# Patent

## Title
Electrically initiated inertial igniters for thermal batteries and the like

## Abstract

A method for electrically initiating an inertial igniter for a munition. The method including: generating a voltage over a duration of an acceleration of the munition; powering a circuit from the generated voltage for determining when the acceleration experienced by the munition is an all-fire condition; electrically activating a reserve battery by the generated voltage when the circuit determines that the acceleration experienced by the munition is the all-fire condition; and electrically initiating pyrotechnic materials at a specified time into a flight of the munition using the reserve battery.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation application of U.S. application Ser. No. 12/164,096 filed on Jun. 29, 2008, which claims the benefit of prior filed U.S. Provisional Application No. 60/958,948 filed on Jul. 10, 2007, the contents of each of which is incorporated herein by reference. This application is related to U.S. Patent Application Publication No. 2008/0129151 filed on Dec. 3, 2007, the contents of which is also incorporated herein by reference.


BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to electrically initiated inertial igniters that require no external batteries for their operation, and more particularly to compact inertial igniters for thermal batteries used in gun-fired munitions and mortars and the like.
2. Prior Art
Thermal batteries represent a class of reserve batteries that operate at high temperatures. Unlike liquid reserve batteries, in thermal batteries the electrolyte is already in the cells and therefore does not require a distribution mechanism such as spinning. The electrolyte is dry, solid and non-conductive, thereby leaving the battery in a non-operational and inert condition. These batteries incorporate pyrotechnic heat sources to melt the electrolyte just prior to use in order to make them electrically conductive and thereby making the battery active. The most common internal pyrotechnic is a blend of Fe and KClO4. 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)/FeS2 or Li(Si)/CoS2 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.
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.
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. Currently, the following two distinct classes of igniters are available for use in thermal batteries.
The first class of igniters operates based on externally provided electrical energy. Such externally powered electrical igniters, however, require an onboard source of electrical energy, such as a battery or other electrical power source with related shelf life and/or complexity and volume requirements to operate and initiate the thermal battery. Currently available electric igniters for thermal batteries require external power source and decision circuitry to identify the launch condition and initiate the pyrotechnic materials, for example by sending an electrical pulse to generate heat in a resistive wire. The electric igniters are generally smaller than the existing inertial igniters, but they require some external power source and decision making circuitry for their operation, which limits their application to larger munitions and those with multiple power sources.
The second class of igniters, commonly called “inertial igniters”, operate based on the firing acceleration. The inertial igniters do not require onboard batteries for their operation and are thereby used often in high-G munitions applications such as in non-spinning gun-fired munitions and mortars. This class of inertial igniters is designed to utilize certain mechanical means to initiate the ignition. Such mechanical means include, for example, the impact pins to initiate a percussion primer or impact or rubbing acting between one or two part pyrotechnic materials. Such mechanical means have been used and are commercially available and other miniaturized versions of them are being developed for thermal battery ignition and the like.
In general, both electrical and 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 all igniters.
In recent years, new and 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 fuzing and other similar applications, and electrical igniters require some external power source and decision making circuitry for their operation, making them impractical for use in small and low power thermal battery applications.
In addition, the existing inertial igniters are not capable of allowing delayed initiation of thermal batteries, i.e., initiation a specified (programmed) and relatively long amount of time after the projectile firing. Such programmable delay time capability would allow thermal batteries, particularly those that are used to power guidance and control actuation devices or other similar electrical and electronic devices onboard gun-fired munitions and mortars to be initiated a significant amount of time into the flight. In such applications, particularly when electrical actuation devices are used, a significant amount of electrical power is usually required later during the flight to aggressively guide the projectile towards the target. Thus, by delaying thermal battery initiation to when the power is needed, the performance of the thermal battery is significantly increased and in most cases it would also become possible to reduce the overall size of the thermal battery and its required thermal insulation.
