Abstract:
A shock sensor has a sensing mass mounted on a metallic reed or spring which, under the influence of a crash-induced acceleration, drives the spring against a contact to close an electrical circuit. The contact end of the spring is twisted to be oriented with respect to the fixed contact at an angle of 60 degrees out of the plane containing the spring. The sensor is oriented such that the acceleration force is approximately normal to the plane containing the spring. The angled contact increases reliability, reduces closure signal noise, and increases contact dwell time. Dwell time can be further enhanced for high shock loads by providing a two stage mass spring system. A second mass/spring combination is arranged so the motion of the second mass, after the first reed has made electrical contact, holds the contact closed.

Description:
FIELD OF THE INVENTION 
     The present invention relates to shock sensors in general and to shock sensors used for engaging or deploying automobile safety devices in particular. 
     BACKGROUND OF THE INVENTION 
     Shock sensors are used in motor vehicles, including cars and aircraft, to detect vehicle collisions. When such a collision occurs, the shock sensor triggers an electronic circuit for the actuation of one or more safety devices. One type of safety device, the deployable air bag, has found widespread acceptance by consumers as improving the general safety of automobile operation. Air bags have gone from an expensive option to standard equipment in many automobiles. Further, the number of air bags has increased from a single driver&#39;s side air bag to passenger air bags with future use of multiple air bags a distinct possibility. 
     With the ever increasing utilization of air bags, research and development has continued with efforts to make air bags and the electronics and sensors which control their deployment both more reliable and of lower cost. A key aspect of reliability with respect to air bags involves the twin, somewhat conflicting requirements that the air bag deploy in every situation where their deployment would be advantageous to the passengers but, at the same time, not deploy except when actually needed. Reliable deployment of an air bag without unwanted deployments is facilitated by use of multiple sensors in combination with actuation logic which can assess the nature and direction of the crash as it is occurring and, based on preprogrammed logic, make the decision whether or not to deploy the air bag. This increase in reliability tends to lead to a greater number of sensors as well as increased use of electronic logic. 
     The desire to hold down sensor cost and to keep the sensor integrated with the logic circuits has led to the use of solid state shock sensors. However, solid state shock sensors are prone to losing touch with the real world and may occasionally indicate a crash is occurring due to radio frequency interference, electronic noise, cross-talk within the electronics, etc. 
     The suitability of mechanical shock sensors as an integral part of bag deployment systems which prevent unnecessary bag deployment has kept up the demand for mechanical shock sensors. 
     A number of types of shock sensors employing reed switches have been particularly advantageous in combining a mechanical shock sensor with an extremely reliable electronic switch which, through design, can be made to have the necessary dwell times required for reliable operation of vehicle safety equipment. The reed switch designs have also been of a compact nature such that the switches may be readily mounted on particular portions of the vehicle which, in a crash, will experience a representative shock which is indicative of the magnitude and even the direction of the shock inducing crash. 
     One type of shock sensor, shown in German Patent No. DE 35 09 054, employs a sensing mass mounted on a spring with a second less-rigid spring spaced from the first spring in a glass housing. An acceleration sensing mass of less than three grams is mounted to the less rigid spring. 
     A need remains, however, for shock sensors having lower cost, high repeatability, and small packaging, which at the same time have the advantages of a mechanical sensor in providing relatively long switch closure or dwell time in combination with insensitivity to electronic noise or interference. 
