Abstract:
An apparatus for use in low frequency vibration energy harvesting (VEH) and with actuators requiring a low deflection force. The apparatus includes a piezo flexure that is loaded with a compressive pre-load force to place the piezo flexure under compression. The piezo flexure may be supported at an intermediate point or at one end thereof. The compressive pre-load force produces flexes the piezo flexure into one or the other of two stable positions, these positions being offset on opposite sides of a longitudinal centerline representing the position of the piezo flexure that would be produced without the compressive pre-load force applied thereto. The compressive pre-load effectively provides a negative spring constant which “softens” the piezo flexure and enhances a responsiveness of the piezo flexure to low frequency vibration energy. The piezo flexure also operates over a much wider frequency bandwidth than conventional systems incorporating a tip mass.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of International Patent Application Ser. No. PCT/US/2004/025049 filed Jul. 30, 2004, which in turn claims priority from U.S. Patent Provisional Application No. 60/491,122 filed Jul. 30, 2003, the disclosures of which are incorporated herein by reference. 

   FIELD OF THE INVENTION 
   This invention relates to vibration energy harvesting devices, and more particularly to an apparatus which is ideally suited for harvesting low frequency vibration energy from a vibrating structure. 
   BACKGROUND OF THE INVENTION 
   Many environments offer a vibration rich environment that is ideal for harvesting vibration energy. Such environments often exist in aircraft and automotive applications where the vibration experienced by an aircraft or automotive vehicle represents energy that could be used to power sensors or other remotely located devices, provided such energy can be harvested by a suitable device. 
   Vibration energy harvesting (VEH) can be accomplished by developing relative motion, and hence energy, between a vibrating structure and a reaction mass coupled to the structure. This mechanical energy can be converted into electrical energy by developing cyclic stress in a piezo electric structure. A simple form of this device is a cantilever beam that has piezo material attached to the surface. This is illustrated in  FIG. 1 . A reaction mass is attached to the tip of the beam to increase performance. When subjected to vibration, the tip of the beam tends to resist motion, thus placing the piezo material under stress. This stress results in electrical charge accumulation in the piezo material that results in an increase in voltage potential between two points of the material. However, for this topology to work efficiently, vibration energy must occur at or above the beam resonance frequency. Vibration energy with frequency content below the resonance frequency will produce very little motion between the tip mass and the base. 
   When the cantilever beam shown in  FIG. 1  is used, the stiffness of the beam, including the piezo material, beam length (L) and the tip mass determine the lowest frequency where the VEH device will work. Additionally, piezo material is usually a ceramic and is fragile when subjected to tension loading which will limit the robustness of the VEH device and its life.  FIG. 2  illustrates the relative tip displacement as a function of frequency for this device. At low frequency, the tip beam moves very little relative to the vibrating structure. Accordingly, the VEH device shown in  FIG. 1 , at low frequency, will provide very little power from low frequency vibrations. Vibration energy at resonance frequency will provide maximum VEH, but utility is limited by a very narrow bandwidth. 
   The limitation of needing to “tune” the system around the resonant frequency of the cantilever beam imposes a significant limitation in terms of efficient operation of the system shown in  FIG. 1 . This is because various structures often produce vibration energy over a much wider frequency bandwidth than what the system can be tuned for. The selection of the tip mass, to essentially tune the system to operate efficiently at the resonant beam frequency, means that the system will not be efficient in harvesting energy at other frequencies above and below the resonant frequency of the cantilever beam. Accordingly, a system that is not limited to efficient harvesting of vibration energy at only the resonant beam frequency, but that is able to harvest energy over a relatively wide frequency range, would be much more effective in generating electrical power from a vibrating structure. 
   Accordingly, there still exists a need for an apparatus able to be used with a piezo material to improve the harvesting of vibration energy at low frequencies, and also at frequencies above and below the resonant frequency of the structure from which vibration energy is being harvested. Such an apparatus would be extremely useful for powering remotely located sensors and various other components from low frequency vibration energy experienced by a piezo, beam-like structure. Such an apparatus would effectively make it possible to provide energy harvesting from a wide variety of structures experiencing low frequency vibration where such energy harvesting would have previously not been practicable. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to a strain energy shuttle apparatus and method. The apparatus of the present invention is useful for providing low frequency vibration energy harvesting (VEH) from a piezo beam-like structure experiencing low frequency vibration energy. In one preferred form the apparatus includes a biasing element for mechanically generating a negative spring force that is added in parallel with a piezo flexure (i.e., a piezo beam-like component). The negative spring force provided by the biasing element effectively “softens” the piezo flexure and provides the piezo flexure with substantially a zero stiffness at zero frequency. The apparatus provides important benefits for applications where softening of a piezo structure is desirable, such as where the piezo structure is being used to harvest low frequency vibration energy or as an actuator to move a separate component. In either application, the apparatus of the present invention operates to overcome the inherent structural stiffness of the piezo flexure to allow much easier flexing thereof. 
   In one preferred embodiment the biasing element is coupled to a cantilever beam which is in turn pivotally coupled to a supporting structure. The piezo flexure is also coupled to the structure in a manner that places the piezo flexure generally in a common plane with the biasing element. A flexure couples a free end of the piezo flexure to a free end of the biasing element. This produces two stable positions, each being laterally offset from a line extending from the point of attachment of the piezo flexure to its associated structure and the point of attachment of the cantilever beam to the structure. Precise tailoring of the force provided by the biasing element enables the piezo flexure to be moved very easily by low frequency vibration energy from one of the two stable positions to the other and back again. Once moved from one stable position to the other, the piezo flexure will oscillate around the new stable position, thereby generating additional power. 
