Abstract:
A power source including: a power generation device; a mass-spring unit having a mass and an elastic element operatively connected to the power generation device; and one or more retention fingers releasably engaged with the mass-spring unit for retaining the mass-spring unit in a position such that potential energy is stored therein and for releasing the potential energy upon occurrence of an event to generate electrical energy in the power generation device, the one or more retention fingers having a first end fixed at a base and a second end releasably engaged with the mass-spring unit. The occurrence of the event can be one or more of an acceleration and spinning of the base. Also disclosed is a power source having one or more retention fingers that are slidable with respect to a base such that the engagement of the first end is released upon a spinning of the base.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present disclosure relates generally to reserve electrical power sources, and more particularly, to reserve power sources for munitions such as air dropped weapons and projectiles fired by guns, mortars and the like, that are initiated during the deployment of munitions to generate power from internally stored mechanical potential energy and when applicable, used to indicate certain events that can be used to achieve safe and arm functionalities or the like. 
     2. Prior Art 
     Chemical reserve batteries have long been used in various munitions, weapon systems and other similar applications in which electrical energy is required over relatively short periods of times. In addition, unique to the military is the need for munitions batteries that may be stored for up to twenty years without maintenance. Reserve batteries are batteries designed to be stored for years, even decades, without performance degradation. Reserve batteries are stored in an inert state and can be activated within a fraction of a second with no degradation of battery capacity or power. Typical Reserve batteries are thermal batteries and liquid reserve batteries. 
     The typical liquid reserve battery is kept inert during storage by keeping the electrolyte separate from the electrodes. The electrolyte is kept in a glass or metal ampoule inside the battery case. Prior to use, the battery is activated by breaking the ampoule and allowing the electrolyte to flood the electrodes. The ampoule is broken either mechanically or by the high g shock experienced from being shot from the cannon. 
     Thermal batteries represent a class of reserve batteries that operate at high temperatures. Unlike liquid reserve batteries, in thermal batteries the electrolyte is already in the cells and therefore does not require a distribution mechanism such as spinning. The electrolyte is dry, solid and non-conductive, thereby leaving the battery in a non-operational and inert condition. These batteries incorporate pyrotechnic heat sources to melt the electrolyte just prior to use in order to make them electrically conductive and thereby making the battery active. Thermal batteries have long been used in munitions and other similar applications to provide a relatively large amount of power during a relatively short period of time, mainly during the munitions flight. Thermal batteries have high power density and can provide a large amount of power as long as the electrolyte of the thermal battery stays liquid, thereby conductive. 
     Reserve batteries are expensive to produce, primarily since the process of their manufacture is highly labor intensive and involve mostly manual assembly. For example, the process of manufacturing thermal batteries is highly labor intensive and requires relatively expensive facilities. Fabrication usually involves costly batch processes, including pressing electrodes and electrolytes into rigid wafers, and assembling batteries by hand. The reserve batteries are encased in a hermetically-sealed metal container that is usually cylindrical in shape. In munitions, thermal batteries may be initiated during launch via inertial or electrical igniters, or may be initiated later during the flight via electrical igniters. The liquid reserve batteries are usually activated during launch by breaking the electrolyte ampoule. 
     Chemical reserve batteries, including thermal batteries and liquid reserve batteries, are generally very expensive to produce, require specialized manufacturing processes and equipment and quality control, and are generally required to be developed for each application at hand. 
     All existing and future smart and guided weapons, including gun-fired projectiles, mortars, and small and large gravity dropped weapons, require electric energy for their operation. For many fuzing operations such as fuzing “safe” and “arm” (S&amp;A) and sensory functionalities and many other “smart” fuzing and initiation functionalities, the amount of electrical energy that is needed is low and may be as low as 10-50 mJ, and even less. In fact, with such electrical energy levels, low-power electronics could be easily powered to provide the above fuzing or the like functionalities. The amount of power required to operate many other electronic components, for example those used for diagnostics and health monitoring purposes, or for receiving a communicated signal or the like is also very small and can be readily achieved with electrical energy in the above range. In all such applications, particularly for powering electronics for fuzing and other similar “safe” and “arm” functionalities, it is highly desirable to have low-cost and safe alternatives to chemical reserve batteries. This is particularly the case for the above applications since it is generally difficult to produce very small, miniature, reserve batteries of any kind. 
     A need therefore exists for alternatives to chemical reserve batteries for low power applications such as fuzing electronics for “safe” and “arm” and other functionalities, and other similar low power applications. For munitions applications, such “reserve” type power sources have to have a very long shelf life of up to 20 years; be low cost; and be capable of being scaled to the required power level requirements, shape and size, with minimal design and manufacturing change efforts. 
     An objective is to provide non-chemical “reserve” type of power sources for the aforementioned and the like low power applications. In these power sources, mechanical potential energy can be stored in the power source and used to generate electrical energy upon occurrence of certain events, such as firing of a projectile by a gun or by the release (or ejection) of a gravity dropped weapon. This is in contrast to chemical reserve batteries in which stored chemical energy is released upon a certain event (such as firing by a gun or by an electrical charge), thereby allowing the battery to provide electrical energy. 
     Hereinafter, and since the source of energy in the disclosed power sources can be mechanical potential energy, these power sources are referred to as “mechanical reserve power sources”. 
     Here, a means of storing potential mechanical energy can be elastic deformation, such as in various types of spring elements and/or the structural flexibility of the structure of the projectile or gravity dropped weapon or the like, and not potential energy due to gravity. It is, however, appreciated by those skilled in the art that potential energy may also be stored by other means such as by pressurizing compressible fluids such as air. The mechanical potential energy stored in the “mechanical reserve power sources” can then be released via certain mechanisms to be described later in this disclosure upon the occurrence of certain intended event(s), such as firing and/or spinning of a projectile or releasing of a gravity-dropped weapon or other events appropriate to the device employing the power source. The released potential energy can then be used to generate electrical energy using well known methods such as by the use of active materials based elements such as piezoelectric elements or magnet and coil type generators. To this end, the mechanical stored potential energy is preferably used to generate vibration of certain mass-spring element(s). The vibration energy is then transformed into electrical energy by one of the aforementioned piezoelectric, coil and magnet or the like elements. Alternatively, stored mechanical potential energy is used to cause a continuous (such as rotary) motion of an inertial element (e.g., an inertial wheel type element) in the form of kinetic energy. The kinetic energy can then be converted to electrical energy using well known magnet and coil type generators or any other type of available mechanical to electrical energy conversion devices (generators). 
     A second object is to provide methods and apparatus for releasing the stored potential energy in the disclosed “mechanical reserve power sources” using various events such as gun firing acceleration (the so-called setback acceleration) of a projectile; deceleration of gun-fired projectile (the so-called set-forward acceleration); the process and/or mechanism of releasing (e.g., gravity dropping) the weapon from its mounting rack or the like; pulling out or ejection of a releasing element (e.g., a releasing pin or wire); etc. 
     For the mechanical reserve power sources employing piezoelectric elements for converting mechanical energy of vibration to electrical energy, methods described for mass-spring systems used in the piezoelectric based power generators described in the U.S. Pat. Nos. 7,231,874 and 7,312,557 can generally be used in the construction of the disclosed mechanical reserve power sources, particularly for those mechanical reserve power sources to be used in gun-fired projectiles and mortars which are subject to very high-G firing acceleration levels. 
