Saturday, 20 July 2013

Headcrab quadcopter - Successful PID tuning!

So the last outing the 'Headcrab' had showed I had some vibration issues that may have been affecting the gyro performance... On reflection I think that the ESCs were not properly calibrated to the new motors I'd paired them with.

I wasn't very satisfied with the double sided foam attachment method for the flight controller board, so I made up a stand-off board with some scrap laser-cut plywood and used vibration damping foam to isolate it from the bolts holding the board up. Hopefully this will reduce the gyro interference to a manageable level using low-pass filtering in software. I also calibrated each ESC, just in case.

Vibration testing didn't look that much better, but I strung the quad' up to test its stability in acro mode and level mode:

This time the quad' looks rock solid at all power settings... the new Turnigy motors may be a tad overpowered :) I hope the ESCs and batteries can cope with the amps!

Batteries are on charge now. If I'm feeling brave tomorrow there may be a free-fright test :]

Wednesday, 17 July 2013

Tuning the 'Headcrab' quadcopter - bad vibrations

The 'Headcrab' quadcopter design now has an underslung battery carrier and vibration-damped camera mounting, thanks to some more laser-cut plywood from RazorLab. It's ready to fly! Almost!

But first, I needed to spend some quality time with the PID controller built into the MultiWii code. Basically, if this bit isn't right then the 'copter will be an un-flyable, rogue manhack that will not stop until it has lacerated its maker into mincemeat... or it crashes, or whatever...

PID tuning is a bit hit and miss (for me) but a nice safe way is to pop the 'copter on strings that hold it in one place and let it rotate about either the pitch or roll axis. This was working great, but I noticed that the quadcopter needed a constant pitch down input to stay level. As the throttle went up, more pitch down was needed to hold the drift in check. At this point I did the only sensible thing: put the quad away and thought about the problem for a few weeks (don't rush me!).

[Video of the gyro noise at different power settings - sad face]

I concluded that the likely cause was vibration (despite using foam tape and a big mass on the flight controller to try and damp things down). This is not a good sign. I already had concerns that the lovely body shape I'd created in Sketchup might act like a multi-modal resonator (kinda like how a guitar or violin amplifies vibrations).

Next stop is some serious damping efforts before I give up and try a different body plate design. I really wish sometimes that I could remember my lectures from university, especially the ones with control laws...