Patent Publication Number: US-11031857-B2

Title: Electromechanical generator for converting mechanical vibrational energy into electrical energy

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a US 371 application from PCT/EP2019/057200 entitled “An Electromechanical Generator for Converting Mechanical Vibrational Energy into Electrical Energy” filed on Mar. 22, 2019 and published as WO 2019/185465 A1 on Oct. 3, 2019, which claims priority to GB Application 1804872.8 filed on Mar. 27, 2018. The technical disclosures of every application and publication listed in this paragraph are hereby incorporated herein by reference. 
     BACKGROUND TO THE INVENTION 
     The present invention relates to an electromechanical generator for converting mechanical vibrational energy into electrical energy, i.e. a vibration energy harvester. In particular, the present invention relates to such a vibration energy harvester which is a miniature generator capable of converting ambient vibration energy into electrical energy for use, for example, in powering intelligent sensor systems. Such a vibration energy harvester can be used in many areas where there is an economical or operational advantage in the elimination of power cables or batteries. 
     DESCRIPTION OF THE PRIOR ART 
     It is known to use vibration energy harvesters comprising an electromechanical generator for harvesting useful electrical power from ambient vibrations, e.g. for powering wireless sensors. A typical magnet-coil generator consists of a spring-mass combination attached to a magnet or coil in such a manner that when the system vibrates, a coil cuts through the flux formed by a magnetic core. 
     The Applicant&#39;s earlier U.S. Pat. Nos. 7,586,220 and 8,492,937 disclose an energy harvester in the form of an electromechanical generator which includes a coil mounting portion, also known as a bobbin, which extends radially inwardly from a coil in a coil support and is mounted to a central tubular body. A magnetic core assembly surrounds the coil, and vibrational motion of the magnetic core assembly in a direction along the axis of the central tubular body induces an electrical current in the coil. It is highly desirable to form the bobbin by a low conductivity material, such as glass-loaded plastic as disclosed in U.S. Pat. No. 8,492,937. The plastic bobbin could be formed by injection moulding, but this process limits the range of achievable wall-thickness. 
     The known bobbin could be strengthened by adding additional material to the bobbin in a separate process after injection moulding, or by modifying the bobbin design by incorporating large internal radii or by adding radial strengthening ribs to either face of the coil support. These options would increase cost and/or complexity of manufacture and would potentially reduce the level of magnetic flux passing through the coil. 
     There is a need in the art to provide a bobbin which minimizes the amount of material, such as plastic, around the coil used to form the bobbin, so that maximum magnetic flux passes through the coil, yet provides the bobbin with sufficient strength, particularly in the axial direction, to maintain the coil in the desired position when the electromechanical generator is subjected to input mechanical vibration. 
     SUMMARY OF THE INVENTION 
     The present invention aims at least partially to provide an energy harvester in the form of electromechanical generator which can reliably provide a bobbin which minimizes the amount of material, such as plastic, around the coil used to form the bobbin, so that maximum magnetic flux passes through the coil, yet provides the bobbin with sufficient strength, particularly in the axial direction, to maintain the coil in the desired position when the electromechanical generator is subjected to input mechanical vibration. 
     The present invention accordingly provides an electromechanical generator for converting mechanical vibrational energy into electrical energy, the electromechanical generator comprising: a central mast, an electrically conductive coil assembly fixedly mounted to the mast, the coil assembly at least partly surrounding the mast, a mount for the coil assembly extending radially inwardly of the coil assembly and fixing the coil assembly to the mast, wherein the mount comprises a conical wall extending between the coil assembly and the mast, a magnetic core assembly movably mounted to the mast for linear vibrational motion along an axis about an equilibrium position on the axis, the magnetic core assembly at least partly surrounding the coil assembly and the mast, and a biasing device mounted between the mast and the magnetic core assembly to bias the magnetic core assembly in opposed directions along the axis towards the equilibrium position. 
     Preferred features are defined in the dependent claims. 
