U.S. patent application number 11/527044 was filed with the patent office on 2008-03-27 for piezoelectric energy harvester.
Invention is credited to Robert D. Myers, Shashank Priya.
Application Number | 20080074002 11/527044 |
Document ID | / |
Family ID | 39224186 |
Filed Date | 2008-03-27 |
United States Patent
Application |
20080074002 |
Kind Code |
A1 |
Priya; Shashank ; et
al. |
March 27, 2008 |
Piezoelectric energy harvester
Abstract
A mechanism for capturing mechanical energy and converting it to
electrical energy for use continually charging or providing
emergency power to mobile, battery-powered devices comprises a
plurality of elongated piezoelectric elements mounted at one or
support points to one or more support structures. The plurality of
piezoelectric elements are preferably structured and arranged so
that at least each three-dimensional coordinate axis has at least
one element with a dominant mode of deflection in a plane normal to
the axis, in order to permit harvesting energy from forces applied
in any direction without regard to the orientation of the energy
harvesting mechanism to the source of forces. This results in
improved coupling of the transducer with the random movements or
vibrations that may not confined to any particular plane or in a
plane that is not necessarily aligned with the plane in which a
piezoelectric element is designed to bend, thus improving the
efficiency of energy capture.
Inventors: |
Priya; Shashank; (Arlington,
TX) ; Myers; Robert D.; (Arlington, TX) |
Correspondence
Address: |
GARDERE WYNNE SEWELL LLP;INTELLECTUAL PROPERTY SECTION
3000 THANKSGIVING TOWER, 1601 ELM ST
DALLAS
TX
75201-4761
US
|
Family ID: |
39224186 |
Appl. No.: |
11/527044 |
Filed: |
September 26, 2006 |
Current U.S.
Class: |
310/339 |
Current CPC
Class: |
H02N 2/188 20130101;
Y02B 40/90 20130101; Y02B 40/00 20130101; H01L 41/1136 20130101;
H02J 7/32 20130101 |
Class at
Publication: |
310/339 |
International
Class: |
H01L 41/113 20060101
H01L041/113 |
Claims
1. An energy harvesting apparatus comprising at least three
elongated piezoelectric elements for generating a voltage when
strained, each element having an axis extending along its length;
each element supported for flexing comparatively easily within a
predetermined bending plane as compared to other planes in which
its axis lays; a first one of the at least three piezoelectric
elements being mounted with its predetermined bending plane facing
a first direction, a second one of the at least three piezoelectric
elements being mounted with its predetermined bending plane facing
in a second direction, and a third one of the at least three
piezoelectric elements being mounted with its predetermined bending
plane facing a third direction; the first, second and third
directions being non-parallel.
2. The energy harvesting apparatus of claim 1, wherein the first,
second and third directions are mutually orthogonal.
3. The energy harvesting apparatus of claim 1, wherein each of the
at least three elongated piezoelectric elements has a width greater
than its thickness along at least a majority of its length.
4. The energy harvesting apparatus of claim 3, wherein the mass of
at least one of the at least three elongated piezoelectric elements
is not evenly distributed along its length.
5. The energy harvesting apparatus of claim 1, wherein at least one
of the at least three piezoelectric elements is supported in a
cantilevered fashion, with one end fixed and one end free to move,
and wherein relatively more mass of the at least one of the at
least three elongated piezoelectric elements is distributed near
its free end as compared to other locations along its length.
6. The energy harvesting apparatus of claim 1, further comprising
at least three AC to DC conversion circuits; a first one of the at
least three AC to DC conversion circuits coupled with the first of
the at least three piezoelectric elements, a second one of the at
least three AC to DC conversion circuits coupled with the second of
the at least three piezoelectric elements, and a third one of the
at least three AC to DC conversion circuits coupled with the third
of the at least three piezoelectric elements.
7. The energy harvesting apparatus of claim 1, wherein the at least
three elongated, elements comprise a first array of cantilevered,
elongated piezoelectric elements with axes oriented in the first
direction, a second array of cantilevered, elongated piezoelectric
elements with axes oriented in the second direction, and a third
array of cantilevered, elongated piezoelectric elements.
8. The energy harvesting apparatus of claim 7, wherein each of the
first, second and third arrays include at least two cantilevered,
elongated piezoelectric elements arranged side-by-side.
