Hello, I am trying to better understand the actuation model in Mujoco, and I am specifically referring to this section of the documentation: Apologies if I am missing something obvious, but I have some trouble understanding Li, grad_Li and Li_dot (what they mean, and the difference between them). Here is my understanding/ confusion regarding those quantities: 1) Li is the length or position of the actuator (from http://www.mujoco.org/forum/index.php?threads/simple-position-servo.3389/#post-4715) : What does this mean? What is the difference from say the joint position qpos for instance? I am confused because in the XML we do not define a position for the actuator, so what does position mean here? 2) Li_dot is the velocity of the actuator? Again not sure what this means here? So if we just create a velocity actuator this will be a propositional controller in velocity with gain k_v? 3) Grad_Li(q) is a scalar that will scale the actuator force pi of actuator i to give a force applied to the joint q, and which depends on the geometric properties of your model: Is that correct? I would appreciate any help with this very much, and please let me know if I am mistaken in some of my understanding above! Thanks!