You can read about auto update state variables in the doxygen for State::allocateAutoUpdateStateVariable() . Basically you get a state variable and a matching cache entry. You write to the cache entry and then that gets used to update the state variable at the end of every step. There are several ex...

Hi, Simone. A few thoughts: - To get reaction force dependent friction, it makes sense to use the previous-step reactions to avoid an intractable mutual dependence among forces & accelerations. That means you need to remember the previous-step reaction forces, and memory == state! The best way to do...

Right!

Yes, the coordinate coupler will only produce mobility forces. Those forces will already be included in the mobilizer reaction forces (converted to spatial forces) so you might not need to get them explicitly. (Mobility forces from any source are always included in reaction forces.) If you want to s...

Hi, Simone. I don't think there is a pre-existing function that would provide exactly the quantity you want. That might be a nice addition! However I can think of a few ways to get it: - thinking of a free-body diagram, use F=MA to obtain the net spatial force F from the body's spatial inertia M and...

:facepalm: yes of course you're right. You want partial(acceleration)/partial(state). Simbody can provide acceleration(state) (fast) but you would have to differentiate that numerically to get the matrix. That's easy to do by hand or using Simbody's Differentiator class.

Sherm

You can obtain the system Jacobian using

SimbodyMatterSubsystem::calcSystemJacobian(). I believe the manual is referring to the eigenvalues of this matrix as the system modes.

Sherm

Hi, Amit. Good question! Terminology for transforms and rotation matrices is tricky, because they serve as both "verb" and "noun" depending on your point of view. As a verb, we have v_B=R_BG * v_G, mapping G to B (as long as you multiply from the left!). The notation ensures you get that correct bec...

According to the OpenSim documentation here it uses a Simbody PointInPlane constraint for the normal contact. That is described here . I'm not sure why it wasn't implemented by a Simbody SphereOnPlaneContact constraint, but I believe it produces a similar result. Ajay Seth might have further insight...

The constraint is applied at the point of contact.
- Sherm