Patent Publication Number: US-2005134149-A1

Title: Piezoelectric vibration energy harvesting device

Description:
RELATED APPLICATIONS  
      This application is a continuation-in-part of co-pending U.S. patent application Ser. No. 10/887,216 to Ken K. Deng, filed Jul. 9, 2004, which claims the benefit of U.S. Provisional Patent Application Ser. No. 60/486,172, filed Jul. 11, 2003, the subject matter of both of which is incorporated by reference herein. 
    
    
     STATEMENT OF GOVERNMENT INTEREST  
      The work leading to the present invention was supported in part by Naval Surface Warfare Center Dahlgren Division (NSWCDD) Contract Number: N00178-03-C-3056. The government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION  
      The present invention is directed to a highly efficient, small size, vibration harvesting and electric energy storage device. The energy level is high enough to power a wireless sensor.  
     BACKGROUND OF THE INVENTION  
      Current technology for harvesting energy utilizes a flexural, piezoelectric composite bending structure as a vibration energy to electric energy transducer. Most conventional harvesting devices are single degree-of-freedom (SDOF) systems. The selected piezoelectric materials are PZT ceramics or PVDF polymer. The output of this device is connected to an AC-DC converter which is typically composed of a diode rectifier with a storage capacitor.  
      A conventional flexural mode piezoelectric effect (d 31  mode) is very inefficient resulting in a low conversion efficiency from vibrational energy to electrical energy (less than 10%). Additionally, flexural mode piezoelectric structures are bulky and not suitable for a high frequency vibration condition. SDOF devices have a single resonance peak, at which the harvested energy reaches the highest conversion efficiency. However, the bandwidth of a SDOF system is narrow, thereby limiting its applications. Additionally, most conventional harvesting devices use bulky discrete inductors for impedance matching with the capacitive piezoelectric element of the device. These drawbacks make the conventional devices impractical for many applications.  
     SUMMARY OF THE INVENTION  
      It is therefore an object of the invention to efficiently harvest vibrational kinetic energy from the ambient environment or machinery and store it in the form of electrical energy, which later is used to power an electronic device. A highly efficient, small size vibration harvesting device will enable a self-powered, truly wireless transducer system.  
      In accordance with an embodiment of the present invention, by using the state-of-the-art relaxor single crystal, which exhibits the highest piezoelectric coupling coefficient, and a compression-tension piezoelectric composite, cymbal structure, a compact, highly efficient vibration energy extracting device is accomplished. Moreover, before connecting a stack including a piezoelectric element disposed between two cymbal-shaped caps, with a rectifier/storage circuit, an inductor L is introduced which is parallel with the piezoelectric stack. The resonance of the LC loop is tuned around the resonance of the stack. This inductor will greatly improve the electrical energy transferring efficiency.  
      A major difference between the prior art and the above design is in the piezoelectric transduction structure. Instead of using a flexural plate or beam, the new vibration energy harvesting device uses a composite cymbal stack with a proof mass on top. During vibration, the inertial force is transmitted to the piezoelectric disk through the circular cymbal caps. Then the piezoelectric disk is under both compression and tension stresses (d 33 +d 31  mode). The present invention is therefore more efficient than the prior art where the piezoelectric layer is only subject to in-plane stress (d 31  mode). Another major change is the transduction material; a relaxor crystal, which has the highest piezoelectric property, is incorporated in the device. In addition, the electric output from the cymbal stack is connected to an inductor before it is linked to a rectifier. The resonance frequency of the inductor L and piezoelectric crystal C x  is tuned to be approximately the same as the mechanical resonance of the cymbal stack. Doing so, the electrical energy flows much efficiently from the harvesting device to the storage capacitor.  
      The invention allows for a much more efficient vibrational energy harvesting device. It also allows for a very small size.  
      In accordance with another embodiment of the present invention a multiple degree of freedom dynamic system is provided that has a wide band peak. The wider band of resonating frequency range combined with a more efficient compression mode of piezoelectric material and impedance matching electronics, creates a more versatile and efficient energy harvesting device. In addition, the utilization of a gyrator to synthesize an inductor allows maximum power to be stored into the storage element. A gyrator simulates large coils electronically. A gyrator converts an impedance into its inverse. This allows for replacement of an inductor with a capacitor, two or more amplifiers, and some resistors. The synthesized inductor or gyrator also allows an electronically tunable harvester, in which the harvester can automatically tune itself around the bandwidth where vibrational energy is mostly concentrated by changing the value of the synthesized inductor.  
      Other objects, advantages and salient features of the invention will become apparent from the following detailed description, which, taken in conjunction with the annexed drawings, discloses preferred embodiments of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:  
       FIG. 1  shows an elevational view of a device with a cymbal stack in accordance with a first embodiment of the present invention;  
       FIGS. 2 and 3  show circuit diagrams of the device of  FIG. 1  connected to different rectifiers, with the device of  FIG. 1  being represented by an equivalent circuit to the left of the dashed line;  
       FIG. 4  is a diagram of a device in accordance with a second embodiment of the present invention, showing a two degree of freedom system;  
       FIG. 5  is a graph showing the frequency response of the electric output from a device utilizing the two degree of freedom system illustrated in  FIG. 4 ;  
       FIGS. 6-9  are elevational views of different devices incorporating the two degree of freedom system illustrated in  FIG. 4 ;  
       FIG. 10  is a circuit diagram of a device in accordance with a third embodiment of the present invention showing the addition of a gyrator; and  
