PNI FieldForce Trax - Manual (Page 60)

PNI Sensor Corporation DOC#1016505 r06
TRAX User Manual Page 56
7.7.2 FIR Filters
When operating in Compass Mode, the TRAX incorporates a finite impulse response
(FIR) filter to provide a more stable heading reading. The number of taps (or samples)
represents the amount of filtering to be performed. The number of taps directly affects
the time for the initial sample reading, as all the taps must be populated before data is
output. The FIR filter settings have no affect when operating in AHRS Mode.
The TRAX can be configured to clear, or flush, the filters after each measurement, as
discussed in Section 7.5.2. Flushing the filter clears all tap values, thus purging old data.
This can be useful if a significant change in heading has occurred since the last reading,
as the old heading data would be in the filter. Once the taps are cleared, it is necessary to
fully repopulate the filter before data is output. For example, if 32 FIR taps is set, 32 new
samples must be taken before a reading will be output. The length of the delay before
outputting data is directly correlated to the number of FIR taps.
kSetFIRFilters (frame ID 12
d
)
The payload for kSetFIRFilters is given below.
Byte 1 should be set to 3 and Byte 2 should be set to 1. The third payload byte
indicates the number of FIR taps to use, which can be 0 (no filtering), 4, 8, 16, or 32.
This is followed by the tap values (0 to 32 total Values can be in the payload), with
each Value being a Float64, and suggested values given in Table 7-6.
Table 7-6: Recommended FIR Filter Tap Values
Count 4-Tap Filter 8-Tap Filter 16-Tap Filter 32-Tap Filter
1 04.6708657655334e-2 01.9875512449729e-2 07.9724971069144e-3 01.4823725958818e-3
2 04.5329134234467e-1 06.4500864832660e-2 01.2710056429342e-2 02.0737124095482e-3
3 04.5329134234467e-1 01.6637325898141e-1 02.5971390034516e-2 03.2757326624196e-3
4 04.6708657655334e-2 02.4925036373620e-1 04.6451949792704e-2 05.3097803863757e-3
5 02.4925036373620e-1 07.1024151197772e-2 08.3414139286254e-3
6 01.6637325898141e-1 09.5354386848804e-2 01.2456836057785e-2
7 06.4500864832660e-2 01.1484431942626e-1 01.7646051430536e-2
8 01.9875512449729e-2 01.2567124916369e-1 02.3794805168613e-2
9 01.2567124916369e-1 03.0686505921968e-2
Byte 1
UInt8 UInt8
Payload
ID Specific ID Specific ID Specific ID Specific
Byte 2 Count N Value 2 Value 1 Value N Value 3
UInt8