Abstract:
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.

Description:
RELATED APPLICATIONS AND PRIORITY 
     This application claims priority of Provisional Patent Application 60/898,160, filed Jan. 29, 2007, incorporated herein by reference. 
    
    
     This application is related to the following commonly assigned patent applications: 
     “Energy Harvesting for Wireless Sensor Operation and Data Transmission,” U.S. Pat. No. 7,081,693 to M. Hamel et al., filed Mar. 5, 2003 (“the &#39;693 patent”). 
     “Shaft Mounted Energy Harvesting for Wireless Sensor Operation and Data Transmission,” U.S. patent application Ser. No. 10/769,642 to S. W. Arms et al., filed Jan. 31, 2004 (“the &#39;642 application”). 
     “Slotted Beam Piezoelectric Composite,” U.S. patent application Ser. No. 11/604,117 to D. L. Churchill, filed Nov. 24, 2006, (“the &#39;117 application”). 
     “Energy Harvesting, Wireless Structural Health Monitoring System,” U.S. patent application Ser. No. 11/518,777 to S. W. Arms et al., filed Sep. 11, 2006 (“the &#39;777 application”). 
     “Sensor Powered Event Logger,” U.S. patent application Ser. No. 11/644,038 to D. L. Churchill et al., filed Dec. 22, 2006 (“the &#39;038 application”). 
     “Integrated Piezoelectric Composite and Support Circuit,” U.S. patent application Ser. No. 11/644,334 to D. L. Churchill et al., filed Dec. 22, 2006 (“the &#39;334 application”). 
     “Heat Stress, Plant Stress and Plant Health Monitor System,” U.S. patent application Ser. No. 11/899,840 to C. P. Townsend et al., filed Sep. 7, 2007 (“the &#39;840 application”). 
     “A Capacitive Discharge Energy Harvesting Converter,” U.S. patent application Ser. No. 12/009,945 to M. J. Hamel &amp; D. L. Churchill, filed Jan. 23, 2008 (“the 115-051 application”). 
     All of the above listed patents and patent applications are incorporated herein by reference. 
     BACKGROUND 
     The vibration energy harvesting beam described in the &#39;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 &#39;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&#39;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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1   a ,  1   b  are side views of one embodiment of a wideband energy harvester with a composite cantilever beam that includes a piezoelectric flexure and length-constraining elements located adjacent the piezoelectric flexure that allow the composite cantilever beam to snap between two stable positions; 
         FIG. 2   a  is a top view of the wideband energy harvester shown in  FIGS. 1   a ,  1   b;    
         FIG. 2   b  is a top view of a step in one embodiment of a process for fabricating a wideband energy harvester; 
         FIG. 2   c  is a side view of another step in the embodiment of a process of fabricating a wideband energy harvester; 
         FIG. 3  is another embodiment of a wideband energy harvester with a composite cantilever beam that includes a piezoelectric flexure and length-constraining elements located adjacent the piezoelectric flexure that allow the composite cantilever beam to snap between two stable positions; 
         FIGS. 4   a ,  4   b  are three dimensional views of another embodiment of a wideband energy harvester with a bowl shaped substrate that allow the substrate to snap between two stable positions; 
         FIGS. 5   a ,  5   b  are side and end views of a step in one embodiment of a process for fabricating a bowl shaped wideband energy harvester; 
         FIGS. 6   a ,  6   b  are top views of steps in another embodiment of a process for fabricating a bowl shaped wideband energy harvester; 
         FIGS. 7   a - 7   d  are side views illustrating how a bowl shaped wideband energy harvester may be biased so it can be used with a force provided in only one direction; 
         FIG. 8  is an embodiment of a compliant vibration harvester that uses a tapered flexure element and curved overload constraint; 
         FIG. 9   a  is another embodiment of a compliant vibration harvester with discrete end stops to prevent overload and provides a point that introduces curvature in the piezoelectric flexure; 
         FIG. 9   b  is another embodiment of a compliant piezoelectric flexure with a mechanical pivot that allows the piezoelectric flexure to be highly compliant, allowing it to oscillate at a wide range of frequencies; 
         FIGS. 10   a ,  10   b  are an embodiment of the compliant vibration harvester of  FIGS. 9   a,    9   b  with springs to counter the force of gravity; 
         FIGS. 11   a,    11   b  are embodiments of circuits derived from commonly assigned U.S. patent application Ser. No. 12/009,945 that take advantage of intrinsic capacitance of a piezoelectric device and that provide this storage at the high voltage of the device through a rectifier and a voltage dependent switch to an inductor and capacitor network; and 
         FIGS. 12   a,    12   b  show more detailed embodiments of the circuits of  FIGS. 11   a,    11   b.    
