1、 1 英文 原文 Introduction to Modern Control Theory Several factors provided the stimulus for the development of modern control theory: a. The necessary of dealing with more realistic models of system. b. The shift in emphasis towards optimal control and optimal system design. c. The continuing developme
2、nts in digital computer technology. d. The shortcoming of previous approaches. e. Recognition of the applicability of well-known methods in other fields of knowledge. The transition from simple approximate models, which are easy to work with, to more realistic models, produces two effects. First, a
3、large number of variables must be included in the models. Second, a more realistic model is more likely to contain nonlinearities and time-varying parameters. Previously ignored aspects of the system, such as interactions with feedback through the environment, are more likely to be included. With an
4、 advancing technological society, there is a trend towards more ambitious goals. This also means dealing with complex system with a large number of interacting components. The need for greater accuracy and efficiency has changer the emphasis on control system performance. The classical specification
5、s in terms of percent overshoot, setting time, bandwidth, etc. have in many cases given way to optimal criteria such as mini mum energy, minimum cost, and minimum time operation. Optimization of these criteria makes it even more difficult to avoid dealing with unpleasant nonlinearities. Optimal cont
6、rol theory often dictates that nonlinear time-varying control laws are used, even if the basic system is linear and time-invariant. The continuing advances in computer technology have had three principal effects on the controls field. One of these relates to the gigantic supercomputers. The size and
7、 2 the class of the problems that can now be modeled, analyzed, and controlled are considerably large than they were when the first edition of this book was written. The second impact of the computer technology has to so with the proliferation and wide availability of the microcomputers in homes and
8、 I the work place, classical control theory was dominated by graphical methods because at the time that was the only way to solve certain problems, Now every control designer has easy access to powerful computer packages for systems analysis and design. The old graphical methods have not yet disappe
9、ared, but have been automated. They survive because of the insight and intuition that they can provide, some different techniques are often better suited to a computer. Although a computer can be used to carry out the classical transform-inverse transform methods, it is used usually more efficient f
10、or a computer to integrate differential equations directly. The third major impact of the computers is that they are now so commonly used as just another component in the control systems. This means that the discrete-time and digital system control now deserves much more attention than it did in the
11、 past. Modern control theory is well suited to the above trends because its time-domain techniques and its mathematical language (matrices, linear vector spaces, etc.) are ideal when dealing with a computer. Computers are a major reason for the existence of state variable methods. Most classical con
12、trol techniques were developed for linear constant coefficient systems with one input and one output (perhaps a few inputs and outputs). The language of classical techniques is the Laplace or Z-transform and transfer functions. When nonlinearities ad time variations are present, the very basis for t
13、hese classical techniques is removed. Some successful techniques such as phase-plane methods, describing function s, and other ad hoc methods, have been developed to alleviant this shortcoming. However, the greatest success has been limited to low-order systems. The state variable approach of modern
14、 control theory provides a uniform and powerful method of representing systems of arbitrary order, linear or nonlinear, with time-varying or constant coefficient. It provides an ideal formulation for computer implementation 3 and is responsible for much of the progress in optimization theory. Modern
15、 control theory is a recent development in the field of control. Therefore, the name is justified at least as a descriptive title. However, the foundations of modern control theory are to be found in other well-established fields. Representing a system in terms of state variables is equivalent to th
16、e approach of Hamiltonian mechanics, using generalized coordinates and generalized moment. The advantages of this approach have been well-known I classical physics for many years. The advantages of using matrices when dealing with simultaneous equations of various kinds have long been appreciated in
17、 applied mathematics. The field of linear algebra also contributes heavily to modern control theory. This is due to the concise notation, the generality of the results, and the economy of thought that linear algebra provides. Mechanism of Surface Finish Production There are basically five mechanisms
18、 which contribute to the production of a surface which have been machined. There are: (1) The basic geometry of the cutting process. In, for example, single point turning the tool will advance a constant distance axially per revolution of the work piece and the resultant surface will have on it, whe
19、n viewed perpendicularly to the direction of tool feed motion, a series of cusps which will have a basic form which replicates the shape of the tool in cut. (2) The efficiency of the cutting operation. It has already been mentioned that cutting with unstable built-up-edges will produce a surface whi
20、ch contains hard built-up-edge fragments which will result in a degradation of the surface finish. It can also be demonstrated that cutting under adverse conditions such as apply when using large feeds small rake angles and low cutting speeds, besides producing conditions which continuous shear occurring in the shear zone, tearing takes place, discontinuous chips of uneven thickness are produced, and the resultant surface is poor. This situation is particularly noticeable when machining very ductile materials such as copper and aluminum.