     SUMMARY OF THE INVENTION 
     The shock sensor of this invention has some structural similarities to a reed switch. But, whereas a reed switch, when functioning as part of a shock sensor, requires a moving magnetic mass, the shock sensor of this invention employs a sensing mass mounted on a metallic reed or spring which, under the influence of a crash-induced acceleration, drives the reed against a fixed contact to close an electrical circuit. In order to extend the closure duration to increase the reliability and ease with which a significant event may be detected, a contact surface at the end of the reed and the fixed contact are oriented at an angle 60 degrees out of the plane containing the reed. The 60 degree contact surface on the reed may be formed by twisting a portion of the reed adjacent to the contact end. The sensor is oriented such that the acceleration force is approximately normal to the plane containing the reed. The orientation of the contact area on the reed and the fixed contact allows contact shock to dissipate sufficiently to eliminate most bouncing upon initial closure. The 60 degree contact angle provides a more reliable, less noisy closure signal in the presence of a crash-induced shock. Dwell time of initial contact closure because of the angled contacts is increased five to ten times on even marginal sensor closing events. The dwell time on higher force events is in some instances comparable to magnetically actuated crash-sensing devices. Further, manufacturing imperfections, in achieving alignment of contact interfaces, can actually provide a softer more gradual transition to mating contact The wiping and twisting of the contact surfaces, as they come into full face-to-face contact, increases dwell time. 
     Closure dwell time can be further enhanced for high shock loads by providing a two stage mass spring system. A second mass is mounted to the first reed or spring by a second reed or spring. The second mass/spring combination is arranged so the motion of the second mass, after the first reed has made electrical contact, is such as to hold the contact closed. 
     It is a feature of the present invention to provide a shock sensor for use in triggering safety devices within a moving vehicle. 
     It is another feature of the present invention to utilize the technology for manufacturing reed switches in the construction of a shock sensor. 
     It is a further feature of the present invention to provide a shock sensor in which all electromechanical components are contained within a hermetically sealed volume. 
     It is yet another feature of the present invention to provide a mechanical shock sensor having fewer components. 
     It is a yet further feature of the present invention to provide a reed switch with reduces contact bounce on switch closure. 
     Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side elevational view, broken away in section, of the shock sensor of this invention. 
     FIG. 2 is a cross-sectional view of the reed switch of FIG. 1 taken along section line  2 — 2 . 
     FIG. 3 is a side elevational view, broken away in section, of an alternative embodiment of the shock sensor of this invention employing a two stage mechanical system. 
     FIG. 4 is a cross-sectional view of the shock sensor of FIG. 3 taken along section line  4 — 4 . 
     FIG. 5 is a diagrammatic view of fifty-one different spring mass systems which can be employed to increase the dwell time of the shock sensor of FIG.  1 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring more particularly to FIGS. 1-5, wherein like numbers refer to similar parts, a shock sensor  20  is shown in FIG.  1 . The shock sensor  20  is composed of a glass capsule  22  which defines an internal volume  24 . The internal volume may be filled with an inert gas or gas with a high dielectric breakdown strength. The glass capsule  22  is formed around a short lead  26 , a long lead  28  and a mounting lead  30 . Electrical contact is made between the short and long leads by a reed or spring  32 . 
     The spring  32  has an attachment end  34  which is welded to a raised flange  36  at the free end  38  of generally rigid long lead  28 . The spring  32  has a shock-sensing upper masse  40  and a shock-sensing lower mass  42  which are welded to the spring  32  adjacent the contact end  44 . The contact end  44  is comprised of a twisted portion  46  and a contact flat  48 . As shown in FIG. 2, the spring  32  defines a plane and the centerline of the spring defines a line  52 . The twisted portion  46  is twisted about the line  52  of the spring  32  to bring the contact flat  48  into a plane  54  which is rotated through an angle α of 60 degrees with respect to the plane  50  of the spring  32 . The short lead  26  has a deformed portion  56  which defines a non-moving contact  58  which has a contact surface  60 . 
