What is a Closed Loop Control System & Its Working

The number of elements that are combined to form a system to generate the desired output is known as the control system. The output of any system can be controlled by the control system because each element within this system has an effect on the output. Sometimes, a control system uses a feedback loop to get the stability & consistency and stability of a system for generating the preferred output. Here, a feedback loop is an element of the output signal. The concept of feedback is very essential in any control system to get the output stability. Control systems are classified into two types based on the connection of feedback namely closed-loop control system and open-loop control system

What is a Closed-Loop Control System?

Definition: A closed-loop control system can be defined as, a system that has a feedback loop (or) a control system that uses a feedback signal to generate the output. The stability of this system can be controlled by a feedback system. So by providing a feedback system, any open-loop control system can be changed into a closed loop.


The desired output can be achieved and maintained by evaluating the actual condition & generated output. If the generated output is moved away from the actual output, then this control system produces a faulty signal which is fed to the i/p of the signal. Once the error signal is added to the input signal, then the next loop output can be corrected which is known as automatic control systems.

Block Diagram

The block diagram of the closed-loop system is shown below. The basic elements of the closed-loop control system include error detector, controller, feedback elements & power plant.

Closed Loop Control System Block Diagram
Closed-Loop Control System Block Diagram

When the control system includes a feedback loop, then the systems are known as feedback control systems. So the output can be controlled accurately by providing feedback to the input. This type of control system can include more than one feedback.

In the above diagram, the error detector generates an error signal, so this is the variation of the input as well as the feedback signal. This feedback signal can be obtained from the elements of feedback in the control system by considering the system output as an input. As an alternative of the input, this error signal can be given as an input of a controller.

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Consequently, the controller generates an actuating signal to control the plant. In this arrangement, the control system output can be corrected automatically to get the preferred output. Therefore, these systems are also named as automatic control systems. The best example of a closed-loop control system is a traffic light control system including a sensor at the input.

Types of Closed Loop Control System

Closed-loop control systems are classified into two types depending on the feedback signal nature such as positive feedback signal and the negative feedback signal.

Positive Feedback Signal

The closed-loop system including a positive feedback signal can be connected to the input of the system is known as a positive feedback system. This system is also named as regenerative feedback. The best example of this positive feedback in electronic circuits is an operational amplifier. Because this loop can be achieved by connecting some portion of the output voltage to the input of the non-inverting terminal through a feedback loop using a resistor.

Negative Feedback Signal

The closed-loop system including a negative feedback signal can be connected to the input of the system is named as a negative feedback system. This kind of system is also named as degenerative feedback. These types of systems are very stable and also enhance strength.

These systems are used to control the electronic machines like current generators, voltage generators, and also control the machinery speed. The closed-loop control systems are used for the below-stated requirements.

Transfer Function

The system behavior can be indicated through its transfer function. It can be defined as the mathematical relation of the input and output of the control system. The control system can be calculated through the ratio of o/p to i/p. Therefore, the control system output is the product of input & transfer function.

Control System
Control System

The closed-loop control system example is shown below.

For the above system,

C(S) = E(S) * G(S)

E(S) = R(S) – H(S) * C(S)

Substitute this E(S) value in C(S), then we can get

C(S) = [R(S) – H(S) * C(S)]* G(S)

C(S) = R(S) G(S) – H(S) * C(S) * G(S)

From the above equation

R(S) G(S) = C(S) + H(S) * C(S) * G(S)

R(S) G(S) = C(S) [1 + H(S) * G(S)]

C(S)/ R(S) = G(S)/ [1 + H(S) * G(S)]

This is the transfer function of this system with negative feedback. Similarly, for the positive feedback, the transfer function equation can be written as

C(S)/ R(S) = G(S)/ [1 – H(S) * G(S)]

Closed-Loop Control System Examples

There are different kinds of electronic devices that use a closed-loop control system. So the applications of closed-loop control systems include the following.

  • In servo voltage stabilizer, the voltage stabilization can be attained by giving output voltage feedback to the system
  • In the water level controller, the level of water can be decided by the input water
  • The temperature in the AC can be adjusted depending on the temperature of the room.
  • The motor speed can be controlled using a tachometer or current sensor, where the sensor detects the motor speed and sends feedback to the control system to change its speed.
  • Some more examples of these systems include thermostat heater, solar system. missile launcher, auto engine, automatic toaster, water control system using a turbine.
  • Automatic electric iron can be controlled automatically by the heating element’s temperature in the iron.

Advantages

The advantages of the closed-loop control system include the following.

  • These systems are very precise & less error-prone.
  • Errors can be corrected through the feedback signal
  • High bandwidth
  • It supports automation
  • High noise margin
  • They cannot affect by means of noise.

Disadvantages

The disadvantages of the closed-loop control system include the following.

  • The designing of this system is complicated
  • They are very complex
  • Expensive
  • Huge maintenance is needed
  • The control system oscillates sometimes due to feedback signals.
  • More efforts, as well as time, are required while designing the system.

Thus, this is all about an overview of a closed-loop control system including block diagram, types, transfer function, advantages, disadvantages, and its applications The characteristic equation of closed-loop control system is nothing but the setting of the transfer function denominator to zero. Here is a question for you, what is the open-loop control system?

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