Abstract:
A method for igniting a thermal battery upon a predetermined acceleration event. The method including: rotatably connecting a striker mass to a base; aligning a first projection on the striker mass with a second projection on the base such that when the striker mass is rotated towards the base, the first projection impacts the second projection; and preventing impact of the first and second projections unless the predetermined acceleration event is experienced.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part of U.S. application Ser. No. 12/955,876 filed on Nov. 29, 2010, the entire contents of which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present disclosure relates generally to mechanical igniters, and more particularly to compact, reliable and easy to manufacture mechanical igniters for thermal batteries and the like that are activated by high-G shocks such as by the gun firing setback acceleration. 
         [0004]    2. Prior Art 
         [0005]    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. 
         [0006]    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. 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. 
         [0007]    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. 
         [0008]    In general, the inertial igniters, particularly those that are designed to operate at relatively low impact 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. 
         [0009]    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, the existing 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 particularly the case for thermal batteries used in gun-fired munitions that are subjected to high G accelerations, sometimes 10,000-30,000 G and higher. 
         [0010]    The need to differentiate accidental and initiation accelerations by the resulting impulse level of the event necessitates the employment of a safety system which is capable of allowing initiation of the igniter only during high total 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 pyrotechnics. An inertial igniter that combines such a safety system with an impact based initiation system and its alternative embodiments are described herein together with alternative methods of initiation pyrotechnics. 
         [0011]    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 elements. The function of the safety system is to fix the striker in position until a specified acceleration time profile actuates the safety system and releases the striker, allowing it to accelerate toward its target under the influence of the remaining portion of the specified acceleration time profile. 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. 
         [0012]    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 (in the direction of the acceleration) 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 space that the thermal battery assembly  16  occupies within a munitions housing (usually determined by the total height  15  of the thermal battery), 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 and with currently available inertial igniter, the height of the inertial igniter portion  13  is a significant portion of the thermal battery height  15 . 
         [0013]    It is, therefore, highly desirable to develop inertial igniters that are smaller in height and also preferably in volume for thermal batteries in general and for small thermal batteries in particular. This is particularly the case for inertia igniters for gun-fired munitions that experience high G firing setback accelerations levels, e.g., setback acceleration levels of 10-30,000 Gs or even higher, since such thermal batteries would have significantly higher no-fire and all-fire acceleration requirements, which should allow the development of inertial igniters that are smaller in height and possibly even in volume. 
       SUMMARY OF THE INVENTION 
       [0014]    Accordingly, an inertial igniter for igniting a thermal battery upon a predetermined acceleration event is provided. The inertial igniter comprising: a base having a first projection; a striker mass rotatably connected to the base through a rotatable connection, the base having a second projection aligned with the first projection such that when the striker mass is rotated towards the base, the first projection impacts the second projection; and a rotation prevention mechanism for preventing impact of the first and second projections unless the predetermined acceleration event is experienced. 
         [0015]    The rotation prevention mechanism can comprise a restriction member for restricting rotation of the sticker mass, the restriction member being disposed directly or indirectly between the striker mass and the base. The restriction member can have a weakened portion which fails upon the predetermined acceleration event thereby allowing the striker mass to rotate towards the base. The inertial igniter can further comprise a spring for biasing the striker mass in a biasing direction away from the base. The inertial igniter can further comprise a stop for limiting the movement of the striker mass in the biasing direction. The restriction member can be arranged in shear and the weakened portion can be a reduced cross-sectional portion. The restriction member can be arranged in tension and the weakened portion can be a reduced cross-sectional portion. 
         [0016]    The rotation prevention mechanism can comprise a retaining member movably disposed at least partially in the striker mass and a blocking member movably disposed in a blocking position for blocking the retaining member from moving from the striker mass unless the predetermined acceleration event is experienced. The retaining member can be a ball disposed in a dimple on the striker mass. The blocking member can be a mass biased in the blocking position by a spring member. The blocking member further can have a curved surface for accommodating a portion of the retaining member. The blocking member can be slidingly disposed relative to the base. The blocking member can be rotatably disposed relative to the base. The blocking member can be a flexural spring having a first end connected to one of the base or striker mass and a second end blocking the retaining member, and the second end can include an opening that allows the retaining member to pass when the flexural spring rotates or bends due to the predetermined acceleration event. 
         [0017]    One or more of the base and striker mass can include a pyrotechnic material which ignites upon the second projection striking the first projection. The base can further include one or more openings for allowing a product of the ignited pyrotechnic to exit the opening. 
         [0018]    The rotatable connection can include a pin disposed in at least a portion of the striker mass and base. 
         [0019]    The rotatable connection can include a cylindrical portion on one of the striker mass and base and a corresponding cylindrical recess on the other of the striker mass and base. 
         [0020]    Also provided is an inertial igniter for igniting a thermal battery upon a predetermined acceleration event. The inertial igniter comprising: a base having two or more first projections; two or more striker masses, each rotatably connected to the base through a rotatable connection, the base having two or more second projections aligned with the two or more first projections such that when the striker mass is rotated towards the base, each of the first projections impact a corresponding one of the two or more second projections; and a rotation prevention mechanism for preventing impact of each of the first projections with the corresponding second projections unless the predetermined acceleration event is experienced. 
