Abstract:
An energy harvesting device, system and method are described. The energy harvester collects acoustic energy and transforms it into electrical energy for use by a sensor. The energy harvester utilizes a piezoelectric device, which may be encased, either wholly or partially, within an acoustic chamber. Alternatively, the piezoelectric device may be entirely exterior to the acoustic chamber, which acts to amplify the collected acoustic energy.

Description:
BACKGROUND  
       [0001]     The invention relates generally to a system, apparatus and method for harvesting energy, and more particularly to an energy harvesting system, apparatus and method for harvesting acoustic energy and converting it into electrical energy for running a sensing assembly.  
         [0002]     The U.S. Congress promulgated the Transportation Recall Enhancement, Accountability and Documentation (TREAD) Act in 2000. The TREAD Act provides that by the year 2007, all new motorized vehicles operated on the U.S. transportation system must include a tire pressure sensing system to monitor for and report the occurrence of unsafe tire pressure. The TREAD Act is applicable to all consumer and commercial trucks and automobiles.  
         [0003]     The tire pressure sensing system is to include a tire pressure sensor mounted in a position suitable for determining the tire pressure, such as, for example, on a wheel rim within each tire. Current power sources for sensors include batteries and RF sources. The use of batteries in some applications has disadvantages. Batteries are not environmentally friendly, amounting to millions of discarded batteries per year. For applications where replacement of the battery is problematic, the only battery option is a lithium ion battery (Li/CFx), which is expensive. Further, batteries are subject to a loss in capacity at certain temperatures, such as at minus 40° C. Further, many batteries have a large enough size and mass to render them impracticable for certain applications.  
         [0004]     There exists a need for an applicable power source capable of providing power to sensors in small or crowded environments. For example, there exists a need for a power source for the required tire sensors. The power source must have a small enough profile to fit within wheel rims. Further, the power source should be renewable or, at the very least, have a long lifetime to lessen the need for replacement.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]      FIG. 1  is a perspective view of an energy harvester constructed in accordance with an exemplary embodiment of the invention.  
         [0006]      FIG. 2  is a cross-sectional view taken along line II-II of the energy harvester of  FIG. 1 .  
         [0007]      FIG. 3  is a cross-sectional view of an energy harvester constructed in accordance with another exemplary embodiment of the invention.  
         [0008]      FIG. 4  is a side view illustrating the flexure of the piezoelectric device of  FIG. 3 .  
         [0009]      FIG. 5  is a cross-sectional view of an energy harvester constructed in accordance with another exemplary embodiment of the invention.  
         [0010]      FIG. 6  is a cross-sectional view of an energy harvester constructed in accordance with another exemplary embodiment of the invention.  
         [0011]      FIG. 7  is a cross-sectional view of an energy harvester constructed in accordance with another exemplary embodiment of the invention.  
         [0012]      FIG. 8  illustrates an energy harvesting system in accordance with another exemplary embodiment of the invention.  
         [0013]      FIG. 9  is a partial side view illustrating the mounting of the energy harvesting system of  FIG. 8  on a wheel rim in accordance with one aspect of the invention.  
         [0014]      FIG. 10  is a partial side view illustrating the mounting of the energy harvesting system of  FIG. 8  on a wheel rim in accordance with another aspect of the invention.  
         [0015]      FIG. 11  is a partial side view illustrating the mounting of the energy harvesting system of  FIG. 8  on a wheel rim in accordance with yet another aspect of the invention.  
         [0016]      FIG. 12  is a schematic representation of a vehicle equipped with the system of  FIG. 8 .  
         [0017]      FIG. 13  illustrates process steps for harvesting energy and powering a sensor in accordance with another exemplary embodiment of the invention.  
         [0018]      FIG. 14  is a graph plotting pressure amplifications experienced within a chamber of an exemplary embodiment of the energy harvesting system. 
     
    
     SUMMARY  
       [0019]     The present invention describes an apparatus, a system and a method for harvesting energy for use in powering a sensor.  
         [0020]     One exemplary embodiment of the invention is an energy harvester that includes an acoustic chamber configured to collect acoustic energy, a back plate in connection with the acoustic chamber and configured to convert the collected acoustic energy into mechanical energy, and a piezoelectric device mounted to the back plate and configured to convert the mechanical energy into electrical energy.  