     The present invention is predicated on the finding that a conical wall incorporated in the coil mount, the conical wall extending between the coil assembly and the central mast of the electromechanical generator, can provide a highly rigid, thin, axis-symmetric structure capable of supporting axial loads preferentially. The electromechanical generator can therefore incorporate a bobbin which minimizes the amount of material, such as a thermoplastic, around the coil used to form the bobbin, so that maximum magnetic flux passes through the coil. In addition, the bobbin is provided with sufficient strength, particularly in the axial direction, to maintain the coil in the desired position when the electromechanical generator is subjected to input mechanical vibration. The desired technical effects of enhanced magnetic coupling, and minimum mechanical motion of the bobbin and associated coil when subjected to vibration, are achieved, and can be achieved using an injection moulded thermoplastic one-piece bobbin. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic side section through an electromechanical generator for converting mechanical vibrational energy into electrical energy in accordance with a first embodiment of the present invention; and 
         FIG. 2  is a schematic plan view of a spring in the electromechanical generator of  FIG. 1 ; 
         FIG. 3  is a schematic plan view of an end core part in an electromechanical generator in accordance with a second embodiment of the present invention; and 
         FIG. 4  is a schematic side section through an electromechanical generator for converting mechanical vibrational energy into electrical energy in accordance with a third embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The electromechanical generator of the present invention is a resonant generator known in the art as “velocity-damped” where substantially all of the work done by the movement of the inertial mass relative to the housing is proportional to the instantaneous velocity of that movement. Inevitably, a portion of that work is absorbed overcoming unwanted mechanical or electrical losses, but the remainder of the work may be used to generate an electrical current via a suitable transduction mechanism, such as the electrical coil/magnetic assembly described below. 
       FIGS. 1 and 2  illustrate an electromechanical generator  2  for converting mechanical vibrational energy into electrical energy in accordance with a first embodiment of the present invention. In operation, the electromechanical generator  2  is enclosed within a housing (not shown) and the device is provided with a fitting (not shown) for securely mounting the electromechanical generator  2  to a support (not shown) from which support mechanical vibrational energy is harvested which is converted into electrical energy by the electromechanical generator  2 . 
     The electromechanical generator  2  comprises a central mast  4  extending along a longitudinal axis A-A. In use, the amplitude of the input mechanical vibrational energy is typically along, or has a component extending along, the longitudinal axis A-A. The opposite ends  6 ,  8  of the mast  4  are fitted to the housing (not shown) and one or both ends  6 ,  8  of the mast  4  may be provided with a fitting (not shown), for example a threaded hole, for securely mounting the electromechanical generator  2  to a support, or to a housing. 
     Preferably the mast  4  is made from a low-permeability, low-conductivity, but high-elastic-modulus material such as  316  stainless steel. The mast  4  may be at least partly hollow, with a central hollow bore  5 . 
     An electrically conductive coil assembly  10  is fixedly mounted to the mast  4 . The coil assembly  10  at least partly, preferably wholly, surrounds the mast  4 . The assembly  10  comprises an electrically conductive coil  12  which is circular and is coaxial with the mast  4 . The assembly  10  has radially inner and outer sides  14 ,  16 , the sides  14 ,  16  extending parallel to the axis of rotation A-A. The assembly  10  has first and second (typically upper and lower) opposite edges  18 ,  20 . The coil  12  has first and second coil portions  13   a ,  13   b  respectively located adjacent to the first and second opposite edges  18 ,  20 . 
     A mount  22  for the coil assembly  10  extends radially inwardly of the coil assembly  10  and fixes the coil assembly  10  to the mast  4 . The mount  22  extends radially inwardly of the radially inner side  14 . The coil  12  is mounted within an annular coil support  24  of the mount  22 . In this specification the term “annular” means “ring-like” but does not imply any other specific geometric shape and does not imply that the plan view of any annular element must be rounded; for example the sides of the “annular” or “ring-like” element may be straight. In the illustrated embodiment, the annular coil support  24  is preferably circular in plan, but may be any other ring-like geometric shape. Similarly, the other annular elements described herein are also preferably circular in plan, but may be any other ring-like geometric shape. This assembly  10  mounts the coil  12  in a fixed position within the housing. The coil support  24  is located outwardly in a radial direction from the axis A-A, and also substantially midway in an axial direction between the ends  6 ,  8  of the mast  4 . 
