LibrePilot Forum
Users => Applications - Autonomous Flight => Topic started by: nuggetz on May 12, 2016, 05:24:17 am
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First off I have to say that I'm really impressed so far with LP. I was expecting a lot more trouble getting GPS working. I was previously using different firmware for my GPS craft and decided to give LP another try.
Anyway, position hold seems to be doing its job very well. I'm satisfied with the performance so far except for a minor issue. I'm noticing small oscillations while its maintaining position in the sky. Can't say I ever noticed this before using different firmware but nonetheless I see it now and was wondering if this is normal or related to any tuning parameters that affect PH specifically. The PID's that I'm using are the preset ones from LP. I selected the Flamewheel 450 setup as that is the craft I built. Is there any way I can tune the oscillations out? The quad doesn't oscillate under normal flight or when hovering - only when trying to position hold.
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Balance props.
Balance frame.
Check that the level calibration is a level as possible.
In the direction of oscillation turn down PIDs slightly. The stock #s are a starting point, but no two airframes are ever exactly the same.
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Yeah I'm pretty much well balanced and stock PIDs are fine for normal attitude mode flights and don't exhibit any oscillations whatsoever. Perhaps I'm just seeing the tiny corrections the quad is making to maintain its position in the sky. If there were a separate set of PIDs for GPS modes this would look like P being too high. The corrections are very snappy. I see some settings for tuning altitude hold - would any of that apply here?I haven't tested AH yet and perhaps those settings are more baro related?
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There indeed is yet another set of PIDs that are only used during GPS flight.
First set is Rate mode (inner loop)
Second set is Attitude mode (outer loop)
Several third sets are in System -> Settings -> VtolPathFollower
These should generally be tuned in order, but most normal stab tuning takes care of both inner and outer loops. Also a plug for the new version that is always about to be released (for the last 5 months anyway :) ) it has an autotune feature.
Also, you can get oscillations if your mags are being affected by motor current. This is usually like "toilet bowl". If the mags are really bad you can even get a fly away, so always keep an Attitude mode on your switch and PLEASE set up / test your failsafes (RC and FC) correctly even for non-GPS flight.
I recommend that you use VelocityRoam mode instead of PositionHold mode. It is the same when the stick are centered, but VR allows you to move the hold location around in very entertaining ways.
As always, start simple, and get the simple system working first, for instance get your stabilization PIDs working without GPS, then GPS and mag (mag usually green even with motors running at high power) working and tested in Basic (Complementary) before going to GPS Nav (INS13). Get VelRoam working first and without "Assisted Control" or Braking or Takeoff or Landing, etc.