INTRODUCTION OF CONTROL SYSTEMS

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Introduction

In this chapter we attempt to familiarize the reader with the following subjects:
1. What a control system is
2. Why control systems are important
3. What the basic components of a control system are
4. Why feedback is incorporated into most control systems
5. Types of control systems
Let's begin with a simple question. When did you last use the word " Control" ? Perhaps one may have to think for a while. But the paradox is that one invariably uses this word almost in every walk of life but fails to take notice. The following are a few common phrases we come across all the time.

Definition of control system

A control system can be defined as an interconnection of several components all working together to perform a certain function. In most cases this function is the control of physical variable, such as temperature voltage, frequency, flowrate, current, position, hp speed, illumination, altitude etc., These are called controlled variables.

OPEN LOOP CONTROL SYSTEMS ( NON FEED BACK SYSTEMS)

Those systems in which the output has no effect on the control action are called open loop control systems. In other words, in an open - loop control systems the output is neither measured nor feedback for comparison with the input. Thus to each reference input there corresponds a fixed operating condition; as a result, the accuracy of the system depends on calibration. Open loop control can be used, in practice, only if the relationship between input and output is known, and if there are neither internal nor external disturbances. Note that any control system that operates on a time basis is open loop. We shall go through examples and try to identify the inputs (objectives) and outputs (effects).

CLOSED- LOOP VERSUS OPEN LOOP CONTROL SYSTEMS

An advantage of the closed loop control system is the fact that the use of feedback makes the system response relatively insensitive to external disturbances and internal variations in systems parameters. It is thus possible to use relatively inaccurate and inexpensive components to obtain the accurate control of the given plant, whereas doing so is impossible in the open-loop case.
From the point of view of stability, the open loop control system is easier to build because system stability is not a major problem. On the other hand, stability is a major problem in the closed loop control system, which may tend to overcorrect errors that can cause oscillations of constant or changing amplitude.