Abstract:
An inertial igniter including: a body having a base; a striker release element rotatably disposed on the body, the striker release element having a first surface; a first biasing element for biasing the striker release element away from the base; a striker mass rotatably disposed on the base along a second axis, the striker mass having a second surface corresponding to the first surface of the striker release element, the first surface obstructing rotation of the striker mass; and a second biasing element for biasing the striker mass such that the second surface is biased towards the first surface; wherein when the body experiences an acceleration profile of a predetermined magnitude and duration, the striker release element rotates towards the base to release an engagement between the first and second surfaces and allow the striker mass to rotate under a biasing force of the second biasing element.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates generally to mechanical inertial igniters, and more particularly to compact, low-volume, reliable and easy to manufacture mechanical inertial igniters and ignition systems for thermal batteries and the like used in munitions with relatively short duration firing setback acceleration (shock). 
         [0003]    2. Prior Art 
         [0004]    Thermal batteries represent a class of reserve batteries that operate at high temperature. 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. The most common internal pyrotechnic is a blend of Fe and KClO 4 . Thermal batteries utilize a molten salt to serve as the electrolyte upon activation. The electrolytes are usually mixtures of alkali-halide salts and are used with the Li(Si)/FeS 2  or Li(Si)/CoS 2  couples. Some batteries also employ anodes of Li(Al) in place of the Li(Si) anodes. Insulation and internal heat sinks are used to maintain the electrolyte in its molten and conductive condition during the time of use. Reserve batteries are inactive and inert when manufactured and become active and begin to produce power only when they are activated. 
         [0005]    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. 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 or semi-automatically. Other manufacturing processes have also been recently developed that are more amenable to automation. The batteries are encased in a hermetically-sealed metal container that is usually cylindrical in shape. Thermal batteries, however, have the advantage of very long shelf life of up to 20 years that is required for munitions applications. 
         [0006]    Thermal batteries generally use some type of igniter to provide a controlled pyrotechnic reaction to produce output gas, flame or hot particles to ignite the heating elements of the thermal battery. There are currently two distinct classes of igniters that are available for use in thermal batteries. The first class of igniter operates based on electrical energy. Such electrical igniters, however, require electrical energy, thereby requiring an onboard battery or other power sources with related shelf life and/or complexity and volume requirements to operate and initiate the thermal battery. The second class of igniters, commonly called “inertial igniters”, operates based on the firing acceleration. The inertial igniters do not require onboard batteries for their operation and are thereby often used in high-G munitions applications such as in gun-fired munitions and mortars. 
         [0007]    In general, the inertial igniters, particularly those that are designed to operate at relatively low firing setback or the like acceleration (shock) levels, have to be provided with the means for distinguishing events such as accidental drops or explosions in their vicinity from the firing acceleration levels above which they are designed to be activated. This means that safety in terms of prevention of accidental ignition is one of the main concerns in inertial igniters. 
         [0008]    The need to differentiate accidental and other so-called no-fire events from the so-called all-fire event, i.e., the firing setback acceleration (shock) event necessitates the employment of a safety system which is capable of allowing initiation of the inertial igniter only when the inertial igniter is subjected to the impulse level threshold corresponding to or above the minimum all-fire impulse levels. The safety mechanism can be thought of as a mechanical delay mechanism, after which a separate initiation system is actuated or released to provide ignition of the inertial igniter pyrotechnics. An inertial igniter that combines such a safety system with an impact based initiation system and its alternative embodiments are described herein. 
         [0009]    Inertia-based igniters must therefore comprise two components so that together they provide the aforementioned mechanical safety (delay mechanism) and to provide the required striking action to achieve ignition of the pyrotechnic element(s) of the inertial igniter. The function of the safety system (mechanism) is to hold the striker element fixed to the igniter structure until the inertial igniter is subjected to a high enough acceleration level with long enough duration, i.e., to a prescribed impulse level threshold, corresponding to the firing setback acceleration event. The prescribed impulse level threshold requirement is generally accompanied also with a minimum acceleration level requirement to ensure that the inertial igniter is safe, i.e., the striker element stays fixed to the inertial igniter structure, when subjected to relatively low acceleration levels for relatively long duration. Once the all-fire event, i.e., the said minimum acceleration level and the prescribed impulse level threshold has been reached, the said safety system (mechanism) releases the striker element, allowing it to accelerate toward its target. The ignition itself may take place as a result of striker impact, or simply contact or proximity. For example, the striker may be akin to a firing pin and the target akin to a standard percussion cap primer. Alternately, the striker-target pair may bring together one or more chemical compounds whose combination with or without impact will set off a reaction resulting in the desired ignition. 
