Abstract:
A method of forming an energy harvesting device comprises supporting an outer peripheral edge of a disc spring using a support element that allows oscillations of the disc spring. A first preload force is applied to the disc spring and directed along its axial center. During application of the first preload force, a piezoelectric material is fixedly secured with a surface of the disc spring. A second preload force is applied to the disc spring to thereby provide a predetermined reduction of a stiffness of the disc spring. The reduction of the stiffness corresponds to an increased sensitivity to low-frequency components of vibrational energy received by the energy harvesting device.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 12/792,151, filed on Jun. 2, 2010, which is a continuation-in-part of U.S. patent application Ser. No. 11/672,695 filed on Feb. 8, 2007. The entire disclosures of each of the above applications are incorporated herein by reference. 
    
    
     FIELD 
     The present disclosure relates to energy harvesting apparatus and methods and, more particularly, to an energy harvesting apparatus and method that makes use of a spring disc, commonly known as a “Belleville” spring, to harvest vibration energy from a vibrating structure. 
     BACKGROUND 
     The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. 
     Electrically-powered devices require a power source. Electrical power can be supplied in a variety of ways, including through wiring from a centralized source or from a battery. Many electrical devices are used on mobile platforms, such as aircraft, aerospace vehicles, rotorcraft, etc. The wiring typically used in these applications is heavy and costly. The use of batteries requires periodic replacement and/or recharging. In addition, a battery contains corrosive materials, and this can be a factor in limiting the use of a battery in some applications. Furthermore, in some aerospace and aircraft applications such as flight testing, various forms of sensors are located in areas where it would be costly to route power wiring. 
     Various attempts have been made to use piezoelectric material as a component of an energy harvesting device. When piezoelectric material is strained, an electrical charge is generated through the coupling of the mechanical and electrical states of the material. The charge generated can be useful electrical energy. The development of areas and methods of harnessing this electrical energy is finding considerable interest at the present time for their potential to power various forms of sensors and electrical components, and especially in applications where it is impractical or difficult to make use of a battery and/or wiring leading to the sensor or device. 
     Various forms of piezoelectric devices have attempted to convert vibrating energy from a structure into useful electrical energy. However, many piezoelectric energy harvesting devices have difficulty harvesting vibration energy at low frequencies (i.e., frequencies typically less than 100 Hz). The problem with such piezoelectric devices is their lack of sensitivity to low frequency vibration energy. A device able to convert low frequency vibration energy into useful electrical energy would thus prove highly useful in a wide variety of applications where the need exists to power a remotely located sensor or other form of electronic device. 
     SUMMARY 
     In one aspect the present disclosure relates to a method for forming an energy harvesting device. The method may comprise providing a disc spring and supporting an outer peripheral edge of the disc spring. The method may further comprise applying a pre-load force to an inner peripheral edge of the disc spring directed along an axial center of the disc spring, and while the pre-load force is being applied, using an adhesive compound to adhere a piezoelectric material to a surface of the disc spring. The method may further comprise waiting a predetermined time until the adhesive compound has cured. The disc spring may be secured to a support element using a fastening assembly, and the fastening assembly and the support element used to apply a predetermined preload force to said disc spring. The pre-load force may cause a degree of deflection of the disc spring, with the deflection being sufficient to place the disc spring in a condition of reduced stiffness. 
     In another aspect the present disclosure relates to a method for harvesting vibration energy from a vibrating source. The method may involve securing a pair of disc springs to the vibrating source. The disc springs may be held in opposing relationship and pre-loaded with a force sufficient to substantially soften the disc springs and to make the disc springs sensitive to low frequency, low amplitude vibration energy. A material may be secured to a first one of the disc springs. The material may generate an electrical output signal in response to changes in strain that is experienced as the first disc spring flexes in response to vibration transmitted from said vibrating structure. Electrical output signals may be received from the material as the one disc spring flexes during vibration of the structure. 