     The mount  22  comprises a conical wall  26  extending between the coil assembly  10  and the mast  4 . The conical wall  26  is integral with the annular coil support  24 . The annular coil support  24  includes a radially oriented inner wall  28  which connects to the conical wall  26 . The conical wall  26  is a moulded body, preferably injection moulded, composed of a thermoplastic material, and the moulded body comprises the annular coil support  24  and the conical wall  26 . Preferably the thermoplastic material is a very low-conductivity material, such as glass-loaded plastic. 
     The conical wall  26  has opposite first and second ends  30 ,  32 . The first end  30  has a smaller diameter than the second end  32 . The first end  30  is mounted to the mast  4  and the second end  32  is mounted to the coil assembly  10 . The mount  22  further comprises an inner wall  34  integral with the first end  30 . The inner wall  34  is arcuate, or curved, and fitted around at least a portion of the circumference of a middle portion  36  of the mast  4 . 
     In the illustrated embodiment, the conical wall is inclined at an angle of from 40 to 50 degrees to the axis A-A, typically at an angle of about 45 degrees to the axis A-A. Preferably, a central part  38  of the conical wall  26  is located substantially midway in an axial direction along the mast  4 . 
     The mount  22  for the coil assembly  10  preferably defines a recess (not shown) in which is received circuitry (not shown) for electrically conditioning the electrical output of the coil  12 , for example by voltage regulation. The circuitry is preferably encapsulated by a plastic or rubber sealing material, which seals and protects the circuitry against undesired environmental influences, such as humidity, liquids, etc. The coil  12  is connected the circuitry by wires (not shown) and in turn the circuitry has second wires (not shown) extending therefrom for connecting to external circuitry (not shown). 
     A magnetic core assembly  40  is movably mounted to the mast  4  for linear vibrational motion along the axis A-A about an equilibrium position on the axis A-A, the equilibrium position being illustrated in  FIG. 1 . The magnetic core assembly  40  at least partly, preferably wholly, surrounds the coil assembly  10  and the mast  4 . 
     The magnetic core assembly  40  comprises two opposed magnetic circuits spaced along the axis A-A. In the illustrated embodiment, the magnetic core assembly  40  comprises a pair of axially aligned annular first and second magnets  42 ,  44  spaced along the axis A. The magnets  42 ,  44 , are each typically a rare earth permanent magnet having a high magnetic field strength. Poles of the magnets  42 ,  44  having a first common polarity face towards each other, and poles of the magnets  42 ,  44  facing away from each other are of a second common polarity. 
     A ferromagnetic body  46  contacts and is magnetically coupled to the poles of the magnets  42 ,  44  facing away from each other. Generally, the ferromagnetic body  46  extends radially outwardly of the magnets  42 ,  44  relative to the axis. The ferromagnetic body is generally tubular and has radially inwardly extending arms at each end thereof, each arm mounting a respective magnet  42 ,  44  thereon. 
     The magnets  42 ,  44  are mounted on opposite sides of, in  FIG. 1  above and below, the conical wall  26  and radially inwardly of the coil  12 . The magnets  42 ,  44  are each axially spaced from the conical wall  26 , and define a gap  48  through which the mount  22 , in particular conical wall  26 , extends. The magnets  42 ,  44  are aligned so that their identical poles face each other on opposite sides of the conical wall  26 . 
     The magnetic core assembly  40  comprises an outer core  50 , comprising a one-piece tubular body  52 , which encloses the electrically conductive coil assembly  10  on the radially outer side  16 . The tubular body  52  is cylindrical. 
