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Connor
12-25-2008, 07:13 PM
I just got my Devantech Compass for Christmas, and my question is, does it mater if the compass is mounted in the center of the bot, or can it be mounted further out? I know it can't be mounted near metal or magnetic interference, which is what it might not be possible to mount it in the center of the bot, I just want to know if it will effect it's accuracy or anything.

Thanks, Connor

Adrenalynn
12-25-2008, 07:22 PM
Away from anything is actually ideal. It needs to be as far away from servos and motors (as you guessed) as practical because they generate magnetic fields.

If you think about using a compass - it always points north. If you hold it out in front of you, tuck it into your body, or hold it out to the side, as long as your body is oriented in the same direction you'll be facing magnetic north.

It does need to be as flat and level as possible. Tipping a compass, even a damped electronic compass, messes-up any readings. If you think about it - it's trying to move to point THROUGH itself rather than toward the horizon. No matter what, it's going to try to point at the magnetic pole.

4mem8
12-26-2008, 12:39 AM
Thanks for that explanation Adrenlynn, I also wanted to know where is the best placement for a compass. What would we do without you. :happy:

Adrenalynn
12-26-2008, 01:00 AM
Someone else would answer. :)

Quantum
12-26-2008, 12:11 PM
Cables or wires that are power lines which can come from a motor or a servo are bad as well. As current follows thru a wire it will create a small magnetic field around it as well. It could affect your readings.

Not as much as a motor or servo. But still not great.

Paul

Adrenalynn
12-26-2008, 12:27 PM
Agreed - but most digital compasses (I can't think of one I've seen otherwise) are pretty filtered against the super-low-level EMI. As long as the cables are short and twisted helps cancel the fields too. I prefer putting mine up on a mast and just eliminating any questions. Another thing most people don't think about - putting it on anything magnetically inclined is bad form too. I tend to make my platform out of aluminum or plastic. Little things will gotcha like chain-drives - metal on metal creating EMI. Or using steel pop-rivits with a steel frame - the flex creates mad EMI.

ooops
12-26-2008, 09:19 PM
Someone else would answer. :)

Although that may be true ... your answers are the best:)

Adrenalynn
12-26-2008, 09:21 PM
Well, maybe, but I neglected to note that some of the digital compasses are "tilt compensated" - they won't work at tooo steep an angle, but their better'n analog.

Connor
12-26-2008, 10:27 PM
I'm using the Devantech Compass, no tilt compensation.. Might make a simple gimble system... The bot might tilt's forward/after a degree or two ..

Thanks, Connor.

ooops
12-26-2008, 11:00 PM
You should be ok without the gimble, it should smooth (average) itself out ... I am assuming a walking gate is what you are concerned with?

Connor
12-26-2008, 11:05 PM
No, It's a rover, (based on Irobot Create, with 4th wheel on back) it can teater a little bit back and forth when it starts/stops.

ooops
12-26-2008, 11:26 PM
I would think you should be fine:)

Adrenalynn
12-26-2008, 11:34 PM
From the FAQ:



Q. How many degrees can the module be tilted before the readings become inaccurate?
A. None. Moving the compass off horizontal will result in increasing error. The sensors are sensitive to the vertical component of the Earths magnetic field as well. The angle of the Earths field is not horizontal, it dives into the ground at an angle which varies according to location. It is this which produces an inherent error in the reading, and makes calibration of the compass required at the point where it is to be operated. After calibration you can expect 3-4° accuracy if you keep it horizontal.


That said - I wouldn't worry about it because you're constantly taking readings. In software, just using a sliding window and average out the error.