Hey guys, first post here, but nice to join the community.

I've built up an Align M690L rig that's powered by a Pixhawk. Using the stock Align motors and ESC's, the Align PMU (power unit) to distribute power from battery to ESC's. The Pixhawk provides the PWM signal that drives the ESC's however. The drone is working well, tuned the gains with AutoTune, pretty crisp. However, when I started doing automated waypoint flights, it flipped, and crashed. (Happy to discuss what I had to do to get the Pixhawk to work with Align, mostly a list of random harnessing stuff). This is pretty reproducible behavior.

I've looked at a ton of telemetry, and my theory is that when it's approaching the last waypoint, or when RTL is commanded, it causes a fairly sudden spike in the throttle command from the Pixhawk. This in turn causes a sync issue with the motors, and that is the source of the flip. This is just a working theory, I don't have a whole lot to prove it, except when the hex landed after an almost unsuccessful RTL, one of the motors wasn't spinning.

So my question is: has anyone seen and/or resolved sync issues with Pixhawk and Align ESC's (http://www.aligntrexstore.com/Multicopter-Brushless-ESC-HES04001_p_...)?

On a broader scale, what are some ways that people resolve sync issues? Do you decrease/increase the RC_SPEED parameter? I've seen people also twist the ESC power cables. Any general tips to move forward?

Thanks for your support. Logs are attached, flip at the very end.

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Mmm... we're also doing some tests...
https://www.youtube.com/watch?v=VrGdx_pMu68
Can you share your PID parameters?
S.

The solution was to drastically turn down all PID gains related to acceleration or any of its derivatives. So the loiter gains were tuned down by ~70%, also decreased a bunch of WP_ parameters like acceleration, jerk, etc. It made the copter transition much smoother between waypoints and put us in a regime where I do not seem to be driving the motors out of sync.

1) are you 100% sure that you're going out of sync? I never noticed going out of sync.
2) here is a video of Auto Tuning https://www.youtube.com/watch?v=vN11IhwZT8o&feature=youtu.be everything seems ok, except autotuning never finishes! However pitch/roll P is 0.130 
3) Super-smooth on RTL, Auto. Problems arise at max speed.

Yeah not 100% sure, but I did see it land with one of the motors not spinning, so pretty good indicator. That said, it did do autotune and auto pretty well. At 15m/s though has some weird spikes of pitch however. See http://diydrones.com/forum/topics/strange-regular-spikes-in-desired...

You're jealous. ;) Share your PIDs for an answer! 

RC_SPEED is just a feel question. You should be very careful in the inner PID loop, which is the most important one.

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