     
    
    
     DETAILED DESCRIPTION 
     In one embodiment of the present patent application a flexure element is used that is mechanically bi-stable. Piezoelectric flexure  20  of energy harvester  22  is stable at two extremes of its motion, as shown in  FIGS. 1   a ,  1   b . During the transition in-between these two stable extremes piezoelectric flexure  20  “snaps” suddenly from stable position A to stable position B. 
     If enough inertial load is provided to mass  24  piezoelectric flexure  20  will snap between stable positions A and B at a wide range of ambient load frequencies or vibration frequencies. Energy harvester  22  may 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 flexure  20  from position A to B occurs because piezoelectric flexure  20  is under compressive pre-loading to create a curvature in piezoelectric flexure  20 . In this embodiment, piezoelectric flexure  20  generates 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 beam  26  includes piezoelectric flexure  20 , length-constraining elements  28  located adjacent piezoelectric flexure  20 , and mass  24 , as shown in  FIGS. 1   a ,  1   b , and in  FIG. 2 . 
     In this embodiment piezoelectric flexure  20  is longer than adjacent length-constraining elements  28 . Because they are shorter and mounted between the same support structure  29  and mass  24 , length-constraining elements  28  put piezoelectric flexure  20  under compression, causing piezoelectric flexure  20  to curve. When mass  24  is subjected to a sufficient load from either a directly applied force or from an acceleration due to vibration input, piezoelectric flexure  20  moves from one stable curved position to another. Length-constraining elements  28  are momentarily stretched during the transition. Thus, as mass  24  was deflected away from stable position A in  FIG. 1   a , and length-constraining element  28  was stretched, the curvature in piezoelectric flexure  20  rapidly reversed, and piezoelectric flexure  20  “snapped” its shape from convex to concave, as shown in  FIG. 1   b.    
     Piezoelectric flexure  20  includes substrate  30  and piezoelectric patches  32   a ,  32   b  mounted to substrate  30 . End  34  of composite cantilever beam  26  is fixedly mounted to support structure  29  while end  38  of composite cantilever beam  26  includes mass  24 . Piezoelectric material called “macro fiber composites” are available from Smart Materials, Inc., (Sarasota, Fla.), and piezoelectric fiber composites are available from Advanced Cerametrics, Inc, (Lambertville, N.J.). 
     Adjustment of the mechanical compliance of piezoelectric flexure  20  can be made by changing its stiffness or by changing the amount of mass  24 . Stiffness of piezoelectric flexure  20  depends on the material of which substrate  30  is made and its cross sectional area, as well as the contribution to stiffness from piezoelectric flexure  20 . 
     In one embodiment, if a more compliant composite cantilever beam  26  is used with the same mass, energy harvester  22  can operate reliably in applications where the vibration amplitude includes lower G levels. The present inventors recognized that substrate  30  and length-constraining elements  28  both contribute to the stiffness of composite cantilever beam  26  and that the stiffness of composite cantilever beam  26  can be adjusted to match the expected vibration or loading amplitude. A softer more compliant composite cantilever beam  26  needs 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 beam  26 , allowing it to operate at a lower G level. 
     Support structure  29  moves with vibrating or oscillating component, machine, or structure  40 , providing energy to composite cantilever beam  26 . Mass  24 , connected to free end  38  of composite cantilever beam  26  is free to oscillate when subjected to vibration or movement. When mass  24  is subject to inertial loads or a directly applied force, substrate  30  suddenly snaps from stable position A to new stable position B, because these two positions represent the lowest energy state for substrate  30  with mass  24 . Substrate  30  may be constructed of hardened steel, titanium, or super elastic nickel-titanium. 
     Piezoelectric patches  32   a ,  32   b  bonded to the upper surface  42  and lower surface  44  of substrate  30  respectively are connected by lead wires  46  to energy harvester electronics  47  which receives electricity from piezoelectric patches  32   a ,  32   b  during this sudden snapping event. The electricity can be used to drive light emitting diode  48 . 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 enclosure  49 . Once enough energy has been stored, smart electronics modules, such as those described in the commonly assigned &#39;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 patches  32   a ,  32   b  bonded to either side of substrate  30  produce 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 LED  48 . 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 patches  32   a ,  32   b  exceeded 12 microJoules per pulse. 