     In addition, in such mechanical reserve power sources, the piezoelectric elements (stacks) employed to convert mechanical energy of vibration to electrical energy may also be used as sensors to measure setback and set-forward acceleration levels, target impact impulse levels and direction, the time of such events and more as described in the patent application publication number 2007-0204756 filed on Jan. 17, 2007, the contents of which is incorporated herein by reference. In this regard, it is important to note that all existing and future smart and guided projectiles can be equipped with means for sensing one or preferably more of the firing setback and set-forward accelerations, radial accelerations, flight vibration in the longitudinal and lateral (radial) directions, and terminal point impact induced acceleration. The measurements can include the related acceleration profiles. The sensory information can be used for guidance and control purposes as well as for fuze safety and operation. 
     A third object is to provide methods for using the disclosed mechanical reserve power sources as the means to provide for safety in general, and “safe” and “arm” functionalities in particular, for fuzing and other similar applications in gun-fired projectiles, mortars as well as gravity dropped weapons. 
     A fourth object is to provide methods for allowing the disclosed mechanical reserve type power sources that rely on conversion of the stored potential energy to vibration energy and consequent conversion of the vibration energy to electrical energy to continue to harvest energy from vibration and other oscillatory motions of the weapon, from aerodynamically induced vibrations, etc., during the flight. 
     SUMMARY OF THE INVENTION 
     Accordingly, a method for the development of mechanical reserve power sources is provided. In these power sources, mechanical potential energy can be stored in elastic elements such as spring elements. The potential energy can then be released upon certain events via certain mechanisms, such as gun firing of a projectile or gravity dropping of a weapon. The released energy can then be transformed into vibration energy, which is then harvested by mechanical to electrical energy conversion elements such as piezoelectric elements or magnet and coil elements. 
     Accordingly, methods and apparatus for storing potential energy in the mechanical reserve power sources, and methods and apparatus for releasing the stored potential energy upon the occurrence of several events are provided. Upon the release of the stored potential energy, the potential energy can cause vibration of the power source “mass-spring” elements (or equivalent mass-spring elements when structural flexibility is used for potential energy storage purposes). Mechanical to electrical energy conversion elements, such as piezoelectric elements in stack configuration, can then be used to convert the mechanical energy of vibration to electrical energy which can then be used directly by onboard electrical and electronics components or stored in electrical energy storage devices such as capacitors. 
     The event upon which the stored mechanical potential energy of the disclosed mechanical reserve power sources is released and the start of electrical power generation can be used to provide “safe” and “arm” (S&amp;A) or other similar safety functionality, particularly when the power source is used for powering fuzing means. The generated electrical energy may also be used to power electronic circuitry and/or logics used to provide additional “safe” and “arm” (S&amp;A) functionality for fuzing or other similar applications. Accordingly, methods and apparatus for the “safe” and “arm” (S&amp;A) or other safety functionality with and without electronics circuitry and/or logics are also provided. 
     The power-source “mass-spring” elements may also be configured to be excited by the vibration and rotary oscillations of the munitions during the flight, thereby allowing the power source to generate additional electrical energy. The power source may also be provided with the means to generate vibration of its “mass-spring” element during the flight due to aerodynamics forces, e.g., by the means to generate flutter. 
     The mechanical to electrical energy conversion may also be constructed with at least three piezoelectric elements that are configured to measure acceleration in the longitudinal and two independent radial directions, including such target impact induced accelerations (noting the term acceleration is used to also mean deceleration—or negative acceleration), thereby the level of impact force and its direction. More piezoelectric elements may also be added to measure rotary acceleration, such as spinning acceleration inside the gun barrel for rifled barrels or the like. Methods and apparatus for integrated mechanical to electrical energy converting and acceleration/impact level and direction sensing piezoelectric stacks and their configurations see application serial publication number 2007-0204756 filed on Jan. 17, 2007, the contents of which is incorporated herein by reference. 
     The apparatus can comprise a mass-spring system with stored mechanical energy. The mass can be a portion of the spring element. The mass can be a separate portion from the spring and attached thereto. The mass-spring system can be attached to the structure of the projectile through the aforementioned piezoelectric elements. Upon release, the stored mechanical energy can cause the mass-spring system to vibrate, which exerts a cyclic force on the piezoelectric elements, generating electrical charges in the piezoelectric elements. The magnitude of the generated charge in each piezoelectric element can be proportional to the amount of force being exerted on the said piezoelectric element and can be measured. The distribution of force exerted on the piezoelectric elements can then be used to determine the direction of the applied accelerations to the projectile during the firing or gravity drop, during the flight as a result of vibration and rotary oscillations and during the impact at the terminal point of the flight. 
     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. The preloading can be for the purpose of preventing the piezoelectric elements to be subjected to tensile forces during aforementioned firing accelerations or gravity drops, during flight vibration and rotary oscillations, and as the result of the projectile impact at the terminal point of the flight. Piezoelectric ceramics must generally be protected from tensile stresses since they are highly brittle and can readily fracture with the application of a considerable amount of tensile stress. In general, methods described in the aforementioned U.S. Pat. Nos. 7,231,874 and 7,312,557 can be used to provide such preloading mechanisms in the construction of the disclosed mechanical reserve power sources, particularly for those mechanical reserve power sources to be used in gun-fired projectiles and mortars which are subject to very high-G firing acceleration levels. 
     The apparatus can further comprise a housing having an internal cavity for containing the piezoelectric member and spring and mass elements in the internal cavity. The housing can also comprise means for collapsing in a direction of the acceleration to limit an amount of movement of the spring member. The apparatus can further comprise limiting means for limiting a loading on the piezoelectric member due to firing acceleration and terminal point impact. Examples of such limiting means are disclosed in the U.S. Pat. No. 7,312,557. 
     It is noted that the disclosed mechanical reserve power sources with integrated inertial sensors may also be used in devices that only experience high acceleration levels upon impacting certain object or medium. In such applications, the present power generators with integrated inertial sensors can be used to determine the direction of the impact and the level of impact forces that are experienced, which would also provide information as to the physical characteristics of the impacted medium (e.g., its softness, elasticity and density). The power source could then generate enough energy for onboard electronics to make appropriate decisions and initiate programmed actions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIG. 1  illustrates a schematic of one embodiment of a mass-spring based mechanical reserve power source with piezoelectric stack mechanical to electrical energy conversion. 
         FIG. 2  illustrates an embodiment of a mass-spring unit of a mechanical reserve power source with the energy conversion piezoelectric stacks used to also act as force/moment and torque measuring sensors. 
         FIG. 3  illustrates an embodiment of a mass-spring and piezoelectric based mechanical reserve power source similar to the embodiment of  FIG. 1  with the spring element preloaded in tension. 
         FIG. 4  illustrates a schematic of an embodiment of the mass-spring based mechanical reserve power source with piezoelectric film mechanical to electrical energy conversion elements in which the mass-spring unit includes a vibrating beam. 
         FIG. 5  illustrates a schematic of the embodiment of  FIG. 4  with piezoelectric stacks used at the base of the vibrating beam for mechanical to electrical energy conversion. 
         FIG. 6  illustrates an embodiment of a mass-spring and piezoelectric based mechanical reserve power source similar to the embodiment of  FIG. 1  for activation by spinning. 
         FIG. 7  illustrates an embodiment of a mass-spring and piezoelectric based mechanical reserve power source similar to the embodiment of  FIG. 1  for activation by firing (setback) acceleration and insensitivity to spinning. 
         FIG. 8  illustrates an embodiment of a mass-spring and piezoelectric based mechanical reserve power source similar to the embodiment of  FIG. 1  for activation by firing set-forward acceleration and insensitivity to spinning. 
         FIG. 9  illustrates an embodiment of a mass-spring and piezoelectric based mechanical reserve power source similar to the embodiment of  FIG. 1  for activation by an external actuation (releasing) means. 
         FIG. 10  illustrates an embodiment of a mass-spring and piezoelectric based mechanical reserve power source similar to the embodiment of  FIG. 1  for activation by removal of locking stops. 
