Broadband energy harvester apparatus and method

[0001] The present application is related in general subject matter to the following applications, each of which being filed concurrently with the present application, and each of which is incorporated by reference into the present application:

[0002] U.S. application Ser. No. 11/551,525;

[0003] U.S. application Ser. No. 11/551,515;

[0004] U.S. application Ser. No. 11/584,304;

[0005] U.S. application Ser. No. 11/584,305.

[0006] The present disclosure relates to relates to energy harvesting devices and methods, and more particularly to a broadband energy harvesting apparatus and method that enables energy harvesting of small vibration amplitudes over a wide frequency band.

[0007] The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.

[0008] Low power electronics and sensors are becoming prevalent, as are wireless transceivers, that allow various forms of sensors to be placed in remote and inaccessible locations on various structures, for example on various forms of aircraft and spacecraft. The demand for self-powered sensors and devices that are able to obtain electrical power through environmental energy harvesting is also increasing. Energy in various forms of vibrating structures is typically available over a wide range of frequencies. This is especially so in automotive applications, and particularly on or near wheels or on suspension components used with motor vehicles such as cars, trucks and SUVs. Typically, this energy is lost during normal operation of the structure or platform on which the energy harvesting device is being employed. Thus, an autonomous, inexpensive and reliable energy harvesting device that is able to harvest vibration energy over a wide frequency range, and is responsive to very small amplitudes of vibration energy, would be highly useful in powering remotely located sensors and other forms of devices.

[0009] A principal disadvantage of many present-day energy harvesting devices is that such devices harvest vibration energy primarily at one or more frequencies. It would enhance the utility and efficiency of an energy harvesting device significantly if such a device was responsive to vibration energy over a wide frequency band, as well as responsive to very small amplitudes of vibration energy.

[0010] Still further, a vibration energy harvesting device that is highly compact and lightweight, and which is able to harvest vibration energy over a wide frequency band. would enable electrical power to be generated that could be used to power remotely located sensors and other devices. The ability to provide electrical power to remotely located sensors and various devices would eliminate the need to extend cabling or other forms of conductors to the devices. it would further enable the use of sensors in various forms of electrically powered devices at locations on various structures, for example on aircraft and spacecraft, where it might otherwise be impractical or impossible to place a sensor because of the need to run electrical cabling to the sensor to power the sensor.

[0011] The present disclosure is related to an energy harvesting apparatus and method. In one embodiment an energy harvesting apparatus is provided that includes a beam and a biasing member. The beam includes electrically responsive material. In one implementation the electrically responsive material comprises at least one piezoelectric layer of material. The beam and the biasing member are both supported from a support structure and compressed while being supported. The compression significantly softens the electrically responsive material of the beam, thus promoting ready flexing of the beam in response to small amplitudes of vibration. The beam is secured to an external structure that generates vibration energy. The biasing member operates to apply a biasing force to the beam that enhances flexing motion of the beam in response to vibration.

[0012] As the beam experiences vibration, it flexes, which causes the electrically responsive material on the beam to generate electrical signals. These electrical signals are routed through suitable conductors from the electrically responsive material and used to provide electrical power to an external device, for example a sensor or an actuator.

[0013] In one specific implementation, a pair of proof masses are used to support the beam and the biasing member. A coupling element extends between the two proof masses and urges the proof masses towards one another, thus compressing the beam and the biasing member. In this implementation the biasing member also comprises a beam that is flexible. The proof masses are further coupled via the coupling element such that they are able to pivot as both beams flex in response to vibration.

[0014] In various implementations, the energy harvesting apparatus and method of the present disclosure provide an extremely compact, lightweight system that enables it to be readily secured to various forms of external structures and/or sensors, actuators, etc., to harvest vibration energy being experienced by the structure, sensor, or actuator.

[0015] Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.

[0027] The following description is merely exemplary in nature and is not intended to limit the present disclosure, its application or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features.

