It is a strategy which compensates for specific disturbances at source and largely
Prevents them from affecting process being controlled.
A cascade control scheme has two controllers, the output of the master controller being used to adjust the set point of the slave controller
The effect of steam pressure disturbances can be compensated for using a slave The effect of steam pressure disturbances can be compensated for using a slave flow control loop.
The set point of the flow control loop u
M is manipulated by the level controller output. If the level h is too low the set point of the flow loop will be increased, and vice versa. The flow loop controls the flow of water F w agaiinst this set point in the normal way. Again, suppose that the pressure in the drum Ps suddenly increases, perhaps due to a transient drop in steam load F perhaps due to a transient drop in steam load Fs
, causing the flow of water to causing the flow of water to decrease. The flow loop will respond quickly by opening the valve to maintain the water flow water flow F w at the rate demanded by the level controller In effect the flow loop is at the rate demanded by the level controller. In effect, the flow loop is insulating the level loop from, or rejecting disturbances due to, changes in steam pressure. Temperature control of a reactor or tank via flow of a heating medium (or cooling medium):
TICreactor → FICheat medium → Valve heat medium
Temperature control of the inner reactor temperature via slave controller for the jacket temperature, sometimes with subordinated controller for heating- and cooling medium flow (multi cascade!):
TICreactor → TICjacket → FICheat medium → Valve heat medium
Often in combination with split-range at the slave controller because of heating and cooling.