User manual

PID Loop Operation
(DL450 Only)
Maintenance
and Troubleshooting
8--64
PID Loop Operation (DL450 only)
DL405 User Manual, 4th Edition, Rev. A
In the use of the term “cascaded loops”, we must make an important distinction. Only
the minor loop will actually be in the Cascade Mode. In normal operation, the major
loop must be in Auto Mode. If you have more than two loops cascaded together, the
outer-most (major) loop must be in Auto Mode during normal operation, and all inner
loops in Cascade Mode.
NOTE: Technically, both major and minor loops are “cascaded” in strict process
control terminology. Unfortunately, we are unable to retain this c onvention when
controlling loop modes. Just remember that all minor loops will be in Cascade Mode,
and only the outer-most (major) loop will be in Auto Mode.
You can cascade together as many loops as necessary on the DL450, and you may
have multiple groups of cascaded loops. For proper operation on cascaded loops
you must use the s ame data range (12/15 bit) and polar/bipolar settings on the major
and minor loop.
To prepare a loop for Cascade Mode operation as a minor loop, you must program its
remote Setpoint Pointer in its loop parameter table l ocation V+32, as shown below.
The pointer must be the address of the V+05 location (control output) of the major
loop. In Cascade Mode, the minor loop will ignore the its local SP register (V+02),
and read the major loop’s control output as its SP instead.
Loop Table
V+02 SPXXXX
V+03 PVXXXX
V+32 Remote SP PointerXXXX
V+02 SPXXXX
V+03 PVXXXX
V+05 Control OutputXXXXV+05 Control OutputXXXX
Loop Table
Major Loop
(
A
uto mode
)
Minor Loop
(
Cascade Mode
)
When using DirectSOFT’s PID View to watch the SP value of the minor loop,
DirectSOFT automatically reads the major loop’s control output and displays it for
the minor loop’s SP. The minor loop’s normal SP location, V+02, remains
unchanged.
Now, we use the loop parameter arrangement above and draw its equivalent loop
schematic, shown below.
Process Variable
Σ
+
--
Setpoint
Cascade
Auto/Manual
Control Output V+05
Local SP
Loop
Calculation
Control
Output
Loop
Calculation
Minor Cascaded loopMajor loop
Remote SP
V+02
Remember that a major loop goes to Manual Mode automatically if its minor loop is
taken out of Cascade Mode.
Cascaded Loops in
the DL450 CPU