Shock sensors have been used in motor vehicles to detect a vehicle collision and to activate an electronic circuit for the actuation of safety devices, such as to tension safety belts, to inflate air bags, to shut off fuel systems, to activate radio signals, or to actuate other desired systems. Such shock sensors typically employ a reed switch having ferromagnetic contacts and operate such that the mass of a magnet is accelerated by vehicle impact against a restoring force such as a wire spring or repelling magnet. A change in magnet position causes a change in the magnetic field relative to reed switch sensitivity. The change in magnetic field therefore acts as a means for operating the reed switch contacts.
The shock sensors that employ reed switches typically have a housing that encloses a magnet that slidably travels between a first abutment and a second abutment. See, e.g., U.S. Pat. Nos. 4,484,041 and 4,639,563 issued to Andres et al., and Gunther, respectively. The magnetic reed switch is positioned within the shock sensor so that the reed switch is activated by the change in magnetic field exerted upon the reed switch by the magnet as the magnet travels from the first abutment to the second abutment and back again. The magnet is biased away from the second abutment by a spring or other biasing means.
In an exemplary use, the shock sensor may be mounted in a vehicle with the second abutment oriented forwardly so as to detect a vehicular collision from the front of the vehicle. In such a case, the vehicle and the housing of the sensor would be subject to rapid deceleration resulting from the collision, and the mass of the magnet of the sensor would overcome the biasing means to accelerate relative to the housing toward the second abutment.
Because it is critical to actuate safety devices under emergency conditions resulting from a collision, it is necessary to test the movement of the magnet and electrical operation of the reed switch. The prior art shows a test of the reed switch accomplished by winding an electromagnetic coil around the reed switch, and thereby applying a magnetic field that activates the reed switch in order to test the reed switch. See Zuvela, U.S. Pat. No. 3,737,599. Such a testing device does not, however, test the functional status of the required movement of the magnet against the biasing means. This is accomplished in the prior art by a random sampling of the shock sensors in actual crash conditions, a method inadequate to check the entire lot of shock sensors in any repeatable fashion.