Redundant safety circuit for squib testing

• US 20030218462A1
• Filed: 05/23/2002
• Published: 11/27/2003
• Est. Priority Date: 05/23/2002
• Status: Active Grant

• Current Assignee: Lockheed Martin Corporation (Martin Marietta Corporation)
• Original Assignee: Lockheed Martin Corporation (Martin Marietta Corporation)
• Inventors: Dick, Geoffrey E., Perkins, Darrell L.
• Granted Patent: US 6,943,555 B2
• Time in Patent Office: Days
• US Class Current: 324/502
• CPC Class Codes: B60R 21/0173 Diagnostic or recording means therefor

Abstract

An apparatus for measuring the resistance of a squib, including an input for receiving a power supply and a circuit connected to the input for limiting the power supplied to the squib. The circuit includes a voltage limiter which limits voltage output of the circuit to a level insufficient to ignite the squib, and a current limiter for limiting the current output of the circuit, the current limiter being responsive to a current supplied upon activation of the voltage limiter to limit the total energy supplied by the circuit to a predetermined level following activation of the voltage limiter. The voltage limiter can be a parallel arrangement of diodes connecting first and second conductive leads. The current limiter can include fuses which open when diodes conduct current through the conductive leads. This apparatus prevents squib activation even if an overcurrent or overvoltage occurs in the ac power supply.

Claims

1. An apparatus for measuring the resistance of a squib, comprising:

an input for receiving a power supply;

a circuit electrically connected to the input for limiting the power supply, the circuit including;

voltage limiting means for limiting a voltage output of the circuit to a level insufficient to ignite the squib, and

current limiting means for limiting a current output of the circuit, the current limiting means being responsive to a current supplied upon activation of the voltage limiting means to limit the energy supplied by the circuit to a predetermined level following activation of the voltage limiting means.

2. The apparatus as in claim 1, the input comprising first and second conductive leads, and the voltage limiting means comprising a parallel arrangement of diodes between the first and second conductive leads.

3. The apparatus as in claim 2, wherein the current limiting means comprises:

a first fuse electrically coupled to the first conductive lead;

a second fuse electrically coupled to the second conductive lead,

the first and second fuses being sized so that at least one of the first and second fuses opens when a current in a conductive lead reaches a preset value in a time period less than a time required to ignite the squib.

4. The apparatus as in claim 3, wherein the predetermined time period is less than the time required for the resistance element to heat sufficiently to ignite an adjacent explosive material.

5. An apparatus as in claim 3, wherein the first and second fuses are fast-acting fuses and rated to open upon application of a {fraction (1/16)}th ampere current.

6. An apparatus as in claim 3, wherein each of the first and second fuses open in less than ten milliseconds upon application of a 0.3 ampere current.

7. An apparatus as in claim 3, wherein the first and second diodes are switching diodes and open in less than 35 nanoseconds upon application of a 5 ampere current.

8. An apparatus as in claim 1, in combination with a power supply means for supplying a voltage and a current.

9. An apparatus as in claim 8, wherein the power supply means is a voltmeter.

10. An apparatus as in claim 8, wherein the power supply means has first and second electrical terminals for applying a current to the circuit, and third and fourth electrical terminals for receiving a voltage to be measured from the circuit.

11. The apparatus as in claim 8, wherein the power supply means has a maximum output voltage of 20 millivolts when applying a test current of 1 milliampere for measuring a resistance element having a resistance of about 1 ohm.

12. An apparatus as in claim 1, in combination with a squib having a resistance element.

13. An apparatus as in claim 12, comprising:

an output for providing a voltage and a current to the squib, the output electrically coupled to the squib.

14. An apparatus as in claim 13, wherein the output comprises first and second conductive leads.

15. An apparatus as in claim 1, wherein the circuit comprises:

circuit sensing means for determining a resistance of circuit elements between a squib and a power supply means.

16. An apparatus as in claim 15, wherein the circuit sensing means includes third and fourth conductive leads each having a terminal for electrical connection to the resistance element and another terminal for electrical connection to a power supply means.

17. An apparatus as in claim 16, wherein the circuit comprises a fuse electrically coupled to the third conductive lead, and a fuse electrically coupled to the fourth conductive lead.

18. A method for measuring the resistance of a squib with with a circuit having an input for receiving a power supply, voltage limiting means for limiting a voltage output of the circuit, and current limiting means for limiting a current output of the circuit, the method comprising:

applying a current and voltage to the squib by supplying power to the circuit;

limiting an energy supplied by the circuit to the squib to a predetermined level by activating the voltage limiting means if the voltage across conductive leads of the circuit is greater than a predetermined level and activating the current limiting means responsive to a current supplied on activation of the current limiting means; and

measuring a resulting voltage.

19. A method according to claim 18, further comprising:

sensing the resistance of circuit elements in the circuit, and subtracting the circuit element resistance from a measured resistance of the circuit and the squib.

20. A method according to claim 18, wherein the voltage limiting means includes a parallel arrangement of diodes between a first and a second conductive leads of the circuit, and the current limiting means includes a first fuse electrically coupled to the first conductive lead and a second fuse electrically coupled to the second conductive lead, and wherein upon application of a voltage greater than a preset value, at least one of the diodes conducts current through at least one of the first and second conductive leads sufficient to activate at least one of the first and second fuses.