Effect of Derivative Algorithm and Tuning Selection on the PID Control of Dead-Time Processes

Abstract

Many control engineers try to avoid the use of derivative action in operating units. As Isaksson and Graebe have pointed out (in a presentation at Control Systems 2000, Victoria, British Columbia, May 2000), there is an “industrial myth that derivative action does not work.” They claim that some of this is due to the need to adjust the derivative filter constant, in addition to the usual three parameters of PID controllers. Modern distributed control systems offer a variety of choices for the PID algorithm, including both series (interacting) and parallel (ideal). The many PID tuning methods that have been developed over the years have used different algorithms, and some are limited to a narrow range of parameter values (dead times). Most tuning methods require setting three tuning parameters, but some propose setting four parameters. All this complexity presents a confusing picture to the practitioner who is faced with trying to tune a PID controller. This paper presents a quantitative comparison of alternative derivative algorithms and tuning methods for processes with a wide range of dead-time/lag ratios. Results show that, when the correct algorithm is used, the IMC tuning method proposed by Morari and Zafiriou (Robust Process Control; Prentice Hall: New York, 1989) gives good performance for large-dead-time processes. The correct algorithm has a four-parameter, parallel, output-filtered PID structure.