Patent Publication Number: US-7913623-B1

Title: MEMS fuze assembly

Description:
The present application is a Continuation Application of prior U.S. patent application Ser. No. 11/894,628 filed on Jul. 31, 2007 now U.S. Pat. No. 7,552,681. 
    
    
     STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein may be manufactured and used by or for the Government of the United States of America for Governmental purposes without the payment of any royalties thereon or therefor. 
    
    
     BACKGROUND OF THE INVENTION 
     1) Field of the Invention 
     The invention relates in general relates to MEMS (microelectromechanical systems) devices and more particularly to a MEMS fuze utilized to set off a main charge of a munitions round. 
     2) Description of the Related Art 
     A fuze is a device designed to set off an explosive train in a munitions round such as a mortar round, artillery shell or rocket warhead, by way of example. Conventional mechanical fuzes make use of a detonator, such as an M100, which is cylindrical and approximately 3 mm (millimeters) in diameter and 10 mm in length. These detonators are mounted in a rotor mechanism with mechanical locks, with a typical volume of greater than 10 cc (cubic centimeters). 
     Such detonators are much too large for use in MEMS type fuzes and, in addition, they require assembly of multiple mechanical components resulting in higher complexity, higher costs and lower reliability. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a fuze assembly that is over 100 times smaller than conventional detonators, thus leaving more space for electronics and explosive material. 
     A MEMS fuze for use in a munitions round in accordance with the present invention includes a moveable slider with a microdetonator carried by the slider for positioning relative to a secondary lead to ignite the secondary lead when in position. A plurality of locks are provided, each having a respective locking arm in interlocking engagement with the slider to prevent movement of the slider. The locks are released upon attainment of certain predetermined conditions to move the locking arms out of engagement with the slider whereby when the locking arms are disengaged from the slider, the slider is operable to move the microdetonator into position for igniting the secondary lead. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings, which are not necessarily to scale, like or corresponding parts are denoted by like or corresponding reference numerals. 
         FIGS. 1A and 1B  illustrate an operation of an exemplary microdetonator. 
         FIG. 2  illustrates an exemplary SOI (silicon on insulator) wafer prior to fabrication of the MEMS device of the present invention. 
         FIGS. 3A and 3B  illustrate an exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 1A and 1B  illustrate a microdetonator and its placement for initiating a charge sequence. In  FIG. 1A , a microdetonator  10  is carried by a slider  12  and is in an initial position insufficient to set off a secondary explosive  14 , also known as a secondary lead. 
     When the slider  12  moves to the right as indicated in  FIG. 1B  by arrow  16 , microdetonator  10  may be adjacent an initiator  18  and directly above secondary lead  14 , whereupon the microdetonator  10  may be initiated or detonated by the initiator  18 . Secondary lead  14  may be initiated by the microdetonator  10  and set off a main explosive charge  20 , which is the main charge of the munitions round in which the apparatus is imbedded. Movement of slider  12  may be inertial, such as upon impact with a target, or may be mechanical, as will be described herein. 
       FIG. 2  illustrates a portion of an SOI wafer  24  from which the MEMS fuze assembly of the present invention is fabricated. The structure of  FIG. 2  includes, in an exemplary embodiment, a silicon substrate  26  (also known as a handle layer) covered by an insulating or intermediate layer  28 , such as silicon dioxide, over which is bonded or deposited another silicon layer  30 , also known as the device layer  30 , which is the layer from which the MEMS fuze assembly components are fabricated. The MEMS fuze assembly components may be formed by a DRIE (deep reactive ion etching) process that removes unwanted portions of device layer  30 . The DRIE process is a well developed micromachining process used extensively with silicon based MEMS devices. For this reason silicon is an exemplary material for the MEMS fuze assembly of the present invention, although other materials are possible. In other exemplary embodiments, materials other than silicon may be used as a substrate, including glass, stainless steel, and a plastic material, such as, polycarbonate. 
     An exemplary embodiment of the present invention is illustrated in  FIGS. 3A and 3B . The MEMS fuze  32  in  FIG. 3A  includes slider  12  which, in an exemplary embodiment, is driven mechanically as opposed to inertially. As a safety precaution and in accordance with safety regulations, movement of the slider  12  is initially prevented by a series of locks, which are released upon attainment of certain predetermined conditions. Slider  12  is in the safe position in  FIG. 3A  and in the armed position in  FIG. 3B . By way of example, the arrangement includes a setback activated lock  34  and a spin activated lock  36 . 
     Setback activated lock  34  includes a setback inertial mass  38  having a latching arm  40  that engages with complementary first and second holding arms  42  and  44 , these latter first and second holding arms may be connected to respective anchors  46  and  48 . Setback inertial mass  38  is restrained from movement by spring  50  connected to anchor  52 . Setback activated lock  34  additionally includes a locking arm  54 , which is in interlocking relationship with slider  12 . More particularly, the end of locking arm  54  abuts a projection  56  on slider  12  to prevent movement thereof. 