         FIG. 11  illustrates an embodiment of a mass-spring and piezoelectric based mechanical reserve power source similar to the embodiment of  FIG. 1  for by cutting/releasing of a locking cable. 
         FIG. 12  illustrates an embodiment of the mechanical reserve power source that uses an inertia wheel and torsional spring or the like and piezoelectric stacks for electrical energy generation. 
         FIG. 13  illustrates an alternative of the embodiment of  FIG. 12  in which a magnet and coil (dynamo) generator is used for electrical energy generation. 
         FIG. 14  illustrates another alternative of the embodiment of  FIG. 12  in which the mechanical reserve power source is activated by spinning. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Although this invention is applicable to numerous and various types of devices, it has been found particularly useful in the environment of generating power onboard gun-fired and gravity dropped munitions. Therefore, without limiting the applicability of the invention to generating power onboard such munitions, the invention will be described in such environment. However, those skilled in the art will appreciate that the present methods and devices can also be used in generating power in other devices, including commercial electronic devices for direct powering of such devices and/or for charging appropriate electrical energy storage devices such as rechargeable batteries or capacitors. 
     In the methods and apparatus disclosed herein, the spring end (or the end of an elastic element used for the purpose of storing mechanical potential energy) of a mass-spring (or an equivalent such mass-spring system) 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. In practice, a relatively rigid element can be used as an interface element to distribute the force exerted by the spring element over the surface of one or more piezoelectric elements. 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. In general, the assembly is preferably provided with means to preload the piezoelectric element in compression such that during the operation of the power generation unit, i.e., during the vibration of the mass-spring unit, tensile stressing of the piezoelectric element is substantially avoided. The entire assembly can be in turn attached to the base structure (e.g., gun-fired munitions or the gravity dropped weapon). When used in applications that subject the mechanical reserve power source unit to relatively high acceleration and/or deceleration levels, the spring of the mass-spring unit can be allowed to elongate and/or compress only within a specified limit. Once the applied acceleration and/or deceleration have 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. When the base structure is a gun-fired projectile or mortar or a gravity dropped weapon or the like or any other moving platform, that undergoes vibration and oscillatory motions during the flight, such motion will also excite the mass-spring system and cause it to similarly vibrate and apply a cyclic force to the piezoelectric element, which can similarly be 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 or other energy conversion means. 
     Referring now to the mechanical reserve power sources shown in  FIG. 1  and generally referred to by reference numeral  10 . The mechanical reserve power source is considered to be mounted to the structure  13  of a gun-fired projectile, in which it is intended to start to generate electrical energy upon firing. The firing acceleration is considered to be in the direction of the arrow  14 . In this embodiment, the mass  20  is attached to the piezoelectric stack  11  via the spring  21 . An intermediate rigid element  12 , such as one made out of stainless steel, can be used between the spring  21  and the piezoelectric stack  11  to more uniformly distribute the force applied by the spring  21  to the piezoelectric stack  11 . The intermediate element  12  can be integral to the spring element  21 . Similarly, the mass element  20  can be integral to the spring element  21 . The spring element  21  is preferably made with at least  3  helical strands to minimize the tendency of the mass-spring element to displace laterally or bend to the side during longitudinal displacement and vibration in the direction of the arrow  14 . 
     In its pre-firing position, the spring  21  is compressed to store the desired amount of potential energy, bringing the mass  20  to the position shown with solid lines. The mass  20  is then locked in place by at least one locking element  22  that is provided to lock the spring  21  in its compressed configuration shown by the solid lines in  FIG. 1 . 
     During the firing of the projectile, the munitions structure  13  is accelerated in the direction  14 , causing the firing acceleration to act on the inertia of the at least one locking element  22  and bend it out to the position  23 , thereby forcing the tip  24  of the locking elements out of engagement with the mass (or other portion of the device  10 ) to release the mass  20 . The at least one locking element  22  may be provided with additional eccentrically positioned mass (inertia)  15  to increase the aforementioned force due to the presence of the firing acceleration for bending away the locking element  22  to its position  23  to unlock the mass  20 . Such bending rotating the locking element  22  from engagement with the mass  20 . Such additional mass (inertia) may be required if the firing acceleration levels are relatively low or if higher force (moment or torque) levels are required to unlock the locking element  22 . In general, the locking element  22  is preferably moved and kept away from the mass  20  and spring  21  (such as by plastic deformation of at least a portion of the locking element  22  or a ratchet mechanism) so that it would not interfere with their motion (each of such movements, along with the bending discussed above, being collectively referred to herein as rotation). 
     Once the mass  20  is released, the mechanical potential energy stored in the spring  21 , i.e., the mechanical potential energy stored in the “mechanical reserve power sources”  10 , is released. The released mechanical potential energy will then cause the mass  20  and spring  21  (mass-spring unit) to vibrate. The vibration will then apply a cyclic force (push and pull) to the piezoelectric stack  11 , thereby generating an electrical charge, which is then harvested and used directly or stored in certain electrical energy storage device such as a capacitor using electronic regulation and charging circuitry well known in the art. 
     It is noted that in the schematic of  FIG. 1 , the locking element is shown to be constructed as a single element with bending flexibility. However, in general, the locking mechanism may be constructed with any mechanism type that would provide the desired movement to release the mass  20  as a result of the firing acceleration in the direction of the arrow  14 . Further, although the locking element  24  is shown engaged with the mass  20 , it can be engaged with a portion thereof or with the spring element  21 . Still further, although the spring element  21  is shown as a helical spring, it can be any elastic member that is capable of storing energy which can be released upon the firing acceleration so as to result in a vibration of the mass and/or elastic member further resulting in the application of a cyclic force on the piezoelectric stack  11  or other energy conversion means. 
     It is noted that the above “mechanical reserve power source” design provides for a high level of safety since zero power is provided to the projectile electronics even if the projectile is accidentally dropped over a hard surface. This is the case since the spring element  21  of the “mechanical reserve power source”  10  is preloaded to store mechanical potential energy and is locked in its preloaded configuration. The amount of preload and the locking mechanism release threshold can be readily selected such that during accidental dropping of the projectile, for example if the projectile is accidentally dropped and impacts a hard surface, the locking mechanism is not released and the preloading force is not overcome, thereby no significant amount of charges is generated by the piezoelectric stack. 
     In the embodiment shown in  FIG. 1 , a single piezoelectric stack is used to convert mechanical energy to electrical energy. Alternatively, the piezoelectric element  11  can consist of more than one (preferably stack type) elements as shown in  FIG. 2 . In the schematic of  FIG. 2 , the locking elements  22  ( FIG. 1 ) are not shown. In this alternative embodiment  35 , the spring element  33  is also preferably attached to the piezoelectric elements  34  via a substantially rigid element  36  to distribute the forces applied by the spring element  33  more uniformly to the piezoelectric elements  34 . The piezoelectric elements  14  are in turn attached (directly or via other substantially rigid elements (not shown) to the structure of the projectile  35 . 
     During the firing, during the flight and during the impact at the terminal point of the flight, the projectile is subjected to axial and radial accelerations in the direction of the arrows  30  and  31 , respectively, and rotary accelerations about the axial and radial directions. 
     These linear and rotational accelerations act on the inertia of the mass element  32  and the spring element  33 , thereby resulting in the application of axial forces in the direction of the arrow  30 ; shearing forces in the direction of the arrow  31  (and the direction normal to the arrows  30  and  31 —not shown for clarity); moments about the above two shearing force directions; and a moment (torque) about the direction of the above axial force to the element  36 ,  FIG. 2 . The element  36  in turn transmits the applied axial and shearing forces and moments and torque to the underlying piezoelectric elements  34 . The element  36  can be integral to the spring element  33 . 
