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Mar 05, 2014, 04:39 PM
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Brushless gimbal rebuild

There was a lot of risk in this, tearing down a perfectly good gimbal & making massive unproven changes.

The journey begins with 2 new IMU's, made entirely of recycled parts. Got rid of the MPU9150 & went with ancient ITG3200 + KXTF9's to provide only 6 axes. To eliminate the motor interference, 2 wire I2C was converted to 1 wire UART in microcontrollers near the sensors.

No more heavy insulation was required & the wires were reduced to 3 conductors.

The mane board was reworked to take the 2 UART signals instead of 2 I2C signals.

The frame was cut down & the motor shafts were reworked to reduce inertia. Ideally, there would be welding tools to get everything flush. No budget ever emerged for better motors.

After many days of debugging & tuning, the buffered I2C worked perfectly. The extra latency wasn't an issue. These are the kinds of obvious improvements which take forever for someone to get around to, then instantly become standard.

The only problem was many feedback terms being reversed. When pitch feedback was reversed, it appeared to over shoot the correcting motion. When roll feedback was reversed, it went crazy. There was also a bug where absolute angle was being counted twice.

It required stabilizing 1 axis at a time, starting with pitch. Then, all the terms with roll & yaw reversed had to be calibrated. The 1st terms to calibrate were rate & rate of rate feedback of 1 motor with everything else set to 0. Then came absolute angle.

Getting precise angle readings from the aux IMU was very important for reversing the roll & yaw to work. Being off by only 10 degrees made it go crazy.

Rebuilt gimbal test 1 (4 min 4 sec)

A lot of calibration is still required.
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Mar 09, 2014, 08:44 PM
Multirotor Feen
Nice work

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