Online Guides to Loop Control

June 7, 2012
Control's Monthly Resource Guide. This Month We Tackle Loop Control

Every month, Control's editors take a specific product area, collect all the latest, significant tools we can find, and present them here to make your job easier. If you know of any tools and resources we didn't include, send them to [email protected], and we'll add them to the website.

The manner in which a measured process variable responds over time to changes in the controller output signal is fundamental to the design and tuning of a PID controller. The best way to learn about the dynamic behavior of a process is to perform experiments, commonly referred to as "bump tests." Critical to success is that the process data generated by the bump test be descriptive of actual process behavior. Discussed are the qualities required for "good" dynamic data and methods for modeling the dynamic data for controller design. Parameters from the dynamic model are not only used in correlations to compute tuning values, but also provide insight into controller design parameters such as loop sample time and whether dead time presents a performance challenge. It is becoming increasingly common for dynamic studies to be performed with the controller in automatic (closed loop). For closed-loop studies, the dynamic data is generated by bumping the setpoint. The method for using closed-loop data is illustrated. Concepts in this work are illustrated using a level control simulation.
Control Station

"A Simple, Single-Setting Controller Yields PI Performance" presents a simple velocity control algorithm with output modification that has equivalent PI controller dynamic performance. The controller features a single control setting. The controller can be easily configured in most distributed control systems (DCSs) and programmable logic controllers (PLCs). This white paper describes the controller structure and behavior as well as a control discussion on how to calculate the gain setting to determine the control period. To test the controller on real processes, the algorithm was applied to a level and temperature control loops in a laboratory, pilot-plant setting.

This tutorial covers the basics of PID loop tuning, including definitions of basic terms such as error, setpoint, proportional, integral and derivative. It also discusses control-loop tuning, fine-tuning rules and beginning settings for common control loops.

This downloadable PDF from ControlSoft Inc. contains Version 4 of the company's popular "Pocket Loop-Tuning Guide." The 12-page booklet contains basic loop-tuning information, including definitions and
descriptions, a reference list of common controllers available on the market, cascade, closed-loop and open-loop tuning and more. It's free, but registration is required.
Control Soft

This book, Controller Tuning and Control Loop Performance, 2nd Edition, has sold over 25,000 copies since it was first published in 1990. It treats the subject in a non-mathematical way, but gives math for those interested in pursuing the subject further. Chapter titles include Getting Started, Tuning Rules and Procedures, Tuning Objectives and Expected Loop Performances, Lags and Gains, Examples of Actual Lags, Cascade Control, Derivative Action, Interactions and Nonlinearities, and Potpourri. Price is $25. It is available directly through Omega at
Omega Engineering

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