Abstract:
An energy harvesting device for harvesting energy from a moving structure includes a housing allowing transmission of magnetic fields therethrough. A piezoelectric material capable of a phase transition and a magnetostrictive material capable of a structural change when subjected to a magnetic field are mechanically coupled to each other in the housing. An adjustable pre-stress means is positioned between the housing and the combination of the piezoelectric and magnetostrictive materials. Electrical contacts are positioned on the piezoelectric material. A magnet is mounted on the moving structure. The housing is mounted in sufficient proximity to the magnet for the magnetic field of the magnet to induce the structural change in the magnetostrictive material.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor. 
    
    
     CROSS REFERENCE TO OTHER PATENT APPLICATIONS 
     None. 
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates to an electro-mechanical energy harvesting device and more particularly relates to such a device that has a magnetostrictive and piezoelectric component. 
     (2) Description of the Prior Art 
     It is known that ferroelectric single crystals convert mechanical energy to electrical energy or vice versa. This makes them a candidate as the active material in energy harvesting devices. By utilizing the direct piezoelectric (or pyroelectric) effect when mechanical or thermal energy is available from the environment, the mechanical or thermal energy can be converted to electric charge polarization in relaxor ferroelectric single crystal material and useful amounts of energy can be obtained. 
     Relaxor single crystals display both a linear piezoelectric effect and a non-linear electromechanically coupled phase transition. The linear piezoelectric effect in relaxor single crystals has been well characterized and is extraordinarly large, approximately a factor of six times that of the ceramic lead zirconate titanate (PZT). The non-linear electromechanically coupled phase transition associated with field and stress driven phase transformations has been the subject of extensive study, especially for lead indium niobate-lead magnesium niobate-lead titanate (PIN-PMN-PT) ternaries. Reversible stress and temperature induced phase transformations are associated with spontaneous charge generation in the relaxor single crystals.  FIG. 1  is a graph showing strain versus stress for a representative phase change piezoelectric material.  FIG. 1  clearly shows a large strain jump at the stress and field induced phase transformation. These stress and field driven phase transformations offer significant new approaches to energy harvesting. These results demonstrate that phase transformations can provide more than an order of magnitude increase in energy density per cycle for mechanical energy harvesting. Utilizing this phase transformation behavior suggests that a stress-biased energy harvester would take maximum advantage of the phase transformation in the relaxor single crystal material. 
     Magnetostrictive materials are similar to ferroelectric materials because they convert magnetic energy into mechanical energy. However, magnetostrictive materials utilize a magnetic field rather than an electrical field. An applied magnetic field can alter the direction of the magnetic moments inside the material, and the magnetic moments will tend to align themselves in the direction of the applied magnetic field. This directional change of the magnetic moments is coupled to the material&#39;s lattice via spin-orbit coupling and results in a physical change in the dimension of the material. It is known to utilize this physical change in mechanical applications and control systems. 
     It is known to use piezoelectric materials to harvest energy. Piezoelectric materials will generate electric potential when subjected to some kind of mechanical stress. However, piezoelectric materials have constraints on their ability to function properly, such as temperature, force, and pressure. These constraints, along with the difficulty of attaching piezoelectric materials to rotating or moving machinery, make it difficult to locate piezoelectric material devices in contact with the mechanical stress generator which allows the piezoelectric material to act as an energy harvesting device. 
     It is also known to use magnetostrictive materials to harvest energy. Magnetostrictive materials are able to harvest vibrational energy from vibrating pumps, motors, buildings, ships, etc. since magnetostrictive materials are able to change shape in response to a magnetic field, and it is known to use these changes in magnetic state to induce a voltage in coils, which can then be converted into power. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide an energy harvesting device for a changing magnetic field. 
     It is another object of the present invention to provide an energy harvesting device having maximum efficiency. 
     Accordingly, there is provided an energy harvesting device for harvesting energy from a moving structure. The device has a rigid, hollow housing capable of allowing the transmission of a magnetic field therethrough. A piezoelectric material is positioned in the hollow housing. A magnetostrictive material capable of a structural change when being subjected to a magnetic field is also positioned in the housing mechanically coupled to the piezoelectric material. An adjustable pre-stress means is positioned between the housing and the piezoelectric material and magnetostrictive material combination to apply a pre-stress to the piezoelectric material and the magnetostrictive material. Electrical contacts are joined in contact with the piezoelectric material. A magnet mounted on the moving structure induces the structural change in the magnetostrictive material when the magnet is nearest the housing. The structural change stresses the piezoelectric material which generates electrical potential at the contacts. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects and advantages of the present invention will become apparent from the following description, drawings and claims wherein: 
         FIG. 1  is a graph showing strain versus stress for a representative phase change piezoelectric material; 
         FIG. 2  is a diagram of an energy harvesting device and its operational set up; 
         FIG. 3  is a graph showing test results for output voltage versus magnetic field for the embodiment of  FIG. 2 ; 
         FIG. 4  is a cross-sectional view of a first embodiment of the energy harvesting device. 