         [0021]    Further provided is a method for igniting a thermal battery upon a predetermined acceleration event. The method comprising: rotatably connecting a striker mass to a base; aligning a first projection on the striker mass with a second projection on the base such that when the striker mass is rotated towards the base, the first projection impacts the second projection; and preventing impact of the first and second projections unless the predetermined acceleration event is experienced. 
         [0022]    Still further provided is a switch for opening a circuit upon a predetermined acceleration event. The switch comprising: a base having first and second electrical contacts configured to form a closed electrical circuit; a striker mass rotatably connected to the base through a rotatable connection, the striker mass having a member formed of an electrically insulating material, the first and second electrical contacts being aligned with the member such that when the striker mass is rotated towards the base, the member opens the circuit between the first and second electrical contacts; and a rotation prevention mechanism for preventing the member from opening the circuit unless the predetermined acceleration event is experienced. 
         [0023]    Still further yet provided is a switch for closing a circuit upon a predetermined acceleration event. The switch comprising: a base having first and second electrical contacts configured to form an open electrical circuit; a striker mass rotatably connected to the base through a rotatable connection, the striker mass having a third electrical contact formed of an electrically conductive material, the first and second electrical contacts being aligned with the third electrical contact such that when the striker mass is rotated towards the base, the third electrical contact closes the circuit between the first and second electrical contacts; and a rotation prevention mechanism for preventing the third electrical contact from closing the circuit unless the predetermined acceleration event is experienced. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    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: 
           [0025]      FIG. 1  illustrates a schematic of a cross-section of a thermal battery and inertial igniter assembly of the prior art. 
           [0026]      FIG. 2  illustrates a schematic of a cross-section of a first inertial igniter embodiment. 
           [0027]      FIG. 3  illustrates a schematic of the cross-section of the tensile-mode failure element of a second inertial igniter embodiment. 
           [0028]      FIG. 4  illustrates a schematic of a cross-section of another inertial igniter embodiment. 
           [0029]      FIG. 5  illustrates a schematic of an alternative rotary joint for the inertial igniter embodiment of  FIG. 4 . 
           [0030]      FIG. 6  illustrates a schematic of another alternative rotary joint for the inertial igniter embodiment of  FIG. 4 . 
           [0031]      FIG. 7  illustrates a schematic of a cross-section of yet another inertial igniter embodiment. 
           [0032]      FIG. 8  illustrates a schematic of a partial cross-section of a variation of the embodiment of  FIG. 4 . 
           [0033]      FIG. 9  illustrates a schematic of a cross-section of yet another inertial igniter embodiment. 
           [0034]      FIG. 10  illustrates a side view of the inertial igniter of  FIG. 9 . 
           [0035]      FIG. 11  illustrates a top view of an embodiment employing multiple inertial igniters. 
           [0036]      FIG. 12  illustrates schematic of a partial cross-section of the multiple inertial igniter embodiment of  FIG. 11 . 
           [0037]      FIG. 13  illustrates a schematic of a cross section of a g-switch embodiment. 
           [0038]      FIG. 14  illustrates a schematic of a cross section of another g-switch embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0039]    The safety related no-fire acceleration level requirements for inertial igniters that are used to initiate thermal batteries or other devices in gun-fired munitions, mortars or the like that are subjected to high-G setback (or impact) accelerations during the launch (or events such as target impact) are generally significantly higher than those that could occur accidentally, such as a result of the aforementioned drops from the 7 feet heights over concrete floors. In general, the no-fire safety requirement translates to the requirement of no initiation at acceleration levels of around 2000 Gs with a duration of approximately 0.5 msec. However, for initiation devices that are subjected to setback acceleration levels of 10-30,000 Gs or even higher, the no-fire acceleration levels are set at well above the 2000 G levels that munitions can experience when accidentally dropped over concrete floor from indicated heights of up to 7 feet. As a result, the no-fire acceleration levels for such munitions are set significantly higher than those that can be experienced during accidental drops. 
         [0040]    In the following description and for the purpose of illustrating the methods of designing the disclosed inertial igniter embodiments to satisfy the prescribed no-fire and all-fire requirements of each munitions, a no-fire acceleration level of 3000 G (significantly higher than the accidental acceleration levels that may be actually experienced by the inertial igniter) and an all-fire acceleration level of 6000 G (significantly higher than the prescribed no-fire acceleration level of 3000 G) for a duration exceeding 2 msec will be used. It is, however, noted that as long as the prescribed no-fire acceleration level is significantly higher than those that may be actually experienced during accidental drops or the like and as long as the prescribed all-fire acceleration level is significantly higher than the prescribed no-fire acceleration level and its duration is long enough to cause the striker mass of the inertial igniter to gain enough energy to initiate the igniter pyrotechnic material, then the disclosed novel methods and various embodiments are useful to fabricate highly reliable and low cost inertial igniters for the munitions at hand. Here, two acceleration levels are considered to have a significant difference if considering the existing range of their distributions about the indicated values, their extreme values would still be a significant amount (e.g., at least 500-1000 G) apart. 
