I flew my 3DR-B tonight in startlingly windless conditions.  It was the first time I've really tried to test Loiter.  I got the quad into a nice stable hover (it was hovering practically dead still in these conditions with no control input at all) and flipped to LOITER.  It did a pretty good job of staying in place but started to sort of orbit around the spot it started in.  These orbits slowly got wider, to the point where I got uncomfortable and switched back to stabilize, though it took a minute or so.

Is this normal?  Do I need to tune my LOITER PID?  I noticed that the simulator has a default LOITER D of 0 while the default for Arducopter is 0.4.  Changing the sim to 0.4 seemed to make it oscillate more in windless conditions, so maybe the answer is to change my D to 0?  But will that negatively effect performance in windy conditions?

Here's my log from tonight: 2012-07-09%2023-07%202.log

Thanks for any advice.

Tags: PID, loiter, tuning

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What version of the code?

2.6

That's on APM2 btw, I didn't make that clear.  More info on my quad is here

What your seeing is lag in your GPS input being too much for the rate_P value you have. Try lowering loiter rate P.

The reason it's jumping out of a perfect loiter is the GPS hits a threshold and makes the quad think it's traveling faster than it is. it over-corrects with the High gain and you start an oscillation.

Jason

Thanks Jason, I'll give that a try.

Would the new ublox GPS help with the lag?

I found that with my small quads it works best to reduce Loiter P to 0.2 and Loiter I to 0.05. It also seems to make a difference when I change Nav P to 0.9 and Nav I to 0.05. Finally got Loiter and waypoint flying OK in wind with purple motors, 10x4.7 APC props, 1250g weight and motors on 570mm pitch diameter.

Problem I have though is that when I fly waypoints and choose Loiter turns, it goes into a 45deg bank and drops out of the sky when it starts executing the command. Any suggestions please?

I had some success with reducing the loiter P, I got it to the point where my loiter would stay within a 5m box or so.

Then I got the ublox GPS, which is worlds better in terms of locking on quickly and getting a higher satellite count / lower hdop.  However when I did some loiter tests I got a very strange result where the quad started an expanding spiral that rapidly got out of control.  Anyone know why this would happen?  I haven't had much time to play with it.

Logs from the incident I'm referring to are attached.

Thanks

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Never underestimate the compass miscalibration possibility. 

hi,

i've just install 2.9 rc2 on my apm2.5. i have just tried loiter in stock pids, and i see quite oscilations.

since i'm a noob, i need advice in which PID i should tune ? attached is my loiter log and param.

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