         FIG. 5  is a cross-sectional view of a second embodiment of the energy harvesting device. 
         FIG. 6  is a diagram of a third embodiment of the energy harvesting device and a second operational set up. 
         FIG. 7  is a cross-sectional view of a third embodiment of the energy harvesting device. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a graph that generically depicts hysteresis curves  10  of a single crystal material having a sharp elastic instability near the phase transition point. The curve with a solid line shows the stress/strain response, and the curve with the dashed line shows the stress/polarization response. As compressive stress increases in the material, strain response increases linearly in region  12 A and polarization declines somewhat linearly at  12 B. A phase transition point occurs at  13 A. Polarization transition is shown at  13 B. During phase transition the response is shown as indicated at  14 A and  14 B. Strain response is rapid during phase transition and polarization response is somewhat slower. The phase transition completes as indicated at  15 A and  15 B. A second linear region begins as indicated at  16 A and  16 B. As compressive stress is reduced, strain declines linearly in region  16 C, and polarization increases somewhat linearly at  16 D. Once stress declines sufficiently, a phase transition occurs at  15 C and  15 D. The response during phase transition is shown at  14 C, and phase transition completes at  13 C. The polarization response is shown at  14 D. The response enters the linear region at  12 C and  12 D as stress declines further. 
     For a given material, the position of this curve and its inflection points depend on pressure, temperature and the electrical field to which the material is subjected. In one known material, this compressive pre-stress occurs in the region between about 24 MPa and 21 MPa as indicated in the FIG. The resulting microstrain (μ∈) is between −2,500 and −4,500 as shown in the FIG. The critical stress is a characteristic of the particular material composition, and it can be determined experimentally for a given temperature and operating condition. 
       FIG. 2  depicts an energy harvesting device  20  with a hollow housing  22 . Hollow housing  22  is made from a material that can support tensile loading while allowing the transmission of magnetic fields therethrough, such as aluminum, another non-ferroelectric material, or the like. Hollow housing  22  has terminal exterior threads on a first end and internal threads on a second end. A cap  24  is affixed to hollow housing  22  first end, and a compression bolt  26  is affixed to hollow housing  22  second end. Cap  24  and compression bolt  26  contain appropriate complementary threads and are made from a material such as a non-ferrous metal or the like. A mounting location  28  serves as a mounting location for energy harvesting device  20 . Energy harvesting device  20  can be mounted by cap  24  and/or compression bolt  26 . Access apertures  30  can be added to hollow housing  22 . Wires  32  can go through access apertures  30  to access a piezoelectric material discussed hereinafter. Anti-torsion plug  34  can be located in anti-torsion apertures  36  in hollow housing  22  to prevent twisting. 
     Energy harvesting device  20  is mounted in proximity to a machine  40 . Machine  40  vibrates in direction  42 . Magnet  44  is attached to machine  40 , preferably such that the entirety of a magnetostrictive material discussed hereinafter is in the magnetic field. A lesser response will result if the entirety of the magnetostrictive material is not in the magnetic field. Magnet  44  preferably being capable of generating a changing magnetic field of about at least ±50 Oe on the magnetostrictive material as it vibrates on machine  40 . 
     In operation, magnet  44  joined to machine  40  provides a time varying magnetic field on the magnetostrictive material to actuate the piezoelectric material as a harvester. As the magnetostrictive rod is activated by external magnetic field it expands linearly. Because the magnetostrictive rod is constrained by cap  24  and bolt  26 , rod causes compressive stress that is transmitted to piezoelectric material. Piezoelectric material and magnetostrictive rod have been subjected to a preload stress close to the critical stress required for ferroelectric phase transition. Compressive stress from the rod loads the piezoelectric material with additional force needed to bring the piezoelectric material through the phase transformation (from that point this is equivalent to mechanical energy harvesting process). Preload stress on the magnetostrictive material optimizes the slope of the magnetostriction versus field curve. 
       FIG. 3  is a graph of results obtained using energy harvesting device  20  subjected to a varying magnetic field with a galfenol (FeGa) magnetostrictive material and a PIN-PMN-PT piezoelectric single crystal material. This shows that about 600 volts can be obtained with a magnetic field difference of about 15 Oe. The magnetoelectric coefficient, ME, as estimated from experimental results is: 
                     M   ⁢           ⁢   E     =         ⅆ   E       ⅆ   H       &gt;     40   ⁢           ⁢     V     cm   ×   Oe                   (   1   )               
where E is the electric field per centimeter (V/cm), and H is the magnetic field (Oe). This is a non-resonant mode estimate. Results for known energy harvesters are usually given in the resonant mode and are capable of delivering energy for a narrower band of frequencies than the tested device. These results are much greater than can be obtained using a simple magnet/coil energy harvester arrangement as is known in the art.