[0028] Referring to FIG. 1, there is shown a bimorph. broadband energy harvester apparatus 10 in accordance with one implementation of the present disclosure. In this embodiment, the apparatus 10 includes a first, normally straight. electrically responsive beam, in this instance a piezoelectric beam 12, and a second biasing member in the form of a biasing beam 14. The beams 12 and 14 are held slightly compressed by a supporting structure 15 so as to be normally slightly bowed or “buckled”. The supporting structure 15 includes a plate forming an upper proof mass 16, a plate forming a lower proof mass 18, and a coupling assembly 20 that draws the proof masses 16 and 18 towards one another to compress the beams 12 and 14. Suitable electrical leads are coupled to the piezoelectric beam to receive electrical output signals generated by the beam 12 as the beam flexes. An attachment structure 24 permits the apparatus to be attached to or supported either partially or entirely, from a vibrating structure. Although it will be appreciated that the apparatus 10 is not limited to such an attachment arrangement.

[0029] It will be appreciated that in some applications, the piezoelectric beam 12 may instead incorporate a magnetostrictive material. Thus, other forms of electrically responsive materials may be used to help form the beam 12 rather than piezoelectric material.

[0030] With reference to FIGS. 2 and 6, the piezoelectric beam 12 comprises a multi-layer assembly having a center substrate 26, and piezoelectric material layers 28 and 30 adhered on opposing surfaces of the center substrate 26. The center substrate 26 may be formed from any suitable, slightly flexible material, and in one implementation is formed by a carbon laminate material layer having a zero degree fiber orientation. In one embodiment, the center substrate 26 has dimensions of approximately 2.0 inches (50.8 mm) in length, 0.5 inch (12.7 mm) in width, and 0.05 inch (1.27 mm) in thickness. Piezoelectric material layers 28 and 30, in this example, have dimensions of about 1.45 (36.83 mm) inch in overall length, 0.45 inch (11.43 mm) in width, and 0.05 inch (1.27 mm) in thickness. It will be appreciated, however, that these dimensions may vary significantly depending upon the needs of a specific application. Each piezoelectric material layer 28, 30 may be formed from one or more distinct layers that are adhered or otherwise bonded together.

[0031] With further reference to FIG. 2, each end of the piezoelectric beam 12 includes a support element 32 having a slot 34 that receives an end portion 36 of the piezoelectric material layer 26. End portion 36 may be adhered, soldered, or otherwise secured fixedly within the slot 34. The support element 32 is preferably circular in shape and includes one or more bumps or protrusions 38 that provide bearing alignment and work together with the circular shape of its associated support element 32 to provide extremely low friction bearing pivot surfaces that facilitate flexing movement of the piezoelectric beam 12. It will be appreciated that both ends of the piezoelectric beam 12 are secured to separate support elements, such as support element 32 shown in FIG. 2. Similarly, both ends of the biasing beam 14 are coupled to separate support elements, such as support element 32, shown in FIG. 1. A detailed discussion of one specific construction of one embodiment of the piezoelectric beam 12 and its method of manufacture is disclosed in U.S. application Ser. No. 11/584,304, filed concurrently herewith, and incorporated by reference into the present application. It will be appreciated, however, that the support element 32 can have other forms or configurations which permit relative motion, including only one pivotal end, without departing from the scope of this disclosure.

[0032] Referring to FIG. 4, the lower proof mass 18 is shown in greater detail. The proof mass 18 is formed from any suitably rigid material, for example steel or aluminum, or possibly even high strength plastic. In this embodiment the proof masses 16 and 18 are similar in construction and are sufficiently rigid so that they do not flex during operation of the apparatus 10. The proof mass 18 includes a pair of raised ribs 40 that each has at least one, and more preferably a pair, of hemispherical recesses 42. Recesses 42 are positioned to align with the spacing of the protrusions 38, and each has a diameter generally in accordance with the diameter of the protrusions 38. In this manner, for example, when the support element protrusions 38 (FIG. 2) are situated in one pair of recesses 42, the support element 32 is able to freely rotate in accordance with arrow 44 in FIG. 2. The engagement of protrusions 38 within their respective recesses 42 provides a low friction, bearing-like support arrangement that facilitates smooth rotational movement of the support element 32 relative to the proof mass 18. Other configurations, including a support element without protrusions, are feasible.

