Allen-Bradley 1444-TSCX02-02RB User Manual page 334

Appendix A CIP Objects
334
Phase Data
When the SR_mAG_PHASE bit of the SpecialRequest byte is set the phase
data of the FFT, meaningful or not, is returned following the linear FFT data.
However...
Selected sample data for any "Live" TWF/FFT always starts at the nearest
sample to a tacho event irrespective of how the data is sampled (synchronous or
asynchronous). Therefore, there is usable phase from both synchronous or
asynchronously sampled data. In either case, it must (reasonably) agree with
the tracking filter order phase - all it needs is the once per revolution signal.
On a TWF (if the signal is a simple 1x sinusoid), the phase is visible as the
angle from the start of the trace to the first positive peak. On an FFT phase,
values for every bin/line are provided.
The Dynamic module incorporates phase corrections for any filter on the
alternate path and for the main path LP filter. However, the Primary Path HP
filter is non-linear phase and cannot practically be corrected for. Therefore, the
POST FILTER data source must be avoided if phase data is important (to
capture) and the HP filter is enabled (dependent on Measurement Type).
Asynchronous Measurements
Determine the number of lines that are in the FFT – the number of lines
configured (Nc). You must first consider if the PHASE SPECTRA is also
included in the returned data. Bit 0 of the ucDataSelect byte of the FFT data
header record indicates the returned data. Calculate the number of lines read
(Nr) as follows:
If bit 0 of ucDataSelect is set, then:
Otherwise:
Rockwell Automation Publication 1444-UM001D-EN-P - June 2018
ByteCount
Nr =
8
ByteCount
Nr =
4