     The energy provided by multiple pulses from piezoelectric patches  32   a ,  32   b  and 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). 
     Substrate  30  and length-constraining elements  28  can also be fabricated from a single sheet of material, such as spring steel, as shown in  FIGS. 2   b ,  2   c . First, slots  52  are formed in material  54  by a process, such as machining, stamping, chemical etching, or laser cutting. Next, region  56  between slots  52  is lengthened by a process such as pressure from press  58   a  on rigid hollowed press surface  58   b  as shown in  FIG. 2   c , to provide a bowed shape to region  56 . The bowed shape permits two stable positions of region  56 . Next, piezoelectric patches are bonded to both surfaces of region  56 , a mass is added to one end of material  54 , 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 in  FIG. 3 . Piezoelectric flexure  60  of this embodiment is constrained at both ends  62   a ,  62   b  by V-grooves  64   a ,  64   b  machined into end blocks  66   a ,  66   b  respectively. V-grooves  64   a ,  64   b  are designed to receive edges  68   a ,  68   b  of long curved piezoelectric flexure  60 . Edges  68   a ,  68   b  are machined as knife edges to remain in place in V-grooves  64   a ,  64   b  while maintaining the capability of piezoelectric flexure  60  to quickly change from convex to concave and vice versa upon loading of mass  69 . Loading of curved piezoelectric flexure  60  is from the force generated by the acceleration of mass  69 . 
     Adjustable length rods  70   a ,  70   b  have threaded ends  72   a ,  72   b ,  72   c ,  72   d  that extend through clearance holes  74   a ,  74   b ,  74   c ,  74   d  in end blocks  66   a ,  66   b . Adjustable length rods  70   a ,  70   b  can be shortened or lengthened by changing the position of four threaded fasteners  76   a ,  76   b ,  76   c ,  76   d  at each threaded end  72   a ,  72   b ,  72   c ,  72   d  of adjustable length rods  70   a ,  70   b . Shortening of adjustable length rods  70   a ,  70   b  compresses piezoelectric flexure  60 , causing it to buckle and to have two stable positions. As adjustable length rods  70   a ,  70   b  are shortened, piezoelectric flexure  60  curves more and therefore experiences greater strain when snapping between its two stable positions. The shortening of rods  70   a ,  70   b  also increases the inertial load required to allow mass  69  to snap piezoelectric flexure  60  from one stable position to the other stable position. Inertial loads applied to mass  69  cause piezoelectric flexure  60  to snap from one to the other of the two distinct stable positions. 
     The inertial load required to snap piezoelectric flexure  60  may be adjusted by changing the stiffness of springs  86   a ,  86   b ,  86   c ,  86   d  which are positioned between end blocks  66   a ,  66   b  and threaded fasteners  76   a ,  76   b ,  76   c ,  76   d . Further adjustments may be made by changing the amount of mass  69  and/or the stiffness of piezoelectric flexure  60 . In the embodiment depicted in  FIG. 3 , end blocks  66   a , is fixed relative to the vibrating or oscillating component, machine, or structure and vibrate with that structure. End block  66   b  is free. An inertial load from the vibration applied to mass  69  causes piezoelectric flexure  60  to 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 patches  88  bonded to upper and lower surfaces  90  of substrate  92  provide energy through lead wires  94  to energy harvester electronics module  96 . 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 block  66   b  similar to that shown in  FIGS. 1   a ,  1   b  and  2 . 
     In one embodiment, bowl shaped substrate  100  is bi-stable, as shown in  FIGS. 4   a ,  4   b . Bowl shaped substrate  100  can either have a concave bowl shape, as viewed in  FIG. 4   a , or it can snap to a convex bowl shape, as viewed in  FIG. 4   b . Substrate  100  is mounted to a support structure at substrate end  102  and has mass  104  mounted to free end  106 . Mass  104  is attached in central location  107  allowing substrate  102  to snap from one stable bowl shape to another. Substrate  100  may have center region  108  cut out, facilitating snapping between its two stable positions. Piezoelectric patches  110   a ,  110   b ,  110   c    110   d  are bonded to upper and lower surfaces of substrate  100 , and these patches generate a pulse of electricity every time substrate  100  snaps between its stable positions. 
     Bowl shaped substrate is curved in two planes, as shown by curves  111   a ,  111   b  of  FIG. 4   a  and curves  111   a ′,  111   b ′ of  FIG. 4   b.    