     In still another aspect the present disclosure relates to a method for forming an energy harvesting device. The method may involve providing a disc spring and supporting an outer peripheral edge of the disc spring. A pre-load force may be applied to the disc spring which is directed along an axial center of the disc spring. While the pre-load force is being applied, a piezoelectric material may be fixedly secured to a surface of the disc spring. The disc spring may be supported from a peripheral edge thereof to allow oscillating motion of the disc spring. The support element may be used to apply a predetermined preload force to the disc spring that reduces a stiffness of the disc spring. 
     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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. 
         FIG. 1  is a perspective view an energy harvesting apparatus in accordance with one embodiment of the present disclosure; 
         FIG. 2  is an exploded perspective view of the apparatus of  FIG. 1 ; 
         FIG. 2A  is an enlarged cross sectional view showing the attachment of one of the piezoelectric material rings to its associated spring disc, as taken in accordance with section line  2 A- 2 A in  FIG. 2 ; 
         FIG. 3  is a cross-section of the assembled apparatus in accordance with section  3 - 3  in  FIG. 1 ; 
         FIG. 4  is a simplified side view of one of the spring discs illustrating the geometry of the spring disc; 
         FIG. 5  is a force versus deflection curve for the spring disc of  FIG. 4  illustrating the region of low stiffness which the spring disc of  FIG. 4  is pre-loaded to once fully assembled; 
         FIG. 6  is a graph illustrating the force versus deflection curves of a pair of Bellville springs arranged in facing relationship with one another, such as shown with the apparatus of  FIG. 1 , and illustrating the region of low stiffness within which the springs operate; 
         FIG. 7  is a simplified side view of an arrangement for supporting one of the disc springs by use of a magnetic bearing; and 
         FIG. 8  is a simplified side view of an alternative magnetic bearing arrangement for supporting one of the disc springs. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. 
     Referring to  FIG. 1 , there is shown an exemplary energy harvesting apparatus in accordance with an embodiment of the present disclosure. The energy harvesting apparatus  10  may be mounted to a vibrating structure  12  that vibrates at a frequency over a relatively wide frequency range (e.g., between about 10 Hz-1 KHz). The apparatus  10  is supported from the vibrating structure  12  in this example by mounting arms  15  that are secured in any suitable manner to the vibrating structure  12 . Thus, the apparatus  10  receives the vibration energy from the structure  12  and vibrates in accordance with the structure. Preferably, the apparatus  10  is mounted relative to the structure  12  such that the axis of motion of the apparatus  10  is parallel to the axis of vibration being experienced by the structure  12 , in this example along the axis defined by arrow  16 . 
     The apparatus  10  generates electrical power in response to the vibration energy from the vibrating structure  12  and transmits the electrical power to a suitable power conditioning system  18 , which then supplies an electrical power output  20  to an electronic or electromechanical device requiring electrical power. While the apparatus  10  is especially well suited for providing electrical power to power other electrical, electronic or electromechanical devices, it will be appreciated that the electrical output signals generated by the apparatus  10  could just as readily be used to turn on and off a sensor or other electrical, electronic or electromechanical component or be conditioned and stored in a circuit for later use. 
     Referring to  FIGS. 2 and 3 , the construction of the apparatus  10  is shown in greater detail. The apparatus  10  generally includes a first disc spring  22 , a second disc spring  24  and a support ring  26  for supporting peripheral edge portions  28  and  30  of the first and second disc springs  22  and  24 , respectively. The disc springs  22  and  24  may comprise well known “Belleville” springs. Alternatively, any resilient, frustoconical shaped disc is potentially useable. A power conditioning subsystem  18 , as will be further described in the following paragraphs, processes the electrical signals obtained from the apparatus. 
     The first disc spring  22  further includes a ring of electrically responsive material  32 , which in one preferred form may comprise a piezoelectric material ring. Similarly, the second disc spring  24  includes an electrically responsive material ring  34  secured thereto, which also may comprise a piezoelectric material ring. For convenience, material rings  32  and  34  will be referred to as “piezoelectric” material rings throughout the following discussion. It will be appreciated, however, that any material that is able to generate electrical signals in response to changes in strain may be used in place of a piezoelectric material. Such other materials might include polyvinylidine fluoride (PVDF) film. Each piezoelectric material ring  32 , 34  is further arranged coaxially with the axial center of its associated disc spring  22  or  24 . 
