Abstract:
A vibrational energy harvesting apparatus comprising: a substrate having a plurality of integral compliant regions; at least two ferromagnetic masses each coupled to a corresponding one or more of the integral compliant regions such that at least one of the ferromagnetic masses moves with respect to the substrate responsive to substrate acceleration, each ferromagnetic mass having an inner magnetic pole disposed such that the inner magnetic poles are separated by a gap, wherein the magnetic polarities of the inner magnetic poles on the opposing sides of the gap are similar; wherein the inner magnetic poles form a steep flux gradient region in and around the gap; and a coil coupled to the substrate and disposed within the steep flux gradient region where it is exposed to a changing magnetic flux arising from motion of at least one of the ferromagnetic masses with respect to the substrate.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is a continuation application of U.S. patent application Ser. No. 10/911,748 (NC #84774), filed 27 Jul. 2004 now U.S. Pat. No. 7,501,726, titled “Micro-Electro-Mechanical System (MEMS) and Apparatus for Generating Power Responsive to Mechanical Vibration,”, which is incorporated herein by this reference. 

   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 purpose without the payment of any royalties thereon or therefore. 
   FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT 
   This application is assigned to the United States Government and is available for licensing for commercial purposes. No license is necessary when used for Governmental purposes. Licensing and technical inquiries should be directed to the Office of Patent Counsel, Space and Naval Warfare Systems Center, San Diego, Code 20012, San Diego, Calif., 92152; telephone (619)553-3001, facsimile (619)553-3821. Reference Navy Case #99735. 

   BACKGROUND OF THE INVENTION 
   There is a growing interest in the field of miniature sensors in applications such as medical implants and embedded sensors in buildings. One of the projected goals for micro-electro-mechanical systems (MEMS) technology is to develop low-cost and high-performance distributed sensor systems for medical, automotive, manufacturing, robotics, and household applications. One area that has received little attention is how to effectively supply the required electrical power to such sensor elements. Many applications require the sensors to be completely embedded in a structure with no physical connection to the outside world. Ideally, the elements of these distributed systems have their own integrated power supplies to reduce problems related to interconnection, electronic noise and control system complexity. Efforts are underway to develop integrated chemically-based power supplies with MEMS devices. Chemical power supply (battery) technology is well-developed for such applications but, where shelf life or replacement accessibility is a limiting factor, chemical power supplies may not be suitable for the application. Another approach to supplying power to such systems is to include a renewable power supply within the sensor element, thereby making them self-powered microsystems. 
   Renewable power supplies convert energy harvested from an existing energy source within the environment into electrical energy. The preferred source of energy depends on the application. Some possible energy sources include optical energy from ambient light such as sunlight, thermal energy harvested across a temperature gradient, volume flow energy harvested across a liquid or gas pressure gradient, and mechanical energy harvested from motion and vibration. Of these sources, light and thermal energy have already been exploited for use in micro-power supplies. However, there are many applications where there is an insufficient amount of light or thermal energy such as in medical implants. Therefore, practitioners in the art have proposed many different power supplies that generate electricity from ambient mechanical energy. Ambient mechanical vibrations inherent in the environment, from the movement of our bodies to the hum of a computer, can provide a constant power density of 10 to 50 μW/cc. 
   Several practitioners have proposed rudimentary vibration-based power generators at the University of Sheffield [C. B. Williams, R. B. Yates, “Analysis of a micro-electric generator for micro-systems,” 8 th Intl. Conf. on Solid - State Sens . &amp;  Actuators , Stockholm, Sweden, 25-29 Jun. 1995, 87-B4, pp. 369-72] and Massachusetts Institute of Technology [Scott Meninger, Jose Oscar Mur-Miranda, Rajeevan Amirtharajah, Anantha P. Chandrakasan, and Jeffrey H. Lang, “Vibration-to-Electric Energy Conversion,”  IEEE Trans. on VLSI Systems  Vol. 9, No. 1, pp. 64-76, February 2001] for example. Meninger, et al. describe a micro-generator that harvests vibrational energy by accumulating the voltage created by vibration-induced changes in a variable capacitor. 
