Hi Tom,
Been through a lot with this.
First: as Bill, Phil and Tom said above, yes you can use the combination GPS/Magnetometer module with either the PX4 or Pixhawk and they will work fine due to outputting 3.3 volt signal levels which are compatible with the PX’s.
Second: you should not use the 3DR standalone magnetometer because it outputs 5 volt signal levels.
In addition, your video and my presentation of it have generated a major XXXXstorm of responses which I will attempt to summarize.
You are generally better off using an external magnetometer both to get the sensor away from (some) on board noise and primarily from external DC magnetic fields such as those generated from the batteries, power distribution board and ESC wiring and also from AC fields such as the ESCs, motors and motor wiring.
However, although it is true that your video does illustrate a genuine offset caused by the transformer on the power supply, it is entirely possible it represents a fixed offset and as such, so long as it doesn’t saturate the compass, it is reasonably compensated for with the normal offset setting process.
However if varying power consumption by the PX4 causes that offset to vary then that variance cannot be compensated for and that is something we don’t really know yet (or how severe a problem it might be).
Basically they (Lorenz and Craig) are saying that from current experience, if the PX4 is clear of outside magnetic influences the on-board compass can be used effectively, but they also acknowledge that due to the extreme difficulty of mounting the flight controller free of outside influences as well as the high internal offsets for both the PX4 and Pixhawk an external magnetometer is a superior solution.
As to whether the PX4FMU / PX4IO has a dynamic (and unfixable) offset problem caused by the variable transformers power consumption, the jury is still out as to whether there is one or whether it is significant enough to be a problem in any case. And they are in current use.
In the newer Beta versions of the firmware, the offset numbers that are allowable for the magnetometer have been increased enough to generally allow the PX4 and Pixhawk (which also has high natural offset numbers) to arm and to not fail the checks.
Copter 3.1 firmware will be getting put out for general release within a week or so at which time the normal release code should contain sufficiently high offset numbers to allow the PX4 and Pixhawk to normally pass the magnetometer arming checks too.
Bottom line by all means use the external GPS/magnetometer if you can and keep it as far away from the assorted magnetic influences as possible.
You can move it around the copter while powered up and hooked up by USB and looking at the raw magnetometer output as you were in the video to find the best place with the smallest offset to put it.
I will probably be detailing this on the wiki soon and I will be checking for variable offset caused by varying the load on the power supply / transformer with more to come
Thank you for your input and your video and your input it has resulted in some worthwhile activity.
Best Regards,
Gary
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