Open-loop and closed-loop control systems

There are two types of control systems:

1. Open-loop control system

2. Closed-loop control system.

1. Open-loop control system:

A system in which the output is dependent on input but controlling action is totally independent of the output is called an Open-loop system.

It is also referred to as a non-feedback system, which is a type of continuous control system in which the output has no influence or effect on the control action of the input signal.

The manual system is also an open-loop control system.,

The block diagram of the open-loop control system in which process output is totally independent of the controller action.

The below figure is a basic open-loop system, in which the operator is controlling the tank level by throttling the manual valve by hand by looking at the indication in the transparent hose pipe and controlling the level by a manual hand valve.

Advantages of the open-loop control system:

  • Simple in construction and design.
  • Economical
  • Easy to maintain
  • Generally stable
  • Convenient to use as output is difficult to measure.

Disadvantages

  • They are inaccurate
  • They are unreliable
  • Only changes in output can not be corrected automatically.

2. Closed-Loop Control System:

A system In which the controlling action is dependent on the output is called a Closed-loop system.

In which the output has an effect on input quantity in such a manner that the input quantity will adjust itself based on the output generated.

An open-loop control system can be converted in to close loop control system by adding feedback. this feedback automatically makes satisfactory changes in the output due to external disturbances. in this way, the closed-loop control system is called an automatic control system or feedback control system.

The below block diagram and figure show the closed-loop control system in which feedback is taken from the output and fed into the input.

The below figure is a basic closed-loop system, which shows the tank level is sensed and measured by an electronic transmitter, the signal is fed to the controller and its output signal is regulating the control valve to control the level in the tank automatically.

Basic Closed Loop System

Advantages :

  • The closed-loop control systems are more accurate even in the presence of nonlinearities.
  • Highly accurate as any error arising is corrected due to the presence of a feedback signal.
  • Facilitates automation.
  • The sensitivity of the system may be made small to make the system more stable.
  • This system is less affected by noise.

Disadvantages:

  • They are costlier.
  • They are complicated to design.
  • Require more maintenance.
  • Feedback leads to an oscillatory response.
  • Stability is the main problem.

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