Chapter 1 - Introduction Control System - Notes, Control System, Electrical Engineering Electrical Engineering (EE) Notes | EduRev

CONTROL SYSTEM INTRODUCTION

• A system is an arrangement, set, or collection of things connected or related in such a manner as to form it entirely or whole.
• The word control is usually taken to mean regulate, direct, or command.
• The control system is that means by which any quantity of interest in a machine, mechanism or other equipment is maintained or altered in accordance with a desired manner.

An Example of a Control Action

• Control of a room temperature is achieved by switching ON and OFF of a supply to a heating appliance. Thus power supply to an appliance is switched ON, when the room temperature is felt low and switched OFF, when the desired temperature is reached.
• The above system can be modified, if the duration of application of power is predetermined to achieve the room temperature within desired limits.
• However, a further refinement can be made by measuring the difference between the actual room temperature and the desired room temperature and this difference being the error is used to control the element which in turn controls the output i.e. room temperature.
• The above description indicates that in the former case the output (room temperature) has no control on the input and the control action is purely based on a sort of predetermined calibration only, whereas in the letter case the control action is affected by a feedback received from the output to the input.

OPEN LOOP & CLOSED LOOP SYSTEMS
Open Loop Systems:

• An open-loop control system consists of a controller or control actuator to obtain the desired response.
• It utilizes an actuating device to control the process directly without using device.
• An example of an open-loop control system is an electric toaster.
• In Open Loop Controlling action there is no feedback is present to sense the error in the desired output.

Advantages:
    (a) Simple and economic
    (b) No stability problem
Disadvantages :
    (a) Inaccurate
    (b) Unreliable
    (c) The effect of parameter variation and external noise is more.

NOTE:

• No performance analysis is required for open loop control system.
• Feedback is not used for improving stability.
• An open loop stable system may become unstable when negative feedback is applied. 
• Except oscillators in positive feedback, we have always unstable systems.

CLOSED LOOP SYSTEM OR FEEDBACK SYSTEM

• In a closed-loop control system, it consist of an additional measure of the actual output to compare the actual output with the desired output response.
• The measure of the output is called the feedback signal.
• A feedback control system  that tends to maintain a relationship of one system variable to another by comparing functions of these variables and using the difference as a
  means of control.
• Since as the system is become more complex, the interrelationship of many controlled variables may be considered in the control scheme.
• An example of closed-loop control system is a person steering an automobile by looking at the auto’s location on the road and making  the appropriate adjustments.Fig: A basic Control System Architecture 

Examples of Closed Loop Systems are:

• Electric Iron
• DC motor speed control
• A missile laughing system (direction of missile changes with the location of target)
• Radar tracking system 
• Human respiratory system
• Autopilot system
• Economic inflation
  

Advantages

(a) Accurate and reliable.
(b) Reduces effect of parameter variations.

• Example: RLC network

(c) Bandwidth of the system can be increased.
(d) Reduces effect of non-linearities

Disadvantages
(a) The system is complex and costly.
(b) System may become unstable.
(c) Gain of the system reduced with negative feedback.

COMPARISON BETWEEN OPEN LOOP AND CLOSED LOOP SYSTEM
Open Loop System

• So long as the calibration is good, an open loop system performance will be accurate. 
• Organisation is simple and easy to construct.
• Generally stable in operation. If non-linearity is present, system operation degenerates.

Closed Loop System

• Due to feedback due to feedback, the performance of closed. closed-loop systems is accurate.
• Complicated and difficult.
• Stability depends on system components.
• Comparatively the performance is better than open-loop system, if non-linearity is present.