Introduction

This chapter discusses the design and application of the process control
computers used in industry. The topics covered include the history of process
control computers, the basics of the computers used for process control,
and the characteristics of programmable logic controllers.

History of Process Control Computers

Before the introduction of computers for industrial process control applications,
the standard industrial control system consisted of many single-loop
analog controllers, either pneumatic or electronic, as well as their
associated field instruments and control devices. This stand-alone method
of control provided, and still provides, excellent control of industrial processes.
The main disadvantages of stand-alone controllers are that they
cannot be easily reconfigured and cannot easily communicate with other
plant computers.

Although computer designers long predicted that digital computers
would be used for process control applications, the first practical digital
computer-based control system was designed for the U.S. Air Force by
Hughes Aircraft Company. Using a computer called the DIGITAC, this
system controlled an airplane in flight through computed set-point
changes to an analog-based automatic pilot. This system was flown successfully
in 1954.

The losing company in the competition for the Air Force contract was
Ramo-Wooldridge and its computer, the RW-30. Ramo-Wooldridge
decided to market its computer as a process control computer, repackaging
it as the RW-300. This was a very important entry in the early history
of process computers. It is interesting to note that computer control developed
not as an initiative of the process and manufacturing industries but
as a result of computer and electronics vendors efforts to expand their
markets beyond military applications.

Introduction

This chapter discusses the design and application of the process control
computers used in industry. The topics covered include the history of process
control computers, the basics of the computers used for process control,
and the characteristics of programmable logic controllers.

History of Process Control Computers

Before the introduction of computers for industrial process control applications,
the standard industrial control system consisted of many single-loop
analog controllers, either pneumatic or electronic, as well as their
associated field instruments and control devices. This stand-alone method