Re: Force feedback integration : need help about physics

Hi, Christos, and all.

>> Currently working on force feedback integration in Torcs,
>> I am very close to the moment when I'm able to inject some effects
>> into my wheel.
> 
> Since we already use the Pacejka formula, adding the aligning force is
> trivial.  I hope that it works correctly.
> 
> I guess I can export a couple of variables in some structure..
> How about
> struct SteeringFeedback {
> 	float align; // magnitude of the horizontal force
> 	float bump; // magnitude of the vertical force
> 	float bump_frequency; // in case the force updates are too slow
> };
> 
> Now, there bumps may be a bit tricky.  The simulation's dt is currently
> 0.002 if I remember correctly.  I think that the control feedback is set
> to 0.01, though I could be wrong.  In that case, you wouldn't need a
> bump_frequency, just a magnitude for the vertical forces and then how
> that is handled would depend on the actual feedback wheel used.

If I understand correctly (raceengine::ReUpdate/ReOneStep), the physics engine
call period can't be lower than 0.002s (RCM_MAX_DT_SIMU), and the human input
device read period can't be lower than 0.02 (RCM_MAX_DT_ROBOTS).
As I want to update feedback force in the device each time Torcs reads it
(it's easy to implement), this means that we can't render a vibration
due to a run on a -say - 0.5 m period kerb, if the speed of the car
is higher than 45 km/h (if Shannon is right ;-)
(2 * car speed (m/s) / bump period (m) < feedback force update frequency)

Or 1m period kerb at 90 km/h ...

It'a a bit poor I think (if I am correct).

On the other hand, it seems that my wheel can't render periodic forces
under 50 or 40 ms period ... so, it may be poor, but the device can't do better.

And of course the final question is : what a real car mechanical steering chain
can render, as far as high frequency vibrations are concerned ?
I don't know that ...

So, at the end, you are right : we don't really need the bump frequency ...

> Are there any other forces that you'd feel on the wheel?

After some researches about force feedback over other racing cars simulation
games, I tried rFactor (and found it a dangerous competitor against Torcs (;-).
In rFactor, the aligning force injected to the steering wheel decreases
as the grip on the front wheels loose the grip (with some mysterious damping
formula) ... I don't know if this "effect" will already be in the "Magnitude
of the horizontal force" you would export from Torcs physics engine ...
but if not, it would also be very interesting to export some kind of grip value
on the front (=steering) wheels.

> I think I'll add the structure in interfaces/car.h and
> add it to the tPrivCar structure.

I can imagine you are very very very busy with family, work and many passions
(this is interesting life ;-) ... but could you give us an idea
of when you could be able to release such a patch ?
Or if not, could you find some time to give us enough tips about simuV2/V3
to enable us to try and achieve it ourselves ?

Thanks a lot for your work and help.

Jean-Philippe.




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