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May 18, 2013, 03:55 AM
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3 axis jinx


As soon as all 3 motors were fired up, it immediately became clear that the roll motor needed to be a lot more powerful or the roll assembly needed to be a lot more compact. There's too much inertia in that direction.

It also became clear why a 3 axis gimbal hasn't appeared from Alexmos or open source. The yaw motor fights the roll & pitch motors if it isn't level. The pitch motor handles crosstalk better, since it has less inertia. The roll motor can't handle any crosstalk before it oscillates.

In very little tilting, the yaw motor becomes a roll or pitch motor. The roll motor also becomes a yaw motor when tilted. It could probably be bearable if the motors were powerful enough, but never perfect.

A more complex feedback model is required, which predicts the effect of each motor on the IMU, after translation through the downstream motors. That would require knowing the orientation of each motor.

The DJI Zenmuse does it perfectly. It seems to have potentiometers on all the motors. No-one has ever torn down a DJI. It's only a matter of time before the extra math makes its way into open source. It'll probably use an IMU for each motor, so people can still make their own frames.
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