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Oct 03, 2016, 11:03 AM
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The Feiyu lives

Decided to just put in a standard brushless gimbal program & see if it worked without trying to fix the reluctance cogging. The immediate problem was latency when propagating data from the IMU to the 3 microcontrollers. This was why the stock Feiyu used 1Mhz I2C & 2Mhz UART, but I2C was crashing any higher than 100khz. Tried shorting the series 100R's & replacing the 2k pullups with 1k pullups on their I2C bus. That didn't work. It turned out I2C on the STM32 needs to be polled as fast as possible. Once done, it went all the way to 1Mhz using 2:1 duty cycle.

Captured a waveform to show 1Mhz I2C was really a thing. Helas, 16:9 duty cycle doesn't allow it to do the stock 1Mhz, only 720khz or 1.4Mhz. The 16:9 would be more reliable, but to get 1Mhz would require a clock speed multiple of 10. If the gimbal is fully deflected, it'll break I2C. 2Mhz for the UART wasn't a problem.

It would have been quite valuable to know the frequency of the stock IMU readouts. Set it to 1khz. Used source code from an ancient BruGi project. They ran the gyros at 8khz, but 1khz was the fastest the Feiyu could get the data down all 3 microcontrollers. It started glitching above 1khz, so left it. Better scheduling of I2C & UART is required to max out the rate.

At 1khz, the minimal feedback worked quite well. It seemed to compensate for reluctance cogging on its own. It's a bit more solid to run the motors at higher current than stock, but it overheats after a short time. Considered mounting a fan on the handle to blow air at the entire gimbal.
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