Cylinders having piston position measurement

A cavity such as a chamber (10) of a hydraulic cylinder (2) containing a movable piston (4) is equipped with transducers (12) for transmitting alternating pressure signals into a fluid in the chamber and for receiving signals from it. The fluid chamber acts as a resonant tank circuit for determining the frequency of an external electronic oscillator (24). The frequency of oscillation of the circuit indicates the depth of the cavity, i.e., the position of the piston in the cylinder Multiple modes of oscillation are employed (34, 36) in the same chamber to provide high accuracy and resolution of ambiguities. In one embodiment chambers (10 min , 66) on both sides of the piston (4 min ) of a double-acting cylinder (2 min ) are measured, and the readings are combined (76) to reduce errors that would otherwise result from changes in temperature, pressure, viscosity and other parameters unrelated to the position of the piston.