Abstract:
A method for harvesting energy from an input deflection. The method including: storing mechanical potential energy in at least one spring element resulting from an acceleration of a mass connected to the at least one spring element; transferring the stored potential energy to a flexible element to deflect the flexible element; and converting the deflection of the flexible element to electrical energy by compressing at least one piezoelectric element due to the deflection.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit to U.S. Provisional Application No. 61/444,746 filed on Feb. 20, 2011, the contents of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     1. Field 
     The present application relates generally to energy harvesting devices, and more particularly, to energy-harvesting from input impulse with motion doubling mechanism for generating power from mortar tube firing impulses and the like. 
     2. Prior Art 
     Providing a safe, lightweight, and reliable source of electrical power for ancillary electronic systems for infantry weapon platforms is an ongoing supply chain problem, especially when operating in remote areas. Given recent advances in low-power electronics for components such as fire control systems, energy harvesting technology is seeing renewed attention as a replacement or supplement for power sources such as batteries in many systems. 
     Harvesting energy from the environment to power various devices is not new. The earliest such devices can probably be said to be windmills. The first windmills were developed to mechanize the tasks of grain-grinding and water-pumping. The earliest-known design is the vertical axis system developed in Persia circa 500-900 CE. The first documented design is also of Persian origin, this one with vertical sails made of bundles of reeds or wood which were attached to the central vertical shaft by horizontal struts. 
     In recent years, particularly following the development of low-power electronics, sensors and wireless communications devices, electrical energy generators that harvest energy from the environment have seen renewed attention. In this area, piezoelectric materials have been widely used to generate electrical energy from ambient vibration. Such electrical energy generators and methods of collecting, regulating and storing the generated electrical energy have been the subject of numerous studies. 
     Harvesting energy from transient high-shock events such as gun-firing and converting it to usable electrical energy is also not new. The developed piezoelectric power generators are generally suitable for applications with low to medium power requirements. To date, several such piezoelectric-based power generators have been developed and tested for fuzing applications, which are designed to produce up to 2 Joules of energy from firing accelerations of 20,000 G&#39;s-40,000 G&#39;s. 
     SUMMARY 
     The energy harvesting devices and methods disclosed herein harvest mechanical energy from a recoil perturbation of a weapon platform and convert that energy to electrical power using piezoelectric elements. First disclosed are devices and methods that increase the performance and efficiency of devices which harvest energy from high-shock transient impulses such as gun-firing. Next disclosed is the application of such technology as a power source for the fire control system for a mortar, such as a 60 mm mortar. Still further disclosed are general concepts of energy harvesting from the high-shock environment augmented with considerations of the nature of the recoil perturbation of a particular weapon platform, as well as mathematical simulation results of performance and efficiency. 
     It should be noted that the disclosed generators are not limited to harvesting energy from a weapon recoil perturbation, but that the devices may be parameterized to harvest energy from almost any environment which is characterized by transient or steady-state shock or vibration/oscillation on a wide range of machinery and systems. The disclosed generators may also be configured for a rotational acceleration or rotating input. 
    
    
     
       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 conventional generator which employs a spring-mass system directly connected to a piezoelectric element. 
         FIG. 2   a  illustrates first embodiment of an energy harvesting generator. 
         FIG. 2   b  illustrates a sectional view of the energy harvesting generator of  FIG. 2   a.    
         FIG. 2   c  illustrates the energy harvesting generator of  FIG. 2   a  where a large axial tension is induced in a flexible element. 
         FIG. 3  illustrates the energy harvesting generator of  FIG. 2   a  coupled to a mass/spring system. 
         FIG. 4  illustrates a variation of the system of  FIG. 3  in which a primary spring stores an initial parcel of mechanical potential energy and a secondary spring transmits the force to the piezoelectric elements. 
         FIG. 5   a  illustrates an embodiment of an energy harvesting generator configured for a mortar tube. 
         FIG. 5   b  illustrates a close-up view of the energy harvesting generator of  FIG. 5   a  without a cover thereof. 
         FIG. 5   c  illustrates the energy harvesting generator of  FIG. 5   b  shown without the annular mass. 
         FIG. 6  illustrates a plot of the generator output of  FIGS. 5   a - 5   c  showing spring force, transmitted piezoelectric force, and rectified open-circuit piezoelectric voltage with energy extraction at voltage extremes. 
