Page 33 - Modern Control Systems

Section 1.2 Brief History of Automatic Control 7

self-oscillations over very wide frequency bands when many people doubted such cir-
cuits would be stable? My confidence stemmed from work that I had done two years
earlier on certain novel oscillator circuits and three years earlier in designing the termi-
nal circuits, including the filters, and developing the mathematics for a carrier telephone
system for short toll circuits.
The frequency domain was used primarily to describe the operation of the feed-
back amplifiers in terms of bandwidth and other frequency variables. In contrast,
the eminent mathematicians and applied mechanicians in the former Soviet Union
inspired and dominated the field of control theory. Therefore, the Russian theory
tended to utilize a time-domain formulation using differential equations.
The control of an industrial process (manufacturing, production, and so on) by
automatic rather than manual means is often called automation. Automation is
prevalent in the chemical, electric power, paper, automobile, and steel industries,
among others. The concept of automation is central to our industrial society. Auto-
matic machines are used to increase the production of a plant per worker in order to
offset rising wages and inflationary costs. Thus industries are concerned with the
productivity per worker of their plants. Productivity is defined as the ratio of physi-
cal output to physical input [26]. In this case, we are referring to labor productivity,
which is real output per hour of work.
The transformation of the U.S. labor force in the country's brief history follows
the progressive mechanization of work that attended the evolution of the agrarian
republic into an industrial world power. In 1820, more than 70 percent of the labor
force worked on the farm. By 1900, less than 40 percent were engaged in agriculture.
Today, less than 5 percent works in agriculture [15].
In 1925, some 588,000 people—about 1.3 percent of the nation's labor force—
were needed to mine 520 million tons of bituminous coal and lignite, almost all of it
from underground. By 1980, production was up to 774 million tons, but the work
force had been reduced to 208,000. Furthermore, only 136,000 of that number were
employed in underground mining operations. The highly mechanized and highly
productive surface mines, with just 72,000 workers, produced 482 million tons, or 62
percent of the total [27].
A large impetus to the theory and practice of automatic control occurred during
World War II when it became necessary to design and construct automatic airplane
piloting, gun-positioning systems, radar antenna control systems, and other military
systems based on the feedback control approach. The complexity and expected per-
formance of these military systems necessitated an extension of the available con-
trol techniques and fostered interest in control systems and the development of new
insights and methods. Prior to 1940, for most cases, the design of control systems was
an art involving a trial-and-error approach. During the 1940s, mathematical and an-
alytical methods increased in number and utility, and control engineering became an
engineering discipline in its own right [10-12].
Another example of the discovery of an engineering solution to a control system
problem was the creation of a gun director by David B. Parkinson of Bell Telephone
Laboratories. In the spring of 1940, Parkinson was a 29-year-old engineer intent on
improving the automatic level recorder, an instrument that used strip-chart paper to
plot the record of a voltage. A critical component was a small potentiometer used to
control the pen of the recorder through an actuator.