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 200oF and it takes a 50oF change in temperature to
cause full valve travel, the percent proportional band is 50oF in 200oF, 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.
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
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.