Open Loop and Closed Loop System : Definition, Working Diagram, Examples, Advantages, Disadvantages, Application, PDF

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Open Loop and Closed Loop System:

OPEN LOOP SYSTEM:

An open loop control system is one in which the control action is independent of the output. As the above figure has shown the elements of an open loop control system can be divided into the following two parts:

  • Controller and 
  • Controlled process

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

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 and constant if there are changed in weather condition because no information is fed back to the heating element.

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, but not on the basis of whether the clothes are clean properly.
No information is fed back cleanliness off the clothes.

Advantages / Merits:

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

Disadvantages / De-merits:

  • Presence of non-linearities causes malfunctioning.

CLOSED LOOP SYSTEM:

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. 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.
A closed loop control system is shown in the above figure.
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 take(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 so increase the temperature of the process 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 control or a tank of the 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 and EMF proportional to the output temperature.

Advantages / Merits:

  • High accuracy

Disadvantages / De-merits:

  • Complicated and costly.
  • The system has a tendency to oscillate.

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