I'm having problems with APM 2.6 running 3.1-RC2 on Octocopter. Stabilize works well, but Alt-Hold and any other "automatic mode" like Loiter or RTL is problematic. I've tried default PIDs and played around with them but no luck. The main problem is rather strange motor behavior. 

A picture from datalog is attached, note the throttle on Alt-Hold. I'll also include a photo showing Accel X, Y and Z vibrations, which should be OK.

What could be the problem? Has anyone gotten Alt-Hold to work well on 3.1rc2 Octocopter and is willing to share the values?

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It's quite possible this is the processor loading problem on Octos.  This type of behaviour is what led to my investigation which is when I discovered the problem.

I've tried to keep the CPU load on minimum by not using Simple Mode (& mount) and keeping the datalog on default. Is there anything else to do to lower the CPU load?

Not really, other than I turn the logging right off.

Hello,

I have the same problem with 3.1RC4 on a Quad. How to solve this?

Regards.

Diego,

    Almost all altitude hold problems are caused by vibration.  There are instructions on that link about how to check.  The other thing that might help is reducing the Alt Hold P value from 1.0 to 0.5.  Hope this helps.

Thank you Randy!

Regards.

Hello Randy,

I'm checking the vibrations of my copter, and was on the normal range.

I have exactly the same problem of this user: Shyam Balasubramanian http://diydrones.com/forum/topics/arducopter-3-0-1-released?comment...

Please, can you help us?

Regards.

Diego,

    Sometimes similar symptoms are actually quite different problems.  To be sure, can you provide a dataflash log with the IMU message turned on?  There are details here on how to do this.

Hello Randy,

Thanks for your support!

There were slight traces of wind, but this Up-Down movement is very consistent. Goes up and down by same distance everytime.

I flew later that day on another site, with less wind,  and it worked fine.

Waiting for your reply, best regards,

Diego.

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Diego,

     Looks like you're getting little spikes in the barometer altitude.  Spikes like this are normally caused by not having foam over the barometer.  3DR APM2.5/2.6 come in a case with a sponge over the baro to avoid this type of problem but if you're not using that then you can just rig up your own solution fairly easily.

     The other possibility is light hitting the baro at just the wrong angle.  You wouldn't think that a baro would be sensitive to light but strangely they are.  Normally a piece of black electrical tape over the top of the case will sort this out.  The latest 3dr cases have a piece of electrical tape on the inside of the case to stop this...but again..it's just a piece of tape.

Hello Randy! Thanks for your reply!

Actually I have the 3DR case. Therefore the light hitting the baro isn't a possibility. The only possibility is caused by not having foam over the barometer. I installed the foam, under the electronic, but no over the barometer... I try to change de foam and test it.

Only have this performance with the v.3.1RC4. With the v.3.0.1 it works fine...

Best Regards,

Diego.

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