Vibration Consideration

Ok, so I fully realize that we have been absolutely awful about updating this thing. The other day, as I sent email #5832 to the flight crew updating them on progress with our aircraft, I remembered that avoiding having all of our development information hidden away in email threads is exactly why we created this blog. So, with that in mind, I'm going to try to do a better job. I may also set up a Gmail group, as that seems like a really good way to track/organize routine/mundane communications within the group that don't really justify a blog post.

Anyway, the big news items are:

• The Mugin is flying beautifully in manual and FBW. We haven't tried auto, but will like do that the next time we fly. I'm still concerned about vibration but FBW at least seems to be totally unaffected by it. An FBWB test may be in order
• The Z3 is still excellent. 60% of the time it works all the time.
• My company built 2 Skywalker X8 aircraft for a client and they are really superb machines. They are a little flimsy, but the trade-off you get for that flimsiness is huge: carrying a Nex-5 for 75-90 minutes at 18 m/s. We need to buy 1 or 5. 
• I am building a quad for a client which will have the Swift Nav Piksi on it. I am excited to see how well that performs.

And now, on to what I want to spend this post talking about. I've been developing a new quadcopter which should fall nicely into a niche of good endurance combined with high portability. Spurred on by the excellent work done by the Ecosynth crew and Jonathan Dandois down at the STRI in developing  a quadcopter with 50 minute endurance from spare MK parts, I set out to copy it. I believe this is detailed in an earlier post on here.

Anyway, to make a long story short, I abandoned the MK frame for 2 reasons: (1) rigidity and (2) portability (or, more accurately, lack thereof for both). No matter how much balancing I did I couldn't seem to get the copter to turn 15" props without quaking like a leaf in the wind. The thin aluminum arms would pick up harmonics and shake like crazy. The arm shaking was visible from 5-10' away (read: it was really, really bad). The lack of portability, if I ever got the MK frame functional, was also of concern to me. I know firsthand how hard it is to travel with a large aircraft, and I wasn't looking forward to it. I knew from taking it to Belize that my only real option for transporting the MK was to pull the arms off, meaning that I would have a few hours of work getting it taken apart and put back together every time I took it somewhere. That isn't a big deal if you're basically stationary but if you're moving every few days it is unacceptable.

Ok, so with that in mind I set out to find a better frame for our purposes. I wanted something that folded. I knew I would sacrifice some flight time to get that luxury, but for our uses it is worth it. This is what I came up with:

As you can see, it has a really nice folding design unlike any other I had seen before. Normal folding quads, like our monstrous Tarot 810 hex, have a design where 2 or 3 (depending on quad/hex config) fold away to one side and the others fold away to the other. That is less than ideal as it makes them quite long. For example, our hex is still 810 mm long when folded, it is just long and narrow instead of being a 810 mm diameter circle.

So, I ordered one, got some APC MR 14" props, and put MK 3536 (760 KV) motors on it. It flew really, really nicely but the motors got quite hot and it had a tendency to lose a motor and flip in flight (so, less than ideal flight characteristics). I suspect this was due to the KV/prop size mismatch, although it may also have been partly caused by a wonky ESC.

Regardless, I put some different motors on it, changed the suspect ESC, and it flew really, really nicely. I also moved the ESCs out to the bottom of the motor mounts for cooling. Heat is no longer an issue and was totally locked in in Stabilize after an autotune. And it flies for about 30 minutes easily on 2 10 AHr 4S batteries. When we would switch into Alt Hold or Loiter, however, the issues began...

In still air it flew fine in altitude controlled modes. When a gust came along, though, it launched up like it was trying to reach escape velocity. I initially suspected everything but vibration, as I had never experienced vibration issues like that before, even on the big hex with 6 15" props spinning. I was married to the idea that airflow and/or prop wash around the Pixhawk was causing the problem.

Now would be a good time for me to mention that the dataflash logs are your friend. Sure, I looked at them, but I didn't really see them. They were trying to tell me what the real problem was, I just didn't want to see it. It was vibration.

I finally accepted this a few days ago when I looked back at the logs and saw the dead giveaway indicators. For your viewing pleasure, here they are:

Here we see XY accelerometer data in red and green and Z in blue. XY is definitely acceptable, and Z is on for the most part, but then we get these odd explosions of vibration in Z. These happen to coincide with wind gusts. I'm not sure why the vibration only jumps in the wind, but I suspect that a unique combination of throttle values for each motor leads to some runaway harmonic.

Now we see the effect on altitude of these vibrations. We still have Z acc in blue, but now have barometric altitude in green and calculated altitude (using accelerometers and barometer together) in red. You can see that as vibrations spike the Pixhawk gets confused and thinks it is descending based on accelerometer data. It compensates by throwing the coals to the motors and we see the resultant climb indicated in the barometric altitude (green).

So, there are 2 solutions to this problem. One is to reduce vibration and one is to dampen it. First I spent a few hours balancing props and motors. This quad is now the smoothest operating multirotor I have ever seen. You can see, though, that the baseline vibration is less but the spikes are still there.

Also, Blake had the idea to move the Pixhawk forward about 3/4" which, incredibly, actually had an effect. It may have moved the IMU away from an area that was getting hit hard by a harmonic. Anyway, it seems like it elimintated the big spikes in Z acc.

Finally, I put some damping in the equation. I re-mounted the Pixhawk with Kyosho Zeal Gel. Now vibration is almost nonexistent.

Those vibration levels are extremely low for a big quad and well within the acceptable range for reliable operation. In fact, I didn't really know vibration levels that low were even possible for something this size. Now we can take another look at barometric vs calculated altitude:

As you can see, both altitude values (green and red) track pretty much perfectly. You can see deviations in Z acc (blue), but those aren't vibration, they are actual vertical acceleration which coincides with climbs and descents.

So, now I think we're ready for primetime. About 50 minutes of flight time after making these changes suggests that the problem is solved. Now it is just time for some testing of Auto and then we can press this thing into service.

Hopefully I'll post again soon about something, either the Mugin, the Piksi, or some things I'm learning about Photoscan.