Process Controls PID CONTROL SYSTEMSPROPORTIONAL-INTEGRAL-DERIVATIVECONTROL SYSTEMSProportional plus reset plus rate controllers combine proportional control actionswith integral and derivative actions.EO 1.4 DESCRIBE the characteristics of the following types ofautomatic control systems:f. Proportional plus reset plus rate controlProportional-Integral-DerivativeFor processes that can operate with continuous cycling, the relatively inexpensive two positioncontroller is adequate. For processes that cannot tolerate continuous cycling, a proportionalcontroller is often employed. For processes that can tolerate neither continuous cycling nor offseterror, a proportional plus reset controller can be used. For processes that need improved stabilityand can tolerate an offset error, a proportional plus rate controller is employed.However, there are some processes that cannot tolerate offset error, yet need good stability. Thelogical solution is to use a control mode that combines the advantages of proportional, reset, andrate action. This chapter describes the mode identified as proportional plus reset plus rate,commonly called Proportional-Integral-Derivative (PID).ProportionalPlusResetPlusRateControllerActionsWhen an error is introduced to a PID controller, the controller’s response is a combination of theproportional, integral, and derivative actions, as shown in Figure 30.Assume the error is due to a slowly increasing measured variable. As the error increases, theproportional action of the PID controller produces an output that is proportional to the errorsignal. The reset action of the controller produces an output whose rate of change is determinedby the magnitude of the error. In this case, as the error continues to increase at a steady rate,the reset output continues to increase its rate of change. The rate action of the controllerproduces an output whose magnitude is determined by the rate of change. When combined, theseactions produce an output as shown in Figure 30.Rev. 0 Page 43 IC-07
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