         [0010]    A schematic of a cross-section of a conventional thermal battery and inertial igniter assembly is shown in  FIG. 1 . In thermal battery applications, the inertial igniter  10  (as assembled in a housing) is generally positioned above the thermal battery housing  11  as shown in  FIG. 1 . Upon ignition, the igniter initiates the thermal battery pyrotechnics positioned inside the thermal battery through a provided access  12 . The total volume that the thermal battery assembly  16  occupies within munitions is determined by the diameter  17  of the thermal battery housing  11  (assuming it is cylindrical) and the total height  15  of the thermal battery assembly  16 . The height  14  of the thermal battery for a given battery diameter  17  is generally determined by the amount of energy that it has to produce over the required period of time. For a given thermal battery height  14 , the height  13  of the inertial igniter  10  would therefore determine the total height  15  of the thermal battery assembly  16 . To reduce the total volume that the thermal battery assembly  16  occupies within a munitions housing, it is therefore important to reduce the height of the inertial igniter  10 . This is particularly important for small thermal batteries since in such cases the inertial igniter height with currently available inertial igniters can be almost the same order of magnitude as the thermal battery height. 
         [0011]    The isometric cross-sectional view of a currently available inertia igniter is shown in  FIG. 2 , referred to generally with reference numeral  200 . The full isometric view of the inertial igniter  200  is shown in  FIG. 3 . The inertial igniter  200  is constructed with igniter body  201 , consisting of a base  202  and at least three posts  203 . The base  202  and the at least three posts  203 , can be integrally formed as a single piece but may also be constructed as separate pieces and joined together, for example by welding or press fitting or other methods commonly used in the art. The base  202  of the housing can also be provided with at least one opening  204  (with a corresponding opening(s) in the thermal battery—not shown) to allow ignited sparks and fire to exit the inertial igniter and enter into the thermal battery positioned under the inertial igniter  200  upon initiation of the inertial igniter pyrotechnics  215 , or percussion cap primer when used in place of the pyrotechnics  215  (not shown). Although illustrated with the opening  204  in the base, the opening (or openings) can alternatively be formed in a side wall or in the striker mass as described in U.S. Patent Application Publication No. 2011/0171511 filed on Jul. 13, 2010, the entire contents thereof is incorporated herein by reference. 
         [0012]    A striker mass  205  is shown in its locked position in  FIG. 2 . The striker mass  205  is provided with guides for the posts  203 , such as vertical surfaces  206 , that are used to engage the corresponding (inner) surfaces of the posts  203  and serve as guides to allow the striker mass  205  to ride down along the length of the posts  203  without rotation with an essentially pure up and down translational motion. 
         [0013]    In its illustrated position in  FIGS. 2 and 3 , the striker mass  205  is locked in its axial position to the posts  203  by at least one setback locking ball  207 . The setback locking ball  207  locks the striker mass  205  to the posts  203  of the inertial igniter body  201  through the holes  208  provided in the posts  203  and a concave portion such as a dimple (or groove)  209  on the striker mass  205  as shown in  FIG. 2 . A setback spring  210 , which is preferably in compression, is also provided around but close to the posts  203  as shown in  FIGS. 2 and 3 . In the configuration shown in  FIG. 2 , the locking balls  207  are prevented from moving away from their aforementioned locking position by the collar  211 . The setback spring  210  is preferably a wave spring with rectangular cross-section. The collar  211  is usually provided with partial guide  212  (“pocket”), which are open on the top as indicated by the numeral  213 . The guide  212  may be provided only at the location of the locking balls  207  as shown in  FIGS. 2 and 3 , or may be provided as an internal surface over the entire inner surface of the collar  211  (not shown). 
         [0014]    The collar  211  rides up and down on the posts  203  as can be seen in  FIGS. 2 and 3 , but is biased to stay in its upper most position as shown in  FIGS. 2 and 3  by the setback spring  210 . The guides  212  are provided with bottom ends  214 , so that when the inertial igniter is assembled as shown in  FIGS. 2 and 3 , the setback spring  210  which is biased (preloaded) to push the collar  211  upward away from the igniter base  201 , would “lock” the collar  211  in its uppermost position against the locking balls  207 . As a result, the assembled inertial igniter  200  stays in its assembled state and would not require a top cap to prevent the collar  211  from being pushed up and allowing the locking balls  207  from moving out and releasing the striker mass  205 . 
