Fail-safe sensor system

摘要

A fail-safe sensor system to provide a positive indication of the proper operation of a sensor system by accounting for the return signals from an active zone and a monitor zone beyond. Search signals, provided by a transducer operating in the transmit mode, are transmitted within an active zone within which objects to be detected are expected to be located. Unreturned search signals are permitted to travel beyond the active zone into a monitor zone beyond and objects in such monitor zone cause return signals which are also employed. A counter feed from a free running clock source is permitted to run up a count whose predetermined limit represents the estimated maximum time period during which a sensor system should operate without contacting an object. Return signals, whether from objects in the active zone or the monitor zone, reset the counter back to zero. The absence of a return signal permits the counter to exceed its predetermined limit and produce an output signal which can be directed to control circuitry, an alarm or logic circuitry to determine if the system is operative.