Observation and Compensation of Friction in a Pneumatic Servo Welding System

Pneumatic servo systems are affected by nonlinear factors, especially friction between the inner wall of the cylinder and the piston. The limit-cycle oscillation caused by friction can easily result in dead-zone nonlinearity in a pneumatic servo system, reducing the system performance and repeatability. In this study, the impact of friction, which has a negative influence on a pneumatic servo welding system, is effectively reduced by observing and compensating the friction. The system is treated as a pneumatic position/force servo system before and after contact between the welding gun electrodes. A reduced-order observer and a Kalman filter are applied to observe and estimate dynamic and static friction. Then friction is compensated by using feed-forward control. The experimental results show that a steady-state error of within 1.3% is achieved for a step signal.