         [0015]    In the inertial igniters of the type shown in  FIGS. 2 and 3 , a one part pyrotechnics compound  215  (such as lead styphnate or other similar compound) is used as shown in  FIG. 2 . The striker mass  205  is usually provided with a relatively sharp tip  216  and the igniter base surface  202  is provided with a protruding tip  217  which is covered with the pyrotechnics compound  215 , such that as the striker mass  205  is released during an all-fire event and is accelerated down (opposite to the arrow  218  illustrated in  FIG. 2 ), impact occurs mostly between the surfaces of the tips  216  and  217 , thereby pinching the pyrotechnics compound  215 , thereby providing the means to obtain a reliable initiation of the pyrotechnics compound  215 . Alternatively, a two-part pyrotechnics compound consisting, for example, one being based on potassium chlorate used in place of the pyrotechnics  215  and the other based on red phosphorous which is positions over a (generally larger) tip  216  of the striker mass  206 , may be used. In another alternative design, instead of using the pyrotechnics compound  215 ,  FIG. 2 , a percussion cap primer or the like (not shown) is used. In such inertial igniters, the tip  216  of the striker mass  205  is appropriately sized for initiating the percussion cap primer being used. 
         [0016]    The basic operation of the inertial igniter  200  shown in  FIG. 2  and is as follows. Any non-trivial acceleration in the axial direction  218  which can cause the collar  211  to overcome the resisting force of the setback spring  210  will initiate and sustain some downward motion of the collar  211 . The force due to the acceleration on the striker mass  205  is supported at the dimples  209  by the locking balls  207  which are constrained inside the holes  208  in the posts  203 . If an acceleration time in the axial direction  218  imparts a sufficient impulse to the collar  211  (i.e., if an acceleration time profile is greater than a predetermined threshold), it will translate down along the axis of the assembly until the setback locking balls  205  are no longer constrained to engage the striker mass  205  to the posts  203 . If the acceleration event is not sufficient to provide this motion (i.e., the acceleration time profile provides less impulse than the predetermined threshold), the collar  211  will return to its start (top) position under the force of the setback spring  210 . 
         [0017]    Assuming that the acceleration time profile was at or above the specified “all-fire” profile, the collar  211  will have translated down past the locking balls  207 , allowing the striker mass  205  to accelerate down towards the base  202 . In such a situation, since the locking balls  207  are no longer constrained by the collar  211 , the downward force that the striker mass  205  has been exerting on the locking balls  207  will force the locking balls  207  to move outward in the radial direction. Once the locking balls  207  are out of the way of the dimples  209 , the downward motion of the striker mass  205  is no longer impeded. As a result, the striker mass  205  moves downward, causing the tip  216  of the striker mass  205  to strike the pyrotechnic compound  215  on the surface of the protrusion  217  with the requisite energy to initiate ignition. 
         [0018]    In the inertial igniter  200  of  FIGS. 2 and 3 , following ignition of the pyrotechnics compound  215 , the generated flames and sparks are designed to exit downward through the opening  204  to initiate the thermal battery below. Alternatively, if the thermal battery is positioned above the inertial igniter  200 , the opening  204  can be eliminated and the striker mass could be provided with at least one (not shown) to guide the ignition flame and sparks up through the striker mass  205  to allow the pyrotechnic materials (or the like) of a thermal battery (or the like) positioned above the inertial igniter  200  to be initiated. 
         [0019]    In the inertial igniter  200  of  FIGS. 2 and 3 , by varying the mass of the striker  205 , the mass of the collar  211 , the spring rate of the setback spring  210 , the distance that the collar  211  has to travel downward to release the locking balls  207  and thereby release the striker mass  205 , and the distance between the tip  216  of the striker mass  205  and the pyrotechnic compound  215  (and the tip of the protrusion  217 ), the designer of the disclosed inertial igniter  200  can match the all-fire and no-fire impulse level requirements for various applications as well as the safety (delay or dwell action) protection against accidental dropping of the inertial igniter and/or the munitions or the like within which it is assembled. 
         [0020]    Briefly, the safety system parameters, i.e., the mass of the collar  211 , the spring rate of the setback spring  210  and the dwell stroke (the distance that the collar  210  has to travel downward to release the locking balls  207  and thereby release the striker mass  205 ) must be tuned to provide the required actuation performance characteristics. Similarly, to provide the requisite impact energy, the mass of the striker  205  and the aforementioned separation distance between the tip  216  of the striker mass and the pyrotechnic compound  215  (and the tip of the protrusion  217 ) must work together to provide the specified impact energy to initiate the pyrotechnic compound when subjected to the remaining portion of the prescribed initiation acceleration profile after the safety system has been actuated. 
         [0021]    In general, the required acceleration time profile threshold for inertial igniter initiation, i.e., the so-called all-fire condition, is described in terms of an acceleration pulse of certain amplitude and duration. For example, the all-fire acceleration pulse may be given as being 1000 G for 15 milliseconds. The no-fire (no-initiation) condition may be indicated similarly with certain acceleration pulse (or half-sine) amplitude and duration. For example, the no-fire condition may be indicated as being an acceleration pulse of 2000 G for 0.5 milliseconds. Other no-fire conditions may include transportation induced vibration, usually around 10 G with a range of frequencies. 
