rec.autos.simulators

First off, thanks to everyone who responded to the "slip ratio=0"
thread. I've implemented SAE950311 and it seems to be working well,
I can move forward and backward (no slip angle yet!) with somewhat
appropriate amounts of wheel slip, and it looks good.

Now I'm trying to close the loop and return the road reaction force
back through the drivetrain. However the way things stand now, within
0.1-0.2 secs after the car starts to move, the road reaction force
has climbed to an order of magnitude greater than the engine torque,
and my engine gets slammed down to zero rpms. This may have something
to do with the fact that I haven't implemented a proper clutch yet,
but I was wondering if this is a typical result? I'm pretty sure I've
got the drivetrain modeled properly, as everything spins up at the
proper rates, and I've plugged some sample values into my Pacejka
stuff and that looks okay. It's possible that I'm calculating the
slip ratio wrong, though I've been over it a hundred times now.

Anyway, intuitively it seems wrong to be getting 10x more torque
back from the road, even if all of the formulas seems to be right,
especially considering that everyone seems to be able to get away
with a simple 'torque=engineTorque-reactionTorque'. Any insights
would be appreciated.

Jason

You passing the tyre torque back to the engine through the gear system
properly ie 1/(gear ratio)

I'm not using T/(gear ratio) to pass the torque back, rather I
am calculating the acceleration and resulting torque loss at
each component in the drivetrain - since all the parts are
accelerating at the proper rates, I assumed that I had gotten
that part right. That said, T/(gear ratio) results in a *much*
lower torque arriving at the engine, enough to fix the problem,
but I would have to question the accuracy?

Jason

Surely the drivetrain is one system (ignoring slip at the clutches) so why
calculate all the different speeds when you can calculate one and set the
other speeds from that. You can still model the torque losses due to
friction by adding up the torques with respect to ratios, and summing the
moi's by using the square of the ratios.

Essentially what I'm doing. Regardless, I end up with aw=ae/Ntf for
my engine torque, and ae=aw*Ntf for my reaction force, so it would
appear that my drivetrain is not the problem. I accelerate the wheel,
then (in the next timestep) compute a slip ratio and Fx. Assuming
that my drivetrain is correct, and my Pacejka formulas are correct,
that would leave my slip ratio calculations at fault.

I do notice that the SR I calculate with 950311 is considerably
smaller than what I would get with the traditional slip formula,
but I suppose that would be expected since it takes the relaxation
length of the tire into account. I'll keep investigating.

Jason

On Tue, 13 Nov 2001 20:11:37 -0000, "Carol Bekker"

You'll have to use 2 speeds when you add the clutch; the engine speed
then becomes separate from that of the drivetrain  (which is indeed
only modified by ratios depending on where on the train you are).

Ruud van Gaal
Free car sim  : http://www.marketgraph.nl/gallery/racer/
Pencil art    : http://www.marketgraph.nl/gallery/

Did you add the damping as described in the appendix? Does that work
with your code?

Apart from the ratio thing, yes, this also happens with braking; in my
sim the rpm goes down to 0. With road reaction it doesn't happen as
quick though (because of the ratio). I also make sure my engine
doesn't generate 0 torque at 0 rpm (but instead start at the torque
value for say 1000 rpm).

Also note the double ratio; differential(s) and gears. That's often
quite a factor.

Ruud van Gaal
Free car sim  : http://www.marketgraph.nl/gallery/racer/
Pencil art    : http://www.marketgraph.nl/gallery/

You might want to check with the numbers. Take a reasonable
acceleration of the car, divide that by 4, then you get a reasonable
traction (= road reaction force) per wheel in Newtons. If that's ok,
then it may be something else.

Yes, it acts like a filter somewhat.

Ruud van Gaal
Free car sim  : http://www.marketgraph.nl/gallery/racer/
Pencil art    : http://www.marketgraph.nl/gallery/

Check and make sure that the accelerating force
at the tires is roughly     enginePower / velocity
(in proper units) at various speeds
as you accelerate.

(although you might be tire friction limited at
very low speeds rather than having infinite acceleration ;)

If you aren't reasonably close to this (within 10-20%)
in the middle speeds where the acceleration is less
and the aero drag is still low, then you have some major
problem with units or gear ratios or something
still to work out.

- Matt

It acts like a regularizing, physically sound filter. No reason not to
use it :).

-Gregor

Speaking of relaxation length damping; I have no idea how the damping is
done in the paper, but what I use is that not only do I feed the slip
ratio (SR) into the force calculation, but the variable

SR_enter = SR + tau * (dSR/dt),

where dSR/dt is just the rate of change of SR as found in the basic
equations in the paper, and tau is some time constant that should be
played with. The same goes for the slip angle as well, although the time
constant for that one might be much different. Works well in my sim.

-Gregor

Thanks everyone for your suggestions, but I got it figured out
last night (finally! whew!). It was, typically, a stupid error.
When I recoded my drivetrain to handle the torque coming back
from the road, I accidentally cancelled out one factor of the
gear ratio in my torque/inertia calculations. Because it was
in both calculations, the wheels and engine still accelerated
at the proper ratio. The wheels weren't receiving as much
engine torque as they should have, but the inertia at the
engine was very low, so the car still accelerated at about
the right rate. The return torque wasn't being reduced by
the gear ratio and the change in inertia isn't as big at
the wheels, so it ended up too high.

Lesson learned: always validate your results in as many ways
as possible. When I first wrote the drivetrain code, I checked
it against all the equations in Gillespie ch. 2. When I
modified it later, I only checked to be sure the accel ratios
were correct and didn't catch the bad torque numbers.

Whew, what a relief to have that out of the way. Thanks
again for the suggestions.

Jason

On Thu, 15 Nov 2001 10:09:39 +0100, Gregor Veble

IIRC, the SAE950311 uses tanSR as a state variable, and derives SR
each step from tanSR. tanSR is updated every step, where the
relaxation coefficient comes into play (don't know how that was
exactly).
After all that, an extra damping *force* is applied at low speed
(v<0.15 m/s is suggested), which is derived from the vibration
frequency from the tire (just like a swing). So something like
F_damping=sqrt(tire_longitudinal_spring_stiffness/tire_mass).

Adding it to the other forces (F_long=pacejka_Fx(SR,...)) is where I
go wrong. Must probably always counteract the F_long (so if F_long>0,
F_damping<0 and vice versa).

It looks here like you put the 'relaxation' factor a bit later on, but
other than that its effect may be the same.

Ruud van Gaal
Free car sim  : http://www.marketgraph.nl/gallery/racer/
Pencil art    : http://www.marketgraph.nl/gallery/

I saw you mention this in another post, though I wasn't paying
enough attention at the time. Is tau a function of relaxation
length, or timestep? That is, does tau need to be set per tire
or can one value suffice for the sim? I'll have to go find your
original post. This approach is certainly simpler than what's in
the paper (which I don't have with me right now or I'd try to give
a quick overview).

Jason

Hi Jason,

tau is just some time constant that certainly cannot be larger than your
timestep for the sake of stbility, but the higher it is the higher the
damping. I should do an analysis again of how much it should be to
dampen oscillations perfectly, but usually with a bit of experimentation
a good value can be found for a tyre with given properties. Start with a
tenth of your timestep and move up/down accordingly, while I will try to
stop being lazy and perhaps give an estimate :). Just keep in mind that
indeed the damping needs to be smaller in the longitudinal direction as
the inertial part there is much smaller (the rotation of the wheel is
easier to induce than the lateral car movement).

-Gregor