         [0041]    A schematic of a first embodiment  20  is shown in  FIG. 2 . The inertial igniter  20  is considered to be cylindrical in shape since most thermal batteries are constructed in cylindrical shapes, but may be constructed in any other shape with the general cross-sectional view shown in  FIG. 2  and with its general mode of operation. The inertial igniter  20  consists of a base element  21  (which can be separate from or integral with the thermal battery), which in a thermal battery construction shown in  FIG. 1  would be positioned in the housing  10  with the base element  21  positioned on the top of the thermal battery cap  19 . A striker mass  22  of the inertial igniter is attached to the base element  21  via a rotary joint  23 . In the embodiment  20  of  FIG. 2 , the striker mass  22  is kept separated from the base element  21  by a spring element  24  which biases the striker mass  22  away from the base element  21 . A stop element  25  is also provided to limit the counterclockwise rotation of the striker mass  22  relative to the base element  21  (the stop element opposes the biasing of the striker mass  22  due to the spring element  24 ). The stop element  25  is attached a post  26 , which is in turn attached to the base element  21  of the inertial igniter  20 . 
         [0042]    The spring element  24  can be preloaded in compression such that with the no-fire acceleration acting on the base element  21  of the inertial igniter in the upward direction, as shown by the arrow  27 , the inertia force due to the mass of the striker mass  22  would not overcome (or at most be equal to) the preloading force of the spring element  24 . As a result, the inertial igniter  20  is ensured to satisfy its prescribed no-fire requirement. 
         [0043]    A shearing pin  28  is also provided and is fixed to the post  26  on one end and to a portion, such as an end of the striker mass  21  on the other end as shown in  FIG. 2 . The shearing pin  28  is provided with a narrow neck  29 , which provides for concentrated stress when the striker mass  22  is pressed down towards the base element  21  due to all-fire acceleration in the direction of the arrow  27  acting on the inertia of the striker mass  22 . By properly designing the geometry of the shearing pin  28  and its neck  29  and selection of the proper material for the shearing pin  28 , the shearing pin  28  can be designed to fracture in shear (and in fact in any other mode as described later in this disclosure), thereby releasing the striker mass  22  and allowing it to be accelerated in the clockwise rotation. The free end of the striker mass  22  is sized, shaped and otherwise configured so as not to interfere with any other portions, such as the post  26  when turning about the pivot  23  upon the all-fire acceleration level. As a result, for a properly designed inertial igniter  20  (i.e., by selecting a proper mass and moment of inertial for the striker mass  22 , the required range of clockwise rotation for the striker mass  22  so that it would gain enough energy, considering the all-fire acceleration level and the preloading level of the spring element  24 ), the striker mass  22  will gain enough energy to initiate the pyrotechnic material  30  between the pinching points provided by the protrusions  31  and  32  on the base element  21  and the bottom surface of the striker mass  22 , respectively, as shown in the schematic of  FIG. 2 . The ignition flame and sparks can then travel down through the opening  33  provided in the base element  21 . When assembled in a thermal battery similar to the thermal battery  16  of  FIG. 1 , the inertial igniter is mounted in the housing  10  such that the opening  33  is lined up with the opening  12  into the thermal battery  11  to activate the battery by igniting its heat pallets. 
         [0044]    It is will be appreciated by those skilled in the art that the duration of the all-fire acceleration level is also important for the proper operation of the inertial igniter  20  by ensuring that the all-fire acceleration level is available long enough to accelerate the striker mass  22  towards the base element  21  to gain enough energy to initiate the pyrotechnic material  30  as described above by the pinching action between the protruding elements  31  and  32 . 
         [0045]    It is will be appreciated by those skilled in the art that when the inertial igniter  20  ( FIG. 2 ) is assembled inside the housing  10  of the thermal battery assembly  16  of  FIG. 1 , a cap  18  (or a separate internal cap—not shown) is commonly used to secure the inertial igniter  20  inside the housing  10 . In such assemblies, the stop element  25  is no longer functionally necessary since the striker mass  22  is prevented by the cap from tending to rotate in the counterclockwise direction by the spring element  24 , thereby minimizing the shearing load on the shearing pin in the assembled thermal battery. It is, however, appreciated by those skilled in the art that by proving the stop element  25 , the storage of the inertial igniter  20  and the process of assembling it into the housing  10  is significantly simplified since one does not have to provide secondary means to keep the spring element  24  from applying shearing load to the shearing pin  28 . 
         [0046]    It will be appreciated by those skilled in the art that in place of the shearing pin  28 , other types of elements that are designed to fracture upon the application of the all-firing acceleration as described above and release the striker mass  22  may be used to perform the same function. For example, the mode of fracture may be selected to be in tension, torsion or pure bending. In general, the fracture can be achieved with minimal deformation in the direction that results in a significant clockwise rotation of the striker mass  22  prior to pin fracture and release of the striker mass  22 . This would result in minimum height requirement for the inertial igniter since the clockwise rotation of the striker mass  22  will reduce the terminal (clockwise) rotational speed of the striker mass  22  at the instant of initiation impact between the protruding elements  31  and  32 ,  FIG. 2 , and pinching of the pyrotechnic material  30  to achieve initiation. 
