Process Controls PROPORTIONAL PLUS RESET CONTROL SYSTEMSPROPORTIONAL PLUS RESET CONTROL SYSTEMSProportional plus reset control is a combination of the proportional andintegral control modes.EO 1.4 DESCRIBE the characteristics of the following types ofautomatic control systems:d. Proportional plus reset control systemProportionalPlusResetThis type control is actually aFigure 22 Response of ProportionalPlus Reset Controlcombination of two previouslydiscussed control modes,proportional and integral.Combining the two modes resultsin gaining the advantages andc o m p e n s a t i n g f o r t h edisadvantages of the twoindividual modes.The main advantage of theproportional control mode is thatan immediate proportional outputis produced as soon as an errorsignal exists at the controller asshown in Figure 22. Theproportional controller isconsidered a fast-acting device.This immediate output changeenables the proportional controllerto reposition the final controlelement within a relatively shortperiod of time in response to theerror.The main disadvantage of the proportional control mode is that a residual offset error existsbetween the measured variable and the setpoint for all but one set of system conditions.The main advantage of the integral control mode is that the controller output continues toreposition the final control element until the error is reduced to zero. This results in theelimination of the residual offset error allowed by the proportional mode.Rev. 0 Page 33 IC-07
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