Abstract:
A reed switch based shock sensor provides for passing electrical current through the coil spring used to bias the shock sensing magnetic mass. The spring is wrapped around the reed switch, allowing the coil spring to act as an electrical coil. The coil generates a magnetic field of sufficient strength to cause the reed switch reeds to attract and so close the reed switch, thus allowing the reed switch to be tested without the addition of a test coil.

Description:
FIELD OF THE INVENTION 
     The present invention relates to shock sensors incorporating a reed switch in general, and to shock sensors incorporating self-testing in particular. 
     BACKGROUND OF THE INVENTION 
     A typical automobile manufactured today has a number of active safety systems that function to deploy air bags, and initiate seatbelt retractors and other devices. As the cost of air bags decreases, and the sophistication of air bags increases, the number of air bags provided in each vehicle is increasing. Systems now being installed or under development include multiple air bags to protect the passenger from front, rear, and side impacts, and to position the passenger&#39;s body to withstand acceleration. Deployment of safety systems requires sensors that can detect and characterize a crash as it occurs. The widespread use of safety systems results in ever increasing attention to producing systems that can be economically employed on a large number of vehicles. 
     Typically, the lowest cost sensors are those formed as micro devices on an integrated circuit chip used to form electronic circuitry. This technology is used to fabricate accelerometers that can detect accelerations indicative of a vehicle crash. These sensors are particularly cost effective when the sensor can be fabricated together with the deployment logic circuitry using the same technology which is used cost effectively for large scale integrated circuit chips. However, the very small size of these devices makes them sensitive to electromagnetic interference and the like, which can result in false indications that a crash is taking place. 
     Thus an important role remains for macro scale mechanical devices which are less prone to false readings. Such devices are used to verify the existence of an actual crash event. These macro scale devices employ a sensing mass mounted on a spring or pendulum. Motion of the mass is detected by actuation of a reed switch or a magnetic field sensor. 
     The typical reed switch shock sensor employs a magnet, a spring, and a reed switch mounted in a housing. The three components are arranged so that under an acceleration-induced load the magnet acting as an acceleration sensing mass compresses the spring and moves to a position where the magnetic field of the magnet causes the reeds of the reed switch to attract and thus close the reed switch. 
     The reed switch shock sensor is a highly reliable component. However, many electronic circuits today incorporate built-in test, and the reed switch is indistinguishable from an open circuit unless the circuit board is undergoing the proper acceleration. Thus, in some cases the shock sensor may incorporate some method of self-testing which can verify the presence of the reed switch and which may cause the reed switch to operate. Such self-testing functions typically require additional parts, including the addition of a self-test electrical coil to cause the reed switch to close. 
     What is needed is a shock sensor employing a reed switch that can be self-tested without the addition of a test coil. 
     SUMMARY OF THE INVENTION 
     The reed switch based shock sensor of this invention provides means for passing electrical current through the spring used to bias the shock sensing magnetic mass in the unactuated position. The spring extends between a first stop and a shock sensing magnetic mass that is biased against a second stop. So long as the magnetic mass is held against the second stop, the reed switch remains open. A path for electrical current is created which leads through the coil spring used to bias the sensing mass. The coil spring is wrapped around the reed switch, allowing the coil spring to act as an electrical coil. The electric coil generates a magnetic field of sufficient strength to cause the reed switch reeds to attract and so close the reed switch, thus allowing the reed switch to be tested, without the addition of an electrical test coil. 
     It is a feature of the present invention to provide a shock sensor that facilitates built-in test. 
     It is a further feature of the present invention to provide a shock sensor having a reed switch that can be electrically detected. 
     It is another feature of the present invention to provide a shock sensor that can be actuated electronically for a self-test, without the addition of a test coil. 
    