         [0047]    As an example, the option of replacing the shearing pin  28 ,  FIG. 2 , with a pin that is designed to fracture in tension by when the inertial igniter  20  is subjected to the aforementioned all-fire acceleration is shown in the schematic of  FIG. 3 . Part of the base element  40 , the post  41 , the stop element  42  and the front portion of the striker mass  43  (indicated by numerals  21 ,  26 ,  25  and  22  in  FIG. 2 , respectively) are shown. The stop element  42  is provided with a hole and countersink  44  as shown in  FIG. 3 . An opposite hole and countersink  45  is provided in the striker mass  43  under the stop element  42  as shown in  FIG. 3 . A one piece tension element  46  (which can be cylindrical in shape) with top and bottom flange portions  47  and  48 , respectively, is also provided. The top flange portion  47  of the tension element  46  is assembled seating in the countersink  44  of the stop element  42  and the bottom flange portion  48  of the tension element  46  is assembled seating in the countersink  45  of the striker mass  43 . The stop element  42  and the striker mass  43  can be provided with passages (not shown) for assembling the tension element  46  as shown in  FIG. 3 . Alternatively, the tension element  46  may be a two part element that is assembled in place as shown in  FIG. 3 , such as by riveting , welding or otherwise fastening the flange  47  to the stem portion of the tension element  46 . The tension element  46  is also provided with a narrow neck portion  49 , which provides for concentrated stress when the striker mass  43  is pressed down towards the base element  40  due to all-fire acceleration in the direction of the arrow  27  ( FIG. 2 ) acting on the inertia of the striker mass  43 . By properly designing the geometry of the tension element  46  and its neck portion  49  and selection of the proper material, the tension element  46  can be designed to fracture in tension, thereby releasing the striker mass  43  and allowing it to be accelerated in the clockwise rotation. As a result, for a properly designed inertial igniter (i.e., by selecting a proper mass and moment of inertial for the striker mass  43 , the required range of counterclockwise rotation for the striker mass  43  so that it would gain enough energy, considering the all-fire acceleration level and the preloading level of the spring element  24 , the striker mass  43  will gain enough energy to initiate the pyrotechnic material  30  between the pinching points provided by the protrusions  31  and  32  on the base element  40  and the bottom surface of the striker mass  43 , respectively, as shown in the schematics of  FIGS. 2 and 3 . The ignition flame and sparks can then travel down through the opening  33  provided in the base element  40 . When assembled in a thermal battery similar to the thermal battery  16  of  FIG. 1 , the inertial igniter is mounted in the housing  10  such that the opening  33  is lined up with the opening  12  into the thermal battery  11  to activate the battery by igniting its heat pallets. 
         [0048]    The shearing pin can be a failure member of any configuration having a portion that is weaker than other portions about which the failure member can fail upon experiencing the all-fire acceleration level. Such weaker portion can include a material that has one or more portions having a smaller cross-sectional area than other portions and/or different materials having a weaker strength than other portions as is known in the art. 
         [0049]    Another embodiment  50  is illustrated schematically in  FIG. 4 . Similar to the inertial igniter of embodiment  20  of  FIGS. 2 and 3 , the inertial igniter  50  consists of a base element  51 , which in a thermal battery construction shown in  FIG. 1  would be positioned in the housing  10  with the base element  51  positioned on the top of the thermal battery cap  19 . The striker mass  52  of the inertial igniter  50  is attached to the base element  51  via the rotary joint  53 . A post  54 , which is fixed to the base element  51  is provided with a hole  55 , which in the configuration shown in  FIG. 4  is aligned with a dimple  56  in the striker mass  52 . A ball  57  is positioned in the hole  55 , extending into the dimple  56  of the striker mass  52 . In the configuration of  FIG. 4 , the (up-down) sliding member  58  is shown to block the movement of the ball  57  out of engagement with the dimple  56  of the striker mass  52 , thereby locking the striker mass  52  in the illustrated configuration. A sliding member  58  is free to slide down against a member  60  (the rolling elements  59  are provided for illustrative purposes only to indicate a sliding joint between the sliding member  58  and the surface of the member  60 ). The member  60  is fixed to the base element  51 . A spring element  61  resists downward motion of the sliding member  58 , and is preferably preloaded in compression so that if a downward force that is less than the compressive preload is applied to the sliding member  58 , the applied force would not cause the sliding element  58  to move downwards. A stop  62 , fixed to the member  60 , is provided to allow the spring element  61  to be preloaded in compression by preventing the sliding member  58  from moving further up from the configuration shown in  FIG. 4 . 
         [0050]    During the firing, the inertial igniter  50  is considered to be subjected to setback acceleration in the direction of the arrow  63 . If a level of acceleration in the direction of the arrow  63  acts on the inertia of the sliding element  58 , it would generate a downward force that tends to slide the sliding element  58  downwards (opposite to the direction of acceleration). The compression preloading of the spring element  61  is selected such that with the no-fire acceleration levels, the inertia force acting on the sliding element  58  would not overcome (or at most be equal to) the preloading force of the spring element  61 . As a result, the inertial igniter  50  is ensured to satisfy its prescribed no-fire requirement. 
