Two Position Controller Summary

Figure 14 Relation Between Valve Position and Controlled Variable Under Proportional Mode

PROPORTIONAL CONTROL SYSTEMS

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Process Controls TWO POSITION CONTROL SYSTEMS Three 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 variable that causes full travel of the final control element. Figure 14 shows the relationship between valve position and temperature band for two different proportional bands. The proportional band of a particular instrument is expressed as a percent of full range. For example, if full range of an instrument is 200^oF and it takes a 50^oF change in temperature to cause full valve travel, the percent proportional band is 50^oF in 200^oF, or 25%. Proportional bands 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 element to amount of change in the controlled variable. Mathematically, gain and sensitivity are reciprocal to proportional band. Offset, also called droop, is deviation that remains after a process has stabilized. Offset is an inherent characteristic of the proportional mode of control. In other words, the proportional mode of control will not necessarily return a controlled variable to its setpoint. Summary The important information in this chapter is summarized below: Two Position Controller Summary It 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 above or below the setpoint. Deviation is the difference between the setpoint of a process variable and its actual value. In the proportional (throttling) mode, the amount of valve movement is proportional to the amount of deviation. Gain compares the ratio of amount of change in the final control element to change in the controlled variable, and offset is the deviation that remains after a process has been stabilized. Rev. 0 Page 21 IC-07

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