Wideband vibration energy harvester

In one embodiment a device comprises a composite structure that includes a piezoelectric flexure and a length-constraining element. The length-constraining element provides the piezoelectric flexure with a bowed shape. The piezoelectric flexure has a first stable bowed position and a second stable bowed position. The length-constraining element is one from the group consisting of a planar sheet and a columnar rod. In another embodiment a device comprises a piezoelectric flexure having a bowl shape. The piezoelectric flexure has a first stable bowl-shaped position and a second stable bowl-shaped position.

This application is related to the following commonly assigned patent applications:

All of the above listed patents and patent applications are incorporated herein by reference.

BACKGROUND

The vibration energy harvesting beam described in the '117 application attempts to maximize the strain of bonded piezoelectric patches and maximize the electrical output by providing a slotted, tapered vibrating beam that places the piezoelectric patches away from the neutral axis of the beam. Such a vibrating beam is especially useful when the ambient vibration level is low and if the vibrating beam may be tuned to be resonant at the predominant frequency present in the instrumented component, machine, or structure to which it is mounted. Such an energy harvester was tuned to generate electricity to power a wireless temperature and humidity sensing node from ambient vibration, as described in the '840 application.

However, in many cases the ambient vibration level may be much higher but the predominant frequency may be inconsistent or unpredictable. For example, aboard helicopters the predominant vibration frequency may be the rotational rate of the rotor assembly times the number of rotor blades in the assembly. Thus, the structure of the Sikorsky H-60 helicopter, which has four rotor blades and has a typical rotational rates of about 4.3 Hz has a predominant vibration frequency of about 16-17 Hz. The G levels have been reported to vary significantly with location from about 1 to about 5 G's. Other rotating structures on this helicopter experience fundamental vibration frequencies that may be lower, such as the pitch links or control rods, which vibrate with the rotational rate of the rotor assembly of about 4.3 Hz, but which also contain higher frequency components. What is needed is an energy harvester design that will generate electricity efficiently under a wide range of vibration amplitudes and frequencies.

SUMMARY

One aspect of the present patent application is a device that comprises a composite structure. The composite structure includes a piezoelectric flexure and a length-constraining element. The length-constraining element provides the piezoelectric flexure with a bowed shape. The piezoelectric flexure has a first stable bowed position and a second stable bowed position. The length-constraining element is one from the group consisting of a planar sheet and a columnar rod.

Another aspect of the present patent application is a device, comprising a piezoelectric flexure having a bowl shape. The piezoelectric flexure has a first stable bowl-shaped position and a second stable bowl-shaped position.

Another aspect of the present patent application is a device, comprising a piezoelectric flexure and a first stop. The piezoelectric flexure generates electricity when the piezoelectric flexure strikes the first stop.

Another aspect of the present patent application is a device, comprising a bi-stable piezoelectric flexure and a circuit. The circuit includes a solid state voltage dependent switch and an inductor. The bi-stable piezoelectric flexure, the voltage dependent switch, and the inductor are all electrically connected in series.

Another aspect of the present patent application is a method of fabricating an energy harvesting device, comprising providing a piezoelectric flexure and a length-constraining element. The method also includes connecting the piezoelectric flexure and the length-constraining element, wherein the piezoelectric flexure is bowed and wherein the length-constraining element is one from the group consisting of a planar sheet and a columnar rod.

Another aspect of the present patent application is a method of fabricating an energy harvesting device, comprising providing a bowl shape in a material wherein the bowl shaped material is capable of two stable positions. The method also includes mounting a piezoelectric patch on the material.

Another aspect of the present patent application is a device, comprising a piezoelectric flexure and a restoring spring. The piezoelectric flexure has a first stable position and a second stable position. When the piezoelectric flexure snaps from the first stable position to the second stable position the restoring spring acts to restore the piezoelectric flexure to the first stable position.

DETAILED DESCRIPTION

In one embodiment of the present patent application a flexure element is used that is mechanically bi-stable. Piezoelectric flexure20of energy harvester22is stable at two extremes of its motion, as shown inFIGS. 1a,1b. During the transition in-between these two stable extremes piezoelectric flexure20“snaps” suddenly from stable position A to stable position B.

