### Basics of Control System: Definations

To understand the meaning of the word control system, first we will define the word system and we will try to define the word control system.
System : A system is a combination or an arrangement of different physical components which act together as an entire unit to achieve certain objective.
Every physical object is actually a system. A classroom is a good example of physical system. A room along with the combination of benches, blackboard, fans, lighting arrangement etc. can be called a classroom which acts as an elementary system.
Another example of a system is a lamp. A lamp made up of glass, filament is a physical system. Similarly a kite made up of paper and sticks is an example of  physical system.
Similarly system can be of any type i.e. physical, ecological, biological etc.
Control system : To control means to regulate, to direct or to command. Hence a control system is an arrangement of different physical elements connected in such a manner so as to regulate, direct or command itself or some other system.
For example, if in classroom, professor is delivering his lecture, the combination becomes a control system as; he tries to regulate, direct or command the students in order to achieve the objective which is to impart good knowledge to the students. Similarly if lamp is switched ON or OFF using a switch, the entire system can be called a control system. The concept of physical and a control system is shown in the Fig. 1 and Fig.2.

When a child plays with the kite, he tries to control it with the help of string and entire system cen be considered as a control system.
In short, a control system I in the broadest sense, an interconnection of the physical components to provide a desired function, involving some kind of controlling action in it.
Plant : The portion of a system which is to be controlled or regulated is called the plant or the process.
Controller : the element of the system itself or external to the system which controls the plant or the process is called controller.
For each system, there must be an excitation and system accepts it as an input. And for analyzing the behavior of system for such input, it is necessary to define the output of a system.
Input : it is an applied signal or an excitation signal applied to a control system from an external energy source in order to produce a specified output.
Output : it is the particular signal of interest or the actual response obtained from a control system when input is applied to it.
Disturbances: Disturbance is a signal which tends to adversely affect the value of the output of the system. If such a disturbance is generated within the system itself, it is called an internal disturbance. The disturbance generated outside the system acting as an extra input to the system in addition to its normal input, affecting the output adversely is called an external disturbance.
Control system may have than one input or output. From the information regarding the system, it is possible, it is possible to well define ll the inputs and outputs of the systems.
The input variable is generally referred as the Reference Input and output is generally referred as the Controlled Output.
Cause and effect relationship between input and output for a plant can be shown as in the Fig.3.