How accurate should a Pixhawk be in Althold mode

[Paul-H]

Hi all

How accurate should a Pixhawk be at holding altitude in Altitude hold mode.

Took my new quad out for a spin yesterday and found it did not hold altitude, but I might be expecting too much for a basic Pixhawk with no ad on sensors.

I entered Alt hold at about 20 foot but it quickly ended up at around 60 foot.

Is that to be expected or an indication my Pixhawk has issues.

Thanks

Paul

[hoverfly]

I would check for vibration  and make sure the baro has foam covering it, that is not normal behavior.

[Hands0n]

+1 the above.

Alt Hold relying purely on Barometer is always going to suffer from some drift.  But a 40ft variance is simply too much.  Barometers are sensitive to light, so one thing to check is that they are shielded from that, which I would expect to be of an encased Pixhawk.   Air pressure has an effect also, is the Pixhawk exposed to any large amount of prop wash? 

Have a look at your Logs, see what the variance is being logged for Barometer ... That may help give you a place to look.   See this page here ---> http://ardupilot.org/copter/docs/altholdmode.html   "Verifying AltHold performance with dataflash logs".

[Paul-H]

Thanks for the replies

Will look into the logs when I get home.

It';s mounted on the top frame of one of the generic 450 size eBay quads, I am running 4 off 10x45 props so prop wash shouldn';t be an issue and the Pixhawk is still in its case and I assume the foam is in place over the Baro, but will check to be sure.

Still getting the hang of this so it might still just be me.  I might be too old for all this new fangled stuff. 

Paul

[hoverfly]

If you are still breathing you';re not too old 

[Paul-H]

Just tried downloading and viewing the Logs, but get an FMT error message




Anyone know what it means

Three log files can be found here if anyone can view them and tell me whats going wrong.

Thanks

Paul

https://www.dropbox.com/s/orj87g57tngvx1m/57%2025-07-2016%2016-17-10.bin?dl=0

https://www.dropbox.com/s/2zgsu3y6ui8cim7/58%2025-07-2016%2016-29-14.bin?dl=0

https://www.dropbox.com/s/xos5eh0eyq539tc/59%2025-07-2016%2016-29-22.bin?dl=0

[PaulJC]

Pro wash is most likely the cause of the error here on a smaller size quad, i had nightmare problems with Arducopter and barometer errors, eded up blocking it from light and any possible air flow and it was much better but still not great, changed that particular quad over to a Naza V2 and it was miles better, Pixhawk is very capable but takes a lot of setup time.

[Mslater]

I just tried to view your logs, they are not valid Pixhawk log files so I can';t view them here.

However in answer to your question, ALT HOLD is surprisingly accurate when set up correctly. Your problems more than likely come from vibration in the Z-Axis.