Apparatus and method for performing built-in testing of a squib fire network

The present invention is directed to a squib fire network including built-in testing which is able to safely and efficiently verify the proper operational status of the squib fire network. All squibs in a given squib fire network of a missile can be tested prior to launch, thereby improving the reliability of weapon deployment.

BACKGROUND OF INVENTION
 1. Field of the Invention
 The present invention generally relates to squib fire networks which
 include a built-in testing system, and a method to evaluate the
 operational status of, and to determine faults within, a squib fire
 network.
 2. State of the Art
 A squib fire network includes at least one squib, a squib being an
 explosive device that comprises resistive elements, such as bridge wires.
 The bridge wires can be surrounded by explosive charges. When the
 resistive elements are charged through the passage of current, they
 detonate the explosive charges which explode and create a pressure wave
 that can be used to operate valves, explode larger charges (for use in
 weapons) and move pistons, and so forth. Within a missile for example, the
 firing of twenty-five or more squibs can be required to deliver the
 missile to its intended target. For example, squibs can be used to
 activate batteries, deploy wings, close doors, start engines, and actuate
 fuel valves.
 Authorities have been reluctant to test squibs installed in a device, for
 fear of activating the device in response to an unintended current. This
 is especially true for squibs used in military applications. For example,
 missiles can be held below a ship's deck in storage bunkers. The testing
 of the electronics of these missiles is quite dangerous. A broken switch
 could ignite the missile and cause an explosion. As such, military
 authorities have refrained from passing current through squibs to test
 their operability. In fact, this type of testing has been strictly
 forbidden, as closing any contacts that would normally be used to fire the
 squib during a detonation sequence of a missile presents too much
 potential danger.
 For example, U.S. Pat. No. 3,892,182 describes a squib control circuit
 which has squib filaments connected in series with a relay winding. A
 small amount of current is applied to the squib filaments, which light a
 lamp assciated with the relay, indicating the squib is ready to fire.
 Although the current directly applied to the squib is normally not high
 enough to "fire" the squib, a transient surge or spike in the current
 could inadvertently fire the squib. Thus, restrictions imposed on the
 testing of squib fire networks have rendered this test routine
 inapplicable to weapons because of its susceptibility to inadvertent
 firings.
 U.S. Pat. No. 3,619,792 discloses a self-test circuit for use in a weapons
 drop situation. This system reports a failure of a weapons release
 subsequent to a release sequence. Although partial testing of squibs is
 performed, there is no testing to ensure that a current through the squib
 will ignite the squib before the weapon is actually delivered to its
 target.
 Typically, at most two switches are involved in closing a squib fire
 network to provide fire current to the squib. Two switches are normally
 used so that if one switch fails and allows current to the squib, the
 second switch will block the current and the squib will not be prematurely
 ignited. For example, U.S. Pat. No. 5,621,326 discloses a squib which can
 be used for an airbag placed in an automobile. This system has the squibs
 placed between an acceleration switch and a second switch on the output
 side of the squib. However, should either squib produce a shorted
 condition after its activation, then the remaining squib may be starved of
 current and be unable to be activated. Furthermore, this circuit is not
 able to selectively activate a squib. Because this circuit increases the
 possibility of switch failure and inadvertent squib fire, it is
 inappropriate for use in all military applications.
 The restrictions imposed on the testing of squib fire networks used in
 weaponry have left the reliability of missiles employing these devices in
 question until the missiles are actually deployed. Accordingly, there is a
 need for a squib fire network having a cost effective, built-in testing
 system to detect faults with a high level of accuracy without the
 potential for actuating squibs of the squib fire network. If the built-in
 testing system could determine whether all the squibs are in proper
 working order prior to launch, repairs could be performed on the weapon
 before deploying it, thus reducing weapon malfunctions in the field, and
 the number of weapons that must be expended to accomplish the field
 objective.
 SUMMARY OF THE INVENTION
 The present invention is directed to a squib fire network including
 built-in testing which is able to safely and efficiently verify the proper
 operational status of the squib fire network. For example, all squibs in a
 squib fire network of a missile can be tested prior to launch, thereby
 improving the reliability of weapon deployment.
