Book - School of Science and Technology

638 Automatic controls and building management systemsSet-pointControlledconditionNeutral zoneControldevicepositionTimeFigure 22.21 Floating controldeviation from the set-point persists such that the rate of movement is a function of theamount of deviation from the set-point.Derivative controlThis mode involves a further development of integral action such that the controlleroutput is a function of the rate of change of the controlled variable. This form of control,like the integral mode, would not normally be used alone, but in combination with others.Proportional plus integralSometimes referred to as proportional with reset (or abbreviated to PI) this combinationgives stable control with zero off-set, as shown in Figure 22.22. So long as there isdeviation from the set-point, the controller will continue to signal a change until zeroerror exists. This approach would be applied, for example, to space temperature control incircumstances where the load fluctuated widely over relatively short periods of time andControlledconditionSet-pointTimeFigure 22.22 Proportional plus integral control