The mode of control is the manner in which a control system makes corrections relative to an error that exists between the desired value (setpoint) of a controlled variable and its actual value. The mode of control used for a specific application depends on the characteristics of the process being controlled. For example, some processes can be operated over a wide band, while others must be maintained very close to the setpoint. Also, some processes change relatively slowly, while others change almost immediately.

Deviation is the difference between the setpoint of a process variable and its actual value. This is a key term used when discussing various modes of control.

Four modes of control commonly used for most applications are:

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Each mode of control has characteristic advantages and limitations. The modes of control are discussed in this and the next several sections of this module.

In the proportional (throttling) mode, there is a continuous linear relation between value of the controlled variable and position of the final control element. In other words, amount of valve movement 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 to deviation. Also, the proportional mode responds only to amount of deviation and is insensitive to rate or duration of deviation. At the 2 minute and 4 minute marks, when the temperature returns to its setpoint value, the valve returns to its initial position. There is no valve correction without deviation.

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

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