GB2309288A

GB2309288A - Solid state laser arm/fire device

Info

Publication number: GB2309288A
Application number: GB9700728A
Authority: GB
Inventors: Craig A Kesner; James E Phillips; Fred Smith; Jeffrey T Winebrenner
Original assignee: Alliant Techsystems Inc
Current assignee: Northrop Grumman Innovation Systems LLC
Priority date: 1996-01-16
Filing date: 1997-01-15
Publication date: 1997-07-23
Also published as: NO970173D0; FR2743625A1; ITRM970016A1; NO970173L; GB9700728D0; FR2743625B1; IT1295701B1

Description

Solid State Lascr Arm/Fire Dcvice Background of the Invention 1. Field of the Invention The present invention relates to the field of arm/fire device (AFD), and more particularly a laser AFD (LAFD).

2. Descrintion of the Related Art Conventional arm/fire device (AFD) designs require a mechanical barrier between the electro-explosive device (EED) and the output charge or pyrotechnic material. This mechanical barrier involves moving parts and must have the structural ability to divert the EED output. Moving parts hinder reliability and mechanical barriers add weight and size to the device. Also, EED's can be affected by electromagnetic impulscs (EMI) and electrostatic discharge (ESD).

An example of an arm/fire device is US 5,022,324, entitled Piezoelectric Crystal Powered Ignition Device, which uses pyro flash lamps to trigger a solid state laser. The laser is neodymium glass laser. rod which is "pumped" when light from the flash lamps enters through the sides of the laser.

Like conventional AFD's, this device uses mechanical safety devices. In the preferred embodiment the mechanical safety device is a spring loaded weight assembly (7) which is used to mechanically open a shutter (5). Another embodiment uses a mechanical rotating shaft having a hole which is out of alignment, unless the hole is aligned with the laser and the pyrotechnic material.

What is needed is an AFD which eliminates the mechanical barrier and the moving parts between the EED and the output charge.

Summary of the Invention The purpose of the present invention is to provide an arm/fire device which overcomes the limitations discussed above. The invention overcomes the limitations discussed above by providing a device for ignition of a pyrotechnic material and includes an electronic switch means for triggering ignition of the pyrotechnic material in response to a signal, a laser means electrically connected to the electronic switch means for generating a laser pulse in response to the electronic switch means, and an optical fiber with one end optically coupled to the laser means and the other end embedded in the pyrotechnic material so that the laser pulse heats a small area very quickly, igniting the pyrotechnic material.

The laser means may be a laser diode. The pyrotechnic material includes a metal header and the optical fiber is hermetically sealed to the metal header with a copper or epoxy seal. The copper seal may be comprised of fine copper powder placed in a seal area, which is consolidated with a compression means to create the seal, or the copper seal may be comprised of a copper sleeve placed over the optical fiber in the seal area, which is comprcssed with a compression means to create the seal. The wavelength of the laser pulse is approximately 860 nanometers. The optical fiber is embedded between 0.030 - 0.60 inches into the pyrotechnic material, which is B/KNO, powder sized to sieve 100% through a 25 micron screen and where the pyrotechnic material includes 2% by weight carbon black to enhance absorption of laser cnergy.The B/KNO3 powder is consolidated around the embedded optical fiber under a pressure of between 2000 to 3000 psi for a dwell time of 1 to 10 seconds. The laser pulse generates heat in a small area sufficiently so that ignition occurs in approximately 4 milliseconds. The optical fiber has a diameter of 100 microns.

The laser AFD may be used in military ordnance, commercial blasting or in the ignition of solid propellant rocket motors.

A process of igniting a pyrotechnic material using the device discussed above comprises applying an electrical signal to the electronic switch means.

The invention may also be described as a solid state laser arm/fire device which includes a first electronic switch for arming the device in response to an ARM signal and a second electronic switch for firing the device and triggering ignition of a pyrotechnic material in response to a FIRE signal. A laser diode electrically connected to the second electronic switch generates a laser pulse in response to the FIRE signal. The device also includes an optical fiber with one end optically coupled to the laser diode and the other end embedded in the pyrotechnic material so that the laser pulse ignites the pyrotechnic material. The device includes a visual SAFE/ARM indicator and a Faraday cage housing to house the device. The device includes a diagnostic test means to verify proper laser diode operation without igniting the pyrotechnic material.

A process of igniting a pyrotechnic material of the device discussed above includes the steps of applying an ARM signal to the first electronic switch, and applying a FIRE signal to the second electronic switch.

Brief Descrintion of the Drawings A detailed description of the invention is hereaftcr described with specific reference being made to the drawings in which: Figure 1 is a cross-sectional view of a conventional prior art AFD; Figure 2 is a cross-sectional view of inventive laser AFD; Figure 3 is a circuit diagram of the inventive laser AFD; Figure 4 is a cross-sectional showing how the optical fiber is sealed to a metal header1 and Figure 5 is an alternative embodiment of the circuit diagram of the inventive laser AFD.