     The magnetic core assembly  40  further comprises first and second end cores  54 ,  56  contacting and magnetically coupled to the outer core  50  at respective opposite first and second ends  58 ,  60  of the outer core  50 . The first and second end cores  54 ,  56  extend radially inwardly and enclose the respective first and second opposite edges  18 ,  20  of the coil assembly  10 . The magnetic core assembly  40  further comprises the first and second magnets  42 ,  44  spaced along the axis A-A. The first and second magnets  42 ,  44  contact and are magnetically coupled to the respective first and second end cores  54 ,  56 . The first and second coil portions  13   a ,  13   b  are respectively at least partly located between the outer core  50  of the common ferromagnetic body and one of the magnets  42 ,  44 . 
     The first and second ends  58 ,  60  of the tubular body  52  each comprise a recess  62 ,  64  on an inner side  66  of the tubular body  52 . The first and second end cores  54 ,  56  are fitted in the recess  62 ,  64  of the respective first and second ends  58 ,  60  of the tubular body  52 . 
     The recess  62 ,  64  has a transverse mounting surface  68  facing along the axis A-A away from the equilibrium position and a longitudinal mounting surface  70  facing towards the axis A-A. Radial and circumferential surfaces  72 ,  74  of the respective first and second end cores  54 ,  56  are respectively fitted to the transverse and longitudinal mounting surfaces  68 ,  70 . 
     The first and second end cores  54 ,  56  comprise plates. The first and second end cores  54 ,  56  may be planar or may be provided with some three-dimensional shaping on the outer or inner surfaces. The first and second end cores  54 ,  56  are circular, each having an outer circumferential surface  74  fitted to an inner circumferential surface, which is the longitudinal mounting surface  70 , of the outer core  50  and a central hole  76  surrounding the mast  4 . In the illustrated embodiment the first and second end cores  54 ,  56  are circular discs which are fitted into the ends of the tubular body  52 . The circular circumference of the first and second end cores  54 ,  56  may be axially fitted to shoulders, formed by the transverse and longitudinal mounting surfaces  68 ,  70 , on the inner side  66  of the tubular body  52 . The fitting may be a pressure, relaxation or elastic fit. The first and second end cores  54 ,  56  may be optionally bonded to the tubular body  52 . The resultant structure provides a substantially C-shaped magnetic core with substantially uniform ferromagnetic properties, and an accurate axial length. 
     In a modified embodiment, as shown in  FIG. 3 , the first and second end cores  54 ,  56  are circular and have a small angular segment cut-out or opening  55  which enables the first and second end cores  54 ,  56  to be press-fitted into the recess  62 ,  64  of the respective first and second ends  58 ,  60  of the tubular body  52 . The first and second end cores  54 ,  56  are composed of a compliant material and are oversize relative to the internal dimensions of the recess  62 ,  64 , and the elastic relaxation of the first and second end cores  54 ,  56  in the recess  62 ,  64  ensures axial retention of the first and second end cores  54 ,  56  in the tubular body  52  so that the magnetic core assembly has an accurate axial length. Since the magnetic field in the first and second end cores  54 ,  56  is radial, and a cut-out or opening  55  which is substantially radial only minimally affects the magnetic circuit. The installed first and second end cores  54 ,  56  exert outward pressure on the tubular body  52 , which completes the required magnetic circuit efficiently. 
     In a further embodiment, as shown in  FIG. 4 , one (rather than both as shown in  FIG. 1 ) of the first and second end cores, in the illustrated embodiment the second end core  56 , is integral with the tubular body  52 , and the first end core  54  has the structure described above with reference to the embodiment of  FIG. 1 . At the opposite end, the tubular body  52  is provided with an integral end core part  57 . The non-integral end core  54  may have either construction as described above, in particular either continuous or with a cut-out or opening  55  which is substantially radial. 
     First and second locator elements  78 ,  80  are respectively fitted to the first and second end cores  54 ,  56 . The first and second locator elements  78 ,  80  each extend towards the mount  22 . Each of the first and second locator elements  78 ,  80  has a locating surface  82  which engages a side surface of a respective first and second magnet  42 ,  44 . The first and second locator elements  78 ,  80  are fitted to a fitting surface  84 ,  86  of the respective first and second end cores  54 ,  56 , the fitting surface  84 ,  86  facing towards the axis A-A. 