A review of the aforementioned merits and shortcomings of the currently available electrical and inertial igniters clearly indicates that neither one can satisfy the need of many thermal batteries, particularly the small nor miniature thermal batteries and the like, for small size igniters that are programmable to provide the desired initiation delay time and to operate safely by differentiating all-fire and various no-fire events such as accidental drops and vibration and impact during transportation and loading and even nearby explosions.
A review of the aforementioned merits and shortcomings of the currently available electrical and inertial igniters also clearly indicates the advantages of electrical initiation in terms of its reliability and small size of electrical initiation elements such as electrical matches, the possibility of providing “programmable” decision making circuitry and logic to achieve almost any desired all-fire and no-fire acceleration profiles with the help of an acceleration measuring sensor, and to provide the means to program initiation of the thermal battery or the like a specified amount of time post firing or certain other detected event, but also their main disadvantage in terms of their requirement of external batteries (or other power sources) and electronic and electric circuitry and logic and acceleration sensors for the detection of the all-fire event. On the other hand, the review also indicates the simplicity of the design and operation of inertial igniters in differentiating all-fire conditions from no-fire conditions without the use of external acceleration sensors and external power sources.
SUMMARY OF THE INVENTION
A need therefore exists for miniature electrically initiated igniters for thermal batteries and the like, particularly for use in gun-fired smart munitions, mortars, small missiles and the like, that operate without external power sources and acceleration sensors and circuitry and incorporate the advantages of both electrical igniters and inertial igniters that are currently available. Such miniature electrically initiated igniters are particularly needed for very small, miniature, and low power thermal batteries and other similar applications. For example, flexible and conformal thermal batteries for sub-munitions applications may occupy volumes as small as 0.006 cubic inches (about 100 cubic millimeters). This small thermal battery size is similar in volume to the inertial igniters currently available and used in larger thermal batteries.
An objective of the present invention is to provide a new class of “inertial igniters” that incorporates electrical initiation of the pyrotechnic materials without the need for external batteries (or other power sources). The disclosed igniters are hereinafter referred to as “electrically initiated inertial igniters”. The disclosed “electrically initiated inertial igniters” utilize the firing acceleration to provide electrical power to the igniter electronics and decision making circuitry, start the initiation timing when the all-fire condition is detected, and electrically initiate the pyrotechnic materials at the specified time into the flight. In addition, electrical initiation of pyrotechnic materials is generally more reliable than impact or rubbing type of pyrotechnic initiation. In addition, electronic circuitry and logic are more readily configured to be programmable to the specified all-fire and no-fire conditions.
The method of providing electrical power includes harvesting electrical energy from the firing acceleration by, for example, using active materials such as piezoelectric materials. The method of providing electrical power also includes activation of certain chemical reserve micro-battery using the aforementioned harvested electrical energy, which would in turn provide additional electrical energy to power different components of the “electrically initiated inertial igniter”.
The disclosed “electrically initiated inertial igniters” can be miniaturized and produced using mostly available mass fabrication techniques used in the electronics industry, and should therefore be low cost and reliable.
To ensure safety and reliability, all inertial igniters, including the disclosed “electrically initiated inertial igniters” must 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 the ordinance, i.e., an all-fire condition, the igniter must initiate with high reliability. In many applications, these two requirements compete with respect to acceleration magnitude, but differ greatly in their duration. For example:

    
    
        An accidental drop may well cause very high acceleration levels—even in some cases higher than the firing of a shell from a gun. However, the duration of this accidental acceleration will be short, thereby subjecting the inertial igniter to significantly lower resulting impulse levels.
        It is also conceivable that the igniter will experience incidental long-duration acceleration and deceleration cycles, whether accidental or as part of normal handling or vibration during transportation, during which it must be guarded against initiation. Again, the impulse input to the igniter will have a great disparity with that given by the initiation acceleration profile because the magnitude of the incidental long-duration acceleration will be quite low.
    
    


The need to differentiate accidental and initiation acceleration profiles by their magnitude as well as duration necessitates the employment of a safety system which is capable of allowing initiation of the igniter only during all-fire acceleration profile conditions are experienced.
In addition to having a required acceleration time profile which should initiate the igniter, requirements also commonly exist for non-actuation and survivability. For example, the design requirements for actuation for one application are summarized as:
1. The device must fire when given a [square] pulse acceleration of 900 G ±150 G for 15 ms in the setback direction.
2. The device must not fire when given a [square] pulse acceleration of 2000 G for 0.5 ms in any direction.
3. The device must not actuate when given a 1⁄2-sine pulse acceleration of 490 G (peak) with a maximum duration of 4 ms.
4. The device must be able to survive an acceleration of 16,000 G, and preferably be able to survive an acceleration of 50,000 G.
The electrical and electronic components of the disclosed electrically initiated inertial igniters are preferably fabricated on a single platform (“chip”), and are integrated into either the cap or interior compartment of thermal batteries or the like, in either case preferably in a hermetically sealed environment. The disclosed electrically initiated inertial igniters should therefore be capable of readily satisfying most munitions requirement of 20-year shelf life and operation over the military temperature range of −65 to 165 degrees F., while withstanding high G firing accelerations.
Some of the features of the disclosed “electrically initiated inertial igniters” for thermal batteries for gun-fired projectiles, mortars, sub-munitions, small rockets and the like include:

    
    
        1. The disclosed (miniature) electrically initiated inertial igniters are capable of being readily “programmed” to almost any no-fire and all-fire requirements or multiple predefined setback environments. For these reasons, the disclosed miniature electrically initiated inertial igniters are ideal for almost any thermal battery applications, including conformal small and low power thermal batteries for fuzing and other similar munitions applications.
        2. The disclosed (miniature) electrically initiated inertial igniters can be fabricated entirely on a chip using existing mass fabrication technologies, thereby making them highly cost effective and very small in size and volume.
        3. The disclosed (miniature) electrically initiated inertial igniters do not require any external power sources for their operation.
        4. In those applications in which the thermal battery power is needed for guidance and control close to the target, the disclosed (miniature) electrically initiated igniters can be programmed to initiate ignition long after firing, thereby eliminating the effects of thermal battery cooling.
        5. The disclosed (miniature) electrically initiated inertial igniters are solid-state in design. Their final total volume is therefore expected to be significantly less than those of currently available electrical and inertial igniters.
        6. The disclosed (miniature) electrically initiated inertial igniter is capable of electric initiation of Zr/BaCrO4 heat paper mixtures or their equivalents as is currently practiced in thermal batteries.
        7. The disclosed (miniature) electrically initiated inertial igniters are readily packaged in sealed housings using commonly used mass-manufacturing techniques. As a result, safety and shelf life of the igniter, thermal battery and the projectile is significantly increased.
        8. The solid-state and sealed design of the disclosed (miniature) electrically initiated inertial igniters should easily provide a shelf life of over 20 years and capability to operate within the military temperature range of −65 to 165 degrees F.
        9. The disclosed (miniature) electrically initiated inertial igniters can be designed to withstand very high-G firing accelerations in excess of 50,000 Gs.
        10. The disclosed (miniature) electrically initiated inertial igniters are programmable for any no-fire and all-fire requirements and delayed initiation time following an all-fire event. The disclosed igniters could therefore be used with other electrically activated igniters for thermal batteries, munitions or other similar applications.
        11. The disclosed (miniature) electrically initiated inertial igniters can be designed to conform to any geometrical shape of the available space and thermal batteries.
    
    





