Patent Publication Number: US-7216589-B2

Title: Fuse for projected ordnance

Description:
TECHNICAL FIELD OF THE INVENTION 
   This invention relates generally to a fusing arrangement for a projected ordnance and, more particularly, to a fusing apparatus implemented using a laser and an optical switch to detonate the ordnance. 
   BACKGROUND OF THE INVENTION 
   Fuse systems serve to detonate the main charge (‘secondary’ of military ordnance) of a munition, a cartridge, or an ordnance (collectively referred to herein as ordnance) at the desired time or location. The fuse (or fuze) plays an essential safety role of preventing accidental detonation of the ordnance, making the ordnance safe to handle. There are a variety of technologies used in fuse systems. The fuses considered here are “programmable”: immediately prior to the ordnance being fired from a gun, timing or similar data is loaded into the fuse so that the fuse initiates detonation of the secondary charge of the ordnance at the desired time and/or location. One common approach to such a fuse system is to charge a capacitor, and then discharge it at the desired time across a thin wire to create sufficient local heating or a spark to ignite the primary explosive. On-board electronics or mechanical devices control the discharge timing. Fuses typically incorporate “g-switches” that prevent detonation until the fuse has been exposed to accelerations of a magnitude and time typically only encountered in a gun barrel. There are on-going efforts at fabricating Micro-Electrical Mechanical Switch (MEMS)-based g-switches. 
   Notwithstanding the advances made by these prior fuse systems, there is a continuing need to significantly reduce the size, improve the performance and safety of the overall ordnance fuse system. 
   SUMMARY OF THE INVENTION 
   In accordance with the present invention, an ordnance fuse apparatus is described that uses electrical, mechanical, and optical devices. The ordnance fuse apparatus includes a controller to control an optical switch and a laser to detonate (directly or indirectly) an explosive charge of the ordnance. The resulting ordnance fuse apparatus has significantly reduced size and improved performance and safety. 
   More generally, we disclose a fuse apparatus for igniting an explosive charge of a fired ordnance, comprising
     a laser having a controllable optical power level,   an optical switch device having a first position for preventing a laser optical signal from impinging on the explosive charge when the fuse apparatus is in a pre-firing state and, in response to an arming signal, establishing a second position for unblocking the laser optical signal to enable it to impinge the explosive charge,   a control unit for determining when the ordnance has been fired, for sending the arming signal to the optical switch device, and for increasing the laser power level to a level that detonates the explosive charge.   

   Other embodiments include an accelerometer and/or spin detector for detecting that the ordnance has been fired and an optical detector for detecting the proper operation of the laser. In yet other embodiments the explosive charge is detonated either by ignition (burning) of an ignitor or by a shock wave from the ignitor, where the ignitor is a small (primary) explosive or pyrotechnic charge that is part of the fuze. Another embodiment includes a microlens to focus the laser optical signal onto the ignitor. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be more fully appreciated by consideration of the following Detailed Description, which should be read in light of the accompanying drawing in which: 
       FIG. 1  illustrates, in accordance with the present invention, an ordnance fuse apparatus in its pre-firing state. 
       FIG. 2  illustrates the ordnance fuse apparatus in its post-firing and detonation state. 
       FIG. 3 . Describes the sequence of operations of our fuse apparatus. 
   

   In the following description, identical element designations in different figures represent identical elements. Additionally in the element designations, the first digit refers to the figure in which that element is first located (e.g.,  100  is first located in  FIG. 1 ). 
   DETAILED DESCRIPTION 
   Almost all artillery shells, torpedoes, ordnance incorporate a fuse that serves to detonate the main charge (‘secondary’) at the desired time. The fuse plays an essential safety role of preventing accidental detonation, making the ordnance safe to handle. The ideal fuse would take up a negligible amount of space, is safe to handle, and ignites the main charge at the correct time. In accordance with the present invention, an ordnance fuse apparatus is disclosed that uses electrical, mechanical, and optical devices for improved safety and reliability of the fuse. 
   With reference to  FIG. 1  there is shown, in accordance with the present invention, an illustrative diagram of our ordnance fuse apparatus  100 , which together with explosive charge  142  are part of an ordnance to be fired and detonated. The ordnance fuse apparatus  100  is shown to include five main components including a laser and detector unit  110 , an optical switch or shutter  120 , a microlens  130 , an explosive charge  142  and a “programmable” electronic control chip  150 . Illustratively, the laser/detector unit  110  includes laser  111  and detector  114  mounted on an Indium phosphide (InP) chip  115 , which connects to controller chip  150 . The laser/detector unit  110  may include built-in self-test circuitry to test the operation of laser  111  and pre- and post-firing position of optical switch  120 . 