     Electrical conductor  22   a  is electrically coupled to the piezoelectric material ring  32  by any suitable conductive adhesive or mechanical connection, while electrical conductor  22   b  may be similarly coupled to the first disc spring  22 . Conductors  22   a  and  22   b  feed the potential difference signal generated across the piezoelectric material ring  32  to a first input channel of the power conditioning system  18 . Conductor  24   a  is electrically coupled to the second piezoelectric material ring  34  by any suitable conductive adhesive or mechanical connection, and conductor  24   b  may be similarly conductively coupled to the second disc spring  24 . Conductors  24   a  and  24   b  feed the potential difference signal generated across the second piezoelectric material ring  34  into a second input second channel of the power conditioning circuit  18 . The power output  20  of the power conditioning system  18  represents an electrical signal that may be related to the vibration energy harvested by the apparatus  10 . 
     Referring further to  FIGS. 2 and 3 , the first disc spring  22  includes an aperture  36  at its axial center, while the second disc spring  24  similarly includes an aperture  38  at its axial center. The fastening member  14  is inserted through a washer  40   a , which may be optional, through the apertures  36  and  38 , through an optional washer  40   b , and is engaged with a nut  42  to hold the disc springs  22  and  24  clamped against oppositely facing edge portions  44  and  46  of the support ring  26 , and in an opposing but axially aligned relationship. In this regard, it should be appreciated that edge portions  44  and  46  may include a notch or shoulder formed therein to help maintain the disc springs  22  and  24  axially aligned during an assembly procedure. The fastening member  14  also may have a length that is sufficiently long to enable it to be used to secure the apparatus  10  to the vibrating structure  12 , in place of the arms  15  shown in  FIG. 1 . The fastening member  14  and the threaded nut  42  allow an adjustable degree of preload force to be applied to the disc springs  22  and  24  during assembly. This is advantageous, as will be explained in the following paragraphs. Depending on the thread count of the fastening member  14 , and analytical or computational modelling or empirical testing, it is possible to determine that a specified number of turns of the fastening member  14  will apply a known preload force to the disc springs  22 , 24 . The mass of the fastening member  14  and nut  42  may increase the amplitude of the motion of the apparatus  10 , further increasing the sensitivity of the system to low frequency vibration energy. 
     With further reference to  FIGS. 2 and 3 , the assembly of disc spring  22  and the piezoelectric material ring  32  will be described in greater detail. Disc springs  22  and  24  may be made from spring steel or any other material having suitable resilient properties, such as carbon fiber reinforced plastic. The disc springs  22  and  24  are held by a suitable tool (not shown) in axial alignment with one another, with their outer peripheral edges  28  and  30  against the support ring  26 . Using fastening member  14 , a predetermined preload force is applied to the disc spring. This causes the disc springs  22  and  24  to flex (i.e., deflect) slightly. The precise preload may vary depending upon the geometry of the cross section of the disc springs  22 , 24 . Preferably, the amount of preloading is sufficient to place the disc springs  22 ,  24  at the middle of a range of low stiffness. For a single disc spring  22  or  24 , this range is illustrated in  FIG. 5 . In this example, the preloading force would be sufficient to cause a deflection of one of the disc springs  22  or  24  by at least about 0.03 inch (0.762 mm), which puts it at approximately the beginning point of the “reduced stiffness” range (i.e., point  48 ) on the graph of  FIG. 5 , and more preferably by about 0.042 inch (1.0668 mm) to place it at the midpoint of the reduced stiffness range. 
     With further reference to  FIGS. 2 and 2A , while the disc spring  22  is held with the above-described degree of preloading force, the piezoelectric material ring  32  is adhered thereto. In one specific form of assembly, a plurality of spaced apart drops of conductive adhesive  50  are placed along the undersurface  52  of the piezoelectric material ring  32 , and separated by a layer or nonconductive adhesive  54 . The conductive adhesive drops  50  provide electrical conductivity between the piezoelectric material ring  32  and the disc spring  22 , which allows the disc spring to conveniently act as an electrical connection to the electrode on the piezoelectric material ring  32  that is in contact with the disc spring  22 . Conductors  22   a  and  22   b  electrically coupled to the disc springs  22 ,  24  allow the electrical current generated by the piezoelectric material layers  32 ,  34  to be transmitted to the power conditioning subsystem  18  ( FIG. 1 ). 
