A quick look suggests you need to get a reference to the existing SimbodyMatterSubsystem rather than a copy:
Regards,
Sherm

Right! Sorry I didn't think of that earlier. Mobilizer forces are conceived as being generated by a motor in the joint so act equal and opposite on the connected bodies.

Oh, I think I understand now. If you don't want the generalized force Qa included in the reaction, have you considered applying it as a body force to A rather as a mobility force? Then all you would see in the reaction is the coordinate coupler force and the sliders' reactions to gravity. Otherwise ...

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