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.
9-10
To Next Page
Loading...
Need help?
General
