On Wed, Dec 19, 2012 at 9:48 AM, James Turner wrote:
> Hi,
>
> Once 2.10 has branched (i.e, in a month!) I'm going to make a consistency
> change to a simulation variable: namely to change the body (UVW)
> accelerations JSBsim exposes to FlightGear, to come from a slightly
> different source (FGAccelerations::GetUVWdot instead of
> FGAccelerations::GetBodyAccelerations).
>
> This is because the later includes gravity and some other terms
> (centripetal acceleration, I'm guessing, but I'm not an expert) which the
> YASim equivalents do not. I'm also going to expose the values via new
> properties under /accelerations/body/foo-accel-fps_sec, for both JSBSim and
> YASim.
>
Hi James,
I believe JSBSim does include gravity and centripetal accelerations. I
believe YASim also includes gravity, but centripetal accelerations are
beyond the scope of the YAsim physics engine. This is a tiny difference
that wouldn't show up in most situations, unless you are dealing with
extremely expensive inertial sensors such that might go in a 737 or a space
shuttle.
> This will enable standard simulation of inertial acceleration based
> instruments, which I want to play around with.
>
This should already work for both JSBSim and YASim -- I believe the
following properties work well for both JSBSim and YASim aircraft. (the
only main difference being that JSBSim includes centripetal accelerations
which doesn't matter if you are playing with code that will eventually be
driven by consumer level mems sensors.)
Gyros: /orientation/{p,q,r}-body
Accelerometers: /accelerations/pilot/{x,y,z}-accel-fps_sec
Both of these are expressed in the pilot/body frame of reference. The
accelerometer values include gravity. I've used these to drive a 15 state
kalman filter and observe good attitude estimate convergence for both yasim
and jsbsim aircraft.
Because of the current discrepancy between JSBSim and YASim, the body
> accelerations are currently *not* exposed in any standard location (JSBSim
> exposes them in its private property tree), and code is avoiding using
> them: for example: (from the multiplayer code!)
>
> // accels, set that to zero for now.
> // Angular accelerations are missing from the interface anyway,
> // linear accelerations are screwed up at least for JSBSim.
> // motionInfo.linearAccel =
> SG_FEET_TO_METER*SGVec3f(ifce.get_U_dot_body(), ifce.get_V_dot_body(),
> ifce.get_W_dot_body());
>
> The JSBSim values aren't screwed up, I think, just including other
> non-inertial terms which YASim does not. Since it's the inertial
> accelerations I want to sense in the instruments, that's why I will tweak
> the JSBSim<->FGFS interface and not YASim.
>
> Any comments or observations on this, please let me know. As I said it's
> all only relevant for 2.12 - I won't make any changes until after we branch
> for 2.10
>
See above, I do believe they values of interest are exposed in the
flightgear tree with identical property names and identical units. It's
always possible I'm mistaken, but my kalman filter seems happy enough which
isn't direct forward proof, but says something I think?
Thanks,
Curt.
--
Curtis Olson:
http://www.atiak.com - http://aem.umn.edu/~uav/
http://www.flightgear.org - http://gallinazo.flightgear.org
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