Abstract:
An apparatus for generating an electrical power upon an acceleration of the apparatus is provided. The apparatus includes: a piezoelectric member having at least a portion thereof formed of a piezoelectric material for generating an output power upon an impact; and a spring element configured to have at least one of a portion thereof and a portion attached thereto impact the piezoelectric material upon the acceleration.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority to earlier filed U.S. provisional application Ser. No. 60/600,455 filed on Aug. 11, 2004, the entire contents of which is incorporated herein by its reference. 
     
    
     GOVERNMENTAL RIGHTS  
       [0002]     This invention was made with Government support under Contract No. DAAE30-03-C1077, awarded by the U.S. Army. The Government may have certain rights in this invention. 
     
    
     BACKGROUND OF THE INVENTION  
       [0003]     1. Field of the Invention  
         [0004]     The present invention relates generally to power supplies, and more particularly, to power supplies for projectiles, which generate power due to an acceleration of the projectile.  
         [0005]     2. Prior Art  
         [0006]     All existing and future smart and guided projectiles and those with means of one-way or two-way communications with a command or tracking station or with each other require electric power for their operation. In addition, as munitions are equipped with the means of communicating their type and characteristics with the firing system to ensure that the intended round is being used and for fire control purposes, and for health monitoring and diagnostics runs before loading, they would require a low level of power supply minutes and sometimes even seconds before being loaded into the gun system. The amount of power required for the proper operation of such smart and guided munitions or those equipped with the aforementioned health monitoring and diagnostics capabilities, is dependent on their mode of operation and the on-board devices that have to be powered. The amount of power requirement is fairly small if the projectile is required to only receive a RF or other similar signal and to power sensors such as MEMs types of accelerometers and rate gyros or health monitoring and diagnostics related electronics. The power requirement is increased if the projectile is also required to communicate back to the ground or some mobile station. The power requirement, however, becomes significant when the projectile has to be equipped with electric or smart materials based actuation devices for guidance and control, particularly if the projectile is required to become highly maneuverable over long traveling times and while traveling at relatively high speeds such as supersonic speeds.  
       SUMMARY OF THE INVENTION  
       [0007]     Accordingly, an apparatus for generating an electrical power upon an acceleration of the apparatus is provided. The apparatus comprising: a piezoelectric member having at least a portion thereof formed of a piezoelectric material for generating an output power upon an impact; and a spring element configured to have at least one of a portion thereof and a portion attached thereto impact the piezoelectric material upon the acceleration.  
         [0008]     The apparatus can further comprise a mass associated with the spring element for increasing a magnitude of the impact. The mass can be a portion of the spring element. The mass can be a separate portion from the spring and attached thereto.  
         [0009]     The apparatus can further comprise means for preloading the piezoelectric material in compression. In which case, the apparatus can further comprise means for adjusting an amount of the preloading.  
         [0010]     The apparatus can further comprise a housing having an internal cavity for containing the piezoelectric member and spring element in the internal cavity. The housing can comprises means for collapsing in a direction of the acceleration to limit an amount of movement of the spring member. The means for collapsing can comprise the housing being an additional spring member having a greater spring coefficient than the spring element. The means for collapsing can comprise the housing having a curved shape for facilitating collapse thereof where the acceleration is greater than a predetermined limit.  
         [0011]     The apparatus can further comprise limiting means for limiting a loading on the piezoelectric member due to the impact. The limiting means can comprise sandwiching the piezoelectric member between the spring element and an intermediate member, wherein one of the spring element and intermediate member have a stop for contacting the other of the spring element and intermediate member where the acceleration reaches a predetermined limit. The limiting means can comprise an intermediate element having a tapered surface, wherein the spring element has an opposing tapered surface for mating with the tapered surface of the intermediate element where the acceleration reaches a predetermined limit. The limiting means can comprise the spring element having a flange for contacting a surface of an intermediate element where the acceleration reaches a predetermined limit. The intermediate element can have first and second surfaces and wherein the flange contacts the first surface where the acceleration reaches a predetermined limit and the flange contacts the second surface where a deceleration reaches another predetermined limit.  
         [0012]     The spring element can comprise fist and second spring elements and the piezoelectric member can comprise first and second piezoelectric members corresponding to the first and second spring elements, respectively, wherein the acceleration causes a vibration in the first and second spring members in the direction of the acceleration to cause an reciprocating impact of the first and second piezoelectric members. In which case, the apparatus can further comprise a mass positioned between the first and second spring elements. The first spring element, second spring element and mass can be a single integral member.  
         [0013]     Also provided in an apparatus for generating an electrical power upon an acceleration of the apparatus in which the apparatus comprises: a housing; a piezoelectric member positioned within the housing; a spring element disposed with the housing; and a mass configured to impact the piezoelectric material upon the acceleration. The mass can be a portion of the spring element.  
         [0014]     Still provided is a method for generating an electrical power upon an acceleration of an apparatus. The method comprising: accelerating the apparatus; and impacting a piezoelectric material upon the acceleration. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     These and other features, aspects, and advantages of the apparatus and methods of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where:  
         [0016]      FIG. 1  illustrates a schematic cross section of a power generator according to a first embodiment.  