If enough inertial load is provided to mass24piezoelectric flexure20will snap between stable positions A and B at a wide range of ambient load frequencies or vibration frequencies. Energy harvester22may be considered to be “wideband” since it is capable of efficiently producing electrical energy at a wide range of ambient vibration frequencies. The range can be tailored by adjusting mass, stiffness of the piezoelectric flexure and its dimensions. It can even efficiently generate electricity with a single event provided the single events provide enough force to cause the piezoelectric flexure to snap to the other stable position. In another embodiment it can be combined with a tuned cantilever harvester to provide features of both.

The sudden change in position of piezoelectric flexure20from position A to B occurs because piezoelectric flexure20is under compressive pre-loading to create a curvature in piezoelectric flexure20. In this embodiment, piezoelectric flexure20generates electrical energy when this curvature is reversed based on the applied mechanical energy from vibration or load to the machine or structure to which it is attached. In one embodiment, composite cantilever beam26includes piezoelectric flexure20, length-constraining elements28located adjacent piezoelectric flexure20, and mass24, as shown inFIGS. 1a,1b, and inFIG. 2.

In this embodiment piezoelectric flexure20is longer than adjacent length-constraining elements28. Because they are shorter and mounted between the same support structure29and mass24, length-constraining elements28put piezoelectric flexure20under compression, causing piezoelectric flexure20to curve. When mass24is subjected to a sufficient load from either a directly applied force or from an acceleration due to vibration input, piezoelectric flexure20moves from one stable curved position to another. Length-constraining elements28are momentarily stretched during the transition. Thus, as mass24was deflected away from stable position A inFIG. 1a, and length-constraining element28was stretched, the curvature in piezoelectric flexure20rapidly reversed, and piezoelectric flexure20“snapped” its shape from convex to concave, as shown inFIG. 1b.

Adjustment of the mechanical compliance of piezoelectric flexure20can be made by changing its stiffness or by changing the amount of mass24. Stiffness of piezoelectric flexure20depends on the material of which substrate30is made and its cross sectional area, as well as the contribution to stiffness from piezoelectric flexure20.

In one embodiment, if a more compliant composite cantilever beam26is used with the same mass, energy harvester22can operate reliably in applications where the vibration amplitude includes lower G levels. The present inventors recognized that substrate30and length-constraining elements28both contribute to the stiffness of composite cantilever beam26and that the stiffness of composite cantilever beam26can be adjusted to match the expected vibration or loading amplitude. A softer more compliant composite cantilever beam26needs less mass to snap to the other stable position, given the same force. The mass can also be adjusted, with a larger mass delivering more force to composite cantilever beam26, allowing it to operate at a lower G level.

Support structure29moves with vibrating or oscillating component, machine, or structure40, providing energy to composite cantilever beam26. Mass24, connected to free end38of composite cantilever beam26is free to oscillate when subjected to vibration or movement. When mass24is subject to inertial loads or a directly applied force, substrate30suddenly snaps from stable position A to new stable position B, because these two positions represent the lowest energy state for substrate30with mass24. Substrate30may be constructed of hardened steel, titanium, or super elastic nickel-titanium.

Piezoelectric patches32a,32bbonded to the upper surface42and lower surface44of substrate30respectively are connected by lead wires46to energy harvester electronics47which receives electricity from piezoelectric patches32a,32bduring this sudden snapping event. The electricity can be used to drive light emitting diode48. The energy produced by these harvesters can be stored in one or more capacitors or the energy can be used to charge and re-charge thin film batteries, such as those available from Infinite Power Solutions (Golden, Colo.). The battery may be located within electronics enclosure49. Once enough energy has been stored, smart electronics modules, such as those described in the commonly assigned '693 patent, allow the load to draw from this energy store to perform a task. These tasks may include sampling of sensor data, storage of sensor data, sending data over a wireless link to another location, receiving data or instructions from another location, and/or storing and forwarding information to another location.

Piezoelectric patches32a,32bbonded to either side of substrate30produce large voltage pulses that may exceed 200 volts each time the sudden shape change snapping event occurs. As described in the 115-051 application, Capacitive Discharge Energy Harvesting (CDEH) converters are especially well suited for use with mechanical energy harvesting elements that receive energy from high voltage piezoelectric materials. In one embodiment significant charge is accumulated within the piezoelectric material itself, improving efficiency. In another embodiment, the voltage threshold upon which energy is released from the piezoelectric and into the energy storage elements of the circuit may be adjusted to take advantage of the voltage provided by the piezoelectric in actual operation.

