Abstract:
Disclosed is an energy harvesting device which can produce electrical power from vibrational energy over a wide range of frequencies. The energy harvester includes a housing having opposing slots. A bendable substrate is at least partially positioned within the housing and at least partially extends through the opposing housing slots. A piezoelectric element is positioned on the bendable substrate and a weight cooperates with the bendable substrate. A stopper is positioned on each end of the bendable substrate that extends outside the housing; the stoppers are configured to maintain a portion of the bendable substrate within the housing such that the bendable substrate is freely movable within the housing. Vibrational energy causes collisions between the bendable substrate and the housing such that the forces on the piezoelectric element generate power.

Description:
FIELD OF THE INVENTION 
     The present invention relates to energy harvesting devices in general and, more particularly, to energy harvesting devices that can harvest energy over a range of frequencies, including non-resonant frequencies. 
     BACKGROUND 
     Vibration energy can be used to power a variety of devices such as sensors and transmitters that are located in hard-to-reach positions in apparatus such as vehicles and machinery. Such sensors and transmitters have been conventionally hard-wired or powered by batteries, but such power sources increase the complexity of the overall apparatus or require frequent maintenance (e.g., changing or recharging of batteries). 
     Although devices that generate power from vibration are known in the art, such devices typically are effective at a particular resonant frequency. However, the frequency of the vibration that will be generated by a particular apparatus is not always known in advance. Therefore, it is difficult to design a power-generating device that will work over the wide range of resonant frequencies that are encountered in different apparatus. 
     Various approaches to overcome the resonant frequency issue have been proposed. In U.S. Patent Application Publication 2007/0125176, plural energy harvesting devices, each having a different resonant frequency, are concatenated to provide a system that can produce power of a range of frequencies. 
     Similarly, U.S. Pat. No. 7,667,375 uses a group of harvesters each with a single resonance mode to create a device that can operate in a range of resonant frequencies to produce power. 
     Other approaches are designed to operate in a rotational environment such as the inside of a tire. Each rotation of the tire creates pulses of electrical charge. This approach is shown in U.S. Patent Application Publication 2007/0063621 and U.S. Patent Application Publication 2008/0258581. 
     However, most of the prior art approaches feature high quality factor systems where the quality factor, Q, is defined as:
 
 Q=A/B  
 
where B is the static deformation and A is the amplitude under the resonant mode. This limits the bandwidth in which the device can produce energy.
 
     Thus there is a need in the art for improved energy harvesting devices which can operate in a broad range of frequencies to generate power. 
     SUMMARY OF THE INVENTION 
     The present invention relates to an energy harvesting device which can produce electrical power from vibrational energy over a wide range of frequencies. The energy harvester includes a housing having opposing slots. A bendable substrate is at least partially positioned within the housing and at least partially extends through the opposing housing slots. A piezoelectric element is positioned on the bendable substrate and a weight cooperates with the bendable substrate. 
     A stopper is positioned on each end of the bendable substrate that extends outside the housing; the stoppers are configured to maintain a portion of the bendable substrate within the housing such that the bendable substrate is freely movable within the housing. Vibrational energy causes collisions between the bendable substrate and the housing such that the forces on the piezoelectric element generate power. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically depicts a cross-sectional view of an energy harvesting device according to an embodiment of the present invention. 
         FIG. 2  shows a modification of bendable substrate and weight configuration usable with the energy harvesting devices of the present invention. 
         FIG. 3  depicts a housing modification that can be used with the energy harvesting device of  FIG. 1 . 
         FIG. 4  schematically depicts an energy harvesting device according to another embodiment of the present invention. 
         FIG. 5  depicts plural bendable substrates and weight configurations that can be used in the energy harvesters of the present invention. 
         FIG. 6  depicts several bendable substrates and weight configurations that can be used in the energy harvesters of the present invention. 
         FIGS. 7A and 7B  depict an embodiment of the harvester in which bendable substrates of different spans are supported in a housing. 
     
    
    