[0033] With further reference to FIG. 4, the lower proof mass 18 also includes a generally square shaped opening 46 to enable the coupling assembly 20 to secure to the proof mass 18. In this embodiment proof mass 16 is identical in construction to proof mass 18, as will be explained in connection with FIG. 5.

[0034] Referring to FIG. 5, a substantial portion of the coupling assembly 20 is shown. The coupling assembly 20 includes a coupling element 52, which in one implementation is a steel ribbon. Although there is one coupling assembly 20 on the illustrated embodiment, there could be more. The coupling element 52 is secured such as by soldering or adhesives to an attachment component 54 having a slot 56 for receiving an end of the coupling element 52. The attachment component 54 includes a threaded portion 58 that extends through the hole 48 and engages with a threaded nut 60. The generally square shaped opening 46 in FIG. 4 restricts the attachment component 54 from rotation, allowing the nut 60 to be adjusted without rotating the attachment component 54. The nut 60 enables a desired axial pre-loading force to be imparted to the beams 12 and 14. A second attachment component 54 and second nut 60 is used to secure the opposite distal end of the coupling element 52 in like fashion.

[0035] With reference to FIGS. 1 and 7, the biasing beam 14 also comprises a multi-layer assembly having a center substrate 62 and a pair of outer substrates 64 on outer opposing surfaces of the center substrate 62. In one form, the center substrate 62 comprises a carbon laminate material layer having a fiber orientation of zero degrees and orientated along the beam 14 major axis. This enhances the lengthwise bending toughness of the beam 14 while still allowing the widthwise bending stiffness to remain “soft”. In this implementation, the substrates 64 are also formed by carbon laminate layers, with each having a fiber orientation of between about 25 degrees-30 degrees. The dimension of the center substrate 62 of the biasing beam 14 and the center substrate 26 of the piezoelectric beam 12 may also be identical in dimensions, and the substrates 64 may be identical in dimensions to the piezoelectric material layers 28 and 30. The beams 12 and 14, in one implementation, have preferably the same bending stiffness along their longitudinal lengths. In one embodiment, the beams 12 and 14 also have the same coefficient of thermal expansion.

[0036] An advantage of the coupling assembly 20 and proof masses 16 and 18 is that these components form an especially compact, lightweight and effective arrangement for holding the beams 12 and 14 in an initially compressed orientation, such that each of the beams 12 and 14 is bowed or flexed slightly as shown in FIGS. 1 and 8. Since the apparatus 10 forms a highly compact arrangement, it is well suited for use in a wide variety of applications where space limitations might not permit the use of larger energy harvesting assemblies. The use of a buckled beam for the bias beam 14 is a distinct advantage over other forms of force biasing mechanisms in that the buckled bias beam 14 provides a very low axial stiffness compared to other biasing mechanisms such as compression springs.

[0037] Turning now to FIGS. 8-10. a description of operation of the apparatus 10 will be provided. FIG. 8 illustrates the apparatus 10 before the apparatus has received any vibrating energy from a vibrating structure. In this example, it will be noted that the beams 12 and 14 are compressed so as to be bowed approximately equally. Both beams 12 and 14 can be viewed as being in a stable position. An equally likely orientation is with beam 12 bowed in and beam 14 bowed out. This does not change the function of the apparatus 10.