         [0022]    It is appreciated by those skilled in the art that when the inertial igniter  200  of  FIGS. 2 and 3  is subjected to the aforementioned all-fire acceleration profile threshold, the collar  211  is first caused to be displaced downward under the force caused by the acceleration in the direction of the arrow  218  acting on the inertia (mass) of the collar  211 , until the striker mass  205  is released as was described above and accelerated downward to towards the base  202  of the inertial igniter until the tip  216  of the striker mass  205  strikes the pyrotechnic material  215  over the protruding tip  217  and causing it to ignite. It is also appreciated by those skilled in the art that the process of downward travel of the collar  211  takes a certain amount of time, hereinafter indicated as Δt 1 , the amount of which is dependent on the mass of the collar  211  and the aforementioned preloading level of the compressive spring  210  and the distance that it has to travel downward before the balls  207  and thereby the striker mass  205  is released. Similarly, once the striker mass  205  is released, the process of downward travel of the striker mass  205  until its tip  216  strikes the pyrotechnic material  215  over the protruding tip  217  takes a certain amount of time for, hereinafter indicates as Δt 2 , the amount of which is dependent on the level of acceleration in the direction of the arrow  218 . 
         [0023]    In addition, in recent years new improved chemistries and manufacturing processes have been developed that promise the development of lower cost and higher performance thermal batteries that could be produced in various shapes and sizes, including their small and miniaturized versions. However, inertial igniters are relatively large and not suitable for small and low power thermal batteries, particularly those that are being developed for use in miniaturized fuzing, future smart munitions, and other similar applications. This is general the case for munitions with relatively low firing setback acceleration, particularly those in which the firing setback acceleration pulse (shock) has relatively short duration. 
         [0024]    It is therefore appreciated by those skilled in the art that the duration of the all fire acceleration must at least be the sum of the above two time periods Δt 1  and Δt 2 , hereinafter indicated as Δt=Δt 1 +Δt 2 . For example, for the aforementioned case of all-fire (setback) acceleration being 1000 G for 15 milliseconds, the total time Δt must be less than the indicated acceleration duration of 15 milliseconds. 
         [0025]    In certain applications, the aforementioned total time Δt is small enough that even by optimizing the parameters design of the inertial igniter of the type shown in  FIGS. 2 and 3  to minimize the required aforementioned time periods Δt 1  and Δt 2 , the required total time Δt cannot be reduced to below the all-fire acceleration period. 
         [0026]    In certain other case, due to the small size or geometry of the thermal battery or the like, the height of the inertial igniter that can be used is so small that the striker mass  205  upon its release does not have enough distance to travel downward to gain enough velocity (i.e., enough kinetic energy) before its tip  216  strikes the pyrotechnic material  215  over the protruding tip  217  in order to be able to cause the pyrotechnic material  215  to be reliably ignited. 
       SUMMARY OF THE INVENTION 
       [0027]    A need therefore exists for novel miniature inertial igniters that can be used in munitions or the like for initiation of pyrotechnic materials in thermal batteries or the like in which the aforementioned all-fire acceleration profile is very short in duration as is described above for inertial igniters of the type shown in  FIGS. 2 and 3  to be used. 
         [0028]    A need also exists for small inertial igniters that can initiate thermal batteries used in munitions with relatively low firing setback acceleration levels that may also be of short duration. 
         [0029]    There is also a need for inertial igniters that can be used to initiate thermal batteries or the like in munitions or the like when the height available in munitions is too small as is described above for inertial igniters of the type shown in  FIGS. 2 and 3  to be used. 
         [0030]    Such inertial igniters must be safe and do not initiate when subjected no-fire conditions. In general, such inertial igniters are also required to withstand the harsh firing environment, while being able to be designed to ignite at specified acceleration levels when subjected to such accelerations for a specified amount of time to match the firing acceleration experienced. Very high reliability is also of much concern. The inertial igniters must also usually have a shelf life of up to 20 years and could generally be stored at temperatures of sometimes in the range of −65 to 165 degrees F. This requirement is usually satisfied best if the igniter pyrotechnic is in a sealed compartment. The inertial igniters must also consider the manufacturing costs and simplicity in design to make them cost effective for munitions applications. 
         [0031]    To ensure safety and reliability, inertial igniters should not initiate during acceleration events which may occur during manufacture, assembly, handling, transport, accidental drops, etc. Additionally, once under the influence of an acceleration profile particular to the firing of ordinance from a gun, the device should initiate with high reliability. It is also conceivable that the igniter will experience incidental low but long-duration accelerations, whether accidental or as part of normal handling, which must be guarded against initiation. 