     Nonconductive adhesive  54  is used to provide a strong bond between the piezoelectric material layer  32  and the outer surface  56  of the disc spring  22 . Prior to adhering the piezoelectric material layer  32 , it is also preferred to thoroughly clean the outer surface  56  of the disc spring  22 , and possibly also to sand the surface  56  so that a surface is presented that will enable a strong bond to be achieved. For the conductive adhesive  50 , various forms of adhesive may be used, but one suitable adhesive is CHO-BOND®, a two-part conductive epoxy commercially available from Chomerics, a company of the Parker Hannifin Corporation. The non-conductive adhesive  54  may also take a plurality of forms, but one suitable adhesive is commercially available LOCTITE-HYSOL® 9330 two-part epoxy. 
     Once the adhesives  50  and  54  have cured, any tooling being used to hold the disc springs  22 , 24  in place during the curing process may be removed. Once this manufacturing operation has been completed for both of the disc springs  22  and  24 , the apparatus  10  may be assembled and the nut  42  adjustably tightened on the fastening member  14 . The nut  42  is tightened sufficiently to provide a preload force that deflects each of the disc springs  22  and  24  to approximately a midpoint of its low stiffness region. The low stiffness region for one of the disc springs  22  or  24  is defined by arrow  58  in  FIG. 5 . 
     With further reference to  FIGS. 4 and 5 , it will be appreciated that, when loaded, the disc springs  22  and  24  of apparatus  10  will typically exhibit non-linear stiffness. The extent of this non-linear stiffness is governed primarily by the height-to-thickness ratio (h/t) of the disc spring  22  or  24 . The thickness is denoted by “t” in  FIG. 4 , while the height is denoted by “h” in  FIG. 4 . It will also be appreciated that attaching the piezoelectric material ring  32  to the disc spring  22  essentially increases the effective thickness of the disc spring  22 , 24  and thus decreases its height-to-thickness ratio (h/t), which in turn alters the non-linearity of the force-deflection curve shown in  FIG. 5 . Thus, the dimensions of the piezoelectric material rings  32  and  34  will also need to be considered when tailoring the response of the disc springs  22  and  24 , respectively, to place them each in their low stiffness operating region. 
     Still another factor that must be taken into account is the added stiffness of the piezoelectric material rings  32  and  34 . Preferably, the added stiffness provided by the piezoelectric material rings  32  and  34  is accounted for by selecting disc springs  22  and  24  that have suitably high height-to-thickness ratios. Generally, the higher the height-to-thickness ratio for the disc spring, the more piezoelectric material that can be attached (i.e., the greater the thickness of the piezoelectric material layer  34  that can be used). It is also possible to use disc springs having tapering wall thicknesses. It will also be appreciated that the threaded fastener  14 , the nut  42  and the washer  40  may also impact tuning of the disc springs  22  and  24 , and therefore will likely need to be accounted for when setting the preload force for the disc springs  22 , 24 . 
     Referring briefly to  FIG. 6 , the force versus deflection curves for exemplary spring discs  22  and  24  are indicated by curves  60  and  62 , respectively. The reaction forces from disc springs  22 , 24  are in opposite directions because of the opposing configuration of the springs. The resulting region of low stiffness of the disc spring pair  22 , 24  is defined by portion  64  of curve  66 . Again, ideally, the preload force supplied to the disc spring pair  22 , 24  is such as to deflect the disc springs  22 , 24  to a midpoint of the low stiffness range  64 . 
     In operation, as the apparatus  10  of  FIG. 1  experiences vibration from the vibrating structure, the deflection of each of the disc springs  22  and  24  within the region defined by arrow  64  in  FIG. 6  causes strains to be generated within the disc springs. Analysis indicates that for this exemplary configuration, approximately 1000 microstrain is achieved for a 0.020 inch (0.508 mm) deflection of each disc spring  22  and  24 . These strains are transmitted to their respective piezoelectric material rings  32  and  34  through the epoxy  50 , 54  ( FIG. 2A ) and converted into electrical energy by the piezoelectric material rings as the rings are strained. 