     The spring  32 , in a typical shock sensor  20 , may have a thickness of about 1.5 thousandths of an inch and a width of 30 thousandths of an inch. The overall length of the spring from end to end is about 480 thousandths of an inch. The dimensions of the spring thus render it substantially flexible only in a direction normal to the plane  50  in which the spring  32  lies. The normal direction defines a line  62 . The line  62  normal to the plane  50  of the spring  38  is aligned with the direction of acceleration which it is desired to sense. In use, the sensor  20  may be mounted by the leads  30 ,  26 ,  28  directly to a circuit board containing some or all of the electrical components used to actuate an air bag or similar device. The sensor may also be mounted in a package (not shown) to facilitate orienting and mounting the sensor on a part of a vehicle where, through tests and analysis, it has been determined the response of the structure provides reliable indication of the direction and severity of a car crash. 
     The shock sensor  20  takes advantage of the manufacturing tools and techniques for making reed switches to fabricate a shock sensor. The reed switch manufacturing process has developed around the mass production of components such as leads, springs and contacts with high precision and low cost. The reed switch manufacturing process also facilitates the assembly of the leads and springs—automatically positioning them with high tolerance and sealing a hermetic glass capsule about the switch components. The extremely high reliability, long life and low cost of reed switches has found them wide employment in industry and consumer products. 
     The shock sensor  20 , by utilizing the techniques of a reed switch manufacturer, transfers the advantages of low cost and high reliability to shock sensors suitable for use in automobile safety systems. 
     In operation, the shock sensor  20  is mounted in a vehicle with the line  62 , which is normal to the plane of the spring  50 , oriented along the expected line of action of a shock-inducing event or crash. The shock sensor  20  is further oriented so the upper mass  40  faces the direction shown by arrow  51 , in which the crash load is expected. When the vehicle containing the shock sensor  20  experiences a shock-inducing crash, the vehicle rapidly decelerates, which, in turn, decelerates the glass capsule  22  of the shock sensor  20 . The sensing masses  40 ,  42 , because they are relatively unconstrained by the spring  32 , continue in accordance with Newton&#39;s First Law to move forward and thereby bring the contact end  44  of the spring and the contact flat  48 , formed thereon, into contact with the fixed contact  58  which is rigidly mounted to the short lead  26 . The short lead is held in position by the glass capsule  22 . 
     Because the contact flat  48  on the spring  32  and the fixed contact surface  60  on the short lead  26  engage at an angle α which is oriented sixty degrees from the the plane  50  of the spring  32  or correspondingly 30 degrees from the direction of motion of the spring and the sensing masses  40 ,  42 , the closure between the contacts  48 ,  58  is softer. The soft closure results from the contact  48  on the spring sliding along the fixed contact surface  60  which, in turn, causes a limited deflection of the spring  32  in the plane of the spring  50 . The sliding action between the spring contact  48  and the fixed contact  58  results in a frictional engagement between the spring contact  48  the fixed contact  50 . The frictional engagement dissipates energy, helping to reduce bounce. 
     The spring  32  is much stiffer, in that is has greater resistance to bending, in the plane of the spring  50 , than out of the plane of the spring  50 . Because closure of the switch  47  results in in-plane deflection of the spring  32 , when the contact  48  on the spring  32  begins to lift off the fixed contact  58 , due to elastic bounce, friction between the contacts  48 ,  58  is reduced or eliminated. The reduction of the frictional forces between the contacts  48 ,  58  allows the high momentum forces developed by the in-plane deflection of the spring  32  to move the spring contact  48  back into engagement with the fixed contact  58 . Thus, the tendency of the contacts of a switch to bounce open when subjected to a closing force is significantly decreased or eliminated by having the closing surfaces angled with respect to the direction of closing of the switch. In practice, the exact analysis of the dynamics of the closure of the switch are complicated by cross-coupling between the spring constant of the spring  32  in and out of the plane of the spring, as well as by manufacturing tolerances which introduce imperfections in the alignment of the angled contact surfaces. Experience with the construction of the shock sensors  20  has shown that manufacturing imperfections actually enhance switch closure time by providing a softer, more gradual transition in the mating of contact surfaces from a weak point contact, as the contact surfaces wipe and twist towards a more rigid line or face contact. 