         [0051]    Now if the acceleration level in the direction of the arrow  63  is high enough, then the aforementioned inertia force acting on the sliding element  58  will overcome the preloading force of the spring element  61 , and will begin to travel downward. If the acceleration level is applied over a long enough period of time (duration) as well, i.e., if the all-fire condition is satisfied and the sliding element  58  will have enough time to travel down far enough to allow the ball  57  to be pushed out of the dimple  56 , thereby releasing the striker mass  52  and allowing it to be accelerated in the clockwise rotation. As a result, for a properly designed inertial igniter  50  (i.e., by selecting a proper mass and moment of inertial for the striker mass  52  and the range of clockwise rotation for the striker mass  52  so that it would gain enough energy), the striker mass  52  will gain enough energy to initiate the pyrotechnic material  64  between the pinching points provided by the protrusions  65  and  66  on the base element  51  and the bottom surface of the striker mass  52 , respectively, as shown in the schematic of  FIG. 4 . The ignition flame and sparks can then travel down through the opening  67  provided in the base element  51 . When assembled in a thermal battery similar to the thermal battery  16  of  FIG. 1 , the inertial igniter is mounted in the housing  10  such that the opening  67  is lined up with the opening  12  into the thermal battery  11  to activate the battery by igniting its heat pallets. 
         [0052]    It will be appreciated by those skilled in the art that the duration of the all-fire acceleration level can also be important for the operation of the inertial igniter  50  by ensuring that the all-fire acceleration level is available long enough to accelerate the striker mass  52  towards the base element  51  to gain enough energy to initiate the pyrotechnic material  30  as described above by the pinching action between the protruding elements  65  and  66 . 
         [0053]    It will be appreciated by those skilled in the art that when the inertial igniter  50  ( FIG. 4 ) is assembled inside the housing  10  of the thermal battery assembly  16  of  FIG. 1 , a cap  18  (or a separate internal cap—not shown) is commonly used to secure the inertial igniter  50  inside the housing  10 . In such assemblies, the stop element  62  is no longer functionally necessary since the sliding element  58  is prevented from being pushed upward by the force of the spring element  61  and releasing the striker mass  52 . It will be, however, appreciated by those skilled in the art that by providing the stop element  62 , the storage of the inertial igniter  50  and the process of assembling it into the housing  10  is significantly simplified since one does not have to provide secondary means to keep the spring element  61  from pushing the sliding element  58  further up and passed the locking ball  57  and releasing the striker mass  52 . 
         [0054]    In the embodiment of  FIG. 4 , the sliding and spring elements of the locking ball release mechanism may be configured in numerous ways, e.g., the sliding element  58  may be replaced with a rotating member (which may reduce the possibility of jamming) and the spring member  61  may be combined with the rotating member, i.e., as flexible beam element with the inertia of the beam acting as the mass element of the slider. 
         [0055]    An advantage of the embodiment of  FIG. 4  over those of  FIGS. 2 and 3  is that the amount of force to shear the pin or break in tension may not be reliably estimated, on the other hand, the amount and duration of acceleration to move the sliding element  58  in  FIG. 4  is more predictable. 
         [0056]    The sliding element may also be provided with a cup-like base under the ball (with the ball sticking out into the sliding element and over the lip of the cup) so that a top piece is not needed to prevent the preloaded spring to push the sliding element out (up) (see e.g., U.S. application Ser. No. 12/835,709 filed on Jul. 13, 2010, the contents of which is incorporated herein by reference). 
         [0057]    The rotary hinge  23  ( 53 ) used to attach the striker mass  22 ( 52 ) to the base element  21 ( 51 ) of the inertial igniter does not have to be constructed with a pin passing through the connected rotating parts as shown in FIG.  2 ( 4 ). It may, for example, be constructed with a living joint. Alternatively, the joint may also be constructed with one side (for example the striker mass side) formed as a rolling surface with mating surfaces on the base element surface ( FIG. 5 ); or with an intermediate roller or balls with preloaded springs keeping them in contact ( FIG. 6 ); or other similar methods known in the art. 
         [0058]    In the rotary joint shown in  FIG. 5 , the rotary joint is between the striker mass  71  and the base element  73 . The base element  73  is provided with a preferably half-cylindrical recess  75 . The striker mass  71  is provided with a matching cylindrical base  77 , which allows the striker mass  71  to rotate relative to the base element  73 . The spring element  78 , which is attached to the striker mass  71  at point  79  on one end and to the base element  73  at point  80  on the other end, is preloaded in tension to keep the striker mass  71  and the base element  73  in continuous contact. 
         [0059]    In the rotary joint shown in  FIG. 6 , the rotary joint is between the striker mass  72  and the base element  74 . The base element  74  is provided with a half-cylindrical recess  76 . The striker mass  72  is provided with a matching cylindrical recess  81 , with the roller or balls  82  disposed in the recesses  76  and  81  to form a rotary joint between the striker mass  72  and the base element  74 . Similar to the rotary joint of  FIG. 5 , a spring element  83 , which is attached to the striker mass  72  at point  84  on one end and to the base element  74  at point  85  on the other end, is preloaded in tension to keep the striker mass  72  and the base element  74  in continuous contact. 