BRIEF DESCRIPTION OF THE DRAWINGS
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:
FIG. 1 illustrates the block diagram of the first class of the disclosed piezoelectric element based class of programmable electrically initiated inertial igniter embodiments.
FIG. 2 illustrates the piezoelectric powered programmable event detection and logic circuitry for differentiating all no-fire events from all-fire events and to initiate igniter only when all-fire event is detected.
FIG. 3 illustrates a comparison of an accidental drop from the firing acceleration induced voltages.
FIG. 4 illustrates an alternative piezoelectric powered programmable event detection and logic circuitry for differentiating all no-fire events from all-fire events and to initiate igniter with a programmed time delay following all-fire event detection.
FIG. 5 illustrates an alternative piezoelectric powered programmable event detection and logic circuitry for differentiating all no-fire events from all-fire events and to initiate igniter with a programmed time delay for medium caliber rounds and the like.
FIG. 6 illustrates a piezoelectric powered programmable event detection and logic circuitry design for event detection and initiation for operation over time periods ranging from minutes to days.
FIG. 7 illustrates the block diagram of the second class of the disclosed piezoelectric element based programmable electrically initiated inertial igniter embodiments employing reserve electrically activated micro-batteries for pyrotechnic initiation.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The block diagram of a first embodiment of a programmable electrically initiated inertial igniter is shown in FIG. 1. In this embodiment, an appropriately sized piezoelectric element (different options of which are described later in this disclosure) is used, which responds to the axial accelerations and/or decelerations of the munitions or the like, to which it is affixed via a thermal battery or the like. In response to the aforementioned axial accelerations and/or decelerations of the piezoelectric element, a charge is generated on the piezoelectric element due to the resulting forces acting on the piezoelectric element due to its mass and the mass of other elements acting on the piezoelectric element (if any). As a result, the sign of the corresponding voltage on the piezoelectric element would readily indicate the direction of the axial acceleration that is applied to the munitions due to the firing or accidental dropping or other similar no-fire conditions.
However, the detection of the generated piezoelectric element voltage levels alone is not enough to ensure safety by distinguishing between no-fire and all-fire conditions. This is the case since in certain accidental events such as direct dropping of the igniter, thermal battery and/or the munitions, the acceleration levels that are experienced by the igniter may be well above that of the specified all-fire acceleration level requirements. For example, when an igniter is dropped over a hard surface, it might experience acceleration levels of up to 2000 Gs for an average duration of up to 0.5 msec. However, the all-fire acceleration level may be significantly lower, for example around 500 Gs, with the difference being in its duration, which may be around 8-15 msec.
In addition, it is desired to harvest the electrical energy generated by the piezoelectric elements and store the electrical energy in a storage device such as a capacitor to power the igniter electronics circuitry and logics and to initiate the electrical ignition element when all-fire conditions are detected. Then if the voltage of the storage device such as the capacitor is to be monitored for the detection of the all-fire conditions, then very long term vibration type oscillatory accelerations and decelerations of relatively low levels which may be experienced during transportation or the like may also bring the voltage of the storage capacitor to the level corresponding to the all-fire levels. It is therefore evident that the voltage levels generated by active elements such as piezoelectric elements alone, or total accumulated energy cannot be used to differentiate no-fire conditions from all-fire conditions in all munitions since it may have been generated over relatively long periods of time due to vibration or other oscillatory motions of the device during transportation or the like.
Thus, to achieve one single electrically initiated inertial igniter design that could work for different types of munitions and the like, the igniter has to be capable of differentiating no-fire high-G but low duration acceleration profiles from those of all-fire and significantly longer duration acceleration profiles. The device must also differentiate between low amplitude and long term acceleration profiles due to vibration and all-fire acceleration profiles.
Obviously, if in certain munitions the all-fire acceleration levels were significantly higher than the no-fire acceleration levels, then the aforementioned voltage levels of the piezoelectric element used in an igniter device could be used as a threshold to activate the heating element (wire electrode) to initiate the pyrotechnic material or initiate the initiation “delay timing clock”. However, since the all-fire acceleration levels are lower than the no-fire acceleration levels in some munitions, therefore to achieve one single electrically initiated inertial igniter design that could work for all different types of munitions; the igniter has to be capable of differentiating the two events based on the duration of the experienced acceleration profile. In any case, the igniter device must still differentiate long term low acceleration vibration profiles from those of all-fire acceleration profiles.
