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Topic: Accelerometer and Magnetometer calibration (Read 4 times) previous topic - next topic

michinyon

well the 6-dof and 9-dof are much abused teminology which I don't use.  Because they are not, really.

My approach is for the accelerometer and the magnetometer.   Not for the gyro.    In both those cases,
you are attempting to measure a vector which is a fixed, physical quantity.   in the first case, the gravity
vector and in the second case,  the earth's magnetic field vector.

I collect thousands of samples of that vector.   If you then represent all these vectors as pointing outwards
from the coordinate system origin,   you get a surface.   I then calculate the transform of that surface
which gives the least squares best fit to a sphere,  assuming a model with an offset and scaling parameter
for each axis,  which appears to be sufficient.   So I have 6 calibration parameters,  and I calculate the value
which gives the least sum squared deviation from the sphere.

This is probably the same fundamental approach as the other one you mentioned.  You are mapping something
which can be approximated as an off-centre ellipsoid,  to a sphere.

For the gyro,  I just capture the value when the device is stationary, and use that as the offset.   ation device that spins at a
fixed rate. Rate scaling
difference for the gyro would be very hard to measure,  you would need a calibrIn my experience,  this is best done every time you use the device.





jjspierx

Thanks for the description of your calibration method, interesting stuff indeed.  I purchased components to make my 3 gimbal calibration device already. I am using 1/2 CPVC with 90deg elbows.  Actually ended up being really cheap, about $14 for everything.  I think I am going to write my own calibration program, even if I can get my sensors calibrated with the FreeIMU calibration tool, just so I can gain a better understanding of these sensors.  And because, I find the FreeIMU calibration tool cumbersome to use.   

gdavisiv

If you don't mind my asking what about your project would require you to use a gimbal rig to calibrate your accelerometer?

jjspierx

Nothing specifically about my project, I just have not been successful in calibrating my sensors when turning them by hand 360 degrees around X,Y,Z axis.  Every time I attempt to calibrate I get very different values.  My setup uses BlueTooth, so it should be fairly easy to set it in a gimbal rig and smoothly control the motion about each axis to collect better samples to use in the calibration algorithm.  I need the sensors to to determine rotation, and translation in all axes, so I would like them to be calibrated as accurately as possible.

gdavisiv

Thanks for the response I'm using an acceleromter for the first time and trying to understand how to calibrate it also!  I wish I could be of more help but I'm still learning myself.  The gyro seems like a great idea though

I was also going to suggest if you have a bunch of old plastic container tops lying around you could try using those.. and save a bit more $$$ (Noxzema tops for instance, a small vicks vapor rub top, and something that could fit between them both) thats if you have the tools to cut them approriately

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