Rockwell Automation 1771-PD PID MODULE (+DU) - Manual (Page 129)

Programming
Chapter 3
3Ć86
Cascade With Feedforward Control
Cascade with feedforward control is shown in Figure 3.31. The output of
loop 1 is cascaded into the setpoint of loop 2. Loop 2 feedforward
features are being used.
Figure 3.31
Cascade with Feedforward Control
Process
PID
SP1 E
PV1
BIAS1
V
PID
BIAS2
E SP2
PV2
11136
PV1 is the input to the primary, outer, or master loop.
PV2 is the input to the secondary, inner, or slave loop.
FF Disturbance
FF is the feedforward control algorithm
Decouple Control (Multi-Loop, Multi-Variable)
Decoupling is shown in Figure 3.32. Decoupling is also known as
multi-loop, multi-variable control. This type of control is required in
some applications were there is interaction between loops. A change in
loop 1 directly affects loop 2. Loop interaction is compensated by
forwarding output data from loop 1 into the feedforward/input of loop 2.
A change in the output of loop 1 adjusts the output of loop 2 without
affecting the loop 2 PID calculation. Decoupling and cascade control can
be used simultaneously. To enable decoupling, set W30(W50) B14 to 1.