Copter Specs:

APM 2.6: FW 3.1.3

Frame Type: Quad (X)

Motors: T-motor 4008-18 380kV

Props: APC SlowFly 14x4.7

ESC: Castle Creations Pheonix Edge Lite 50 Amp

LiPo: 6s 6000mAH 30C

Recently upgraded to firmware version 3.1.3 and went out to fly yesterday in moderate winds. The copter has not gone through any AutoTune yet but is flyable in Stabilize and in Altitude Hold (however it is not so smooth in this mode). After noticing that the copter was very unstable in GPS (rolled, pitched and oscillated) when left in position I performed another Magnetometer (Compass) calibration in the field and then flew again.

This flight went as follows: Took off in Stabilize. Switched to Altitude Hold, flew for a few seconds and then switched to loiter where immediately the copter flipped and crashed.

Can anyone tell me what happened here? I would greatly appreciate the help!

Kind regards,

Oliver Volkmann

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GPS was malfunctioning at the end of your flight. Number of satellites went from 11 (which is quite a good number) down to 10 (which should be also good enough number but apparently wasn't) and shortly after that error state followed. So I assume the receiving conditions are not the greatest for your GPS module. Try to lift it above the propellers or further away from other antennas and/or strong current flows on the copter. Seems to me that GPS is on the edge in your case. 

Wish you many happy flights anyway. :-)

Jakub

Haven't looked at logs, but it sounds like "GPS Glitch"? I think you can set a param to switch to another flight mode (non GPS) when it happens. Something to start with...

Just a stab in the dark: Not sure that a GPS glitch would have that instant flipping effect. It looks like maybe this flip was in the direction of roll? Also could you tell if the quad was under power when it rolled and crashed? If not, maybe this was a failsafe activation, or if it was under power it could have been thrust failure at one corner (meaning loose or broken prop,motor fail, motor connection fail, esc fail, esc connection fail, with probably the most likely of those being a connection or a prop). 

Hi Everyone,

So indeed this crash seems to have been from a loose motor arm. I have since tightened it but have made the following discoveries which I hope will somehow help someone to help me figure out what is going wrong with loiter on my quad. (Can you tell I'm begging yet?)

AutoTune changed my parameters as follows:

Stabilize P + R: 4.15

Rate Pitch and Rate Roll to:

P: 0.085

I:  0.085

D: 0.005

Since the autotune, I reduced the Stabilize P and R from 4.1 down gradually trying to remove this sudden extreme pitching and rolling that would start when I gave quick horizontal stick movements (ie. forward, backward or left to right really fast) while in Altitude Hold and seemed to always end in a crash until I figured out to fly higher and switch to Stabilize Mode to eliminate whatever part of the system seems to be over-correcting on its own corrections over and over again. The ride gets quite violent.

My new rates came from experimenting by lowering first the Stabilize values from 4.1 to 3.3 to 2.5 to 2.2 to 2.0. Each time I lowered them, the copter seemed to calm down a bit and not go into these violent rolls and pitches. After seeing no difference between 2.2 and 2.0 and doing a bit of reading online (although this whole tuning thing is really confusing) I came to the conclusion that perhaps I should lower the Rate P and Rate I value a little since it was adjusted from 0.06 to 0.085 by the Autotune procedure.

The new PIDs are as follows:

Stabilize P and R:   2.5

Rate P and Rate R P Value:  0.075

Rate P and Rate R I Value: 0.075

Rate P and Rate R D Value: 0.005

These new settings allow me to fly quite smoothly in both Stabilize Mode and Altitude Hold Mode. One of my tests to make sure it is stable is to rapidly move my lateral movement stick rapidly from one direction to the opposite repeatedly. If the copter stays calm and does not continue the movement by itself with increasing angles after I let go, I consider it a good. This is now the case with my platform... but here is the kicker:

AS SOON AS LOITER is engaged, things take a dramatic turn. It sits quite nicely in light wind (< 3m/s) but as soon as there is a gust or a change in direction, it pitches and/or rolls dramatically like a child on a swing with the hulk pushing! I can fly the platform in loiter however as soon as I let go of the stick, this swinging starts taking place. One thing that I was able to repeat quite consistently is when flying forward in Loiter, it would not fly smoothly but rather more like a bumble bee, pitching back and forth and sometimes, especially when letting go of the controls, pitch really far back and then forward and start behaving like a pendulum. It gets bad enough that I have to immediately switch out of Loiter to Stabilize to get it to calm down. Pitch in VS Pitch does not seem to be consistent while flying in Loiter.

