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What is meant by cascade control?

Cascade control can significantly improve the control quality. This applies in particular to the dynamic action of the control loop, in other words, the transition of the process variable following setpoint changes or disturbances.

Example 1: schematic construction of a cascade

Chocolate has to be heated to vs = 40 °C for processing. The chocolate temperature must nowhere exceed 50 °C (even close to the heater). It is therefore heated on a water bath.
Cascade control is used in order to achieve rapid stabilisation.
Controller 1 is always the master controller, controller 2 always the slave.
The setpoint for the slave controller is produced by output conversion.
The control output y1 is converted to a setpoint using the unit of the process value x2 (here: 0 - 100 % = 0 - 50 °C).

P2_2_Q16_1_DE

List of symbols
O2 - Output 2
I1 - Analogue input 1
I2 - Analogue input 2
C1 - Controller 1
C2 - Controller 2
w1 - Setpoint controller 1
w2 - Setpoint controller 2
x1 - Process value controller 1
x2 - Process value controller 2
xw1 - Deviation controller 1
xw2 - Deviation controller 2
y1 - Control output 1
y2 - Control output 2; output 1 of controller 2
vs - Chocolate temperature
vw - Water bath temperature

Example 2: construction of a trimming cascade

Two charges of chocolate have to be heated to 40 °C and 50 °C. The chocolate temperature must nowhere (not even close to a heater) exceed the setpoint by more than 10 °C. It is therefore heated on a water bath.
Trim cascade control is used to achieve rapid stabilisation without overshoot and without altering the controller configuration (output conversion) at a change of setpoint (batch change).
Controller 1 is always the master controller, controller 2 always the slave controller.
The setpoint for the slave controller is produced by output conversion and the addition of the master controller setpoint (w1).
In setpoint conversion, the control output y1 is converted to a value with the unit of the process value w2. It corresponds to the maximum permitted temperature difference (± | x1 - w1 |; here: 0 - 100 % = -10 to +10 °C).


P2_2_Q16_2_DE

List of symbols

O2 - Output 2
I1 - Analogue input 1
I2 - Analogue input 2
C1 - Controller 1
C2 - Controller 2
w1 - Setpoint controller 1
x1 - Process value controller 1
x2 - Process value controller 2
xw1 - Deviation controller 1
xw2 - Deviation controller 2
y1 - Control output 1
y2 - Control output 2; output 1 o controller 2
vs - Chocolate temperature
vw - Water bath temperature