     The locating surface  82  of the first and second locator elements  78 ,  80  is fitted to a side surface  88 ,  90  of the respective first and second magnets  42 ,  44 , the side surface  88 ,  90  facing towards the axis A-A. The first and second locator elements  78 ,  80  accurately and securely fit the magnets at the correct location in the magnetic core assembly  40 . 
     The magnetic core assembly  40 , comprising the radially outer core  50 , first and second end cores  54 ,  56  and radially inner magnets  42 ,  44 , defines therebetween an annular enclosed cavity  92  in which the coil  12  is received. The magnets  42 ,  44  and the outer core  50  and first and second end cores  54 ,  56  are slightly spaced from the coil  12  to permit relative translational movement therebetween. The magnetic core assembly  40  has a substantially C-shaped cross-section and is rotationally symmetric. 
     The cavity  92  has respective cavity portions between each of the first and second coil portions  13   a ,  13   b  and the central mast  4 , and above or below, respectively, the conical wall  26  of the mount  22 . 
     The outer core  50  and first and second end cores  54 ,  56  are composed of a ferromagnetic material having a high magnetic permeability, and a high mass, such as soft iron. The assembly of the outer core  50 , first and second end cores  54 ,  56  and the magnets  42 ,  44  therefore forms two axially spaced magnetic circuits of the magnetic core assembly  40 . The limits of the lines of magnetic flux of each magnetic circuit are defined by the outer core  50  and the respective first and second end cores  54 ,  56 , which substantially prevents magnetic flux from each magnet  42 ,  44  extending axially or radially outwardly from the common ferromagnetic body formed of the outer core  50  and first and second end cores  54 ,  56 . Since the opposed magnets  42 ,  44  face each other with common poles, at the central region of the magnetic core assembly  40  the magnetic flux of the opposed magnetic circuits are in opposition thereby directing the magnetic flux radially outwardly towards the common ferromagnetic body. 
     The resultant effect is that a single magnetic core assembly  40  comprises two separate magnets  42 ,  44  and each has a respective magnetic circuit in which a very high proportion of the magnetic flux is constrained to pass through the respective coil portion  13   a ,  13   b . This in turn provides a very high degree of magnetic coupling between the magnets  42 ,  44  and the coil  12 . Consequently, any relative movement between the magnets  42 ,  44  and the coil  12 , in particular as described below by linear axial resonant movement of the magnetic core assembly  40  relative to the fixed coil  12 , produces a very high electrical power output at the coil  12 . 
     A biasing device  100  is mounted between the mast  4  and the magnetic core assembly  40  to bias the magnetic core assembly  40  in opposed directions along the axis A-A towards the equilibrium position. The biasing device  100  comprises a pair of first and second plate springs  102 ,  104 . Each of the first and second plate springs  102 ,  104  has an inner edge  106 ,  108  respectively fitted to the first and second opposite ends  6 ,  8  of the mast  4  and an outer edge  114 ,  116  fitted to the magnetic core assembly  40 . The outer edge  114  of the first plate spring  102  is fitted to a first end part  118  of the magnetic core assembly  40  and the outer edge  116  of the second plate spring  104  is fitted to a second end part  120  of the magnetic core assembly  40 . 
     Each of the first and second plate springs  102 ,  104  comprises a spring member  122 ,  124  comprising an inner portion  126 ,  128 , which is substantially orthogonal to the axis A-A and includes the respective inner edge  106 ,  108 , and a cylindrical outer portion  130 ,  132  which is substantially parallel to the axis A-A and includes the respective outer edge  114 ,  116 . 
     The spring member  122 ,  124  is a folded sheet spring and the inner and outer portions  126 ,  128 ;  130 ,  132  are connected by a fold  134 ,  136 . 
     Each outer edge  114 ,  116  is fitted to an outer circumferential surface  138 ,  140  of the magnetic core assembly  40 . In the illustrated embodiment, each outer edge  114 ,  116  is push-fitted onto the outer circumferential surface  138 ,  140  of the magnetic core assembly  40  and fitted thereto by an elastic fit. 
