Abstract:
A substitute solid state device for safely initiating a sustainer motor is provided. The substitute device replaces a mechanism that is integral to a warhead. The substitute device interfaces to a telemetry package and is suitable for insertion into small housings. A specific embodiment is a substitute interface to a telemetry system incorporating a circuit for firing a sustainer motor of a small missile or rocket. The substitute interface replaces the interface and firing circuit associated with the warhead in a missile of 2.75-inch diameter, such as the STINGER missile.

Description:
FIELD OF THE INVENTION 
     The field of the invention is that associated with the provision of a replacement electromagnetic device used to initiate an action. Specifically a preferred embodiment of the present invention is an electromagnetic device that provides the necessary signal to initiate a sustainer motor on a small rocket or missile in which the warhead, with integral circuitry that is connected to a sustainer motor initiator, has been replaced by a telemetry package. 
     BACKGROUND 
     Despite the lack of test instrumentation specifically designed to fit in small volumes, there is continued pressure on the defense complex to deliver smaller, high performance weapon systems with quantifiable performance characteristics. Of course, it is expected that these be procured at a cost comparable to presently available weapon systems. 
     Currently there are no commercially available integrated secure telemetry systems suitable for use on small airframes, e.g., missiles or rockets. There are systems for recording data on board and later recovering the airframe and recorder as evidenced by those built by the U.S. Air Force at Eglin Air Force Base (AFB), Florida. These systems typically enjoy a 50% recovery rate, effectively doubling test requirements. Raytheon Corp. sells a system for use with the STINGER missile, however, it has no encryption capability nor does it have an IMU. The Navy at NAWC, China Lake, Calif. has built systems for use with small airframes but, these are not capable of encryption, have a limited number of channels for data capture and, do not have a fully capable IMU. Refer to U.S. Statutory Invention Registration H1288 , Control and Digital Telemetry Arrangement for an Aerial Missile , issued to Kenneth P. Lusk. 
     In addition to the problem of squeezing a high performance telemetry package into a missile or rocket in place of its warhead, the necessary circuitry to insure reliable firing of the rocket or missile&#39;s sustainer motor must be provided in the same telemetry package in order to replace that firing circuitry packaged with the original warhead. Previous versions of the firing circuit used with the telemetry package used latching relays, a mechanical G-switch and, analog timing circuits. These components were bulky and, would have been difficult to integrate into a high performance telemetry package for use in a small volume. A new design, employing size and energy efficient solid state components, including digital timers, was needed. 
     SUMMARY OF THE INVENTION 
     A preferred embodiment of the present invention is a system interface designed to safely and reliably insure the firing of a sustainer motor about 250 milliseconds (ms) after launch of a rocket or missile. The sequence is: 
     a longitudinal accelerometer, part of an inertial measurement unit (IMU) associated with a telemetry system, is sensed and upon reaching a pre-determined state, initiates timers within a programmable logic device (PLD); 
     the PLD uses internal power from the missile or rocket, e.g., the “fuze power,” to enable an initiator, typically a squib; 
     by removing an electrical short, i.e., a path to electrical ground, across the squib&#39;s input an supplying suitable energy to the squib, the squib is energized, firing the sustainer motor. 
     In the missile&#39;s dormant state, the sustainer squib is shorted to ground in order to enhance safety in handling the missile or rocket prior to launch. The electrical short is removed only when the following sequence occurs: 
     power up of the telemetry (TM) system and initiation of the missile&#39;s fuze battery occurring at launch, and 
     reaching and maintaining an acceleration force of 25 G, or more, for 20 ms in a 40 ms window. 
     Occurrence of this sequence then enables an energizing signal to be delivered to the squib&#39;s input, resulting in the firing of the missile&#39;s sustainer motor at about 250 ms after actual launch. 
     This interface is designed to work in conjunction with a telemetry package having a power distribution board and fitted into a housing designed to replace the warhead in the current 2.75″ family of small missiles and rockets. This integrated capability has not been able to be packaged for use in such small platforms heretofore. 
