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Process Control: Modeling, Design, and Simulation by B. Wayne Bequette

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Module 6. First-Order + Time-Delay Processes

The most commonly used model to describe the dynamics of chemical processes is first order + time delay. In this module, we relate the stability of feedback control of these processes to frequency-response analysis. The important principles that we wish to illustrate are as follows:

  • Gain margin is a measure of gain uncertainty that can be tolerated

  • Phase margin is a measure of the time-delay uncertainty that can be tolerated

  • The “cross-over frequency” is related to the ultimate period from the closed-loop Ziegler-Nichols approach

  • Ziegler-Nichols recommended tuning values do not provide adequate gain and phase margins

  • IMC-based PI is easier to tune for robustness than are the other approaches

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