Fig:Heat Exchanger

Here's how a heat exchanger shown in the above diagram works.
Here CO is the controller output,PV is the process variable i.e temperature of the water.The control valve on the cooling liquid line respond to the temperature variations of the water at the output of the heat exchanger.As the temperature at the outlet begins to rise the controller responds and signals the control valve to open more and let the pressure to drop.

This is how a Control System works.

The working of a controller is achieved through PID.We would soon explore the world of Confusing PID'S in our following posts.


    STAY TUNED.......

What is a CASCADE control loop.?
To shield the  effect of disturbances as in feedback control loop,we have a CASCADE control loop.
A CASCADE control loop is a loop within a loop, Primary loop and a Secondary loop.The Primary loop acts first as soon as it experiences some kind of disturbance before being fedback to the main controller.Hence this provides a corrective action on receiving disturbance and neccessary action is then taken.You can see the difference between the feedback control and the cascade control loop in the block diagram below.Feedback control has only one loop through which the error (SP-PV) is feed back to the controller,the controller output(CO) then drives the final control element which changes the Manipulated variable(MV) and the desired process variable(PV) is then obtained.

Cascade control loop as said has two loop primary and the secondary,the disturbance added in the process produces error and this error signal is fed back to the secondary controller and this is how a corrective action is already taken.This corrective action output is fed to the primary controller and the process continues.

Fig 1.1 .Cascade Loop.

Fig1.2.A feed back Control.

What is a PID?
A Process to work in Automatic mode must be properly tuned for optimum performance.If the parameters tuned are incorrect ,then there would be wastage of resources.The Process we dealt with in the previous page (i.e heat exchanger) forms the base of process control.More complex loops are present in addition to feedback control..It is called feedback control because the feedback control is fedback & an error is calculated as SP-PV and fed to the controller before being fedback to the final control element(FCE).

The feedback control suffers from a disadvantage,its DISTURBANCE affects the process. To shield the effects of disturbances as in feedback control we have CASCADE control loop.

What is a Process?
Well,I have the simplest of the definations.A Process Control is the act of controlling Final Control Element (FCE) such as a Control valve which changes the manipulated variable to control a process value.

Terms related to Process Control.
1.Controller Output(CO):It is the Output of Controller.i.e Setpoint(SP)-Process value(PV).

2.Final Control Element (FCE):Final Control Element controls the process variable such as control valve through is behaviour.

3.Process Variable(PV):The Parameter to be measured.

An Example of  heat exchanger will let you understand  the above terms easily.