A relay feedback based autotuning PID controller and its application to pulp consistency control

An autotuning PID method based on modified relay feedback for stable process is presented. The modified relay is made up of a standard relay followed by a PI controller. The proportional gain of the PI controller is set as a unity and the integral time constant of it is chosen so as to ensure a minimum loop phase margin of π / 6. The PID controller is designed in terms of the limit cycle output data generated by the modified relay. The derivative time constant is determined by maintaining the loop phase margin mentioned above. Minimizing the distance of Nyquist curve of the loop transfer function from the imaginary axis of the complex plane gives the proportional gain. The integral time constant of the PID controller is set equal to the integral time constant of the PI controller of the modified relay. The resulting PID controller is used to pulp consistency control of a papermaking machine and good effectiveness is obtained.