Rockwell Automation 1756-OF8I ControlLogix Eight-channel Isolated Analog I/O Modules - Manual (Page 74)

74 Rockwell Automation Publication 1756-UM540A-EN-P - May 2014
Chapter 4 1756-IRT8I Combined Temperature-sensing Isolated Analog Module
Rate Alarm
The rate alarm triggers if the rate of change between input samples for each
channel
exceeds the specified trigger point for that channel. The actual rate of
change for the last sample is returned in the Ch[x].RateOfChange input tag of
each channel.
To see where to set the Rate Alarm, see page 131 .
Sensor Offset
The sensor offset value compensates for any known error on the sensor or
channel to which the sensor is connected. The value is set in signal units.
You set this value via a module output tags. That is, tag O.Ch[x].SensorOffset.
Where x represents the module channel.
EXAMPLE In normal scaling in Celsius, if you configure a channel’s rate alarm to
100.1 °C/s, the rate alarm triggers only if the difference between measured
input samples changes at a rate > 100.1 °C/s.
Consider the following conditions:
• The module’s RPI is 100 ms, that is, new data is sampled every 100 ms.
• At input sample #1, the channel measures 355 °C.
• At input sample #2, (100 ms later) the channel measures 363 °C.
At this sample instance, the rate alarm is not triggered because the
rate of change is less than 100.1 °C/s.
The rate of change is 80 °C/s [(363 °C- 355 °C) / (100 ms)].
• At input sample #3 (100 ms later) the channel measures 350.3 °C.
At this sample instance, the rate alarm is triggered because the rate
of change is greater than 100.1 °C.
The rate of change is 127 °C. [(350.3 °C - 363 °C) / (100 ms)].
At this sample instance, the absolute value of this result is > 100.1 °C,
so the rate alarm sets. Absolute value is used because rate alarm checks
for the magnitude of the rate of change being beyond the trigger point,
whether a positive or negative excursion.