The present applicants used a CDEH circuit, as described in the 115-051 application, to efficiently provide the voltage required by LED48. In one experiment, applicants combined the sudden pulse of energy from bonded piezoelectric patches with a CDEH circuit to light up a blue LED with every pulse. Preliminary measurements using a digital storage oscilloscope indicated that the energy generated from piezoelectric patches32a,32bexceeded 12 microJoules per pulse.

The energy provided by multiple pulses from piezoelectric patches32a,32band a CDEH circuit can be stored and used to power a wireless sensing node and a radio frequency (RF) communications module, such as an SG-LINK from MicroStrain, Inc. (Williston, Vt.). In preliminary experiments, the present applicants found that approximately 20 seconds of cycling at roughly 2 Hz generated sufficient energy to allow the SG-LINK to sample a 1000 ohm strain gauge and to transmit these data along with a unique radio node identification address (RFID).

Substrate30and length-constraining elements28can also be fabricated from a single sheet of material, such as spring steel, as shown inFIGS. 2b,2c. First, slots52are formed in material54by a process, such as machining, stamping, chemical etching, or laser cutting. Next, region56between slots52is lengthened by a process such as pressure from press58aon rigid hollowed press surface58bas shown inFIG. 2c, to provide a bowed shape to region56. The bowed shape permits two stable positions of region56. Next, piezoelectric patches are bonded to both surfaces of region56, a mass is added to one end of material54, and the other end is ready to be clamped to the support structure from which energy will be harvested.

Another embodiment of a snap action wideband vibration energy harvester is shown inFIG. 3. Piezoelectric flexure60of this embodiment is constrained at both ends62a,62bby V-grooves64a,64bmachined into end blocks66a,66brespectively. V-grooves64a,64bare designed to receive edges68a,68bof long curved piezoelectric flexure60. Edges68a,68bare machined as knife edges to remain in place in V-grooves64a,64bwhile maintaining the capability of piezoelectric flexure60to quickly change from convex to concave and vice versa upon loading of mass69. Loading of curved piezoelectric flexure60is from the force generated by the acceleration of mass69.

Adjustable length rods70a,70bhave threaded ends72a,72b,72c,72dthat extend through clearance holes74a,74b,74c,74din end blocks66a,66b. Adjustable length rods70a,70bcan be shortened or lengthened by changing the position of four threaded fasteners76a,76b,76c,76dat each threaded end72a,72b,72c,72dof adjustable length rods70a,70b. Shortening of adjustable length rods70a,70bcompresses piezoelectric flexure60, causing it to buckle and to have two stable positions. As adjustable length rods70a,70bare shortened, piezoelectric flexure60curves more and therefore experiences greater strain when snapping between its two stable positions. The shortening of rods70a,70balso increases the inertial load required to allow mass69to snap piezoelectric flexure60from one stable position to the other stable position. Inertial loads applied to mass69cause piezoelectric flexure60to snap from one to the other of the two distinct stable positions.

The inertial load required to snap piezoelectric flexure60may be adjusted by changing the stiffness of springs86a,86b,86c,86dwhich are positioned between end blocks66a,66band threaded fasteners76a,76b,76c,76d. Further adjustments may be made by changing the amount of mass69and/or the stiffness of piezoelectric flexure60. In the embodiment depicted inFIG. 3, end blocks66a, is fixed relative to the vibrating or oscillating component, machine, or structure and vibrate with that structure. End block66bis free. An inertial load from the vibration applied to mass69causes piezoelectric flexure60to snap from one to the other stable position. An applied load, such as from a finger or foot, can also be used, and in this case no mass is needed.

Piezoelectric patches88bonded to upper and lower surfaces90of substrate92provide energy through lead wires94to energy harvester electronics module96. Electrical energy provided may be used to illuminate a light emitting diode or may be stored in a battery which may be located in a compartment within an enclosure along with the electronics that may be located in or connected to end block66bsimilar to that shown inFIGS. 1a,1band2.

In one embodiment, bowl shaped substrate100is bi-stable, as shown inFIGS. 4a,4b. Bowl shaped substrate100can either have a concave bowl shape, as viewed inFIG. 4a, or it can snap to a convex bowl shape, as viewed inFIG. 4b. Substrate100is mounted to a support structure at substrate end102and has mass104mounted to free end106. Mass104is attached in central location107allowing substrate102to snap from one stable bowl shape to another. Substrate100may have center region108cut out, facilitating snapping between its two stable positions. Piezoelectric patches110a,110b,110c110dare bonded to upper and lower surfaces of substrate100, and these patches generate a pulse of electricity every time substrate100snaps between its stable positions.

