Despatch MIC1422 Controller - Manual (Page 44)

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6.2 Loop Alarm Enable
This parameter is the means by which the user can enable or disable the Loop Alarm.
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, when enabled, repeatedly checks the control output(s) for being at the
maximum or minimum limit. If an output is found to be at the limit, the Loop Alarm
Mode starts a timer; thereafter, if the high output has not caused the process variable to
be corrected by a predetermined amount V after a time T has elapsed, the Loop Alarm
goes active. Subsequently, the Loop Alarm Mode 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 output comes below the limit, the Loop Alarm is
deactivated.
For PID control, the Loop Alarm Time T is always set to twice the value of the Auto
Reset parameter. For ON/OFF control, the user defined value of the Loop Alarm Time
Set Up parameter is used.
The value of V is dependent upon the input type:
Deg C: 2ºC or 2.0ºC
Deg F: 3ºF or 3.0ºF
Linear Range: 10 least significant display units
For single output instruments, the limits are 0% and Out 1 Max %. For dual
output instruments, the limits are -1 00% and Out 1 Max %.
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 Mode. Upon exit from Manual Mode or after completion
of the Pre-Tune routine, the Loop Alarm is automatically re-enabled.
6.3 Loop Alarm Time
When full ON/OFF control is selected and Loop Alarm is enabled, this parameter
determines the duration of the limit condition after which the Loop Alarm will be
activated. It may be adjusted within the range of 1 second to 99 minutes 59 seconds.
This parameter is omitted from the Tune Mode display sequence if ON/OFF control is
not selected or Loop Alarm is disabled. The default setting 99:59. Display code is LAti.