         [0017]      FIG. 2  illustrates a variation of the power generator of  FIG. 1 .  
         [0018]      FIG. 3  illustrates a schematic cross section of a power generator according to a second embodiment.  
         [0019]      FIG. 4  illustrates a schematic cross section of a power generator according to a third embodiment.  
         [0020]      FIG. 5   a  illustrates a schematic cross section of a power generator according to a fourth embodiment.  
         [0021]      FIG. 5   b  illustrates a first variation of the power generator of  FIG. 5   a.    
         [0022]      FIG. 5   c  illustrates a second variation of the power generator of  FIG. 5   a.    
         [0023]      FIG. 5   d  illustrates a third variation of the power generator of  FIG. 5   a.    
         [0024]      FIG. 5   e  illustrates a fourth variation of the power generator of  FIG. 5   a.    
         [0025]      FIG. 5   f  illustrates a fifth variation of the power generator of  FIG. 5   a.    
         [0026]      FIG. 5   g  illustrates a sixth variation of the power generator of  FIG. 5   a.    
         [0027]      FIG. 6  illustrates a schematic cross section of a power generator according to a fifth embodiment.  
         [0028]      FIG. 7  illustrates a schematic cross section of a power generator according to a sixth embodiment.  
         [0029]      FIG. 8  illustrates a schematic cross section of a power generator according to a seventh embodiment.  
         [0030]      FIG. 9  illustrates a variation of the power generator of  FIG. 8 .  
         [0031]      FIG. 10  illustrates a variation of the power generator of  FIG. 9 .  
         [0032]      FIG. 11  illustrates a schematic cross section of a power generator according to a eighth embodiment.  
         [0033]      FIG. 12  illustrates a variation of the power generator of  FIG. 11 .  
         [0034]      FIG. 13  illustrates a schematic cross section of a power generator according to a ninth embodiment. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0035]     In the methods and apparatus disclosed herein, the spring end of a mass-spring unit is attached to a housing (support) unit via one or more piezoelectric elements, which are positioned between the spring end of the mass-spring and the housing unit. A housing is intended to mean a support structure, which partially or fully encloses the mass-spring and piezoelectric elements. On the other hand, a support unit may be positioned interior to the mass-spring and/or the piezoelectric elements or be a frame structure that is positioned interior and/or exterior to the mass-spring and/or piezoelectric elements. The assembly is provided with the means to preload the piezoelectric element in compression such that during the operation of the power generation unit, tensile stressing of the piezoelectric element is substantially avoided. The entire assembly is in turn attached to the base structure (e.g., gun-fired munitions). When used in applications that subject the power generation unit to relatively high acceleration/deceleration levels, the spring of the mass-spring unit is allowed to elongate and/or compress only within a specified limit. Once the applied acceleration/deceleration has substantially ended, the mass-spring unit begins to vibrate, thereby applying a cyclic force to the piezoelectric element, which in turn is used to generate electrical energy. The housing structure or the base structure or both may be used to provide the limitation in the maximum elongation and/or compression of the spring of the mass-spring unit (i.e., the amplitude of vibration). Each housing unit may be used to house more than one mass-spring unit, each via at least one piezoelectric element.  
         [0036]     In the following schematics of the various embodiments, the firing acceleration is considered to be upwards as indicated by arrow  113 .  
         [0037]     In a first embodiment, power generator  100  includes a spring  105 , a mass  110 , an outer shell  108 , a piezoelectric (stacked and washer type) generator  101 , one socket head cap screw  104  and a stack of Belleville washers  103  (each of the washers  103  in the stack is shown schematically as a single line). Piezoelectric materials are well known in the art. Furthermore, any configuration of one or more of such materials can be used in the power generator  100 . Other fasteners, which may be fixed or removable, may be used and other means for applying a compressive or tensile load on the piezoelectric generator  101  may be used, such as a compression spring. The piezoelectric generator  101  is sandwiched between the outer shell  108  and an end  102  of the spring, and is held in compression by the Belleville washer stack  103  (i.e., preloaded in compression) and the socket head cap screw  104 . The mass  109  is attached (e.g., screwed, bonded using adhesives, press fitted, etc.) to another end  106  of the spring  105 . The piezoelectric element  101  is preferably supported by a relatively flat and rigid surface to achieve a relatively uniform distribution of force over the surface of the element. This might be aided by providing a very thin layer of hard epoxy or other similar type of adhesives on both contacting surfaces of the piezoelectric element. The housing  108  may be attached to the base  107  by the provided flange  111  using well known methods, or any other alternative method commonly used in the art such as screws or by threading the outer housing and screwing it to a tapped base hole, etc. The mass  109  is provided with an access hole  110  for tightening the screw  104  during assembly. Between the free end  106  of the spring and the base  107  (or if the mass  109  projects outside the end  106  of the spring, then between the mass  109  and the base  107 ) a gap  112  is provided to limit the maximum expansion of the spring  105 . Alternatively, the gap  112  may be provided by the housing  108  itself. The gap  112  also limits the maximum amplitude of vibration of the mass-spring unit.  