   In one embodiment, the optical switch  120  may be implemented using a MEMS shutter  121  (including an actuator which is used to move the MEMS shutter  121  upon firing of the ordnance.) and an accelerometer (g-switch)  122 . The g-switch  122  or a spin detector can be used to detect that the ordnance has been fired. MEMS g-switches are described in U.S. Pat. Nos. 6,167,809 and 6,321,654. The MEMS g-switch  122  signals the controller chip  150  to move the shutter into the firing position. 
   In a preferred embodiment, the MEMS shutter  121  may be implemented as described in the concurrently filed patent application of D. S. Greywall entitled “MICROMECHANICAL LATCHING SWITCH,” Ser. No. 10/766,451, which is incorporated by reference herein. It should be noted that the optical switching performed by MEMS shutter  121  may also occur by tilting a reflective element to redirect laser light to the explosive charge unit  140  rather than by moving the shutter to unblock the light (letting light pass) to explosive charge unit  140 . One such tilting MEMS optical switch which may be utilized is a MEMS mirror as described in the article entitled “Monolithic MEMS optical switch with amplified out-of-plane angular motion”, written by “Lopez, D.; Simon, M. E.; Pardo, F.; Aksyuk, V.; Klemens, F.; Cirelli, R.; Neilson, D. T.; Shea, H.; Sorsch, T.; Ferry, E.; Nalamasu, O.; Gammel, P. L”, published in “Optical MEMs, 2002. Conference Digest. 2002 IEEE/LEOS International Conference on, 20-23 Aug. 2002, Page(s): 165-166” on “2002 Aug. 22”. 
   In this embodiment, electronic control chip  150  would receive a signal from an accelerometer (g-switch)  122  and generate a signal to the MEMS blocking mirror which would redirect the laser light from the detector  114  to the explosive charge unit  140 . 
   In its simplest embodiment, the optical switch  120  need not have an accelerometer  122  incorporated therein. The accelerometer  122  could either not be needed or may be located on a chip separate from the optical switch  120  and/or fuse apparatus  100 . Without the accelerometer  122 , the electronic control chip  150  uses timing or similar data loaded into the fuse from a fire control unit to determine the desired time and/or location when the fuse is to detonate the ordnance. Using this data, electronic control chip  150  may either initiate a timer or other control programs to control the turning-on/power level of the laser  111  and moving the shutter  121  to initiate detonation of explosive charge unit  140 . 
   However, when fuse apparatus  100  does not include an accelerometer  120  it is less safe, since accelerometer  122  provides a redundant safeguard, providing a positive indication of the ordnance being fired. Redundancy is provided since the mechanical activation of accelerometer  122  would be used to detect the ordnance firing and signal the electronic control chip  150  to increase the power level of the laser  111  to ignite explosive charge unit  140 . Note for additional safety, a spin-sensor  123  could be incorporated with the fuse apparatus  100  to detect the spin that occurs when the ordnance is fired and signal the electronic control chip  150 . This spin-sensor  123  would provide additional safety that the ordnance would not explode for any g-force, e.g., dropping, not caused by ordnance being fired. 
   The explosive charge unit  140  may include an explosive charge  142  alone or in combination with a Reactive Nano Technologies (RNT) foil  141  (as a primer charge). The RNT foil  142  is a highly energetic nano-metal material that is easily ignited by a focused laser. It should be noted that other types of pyrotechnic or explosive device that can be ignited by a focused laser could be substituted for the RNT foil  141 . When the ordnance includes an explosive charge  142 , but not a RNT foil  141 , the laser  111  power must be made sufficient to directly ignite the explosive charge  142 . When the explosive charge unit  140  includes a RNT foil  141 , the laser  111  ignites RNT foil  141 , which then ignites the explosive charge  142 . When a RNT foil  141  is used, it is implemented as part of the ordnance fuse apparatus  100 , while the explosive charge  142  is not included as part of the ordnance fuse apparatus  100 . 
