Position Control of an Experimental Servo Pneumatic Actuator, Using Sliding Mode Control

In this paper, two position controllers have been developed for a double-acting pneumatic cylinder with two 3/2 on/off solenoid valves, using PWM method. The first one is a PID controller, whose gains have been obtained by Zighler-Nicoltz method. The second one - a Sliding Mode Controller (SMC) - has been designed based on a specific mathematical model, where an appropriate PWM method has been applied to overcome non-linearities such as time delay, during opening or closing of valves, and deadband due to stiction. The results of experimental tests on the closed loop system with the PID controller illustrate that the PWM method is suitable for servo control purposes with high accuracy. Furthermore, high accuracy was also achieved in experimental results in response to step inputs. Small values of maximum and Root Mean Square (RMS) positioning errors in response to sinusoidal inputs with various frequencies show better accuracy of SMC controller in comparison with the PID one. Good performance of the developed SMC is even more evident as frequency increases. These results also show considerable improvements in accuracy in comparison with the results of previous works on costly proportional valves.