9. The energy harvesting apparatus of claim 7, wherein at least one
of the first, second and third arrays of cantilevered, elongated
piezoelectric elements are formed on a monolithic semiconductor
substrate.
10. The energy harvesting apparatus of claim 1, wherein each of the
at least three elongated piezoelectric elements are formed on a
monolithic semiconductor substrate.
11. The energy harvesting apparatus of claim 1, wherein each of the
at least three elongated piezoelectric elements are comprised of a
piezoelectric bimorph.
12. The energy harvesting apparatus of claim 1, wherein each of the
at least three piezoelectric elements has dimensions and
distribution of mass causing it to have a resonance frequency of
between 10 and 30 hertz.
13. The energy harvesting apparatus of claim 1, wherein each of the
at least three elongated piezoelectric elements is supported at one
fixed point in a cantilevered fashion, with one end free to
move.
14. The energy harvesting apparatus of claim 1, wherein each of the
at least three elongated piezoelectric elements is supported by at
least two fixed points, with segments of the element between the
fixed points free to flex.
15. A portable device comprising a circuit and a battery for
powering at least the circuit, the device electrically coupled with
an energy harvester for charging the battery, the energy harvester
comprising at least three elongated piezoelectric elements for
generating a voltage when strained, each element having an axis
extending along its length; each element supported for flexing
comparatively easily within a predetermined bending plane as
compared to other planes in which its axis lays; a first one of the
at least three piezoelectric elements being mounted with its
predetermined bending plane facing a first direction, a second one
of the at least three piezoelectric elements being mounted with its
predetermined bending plane facing in a second direction, and a
third one of the at least three piezoelectric elements being
mounted with its predetermined bending plane facing a third
direction; the first, second and third directions being
non-parallel.
16. The portable device of claim 15 wherein the energy harvester
further comprises at least three AC to DC conversion circuits; a
first one of the at least three AC to DC conversion circuits
coupled with the first of the at least three piezoelectric
elements, a second one of the at least three AC to DC conversion
circuits coupled with the second of the at least three
piezoelectric elements, and a third one of the at least three AC to
DC conversion circuits coupled with the third of the at least three
piezoelectric elements.
17. The portable device of claim 15, wherein the device comprises a
mobile telephone.
Description
BACKGROUND OF THE INVENTION
[0001] Unused power exists in various forms such as industrial
machines, human activity, vehicles, structures and environment
sources. Among these, some of the promising sources for recovering
energy are periodic vibrations generated by rotating machinery or
engines. Primarily, the selection of the energy harvester as
compared to other alternatives such as battery depends on two main
factors cost effectiveness and reliability. In recent years,
several energy harvesting approaches have been proposed using
solar, thermoelectric, electromagnetic, piezoelectric, and
capacitive schemes which can be simply classified in two categories
(i) power harvesting for sensor networks using MEMS/thin/thick film
approach, and (ii) power harvesting for electronic devices using
bulk approach.
[0002] Promising applications for piezoelectric energy harvesting
have inherent forms of energy to capture, store and use. Examples
include "active" sports equipment such as tennis racquets and skis
that use strain to power actuators for feedback control loops, and
watches that use body motion to supply power. Other applications
which have been suggested include the use of aircraft engine
vibrations, airflow over wings, vibrations induced by driving on a
road, and periodic vibrations generated by rotating machinery or
engines. Primarily, the selection of the energy harvester as
compared to other alternatives such as battery depends on three
main factors, cost effectiveness, profile and reliability. In an
other form, the energy harvester can supplement the other energy
alternatives such as battery and prolong their lifetime.
[0003] Conversion of mechanical low frequency stress into
electrical energy is obtained through the direct piezoelectric
effect, using a rectifier and DC-DC converter circuit to store the
generated electrical energy. There are three primary steps in power
generation: (a) trapping mechanical AC stress from available
source, (b) converting the mechanical energy into electrical energy
with piezoelectric transducer and (c) processing and storing the
generated electrical energy. The mechanical output can be in the
form of a burst or continuous signal depending on the cyclic
mechanical amplifier assembly. Depending on the frequency and
amplitude of the mechanical stress, one can design the required
transducer, its dimensions, vibration mode and desired
piezoelectric material. The energy generated is proportional to
frequency and strain and higher energy can be obtained by operating
at the resonance of the system.