         [0032]    Those skilled in the art will appreciate that the inertial igniters disclosed herein may provide one or more of the following advantages over prior art inertial igniters: 
         [0033]    provide small inertial igniters that can be initiated when subjected to very short duration firing setback acceleration (shock); 
         [0034]    provide small inertial igniters that can be initiated when subjected to relatively low firing setback acceleration (shock); 
         [0035]    provide small inertial igniters that can be initiated when subjected to relatively low firing setback acceleration (shock) with relatively short duration; 
         [0036]    provide inertial igniters that are significantly shorter than currently available inertial igniters for thermal batteries or the like; 
         [0037]    provide inertia igniters that could be constructed to guide the pyrotechnic flame essentially downward (in the direction opposite to the direction of the firing acceleration—usually for mounting on the top of the thermal battery as shown in  FIG. 1 ), or essentially upward (in the direction opposite of the firing acceleration—usually for mounting at the bottom of the thermal battery); 
         [0038]    Accordingly, inertial igniters and ignition systems for use with thermal batteries or the like upon subjection to firing setback acceleration, in particular short duration and/or relatively low peak acceleration levels, are provided. Provided are also inertial igniters that are very low height for small thermal batteries. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0039]    These and other features, aspects, and advantages of the apparatus of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where: 
           [0040]      FIG. 1  illustrates a schematic of a cross-section of a thermal battery and inertial igniter assembly. 
           [0041]      FIG. 2  illustrates an isometric cut away view of an inertial igniter assembly known in the art. 
           [0042]      FIG. 3  illustrates a full isometric view of the prior art inertial igniter of  FIG. 2 . 
           [0043]      FIG. 4  illustrates a full isometric view of a first embodiment of an inertial igniter in a locked position. 
           [0044]      FIG. 5  illustrates a blow up view of the first embodiment of the inertial igniter of  FIG. 4  showing all its individual components. 
           [0045]      FIGS. 6   a  and  6   b  illustrate first and second variations of thermal battery and inertial igniter assemblies. 
           [0046]      FIG. 7  illustrates the alternative options for the biasing compressive springs for the striker release element of the inertial igniter embodiment of  FIG. 4 . 
           [0047]      FIG. 8  illustrates the pyrotechnic region of the inertial igniter of  FIG. 4  with impacting ridges that ensure reliable initiation of the pyrotechnic material. 
           [0048]      FIG. 9  illustrates the inertial igniter embodiment of  FIG. 4  with a provided cover element with a ignition flame and spark exit hole. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0049]    A schematic of the isometric view of a first embodiment of an inertia igniter is shown in  FIG. 4 , referred to generally with reference numeral  250 . In the isometric view of  FIG. 4  the inertial igniter body  251  of the inertial igniter  250  is shown as being transparent to enable the internal components of the device to be seen. A lever type striker release element  252  is provided which is rotationally hinged to the inertial igniter body  251  by the pins  253  and  254 . One or both pins  253  and  254  may be fixed to the inertial igniter body  251 , preferably through press fitting or otherwise using adhesives such as epoxy or by soldering or brazing or by welding or the like, particularly if the joint needs to be hermetically sealed. When any one of the pins  253  or  254  is fixed to the inertial igniter body, then the corresponding hole  252   a  in the striker release element  252  is provided with enough clearance to allow free rotation of the striker release element  252  relative to the inertial igniter body about the long axes of the pins  253  and  254 . In an embodiment, the pins  253  and  254  are fixed to the inertial igniter body, where the fixing process can be achieved by press fitting the pins into holes  256  provided in the inertial igniter body  251  during the inertial igniter assembly process. Alternatively, one or both pins  253  and  254  are fixed to the striker release element  252  using one of the aforementioned methods and enough clearance is provided in the holes  256  in the inertial igniter body to allow free rotation of the striker release element  252  relative to the inertial igniter body about the long axes of the pins  253  and  254 . 
         [0050]    The striker release element  252  is rotationally biased upward by at least one preloaded torsion spring  255 , which is positioned at one or both rotating joints with pins  253  and/or  254  as shown in  FIG. 4 . The upward rotation of the striker release element  252  past the top surface  257  of the inertial igniter  250  can be prevented by a stop (not shown) for ease of inertial igniter assembly into the intended device (usually a thermal battery or the like), or by a top inertial igniter cover (not shown), which can be provided by the thermal battery assembly itself to minimize the total height of the inertial igniter. 
         [0051]    The inertial igniter  250  is provided with a rotating striker mass  258 , which is free to rotate about the cylindrical post  259 , which is provided on the base  260  of the inertial igniter body  251  as shown in  FIGS. 4 and 5 . 