         [0038]     During firing of a projectile (the base structure  107 ) containing such power generator  100 , the firing acceleration is considered to be in the direction  113 . The firing acceleration acts of the mass  109  (and the mass of the spring  105 ), generating a force in a direction opposite to the direction of the acceleration that tends to elongate the spring  105  until the end  106  of the spring (or the mass  109  if it is protruding from the end  106  of the spring) closes the gap  112 . For a given power generator  100 , the amount of gap  112  defines the maximum spring extension, thereby the maximum (tensile) force applied to the piezoelectric element  101 . As a result, the piezoelectric element is protected from being damaged by tensile loading. The gap  112  also defines the maximum level of firing acceleration that is going to be utilized by the power generator  100 .  
         [0039]     In applications where high levels of acceleration (deceleration) are present in both directions (direction  113  and in its opposite direction), then similar stops may be provided to limit spring compression. This can be achieved by providing flanges on the end  106  of the spring  105  and stops to prevent compression of the spring element  105  over a predetermined limit, for example as shown in  FIG. 2 .  FIG. 2  shows a partial cross-section of the side of the power generator assembly that is connected to the base structure  107 , with the remaining part of the assembly being identical to that shown in  FIG. 1 . A free end  122  of the spring unit (with or without the mass  121 ) is provided with a flange  127 . A flanged ring  123  is then positioned around the flange  127  before assembling the unit inside the housing  124 . Once attached to the base structure  107 , a housing lip  128  keeps the flanged ring  123  in contact with the base structure  127 , thereby limiting the motion of the free end  122  of the spring unit within the distances  125  and  126 , up and down, respectively.  
         [0040]     When the firing acceleration has ended, i.e., after the projectile has exited the gun barrel, the mechanical (potential) energy stored in the elongated spring is available for conversion into electrical energy. In all the present power generators, this is accomplished by harvesting the varying voltage generated by the piezoelectric element  101  as the mass-spring element vibrates. The spring rate and the maximum allowed deflection determine the amount of mechanical energy that is stored in the spring  105 . The effective mass and spring rate of the mass-spring unit determine the frequency (natural frequency) with which the mass-spring element vibrates. By increasing (decreasing) the mass or by decreasing (increasing) the spring rate of the mass-spring unit, the frequency of vibration is decreased (increased). In general, by increasing the frequency of vibration, the mechanical energy stored in the spring  105  can be harvested at a faster rate. Thus, by selecting appropriate spring  105 , mass  109  and gap  112 , the amount of electrical energy that can be generated and the rate of electrical energy generation can be matched with the requirements of a projectile.  
         [0041]     In  FIG. 1 , the spring  105  is shown to be a helical spring. The preferred helical spring, however, has three or more equally spaced helical strands to minimize the sideways bending and twisting of the spring during vibration. In general, any other type of spring may be used as long as they provide for vibration in the direction of providing cyclic tensile-compressive loading of the piezoelectric element.  
         [0042]     In a second embodiment, as shown in  FIG. 3 , the power generator is very similar to that of the previous embodiment, with the difference being that the socket head cap screw  104  ( FIG. 1 ) is eliminated, and the preloading of the piezoelectric element  101  is achieved by means of a pin  116 , which is attached to or an integral part of a cap  115 . The cap  115  is connected to the housing  118 , for example by means of threads, potting, press fitting, flange or other methods known in the art. The cap  115  is in turn attached to the base structure using one of the means described in the previous embodiment. The gap  117 , which allows vibration of the mass-spring unit as described for the previous embodiment is provided between the free end  119  of the spring  105  and the cap  115 . In addition, the mass  109  is preferably eliminated and a required mass is added to the free end  119  of the spring by making it larger. Alternatively, and if it is allowed by the size of the power generator, the hole  110  in the mass  109  ( FIG. 1 ) is made large enough to accommodate the pin  116 . The hole  110  must obviously be large enough to allow vibration of the mass-spring unit without the interference of the pin  115 . This embodiment has the advantage of eliminating the possibility of failure of threads of the screw  104  as a result of high firing accelerations or fatigue during vibration, and the possibility that the screw loosening up as a result of acceleration and decelerations and vibration of the mass-spring unit.  
         [0043]     In a third embodiment, as shown in  FIG. 4 , the power generator is very similar to that of the first embodiment, with a difference being that the housing  131  is provided with a significant flexibility in the axial direction, i.e., along the length of the housing  131 . In  FIG. 4 , the housing  131  is shown as a helical spring (preferably with three or more strands). However, any other housing design that provides the desired axial flexibility may also be used. By providing a housing that is flexible in the axial (parallel to the spring  105 ) direction, the electric power generator  130  has the ability to collapse in the axial direction due to the firing acceleration and limiting the stretching of the spring  105 . By making the housing spring  131  much stiffer than the spring  105 , the electric power generator  130  can still vibrate and generate electricity at lower acceleration levels and collapse and protect the spring  105  and the piezoelectric element at extremely high accelerations. In  FIG. 4 , a washer  132  is shown to be positioned between the piezoelectric element and the housing  131 . Such washers are preferably bonded to one or both surfaces of the piezoelectric element  101  to better distribute load over its top and bottom surfaces.  
