Control Engineering Practice

Introduction to Control Systems

Introduction

Control engineering is based on the foundations of the feedback theory and the linear systems analysis, and integrates the concepts of network theory and communication theory. Therefore control engineering is not limited to any engineering discipline but is equally applicable for aeronautical, chemical, mechanical, environmental, civil, and electrical engineering. For example, quite often the control system includes electrical, mechanical, and chemical components. Furthermore, as the understanding of the dynamics of business, social, and political systems increases, the ability to control these systems will increase too.

A control system is an interconnection of components forming a system configuration that provides a system response. The basis for analysis of a system is a foundation provided by linear system theory, which assumes a cause- effect relationship for the components of a system. Therefore a component or process to be controlled can be represented by a block as shown in Fig. 1.1.

Those systems in which output quantity has no effect upon the input quantity are defined as open-loop control systems. An open-loop control system utilizes a controller or control actuator in order to obtain the desired response as shown in Fig. 1.2.

It is clear, that in this case the desirable control quality could not be achieved at all or only imperfectly by manual correction made by operator who always had to supervise the process and make changes where it is necessary. This can be realized by feedback control loop. Those systems in which the output has an effect upon the input quantity are defined as closed-loop control systems.

In contrast to an open-loop control system, a closed-loop system utilizes an additional measure of the actual output in order to compare the actual output with the desired output response. The measure of the output is called the feedback signal . A simple closed-loop system is shown in Fig. 1.3.

A standard definition of a feedback control system is as follows: a feedback control system is a control system that tends to maintain a prescribed relationship of one system variable to another by comparing functions of these variables and using the difference as a means of control. Often the difference between output of the process under control and the reference input is amplified and used to control the process so that the difference is continually reduced. The feedback concept has been the foundation for control system analysis and design.

Note that a feedback action can be continuous and discontinuous. Continuous control means that output is continuously being fed back, in time, and compared with the reference output. In one form of discontinuous control the input and output quantities are periodically sampled and compared. Another type of discontinuous control system is so-called an “on - off” or relay feedback control system.

Due to the increasing complexity of the system under control and the interest in achieving optimum performance, the importance of control system engineering has grown in this decade. Furthermore, as the systems become more complex, the interrelationship of many controlled variables must be considered in the control scheme. A block diagram depicting a multivariable control system is shown in Fig. 1.4.

Control Engineering Practice.