Allen-Bradley LOGIX 5000 Reference Manual page 93

Process Control Instructions
PV Rate-of-Change Alarming
PV rate-of-change (ROC) alarming compares the change in the value of PV over
the PVROCPeriod against the PV positive and negative rate-of-change limits. The
PVROCPeriod provides a type of deadband for the rate-of-change alarm. For
example, if you use a ROC alarm limit of 2°F/second with a period of execution of
100 ms, and an analog input module with a resolution of 1°F, then every time the
input value changes, a ROC alarm is generated because the instruction sees a rate
of 10°F/second. However, by entering a PVROCPeriod of at least 1 sec, the ROC
alarm is only generated if the rate truly exceeds the 2°F/second limit.
The ROC calculation is only performed when the PVROCPeriod has expired.
The rate-of-change is calculated as:
ElapsedROCPeriod = ElapsedROCPeriod + ElapsedTimeSinceLastExecution
If ElapsedROCPeriod ≥ PVROCPeriod then:
This value:
PVROC
PVROC
N-1
ElapsedROCPeriod
Once PVROC has been calculated, the PV ROC alarms are determined as follows:
(1) During instruction first scan, the instruction clears the PV ROC alarm
outputs. The instruction also clears the PVROC alarm outputs and disables the
PV ROC alarming algorithm when PVFaulted is set.
Rockwell Automation Publication 1756-RM006K-EN-P - November 2018
Is:
PVROC = PV
N-1
ElapsedROCPeriod = 0
Chapter 1
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