The block diagram of FIG. 1 shows the general schematics of an embodiment of an electrically initiated inertial igniter. In the igniter of FIG. 1, at least one piezoelectric element is used to generate a charge (electrical energy) in response to the acceleration and/or deceleration profile that it experiences due to all no-fire and all-fire events. The charge generated by the piezoelectric element is then used to power the detection and safety electronics and logic circuitry and the detonation capacitor and its activation circuitry, as described later in this disclosure. In one embodiment, the electrical energy from the piezoelectric element is stored in a separate and relatively small capacitor that would act as a controlled power source to power the logic circuit. This power, supplied by the charged capacitor, would be used to activate the monitoring circuit logic to provide functionality, allowing for a range of triggering events to be detected from the piezoelectric element that are not directly coupled to peak voltage or energy detection of the piezoelectric element. In this way, circuits can be designed as described below to prevent detection of momentary spike voltage that could be accidentally generated by random vibrations or accidental droppings or other similar accidental events, indicating a false ignition condition.
The design of the electronics of a programmable electrically initiated inertial igniter is intended to address the following two basic requirements. The first requirement is to ensure safety and reliability of the thermal battery which must not be initiated during accidental drops, transportation vibration, manufacturing or other handling, miss-fire conditions and the like. The second requirement, which is achievable in a miniature igniter only with electronics circuitry, is related to one of the key benefits added by electrically operated ignition systems, i.e., the control of the time of battery initiation, which would allow munitions design engineer to have better control over the power budget and the mission profile of the guided rounds. Furthermore, by having the ability to initiate thermal battery at any point of time during the flight of a round allows munitions designer to optimize the size and efficiency of the thermal battery by operating it at optimum temperature and thereby reduce its required size.
The following two basic and general event detection, safety and ignition electronics and logic circuitry options may be used in the various embodiments disclosed herein. It is, however, appreciated by those skilled in the relevant art that other variations of the present detection and logic circuitry may also be constructed to perform the desired functions, which are intended to be within the scope and spirit of the present disclosure.
FIG. 2 shows the basic diagram of one possible design of the electronics circuitry for use in a piezoelectric element powered electrically initiated inertial igniter. The circuitry shown in FIG. 2 is not designed to provide a programmable initiation time delay. This feature is shown in a subsequent embodiment described below. The circuitry functions as a reusable power source based on harvesting energy from the at least one piezoelectric element and storing the harvested energy in the capacitor C1. A dedicated safety feature function (Safety Programming Feature) detects accidental drop or other accidental vibration or impact and determines when it is safe to initiate the battery. A third dedicated function (Initiation Trigger Mode) operates the initiation device which starts the battery initiation process, i.e., to ignite the igniter pyrotechnic material. The circuit incorporates circuitry to compare thresholds of energy generated by events and compares these thresholds with appropriately selected reference voltages at IC1 and IC2 to operate logic that drives the output switching stages T1 and T2.
The circuitry in FIG. 2 receives energy from at least one piezoelectric element that converts mechanical energy harvested from the firing acceleration into electrical charge. Diode bridge B1, rectifies this energy and dumps it into the capacitor C1 which is sufficiently large to serve as a power supply to the rest of the circuitry. The diode bridge B2 converts a very small portion of the energy generated by the piezoelectric generator to operate the Safety Programmable Feature and charges the capacitor C2. The energy stored in the capacitor C2 is measured by the resistor R2 and discharge resistor R16. The voltage at C2 (VC2) is compared with (VT1) at the midpoint of R4 and R5. When VC2 is higher than VT1, the output of IC1 become transitions to a high state and sets flip-flop IC3 and the flip-flop output Q transitions to a high state which causes switching transistor T1 to open and not allow power from reaching the initiator.
The initiator trigger mode operates in a similar fashion except that the time constant of R3 and C3 and bleed resistor R15 is significantly greater than the time constant of the Safety Programmable Feature. Similar to the operation of IC1, IC2 verifies that the voltage at C3 (VC3) is greater than the voltage VT2. When this occurs the output of IC2 transitions to a high state and causes switching transistor T2 to conduct and power the initiator. Note that this could only happen if the transistor T1 is enabled to conduct (IC1 output, Q, is low).
The logic circuits IC3 and IC4 operate to ensure that the initiator cannot be activated when accidental energy is generated by the piezoelectric element, such as during an accidental drop, transportation vibration or other handling situations. The sequence of operation is as follows: when the power first turns on, IC3 is reset by the OR circuit, this ensures that IC3 is now ready to detect accidental energy. Note that this enables T1 to provide power to T2. However, switching transistor T2 is open which prevents T2 from powering the initiator of the battery. The function of the OR circuit is to initialize IC3 when the power first turns on and also to initialize IC3 when an all-fire signal occurs. Initializing IC3 will allow the firing circuit comprised of switching transistor T1 and T2 to be able to power the initiator.