         [0044]     In the embodiment shown in  FIG. 4 , the housing  131  is providing the axial flexibility that is desired in the axial direction. Alternatively, the pin  116  ( FIG. 3 ) may be provided with the desired axial flexibility while keeping the housing  131  rigid.  
         [0045]     In a fourth embodiment, as shown in  FIG. 5   a , the housing shell or support (frame) structure  141  is designed to buckle when the firing acceleration increases beyond a certain predetermined range, thereby helping to provide added protection against damage to the piezoelectric and/or mass-spring and/or other elements of the power generation unit  140 . The housing shell or support structure  141  may be designed to be prone to buckling instability in any of the ways known in the art. In  FIG. 5   a , the buckling instability of the power generator  140  is due to a bowed geometry in its housing shell or support structure  141 . At low accelerations and during vibration of the mass-spring unit, the instability will not be noticeable. However, if the acceleration exceeds a critical value, the housing shell (support structure)  141  will become unstable and buckle. The buckling of the housing shell or support structure  141  can be designed to provide protection for the mass-spring unit, piezoelectric element, assembly screw and other elements of the assembly from excessive loading. The buckling may be limited to its elastic range, in which case the housing shell or support structure returns to its original shape once the critical acceleration level has subsided. Alternatively, the housing shell or support structure could be designed to permanently deform during buckling. The housing shell or support structure could also be designed to achieve a combination of elastic and plastic deformation.  
         [0046]     In either one of the above cases, the total amount of buckling deformation must be limited to prevent a total collapse of the housing shell or support structure during high acceleration (firing) periods and excessive loading of the piezoelectric and/or the mass-spring and/or the assembly screw or other elements of the assembly. In addition, the total amount of buckling deformation must be limited to prevent a total and permanent collapse of the housing shell or support structure, in order to allow the mass-spring unit to vibrate with the desired amplitude following the high acceleration period(s).  
         [0047]     The means of limiting the maximum buckling deformation of the housing shell or support structure  141  may be an integral part of the housing shell or support structure as shown in  FIG. 5   b . In the embodiment of  FIG. 5   b , the housing shell or support structure is provided with pairs of axially positioned steps  142  (preferably three or more that are positioned symmetrically around the periphery of the housing shell or support structure) are used to limit the axial buckling of the housing shell or support structure  141  to the provided gap  143 . The steps  142  may be internal and/or external to the housing shell or support structure  141 . The steps  142  are preferably integral to the housing shell or the support structure.  
         [0048]     Alternatively, the means of limiting the maximum buckling deformation of the housing shell or the support structure  141  may be provided by a space  148  between a head of the assembly screw  144  and a pin  145  attached to the base structure  107  as shown in  FIG. 5   c  (the pin may also be attached to the housing shell or the support structure base  141 , not shown).  
         [0049]     Alternatively, the means of limiting the maximum buckling deformation of the housing shell or the support structure  141  may be provided by a space  149  between the head of the assembly screw  144  and the mass  147  (of the mass-spring unit) and the space  150  between the mass  147  and the base structure  107  as shown in  FIG. 5   d  (the space  150  may also be between the mass  147  and the base of the housing shell or the support structure  141 , not shown).  
         [0050]     Alternatively, the means of limiting the maximum buckling deformation of the housing shell or the support structure  141  may be provided by a space  152  between the head of an assembly screw  151  and the base structure  107  as shown in  FIG. 5   e  (the space  152  may also be between the head of an assembly screw  151  and the base of the housing shell or the support structure  141 , not shown).  
         [0051]     Alternatively, the means of limiting the maximum buckling deformation of the housing shell or the support structure  141  may be provided by a space  153  between a cylindrical sleeve  154  and the base structure  107  as shown in  FIG. 5   f  (the space  153  may also be between the sleeve  153  and the base of the housing shell or the support structure  141 , not shown). In  FIG. 5   f , the sleeve  154  is shown to be press fitted into the top portion of the housing shell or support structure  141 . Alternatively, the sleeve  154  may be press fitted to the bottom portion of the housing shell or support structure  141  or even be loosely assembled inside of the housing shell or support structure  141 .  
         [0052]     Alternatively, the sleeve  154  may be positioned exterior to a housing shell or support structure  155  that has a top flange  156  as shown in  FIG. 5   g , and limit its maximum buckling by either the space  157  between the flange  156  and the sleeve  154  or by limiting the outward radial expansion of the housing shell or support structure  155 .  
         [0053]     In the  FIGS. 5   a - 5   g , the buckling under high (firing) acceleration is shown to be in the housing shell or the support structure (columns of a support structure frame) in the axial direction. However, the buckling may be designed to occur in other modes, and in other elements of the structure of the power generation assembly. For example, the pin  116  of the embodiment shown in  FIG. 3  may be designed to buckle in the elastic range to reduce the peak loading of the piezoelectric element  101  during peak acceleration period(s), and return to essentially its original shape and position to allow unhindered operation of the power generator.  
