PROPORTIONAL PLUS RESET CONTROL SYSTEMSProcess ControlsThe main disadvantage of the integral mode is that the controller output does not immediatelydirect the final control element to a new position in response to an error signal. The controlleroutput changes at a defined rate of change, and time is needed for the final control element tobe repositioned.The combination of the two control modes is called the proportional plus reset (PI) control mode.It combines the immediate output characteristics of a proportional control mode with the zeroresidual offset characteristics of the integral mode.ExampleofProportionalPlusResetControlLet’s once more refer to our heat exchanger example (see Figure 23). This time we will applya proportional plus reset controller to the process system.Figure 23 Heat Exchanger Process SystemThe response curves shown in Figure 24 illustrate only the demand and the measured variablewhich represents the hot water outlet temperature.Assume the process undergoes a demand disturbance which reduces the flow of the hot water outof the heat exchanger. The temperature and flow rate of the steam into the heat exchangerremain the same. As a result, the temperature of the hot water out will begin to rise.The proportional action of the proportional plus reset controller, if acting alone, would respondto the disturbance and reposition the control valve to a position that would return the hot waterout to a new control point, as illustrated by the response curves. You’ll note that a residual errorwould still exist.IC-07 Page 34 Rev. 0
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