   In another preferred embodiment, an apparatus is disclosed that makes use of a piezo flexure having a support substrate with at least one piezoceramic wafer secured thereon. The support substrate is supported at an intermediate point of its length. A biasing element is operatively associated with the support substrate to apply a compressive force to the substrate sufficient to deflect the substrate and the piezoceramic wafer mounted thereon into one of two stable positions on opposite sides of a centerline that extends through the substrate when it is not being subjected to a compressive pre-stress force. The compressive force provided by the biasing element serves to “soften” the inherent structural stiffness of the piezoceramic wafer and enhance its responsiveness to vibration energy, and particularly to low frequency vibration energy. 
   In one preferred form a plurality of biasing elements are disposed longitudinally parallel to and along a longitudinal axis of the support substrate on opposite sides of the piezoceramic wafer. The support substrate is also supported at an approximate mid-point thereof on a base assembly that experiences the vibration energy. Opposite ends of the support substrate are secured to link arms, with each of the biasing elements being attached between the link arms on opposite longitudinal sides of the support substrate. This arrangement imparts a compressive stress to the piezoceramic wafer that softens the wafer and deflects the opposite ends of the wafer into one of two stable positions. Most importantly, the compressive force significantly enhances the response of the piezo electric material to low frequency vibration energy. 
   The present invention thus significantly assists in overcoming the inherent structural stiffness of a piezo flexure. The apparatus makes the piezo flexure highly susceptible to very low frequency vibration energy which would otherwise not be sufficient to cause sufficient flexing or bending of the piezo flexure for low frequency vibration energy harvesting applications. A significant advantage of the present invention is that vibration energy harvesting can be accomplished over a significantly wider frequency bandwidth than what is possible with conventional cantilever beam VEH devices using a tip mass. 
   Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
       FIG. 1  is a view of a prior art energy harvesting device incorporating a conventional tip mass disposed at a free end of a piezo beam-like structure; 
       FIG. 2  is a graph of the tip displacement of the beam shown in  FIG. 1  relative to frequency; 
       FIG. 3  is a view of a preferred embodiment of the present invention incorporated for use with a piezo flexure; 
       FIG. 4  is a perspective view of an alternative preferred embodiment of the present invention, making use of a pair of apparent biasing elements to soften the piezoelectric flexure of the assembly; and 
       FIG. 5  is a side view of the assembly of  FIG. 4 ; and 
       FIG. 6  is a side view of the assembly of  FIG. 4  illustrating the two stable positions. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
   Referring to  FIG. 3 , an apparatus  10  in accordance with a preferred embodiment of the present invention is shown. The apparatus is used for enabling low frequency vibration energy harvesting (VEH) through the use of a piezo flexure  12  which is supported fixedly from a vibrating structure  14 . The piezo flexure essentially forms a beam-like structure, and in one preferred form comprises a piezo bimorph flexure. The piezo flexure  12  includes piezo layers  16  and  18  formed on opposite sides of a flexible supporting substrate  20 . The substrate  20  includes an end  22  which is fixedly coupled to the structure  14 . The substrate  20  can be plastic, metal or any other flexible material that allows the piezo layers  16  and  18  to be bonded thereto. 
   The apparatus  10  further includes a link or cantilever beam  24  which is pivotally coupled at a first end  26  via a pivot pin  28  or other suitable coupling element such that the beam  24  is able to pivot about pin  28  in response to vibration experienced by the structure  14 . A biasing element  30  is fixedly coupled at one end  32  to a free end  34  of the beam  24  and at an opposite end  36  to a flexure  38 . The flexure  38  is in turn coupled to a free end  40  of the piezo flexure  12 . Flexure  38  may comprise any suitable coupling element or material which enables relative movement between the end  36  of the biasing element  30  and the free end  40  of the piezo flexure  12 . In one preferred form the biasing element  30  comprises a compression coil spring, but it will be appreciated that any biasing element capable of providing a force directed against the piezo flexure  12  could be incorporated. 
   The coupling of the cantilever beam  24  to the free end  40  of the piezo flexure  12  (via the biasing element  30 ) produces an arrangement wherein the piezo flexure  12  has two stable positions, with one being shown in solid lines in  FIG. 3  and the other being indicated in phantom. Each of the stable positions of the piezo flexure  12  are laterally offset from a longitudinal mid line  42  extending between the point of attachment of the fixed end  22  of the piezo flexure  12  and the pivot pin  28  supporting the cantilever beam  24 . The mid-line  42  can be viewed as a position of equilibrium, albeit an unstable one. This geometry essentially produces a geometric cotangent function between the piezo flexure  12  and the biasing element  30 . In effect, the piezo flexure  12  experiences a spring constant that is the negative of the spring constant provided by biasing element  30 . 
   The apparatus  10  significantly reduces the force required to move the piezo flexure  12  between the two stable positions shown in  FIG. 3 . Put differently, the biasing element  30  and cantilever beam  24  operate to significantly “soften” the piezo layers  16  and  18  to overcome the inherent structural stiffness of each. By tailoring the spring constants of the biasing element  30  and the piezo flexure  12 , the energy required to switch conditions and cause movement of the piezo flexure  12  out of one stable position and into the other stable position can be matched to the electrical load which the piezo flexure  12  is electrically coupled to. When properly matched, the apparatus  10  is highly efficient. Under such a condition, the energy delivered to the electrical load is equal to the stress-strain hysteresis observed in the piezo flexure  12 . 
   The apparatus  10  has a significantly lower frequency of operation than a conventional energy harvesting device, such as that shown in  FIG. 1 . The frequency of operation of the apparatus  10  is determined in part by the stiffness of the piezo flexure  12  and the mass of the piezo flexure. The relationship between the angular tip deflection (i.e., of tip  38 ) of the piezo flexure  12  and an applied torque is given by equation 1 below:
 