     As described in the U.S. Provisional Patent application No. 61/158,387 filed on Mar. 8, 2009 (the contents of which are incorporated herein by reference), the level of charges (voltages) generated by the individual piezoelectric elements  34  as a result of the application of the aforementioned axial and shearing forces and moments and torque are measured and used to determine the level of at least one of the said applied forces, moments and torque. These measurements are made while the said charges are harvested. Noting that the said forces, moments and torque are proportional to the aforementioned linear and rotary accelerations that are experienced by the projectile, the said levels of measured forces and/or moments and/or torque would also provide the levels of at least one of the related aforementioned linear and/or rotary accelerations. 
     As a result, the device  35  can function both as a mechanical reserve power source and an accelerometer and/or force (moment and/or torque) sensor. Such an integrated power source and acceleration and/or force (moment and/or torque) sensor device, will significantly reduce the overall size and volume that would have been occupied by currently available and separate power source units and acceleration and force (moment and torque) sensor units. Such integrated power source and acceleration and force (moment and torque) sensor units are of particular need in applications such as gun-fired munitions, mortars and the like where such devices have to occupy minimal volume in order to allow room in the projectile for other components of the munitions that are required to make the projectile effective. 
     It is noted that in gun-fired munitions applications, the piezoelectric based power generators can be designed as described in the U.S. Pat. Nos. 7,231,874 and 7,312,557 so that they could withstand high firing accelerations and target impact forces that are generally experienced by gun-fired munitions, mortars and the like. 
     In the embodiment shown in  FIG. 1 , the mechanical potential energy is stored in the spring element  21  of the mechanical reserve power source  10  by preloading the spring element in compression. Alternatively, the mechanical reserve power source may be designed such that the mechanical potential energy is stored in a spring element which is preloaded in tension. The schematic of such an embodiment  40  is shown in the schematic of  FIG. 3 . The mechanical reserve power source  40  is considered to be mounted to the structure  41  of a gun-fired projectile, in which it is intended to start to generate electrical energy upon firing. The firing acceleration is considered to be in the direction of the arrow  42 . The mass element  43  is attached to the piezoelectric stack  44  via the spring  45 , via an intermediate rigid element  46  to more uniformly distribute the force applied by the spring element  45  to the piezoelectric stack  44 . The intermediate element  46  and the mass element  43  can be integral to the spring element  45 . The spring element  45  can be formed with at least  3  helical strands to minimize the tendency of the mass-spring element to displace laterally or bend to the side during longitudinal displacement and vibration in the direction of the arrow  42 . 
     In its pre-firing position, the spring element  45  is preloaded in tension to store the desired amount of mechanical potential energy. This is done by bringing the mass element  43  to the position shown in  FIG. 3 , and locking it in place with at least one locking element  47 , in this case by preventing the mass element  43  from traveling downwards (opposite the direction of acceleration). 
     During the firing of the projectile, the munitions structure  41  is accelerated in the direction  42 , causing the firing acceleration to act on the inertia of the at least one locking element  47  and bend it out of engagement with the mass  43  to the position  48 , thereby forcing the tip  49  of the locking elements to release the mass  43 . The at least one locking element  47  may be provided with additional eccentrically positioned mass (inertia)  50  to increase the aforementioned force moving the locking element  47  to its position  48  to release the mass element  43 . Such additional mass (inertia) may be required if the firing acceleration levels are relatively low or if higher force (moment or torque) levels are required to displace the locking element  47 . In general, the locking element  47  can be moved and kept away from the mass element  43  and spring element  45  (such as by plastic deformation of at least a portion of the locking element  47  or a ratchet mechanism) so that it would not interfere with their subsequent vibration. Once the mass element  43  is released, the mechanical potential energy stored in the spring element  45 , i.e., the mechanical potential energy stored in the mechanical reserve power sources  40 , is released. The released mechanical potential energy will then cause the mass element and spring element  45  (mass-spring unit) to vibrate. The vibration will then apply a cyclic force to the piezoelectric stack  44 , thereby generating an electrical charge, which is then harvested and used directly or stored in certain electrical energy storage device such as a capacitor using electronic regulation and charging circuitry well known in the art. 
     In the embodiments  10  and  40  shown in  FIGS. 1 and 3 , respectively, the locking elements  22  and  47  are shown to be released by the (axial) acceleration  14  and  42 . Alternatively, the locking mechanisms of the disclosed mechanical reserve power sources may be designed to be released by accelerations in the lateral directions or due to rotational accelerations. In all such cases, the locking mechanism can be provided with mass (inertia) elements similar to mass  15  ( FIG. 1 ) or mass  50  ( FIG. 3 ), such that the applied acceleration acts on the indicated mass, generating forces (moments or torques) that would act to release the locking mechanism, such as was described for the aforementioned embodiments  10  and  40 . 
     In the particular case of the embodiments  10  and  40  shown in  FIGS. 1 and 3 , respectively, the rotational spin (shown by the arrows  52  and  51 , respectively) of the base structure  13  and  41  would also generate a centripetal acceleration that acts on the masses  15  and  50 , forcing the locking elements to release the locked masses  20  and  43  to begin to vibrate. Such rotational spin is commonly applied to gun-fired projectiles and in many cases to mortars and missiles for stabilization purposes during the flight. In such applications, the projectile spinning—alone or in combination of the aforementioned axial acceleration—may be used to release the locking mechanism in the disclosed embodiments of the mechanical reserve power sources. 
     In general, the locking mechanisms are preloaded in the direction opposing their release. For example in the embodiment  10  of  FIG. 1 , the locking elements  22  (acting as flexural spring element) are preferably preloaded such that normally they would press against the mass element  20 . The purpose of this preloading and the threshold force for the release of the locking element  22  is to prevent accidental release of the locking mechanism such as in the case of accidental drops or other unintended acceleration or spinning of the projectile  13 . 
     The amount of preload of the springs  21  and  45  of the mechanical reserve power sources of the embodiments of  FIGS. 1 and 3  and the locking mechanism release threshold can be selected such that during accidental dropping of the power source and/or projectile (device) in which they are mounted, the springs  21  and  45  do not transmit any significant amount of force to the piezoelectric stack elements  11  and  44 , respectively, thereby no significant amount of charges is generated by the said piezoelectric stacks, thereby providing for a high level of safety for the system employing these power sources. 
     In another embodiment  60  shown schematically in  FIG. 4 , the mechanical potential energy storage element (spring element in the embodiments of  FIGS. 1 and 3 ) of the mechanical reserve power source is a relatively flexible beam  61 . The base structure of the mechanical reserve power source  62  is attached to base structure of the munitions  70 , such as a gun-fired projectile in which it is intended to start to generate electrical energy upon firing. The flexible beam  61  is then in turn attached to the power source base structure  62  at the point  71 . 
     The firing acceleration is considered to be in the direction of the arrow  66 . At least one piezoelectric element  68 , preferably a relatively thin element designed to generate a charge when subjected to tensile and compressive stresses in the longitudinal direction of the beam is then attached to at lease one side (and can also be attached to more than one side) of the beam top and bottom surfaces. The piezoelectric elements can be attached closer to the fixed end of the beam and in their normal position (substantially straight), i.e., when the beam is not subjected to flexural bending, are preloaded in compressive stress such that as the beam vibrates up and down as shown in the general lower position  61  and general upper position  69 . 
     In its pre-firing position, the flexible beam is preloaded in bending to the position  61  from its unloaded (normal) position (not shown) to store the desired amount of mechanical potential energy. The preloaded flexible beam  61  is then locked in its position  61  by the tip  64  of at least one locking element  63  as shown in  FIG. 4 . 
