Help in finding the right qpos and ctrl values while defining the keyframe about mujoco

[nimrod-gileadi]

qpos is an array that holds all the joint position values for all joints in the model.

For normal robot joints (e.g. hinges, sliders), there's one entry per joint. For free joints (the "joint" between a free floating object and the world), there are 7 entries: 3 for the position and 4 for the quaternion representing the object's orientation, and the order in the array is the same order that they're specified in the MuJoCo model XML.

ctrl is an array that holds the control commands for the robot. The length of the array is the number of actuators in the model.

See the Computation chapter in the docs.

[yuvaltassa]

model.actuator("actuator_name" or actuator_id).trnid[0] will give you the id of the joint actuated by this actuator, assuming it is connected to a joint. I hope this answers your question.

[rs545837]

@yuvaltassa Here you mentioned that qpos has values of joint angles. Also, Is there a way to access the actuators directly or I have to access it through mjData.joint(i).name, where i is the index for actuators.
I didn't find anything about just accessing values like qpos, qvel, xpos etc. for just actuators.

[yuvaltassa]

Your question doesn't make sense. Actuators are not directly associated with dofs. For example an actuator can be attached to a tendon that will pull on multiple joints. Certainly an actuator doesn't have a position.

Try to say what you are trying to achieve, rather than where you got stuck trying to achieve it.

[rs545837]

I am sorry for my stupid way of asking. I can very well see I was not clear in the question. So basically I have two questions as follows:

First One
qpos stores the value of joint position or joint angles.
I want to define functions that can access joint positions and joint angles, so how to do that.

Second One
I just noticed that the tutorial.py for MJX got changed. I was basically trying to make some functions in the MjxEnv class according to the prev tutorial, that returns things like joint_torques, joint_positions, joint_velocities but for only actuated joints. For accessing those particular joints, I used an array of all the actuated joints.

Then I am trying to return these stored values as:

My question was, we know that information about control commands is stored in ctrl, so is there also an option of array as well where the actuated joints are directly stored, and I can find info just about them, or is this the only way.