Pixhawk Pitch and Roll Sensitivity in FBWA Mode

Posted by Jacques Eloff on September 7, 2015 at 12:54pm

Me again with another query - hoping someone can help me out again!

I have my X-5 wing flying nicely baselined in manual mode but every time I try FBWA mode I have almost no response and have to revert to manual just to complete a turn at my local model club. Same thing goes for Autotune mode. I have double checked the LIM_ROL_CD, LIM_PITCH_MAX and LIM_PITCH_MIN parameters and they seem to be high enough (4500 and 2000/2500 respectively) so what could be wrong?

I also did a little ground check by holding over aileron all the way to one side, then slowly banking the aircraft over (in my hand). Sure enough, when it gets to around 20 degrees bank angle, the elevons are level with each other which would seem to indicate that LIM_ROL_CD is too low (certainly well below 45 degrees!).

I have done some searches on this issue and the reason for sluggishness for others was too low settings for these parameters, but I'm at a loss for my problems!

Thanks,

Jacques

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Replies

    • Jean-Marie PRIGENT > Jacques Eloff September 14, 2015 at 2:00pm

      I'm sorry - I don't know what could be the 'problem' if it's ok in manual and other mode. For me too, aileron/elev are less than in manual and stabilize but it's not a big issue on my x61. I'm using stabilize mode more often if there to much wind and need lot of responsivness.

    • Jean-Marie PRIGENT > moglos September 13, 2015 at 2:37pm

      I've got similar problem with my FX-61 flying wing : 1 year ago and when I discover 'elevon mixing' > I set it from 0.5 default value to 0.6 then 0.7 > performance of stabilsiation and flying had increase a lot. 

      Default 'elevon mixing' of 0.5, avoid saturation of mixing but it's clearly not enough to have good performance in windy condition for example.

  • DougB September 12, 2015 at 10:55am

    I have an RVjet doing the same thing. I'll try some of these solutions and see if I can get it to work.

  • Jacques Eloff September 12, 2015 at 4:45am

    So I fiddled around a little more with my X-5 on the work bench by holding full aileron and banking the model over by hand. In FBWA mode I’m definitely getting no more than 20 degrees of bank angle by the time the elevons are level with each other. I then enabled Auto mode and found that it allows the full 45 degrees of bank so this issue is unique to FBWA mode (just as Martin Poller found). I also tried what Martin did and disabled stall prevention and got the full 45 degrees in FBWA mode but really don’t want to be flying without stall prenetion.

    So what could be limiting my bank angle???

     

    As moglos suggested, I did check that airspeed sensor is disabled as I don’t have one installed. I actually just purchased one and may give it a try later.

    Jacques

  • Ben Harris September 8, 2015 at 11:41pm
    Jacques

    I remember reading this a few weeks ago from someone who was having a similar problem, and wonder if it could be the same or a similar issue?


    http://flitetest.com/articles/pixhawk-fbw-a-autotune-lethargic-reso...
    • Martin Poller > Ben Harris September 9, 2015 at 7:20am

      Ben/Jacques

      I'm having the same issues, limited roll in FBWA mode, with the roll limited to about 15 degrees of bank despite the LIM_ROLL_CD parameter being set at 4500. Giant slow circles with limited climb on take off.

      Yet if I change to AUTO mode the roll limit is exactly as expected at bang on 45 degrees.

      If I increase LIM_ROLL_CD to 6000 the roll limit does increase in FBWA but then it is at 60 degrees when I change to auto mode, which is too much.

      Ben, the link that you have posted seems to say that the ppm values are being reduced by the FBWA mode and the solution is to increase the ppm values, could someone tell me how to do this without affecting the other flight modes.

      I have only referred to roll but pitch is similarly affected.

      Thanks.

    • Martin Poller > Martin Poller September 9, 2015 at 2:34pm

      Stand down guys,

      After a bit of reading I disabled stall prevention and in FBWA mode the roll limit now achieves the full LIM_ROLL CD parameter value.

      Happy flying.

    • Jakob Schmidt > Martin Poller September 9, 2015 at 6:27pm

      You might be better off lowering the arsp_fbw_min:

      http://plane.ardupilot.com/wiki/arduplane-parameters/#minimum_airsp...

    • Martin Poller > Jakob Schmidt September 10, 2015 at 6:07am

      Thanks Jakob, I'll try that, would that mean that I can keep stall prevention active?

    • Jakob Schmidt > Martin Poller September 10, 2015 at 10:44am

      That's the idea. Ideally you should figure out the stall speed first and then set the fbw_min above that.  It's a little tricky to find precisely, but to my best estimates my Talon has a stall speed of ~10m/s, so I have fbw_min set to 13m/s. This gives me a nice buffer, so along with the stall prevention code, it should never stall while in any kind of auto-mode.

      Having said that, a gusting tail wind can still throw off the airspeed estimation code,  so if you want to rely on the stall prevention, you should probably get an airspeed sensor.

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