 T   PiezoFlexure =θ PiezoFlexure   *K   PiezoFlexure   Equation 1
 
 T   PiezoFlexure   =σ   PiezoFlexure   *K   PiezoFlexure  
 
   The biasing element  30  (i.e., compression spring) applies a non-linear torque to the piezo flexure  12  which is represented by equation 2 below:
 
 T   PiezoFlexure/Spring   =L*F   Spring *sin(θ PiezoFlexure )  Equation 2
 
   The torque applied to the piezo flexure  12  deforms the piezo flexure  12  to an angle whereat the piezo flexure  12  is stable at two locations as shown in  FIG. 3 . The torque required to change the state of the piezo flexure  12  from +θ to −θ is the difference between the torque generated by the biasing element in equation 1 and the torque given in equation 2, and is represented by the following Equation 3:
 
 T   Tot   =T   PiezoFlexure   −T   PiezoFlexure/Spring=θ   PiexoFlexure   *K   PiezoFlexure   −L *Sin(θ PiexoFlexure ) ( F   max −2* L *(1−Cost(θ PiexoFlexure )))* K   spring  
 
   The difference in torque is the effective “softening” of the piezo flexure  12 . Equation 4 represents the deflection of the biasing element  30  (i.e., the spring) as a function of piezo flexure angle θ:
 
δ spring =2 *L (1−cost(θ PiezoFlexure   ))   Equation 4
 
   Equation 5 defines the maximum spring force F max :
 
 F   max   =[F   Spring ]θ=0
 
   Also, a compression spring exhibits a force-distance relationship expressed by equation 6:
 
