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

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7.8. Summary

In this chapter, we developed the Bode and Nyquist frequency response techniques for control-system design. Gain and phase margins indicate the amount of uncertainty a feedback system can tolerate before becoming unstable. The gain margin is related to the change in gain (process or controller) that can be tolerated, while the phase margin is related to the change in time delay that can be tolerated. From a practical perspective, we recommend a gain margin of at least 2.5 and phase margin of at least 60° to assure robustness.

The basic concepts covered were as follows:

Amplitude ratio (AR): Ratio of the output to input amplitudes

Phase angle (φ): Peak-to-peak shift between input and output curves

All-pass: A transfer function with ...

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