 In accordance with exemplary embodiments, an apparatus for controlling a
 squib fire network is disclosed which comprises: at least one means for
 supplying a current to at least one squib of said squib fire network;
 means for selecting said at least one squib to fire; and means for testing
 the squib fire network, said means for testing further including: means
 for selectively gating said current to said at least one squib; and means
 to attenuate said current to a value insufficient to activate said at
 least one squib.
 In accordance with another exemplary embodiment of the invention, a squib
 fire network is disclosed which comprises: at least one squib; and a first
 switch alternately connectable between an open condition and an output of
 a current source, wherein said first switch is in operative connection
 with at least one additional switch, said at least one additional switch
 being alternately connectable between an open condition and an output of
 said first switch, and wherein said at least one additional switch is in
 operative connection with said at least one squib; a third switch
 alternately connectable between an open condition and an output of said at
 least one squib, wherein said third switch is in operative connection with
 a return current line; and a processor operatively connected to
 selectively open and close each of said first switch, said at least one
 additional switch, and said third switch.
 In alternate embodiments of the present invention, an apparatus for firing
 a squib is disclosed which comprises: a squib firing means, said squib
 firing means further including a plurality of squibs and a switch bank
 means; a current supply means for providing a current supply to said squib
 firing means; switching means for selectively interrupting said current
 supply to and from said squib firing means; and control means to control
 said switch means and said switch bank means to selectively provide said
 current supply to said squib firing means.
 In other embodiments of the present invention, a method for testing and
 firing a squib fire network is disclosed which comprises the steps of:
 selecting at least one squib; supplying a first current to said at least
 one squib; testing said squib fire network by selectively gating said
 first current to said at least one squib and attenuating said current to a
 value insufficient to activate said at least one squib.
 In yet other embodiments of the present invention, a method for testing
 operability of a squib fire network is disclosed which comprises the steps
 of: supplying a current to at least one squib of the squib fire network;
 and providing redundant attenuation of said current to a value
 insufficient to actuate said at least one squib in the presence of a
 single point failure in said squib fire network.

DETAILED DESCRIPTION OF THE INVENTION
 The present invention is directed to a squib fire network having at least
 one squib, and to the testing of squib fire networks. To facilitate an
 understanding of the present invention, an exemplary embodiment will be
 described with reference to the squib fire network of a weapon. It will be
 appreciated, however, that practical applications of principles which
 underlie the invention are not limited to weaponry. Rather, the invention
 will find utility in any environment where it is desirable to ensure the
 proper firing of squibs. This invention, for example, can be used in
 applications, such as an automobile air bag system or the like.
 FIG. 1 illustrates an exemplary apparatus for controlling a squib included
 in a squib fire network 100, for a missile or the like. The squib fire
 network can, for example, be used in an aircraft. A voltage supply 102
 (such as a 28 volt supply) powers-on the missile. Normally, this voltage
 supply is available on the aircraft where the missile and its associated
 squibs are located. However, it is also possible, if squibs are not being
 tested on an aircraft, to utilize a test cart having voltage supplies
 thereon.
 An output of voltage supply 102 is connected with redundant attenuators,
 represented in FIG. 1 by two current limiting resistances 112 and 110, on
 the output side of the supply. The current limiting resistance 110, which
 can be one or more (e.g., two) individual resistors in series, limits the
 current (e.g., to milli-amps). The second current limiting resistance 112
 can be placed between the current limiting resistance 110 and the first
 voltage supply 102. The small amount of current generated by the voltage
 supply 102 can be used to test the squib fire network without accidently
 firing the squibs.
 The operational status of both of the current limiting resistances 110 and
 112 can be determined through the use of a resistive load 120. If either
 of the current limiting resistances 110 and 112 has failed, the current
 measurable across resistive load 120 will yield different current levels
 based upon which current limiting resistance has failed. However, both of
 the current limiting resistances 110 and 112 are selected to sufficiently
 restrict the current to a safe level in the presence of the other resistor
 constituting a single point failure within the squib fire network (e.g.,
 having a failed short). A single point failure is the failure of any one
 element of the squib fire network 100. For example, the individual failure
 of any switch can also represent a single point failure within the squib
 fire network.
 A second voltage supply 104, or squib fire voltage source, supplies a squib
 fire voltage, which can vary from 15 to 82 volts, to the squib fire
 network 100. The output voltage of the second voltage supply 104 is used
 to fire a squib located in the squib fire network 100.