         [0052]    The rotating striker mass  258  is provided with a tip portion  261  with a vertical face  262 , which faces a matching (vertical) face  263  provided in the recess  265  on the striker release element  252 . In the pre-activation state, the two surfaces  262  and  263  are pressed against each other (sometimes via a ball element  264 —as later described) by a preloaded torsion spring  266 . A dimple  275  is provided on the contact surface  263  of the striker release element  252  to keep the ball  264  in its indicated position on the contact surface  263 . The dimple  275  can be provided on the contact surface  263  of the striker release element  252 , but could alternatively be provided on the contact surface  262  of the rotating striker mass  258 . The inner end of the spring  266  is fixed to the cylindrical post  259 , by fitting its extended end  267 ,  FIG. 5 , inside the slot  268  provided on the cylindrical post  259  as can be seen in  FIGS. 4 and 5 . The other end  269  of the torsion spring  266  is positioned against a vertical surface  270  that is provided under the rotating striker mass  258 . In the pre-activation state shown in  FIG. 4 , the torsion spring is preloaded (wound) such that it would tend to rotate the rotating striker mass in the counterclockwise direction as seen in  FIG. 4 , thereby causing the surfaces  262  and  263  to be pressed against each other. In an embodiment, the torsion spring  266  is designed and assembled in the inertial igniter  250  such that the preloading action causes the torsion spring spiral to close. Such a direction of preloading of the torsion spring  266  is preferred since in such a preloading state the spring element is more stable. 
         [0053]    As shown in  FIGS. 4 and 5 , the rotating striker mass  258  is also provided with a sharp vertical ridge  271 , with a relatively small flat face, which can run along an entire length (downward) of the rotating striker mass  258 . Inside the igniter body  251  is also provided with an opposing and preferably horizontal ridge  272 , which is also provided with a relatively small flat face. The inertial igniter (one part) pyrotechnic material  273  (shown with dashed lines in  FIG. 8 ) is used to cover the surface of the horizontal ridge  272  with a relatively thin layer, with the bulk of pyrotechnic material being deposited on the surfaces around the horizontal ridge  272  shown in  FIG. 5 . 
         [0054]    The basic operation of the inertial igniter  250  will now be described with reference to  FIGS. 4 and 5 . Any non-trivial acceleration in the axial direction in the direction or opposite to the direction of the arrow  274  acts on the inertia of the striker release element  252 , generating a torque that would tend to rotate the striker release element  252  downward or upward, respectively. If the acceleration in the direction of the arrow  274  is high enough to generate a torque that overcomes the preloaded torque of the torsion spring  255 , then the striker release element  252  would rotate certain amount downwards. The upward rotation of the striker release element  252  is prevented by the aforementioned stop element (not shown) or the top cover of the inertial igniter  250  (not shown). However, if the non-trivial acceleration in the direction of the arrow  274  is not high enough and its duration is not long enough, i.e., if it is not at or above the prescribed all-fire event, then the striker release element  252  would return to its pre-acceleration (original) position shown in  FIG. 4 . 
         [0055]    If an acceleration in the direction of the arrow  274  at or above the all-fire acceleration level and its duration is also at or above the all-fire acceleration duration, then a sufficient impulse is imparted to rotate the striker release element  252  downward enough to cause the contact surface  263  of the striker release element  252  to move below the contact surface  262  of the rotating striker mass  258 . The torque of the preloaded torsion spring  266  will then cause the rotating striker mass  258  to be accelerated rotationally in the counterclockwise direction as observed from the top of the inertial igniter  250 ,  FIG. 4 . The rotating striker mass will keep gaining rotational velocity, thereby rotational energy, until its sharp vertical ridge  271  strikes the pyrotechnic material  273  covering the horizontal ridge  272  provided inside the igniter body  251 . The level of preloading of the torsion spring  266  and the moment of inertia of the rotating striker mass  258  are selected such that as the sharp vertical ridge  271  strikes the pyrotechnic material  273  covering the horizontal ridge  272 , it has an appropriate level of energy to ignite the pyrotechnic material. The resulting flames and sparks will then exit from the provided exit hole  278 . 
         [0056]    In general, a recess  301  is provided in the top surface of the striker release element  252  over which the released rotating striker mass  258  travels as shown in  FIGS. 4 and 5  to minimize the total height of the inertial igniter  250 . 