Bowl shaped substrate is curved in two planes, as shown by curves111a,111bofFIG. 4aand curves111a′,111b′ ofFIG. 4b.

Bowl shaped substrate100may be fabricated of a material such as spring steel. Using press116that has curvature in two planes, as shown inFIGS. 5a,5b, spring steel substrate100is pressed against rigid form118with press116to provide substrate100with concave curvature in two planes: a bowl shape. With this bowl shaped curvature provided, substrate100now can snap between two stable positions, as shown inFIGS. 4a,4b.

Bowl shaped substrate122can also be fabricated by cutting out slot124in flat substrate126, as shown inFIG. 6a. Ends128a,128bare then connected together to provide substrate122with a bowl shape with tear drop shaped slot124′, as shown inFIG. 6b. Ends128a,128bmay be connected with a weld or rivet. Piezoelectric patches129are bonded to upper and lower surfaces of substrate122. With this bowl shaped curvature, substrate122now can snap between two stable positions, as shown inFIG. 4a,4b.

In many uses, energy may be obtained from the bowl shaped piezoelectric flexure so formed when it snaps in each direction. For example, when mounted on a vibrating machine, the vibration may equally force bowl shaped piezoelectric flexure from one stable position to the other and back again to the first due to the inertial load created by acceleration of the mass which is affixed to the bowl shaped piezoelectric flexure. However, in some applications, a force is available primarily in one direction. For example, a force may be provided to piezoelectric flexure130by a person's foot primarily in a downward direction when the person is walking, as shown inFIG. 7a,7b. In this embodiment, after bowl shaped piezoelectric flexure130snaps toward stable position2from stable position1, bowl shaped piezoelectric flexure130comes in contact with spring132located in recessed area134, as shown inFIGS. 7b-7c, to restore piezoelectric flexure130to its ready position between steps when the force is removed, as shown inFIG. 7d.

While electricity is generated when piezoelectric flexure130snaps in either direction, the substantially greater downward force needed to overcome both the tension in bowl shaped piezoelectric flexure130and to generate a restoring spring force means that the mechanical energy in both directions ultimately comes from the stepping action.

An embodiment of an energy harvesting device that has cantilever beam200well protected from overloads, allows cantilever beam200to be very compliant, as shown inFIG. 8. Tapered cantilever beam200may be constructed of hardened steel, titanium, or super elastic nickel-titanium (Nitinol, Memry Corp). The taper provides a constant strain field in the area where piezoelectric patches202a,202bare bonded to the cantilever beam200, as described in the 115-002 application.

Vibration and/or inertial loads applied to mass204cause cantilever beam200to move within upper and lower constraints defined by curved surfaces206a,206bof housing208. Curved surfaces206a,206ballow cantilever beam200to oscillate over a wide range of vibration levels without risk of failure due to fatigue of cantilever beam200or damage to piezoelectric patches202a,202bbonded to cantilever beam200. Thus, cantilever beam200can be very compliant and cantilever beam200will still generate electrical energy without breaking even when vibration amplitude is high.

Cantilever beam200is clamped within housing208in area210and is free to oscillate and vibrate from clamped line A-A′ to free end212. Mass204on free end212of cantilever beam200oscillates due to vibration of the component, machine, or structure to which housing208is affixed.

Housing208also contains energy harvesting electronic module214which is wired to piezoelectric patches202a,202band to a battery in battery compartment216.

In another embodiment, compliant energy harvesting device218provides protection from overloads and provides electrical generation over a wide range of excitation frequencies, as shown inFIG. 9a. Discrete stops220a,220band222a,222bprovide fulcra around which tapered cantilever beam224rotates while limiting the strain experienced by tapered cantilever beam224, preventing overload. Stops220a,220band222a,222balso provide higher frequency resonance points as the effective length of tapered cantilever beam224is reduced when it encounters each stop. The reduction in effective length of tapered cantilever beam224will be accompanied by an increase in its natural (resonant) frequency as dictated by the following equation for a cantilever beam.
Wn2=3EI/l3
Where Wn is the natural frequency of tapered cantilever beam224, E is its Young's modulus, I is its moment of inertia, and l is its length.

