Fail-safe device for electronic control circuitry.

摘要

A fail-safe device (10) is disclosed for removing power from circuitry (12) to be controlled upon the occurance of an undesired operating condition. The device includes circuitry (16) for generating a fail-safe signal that is normally within a predetermined frequency range unless there is an undesired operating condition. It also includes power control circuitry (18) for supplying power to the circuitry (12) to be controlled only when the fail-safe signal is within the predetermined frequency range. In a preferred embodiment, the circuitry for generating the fail-safe signal includes a programmable computer (14) which regulates the circuitry to be controlled and which, unless there is an undesired operating condition, such as a failure of the computer, also generates a sequence of bits in which a predetermined pattern occurs at a known frequency. A comparator circuit (68) compares this sequence of bits with the predetermined pattern and generates a variation in the fail-safe signal each time there is a match. In the preferred embodiment, the power control means includes an electromechanical relay (22) for supplying power to the circuitry (12) to be controlled when that relay is turned on and relay actuating circuitry (32) for keeping the relay turned on when fail-safe signal is within the predetermined frequency range. The power control circuitry (18) may also include latchout circuitry (40, 30-34) for keeping the relay. (22) off once it has been turned off and reset circuitry (44) for enabling the relay to be selectively turned on once it has been turned off.