 A current source 106 is placed on the output of the second voltage supply
 104 to regulate current entering a switch 108. The squib fire current can,
 for example, be less than 9 amps and greater than 2 amps to properly fire
 a squib (e.g., approximately 4.0 A), or can be any desired amperage.
 The switch 108 is a two position switch with a first contact connected to
 the current source 106, and a second open contact. Exemplary embodiments
 can employ, as a switching mechanism, a combination of opto-couplers and
 amplifiers in an integrated circuit. However, those skilled in the art
 will appreciate that any element which can perform the switching operation
 can be used.
 The switch 108 is controlled by a logic device 114, such as a
 field-programmable-gate-array (FPGA) or
 application-specific-integrated-circuit (ASIC), which is controlled by a
 microprocessor 116, and used to gate the squib fire current output from
 the current source 106. The microprocessor 116 controls the test and/or
 tactical operation through the use of the logic device 114, which controls
 all switches in the squib fire network 100.
 The output of the switch 108 is operably connected with a switch 118 and
 the output of the current limiting resistance 110. Because current from
 the voltage supply 102 has been limited by current limiting resistances
 112 and 110, the voltage supply 102 adds only a very small amount of
 current to the squib fire current output by the current source 106 during
 normal operation. Switch 118 is also controlled by logic device 114 to
 switch between a first open contact and a second squib fire line.
 The output of the second switch 118 is directly connected with a resistive
 load 120 which is used during testing to represent a squib. Those skilled
 in the art will appreciate that the load resistance 120 need not be a
 permanent element of the squib fire network. For example, a test cart can
 have a variable load resistor, that can be used as the resistive load 120
 when the squib fire network 100 is being tested. Also, microprocessor 116
 is able to detect the amount of current sent through the resistive load
 120.
 The switch 118 is connected in parallel with the switches of a switch bank
 122. Each of the switches in the switch bank 122 is connected to a bank of
 squibs 124.sub.1 to 124.sub.N. In an alternate exemplary embodiment, the
 second switch 118 can be a switch within the switch bank 122 that is
 connected with the load resistance 120 instead of a squib.
 Each squib 124.sub.1 to 124.sub.N is configured using one or more (e.g.,
 two) bridge wire circuits. The bridge wire circuit(s) include bridge wires
 surrounded by explosive charges. When the resistive elements of the bridge
 wires are charged through the passage of a predetermined amount of
 current, they detonate the explosive charges which explode to create a
 pressure wave that can be used to operate valves, explode larger charges,
 and/or move pistons and other mechanical elements.
 The output sides of the squibs 124.sub.1 to 124.sub.N are tied to a common
 node. A switch 126 is connected to this node in parallel with the series
 connection of a current limiting resistance 128 and a current detection
 device 130. Actuation of the switch 126 is controlled by the logic device
 114. When the switch 126 is open, current, if present, will flow through
 the current limiting resistance 128 and the current detection device 130.
 The current, if present, is severely reduced by the current limiting
 resistor 128, and flows through the current detection device 130, which
 sends a signal to the logic device 114. When the switch 126 is closed,
 current will flow through the switch 126 and back through a return line to
 the voltage supply 102. The resistance of switch 126 is such that
 insufficient current will flow through current detection device 130 to
 detect current.
 The embodiment shown in FIG. 1 can, for example, be expanded to include
 multiple switch banks. If another bank of switches is used, an additional
 pair of switches 108 and 126 can be used. In this case, as opposed to a
 single switch bank situation, multiple squibs can be fired at the same
 time.
 An actual squib fire operation is shown in the flow diagram of FIG. 2. An
 actual squib fire operation includes a precheck operation of various
 components in the squib fire network that is separate and distinct from a
 built-in test operation of the entire squib fire network.
 In step 210, a squib to be fired is selected by the FIG. 1 logic device 114
 under control of the microprocessor 116. In step 220 of FIG. 2, the
 improper presence of current prior to closure of the FIG. 1 switch 126 is
 detected by the current detection device 130.
 In step 225 of FIG. 2, if current is detected, the firing sequence is
 aborted, and an indication can be provided that the FIG. 1 switch 118
 and/or one of the switches in switch bank 122 is faulty. The built-in test
 operation can be then used to indicate which switch has failed.
