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Magnetic Compass Tilt Errors

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dknguyen

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Hi. Has anyone here used a 2-axis non-tilt compensated magnetic compass on anything and bothered to try and calculate the heading errors? it seems where I am you get an error of about 20 degrees with just 5 degrees of tilt! Maybe the compasses are not supposed to super accurate as I had in mind, but rather to bound the error on gyroscope drift?
 
dknguyen said:
Hi. Has anyone here used a 2-axis non-tilt compensated magnetic compass on anything and bothered to try and calculate the heading errors? it seems where I am you get an error of about 20 degrees with just 5 degrees of tilt! Maybe the compasses are not supposed to super accurate as I had in mind, but rather to bound the error on gyroscope drift?
As any pilot knows a magnetic compass has "turning" errors. The amount of the error depends on your initial heading and your final heading. It generally takes several tens of seconds of straight and level flight to establish a new heading. That is why for precision maneuvers and navigation the heading gyro is preferred with the magnetic compass as a cross check.
 
I was going to use the compass to correct long-term drift errors in my yaw gyro, but my current robot is already overbudget in terms of money effort so I was going to cheap out and go for the 2-axis compass ($80). The 3-axis compass is expensive ($1000-1500) and I don't feel like putting in the effort of getting the 3-axis magnetic sensor ($50) and calibrating it myself (already overbudget in effort).

It's an outdoor ground robot so I can't really "drive level" since I'm at the mercy of terrain. With an error of 30 degrees with just 5 degrees of tilt, I'm wondering if I'm better off without it.
 
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I'm thinking that you should be good for up to half an hour without correction if the gyros are at least as good as the ones in GA aircraft. That was about the time between heading gyro corrections IIRC. Those corrections were seldom more than a couple of degrees.
 
Nah, they're relatively crummy. They're the Analog Devices ones. I don't know what their drift vs time is though. It's not listed. FOr the silicon sensing gyros it's 0.55 degrees/s per 30 second period which seems pretty good, just their bandwidt is 10Hz which I think is way too low.
 
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