Book - School of Science and Technology

Controller modes of operation 637SwitchingvaluesSet-pointControlledconditionControllersignalTimeFigure 22.19 Step controlSet-pointControlledconditionTimeOff-setFigure 22.20 Proportional controlFloating controlWith floating control, there is normally a neutral zone around the set-point, within whichno control action occurs: the control device remains in the last controlled position. Whenthe variable moves outside the neutral zone, a signal causes the actuator to move at aconstant rate in a direction corresponding to whether the variable is above or below theset-point, see Figure 22.21. An example would be a control device motor running at aconstant but slow speed; the full stroke being at least two minutes. A multi-step controllermight also be used to initiate a multi-speed actuator action.Other forms of modulating control avoid off-set and may be preferred to proportionalor floating control in some applications.Integral controlSeldom used alone, this is an important addition to other forms of control, particularlyto the proportional mode. With integral action there is continuous movement whilst