         [0054]     In other apparatus and methods disclosed herein, a mass-spring unit is attached directly or via an intermediate element to the base structure. A piezoelectric element is positioned between the spring of the mass-spring unit and the base structure or between the mass-spring unit and an intermediate element. The means of attaching the spring of the mass-spring unit to the base structure (or the aforementioned intermediate element) is preferably provided with the means to preload the piezoelectric element in compression so that during the vibration of the mass-spring unit, the piezoelectric element is not subjected to tensile loading. When an intermediate element is present, it may be attached directly to the base structure by any one of the methods commonly used in the art, e.g., by constructing the intermediate element as a cylinder and threading it and the base structure; or by using screws or bonding using various methods known in the art, including adhesives; by providing a flange on the intermediate element and then attaching the flange to the base structure using methods known in the art, including the use of clamps; etc.  
         [0055]     An advantage of this method is that it leads to designs that are very simple and easy to manufacture, assemble and mount on the base structure. However, a disadvantage of this method is that during acceleration of the base structure (in the axial direction), the force generated by the entire mass of the mass-spring unit, the attachment means (e.g., screw), the preloading means (e.g., Belleville washers), etc., act directly on the piezoelectric element. As a result, the piezoelectric element has to be designed to resist the maximum possible (shock) loads, thereby leading to a power generator that is difficult to be optimally designed for the actual (working) acceleration levels of the base structure and produce the maximum possible power for a specified (available) power generator volume. This shortcoming of the present method can, however, be substantially overcome using a number of modifications that are described in the following embodiments.  
         [0056]     A schematic of the fifth embodiment  160  is shown in  FIG. 6 . The unit  160  primarily consists of a spring  161 , preferably made of 3 or more helices to minimize bending and other rotations and lateral displacement during vibration; mass  162 , which may have a top piece  163  to prevent it from traveling into the spring element  161 , and noting that the free (top) portion  168  of the spring  161  may partly or wholly constitute the mass  162  and  163 ; and a piezoelectric element  165 . The mass  168  is preferably press fitted and/or potted into the open end of the spring  161 . A screw  167  is used to attach the spring  161  to an intermediate element  166 , with the piezoelectric element  165  being positioned between the two. One or more stacks of Belleville washers  164  are positioned between the screw head and the spring  161  to provide the required preloading force on the piezoelectric element  165 . The preloading load is adjusted by adjusting the tightness of the screw  167 . The intermediate element  166  may then be attached to the base structure  107  using any one of the aforementioned means, including by a longer assembly screw  167  that taps into the base  107 .  
         [0057]     In an alternative embodiment of the electrical power generator  160 , the intermediate element  166  can be eliminated and the piezoelectric element can be attached directly to the base structure  107 .  
         [0058]     The embodiment  160  provides a very simple design, which, however, does not offer any protection for the piezoelectric  165  against excessive high accelerations of the base structure. The spring  161  is preferably designed such that during firing it is compressed until it reaches its solid height, which indicates the total potential energy that is stored in the spring  161 . Once the firing (high) acceleration period has ended, the spring-mass unit is free to oscillate. Meanwhile the mechanical energy stored in the spring element  161  can be converted into electrical energy by the piezoelectric element.  
         [0059]     In yet other methods and apparatus disclosed herein, a mechanical mechanism is provided to limit the deformation of the spring element of the mass-spring units in compression, tension or both tension and compression. The purpose of such spring deformation limiting mechanisms is to limit the compressive and/or the tensile loading applied to the piezoelectric elements and also prevent overloading of the spring element when the base acceleration and/or deceleration passes certain limits. The embodiments of this method are otherwise similar to those presented for the fifth embodiment.  
         [0060]     A sixth embodiment, generally referred to by reference numeral  170 , is shown in  FIG. 7 . The power generator  170  of the sixth embodiment is very similar to the previous embodiment  160 , with the addition of a protective stop  172  located around a base of the spring  171 . The stop  172  is designed to bottom out against the intermediate element  174 , closing the gap  173 , if the vertical acceleration of the base structure  107  exceeds a specified level. As a result, by proper selection of the amount of the gap  173 , the piezoelectric element  175  is protected from overloading in compression. The gap  173  must still be large enough to allow the piezoelectric generator  175  to deform during the oscillations of the spring  171 .  
         [0061]     For a realistic thickness of the piezoelectric stack  175 , for example for heights of around 5 mm, the gap  173  needs to be less than 10 microns, depending on the level of the working acceleration, which requires precision manufacture of the spring element or employment of a simplifying manufacturing/assembly technique. As an example, the gap  173  may be made larger than required without requiring high precision, then during the assembly, the gap  173  is filled with hard epoxy, while taking steps to allow the epoxy to bond to only one of the surfaces of the gap  173 . The unit is then loaded in compression to the desired compression limit of the piezoelectric element and the epoxy is allowed to cure. This assembly procedure ensures that the desired gap height is achieved.  