     Advantages of the solid state replacement firing circuit representing a preferred embodiment of the present invention are: 
     compact configuration suitable for installation in small volumes 
     capable of interfacing to both foreign and domestic systems 
     low cost 
     compatible with existing instrumentation 
     easily upgraded with removable boards 
     simple to maintain 
     easy to program 
     ruggedized 
     easy interface to existing and planned systems 
     reliable 
     low power consumption 
     With this replacement circuit designed to interface to an integrated telemetry system, test engineers and range instrumentation personnel will no longer have to provide work-arounds or, otherwise estimate performance of small weapons systems such as 2.75″ missiles or rockets. A rocket or missile will fly as if it had the actual warhead installed, enabling the vehicle&#39;s sustainer motor at the correct time after actual launch. Test data will be taken onboard, processed and, transmitted over a secure link to locations at which it can be properly analyzed, in near real time, for input to formal evaluations of the weapon system as it flies an actual test mission. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 represents the timing sequence of a preferred embodiment of the present invention. 
     FIG. 2 is a circuit diagram of a preferred embodiment of the present invention. 
     FIG. 3 is a circuit diagram of the timers within the PLD of a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The sequence of events pertaining to the operation of the circuit representing a preferred embodiment of the present invention follows. When power is applied to the system containing the circuit, a power-up reset circuit sets all outputs of a programmable logic device (PLD) to a pre-determined state. The timing of the reset circuit is mathematically described by:                V   C     =     S   -     S   *          -     t   RC                     (   1   )                                
     where:                                    V   C     =     voltage                 drop                 across                 the                 capacitor       ,     volts                   (   V   )                     S   =     charging                   (   source   )                   voltage       ,   V                       t   =   time     ,     seconds                   (   s   )                           R   =     resistance                 of                 an                 RC                 circuit       ,     ohms                   (   Ω   )                           C   =     capacitance                 of                 an                 RC                 circuit       ,     farads                   (   f   )                                    
     After reset, a clock is generated using an RC circuit and feedback. The clock frequency, f, in Hertz (Hz), can be approximated by:              f   =     1     2.2                 RC               (   2   )                                
     from Altera Corporation&#39;s  Application Handbook , July 1998, p. 66. This clock is used as reference for timing as shown in the timelines of FIG.  1 . 
     At launch, a launch accelerometer in the IMU of the onboard telemetry system sends a signal to the power/sustainer board, also more generally termed the power distribution board, from whence it is attenuated by an signal conditioning and, attenuator circuit consisting of a simple resistor divider network; a comparator having a reference voltage source, and a capacitor. When the accelerometer output attains a pre-determined value, the input level to the comparator rises causing the state of the comparator to switch. The output of the comparator is sent to an input of a PLD, enabling two timers. The first timer measures the time period that the launch accelerometer remains at or above a predetermined level. The second timer, termed the sustainer timer in a missile application, is used for control functions. When a pre-determined time has elapsed and, the first timer has determined that the accelerometer has remained at or, above its pre-determined level for a second pre-determined time, the second timer (sustainer timer) toggles a switch signal, sometimes referred to as “G-switch” in keeping with the nature of the output from the accelerometer. 
     The second timer has instituted a “guarantee” or safety signal that the missile has been launched. This status was determined by integrating the timer&#39;s received signal over the first predetermined time. Once the switch signal (G-switch) has toggled, the firing sequence for the sustainer motor is irreversible, since this G-switch signal toggles a physical switch that removes the “safety short” across the sustainer squib and, enables the connection that provides energy to the squib for firing the sustainer motor. However, if the launch accelerometer signal does not indicate a level at, or above, the predetermined level for the first predetermined time period, before the end of the second predetermined time period, the circuit is reset. 
     The sustainer motor&#39;s squib is shorted with a thermal relay. The current needed to fire the relay is provided by the missile&#39;s thermal battery. If the missile battery has not been initiated, such as by an actual launch, then the short to ground across the squib can not be removed. Under normal operation, the electrical short is removed at the end of the second pre-determined period after launch. At this time, the switch signal generated within the PLD, i.e., G-switch, is ANDed with a feedback signal from the sustainer timer&#39;s counters, causing an output on a physical switch, typically a MOSFET, allowing current through the thermal relay, thus interrupting the short to ground across the squib&#39;s input. 
