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Multiple compasses in firmware 3.3 for PX4?

Posted by Paul Atkin on June 8, 2015 at 12:09pm

Hi,

I have stumbled a bit on this topic.

I started building new platform on the AUAV-X2 px4 clone FC, and it has an embedded compass in it.

I will use M8N GPS/Compass units with an external compass, but I do not see in the Mission Planner an option similar to APM where we could specify arbitrarily if we use external or internal compass.

How it should be setup?

Also, if we have a possibility to use dual GPS sensors on 2 different UART ports - is there a same method to use multiple compasses?

Can be 3.3 firmware setup to use external compass as a priority 1 unit and correlate it with a secondary compass reading for a redundancy? Or if I have 2 M8N GPS units connected - I will get 3 compasses total at same time this way, how will it work and how can it be configured?

Regards, Paul.

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Replies

  • Mike Kelly June 8, 2015 at 3:51pm

    I just did research on this myself.

    The compass interface on Pixhawk uses I2C interface. The chips that are commonly used come wtih the same I2C address. You can't have two devices on the I2C bus with the same adddress. So no you can't use the extra compass in your second GPS. It is a little confusing because there are parameters in Mission Planner for a compass3.

    If you look at the parameters in Mission Planner and search for "compass" you will see the answers to all your questions.

    There is a parameter to use the external compass as the primary.

    • Paul Atkin > Mike Kelly June 8, 2015 at 4:27pm

      I see, it makes sense. I just spent some time trying to do exactly same thing and got to the same result.

      Still, I have an internal compass in the unit and one in the GPS module, so it is dual system, and will just decide if I want to run second GPS or not.

      It did not seem to have any positive effect to have 2 same units attached, I did not get better/faster HDOP drop yet, but will try to experiment a bit more tomorrow.

      • Mike Kelly > Paul Atkin June 8, 2015 at 4:53pm

        Well my ignorant guess at the matter is that the M8N must have a wider bandwidth antenna to cover the multiple GPS bands that it does, which means it will be noisier. The older 6 series have more narrow bandwidth antennas and could have a better hdop. ( I have tested this and indeed my 6 can have a better hdops even when the M8N has more satellites)

        So I am using an M8N for more satellites, to cover when the M6 loses a sat or two and the M6 for the best hdop when it has a good lock.

        GPS performance is so very important for the GPS flight modes that I think having redundent GPS is a good idea.

        Cheers

        • Paul Atkin > Mike Kelly June 8, 2015 at 5:59pm
          I just tried an experiment - unsoldered antenna from old 5q unit i have and soldered it into this m8n but it did not produce anything good, it waited forever and got some wrong coordinates and locked into no sats at all. So i soldered original one back and it is still same story - bouncing between 5-7 sats... Will try tomorrow with an another unit while waitng for an USA made m8n to arrive. Those i got are all from ebay and act, well, erratic.
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