[0038] In FIG. 9, a vibration force directed in accordance with arrow 70 is imparted to the attachment structure 24, which causes the piezoelectric beam 12 to flex into a substantially flat orientation. This causes a simultaneous flexing of the biasing beam 14. The proof masses 16 and 18 pivot freely as the beams 12 and 14 are flexed. As the piezoelectric beam 12 flexes from the position shown in FIG. 8 to that shown in FIG. 9, it generates electrical signals that can be used to power an external sensor, actuator, or other forms of electrical components. The compressive force exerted by the proof masses 16 and 18 serve to buckle the piezoelectric beam 12 and reduce stiffness or substantially “soften” the beam. That enables broadband energy harvesting because the beam 12 readily flexes in response to small amplitude forces and at a relative wide range of frequencies.

[0039] Referring to FIGS. 9 and 10, as the vibration force continues to be directed along arrow 72 in FIG. 10, the piezoelectric beam moves past an over center position (essentially defined by the orientation of the piezoelectric beam 12 in FIG. 9), into another bowed orientation, as shown in FIG. 10. As the vibrating force changes direction, the piezoelectric beam 12 will again begin to flex towards the flattened out position shown in FIG. 9. During this opposite flexing motion, the piezoelectric beam 12 will generate another electrical signal or pulse. This signal or pulse, however, will have a polarity opposite to that of the signal or pulse that was generated during its initial movement from the position shown in FIG. 8 into the position of FIG. 9. Thus, the alternate flexing movement of the piezoelectric beam 12 will be assisted by the biasing member 14. and the piezoelectric beam 12 will generate two electrical pulses or signals for each flexing cycle. This construction also operates to restrain the motions of beams 12 and 14 due to high amplitude vibrations. When the piezoelectric beam 12 flexes to a displacement where the biasing beam 14 becomes straight, the biasing beam 14 will no longer provide a bias force and the piezoelectric beam 12 will become stiff, thus restraining further motions.

[0040] In manufacturing the piezoelectric beam 12, one method of manufacture may employ using a pair of piezoelectric material sheets that are positioned on opposing surfaces of a prepreg material at room temperature. The laminate may be cured at 200° C. Since the coefficient of thermal expansion for the carbon fibers is greater than that of the piezoelectric sheets, when the laminate cools the piezoelectric sheets are in compression. This is advantageous because piezoelectric material is better able to convert strain energy into electrical energy when it operates in compression.

[0041] The apparatus 10 is especially well suited for aerospace and automotive applications. In both aerospace and automotive applications, there is a need to locate electronic sensors or actuators on remote areas of a vehicle or structure. In such instances, it is often impractical to route wiring to the sensor or actuator for the purpose of providing power to the component. Often, the use of batteries to power the component is impractical or impossible. Since the apparatus 10 is able to produce electrical signals that can be used to power an electrical powered component, the elimination of a battery provides the additional advantage that environment disposal concerns are eliminated, which would otherwise be present with the use and replacement of a battery. The ability to place the apparatus 10 in virtually any location on a vehicle adds significant flexibility in allowing various forms of electrically powered components to be placed in locations that would otherwise not be practical or possible if batteries and electrical wiring was required for communicating power to the component(s).

[0042] By axially preloading both of the beams 12 and 14 to the point where both beams are slightly buckled, the piezoelectric beam 12 is significantly softened, and the overall stiffness of both of the beams 12 and 14 is significantly decreased. When the attachment structure 24 is secured to a vibrating structure, the piezoelectric beam 12 is highly responsive to even small amplitudes of vibration, and at low frequencies (typically above 0.5 Hz. Thus, very low vibration forces are able to be sensed by the apparatus 10, and such small forces generate high strains on the piezoelectric beam 12. This is in contrast to many present day energy harvesting systems, which rely on harvesting energy at one or more discrete resonant frequencies. The apparatus 10, being especially sensitive to very small vibration amplitudes, enables vibration energy harvesting over a much wider frequency range than is typically possible with existing resonant frequency energy harvesting devices.

[0043] While various embodiments have been described, those skilled in the art will recognize modifications or variations which might be made without departing from the present disclosure. The examples illustrate the various embodiments and are not intended to limit the present disclosure. Therefore, the description and claims should be interpreted liberally with only such limitation as is necessary in view of the pertinent prior art.