Introduction to Control System

1.0       THE BASIC CONCEPT OF CONTROL SYSTEM

Automatic control has played a vital role in the advance of engineering and science. In addition to its extreme importance in space-vehicle system, missile-guidance systems, air-craft-autopiloting systems, robotic systems and the likes, automatic control has become an important and integral part of modern manufacturing and industrial processes. For example, automatic control is essential in the numerical of machine tools in the manufacturing industries.

The controlled  variable is the quantity or condition that is measured and controlled. The manipulated  variable is the quantity or condition that is varied by the controller so as to affect the value of the controlled variable. Normally, the controlled variable is the output of the system. Control  means measuring the value of the controlled variable of the system and applying the manipulated variable to the system to correct or limit deviation of the measured value from a desired value.

In studying control engineering, we first need to define additional terms that are necessary to describe control systems, such as plants, disturbances, processes, feedback control systems and etc. Then a description of closed-loop and open-loop control systems  and their advantages and disadvantages will be given in the following sections.

1.1       IDENTIFYING THE DEFINITION AND TERMINOLOGY RELATED TO CONTROL SYSTEM

The various definitions of the system variables and components are as mentioned below:

            1.1.1   Pneumatic Control Systems

            The working medium in a pneumatic control systems uses a compressible fluid, such as air because it may be exhausted to the atmosphere at the end of the device’s work cycle, thus eliminating the need for return lines.

1.1.2     Hydraulic Control Systems

Hydraulic control systems is the study of incompressible liquids, and hydraulic devices use an incompressible fluid, such as oil, for their working medium. Liquid level systems consisting of storage tanks and connecting pipes are a class of hydraulic systems whose driving force is due to relative difference in the liquid heights in the tanks.

1.1.3     Reference Input

It is the actual signal input to the control system.

1.1.4     Process

Any operation to be controlled. For example, chemical, economic, and biological processes.

1.1.5     Feedback Element

It is the unit which provides the means for feeding back to the output quantity in order to compare it with the reference input.

            1.1.6   Disturbances

           

            A disturbance is a signal that tends to adversely affect the value of the output of a system. If the disturbance is generated within the system, it is called internal, while an external disturbance is generated outside the system and is an input.

1.2       EXPLANATION OF CONTROL SYSTEM TYPES

A control system may consists of a number of components. In order to show the functions performed by each component, in control engineering, we commonly use a diagram called the block diagram.

A block diagram of a system is a pictorial representation of the functions performed by each component and of the flow of signals. In a block diagram all system variables are linked to each other through functional blocks. The functional block or simply block is a symbol for the mathematical operation on the input signal to the block that produces the output.

Figure 1.1 shows an element of the block diagram. Such arrows are referred to as signals.

                                                          

Figure 1.1      Element of a block diagram

(Source : Katsuhiko Ogata (1990), Modern Control Engineering)

The advantages of the block diagram representation of a system lies in the fact that it is easy to form the overall block diagram for the entire system by merely connecting the blocks of the components according to the signal flow and that it is possible to evaluate the contribution of each component to the overall performance of the system.

                        Summing Point

Referring to Figure 1.2, a circle with a cross is the symbol that indicates a summing operation. The plus or minus sign at each arrowhead indicates whether that signal is to be added or subtracted.

                   

       

           

Figure 1.2      Summing point

(Source : Katsuhiko Ogata (1990), Modern Control Engineering)

Branch point

            A branch point is a point from which the signal from a block goes concurrently to other blocks or summing points.

Control systems are classified into  two general categories:

                        ☺        open-loop system

                        ☺        closed-loop system

The distinction is determined by the control action, which is that quantity responsible for activating the system to produce the output. 

1.2.1   Open-loop Control System

           

An open-loop control system is one in which the control action is independent of the output. Figure 1.3 shows the block diagram of an open-loop control system (basic system) and Figure 1.4 shows the block diagram of an open-loop control system (automobile driving system).

                        

 Figure 1.3      An open-loop control systems (basic system)

(Source : S.P. Eugene Xarier & Joseph Cyril Babu.J (1999), Principles of Control System)

 

Figure 1.4      An open-loop control systems (Automobile driving system)

(Source : S.P. Eugene Xarier & Joseph Cyril Babu.J (1999), Principles of Control System)

1.2.2   Closed-loop Control System

            Closed-loop control systems are more commonly called feedback control systems. Feedback is the characteristic of closed-loop control systems which distinguishes them from open-loop systems. It is the property of closed-loop control systems which permits the output to be compared with the input of the system so that the appropriate control action may be formed as a function of the output and input. In general, feedback is said to exist in a system when closed sequence of cause-and-effect relation exists between system variables.

Figure 1.5      General block diagram of an automatic control system

(Source : Katsuhiko Ogata (1990), Modern Control Engineering

1.2.3     Comparison between open-loop and closed-loop control systems

Open-loop Control System

The important features of open-loop control systems are :

i.              Their ability to perform accurately is determined by their calibration, which simply implies, to establish the input-output relation to obtain a desired system accuracy.

ii.            They are not generally troubled with problems of instability.

Closed-loop Control System

The important features of feedback are :

i.              Reduced effects of  nonlinearities and distortion

ii.            Increased accuracy

iii.           Increased bandwidth

iv.           Reduced sensitivity of the ratio of the output to input to variations in system characteristics.

v.            Tendency towards oscillation or instability.