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Alarm hysteresis is the deadband on the safe side of an alarm (that is, the side
of the alarm that is below the high alarm value or above the low alarm value).
The signal must pass through this deadband before the alarm deactivates.
Alarm Inhibit prevents unwanted process or deviation alarm activation at
power-up or when the controller setpoint is changed. The alarm activation is
inhibited until a safe condition is present. The alarm operates normally from
that point on.
For example, when the alarm is inhibited, a low alarm will not activate at
power-up until the process value first rises above the alarm point than falls
back below.
A loop alarm detects faults in the control feedback loop by continuously
monitoring process variable response to the control output(s). If one of the
five alarms is defined to be a loop alarm, it repeatedly checks if the PID
control output is at saturation. If saturation is reached (0% or 100% power for
single control type), an internal timer is started. Thereafter, if the output has
not caused the process variable to be corrected by a predetermined amount
(V) after time (T) has elapsed, the alarm becomes active.
Subsequently, the alarm repeatedly checks the process variable and the PID
output. When the process variable starts to change value in the correct sense
or when the PID output is no longer at the limit, the alarm is deactivated.
For PID control, the loop alarm time (T) can be automatic (twice the Integral
Time value) or set to a user defined value. Correct operation with the
automatic loop alarm time depends upon reasonably accurate PID tuning. The
user defined value is always used for On-Off control, and the timer starts as
soon as an output turns on.
The value of V depends on the input type. For Temperature inputs, V = 2°C or
3°F. For Linear inputs, V = 10 x LSD (Least Significant Digit—smallest
incremental value that can be show at the defined display resolution).
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