On Nov 30, 2007 10:11 PM, Jon S. Berndt <> wrote:
> Some of the engineering sims I use at work (space shuttle) have very
> detailed models of landing gear and tire spinup, etc. Of course, we don't
> ever see power trains driving the wheels (at least we don't in JSBSim).
> There are some simplifications made in our gear model that suffice for
> modeling what planes do. However, out of curiosity, I am interested to see
> how ground reactions are modeled for autos - particularly for the case
> when
> the vehicle is at rest.
>
> I am familiar with Pacejka's magic formula, etc. It can be a complicated
> problem, so it's useful and interesting to see how others approach the
> problem.
I think it's worth pointing out that in FlightGear cvs we have models of a
jeep and of a snowplow (truck). The dynamics are based on YAsim and the
wheel/suspension modeling seems to work really well. I was actually very
impressed at the sorts of things that YAsim does ... Andy cooked a little
bit of physics magic in there some how! Things I notice when playing around
with the snowplow:
- The suspension at each tire is modeled independently.
- The suspension reacts to surface properties ... like smooth pavement,
rougher grass, etc.
- If you drive into a lake or ocean you sink.
- If you drive off a bridge you dive end over end pretty realistically.
- When you corner, the individual suspension elements seem to react
correctly. The front outside tire seems to dig in as you turn sharper and
sharper.
- As you corner more and more sharply, you need more power to maintain the
same speed.
- If you turn too sharply, you can actually roll the vehicle ... and
visually, it looks very realistic.
- The vehicle reacts to wind.
- There is great interaction between the larger vehicle/body dynamics, the
individual suspension components, and the surface. The vehicle reacts
correctly to slopes and change in terrain. I caught one view where I was
driving over the edge of some detailed road I created for a day job project
and there was a lot of slope/surface variation in the triangle mesh.
Watching this big truck barrel over that with the body and suspension all
working together ... visually it looked "right on". I wish I would have
been able to capture that particular sequence as a movie, but it's one of
those sorts of fleeting things and it's difficult to reproduce the exact
same sequence of speed, vehicle path, and view point.
- So then if you poke around our aircraft fleet, you find a catalina and a
beaver on amphibs ... you can literally take off on wheels, retract them,
and land on the pontoons, take off and land back on wheels. Oh, and there's
a few helicopters available too. So I'm not saying everything is perfect,
but it's a pretty darn good little general purpose physics engine.
I would also comment that my day job (well until my contract expires in
June) [sniff, hand me another box of kleenex ... actually more like break
out the champaign] :-) involves taking care of a very expensive commercial
driving simulator. In my best estimation, the YAsim based snowplow captures
or models many more dynamics effects at a much better detail level and
realism than this big fancy driving simulator we use for human factors
research. (And we spent close to $250k when it was first installed and
probably a couple more $100k in the subsequent years on improved hardware
and software.)
I think a person could do a lot worse than looking over Andy's shoulder to
see how he took care of the gear/suspension/wheel dynamics portion of YAsim
... it's really pretty darn good. Now I'm going to guess he's not modeling
things like tire flex and some of the really subtle details some people get
into ... there's always room to nitpick anything. I don't bring this up to
nitpick, but to fend off the potential nitpickers in advance. :-)
Regards,
Curt.
--
Curtis Olson: http://baron.flightgear.org/~curt/
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