[Question] Need some clarity on reference frames usage in ArticulationData

[kbkartik]

Question

newbie to sim2real world. I am trying to understand how reference frames are setup in ArticulationData. My doubt is w.r.t the following 6 methods:

1. root_link_state_w()
2. root_com_state_w()
3. body_link_state_w()
4. body_com_state_w()
5. com_pos_b()
6. com_quat_b()
7. root_com_lin_vel_b()
8. root_com_ang_vel_b()

Among all these methods, I am seeing multiple definitions of reference frame such as:

1. simulation world frame
2. body frame
3. center of mass frame
4. actor frame
5. orientation of principle inertia (or principle axes of inertia)

I know what reference frames are. What I don't understand is how its setup in IsaacLab? For instance,

1. Why do we adjust linear velocity in root_link_state_w() and (pos, quat) in root_com_state_w()?
2. Why do we use com_pos_b and com_quat_b in root_link_state_w() and root_com_state_w()? Are we transforming from body frame to world frame?
3. Why do we do update_articulations_kinematics() in body_link_state_w() and body_com_state_w()?
4. What is the mapping/ordering of bodies in body_link_state_w/body_com_state_w's shape: (num_instances, num_bodies, 13)?
5. In root_com_lin_vel_b() and root_com_ang_vel_b(), why are we using root_link_quat_w to inverse rotate root_com_lin_vel_b and root_com_ang_vel_b? Shouldn't we be using root_com_quat_w? Is this a bug?

Some clarity on the above questions will definitely be helpful to understand the code and the documentations.

best,
kb

[RandomOakForest]

Thank you for submitting these questions. We are working on improving our documentation. I will move this to a discussion for the team to follow up.