Closed-Loop Control System: Definition, Block Diagram, Types, Transfer Function, Advantages & Disadvantages

A Closed-Loop Control System is one of the most important concepts in control systems and automation. These systems use feedback to automatically control and maintain the desired output.

What is a Closed-Loop Control System?

A closed-loop control system is defined as a system that uses a feedback loop to control its output. The output is continuously monitored and compared with the desired input. If there is any difference (error), the system automatically corrects it. Any open-loop control system can be converted into a closed-loop system by adding a feedback mechanism.

In this system:

• The actual output is compared with the desired output.
• An error signal is generated if a difference exists.
• The error signal is fed back to the input.
• The system automatically corrects itself.

Because of this automatic correction, closed-loop systems are also called Automatic Control Systems.

Block Diagram of Closed-Loop Control System

The basic components of a closed-loop control system are:

• Error Detector
• Controller
• Plant (Power Plant)
• Feedback Element

Working principle:

• The error detector compares input and feedback signal.
• The difference generates an error signal.
• The controller processes this error signal.
• The plant produces the corrected output.
• The feedback element sends part of the output back to the input.

A traffic light control system with sensors is a common example of a closed-loop system.

Types of Closed-Loop Control System

Closed-loop systems are classified based on the nature of feedback:

1. Positive Feedback System

A positive feedback system is a closed-loop system where the feedback signal is added to the input signal. This is also known as Regenerative Feedback.

Example: Operational amplifier circuits where part of the output is connected to the non-inverting terminal through a resistor.

2. Negative Feedback System

A negative feedback system is a closed-loop system where the feedback signal is subtracted from the input signal. This is also known as Degenerative Feedback.

Negative feedback systems are:

• More stable
• More accurate
• Less sensitive to noise

Applications include motor speed control, voltage generators, current generators, and temperature control systems.

Transfer Function of Closed-Loop Control System

The transfer function represents the mathematical relationship between the input and output of a control system. It is defined as:

Transfer Function = Output / Input

For Negative Feedback System:

The closed-loop transfer function is:

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Where:

• R(s) = Input
• C(s) = Output
• G(s) = Forward path gain
• H(s) = Feedback gain

For Positive Feedback System:

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Examples of Closed-Loop Control Systems

• Servo Voltage Stabilizer (Voltage stabilization using feedback)
• Water Level Controller
• Air Conditioner (Temperature control)
• Motor Speed Control using tachometer
• Thermostat Heater
• Automatic Electric Iron
• Solar Tracking System
• Automatic Engine Control
• Missile Guidance System

Advantages of Closed-Loop Control System

• High accuracy
• Automatic error correction
• Better stability
• Large noise margin
• Suitable for automation
• Less affected by external disturbances

Disadvantages of Closed-Loop Control System

• Complex design
• High cost
• Requires maintenance
• Possibility of oscillations due to feedback
• More time and effort required for design

Conclusion

A closed-loop control system is essential in modern automation and industrial applications. By using feedback, it ensures accuracy, stability, and automatic correction of errors. Although it is complex and costly, its advantages make it suitable for advanced electronic and mechanical systems.