Descrintion of the Preferred Embodimcnts While this invention may be embodied in many different forms, there are shown in the drawings and described in detail herein specific preferred embodiments of the invention. The present disclosure is an exemplification of the principles of the invention and is not intended to limit the invention to the particular embodiments illustrated.

Referring now to Figure 1 a prior art AFD is shown generally at 10.

The mechanical safe/arm indicator is shown at 12 and the solenoid, which is a mechanical moving part is shown at 14. The detonator is shown at 16, the donor charge is shown at 18, the pickup charge is shown at 20 and the B/KNOZ output charge is shown at 22.

Referring now to Figure 2, the inventive laser AFD (LAFD) is shown generally at 30. An electrical and visual safe/arm indicator is shown at 32. Two independent electronic switches are shown at 34 and 36, respectively. A laser diode is shown at 38. A fiber optic cable optically coupled to the laser diode is shown at 40. The fiber optic cable 40 is embedded into the B/KNO, output charge 42.

The LAFD provides for the same electrical inputs and the same pressure outputs as those of the conventional clectro-mcchanical AFD shown in Figure 1. The electrical interface accepts standard AFD signals for ARM and FIRE commands.

Referring now to Figure 2 and 3, the electrical schematic for the LAFD is shown. Upon receipt of an ARM signal, shown at pin 7 and reference numeral 50, the electrical and visual indicator 32 is illuminated by a high intensity red LED 52. This light is focuscd through a film that is screened with the green background/white 'S' (safe) and the 'A' (arm). With no light input the indicator shows the green background with the white 'S' superimposed. Red LED 52 is in series with two optocouplers 54 and 56. Optocoupler 54 provides the electronic SAFE/ARM indicator, pin 6 shown at 58. Optocoupler 56 represents the first electronic switch and enables the firing circuit. If any one of the two components 52 or 56 fails, the arming function will not be completed and the visual indicator will not function.A wide range of ARM signals can be accommodated based on the value selection of the cqmponents, and selection of the various values is considered routine engineering. The value selection shown in Figure 3 sets the LAFD for a 30 volt ARM signal.

When the LAFD is armed it is ready to be fired. Upon receipt of a FIRE signal, pins 4 and 1 at 60, the second electronic switch 64 is closed and current passes through laser diode 62. The emitting facet of laser diode 62 generates a laser pulse at approximately 860 nanometers which is launched into the 100 micron optical fiber 40. Optical fiber 40 is embedded into the output charge 42 of B/KNO3. The optical fiber 40 projects or extends between 0.030 - 0.060 inches into the output charge 42.

The output charge 42 is sized to sieve 100% through a 25 micron screen. A 2-3% by weight quantity of carbon black is added to enhance absorption of laser energy.

Referring now to Figure 4, a cross-sectional view of the seal is shown. Optical fiber 40 is sealed to a metal header 70 of the output charge 42 using a consolidated copper seal 72. The preferred embodiment of the consolidated copper seal involves placing fine copper powder in the seal area 72 and consolidating the powder by tightening a set screw or pressing a ram down onto the seal area. An alternative embodiment of the consolidated copper seal resembles a copper compression fitting using a small copper sleeve which is placed over the fiber and into the seal area. A set screw is tightened or a ram is pressed onto the seal area 72, which expands the sleeve and creates the seal. Both seal embodiments are soldered after compression to achieve a hermetic seal. An epoxy seal is added to prevent corrosion and contamination.In anothcr alternative embodiment, a pure epoxy seal can be substituted for the copper scal for applications which do not require a high temperature, long duration scal.

The B/KNO3 powder is consolidated around the optical fiber 40 under a pressure of 2000 - 3000 psi for a dwell time of 1 to 10 seconds. The quantity of powder is adjusted to achicve the required output pressure, as is well known in the prior art. The charge cavity is sealed by installing a non-conductive insulating disk 74, followed by a stainless steel cup 76 (best seen in Figure 4). The cup is TIG welded in place and the assembly is leak tested to ensure integrity. When laser energy is received at the fiber tip of optical fiber 40, the temperature of the output charge 42 is quickly increased until ignition occurs. First indication of pressure typically occurs in 4 milliseconds.

Applicants' have found that using an embedded optical fiber eliminates the effect of operating temperature on ignition delay. Even though the charge is harder to ignite at the cold extreme, the electronics are more efficicnt and provide more laser energy. Likewise, at the hot extreme, the charge is easier to ignite but the electronics are less efficient. The variables appear to balance out and no statistically significant difference exists between the ignition delays across the tactical temperature range.

Another advantage of embedding the fiber 40 directly in the output charge 40 is that it allows compliance with MIL-STD-1901, eliminating the need for gas containment/mechanical diversion systems. Utilizing a con-conductive optical fiber 40 also eliminates any possibility of inadvcrtcnt ignition response through EMI or ESD. The AFD can be housed in a complete Faraday cage for additional protection from EMI/ESD while providing the lowest weight of competing designs.