     
    
    
     DETAILED DESCRIPTION 
     Use of a Motion-Doubling Mechanism to Transmit Forces to the Piezoelectric Element 
     Generators which employ a spring-mass system directly connected to a piezoelectric element have been employed with great success to harvest energy from various acceleration events such as the firing of a projectile from a gun. Such a system  100  is shown schematically in  FIG. 1  where the basic principle is that the mass element  102  reacts to an (upward) acceleration event (A), deflecting a generator spring  104  and storing some amount of mechanical potential energy. After the acceleration event subsides, the mass/spring unit  106  (consisting of the spring  104  and mass element  102 ) is free to vibrate at its natural frequency and it applies a cyclical force to an piezoelectric element  108  where the mechanical strain is converted into electrical potential and may be extracted for storage or immediate use. 
     The energy harvesting generators described below utilize a novel approach to transmitting the spring force to the piezoelectric element. Such energy harvesting devices exploit a large axial tension induced in a cable or similar flexible element whose ends are fixed and whose midpoint is deflected laterally by a relatively small force. 
     Referring now to  FIGS. 2   a  and  2   b , a first embodiment of an energy harvesting generator  200  is illustrated where the transverse force and resultant axial tension applies a compressive load to stacks of piezoelectric elements  202  on either end of the generator  200 . The generator  200  includes a pair of such piezoelectric elements  202 , each sandwiched between a base member  204  connected to a base structure  206  (such as a base structure of a mortar) and a stop clamp  208 . A flexible element  210  (such element is described a “flexible” in that it is flexible in bending but relatively inextensible longitudinally, i.e., along its length), such as a cable or wire rope, is disposed between the pair of piezoelectric elements  202 . The flexible element  210  is loosely fitted in a bore  202   a  in each of the piezoelectric elements  202  and corresponding bore  204   a  in each of the base members  204 , as shown in  FIG. 2   b . The flexible element  210  is fixed to each of the stop clamps  208  by any means known in the art, such as welding or by being crimped about a bore  208   a  in the stop clamp  208 . The flexible element  210  can be fixed to the stop clamps  208  such that no or very little force is applied to the piezoelectric elements  202  or the flexible element  210  can be fixed to the stop clamps  208  such that the same is in tension so as to preload the piezoelectric elements  202 . As can be seen by  FIG. 2   c , an axial tension is induced along the length of the flexible element  210  whose ends are fixed by the stop clamps  208  by laterally deflecting (D) a midpoint of the flexible element  210  by a force (F). As will be apparent to those skilled in the art, a large axial tension can be induced in the flexible element (and applied to the piezoelectric elements due to the stop clamp  208  compressing the piezoelectric elements  202  against the base member  204 ) by application of a relatively small force (F). 
     Referring now to  FIG. 3 , there is illustrated the generator  200  of  FIG. 2   a  mated to a mass/spring system  300  to configure a shock/vibration energy harvesting system, which in this case is configured for a lateral acceleration event (A). The mass/spring system  300  includes a mass  302  movable along the direction of the acceleration (A) relative to the base structure  206 . The mass  302  is shown schematically in  FIG. 3  as being movable on wheels  304  or other rolling elements. However, the mass  302  can be movable by any other means, such as by sliding. The mass  302  is coupled to a compression spring  304  or other spring element at a first end  304   a  of the spring  304  and to the flexible element  210  at a second end  304   b . The second end  304   b  of the spring  304  can be attached directly to the flexible element  210  or indirectly through a fastening element  306 . The fastening element  306  can be attached to the flexible element  210  by any means know in the art, such as by crimping, welding and the like and can be positioned at any point along the axial length of the flexible element  210 , such as at a midpoint between the two stop clamps  208 . 
     As described above with regard to  FIG. 1 , the acceleration event (A) causes the mass  302  to compress the spring  304  to store mechanical potential energy therein. After the acceleration event (A) subsides, the mass/spring unit (consisting of the spring  304  and mass element  302 ) is free to vibrate at its natural frequency and applies a cyclical force to the flexible element  210 , which in turn applies a cyclic compressive force to the piezoelectric elements  202  where the mechanical strain is converted into electrical potential and may be extracted for storage or immediate use. 
