Dear all,

thanks to this forum I was able to build my first copter. It is a Hexa +, with 360kv motors, 15*5.5 props, an AUW of about 3.1 kg, and about 30 min loiter time. You can find some more details on the Ecosynth webpage

Basically, it now flies and loiters rock solid.

However, the copter yaws CCW when I raise throttle - at take off and in flight. There is no problem in loiter mode. As soon as there is some wind there is some CCW yaw in loiter too. Initially, I thought it were the ESCs, but after replacing all of them (different brand and firmware) as well as further PID tuning the problem stays the same. Magnetic declination is set fixed to the correct value.

Compassmot is at 2% and vibration seems to be ok:

The corresponding motor log however looks rather strange with all CCW props generally requiring more power after take off:

From a longer flight (mostly in loiter):

One idea is that the CW props are more efficient, causing a CCW momentum that cannot be compensated by the less efficient CCW props!?

It seems that the motors caused the problem, or more precisely the direction of rotation. Somehow, they prefer to run CCW as suggested here. So I mounted the CCW motors and props on top of the arm. Looks strange but works. This is a log analysis from a short "flight" on my "test stand":

If it ever stops raining I'll make some more test outside. 

Test done: same as before! Back to the props?

New test: I did some quick and dirty correction of the tilt of the motors/props like this using some tape and shrinking tube:

This is the result:

Not perfect but much much better! There was not that much wind so I have to test it again. However, this seems to be part of the problem.

I made another test under similar conditions as in the third figure of this post:

There are still differences between the motors/props but much smaller. Apart from the differences in thrust between the props my main problem seems to be solved - no more yaw. The hexa is also more responsive now.

Any kind of help, idea, or suggestion on this problem is greatly appreciated!

Thanks in advance,

Thorsten

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Update 1: I added another screenshot of a longer flight and added some additional information.

As discussed here I moved all three feet of the landing gear which I had mounted under the CCW props under the CW props to test the effect of interference with prop wash. No influence.

Thorsten, excellent improvements. I just came back and did some preliminary measurements, but I need a better level surface to make sure. Another thing I measured is the distance of the prop from the arm. With a couple of arms, there's some difference there too of about 2mm, so there's also some inward thrust.

Motor 3 and 4 are opposites, suggesting a slight CG offset to the back and left. Should be easy to check by lifting the hexa on each opposite arm pair and see if it turns over, rather than finding the actual CG to support the whole frame on.

I'm also seeing more dip in the arms where the landing gear is not mounted. Likely those arms are more flexible, which under load probably go up a bit more.

Good to see such progress. Tomorrow I'll measure my hexa and probably will find similar issues.

Did you find offsets on all motors and in the same direction?

Gerard, I mounted the motors back under the arms again and did some further leveling. The offset is on all motors in the same direction! That's very systematic. I can't believe it is the arms. Maybe I made some mistake. However: Gusty wind, no more yaw, 37min flight time!

The dip in the arms comes from the center plates. The CG is ok. 

Confirmed here too. My test flight after levelling out the prop tilt on a leveled surface. Only modified 3&4, which were 0.5 and 0.7mm respectively. Other motors still have ~0.2mm offset or less. This equals about 0.8-1 degree offset only. I'm using folded paper instead, the corrections are that thin. Indeed, if an offset exists, it's always causing CCW movement.

The throttle response really improved a lot. It's not dropping down like it used to, even in moderate winds it's pretty easy to control. Yaw movement however reduces thrust significantly, so pay attention to its altitude / behavior when autopiloted and it's changing direction to head to another waypoint. On regular machines you'd just mix throttle in with yaw to compensate, but I think this would mess up AP or loiter on this machine. Not sure yet if APM has an option for that, but it may. There exists thus also a possibility that a very fast yaw destabilizes the vehicle, which is something to look out for in auto mode.

The dip in the arms is because those are the ones without landing gear. There are two holes on the outer ring though which do not have standoffs, these can help to improve rigidity just a little bit more.

I'm going to take it apart, flip the arms again, reconstruct it and loctite everything again and then look for an opportunity for another test, this time autopilot+camera trigger.

Great! I also reconstructed mine trying to save some grams. What kind of camera/trigger are you using? 

CHDK with an opto-isolator from sparkfun: https://www.sparkfun.com/products/9118 with a 370mAh 2S battery. This perfectly separates noisy grounds from cheap cameras from apm, which should only be really important for accurate analog sensor measurements, but anyway. My camera is a Canon IXUS220HS, total weight of setup 168g. Camera = 143g. Very reasonable image quality, consistent focus throughout image and compact. Only issue is that it doesn't remember all settings when turning it off.

I attempted to use a very lightweight 5V BEC, since I was concerned about the 2S voltage being too high for the camera. I think however the current going through the opto is too constrained, so it drops the voltage too much perhaps. At first the trigger didn't work, but then I removed the BEC and used the 2S directly. Now I get very consistent triggers. I'd insert the BEC first to see how your cam reacts. The benefit is that you don't have to go through the flightplan and switch all the servo commands so something sensible. Just use V2 grid and you're off.

The opto IN is just connected to A9 on the apm (5th from the end on APM 2.6+) with signal and GND. The opto out has V+/GND connected to 2S with GND and OUTx to some cut off USB lead. The Canon usually has a regular mini-B USB input. In my case I used an angled connector, they help to remove unnecessary stress caused by straight leads.

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