Evaluation of open-loop frequency indicators in a closed loop for PID controller autotuning

In industrial practice, PID controllers are extensively used because they can be successfully implemented without knowing a model of the object or process which is controlled. Despite of the fact that many PID controller tuning algorithms have been developed since the Ziegler and Nichols tuning procedures were published in 1942, their experimental method continues to be used up to now. There are two main reasons why newer methods are not widely-used: the methods are based on a model of the controlled plant, and/or it is necessary to stop the control process during controller retuning. Our method presented in this paper is based on an experimentally performed evaluation of excited frequency responses with the aim of achieving recommended values of one or more control quality indicators known from the course of the Nyquist plot, e.g. Frequency response evaluation can be performed in a closed loop with no controller function degradation, and no model is necessary for frequency response evaluation. In this way, the main problems of other tuning methods are eliminated. Since there is no need for a model, the tuning method can be used in a control loop involving nonlinearities even in the controller.