     During the firing of the projectile, the munitions structure  70  is accelerated in the direction  66 , causing the firing acceleration to act on the inertia of the at least one locking element  63  and bend it out to the position  67 , thereby forcing the tip  64  of the locking element  63  to release the flexible beam  61 . The at least one locking element  63  may be provided with additional eccentrically positioned mass (inertia)  65  to increase the aforementioned force that acts on the locking element  63  and tend to move it to the position  67  to release the flexible beam  61 . Such additional mass (inertia) may be required if the firing acceleration levels are relatively low or if higher force (moment or torque) levels are required to displace the locking element  63 . In general, the locking element  63  can be moved towards the position  67  and kept away from the flexible beam  61  (such as by plastic deformation of at least a portion of the locking element  63  or a ratchet mechanism) so that it would not interfere with its subsequent vibration. Once the flexible beam  61  is released, the mechanical potential energy stored in the flexible beam  61 , i.e., the mechanical potential energy stored in the present embodiment of the mechanical reserve power sources  60 , is released. The released mechanical potential energy will then cause the flexible beam  61  to vibrate. The vibration will then apply a cyclic tensile and compressive stresses to the at least one piezoelectric  68 , thereby generating an electrical charge, which is then harvested and used directly or stored in certain electrical energy storage device such as a capacitor using electronic regulation and charging circuitry well known in the art. 
     In an alternative embodiment of the embodiment of  FIG. 4 , the piezoelectric element used to transform mechanical vibration energy of the vibrating flexible beam  61  to electrical energy is positioned at the base  71  of the flexible beam, between the flexible beam and the base structure of the mechanical reserve power source. The schematic of such an embodiment  80  is shown in  FIG. 5 . In this embodiment, the base structure of the mechanical reserve power source  82  is attached to base structure of the munitions  81 , such as a gun-fired projectile in which it is intended to start to generate electrical energy upon firing. The flexible beam  83  is attached to the power source base structure  82  through at least one piezoelectric element  85  (which can be at least two piezoelectric stacks  85  as shown in  FIG. 5 ). The forces/moments transmitted from the flexible beam  83  to the at least one piezoelectric element  85  is preferably through a relatively rigid intermediate element  84  to better distribute the applied forces over the surface of the at least one piezoelectric element  85 . At least one spring element  86  can be used to attach the intermediate element  84  to the power source base structure  82 . The at least one spring element  86  is preloaded in tension such that as the flexible beam  83  vibrates as shown between its general upper position  88  and its general lower position  83 , piezoelectric elements  85  are not subjected to a significant amount of tensile stresses. 
     The firing acceleration is considered to be in the direction of the arrow  87 . In its pre-firing position, the flexible beam is preloaded in bending to the position  83  from its unloaded (normal) position (not shown) to store the desired amount of mechanical potential energy. The preload beam is then locked in its position  83  by the tip  90  of at least one locking element  89  as shown in  FIG. 5 . 
     During the firing of the projectile, the munitions structure  81  is accelerated in the direction  87 , causing the firing acceleration to act on the inertia of the at least one locking element  89  and bend it out to the position  91 , thereby forcing the tip  90  of the locking element  89  to release the flexible beam  83 . The at least one locking element  89  may be provided with additional eccentrically positioned mass (inertia)  92  to increase the aforementioned force that acts on the locking element  89  and tend to move it to the position  91  to release the flexible beam  83 . Such additional mass (inertia) may be required if the firing acceleration levels are relatively low or if higher force (moment or torque) levels are required to displace the locking element  89 . In general, the locking element  89  is preferably moved towards the position  91  and kept away from the flexible beam  83  (such as by plastic deformation of at least a portion of the locking element  89  or a ratchet mechanism) so that it would not interfere with its subsequent vibration. Once the flexible beam  83  is released, the mechanical potential energy stored in the flexible beam  83 , i.e., the mechanical potential energy stored in the present embodiment of the mechanical reserve power sources  80 , is released. The released mechanical potential energy will then cause the flexible beam  83  to vibrate. The vibration will then apply a cyclic force/moment to the at least one piezoelectric element  85 , thereby generating an electrical charge in the piezoelectric elements, which is then harvested and used directly or stored in certain electrical energy storage device such as a capacitor using electronic regulation and charging circuitry well known in the art. 
     In general, an additional mass  93  may also be attached to the flexible beam  83 , preferably as close as possible to its free end, for the general purpose of reducing the natural frequency of vibration of the beam element to optimize the amount of mechanical energy that is converted to electrical energy. The mass  93  can also be integral to the flexible beam  83 . 
     It is noted that for the embodiment  60  ( 80 ) shown in  FIG. 4  ( 5 ), the spinning of the round, if high enough, about an axis indicated by the arrow  94  ( 95 ) would also act on the inertia of the locking element  63  ( 89 ) and the mass  65  ( 92 ) and generate a force to release the locking element as previously described for these embodiments. 
     In the embodiment  10  shown in  FIG. 1 , the locking element  22  is designed to release as the base structure  13  (projectile) is accelerated in the direction of the arrow  14  as the projectile is launched. In this embodiment, the (firing) acceleration in the direction of the arrow  14  acts on the inertia (mass) of the locking element  22  (and the additional mass  15 , if present) to generate a force (moment or torque) to release the mass element  20 , thereby allowing the potential energy stored in the spring element  21  to cause the mass-spring system to vibrate, thereby generate electrical energy as described earlier. It was also shown that similar releasing forces (moments or torques) are generated by the spinning of the projectile at a high enough rate in the direction of the arrow  52 . In this embodiment, both firing acceleration in the direction of the arrow  14  and the spinning about the axial direction  52  tend to release the mass element  20 . 
     In certain applications, however, the locking mechanism may be desired not to be released during the firing acceleration but later during the so-called set-forward acceleration, i.e., the acceleration in the direction opposite to that of the firing (set forward) acceleration, i.e., in the direction opposite to the arrow  14  in  FIG. 1 . Accelerations in the direction opposite to the direction of the firing acceleration (the direction of the arrow  14 ) are also experienced by sub-munitions or the like during expulsion from projectiles during the flight. An alternative embodiment  100  of the embodiment  10  that satisfied this requirement is shown in the schematic of  FIG. 6 . In this schematic, the elements not indicated by numerals are identical to those shown in the schematic of  FIG. 1 . The mechanical reserve power source  100  is similarly fixed to the structure  101  of the projectile. The firing (setback) acceleration is considered to be in the direction opposite to the arrow  102 , while the set-forward acceleration is in the direction of the arrow  102 . In this embodiment, the tip  103  of the locking element  104  is similarly used to keep the mass element from being released. A mass element  105  is attached as shown to the locking elements  104 . During the firing, the setback acceleration will act on the mass  105  (the mass of the locking element is considered to be small relative to the mass of the element  105 ), and generate a force that tends to rotate the locking elements inwards, i.e., tend to bring the tips  103  of the locking elements  104  closer to each other. As a result, the mass-spring unit of the mechanical reserve power source  100  is held locked in its pre-firing (preloaded) position. During the set-forward acceleration in the direction of the arrow  102 , however, the set forward acceleration acts on the mass  105  of the locking elements  104 , and generate a force in the opposite direction, which would tend to rotate the locking elements away from the mass-spring unit of the mechanical reserve power source  100 , i.e., tend to move the tips  103  of the locking elements  104  away from each other towards the position indicated as  106 , thereby releasing the mass-spring unit of the mechanical reserve power source  100 . The released mass-spring unit of the mechanical reserve power source  100  will then begin to vibrate and generate electrical energy as previously described above. 
     It is noted that for the embodiment  100  shown in  FIG. 6 , the spinning of the round about its longitudinal axis as indicated by the arrow  107 , if high enough, would also act on the inertia of the locking element  104  and the mass  105  and generate a force to release the locking element as previously described for the embodiments  10  and  40 . 