     Bowl shaped substrate  100  may be fabricated of a material such as spring steel. Using press  116  that has curvature in two planes, as shown in  FIGS. 5   a ,  5   b , spring steel substrate  100  is pressed against rigid form  118  with press  116  to provide substrate  100  with concave curvature in two planes: a bowl shape. With this bowl shaped curvature provided, substrate  100  now can snap between two stable positions, as shown in  FIGS. 4   a ,  4   b.    
     Bowl shaped substrate  122  can also be fabricated by cutting out slot  124  in flat substrate  126 , as shown in  FIG. 6   a . Ends  128   a ,  128   b  are then connected together to provide substrate  122  with a bowl shape with tear drop shaped slot  124 ′, as shown in  FIG. 6   b . Ends  128   a ,  128   b  may be connected with a weld or rivet. Piezoelectric patches  129  are bonded to upper and lower surfaces of substrate  122 . With this bowl shaped curvature, substrate  122  now can snap between two stable positions, as shown in  FIG. 4   a ,  4   b.    
     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 flexure  130  by a person&#39;s foot primarily in a downward direction when the person is walking, as shown in  FIG. 7   a ,  7   b . In this embodiment, after bowl shaped piezoelectric flexure  130  snaps toward stable position  2  from stable position  1 , bowl shaped piezoelectric flexure  130  comes in contact with spring  132  located in recessed area  134 , as shown in  FIGS. 7   b - 7   c , to restore piezoelectric flexure  130  to its ready position between steps when the force is removed, as shown in  FIG. 7   d.    
     While electricity is generated when piezoelectric flexure  130  snaps in either direction, the substantially greater downward force needed to overcome both the tension in bowl shaped piezoelectric flexure  130  and 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 beam  200  well protected from overloads, allows cantilever beam  200  to be very compliant, as shown in  FIG. 8 . Tapered cantilever beam  200  may 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 patches  202   a ,  202   b  are bonded to the cantilever beam  200 , as described in the 115-002 application. 
     Vibration and/or inertial loads applied to mass  204  cause cantilever beam  200  to move within upper and lower constraints defined by curved surfaces  206   a ,  206   b  of housing  208 . Curved surfaces  206   a ,  206   b  allow cantilever beam  200  to oscillate over a wide range of vibration levels without risk of failure due to fatigue of cantilever beam  200  or damage to piezoelectric patches  202   a ,  202   b  bonded to cantilever beam  200 . Thus, cantilever beam  200  can be very compliant and cantilever beam  200  will still generate electrical energy without breaking even when vibration amplitude is high. 
     Cantilever beam  200  is clamped within housing  208  in area  210  and is free to oscillate and vibrate from clamped line A-A′ to free end  212 . Mass  204  on free end  212  of cantilever beam  200  oscillates due to vibration of the component, machine, or structure to which housing  208  is affixed. 
     Housing  208  also contains energy harvesting electronic module  214  which is wired to piezoelectric patches  202   a ,  202   b  and to a battery in battery compartment  216 . 
     In another embodiment, compliant energy harvesting device  218  provides protection from overloads and provides electrical generation over a wide range of excitation frequencies, as shown in  FIG. 9   a . Discrete stops  220   a ,  220   b  and  222   a ,  222   b  provide fulcra around which tapered cantilever beam  224  rotates while limiting the strain experienced by tapered cantilever beam  224 , preventing overload. Stops  220   a ,  220   b  and  222   a ,  222   b  also provide higher frequency resonance points as the effective length of tapered cantilever beam  224  is reduced when it encounters each stop. The reduction in effective length of tapered cantilever beam  224  will be accompanied by an increase in its natural (resonant) frequency as dictated by the following equation for a cantilever beam.
 
 Wn   2 =3 EI/l   3  
 
Where Wn is the natural frequency of tapered cantilever beam  224 , E is its Young&#39;s modulus, I is its moment of inertia, and l is its length.
 
     In this embodiment, electrical energy is collected by piezoelectric patches  226   a ,  226   b  bonded to the upper and lower surfaces of cantilever beam  224  each time cantilever beam  224  strikes stops  220   a ,  220   b  and  222   a ,  222   b.    
     In one embodiment, wideband energy harvester may include pivot  230  through section A-A′. Pivot  230  may include pinned joint  232 , allowing cantilever beam  224  to freely move between stops  220   a ,  220   b  and  222   a ,  222   b . Pinned joint  232  can be thinned-down section  234  within cantilever beam  224 , as shown in the detail of section A-A′ in  FIG. 9   b . Lead wires  238  emanating from piezoelectric patches,  226   a ,  226   b  cross over pivot  230  and connect to electronics module  240 . Lead wires  238  are coiled in order to prevent fatigue due to cyclic motion in the area of pivot  230 . 
