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Forgive me, I'm sure this is a total noob question, but I have never seen it actually addressed in any example or demo.

When using something like a MinIMU-9 board (https://www.pololu.com/product/2738/), does it matter what position you mount the board on your project? I mean, does it have to be positioned so the proper axes line up with the axes of your vehicle, or is there some calibration to set that alignment?

For a ground vehicle where I will concerned primarily with only one axis, are all axes interchangeable?

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    Single rotor helicopters inherently hover tilted at a slight angle. There's a popular micro RC helicopter with 6-axis flybarless electronics where the PCB faces forward, and the MEMS sensor is aligned on the board at the approximate opposite of that angle of rotation, so that the chip is actually standing vertically on edge in flight. Presumably this helps prevent yaw from cross coupling to the pitch and roll gyro readings in a way that would require extra math to untangle.
    – Chris Stratton
    Feb 28, 2018 at 0:53

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If the axes of measurement don't line up with the axes of motion, then comparing the IMU outputs to vehicle-relative (VR) coordinates will require some vector arithmetic to convert one to the other. It can be done, but requires some engineering time to code it and processor time to run.

It wasn't clear to me at a quick read whether your chosen IMU can do the coordinate translation itself, or your MCU has to do it. In either case, the computation will always reduce the accuracy of the original data, maybe a lot, maybe inconsequential, but expect some degradation.

Bottom line is: if you can line them up, things get simpler.

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