     In other applications, the locking mechanism is not desired to operate and release the vibrating mass of the mechanical reserve power source (e.g., mass  20  in the embodiment  10  shown in  FIG. 1 ) due to the spinning of the projectile, even if the spinning rate is relatively high. In all the above embodiments, the aforementioned locking elements and the added mass element of the locking element (elements  22  and  15 , respectively, in the embodiment  10  shown in  FIG. 1 ) may be configured such that the aforementioned vibrating mass is not released as a result of projectile spinning. Here, such locking element and added mass element configuration will be shown as applied to the embodiment  10  of  FIG. 1 . However, it is appreciated by those familiar with the art that other disclosed embodiments, i.e., embodiments  40 ,  60 ,  80  and  100  shown in  FIGS. 3-6 , and other similar embodiments may also be constructed with the “spin resistant” release mechanism described below. 
     In this alternative embodiment  110  shown in  FIG. 7 , the elements not indicated by numerals are identical to those shown in the schematic of  FIG. 1 . The mechanical reserve power source  110  is similarly fixed to the structure  111  of the projectile. The firing (setback) acceleration is considered to be in the direction of the arrow  112 , while the set-forward acceleration is in the direction opposite to the arrow  112 . In this embodiment, the tip  113  of the locking element  114  is similarly used to keep the mass element of the mechanical reserve power source from being released. The locking element  114  is attached to the base structure  111  via the hinge joints  115 , which can be living joints. A mass element  116  is attached as shown to each locking elements  114 . During the firing, the setback acceleration will act on the mass element  116  (the mass of the locking element is considered to be small relative to the mass of the element  116 ), and generate a force that tends to rotate the locking elements outwards, i.e., tend to move the tips  113  of the locking elements  114  apart and move the locking element  114  and its attached mass element  116  to the position indicated as  117 , thereby releasing the mass-spring unit of the mechanical reserve power source  110  and allowing the unit to begin to vibrate. The spinning of the projectile about the axial direction shown by the arrow  118 , however, generates centripetal forces shown by the arrows  119  that act laterally on the mass elements  116 . However, since the generated centripetal forces  119  act through the hinges  115 , they do not generate a moment about these joints and therefore would not tend to rotate the locking elements  114  to release the mass-spring unit of the mechanical reserve power source  110 . In general, certain amount of spring preloading (not shown) to bias the locking elements in the direction of locking the mass-spring unit of the mechanical reserve power source  110  and/or frictional force at the joints  115  are provided to provide stable locking of the said mass-spring unit so that it is not accidentally released with a minimal amount of axial acceleration in the direction of the arrow  112  or the like. 
     The locking element  114  and its attached mass  116  of the embodiment  110  of  FIG. 7  may be readily configured for releasing of the mass-spring unit of the mechanical reserve power source during the set forward acceleration of the projectile, i.e., acceleration in the direction opposite to the arrow  112  similar to the embodiment  100  shown in  FIG. 6 . This is done by moving the mass  116  to the opposite side of the hinge  115 , as shown in the schematic of the embodiment  120  of  FIG. 8 . In  FIG. 8 , the elements not indicated by numerals are identical to those shown in the schematic of embodiment  110  of  FIG. 7 . Here, as shown in  FIG. 8 , the locking elements  121  are provided with mass elements  122  that are attached as shown on the opposite side of the hinges as compared to the embodiment  110  of  FIG. 7 . As a result, the mass-spring unit of the mechanical reserve power source is released only during the set forward acceleration of the projectile, i.e., acceleration in the direction of the arrow  123 . In addition, the spinning of the projectile about its axial direction as shown with the arrow  124  would not release the mass-spring unit of the mechanical reserve power source for the same reason described previously for the embodiment  110  of  FIG. 7 . 
     It is noted that in the above disclosed embodiments, the locking mechanism is shown to be a simple rotating link (beam), which is fixed to the base structure either via a hinge (preferably a living joint), embodiments of  FIGS. 7 and 8 , or as a cantilever beam, embodiments of  FIGS. 1 ,  3 - 6 , with the beam being provided with an appropriate amount of flexural (bending) flexibility. The mass-spring (vibrating) unit of the mechanical reserve power source is then released due to the movement of the tip of the said locking elements, i.e., the rotation of the aforementioned beam elements, away from the position in which they interfere with the release of the said mass-spring (vibrating) units, upon the application of the setback (firing) acceleration, or set forward acceleration, or the spinning of the projectile to which the power source is attached. In general, such locking mechanisms are among the least complex mechanisms with which the aforementioned desired locking and releasing functionalities can be performed. It is, however, appreciated by those skilled in the art that other acceleration or spin actuated locking mechanisms may also be employed, and that the use of the above rotating link (beam) type of locking mechanisms in the disclosed embodiments is not meant to exclude any other mechanisms that could provide such functionalities, including all those available in the art. 
     Another embodiment  130  is shown in the schematic of  FIG. 9 . The embodiment  130  is similar to the embodiment  10  of  FIG. 1 , except for its locking mechanism and method of its release. In the embodiment  130  shown in  FIG. 9 , the elements not indicated by numerals are identical to those shown in the schematic of  FIG. 1 . In the embodiment  130 , the locking mechanism consists of relatively rigid links  131  which are fixed to the munitions or the like structure  132  via joints  133 , which can be living joints. In the configuration shown in  FIG. 9 , the tips  134  of the locking links  131  hold the mass-spring unit of the mechanical reserve power source in its preloaded position as described for the embodiment  10  of  FIG. 1 . The locking links  131  can be attached together via a spring element  136  which is preloaded in tension so that the mass-spring unit of the mechanical reserve power source may not be accidentally released. The mass-spring unit can be released by applying external forces in the direction of the arrows  135  that would overcome the spring  136  preload and other resisting forces such as friction forces and separate the tips  134  of the links  131  apart to release the mass-spring unit of the mechanical reserve power source, allowing the mass-spring unit to begin to vibrate and generate electrical energy as previously described for the embodiment  10  of  FIG. 1 . The applied forces  135  are preferably either kept during the vibration of the mass-spring unit or are large enough to deform the links  131  and/or break the spring  136  or both to keep the links  131  from interfering with free vibration of the said mass-spring unit. 
     Another embodiment  140  is shown in the schematic of  FIG. 10 . The embodiment  140  is similar to the embodiment  10  of  FIG. 1 , except for its locking mechanism and method of its release. In the embodiment  140  shown in  FIG. 10 , the elements not indicated by numerals are identical to those shown in the schematic of  FIG. 1 . In the embodiment  140 , the locking mechanism consists of relatively rigid locking elements  141  which are provided with the guides  142  along which the can move back and forth in the direction of the arrow  143 . The embodiment  140  is fixed to the munitions or the like structure  144 . The guides  142  are also similarly fixed to the munitions or the like structure  144 . In the configuration shown in  FIG. 10 , the tips  145  of the locking elements  141  hold the mass-spring unit of the mechanical reserve power source in its preloaded position as described for the embodiment  10  of  FIG. 1 . The locking elements  141  are preferably provided with means such as friction or springs (not shown) within the guides  142  that would prevent them from accidentally releasing the mass-spring unit of the mechanical reserve power sources. The mass-spring unit of the mechanical reserve power source can then be released by displacing the locking elements  141  back away from the mass-spring unit, thereby allowing the mass-spring unit to begin to vibrate and generate electrical energy as previously described for the embodiment  10  of  FIG. 1 . Appropriate means such as friction forces or well known locking elements (not shown) are preferably provided to keep the locking elements  141  from interfering with free vibration of the said mass-spring unit. 
