Module 1.5 - Control Actions

The Controller

The controller plays an essential role in the control system. Of the four basic functions of a control system, (measurement, comparison, computation, and correction) comparison and computation are solely achieved by the controller. The correction is materialized by the final control element, but this is done according to the controller's calculation.

The control mechanism in the controller may be considered as consisting of two sections

The purpose of the first is to compare the measured and the desired values of the controlled variable and then compute the difference between them as the error. If there is no error, i.e. the controlled variable is at the setpoint, then no action is taken.

If an error is detected, the second section of the controller operates to alter the setting of the final control element in such a way as to minimize the error in the least possible time with the minimum disturbance to the system. To achieve this objective, different actions could be taken by the controller and hence different signals are sent to the final control element.

The remainder of this section is concerned with the different control actions that the feedback controller could take.

On-off or two-position action

This is the simplest and most commonly experienced type of control. A typical example is the thermostatically controlled domestic immersion heater. Depending on the temperature of the water in the tank, the power supply to the heater is either on or off. In The Virtual Control Laboratory and Case Study Section there is an experiment which shows on-off control of the temperature in a hot water tank.

Proportional Control Action

Proportional action is the simplest and most commonly encountered of all continuous control modes. In this type of action, the controller produces an output signal which is proportional to the error. Hence, the greater the magnitude of the error, the larger is the corrective action applied.