Control system is that means by which any quantity of interest in a process/machine/mechanism/equipment
etc. can be maintained or altered in accordance with desired manner. For example; Temperature control
system (Air conditioner; AC), traffic light controller, washing machine, heating iron rod or immersion rod etc.
Control system is any collection of components forming a system configuration that have cause and effect
relationships among the system variable. It can also be defined as an interconnection of components that
will provide a desired system response.
There are mainly two types of control system.
i) Open Loop Control System(OLCS)
ii) Closed Loop Control System(CLCS)
According to history of control system, The first significant control device was James watt’s fly ball governor
was invented in 1767 to keep the speed of engine constant by regulating the supply of the steam to the
engine.
In 1922, Minorsky developed automatic controller for ship steering. He also developed that behavior of any
control system can be described with the help of set of differential equations and so as its ability.
In 1932, Nyquist confirmed that stability of close loop control system can be defined in easy way by observing
response of open loop system with steady state sinusoidal inputs.
In 1934, Hazen, who introduced the term Servo mechanism for positive control system, discussed the design
of relay servomechanisms capable of closely following a changing input.
In 1940, Frequency response technique using Bode diagram. Here, we design a linear closed loop control
system with related to specific performance. Many industrial control systems in 1940s used PID controllers
to control pressure, temperature, humidity etc.
In 1950, Root locus method for design and stability. Frequency response technique and root locus technique
for Single Input Single Output (SISO). The frequency-response and root-locus methods, which are the core of
classical control theory, lead to systems that are stable and satisfy a set of more or less arbitrary performance
requirements. Such systems are, in general, acceptable but not optimal in any meaningful sense. Since the
late 1950s, the emphasis in control design problems has been shifted from the design of one of many systems
that work to the design of one optimal system in some meaningful sense.
As modern plants with many inputs and outputs become more and more complex, the description of a modern
control system requires a large number of equations. Classical control theory, which deals only with singleinput, single-output systems, becomes powerless for multiple-input, multiple-output systems. Since about
1960, because the availability of digital computers made possible time-domain analysis of complex systems,
modern control theory, based on time-domain analysis and synthesis using state variables, has been
developed to cope with the increased complexity of modern plants and the stringent requirements on
accuracy, weight, and cost in military, space, and industrial applications
From 1960 to present, Complex control system is developing stage, In Automatic control system, the existing
values of a quantity or condition are measured and compare it with the desired value and the different of
these two values is used to initiate the action for reducing the difference. There are many advantages of
automatic control system such as cost of energy or power reduces, Cost of processing materials in industries
reduces, Quality of products improve, Productivity increases etc.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 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.
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.
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 . 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 labor 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.
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