     Such a system  200 , 300  has the advantage of a force-multiplying effect as discussed above. The relatively small spring force (F) and resultant lateral deflection (D) of the flexible element  210  is transferred to a much larger axial tension. For example, if the length of the flexible element is 2 L as shown in  FIG. 2   c , then the deflection angle Θ is tan −1  (D/L). Then the tensile force T generated in the flexible element (cable)  210  due to the applied force F is T=F/(2 Sin Θ). Therefore, if the cable  210  is relatively inextensible longitudinally and is initially preloaded in tension, then a relatively small force F applied laterally as shown in  FIG. 3   c , would cause a relatively small deflection D, thereby a small deflection angle Θ, thereby generate a relatively large tensile force T to balance the applied lateral force F. For example, if the deflection angle Θ=10 degrees, then the tensile force T becomes 1/(2 Sin Θ)=1/(2 Sin 10 deg.)=2.9 times larger than the applied lateral force F. Similarly, for a deflection angle of 5 degrees, the induced (axial) tensile force T is about 11.4 times the applied lateral applied force F. A proof-of-concept prototype indicates that deflection angles between 4 and 11 degrees are realistic for practical devices using “aircraft cable” wire rope as the flexible element  210 . 
     Another advantage is a doubling of the frequency of compressive forces applied to the piezoelectric elements  202  as compared to the direct mass-spring-piezoelectric generators known in the art, and discussed above with regard to  FIG. 1 . In the system  200 ,  300  illustrated in  FIG. 3 , each half-cycle of the mass/spring unit  302 / 304  motion (vibration) produces a full-cycle compression and unloading to the piezoelectric stacks (thus, each full cycle of the mass/spring unit  302 / 304  motion produces two full-cycles compression and unloading to the piezoelectric stacks). It is appreciated by those skilled in the art that in mass/spring unit based generators such as the one shown in  FIG. 1 , a substantial and sometimes significantly more than half of the stored mechanical energy in the unit spring is lost due to damping losses in the spring material. The aforementioned doubling motion characteristic of the present invention reduces the number of cycles of mass/spring unit vibration as the mechanical energy is harvested by the piezoelectric element of the said generator. As a result, essentially half as much mechanical energy is lost due to spring material damping, thereby significantly increasing the efficiency of such mass/spring based energy harvesting generators. For example, if around 50% of the mechanical energy is lost due to spring element damping, a value that is probably more on the low side than the high, the efficiency with which such generators would harvest the stored mechanical energy is essentially increased by more than 25%. 
     These advantages, when combined, increase the efficiency of mass/spring type piezoelectric generators by an order of magnitude or more. The force-multiplying effect increases the energy extracted from the mechanical system quadratically since the electrical potential developed in the piezoelectric element is generally a linear function of applied force and the energy stored in the piezoelectric element (for extraction) is proportional to the square of the voltage. While all piezoelectric generators should be designed to maximize the voltage on the piezoelectric elements, the system  200 ,  300  illustrated in  FIG. 3  allows for much larger piezoelectric elements to be used—ones which in order to reach their maximum voltage would require more force than the acceleration-loaded mass/spring system could provide directly. 
     The frequency-doubling may seem at first to only extract the energy from the spring-mass system faster and have no net benefit on total harvested energy. However, as previously indicated and as mathematical models of various mass/spring type piezoelectric generators have also indicated, the bulk of the losses in the conversion of the mechanical energy stored in the spring element of mass/spring units as they vibrate are from structural damping in the generator spring, which is a function of the number of cycles of oscillation of the mass/spring system. By doubling the rate at which compressive forces are applied to the piezoelectric elements while holding the natural frequency of the mass/spring system constant, the losses in the system is thereby reduced considerably. 
     Use of Separate Generation and Transmission Springs in a Discontinuous System 
     As discussed above, maximization of the voltage on the piezoelectric elements  202  achieves considerable electrical energy output. In the systems discussed thus far, the spring which stores the initial amount of mechanical potential energy is connected to the piezoelectric element through a continuous system. Therefore, as the vibration amplitude of the mass/spring system decays, so does the force applied to the piezoelectric element, no matter the transmission mechanism. 
     Referring now to  FIG. 4 , there is illustrated a system  400 , which can provide a constant force amplitude to the piezoelectric elements  202  over many cycles by employing two separate compression springs (or other spring elements): a primary spring  304  to store the initial parcel of mechanical potential energy, and a secondary spring  402  to transmit the force to the piezoelectric elements  202  (in this case through a motion-doubling mechanism). 