    
     Further 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 cross-sectional view of the shock sensor of this invention. 
     FIG. 2 is a cross-sectional view of an alternative embodiment of the shock sensor of FIG.  1 . 
     FIG. 3 in an enlarged detail view of the electrical connection between the housing and the spring of FIG.  1 . 
     FIG. 4 in an enlarged detail view of the electrical connection between the housing and the spring of FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring more particularly to FIGS. 1-4 wherein like numbers refer to similar parts, a shock sensor  20  is shown in FIG.  1 . The shock sensor  20  has a housing  22 . A reed switch  24  is mounted on the housing  22 , and a shock sensing magnet  26  is positioned for movement on the housing. The shock sensing magnet  26  is in the shape of a ring which is positioned coaxially about the reed switch  24 . A spring  32  biases a shock sensing magnet  26  against a first stop  28  formed by portions  30  of the housing  22 . The spring  32  extends between the magnet  26  and a second stop  34  spaced from the first stop  28  and spaced axially along the reed switch  24 . 
     When the shock sensor  20  undergoes acceleration due to a crash event, the magnet  26  compresses the spring  32  until the magnet moves to a second position adjacent the overlapping portions  36  of the reed switch reeds  38 . Properly positioned, the magnet will cause the reeds to take on opposite magnetic polarities and so attract to close the switch formed by the reed switch  24 . 
     It is generally not practical or desirable to test a reed switch shock sensor by subjecting it to shock levels simulative of a crash event. It is known in the prior art to place an electrical coil around the reed switch so that when the coil is energized the reed switch closes. It is also known to use an electric coil to cause the shock sensing magnet  26  to move so as to close the reed switch  24 . Such prior art solutions require the addition of an electrical coil, resulting in some increase in cost, size and part count. The shock sensor of this invention  20  is arranged to pass a current through the spring  32  which is used to bias the shock sensing magnet against the first stop  28 . A typical coil used to actuate a reed switch will employ a coil having thousands to tens of thousands of turns, and operation of the reed switch by energizing the coil will typically require a power of a small fraction of one Watt. 
     Through experimentation it has been shown that coil springs having, for example, between 26 and 33 turns, can support sufficient current to cause actuation of a reed switch in a shock sensor configuration. Table 1 provides test results for two coil springs: part number 251-90-226-00 which has  26  coils and a resistance of 7.3 ohms; and part number 251-90-084-00 which has 27 coils and a resistance of 10.8 ohms. Each coil was positioned about a series of reed switches (Hamlin type MLRR-4) with different ampere turn requirements, as shown in column one of Table 1, the reed switch having ampere turn requirements of 14, 15, 16 and 23 ampere turns. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 251-90- 
                 Theoretical 
                   
                 Theoretical 
               
               
                 Coil Spring 
                 226-00 
                 AT 
                 251-90-084-00 
                 AT 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Coil Turns 
                 26 
                   
                 27 
                   
               
               
                 Resistance (Ω) 
                 7.3 
                   
                 10.8 
               
               
                 Switch AT 
                 Volts 
                   
                 Volts 
               
               
                 14 
                 3.90 
                 13.89 
                 7.10 
                 17.75 
               
               
                 15 
                 4.25 
                 15.14 
                 7.57 
                 18.93 
               
               
                 16 
                 4.48 
                 15.96 
                 8.50 
                 21.25 
               
               
                 23 
                 8.20 
                 29.21 
                 14.10 
                 35.25 
               
               
                 24 
               
               
                 25 
               
               
                 29 
               
               
                   
               
             
          
         
       
     
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                 251-90- 
                 Theoretical 
                   
                 Theoretical 
               
               
                 Coil Spring 
                 018-00 
                 AT 
                 251-90-071-00 
                 AT 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Coil Turns 
                 29 
                   
                 33 
                   
               
               
                 Resistance (Ω) 
                 6.9 
                   
                 10.6 
               
               
                 Switch AT 
                 Volts 
                   
                 Volts 
               
               
                 14 
                 1.72 
                 7.23 
                 5.40 
                 16.81 
               
               
                 15 
                 2.94 
                 12.36 
                 5.90 
                 18.37 
               
               
                 16 
                 3.85 
                 16.18 
                 6.40 
                 19.92 
               
               
                 23 
                 6.20 
                 26.06 
                 10.80 
                 33.62 
               
               
                 24 
                 7.63 
                 32.07 
               
               
                 25 
                 7.92 
                 33.29 
               
               
                 29 
                 10.30 
                 43.29 
               
               
                   
               
             
          
         
       
     