SUMMARY OF THE INVENTION
[0004] The invention pertains generally to a mechanism for
capturing mechanical energy and converting it to electrical energy,
and is particularly useful for continually charging or providing
emergency power to mobile, battery-powered devices that are handled
or carried by persons. The mechanism comprises a plurality of
elongated piezoelectric elements for generating electric energy
from mechanical energy.
[0005] In one exemplary embodiment, the piezoelectric elements are
mounted to one or more support structures in a cantilevered
fashion, with a single point of support, such as one end supported
or fixed and an opposite end free to move relative to the fixed end
within a predetermined plane relative to the support structures. In
another exemplary embodiment, the piezoelectric elements are
supported at two or more fixed points of support, with the element
free to bend between the points of support and within a
predetermined plane relative to the fixed points of support.
[0006] In each exemplary embodiment, the plurality of piezoelectric
elements are preferably arranged so that at least each
three-dimensional coordinate axis (e.g. x, y and z) has at least
one element is structured and oriented to bend or flex
predominantly in a plane normal to the axis, allowing harvesting of
energy from forces applied in any direction without regard to the
orientation of the energy harvesting mechanism to the forces. This
arrangement results in improved coupling of the transducer with the
random movements or vibrations that may not be confined to any
particular plane or in a plane that is not necessarily aligned with
the plane in which a piezoelectric element is designed to bend,
thus improving the efficiency of energy capture.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] FIG. 1 is a schematic diagram of a cantilevered
piezoelectric parallel bimorph element.
[0008] FIG. 2 is a schematic diagram of a cantilevered
piezoelectric serial bimorph element.
[0009] FIG. 3 illustrates a grouping of commonly oriented
piezoelectric elements in a clamp.
[0010] FIG. 4 illustrates an example of a energy harvesting
mechanism in a single enclosure using the groupings of
piezoelectric elements shown in FIG. 3.
[0011] FIG. 5 illustrates an example of a micromachined array of
cantilevered piezoelectric elements.
[0012] FIG. 6 illustrates a second example of a micromachined array
of cantilevered piezoelectric elements.
[0013] FIG. 7 illustrates assembly of the arrays of FIGS. 5 and 6
into a compact energy harvesting mechanism or generator.
[0014] FIG. 8 illustrates further assembly of the generator of FIG.
7.
[0015] FIG. 9 illustrates a nearly complete assembly of the
generator of FIGS. 7 and 8, without a cap enclosing the
assembly.
[0016] FIG. 10 is a schematic illustration of a generator, such as
the one shown in FIGS. 7-9, as part of a charging apparatus for a
battery for a mobile device.
[0017] FIG. 11A is an electrical schematic of a circuit for
charging the battery using a piezoelectric energy harvesting
mechanism, with a signal conditioning element.
[0018] FIG. 11B is an electrical schematic of a circuit for
charging the battery using a piezoelectric energy harvesting
mechanism, with a switching diode element.
[0019] FIG. 12 schematically illustrates another example of a
piezoelectric energy harvester.
[0020] FIG. 13 illustrates deployment of multiple numbers of the
piezoelectric energy harvester of FIG. 12 in a mobile device, which
is indicated in phantom.
[0021] FIG. 14 illustrates schematically an orthographic view of an
example of a piezoelectric energy harvesting mechanism formed on a
monolithic substrate.
[0022] FIG. 15 is a cross-sectional view of the energy harvesting
mechanism of FIG. 14, taken along section lines 15-15.
[0023] FIG. 16 is a cross-sectional view of the energy harvesting
mechanism of FIG. 14, taken along section lines 16-16.
[0024] FIG. 17 is a cross-sectional view of one piezoelectric
element of the energy harvesting mechanism of FIG. 14.
[0025] FIGS. 18A-18E illustrate a series of steps of forming
piezoelectric elements on the monolithic substrate for the energy
harvesting mechanism of FIG. 14.
[0026] FIG. 19 illustrates schematically a stacked wafer or stacked
die embodiment for an energy harvesting mechanism.
[0027] FIG. 20 is a cross-section of FIG. 19 taken along section
line 20-20.
[0028] FIG. 21 is an example of an energy harvesting mechanism
formed on monolithic substrate using piezoelectric elements
supported at two or more points.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0029] In the following description, like numbers refer to like
parts.
