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Closed-Loop Control System

Types of Control Systems

In this article, I’m going to talk about the types of control systems. Now why is that important? The importance of a Control System comes into play when we’re concerned about the design of a control system. A System Design Engineer must keep in mind the following before designing any system.

  • Robustness
  • Simplicity
  • Economy
  • Sensitivity to disturbances

At the end of this article, we’ll find out how the above mentioned factors are affected by the type of control system we choose.

There are two basic types of Control Systems

  • Open-Loop System
  • Closed-Loop System

Open-Loop System

The open-loop system (also referred to as a non-feedback system) is a system in which the output has no effect or influence on the control action of the input signal. Such sort of system will work according to the input (set point) provided, regardless of the output (result). The drawback of such system is that it cannot compensate for the disturbances that occurs during the process.

Let’s consider an example with the help of the figure given below. If the controller block is considered to be an Electronic Amplifier and a noise signal is considered as disturbance (although there are several other sources of disturbances), then it seems for sure that the output of the process will be corrupted (having noise). So this system cannot compensate for these disturbances as it is only commanded by the input and desired output cannot be achieved.

Such kind of control system is not desirable in most of the applications. It can only be used in systems which are immune to any kind of disturbances.


Open-Loop Control System
Block diagram of an Open-Loop Control System


Closed-Loop System

The Closed-loop system (also referred to as a feedback system) is a system in which the errors/disturbances in the output are compensated by using a feedback path. i.e. The input/desired response is achieved by feeding back a portion of the output/actual response for compensation of errors/disturbances added along the way.

The primary goal for designing a closed-loop control system is its ability to automatically achieve and maintain the desired response. It is done by comparing the output signal (which is compromised by the error signal) and the input signal. The result would be an error/actuating signal which will be fed to the controller.

Now let us consider the figure given below. It shows how a closed loop system compensates for disturbances. We can see that the output signal is added to the first summing junction via feedback path and the output signal is subtracted from the input signal. The result is formation of an error/actuating signal. This signal is fed to the controller. If there is any difference between the input and output signals, the system drives the plant via the error/actuating signal, to compensate for the error. If the error/actuating signal is zero, the system won’t make any changes.


Closed-Loop Control System
Block diagram of an Closed-Loop Control System



From the above discussion we can conclude that a closed-loop system has greater accuracy, less sensitive to noise, disturbances and changes in the environment. But such a system is complex and expensive. On the contrary, open-loop systems are simple and economical although they are prone to noise and disturbances. To decide between these two, one must consider the trade-off between the simplicity and low cost of an open-loop system and the accuracy and higher cost of a closed-loop system before designing a control system.


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