         [0062]     A seventh embodiment is similar to the embodiment of  FIG. 7 , with the difference being in the method of stopping the spring element against the intermediate element. The schematic of such an embodiment  180  is shown in  FIG. 8 . The spring element  183  is made with a tapered outer diameter, while the intermediate element  181  is provided with a matching tapered surface  182 . As the base structure  107  accelerates upward, e.g., during firing by a gun, the spring  183  begins to compress, until it comes to rest against the tapered surface  182  of the intermediate element  181  when a specified acceleration level is reached. If the acceleration of the base structure exceeds the specified level, the contacting tapered surfaces prevent overloading of the piezoelectric element  186 , and also protects the spring element  183  from excessive deformation axially and in other modes such as bending or lateral displacement, thereby protecting it from failure. Similar to the previous embodiments, the spring element  183  is attached to the intermediate element  181  with the screw  187  and with the Belleville washers  188  to provide the means to preload the piezoelectric element for its protection from tensile loading during vibration of the mass-spring unit. The intermediate element  181  is in turn attached to the base structure  107  using one of the means previously described. This embodiment therefore provides protection against over-stressing of both the spring element  183  and the piezoelectric element  186 .  
         [0063]     In a variation of the seventh embodiment, the spring element  183  is also provided with a top flange  184 . In the absence of the acceleration of the base structure  107 , a gap  189  is provided between the flange  184  and a top surface  185  of the intermediate element  181 . When the acceleration of the base structure  107  reaches a certain specified level, the spring  183  is compressed enough to close the gap  189 , thereby preventing the top flange  184  of the spring element to move down any further. As a result, the maximum compressive load of the piezoelectric element  186  can be limited, thereby providing the means to protect it from failure.  
         [0064]     In another alternative of the seventh embodiment, no taper is provided on either the spring element  183  or the intermediate element  181 . The spring element is provided with the flange  184 ,  FIG. 8 , which comes to a stop against the top  185  of the intermediate element  181  at a specified level of the acceleration of the base structure  107 , thereby providing protection for both the piezoelectric element  186  and the spring  183 .  
         [0065]     In another alternative of the seventh embodiment, the spring flange  184  can be positioned along the length of the spring. Such an embodiment  190  is shown in  FIG. 9 . The intermediate element  191  of this embodiment is shown to have an internal groove  192 , in which the flange  194  of the spring element  193  is positioned. The flange  194  may be an integral part of the spring  193 , in which case to make the unit assembly possible, either the intermediate element has to be made out of two parts with a common surface at the groove  192  (the two parts, longitudinal or transverse, have to be then joined using any one of the methods known in the art); or the flange  194  may be a retaining ring, which is assembled in a groove (not shown) in the spring  193 . The spring element  193  is then attached to the intermediate element  191  by a screw  196  as shown for the previous embodiments, with the piezoelectric element  195  positioned between the two as shown in  FIG. 9 . Preloading Belleville washers (not shown) are preferably used with the screw  196  as shown in  FIG. 8 . As can be appreciated, the total axial compressive and tensile deformation of the spring is thereby protected at high accelerations and decelerations of the base structure  107 . The total amount of compressive and tensile deformation of the spring is determined by the gaps  198  and  197 , respectively, between the lower and upper surfaces of the flange  194  and the lower and upper surfaces of the groove  192 . The piezoelectric element  195  and the spring element  193  are thereby protected from overloading due to high levels of base structure acceleration and deceleration.  
         [0066]     In a variation of the embodiment shown in  FIG. 10 , the spring element  203  has a flange  204  at its free (upper) end. The intermediate element  201  is the provided with a counter bore  202 , in which the flange  204  is positioned in the assembled unit  200 . A cap  205  is then fixed to the top of the intermediate element  201 , for example by screws  206  (shown schematically by dashed lines). The remaining elements of this embodiment are the same as those of the embodiments shown in  FIGS. 8-9 . The flange  204 , thereby, protects both the piezoelectric element and the spring element  203  as was described for the previous embodiment.  
         [0067]     In the embodiments shown in the  FIGS. 8-10 , the spring deformation limiting taper surfaces and spring flanges are positioned external to the spring element. Alternatively, the taper surfaces and/or flanges may be positioned internal to the spring, with the mating taper surfaces and/or flange accommodating grooves positioned on an internal pin (such as a pin similar to the pin  151  in the embodiment of  FIG. 5   e , with an external taper surface and/or groove used in place of the screws  187  or  196  in the embodiments of  FIGS. 8-10 ).  
         [0068]     In the embodiments shown in  FIGS. 9 and 10 , the spring deformation limiting flanges are provided on the spring elements and the mating grooves are provided on the intermediate elements. Alternatively, the flanges may be provided on the intermediate elements and the mating grooves on the spring elements.  
         [0069]     In all the above embodiments of this method, one part of the spring deformation limiting mechanism (for example a groove or its mating flange, or one of the tapered mating surfaces) is provided on the intermediate element, which is in turn fixedly attached to the base structure. It is, therefore, possible for the intermediate element to be an integral part of the base structure.  
         [0070]     In still yet further apparatus and methods disclosed herein, double spring-mass (mass positioned in between two springs) unit(s) are packaged such that: (a) there is no need for separate preloading elements; (b) the internal attachment screws or the like are eliminated; (c) fewer internal components are needed; and (d) the assembly process is greatly simplified and the need for a preload adjustment step is eliminated. The electric power generators using this method can be constructed with three basic parts; a double spring and mass unit, which can be constructed as a single integral unit; piezoelectric generator(s); and an outer (or inner) support structure, which may be in the form of a shell housing. In this method, the mass-spring unit is compressed and positioned within a gap provided with a relatively rigid housing shell or support structure. Piezoelectric elements are positioned between at least one of the springs and the gap surfaces. The unit is then attached to the base structure using one of the methods described for the previous embodiments.  