[0030] FIGS. 1 and 2 each illustrate schematically cantilevered
piezoelectric elements, each comprised of a piezoelectric
transducer for generating a voltage in response to mechanical
strain of the transducer. Each piezoelectric element 100 and 200 is
elongated and is mounted to a structure 102 and 202, respectively
in a cantilevered fashion. Each has a length L and an axis 103 and
203, respectively, extending along its length. Each element is
anchored near one end of its length to a structure, and its
opposite end left free. Forces applied to the element, in a
direction generally normal to its axis, will tend to bend and thus
strain the element, generating a voltage across the element
generally in a plane defined by the force and the axis of the
element. Although not shown, each element has substantially larger
width as compared to its thickness so that it tends to bend more
easily in a plane indicated by arrows 104 and 204, as compared to
other directions, thus making it more sensitive to vector forces
within that plane, particularly ones that are normal or near normal
to the axis.
[0031] Each of the piezoelectric elements shown in FIGS. 1 and 2,
as well as those shown in the remaining figures, is preferably
elongated and relatively flat, so that it deflects and resonates
predominately in a single plane that contains the axis of its
predominate dimension and the vector forces that cause deflection
of the element. This mode of deflection corresponds to the d.sub.31
piezoelectric strain constant. It is generally preferred that the
length should be at least 10 times the width, and the width should
be at least 5 times the thickness of the element. This provides one
dominant resonance mode associated with the length of the
piezoelectric element, with other resonance modes associated with
length and width relatively suppressed. This ratio of 10 to 5 to 1
ensures good frequency spacing for the resonances (including
overtones) in each possible direction of deflection, which tends to
avoid signals from subordinate resonance modes interfering with
signals for the dominant resonant mode, resulting in more efficient
conversion of kinetic energy to electrical energy.
[0032] Several types of piezoelectric materials can be employed to
fabricate the piezoelectric elements of the type shown in the
figures, including BaTiO3, Pb(ZrxTi1-x)O3,
(Na.sub.xK.sub.1-x)NbO.sub.3, (Na.sub.1/2Bi.sub.1/2)TiO.sub.3,
(K.sub.1/2Bi.sub.1/2)TiO.sub.3,
Pb(A.sub.1/3B.sub.2/3)O.sub.3--PbTiO.sub.3 (where A=Zn, Mg, Ni, and
B=Nb, Ta), Pb(Yb.sub.1/2Nb.sub.1/2)O.sub.3--PbTiO.sub.3,
LiNbO.sub.3, AgNbO.sub.3, and Bi-layered structures. Solid
solutions and composite formulations of these materials can also be
employed to fabricate the piezoelectric elements.
[0033] In the example of FIGS. 1 and 2, piezoelectric elements 100
and 200 possess a bimorph structure, meaning that they are
comprised of two layers of piezoelectric material 106 and 108, and
206 and 208, respectively. Bimorph elements tend to provide high
power density and have lower resonance frequency as compared to
single layer, or unimorph, piezoelectric elements. Each layer has a
piezoelectric coefficient labeled as "P," which is equal to
piezoelectric coefficient d.sub.31 for the material and corresponds
to a bending motion. Bimorph piezoelectrics have sufficient
mechanical strength for high amplitude vibrations in the range of
1-10 Hz. The applied load on the bimorph can be of the order of
several Newtons of force. Laboratory scale measurements have shown
that a bimorph vibrating under a force of 10 N at low frequencies
of 10 Hz typically do not suffer from mechanical degradation. The
piezoelectric voltage coefficient of a bimorph is high so the
charge developed under fully loaded condition is high. The maximum
displacement of the bimorph is significant due to the high level of
bending force that can be applied. Hence, the mechanical energy
obtained from the bimorph is high.
[0034] Bimorphs can be electrically driven in order to obtain the
bending movement by polarizing each layer in opposite direction. In
the figures, the arrows next to the "P" indicate the direction of
polarization. Usually, a 3-terminal input is used for a driving
power supply. FIG. 1 illustrates a parallel configuration, in which
the polarization of each layer is in the same direction, and FIG. 2
illustrates a serial configuration, in which the polarization in
each layer is in opposite directions. A positive side of the driver
input is connected to a positive terminal of the piezoelectric
transducer, a negative terminal is connected to zero, and an
electrode layer is connected to alternating positive voltage or
zero. In the parallel configuration, ground input 110 is connected
to a bottom electrode of layer 106 and output 112 from op amp 213
is connected to middle electrode layer, between the layers. Supply
voltage input 114 for the op amp is also connected to the top
electrode of layer 108. In the serial configuration of FIG. 2,
ground line 210 is connected to a bottom electrode of layer 206 and
output 212 of op amp 213 is connected to the middle electrode.
