Abstract: An opto-electronic position sensing system comprising a series of radiation sources, a series of radiation detectors, a moveable member having a range of movement over which it controls irradiation of various ones of said detectors by various ones of said sources according to the position of said member, each detector being adapted to produce an analog signal which varies from a minimum to a maximum with movement of the said member over a corresponding parts of its range in such a manner that the output signal of at least two of said detectors varies with movement of said member whatever its position in the said range of movement, and first means which determine the position of the member by digital analysis of outputs from the detectors and second means which subsequently better determine the position of the member by analog analysis of output from those detectors identified by the first means as indicating the position of the member.

Abstract: A fail safe control circuit in which an input signal for initiating control of a potentially dangerous system is amplified only if it lies between given limits and also only if electrical power is being supplied to the system. The outputs of the amplifiers are monitored to check their correct operation, and two independent optical output signals obtained therefrom. The optical signals are arranged independently to operate switches in the system, such that both switches have to operate before the system can be actuated.