         [0060]    It was noted that the embodiment  50  of  FIG. 4  requires the stop element  62  to prevent further upward motion of the sliding element  58  by the force of the compressively loaded spring element  61 . In an alternative design of this portion of the inertial igniter  50  shown in  FIG. 8 , the sliding element is provided with a recessed surface  100  that in the configuration of the inertial igniter  50  shown in  FIG. 4  is pushed against the lower surface of the locking ball  57  as shown in the schematic of  FIG. 8  by the compressively loaded spring element  61 . As a result, the sliding element  58  is prevented from further upward motion. 
         [0061]    It is appreciated by those skilled in the art that in the embodiment  50  of  FIG. 4  the locking ball  57  release mechanism (consisting of sliding element  58  and the spring element  61 ) could be replaced with many other types of mechanisms. One such release mechanism embodiment is shown in the schematic of  FIG. 7 . 
         [0062]    In the embodiment of  FIG. 7 , the components of the inertial igniter  90  are identical to those of the embodiment  50  of  FIG. 4  except the locking ball  57  release mechanism components (the sliding element  58  and its related elements  59 - 62 ), which are all replaced by the components of the present embodiment. In this embodiment  90  of the inertial igniter, a lever element  91 , attached to the base element  51  by a rotary joint  92  is provided as shown in  FIG. 7 . The rotary joint  92  can be the same or a different rotary joint from rotary joint  53 . On the free end of the lever element  91  is provided with an end  93  with the geometry that provides a surface, such as a planar surface  94  facing the locking ball  57 . In normal conditions, the lever element  91  is held in the configuration shown in  FIG. 7 , i.e., with the flat surface  94  facing the locking ball  57 , thereby locking the striker mass  52  to the post  54  (i.e., the base element  51 ). A spring element  95 , which is preloaded in compression, is used to keep the lever element  91  in the configuration of  FIG. 7 . It is noted that in this embodiment, there is no need for the stop element  62  shown in  FIG. 4  since the compressively preloaded spring element  95  pushed the surface  94  against the surface of the post  54 , thereby preventing the lever element  91  to rotate any further in the counterclockwise direction to and release the locking ball. 
         [0063]    During the firing, the inertial igniter  90  is considered to be subjected to setback acceleration in the direction of the arrow  96 . Acceleration in the direction of the arrow  96  will act on the inertia of the inertia of the lever element  91 , and generate a downward force that would tend to rotate the lever element  91  in the clockwise direction. The compression preloading of the spring element  95  will, however, resists the clockwise rotation of the lever element  91 . The level of compressive preloading of the spring element  95  is selected such that with the no-fire acceleration levels, the inertia force acting on the lever element  91  would not overcome the preloading force of the spring element  95 . As a result, the inertial igniter  90  is ensured to satisfy its prescribed no-fire requirement. 
         [0064]    Now if the acceleration level in the direction of the arrow  96  is high enough, then the aforementioned inertia force acting on the lever element  91  will overcome the preloading force of the spring element  95 , and will begin rotate in the clockwise direction. Now if the acceleration level is applied over a long enough period of time as well, i.e., if the all-fire condition is satisfied, then the lever element  91  will have enough time to rotate enough in the clockwise direction to allow the locking ball  57  to be pushed out of the dimple  56 , thereby releasing the striker mass  52  and allowing it to be accelerated in the clockwise rotation. As a result, for a properly designed inertial igniter  90  (i.e., by selecting a proper mass and moment of inertial for the striker mass  52  and range of clockwise rotation for the striker mass  52  so that it would gain enough energy), the striker mass  52  will gain enough energy to initiate the pyrotechnic material  64  between the pinching points provided by the protrusions  65  and  66  on the base element  51  and the bottom surface of the striker mass  52 , respectively, as shown in the schematic of  FIG. 4 . The ignition flame and sparks can then travel down through the opening  67  provided in the base element  51 . When assembled in a thermal battery similar to the thermal battery  16  of  FIG. 1 , the inertial igniter is mounted in the housing  10  such that the opening  67  is lined up with the opening  12  into the thermal battery  11  to activate the battery by igniting its heat pallets. 
         [0065]    It is appreciated by those skilled in the art that the duration of the all-fire acceleration level is also important for the proper operation of the inertial igniter  50  by ensuring that the all-fire acceleration level is available long enough to accelerate the striker mass  52  towards the base element  51  to gain enough energy to initiate the pyrotechnic material  30  as described above by the pinching action between the protruding elements  65  and  66 . 
         [0066]    Referring now to  FIG. 9 , there is shown another embodiment of an inertial igniter, referred to generally by reference numeral  150 . The inertial igniter  150  is similar to that illustrated in  FIG. 7 , except that link  93  (with hinge  92 ) and spring are replaced by a flexural spring  151 , which in the embodiment of  FIG. 9  is flat shaped. The spring  151  is fixed to the striker element  52 , such as with fasteners  152  or any type of fastening method known in the art. Alternatively, the spring  151  may be fixed to the base of the inertial igniter  51 . The spring  151  extends at least partly over the striker element  52  and bends over the front area to cover the front portion of the release ball  57  (this portion of the spring  151  is indicated by numeral  154 ) and prevent it from moving forward and releasing the striker element  52 . The spring  151  has an opening  153  as seen in the frontal view of  FIG. 10 , as observed in the direction of the arrow  155  of  FIG. 9 . 
