Page 453 - Mechanical Engineers' Handbook (Volume 2)

444 Closed-Loop Control System Analysis

1 INTRODUCTION
The field of control has a rich heritage of intellectual depth and practical achievement. From
the water clock of Ctesibius in ancient Alexandria, where feedback control was used to
regulate the flow of water, to the space exploration and the automated manufacturing plants
of today, control systems have played a very significant role in technological and scientific
development. James Watt’s flyball governor (1769) was essential for the operation of the
steam engine, which was, in turn, a technology fueling the Industrial Revolution. The fun-
damental study of feedback begins with James Clerk Maxwell’s analysis of system stability
of steam engines with governors (1868). Giant strides in our understanding of feedback and
its use in design resulted from the pioneering work of Black, Nyquist, and Bode at Bell
Labs in the l920s. Minorsky’s work on ship steering was of exceptional practical and theo-
retical importance. Tremendous advances occurred during World War II in response to the
pressing problems of that period. The technology developed during the war years led, over
the next 20 years, to practical applications in many fields.
Since the 1960s, there have been many challenges and spectacular achievements in
space. The guidance of the Apollo spacecraft on an optimized trajectory from the earth to
the moon and the soft landing on the moon depended heavily on control engineering. Today,
the shuttle relies on automatic control in all phases of its flight. In aeronautics, the aircraft
autopilot, the control of high-performance jet engines, and ascent/descent trajectory optim-
ization to conserve fuel are typical examples of control applications. Currently, feedback
control makes it possible to design aircraft that are aerodynamically unstable (such as the
X-29) so as to achieve high performance. The National Aerospace Plane will rely on ad-
vanced control algorithms to fly its demanding missions.
Control systems are providing dramatic new opportunities in the automotive industry.
Feedback controls for engines permit federal emission levels to be met, while antiskid brak-
ing control systems provide enhanced levels of passenger safety. In consumer products,
control systems are often a critical factor in performance and thus economic success. From
simple thermostats that regulate temperature in buildings to the control of the optics for
compact disk systems, from garage door openers to the head servos for computer hard disk
drives, and from artificial hearts to remote manipulators, control applications have permeated
every aspect of life in industrialized societies.
In process control, where systems may contain hundreds of control loops, adaptive
controllers have been available commercially since 1983. Typically, even a small improve-
ment in yield can be quite significant economically. Multivariable control algorithms are now
being implemented by several large companies. Moreover, improved control algorithms also
permit inventories to be reduced, a particularly important consideration in processing dan-
gerous material. In nuclear reactor control, improved control algorithms can have significant
safety and economic consequences. In power systems, coordinated computer control of a
large number of variables is becoming common. Over 30,000 computer control systems have
been installed in the United States alone. Again, the economic impact of control is vast.
Accomplishments in the defense area are legion. The accomplishments range from the
antiaircraft gunsights and the bombsights of World War II to the missile autopilots of today
and to the identification and estimation techniques used to track and designate multiple
targets.
A large body of knowledge has come into existence as a result of these developments
and continues to grow at a very rapid rate. A cross section of this body of knowledge is
collected in this chapter. It includes analysis tools and design methodologies based on clas-
sical techniques. In presenting this material, some familiarity with general control system
design principles is assumed. As a result, detailed derivations have been kept to a minimum
with appropriate references.