Photoelectric measuring ruler including fault detection apparatus

A displacement measuring system comprising a pair of sensors arranged in fixed relationship to each other;

• a member which is relatively movable in relation to each sensor and which is arranged to be sensed by each sensor so that relative movement of the member in a given direction produces a number of cyclical variations of a signal output from each sensor, the number being proportional to the magnitude of the relative movement;
• the sensors being substantially 180.degree. out of phase with each other so that when the signal from each sensor is at its high value the signal from the other sensor is at its low value and vice versa;
• difference measuring means connected to the sensors for producing a difference signal in dependence on the signals of the sensors;
• counting means connected to the difference measuring means and responsive to the difference signal for counting its cycles to determine the magnitude of relative movement of said member;
• and sampling means for sampling the signals from the two sensors during the time that one is at its high value and the other is at its low value and vice versa and for generating a fault signal if the ratio of the magnitudes of the two signals at a sample lies outside a predetermined range.