# Types of control system – Open and Close Loop System

CONTROL SYSTEM:
A control system can be defined as a block box which can be used to control its output to some particular value.

Example: a domestic central heating control system. We can set the required temperature on the thermostat or controller and the pump can be adjusted to supply water through radiators. So the required temperature can be maintained in the house.

In a system when the output quantity is controlled by varying the input quantity then the system is called as Control system. The output quantity is called as controlled variable or response and the input quantity is called as command signal or excitation.

In Control system, we have two types

1. Open loop control system.

2. Closed loop control system.

OPEN AND CLOSED-LOOP SYSTEMS:
There are two basic forms of control system one being called and Open loop and other closed-loop systems. The difference between these can be illustrated by a simple example.

Consider an electric fire which has a selection switch which allows a 1 KW or a 2 kW heating element to be selected. If a person used the heating element to heat a room, he or she might just switch on the 1 kW element if the room is not required to be at too high a temperature. The room will heat up and reach a temperature which is only determined by the fact the 1 kW element was switched on, and not the 2 kW elements. If there are changes in the conditions perhaps someone opening a window, there is no way the heat output is adjusted to compensate.
This is an example of open loop control in that there is no information fed back to the element to adjust it and maintain a constant temperature.

The heating system with the heating element could be made a closed loop system if the person has a thermometer and switches the 1 kW and 2 kW elements on or off, according to the difference between the actual temperature and the required temperature, to maintain the temperature of the room constant.
In this situation there is feedback, the input to the system being adjusted according to whether its output is the required temperature. This means that the input to the switch depends on the deviation of the actual temperature from the required temperature.

In an open-loop control system the output from the system has no effect on the input signal. In a closed-loop control system the output does have an effect on the input signal, modifying it to maintain an output signal at the required value.

OPEN-LOOP SYSTEMS have the advantage of being

1. Relatively simple and
2. Consequently low cost with generally good reliability.

• Inaccurate since there is no correction for error.

CLOSED-LOOP SYSTEMS have the advantage of being

• Relatively accurate in matching the actual to the required values.