Rockwell Automation 1771-PD PID MODULE (+DU) - Manual (Page 127)

Programming
Chapter 3
3Ć84
Figure 3.28
Closed Loop Control
Process
PID
SP E
PV
BIAS
V
V = Controll Variable (Analog Output)
SP = Set Point
PV = Process Variable (Analog Input)
E = Error
11133
Cascade Control
Cascade control is shown in Figure 3.29. There are two separate PID
loops. The output of loop 1 is cascaded into the setpoint of loop 2. To
enable direct cascade, set B00 = 1 in word W30(W50). Loop 1 is called
the outer loop, master loop or primary loop. Loop 2 is called the inner
loop, slave loop or the secondary loop. The inner loop senses a change
and compensated for it before the outer loop is affected. This type of
control is used to reduce the response time of the secondary loop when
controlling a primary loop with large inertia.