     Pivot  230  introduces an extremely high compliance to rotation of cantilever beam  224 . In this energy harvesting system, cantilever beam  224  is unconstrained in all positions, except when it bangs against a stop. Under conditions of vibration or cyclic loading, beam  224  will rock or bang between stops  220   a ,  220   b  and  222   a ,  222   b . When beam  224  encounters these stops  220   a ,  220   b  and  222   a ,  222   b , strain is created in piezoelectric  226   a ,  226   b  which in turn generates energy that is harvested by electronics module  240 . 
     Harvesting energy with energy harvesting device  218  with pivot  230  begins when enough vibration amplitude is present to cause the mechanically unstable mass  246  to oscillate and thereby cause cantilever beam  224  to cycle between the two mechanically stable end stop positions  220   a ,  220   b  and  222   a ,  222   b . Because pivot  230  is designed to be extremely compliant torsionally, mass  246  will move under conditions of low frequency vibration as well as higher frequencies. As shown, pivot  230  has a very thin section within cantilever beam  224 . This thin section can be machined or formed with a press, allowing pivot  230  to act as a pinned joint that little resists rotation of cantilever beam  224 . 
     Compliant energy harvesting device  218  can be optimized for a given application by adjusting the compliance of pivot  230 , the length of cantilever beam  224 , the position of stops  220   a ,  220   b  and  222   a ,  222   b  relative to pivot  230 , and the compliance of stops  220   a ,  220   b  and  222   a ,  222   b.    
     Compliant wideband energy harvesting device  218  can be mounted in any position. For example, it can be mounted in a vertical orientation relative to gravity, so that mass  246  hangs downward like a pendulum, with pivot  230  located above mass  246 . In this orientation, side to side motion of the component, structure, or machine to which housing  248  is affixed will cause cantilever beam  224  to encounter stops  220   a ,  220   b  and  222   a ,  222   b  and generate energy. 
     Alternatively, pivot and cantilever beam may be located below mass  246 . Cantilever beam  224  will have stable positions when resting against stops  220   a ,  220   b  and  222   a ,  222   b . In this case, two very compliant springs  244  may used to maintain cantilever beam  224  and its mass  246  in a midline relative to stops  220   a ,  220   b  and  222   a ,  222   b  under conditions of no vibration, as shown in  FIGS. 10   a ,  10   b.    
     Cantilever beam  224  can also be mounted in a horizontal orientation, as shown in  FIG. 9   a . In this case, a single light spring  244  may be used to counteract the moment created by the weight of mass  246 . One end of spring  244  would be connected to cantilever beam  224  and the other end to housing  248 . Spring  244  would be placed so that cantilever beam  224  will remain in a mid position under conditions of no vibration. When placed in a vibrating environment, cantilever beam  224  will move rapidly between the stops  220   a ,  220   b  and  222   a ,  222   b , resulting in strain in cantilever beam  224  and in piezoelectric patches mounted to cantilever beam  224 , 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 &#39;945 application”), the present applicants designed circuit  300   a ,  300   b  that substantially improves energy conversion efficiency, as shown in  FIGS. 11   a ,  11   b  that are derived from that patent application. Circuits  300   a ,  300   b  take advantage of intrinsic capacitance  302  of piezoelectric device  304  to 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 device  304  is 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 switch  306  in the circuit rapidly discharges that stored charge through a rectifier and through a high speed switch to inductor and capacitor network  308   a ,  308   b  that converts to a lower DC voltage suitable for use powering electronic circuits. Because the entire charge on intrinsic capacitance  302  of piezoelectric device  304  is 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 switch  306  is off piezoelectric device  304  is 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 device  304 . Only when the voltage across piezoelectric device  304  has risen to the threshold of voltage dependent switch  306 , and voltage dependent switch  306  turns on, is energy first drawn from piezoelectric device  304  to ultimately charge storage capacitor  310 . A battery can be used in place of or in addition to capacitor  310 . 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 &#39;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 of  FIGS. 2   a ,  2   b  of the &#39;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 =½ CV   2  
 
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 capacitance  302  of piezoelectric device  304  instead of providing a separate capacitor, as in the &#39;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 device  304  is modeled as generator  320  with intrinsic capacitance  302  in parallel, as shown in  FIGS. 11   a ,  11   b . As mechanical energy is applied to piezoelectric device  304  on its dependent axis, intrinsic capacitance  302  is charged to a voltage proportional to the applied mechanical energy. One embodiment, further described herein below, provides that when the voltage on capacitance  302  reaches a preset threshold, switch  306  closes, allowing the charge on capacitance  302  to flow into inductor  322 . Inductor  322  stores energy in a magnetic field while switch  306  is closed and current is flowing from intrinsic capacitor  302  in piezoelectric device  304 . When intrinsic capacitor  302  has discharged to a second threshold voltage, voltage dependent switch  306  opens, current through inductor  322  decreases rapidly, and this magnetic field around inductor  322  collapses. The second threshold voltage may be set to provide for nearly complete discharge of intrinsic capacitor  302 . The rapid reduction in current and rapid collapse of the magnetic field when switch  306  opens induces a voltage across inductor  322  according to the equation
 
 V=LDi/DT  
 
     This induced voltage across inductor  322  provides a current through diode  324 ,  324 ′ charging large storage capacitor  310 . This voltage across storage capacitor  310  is substantially lower than the voltage across piezoelectric device  304 . A correspondingly higher charge is stored on capacitor  310 . 