The overall functionality of the electrically initiated inertial igniter circuitry is controlled by the Safety Programmable Feature (SPF) time constant and by the Initiation Trigger Mode (ITM) time function. For example, for the aforementioned no-fire and all-fire requirements, the SPF time constant is 0.5 msec and the ITM time constant is 15 msec. Thus the safety feature will always occur first as shown in FIG. 3. In situations such as transportation of the device in which the thermal battery or the like is mounted, the device will be subjected to continuing vibration or vibration like oscillatory loading. In such situations, when the vibration continues, the present device would still provide for safety and prevents the initiator from being powered. The safety cushion is governed by a time constant of 14.5 msec, which is controlled by both R2 and R3.
FIG. 4 shows the diagram of another possible design of the piezoelectric element powered electronics circuitry with programmable initiation time delay feature for use in the disclosed electrically initiated inertial igniters. This design includes an integrated capability to delay the initiation signal by a selected (programmed) amount of time, which could be in seconds and even minutes or more.
In the design shown in FIG. 4, power stored in power supply capacitor C1 is harvested similarly from the at least one piezoelectric element and rectified by the bridge rectifier B1. The voltage at C1 rises to the operational value and it is now ready to start powering the electronics, however, during the transitional state it is very important that the comparator IC1 and IC2, and the OR gate be reset to its desired output value. Capacitors C6 and C7, stabilize and reset IC1 and IC2, respectively, and capacitor C4 resets the IC3, which ensures that switching transistor T1 is ready for operation. A second enhancement of the design shown in FIG. 4 compared to that of the design shown in FIG. 2 is related to the safe operation of the rectified output of the at least one piezoelectric element at the bridge rectifiers output. Diodes D1, D3 and D4 are clamping and transient suppression diodes. These devices ensure that high transient values of voltages produced by the piezoelectric elements do not reach the electronic circuits.
In the event detection and logic circuitry of FIG. 4, a programmable time delay capability to delay the signal to initiate the igniter is also incorporated. In this circuitry design, IC4, the resistor R17 and the capacitor C9 provide the time constant for the output of IC4 at R18 to provide a delayed output to the igniter initiator circuit. The delayed output is determined by the values of R17 and C9. This circuitry obviously offers for both non-delayed as well as delayed output depending on the application. Obviously any other programmable timing device may be used instead.
In certain applications such as medium caliber projectiles, the firing acceleration is very high, for example up to 55,000 Gs and even higher, therefore significantly higher than any accidental accelerations that may be experienced due to dropping. In addition, the volume available for the thermal battery and its igniter is very small.
For such applications, it is preferable that the battery be kept in its inactive state throughout the gun launch and until the acceleration forces resulting from setback and set forward have been significantly abated. For this reason, it is advantageous that initiation of the thermal battery be delayed after launch until the projectile has exited the gun barrel. For such applications, the event detection, safety and ignition electronics and logic and initiation time delay circuitry can be significantly simplified.
FIG. 5 shows a design of a circuit that will measure the setback acceleration by means of the at least one piezoelectric element. The signal produced by the piezoelectric element due to the setback acceleration is rectified and monitored by IC1 for peak amplitude and duration. These two parameters create a voltage (VC2) which will be compared by IC1. When voltage VC2 becomes higher than voltage VT1, IC1 will output a voltage which will reset IC2. At reset, IC2 will initiate a count of time which will be governed by the value of resistor R6 and capacitor C3. The output of IC2 will be buffered by switching transistor T1 which powers the initiator.
There are also military and civilian applications that require certain sensors be deployed and remain waiting for certain events for relatively long periods of time, ranging from minutes to hours or even days. To accomplish this purpose, a new type of timer will be employed to provide such a dynamic range (minutes to days) as shown in FIG. 6. IC2 can be programmed to deliver delay times from minutes to days by the use of a binary type counter which uses the clock generated by the parallel combination of R6 and C3 and multiplying it by a binary count depending on which output 2′′ is used.
In the circuitry shown in FIG. 6, the piezoelectric element will detect a launch or impact induced acceleration and/or deceleration, and the signal produced by the launch and/or impact forces will be rectified and detected by R1 and C2. The time constant provided by R1 and C2 will test the signal from the piezoelectric element for duration, and the comparison of the threshold voltage VC2 compared with VT1 will test the signal for amplitude threshold. When the threshold has been detected, IC1 will reset the binary counter IC2 which will start counting time. When the selected time delay has been reached, the output of counter will switch T1, upon which the initiator is powered.