     In the embodiments  130  and  140 , the locking elements  131  and  141  may be actuated to release the mass-spring units of the mechanical reserve power sources by any external means depending on the application at hand, including the following: 
     a) Manually, by pulling a cable, lever or the like attached to the said locking elements. 
     b) By pulling a cable or the like attached on one end to the locking elements and on the other end to the structure of the system, e.g., an aircraft, from which the weapon to which the mechanical reserve power source is attached is released. 
     c) By spinning of the munition and the resulting centripetal forces. 
     It will be appreciated by those skilled in the art that many possible means can be used to actuate the locking mechanisms used in the various embodiments (for the embodiments shown in  FIGS. 1 ,  3 - 8 , in addition to the firing setback and set forward accelerations and spinning of the projectiles). One method of such actuation which is appropriate for many munitions is illustrated by its application to the embodiment  150  of  FIG. 11 . In this method, the locking mechanism is actuated by preloaded springs or the like by cutting a cable or moving a stop, manually or by certain externally applied force or via detonation of a properly positioned charge. An illustrative example of the application of this method to the embodiment  150  is shown in  FIG. 11 . The embodiment  150  is similar to the embodiment  10  of  FIG. 1 , except for its locking mechanism and method of its release. In the embodiment  150  shown in  FIG. 11 , the elements not indicated by numerals are identical to those shown in the schematic of  FIG. 1 . In the embodiment  150 , the locking mechanism consists of relatively rigid locking elements  151  which are fixed to the structure of the munitions  152  via joints  153 , which can be living joints. The embodiment  150  is fixed to the munitions or the like structure  152 . In the configurations shown in  FIG. 11 , the tips  154  of the locking elements  151  hold the mass-spring unit of the mechanical reserve power source in its preloaded position as shown in  FIG. 11  and described for the embodiment  10  of  FIG. 1 . The element  155 , which can be a cable or the like, and can be relatively inextensible, is used to connect the two locking elements by being fixed to each locking element  151  at points  156 , which may be an extension of the locking element  151 . The element (cable)  156  prevents the tips  154  of the locking elements  151  from being separated and release the mass-spring unit of the mechanical reserve power source. The springs  157 , preloaded in tension, are attached on one end to the locking elements  151  and fixed to the structure of the munitions  152  on the other end. The element  158  is intended to indicate a means of cutting the cable  156 , thereby allowing the springs  157  to pull back the locking elements and release the mass-spring unit of the mechanical reserve power source. For munitions, such means  158  may be a detonation charge that is initiated to cut the cable directly or by cutting or pulling of a pin holding a two piece cable together or the like as commonly known in the art. In certain munitions such as in small gravity dropped weapons, an “arming” wire attached to a pin holding a two piece cable together or the like may be used, which is pulled as the weapon is released to free the locking elements  151  to be pulled back by the springs  157 , thereby releasing the mass-spring unit of the mechanical reserve power source, thereby allowing the mass-spring unit to begin to vibrate and generate electrical energy as previously described for the embodiment  10  of  FIG. 1 . 
     The locking elements  151  can be provided with means such as friction in the joints  153 , however, springs  157  act to bias them away from the mass-spring unit and prevent them from interfering with free vibration of the said mass-spring unit. 
     It is noted that in the embodiments of  FIGS. 1-3  and  6 - 11 , the spring element may be of any type, such as helical or any other machined axial springs of appropriate pattern. In particular, machined springs with integrated mass element (as a separate section of the spring—similar to the mass elements shown in the aforementioned embodiments—or utilizing the effective mass of the spring itself) can be used to eliminate the need for mass-to-spring attachment procedures. In either case, the spring element can be resistant to lateral bending. For helical springs or helical-type machined springs, this can be accomplished by using helical springs with more than one strand, preferably at least three strands to make it resistant to bending in all lateral directions. 
     In the above embodiments, the mechanical energy is stored either in linear (such as helical) springs or in relatively flexible beams. The present mechanical reserve power sources may, however, be designed for rotational vibration of an inertia element (such as a wheel). The mechanical to electrical energy conversion can then be achieved using commonly used magnet and coil (dynamo type) or the like generators or as described for the previous embodiments, using piezoelectric elements. Alternatively, the stored mechanical energy in such mechanical reserve power sources may be transferred to a similar but continuously rotating wheel, essentially as kinetic energy, and using a magnet and coil (dynamo type) or the like generator that is attached (directly or through certain other mechanisms such as a gearing mechanism) to convert the kinetic energy to electrical energy. 
     One such embodiment  160  is shown in the schematic of  FIG. 12 . The mechanical reserve power source  160  consists of a wheel  161 , which is attached to the ground  166  (e.g., the structure of a projectile) by at least one shaft  162  via the bearing  163 , in which the at least one shaft  162  is free to rotate. The shaft  162  is fixed to the wheel. A torsional spring  164  is attached on one end to the wheel  161  (or the shaft  162 ) and on the other end  165  to the ground  166  via the piezoelectric element  173 . In this configuration, by rotating the wheel  161  in either direction, the torsional spring  164  is preloaded and stores certain amount of potential energy, and when the wheel is released, the wheel  161  and torsional spring  164  unit vibrates (oscillate back and forth) in rotation. 
     The embodiment is provided with a link  167 , which is attached to the support  169  by the rotary joint  168 . The support  169  is attached to the structure of the projectile  166 . When the link  167  is in the configuration shown in  FIG. 12 , its tip  171  can engage the extension  170  provided on the wheel  161 , preventing it from rotating further in the clockwise direction. Mechanical energy is stored in the torsional spring by rotating the wheel counterclockwise (or clockwise but the tip  171  would then need to be positioned on the opposite side of the extension  170 ) and bringing the tip  171  to the position shown in  FIG. 12  to prevent the wheel  161  from being released. The link  167  is preferably biased to prevent the wheel  161  from being released by minor vibrations and motions of the projectile, for example by friction at the joint  168  or by certain mechanical interference means such as by the engagement of protruding point and a matching dimple (not shown) on the engaging surface of the extension  170  and tip  171 . 
     A mass  172  is provided on the link  167 , as shown in  FIG. 12 , or integral therewith. Then during the firing of the projectile, the munitions structure  166  is accelerated in the direction  174 , causing the firing acceleration to act on the mass  172  (the link is considered to be balanced relative to the joint  168 ), thereby causing the link  167  to rotate clockwise. At some point, the tip  171  clears the extension  170  of the wheel  161 , thereby releasing the wheel. In general, link  167  can be rotated clockwise to release the wheel  161  and is kept away so that it would not interfere with their subsequent rotational motion of the wheel  161 . Once the wheel  161  is released, the mechanical potential energy stored in the torsional spring  164 , i.e., the mechanical potential energy stored in the mechanical reserve power sources  160 , is released. The released mechanical potential energy will then cause the wheel  161  and torsional spring  164  unit to begin to vibrate. The vibration will then apply a cyclic force to the piezoelectric stack  173 , thereby generating an electrical charge, which is then harvested and used directly or stored in certain electrical energy storage device such as a capacitor using electronic regulation and charging circuitry well known in the art. The piezoelectric stack element  173  is preferably preloaded in compression using one of the methods previously discussed so that it is not subjected to a substantial net tensile stress. 
     Alternatively, the mechanical reserve power source embodiment  160  of  FIG. 12  may be configured not to release during the firing setback but release during the firing set-forward, i.e., as a result of an acceleration of appropriate magnitude in the direction opposite to that of the arrow  174 . This is readily accomplished by preventing the link  167  from rotating in the counterclockwise direction, e.g., by providing a stop (not shown) under the link  167 . 
