PID Loop Operation
Maintenance
and Troubleshooting
8–24
PID Loop Operation
Since the modes Manual, Auto, and Cascade are the most fundamental and
important PID loop controls, you may want to “hard-wire” mode control switches to
an operator’s panel. Most applications will need only Manual and Auto selections
(Cascade is used in a few advanced applications). Remember that mode controls
are really mode request bits, and the actual loop mode is indicated elsewhere.
The following figure shows an operator’s panel using momentary push-buttons to
request PID mode changes. The panel’s mode indicators do not connect to the
switches, but interface to the corresponding data locations.
PID Mode 1 Setting V+00
013456789101112131415 2Bit
Operator’s Panel
Loop Mode and Alarm Status V+06
013456789101112131415 2Bit
Mode Request Mode Monitoring
Auto
Cascade
Manual
If you have selected the option for the loops to follow the PLC mode, the PLC modes
(Program, Run) interact with the loops as a group. The following summarizes this
interaction:
S When the PLC is in Program Mode, all loops are placed in Manual Mode
and no loop calculations occur. However, note that output modules
(including analog outputs) turn off in PLC Program Mode. So, actual
manual control is not possible when the PLC is in Program Mode.
S The only time the CPU will allow a loop mode change is during PLC run
Mode operation. As such, the CPU records the modes of all 16 loops as
the desired mode of operation. If power failure and restoration occurs
during PLC Run Mode, the CPU returns all loops to their prior mode
(which could be Manual, Auto, or Cascade).
S On a Program-to-Run mode transition, the CPU forces each loop to
return to its prior mode recorded during the last PLC Run Mode.
S You can add and configure new loops only when the PLC is in Program
Mode. New loops automatically begin in Manual Mode.
In normal conditions the mode of a loop is determined by the request to V+00, bits 0,
1, and 2. However, some conditions exist which will prevent a requested mode
change from occurring:
S A loop that is not set independent of PLC mode cannot change modes
when the PLC is in Program mode.
S A major loop of a cascaded pair of loops cannot go from Manual to Auto
until its minor loop is in Cascade mode.
In other situations, the PID loop controller will automatically change the mode of the
loop to ensure safe operation:
S A loop which develops an error condition automatically goes to Manual.
S If the minor loop of a cascaded pair of loops leaves Cascade Mode for
any reason, its major loop automatically goes to Manual Mode.
Operator Panel
Control of
PID Modes
PLC Modes’ Effect
on Loop Modes
Loop Mode
Override
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