 In step 230 of FIG. 2, the logic device 114 effects squib fire via closure
 of an appropriate select switch in switch bank 122. That is, if no current
 is detected at the current detection device 130, the appropriate select
 switch in the switch bank 122 is closed.
 In step 235, after closure of a switch in the switch bank 122, if a current
 is detected by the current detection device 130, the switch has been
 properly closed. Otherwise, as shown in step 240, successful switch
 closure has not been achieved. The process can, at this point, abort or
 can continue at the discretion of the microprocessor 116. For example, if
 the failed switch in switch bank 122 is associated with a redundant squib
 bridge wire associated with a squib separately selected and detonated, the
 microprocessor can be programmed such that the actual squib fire operation
 will continue.
 In step 250, after having verified correct operation of switch 118 and a
 given switch in switch bank 122 in step 245, switch 126 is closed on a
 command from the logic device 114. In step 255, the current detection
 device 130 is checked to verify that it does not detect any current. If no
 current is detected, then the switch 126 has been properly closed, as
 shown in step 265. Otherwise, switch 126 has not been properly closed, as
 shown in step 260.
 In step 270, the selected squib is fired by momentarily closing switch 108
 which permits a predetermined current (e.g., about 4.0 A) to run from the
 FIG. 1 current source 106 through the switch 108, through a given switch
 activated in the switch bank 122, through the selected squib, and then
 back through closed switch 126. This current ignites the selected squib in
 step 270. An alteration of switch selection in the switch bank 122 and an
 opening of switch 126 are inhibited during the closure of switch 108.
 Alternatively, step 270 can be performed prior to step 250 without
 changing the end results.
 In step 275, switch 108 is re-opened by the logic device 114 following a
 predetermined time interval (e.g., a predetermined pulse). Following the
 predetermined time interval, switch 126 can also be opened and successful
 squib firing can be validated by an absence of current detected by the
 current detector 126. However, this is not an entirely accurate indicator
 in all cases because successful squib firing can, on occasion, result in
 an electrical short circuit condition at the squib instead of an open
 circuit condition.
 Another exemplary embodiment of the invention is illustrated in FIG. 3A as
 a block diagram of a dual redundant power monitoring system used in
 combination with the FIG. 1 squib fire network 100. Within the squib fire
 network 100, various voltages are required for proper network operation.
 In the FIG. 1 embodiment, the voltages are 5 V, 3.3V and 2.5V, which
 operate to control the logic device 114 and microprocessor 116. When the
 microprocessor 116 performs a squib fire sequence, it commands the logic
 device 114 to perform the proper switch closures. The microprocessor 116
 can be any computer system including, but not limited to a Power PC.TM.
 available from International Business Machines (IBM), Inc. or Motorola
 Inc. The power PC.TM. can determine the sequence of squib firings.
 A typical Power PC.TM. has a 3.3 V device and a 2.5 V internal logic core,
 although any microprocessor, having any specified voltage requirements can
 be used. Both of those voltages should be within 5 percent of the rated
 voltage, or any other specified tolerance, for the squib firing network to
 operate properly. In an exemplary embodiment, the logic device 114 is
 powered by a 5 V source which should be within 10 percent of its rated
 output for a squib to operate properly. The voltages used by the logic
 device 114 must be within their operable ranges to ensure proper
 operation.
 Two power monitor circuits 332 and 334 are connected to switches 108 and
 126, respectively. In addition to previously stated processes, both of
 those switches must be closed for a squib to fire. If, for example, a
 brown out situation were to occur (i.e., power available to the squib fire
 network is reduced), the logic device 114, which controls the switches,
 could erroneously attempt to close one or more of the switches 108, 118
 and 126 when there is no actual command to do so. If the squib fire
 current were accidently sent through the squibs 124, the results could be
 disastrous.
 As illustrated schematically in FIG. 3B, the power monitor circuit 332
 detects and monitors the voltages used by the FIG. 3A logic device 114 and
 microprocessor 116. If an out-of-tolerance voltage is sensed, either of
 the power monitor circuits 332 and 334 can effectively gate (i.e., shunt)
 current input from the logic device 114 into the switch 108. For example,
 the power monitor circuit 332 of FIG. 3B starves the anode of the diode
 350 in the opto-coupler 352 of switch 108 so that no closure of the switch
 108 occurs. As a result, a squib fire gate current cannot pass through the
 switch 108.
