Severe Bucking of Copter in all modes

We have been attempting to Tune out a steadidrone Mavrik X8 for some time now and have made major progress however we have one major hurdle left to deal with. The Copter will begin to buck wildly in pitch every so often seemingly without an apparent cause. Other times it exhibits the same behavior with minor throttle changes. The PIDS seem to be fairly well tuned and we can make aggressive maneuvers without this happening. I have attached a DATA Flash Log from testing that we did last week the most obvious occurrence is near the end of the log however it occurred several times throughout the flight. Any Ideas would be greatly appreciated. We are still having compass interference issues however I believe we have found a way around that and from what I can tell that shouldn’t have any bearing on this issue.

Your problem looks similar to one I had for a long time. Like mine, your copter is over-powered. The log file shows hover throttle to be in the low-mid 20s. If you look at RCOU, your C3 out pits the min PWM repeatedly and flatlines along the min. At the same time, other channels are not spiking to offset, and this creates some stability challenges. Wind is the primary cause of this in my experience.

The workaround is to give the autopilot more PWM range to work with to maintain attitude by (1) increasing ATC_THR_MIX_MAX from 0.5 default to as high as 2 (you need fw 3.4.3-rc1 to do this). If you work your way up to 2 and still get see large unexpected pitch and roll moves, then (2) set MOT_HOVER_LEARN to 0 (you have it at 1 right now) and set MOT_THST_HOVER to .35 or .4. If you do this last part, keep ATC_THR_MIX_MAX at 2. Also, if you do this last part, be aware that when you go to lift off, the copter will likely take off with just a slight touch of your throttle. once you get it into one of the auto modes, though, you’ll be able to put the throttle at the mid-point, and you’ll be able to fly like you usually do. but if you switch back to stabilize, keep in mind the copter will move higher, potentially quickly. So you may want to work up that MOT_THT_HOVER slowly until the bucking problem goes away.

These changes eliminated the problem for me, and your log file looks a lot like mine did.

Thank you so much for the input! I will give this a crack and let you know how it goes! I hand’t even thought about the power being an issue but that makes complete sense when thinking about the behavior. Thanks again and I’ll let you know how it works out!


Dan were you able to resolve your problem following Roger’s advice? I’m going thru a similar situation with a new Long-range quad copter flying with Pixhawk 2.