         [0021]     One aspect of the energy harvester embodiment includes an acoustic chamber that amplifies the collected acoustic energy. Further, the energy harvester includes a low modulus material connecting the back plate with the acoustic chamber.  
         [0022]     Another exemplary embodiment of the invention is an energy harvester that includes an acoustic chamber configured to collect acoustic energy and a piezoelectric device in connection with the acoustic chamber and configured to convert the collected acoustic energy into mechanical energy and to convert the mechanical energy into electrical energy.  
         [0023]     Another exemplary embodiment of the invention is an energy harvesting system that includes a sensing assembly with a sensor for sensing a physical condition. The energy harvesting system includes an energy harvester with an acoustic chamber and configured to collect acoustic energy and transform the collected acoustic energy into electrical energy.  
         [0024]     One aspect of the system embodiment is an air pressure sensing system for sensing the air pressure in an individual tire mounted on a wheel rim of a motorized vehicle. The air pressure sensing system includes a sensor mounted on the wheel rim and an energy harvester mounted on the wheel rim. The energy harvester includes an acoustic chamber and is configured to collect acoustic energy and transform the collected acoustic energy into electrical energy.  
         [0025]     Another exemplary embodiment of the invention is a motorized vehicle that includes at least one wheel rim upon which is mounted a tire, a sensor mounted on the at least one wheel rim, and an energy harvester mounted on the at least one wheel rim and adapted to harvest acoustic energy and convert the acoustic energy to electrical energy.  
         [0026]     Another exemplary embodiment of the invention is a method for powering a sensor that includes the steps of transforming acoustic energy into mechanical energy, converting the mechanical energy into electrical energy, and supplying the electrical energy to a sensor.  
         [0027]     These and other advantages and features will be more readily understood from the following detailed description of preferred embodiments of the invention that is provided in connection with the accompanying drawings.  
       DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0028]     Referring now to  FIGS. 1 and 2 , there is illustrated an energy harvester  10  including an acoustic chamber  12  and a back plate  30 . The illustrated acoustic chamber  12  has a generally circular profile and includes a first and a second chamber portion  14 ,  16  defining a chamber space  18 . It should be appreciated, however, that any geometric profile may be utilized for the chamber  12 . The first chamber portion  14  includes a cavity  31  into which fits the back plate  30 . A soft, low modulus material  31  connects the back plate  30  to an edge of the first chamber portion  14 . The material  31  allows the back plate  30  to move, while concurrently providing a leak-free chamber  12 .  
         [0029]     The back plate  30  is attached to a piezoelectric device  26  through a support beam  28 . The piezoelectric device  26  is supported at one end by the wall  17  of the second chamber portion  16 , the other end supporting the back plate  30 . The second chamber portion  16  includes an aperture  20  through which a channel  22  extends. With the exception of the channel  22 , the chamber  12  is acoustically sealed (air tight).  
         [0030]     The acoustic chamber  12  functions to collect acoustic energy, and the back plate  30  is forced to move by the acoustic energy. Thus, the acoustic energy is transformed into mechanical energy. Then, the piezoelectric device  26  attached to the back plate  30  serves to transform the mechanical energy into electrical energy. The acoustic chamber  12  also serves as a Helmholtz resonator, amplifying the collected acoustic energy.  
         [0031]     Next will be described the functioning of the energy harvester  10 . For illustration purposes only, the energy harvester  10  will be described in an application whereby a sensor being powered thereby is mounted on a wheel rim within a tire. Tires, which are shells that enclose a cavity, are continually compressed and relaxed as a result of the rolling motion of the tires on the ground. The physical properties of the shell, the air or other fluid medium within the shell, and the dimensions of the cavity dictate that the natural frequency of the shell and the rolling of the vehicle will induce a forcing function. When a vehicle rolls, the acoustic modes of the tires are excited. For example, a typical fifteen-inch tire has its first acoustic mode at about 220 Hertz, at which the pressure variation ranges from 0.005 to 0.01 psi. This pressure variation is the ultimate source of energy for the energy harvester  10 .  