     DETAILED DESCRIPTION 
     Turning now to the drawings in detail,  FIG. 1  depicts a cross-sectional view of an energy harvester  10  according to one embodiment of the present invention. The harvester shape is not critical; it can be cylindrical, a rectangular parallelepiped, or any other three-dimensional shape that can accommodate the various harvester elements. 
     Harvester  10  includes housing  20  having opposing slots  22  formed therein. Extending from slots  22  is bendable substrate  30 . To maintain bendable substrate  30  within housing  20 , stoppers  32  are positioned adjacent either end of the substrate. Stoppers  32  are sized such that they are larger than housing slots  32 . Therefore, even though the position of the bendable substrate is not fixed (that is, it is freely movable within housing  20 ), it cannot escape from the housing. 
     Depending upon the desired bending stiffness of bendable substrate  30 , various materials are selected. Exemplary materials include, but are not limited to, bendable metallic materials such as copper alloys and stainless steel, and polymeric materials such as PVC. The bendable substrate has a slender, shim-like shape, with the selected thickness based on the selected material, the overall dimensions of the harvester and the desired bending stiffness of the bendable substrate. The desired bending stiffness is determined, in part, by the application environment of the harvester with strong vibration environments typically using a higher bending stiffness than weaker vibration environments. 
     On a first side of bendable substrate  30  is piezoelectric element  40 . Piezoelectric element  40  may be selected from any piezoelectric material that produces sufficient electric power to run a selected device, such as a sensor. Suitable piezoelectric materials are typically piezo-ceramic materials such as lead zirconate titanate (PZT) and lanthanum-doped lead zirconate titanate (PLZT). An electrode pattern is coated on the outer surface piezoelectric element  40 ; the electrode material is selected from conductive materials such as gold, silver, nickel, and conductive polymers such as conductive epoxies. A variety of wire bonding techniques know in the art for use with electronics packaging may be used to attach leads to the electrodes. Attachment points are typically reinforced with materials such as epoxy resins to prevent fatigue of the wires and protect the joint between the wire and the electrode from damage or separation during harvester operation. 
     On the opposite side of bendable substrate  30  is deadweight  50 . The momentum provided by weight  50  enhances the contacts and impacts of the bendable substrate  30  with housing  20 . For example, if energy harvester  10  is positioned within a tire in order to power a pressure sensor, vibration plus the rotation of the tire causes bendable substrate  30  with piezoelectric element  40  and deadweight  50  to contact housing  20  and causes substrate  30  to impact the housing at the edges of the slots. When bendable substrate  30  bends, a force is applied to the attached piezoelectric element  40 , resulting in power generation. Typically, the alternating current output is sent to a rectifier for conversion to direct current and stored in capacitors to power an associated device. 
     The energy harvester  10  is capable of frequency spectrum transformation due to the impacts between bendable substrate  30  and housing  20 . The contact force caused by an impact is typically characterized by a sharp peak followed by oscillations. The design of harvester  10  results in even low frequency excitations causing the harvester to oscillate; these oscillations result in impacts between bendable substrate  30  and housing slots  22 . Those impacts will introduce high frequency vibration excitations to the bendable substrate so that more energy will be generated by the piezoelectric element. This process is called “frequency spectrum transformation.” 
     In the environment of a rotating tire, the vibration spectrum includes two components: a low frequency component from the rotation of the tire (typically less than 20 Hz and its energy increases with increasing vehicle speed) and a component from the random vibration of the tire. This latter component is white noise which is independent of vehicle speed. The design of harvester  10  permits both types of vibrational energy to be converted into electrical energy. 
     In order to minimize the possibility of damage to piezoelectric element  40 , housing stoppers  60  made from a resilient material such as an elastomeric polymer (e.g., rubber) can be optionally positioned in the housing  20 . Housing stoppers  60  can include hemispherical portions  62  as seen in  FIG. 3  to further cushion the contact between the substrate/piezoelectric element/deadweight structure and the housing. 
     To further protect the piezoelectric element  40 , e.g., from over-deformation of substrate  30  and from direct contact with the housing  20  or housing stoppers  60 , an alternate embodiment is depicted in  FIG. 2 . In the  FIG. 2  embodiment, a pair of deadweights  52  are positioned on either side of bendable substrate  30 , connected by a narrow connector  54  (thus providing minimum interference to the stiffness of bendable substrate  30 ). 
     As shown in  FIGS. 5 and 6 , multiple bendable substrates  30  and piezoelectric layers  40  can be combined with deadweights  52  to increase the energy to be harvested from the device. The edges of bendable substrate  30  include projecting stoppers  34  (made from portions of the substrate itself or bonded to the substrate) to prevent the substrate from escaping from within housing  20  through housing slots  22 . To reduce the overall bending stiffness of the structure, blocks  70  are each bonded to a single substrate  30  with sufficient clearance to permit relative motion of the bendable substrates  30 . Blocks  70  are made from a material with low coefficient of friction or are covered with a material having a low coefficient of friction such as PTFE. The blocks provide additional impact points for the bendable substrates, contributing to the bending of bendable substrates  30 . As the bendable substrates  30  with piezoelectric elements  40  are bonded together through connectors  54 , they will deform in-phase, resulting in a compact device that produces more power than with a single bendable substrate/piezoelectric element and without the complexity and added weight of plural devices used together. 
     As seen in  FIG. 4 , a vibration source may be directly connected to bendable substrate  130  via support rod  180 . Here, housing  120  also acts as the deadweight such that relative motion between the housing and the bendable substrate  130  will deform the piezoelectric element  140  to generate power. Housing  120  includes slots  122  and deformable substrate  130  includes stoppers  132  to prevent escape from the housing. Housing further includes slot  128  to permit relative motion between the rod  180  and the housing  120 . Stoppers  170  may optionally be included to cushion the contact between the housing and the bendable substrate/piezoelectric element combination. It is noted that multiple bendable substrate/piezoelectric material combinations can be bonded together as in  FIGS. 5 and 6  to generate increased power. Using the housing as the weight can reduce the total weight of the energy harvester as well as reducing the overall dimensions of the energy harvester. 
       FIGS. 7A and 7B  depict an energy harvester  210  in which plural bendable substrates  230  are provided, each bendable substrate having a different span width. By providing bendable substrates  230  with different span widths, the overall response frequency range of harvester  210  is increased. Note that although  FIG. 7  depicts piezoelectric elements  240  and deadweights  250  attached to bendable substrates  230 , the application of multiple span width substrates is not limited to this configuration. The use of multiple span width bendable substrates may also be used in embodiments in which the housing itself acts as the dead weight (as depicted in  FIG. 4 ). Additionally, for each span width, multiple bendable substrates may be joined together as depicted in  FIGS. 5 and 6 . 
     The present invention is applicable in a variety of industrial applications. The energy harvester may be positioned within a rotating tire to power a pressure sensor which transmits tire pressure information to a receiver positioned remotely. For vehicle applications, such a receiver is positioned elsewhere on the vehicle such that tire pressure information can be viewed by the operator of the vehicle. However, the present invention is not limited to vibration sources which include rotation. For example, the energy harvesters may be used to power strain gauges on components in high vibration environments (e.g., aircraft wings, power equipment) such that the strain gauges transmit strain information to a monitoring location. 
     While the foregoing invention has been described with respect to various embodiments, it is understood that other embodiments are within the scope of the present invention as expressed in the following claims and their equivalents.