Despatch Protocol 3 Controller - Manual (Page 52)

W ORKING WITH O PERATING M ODES Protocol 3 Controller Owner’s Manual
52 Version 1.2
Copyright © 2013 by Despatch Industries.
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5.6.1.5. Glossary of Alarm Terms
Term Description
Alarm
Hysteresis 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
(Not functional
on Firmware
2.3 or lower)
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.
Input Signal
Break Alarm If the signal from the process value input is broken the process value will
display OPEN and the alarm will activate.
Loop Alarm 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).
% Memory
Used Alarm The Protocol 3 controller can record data at a set interval. If the percentage
selected of the memory capacity of the recorded data is exceeded the alarm will
activate.
Process Value
Alarms Three types of process alarms are implemented in the Protocol 3: High, Low,
Band. If the process variable passes the boundary, the alarm activates as
depicted in
Figure 19
PV-SP
Deviation
Alarm The control subtracts the process value from the setpoint. If the resulting value
exceeds the alarm value setting the alarm will activate.
Signal Change
Per Min Alarm An alarm based on the rate of change in the measured process variable. If the
PV changes at a rate greater than the alarm level per minute, the alarm will
activate. The rate of change must be above the alarm threshold for longer than
the Minimum Duration Of Change time, before the alarm will change state.
Caution: If the duration is less than this time, the alarm will not activate no
matter how fast the rate of rise.