What is the purpose of a process loop? What is the controllers responsibily?

What is the purpose of a process loop? What is the
controllers responsibily?

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What is the purpose of a process loop? What is the controllers responsibily?..

The above question answer is explained by an example-
suppose we have to control the temperature of the water from "Heat Exchanger", for these we required a following instruments-
Temperature Transmitter=The transmitter sense the temperature of the boiler and convert these signal to standard signal 4-20 mAh.
Temperature Indicator and controller= The indicator use in the process has a facility of setting a SP(set point) for the desire process, these indicator with its inbuilt technology find out weather to increase the flow of steam or decrease the flow of steam. these condition depend on the logic E(error)=SP(set point)-PV(process variable).

suppose the output water has to maintain the temperature 200 degree cent. these is the SP to the controller.The controller will receive the signal from the temperature transmitter which is process variable(PV) and compare it with the SP i.e. let x be the PV(process value) then,
E=SP-PV
E=200-x
let x=150
then E=200-150
E=50
the value indicate that the inlet flow of the water should be OFF so that the outlet water temperature will reach to SP.

Note- Positive value OFF the inlet flow.
Negative value ON the inlet flow.

Temperature converter(I to P converter)= The signal send by the controller logically is converted into pneumatic by I to P converter with standard signal 3-15psi.
i.e. 3psi =4mAh &
15psi=20mAh respectively.

Valve= The valve can be air to open or air to close depend on the site selection and safety aspect.

*The controller responsibility is to maintain the process in SP and take the proper decision.
*Beside these was short or easy process so we can control it manually but when there is a large process parameter or it is not possible for a human to pay attention or also in hazardous area then at these place the controller play a vital roll in industries.