     In the system  400  of  FIG. 4 , the primary spring  304 , having a higher spring constant than a spring constant of the secondary spring, when deflected by the acceleration event (A), exerts a force much greater than the piezoelectric element could withstand without failing. The secondary spring  402  and its accompanying mechanical stop  404  act to transmit the force to the piezoelectric element while limiting the magnitude of the force to the upper limit which the piezoelectric element can withstand due to the mass stop  404  limiting the amount of deflection (D) of the flexible element  210  to produce the maximum voltage. As the vibration amplitude of the primary spring  304  decays, the force transmitted to the piezoelectric elements  202  will remain at the upper limit for many cycles (until the force amplitude of the primary spring  304  drops below the upper limit of the piezoelectric elements  202 ). 
     Use of a Motion-Doubling Mechanism in an Energy Harvester for a Mortar Tube 
     Referring now to  FIGS. 5   a - 5   c , the systems disclosed above may be employed in an electrical energy harvester system  500  for a mortar tube  502 . When a mortar is fired, the perturbation of the weapon platform may be used as a mechanical input to a mass/spring piezoelectric generator. As an example, when fired, a 60 mm mortar tube undergoes a chaotic lateral vibration with a magnitude of several thousand G&#39;s. The non-directional nature of the vibration and the geometry of the tube are well-suited to a generator design such as the one shown in  FIGS. 5   a - 5   c .  FIG. 5   a  shows such system  500  with a covering  501 , which is removed for clarity in  FIGS. 5   b  and  5   c.    
     In the system of  FIGS. 5   a - 5   c , a single annular mass  504  element is connected to flexible elements  210  (e.g., three force transmission cables) through generator springs  506  (or other spring elements). The single annular mass  504  can be replaced with separate masses corresponding to each of the spring  506 . The springs  506 , shown in  FIG. 5   c  without the annular mass  504  for clarity, are connected at one end to the flexible element  210  and at another end to the mass  504 . The springs  506  can be accommodated in cut-out portions  508  formed in the annular mass. When a lateral acceleration from any direction is experienced, the deflected mass  504  and springs  506  will store an initial amount of mechanical potential energy, as discussed above with regard to  FIGS. 2   a - 2   c . After the perturbation subsides, the system  500  will vibrate and pass cyclic transverse forces to the three flexible elements  210 , such as at a midpoint thereof. As discussed above, the force multiplication and frequency doubling effects of the cables will allow for highly efficient extraction of energy from the system. 
     Mathematical simulations of the system of  FIGS. 5   a - 5   c  have been performed using first-iteration, un-optimized figures for parameters such as mass, spring rate, and piezoelectric characteristics. Consider a generator which occupies an annulus with an inside diameter of 70 mm, an outside diameter of 120 mm, and a length of 100 mm. The simulation was conducted using parameters of off-the-shelf components which physically fit into the design volume generally as shown in  FIG. 5   a  (by cover  201 ). For such simulation, a ⅓ equivalent mass paired to one of the three generator modules is considered. For this “⅓-system”, a mass of 500 grams is mated to an equivalent generator spring which has a rate of 112 N/mm and a maximum deflection of about 4 mm. In response to the recoil perturbation, such a system would store an initial mechanical potential energy of approximately 900 mJ, and afterward oscillate with a frequency of about 75 Hz. Several cycles of such a generator output are plotted in  FIG. 6 , which shows the spring force, transmitted piezoelectric force, and the rectified open-circuit piezoelectric voltage with energy extraction at voltage extremes. The energy output for the modeled sub-system was approximately 350 mJ, which indicates an overall efficiency of about 39% based on the 900 mJ initially stored in the generator springs. 
     The particular geometry of the single annular mass element and the discrete generator modules will not allow for a full 4 mm deflection of all three generator modules; the initial stored energy actually being approximately 1.3 J for the system as a whole. Also, the chaotic nature of the recoil perturbation will further muddy the waters in trying to estimate the energy output of the device, but as a rough-order-of-magnitude, designs such as this can clearly provide electrical energy in the hundreds of millijoules. Considering this, and some initial testing of a proof-of-concept prototype, efficiencies of at least between 20% and 40% are achievable. 
     Although not shown, a discontinuous dual-stage generator such as the concept illustrated in  FIG. 4  may be employed in the system of  FIGS. 5   a - 5   c  to further increase efficiency. 
     Thus, the energy harvesting power source generators disclosed herein provide a novel motion-doubling mechanism forming a two-stage force transmission system. Such features represent major advances in the potential performance and efficiency of piezoelectric type energy harvesters, thereby also expanding the potential field of application of such technology. 
     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.