     The inner edge  106 ,  108  of each of the first and second plate springs  102 ,  104  is fitted to the mast  4  by a riveted joint  142 ,  144  between the inner edge  106 ,  108  and the mast  4 . 
     The first and second plate springs  102 ,  104  each apply the same mechanical biasing force against the magnetic core assembly  40  when the magnetic core assembly  40  is moved away from the central equilibrium position. The first and second plate springs  102 ,  104  preferably have the same spring constant. 
     The provision of a pair of first and second plate springs  102 ,  104  at opposed axial ends of the movable magnetic core assembly  40  provides a structure that can not only provide a sufficient spring biased restoring force on the magnetic core assembly  40  to bias it towards an axially central position with respect to the coil  12 , but also takes up substantially minimum volume within the housing. In particular, the location of the first and second plate springs  102 ,  104  at opposed axial ends of the movable magnetic core assembly  40  can help to maximize the size of the magnetic core assembly  40  for a given device volume which not only maximizes the magnetic coupling, but also importantly permits the mass of the movable magnetic core assembly to be correspondingly maximized. As known in the art, there is a desire to maximize the mass of the movable magnetic core assembly in a resonant vibrational electromagnetic energy harvester because this increases the output electrical power. 
     The provision of a pair of first and second plate springs  102 ,  104  also avoids the need for expensive and cumbersome helical springs surrounding the movable magnetic core assembly. This decreases the manufacturing cost by reducing the component cost. 
     Referring now to  FIG. 2  which shows a plan view of the first and second plate springs  102 ,  104 , the inner portion  126 ,  128  is substantially circular. In the illustrated embodiment, each inner portion  126 ,  128  has an outer circumferential part  146  adjacent to the fold  134 ,  136 , an inner circumferential part  148  adjacent to the inner edge  106 ,  108 , and at least three arms  150 ,  152 ,  154  connecting together the outer and inner circumferential parts  146 ,  148 . The arms  150 ,  152 ,  154  are mutually spaced around the axis A-A and each pair of adjacent arms  150 ,  152 ,  154  is separated by a respective opening  156 ,  158 ,  160  therebetween. The arms  150 ,  152 ,  154  are equally mutually spaced around the axis A-A. With three arms  150 ,  152 ,  154 , the angular separation between the same parts of adjacent arms  150 ,  152 ,  154  is 120 degrees. 
     Each arm  150 ,  152 ,  154  comprises a radial outer part  162  connected to the outer circumferential part  146 , a middle circumferential part  164  connected to the radial outer part  162 , and a radial inner part  166  connected between the middle circumferential part  164  and the inner circumferential part  148 . 
     Each opening  156 ,  158 ,  160  comprises an outer circumferential region  168  adjacent to the outer circumferential part  146 , a middle radial region  170  connected to the outer circumferential region  168 , and an inner circumferential region  172  connected to the middle radial region  170  and adjacent to the inner circumferential part  148 . 
     Each opening  156 ,  158 ,  160  extends between outer and inner opening ends  174 ,  176 , each of the opening ends  174 ,  176  having an enlarged width as compared to the adjacent portion of the opening  156 ,  158 ,  160 . This reduces stress concentrations in the spring. 
     In the illustrated embodiment, there are exactly three arms  150 ,  152 ,  154  and exactly three openings  156 ,  158 ,  160 . This provides an angular separation of 120 degrees between the arms, and between the openings. In other embodiments, more than three arms and correspondingly more than three openings, are provided. For example, four, five or six arms/openings may be provided, with respective angular separations of 90, 72 and 60 degrees between the arms, and between the openings. 
     Preferably, as in the illustrated embodiment, the arms  150 ,  152 ,  154  have the same shape and dimensions and correspondingly the openings  156 ,  158 ,  160  have the same shape and dimensions. 
     The electromechanical generator  2  further comprises a pair of first and second spacers  178 ,  180 . The first spacer  178  is fitted between the first plate spring  102  and a first surface  182  of the mast  4 , and the second spacer  180  is fitted between the second plate spring  104  and a second surface  184  of the mast  4 . The first and second surfaces  182 ,  184  are located at the respective first and second opposite ends  6 ,  8  of the mast  4 . 
