Add new ViscoElasticContacts

[diegoferigo]

WIP

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📚 Documentation preview 📚: https://jaxsim--248.org.readthedocs.build//248/

[xela-95]

Hi @diegoferigo, another early comment since I'm testing this contact model: in jaxsim.api.ode.system_dynamics @

jaxsim/src/jaxsim/api/ode.py

Lines 378 to 387 in 986854e

	case SoftContacts():
	ode_state_kwargs["tangential_deformation"] = aux_dict["m_dot"]
	case RigidContacts() | RelaxedRigidContacts():
	pass
	case _:
	raise ValueError("Unable to determine contact state class prefix.")
	# Extract the velocities.

we should add a case for this class of contact model.

From collidable_point_dynamics I can see this model outputs some auxiliary data, that should be extracted:

jaxsim/src/jaxsim/api/contact.py

Lines 235 to 236 in 986854e

	aux_data = dict(W_f_avg2_C=W_f̿_Ci, m_tf=m_tf)

[diegoferigo]

[...]
we should add a case for this class of contact model.

From collidable_point_dynamics I can see this model outputs some auxiliary data, that should be extracted: [...]

Thanks @xela-95 for the feedback. The dictionary returned by the visco-elastic contact model is somewhat different then the one returned by the soft contact model. The latter returns $\dot{\mathbf{m}}$ (the derivative of the material deformation) that needs to be integrated together with the generalized state $(\mathbf{q}, \, \boldsymbol{\nu})$. The visco-elastic contacts, instead, directly compute $\mathbf{m}(t+\Delta t)$, therefore we need to override $\mathbf{m}$ of the extended state similarly than the velocity reset done in by the rigid contacts.

Regardless, I'm still unsure how to integrate the new visco-elastic contacts with the default jaxsim.model.step function. What I just wrote is only necessary when the default integrators are used. The best way to exploit this new contact model, however, is to use its own integrator jaxsim.rbda.contacts.visco_elastic.step, that integrates both the generalized position and velocity using the contact forces computed explicitly with the continuous exponential integrator.