TWO POSITION CONTROL SYSTEMSProcess ControlsDeviation is the difference between the setpoint of a process variable and its actual value. Thisis a key term used when discussing various modes of control.Four modes of control commonly used for most applications are:proportionalproportional plus reset (PI)proportional plus rate (PD)proportional plus reset plus rate (PID)Each mode of control has characteristic advantages and limitations. The modes of control arediscussed in this and the next several sections of this module.In the proportional (throttling) mode, there is a continuous linear relation between value of thecontrolled variable and position of the final control element. In other words, amount of valvemovement is proportional to amount of deviation.Figure 14 shows the relationship between valve position and controlled variable (temperature)characteristics of proportional mode. Notice that valve position changes in exact proportion todeviation. Also, the proportional mode responds only to amount of deviation and is insensitiveto rate or duration of deviation. At the 2 minute and 4 minute marks, when the temperaturereturns to its setpoint value, the valve returns to its initial position. There is no valve correctionwithout deviation.Figure 14 Relation Between Valve Position and ControlledVariable Under Proportional ModeIC-07 Page 20 Rev. 0
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