Automatic Controller Tuning using Relay-based Model Identification

Abstract: Proportional-integral-derivative (PID) controllers are very common in the process industry. In a regular factory there may be hundreds or thousands of them in use. Each of these controllers needs to be tuned, and even though the PID controller is simple, tuning the controllers still requires several hours of work and adequate knowledge in order to achieve a desired performance. Because of that, many of the operating PID controllers today are poorly tuned or even running in manual mode. Methods for tuning the controllers in an automated fashion are therefore highly beneficial, and the relay autotuner, that was introduced on the market in the 1980’s, has been listed as one of the great success stories of control. The technology development since the 1980’s, both concerning PID control and available computing power, gives opportunities for improvements of the autotuner. In this thesis three new autotuners are presented. They are all based on asymmetric relay feedback tests, providing process excitation at the frequency intervals relevant for PID control. One of the proposed autotuners is similar to the classic relay autotuner, but provides low-order models from which the controllers are tuned by simple formulas. The second autotuner uses the data from a very short relay test as input to an optimization method. This method provides more accurate model estimations, but to the cost of more omputing. The controller is then tuned by another optimization method, using the estimated model as input. The principle of the third autotuner is similar to the second one, but it is used to tune multivariable PID controllers for interacting processes. In this case a relay feedback experiment is performed on all loops simultaneously, and the data is used to identify the process transfer function matrix. All of the proposed autotuners strive to be user-friendly and practically applicable. Evaluation of the three autotuning strategies are done both through simulation examples and on experimental processes. The developed autotuners are also compared to commercially available ones, and the study shows that an upgrade of the industry standard to the newly available autotuners will yield a significant performance improvement.