Cascade Control Basics.

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.