         [0071]     A schematic of an eighth embodiment  210  is shown in  FIG. 11 . It comprises a mass  212 , which is positioned between two springs  213  and  214 . In  FIG. 11 , the mass  212  and the two springs  213 ,  214  are constructed as a single unit, however, they may also be individual components. The mass-spring unit is then positioned inside a relatively rigid shell housing  211 . Piezoelectric elements  215  are placed between each spring  213 ,  214  and the housing on one or both ends. In the embodiment shown in  FIG. 11 , the opening through which the mass and spring unit and the piezoelectric elements  215  are entered into the housing shell  211  is positioned on a side of the housing shell  211 . Alternatively, all internal elements may be entered from a top or bottom opening, and then sealed by a cap. When the loading opening is on the bottom of the housing, which is directly attached to the base structure  107 , no cap may be required.  
         [0072]     The mass  212  or the spring elements  213  and/or  214  (preferably only one of the two) can be provided with a flange similar to the flange  194  in  FIG. 9 , and the housing shell can be provided with a mating groove  192  (alternatively, the position of the flange and the mating groove may be exchanged). As a result, the total deformation of the springs, thereby the compressive and tensile force exerted on the piezoelectric element(s) is limited. This provides protection for both piezoelectrics  215  and the spring elements  213 ,  214  when the acceleration or deceleration of the base structure  107  exceeds the specified amount.  
         [0073]     The spring  213  and/or spring  214  can be provided with outside taper and mating taper surfaces on the inner surfaces of the housing shell, both similar to that shown in  FIG. 8 . As a result, the compressive and/or tensile deformation of the springs  213  and  214  is/are limited. This provides protection for both piezoelectric  215  and the spring elements  213 ,  214  when the acceleration or deceleration of the base structure  107  exceeds the specified amount.  
         [0074]     Instead of an exterior shell housing or support structure, an interior structure can be used to keep the distance between the top surface of the interior assembly (top surface of the spring or the piezoelectric element, if any) and the bottom surface of the interior assembly (bottom surface of the spring or the piezoelectric element, if any) relatively constant.  
         [0075]     The schematics of a typical such embodiment  220  is shown in  FIG. 12 . The support structure is shown as a cylinder  221 , with top  222  and bottom  223  ends (one of the ends  222  or  223 , alone or with certain portion of the cylinder  221 , is a separate piece and is fastened to the main piece to allow assembly). Two springs  224  and  225 , with a mass  226  that is positioned between the two springs are assembled as shown around the interior cylinder  221 . The mass and the two springs are preferably constructed as a one integral piece. Piezoelectric elements  227  are positioned on at least one side of the mass and spring unit. The spring is preferably preloaded to prevent the piezoelectric element(s) from being loaded with a considerable tensile loading to prevent its failure.  
         [0076]     In the above embodiments, the springs are preloaded to prevent excessive loading of the piezoelectric elements in tension. Alternatively, by providing little or no preloading, and by firmly attaching the piezoelectric element(s) to the housing shell, the spring is allowed to bounce back and forth inside the housing shell cavity. The advantage of such a design is that the piezoelectric elements are never subject to tensile loads, which can easily fracture such brittle materials. However, the resulting impact loading can cause problems. In addition, the impulsive loading of the piezoelectric element(s) result in high but short duration charges that has to be harvested rather quickly, which can be difficult to accomplish efficiently.  
         [0077]     In still yet other apparatus and methods disclosed herein, the piezoelectric based power generators are constructed with two modular units. The first module is a mass-spring unit and the second module is a packaged preloaded and high acceleration and shock resistant piezoelectric unit. The two modules are then connected to each other by a screw or by using any one of the methods known in the art.  
         [0078]     The spring of the mass-spring unit is preferably designed such that it could compress essentially elastically to a solid length, thereby providing a means of protecting the spring from failure in compression. When necessary, relatively solid stops (provided by a housing shell or internal or external support structure) are preferably provided to limit tensile deformation (elongation) of the spring, thereby providing a means of protecting the spring from failure in tension. As a result, the spring of the mass-spring unit can readily be protected from excessive acceleration and/or deceleration of the base structure.  
         [0079]     The piezoelectric unit (module) comprises a housing or support structure, within which the piezoelectric element is assembled with two sets of preloading springs (preferably of Belleville washer type), separated by a relatively solid separating element, to which the spring-mass module is attached. The piezoelectric element is positioned between the base of the housing and one of the two sets of preloading springs, opposite to the separating element.  