     It will also be appreciated by those skilled in the art that the mass  172  could have been placed on the link but on the opposite side of the joint  169 , indicated as the element  175  in dotted lines in  FIG. 12 . The resulting embodiment will then release the wheel  161  (activate the mechanical reserve power source  160 ) when subjected to setback acceleration. For activation under set-forward acceleration, the link  167  needs to be prevented from rotating in the clockwise direction during the setback acceleration (i.e., acceleration in the direction of the arrow  174 ), which is readily accomplished by placing the aforementioned stop (not shown) above the link  167 . 
     In the embodiment of  FIG. 12 , the mass element  172  and  175  are shown to be separate elements that are attached to the link  167 . However, the mass elements  172  and  175  can be integral parts of the link  167 . Similarly, the rotary  169  can be a living joint and be plastically deformed away from the wheel  161  following the release of the wheel  161  (activation of the mechanical reserve power source) so that it would not interfere with free vibration of the system. 
     In the embodiment of  FIG. 12 , once the wheel  161  is release and the wheel-torsional spring unit begins to vibrate, the mechanical energy stored in the torsional spring  161  is converted to electrical energy as previously described via the piezoelectric (stack) element  173 . Alternatively, the mechanical energy can be converted by commonly used magnet and coil generators (dynamos) or any other means of mechanical energy to electrical energy conversion known in the art. In the schematic of  FIG. 13 , indicated as the embodiment  180 , such a magnet and coil (dynamo) type generator  176  is shown to have been positioned along the shaft  162  and its outer housing grounded (attached to the projectile structure  166 ). As a result, as the wheel-torsional spring unit vibrates, the generator shaft is rotated back and forth, thereby allowing the generator  176  to generate electrical energy, i.e., convert the mechanical energy stored in the mechanical reserve power source to electrical energy. The harvested energy can then be used directly or stored in certain electrical energy storage device such as a capacitor using electronic regulation and charging circuitry well known in the art. 
     In the embodiment of  FIG. 13 , the wheel  161  and the generator  176  are separately attached to the shaft  162 . It will, however, be appreciated by those skilled in the art that the two units may be combined into a single unit with the wheel  161  constituting the rotor of the generator  176 . The extension  170  used for locking the preloaded torsional spring will still be fixed to the shaft  162  and the release link  167  would similarly be rotated by either the setback or set-forward acceleration of the projectile to release the shaft  162  and initiate the aforementioned rotational vibration of the wheel-torsional spring unit to generate electrical energy ( FIG. 12 ). 
     In the embodiments of  FIGS. 12 and 13 , the mass of the link  167  and the added mass elements  172  or  175  are shaped such that the center of mass of the resulting structure (link  167  and mass element  172  or  175 ) lies in line with the rotary joint  169 . As a result, if the projectile (indicated by its structure  166 ) is spinning during its flight (about the firing direction  174  about the longitudinal axis of the projectile), then the centripetal acceleration acting on the mass element  172  or  175  would substantially pass through the rotary joint  169 , and would thereby generate no moment about the said rotary joint  169  to tend to rotate the link and release the wheel  161 , i.e., activate the mechanical reserve power source  160  or  180 . 
     Alternatively, the release link assembly may be configured such that the inertia wheel  161  ( FIG. 12 ) is not released as a result of firing acceleration (setback and/or set-forward), but be released only due to the spinning of the projectile along its long axis. One such embodiment  200  is shown in the schematic of  FIG. 14 . In  FIG. 14 , all the elements except those of the locking/releasing mechanism (link  167  and associated components) are identical to those of the embodiment of  FIG. 12 ). In the embodiment  200 , the locking/releasing mechanism consists of a similar link element  202 , which is similarly attached to the projectile structure  201  via a support  203  by the rotary joint  204 . The tip  205  of the link  202  similarly engages the extension  170  of the wheel  161 , thereby preventing the preloaded torsional spring from unwinding. A mass  206  is also fixed to the link  202 . The mass of the link together with the mass of the element  206  are distributed such that the center of mass of their combined body is substantially located on a vertical line passing through the hinge  204 . As a result, the firing acceleration (setback or set-forward), which is in the direction of the arrow  207 , does not generate a torque on the link  202  and mass  206  assembly about the axis of rotation of the hinge  204 , and would thereby not act to rotate the link  202  to release the wheel  161 , i.e., to activate the mechanical reserve power source  200 . On the other hand, the spinning of the projectile (about the long axis of the projectile, i.e., about a vertical axis parallel to the direction of the arrow  207 ) would generate a net centrifugal force on the link  202  and when the rate of spinning is above a certain threshold, the torque generated by the centrifugal force about the axis of the hinge  204  overcomes the aforementioned friction and mechanical locking forces that are provided to prevent accidental rotation of the link  202 , and rotates the link  202  in the clockwise direction, thereby releasing the wheel  161 , i.e., activating the mechanical reserve power source  200 . As a result, the wheel-torsional spring unit begins to vibrate. The released mechanical energy can then be converted to electrical energy by either the piezoelectric (stack) element  173  or the magnet and coil generator  176  or both as was described in the previous embodiments. The harvested energy can then be used directly or stored in certain electrical energy storage device such as a capacitor using electronic regulation and charging circuitry well known in the art. 
     It will also be appreciated by those skilled in the art that the embodiments of  FIGS. 12-14  may be configured such that the resulting mechanical reserve power source is activated (i.e., the vibrating wheel is released) by, for example, both setback and set-forward accelerations; or by both setback acceleration and spinning of the projectile; or by both set-forward acceleration and spinning of the projectile; or any other similar combinations of events. 
     In addition, torsional springs are used in the embodiments of  FIGS. 12-14 . However, it is appreciated by those familiar with the art that any other type of spring elements such as helical springs, flexible bending type springs, and the like may also be used. In fact, the spring elements (for example of bending or similar types, attached on one end to the wheel and on the other end to the projectile structure) may even be integral to the wheel element of the disclosed mechanical reserve power sources. In a similar manner, any other type of elastic elements may be used in the previous embodiments. In fact, in certain applications, the structure of the projectile itself may be used (entirely or partially) as the elastic element of the mechanical reserve power source for any one of the disclosed embodiments. 
     In the embodiments of  FIGS. 13-14 , following the release of the inertia wheel  161 , the inertia wheel-torsional spring unit begins to undergo rotary vibration. The system mechanical energy is then transformed to electrical energy by the indicated piezoelectric elements and/or magnet and coil generator. In an alternative embodiment, the torsional spring is attached to the shaft  162  via a one-way clutch. The wheel  161  can then be rotated in the free direction of rotation of the one-way clutch to preload (wind) the torsional spring, and lock the wheel in place at its extension  170  as described for the embodiments of  FIGS. 13-14  by the tip  171  ( 205  in  FIG. 14 ) of the lever  167  ( 202  in  FIG. 14 ). Then upon the release of the inertia wheel  161  (due to firing accelerations and/or spinning of the round, etc.), the potential energy stored in the torsional spring is transferred to the inertia wheel as kinetic energy and the wheel will start to continuously rotate at certain angular velocity. The magnet and coil type generator  176  can then be used to convert the mechanical (kinetic) energy stored in the inertia wheel  161  to electrical energy. The harvested energy can then be used directly or stored in certain electrical energy storage device such as a capacitor using electronic regulation and charging circuitry well known in the art. 
     It is noted that in the embodiments of  FIGS. 1 ,  3 - 8  and  12 - 14 , the mechanical reserve power sources are activated by the firing or centripetal acceleration acting on an inertia element. It will be, however, appreciated by those skilled in the art that release mechanisms of these embodiments could also be activated manually, or by other externally applied forces, such as by a mechanism shown in  FIG. 11 , i.e., by providing preloaded springs that are released to actuate the release mechanisms of the mechanical reserve power sources methods such as those described for releasing the cable  155  in the embodiment of  FIG. 11 .