   Others have recently improved on the earlier efforts. For example, Ching et al. [Neil N. H. Ching, H. Y Wong, Wen J. Li, Philip H. W. Leong, and Zhiyu Wen, “A laser-micro-machined multi-modal resonating power transducer for wireless sensing systems,”  Sensors and Actuators A: Physical , Vol. 97-98, pp. 685-690, 2002.] describe a micromachined generator with enough power to drive an off-the-shelf circuit. For this work, Ching et al. prefer micromachining methods to build their vibration-induced power generator because the methods afford precise control of the mechanical resonance necessary for generator efficiency, and batch fabricability for low-cost mass production of commercially viable generators. Similarly, Williams et al. later describe [C. B. Williams, C. Shearwood, M. A. Harradine, P. H. Mellor, T. S. Birch and R. B. Yates, “Development of an electromagnetic micro-generator,”  IEE Proc.—Circuits Devices Syst ., Vol. 148, No. 6, pp. 337-342, December 2001] a simple inertial generator built according to their earlier theoretical analysis that is also fabricated by means of micromachining. Other examples include the laser-micro-machined electromagnetic generator described by Li et al. [Wen J. Li, Terry C. H. Ho, Gordon M. H. Chan, Philip H. W. Leong and Hui Yung Wong, “Infrared Signal Transmission by a Laser-Micro-machined Vibration-Induced Power Generator,”  Proc.  43 rd IEEE Midwest Symp. on Circuits and Systems , Lansing Mich., 08-11 Aug. 2000, pp 236-9], which provides 2 VDC power sufficient to send 140 ms pulse trains every minute when subjected to 250 micron vibrations in the 64-120 Hz region. 
   In U.S. Pat. No. 6,127,812, Ghezzo et al. describe an energy extractor that includes a capacitor that experiences capacitance and voltage changes in response to movement of a capacitor plate or of a dielectric material. In one embodiment, a third plate is positioned between first and second plates to create two capacitors of varying capacitances. In another embodiment, one capacitor plate is attached by flexible arms which permit movement across another capacitor plate. The above capacitors can be used singularly or with one or more other capacitors and are rectified either individually or in a cascaded arrangement for supplying power to a rechargeable energy source. The above capacitors can be fabricated on a substrate along with supporting electronics such as diodes. Ghezzo et al. employ varying capacitance and neither consider nor suggest any solution the problem of fabricating an electromagnetic micro-generator. 
   In U.S. Pat. No. 6,722,206 B2, Takeda describes a force sensing device having an element of magnetic material mounted to a substrate such that another magneto-electrical material element is subjected to the magnetic field generated by the magnetic member. A movable member is mounted for oscillation in response to vibration and such oscillation changes the magnetic field experienced by the magneto-electrical material, which in turn changes an electrical property of the magneto-electrical material. Takeda neither considers nor suggests any solution to the problem of fabricating an electromagnetic micro-generator. 
   Despite the efforts of several practitioners in the art, there still exists a need in the art for an electromagnetic micro-generator suitable for inexpensive fabrication in volume at the MEMS scale that can generate power sufficient for operating today&#39;s microchips. The electromagnetic devices known in the art all generally employ a single magnetic mass which oscillates on a spring element to change the magnetic flux at a nearby stationary coil. These devices are thereby limited in power output capacity by the limited mass of the single magnet, the limited room for a number of coils in the flux field of the single magnet and the limited flux slope available at the coils because of the single magnetic pole exposed thereto. These unresolved problems and deficiencies are clearly felt in the art and are solved by this invention in the manner described below. 
   SUMMARY OF THE INVENTION 
   A vibrational energy harvesting apparatus is disclosed herein that comprises a substrate with a plurality of integral compliant regions, at least two ferromagnetic masses, and a coil. Each ferromagnetic mass is coupled to a corresponding one or more of the integral compliant regions such that at least one of the ferromagnetic masses moves with respect to the substrate responsive to substrate acceleration. Each ferromagnetic mass has an inner magnetic pole disposed such that the inner magnetic poles are separated from one another by a flux gap. The magnetic polarity of each inner magnetic pole is similar to the magnetic polarity of the inner magnetic pole on the opposing side of the flux gap. The inner magnetic poles form a steep flux gradient region in and around the flux gap. The coil is coupled to the substrate and disposed within the steep flux gradient region where it is exposed to a changing magnetic flux arising from motion of at least one of the ferromagnetic masses with respect to the substrate. 