         [0067]    When the device is subjected to acceleration in the direction of arrow  96 , the acceleration acts on the inertia of the spring  151  and tend to rotate (bend) it down in the direction of the position  156 , as shown with a broken line. The aforementioned portion  154  ( FIGS. 9 and 10 ) will thereby move down from the position of blocking the release ball  57 , thereby allowing the ball  57  to be pushed through the opening  153  to release the striker element  52 , which is then accelerated down to strike and ignite the pyrotechnic material of the inertial igniter as was previously described for the embodiment of  FIG. 7 . 
         [0068]    In general, for the spring  151  to rotate (bend) enough to release the striker element  52 , the inertia of the spring  151  must be enough to overcome its stiffness to achieve the required amount of downward rotation (bending). However, if the inertial of the spring  151  is not enough for a given level of acceleration in the direction of the arrow  96 , the additional mass  157  ( FIG. 9 ) may be attached to the spring  151 . The size of the mass  157  and position of the mass  157  can be varied to achieve the desired spring  151  rotation (bending). 
         [0069]    In addition, the amount of acceleration in the direction of the arrow  96  that is required to allow the release ball  57  to be released should be at least equal to the specified no-fire acceleration of the inertial igniter  150  to ensure for safety. 
         [0070]    Referring now to  FIGS. 11 and 12 , therein is illustrated a multiple inertial igniter embodiment, generally referred to by reference numeral  300  in which similar elements are referred to with similar reference numerals from previous embodiments. Although the inertial igniter  90  of  FIG. 7  is used to describe such multiple inertial igniter embodiment, it will be appreciated that any of the previous embodiments described above can be used, and each of the individual inertial igniters can be the same or more than one type of inertial igniter discussed above can be employed. Further, while the inertial igniter  300  of  FIGS. 11 and 12  is described with regard to four inertial igniters, it will also be appreciated that any number more than one can be employed. The inertial igniter  300  is illustrated in  FIG. 11  without a top cover  312  (which optional, but nonetheless not shown in  FIG. 11  so as to be able to view the components therein). 
         [0071]    The inertial igniter  300  of  FIGS. 11 and 12  is configured as a cylinder, but can be any shape or size. The inertial igniter  300  includes a first cylinder  302  and second cylinder  304 , where the first cylinder  302  has a larger diameter than the second cylinder  304 . For ease of manufacturing, each of the first and second cylinders  302 ,  304  have a closed bottom  306 ,  308 , respectively. However, they can share a common bottom or use a surface of the thermal battery as a bottom. 
         [0072]    The inertial igniters  90 , are distributed about a central post  310  about which the striker mass  52  and lever element  91  are pivotably connected (about pivots  53  and  92 , respectively). The spring element  95  is disposed in a space between the first and second cylinders  302 ,  304  to bias the lever element in the position shown in  FIG. 12 . The lever element is disposed in a slot  312  formed in the second cylinder so as to be able to rotate about the pivot  92 . The lever element can be biased directly against the ball  57 , as shown in  FIG. 7 , or spaced therefrom, as shown in  FIG. 12 . 
         [0073]    During the firing, the inertial igniters  90  are considered to be subjected to setback acceleration in the direction of the arrow  96 . Acceleration in the direction of the arrow  96  will act on the inertia of the inertia of the lever element  91 , and generate a downward force that would tend to rotate the lever element  91  in the clockwise direction. The compression preloading of the spring element  95  will, however, resists the clockwise rotation of the lever element  91 . The level of compressive preloading of the spring element  95  is selected such that with the no-fire acceleration levels, the inertia force acting on the lever element  91  would not overcome the preloading force of the spring element  95 . As a result, the inertial igniter  90  is ensured to satisfy its prescribed no-fire requirement. 
         [0074]    Now if the acceleration level in the direction of the arrow  96  is high enough, then the aforementioned inertia force acting on the lever element  91  will overcome the preloading force of the spring element  95 , and will begin rotate in the clockwise direction. Now if the acceleration level is applied over a long enough period of time as well, i.e., if the all-fire condition is satisfied, then the lever element  91  will have enough time to rotate enough in the clockwise direction to allow the locking ball  57  to be pushed out of the dimple  56 , thereby releasing the striker mass  52  and allowing it to be accelerated in the clockwise rotation. As a result, for a properly designed inertial igniter  90  (i.e., by selecting a proper mass and moment of inertial for the striker mass  52  and range of clockwise rotation for the striker mass  52  so that it would gain enough energy), the striker mass  52  will gain enough energy to initiate the pyrotechnic material  64  between the pinching points provided by the protrusions  65  and  66  on the base element  51  and the bottom surface of the striker mass  52 , respectively, as shown in the schematic of  FIG. 4 . The ignition flame and sparks can then travel down through the opening  67  provided in the base element  51 . When assembled in a thermal battery similar to the thermal battery  16  of  FIG. 1 , the inertial igniter is mounted in the housing  10  such that the openings  67  are lined up with corresponding openings  12  into the thermal battery  11  to activate the battery by igniting its heat pallets. 
