In cascade control system two controllers are cascaded to each other as shown in the diagram below. Primary controllers output is fed to secondary controller as a Set point. Both the controllers has their own sensing elements but only secondary controller gives output to the control valve. Primary controller receives set point from operator
For example in Batch reactor temperature is controlled using Cascade control strategy. The output of temperature controller TC-1 adjusts the set point of TC-2 in the batch reactor control system. The primary variable batch temperature is controlled by the master controller, but not by directly adjusting the cooling water valve. Instead, it manipulates the set point of the slave or secondary controller, which in turn supplies jacket water at the asked for temperature. There are two controlled variables, one manipulated and one independent set point.
Cascade controller tuning -
1.First,tune the secondary controller (with the primary controller in manual mode)
2.Then,tune the primary controller (with the secondary controller in cascade mode)
Cascade control has two functions :
1. Reduce the effect on the total control system of the dynamic elements in the secondary loop.
2. Correct for disturbances which occur within the secondary loop before they affect the master loop (such as cooling water temperature changes).
The slave controller need have only proportional response. The major time constant must not be in the slave loop.
Cascade control is normally not applied in fast control loops - flow, pressure, etc. It is more useful in temperature or composition control systems.