   In an alternate embodiment of the energy harvesting apparatus described above, the two ferromagnetic masses may be rigidly coupled to one another and disposed to move synchronously. 
   In another alternate embodiment of the energy harvesting apparatus described above, the coupled ferromagnetic masses may be configured to move linearly with respect to the substrate responsive to substrate acceleration. 
   In another alternate embodiment of the energy harvesting apparatus described above, conductors may be coupled to the coil for conducting electrical current flowing in response to the changing magnetic flux. 
   In another alternate embodiment of the energy harvesting apparatus described above, the coil may comprise a plurality of independent coils coupled to the substrate and disposed within the steep flux gradient region where the plurality of independent coils are exposed to the changing magnetic flux. 
   In another alternate embodiment of the energy harvesting apparatus described above, the coil may be disposed within the steep flux gradient region within the flux gap. 
   In another alternate embodiment of the energy harvesting apparatus described above, the coil may be disposed within the steep flux gradient region outside the flux gap. 
   The vibrational energy harvesting apparatus may be constructed as a micro-electro-mechanical system (MEMS) power generator comprising: a substrate having a plurality of integral compliant regions; at least one monolithic micro-generator, a coil, and conductors. In this embodiment, each monolithic micro-generator comprises at least two ferromagnetic masses. Each ferromagnetic mass may be coupled to a corresponding one or more of the integral compliant regions such that at least one of the ferromagnetic masses moves with respect to the substrate responsive to substrate acceleration. Each ferromagnetic mass has an inner magnetic pole disposed such that the inner magnetic poles of the ferromagnetic masses are of the same magnetic polarity and are separated from one another by a flux gap. The inner magnetic poles form a steep flux gradient region in and around the flux gap. The coil is coupled to the substrate and disposed within the steep flux gradient region where it is exposed to a changing magnetic flux arising from motion of at least one of the ferromagnetic masses with respect to the substrate. The conductors are coupled to each micro-generator coil for conducting electrical current flowing in response to the magnetic flux changes. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of this invention, reference is now made to the following detailed description of the embodiments as illustrated in the accompanying drawing, in which like reference designations represent like features throughout the several views. 
       FIG. 1  is a schematic diagram illustrating a damped mass-spring model representative of the micro-generator system of this invention. 
       FIG. 2  illustrates the theoretical relationship between coil voltage, flux density and relative displacement according to classical electromagnetic theory for the model of  FIG. 1 . 
       FIG. 3  is a diagram illustrating an edge view of several different coil/flux configurations available for use in the micro-generator system of this invention. 
       FIG. 4  is a diagram illustrating an edge perspective of an exemplary embodiment of the micro-generator of this invention. 
       FIG. 5  is a diagram illustrating an edge perspective of an exemplary embodiment of the micro-electro-mechanical system (MEMS) power generator system of this invention. 
       FIG. 6 , comprising  FIGS. 6(   a )-( d ), is a diagram illustrating an edge view of an exemplary magnet layer fabrication process of this invention. 
       FIG. 7 , comprising  FIGS. 7(   a )-( e ), is a diagram illustrating an edge view of an exemplary magnet layer fabrication process of this invention. 
       FIG. 8  is a diagram illustrating a facial view of the exemplary magnet layer embodiments of  FIGS. 6 and 7 . 
       FIG. 9 , comprising  FIGS. 9(   a )-( d ), is a diagram illustrating an edge view of an exemplary coil layer fabrication process of this invention. 
       FIG. 10  illustrates a facial view of the exemplary coil layer embodiment of  FIG. 9 . 
       FIG. 11 , comprising  FIGS. 11(   a )-( c ), is a diagram illustrating an edge view of a first exemplary micro-generator fabrication process of this invention using the magnet layer embodiment of  FIG. 6 . 
