Control variable: is a process variable that is measured and/or controlled by a control system.

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Is the amount or condition modified by the controller in order to affect the controlled variable. These affect the course of the process and can be measured and changed at will.

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For example: 
– The steam flow in the heater 
– The input composition in a reaction system.

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Controlled variable: It is the direct variable to be controlled, on which an inspection is constantly carried out. 

is the amount or condition that is measured and controlled. Normally the controlled variable is the output of the system and changes with the progress of the process. 

For example: 

– output temperature of the process current in the heater 

– The output composition in a reaction system.

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Uncontrolled variable: Is the one that modifies to directly affect the controlled variable. It is the amount that is charged to vary the final instruments of control. 

Is a signal that tends to adversely affect the output value of the system. These directly affect the course of the process but cannot be changed at will.

 

For example: 
– Sudden change in flow of input in a reaction system. 

Disturbances may be: 
– internal disturbances: When generated within the 
system – external disturbances: When generated outside the system and constitutes an input.

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Variables and perturbations:

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Intermediate Variables. They are variables related to the course of the process only indirectly. For example, the steam temperature in the heating tank or the cooling water temperature in a reaction system.

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Parameters. Are the variables that take a fixed value during the process. For example, the operating pressure in a reactor. 

Control. It means measuring the value of the controlled variable of the system and applying the manipulated variable to the system to correct or limit the deviation of the measured value, relative to the desired value.