       FIGS. 11 and 12  are exemplary circuit diagrams of the gyrator of the circuit diagram illustrated in  FIG. 10 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Referring to  FIGS. 1-3 ,  FIG. 1  shows an energy harvesting device  100 . The device  100  includes a base  102  and a proof mass  104 . Disposed between the base  102  and the proof mass  104  is a cymbal stack  106  including top and bottom cymbal-shaped caps  108 ,  110  sandwiching a relaxor single crystal  112 . The cymbal-shaped caps  108 ,  110  are connected to electrodes  114 ,  116  forming an electric output.  
       FIG. 2  shows a first circuit  200  incorporating the energy harvesting device  100 . In the circuit diagram of  FIG. 2 , the cymbal stack  106  is represented by an electrical circuit comprising a current source  202  and a capacitor  204 . Connected in parallel across the output of the cymbal stack  106  is an inductor  206 . A single diode rectifier  208 , a storage capacitor  210  and output electrodes  212 ,  214  complete the circ  200 .  
       FIG. 3  shows a second circuit  300  incorporating the energy harvesting device  100 . The single diode rectifier  208  is replaced with a low forward voltage, low leakage current rectifier  302 .  
      Referring to  FIGS. 4-5 , a multiple degree of freedom dynamic harvesting system, such as a two degree of freedom system (2DOF) piezoelectric resonator or device  400 , is shown that has a wide band peak. As seen in  FIG. 4 , the system  400  includes a first inertial mass  402  and a second inertial mass  404 . Between a base  406  of the device  400  and the first mass  402  is a first spring element  408  and a first damper  410 . Between the first mass  402  and the second mass  404  is a second spring element  412  and a second damper  414 , so that the piezoelectric element (described below) is stressed whenever the second spring element  412  is stressed. The first and second masses  402  and  404  are attached to the first and second spring elements  408  and  412 , respectively. As seen in  FIG. 5 , there are two resonance peaks  502  and  504  created by the 2DOF system  400 . Once the two resonance peaks  502  and  504  are tuned close to each other, the resulting response from the piezoelectric element of the harvester is a much wider resonant range or band of high efficiency compared to a SDOF system. The solid curve of  FIG. 5  is the projected spectral response from the piezoelectric element which is coupled with the second spring element  412 . The bandwidth of the 2DOF system  400  could be as wide was 2-3 kHz.  
       FIGS. 6-9  are examples of harvesting devices that use the 2DOF system  400  of  FIGS. 4 and 5 . Referring to  FIG. 6 , a harvesting device  600  is a tension-compression mode cymbal design similar to the harvesting device  100  of the first embodiment, except the harvesting device  600  incorporates the 2DOF system  400 . Specifically, a piezoelectric element or plate  601  of the harvesting device  600  is disposed between cymbal-shaped caps or spring elements  612 . Another spring element  608  is connected to the base  606  of the device  600 . A first mass  602  is located below the cymbal-shaped spring element  612  such that the piezoelectric element  601  is bonded between the first mass  602  and the second spring element  612 . A second mass  604  is disposed on top of the cymbal-shaped spring elements  612 .  
      Referring to  FIG. 7 , a harvesting device  700  is a compression mode piezoelectric plate design that has a first spring element  708  connected to a base  706  of the device  700 . A piezoelectric element  701  is bonded between a first mass  702  and a second compression spring element  712 . A second mass  704  is disposed on top of the second spring element  712 .  
      Referring to  FIG. 8 , a harvesting device  800  is a shear mode piezoelectric ring design that includes a first spring element  808  disposed on a base  806  of the device  800 . A first mass  802  is disposed on top of the first spring element  808 . A piezoelectric ring element  801  is disposed around a portion of the first mass  802  inside of a second spring element  812 , such that piezoelectric ring element  801  is bonded between the first mass  802  and the second spring  812 . A second mass  804  is disposed on top of the second spring element  812 .  
      Referring to  FIG. 9 , a harvesting device  900  is a bending mode flexural beam design that includes a base  906  with a first spring element  908  disposed thereon. A first mass  902  is disposed on the first spring element  908 . Between the first mass  902  and a cantilever beam second spring element  912  is a supporting post  915 . First and second parts  905  and  907  of a second mass  904  are disposed on opposite ends of the second spring element  912 . A piezoelectric element or plate  901  is bonded to the second spring element  912 .  
      The piezoelectric elements  601 ,  701 ,  801  and  901  can be made of any piezoelectric material, including a single crystalline, such as a diamond, or a multi-crystalline, such as a ceramic.  
      Referring to  FIGS. 10-12 , a harvesting device  1000  is similar to the device  100  of  FIGS. 2 and 3 , except the device  1000  uses a synthesized inductor or gyrator  1010  instead of a conventional metal coil inductor. The gyrator  1010  is placed in parallel with the piezoelectric element (represented by circuit  1001 ) prior to the rectifier  1020  and  1030  storage circuitry. Because of the very high value inductor (in the hundreds of Henry) required to resonate at low frequencies, the use of a conventional metal inductor can be impractical. The gyrator  1010  is an electronic circuit that simulates an inductor. Therefore, the conventional heavy and bulky inductor can be replaced with a smaller lighter weight synthesized inductor.  
       FIGS. 11 and 12  represent two alternative circuits  1110  and  1210  for the gyrator  1010 . The gyrator  1010  can synthesize very large inductors at very low power, which is preferably taken from the storage element. The input  1102  (IN) and the ground  1104  of both circuits  1110  and  1210  simulate the two ends of a conventional metal inductor. The gyrator  1010  is made of a ultra low power, low voltage operation amplifier  1106  along with a capacitor  1108  (C 1 ) and uses the gyration effects to convert the capacitor into an inductor. The inductance at the input  1102  of both circuits  1110  and  1210  is L synthesized =C 1 *R 1 *R 2 .  
      While particular embodiments have been chosen to illustrate the invention, it will be understood by those skilled in the art that various changes and modification can be made therein without departing from the scope of the invention as defined in the appended claims.