         [0032]     A Helmholtz resonator is essentially an enclosed cylinder, such as the acoustic chamber  12 , with an opening, such as the channel  22 . The dimensions of the acoustic chamber  12 , as with the tire, determine its acoustic properties. The acoustic modes of a Helmholtz resonator can be excited by outside pressure fluctuation and even by a steady flow over the opening. At the Helmholtz resonator&#39;s resonance frequency, the pressure inside the resonator can be amplified many times over that of the outside pressure. Thus, the acoustic chamber  12 , if designed with a resonance frequency similar to that of the tire&#39;s, will allow for an amplification of the pressure variation in the tire of from one to ten times.  
         [0033]     The acoustic energy collected in the acoustic chamber  12  is changed into mechanical energy through the movement of the back plate  30 . The variation of pressure inside the acoustic chamber  12  applies a force across the back plate  30 , thereby flexing the attached piezoelectric device  26  under strain. The piezoelectric device  26  converts this mechanical energy into electrical energy and outputs a voltage at its electrodes (not shown). For the illustrated energy harvester  10 , the pressure variations at or near the resonance frequency of the acoustic chamber  12  push on the back plate  30 , which in turn forces a strain through the support beam  28  onto the piezoelectric device  26 , causing an output of voltage. The material  31  provides that the chamber  12  is leak-free. The output voltage is attached to an electrical circuit (not shown) that properly rectifies the signal, allowing it to power the sensor.  
         [0034]     Next, with reference to  FIGS. 3 and 4 , will be described an energy harvester  110 . Unlike the energy harvester  10 , the energy harvester  110  has a unitary acoustic chamber  112  and a piezoelectric device  126  positioned entirely exterior to the acoustic chamber  112 . A channel  122  extends through one surface of the acoustic chamber  112 , and a back plate  130  is located within an opening on an opposite surface thereof. The back plate  130  is connected to the surface with a soft, low modulus material  131 , which serves to inhibit leaks from the chamber  112 . The back plate  130  is mounted onto the piezoelectric device  126  through a support beam  128 . The acoustic chamber  112  is mounted on the piezoelectric device  126  through one or more simple supports  132 . The piezoelectric device  126  is itself mounted on a mounting surface, such as a wheel rim, through one or more simple supports  134 . The simple supports  132 ,  134  each may be a single support mechanism, such as a ring, or separate support mechanisms, such as support beams.  FIG. 4  illustrates the flexure of the piezoelectric device  126  while under strain. The simple supports  132 ,  134  are illustrated as such for simplicity sake, and it should be appreciated that the simple supports  132 ,  134  are to be configured to allow for necessary strain of the piezoelectric device  126 .  
         [0035]      FIG. 5  illustrates another embodiment of the invention, specifically an energy harvester  210  that includes an acoustic chamber  212  formed of a first chamber portion  214  and a second chamber portion  216 . The chamber portions  214 ,  216  define a chamber space  218 , in which is positioned a back plate  230  and a piezoelectric device  226 . The energy harvester  210  differs from previous energy harvester embodiments  10 ,  110  ( FIGS. 1-4 ) in that the mechanism for converting the acoustic energy to electrical energy, i.e., the piezoelectric device  226 , is housed entirely within the acoustic chamber  212 . Further, a phase difference and pressure difference is introduced across the piezoelectric device  226  within the acoustic chamber  212 .  
         [0036]     The piezoelectric device  226  is mounted on the back plate  230  through a support beam  228 . The piezoelectric device  226  is held in place between simple supports  232 ,  234 . The simple supports  234  are positioned on a flange  236  in the second chamber portion  216 . A soft, low modulus material  231  connects the back plate  230  with a second flange  237  to inhibit leakage from the acoustic chamber  212 .  
         [0037]     Next, with specific reference to  FIG. 6 , an energy harvester  310  is described. The energy harvester  310  includes an acoustic chamber  312  having a surface through which a channel  322  extends. A large opening extends through an opposing surface  337 . A piezoelectric device  326  is positioned such that it forms one wall of the acoustic chamber  312 . Specifically, the piezoelectric device  326  is pinioned between the pointed base  315  of wall  314  of the acoustic chamber  312  and a simple support  334 . Piezoelectric device  326  functions to transform the acoustic energy to mechanical energy, and to transform the mechanical energy to electrical energy. The energy harvester  310  is mounted on a mounting surface with simple support  334 .  
