Modern control practices.
There are various cases in industrial control
practice in which theoretical automatic control methods
are not yet sufficiently advanced to design an automatic
control system or completely to predict its effects.
This situation is true of the very large, highly
interconnected systems such as occur in many industrial
plants. In this case, operations research, a
mathematical technique for evaluating possible
procedures in a given situation, can be of value.
In determining the actual physical control system to
be installed in an industrial plant, the instrumentation
or control-system engineer has a wide range of possible
equipment and methods to use. He may choose to use a set
of analogue-type instruments, those that use a
continuously varying physical representation of the
signal involved--i.e., a current, a voltage, or
an air pressure. Devices built to handle such signals,
generally called conventional devices, are capable of
receiving only one input signal and delivering one
output correction. Hence they are usually considered
single-loop systems, and the total control system is
built up of a collection of such devices. Analogue-type
computers are available that can consider several
variables at once for more complex control functions.
These are very specific in their applications, however,
and thus are not commonly used.
The number of control devices added to an industrial
plant may vary widely from plant to plant. They may
comprise only a few instruments that are used mainly as
indicators of plant-operating conditions. The operator
is thus made aware of off-normal conditions and he
himself manually adjusts such plant operational devices
as valves and speed regulators to maintain control. On
the other hand, there may be devices of sufficient
quantity and complexity so that nearly all the possible
occurrences may be covered by a control-system action
ensuring automatic control of any foreseeable failure or
upset and thus making possible unattended control of the
process.
With the development of very reliable models in the
late 1960s, digital computers quickly became popular
elements of industrial-plant-control systems. Computers
are applied to industrial control problems in three
ways: for supervisory or optimizing control; direct
digital control; and hierarchy control.
In supervisory or optimizing control the computer
operates in an external or secondary capacity, changing
the set points in the primary plant-control system
either directly or through manual intervention. A
chemical process, for example, may take place in a vat
the temperature of which is thermostatically regulated.
For various reasons, the supervisory control system
might intervene to reset the thermostat to a different
level. The task of supervisory control is thus to
"trim" the plant operation, thereby lowering
costs or increasing production. Though the overall
potential for gain from supervisory control is sharply
limited, a malfunction of the computer cannot
adversely affect the plant.
In direct-digital control a single digital computer
replaces a group of single-loop analogue controllers.
Its greater computational ability makes the substitution
possible and also permits the application of more
complex advanced-control techniques.
Hierarchy control attempts to apply computers to all
the plant-control situations simultaneously. As such, it
requires the most advanced computers and most
sophisticated automatic-control devices to integrate the
plant operation at every level from top-management
decision to the movement of a valve.
The advantage offered by the digital computer
over the conventional control system described earlier,
costs being equal, is that the computer can be
programmed readily to carry out a wide variety of
separate tasks. In addition, it is fairly easy to change
the program so as to carry out a new or revised set of
tasks should the nature of the process change or the
previously proposed system prove to be inadequate for
the proposed task. With digital computers, this can
usually be done with no change to the physical equipment
of the control system. For the conventional control
case, some of the physical hardware apparatus of the
control system must be replaced in order to achieve new
functions or new implementations of them.
Control systems have become a major component of the
automation of production lines in modern factories.
Automation began in the late 1940s with the development
of the transfer machine, a mechanical device for moving
and positioning large objects on a production line (e.g.,
partly finished automobile engine blocks). These
early machines had no feedback control as described
above. Instead, manual intervention was required for any
final adjustment of position or other corrective action
necessary. Because of their large size and cost, long
production runs were necessary to justify the use of
transfer machines.
The need to reduce the high labour content of
manufactured goods, the requirement to handle much
smaller production runs, the desire to gain increased
accuracy of manufacture, combined with the need for
sophisticated tests of the product during manufacture,
have resulted in the recent development of computerized
production monitors, testing devices, and
feedback-controlled production robots. The
programmability of the digital computer to handle
a wide range of tasks along with the capability of rapid
change to a new program has made it invaluable for these
purposes. Similarly, the need to compensate for the
effect of tool wear and other variations in automatic
machining operations has required the institution of a
feedback control of tool positioning and cutting rate in
place of the formerly used direct mechanical motion.
Again, the result is a more accurately finished final
product with less chance for tool or manufacturing
machine damage.
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