Non-linear Control of a Piezoelectric Two Stage Servovalve

This paper describes an algorithm to control a two stage hydraulic servovalve designed for aerospace applications. The valve has a piezoelectric ring bender actuating a first stage spool with a significant amount of overlap to reduce internal leakage. The piezoelectric ring bender is a less complex and lighter alternative to a conventional torque motor. The second stage has electrical instead of the conventional mechanical feedback. The control algorithm includes compensation for the first stage spool overlap, piezoelectric hysteresis compensation and a feed forward term. The hysteresis compensation is based on a relatively simple Bouc-Wen hysteresis model that is able to significantly reduce the amount of first stage hysteresis. The overlap compensation, increasing the gain in the overlap region, reduces the impact of amplitude change and increases performance. It can also reduce any asymmetry in the system. The controller has a superior performance compared to a PI controller, as demonstrated experimentally using step and frequency responses.