     A shock sensor  120 , shown in FIGS. 3 and 4, has improved dwell time through the employment of a two-stage mechanical system. The shock sensor  120  has a glass capsule  122  or housing which encloses an internal volume  124 . The internal volume  124  may be filled with an inert gas or gas with a high dielectric breakdown strength. As in the shock sensor  20 , the shock sensor  120  has a short lead  126  and a long lead  128  positioned at a first end  129  of the capsule  122  as well as a mounting lead  130  positioned at the opposite end  131 . 
     The long lead  128  has a raised flat  136  at its free end  138 . A first spring  132  is welded at an attachment end  134  to the raised flat  136 . The spring has a contact end  144  which has a contact flat or surface  148  which is movable against a fixed contact  158  which has a contact surface  160 . The fixed contact  158  is formed of a deformed portion  156  of the short lead  126 . The switch  120  has a first acceleration sensing mass  142  mounted to the spring  132  near the contact end  144 . Between the first mass  142  and the contact end portion of the spring  132  there is a twisted portion  146  so the contact flat or surface  148  is rotated sixty degrees out of the plane  150  in which the spring lies. As thus described, the shock sensor  120  is similar to the shock sensor  20 . However, a second mechanical stage is formed by joining a second spring  164  to the first spring  132 . 
     The second spring  164  is joined to the upper surface  166  of the spring  132  overlying the first mass  142 . The second spring  164  has a free end  168  on which is mounted a second mass  140 . During a crash the acceleration sensing masses  142 ,  140  experience an apparent acceleration in the direction of arrows  170 ,  172 , causing them to move in the direction of the arrows  170 ,  172 , which, in turn, causes the contact surfaces  148  and  160  to touch and close the switch  147 . 
     Because the first acceleration sensing mass  142  is closely spaced from the contact surfaces  148 , 160  the engagement of the contacts brings the first mass  142  to rest with respect to the contacts. On the other hand, the second mass  140  continues to deflect even after the contacts have become fully engaged. This continual deflection continues to move the second spring  164  and thereby holds the first spring  132  against the contact  158  increasing dwell time. 
     FIG. 5 shows fifty-one mechanical systems all except systems  211 ,  228  of which can be used to extend or improve the dwell or closure time of a switch. Each of the mechanical systems  201 - 251 , illustrated in FIG. 4, has some or all of the following: a fixed mount  1 , a stationary contact  2 , a first spring  3 , a first acceleration sensing mass  4 , a second spring  5 , and a second acceleration sensing mass  6 . 
     Dynamic system  226  is closely representative of the dynamic system employed by the shock sensor  120 . 
     The dynamic system  211 , and to some extent,  204 ,  221 ,  238 , is representative of the dynamic system employed by the shock sensor  20 . 
     Dynamic systems  201 - 203 ,  205 - 210  and  212 - 217  are characterized in that the second spring  5  and second masses  6  are mounted to the first spring  3 , opposite the fixed contact  2 . Systems  218 - 220 ,  222 - 227 , and  230 - 234  have the secondary spring  5  and secondary mass  6  attached to the side of the first spring  3  which faces the fixed contact  2 . 
     Systems  235 - 251  are systems which employ secondary masses  6  on secondary springs which are parallel to and offset from the first spring  3  and first masses  4 . Thus, for example, a shock sensor employing dynamic system  236  might employ a spring in the shape of a three tined fork with the first mass  4  located on the central tine and secondary masses  6  located on the outer tines. Thus, dynamic systems  235 - 251  may have one or two secondary masses  6  positioned on one or both sides of the primary spring  3 . 
     Within the fifty-one mass systems disclosed in FIG. 5 some systems have greater potential for increasing switch closure dwell times and providing reliable long-term operation. Systems  204 ,  221  and  238  generally have a basic mass distribution. Systems  201 - 203 ,  205 - 207 ,  218 - 220 ,  222 - 224 ,  235 - 237 , and  239 - 241  can produce stress concentrations at mid-spring and, for this reason, may be undesirable. On the other hand, in systems  208 - 210 ,  225 - 227  and  242 - 244 , the second mass  6  and second spring  5  function to reinforce the natural function of the primary spring  3  and primary mass  4 . 
