User manual

PID Loop Operation
(DL450 Only)
Maintenance
8--29
PID Loop Operation (DL450 only)
DL405 User Manual, 4th Edition, Rev. A
In Cascade Mode, the loop operates just as in Automatic Mode, with one important
change. The data source for the SP changes from its normal location at V+02, using
the control output value from another loop (the purpose of cascading loops is
covered later in this chapter). So in Auto or Manual modes, the loop calculation uses
the data at V+02. In Cascade Mode, the loop calculation reads the control output
from another loop’s parameter table.
Process Variable
Σ
+
--
Setpoint
Cascade
Auto/Manual
Control Output V+05
Normal SP V+02
Loop
Calculation
Control Output
Loop
Calculation
Cascaded loopAnother loop
As pictured below, a loop can change from one mode to another, but cannot go from
Manual Mode to Cascade. This mode change is prohibited because a loop would be
changing two data sources at the same time, and could cause a loss of control.
Manual Automatic Cascade
When the CPU is operating in the Run Mode, the normal operation of the PID loop
controller is to read the loop data and perform calculations on each scan of the RLL
program. When the CPU is placed in the Program Mode, the RLL program halts
operation and all PID loops are automatically put into the Manual Mode. The PID
parameters can then be changed if desired. Similarly, by placing the CPU in the Run
mode, the PID loops are returned to the operational mode which they were
previously in, i.e., Manual, Automatic and Cascade. With this selection you
automatically affect the modes by changing the CPU mode.