Supply voltage 214 for the op amp is not connected to the bimorph.
When the middle electrode layer is at zero voltage, the
middle-layer and negative terminal are isopotential and the
piezoceramic at the negative side does not operate. The
piezoceramic at the positive side is at the positive electric
field. In this condition, by the action of converse piezoelectric
effect, the ceramics is lengthened along the polarization direction
(thickness or 33 orientation), which causes shortening in the
transversal direction. Since the negative terminal side tightly
bonded to the ceramics restricts this shortening, the positive
terminal can only bend to its own side. Similarly, when the middle
electrode is at the positive voltage, the negative terminal
operates, which results in bending to the negative terminal side.
This power-connected driving mode is referred to as a serial
connection. Since a piezoceramic is a capacitive element, the
electric energy absorbed in the piezoelectric ceramics will be
slowly released in a short period after power off. Resistors (not
shown) are connected in parallel between two electrodes to consume
the absorbed energy, so that the bimorph can rapidly return to its
original position after power off. A series bimorph transducer will
perform better for energy harvesting application than a parallel
bimorph.
[0035] The bimorph or layered piezoelectric may be fabricated using
a dry sheet process or a wet build-up process. In a typical dry
process, powdered ceramic material is mixed with a polymeric binder
and cast onto moving belts to form green ceramic tapes. The tapes
are then coated with a film of the electrode material, usually
silver or a silver palladium alloy. The coated tapes are next
stacked upon each other and pressed together. The final structure
is sandwiched between top and bottom ceramic layers without
electrodes to form a `pad`, which is diced into individual
components. The components are sintered at elevated temperatures of
900 to 1,100.degree. C. After cooling, the components are poled at
high temperature and field. The poled components are then mounted
on a metallic sheet in the bimorph configuration.
[0036] Each cantilevered piezoelectric element possesses a natural
resonance frequency dictated at least in part by how it is
supported, the materials(s) used in the element, and its length,
thickness, shape, mass, and distribution of mass. Although the
cross sectional shape of the element in the examples illustrated in
FIGS. 1 and 2, as well as most of the other examples described
below, is predominately rectangular, other cross sections could be
employed. Furthermore, the mass of the element need not be
distributed evenly along its length. For example, more mass, which
need not necessarily be of piezoelectric material, can be
concentrated in the free end or tip, thereby increasing the moment
of inertia and lowering the resonance frequency. The element could
also be shaped differently along its length to alter resonant
characteristics or for other reasons.
[0037] Tuning the resonant frequency of a piezoelectric element to
correspond generally to the expected frequency band of forces to be
applied improves coupling and leads to higher efficiency. The
tuning of the resonant frequency of the energy harvester to the
available vibration band can be done by several ways, depending on
the type piezoelectric element and how it is supported, including
changing the length of the piezoelectric element, increasing the
number of layers of the piezoelectric element, adding the mass at
the tip of the piezoelectric element, changing the thickness of the
electrode layer, changing the thickness of an intermediate metal
layer and mounting mechanism.
[0038] Orienting within an energy harvesting mechanism the planes
of the dominate modes of deflection or resonance of at least three
piezoelectric elements in a mutually orthogonal fashion, thereby
defining three coordinate axes, such that there is for each three
dimensional coordinate axis at least one of the elements is
sensitive to a vector force applied along each coordinate axis,
creates a mechanism sensitive to a force applied in any direction.
Such a mechanism is thus able to harvest energy from movement of
the mechanism in any direction. It is advantageous for use in
applications, for example, in which forces applied by movement of
the mechanism, are unpredictable, or in which the orientation of
the energy harvesting mechanism cannot be known, set or maintained.
Additional piezoelectric elements which are not aligned with three
coordinate axes could be included, if found desirable.