         [0057]    In  FIG. 4 , the inertial igniter embodiment  250  is shown without any outside housing. In many applications, as shown in the schematics of  FIG. 6   a,  the inertial igniter  250  ( FIG. 4 ) is placed securely inside a top housing  283  of the thermal battery  281 . Here, the thermal battery is considered to be subjected to all-fire setback firing acceleration in the direction of the arrow  276 . In such a thermal battery assembly, the top surface of the inertial igniter is covered (either by the top cap  277  of the thermal battery,  FIG. 6   a,  or an inertial igniter top cover—not shown in  FIG. 4 ), and the ignition flame and sparks are routed through the opening  278  provided on the bottom surface  260  of the inertial igniter  250  as shown in  FIG. 4 . In addition, depending on the location of the opening  285  in the bottom surface  284  of the inertial igniter compartment  283  relative to the inertial igniter flame and spark exit opening  278 , a strip of intermediate ignitable material  279  such as so-called heat paper may be used to facilitate ignition of the thermal battery heat generating pyrotechnic material inside the housing  282  of the thermal battery cell  286 . 
         [0058]    In other applications, as shown in the schematics of  FIG. 6   b,  the inertial igniter  250  ( FIG. 4 ) is placed securely inside a bottom housing  293  of the thermal battery  291 . Here, the thermal battery is also considered to be subjected to all-fire setback firing acceleration in the direction of the arrow  276 . In such a thermal battery assembly, the top surface of the inertial igniter is covered by bottom surface  297  of the thermal battery,  FIG. 6   b,  and the ignition flame and sparks are routed through an opening provided  298  on the inertial igniter top cover  299  (shown in  FIG. 9 ). In addition, depending on the location of the opening  295  on the surface  294  of the inertial igniter compartment  293  relative to the inertial igniter flame and spark exit opening  298 , a strip of intermediate ignitable material  300  such as so-called heat paper may be used to facilitate ignition of the thermal battery heat generating pyrotechnic material inside the housing  292  of the thermal battery cell  296 . 
         [0059]    In the inertial igniter embodiment  250  of  FIG. 4 , the at least one preloaded torsion spring  255 , which is positioned at one or both rotating joints with pins  253  and/or  254 , was described as being used to bias the striker release element  252  upward rotation against a stop (not shown) for ease of inertial igniter assembly into the intended device (usually a thermal battery or the like), or against a top inertial igniter cover (not shown). It is, however, appreciated by those skilled in the art that alternatively, the torsion spring  255  may be replaced by a compressively preloaded spring as is shown in  FIG. 7 . In  FIG. 7 , a simplified side view (as viewed in the direction of the axis of rotation of the rotary joints with pins  253  and  254 ) is shown with only a partial view of the housing  251  ( 302  in  FIG. 7 ) of the inertial igniter  250  of  FIG. 4 , with most of the housing wall removed except the portion containing the rotary joint accommodating the joint pin  253  ( 303  in  FIG. 7 ) for simplification of the view. In  FIG. 7 , the simplified view of the striker release element  304  ( 252  in  FIG. 4 ) is shown in its normal (in non-initiated inertial igniter) position. The striker release element  304  attached to the inertial igniter housing side wall  309  by the rotary joint pin  303 . The stop element that prevents further clockwise rotation of the striker release element  304  from its position seen in  FIG. 7  is not shown for clarity. 
         [0060]    The aforementioned upward biasing compressively loaded spring may be a regular helical spring (which can be a wave spring type)  306  or a flat spring  305  formed of a strip of spring steel or the like. Either compressively preloaded springs  305  or  306  are positioned between the bottom surface  307  of the striker release element  304  and the top surface  308  of the inertial igniter housing  302 . In general, the compressively preloaded springs  305  or  306  are mounted within provided detents and/or protrusions on one or both surfaces  307  and  308  (not shown) to keep the springs  305  or  306  in place and prevent them from moving inside the inertial igniter assembly. An advantage of using such compressively preloaded biasing springs  305  or  306  (such as a formed flat spring  305  type) is that they would exert an upward force to the bottom surface  307  of the striker release element  304 , thereby generating a nearly pure rotating torque to the striker release element  304 , thereby minimizing the chances of generating increased friction forces at its rotating joints. The other advantage is that it significantly reduces assembling complexity, thereby the production cost of the inertial igniter. 
         [0061]    In  FIG. 4 , in the schematic of the inertial igniter  250 , the rotating striker mass  258  is shown to be provided with a tip portion  261  with a vertical face  262 , which faces the matching (vertical) face  263  provided in the recess  265  on the striker release element  252 . As it was previously described, in the pre-activation state, the two surfaces  262  and  263  are pressed against each other by the preloaded torsion spring  266 . In the schematic of  FIG. 4 , a ball  264  is shown to be positioned (on one side within the dimple  275 ) between the surfaces  262  and  263 , the reason of which is to facilitate the relative sliding motion between the two surfaces by minimizing friction between the two surfaces as the inertial igniter is subjected to all-fire condition. It is, however, appreciated by those skilled in the art that other means and methods may also be used to minimize friction between the sliding surfaces  262  and  263  to facilitate downward rotation of the striker release element  252 , including the following. 