         [0038]     With reference to  FIG. 7 , an energy harvester  410  is shown including an acoustic chamber  412 . The acoustic chamber  412  includes a pair of channels  422   a ,  422   b , each extending through an opposing surface of the acoustic chamber. Positioned within the acoustic chamber  412  is a piezoelectric device  426 . The piezoelectric device  426  is mounted in the acoustic chamber  412  on a pair of simple supports  434 . The simple supports  434  may be configured as O-rings.  
         [0039]     The channels  22 ,  122 ,  222 ,  322 ,  422   a  and  422   b  have all been shown as being straight. It should be appreciated that the dimensions of both the channel and the chamber determine the resonant frequency of the acoustic chamber. Thus, the channels  22 ,  122 ,  222 ,  322 ,  422   a  and  422   b  may take on another suitable profile. For example, each channel may be coiled, bent, angular, or labyrinthine. The coupled response of the Helmholtz resonator (the acoustic chamber) with the compliant back plate should match the frequency of the acoustic energy source, such as, for example, the tires of a vehicle. Preferably, the two coupled system resonant peaks should be in the range of about 150 to about 300 Hertz.  FIG. 14  shows a coupled response at various frequencies. Specifically, a first peak is shown at about 230 Hz, while a second, smaller peak is shown at about 500 Hz.  
         [0040]     It should be appreciated that, for some applications, such as, for example, powering a sensing assembly located on a wheel rim within a tire cavity, the dimensions of the acoustic chamber  12 ,  112 ,  212 ,  312 ,  412  need to be rather small, on the order of about a half inch in height and about two inches in diameter. With such dimensions, the energy harvester  10 ,  110 ,  210 ,  310 ,  410  may fit within housings for sensing assemblies that currently utilize a battery as an energy source.  
         [0041]     Next will be described, with reference to  FIGS. 8-11 , an energy harvesting system  500 . The energy harvesting system  500  includes a pressure sensor  502 , an ASIC  504 , a temperature sensor  506 , and an RF transmitter  510 . An energy harvester  10 ,  110 ,  210 ,  310 , or  410  powers all the electronic components, the ASIC  504 , the pressure sensor  502 , the RF transmitter  510 , etc. The sensor  502  is configured to transmit a signal to the RF antenna  510  through the ASIC  504 , which in turn wirelessly transmits the signal to a display device  512 . For a tire pressure monitoring application, the sensor  502  and the energy harvester  10 ,  110 ,  210 ,  310 , or  410  are mounted on a wheel rim and the display  512  is located within the vehicle, such as on the dashboard. Although an ASIC  504  is shown and described, it should be appreciated that any apparatus capable of signal condition and micro-processing or micro-controlling may be utilized.  
         [0042]     Providing an energy harvester  10 ,  110 ,  210 ,  310 , or  410  with a sensor  502  for a tire pressure monitoring application will allow the sensor  502  to monitor and signal information pertaining to tire pressure while the vehicle is in motion and for a short period thereafter. This is due to the energy harvester  10 ,  110 ,  210 ,  310 , or  410  deriving its energy from the pressure fluctuations experienced by the tire during rotation. If it is desired to maintain the ability to monitor and signal tire pressure information while the vehicle is motionless, an optional battery  508  may further be included. The battery  508  may be smaller, since its sole function would be to provide power to the sensor  502  only when energy from the energy harvester  10 ,  110 ,  210 ,  310 , or  410  is insufficient to power the sensor  502 .  
         [0043]     The electronic signal from the energy harvester  10 ,  110 ,  210 ,  310 , or  410  may be used by the electronics as a motion detector. Specifically, the energy harvester  10 ,  110 ,  210 ,  310 , or  410  in conjunction with the battery  508  may signal a switch from one type of data collection that occurs during motion to a second, lower rate type of data collection that occurs in period of no motion.  
         [0044]     With specific reference to  FIGS. 9-11 , the energy harvesting system  500  is shown mounted on a wheel rim  50 . It should be appreciated that the system  500  may instead be mounted within each tire  52  ( FIG. 12 ). In such an arrangement, vibration and/or strain, and not acoustic energy, would be used as the mechanical energy transformed into electrical energy to power the sensor. In  FIGS. 9 and 10 , the energy harvesting system  500  is within a housing  514 . The system  500  may be mounted near and attached to a tire valve  53  ( FIG. 9 ). Alternatively, the system  500  may be mounted on the wheel rim  50  through the use of a strap  516  ( FIG. 10 ). Or, as shown in  FIG. 11 , the energy harvesting system  500  may be the sensor  502  and the energy harvester  10 ,  110 ,  210 ,  310 , or  410  separately housed.  
