Re: [Jsbsim-devel] Slaved FDM

[Jon S. B. <jon...@co...>]

Hi Dave and Jon,

 

I have another potential usage case that might benefit from something
similar.   Let me state up front this isn't fully thought out!

 

I do quite a bit of UAV work these days.  Typically we use inertial sensors
(gyros and accelerometers) in combination with gps to estimate the current
location and attitude of the vehicle.  Gps arrives at a slower rate than the
inertial data.  Given an initial state (location, velocity, attitude) we can
estimate our new location, velocity vector and attitude moving forward in
time based on the inertial data alone, and sometime in the future (i.e. once
a second) we receive a gps reading and that combined with a few tricks
allows us to correct our location, velocity, and attitude estimate.


What we think might be interesting to investigate is to build up a flight
dynamics model of our vehicle and use that in real time (with knowledge of
the control surface locations) to provide another way to estimate our
location, velocity, and attitude as time progresses.  The results of
crunching the equations of motion could be used to help constrain potential
errors, the results could be rolled into the kalman filter along with
everything else, we could compute an attitude estimate at a higher rate than
the sampling rate of our inertial sensors.

 

I think in basic theory, what I would like to do is similar to what Dave is
proposing ... the idea of running the equations of motion and keeping the
pipeline full of good data, but slaving the position, velocity vector and
attitude to some external reference and being able to update/reset these
basic location/attitude state values without blowing everything up
internally.

 

I know Dave is talking about executing the inverse of the EOM, but I'm not
able to turn my head inside out on a holiday. :-)


Curt.

-- 
Curtis Olson: http://baron.flightgear.org/~curt/

 

 

Curt,

 

It sounds like what you want is the ability to set (or reset) the state
vector for a simulated vehicle model at will. At that time you could fly out
the model for as long as desired.

 

There are a few functions calls in JSBSim that could be helpful, but I don't
think they have been applied exactly to what you want to do, yet.

 

FGFDMExec::RunIC()  // Requires the FGFDMExec::IC structure to be populated.

FGState::Initialize(FGInitialCondition *FGIC)

FGPropagate::Initialize(FGInitialCondition *FGIC)

 

Jon