A frequency domain procedure is proposed for the automatic tuning of proportional integral derivative (PID) controllers. The nonparametric open-loop frequency response function is estimated online (in closed loop), and simple discriminatory parameters are obtained, which enable appropriate tuning of the controller. For processes with some prior knowledge of the dynamics, the method removes the need for relay tuning or open-loop step response testing. If there is little or no prior knowledge of the process dynamics, relay or step response testing can be used initially, with the method then being used for the subsequent tuning. As part of the paper, a comprehensive comparison of existing rule-based tuning formulas is given using realistic plant examples.
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