TO MODEL BASED RULES
Michael G. Ott and Willy K. Wojsznis
Fisher-Rosemount Systems, Inc.
1712 Centre Creek Drive
Austin, Texas 78754
Process Control, Control Loop Tuning, Ziegler-Nichols Tuning, Model Based Tuning, PID
Automatic control loop tuning using the relay oscillation tuning method has been used to identify the process parameters of ultimate gain and ultimate period. Ziegler-Nichols or modified Ziegler-Nichols tuning rules may be used to determine PID loop settings. The same relay oscillation technique has been enhanced to identify a first-order plus deadtime model for self-regulating processes and an integrating process model for integrating processes. Model based tuning rules, such as Internal Model Control or Lambda tuning rules, may be used to determine tuning parameters for various feedback controllers.
Determining the tuning parameters of a PID controller based on ultimate gain and ultimate period is known as Ziegler-Nichols frequency response tuning , . The technique, developed more than 50 years ago, has been used extensively to tune loops in the process industries. The original Ziegler-Nichols tuning rules were designed to provide a quarter amplitude damped response to a load disturbance. Once considered ideal, the underdamped and oscillatory nature of Ziegler-Nichols tuning has been criticized for destabilizing control loops, i.e. increasing variability instead of reducing it .
An improvement in performance is achieved by modifying the tuning rules in such a way to get a desired phase or amplitude margin in the loop . Modified Ziegler-Nichols tuning rules are more conservative than the original rules, reducing the overshoot and oscillation following a setpoint change or load disturbance. Once the ultimate gain and ultimate period of a loop are known, published tuning rules can be used to get initial controller settings.
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