[0039] For example, by tuning the resonance frequency of the
cantilevered piezoelectric elements to relatively low frequencies,
typically less than 30 hertz, movement associated with a person
moving, which is typically less then 10 hertz, can be harvested and
stored. The energy harvesting mechanism can be placed in a powered
device carried by the person, without concern for the orientation
of the device, or it can be placed on the person, such as in an
article of clothing, including a belt or shoe, or placed in or on
an item carried by the person, such as a bag, backpack, briefcase,
belt clip or holster.
[0040] FIGS. 3 and 4 illustrate one example of energy harvesting
mechanism or generator. An array of multiple cantilevered
piezoelectric elements 300 are anchored or fixed by, for example, a
clamp 302 into a grouping 304. All of the elements in the grouping
are oriented in the same direction. The piezoelectric elements can
be unimorph or bimorph. However, in this example they are thick
film bimorphs. Selecting elements with substantially equal
resonance frequencies for a grouping and orienting them in the same
direction as shown in the drawings permits closer spacing. In such
an arrangement, the piezoelectric elements will typically move in
unison, in the same direction, in response to a force applied to
the structure to which they are fixed or mechanically coupled.
Closer spacing permits greater density, resulting in more energy
harvested per unit volume. In this example, a lower portion 402 of
a case is formed to receive three groupings 404, 406 and 408 of
cantilevered piezoelectric elements. Each is substantially the same
as the one shown in FIG. 3, but oriented differently. Arrows 410,
412, and 414, respectively, indicate the direction of sensitivity
of the cantilevered piezoelectric elements, which correspond to
three coordinate axes. A cover mates with the lower portion 402 to
form a protective case. Additional energy can be generated by
adding additional groupings, and/or by adding more piezoelectric
elements to each grouping. Electrical connections and rectifying
circuitry are omitted in these figures.
[0041] The lower portion of the case 402 serves, along clamps 302,
as a fixture for maintaining the respective orientations of the
groupings of piezoelectric elements and to couple mechanically the
piezoelectric elements to forces from which energy is to be
harvested. The case or other fixture or package in which the
piezoelectric elements are mounted may also serve to protect the
piezoelectric elements and associated circuitry.
[0042] Referring to FIGS. 5, 6, 7, 8, and 9, increased density and
improved manufacturability can be achieved by creating arrays of
cantilevered piezoelectric elements from monolithic pieces of
ceramic material. As shown in FIGS. 5 and 6, arrays of cantilevered
piezoelectric elements 500 and 600 extending, respectively, from
frames 502 and 602, are cut from a single slab of piezoelectric
ceramic material. A laser, 3-D cutting tools or other micromaching
process may be used to cut the design. Before micromaching an
electrode pattern is printed on the plates and the ceramic is
poled. Frames 502 and 602 differ on the type of structure used for
registering the plates to each other and to a base structure. On
frame 602 are four extensions or legs 603 on frame 600. Frame 502
includes holes 503. However, different frames or different
groupings of frames within a stack can be tuned to be sensitive to
different frequencies of force in order to improve coupling. Each
of the cantilevered piezoelectric elements 500 and 600 has an added
mass 504 and 604, respectively, on its free or tip end in order to
lower the resonance frequency. The array of cantilevered elements
in each frame includes elements of different lengths.
[0043] As shown in FIGS. 7, 8, and 9, frames 500 and 600 are
assembled into larger arrays by stacking them on a base 700 in a
box-like configuration. Stack 800 is oriented so that its
cantilevered piezoelectric elements are most sensitive to vector
forces in the direction indicated by arrows 704. Stack 802 is
oriented so that its cantilevered piezoelectric elements are most
sensitive to vector forces in the direction indicated by arrows
706. Stack 806 is oriented so that its cantilevered piezoelectric
elements are most sensitive to vector forces in the direction
indicated by arrows 708. Legs 604 are inserted into grooves 702.
Bands 900, shown in FIG. 9, hold stack 806 together. A top plate,
not shown, is added. The entire structure is preferably encased.
Electrical connections are made by strips of silver electrode paint
applied to the frames. An electrode on the side of a frame from
which the cantilevered elements extend serves as the negative
electrode. An electrode on the opposite side of the frame services
as the positive electrode. To prevent cantilevered piezoelectric
devices in adjacent frames from rubbing against each, each adjacent
frame is preferably separated by a small amount. A thin layer of
conductive material, such as electrode paint, can be used for this
purpose. Since the currents generated by each of the stacks will
not be in phase, an insulating material is preferably placed
between each of the stacks in order to isolate the circuit for each
stack.