         [0075]    The multiple inertial igniters  90  increase the reliability of the overall igniter  200  since only one has to initiate in order to produce the required spark to ignite the thermal battery. Furthermore, the springs and/or striker masses can be the same for each of the inertial igniters  90  in the multiple inertial igniter  300  of vary between inertial igniters  90 . 
         [0076]    In the above embodiments, the disclosed devices are intended to actuate, i.e., release their striker mass (element  22  in the embodiment of  FIG. 2  and element  52  in the embodiments of  FIGS. 4 ,  7 ,  9  and  12 ) in response to an all-fire acceleration level in the direction of the indicated arrow and accelerate downwards to impact the provided pyrotechnics materials causing them to ignite. The same mechanism used for the release of the striker mass due to an all-fire acceleration can be used to provide the means of opening or closing an electrical circuit, i.e., act as a so-called G-switch, that is actuated only if it is subjected to an all-fire acceleration profile, while staying inactive during all no-fire conditions, even if the acceleration level is higher than the all-fire acceleration level but significantly shorter in duration. As a result, this novel G-switch device would satisfy all no-fire (safety) requirements of the device in which it is used while activating in the prescribed all-fire condition. 
         [0077]    A schematic of such an embodiment is shown in  FIG. 13 . The G-switch  350  is similar to the inertial igniter illustrated in  FIG. 9 , except that its pyrotechnic material and initiation elements (elements  64  and  65 - 67  in  FIG. 4  and shown without the indicating numerals in  FIG. 9 ) are removed. An element  355  which is constructed of an electrically non-conductive material is fixed to the base  51  of the device as shown in  FIG. 13 . The element  355  is provided with two electrically conductive elements  361  and  362  with contact ends  356  and  357 , respectively. The electrical wires  358  and  359  are in turn attached to the electrically conductive elements  361  and  362 , respectively. As it was described for the embodiment  150  of  FIG. 9 , when the device is subjected to an all-fire acceleration in the direction of arrow  351 , the acceleration acts on the inertia of the spring  151  and tend to rotate (bend) it down in the direction of the position  156 , as shown with a broken line. The portion  154  ( FIGS. 9 and 10 ) will thereby move down from the position of blocking the release ball  57 , thereby allowing the ball  57  to be pushed through the opening  153  to release the element  352  (striker element  52  in  FIG. 9 ), which is then accelerated downward. The element  352  is provided with a flexible strip of electrically conductive material  353  which is fixed to the bottom surface of the element  352  (such as by being soldered or attached with fasteners  354 ). Therefore, as the element  352  moves downward towards the base  51  of the device, it would cause the flexible electrically conductive strip  353  to come into contact with the contacts  356  and  357 , thereby causing the circuit through the wires  358  and  359  to close. The element  352  can be provided with a biasing tensile spring  363  (or torsional spring positioned at its rotating joint  53 ,  FIG. 7 ), to ensure that the flexible electrically conductive strip  353  stays in contact with the contacts  356  and  357 . It is noted that in the schematic of  FIG. 13 , the biasing tensile spring is shown to be attached to base  51  for the sake of simplicity only, and alternatively a compressively biased spring (helical or flexural type—not shown) may be positioned between the elements  151  and  352  to serve the same purpose. 
         [0078]    It is appreciated by those skilled in the art that the “normally open” (G-switch) device  350  may be readily modified to open an already closed (“normally closed”) electrical circuit, or provide the means to close (open) the electrical circuit and open (close) it after the all-fire acceleration event. 
         [0079]    The latter goal is achieved by simply changing the biasing tensile spring  363  into a biasing compressive spring (converting the aforementioned compressively biased spring between the elements  151  and  352  into a biased tensile spring). As a result, after the all-fire acceleration has ended, the biasing spring would push (pull) the element  352  and thereby the flexible electrically conductive strip  353  away from the contacts  356  and  357 . 
         [0080]    The G-switch  350  of  FIG. 13  can also be readily modified to provide a “normally close” switching configuration. As an example, the contact components of the G-switch  350  may be modified to that shown in the schematic of  FIG. 14 . This embodiment  370  of the G-switch has all its other components being the same as those of the embodiment  350  of  FIG. 13 . The “normally closed” G-switch  370  is provided with two flexible contact elements  371  and  372 , which are fixed to the electrically non-conductive member  375 , which is fixed to the base  51  of the device  371 . The flexible contact elements  371  and  372  are provided with contact points  373  and  374 , which are normally in contact (such as by being biased towards each other), thereby causing the wires  356  and  357  that are attached to the contact elements  371  and  372  to close the electrical circuit to which they are connected to. The element  352  is provided with a non-conductive member  378  as shown in  FIG. 14 . 
         [0081]    As was described for the embodiment  150  of  FIG. 9 , when the device is subjected to an all-fire acceleration in the direction of arrow  351 , the element  352  (striker element  52  in  FIG. 9 ), is released and is accelerated downward. As the non-conductive member  378  reaches the contact points  373  and  374 , the force of the acceleration acting on the inertia of the element  372  causes the member  378  to be inserted between the contact points  373  and  374 , thereby rendering their contacts open and opening the aforementioned electrical circuit to which the wires  376  and  377  are connected. 
         [0082]    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.