KING FAHD UNIVERSITY CHEMICAL ENGINEERING COURSE NOTES (Process Control)-Lec20

KING FAHD UNIVERSITY CHEMICAL ENGINEERING COURSE NOTES (Process Control)-Lec20

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1 Chapter 8 Figure 8.1 Schematic diagram for a stirred-tank blending system. Feedback Controllers
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2 Chapter 8 Basic Control Modes Next we consider the three basic control modes starting with the simplest mode, proportional control . Proportional Control In feedback control, the objective is to reduce the error signal to zero where ( 29 ( 29 ( 29 (8-1) sp m e t y t y t = - and ( 29 ( 29 ( 29 error signal set point measured value of the controlled variable (or equivalent signal from the sensor/transmitter) sp m e t y t y t = = =
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3 Chapter 8 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, ( 29 ( 29 (8-2) c p t p K e t = + where: ( 29 controller output bias (steady-state) value controller gain (usually dimensionless) c p t p K = = =
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4 Chapter 8
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5 Chapter 8 The key concepts behind proportional control are the following: 1. The controller gain can be adjusted to make the controller output changes as sensitive as desired to deviations between set point and controlled variable; 2. the sign of K c can be chosed to make the controller output increase (or decrease) as the error signal increases. For proportional controllers, bias can be adjusted, a procedure referred to as manual reset . Some controllers have a proportional band setting instead of a controller gain. The proportional band PB (in %) is defined as p 100% (8-3) c PB K @
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6 Chapter 8 In order to derive the transfer function for an ideal proportional controller (without saturation limits), define a deviation variable
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This note was uploaded on 09/22/2009 for the course CHEMICAL CHE 401 taught by Professor Dr.muhammadal-arfaj during the Spring '09 term at King Fahd University of Petroleum & Minerals.

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KING FAHD UNIVERSITY CHEMICAL ENGINEERING COURSE NOTES (Process Control)-Lec20

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