[0044] Referring now to FIG. 10, in one application the energy
harvesting mechanism comprised of the arrays of piezoelectric
elements such as shown in FIGS. 3-9 is as a generator 1000 for
charging battery 1002. Accumulator 1004 includes circuitry for
rectifying the voltage and interfacing with battery 1004 in order
to charge the battery.
[0045] FIGS. 11A and 11B schematically illustrate examples of
circuits for use in charging a battery using a generator or energy
harvester. Because piezoelectric elements oriented in different
directions will generate out of phase currents in response to a
given vibration, it is preferred that at least three rectifiers or
AC to DC circuits be used, one for each orientation, with the
resulting DC currents combined. Piezoelectric generators 1102,
1104, and 1106, each represent all of the piezoelectric elements
for the X, Y and Z axes, respectively. Each is coupled to a
separate rectifier 1108, 1110 and 1112, respectively. These are
preferably full wave rectifiers. If piezoelectric elements for a
particular axis have different resonant frequencies, it may be
desirable to rectify separately current from piezoelectric elements
having different resonant frequencies in order to avoid phase
cancellations. The outputs of the rectifiers, which is a DC
current, are summed before being used to charge battery 1114.
[0046] In FIG. 11A, the DC current is conditioned by conditioning
circuit 1116 prior to delivery to the battery. The conditioning
circuit generates a charging profile that improves battery
charging. The conditioning circuit may comprise converter circuits,
including buck-buck converter and/or buck boost converter for
modulating the impedance of the circuit. However, in applications
in which the current being generated is not relatively consistent
or continuous, the conditioning circuit may not provide
satisfactory charging. In FIG. 11B, the current is passed through a
diode that allows only flow of current to the battery and
preferably includes a diode switch 1118 to cut off current below a
certain threshold.
[0047] Multiple generators, each with piezoelectric elements
mounted along one or two axes can be fixed in a portable device in
different orientations. For example, in generator 1200 of FIGS. 12
and 13, two piezoelectric elements 1202 and 1204 are supported in a
cantilevered fashion in an "L" arrangement in side enclosures 1206.
They are supported by a common base 1208, which supplies the
electrical connections to the electrodes on the elements (not
shown). In this example, the piezoelectric elements are preferably
bimorphs. They are oriented such that their axes are mutually
orthogonal and each element's dominant mode of deflection lies in
mutually orthogonal planes. Fixing at least two generators 1200 to
a portable device, in this example in corners of mobile telephone
1300, in mutually orthogonal orientations, provides for efficient
coupling to vector forces from any direction. If a generator
includes only piezoelectric elements oriented for bending in the
same plane, at least three such generators could be mounted
discretely within a device in mutually orthogonal orientations.
Distributing multiple numbers of smaller generators in this fashion
can permit more efficient utilization of space within a portable
device, and avoids a requirement for a single, relatively large
volume.
[0048] Other techniques for fabricating arrays of piezoelectric
elements can be used. In the example of FIGS. 14-17 and 19A-19E,
arrays of cantilevered piezoelectric elements, arranged in X, Y and
Z coordinate directions, are fabricated from a monolithic
semiconductor substrate, such as silicon crystal, using
conventional photolithographic and other integrated circuit
fabrication techniques. Similar techniques can be used to fabricate
piezoelectric elements supported at two or more points.
[0049] Referring to FIGS. 14-17, monolithic piezoelectric generator
1400 includes a plurality of arrays 1402 of cantilevered
piezoelectric elements 1403 formed to bend primarily along an X
axis indicated by arrow 1404, a plurality of arrays 1406 of
cantilevered piezoelectric elements 1407 formed to bend primarily
along a Y axis indicated by arrow 1408, and a plurality of arrays
1410 of cantilevered piezoelectric elements 1411 formed to bend
primarily along a Z axis indicted by arrow 1412. To improve
coupling with random movements and vibration of the generator, the
arrays in each of the plurality of arrays 1402, 1406, and 1410 may
optionally be fabricated with piezoelectric elements tuned to
different resonant frequencies. Thus, for example, one "X" axis
array would have piezoelectric elements tuned to a one resonant
frequency and the other "X" axis would have piezoelectric elements
tuned to a different resonant frequency.
