Sparkfun has released a lovely (if expensive) bit of silicon: a tilt-compensated magnetometer.
"The HMC6343 has three axis of magneto, three axis of accelerometer, and a PIC core running all the calculations. What you get is a compass heading over I2C that stays the same even as you tilt the board."
Not cheap at $250, but if you understand what a pain it would be do to that three-axis tilt-compensation yourself, you'll see why it's worth it.
Boomslang has a point - if you place a horizon detect on a moving platform; then simple mag compass is always correct. Assuming your roll is the most problematic, this is the axis you stabilize - also hint-hint - a good platform to use for your camera (as Chris has posted before).
Unless you've got a full IMU running and can use that to correct the magnetometer, I can't think of a good way to ensure it will be right. Maybe switch to a 4Hz GPS, like the UBlox 5, instead?
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.
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.
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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.
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
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?
Comments
Unless you've got a full IMU running and can use that to correct the magnetometer, I can't think of a good way to ensure it will be right. Maybe switch to a 4Hz GPS, like the UBlox 5, instead?
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.
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.
For a plane, you could wait to take readings until you are level.
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