PROPORTIONAL CONTROL SYSTEMSProcess ControlsPROPORTIONAL CONTROL SYSTEMSProportional control is also referred to as throttling control.EO 1.4 DESCRIBE the characteristics of the following types ofautomatic control systems:b. Proportional control systemControlModeIn the proportional control mode, the final control element is throttled to various positions thatare dependent on the process system conditions. For example, a proportional controller providesa linear stepless output that can position a valve at intermediate positions, as well as "full open"or "full shut." The controller operates within a band that is between the 0% output point and the100% output point and where the output of the controller is proportional to the input signal.ProportionalBandWith proportional control, the final control element has a definite position for each value of themeasured variable. In other words, the output has a linear relationship with the input.Proportional band is the change in input required to produce a full range of change in the outputdue to the proportional control action. Or simply, it is the percent change of the input signalrequired to change the output signal from 0% to 100%.The proportional band determines theFigure 15 Proportional System Controllerrange of output values from thecontroller that operate the final controlelement. The final control element actson the manipulated variable todetermine the value of the controlledvariable. The controlled variable ismaintained within a specified band ofcontrol points around a setpoint.To demonstrate, let’s look at Figure 15.In this example of a proportional levelcontrol system, the flow of supply waterinto the tank is controlled to maintainthe tank water level within prescribedlimits. The demand that disturbancesplaced on the process system are suchIC-07 Page 22 Rev. 0
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