     Voltage across the coil spring was increased until the switch closed and the voltage at which the switch closed was recorded. The number of ampere turns (Theoretical AT) required was calculated by taking the voltage value at switch pull in, dividing that value by the resistance of the spring to get a value for the current and finally multiplying the value of the current by the number of turns on the spring. 
     Similarly, in Table 2, coil spring part No. 251-90-018-00 having 29 coil turns and resistance of 6.9 ohms, and part number 251-90-071-00 having 33 coil turns and resistance of 10.6 ohms were tested with switches having ampere turn requirements between 14 and as high as 29. Again the number of ampere turns (Theoretical AT) required was calculated by taking the voltage value at switch pull in, dividing that value by the resistance of the spring to get a value for the current and finally multiplying the value of the current by the number of turns on the spring. 
     Voltage values for switches with higher ampere turn requirements are not entered in the tables where the high voltages caused warping of the springs. Generally, a burning smell was noticed around 5-6 volts when the voltage was left on for around 25 seconds. Therefore it is concluded that reed switches should be used which are sensitive enough to respond to the ampere turns which can be achieved with four volts. 
     Looking at the power dissipated, it is evident that 4 Volts corresponds to about two Watts of dissipated power. As evidenced by the Theoretical AT becoming substantially greater than the Switch AT at the higher voltages, the resistance of the coil is increasing due to the increased coil temperature. If greater ampere turn values are required in any shock sensor which utilizes the coil spring as a test coil, increasing the number of turns in the coil and/or decreasing the resistance of the coil will be necessary to avoid excessive power dissipation with the attendant undesirable heating of the coil spring. 
     Referring to FIGS. 1 and 3, it is illustrated how an electrical voltage source  40  can be connected across the spring  32 , which extends between the magnet  26  and a portion of the housing forming a second stop  34 . Referring particularly to FIG. 3, the magnet  26  is shown plated with a conductive material  42  such as copper or silver so that current can readily flow between a contact  44  attached to the portion of the housing  30  forming the first stop  28  and a first end  46  of the spring  32 . Similarly a contact  48  is formed on the second stop  34  completing the electrical circuit from the electrical voltage source  40  to the second end  49  of the spring  32 . Although movement of the magnet  26  breaks the electrical connection between the spring and the contact  44 , this occurs only during crash induced acceleration. 
     Referring to FIGS. 2 and 4, an alternative embodiment shock sensor  50  is shown in FIG.  2 . The shock sensor  50  employs a reed switch  52  mounted on a housing  54 . A magnet  56  is movable on the housing and is positioned coaxially about the reed switch  52 . This shock sensor  50  has the overall configuration of the shock sensor shown in U.S. Pat. No. 5,212,357 to Reneau which is incorporated herein by reference. The housing  54  has a first stop  58  and a second stop  60  spaced a fixed distance from the first stop  58 . 
     The activation magnet  56 , being slidably mounted on the housing  54 , has a first portion  62  engaged against the first stop  58  and a second portion  64  which engages against the second stop. The magnet first portion  62  has a greater magnetic flux than the second portion  64 . The reed switch  52  is responsive to the position of the activation magnet  56  such that the reed switch is activated when the magnet travels to a preselected activation position during movement of the magnet in response to acceleration applied to the sensor. A coil spring  66  biases the magnet  56  such that the first portion  62  engages against the first stop  58 , and the coil spring  66  extends between the magnet  56  and the second stop  60 . 
     FIG. 3 shows how a voltage source  68  is connected across the spring  66  by an electrically conducting portion  70  of the magnet, which abuts a contact  72  fixed to the portion of the housing  74  forming the first stop  58 . A first end  78  of the spring  66  is thus in electrical engagement with the magnet  56 . The electrical circuit is completed by a second contact  76  affixed to the second stop  60  which engages a second end  80  of the spring  66 . 
     It should be understood that the magnet could be conducting or other means for applying electrical current to the coil spring could be employed. 
     It should be understood that the coil spring through which the current passes must be positioned so as to result in a magnetic field that causes the reeds of the switch to attract, thus closing the reed switch. 
     It should be understood that the number of ampere turns required to activate a given reed switch is dependent on the detail configuration of the coil, and so the rated ampere turns is to some extent a relative measurement. 
     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.