Abstract:
A MEMS inertial delay device having a substrate layer, an intermediate layer and a device layer. A plurality of freely moveable interlocking masses are formed in the device layer along with springs which connect the masses to first and second supports. Movement of a first one of the interlocked masses, due to a shock event, allows subsequent masses to move, with a last mass including an activation member, movement of which causes operation of a mechanism, such as movement of a lock in a safe/arm arrangement in a munition round.

Description:
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 government purposes without the payment of any royalties therefor. 

   BACKGROUND OF THE INVENTION 
   Various scenarios exist where it is desirable to delay the initiation of an event until some time after an initial shock or acceleration. By way of example, in order to prevent premature detonation, many munition rounds, such as artillery shells, go through a multi-stage arming sequence after being fired. It is required that the sequence commence only after the shell has been fired, and for this purpose a delay after firing is imposed in the procedure. 
   One way of providing the necessary delay is by the use of an accelerometer. One problem with the accelerometer, however, is that it requires not only a power supply but a signal processor as well. Such arrangement needs a significant volume to package the necessary components, which is impractical for various situations, including use in a munition round. 
   The delay may also be accomplished by an inertial delay mechanism, one of which is known as a falling leaf delay mechanism comprised of a plurality of interlocking masses wherein a subsequent mass is prevented from moving until the previous mass has moved out of the way. The typical falling leaf delay mechanism is comprised of a first series of masses rotatable about a first post and a second series of masses, interlocked with the first series, rotatable about a second post. Each mass, except for the first, occupies a plane above a previous mass. When the mechanism is subjected to a shock, a first mass of the first series is moved out of position allowing a first mass of the second series of masses to move out of position. That is, movement of a mass allows the next interconnected mass to move out of position. A last of the masses to move includes an activation member to activate some event, the activation occurring after a time delay imposed by movement of the totality of all the masses, subsequent to the initial shock. 
   Existing falling leaf designs require masses, which are individually machined, followed by an assembly process. The required assembly is either by hand, a time consuming process, or by expensive machine placement. In addition, present designs are relatively large for placement in munition rounds and do not respond to relatively low acceleration environments. 
   It is an object of the present invention to provide an inexpensive miniature inertial delay device which can respond to low accelerations and which is fabricated utilizing MEMS (micro electromechanical systems) techniques. 
   SUMMARY OF THE INVENTION 
   A MEMS inertial delay device is provided which includes a substrate, an intermediate layer on the substrate, and a device layer on the intermediate layer. The device layer includes first and second spaced-apart support members secured to the substrate by the intermediate layer, and a plurality of interlocked masses, each connected to one of the first or second support members by a spring arrangement. The interlocked masses and the spring arrangement are devoid of any underlying intermediate layer so that the interlocked masses and the spring arrangement are freely moveable. The principles of the present invention may be applied to a free-standing inertial delay device which includes first and second spaced-apart support members with a plurality of interlocked masses, each connected to one of the first or second support members by a spring arrangement. The masses are all in the same plane and are positioned between the first and second support members. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood, and further objects, features and advantages thereof will become more apparent from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings, in which: 
       FIGS. 1A ,  1 B, and  1 C conceptually illustrate an arming sequence for a munition round. 
       FIG. 2  is a view of a SOI (silicon on insulator) wafer prior to fabrication of the inertial delay device. 
       FIG. 3  is a perspective view of one embodiment of a fabricated inertial delay device in a locked condition. 
       FIG. 4  is a perspective view of the fabricated inertial delay device of  FIG. 3  in an unlocked condition. 
       FIG. 5  is a plan view of another embodiment of a fabricated inertial delay device in a locked condition. 
       FIG. 6  is a perspective view of the fabricated inertial delay device of  FIG. 5  in an unlocked condition. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In the drawings, which are not necessarily to scale, like or corresponding parts are denoted by like or corresponding reference numerals. 
   Although the present invention is applicable for use in a variety of situations, it will be described, by way of example, with respect to the arming of a munitions round, such as an artillery shell.  FIGS. 1A ,  1 B and  1 C conceptually illustrate the components and process involved. In  FIG. 1A  the artillery shell includes a primary charge  10 , which, when detonated will activate a secondary charge  11 , which, in turn, will set off a main charge  12 . The explosive detonation is commenced by a fire command  14 . The explosive train is interrupted however, by intervention of a barrier such as slider  16 . 
   The slider  16  may move out of position to allow firing by means of an arm command  18 , however the slider is constrained from movement by lock  20 , and lock  21 , acting as a back-up safety, engaging respective projections  23  and  24 . Once lock  20  is moved out of position, lock  21  may thereafter may be moved out of position to allow slider  16  to move whereby detonation may subsequently take place. Lock  20  is moved out of position after a certain delay after the firing of the artillery shell, and the lock movement is accomplished by an inertial delay device of the present invention. 