A compass calibration was performed (my GPS/MAG combo is mounted about 10cm above the electronics and off to the side of my platform) and the Compass Learn feature is enabled. When I instruct the platform to fly forward ONLY while in loiter it "crabs" off to the left consistently. The heading does not match the course even though the GPS arrow is pointing to the front of the craft (x configuration) parallel to the APM which is also mounted correctly. The red and the black lines in MP confirm this.

Please find attached to this post the log files from this last flight in which this behavior is observed. If anyone could help me get this sorted out, I'd be extremely grateful!

NOTE: The attached log files are with the STABILIZE P and R values at 2.2. I have since increased them to 2.5 will post those logs next.

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This is the Pitch vs Pitch In issue I see in Loiter mode.

And here is the other flight log with the Stabilize Pitch and Stabilize Roll values at 2.5. I'm hesitant to increase those back up to where the AutoTune did because of how crazy this beast gets when it is subject to strong inputs either from the remote control or from the wind. I'm very confused as to how to solve this issue when Autotune doesn't seem to get it right itself.

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yeah, it was upside down when I lost the GPS.

Seems to be the cause. Hard to tell because of the crash damage but I did notice that one of the folding arms of the SteadiDrone QU4D that I am using was loose. Very embarrassing but weird that it happened at the exact moment of switching to loiter. Anyway, repairs have been done and now I've noticed more issues. as noted in my post below.

Thanks to all of you who have tried to come up with some reasoning as to what is going on. Unfortunately, the saga continues!

Here is my Parameter File:

Attachments:

Hmm, nice to know that the fatal problem was mechanical, as I speculated - a lot of times that's overlooked as we all tend to bury ourselves in the electronics.

I notice now that yours is a really big quad (well, by most standards). Something I learned the long hard way with my now decommissioned "heavy" hex is that a great deal of the APM firmware and software has evolved to "fit" (as it were) a medium-size medium-power quad. It seems that the further one goes from the dimensions/configurations of, say, an Iris, the less straightforward a lot of tuning issues become.

I suggest that you try to find someone with a quad similar to yours and compare parameters. Also, I've been hearing that 3.2 will be more accommodating for non "standard" birds, I myself am waiting for that release before bothering with some minor tuning issues on  my current "dead cat" pattern slightly up-powered quad.

Otherwise, you might double-check that you selected the correct configuration on firmware upload.

But I think the root of the problem may be that it just isn't tuned for its size.

Sorry not to be of more specific help.

Hi Oliver,

Thanks for your response. I'm unfortunately very stubborn and cannot believe that the APM is unable to deal with large platforms. This is a 650mm quad albeit very powerful hence I am loading it with 700grams of dead weight while doing my tuning to keep it calm and semi realistic to real operating conditions. I've used the same motor/esc/prop configuration on a hexacopter and it performed flawlessly. If it can work with an 800mm class hexa, it has to work with this quad.

It flies nicely now in Altitude Hold and Stabilize but the real issue is whenever I engage loiter/GPS which bothers me a lot considering that I am trying to use this for mapping applications which will require extensive flying in Auto Mode.

Are there any developers who can chime in on this issue? I would like to use this platform for a project I have overseas where I have to map 200Ha of a peri-urban environment.

Going to do a compass-mot now and will see (and post) the results there.

Also, can anyone explain, as if to a two year old, what the Stabilize P and Stabilize R rates really do and what the relationship to the Rate Roll and Rate Pitch are really for? Am I wrong in assuming that the Stabilize values are used to control the responsiveness of the platform and the Rate P and Rate R are used to dampen/control the Stabilize values?

Okay, so CompassMot test is done and the results are good...

Interference at ful throttle is 7% of mag field.

Compass

--------------------------------------

enabled

Mag Dec: 0.0000

Mag Off: -76.1009, -21.5024, -81.3041

Motor Comp: Throttle

Comp Vec: -21.53, 11.95, -7.88

So what would be causing the instability (dramatic roll and pitch) while flying in Loiter or being hit by a gust of wind while in Altitude Hold/Loiter?

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