     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 in  FIGS. 12   a ,  12   b  that are derived from the &#39;945 application. Because piezoelectric device  304 ,  304 ′ voltage dependent switch  306 ,  306 ′ and inductor  322  are all in series, leakage current through voltage dependent switch  306 ,  306 ′ does not detract from the efficiency of the circuit. Leakage current just goes to charge storage capacitor  310 . 
     To operate most efficiently, switch  306 ,  306 ′ closes at a first threshold when the voltage on intrinsic capacitance  302  is slightly less than the expected maximum open circuit voltage piezoelectric device  304 ,  304 ′ will attain for the mechanical energy input. Switch  306 ,  306 ′ later opens at a second threshold when intrinsic capacitance  302  is nearly discharged. Switch  306  has 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 of  FIGS. 11   a ,  11   b  shown in  FIGS. 12   a ,  12   b  includes voltage dependent switch  306 ′ that includes Darlington transistors  330  and  340 . 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 transistors  330  and  340  are arranged in the circuit so that the turning on one causes the other to also turn on and vice versa. The two Darlington transistors  330 ,  340  remain latched up until intrinsic capacitance  302  of piezoelectric element  304 ′ has nearly discharged and the voltage provided from intrinsic capacitance  302  has declined to close to zero. At that point Darlington transistors  330  and  340  turn off and reset for the next time charge is available from piezoelectric device  304 ′. P.P. Darlington transistor  330  has part number FZT705 and NPN Darlington transistor  340  has part number FZT605. Both are available from Exodus, Manchester, UK. 
     Darlington transistor  340  remains off while the voltage across its base emitter junction  1 - 3  remains below its 1.2 volt turn on threshold. This voltage is controlled by a voltage divider formed by resistors  342  and  344 . In practice, any leakage current through Darlington transistor  330  from collector to emitter adds to the current through resistor  342  and forms part of this voltage divider. When a threshold of approximately 150 volts is provided by piezoelectric device  304 ′ and applied across voltage dependent switch  306 ′, the voltage at transistor  340  base emitter junction, reaches the 1.2 volt turn-on threshold, and transistor  340  turns on. The voltage across resistor  346  and across the base-emitter junction from pins  2 - 3  of Darlington transistor  330  now also equals at least 1.2 volts, and transistor  330  turns on. This provides a high voltage to the base at pin  1  of Darlington transistor  340 , keeping the transistor on. While the two Darlington transistors  330 ,  340  remain thus latched up, intrinsic capacitance of piezoelectric element  304 ′ is nearly completely discharged into inductor  322  through diode  360 . Voltage dependent switch  306 ′ continues to conduct until the intrinsic capacitance of piezoelectric element  304 ′ is nearly completely discharged. 
     Since voltage dependent switch  306 ′ always turns on at the same threshold voltage, and since the intrinsic capacitance of the piezoelectric device is also a constant, every closure of switch  306 ′ 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 &#39;693 patent, one side of piezoelectric device  304 ′ is connected to ground and shunt diode  365  is used to provide that the entire signal from piezoelectric element  304 ′ and its intrinsic capacitance  302  is positive. Thus, the peak voltage provided by piezoelectric element  304 ′ 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.