Modelling and microcomputer control of a nonlinear pneumatic servomechanism

A mathematical model is proposed for a low-pressure experimental pneumatic servomechanism which exhibits hysteresis characteristics at the torque motor and the servo spool valve assembly, nonlinear mass flow through the spool valve opening, and friction at piston and rod-end seals. Systems parameters are determined experimentally and correlated empirically. A single-board microcomputer is used for both servomechanism control and data acquisition purposes. The model predicts the experimental responses of three control algorithms with good accuracy. In particular, the response of PD with hysteresis compensation control shows no overshoot and minimal steady-state error when a step change of reference input is applied.