User manual
PID Loop Operation
(DL450 Only)
Maintenance
8--51
PID Loop Operation (DL450 only)
DL405 User Manual, 4th Edition, Rev. A
Using Other PID Features
The
f
irst three bits o
f
thePIDMode1word
V+00 requests the operating mode of the
corresponding loop. Note: These bits are
mode change requests, not commands
(certain conditions can prohibit a
particular mode change -- see next page).
PID Mode 1 Setting V+00
013456789101112131415 2Bit
Automatic
Cascade
Manual
The normal s tate of these mode request bits is “000”. To request a mode change, you
must SET the corresponding bit to a “1”, for one scan. The PID loop controller
automatically resets the bits back to “000” after it reads the mode change request.
Methods of requesting mode changes are:
S
DirectSOFT’s PID View -- this is the easiest method. Use the drop--down
menu, or click on one of the radio buttons if using older DirectSOFT versions,
and the appropriate bit will be set.
S Ladder program-- ladder logic can request any loop mode when the PLC is in
Run Mode. This will be necessary after application startup.
Use the rung shown to the right to SET the
mode bit on (do not use an out coil). On a
0--1 transition of X0, the rung sets the Auto
bit = 1. The loop controller resets it.
X0
SET
B2000.1
Go to Auto Mode
S Operator panel -- interface the operator’s panel to ladder logic using standard
methods, then use the technique above to set the mode bit.
Since we can only request mode changes, the PID loop controller decides when to
permit mode changes and provides the loop mode status.It reports the current mode
on bits 0, 1, and 2 of the Loop Mode and Alarm Status word, location V+06 in the loop
table. The parallel request / monitoring functions are shown in the figure below. The
figure also shows the mode -dependent two possible SP sources, and the two
possible Control Output sources.
Process Variable
Σ
Error Term
+
--
Input from Operator
Control Output
Setpoint
Manual
Auto/Cascade
Cascade
Auto/Manual
PID Mode 1 Setting V+00
013456789101112131415 2Bit
Automatic
Cascade Manual
Control Output
from another loop
Normal Source
Loop
Calculation
PID Mode
Control
Mode Select
Loop Mode and Alarm Status V+06
013456789101112131415 2Bit
Automatic
Cascade Manual
Mode Request
Mode Monitoring
How to Change
Loop Modes