         [0045]     Shown schematically in  FIG. 12  is a motorized vehicle  520  incorporating the energy harvesting system  500 . As shown, the energy harvesting system  500  is mounted on a left front wheel rim  50 . For simplicity of illustration, the remaining energy harvesting systems  500  on the mounted wheels, as well as on the spare tire wheel are not shown. The energy harvesting system  500  may include the optional battery  508  ( FIG. 8 ). The energy harvesting system  500  wirelessly reports data from the sensor  502  through the RF transmitter  510  ( FIG. 8 ) to the display  512 , shown in  FIG. 12  to be located on the dashboard of the vehicle  520 . Although a passenger vehicle is shown in  FIG. 12 , it should be appreciated that the energy harvesting system  500  may be incorporated on any motorized vehicle traveling on roadways, including, for example, commercial and consumer trucks, commercially-operated and municipality-operated (including school) buses, commercially-operated automobiles, and motorcycles and all-terrain vehicles.  
         [0046]     It should be further appreciated that the foregoing is not an exhaustive list of potential applications for the energy harvesting system  500 . For example, an energy harvester  10 ,  110 ,  210 ,  310 , or  410  may be positioned within the engine well  524  ( FIG. 12 ). The energy harvester  10 ,  110 ,  210 ,  310 , or  410  may provide power to a sensor used for wireless sensing of vibration, temperature, pressure, or other physical parameters associated with the engine and its performance. For example, the energy harvester  10 ,  110 ,  210 ,  310 , or  410  may be used to provide power to a sensor or sensors monitoring the health of the engine of a motorized vehicle (automobile, truck, aircraft, marine, etc.). Alternatively, the energy harvester  10 ,  110 ,  210 ,  310 , or  410  may provide power to a sensor to monitor something unrelated to the engine performance, such as, for example, current in a wire running near the engine. It also should be appreciated that the energy harvester  10 ,  110 ,  210 ,  310 , or  410  may be utilized to provide power to sensors used in turbines (power generation, aircraft, or marine) or any other noise source with sufficient noise to produce acoustic energy in abundant supply. Obviously, each source of noise will have a unique frequency spectrum, and the energy harvester  10 ,  110 ,  210 ,  310 , or  410  will need to be designed accordingly.  
         [0047]     With reference to  FIG. 13 , next will be described a method for powering a sensing assembly, such as the assembly including sensor  502 , ASIC  504  and RF transmitter  510 . At Step  600 , acoustic energy is transformed into mechanical energy. In the tire pressure monitoring application, acoustic energy is created due to the continuously fluctuating pressure within a tire in motion, and that acoustic energy may be collected within an acoustic chamber  12 ,  112 ,  212 ,  312 ,  412  which functions as a Helmholtz resonator. The back plate  30 ,  130 ,  230 , or the piezoelectric device  326 ,  426  transforms the acoustic energy to mechanical energy, as evidenced by flexure caused by strain induced by a change in pressure in the acoustic chamber  12 ,  112 ,  212 ,  312 ,  412 .  
         [0048]     At Step  605 , the mechanical energy is converted into electrical energy. The piezoelectric device  26 ,  126 ,  226 ,  326 ,  426  is flexed under strain due to the change in pressure in the acoustic chamber  12 ,  112 ,  212 ,  312 ,  412  and converts the strain into electrical energy. At Step  610 , the sensor  502  is powered with the electrical energy. The piezoelectric device  26 ,  126 ,  226 ,  326 ,  426  outputs the voltage through electrodes, which are in connection with an electrical circuit that rectifies the voltage (altering the voltage from alternate current to direct current).  
         [0049]     While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. For example, while the energy harvester and the energy harvesting system have been described in conjunction with a tire pressure monitoring application and a motor health monitoring application, it should be appreciated that the energy harvester and energy harvesting system may find utility for any application in which acoustic energy can be collected and transformed into electrical energy, such as, for example, monitoring physical parameters of a machine that produces vibrations or industrial process monitoring. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.