Control Modes in Automation Systems

The mode of control is the way in which a control system makes corrections relative to an error that exists between the setpoint of a controlled variable and its actual value.

We have a number of ways (i.e. control modes) by which a control unit can react to an error signal and supply an output for correcting elements:

Two-Step Mode

This is often referred to as the ON/OFF mode, in which the controller is essentially just a switch that is activated by the error signal and supplies an ON/OFF correcting signal.

Related: Two Position Controller

Proportional Mode (P)

This control mode produces a control action that is proportional to the error. The correcting signal thus becomes bigger, as the error increases. Therefore, as the error is reduced, the amount of correction is reduced and the correcting process slows down.

Related: Proportional Control System

Derivative Mode (D)         

The derivative mode produces a control action that is proportional to the rate at which the error is changing. When there is a sudden change in the error signal, the controller gives a large correcting signal; when there is a gradual change only, a small correcting signal is produced. Derivative control can be considered to be a form of anticipatory control in that, the existing rate of change of error is measured, a coming larger error is anticipated, and correction is applied before the larger error has arrived. The derivative control is typically not used alone but often in conjunction with proportional control and, often, integral control.

Also read: What is a Controller?

Integral Mode (I)

Integral mode produces a control action that is proportional to the integral of the error with time. Thus, a constant error signal will produce an increasing correcting signal. The correction continues to increase as long as the error persists. The integral controller can be considered to be ‘looking back’, summing all the errors and thus responding to changes that have occurred.

Also read: Analog Proportional Controller

Combination of Modes

The above control modes can be combined to form different arrangements to achieve a certain control action: Proportional plus derivative (PD) modes, proportional plus integral (PI) modes, proportional plus integral plus derivative (PID) modes.