Position Control of a Pneumatic Actuator using Digital Valves and Fuzzy PI Controller

Inexpensive digital valves are implemented to accurately control the position of a pneumatic cylinder using pulse width modulation (PWM) technique. A fuzzy PI controller is designed and implemented to overcome the nonlinearity inherited in the system due to air compressibility, dry friction and the nature of the digital valves. The actuator under control is a vertical double-acting single-rod pneumatic cylinder of 158 mm stroke and 20 mm bore diameter. Four one way digital valves are employed in such a way that two valves supply the pressurized air to the cylinder while the others are used to exhaust the outlet air. The fuzzy controller is designed in Labview environment and coded by the graphical language. The fuzzy rules along with the membership functions are carefully adjusted to produce a satisfactory fuzzy surface capable of controlling the system nonlinearities. Results of the open loop experiments showed a considerable delay at the starting of the piston motion associated with a non-linear behaviour which are dependent on the duty cycle (signal ON time over cycle time). Also the valve operating range is found to be between 12.5% and 60% cycle duty at 95 Hz. The gain selection of the fuzzy PI parameters (Ke, K∆e, K∆u) is investigated experimentally based on root mean square error minimization, and found to be Ke=1, K∆e=4, K∆u=5. The capability of the proposed controller in tracking is tested in response to a square wave reference input with different frequencies of 0.05 Hz, 0.1 Hz, 0.2 Hz and 0.3 Hz and constant duty cycle of 50%. The results shows a good transient behavior with no overshoot with max absolute steady state error of 0.8 mm and a rise time of 265 msec. The controller performance is also tested with sinusoidal, triangular and sawtooth reference wave input at a frequency of 0.1 Hz. A good response is also achieved with no overshoot and max absolute peak to peak error of 1.5