2002---2003 年度下学期《专业英语》期末考试试卷
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请写出下文英文摘要（字数要求在 150－200），且把摘要翻译为中文。
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 developments in digital computer technology.
d. The shortcoming of previous approaches.
e. A 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 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 advancing technological society, there is a trend towards more ambitions goals. This
also means dealing with complex system with a large number of interacting components. The need
for greater accuracy and efficiency has changed the emphasis on control system performance. The
classical specifications in terms of percent overshoot, setting time, bandwidth, etc., have in many
cases given way to optimal criteria such as minimum energy, minimum cost, and minimum time
operation. Optimization of these criteria makes it even more difficult to avoid dealing with
unpleasant nonlinearities. Optimal control theory often dictates that nonlinear time-varying control
laws be 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 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 do with the proliferation and wide
availability of the microcomputers in homes and in 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 disappeared, 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 for a computer to
integrate differential equations directly.
The third major impact of the computers in that they are now so commonly used as just
another component in the control system. Their cost ,size ,and reliability make it possible to use
them routinely in many systems. This means that the discrete-time and digital system control now
deserves much more attention than it did in the 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 variable methods.
Most classical control 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 and time variations are
present, the very basis for these classical techniques is removed. Some successful techniques such
as phase-plane methods, describing functions, and other methods, have been developed to
alleviate this shortcoming.
However, the greatest success has been limited to low-order systems. The state variable
approach of modern control theory provides a uniform and powerful methods of representing
systems of arbitrary order, linear or nonlinear, with time-varying or constant coefficients. It
provides an ideal formulation for computer implementation and is responsible for much of the
progress in optimization theory.
Modern 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 the approach of Hamiltonian mechanics, using generalized coordinates and
generalized moment. The advantages of this approach have been well-known in classical physics
for many years. The advantages of using matrices when dealing with simultaneous equations of
various kinds have long been appreciated in applied mathematics. The field of linear algedra 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.