Method for Tuning Level Control Loops Based on Internal Model Control and Closed Loop Step Test Data

Authors: Arnaud Nougues

Abstract:

This paper describes a two-stage methodology derived from IMC (Internal Model Control) for tuning a PID (Proportional-Integral-Derivative) controller for levels or other integrating processes in an industrial environment. Focus is ease of use and implementation speed which are critical for an industrial application. Tuning can be done with minimum effort and without the need of time-consuming open-loop step tests on the plant. The first stage of the method applies to levels only: the vessel residence time is calculated from equipment dimensions and used to derive a set of preliminary PI (Proportional-Integral) settings with IMC. The second stage, re-tuning in closed-loop, applies to levels as well as other integrating processes: a tuning correction mechanism has been developed based on a series of closed-loop simulations with model errors. The tuning correction is done from a simple closed-loop step test and application of a generic correlation between observed overshoot and integral time correction. A spin-off of the method is that an estimate of the vessel residence time (levels) or open-loop process gain (other integrating process) is obtained from the closed-loop data.

Keywords: closed-loop model identification, IMC-PID tuning method, integrating process control, on-line PID tuning adaptation

References:

[1] Bernardo Soares Torres, Fábio Barros de Carvalho, Marcos de Oliveira Fonseca, Constantino Seixas Filho, Performance assessment of control loops, 2006 IFAC.
[2] D. E. Rivera, M. Morari, S. Skogestad (1986), Internal Model Control, 4. PID controller design, Ind. Eng. Chem. Res. 25(1), 252-265.
[3] Patwardhan R. S. and Ruel M., Best Practices for Monitoring your PID Loops – The Key to Optimizing Control Assets, Proceedings of the ISA EXPO 2008, Houston,TX, November 2008
[4] Brisk M. L., Process Control: Potential Benefits and Wasted Opportunities, 5th Asian Control Conference, vol. 1, 2004, pp.20-23
[5] Ruel, Loop Optimization: Before You Tune - Troubleshooting - How to Tune a Loop, Control Magazine, March/April/May 1999