The block diagram of FIG. 7 shows the general schematics of another embodiment of electrically initiated inertial igniters. In this class of igniters, at least one piezoelectric element is used to generate a charge (electrical energy) in response to the acceleration and/or deceleration profile that it experiences due to all no-fire and all-fire events. The charge generated by the piezoelectric element is then used to power the detection and safety electronics and logic circuitry and possibly partially the detonation capacitor and its activation circuitry, as described later in this disclosure. This class of concepts are similar to the previous class of electrically initiated inertial igniter embodiments shown in FIG. 1, with the main difference being that the electrical energy required to heat the wire electrode probe to initiate ignition of the pyrotechnic paper is provided mainly by a reserve micro-power battery, preferably fabricated on the aforementioned logic-based detection and switching circuitry chip, thereby significantly reducing the amount of power that the at least one piezoelectric element has to produce. In addition, since the energy density of the reserve battery is generally significantly higher than that of the piezoelectric elements, the resulting electrically initiated inertial battery is also expected to be smaller.
In this class of electrically initiated inertial igniter embodiments, essentially the same event detection, safety and ignition initiation electronics and logic circuitry described for the aforementioned first class of electrically initiated inertial igniters shown in FIG. 1 is employed with the exception that the power to initiate the ignition of the pyrotechnics comes mostly from the micro-power battery rather than the piezoelectric generator. As a result, more piezoelectric generated power is available to power the electronics and logic circuitry; thereby it is possible to add more safety features and even active elements to the circuitry. More sophisticated detection schemes and more layers of safety may also become possible to add to the igniter electronics.
One type of reserve micro-power battery that is suitable for the present application is micro-batteries in which the electrode assembly is kept dry and away from the active liquid electrolyte by means of a nano-structured and super-hydrophobic membrane from mPhase Technologies, Inc., 150 Clove Road 11th Floor, Little Falls, N.J. 07424. Then using a phenomenon called electro-wetting the electrolyte can be triggered by a voltage pulse to flow through the membrane and initiate the electrochemical energy generation. Such batteries have been fabricated with different chemistries.
In this class of electrically initiated inertial igniter embodiments, when the aforementioned event detection electronics circuitry and logic (such as those shown in FIGS. 2 and 4-6) detects the all-fire event, the circuit would then switch the required voltage to trigger and activate the reserve micro-power cell. In this concept, the piezoelectric element must only provide enough energy to the capacitor so that the required voltage is generated in the capacitor for activation of the reserve battery. For this purpose and for the aforementioned reserve micro-power cell, the capacitor may have to provide a brief voltage pulse of approximately 50 milliseconds duration of between 30-70 volts. It is important to note that the triggering activation voltages required for electrowetting technique to activate the reserve power cell requires negligible current from the storage capacitor.
The expected size and volume of the class of electrically initiated inertial igniter embodiments shown in the block diagram of FIG. 7 is expected to be less than those for the embodiments constructed based on the block diagram of FIG. 1. This is expected to be the case since a significantly smaller piezoelectric element will be needed for the activation of the aforementioned reserve micro-power battery, which could be of the order of 1 mm2 surface area and integrated onto the logic and switching circuitry. In addition, the capacitor used for triggering the reserve micro-power battery is expected to be significantly smaller than that of the class of igniters shown in the block diagram of FIG. 1. In addition, the power required to activate the reserve micro-power battery is minimal.
The use of piezoelectric elements (preferably in stacked configuration) for energy harvesting in gun-fired munitions, mortars and the like is well known in the art, such as at Rastegar, J., Murray, R., Pereira, C., and Nguyen, H-L., “Novel Piezoelectric-Based Energy-Harvesting Power Sources for Gun-Fired Munitions,” SPIE 14th Annual International Symposium on Smart Structures and Materials 6527-32 (2007); Rastegar, J., Murray, R., Pereira, C., and Nguyen, H-L., “Novel Impact-Based Peak-Energy Locking Piezoelectric Generators for Munitions,” SPIE 14th Annual International Symposium on Smart Structures and Materials 6527-31 (2007); Rastegar, J., and Murray, R., “Novel Vibration-Based Electrical Energy Generators for Low and Variable Speed Turbo-Machinery,” SPIE 14th Annual International Symposium on Smart Structures and Materials 6527-33 (2007). Rastegar, J., Pereira, C., and H-L.; Nguyen, “Piezoelectric-Based Power Sources for Harvesting Energy from Platforms with Low Frequency Vibration,” SPIE 13th Annual International Symposium on Smart Structures and Materials 6171-1 (2006) and U.S. Patent Application Publication No. 2008/0129151 filed on Dec. 3, 2007. In such energy harvesting power sources that use piezoelectric elements, the protection of the piezoelectric element from the harsh firing environment is essential and such methods are fully described in the above provided references.
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.