 The second power monitor circuit 334 of FIG. 3A gates current into switch
 126 in the same manner as discussed above with respect to the first power
 monitor circuit 332 of FIG. 3B. The second power monitor circuit 334
 prevents the failure of a single power monitor circuit from risking the
 safety of the entire squib fire network. While adding slightly to the
 expense of the squib fire network, the fail safe precautions provide a
 safe squib fire environment.
 The power monitor circuits shown in FIGS. 3A and 3B can be powered by the
 voltage supply 102, such that they are powered independent of regulated
 supplies. While in this exemplary embodiment the power monitor circuits
 332 and 334 were shown controlling switches 108 and 126, it will be
 understood that, as long as two power monitor circuits are monitoring two
 of switches 108 or 126 or switch bank 122, dual redundancy has been
 provided.
 The predetermined time interval and the power monitor circuits 332 and 334,
 are part of an interlock system illustrated in FIG. 4. The interlock
 system provides further protection against unwanted switch closure and
 squib firing. When a switch in the switch bank 122 is to be closed to, for
 example, pop out wings or fire an engine of a missile, the interlock
 system requires certain conditions to be met before the switch in the
 switch bank 122 can be closed. Switches 108, 122 and 126 are shown in FIG.
 4 along with various FIG. 1 components. Combinatorial blocks (e.g., analog
 or digital and gates) 405, 410, 415, 430, 425 and 415 represent the
 individual system requirements that must be met before closure of the
 switches can be enabled.
 Block 405 represents conditions required for the closure of the switches of
 the switch bank 122 which are, for example, dedicated to battery squibs.
 While most squibs are to be fired with a stringent interlock system,
 certain squibs are allowed more simplified or more complex interlock
 systems. For example, squibs which fire to enable chemicals to be combined
 to form a battery are typically required to be fired before any other
 squibs. The battery can, for example, thereafter be used to provide firing
 current to the remaining squibs.
 To fire the battery squibs, an exemplary embodiment is configured such that
 the power monitoring system must indicate that power throughout the system
 is within specified tolerance ranges. A command must then be issued from
 the microprocessor 116, and when a given squib switch is selected, it must
 be the only switch in the switch bank 122 selected. If all of these
 requirements are met, then the switch used to fire the selected battery
 squib will close.
 For the remaining switches in the squib switch bank 122 and switch 118, the
 interlock requirements can be configured to be much more demanding, as
 shown by blocks 420 (a logical OR block), 425 and 415. Block 425 will
 enable a remaining switch of the switch bank 122 if the following
 exemplary requirements are met: the requirements of the battery squibs
 described above are met; the squib fire voltage is deactivated (if in
 built-in testing condition) or the squib fire voitage is activated, an
 umbilical (e.g., used to electrically attach to a host, such as an
 airframe) is severed; and batteries used to provide various voltages to
 the squib fire network (e.g., the supply voltage 102 and any other voltage
 supplies such as a 135V supply used to power elevons of the missile) are
 activated (if in a squib fire condition).
 For switch 126, interlock 430 requires that the power monitor circuit 334
 ensure that all operating voltages are within proper tolerance ranges,
 that only one squib is selected, and that a software command has been
 properly issued from microprocessor 116.
 To enable switch 108, the interlock 410 requires that the power monitor
 circuit 332 ensure that all operating voltages are within proper tolerance
 ranges, that only one squib is selected, that a software command has been
 issued from microprocessor 116 and that the switch 108, when closed, is
 only closed for a predetermined time (e.g., 50 msec). Those skilled in the
 art will appreciate that the duration of closure of the switch 108 can be
 programmed to any length of time according to system specifications and
 user requirements and that this time duration could have been applied to
 switch 126 instead of switch 108.
 To fire a squib, the switches 108 and 126, and a selected one of the
 switches in switch bank 122, must be closed. The requirements for closure
 of these switches are quite stringent, and do not allow for system error
 or transient errors to mistakenly fire a squib when the squib is not
 intended to be fired. It should be noted, however, that the interlocks can
 be reconfigured to be more or less demanding based upon the type of squib
 being fired. For example, if a squib fire network were employed for air
 bags within an automobile, the interlock requirements could be more or
 less stringent.
