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Re: [Paparazzi-devel] JSBSim for fixedwing example

From: Stephen Dwyer
Subject: Re: [Paparazzi-devel] JSBSim for fixedwing example
Date: Tue, 15 Feb 2011 22:16:00 -0700

Hello Andreas,

Just wanted to thank you for your help. I have got JSBSim simulations
working now, the speedup patch really helped. It is now merged into
the paparazzi master branch. I will be updating the wiki to better
explain the process as well.


On Thu, Feb 10, 2011 at 1:50 AM, Andreas Gaeb <address@hidden> wrote:
> Am 10.02.2011 04:14, schrieb Stephen Dwyer:
>> [...]
>> alt= 668.773061 time= 0.187000 lat=0.934216 lon=1.766669
>> [...]
>> alt= 664.905993 time= 0.238000 lat=0.934217 lon=1.766668
>> gspeed=166771165678573428146176.000000
>> Something clearly is not working out well.
> looks like the airframe drops below the ground and the gear divergence
> occurs. I noticed the call to check_crash_jsbsim in sim_ac_jsbsim.c:82
> is commented out in the master branch, so that's why the divergence is
> not caught. Commenting it in should at least stop the simulation when
> contacting ground.
>> I believe the Invalid_argument("Latlong.of_utm") originates from the
>> latlong.mli file in paparazzi/sw/lib/ocaml/ right around lines
>> 107-108. But I could be wrong, I really don't understand OCAML at all
>> unfortunately (I can barely read C).
> the error message may be caused by a missing GPS_USE_LATLONG flag, but I
> don't think that's the case here. If the message only showed after the
> divergence set in, the reason would be clear, but as it already shows
> with the airframe waiting for takeoff, I have no idea what's causing it.
>> Another thing, changed one other thing in sim_ac_jsbsim.c, I switched
>> the SetDebugLevel for JSBSim to 4 and I notice that will the frames of
>> the simulation continue to propagate, the time of the simulation never
>> makes it past 0.2 seconds (frames keep growing time goes
>> 0.0,0.1,0.2,0.2,......,0.2,..........).
> this might be due to the divergence. You could check this by printing
> out FDMExec->GetSimTime() with more digits. JSBSim also freezes if it
> detects a crash (hitting the ground too fast) internally, which means
> integration is suspended but the program does not exit.
>> Are you using the current github code or have you made significant
>> changes to your code along the way to make jsbsim work?
> I have made a few changes here and there, though nothing should touch
> basic functionality.
> One thing that might prove helpful is attached as a patch: run JSBSim
> multiple times per control loop execution. 60 Hz is quite a small
> integration rate for RC model sized aircraft. I use a speedup factor of 4.
>> Any other ideas on what the matter may be?
> you might try playing around with the gear spring constants. Physically
> meaningful values usually require integration rates of several kHz,
> though, so you'll have to look for values that work without too much
> divergence. Speeding up JSBSim integration will help as you won't drop
> so far below the surface during a single time step. However, if ground
> contact is detected, the sim should stop anyway, so I wouldn't spend too
> much time on this.
> Best regards,
>        Andreas
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