## Claims

1. An electrically initiated inertial igniter for a munition, the electrically initiated inertial igniter comprising:
an electrical energy harvesting device for generating a voltage over a duration of an acceleration of the munition;
a circuit powered by the electrical energy storage device for determining when the acceleration experienced by the munition is an all-fire condition; and
an electrically activated reserve battery at least partially activated by the electrical energy harvesting device when the circuit determines that the acceleration experienced by the munition is the all-fire condition to electrically initiate pyrotechnic materials at a specified time into a flight of the munition;
wherein the circuit having a sub circuit having at least a capacitor for directing a portion of the charge resulting from the voltage and duration to the capacitor, wherein the all-fire condition is determined based on both the voltage and duration generated by the electrical energy harvesting device and a predetermined accumulated voltage in the capacitor.

2. The electrically initiated inertial igniter of claim 1, wherein the sub circuit further includes a draining element for draining any charge accumulated in the capacitor resulting from non-firing events.

3. The electrically initiated inertial igniter of claim 2, wherein the draining element is a resistor.

4. The electrically initiated inertial igniter of claim 1, wherein the electrical energy harvesting device is a piezoelectric generator.

5. The electrically initiated inertial igniter of claim 1, wherein the circuit is capable of being programmed to predetermined all-fire requirements.

6. The electrically initiated inertial igniter of claim 1, wherein the electrical harvesting device is configured to provide a voltage pulse to the electrically activated reserve battery to activate the electrically activated reserve battery.

7. The electrically initiated inertial igniter of claim 6, wherein the electrically activated reserve battery is configured to initiate electrowetting upon activation by the electrical harvesting device.

8. The electrically initiated inertial igniter of claim 1, wherein the electrically activated reserve battery at least partially provides power to a heating wire to initiate ignition in the pyrotechnic materials upon activation by the electrical harvesting device.

9. A method for electrically initiating an inertial igniter for a munition, the method comprising:
generating a voltage over a duration of an acceleration of the munition;
powering a circuit from the generated voltage for determining when the acceleration experienced by the munition is an all-fire condition;
electrically activating a reserve battery by the generated voltage when the circuit determines that the acceleration experienced by the munition is the all-fire condition;
electrically initiating pyrotechnic materials at a specified time into a flight of the munition at least partially using the reserve battery;
directing a portion of the charge resulting from the voltage and duration to a sub circuit having at least a capacitor; and
determining the all-fire condition based on both the generated voltage and duration and a predetermined accumulated voltage in the capacitor.

10. The method of claim 9, further comprising draining any charge accumulated in the capacitor resulting from non-firing events.

11. The method of claim 9, further comprising programming the circuit to predetermined all-fire requirements.

12. The method of claim 9, wherein the electrically activating comprises providing a voltage pulse to the reserve battery.

13. The method of claim 12, wherein the voltage pulse initiates electrowetting in the reserve battery.

14. The method of claim 9, wherein the electrically initiating comprises at least partially providing power from the reserve battery to a heating wire to initiate the pyrotechnic materials.