 FIGS. 5A and 5B show an exemplary built-in test operation that can be
 performed using the system architecture of FIGS. 1-4, to ensure the
 ability of the squibs fire network to operate properly. The built-in test
 can, for example, be performed prior to an actual squib fire operation.
 The built-in test operation ensures that an actual squib fire current can
 never be supplied to a squib. However, the FIG. 1 squib fire network can
 verify the relatively high magnitude of an actual squib fire current using
 the FIG. 1 resistive load 120, without concern that a squib will
 inadvertently be fired. In addition, circuit continuity through each squib
 can be verified using a relatively low, redundantly attenuated current
 from voltage supply 102, without concern that a squib will inadvertently
 be fired.
 In step 505, when a squib fire network built-in test is performed, the
 system starts from an initialized position. The FIG. 4 safety interlocks
 require switches 108, 118, 126 and switch bank 122 to all be open prior to
 a built-in test operation. The presence of current across the FIG. 1
 resistive load 120 and/or detection of a current by the current detection
 device 130 denotes a switch failure.
 Once an initialized position has been established, switch 118 is closed in
 FIG. 5A step 510. When switch 118 is closed during testing, the current
 magnitude is measured to determine if it is correct. When a test takes
 place, only the voltage supply 102 should be available to switch 118.
 In FIG. 5A, step 520, the presence of an expected current magnitude through
 the resistive load 120 of FIG. 1 is a positive indication that the switch
 118 has closed and that switch 108 is still open. The current detection
 device 130 provides an additional indication of the closure of switch 118.
 The absence of either or both indications represents a failure of the
 system in FIG. 5A, step 525.
 If the switch 118 has closed properly, switch 126 is closed in step 530. In
 step 535, the current at the current detection device 130 is monitored to
 indicate that there was a closure of switch 126 (e.g., as indicated by a
 significant reduction in current magnitude to a negligible amount). In
 step 540, if there is not a loss of current in the current detection
 device 130, there is a failure of switch 126.
 After having refined the operability of switches in switch 118 and switch
 126, a verification that switch 108 is still open is performed. In step
 545, a current across the resistive load 120 is measured. Switch 126 is
 closed, and current through resistive load 120 is monitored. A current
 measurement of approximately 4 A indicates a failure of switch 108 as
 shown in step 550. Otherwise, switch 108 is determined to still be
 properly opened.
 Having verified that switch 108 is open, that all switches in switch bank
 122 are open, and that switches 118 and 126 are closed, an actual squib
 fire current can be verified by closing switch 108 and monitoring current
 through resistive load 120. The high magnitude current from current source
 106 is tested independently of switches in switch bank 122 to ensure that
 the circuit will not inadvertently apply the high current value to the
 squibs. Switch 108 is closed in step 555. In step 560, measurement of
 current through the resistive load 120 indicates, via the magnitude,
 whether the test has passed or failed. In an exemplary embodiment, the
 squib fire current is approximately 4 A. In step 565, if there is not
 approximately 4 A across resistive load 120 there is a failure within the
 system. In step 570 switch 108 is opened to remove the squib fire current.
 In step 575 of FIG. 5B, after the squib fire current is removed (i.e.,
 switch 108 is opened) the current should measure much less than the 4 A
 fire current. Otherwise, as shown in step 580, switch 108 has failed to
 open. In step 585, switch 126 is opened. Existence of current at the
 current detection device 130 verifies the open condition of switch 126. In
 step 590 of FIG. 5B, switch 118 is opened, and an absence of current at
 the current detection device 130 verifies the open condition of switch
 118.
 The current must be returned through FIG. 1 switch 126 to supply proper
 current through the bridge wire of a given squib. When closing switch 126,
 the current detection device 130 can be used to indicate an absence of
 current, therefore verifying the closure of switch 126. When the switch
 126 is open, the current passes through current detection device 130, that
 can be configured with enough impedance to protect against an inadvertent
 activation. For example, the current detection device 130 can be in series
 with the current limiting resistance 128.
