Automatic Control Systems

!±8± Automatic Control Systems

With the development of technology, man has learnt to use very reliable and accurate systems that require least manpower. Automatic control systems are one such development. Control is defined as the science of regulation of a parameter by comparing it with a standard value. This is the aim of an automatic control system. They constantly monitor the output of system and if this output is found to deviate from the desired value, it produces a control signal that server to bring down the change by providing a driving energy to the component that is responsible for the deviation.

A system is said to be made of a number of components such that the behavior of the overall combination can be predicted if

1. The behavior of the each component can be predicted, and
2. The interaction between each component is known.

Hence, a system is obtained when a number of components are connected in a sequence to perform a specific function. Suppose in a system, the output quantity is controlled by altering the input quantity, and then such a system is called control system.

The output quantity is called the controlled variable and the input quantity is called the input signal. Automatic control systems have become an integral part of modern manufacturing and industrial processes.

Examples:

-Manufacturing industry: numerical control of machine tool.
-Industrial process: control of pressure, temperature and flow.

Basic control system definitions

Controlled variable:

The quality or condition characterizing a process whose value is held constant by a controller or is changed according to a certain algorithm designed with the interests of the nature of the function the system is performing.

Controlled medium:

The process material in the control system in which the variable to be controlled exists.

Command

An input that is established or varied by some means external to and independent of the feedback system.

Set point or Reference input:

A signal established as a standard of comparison for a feedback control system by virtue of it's relation to command. The set point either remains the same or is varied with respect to time depending on a preset algorithm.

Actuating signal:

An algebraic sum of reference input and primary feedback. It is also called error or control signal.

Manipulated variable:

The quality or condition that is varied as a function of the actuating signal so as to change the value of the controlled variable.

Primary feedback signal:

The function of the controlled output which is compared to reference input to obtain the actuating signal.

Error detector:

An element that detects feedback; essentially a summing point which gives the algebraic sum of its inputs.

Disturbance:

An unwanted variable in the system which tends to affect the system adversely by changing the controlled variable. Disturbance may due to change in set point, supply, demand, environment and the other associated variables.

Feedback element:

An element of the feedback control system which establishes a functional relationship between controlled variable and feedback signal.

Requirements of a Control System

Stability, accuracy and speed of response are the three requirement s of a control system.

Stability:
A system is to be stable if the output of the system after fluctuations, variation or oscillation, if any, settles at a reasonable value for any change in input or change in disturbance.

Accuracy:
A system is said to be 100 percent accurate if the error (different between input and output) is zero. An accurate system is costly. There is no point in going for 100 percent accurate system when that much of accuracy is not really required.

Example of accuracy:
When a variation of say 0.2 degree centigrade cannot be sensed by a human being, there is no need to have a home heating system of temperature variation equal to zero.

Speed of Response
This refers to time taken by the system to respond to the given input and give that as the output. Theoretically the speed of response should be infinity, that is, the system should have an instantaneous response. This requirement is prime concern with follow-up systems.

Any ideal system is perfectly stable, 100 percent accurate and has instantaneous speed of response. Unfortunately, the requirements are incompatible. Hence there should be a compromise between these requirements.