     FIG. 1  shows ordnance fuse apparatus  100  during it pre-fire state. During the pre-fire state and immediately prior to the ordnance being fired from a gun, controller  150  receives timing or similar data, via Data input leads  117 . This data is used to program the controller  150  to static test the ordnance fuse apparatus  100  and to control the detonation of the explosive charge  140  of the ordnance at the desired time and/or location. Note that controller  150  may be powered by an included battery  151  that is turned-on by a signal on one of the Data leads or by a capacitor  152  that is charged via one of the Data leads, or by a separate power lead, during the pre-fire state. 
   With joint reference to  FIGS. 1-3 , we describe the sequence of operations of our ordnance fuse apparatus  100  for use by a gun apparatus. The description assumes that the optical switch  120  is implemented using a MEMS shutter including an accelerometer  122 . In step  301  the ordnance (containing our fuse apparatus  100  of  FIG. 1 ) is loaded in the gun barrel and coupled to the Data leads from the gun fire-control unit (not shown). In step  302 , the capacitor(s)  152  is charged or the internal battery is “turned-on” to provide power to operate the fuse apparatus  100 . Controller  150  then receives fire-control programs and/or data via Data leads  117 , in a well-known manner from the fire control unit of the gun. 
   In step  303 , controller  150  performs self-testing to check that the MEMS shutter  120  position is in the closed (blocking) position, preventing laser light from reaching the explosive charge unit  140 . The MEMS shutter  121  position may be determined using a mechanical position sensor. If the MEMS shutter position is not correct, the procedure is aborted, in step  306 , and an Abort signal is sent back to the fire control unit to prevent the ordnance from being fired. If the position is correct, then in step  304  controller  150  checks the operation of the laser  111  and detector  114 , by detecting low-power pulses (&lt;1 mW) from the laser  111  which are reflected by the shutter  120  onto the detector  114 . In step  305 , if it is determined that the MEMS shutter position is not safe, then in step  306  an Abort signal is sent back to the fire control unit to prevent the ordnance from being fired. Note the low power laser pulses are of such a low power that they cannot ignite the explosive even if the shutter somehow were open. 
   If the position is safe, the self-test passed and the fire control unit is notified, in step  307 , that the ordnance can be fired. This information is transmitted back to the fire control unit during a talkback phase of the pre-firing state, to confirm data decoding and correct ordnance fuse apparatus  100  operation. The steps  301 - 307  complete the pre-firing state. 
   In step  308  the ordnance is fired and the rapid ordnance acceleration causes accelerometer (g-switch)  122  to move MEMS shutter  121  to the partially armed position in step  309 . In step  310 , a separate sensor (e.g., a timer or shock sensor) determines when to initiate detonation. That is, the fuse may be programmed by controller  150  to detonate after a certain time from firing or there may be some other means to determine when the fuse should go off, for example another shock sensor to detect when it has hit a wall or tank, or a proximity sensor or an altimeter, etc. In step  311 , the MEMS shutter enters a fully armed state. This may be accomplished by having the MEMS shutter position moved again electrically or thermally in response to a shutter control signal from controller  150 . The shutter control signal is applied after a predetermined programmed time has elapsed or in response to the shock sensor signal. The ordnance is then ready to detonate and, in step  312 , the laser  111  power is ramped up to its maximum value. In the fully armed state step  313 , the MEMS shutter  121  either unblocks or redirects the laser  111  light enabling it to impact and ignite the RNT foil  141 . In step  314 , the ignited RNT foil  141  rapidly heats up to over 1000° C., igniting the primary explosive (or pyrotechnic) charge  142  ( 201  of  FIG. 2 ). Or in an alternative design, the explosive charge unit  140  does not include RNT foil  141  and laser  111  directly ignites the primary explosive charge  142 . 
   The ordnance fuse apparatus  100  is implemented as an integrated system that includes a specially built chip ( 110 ,  130 ) that includes laser  111 , with an integrated detector  114 , and a micromachined lens  130 . Illustratively, this laser/detector/lens chip ( 110  and  130 ) may be implemented as an Indium Phosphide (InP) chip. The laser/detector/lens chip and MEMS unit  120  (including an optical shutter/switch and an accelerometer g-switch) may be bonded to a conventional “micro” core unit. An integrated thin film of energetic, nano-metal foil  141  is attached to the micro-core unit. The sensitivity of the RNT foil  141  is selected to safely and reliably operate in the hostile environment of the ordnance. The RNT foil (or pyrotechnic or explosive charge)  141  may be encapsulated in a glass for passivation and protection. The glass could be a spin-on or sol-gel like glass. The glass envelope protects the nano metal from heat or chemical attack. However, the glass is easily penetrated by a laser pulse; the heat of that laser pulse is contained within the “oven” like chamber created by the glass encapsulation and detonation can occur rapidly and reliably. Thus the glass coating both protects the foil from oxidation or contamination, and enhances its explosive performance. So the heat from a focused laser pulse (which readily penetrates the glass envelope, if present) starts a reaction in the RNT foil  141  that quickly heats up to over 1000° C., thus detonating the explosive charge  142  rapidly and reliably. 