         [0062]    In one alternative embodiment, a rolling element (shown in dashed lines in  FIG. 5  and enumerated as  310 ) is used in place of the aforementioned ball  264 . A dimple similar to the dimple  275  shown in  FIG. 5  but shaped to accommodate the roller  300  is also provided to secure the roller in the inertial igniter assembly. 
         [0063]    In another alternative embodiment, the aforementioned ball  264  is not used and the two surfaces  262  and  263 ,  FIG. 4 , are allowed to come into contact. In this embodiment, the two surfaces  262  and  263  can be provided with certain curvature (not shown) to avoid sharp corners scraping between the two surfaces as the striker release element  252  rotates downward to release the rotating striker mass  258 . The contacting surfaces may further be coated by friction reducing materials (lubricants) such as graphite, Teflon or the like (liquid lubricants are usually not desirable due to the required very long shelf life of up to 20 years). One or both surfaces may also be coated with hard materials such as tungsten or the like. 
         [0064]    In yet another alternative embodiment, the aforementioned ball  264  is not used between the two surfaces  262  and  263 ,  FIG. 4 . To facilitate sliding action between the two surfaces, a thin sheet of friction reducing material (not shown) such as one made out of Teflon or a hard and polished metal or ceramic or the like is provided between the two surfaces  262  and  263 . The provided friction reducing material may be fixed to one of the surfaces  262  or  263  to prevent it from being pushed out or fall off. 
         [0065]    The alternative embodiments of the inertial igniter  250  designs have the purpose of reducing friction to the downward rotation of the striker release element  252  as it is rotated under the prescribed all-fire condition to release the rotating striker mass  258 . Other sources of friction that resist the downward rotation of the striker release element  252  are friction at the rotating joints with pins  253  and  254 , where friction exists between the pin surfaces and the mating joint surfaces as well as between the side surfaces of the striker release element  252  and their contacting surfaces on the inertial igniter housing. To reduce the effects (i.e., the generated resisting torque to the downward rotation of the striker release element  252 ), the diameters of the pins  253  and  254  can be small and the contacting surfaces can be coated with friction reducing “lubricating” materials and/or provided with intermediate low friction “washer” type relatively thin members. 
         [0066]    As is shown in  FIGS. 4 and 5 , the rotating striker mass  258  is provided with a sharp vertical ridge  271 , which can have a relatively small flat face  311 , which can run along the entire length of the rotating striker mass  258  as shown in the partial view  FIG. 8 . Inside the igniter body  251  was also shown to be provided with an opposing and preferably horizontal ridge  272 , which is also provided with a relatively small flat face  312 . In  FIG. 8 , a partial view of the inertial igniter  250 ,  FIGS. 4 and 5 , showing the ridges  271  and  272  with their frontal flat surface  311  and  312 , respectively, is shown. In the schematic of  FIG. 8  the one part pyrotechnic material  273 , which can be based on lead styphnate or other similar compounds, and is used to cover the surface of the horizontal ridge  272  (shown in  FIG. 5  but not shown in  FIG. 4  for clarity) is not shown. In general, the portion of the pyrotechnic material covering the flat surface portion  312  of the horizontal ridge  272  is in a relatively thin layer. Then as the rotating striker mass  258  is released, its ridge  271  portion is accelerated towards the ridge  272  and impacts it at a certain point. In this design, since the two flat surfaces  311  and  312  are positioned at about 90 degrees relative to each other, the resulting impacting surface is always close to a rectangle with sides equal to the widths of the two flat surfaces  311  and  312 . As a result, the inertial igniter parts do not have to have extremely high precision to allow the pyrotechnic igniting impact to occur over a relatively small area. In general, it is highly desirable to have a relatively small area of impact, within which a thin layer of pyrotechnic material is impinged during impact to ensure reliable pyrotechnic initiation. 
         [0067]    In the schematics of  FIGS. 4 ,  5  and  8 , the impacting ridges  271  and  272  of the inertial igniter  250  were shown to be vertical and horizontal, respectively, as viewed in the drawings, to ensure impact over a relatively small area without requiring extremely high manufacturing precision of the inertial igniter parts. It is, however, appreciated by those skilled in the art that the flat ridge surface  311  and  312  of the impacting ridges  271  and  272 , respectively, do not have to be vertically and horizontally directed to achieve the goal of small impact surfaces even when the inertial igniter parts are not very high in geometrical precision. The only requirement to achieve the goal is that the two surface strips  311  and  312  are not parallel and make a considerable angle (such as 90 degrees) with each other. 
         [0068]    While the one-part pyrotechnic material  273  is shown the body  251 , it can alternatively be provided on the striker mass  258 . Alternatively, a two-part pyrotechnic can be used in which one part is provided on each of the body  251  and striker mass  258 . 
         [0069]    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.