Process Controls TWO POSITION CONTROL SYSTEMSThree terms commonly used to describe the proportional mode of control are proportional band,gain, and offset.Proportional band, (also called throttling range), is the change in value of the controlled variablethat causes full travel of the final control element. Figure 14 shows the relationship betweenvalve position and temperature band for two different proportional bands.The proportional band of a particular instrument is expressed as a percent of full range. Forexample, if full range of an instrument is 200oF and it takes a 50oF change in temperature tocause full valve travel, the percent proportional band is 50oF in 200oF, or 25%. Proportionalbands may range from less than 1% to well over 200%. However, proportional bands over 100%cannot cause full valve travel even for full range change of the controlled variable.Gain, also called sensitivity, compares the ratio of amount of change in the final control elementto amount of change in the controlled variable. Mathematically, gain and sensitivity arereciprocal to proportional band.Offset, also called droop, is deviation that remains after a process has stabilized. Offset is aninherent characteristic of the proportional mode of control. In other words, the proportional modeof control will not necessarily return a controlled variable to its setpoint.SummaryThe important information in this chapter is summarized below:Two Position Controller SummaryIt is a device that has two operating conditions: completely on or completely off.This device provides an output determined by whether the error signal is aboveor below the setpoint.Deviation is the difference between the setpoint of a process variable and itsactual value.In the proportional (throttling) mode, the amount of valve movement isproportional to the amount of deviation. Gain compares the ratio of amount ofchange in the final control element to change in the controlled variable, andoffset is the deviation that remains after a process has been stabilized.Rev. 0 Page 21 IC-07
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