 F   Spring   =−K   spring *δ spring   +F   max  
 
   This relationship, when applied to the apparatus  10 , results in two stable angles. The torque necessary to change the positions is a strong function of the spring constant, where a softer spring produces a lower reset force (i.e., a force required to move the piezo flexure  12  from one stable position to the other). Equation 3, above, clearly shows the non-linear nature of the torque versus θ relationship. 
   An additional advantage of the apparatus  10  is that the two stable positions produce a frequency conversion between low frequency vibration and the high frequency nature of the stable angle locations. Put differently, once external vibration energy has caused the piezo flexure  12  to move from one stable position to the other, the piezo flexure  12  will oscillate around the stable location that it has just moved to, allowing the piezo material  16  and  18  of the piezo flexure  12  to harvest the strain energy over many cycles. 
   While the apparatus  10  has been described in connection with “softening” a piezo flexure for vibration energy harvesting applications, it will also be appreciated that the apparatus  10  can be readily incorporated in an actuator. For example, the apparatus  10  is extremely well suited to applications requiring large deflections of the piezo flexure and low force, such as with an aerodynamic flow control synthetic jet actuator or a low frequency audio speaker. In this embodiment the apparatus  10  is designed such that the torque required to change the state of the apparatus  10  is within the capability of the piezo material to provide torque. Applying a voltage to the piezo materials  16  and  18  on the piezo flexure  12  causes the apparatus  10  to change states producing significantly larger displacements than would occur if the piezo flexure was energized without the biasing element  30 . Typical increases in motion of the piezo flexure  12  with the present invention can be ten to twenty times that obtained with a simple, conventional piezo flexure unassisted by any spring force. 
   It will also be appreciated that for both energy harvesting and actuation applications, the use of other materials besides piezo electric materials is possible. For example, electromagnetic, electrostatic and magnostictive transduction technology can be used. 
   Referring to  FIGS. 4 and 5 , a VEH apparatus  100  is illustrated in accordance with another preferred embodiment of the present invention. VEH apparatus  100  generally comprises a piezo flexure assembly  102  supported on a base  103 . The flexure assembly  102  includes a pair of piezoelectric wafers  104  and  106  disposed on opposite sides of a support substrate  108 . In one preferred form the piezoelectric wafers  104 ,  106  each comprise piezoceramic wafers. The flexure assembly  102  further includes a pair of link arms  110  and  112  and a pair of biasing elements  114  and  116  coupled between the link arms  110 ,  112 . 
   The entire flexure assembly  102  is supported on the base  103  from a pair of upstanding boss portions  115  and  117 . Conventional threaded fasteners  118  and  120  extend through openings  122 , and  124  in support arms  126  and  128  of the support substrate  108  to thus suspend the entire flexure assembly  102  above the base  103 . The support arms  126  and  128  are located at an approximate midpoint of the support substrate  108 , but the support arms could also be offset so as to be closer to one or the other of the link arms  110 ,  112  and thus not disposed at the approximate longitudinal midpoint. The substrate  108  can be made of spring steel, beryllium copper, brass, glass epoxy composite or graphite epoxy composite, or any other suitable material. 
   With specific reference to  FIG. 5 , the attachment of the biasing element  114  to each link arm  110  and  112  can be seen in greater detail. The same arrangement is used to couple the opposite ends of biasing element  116  to the link arms  110  and  112 . Each link arm  110  and  112  includes a slot  130 ,  131  formed along a portion of its length. Each slot  130 ,  131  is further disposed along a midpoint of the overall thickness of its respective link arm  110  or  112 . Slot  130  has a height sufficient to receive an end  132  of the support substrate  108  therein. Slot  131  similarly is sized to receive end  133  of the support substrate  108 . Ends  132  and  133  are each adhered or otherwise secured in their respective slots  130  or  131  such that they are not removable from either link arm  110  or  112 . 
   The link arm  110  further includes a pair of pins  134  that extend through spaced apart openings  136  in the link arm  110 . Link arm  112  similarly includes a pair of pins  135  that extend through openings  137  in link arm  112 . The pins  134  essentially form a channel through which one end  138 A of a flexure component  138  can be secured. Pins  135  similarly form a channel for securing an end  139 A of a second flexure component  139 . End  138 A of the flexure component has a cross-sectional thickness that is greater than an end  138 B so that end  138 A cannot simply be pulled out from between the pins  134 . End  139 A of flexure component  139  is constructed in identical fashion with a thickness greater than end  139 B so that it cannot be removed from between pins  135 . However, this coupling arrangement allows free pivoting movement of ends  138 A and  139 A about their respective pins  134  and  135  with a minimal degree of friction and while limiting stress at this area of its associated flexure component  138  or  139 . 
   Referring further to  FIGS. 4 and 5 , each end  138 B,  139 B of the flexure components  138  and  139  include a opening  140 A,  140 B which receives an end  142 ,  143  of the biasing element  114 . In practice, any suitable means for attaching the ends.  142 ,  143  of biasing element  114  can be employed. Biasing element  116  is coupled in the same fashion. The link arms  110  and  112  may be made from a variety of materials but preferably are comprised of aluminum, steel, glass or graphite epoxy. Biasing element  114  is illustrated as a coil spring, but in practice any form of spring that is coupled between the link arms  110  and  112  that serves to place the piezoelectric wafers  104  and  106  in compression may be employed. 
   With further specific reference to  FIG. 4 , link arm  110  includes a notch forming ears  110 A, while link arm  112  similarly includes a notch forming ears  112 A. The ears  110 A and  112 A provide stress transition regions that maintain stiffness across those areas where the opposite ends of the support substrate  108  are coupled to their respective link arms  110 ,  112 . The gap formed between each link arm  110  and  112  and the adjacent ends of the piezoceramic wafers  104 ,  106 , produces a low bending stiffness region along the flexure  102  that would otherwise reduce performance of the apparatus  100 . The ears  110 A and  112 A of each link arm  110  and  112 , respectively, thus provide increased bending stiffness to offset this. 
   With further reference to  FIG. 5 , each of the piezoelectric wafers  104 ,  106  may vary significantly in thickness, length and width to suit the needs of a particular application. However, in one preferred form, each piezoelectric wafer  104  and  106  has a thickness of about 0.005″-0.02″ (0.127 mm-0.508 mm). In one preferred form, the length and width of the piezoelectric wafers  104  and  106  is about 1.8″ (45.72 mm) and about 0.6″ (15.24 mm) respectively. The actual energy per bending cycle generated by the apparatus  100  is a function of piezoceramic volume that is under stress. A thicker piezoceramic wafer will provide a higher energy content, but this energy will be in the form of high voltage and low charge as compared to a thinner material. Although more energy is available from thicker piezoceramic material, the conversion electronics that are needed to capture this energy are significantly more complex and less efficient than that required for use with a thinner piezoceramic wafer. Thinner piezoceramic material produces a lower voltage but much higher charge. Merely as an example, typical capacitance and voltages produced by a piezoceramic wafer, per cycle of oscillation, are as follows:
         0.02″ thick PZT 5A-10 nF at 200 volts (harder to convert);   0.005″ thick PZT 5A-100 nF at 30 volts (easily converted);       