     The sustainer timer is initiated by the comparator that switches state when the launch accelerometer signal indicates a G-force at, or above, a pre-determined value. The counters associated with the sustainer timer count to a third pre-determined time period at which time the signal to remove the short across the squib is toggled off and, the signal to fire the sustainer motor is sent to a second switch, typically a MOSFET, connected to the squib&#39;s input. The current for energizing the squib to fire the sustainer motor also comes from the missile&#39;s thermal battery, through the same line as the current, to remove the short across the squib. 
     At a fourth pre-determined time after launch, the squib&#39;s energizing current is removed by a signal from the sustainer timer by having the timer&#39;s counters disable the command signal to energize the squib. At this time these counters are also disabled from cycling again. To reset the counters, the power to the telemetry package has to be cycled. Under normal flight conditions, the firing circuit is inactivated for the remainder of the missile&#39;s flight. 
     The timing sequence for the key actions delineated above is provided in FIG.  1 . The timeline of activities  100  is plotted as a representation of activity start  101  versus time  102  relative to initiation of a firing command for a missile, designated as “trigger pull”  108  on the timeline  100 . All time and voltage values are provided as nominal values for relative comparison only and are not intended to be wholly representative of the invention. A launch signal  109  after trigger pull  108  may require  100  μs to get to the circuit to allow for rise time in the missile&#39;s thermal battery. At launch  109 , time t=0, the fuze voltage  103  has reached a suitable level to enable an attenuated trigger signal from the accelerometer  104 . At 40 ms after launch initiation, assuming the accelerometer still indicates a good launch, e.g., a 25 G force for at least 20 ms during the above 40 ms window, the G-switch  105  is toggled and the short across the sustainer motor&#39;s squib  106  is removed. At 250 ms after launch the sustainer motor&#39;s squib is energized  107 , firing the sustainer motor. 
     EXAMPLE 
     A preferred embodiment of the present invention has been configured for installation in a STINGER missile of nominal 2.75″ diameter. The missile&#39;s warhead has been removed and a telemetry system interfaced to a preferred embodiment of the present invention is installed in its place. Refer to FIG. 2 for the following discussion. 
     Upon powering up the telemetry system, a power up reset signal, nominally 5 V is input to the RC circuit  203  from which it is sent to the PLD  201 , being inserted on pin  202 . Values for resistor  203 A of 75 KΩ and capacitor  203 B of 0.47 μf are chosen so that the RC circuit  203  has a unique time constant compatible with the required setting of timers (not shown in FIG. 2) internal to the PLD  201 . Applying Eqn. 1, for a value of 0.8 V available at pin  202 , timing of the reset circuit, i.e., the time the voltage stays low after power is applied, is a guaranteed 6.15 ms. Therefore, the minimum guaranteed power-up reset pulse is one that is 6.15 ms in duration. 
     Upon power-up reset, a clock is generated using the RC circuit  204  comprising resistor  204 A of 976 Ω and capacitor  204 B of 0,47 μf. Inserting these values into Eqn. 2 yields a clock frequency of 990 Hz and a corresponding period of 1.009 ms. This clock controls the timing to carry out the timeline of FIG.  1 . 
     A launch accelerometer (not separately shown), located in the inertial measurement unit (IMU) (not separately shown) of the telemetry system (not separately shown), is the sensor that will provide necessary triggering data for a preferred embodiment of the present invention. It is an ANALOG DEVICE MODEL ADXL 190 having an input range of +125 G to −75 G. At zero G the output voltage of the accelerometer is 1.80 V and at +25 G the output is 2.207 V. The signal from the accelerometer is sent to the power distribution board  205 , inserted at pin  206 . From pin  206  the accelerometer output is attenuated by the resistor divider network  207  comprised of resistor  207 A of 143 KΩ and resistor  207 B of 121 KΩ. This provides an input voltage to the comparator  208  of 1.2 V when the output level from the accelerometer is at 2.207 V. The comparator is provided with its own reference voltage source  208 A. Once the comparator  208  reaches 1.2 V it switches states. The output of the comparator  208  is inserted at pin  209  of the PLD  201  whereupon it enables two timers (not separately shown in FIG.  2 ). The first timer tracks the time period in which the accelerometer output indicates a force at or above 25 G. The second timer, or sustainer timer, is used as the control timer for all functions of the interface representing a preferred embodiment of the present invention. 