 
       FIG. 4  provides a sectional diagram of energy harvesting device  20  with hollow housing  22 . A magnetostrictive material  46 , a piezoelectric material  48 , and dielectric plugs  50  are located inside the cavity of hollow housing  22 . Magnetostrictive material  46  can be a magnetostrictive material such as galfenol, Terfenol-D®, Metglas®, ferrite, cobalt, nickel, or the like. Piezoelectric material  48  is preferably a piezoelectric or ferroelectric crystal such as PIN-PMN-PT or the like. Non-crystalline and composite materials can be used but will give significantly lower output. Dielectric plugs  50  can be made from an electrically insulating material capable of supporting and aligning the load, such as ceramic or the like. Hollow housing  22  also has access apertures  30  which allow for electrical contact with piezoelectric material  48 . There is an optional friction reducing material  52  between hollow housing  22  and magnetostrictive material  46 . Friction reducing material  52  is made of a material that can reduce the friction between magnetostrictive material  46  and hollow housing  22 , such as polytetrafluoroethylene (such as Teflon® tape), poly (4,4′-oxydiphenylene-pyromellitimide) (such as Kapton®), or the like. Anti-torsion plug  34  has a shaft  54  capable of bearing compressive stress and arms  56 . Arms  56  are positioned in anti-torsion apertures  36  formed in hollow housing  22 . (If the components located in housing  22  twist, arms  56  interfere with housing  22  at apertures  36  to prevent twisting). 
     Compression bolt  26  is adjusted to place the piezoelectric material  48  near the phase boundary. This can also be done electrically. If necessary, a magnetic bias field can be applied to the magnetostrictive material  46  to move piezoelectric material  48  closer to the phase transition. The oscillating magnetic field created by magnet  44  and vibrating machine  40  causes magnetostrictive material  46  to expand and contract. If the resulting stress on piezoelectric material  48  is sufficient to cause a phase transition (ex. orthorhombic phase and rhombohedral phase), large amounts of energy can be generated. Direction of vibration  42  can be longitudinal, lateral, and/or axial depending on the polarization orientation of piezoelectric material  48  and the design constraints of energy harvesting device  20 . 
     The changing magnetic field created by magnet  44  and vibrating machine  40  causes a rearrangement of dipoles in magnetostrictive material  46  which causes magnetostrictive material  46  to change shape. This change in shape applies a stress on piezoelectric material  48  which causes a phase transformation in piezoelectric material  48 . This phase transformation causes piezoelectric material  48  to generate energy, which can then flow into wires  32 . The energy can then flow to circuitry  38 , which can preferably do the signal conditioning. Circuitry  38  can be a load, a battery, or most likely conditioning circuitry which then is connected to a battery. 
       FIG. 5  depicts an energy harvesting device  60  with a hollow housing  62 . Hollow housing  62  is made from a material that can support tensile loading while allowing the transmission of magnetic fields therethrough, such as aluminum, another non-ferroelectric material, or the like. A magnetostrictive material  64 , a piezoelectric material  66 , and anti-friction guides  68  can be located inside the cavity of hollow housing  62 . Magnetostrictive material  64  can be a magnetostrictive material such as galfenol, Terfenol-D®, Metglas®, ferrite, cobalt, nickel, or the like. Piezoelectric material  66  is preferably a piezoelectric or ferroelectric material such as PIN-PMN-PT or the like. Guides  68  are preferably made out of a material that would allow magnetostrictive material  64  to slide along hollow housing  62  with minimal friction, such as rubber, polytetrafluoroethylene (such as Teflon®), or the like. Compression bolt  70  is preferably made from a material such as a non-ferrous metal or the like. Hollow housing  62  and compression bolt  70  contain appropriate complementary threads to allow the two to be attached. 
