3. Controllers and controller types

3.3 Cascade control

This special type of control using an auxiliary controlled variable uses two controllers connected in series.

The control system is made up of a primary or master controller and a secondary or slave controller. At the input of the master controller, the controlled variable x1 is compared with the reference value w1. The controller output variable y from the master controller, the controller output variable y1, acts as the reference value w2 for the slave controller and is compared with the auxiliary controlled variable x2 at its input.

The aim of dividing the control loop into a slave control loop and a master control loop is to improve the quality of the control of x1. In the event of a disturbance, a control process is already triggered by the change of x2 at an earlier time via the slave controller, thus supporting the overall control.

In general, control dynamics can be improved by introducing subordinate control loops into the control loop.

Block diagram of cascade control