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Compass - why?

Discussion in 'Phantom 2 Vision Discussion' started by CallMeAlan, Jun 1, 2014.

  1. CallMeAlan

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    I wonder if somebody could explain why we have a separate compass and why we have to calibrate it?

    I have a satnav in my car, and it always knows which way I'm going and pointing. I also have a Garmin and it also seems to know where I am and where I'm going. So why does the Phantom need a separate compass, which carries the overhead of needing to be calibrated, when it has what amounts to a satnav? (Mind you, this morning, at least a mile away from my previous location, I had the all-clear for takeoff LED indications before calibration. 9 satellites.)

    Thanks.
     
  2. jimre

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    Is your car's "satnav" system actually steering the car? Making sure it's pointed in the right direction? No, I didn't think so...
     
  3. CallMeAlan

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    Thank you for a most (not) useful reply jlmre. No, my satnav is not pointing my car in the right direction at all. But the compass in my p2v is not either. In both cases I am responsible for that. So, let me see, you are saying the compass in the p2v is pointing it in the right direction? Can I ask you how it knows what is the 'right direction'? Telepathy?
     
  4. John Shaw

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    The GPS system is not designed to give heading. Each of the satellites is constantly sending out orbital and timing information. Your GPS receiver gets this info from as many satellites as it can see and computes position and altitude. Effectively it knows where each satellite is and how far it is form it. If your moving it can compute track angle from the successive positions but it can't compute heading. Track angle might look like heading but it isn't. Most if not all cars with a nav system do have a compass. Some handhelds have a compass so they know heading. To test your GARMIN go outside and turn it on. Once it has position turn in place without moving. Is the display oriented properly to the real world? Then turn it place, does it turn with you? If it does both of these it has to have a compass. If it doesn't you'll confirm that GPS doesn't know heading.
    Hope that helps.
     
  5. jimre

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    It knows the "right direction" based on your stick input. When you push the stick forward to make the P2V go straight, the NAZA auto-pilot system uses both the compass and GPS to keep the craft going in a straight line, adjusting for wind and other effects. It needs the compass to know exactly which way the aircraft is pointing at any given moment, so it can apply speed corrections to the proper motor(s).

    Same thing with just hovering in place. It has to precisely know the aircraft's orientation to make the proper corrections. And especially when it's hovering in place, it can't "guess" the approximate orientation based on movement from previous GPS position - like your car does.
     
  6. jimre

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    In other words, the compass is an integral part of the steering system for a multicopter. There's no wheels or rudder to turn. Steering is done by adjusting speed on one or more motors. The computer needs to know the craft's orientation - in real-time - to know which motor(s) to adjust in order to execute a steering command.
     
  7. ElGuano

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    It's a good question, and one that's been discussed a few times before.

    For real-time control, your phantom needs to know what direction it is pointing in order to know what direction to correct drift. The GPS doesn't tell you orientation, only position. If the gps says you are drifting south, which motor do you activate to head north? It depends on what direction your quad is facing, right? Only the compass can give you that information-absolute cardinal direction to square against absolute position from the gps.

    Another point is your sat nav only had to assume your car goes straight ahead. So it can track historical positions over a few seconds and intimate a workable heading. A quad *could* be programmed to do this, but none are because it's not real-time (it needs historical sampling) and only works really well when you have a good amount of speed and runway to swamp out sampling errors.