Modes of Automatic Control

The mode of control can be defined as the way or 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.

Any particular mode of control used for a specific application depends on the characteristic of the process being controlled e.g. some processes can be operated over a wide band, while others must be maintained very close to the setpoint, in addition, some processes change relatively slowly while others change almost immediately.

We have four modes of control commonly used for most industrial control applications, namely:

1. Proportional
2. Proportional plus Reset/Integral (PI)
3. Proportional plus Rate/Derivative (PD)
4. Proportional plus Reset (Integral) plus Rate (Derivative) -(PID)

Each mode of control has characteristic advantages and limitations.

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

If we consider proportional (throttling) mode, there is a continuous linear relation between value of the controlled variable and the position of the final control element i.e. the amount of valve movement is proportional to amount of deviation.

The figure above shows the relationship between valve position and controlled variable (Temperature) characteristic of the proportional mode. From the diagram, you can clearly see the valve position changes in exact proportional to deviation. Also, the proportional mode responds only to the 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 value returns to its initial position.

There is no valve correction without deviation. 3 Common terms are used to describe the proportional mode:

1. Proportional band
2. Gain
3. Offset

Proportional band

Proportional band (throttling range) is the change in value of the controlled variable that causes full travel of the final control element. The figure above shows the relationship valve position and temperature band for two different proportional bands. The proportional band is usually expressed as a percent of full range e.g. if an instrument is 250 °F and it takes 50 °F change in temperature to cause a full valve travel, the percentage proportional band is 50 °F in 250 °F or 20 %. Proportional bands may range from less than 1 % to well over 200 %. However, proportional bands that are over 100 % cannot cause full valve travel even for full range change of the controlled variable.

Related: Process time lags in Control systems

Gain (also called Sensitivity)

Gain 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

You can also read: Controller

Offset

This is also termed as droop, which is the deviation that remains after a process has stabilized. Offset is an inherent characteristic of the proportional mode control. That is to say, the proportional mode of control will not necessarily return a controlled variable to its setpoint.