[0050] Referring now also to FIGS. 18A-18E, which show in detail a
cross section of a single cantilever of the monolithic generator
1400 of FIGS. 14-17, the generator is formed on a substrate of
silicon crystal 1802 having oxide layer 1804 formed on the bottom
and an oxide layer 1806 formed on the top of the substrate, each
acting as an electrical insulator. Similar methods have been
described in P. Muralt, J. Baborowski, and N. Ledermann,
"Piezoelectric MEMS with PZT Thin Films: Integration and
Application Issues", pp. 231-260, in Piezoelectric Materials in
Devices Ed. Nava Setter, ISBN 2-9700346-0-3, May (2002), Lausanne
1015, Switzerland. A layer of platinum (Pt) 1808 is formed on top
of oxide layer 1806. A film of barium titanate (BaTiO.sub.3) 1810
is formed on the Pt layer. One method of forming the layer is by
spin coating a solution of BaTiO.sub.3 on to the Pt layer. The
solution is a mixture of a calculated amount of Ba alkoxides
dissolved into 2-butoxyethanol and titanium tetra-n-butoxide
[Ti(O--C.sub.4H.sub.9).sub.4], which is separately stabilized with
acetylacetone [CH.sub.3COCH.sub.2COCH.sub.3] and refluxed. After
the solution is spin coated onto the wafer, the resulting film is
crystallized directly in the diffusion furnace under oxygen
atmosphere at various temperatures between 550 and 700.degree. C.
The film is then annealed under an oxygen atmosphere.
[0051] A layer of Au/Cr electrodes 1812 are evaporated and
patterned by lift off. Vias 1813 are opened through the BaTiO.sub.3
film to give access to the bottom electrode. The narrow slit
surrounding the cantilevered section is patterned through the
layers of BaTiO.sub.3, Pt, and SiO.sub.2. The BaTiO.sub.3 and Pt
films are etched as shown in FIGS. 18C by means of, for example, an
etchant and ion-beam with photoresist mask. The underlying
SiO.sub.2 film 1806 is etched with chemical process. The cantilever
and beam are released by deep reactive ion etching silicon from the
front side, as shown in FIG. 18E. Deep reactive ion etching silicon
from the backside is then used to define the thickness of each
cantilever section (1-20 .mu.m) 1814.
[0052] As an alternative to fabricating X, Y and Z arrays on a
single die of a wafer, these arrays can be fabricated on separate
die and stacked, as shown in the example stacked die generator 1900
illustrated in FIGS. 19 and 20. Die 1902 includes only arrays 1410
of cantilevered piezoelectric elements 1411 formed to resonate
primarily along the "Z" axis 1903. Similarly, die 1904 includes
only arrays 1406 of cantilevered piezoelectric elements 1407 formed
to resonate primarily along the "Y" axis 1905. And, die 1906
includes only arrays 1402 of cantilevered piezoelectric elements
1403 formed to resonate primarily along the "Z" axis 1907. X and Y
axes die 1904 and 1906 are essentially formed identically, but are
assembled with one rotated 90 degrees to the other.
[0053] FIG. 21 illustrates an energy harvester 2100 formed on a
single, monolithic substrate 2102 having a plurality of
piezoelectric elements supported at two or more points.
Piezoelectric elements 2104 are elongated and generally
comparatively wide and thin. They are mounted in a bridge-like
fashion, supported at each end by a support pad 2106. Each is
fabricated or mounted in an orientation that is sensitive to vector
forces along either the "X" or "Y" axis, with dominant modes of
resonance in a plane defined by their respective axes and the X or
Y axis. Elongated piezoelectric element 2108 is supported at three
points by pads 2106 and is sensitive to forces in the "Z"
direction. As compared to cantilevered or single point mounting,
the span of a piezoelectric element extending between two points
does not require as much deflection to create the same amount of
strain in the material and, thus, the same amount of voltage.
Electrodes (not shown) are placed on opposite sides of the
piezoelectric element within the plane of the dominant resonance
mode.
[0054] The foregoing is a description of exemplary embodiments of
the invention as set forth in the appended claims. The scope of the
invention is not limited to these embodiments and extends to and
includes modifications or improvements, whether by substitution,
addition, rearrangement, omission or otherwise, to these
embodiments and other structures, processes and mechanisms coming
within the scope of the claims.
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