Open Loop and Closed Loop System-Definition, Working, Advantages, Disadvantages, Examples [PDF]


Studying about Open Loop and Closed Loop System can help understand different systems such as a heater, electric washing machine, signal analysis, etc. Let us discuss each type in detail.

Open Loop System Definition

An open-loop control system is the one in which the control action is independent of the output. An open-loop control system can be divided into the following two parts:

  • Controller  
  • Controlled process

A reference input signal is applied to the controller, whose output access the actuating signal. The actuating signal then controls the control process so that the control variable will perform as per the prescribed standards.

open loop control system
Open loop control system

Consider an electric room heater operated by a switch. If a person turns on the switch, the room will heat up and reach the temperature which is only determined by the wattage rating electric heater. The heat output cannot be adjusted. It remains constant if there are changes in weather condition because no information is fed back to the heating element.

Open Loop System Example

The conventional electric washing machine is an example of an open-loop control system because the wash time is set by the estimation of the human operator and not based on whether the clothes are cleaned properly. No information is fed back regarding the cleanliness of the clothes.

Open Loop System Advantages

  • The input command is the sole factor responsible for providing input to the control system.
  • It has the ability to perform accurately which is determined by its calibration.
  • It is easier to build.
  • It is not troubled with the problem of instability.

Open Loop System Disadvantages

  • Presence of non-linearities causing malfunctioning.


The closed-loop system is defined as a system in which Feedback from the output to the input is missing in the open-loop control system. To obtain More accurate Control, the controlled variable should be fed back and compared with the reference input.

closed loop control system
Closed loop control system

An Actuating signal proportional to the difference of the input and output is used to correct the error. A System with one or more feedback path is called a closed loop system.
It consists of the following five elements.:

1. Comparison element

Comparison element gives the difference between the reference Input and feedback signal.

Error signal = (Reference value signal – Measured value signal)

2. Control element

Control element decides what action to perform(for example, switch on or off when it receives an error signal)

3. Correction element

The correction element produces a change in the process to correct or change the controlled condition. Thus it might be a switch which switches on a heater and hence increase the temperature or a valve which opens and allow more liquid to enter the process. The term actuator is for the element of a correction unit that provides the power to carry out the control action.

4. Process element

The process being controlled could be a room in a house with its temperature being controlled or a tank full of water with its level being controlled.

5. Measuring element

The measuring element produces a signal related to the control variable(output). For example, it might be a Thermocouple which gives an EMF proportional to the output temperature.

Closed-loop system Advantages

  1. High accuracy

Closed-loop system Disadvantages

  1. Complicated and costly.
  2. The system has a tendency to oscillate.
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