   After lock  20  is moved out of the way, as illustrated in  FIG. 1B , lock  21  may then next be removed. Arrow  28  represents activation for moving lock  21  and may include such devices as a timer, an airflow sensor or even another inertial delay device, by way of example. In  FIG. 1C  both locks  20  and  21  are out of their initial position thereby unblocking projections  23  and  24  thus allowing arm command  18  to move slider  16  to move in the direction of arrow  30 . Fire command  14  is then free to initiate the explosive portion of the sequence, such as upon target impact, or upon a predetermined delay after impact. 
     FIG. 2  illustrates a portion of an SOI wafer  36  from which the inertial delay device of the present invention will be fabricated. The structure of  FIG. 2  includes a silicon substrate  38  (also known as a handle layer) covered by an insulating, or intermediate layer  40 , such as silicon dioxide, over which is deposited another silicon layer  42 , also known as the device layer, which is the layer from which the inertial delay device will be fabricated. 
     FIG. 3  is a view of an inertial delay device  44 , of a falling leaf design, formed from the wafer  36  of  FIG. 2 . The inertial delay device is formed by a DRIE (deep reactive ion etching) process, which removes unwanted portions of device layer  42 . The DRIE process is a well developed and known micromachining process used extensively with silicon based MEMS (micro electromechanical systems) devices. For this reason silicon is the preferred material for the inertial delay device of the present invention, although other materials are possible. 
   Inertial delay device  44  is one of a multitude of identical inertial delay devices fabricated on the same wafer  36 , with all of the inertial delay devices being separated after fabrication for use as individual inertial delay devices. As illustrated in  FIG. 3 , inertial delay device  44 , etched in the device silicon layer  42  includes first and second spaced-apart supports  47  and  48 . Each of a plurality of interlocked masses  50  to  57 , all in the same plane, is connected to one of the supports  47  or  48  by a spring arrangement, which, in the embodiment of  FIG. 3  consists of a single spring  60 . 
   The last mass  57  includes an extension  62  which functions to move an activator  64  which is connected to the first lock  20  in  FIG. 1  (or to any other mechanical or electrical mechanism for which delayed operation is desired). In order to operate as an inertial delay device, masses  50  to  57 , as well as springs  60  must be free to move and therefore must be free of any underlying silicon dioxide insulating layer  40 . 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 will, in a relatively short period of time, dissolve the insulation beneath the springs  60 , since they are of small width, thus freeing them for movement. In order to shorten the time for dissolving the silicon dioxide under masses  50  to  57 , they are provided with a series of apertures  66  which extend from the top surface down to the insulating layer  40 , thereby allowing the etchant direct access to the undersurface of the masses. Although some of the etchant dissolves the insulation under the supports  47  and  48 , the process of freeing the masses  50  to  57  and springs  60  is completed before the supports are completely freed so that they remain immovable. 
   In response to an initial shock in the direction of arrow  68 , the first mass  50  will move out of position, in the direction of arrow  69 . This movement frees the second mass  51  for movement and the process is repeated until all the masses  50  to  57  are unlocked, as in  FIG. 4 . Movement of the last mass  57  together with extension  62  and activator  64  therefore moves lock  20  out of the way at a time subsequent to the shock causing event. This time delay may, by way of example, be on the order of a fraction of a second up to several seconds. Thus, a shock event such as a dropping of the artillery shell, lasting a period of time measurable in milliseconds will not cause a removal of the lock. 
     FIG. 5  is a plan view of another embodiment of the present invention. Inertial delay device  72  includes four interlocked masses  74  to  77 , each being connected to both supports  80  and  81  by a spring arrangement. More particularly, the spring arrangement is comprised of springs  83  and  84  connecting a respective mass  74  to  77  to first support  80 , and springs  85  and  86  connecting a respective mass  74  to  77 , to second support  81 . Each spring  83  to  86  is of a serpentine shape and, in particular, the serpentine shape is a substantially “S” shape, including two (2) curves, to give flexibility to the springs. As is the case with respect to masses  50  to  57  of  FIG. 3 , masses  74  to  77  include a series of apertures  88  to facilitate the etchant removal of underlying insulating material during the fabrication process. 
   In the absence of any shock, mass  74  remains immobile and prevents movement of the subsequent masses  75  to  77 . If the device is subject to a shock in the direction of arrow  90 , mass  74  will move in the direction of arrow  91 , thus freeing the remaining masses for movement. The last mass  77  includes an activator  92  for operating a mechanism, such as the removal of a lock, as previously described. The unlatched condition of the inertial delay device  72  is illustrated in  FIG. 6 . 
   It will be readily seen by one of ordinary skill in the art that the present invention fulfills the objects set forth herein. After reading the foregoing specification, one of ordinary skill in the art will be able to effect various changes, substitutions of equivalents and various other aspects of the present invention as broadly disclosed herein. It is therefore intended that the protection granted hereon be limited only by the definition contained in the appended claims and equivalents. Having thus shown and described what is at present considered to be the preferred embodiment of the present invention, it should be noted that the same has been made by way of illustration and not limitation. Accordingly, all modifications, alterations and changes coming within the spirit and scope of the present invention are herein meant to be included.