with a price on the piezoelectric
considered this project a success on the               cable and said he could supply us with some
grounds that it proved that piezo-                     free samples if necessary.
transducers can in fact act as voltage
sources. It also showed that the voltage               Future Direction
provided by the transducer can be                              The next step would be to team up
multiplied by several orders of magnitude              with Dr. Gavin and work with the energy
                                                       harvesting setup he has. Once you
8
                                                       understood the fundamentals of it, you
  Ott man, Geoffrey K. and Lesieutre, George A.        could model some of your own circuits on a
“Optimized Piezoelectric Energy Harvesting Circuit.”
9 Horowitz, Paul and Winfield Hill, The Art of         simulation program. Right now the most
Electronics. p.1039                                    likely simulation to use would be Ansys.
This simulation program is installed on the    maximize the power you get out of the
acpub machines as well as several civil        piezo-source. The next step would be to
engineering computers. Prof. Dolbow            start building the actual prototype. You
would be the person to see about learning to   would need to continue contacting
use this program. With this simulation         companies in order to get their support and
program, you could customize the circuit to    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)

				
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