       FIG. 12 , comprising  FIGS. 12(   a )-( c ), is a diagram illustrating an edge view of a second and a third exemplary micro-generator fabrication process of this invention using the magnet layer embodiments of  FIG. 7  and  FIG. 3(   a ). 
   

   DETAILED DESCRIPTION OF EMBODIMENTS 
     FIG. 1  is a schematic diagram illustrating a damped mass-spring model representative of the micro-generator system of this invention. Both electrical and mechanical damping must be considered in analyzing and optimizing the design for particular ambient vibration spectra. Referring to  FIG. 1 , for time t, a mass m, a spring constant k, an electrical damping factor b e , a mechanical damping factor b m , and a displacement function z(t), the power P available from the coil current may be expressed as shown in Eqn. 1: 
                       P   =       ∫   0   v     ⁢     F   ⁢           ⁢     ⅆ   v                     =       ∫   0   v     ⁢       b   e     ⁢     z   .     ⁢           ⁢     ⅆ   v                     =       b   e     ⁢       ∫   0   v     ⁢     v   ⁢           ⁢     ⅆ   v                       =       1   2     ⁢     b   e     ⁢     v   2                   =       1   2     ⁢     b   e     ⁢       z   .     2                     [     Eqn   .           ⁢   1     ]               
Conservation of energy leads to Eqn. 2:
   m{umlaut over (z)} +( b   e   +b   m ) ż+kz=−mÿ   [Eqn. 2] 
Laplacian transformation and the substitution of variables can be shown to provide the following Eqns. 3-10:
 
                 Z   =         -     ms   2       ⁢   Y         ms   2     +       (       b   e     +     b   m       )     ⁢   s     +   k               [     Eqn   .           ⁢   3     ]                 Let   ⁢     :       ⁢     
     ⁢       b   e     =     2   ⁢           ⁢   m   ⁢           ⁢     ξ   e     ⁢     ω   n         ⁢     
     ⁢       b   m     =     2   ⁢           ⁢   m   ⁢           ⁢     ξ   m     ⁢     ω   n                 [     Eqns   .           ⁢   4     ]               
where ω n   2 =k/m.
 
   
     
       
         
           
             
               
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   This is a non-linear problem and, because of the nonlinear nature of the reaction force from the coil current, the system resonance may be optimized with reference to Eqn. 7 for a given application without undue experimentation. In general, the inventors have discovered that a higher electrical damping b e  improves power output performance at frequencies below the mechanical resonant frequency f r =2πω n  of the system. 
     FIG. 2  is a chart illustrating the expected coil voltage, flux density and relative displacement for various electrical and mechanical assumptions. The acceleration is assumed to be a constant 1.0 m/sec 2  over the entire frequency range, B max =1 Tesla, k=1 N/m, velocity=50 mm/sec, mass=1 mg, and x=1 mm. The inventors have conducted both experimental and theoretical tests and have found that the predictions disclosed in  FIG. 2  agree well with experimental measurements implemented on a larger physical scale. 
   A macro-scale version of the energy harvesting device was fabricated to verify the expected voltage output per coil. The experimental setup consisted of a one Tesla magnet measuring one inch in diameter and 3/16 inches in thickness. It was attached to a spring with sufficient spring force to result in a displacement of 2.5 mm under accelerations of 1.0 m/s 2  at a frequency of 20 Hz. The number of turns in the coil was varied sequentially from 5 to 40 in increments of 5 and voltage output measurements were made for each configuration. It was observed that the voltage generated per turn of the coil was very close to the expected value of 1 mV/turn using the simple one-dimensional (1-D) model described above. 
   A detailed analysis was performed by modeling the magnetic flux density in two dimensions and summing the total flux density normal to the surface of the coil. The input was once again assumed to be a 20 Hz sinusoidal input at 1.0 m/s 2 . At each time step, the velocity, displacement from the coil to the magnet and total magnetic flux density normal to the surface were calculated. The results of this detailed analysis confirmed the simple 1-D calculations and the macro-scale experimental observations of 1 mV/turn. 