In this embodiment, electrical energy is collected by piezoelectric patches226a,226bbonded to the upper and lower surfaces of cantilever beam224each time cantilever beam224strikes stops220a,220band222a,222b.

In one embodiment, wideband energy harvester may include pivot230through section A-A′. Pivot230may include pinned joint232, allowing cantilever beam224to freely move between stops220a,220band222a,222b. Pinned joint232can be thinned-down section234within cantilever beam224, as shown in the detail of section A-A′ inFIG. 9b. Lead wires238emanating from piezoelectric patches,226a,226bcross over pivot230and connect to electronics module240. Lead wires238are coiled in order to prevent fatigue due to cyclic motion in the area of pivot230.

Pivot230introduces an extremely high compliance to rotation of cantilever beam224. In this energy harvesting system, cantilever beam224is unconstrained in all positions, except when it bangs against a stop. Under conditions of vibration or cyclic loading, beam224will rock or bang between stops220a,220band222a,222b. When beam224encounters these stops220a,220band222a,222b, strain is created in piezoelectric226a,226bwhich in turn generates energy that is harvested by electronics module240.

Harvesting energy with energy harvesting device218with pivot230begins when enough vibration amplitude is present to cause the mechanically unstable mass246to oscillate and thereby cause cantilever beam224to cycle between the two mechanically stable end stop positions220a,220band222a,222b. Because pivot230is designed to be extremely compliant torsionally, mass246will move under conditions of low frequency vibration as well as higher frequencies. As shown, pivot230has a very thin section within cantilever beam224. This thin section can be machined or formed with a press, allowing pivot230to act as a pinned joint that little resists rotation of cantilever beam224.

Compliant energy harvesting device218can be optimized for a given application by adjusting the compliance of pivot230, the length of cantilever beam224, the position of stops220a,220band222a,222brelative to pivot230, and the compliance of stops220a,220band222a,222b.

Compliant wideband energy harvesting device218can be mounted in any position. For example, it can be mounted in a vertical orientation relative to gravity, so that mass246hangs downward like a pendulum, with pivot230located above mass246. In this orientation, side to side motion of the component, structure, or machine to which housing248is affixed will cause cantilever beam224to encounter stops220a,220band222a,222band generate energy.

Alternatively, pivot and cantilever beam may be located below mass246. Cantilever beam224will have stable positions when resting against stops220a,220band222a,222b. In this case, two very compliant springs244may used to maintain cantilever beam224and its mass246in a midline relative to stops220a,220band222a,222bunder conditions of no vibration, as shown inFIGS. 10a,10b.

Cantilever beam224can also be mounted in a horizontal orientation, as shown inFIG. 9a. In this case, a single light spring244may be used to counteract the moment created by the weight of mass246. One end of spring244would be connected to cantilever beam224and the other end to housing248. Spring244would be placed so that cantilever beam224will remain in a mid position under conditions of no vibration. When placed in a vibrating environment, cantilever beam224will move rapidly between the stops220a,220band222a,222b, resulting in strain in cantilever beam224and in piezoelectric patches mounted to cantilever beam224, which generates energy which is harvested by the electronics module.

The energy harvesting devices of the present application can be used for radio frequency identification tags for tracking inventoried items, pallets, components, subassemblies, and assemblies. With the energy harvesting devices described herein, consumable batteries would no longer be needed for operation, and all energy could be derived from movement or from a direct force input, such as a push button snap action switch. The push button switch generates energy by direct application of force to snap the beam from one curved shape to another curved shape.

The energy harvesting devices can also be used in shoes for children, runners, and bicycle riders to provide electrical energy. For example the shoes may include a light that lights up or flashes when subject to direct pressure from walking, or from the changing inertial load of running, thereby making the wearer more visible to vehicles and increasing the safety of the wearer.

Toys, such as a handheld shaker that lights up when shaken, also could be used with the energy harvesting device of the present application. All energy could be derived from mechanical movement, such as shaking.

A wireless switch also could be used with the energy harvesting device of the present application that in which pressing the button of the switch provides a force that causes the bi-stable element to snap, generating enough electrical energy to wirelessly transmit an RFID signal. When received by a processor, the processor switches a relay that controls a light or any other device.

The energy harvesting device of the present application can also be mounted on a fishing lure such that sufficient energy is harvested to light up an LED when the lure is moved through the water.