     With the apparatus  10 , the opposed arrangement of the disc springs  22  and  24  allows each of the disc springs to be preloaded to its low stiffness region and the deflecting motion of the disc springs is not in anyway impeded by the motion of the other. In certain geometries and/or applications, it may be preferable to provide the support ring  26  with a height that enables each of the disc springs  22  and  24  to flex beyond its flattened position. 
     An alternative implementation of the apparatus  10  involves securing the apparatus  10  to a vibrating structure by using a portion of the threaded fastening member  14 . The fastening member  14  would need to have a length sufficient to allow for this. With this arrangement, the “input” vibration energy would be applied to the fastening member  14 , which would then cause flexing of the disc springs  22  and  24 . One advantage of this implementation would be that the mass of the support ring  26  ( FIG. 1 ) could be used to enhance the amplitude of the vibrating motion of the apparatus  10 , and thus even further increase the sensitivity of the apparatus  10  to low frequency vibration energy. 
     The disc springs  22  and  24  are able to respond to a wide frequency range of low amplitude vibration energy. The apparatus  10  is responsive to a vibration energy having a frequency as low as about 5 Hz or potentially even lower. This is due in part to the low stiffness of the disc springs  22 , 24  when they are preloaded. Some forms of vibration energy harvesting devices have relied on biasing a support member to a “buckling” point to soften the biasing member, and thus heighten its responsiveness to vibration energy. However, buckling is highly sensitive to boundary conditions that can sometimes be difficult to closely manage during a manufacturing process. The low stiffness of the disc springs  22  and  24  can be achieved in large part because of their natural force-deflection characteristics, arising from their axisymmetric geometry. This helps to make the disc springs  22  and  24  less sensitive to boundary conditions than devices that employ buckling to soften the support element. 
     Referring now to  FIGS. 7 and 8 , two alternative arrangements are shown for supporting the disc springs  22 , 24  to minimize the friction between the inner and outer edges of the disc springs with the components with which they are in contact. It will be appreciated that minimizing the friction enhances the ability of the disc springs  22 , 24  to respond to low frequency and/or lower amplitude vibration from a vibrating structure. In  FIG. 7  a permanent magnet  70  is bonded or otherwise secured in an outer peripheral edge  72  of a disc spring  74 , which may be identical or similar to disc springs  22  and  24 . More preferably, a plurality of permanent magnets  70  will be secured about the peripheral edge  72  of the disc spring  74  and spaced apart from one another. An inner peripheral edge  76  may correspond to an edge of disc spring  22  immediately adjacent the aperture  36 . A magnet  78  is attached to a supporting structure  80 , which may or may not correspond to the support ring  26  shown in  FIGS. 1 and 2 , while another permanent magnet  82  is coupled to a structure  84  adjacent the inner peripheral edge  76 . The permanent magnets  70  and  76  are further arranged such their negative poles face the negative poles of magnets  78  and  82 , respectively. In this manner the magnetic forces from the magnets pairs  70 / 78  and  76 / 82  repel, thus preventing physical contact of the magnets of each pair  70 / 78  and  76 / 82  when the disc spring  74  is preloaded. 
     Another arrangement for forming a magnetic bearing is shown in  FIG. 8 . The disc spring  74  includes two permanent magnets  70 A and  70 B formed in its outer peripheral edge  72 , while the inner peripheral edge  75  includes a permanent magnet  76 ′. A fastening structure  84  includes a permanent magnet  82 ′ formed therein. Preferably, a plurality of pairs of magnets  70 A, 70 B, and a plurality of magnets  76 ′ will be spaced apart around the peripheral edges  72  and  75 , respectively, of the disc spring  74 . The magnets  70 A, 70 B and magnets  76 A, 76 B are arranged so that their magnetic lines of flux repel. Furthermore, magnets  76 ′/ 82 ′ have their magnetic poles arranged so that they repel. Peripheral edges  72  and  75  will thus be supported in a non-contact arrangement. 
     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.