Optical torque sensor incorporating sensor failure diagnostics

A torque sensor comprising an input member, an output member, emitter means, first receiver means for receiving a first signal and producing a first output signal, second receiver means for receiving a second signal and producing a second output signal, the output signals from the receiver means being dependent upon the first and second signals which they receive, and signal processing means for receiving said output signals and processing said output signals from the first and second receiver means so as to produce a modified output signal indicative of the relative angular displacement between, or torque applied between, the input and output members, wherein diagnostic means is provided for continuously monitoring a sum signal .SIGMA. comprising the sum of the output signals from the first and second receiver means, the diagnostic means comprising a low pass filtering means through which said sum signal .SIGMA. is passed to produce a steady signal and said diagnostic means further comprises a comparator means which compares said steady signal to a reference signal in order to detect variation in the sum signal due to the failure of the sensor.