[ODE] proportional derivative controller

[Martin C. Martin]

 > My main question is this: Can I simply call
 > dHingeAddTorque with the torque calculated above?

Yes, that's what I created it for.

 > Do you have any advice?

Standard debugging stuff.  Look at the values you're passing to 
dHingeAddTorque just before/as it explodes.  Are they increasing?  Does 
the increase cause the explosion or the other way around?

- Try limiting the difference in velocity, and/or the difference in 
position, and/or a maximum overall torque.

- For the velocity difference: see if the torque, over the timestep, 
would cause the joint to not only stop rotating, but start rotating the 
other way.  If so, limit it to the amount needed to stop in one timestep.

Cheers,
Martin