     Setback inertial mass  38  prevents movement of locking arm  54  until setback inertial mass  38  is moved out of the way. This movement occurs during launch of the munitions round when the axial acceleration force allows setback inertial mass  38  to overcome action of spring  50  such that latching arm  40  may become latched with holding arms  42  and  44 . With setback inertial mass  38  out of the way, locking arm  54  is free to disengage from projection  56  of slider  12 . 
     The disengagement is accomplished with the provision of a thermoelectric actuator such as V-beam actuator  58 . V-beam actuator  58  includes first and second sets of actuator beams  60  and  62 . One end of set  60  is connected to anchor  64 , while the other end is connected to locking arm  54 . One end of set  62  is connected to a second anchor  66 , with the other end connected to locking arm  54 . The first and second set of beams  60  and  62  are of a conductive elastic material with a high melting point, such as silicon. When a current is applied to anchor  64 , the beams  60 ,  62  expand, causing the locking arm  54  to move in the direction of arrow  68 . This current may be applied prior to unlocking of spin activated lock  36  or subsequent thereto. 
     Spin activated lock  36  includes a spin inertial mass  70  having a latching arm  72  which engages with complementary third and fourth holding arms  74  and  76 , these latter third and fourth holding arms may be connected to respective anchors  78  and  80 . Spin inertial mass  70  is restrained from movement by spring  82  connected to anchor  84 . Spin activated lock  36  additionally includes a locking arm  86 , connected to spin inertial mass  70 , with the locking arm  86  in interlocking relationship with slider  12 . More particularly, the end of locking arm  86  abuts a projection  88  on slider  12  to prevent movement thereof. A sufficiently high centrifugal acceleration allows spin inertial mass  70  to overcome action of spring  82  such that latching arm  72  becomes latched, drawing locking arm  86  out of engagement with projection  88  to allow slider  12  to move. 
     A thermoelectric actuator in the form of V-beam actuator  90 , similar to V-beam actuator  58 , is used to move the slider  12  against action of springs  92  and  94 , connected to respective anchors  96  and  98 . Slider  12  includes an enlarged end portion  100  in which is located the microdetonator  10 . 
     To operate as a MEMS fuze, the various springs, locking arms and beam sets of the V-beam actuators must be free to move and therefore must be free of any underlying silicon dioxide insulating layer  28  ( FIG. 2 ). One way to accomplish the removal of the underlying insulating layer is by applying an etchant, such as, hydrofluoric acid, which will dissolve the silicon dioxide. The etchant may, in a relatively short period of time, dissolve the insulation beneath the locking arms and the beam sets of the V-beam actuators, as well as the springs and slider because these components have small widths. The setback inertial mass  38  and spin inertial mass  70  must be free to move and therefore must be free of any underlying silicon dioxide insulating layer  28  ( FIG. 2 ). 
     To shorten the time for dissolving the silicon dioxide under these relatively larger components (masses  38 ,  70 ), each is provided with a series of apertures  102 , which extend from the top surface  30  down to the insulating layer  28 , thereby allowing the etchant direct access to the silicon substrate  26 . Although some of the etchant may dissolve the insulation under the anchors, the process of freeing the other components is generally completed before the anchors are completely freed so that they, that is, the anchors, remain immovable. 
     An actuator arm  104  of V-beam actuator  90  carries one or more teeth  106  at its end which are engageable with teeth  108  on the bottom of slider  12 . When V-beam actuator  90  is provided with current, actuator arm  104  moves to the left, and teeth  106  on actuator arm  104  slide over teeth  108  on slider  12 . When current is removed, V-beam actuator  90  reverts to its original position such that actuator arm  104  will move back to the right. In so doing, teeth  106  engage with teeth  108  to move the slider  12  to the right. 
     A keeper arrangement prevents the slider  12  from moving back under spring action once the slider  12  has been advanced. Such a keeper arrangement includes a keeper arm  110  secured to anchor  112 . Keeper arm  110  includes a set of teeth  114 , which are engageable with teeth  116  on the top of slider  12 . After slider  12  is advanced, teeth  114  engage teeth  116  to prevent backward movement of slider  12 . 
     The process of providing current to, and removing current from, V-beam actuator  90  is repeated until slider  12  has moved a sufficient distance such that microdetonator  10  is adjacent initiator  18 , as illustrated in  FIG. 3B . When in position, and at the proper time, current may be supplied to initiator  18  to initiate microdetonator  10  and start the explosive train. 
     Current is supplied to initiator  18 , as well as to V-beam actuators  58  and  90  by means of current sources (not illustrated) via electrical connections depicted by double ended arrow  118 . A microprocessor (not illustrated) is operable to receive signals via electrical connections when latching arms  40  and  72  latch, and when microdetonator  10  is in position, to command the current sources to provide the respective currents used in the operation. 
     It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. 
     Finally, any numerical parameters set forth in the specification and attached claims are approximations (for example, by using the term “about”) that may vary depending upon the desired properties sought to be obtained by the present invention. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of significant digits and by applying ordinary rounding.