The energy harvesting device of the present application can also be mounted on a rotating part, such as a drive shaft, for powering sensors that sample and store the operating load of the drive shaft, and that record its loading history.

The energy harvesting device of the present application can also be mounted on a structure or vehicle, such as an airframe, earth moving equipment, a bridge, dam, building, or other civil structure for powering sensors that sample and store operating strain, and/or loads and record strain and/or loading history. Networks of such wireless energy harvesting nodes may be deployed as appropriate in order to gain insight and knowledge about the overall behavior of the structure or vehicle.

In each of the above applications a battery can be used for storing energy harvested by the energy harvesting device, and the batteries can be automatically recharged without user intervention or maintenance.

As described in commonly assigned U.S. patent application Ser. No. 12/009,945 (“the '945 application”), the present applicants designed circuit300a,300bthat substantially improves energy conversion efficiency, as shown inFIGS. 11a,11bthat are derived from that patent application. Circuits300a,300btake advantage of intrinsic capacitance302of piezoelectric device304to store charge generated from mechanical strain or vibration, providing this storage at the high voltage of the piezoelectric device and eliminating loss from charging another potentially mismatched capacitor. One side of piezoelectric device304is connected to ground. Diodes provide a positive polarity to the entire electrical signal generated from the back and forth movement of the piezoelectric device. Once a threshold voltage has been reached voltage dependent switch306in the circuit rapidly discharges that stored charge through a rectifier and through a high speed switch to inductor and capacitor network308a,308bthat converts to a lower DC voltage suitable for use powering electronic circuits. Because the entire charge on intrinsic capacitance302of piezoelectric device304is rapidly discharged no oscillator is needed for this DC-DC conversion. Eliminating the oscillator removes an important source of power consumption while maintaining a high efficiency energy transfer.

Unlike previous converter designs, in the present embodiment, when switch306is off piezoelectric device304is not substantially loaded, and is disconnected from almost all sources of loss. Thus, its voltage can rise quickly to a high value when mechanical energy is applied to piezoelectric device304. Only when the voltage across piezoelectric device304has risen to the threshold of voltage dependent switch306, and voltage dependent switch306turns on, is energy first drawn from piezoelectric device304to ultimately charge storage capacitor310. A battery can be used in place of or in addition to capacitor310. Threshold is chosen to be slightly less than the expected open circuit voltage for expected mechanical excitations. In one embodiment threshold was set to 140 volts. In previous designs, such as the embodiments described in the '693 patent, current was drawn from the piezoelectric device as soon as the generated voltage exceeded the two diode forward drops of the full wave rectifier plus the voltage from charge already stored in the storage capacitor from previous energy conversions. These previous designs wasted energy because they did not allow voltage to rise to a high value. By contrast, in the circuit ofFIGS. 2a,2bof the '945 application, by delaying transfer of charge until the threshold voltage is reached, the present circuit design can achieve substantially higher energy conversion efficiency. The threshold voltage is set to be slightly less than the expected open circuit voltage to achieve greatest efficiency.

Energy stored in a capacitance can be described as
E=½CV2
where C is the capacitance, and V is the voltage across the capacitance. Because the energy stored depends on the square of the voltage, high voltage type piezoelectric materials provide substantial advantage. However, the high voltage and high impedance of such materials also introduces difficulty in converting to the low voltage and low impedance needed by typical electronic circuits. By using intrinsic capacitance302of piezoelectric device304instead of providing a separate capacitor, as in the '693 patent, the present inventors found a way to retain the high voltage and high impedance through this first stage of charge storage, significantly improving energy conversion efficiency.

Piezoelectric device304is modeled as generator320with intrinsic capacitance302in parallel, as shown inFIGS. 11a,11b. As mechanical energy is applied to piezoelectric device304on its dependent axis, intrinsic capacitance302is charged to a voltage proportional to the applied mechanical energy. One embodiment, further described herein below, provides that when the voltage on capacitance302reaches a preset threshold, switch306closes, allowing the charge on capacitance302to flow into inductor322. Inductor322stores energy in a magnetic field while switch306is closed and current is flowing from intrinsic capacitor302in piezoelectric device304. When intrinsic capacitor302has discharged to a second threshold voltage, voltage dependent switch306opens, current through inductor322decreases rapidly, and this magnetic field around inductor322collapses. The second threshold voltage may be set to provide for nearly complete discharge of intrinsic capacitor302. The rapid reduction in current and rapid collapse of the magnetic field when switch306opens induces a voltage across inductor322according to the equation
V=LDi/DT

This induced voltage across inductor322provides a current through diode324,324′ charging large storage capacitor310. This voltage across storage capacitor310is substantially lower than the voltage across piezoelectric device304. A correspondingly higher charge is stored on capacitor310.

