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Process Dynamics and PID Controller Tuning

To tune a feedback control system means to adjust parameters in the controller to achieve robust control over the process. “Robust” in this context is usually defined as stability of the process variable despite changes in load, fast response to changes in setpoint, minimal oscillation following either type of change, and minimal offset (error between setpoint and process variable) over time.  “Robust control” is far easier to define than it is to achieve. With PID (Proportional-Integral- Derivative) control being the most common feedback control algorithm used in industry, it is important for the instrument technician (and engineer!) to understand how to tune these controllers effectively and with a minimum investment of time.

Different types of processes, having different dynamic (time-dependent) behaviors, require different levels of proportional, integral, and derivative control action to achieve stability and robust response. It is therefore imperative for anyone seeking to tune a PID controller to understand the dynamic nature of the process being controlled. For this reason, the chapter begins with an exploration of common process characteristics before introducing techniques useful in choosing practical P, I, and D tuning parameter values.

 

1 Process characterization

  1.1 Self-regulating processes

  1.2 Integrating processes

  1.3 Runaway processes

  1.4 Steady-state process gain

  1.5 Lag time

  1.6 Multiple lags (orders)

  1.7 Dead time

  1.8 Hysteresis

 

2 Before you tune . . .

  2.1 Identifying operational needs

  2.2 Identifying process and system hazards

  2.3 Identifying the problem(s)

  2.4 Final precautions

 

3 Quantitative PID tuning procedures

  3.1 Ziegler-Nichols closed-loop (“Ultimate Gain”)

  3.2 Ziegler-Nichols open-loop

  

4 Heuristic PID tuning procedures

  4.1 Features of P, I, and D actions

  4.2 Tuning recommendations based on process dynamics

 

5 Tuning techniques compared

  5.1 Tuning a “generic” process

  5.2 Tuning a liquid level process

  5.3 Tuning a temperature process

 

6 Note to students

 

References

Lipt´ak, B´ela G., Instrument Engineers’ Handbook – Process Control Volume II, Third Edition, CRC Press, Boca Raton, FL, 1999.

Mollenkamp, Robert A., Introduction to Automatic Process Control, Instrument Society of America, Research Triangle Park, NC, 1984.

Palm, William J., Control Systems Engineering, John Wiley & Sons, Inc., New York, NY, 1986.

Shinskey, Francis G., Energy Conservation through Control, Academic Press, New York, NY, 1978.

Shinskey, Francis G., Process-Control Systems – Application / Design / Adjustment, Second Edition, McGraw-Hill Book Company, New York, NY, 1979.

St. Clair, David W., Controller Tuning and Control Loop Performance, a primer, Straight-Line Control Company, Newark, DE, 1989.

Ziegler, J. G., and Nichols, N. B., Optimum Settings for Automatic Controllers, Transactions of the American Society of Mechanical Engineers (ASME), Volume 64, pages 759-768, Rochester, NY, November 1942.

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