6. Function blocks6.1 Manufacturer function blocks6.1.4.4 TC - Three step controllerGeneralTC three-step controllers feature three states for the manipulatedvariable. These states are implemented with two function block out-puts Q1, Q2, of which either none or only one can be closed. I1 is thesetpoint and I2 is the actual value. X = I1 - I2 yields control deviationX, which is applied on the actual controller. The controller will thendetermine the manipulated variable for function block outputs Q1, Q2.Figure 157: Three-step controller schematic diagramI1: SetpointI2: Actual valueOperating principleThe following timing diagram shows how the three-step controller behaves.Figure 158: Timing diagram for three-step controllerXH/ -XH: Distance X from switching pointH1: Hysteresis 1 for XHH2: Hysteresis 2 for -XHY(t): Operating points for Q1/ Q2Q1: Switch output X = positiveQ2: Switch output X = negativeeasyE4 11/18 MN050009 EN www.eaton.com 297
