KING FAHD UNIVERSITY CHEMICAL ENGINEERING COURSE NOTES (Process Control)-Lec21

KING FAHD UNIVERSITY CHEMICAL ENGINEERING COURSE NOTES (Process Control)-Lec21

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1 Chapter 8 Features of PID Controllers Elimination of Derivative and Proportional Kick One disadvantage of the previous PID controllers is that a sudden change in set point (and hence the error, e ) will cause the derivative term momentarily to become very large and thus provide a derivative kick to the final control element.
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2 Chapter 8 This sudden change is undesirable and can be avoided by basing the derivative action on the measurement, y m , rather than on the error signal, e . We illustrate the elimination of derivative kick by considering the parallel form of PID control in Eq. 8-13. • Replacing de / dt by – dy m / dt gives ( 29 ( 29 ( 29 ( 29 0 1 * (8-17) τ t m c D I dy t p t p K e t e t dt dt = + + - Reverse or Direct Action The controller gain can be made either negative or positive.
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3 Chapter 8 • For proportional control, when K c > 0, the controller output p ( t ) increases as its input signal y m ( t ) decreases, as can be seen by combining Eqs. 8-2 and 8-1: ( 29 ( 29 ( 29 (8-22) c sp m p t p K y t y t - = - This controller is an example of a reverse-acting controller. • When K c < 0, the controller is said to be direct acting because the controller output increases as the input increases. Equations 8-2 through 8-16 describe how controllers perform during the automatic mode of operation. However, in certain situations the plant operator may decide to override the automatic mode and adjust the controller output manually.
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4 Chapter 8 Figure 8.11 Reverse and direct-acting proportional controllers. (a) reverse acting ( K c > 0. (b) direct acting ( K c < 0)
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Example: Example: Flow Control Loop Assume FT is direct-acting. 1. Air-to-open (fail close) valve ==> ? 2. Air-to-close (fail open) valve ==>
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KING FAHD UNIVERSITY CHEMICAL ENGINEERING COURSE NOTES (Process Control)-Lec21

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