What is Control systems? | Types of Control systems in Instrumentation


Introduction of Control systems:

In modern times, control systems play a very important role in our life daily. The concepts of automatic control are fundamental and well mixed in every aspect of our life touches. 

From a simple bread toaster to a complex modern power plant, there is a series of control principles that affect our life. 

The principle of the control systems is applicable to engineering as well as non-engineering fields.

In this lecture, we will study the definition of some important terms, types of control systems, applications, and advantages and disadvantages of control systems.

Important Definitions:

1. System: A system is an arrangement of or a combination of different physical components connected or related n such a manner so as to form an entire unit to attain a certain objective.

2. Output: The actual response obtained from the system is called output.

3. Input: The stimulus or excitation applied to control systems from external sources in order to produce the output is called input.

4. Control: It means to regulate, direct or command a system so that the desired objective is attained.

5. Control Systems: It is an arrangement of different physical elements connected in such a manner sp as to regulate, direct or command itself to achieve a certain objective (or command some other systems to achieve a certain objective) 

Thus control systems must have the following:

  • An input or inputs
  • An output or outputs
  • An arrangement to achieve these input-output combinations.

what is a requirement of Good Control systems:

1. Accuracy:

  • Accuracy is very high as any error is arising should be corrected. 
  • Accuracy can be improved by using the feedback element. Because of the feedback element, the system becomes a closed-loop system.
  • In closed-loop control systems, the steady-state error tends to zero.

2. Sensitivity:

  • The control systems sense changes in output due to environmental or parametric changes, internal disturbances, or any other parameters and correct the same.
  • Any control system would be insensitive to such parameters but very sensitive to the input signals. 

3. Noise: 

  • Noise is an undesired input signal.
  • A good control system would be insensitive to such input signals.
  • A good control system should be able to reduce the effect of wise or undesired inputs.

4. Stability:

  • Stability means bounded input and bounded output.
  • In the absence of inputs, the output should be tends to zero as time increases, 
  • A good control system response is stable for all variations.

5. Bandwidth:

  • The operating frequency range decides the bandwidth of the control systems.
  • For the frequency response of a good control system, bandwidth should be large.

6. Speed:

  • A good control system would have high speed.
  • That is the output of the control system should be as fast as possible.

7. Oscillations: 

  • For a good control system, oscillations of output should be constant air sustained oscillations that follow the Barkhauseein’s criteria.

what are the types of  Control systems:

“Control action is that quantity responsible for activating the system to produce the output.”

Depending on whether such control actions s dependent on the status of output, control systems are classified into two categories:

  1. Open-loop Control System
  2. Closed-loop Control System 

What is Open-loop Control system:

Definition: “A system in which the control action is totally independent of the output of the system is called as open-loop control system”.

open loop control system

Example of Open-loop Control system:

  • Automatic hand driver
  • Automatic washing machine
  • Bread toaster
  • automatic coffee server
  • electric lift
  • traffic signal
  • theater lamp dimer etc.

Advantages of Open-loop Control system:

  • They are simple in construction and design
  • They are economic
  • Easy for maintenance
  • Not much problem of stability
  • Convenient to use when output is difficult to measure

Disadvantages of Open-loop Control system:

  • Inaccurate and unreliable because accuracy is dependent on the accuracy of the calibration.
  • Inaccurate results are obtained with parameter variations, internal disturbances.
  • To maintain quality and accuracy, Recalibration of the controller is necessary from time to time.

What is Closed-loop Control system:

Definition 1: “A system in which the control action is somehow dependent on the output is called as closed-loop control system”. 

closed loop control system

Definition 2: “Feedback is that property of the system which permits the output to e compared the reference input so that appropriate control action is formed”.

Example of Closed-loop Control system:

  • Automatic electric iron
  • DC motor speed control by tacho feedback
  • railway reservation status
  • missile launched 
  • servo voltage stabiliser
  • human perspiration system
  • water level controller etc.

Advantages of Closed-loop Control system:

  • Accuracy is very high as any error arising is corrected 
  • It senses changes in output due to environmental or parametric changes, internal disturbances, etc., and corrects the same.
  • Reduced effect of non-linearities
  • High bandwidth
  • Facilitates Automation

disadvantages of Closed-loop Control system:

  • Complicated in design and maintenance costlier
  • The system may become unstable

Comparison of Open and  Closed-loop Control systems:

Sr. No Open-Loop System Closed-Loop System
1. No feedback. Hence feedback elements absent Feedback exists. Hence feedback elements present.
2. No error detector Error detector is present
3. It is inaccurate It is accurate
4. Highly sensitive to parameter changes Less sensitive to parameter changes
5. Small bandwidth large bandwidth
6. Stable May become unstable
7. Economical Costly
8. Example: Coffee maker, toaster, hand drier, etc. Example: Guided-missile, Oven, stabilizers, etc.


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