   Total energy is CV^2/2 per bending cycle. 
   Referring to  FIG. 6 , the oscillating motion of the flexure assembly  102  can be seen in response to a vibration force. The compressive force generated by the biasing elements  114 ,  116  is sufficient to maintain the flexure assembly  102  in a deflected (i.e., bowed) orientation at one of two stable positions  144  or  146 . Longitudinal line  148  represents the plane the flexure assembly  102  would reside in without the compressive force from the biasing elements  114 ,  116 . When in either of positions  144  or  146 , the flexure assembly  102  is highly responsive to low frequency vibration energy and is easily deflected to the other stable position by such energy. The spring force provided by the biasing elements can be tailored to provide the required sensitivity for a specific VEH application. 
   The present invention thus forms a means to significantly “soften” a piezo flexure which enables the piezo flexure to be used in low frequency vibration energy harvesting applications where such a flexure would otherwise be too structurally stiff to harvest the vibration energy. A significant benefit of the present invention is that it can be used over a wider frequency bandwidth than previously developed VEH devices incorporating a tip mass on the free end of the piezo flexure. The capability of operating over a wider bandwidth allows the invention  10  to more efficiently harvest vibration energy from the structure and to generate a greater voltage output from the vibration energy than would be possible with a conventional piezo flexure. 
   While various preferred embodiments have been described, those skilled in the art will recognize modifications or variations which might be made without departing from the inventive concept. The examples illustrate the invention and are not intended to limit it. Therefore, the description and claims should be interpreted liberally with only such limitation as is necessary in view of the pertinent prior art.