     Upon counting a period of 40 ms, given that the accelerometer has indicated a force at or above 25 G for a period of 20 ms during the above 40 ms period, a signal termed “G-switch” is toggled. (If the accelerometer does not indicate a 25 G force for the entire 20 ms time period during the 40 ms time period, the circuit is reset.) This is provided at pin  210  of the PLD  201  and sent to pin  211  of the power distribution board  205 . The sustainer timer “guarantees” that the missile has launched, hence the signal indicating a 25 G force as measured by the accelerometer and provided by the comparator  208 , is integrated over time for the 40 ms period. Once the G-switch (not separately shown) has toggled, the firing sequence is irreversible. 
     The G-switch performs two functions. ANDed with a feedback signal from the sustainer timer, it initiates the removal of the short  212  across the squib&#39;s input  213 . It also starts the sustainer timer for energizing the squib to fire the sustainer motor. 
     The squib input  213  is shorted with a thermal relay  212 . Current (not separately shown) for firing the thermal relay  212  is provided by the missile&#39;s thermal battery (not separately shown) and is inserted at pin  214  on the power distribution board  205 . The missile&#39;s thermal battery is not activated unless the missile has been launched, hence the short across the squib can not be removed absent missile launch. 
     Once a “good” 40 ms post-launch period is determined, the G-switch signal is ANDed in the PLD  201  with feedback from the sustainer timer (not separately shown), causing an output at pin  215  that is provided to MOSFET switch  216 , allowing current to flow through thermal relay  212 . 
     In the case of the STINGER missile, the fuze power is provided at 20 V from pin  214  and the current limiting resistor  217  is zero, i.e., a short. The “on” resistance of MOSFET  216  is 0.028 Ω, the fuze power impedance is 3.45 Ω, and the total resistance of the thermal relay  212  is 4.828 Ω. This yields an average current of 4.14 amps (A), given a 20 V input. The response time for the relay (not separately shown), an M999, is about 60 ms. Thus, at about 110 ms after launch, the squib can be enabled. 
     The sustainer timer is initiated by the comparator  208 . The counters of the sustainer timer count to 250, representing 250 ms, at which time the signal to remove the short across the squib is toggled off at pin  218  of the PLD  201  to pin  219  of the power distribution board  205 , and the signal to energize the squib to fire the sustainer is inserted at pin  220  of the PLD  201  to MOSFET switch  221 . The current used to fire the sustainer also comes from the fuze power line connected to pin  214 , described above. For the example of a STINGER missile, the battery voltage available is 20 V and its impedance is 3.45 Ω. MOSFET switch  221  has an “on” resistance of 0.028 Ω and the sustainer squib (not separately shown) has a resistance of 1.70 Ω. Thus, the total resistance is 5.17 Ω, resulting in a firing current of 3.86 A until about 500 ms after launch. The counters (not separately shown) associated with the sustainer timer in the PLD  201  then initiate a signal to disable the “fire sustainer” command from pin  221 , simultaneously disabling these counters from initiating another count. To reset these counters, telemetry power must be cycled. Thus, with a successful missile launch and sustainer motor firing, this circuit becomes inactive. 
     All the circuits activated by the PLD are monitored by the telemetry system. They are all discrete signals provided at pins  222 ,  218 ,  221 ,  225 ,  210 ,  224 , and  223 . 
     FIG. 3 depicts the layout of the digital timer configuration  300  for the two counters  301  and  302  internal to the PLD  201 . The counter  301  counts the 20 ms period during which the longitudinal acceleration, input as signal longacc  303 , remains at or above 25 G. A feedback signal  304  is also provided to the counters to insure correlation. The counter  302  counts the 40 ms period during which the 20 ms period of acceleration is experienced, the 250 ms period during which the squib is energized for firing the sustainer, and the 500 ms period at the end of which the signal is inactivated. Upon initiation of an enable signal  305  and occurrence of a proper timing sequence, a firing signal  306  is output together with a firing monitoring signal  307  and a clock signal  308 . 
     Prior to an enabling signal  305 , signal GSWITCH  311 , ACCTRIG  312 , SQUIB  309 , and SQUIBMON  310 , are sent to insure the proper firing sequence once an enabling signal is called for by the initiation of a proper timing sequence. 
     Although a specific embodiment has been described in the specification and further represented in drawings, these are not to be taken as limiting. Rather, the full scope and meaning of the invention is to be as interpreted from the following claims.