Fiber optic displacement sensor with built-in reference

Abstract

An apparatus usable in process control applications for detecting the position of a movable element in a sensor wherein first and second beams of light, each having different wavelengths and each preferably being alternately generated with respect to one another by a light source, are transmitted to a filter by means of an optical fiber cable connected between the light source and filter. The filter and the movable element cooperate for modulating the intensity of the first beam in accordance with the position of the movable element and thereby transforming that first beam into a measurement beam which appears at an output stage of the filter. The filter also transmits the second beam therethrough unaffected by the movable element and produces a reference beam appearing at the output stage. A first optical detector coupled to the light source the respective intensities of the two light beams being transmitted towards the filter. A second optical detector connected to the output stage of the filter measures the respective intensities of the measurement and reference beams. The four intensity measurements made by the two optical detectors are combined in a prescribed manner for producing a position signal which is representative of the position of the movable element and which is substantially independent of the transmitting factors for the optical paths transmitting the first, second, measurement and reference beams as well as substantially independent of output variations in the light source or responsivity changes in the detectors.