Automationdirect.com DL05, User Manual

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PID Loop Operation
Maintenance
and T
roubleshooting
8–52
PID Loop Operation
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 convention when
controlling loop modes. 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 DL05, and you may
have multiple groups of cascaded loops. For proper operation on cascaded loops
you must use the same data range (12/15 bit) and unipolar/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 location 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 SP XXXX
V+03 PV XXXX
V+32 Remote SP Pointer XXXX
V+02 SP XXXX
V+03 PV XXXX
V+05 Control Output XXXX V+05 Control Output XXXX
Loop Table
Major Loop (Auto mode) Minor Loop (Cascade Mode)
When using Direct SOFT32’s PID View to watch the SP value of the minor loop,
Direct SOFT32 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
S
+
–
Setpoint
Cascade
Auto/Manual
Control Output V+05
Local SP
Loop
Calculation
Control
Output
Loop
Calculation
Minor Cascaded loop Major 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 DL05 CPU