   Note the RNT foil  141  produces heat but no shock wave when ignited. Many ordnance applications require a shock wave of expanding gas to initiate an explosive chain. In accordance with another feature, our ordnance fuse apparatus  100  may be implemented to layer the RNT foil  141  with a thin layer or coating  143  of an explosive compound, such as silver azide or lead azide, that will be ignited by the heat of the ignited RNT foil  141  and generate the shock wave needed to initiate an explosion in the primary explosive charge  142 . The thin explosive layer  143  could be for example sputtered or painted onto the RNT foil  141 . This approach combines the laser ignition of the RNT foil  141  with the shock wave generation utilized to initiate a conventional explosive. 
   Our ordnance fuse apparatus  100  incorporates a number of unique safety features including: 
   a) In one embodiment, the MEMS unit  120  contains a movable shutter, a shutter position sensor, and an accelerometer switch. Note in its simplest embodiment, the MEMS unit  121  contains only a movable shutter. This shutter is initially in the closed position, blocking any light from the laser from reaching the RNT foil  141 . When controller  150  receives data and power, the laser  111  outputs a low-power signal, which is reflected or passed by the shutter  121  onto a detector  114 . When operating in low-power mode, the laser  111  intensity is set at a level that is too weak to ignite the RNT foil  141 : even if the shutter  121  were to accidentally be open, the RNT foil  141  could not ignite. Signals from detector  114  and from the shutter position sensor are used to confirm correct device operation (self-test). This information is sent back by controller  150  to the fire control box along with the decoded data. 
   b) When the ordnance is fired a MEMS accelerometer  122  is irreversibly moved by the rapid acceleration: only then is the MEMS shutter  121  free to move in response to a control signal from controller  150 , which is applied after the predetermined programmed time has elapsed or a signal received from a shock sensor. The ordnance fuse apparatus  100  thus cannot ignite the RNT foil  141  or explosive charge  142  unless the MEMS shutter  121  has been exposed to a sufficient acceleration for a sufficient time: The ordnance fuse apparatus  100  cannot be detonated prior to being fired. 
   c) Once the MEMS shutter  121  is in its fully armed position, the laser  111  power is ramped up to its maximum value. The laser radiation ignites the RNT foil  141 , which heats up to over 1000° C., igniting the explosive charge. By separating the RNT foil  141  and explosive charge  142  from the electrical signals of controller  150  (using laser  111  light as the source of energy for ignition), our ordnance fuse apparatus  100  is immune from detonating due to electro-static discharge or electrical failure. The laser  111  acts like an opto-isolator, preventing accidental electrical ignition. 
   In a more simplified embodiment, our ordnance fuse apparatus  100  includes only a laser  111 , a MEMS shutter  121 , RNT foil  141 , and controller  150 . In this arrangement, safety features are reduced since controller  150  cannot determine whether laser  111  is operating at all or at what power level and cannot electrically determine that MEMS shutter  121  is in the correct position. Moreover, since no microlens  130  is used, laser  111  must have sufficient unfocused power to ignite the RNT foil  142 . 
   Because of the “integrated circuit” type embodiment of our ordnance fuse apparatus  100 , its very small size is approximately 1 to 4 cubic millimeter “monolithic cube.” Such a monolithic cube would include all control, electronics, primer and a provision for wire termination, by ordinary means, to the power supply and trigger mechanism. Nano-engineered materials combined with micromachining techniques and advanced packaging technology enable this dramatic reduction in size, while increasing performance and reliability. 
   Various modifications of our invention will occur to those skilled in the art. Nevertheless all deviations from the specific teachings of this specification that basically rely upon the principles and their equivalents through which the art has been advanced are properly considered within the scope of the invention as described and claimed.