West Control Solutions N4400 - Manual (Page 35)

9.2 BASE MODE DISPLAYS
Once the complete cycle of Controller Define Mode parameters has been displayed, the
user may then step through the Base Mode displays (controller setpoint - alarm status -
Self Tune - Pre-Tune), making adjustments where required, before re-starting the
Controller Define Mode parameter cycle.
9.3 LOOP ALARM AND LOOP ALARM TIME
The Loop Alarm is a special alarm which detects faults in the control feedback loop by
continuously monitoring process variable response to the control output(s).
The Loop Alarm facility, when enabled, repeatedly checks the control output(s) for
saturation i.e. either or both outputs being at the maximum or minimum limit. If an output is
found to be in saturation, the Loop Alarm facility starts a timer; thereafter, if the saturated
output has not caused the process variable to be corrected by a pre-determined amount V
after a time T has elapsed, the Loop Alarm goes active. Subsequently, the Loop Alarm
facility repeatedly checks the process variable and the control output(s). When the process
variable starts to change value in the correct sense or when the saturated output comes out
of saturation, the Loop Alarm is de-activated.
For PID control, the Loop Alarm Time T is always set to twice the value of the Reset
(Integral Time Constant) parameter. For On/Off control, the user-defined value of the Loop
Alarm Time parameter is used.
The value of V is dependent upon the input type:
o
C ranges: 2
o
C or 2.0
o
C
o
F ranges: 3
o
F or 3.0
o
F
Lin ear ranges: 10 least sig nifi cant dis play units
For single output Controllers, the saturation limits are 0% and Output Power Limit. For dual
output Controllers, the saturation limits are –100% and Output Power Limit.
NOTES
1. Correct operation of the Loop Alarm depends upon reasonably
accurate PID tuning.
2. The Loop Alarm is automatically disabled during Manual Control
Mode and during execution of the Pre-Tune facility. Upon exit from
Manual Control Mode or after completion of the Pre-Tune routine,
the Loop Alarm is automatically re-enabled.
When full ON/OFF con trol is se lected (i.e. Pro por tional Band 1 is set to 0) and Loop Alarm
is en abled, the Loop Alarm Time pa rame ter de ter mines the du ra tion of the satu ra tion
con di tion af ter which the Loop Alarm will be ac ti vated. It may be ad justed within the range 1
sec ond to 99minutes 59 sec onds. This pa rame ter is omit ted from the dis play se quence if
ON/OFF con trol is not se lected or Loop Alarm is dis abled. The de fault set ting is 99:59.
59215-9 Page 9-10
59215