Optical fiber rotary displacement sensor.

Shafts are needed in the operation and control of a broad range of manufacturing systems. Angular position of a shaft is one of the most common parameters measured in industrial and manufacturing systems. In this work, we report a class of a simple, multimode fiber intensity sensors developed to serve as a viable alternative to conventional rotary sensors. The basic structure of the twist sensor is a multimode fiber with a large core and/or a high numerical aperture (N.A.) sandwiched between two fibers with a small core and/or a low N.A. The sensor works by exciting selected modes in the multimode section of fiber. As the multimode section is twisted, mode coupling occurs, resulting in a redistribution of power in the multimode fiber. Power remaining in the selected modes is received by a pickup fiber. Fibers are joined with the aid of small capillary tubes. The usable range is about 135 degrees to 160 degrees depending on the length of twisted section. No evidence of hysteresis has been found. Based on the noise level of the detector used in the experiment and the slope of the measured sensor response, a sensitivity of 0.029' is estimated for the prototype twist sensors.