Section 2
Functional Description
10
FT205EV Wind Sensors
– User Manual
2.4 Electronically Offsetting the Wind Direction Datum
For applications where the compass is not being used, the wind direction datum can be ‘electronically’ rotated
using the CF command. The declination angle function normally used in conjunction with the compass function
can be used to realign the datum position anywhere in the range 0-359.9°.
See Section 6.4.6 for further details.
2.5 Compass & Declination Angle Operation
The FT205EV is fitted with a solid-state electronic compass so that the wind direction can be measured relative
to magnetic North if required.
Since the FT205EV is intended for system integration it is necessary to calibrate the compass before operation
and testing is recommended. Refer to Section 6.4.6 for calibration software commands. Enabling the compass
will require approximately 5mA of additional current draw.
The wind direction can be measured either relative to magnetic North, or by programming the sensor with the
geographical location declination angle, relative to true North. The compass function can also be disabled if
required so that the wind direction is measured relative to the datum mark on the wind sensor body (as per Figure
8). The compass heading can also be read back at any time using the CF command. See Section 6.4.6 for full
details of the CF command. Both the Polar format (P) and NMEA 0183 format (N) outputs provide wind direction
data relative to North (if the compass function is enabled). The NMEA 0183 message also has a field that indicates
whether the compass module is enabled or disabled.
The effects of fixed local magnetic fields (i.e. those caused by magnetised ferrous material in proximity to the
compass) can be compensated for by performing an in situ compass calibration. It is important that the calibration
is performed with the FT205EV mounted in its final position within the host system. The calibration can only
compensate for fixed magnetic fields generated by the host system. Varying fields caused, for example, by ferrous
objects passing near the FT205EV (say by a car or truck) or high current electric circuits will not be corrected.
Should the magnetic signature of the host system change significantly (because component parts have been
added or removed), then a recalibration may be required.
2.5.1
Compass Calibration
Compass calibration should be performed away from any external stray magnetic fields. To calibrate the FT205EV
compass proceed as follows:
1) Mount the FT205EV in its final position within the host system
2) Ensure the compass is enabled (send the CFE ‘enable compass’ command, see Section 6.4.6).
3) Send
the CFC ‘enable calibration’ command (see Section 6.4.6). Important: No further commands should be
sent to the FT205EV after the CFC command has been sent.
4) Slowly rotate the host system 1 complete revolution in the horizontal plane. The revolution should take between
40 and 60 seconds.
5
) To complete the calibration send the CFE ‘enable compass’ command (see Section 6.4.6).
6) Store the new calibration Parameters in the compass module using the CFMS command. The save command,
for example $01,CFMS*//<cr><lf>, will save the new compass calibration parameters calculated during the
calibration process to the Interface Board Flash memory. This command is required.
2.6 Selective Filter Scheme
In addition to the averaging filter described in Section 2.3, the sensor has a feature called the Selective Filter. The
scheme allows the user to set a “validity period”, during which the sensor will exclude invalid readings from
entering the averaging filter. The output will freeze on the last
previous “good” reading and only raise an error flag
once the number of bad readings exceeds the validity period. This scheme can be enabled by factory configuration
or by using the FL command (see Section 6.4.15). The filter is turned off by default to match legacy behaviour.
