Reducing wear of sticky pneumatic control valves using compensation pulses with variable amplitude

The presence of friction in control valves produces limit cycles that reduces the performance of control loops. Once friction is detected, its compensation avoids process stops until the next maintenance stop is performed. Model free methods adding feed-forward pulses to PID controller output have been proposed to reduce the effect of friction. A common drawback in such methods is the increase in the movement of the valve stem. A new method is here proposed to overcome this issue. The amplitude of the pulses vary and becomes zero when a specified limit for the error on the process variable is achieved. Another advantage of this method is its ability to cope with uncertainty in friction, since the amplitude of the pulses pulses vary between limits. The method is illustrated via simulation, and its implementation in a real industrial controller is shown and discussed.