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KING FAHD UNIVERSITY CHEMICAL ENGINEERING COURSE NOTES (Process Control)-Lec33

# KING FAHD UNIVERSITY CHEMICAL ENGINEERING COURSE NOTES (Process Control)-Lec33

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1 Chapter 12 Controllers With Two Degrees of Freedom The specification of controller settings for a standard PID controller typically requires a tradeoff between set-point tracking and disturbance rejection. These strategies are referred to as controllers with two- degrees-of-freedom. The first strategy is very simple. Set-point changes are introduced gradually rather than as abrupt step changes. For example, the set point can be ramped as shown in Fig. 12.10 or “filtered” by passing it through a first-order transfer function, * 1 (12-38) τ 1 sp sp f Y Y s = +

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2 Chapter 12 where denotes the filtered set point that is used in the control calculations. The filter time constant, determines how quickly the filtered set point will attain the new value, as shown in Fig. 12.10. * sp Y τ f Figure 12.10 Implementation of set-point changes.
3 Chapter 12 A second strategy for independently adjusting the set-point response is based on a simple modification of the PID control law in Chapter 8, ( 29 ( 29 ( 29 * * 0 1 (8-7) t m c D I dy p t p K e t e t dt dt τ = + + - @ where y m is the measured value of y and e is the error signal. . The control law modification consists of multiplying the set point in the proportional term by a set-point weighting factor, : sp m e y y - @ β ( 29 ( 29 ( 29 ( 29 * * 0 β 1 τ (12-39) τ c sp m t m c D I p t p K y t y t dy K e t dt dt = + - + - @ The set-point weighting factor is bounded, 0 < ß < 1, and serves as a convenient tuning factor.

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4 Chapter 12 Figure 12.11 Influence of set-point weighting on closed-loop responses for Example 12.6.
5 Chapter 12 On-Line Controller Tuning 1. Controller tuning inevitably involves a tradeoff between performance and robustness. 2. Controller settings do not have to be precisely determined. In general, a small change in a controller setting from its best value (for example, ±10%) has little effect on closed-loop responses. 3. For most plants, it is not feasible to manually tune each controller. Tuning is usually done by a control specialist (engineer or technician) or by a plant operator. Because each person is typically responsible for 300 to 1000 control loops, it is not feasible to tune every controller. 4. Diagnostic techniques for monitoring control system performance are available.

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6 Chapter 12 Continuous Cycling Method Over 60 years ago, Ziegler and Nichols (1942) published a classic paper that introduced the continuous cycling method for controller tuning. It is based on the following trial-and-error procedure: Step 1. After the process has reached steady state (at least approximately), eliminate the integral and derivative control action by setting to zero and to the largest possible value. Step 2.
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KING FAHD UNIVERSITY CHEMICAL ENGINEERING COURSE NOTES (Process Control)-Lec33

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