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06_Feedback_Chap8S10_4

06_Feedback_Chap8S10_4 - Feedback Controllers Week 1 P D I...

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1 Feedback Controllers: Week 1 Chapter 8 1 Figure 8.1 Schematic diagram for a stirred-tank blending system. P D I 2 “Normal”, K c < 0 “Reverse”, K c > 0 p change wrt y m not wrt e = error Basic Control Modes Next we consider the three basic control modes starting with the simplest mode, proportional control . Proportional Control Chapter 8 In feedback control, the objective is to reduce the error signal to zero where ( ) ( ) ( ) (8-1) sp m e t y t y t = and ( ) error signal e t = 3 ( ) ( ) set point measured value of the controlled variable (or equivalent signal from the sensor/transmitter) sp m y t y t = = ( y sp and y m must have same units) Although Eq. 8-1 indicates that the set point can be time-varying, in many process control problems it is kept constant for long periods of time. For proportional control, the controller output is proportional to the error signal, Chapter 8 ( ) ( ) (8-2) c p t p K e t = + where: ( ) controller output bias (steady-state) value p t p = = 4 bias (steady state) value controller gain (usually dimensionless) c K =
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