         [0080]     By assembling mass-spring units with various equivalent masses and spring rates with various piezoelectric unit modules with appropriate preloads and piezoelectric elements, a wide range of power generator units that can operate in various acceleration/deceleration and shock loading environment and various power generation requirements can be constructed. When subjected to higher than operating base structure accelerations, the spring of the mass-spring can be made to come in contact with the piezoelectric unit housing or support structure, thereby preventing the piezoelectric from damage. When subjected to higher than operating base structure deceleration, the mass-spring unit pulls the aforementioned separating element away until it is stopped by the housing element. By having provided enough of a preloading force and by matching the deformation of the preloading springs to the allowed displacement of the separating element, the preloading spring stays in contact with the piezoelectric element at all times, thereby preventing any impact loading of the piezoelectric element during subsequent acceleration (or significant reduction in the deceleration level) of the base structure.  
         [0081]     A schematic of a ninth embodiment  230  is shown in  FIG. 13 , and comprises the mass-spring module  231  and a piezoelectric assembly module  232 . In the schematic of  FIG. 13 , the mass of the mass-spring module is incorporated into the mass of the spring element  233 . However, additional mass may also be added (preferably to the free end) of the spring element  233  to vary (decrease) the natural frequency of the mass-spring module  231 . The two modules ( 231  and  232 ) are attached together by the screw  234 . The two modules may be attached together in numerous ways known in the art. For example, a stem may be provided on the attaching side of the spring, which can then be press fit into a provided hole in the attachment element  235 , or the stem may be threaded and screwed in a tap provided in the element  235 , instead of the screw  234 .  
         [0082]     The piezoelectric assembly module  232  consists of a housing  236 , at the bottom of which the piezoelectric  242  (preferably stack) element is positioned (preferably adhered by a relatively hard epoxy or other similar material to help to distribute the load more uniformly on the piezoelectric element surface at its interface with the housing  236 ). A washer  241  is positioned (preferably similarly adhered) to the piezoelectric element  242 . The separating element (plunger)  238  with at least one preloading (preferably of Belleville washer type) springs  239  and  240 , above and below its flange  238 , respectively, is positioned above the piezoelectric washer  241 . The preloading springs  239  are held in place by the retaining ring  237 . To prevent the retaining ring  237  from being dislodged during impact loading or high acceleration/deceleration of the base structure, a sleeve (not shown) may be placed on the piston  235 , between the piston  235  and the retaining ring (with a slight clearance between the sleeve and the retaining ring).  
         [0083]     In the piezoelectric module  232  shown in  FIG. 13 , the preloading assembly is held in place by the retaining ring  237 . Alternatively, the retaining ring may be integral to the housing  236 , i.e., a step may have been provided to seat the preloading springs  239 . The piezoelectric module is then assembled from the bottom (constructed open) end and is then capped following the assembly.  
         [0084]     In another alternative, at least one side of the housing  236  is open and the parts are assembled from this open side of the housing.  
         [0085]     The piston  235  is designed to be long enough (alternatively, the spring  233  may have been constructed with an appropriate shoulder or a space may be used) to provide the gap  243  between the spring  233  and the top surface of the housing  236 . During acceleration of the base structure  107 , once a specified design acceleration limit is reached, the gap  243  is closed, thereby preventing further loading of the piezoelectric element  242 . During deceleration of the base structure  107 , once a specified design acceleration limit is reached, the gap  245  between the top surface of the spring  233  and the outer shell or frame  244  (which together with the housing  236  is fixed to the base structure using one of the aforementioned methods), is closed, thereby preventing further elongation of the spring  233  and its damage. Meanwhile, the piston  235  is pulled away from the piezoelectric element until the preloading springs  239  are have reached their near rigid (compressed) length, thereby preventing further movement of the piston. As a result, the piezoelectric element is protected from tensile loading.  
         [0086]     In an alternative embodiment, the spring  233  may be protected from excessive levels of deceleration by elongating the head of the screw  234  past the top of the spring  233 , and providing it with a head with the gap  245  with the top surface of the spring to act as a stop against excessive elongation of the spring.  
         [0087]     Other variations of the embodiments disclosed above are also possible. For example, in all cases, the housing may be integral to the structure of the base structure (projectile); the housing may be a structure to support the generated loads or may encapsulate most or all the components of the generator and may even be hermetically sealed; the mode of vibration may be essentially axial, in torsion, in bending or in any of their combination; the piezoelectric element(s) may be of any shape and geometry and may or may not be of stacked construction (however, by using a stacked piezoelectric element, a lower voltage level but larger current can be achieved); the electrical characteristics of the piezoelectric element are also desired to be selected such that it allows efficient transfer of electrical energy to collection circuitry (such collection circuitry being well known in the art and not shown herein) which can mean that the impedance of the piezoelectric element is matched with the collecting circuitry to maximize the rate of energy transfer, e.g., to the storage capacitors; the taper and flange stops shown in  FIGS. 8-10  may also be incorporated into any of the other embodiments; in all cases, the spring element may be designed to elastically (or partly elastically and partly plastically) collapse to its solid length in compression, thereby being protected from higher acceleration/deceleration that produces spring elongation.  
         [0088]     While there has been shown and described what is considered to be preferred embodiments of the invention, it will, of course, be understood that various modifications and changes in form or detail could readily be made without departing from the spirit of the invention. It is therefore intended that the invention be not limited to the exact forms described and illustrated, but should be constructed to cover all modifications that may fall within the scope of the appended claims.