 Each of the switches in the FIG. 1 switch bank 122 can be sequentially
 tested with switch 126 open. When switch 126 is open and at least one
 switch in switch bank 122 closed, a small amount of current should be
 sensed by the current detection device 130 which verifies the closure of
 the switch in the switch bank. Each of the selector switches in the switch
 bank 122 is then sequentially cycled. Switch 126 may or may not be tested
 with respect to each switch of the switch bank 122.
 Those skilled in the art will appreciate that it is typically desired to
 test as many components of the squib fire network using the actual squib
 fire current from current source 106. However, those skilled in the art
 will also appreciate that with respect to any nondestructive components
 deemed inappropriate to conduct a test of the high current supply from
 current source 106 (such as any of the squibs in FIG. 1), the testing
 procedure can be modified accordingly. In addition, any testing of the
 current from current source 106 can be eliminated to shorten the testing
 operation if desired.
 As illustrated in FIG. 6, the switches and squibs associated with the FIG.
 1 switch bank 122 can each be tested in the following manner: In step 600,
 each switch of the switch bank 122 is again initialized to an open
 position. The first selected switch in the switch bank 122 is then closed
 in step 610. In step 620, the continuity of the switch and squib are
 verified by detection of current at the FIG. 1 current detection device
 130. In step 630, if there is no indication of current through the current
 detection device 130, then there is a break in continuity. In step 640,
 the previously closed switch in the switch bank 122 is opened. In step
 650, an absence of current at the current detection device 130 verifies
 the open condition. In step 660, if current is present, the switch has not
 properly opened. As shown in step 670, this process is sequentially
 repeated for each switch in the switch bank 122.
 FIG. 7 illustrates an exemplary schematic implementation of portions of the
 FIG. 1 embodiment. As shown in FIG. 7, current limiting resistances 110
 and 112 (the series of two or more resistance elements) are represented as
 a diode and a current limiting resistor. In FIG. 7, the switch 108 is
 represented by an opto-coupler 352 in combination with a Darlington
 transistor arrangement 754 and bipolar transistor 755. Various voltage
 dividing and current limiting resistances 761-765 are also included, as
 are a Zener diodes 766 and a diode 767. Switch 118 and switch 126 are each
 represented by an opto-coupler 352 in combination with field effect
 transistors (FETs) 756. Switch 118 also includes a bipolar transistor 768
 and various resistors 769-772. Individual switches 730.sub.1 to 730.sub.N
 of switch bank 122 can be configured identically to switch 118. In an
 exemplary embodiment, an individual switch 730.sub.1 to 730.sub.N is
 provided for each squib 124.sub.1 to 124.sub.N. The switch 126, as shown
 in FIG. 7, also includes various resistors 774-775. In addition, outputs
 780-782 from the logic device 114 are directed to opto-couplers 352 of
 each switch in the squib fire network 100 to open and close each switch.
 The logic device 114 also receives an output 783 from the current
 detection device. The microprocessor 116 in conjunction an
 analog-to-digital converter receives an output 784 from the resistive load
 120.
 The resistors used throughout the circuit can, for example, be discrete.
 Resistance values shown in FIG. 7 are by way of example only. It is also
 possible to use resistor packs (e.g., up to 15 resistors on an integrated
 circuit). The output of the opto-coupler 352 in switch 108 can, for
 example be connected to a common pin of the resistor pack. Resistor packs
 can be used to increase the accuracy and reduce the cost of the squib fire
 network. Instead of 15 or 20 discrete resistors, a resistor network can
 also be used which is easier to test and assemble. In addition to digital
 components, analog components can, of course, be used. FIG. 7 also depicts
 the current detection device as an optocoupler. However, those skilled in
 the art will appreciate that any current monitoring device can be used.
 While the present invention has been described in the context of exemplary
 applications, those skilled in the art will appreciate that numerous
 variations exist. For example, while the previous embodiment has been
 described with respect to military applications, other applications
 including automobile airbags and oxygen mask ejection systems in airplanes
 can be employed.
 While the present invention has been described with respect to its
 preferred embodiments, those skilled in the art will recognize that the
 present invention is not limited to the specific embodiments described and
 illustrated herein. Different embodiments and adaptations besides those
 shown herein and described, as well as many variations, modifications and
 equivalent arrangements will be apparent or reasonably suggested by the
 foregoing specification and drawings without departing from the substance
 of scope of the invention.