1、 4 Introduction to Control System 1. 1 HISTORICAL PERSPECTIVE The desire to control the forces of nature has been with man since early civilizations. Although many examples of control systems existed in early times, it was not until the mid-eighteenth century that several steam engine, and perhaps t
2、he most noteworthy invention was the speed control flyball governor invented by James Watt. The period biginnign about twenty-five years before World War Two saw rapid advances in electronics and especially in circuit theory, aided by the now classical work of Nyquist in the area of stability theory
3、, The requirements of sophisticated weapon systems, submarines, aircraft and the like gave new impetus to the work in control systems before and after the war The advent of the analog computer coupled with advances in electronics saw the beginning of the establishment of control systems as a science
4、. By The mid-fifties, the progress in digital computers had given the engineers a new tool that greatly enhanced their capability to study large and complex systems. The availability of computers also opened the era of data-logging, computer control, and the state space of modern method of analysis.
5、 The sputnik began the space race and large governmental expenditures in the space as well as military effort. During this time. circuits became miniaturized and large sophisticated systems could be put together very compactly thereby allowing a computational and control advantage coupled with syste
6、ms of small physical dimensions. We were now capable of designing and flying minicomputers and landing men on the moon. The post sputnik age saw much effort in system optimization and adaptive systems. Finally, the refinement of the chip and related computer development has created an explosion in c
7、omputational capability and computer-controlled devices. This has led to many innovative methods in manufacturing methods. such as computer-aided design and manufacturing, and the possibility of unprecedented increases in 5 industrial productivity via the use of computer-controlled machinery, manipu
8、lators and robotics. Today control systems is a science with the art still playing an important role. Much mathematical sophistication has been achieved with considerable interest in optimal control system. The modern approach, having been established as a science, is being applied not only to the t
9、raditional control systems, but to newer problems like urban analysis, econometrics, transportation, biomedical problems, energy analysis, and a host of similar problems that affect modern man. 1.2 BIASIC CONCEPTS Control system analysis is concerned with the study of the behavior of dynamic systems
10、. The analysis relies upon the fundamentals of system theory where the governing differential equations assume a cause-effect relationship. A physical system may be represented as shown in Fig. where the excitation or input is x(t) and the response or output is y(t) . A simple control system is show
11、n in Fig. Here the output is compared to the input signal, and the difference of these two signals becomes the excitation to the physical system, and we speak of the control system , such as described in Fig . involves the obtaining of y(t) given the input and output are specified and we wish to des
12、ign the system characteristics, then this is known as synthesis. 1.3 SYSTEMS DESCRIPTION Because control systems occur so frequently in our lives, their study is quite important. Generally, a control system is composed of several subsystems connected in such a way as to yield the proper cause-effect
13、 relationship. Since the various subsystems can be electrical, mechanical, pneumatic, biological, etc., the complete description of the entire system requires the understanding of fundamental relationships in many different disciplines. Fortunately, the similarity in the dynamic behavior of differen
14、t physical systems makes this task easier and more interesting. As an example of a control system consider the simplified version of the attitude control of a spacecraft illustrated in Fig.1-4. We wish the satellite to have some specific attitude relative to an inertial coordinate 6 system. The actu
15、al attitude is measured by an attitude sensor on board the satellite. If the desired and actual attitudes are not the same, then the comparator sends a signal to the valves which open and cause gas jet firings. These jet firings give the necessary corrective signal to the satellite dynamics thereby
16、it under control .A control system represented this way is said to be represented by block diagrams. Such a representation is helpful in the partitioning of a large system into subsystems and thereby allowing the study of one subsystem at a time. If we have many inputs and outputs that are monitored
17、 and controlled, the block diagram appears as illustrated in Fig.1-5. Systems where several variables are monitored and controlled are called multivariable systems. Examples of multivariable systems are found in chemical processing, guidance and control of vehicles, the national economy, urban probl
18、ems. The number of control systems that surround us is indeed very large. The essential feature of all these systems is in general the same . They all have input ,control ,output, and disturbance variables. They all describe a controller and a plant . They all have some type of a comparator. Finally
19、, in all cases we want to drive the control system to follow a set preconceived commands. 1.4 DESIGN, MODELING ,AND ANALYSIS Prior to the building of a piece of hardware, a system must be designed, modeled, and analyzed. Actually the analysis is an important and essential feature of the design proce
20、ss. In general, when we design a control system we do so conceptually. Then we generate a mathematical model which is analyzed. The results of this analysis are compared to the performance specifications that are design a control system we do so conceptually. Then we generate a mathematical model wh
21、ich is analysis are compared to the performance specifications that are desired of the proposed system. The accuracy of the results depends upon the quality of the original model of the proposed design. We shall show , in Chapter7, how it is analyzed and then modified so that its performance satisfies the system specifications. The objective then may be considered to be the prediction , prior to construction, of the dynamic behavior that a physical system exhibits, i.e. its natural motion when disturbed from an
