People have different goals when they start on a project. Lately, mine has been getting in the air while spending the absolute minimum cash.

That's lead me to trim costs in a lot of ways. Experimenting with MegapirateNG, playing with cheap props, and flashing my own ESC's.

I've had a lot of success, but one problem has continued to plague me: I could not make Arducopter (or MegapirateNG) fly with cheap props. It would fly like a dream using DJI's excellent 1038 prop (still my favorite for F450-style quads), but with any other prop, my model would bounce up and down in any altitude-hold mode.

Most frighteningly, these oscillations would become fatal if I changed modes while flying at even moderate speeds. I wrote about that particular problem here: http://diydrones.com/forum/topics/violent-over-correction-on-mode-c...

Over the weekend, I finally discovered the source of the problem: I was using simonk-flashed ESC's.

I went back to using stock F-20A ESC's from HobbyKing, and now my quad flies great with any prop in my toolbox. I tried super-cheap 1045's, 9047's, 9443's, so-called "carbon mixed" 1038's, and of course my favorite DJI 1038's. The quad still flies best with DJI 1038's, but all the other props fly quite well in all modes.

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Try dropping the value of THR_ACCEL_P. I had a similar problem with a big hexacopter, the motors would keep surging in loiter. I had to drop the value to 0.25. 

I think this is an oscillation in the z-direction, similar to an oscillation in roll or pitch when the P value is too high.

I thought my copters flew well on Hobbyking F-30As on stock firmware until I got them dialed in with SimonK (same ESCs reflashed), much crisper with SimonK. Also everything seems to run much cooler, even after giving the motors a hard time they are still at ambient temperature, ESCs or only a little warm.

I tried putting THR_ACCEL_P down to 0.5, which I thought was the lowest it could go. It's OK to go below 0.5?

OK, I tried lowering THR_ACCEL_P to 0.25 and THR_ACCEL_I to 0.5. It did smooth out the altitude oscillations in loiter, but it seemed to exacerbate the violent over-correction I wrote about in the other thread. I have nearly identical F450's with and without simonk, so I'm good to experiment.. but the search continues.

I do use simonk on my multiwii setup, and it's great.

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