Control System: A medium by which a quantity of interest such as a process or machine or mechanism is maintained or controlled in a desired manner is known as a control system. A control system is a device or process that is used to maintain or control a quantity of interest.

Control systems are often used to implement processes or systems in the industry. These components are typically composed of various control loops.

Examples of the control system include Air Conditioner (AC), washing machine, automatic coffee maker machine, tank water level controller, etc.

A basic block diagram of the control system can be illustrated as.

Control System Basic Block Diagram

The types of the control system are:

  1. Open-loop control system
  2. Closed-loop control system


In modern days and day to day applications, the control system has a great advantage and importance. The importance of the control system can be summarized as:

  1. Its application in the industries reduced the cost per product.
  2. Due to quicker, faster, and accurate responses, productivity in work increases.
  3. The quality of the goods produced also increases with the aid of a control system.

Features of Control System

The figure below shows the basic block diagram of the closed-loop control system. Here we will be discussing each element of the block diagram.

Block Diagram

Closed-Loop Control System

i) Input Command

The input command signal generates a reference signal to the system and is externally given. This input to the system is independent of the feedback path.

ii)  Reference Input

The reference input signal is the desired parameter for which we will be designing the control system. This is also called desired value for the variable which is to be controlled i.e. for the controlled variable.

iii) Input Transducer

The input signal to the system may be a physical signal such as temperature or pressure that needs to be converted into an equivalent electrical signal.

iv) Error Detector

Error detector produces an error signal by comparing the actual output and the desired output. Our main goal is to minimize the error as minimum as possible.

v) Controller

The controller helps in regulating the error signal generated by the error detector. Generally, Op-amp, relay, filter circuit are used as a controller.

vi) Noise or Disturbance

These are the unwanted signals present in the system. This unwanted signal affects the system output. A common example of a noise signal is the presence of noise in the op-amp.

vii) Plant or Process

The plant or process is also called a controlled system. This is the part of the system which helps in controlling application.

viii) Controlled Variable

This is the quantity that is to be measured and controlled.

ix) Manipulated Variable

The controller varies or manipulates the manipulated variable in order to affect the value of the controlled variable.

x) Feedback

An output transducer is placed in the feedback path which senses the actual output of the system. The value sensed by this sensor is fed to the error detector. The feedback in the system can be positive feedback or negative feedback.

Types of Control System

Here we will be discussing the two basic types of the control system and they are as follows:

1) Open-Loop Control System

Open-loop systems are typically used for controlling systems that rely on input or a controller. They do not have a feedback loop. The output is not measured nor sent back to the input for additional evaluation in open-loop systems. 

It is desired to use an open-loop system if we have the proper knowledge of the relationship between the input and the system. The open-loop systems are time-based controlled systems.

Open-Loop Control System

Examples of Open-Loop System

Traffic Light Controller

This is an open-loop system where the traffic lights are controlled or operated based on time.

Washing Machine

The washing machine is an example of an open-loop system that operates based on operating time. Here the machine operates only on the time, not on the cleanliness of the cloth is not considered as this feedback is not taken by the system.

Advantages of Open-Loop System

The advantages of an open-loop system are as follows:

  • It is an economical system with very little maintenance required.
  • Proper calibration of the system is not a consideration making its design simple.

Disadvantages of Open-Loop System

The disadvantages of an open-loop system are as follows:

  • They have a slow response and are less accurate.
  • Open-loop systems are less reliable.

2) Closed Loop Control System

A closed-loop control system works by automatically controlling a process variable. The closed-loop system is a feedback control system where is output is again fed back to the input and the control action is dependent on the desired output of the system. 

The feedback path in the system helps to reduce the noise and disturbance a lot. The feedback path has a sensor that senses the output, feeds the output back into the system, compares it with the reference input, and then generates an error signal. The controller is designed to minimize the error as small as possible.

The closed-loop systems are parameter-based control systems. The figure below shows a block diagram of a closed-loop system.

Closed-Loop Control System

Examples of Closed-Loop System

Air Conditioner (AC)

The air conditioner measures the actual temperature of the room, compares it with the desired input temperature, and maintains the temperature of the room by turning the compressor ON or OFF as per the requirement.

Governor in Hydroelectric Power Plant

The governer system used in the hydroelectric power plant is an example of a closed-loop system. The governor measures the frequency from the output of the generator, compares it with the reference value, and maintains the frequency of the generator by regulating the water flow to the turbine through a valve.

Advantages of Closed-Loop System

The advantages of a closed-loop system are as follows:

  • A closed-loop system can be optimized and it has a faster response.
  • Closed-loop systems are more reliable.

Disadvantages of Closed-Loop System

The disadvantages of a closed-loop control system are as follows:

  • Closed-loop systems have a complex design and are expensive.
  • The maintenance of this system is difficult.