Rockwell Automation 1771-PD PID MODULE (+DU) - Manual (Page 128)

Programming
Chapter 3
3Ć85
Figure 3.29
Cascade Control
Process
PID
SP1 E
PV1
BIAS1
V
11134
PID
BIAS2
E SP2
PV2
PV1 is the input to the primary, outer, or master loop.
PV2 is the input to the secondary, inner, or slave loop.
Feedforward Control
Feedforward control is shown in Figure 3.30. A disturbance is fed
forward to the PID module to change the output before the disturbance
changes the process variable. The PID module can perform one or more
of the following functions on the feedforward input: square root, lead/lag,
multiplication by a gain constant, and add an offset. Feedforward is often
used to control processes with transportation lag.
Figure 3.30
Feedforward Control
Process
PID
SP E
PV
BIAS
V
11135
FF
FF represents the Feedforward Control algorithm
Disturbance