       FIGS. 6 and 7  depict another embodiment of an energy harvesting device  72  with a hollow housing  74 . Hollow housing  74  is made from a material that can support tensile loading while allowing the transmission of magnetic fields therethrough, such as aluminum, another non-ferroelectric material, or the like. Hollow housing  74  has terminal exterior threads on a first end and internal threads on a second end. A cap  76  is affixed to hollow housing  74  first end, and a compression bolt  78  is affixed to hollow housing  74  second end. Cap  76  and compression bolt  78  contain appropriate complementary threads and are made from a material such as a non-ferrous metal or the like. A mounting location  80  serves as a mounting location for energy harvesting device  72 . Energy harvesting device  72  can be mounted by cap  76  and/or compression bolt  78 . Access apertures  82  can be added to hollow housing  74 . Wires  84  can go through access apertures  82  to access a piezoelectric material discussed hereinafter. Circuitry  86  can be connected to wires  84  to create a complete circuit. Anti-torsion plug  88  can be located in anti-torsion apertures  90  in hollow housing  74  to prevent twisting. 
     Energy harvesting device  72  is mounted in proximity to a machine  92 . Machine  92  rotates in direction  94 . Magnet  96  is attached to machine  92 , preferably such that the entirety of a magnetostrictive material discussed hereinafter is in the magnetic field. A lesser response will result if the entirety of the magnetostrictive material is not in the magnetic field. Magnet  96  preferably being capable of generating a changing magnetic field of about at least ±50 Oe on the magnetostrictive material as it rotates on machine  92 . 
     In operation, magnet  96  joined to machine  92  provides a time varying magnetic field to actuate the piezoelectric material as a harvester. As the magnetostrictive rod is activated by external magnetic field it expands linearly. Because the magnetostrictive rod is constrained by cap  76  and bolt  78 , rod causes compressive stress that is transmitted to piezoelectric material. Piezoelectric material and magnetostrictive rod have been subjected to a preload stress close to the critical stress required for ferroelectric phase transition. Compressive stress from the magnetostrictive rod loads the piezoelectric material with additional force needed to bring the piezoelectric material through the phase transformation (from that point this is equivalent to mechanical energy harvesting process). 
       FIG. 7  provides a sectional diagram of energy harvesting device  72  with hollow housing  74 . A magnetostrictive material  98 , a piezoelectric material  100 , and dielectric plugs  102  are located inside the cavity of hollow housing  74 . Magnetostrictive material  98  can be a magnetostrictive material such as galfenol, Terfenol-D®, Metglas®, ferrite, cobalt, nickel, or the like. Piezoelectric material  100  is preferably a piezoelectric or ferroelectric crystal such as PIN-PMN-PT or the like. Non-crystalline and composite materials can be used but will give significantly lower output. Dielectric plugs  102  can be made from an electrically insulating material capable of supporting and aligning the load, such as ceramic or the like. Hollow housing  74  also has access apertures  82  which allow for electrical contact with piezoelectric material  100 . There is an optional friction reducing material  104  between hollow housing  74  and magnetostrictive material  98 . Friction reducing material  104  is made of a material that can reduce the friction between magnetostrictive material  98  and hollow housing  74 , such as polytetrafluoroethylene (such as Teflon® tape), poly (4,4′-oxydiphenylene-pyromellitimide) (such as Kapton®), or the like. Anti-torsion plug  88  has a shaft  106  capable of bearing compressive stress and arms  108 . Arms  108  are positioned in anti-torsion apertures  90  formed in hollow housing  74 . (If the components located in housing  74  twist, arms  108  interfere with housing  74  at apertures  90  to prevent twisting). 
     Compression bolt  78  is adjusted to place the piezoelectric material  100  near the phase boundary. This can also be done electrically. If necessary, a magnetic bias field can be applied to the magnetostrictive material to move piezoelectric material  100  closer to the phase transition. The oscillating magnetic field created by magnet  96  and vibrating machine  92  causes magnetostrictive material  98  to expand and contract. If the resulting stress on piezoelectric material  100  is sufficient to cause a phase transition (ex. orthorhombic phase and rhombohedral phase), large amounts of energy can be generated. Direction of vibration  94  can be longitudinal, lateral, and/or axial depending on the polarization orientation of piezoelectric material  100  and the design constraints of energy harvesting device  72 . 
     The changing magnetic field created by magnet  96  and vibrating machine  92  causes a rearrangement of dipoles in magnetostrictive material  98  which causes magnetostrictive material  98  to change shape. This change in shape applies a stress on piezoelectric material  100  which causes a phase transformation in piezoelectric material  100 . This phase transformation causes piezoelectric material  100  to generate energy, which can then flow into wires  84 . The energy can then flow to circuitry  86 , which can preferably do the signal conditioning. Circuitry  86  can be a load, a battery, or most likely conditioning circuitry which then is connected to a battery. 
     The foregoing description of the preferred embodiments of the invention has been presented for purposes of illustration and description only. It is not intended to be exhaustive nor to limit the invention to the precise form disclosed; and obviously many modifications and variations are possible in light of the above teaching. Such modifications and variations that may be apparent to a person skilled in the art are intended to be included within the scope of this invention as defined by the accompanying claims.