Control System

A control system is a system that manages, directs, or regulates the behaviour of another device or system in order to achieve a desired result.

It is a system of devices (or a set of devices) that accepts an input, processes it, and produces an output.

A control system is also defined as a set of mechanical or electronic devices interconnected to control a process.

(i) Accuracy

Accuracy is the measurement capability of an instrument and defines the limit of errors when the instrument is used under normal operating conditions.

Key Point: Accuracy can be improved by using a feedback system.

(ii) Sensitivity

The parameters of a control system are always changing due to surrounding conditions, internal disturbances, or any other parameter variation.

This change can be expressed as a term called sensitivity. Any control system should have low sensitivity to parameter changes — meaning its output should remain stable even when parameters vary.

(iii) Stability — BIBO Stability

Stability is the most important characteristic of a control system.

A control system is said to be stable if, for every bounded (limited) input, it produces a bounded (limited) output. This is known as BIBO Stability (Bounded Input Bounded Output).

• If input is zero, output must also be zero — such a system is called a stable control system.
• In practical life, no control system is 100% stable.

(iv) Speed

Speed is the time taken by a control system to achieve a stable (steady-state) output from an input signal.

A good control system should have a high speed — meaning the transient period (the time taken to settle after a change in input) should be very small.

There are 2 types of control system:

1. Open Loop Control System

2. Close Loop Control System

(Manual Control System)

(Automatic Control System)

(i) Open Loop Control System

An open loop control system is a system in which the control action is independent of the desired output signal.

In this system, the output signal is NOT compared with the input signal — which means there is NO feedback signal. That is why it is also called a No-Feedback Control System.

Input

→

Controller

→

Output

Examples: Car control system, Washing machine, Electric iron, Traffic lights

Advantages: Simple design, low cost, easy to maintain.

Disadvantages: Less accurate, cannot correct disturbances automatically.

(ii) Close Loop Control System

A close loop control system is a system in which the control action is dependent on the desired output. The output is continuously compared with the input through a feedback mechanism to reduce error.

It is also called an Automatic Control System or Feedback Control System.

Input

→ ⊗ →

Controller

→ Process →

Output

Examples: Air conditioner (A.C.), Refrigerator, Automatic hand wash system, Automatic missile launch system

Advantages: More accurate, can self-correct disturbances, more reliable.

Disadvantages: More complex, higher cost.

Open Loop vs Close Loop — Comparison

Open Loop	Close Loop
Control action is independent of output	Control action depends on output
No feedback signal used	Feedback signal is used
Less accurate	More accurate
Simple and low cost	Complex and higher cost
Cannot self-correct disturbances	Can self-correct disturbances
Ex: Washing machine, Electric iron	Ex: A.C., Refrigerator