     A resilient device  186  is mounted between the biasing device  100  and the magnetic core assembly  40 . The resilient device  186  is configured to be deformed between the biasing device  100  and the magnetic core  40  when the magnetic core assembly  40  has moved, by the linear vibrational motion, away from the equilibrium position by a predetermined non-zero threshold amplitude. 
     The resilient device  186  comprises a pair of first and second flat spring elements  188 ,  190 . Each of the first and second flat spring elements  188 ,  190  has an outer edge  192 ,  194  fitted to the magnetic core assembly  40  and a free inner edge  196 ,  198  spaced radially outwardly from the mast  4  and spaced axially inwardly of the respective first and second plate spring  102 ,  104 . The outer edge  192  of the first flat spring element  188  is fitted to the first end part  118  of the magnetic core assembly  40  and the outer edge  194  of the second flat spring element  190  is fitted to the second end part  120  of the magnetic core assembly  40 . 
     Typically, the outer edge  192 ,  194  of each of the first and second flat spring elements  188 ,  190  is fitted to the magnetic core assembly  40  by being urged by a spring so as to be securely retained in position against the magnetic core assembly  40 . As shown schematically in  FIG. 1 , a spring bias element  191  is provided between the outer edge  192 ,  194  and respective first or second plate spring  102 ,  104  which urges the outer edge  192 ,  194  firmly against the first or second end part  118 ,  120  of the magnetic core assembly  40 . In an alternative, although less preferred, embodiment, the outer edge  192 ,  194  of each of the first and second flat spring elements  188 ,  190  may be otherwise fitted, for example directly fixed, to the magnetic core assembly  40 . 
     The first and second spacers  178 ,  180  extend radially outwardly of the mast  4  and define respective first and second faces  200 ,  202  each of which is oriented inwardly towards, and spaced from, in a direction along the axis A-A, the respective free inner edge  196 ,  198  of the respective first and second flat spring element  188 ,  190 . In the illustrated embodiment, the first and second faces  200 ,  202  are spaced from, in the direction along the axis A-A, the respective free inner edge  196 ,  198  of the respective first and second flat spring element  188 ,  190  by a respective gap  204 ,  206  having a predetermined length. 
     Preferably, only the outer edge  192 ,  194  of each of the first and second flat spring elements  188 ,  190  is in contact with any other part of the electromechanical generator  2 . 
     Each of the first and second flat spring elements  188 ,  190  has an inner surface  208 ,  210  which faces the magnetic core assembly  40 , and a peripheral portion  212 ,  214  of each inner surface contacts the magnetic core assembly  40 . The peripheral portion  212 ,  214  of each inner surface  208 ,  210  contacts an upstanding peripheral edge  216 ,  218  of the magnetic core assembly  40 . 
     The high degree of magnetic coupling between the movable magnetic core assembly  40  and the coil  12 , and the high mass of the movable magnetic core assembly  40 , enables the resonant frequency readily to be tuned accurately to a desired value, and also permits a high self-restoring force to be applied to the movable magnetic core assembly  40  during its resonant oscillation to minimize the amplitude of the oscillation. Since the amplitude is limited, the springs  102 ,  104  are only ever deformed by a very small degree, well within their linear spring characteristics. Typically, under normal operation the maximum amplitude is less than about 1 mm. 
     The springs  102 ,  104  bias, back towards the central equilibrium position, the magnetic core assembly  40  which can move axially along the axis A-A when the electromechanical generator  2  is subjected to an applied mechanical force, in particular a mechanical vibration, having at least a component along the axis A-A. The springs  102 ,  104  have a high stiffness in the lateral, i.e. radial, direction so as substantially to prevent non-axial movement of the magnetic core assembly  40 . 
     The generator  2  is configured such that the mass of the magnetic core assembly  40  is permitted to oscillate about the equilibrium point relative to the mast  4  with an oscillation amplitude no more than the predetermined threshold amplitude without the resilient device  186 , comprising the first and second flat spring elements  188 ,  190 , being deformed, i.e. flexed, between the biasing device  100  and the mass. Accordingly, in such a scenario of “normal operation”, the resilient device  186  does not cause any power loss from the generator  2  when the oscillation amplitude of the mass is no more than the particular or predetermined threshold amplitude. 