     FIG. 3  is a diagram illustrating an edge view of several different coil/flux configurations. In  FIG. 3 , a coil  20  is disposed at a flux gap  22  formed by the two magnetic masses  24  and  26 . In  FIGS. 3(   a ) and  3 ( b ), a “steep” flux gradient region is formed in flux gap  22  by virtue of the similar magnetic poles on each edge of flux gap  22 . In  FIGS. 3(   c ) and  3 ( d ), a “shallow” flux gradient region is formed in flux gap  22  by virtue of the dissimilar magnetic poles on each edge of flux gap  22 . In  FIG. 3(   a ), coil  20  is disposed in flux gap  22  such that any vertical motion Z(t) of mass  26  with respect to mass  24  and coil  20  produces a rapid change in magnetic flux at coil  20 . Similarly, in  FIG. 3(   b ) coil  20  is disposed in flux gap  22  such that any synchronous vertical motion Z(t) of both masses  24 - 26  together with respect to coil  20  produces a rapid change in magnetic flux at coil  20 . In contrast, in  FIG. 3(   c ) coil  20  is disposed in flux gap  22  such that any vertical motion Z(t) of mass  26  with respect to mass  24  and coil  20  produces a limited change in magnetic flux at coil  20 . Similarly, in  FIG. 3(   d ) coil  20  is disposed in flux gap  22  such that any synchronous horizontal motion Y(t) of both masses  24 - 26  together with respect to coil  20  produces a limited change in magnetic flux at coil  20 . Clearly, the coil/flux configurations illustrated in  FIGS. 3(   a ) and  3 ( b ) are preferred and, in particular, the configuration in  FIG. 3(   b ) is preferred for implementation of the micro-generator of this invention. Moreover, additional magnetic masses may also be added and the present masses reorganized to form other useful geometric configurations are well-suited for implementation as alternative embodiments of the micro-generator of this invention. 
     FIG. 4  is a diagram illustrating an edge perspective of an exemplary embodiment  28  of the micro-generator of this invention. Micro-generator  28  includes a coil  30  consisting of a plurality of turns of electrically-conductive material coupled to the coil terminals  32  and  34 . Coil  30  is disposed in the flux gap  36  bounded by the inner surfaces  38  and  40  of the magnetic masses  42  and  44 , respectively. Inner surfaces  38  and  40  are shown as the N-poles of magnetic masses  42  and  44  but may be either polarity provided that both inner surfaces  38  and  40  have the same magnetic polarity. Magnetic mass  42  is supported by a plurality of compliant elements (springs) exemplified by the compliant element  46 . Similarly, magnetic mass  44  is supported by a plurality of compliant elements exemplified by the compliant element  48 . The free ends of compliant elements  46  and  48  are fixed in any useful manner (not shown) with respect to coil  30 , thereby allowing magnetic masses  42  and  44  to move in the Z(t) direction with respect to coil  30  in response to external mechanical vibration. 
     FIG. 5  is a diagram illustrating an edge perspective of an exemplary embodiment  50  of the micro-electro-mechanical system (MEMS) power generator system of this invention. MEMS power generator  50  includes a plurality of the micro-generators of this invention, exemplified by micro-generator  28 , with the individual coil terminals interconnected such that the electrical power generated by each micro-generator is aggregated at the MEMS power generator terminals  52  and  54 . Preferably, the plurality of micro-generators composing MEMS generator  50  are coupled together for fixed exposure to the same ambient vibration. 
