Automationdirect.com DL05, User Manual

Page: 366 / 480

PID Loop Operation
Maintenance
and T
roubleshooting
8–22
PID Loop Operation
The DL05 PLC has the ability to run PID calculations while the CPU is in Program
Mode. Usually, a CPU in Program Mode has halted all operations. However, a DL05
PLC in Program Mode may or may not be running PID calculations (and providing
PID control output), depending on your configuration settings. Having the ability to
run loops independent of the ladder logic makes it feasible to make a ladder logic
change while the process is still running. This is especially beneficial for large-mass
continuous processes that are difficult or costly to interrupt.
Loops that run independent of the ladder scan must have the ability to directly
access the analog module channels for the PV and control output values. The loop
controller does have this capability, which is covered in the section on direct access
of analog I/O (located prior to this section in this chapter).
The relationship between CPU modes and loop modes is depicted in the figure
below. The vertical dashed line shows the optional relationship between the mode
changes. Bit 15 of PID Mode 1 setting word (V+00) determines the selection. If set to
zero so the loop follows the CPU mode, then placing the CPU in Program Mode will
force all loops into Manual Mode. Similarly, placing the CPU in Run mode will allow
each loop to return to the mode it was in previously (which includes Manual,
Automatic, and Cascade). With this selection you automatically affect the modes of
the loops by changing the CPU mode.
Manual Automatic Cascade
Mode change
0 = loop follows PLC mode
Mode change
Program Run
Mode change
CPU Modes:
Loop
Modes:
PID Mode 1 Setting V+00
0 1 3 4 5 6 7 8 9 10 11 12 13 14 15 2 Bit
Loop Mode Linking
1 = loop is independent
of PLC mode
If Bit 15 is set to one, then the loops will run independent of the CPU mode. It is like
having two independent processors in the CPU... one is running ladders and the
other is running the process loops.
NOTE : To make changes to any loop table parameter values , the PID loop must be
in Manual Mode and the PLC must be stopped. If you have selected (as shown
above) to operate the PID loop independent of the CPU mode, then you must take
certain steps to make it possible to make loop parameter changes. You can
temporarily make the loops follow the CPU mode by changing bit 15 to 0. Then your
programming device (such as
Direct SOFT32) will be able to place the loop into
Manual Mode. After you change the loop’s parameter setting, just restore bit 15 to a
value of 1 to re-establish PID operation independent of the CPU.
CPU Modes and
Loop Modes