A displacement sensor for nonmetallic hydraulic cylinders

A sensor was developed to provide signals proportional to the position of the rod in a hydraulic cylinder. The sensor has a coil winding in the shell of the cylinder. The sensor operates by sensing the change in the coil impedance as the rod moves in the cylinder. This paper describes the construction of the cylinder and the sensor, and the performance evaluation of the sensor. Several different materials were placed in the rod and tested. For the cylinder tested, an aluminum core provided the highest precision, about 0.38 Ω. The full-scale output was 19.90 Ω over a stroke length of 193 mm. The linearity was about 1.64% of full-scale. Shielding from electromagnetic interference can improve the precision dramatically. A ferrite-cored rod assembly yielded a much greater full-scale output, 1300 Ω, but exhibited significant hysteresis.