     FIG. 6 , comprising  FIGS. 6(   a )-( d ), is a diagram illustrating an edge view of an exemplary magnet layer fabrication process of this invention. This process begins as shown in  FIG. 6(   a ) with a semiconductor wafer  56 . The material may be crystalline silicon or any other useful semiconductor material. Although the following discussion is limited to the preparation of a single magnet layer, practitioners in the art can readily appreciate that many such magnet layer elements may be simultaneously fabricated on a single semiconductor wafer in a single process and separated from the wafer in a wafer dicing process well known in the art.  FIG. 6(   a ) illustrates the results of the first step in this process, which is the preparation of the upper surface  58  and the lower surface  60  for processing in the usual fashion by cleaning and polishing as necessary.  FIG. 6(   b ) illustrates the results of the next step of this process, which is the masking and deep reactive ion etching (DRIE) of lower surface  60  to define the magnet well  62 .  FIG. 6(   c ) illustrates the results of the next step of this process, which is the masking and DRIE of upper surface  58  to define the coil layer recesses  64 .  FIG. 6(   d ) illustrates the results of the next two steps of this process, which is the masking and DRIE of upper surface  58  to define the integral compliant regions  66  and the bonding posts  68 , thereby completing the magnet layer sub-element  69  substantially as shown. Bonding posts  68  are also shown in  FIG. 8  in a wafer facial view (magnet well  62  should be demarcated with hidden lines to illustrate the exemplary process of  FIG. 6  and in solid lines for the exemplary process of  FIG. 7) . The final thickness of integral compliant regions  66  is established to provide the spring constant necessary for the desired resonant frequency of the final micro-generator ( FIG. 11  below). The open region  71  in  FIG. 8  is etched away completely to leave magnet well  62  coupled only by compliant regions  66 . The final step of this magnet layer fabrication process is the disposition of a ferromagnetic mass  70  into magnet well  62  of magnet layer sub-element  69  (shown in  FIG. 11(   c )), which may be accomplished immediately following the completion of magnet layer sub-element  69  shown in  FIG. 6(   d ) or, as illustrated herein, may be deferred until after the assembly of the micro-generator magnet layer and coil layer elements ( FIG. 11) . 
     FIG. 7 , comprising  FIGS. 7(   a )-( e ), is a diagram illustrating an edge view of an alternative magnet layer fabrication process of this invention. This process also begins as shown in  FIG. 7(   a ) with semiconductor wafer  56 .  FIG. 7(   a ) illustrates the results of the first step in this process, which is the preparation of upper surface  58  and lower surface  60  for processing in the usual fashion by cleaning and polishing as necessary.  FIG. 7(   b ) illustrates the results of the next step of this process, which is the masking and DRIE of upper surface  58  to define the coil layer recesses  64 .  FIG. 7(   c ) illustrates the results of the next step of this process, which is the masking and DRIE of upper surface  58  to define the magnet well  62 .  FIG. 7(   d ) illustrates the results of the next two steps of this process, which is the masking and DRIE of upper surface  58  to define the integral compliant regions  66  and the bonding posts  68 , which are also shown in  FIG. 8  in a wafer facial view (magnet well  62  should be demarcated with hidden lines to illustrate the exemplary process of  FIG. 6  and in solid lines for the exemplary process of  FIG. 7) . The final thickness of integral compliant regions  66  is established to provide the spring constant necessary for the desired resonant frequency of the final micro-generator ( FIG. 12  below). 
     FIG. 8  shows the open region  71 , which may be etched away completely to leave magnet well  62  coupled only by compliant regions  66 .  FIG. 7(   e ) illustrates the results of the final step of this process, which is the disposition of ferromagnetic mass  70  into magnet well  62 . Ferromagnetic mass  70  should include a suitably “hard” ferromagnetic material, for example, sputtered CoPtCr having a 40 KOe field, and must be disposed with one magnetic pole bonded to the bottom of magnet well  62  and the other pole exposed at the top of mass  70 , thereby completing the magnet layer element  72  substantially as shown. 
     FIG. 9 , comprising  FIGS. 9(   a )-( d ), is a diagram illustrating an edge view of an exemplary coil layer fabrication process of this invention. This process begins as shown in  FIG. 9(   a ) with a semiconductor wafer  74 . The material may be crystalline silicon or any other useful semiconductor material. Although the following discussion is limited to the preparation of a single coil layer, practitioners in the art can readily appreciate that many such coil elements may be simultaneously fabricated on a single semiconductor wafer in a single process and separated from the wafer in a wafer dicing process well known in the art.  FIG. 9(   a ) illustrates the results of the first step in this process, which is the preparation of the upper surface  76  and the lower surface  78  for processing in the usual fashion by cleaning and polishing as necessary.  FIG. 9(   b ) illustrates the results of the next step of this process, which is the masking and DRIE of upper surface  76  to define the coil well  80 .  FIG. 9(   c ) illustrates the results of the next step of this process, which is the disposition of a conductive coil  82  within coil well  80 . The disposition of coil  82  may be accomplished using any of several useful techniques well known in the art, such as, for example, ion deposition of copper or aluminum conductors in a masked pattern, or by bonding a conductive layer (not shown) to the bottom of coil well  80  and masking and etching the conductive layer to create the desired coil geometry, for example. The coil may, for example include 2,500 turns in a radius of 1 mm.  FIG. 9(   d ) illustrates the results of the final step of this process, which is the masking and DRIE of either upper surface  76  or lower surface  78  to define the bonding post through holes  84  thereby completing the coil layer element  86  substantially as shown. 