Eliminating moving parts in the AFD also reduces size and weight of the device, and also increases tlle reliability of the device.

Another advantage of the inventive LAFD is that it allows for diagnostic test capability to verify proper diode operation. In bridgewire devices this feature is not available because energizing a bridgewire will result in ignition.

Another advantage of the inventive LAFD is that by using a high pressure, high temperature copper seal, the costly and complicated manufacturing process for glass-to-metal seals is eliminated. The ability to use a pure epoxy seal for low temperature applications further reduces manufacturing costs.

Referring now to Figure 5, an alternative embodiment of the inventive circuit of Figure 3 is shown which adds electrostatic discharge (ESD) protection and electromagnetic interference (EMI) protection to the input conncctor via filters 66 and 68 respectively.

This completes the description of the preferred and alternate embodiments of the invention. It is to be understood that even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with the details of the structure and function of the invention, the disclosure is illustrative only and changes may be made in detail, especially in matters of shape, size and arrangement of parts within the principals of the invention, to the full extent indicated by the broad, general meaning of the terms in which the appended claims are expressed. Those skilled in the art may recognize other equivalents to the specific embodiment described herein which are intended to be encompassed by the claims attached hereto.

Claims

WHAT IS CLAIMED IS:

1. A device for ignition of a pyrotechnic material, comprising: electronic switch means for triggering ignition of the pyrotechnic material in response to a signal; laser means electrically connected to the electronic switch means for generating a laser pulse in response to the electronic switch means; an optical fiber with one end optically coupled to the laser means and the other end embedded in the pyrotechnic material so that the laser pulse concentrates heat at the embedded end of the optical fiber, igniting the pyrotcchnic material.

2. The device of claim 1 wherein the laser means is a laser diode.

3. The device of claim 2 wherein the pyrotechnic material includes a metal header and the optical fiber is hermetically sealed to the metal header with a copper seal.

4. The device of claim 3 wherein the copper seal is comprised of fine copper powder placed in a seal area, which is consolidated with a compression means to create the seal.

5. The device of claim 3 wherein the copper seal is comprised of a copper sleeve placed over the optical fiber in the seal area, which is compressed with a compression means to create the seal.

6. The device of claim 2 wherein the pyrotechnic material includes a metal header and the optical fiber is hermetically sealed to the metal header with a solder/epoxy seal.

7. The device of claim 1 wherein the wavelength of the laser pulse is approximately 860 nanometers.

8. The device of claim 1 wherein the optical fiber is embedded between 0.030 0.60 inches into the pyrotechnic material.

9. The device of claim 1 wherein the pytotechnic material is B/KNO3 powder.

10. The device of claim 9 wherein the B/KNO3 powder is sized to sieve 100% through a 25 micron screen and where the pyrotechnic material includes 2-3 % by weight carbon black to enhance absorption of laser energy.

11. The device of claim 10 wherein the B/KNO3 powder is consolidated around the embedded optical fiber under a pressure of between 2000 to 3000 psi for a dwell time of 1 to 10 seconds.

12. The device of claim 11 wherein the laser pulse concentrates heat at the embedded end of the optical fiber sufficiently so that ignition occurs in approximately 4 milliseconds.

13. The device of claim 1 wherein the pyrotechnic material is used in military ordnance.

14. The device of claim 1 wherein the pyrnteehnic material is used in commercial blasting.

15. The device of claim 1 wherein the pyrotechnic material is used in the ignition of solid propellant rocket motors.

16. The device of claim 1 wherein the optical fiber has a diameter of 100 microns.

17. A process of igniting a pyrotechnic material using the device of claim 1 comprising applying an electrical signal to the electronic switch means.

18. A solid state laser arm/fire device comprising: a first electronic switch for arming the device in response to an ARM signal; a second electronic switch for firing the device and triggering ignition of a pyrotechnic material in response to a FIRE signal; a laser diode electrically connected to the second electronic switch for generating a laser pulse in response to the FIRE signal; an optical fiber with one end optically coupled to the laser diode and the other end embedded in the pyrotcchnic material so that the laser pulse concentrates heat at the embedded end of the optical fiber, igniting the pyrotechnic material.

19. The device of claim 18 including a visual SAFE/ARM indicator.

20. The device of claim 19 including a Faraday cage housing to house the device.

21. The device of claim 18 including ESD and EMI protection filters.

22. The device of claim 18 including a diagnostic test means to verify proper laser diode operation without igniting the pyrotechnic material.

23. A process of igniting a pyrotechnic material using the device of claim 18 comprising the steps of: applying an ARM signal to the first electronic switch, and applying a FIRE signal to the second electronic switch.

24. A device for ignition of a pyrotechnic material substantially as hereinbefore described with reference to and as shown in any one of Figures 2 to 5 of the accompanying drawings.

25. A solid state laser arm/fire device substantially as hereinbefore described with reference to and as shown in any one of Figures 2 to 5 of the accompanying drawings.