     Systems  212 ,  229  and  246  represent good mass distributions where a single structural member forms both the first spring  3  and the second spring  5 . In systems  213 - 217  and  230 - 234  and  247 - 251  the mass system increases the natural frequency of the dynamic systems and the stress concentrations induced by the flexure of the primary spring  3  about the contact point  2  occurs in homogenous material influencing concern with respect to system integrity. 
     It should be understood that in mass systems  201 - 251 , systems  211 ,  228  and  245 , do not provide a two-stage mechanical system. 
     Mass systems  212 ,  229  and  246  have some similarity to the mechanical system disclosed in German patent DE 3509054, particularly FIGS. 2 and 4 of that patent. They differ, however in that the mass distribution of systems  212 ,  229  and  246  are distributed such that some of the mass is directly opposite the contact point  2  and some of it is spaced beyond the contact. Also, the contact  2  is stationary as opposed to being mounted on a flexible support. Thus, it will be understood that the mass systems  201 - 251 , with the exception of mass systems  211 ,  228  and  245 , form means for increasing the switch dwell time by forming a two-stage mechanical system so the switch has a greater dwell time. 
     It will also be understood that the mass systems of FIG. 5 are illustrative of the mechanical principals and that actual shock sensors constructed in accordance with those mass systems will have minor variations necessary to accommodate actual systems. For example, in mass system  226 , which is representative of shock sensor  120  of FIG. 4, the primary mass  4 , which corresponds to first mass  142  in FIG. 4, is not located directly beneath the fixed contact  158  because of the practical necessity of orienting the contact surfaces  148 ,  160  at the sixty degree angle to the plane containing the spring  132 . Thus, it will be understood that the dynamic systems  201 - 251 , when employed in an actual shock sensor, may require some modifications to incorporate actual design constraints and additional features such as the angled contact surface between the moving contact and the stationary contact. 
     It should be understood that a permanent magnet or electromagnetic induced field may be used in conjunction with springs  32 ,  132  when they are constructed of a ferromagnetic material to induce the shock sensor  20 ,  120  to latch when activated. It should also be understood that an electromagnetic field could be used to induce closure of the shock sensors  20 ,  120  in order to provide a self-testing function. 
     It will also be understood that wherein a sixty degree angle is disclosed between the plane containing the spring  32 ,  132  and the contact surfaces  40 ,  48 ,  60 ,  160 , displacement of the contact surfaces by angles greater than or less than sixty degrees could be used. 
     It should be understood that a reed switch having contacts angled with respect to the plane of the ferromagnetic reed could be constructed similar to the shock sensor  20  of FIG. 1 and 2 but without the mass  40 ,  42  mounted on the reed  32 . A source of electromagnetic force such as a magnet or electrical coil located near the reed switch could thus cause the switch to close; the magnet by moving closer to the activation region of the switch, the coil by being energized by an electrical current. Such a reed switch should have reduced contact bounce. 
     It should be understood that features to prevent overtravel of spring/mass elements or contact faces may be presented by dimensional restrictions present in shock sensor  20  or  120 . Other packaging approaches may make specific travel limit features necessary. 
     It should be understood that where the masses  40 ,  42  are shown as two separate pieces they could be a single mass wrapped around the spring  32  or all the mass could be mounted on one side of the spring. 
     It should be understood that in FIG. 1 the spring end  34  could be mounted to the free end  38  of the long leed  28  with out the formation of a raised portion  36  on the leed  28 . 
     It is understood that the invention is not limited to the particular construction and arrangement of parts herein illustrated and described, but embraces such modified forms thereof as come within the scope of the following claims.