     FIG. 10  shows the bonding post through holes  84  in a wafer facial view.  FIG. 10  also illustrates the two conductive terminals  88  and  90  disposed to permit electrical connection to coil  82 . 
     FIG. 11 , comprising  FIGS. 11(   a )-( c ), is a diagram illustrating an edge view of the fabrication of a first exemplary embodiment  92  of the micro-generator of this invention, which is shown in  FIG. 11(   c ).  FIG. 11(   a ) illustrates the results of the first step in this process, which is the bonding of a coil layer element  86  to a first magnet layer sub-element  69 A at the bonding surfaces  94 A.  FIG. 11(   b ) illustrates the results of the second step in this process, which is the bonding of a second magnet layer sub-element  69 B to coil layer element  86  at the bonding surfaces  94 B and to first magnet layer sub-element  69 A at the bonding post surfaces  96 . Note that sufficient clearance is provided to permit coil  82  to remain mechanically isolated from bonding post surfaces  96  except for the mechanical coupling provided by compliant regions  66 . The final step of this micro-generator fabrication process is the disposition of ferromagnetic masses  70 A and  70 B into magnet well  62  of magnet layer sub-elements  69 A and  69 B, respectively, which may instead be accomplished immediately following the completion of magnet layer sub-element  69  before beginning the assembly of micro-generator  92 . 
     FIG. 12 , comprising  FIGS. 12(   a )-( c ), is a diagram illustrating an edge view of the fabrication of a second exemplary embodiment  98  of the micro-generator of this invention, which is shown in  FIG. 12(   b ) and a third example embodiment of the micro-generator, which is shown in  FIG. 12(   c ), with a coil  82  and ferromagnetic mass  70  configuration as shown in  FIG. 3(   a ).  FIG. 12(   a ) illustrates the results of the first step in this process, which is the bonding of a coil layer element  86  to a first magnet layer element  72 A at the bonding surface  100 A.  FIG. 12(   b ) illustrates the results of the second step in this process, which is the bonding of a second magnet layer element  72 B to coil layer element  86  at the bonding surfaces  100 B and to first magnet layer element  72 A at the bonding post surfaces  102 . Note that sufficient clearance is provided to permit coil  82  to remain mechanically isolated from bonding post surfaces  102  except for the mechanical coupling provided by compliant regions  66 .  FIG. 12(   c ) illustrates an example embodiment of the micro-generator where the coil  82  is disposed within the steep flux gradient region created by the opposing ferromagnetic masses  70  and wherein the coil  82  is outside a volume defined by the perimeters and opposing faces of the ferromagnetic masses  70  as depicted in  FIG. 3(   a ). 
   Based on measurements and calculations, the inventors suggest that the MEMS power generator of this invention can provide an output power from 10 to 500 mW/cc at an output voltage from 100 mV to 5,000 mV. 
   Clearly, other embodiments and modifications of this invention may occur readily to those of ordinary skill in the art in view of these teachings. Therefore, this invention is to be limited only by the following claims, which include all such embodiments and modifications when viewed in conjunction with the above specification and accompanying drawing. 
   It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described and illustrated to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. 
   From the above description of the System and Method for Detecting an Object in a Search Space, it is manifest that various techniques may be used for implementing the concepts of system  10  without departing from its scope. The described embodiments are to be considered in all respects as illustrative and not restrictive. It should also be understood that system  10  is not limited to the particular embodiments described herein, but is capable of many embodiments without departing from the scope of the claims.