ABB COMMANDER 351 User Manual page 18

...2 OPERATOR LEVEL
2.4 Analog Backup (Templates 5 and 6)
Note. Refer also to Appendix A2.1 – Series and Parallel Operation.
The analog backup station provides a backup for a master controller. In normal operation (remote
control mode selected) the COMMANDER 350's current output follows the master controller's output
value. A fault in the master system can be identified either by detecting a low signal on the master
output (template 5) or via a digital signal (template 6). When a fault is detected the COMMANDER
350 switches into local control mode and the process is controlled by the PID output of the
COMMANDER 350. The COMMANDER 350 PID algorithm tracks the master output value
continuously, in order to ensure bumpless transfer from remote to local mode operation. When the
master output is restored or the digital input returns to its inactive state, the COMMANDER 350
switches back to remote control mode.
Note. The Alarm A1 Trip value must be set when using template 5.
Local Set Point
LSPt
Process Variable
I/P1
Master Output
I/P2
Digital Select
di1
•1 Template 5 Only
•2 Template 6 Only
16
Manual Output
CSPt
PID
Control Loop
PV
Low Signal Select
(Alarm 1)
L
L R
Fig. 2.4 Analog Backup Station
Analog Output
ao1
•1
Local/
•2
Remote
Select