Open-loop control systems

In an open-loop control system, an input known as reference input is applied, most often through an amplifier and an output called the Controlled output is obtained.
In this system, the input to the controller is in no way affected by the value of the controlled output.
The input is applied or fixed by human judgment and estimate.

The block diagrammatic representation of an open-loop control system is shown in Figure.
This type of control system is simple and inexpensive, but it is inaccurate.
A typical example of an open-loop control system is an electric heating device without a thermostat.

The heating device will be switched ‘ON’ by a timing device while the duration of heating time will be estimated by a human operator.

After a set time, the heating device will get switched OFF.
In this system, it is quite likely that the room gets overheated or remains underheated.

Further, the heating time, once set, remains independent of the outside temperature and other conditions.
The disadvantage of the open-loop control system is that the system does not adapt itself to the environmental conditions and other related functions.
For example, in addition to the external temperature, if the doors and windows are opened and closed frequently, the heating time allowed may not be sufficient to bring the room temperature to the desired level.

Improvement in this tool system can be made if the human operator or an appropriate control system can be made if the human operator or an appropriate control element continuously measures the room temperature.
In such a system, the output, i.e., the room temperature, is measured continuously and the input, i.e., the time of heating, is varied accordingly.
This is achieved through the use of feedback systems. Such control systems are therefore known as feedback control systems or closed-loop control systems.

The open-loop control systems are not intelligent enough to compare the input signal (reference signal) with the output continually for providing the desired output.
Any system which operates only on the basis of time falls under the category of an open-loop control system.

Closed-Loop System

In closed-loop control systems, the output signal has a direct effect on the control action.

A corrective signal which drives the controller is derived from some kind of comparison between the input variable and the control variable.
The actuating signal also called the ‘error signal is the difference between the input reference signal and the feedback signal.
The feedback signal is a function of the output signal. It is fed to the controller so as to reduce the error, i.e., to bring the output of the system to the desired value.

A closed-loop feedback control system is shown in a block diagram form in Figure. The system uses feedback action to reduce the system error.
The feedback used is called ‘negative feedback’ as the feedback action aims at reducing the system error.

Examples of closed-loop control systems:

1. Room heating system with feedback control.

2. Autopiloting system for aircraft cruise control.

3. Machine tool positioning system etc.,

Room heating system with feedback control:

In a feedback room heating system, there is a thermostat that adjusts the burner in accordance with the room temperature.
The fuel oil input to the furnace depends on a comparison of the actual room temperature with the desired temperature.

This comparison produces an error signal.
The controller increases the fuel oil valve aperture if the error signal indicates that the room temperature is lower than the desired value. Thus maintaining the desired level of temperature in the room.