Section 2
Functional Description
11
FT742-SM (RS422 & RS485) Sensors
– User Manual
2.4 Electronically Offsetting the Wind Direction Datum
For applications where the compass is not being use
d, 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 7.4.2 for further details.
2.5 Compass & Declination Angle Operation
The FT742-SM is fitted with a solid-state electronic compass so that the wind direction can be measured relative
to magnetic North. 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 4). The compass heading can also be read back at any time using the CF command. See
Section 7.4.2 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 close to the
compass) can be compensated for by performing an in situ compass calibration. It is important that the
calibration is performed with the FT742-SM 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 FT742-SM (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 FT742-
SM compass proceed as follows:
1) Mount the FT742-SM in its final position within the host system
2) Ensure the compass is enabled (send the CFE ‘enable compass’ command, see Section 7.4.2).
3
) Send the CFC ‘enable calibration’ command (see Section 7.4.2). Important: No further commands should be
sent to the FT702LM 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 7.4.2).
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 7.4.10). The filter is turned off by default to match
legacy behaviour.
2.7 Error Detection
The sensor has a self-checking mechanism which can detect if a reading is invalid. On very rare occasions
where an invalid reading may have been detected, this is signalled to the computer or data logger by setting an
error flag character within the wind velocity output message (see Sections 7.4.26 and 7.4.27).
