Comment by Marc Ramsey on July 8, 2008 at 11:57pm

Unfortunately, as discussed earlier, the tilt compensation will be incorrect if the UAV is turning or otherwise undergoing acceleration. However,oddly enough, a 3-axis magnetometer, combined with a mathematical model of the Earths magnetic field and the current position (from GPS), can allow determination of heading, pitch, and roll to some degree of accuracy without use of the accelerometer data...

Comment by Marty on July 9, 2008 at 5:39am

I just saw this on the Sparkfun website, I jump over to DIYDrones and Chris has already made a post on it :) Is there currently a compass of choice that's a little cheaper for us DIYers ?

Comment by Marc Ramsey on July 9, 2008 at 9:27am

After sleeping on it, I realize I was wrong on my second point above, given the inclination and declination of the Earth's magnetic field (from the field model and position) and a measured magnetic field vector (from a 3-axis magnetometer), you ca determine that you are parallel to a plane in 3D space, but you can't unambiguously determine what direction you are pointing. Additional data (or assumptions) is required to determine heading, pitch, and roll.

For a less expensive device, look at the PNI MicroMag 3-Axis magnetometer. You'll need some additional means of compensating for tilt.

3D Robotics Comment by Chris Anderson on July 9, 2008 at 9:38am

Good point about the unsuitability to inertial environments like a plane. But this would be awesome for a slow-moving blimp, where the inertial forces can be safely ignored. If only the chip were 1/10th the price. I guess we just need to wait 5 years and it will be.

Comment by Jack Crossfire on July 9, 2008 at 10:25am

If enough diydroners buy the latest & greatest for $250, maybe they'll roll back MicroMag3's to their 2006 prices.

Comment by Boomslang on July 9, 2008 at 11:36am

I see another way to mount the FMA co-pilot is on a movable plate, so you move the plate to control the plane. Up to a point the plate should the stay parallel to the earth when the plane rolls or pitches. Would this sensor (or other compasses) not work if you mount it on the plate?

Comment by Marty on July 9, 2008 at 7:21pm

Boomslang, are youtalking about mounting it on some sort of gimbal with a weight to keep it 'pointing down' ? The co-pilot needs to be fixed to be able to tell the degree of pitch or roll the aircraft is currently in. If it was always parallel to the ground it wouldn't be able to calculate the movements necessary for the aircraft to remain level. Maybe I'm missing something here but could a tilt compensated 3 axis Magnetometer be used to develop an IMU ? The slightly more expensive OS5000 from Sparkfun even comes with software that has an artificial horizon

Comment by Xander on July 9, 2008 at 9:59pm

For a slow moving blimp, won't the platform always be parallel to the ground anyway? If that is true, just use a simple 3-axis mag sensor, or a 2-axis compass unit (like this) that assumes it is level and does some calculations to give you more manageable data.

For a plane, you could wait to take readings until you are level.

Comment by Boomslang on July 9, 2008 at 10:21pm

Hi Marty
Have a read here: http://www.uavs.net/horizon_sensing_autopilot.pdf on page 6. You set the sensor up so that you move the plate when you want to turn, so in normal flight the plate should always be close to horizontal. When you want to turn, you tilt the plate and the autopilot will think the plane is tilted and correct it.. so the plate will still he level in a turn.

Comment by Boomslang on July 11, 2008 at 6:06am

Does anyone else have any info if the setup suggested in the link I posted will work with a simple 3 axis mag sensor to get the heading...1Hz heading updates from my GPS is a bit too slow to get my auto landings right.
I will also have to change my code, as with the FMA sensor on the movable plate, it should not fight the imputs from the pilot anymore... suppose just lowering the gain will also work.

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