     However, the generator  2  is configured such that, when the oscillation amplitude exceeds the predetermined threshold amplitude, such as when it is subjected to a severe shock, the resilient device  186  is then deformed, i.e. flexed, between the biasing device  100  and the mass to act as a limiter that limits the oscillation amplitude. Accordingly, the electromechanical generator  2  according to the preferred embodiments of the present invention has particular utility in environments where it may be subjected to occasional severe shocks. 
     The first and second flat spring elements  188 ,  190  respectively impact the first and second spacers  178 ,  180  to provide the amplitude limitation. The first and second spacers  178 ,  180  provide the advantage that the initial gap  204 ,  206  between the spacers  178 ,  180  and the flat spring elements  188 ,  190  can be accurately set. Therefore the first spacer  178  and the second spacer  180  can function as shims between the respective first and second plate springs  102 ,  104  and the mast  4 , to define a predetermined distance between the first and second spaces  178 ,  180  and the first and second flat spring elements  188 ,  190 . Also, the amplitude limiting motion of the first and second flat spring elements  188 ,  190  against the first and second spacers  178 ,  180  avoids or minimizes sliding motion, which eliminates or minimizes wear. The first and second flat spring elements  188 ,  190  may be made of phosphor-bronze and the first and second spacers  178 ,  180  may be made from steel. These materials can provide the required high spring constant to the flat spring elements  188 ,  190 , which is preferably higher than the spring constant for the first and second plate springs  102 . 
     The electromechanical generator  2  may be disposed with in a housing, which may be hermetically sealed to protect the mechanical and electrical parts of the electromechanical generator  2 . The interior volume of the housing may include an inert gas. 
     The electromechanical generator  2  uses a resonant mass-spring arrangement. If the electromechanical generator  2  is subject to a source of external vibration that causes it to move along the direction A-A, then the magnetic core assembly  40  comprises an inertial mass which may move relative to the mast  4 , also along the direction A-A. In doing so, the springs  102 ,  104  are deformed axially, and work is done against a damper comprising the static electrical coil  12  and the movable magnetic core assembly  40  that generates a region of magnetic flux within which the electrical coil  12  is disposed. Movement of the electrical coil  12  within the magnetic flux causes an electrical current to be induced in the electrical coil  12  which can be used as a source of electrical power for driving an external device (not shown). 
     By increasing the electrical output, as a result of increased magnetic coupling, the operating band width of the device can be greatly increased. This in turn greatly enhances the ability of the device to be used in many new energy harvesting applications. 
     Simple plate springs  102 ,  104  can be employed in the electromechanical generator  2 . This provides a reliable and simple structure to prevent lateral movement on the magnetic core assembly  40 , with low friction and avoiding complicated, intricate and/or expensive manufacturing techniques. The resultant structure is robust and compact. Since the plate springs  102 ,  104  are subjected to a very low amplitude of deformation, their mechanical properties are not especially critical, because they are never deformed anywhere near their mechanical limits of linear elastic movement, and so they can accordingly be of relatively conventional quality, and consequently have a low component cost. 
     In the electromechanical generator of the preferred embodiment of the present invention a high moving mass can be achieved by filling almost all of the internal space of a housing of the device with a metallic magnetic core assembly. This can be achieved at least partly because flat springs at opposed ends of the magnetic core assembly are volume efficient. In addition, a high Q comes from the fact that the “enclosed” structure of the magnetic core assembly leaks very little flux, and so there is very little eddy current in the surrounding material of the stationary housing. Accordingly, little clearance needs to be kept between the moving magnetic core assembly and the housing, allowing more moving mass. A high magnetic coupling comes also from the enclosed nature of the magnetic core assembly where very little flux leaks out—almost all the magnetic flux gets channeled through the coil. 
     Other modifications and embodiments of the present invention as defined in the appended claims will be apparent to those skilled in the art.