The compass, what Mavic uses it for?

borislip · Mar 26, 2017

It uses the GPS for its position, or IMU if GPS isn't available, but whats the purpose of the compass and what would happen if its badly miss-calibrated or faulty?

The only thing i can think of is knowing the heading while standstill, but even if this heading is wrong, and it starts to move in a wrong direction on, lets say, RTH - it could always detect and self-correct by GPS, hence, this would be no reason for flyaways.

Anyone dares to put some magnets on and test what it does? ;-)

Cyberpower678 · Mar 26, 2017

RTH will essentially fail since it relies on the compass heading to point in the right direction to move. It may try to adjust, but it won't get itself in the right position.

borislip · Mar 26, 2017

It should be able to derive its heading from GPS the moment it starts moving, so i really don't see why losing a compass should result in a catastrophe. RTH won't work without the GPS anyway, right?

Dragonfly · Mar 26, 2017

Stand in the middle of your lounge and pick a corner of the room. Now walk backwards to that corner. Do it again and walk sideways to that corner. Both times you moved along an exact course or direction, a GPS heading but both times you were pointing differnt directions. The GPS DOESNT KNOW what way the drone is pointing, it only knows a giving point in space and a giving point to go to and creates a bearing line from poit A to Point B so the drone needs a compass so it can line up its front/nose acording to that bearing line.

borislip · Mar 26, 2017

The drone knows it moves forward because it pitches forward to do so. Or sideways because it used its motors and rolled to that side. Just like i know which way i move my feets. So if it tries to move forward, and deducts actual physical direction from the GPS, it knows where its nose is pointing as well. So, yes, it wouldn't know which way it is oriented without the compass while standing still, but the moment it starts to move, it should know. Just like many old, stand alone, car GPS devices know, those don't even have a compass to begin from.

borislip · Mar 26, 2017

And assuming it HAD the GPS before it went hovering in place, it should know its orientation at that point too. The only use for a compass i can think of is to stabilize the yaw while not in motion, in a better way just IMU can.

borislip · Mar 26, 2017

But if that what it does, it would probably start rotating in place if you put a magnet on it. Anyone dares to try?

AnzacJack · Mar 26, 2017

The gps system alone doesn't know speed or direction. It only knows where you are. Other algorithms are used onboard to work out speed and direction from gps history. Think about how the mavic behaves with wind and other factors.only by knowing it's heading can it keeps itself standard. Sure enough, with fast enough processing power it could do this with out the compass, but then we start hardware to do this and this will increase power draw and weight. So much bout the technology driving our hobby/profession is related to finding that sweet spot, and I guess having the compass to help with the load does that.

borislip · Mar 26, 2017

Your run of the mill mobile phone does those calculation, they aren't as complex as you describe, really. Its actually as simple as knowing your position now, knowing your position 1 second later, and doing a simle math. Or, indeed, using longer history and filtering it to some extent. I am not saying having a compass is a bad thing!!! What i am saying is that i don't understand why not having it is considered potentially deadly for the bird!

AnzacJack · Mar 26, 2017

Yes, but your mobile phone cant hover in place or fly in a straight line with a reasonable cross wind without so much as a flutter and keeping the camera pointed at a POI
At the end of the day, the compass is an integral part of a system that works to fly our machines they way we tell it to. (We aren't doing the flying, just giving the system commands)

borislip · Mar 26, 2017

This doesnt mean there shouldn't be a failover mode, which detects the singnal from the compass going fubar, and still flying in the right direction, as well as it can, in a not-so-straight line anymore, still allowing for sticks input to correct. This shouldn't be a flyway by any means. Yet the stereotype is - you don't have your compass perfectly calibrated? You can easily have a flyaway!

AnzacJack · Mar 26, 2017

I don't know what you want. You asked a question and I tried to answer as best I can.

You may need to talk to the coders that put these systems together.

borislip · Mar 26, 2017

Well, if your answer is, indeed, complete, then either the stereotype of "compass miss-calibration == flyaway" is wrong, or the coders mentioned didn't do a very good job programming fallbacks (which i doubt, if there is any level of QA in there). I wish one could actually speak to those coders. Or see the sources. Or both, lol.

HFMan · Mar 26, 2017

You can't get immediate course from GPS data... there is latency and it takes eons compared to how quickly a quad IMU needs to make a motor correction to change heading of the craft. If you relied solely on GPS, you would end up flying in a drunken erratic fashion. Magnetometers are very precise (not neccessarily accurate) and very fast changing. Without a compass, the flight controller would be constantly making an adjustment, wait for course correction, make another adjustment, wait for course correction, and so on. This is exactly what leads to the toilet bowl effect. It's not like an airplane with thrust in only one direction- you have infinite directions of thrust in a quad.

I would have to agree that perhaps the way the DJI software handles compass errors is not very elegant- but not only is the flight controller at play but the IMU as well. This adds additional layers of complexity. I'm still not convinced by any of the stories of flyaways yet- nothing has been proven to me with regard to logs, pilot actions, and the facts at hand.

AnzacJack · Mar 27, 2017

Actually I was just wondering, sure, flyaway was certainly a big problem in the past, but have there been many mavic flyaway? Most of the fly away claims here seemed to have been found to be a pilot error of some sort when the logs have been examined. Not trying to be a smart ***, just wondering how many unexplained fly always we have had with the mavic?

borislip · Mar 27, 2017

Thats a good question, don't think i've seen any flyway threads. The "fly away with the tail wind" kind of threads do not count.

Logger · Mar 27, 2017

HFMan said:
]... I'm still not convinced by any of the stories of flyaways yet- nothing has been proven to me with regard to logs, pilot actions, and the facts at hand.

By this I take it you mean literal fly aways into the yonder never to return, as compare to spontaneously flying away laterally for no apparent reason, like this. I have DJI looking at the log of this. So they will soon tell me if is Pilots error.

This is a RTH precision Landing. Arrives back over home point, aligns heading, descends, precision lands bombsiting the target - then rapidly spears off 40' in the last few feet of the descent when it all looked home and hosed. Logs cleary show I was hands off through out, with my first input being to toggle the mode switch or press pause (I dont recall which) to prevent a crash 30' away. I am puzzled as to why DJI are allowing autoland to continue with horizontal speeds like this.