The present applicants designed an efficient voltage dependent switch with very low off state leakage current and a very low on state resistance to enable operation of this circuit, as shown inFIGS. 12a,12bthat are derived from the '945 application. Because piezoelectric device304,304′ voltage dependent switch306,306′ and inductor322are all in series, leakage current through voltage dependent switch306,306′ does not detract from the efficiency of the circuit. Leakage current just goes to charge storage capacitor310.

To operate most efficiently, switch306,306′ closes at a first threshold when the voltage on intrinsic capacitance302is slightly less than the expected maximum open circuit voltage piezoelectric device304,304′ will attain for the mechanical energy input. Switch306,306′ later opens at a second threshold when intrinsic capacitance302is nearly discharged. Switch306has been designed to attain a very low resistance quickly when closed to avoid resistive losses. It also has a very high resistance when open, allowing very little leakage current.

The more detailed embodiment of the circuit ofFIGS. 11a,11bshown inFIGS. 12a,12bincludes voltage dependent switch306′ that includes Darlington transistors330and340. Each of these transistors needs only micro-ampere base currents to turn on, and the Darlington arrangement provides a very high gain. The two Darlington transistors330and340are arranged in the circuit so that the turning on one causes the other to also turn on and vice versa. The two Darlington transistors330,340remain latched up until intrinsic capacitance302of piezoelectric element304′ has nearly discharged and the voltage provided from intrinsic capacitance302has declined to close to zero. At that point Darlington transistors330and340turn off and reset for the next time charge is available from piezoelectric device304′. P.P. Darlington transistor330has part number FZT705 and NPN Darlington transistor340has part number FZT605. Both are available from Exodus, Manchester, UK.

Darlington transistor340remains off while the voltage across its base emitter junction1-3remains below its 1.2 volt turn on threshold. This voltage is controlled by a voltage divider formed by resistors342and344. In practice, any leakage current through Darlington transistor330from collector to emitter adds to the current through resistor342and forms part of this voltage divider. When a threshold of approximately 150 volts is provided by piezoelectric device304′ and applied across voltage dependent switch306′, the voltage at transistor340base emitter junction, reaches the 1.2 volt turn-on threshold, and transistor340turns on. The voltage across resistor346and across the base-emitter junction from pins2-3of Darlington transistor330now also equals at least 1.2 volts, and transistor330turns on. This provides a high voltage to the base at pin1of Darlington transistor340, keeping the transistor on. While the two Darlington transistors330,340remain thus latched up, intrinsic capacitance of piezoelectric element304′ is nearly completely discharged into inductor322through diode360. Voltage dependent switch306′ continues to conduct until the intrinsic capacitance of piezoelectric element304′ is nearly completely discharged.

Since voltage dependent switch306′ always turns on at the same threshold voltage, and since the intrinsic capacitance of the piezoelectric device is also a constant, every closure of switch306′ transfers the same amount of energy, independent of the energy of the mechanical event producing it, so long as the energy of the mechanical event is sufficient to reach the threshold.

Rather than using a full wave bridge rectifier as in the embodiments of the '693 patent, one side of piezoelectric device304′ is connected to ground and shunt diode365is used to provide that the entire signal from piezoelectric element304′ and its intrinsic capacitance302is positive. Thus, the peak voltage provided by piezoelectric element304′ is twice the value that would be provided from the same mechanical excitation applied to a circuit using a full wave bridge rectifier that provides a signal centered at 0 volts.

While this half wave rectifier configuration is desirable for applications where mechanical energy input is cyclic, a full wave bridge rectifier can be used where mechanical energy input is random in frequency or is of unknown direction. With a full wave rectifier, half the voltage is reached but twice as often. Thus, the type of rectifier used determines both the magnitude of the voltage achieved and how often the switch fires.

While the disclosed methods and systems have been shown and described in connection with illustrated embodiments, various changes may be made therein without departing from the spirit and scope of the invention as defined in the appended claims.