How to get global position, linear velocity, orientation and angular velocity

[Hardwarize]

Intro

Hi!

I am a student at UNED, I use MuJoCo for my research on human motion generation.

I'm trying to replicate Control-VAE using MuJoCo, but I'm yet very noob with it.

My setup

Py-MuJoCo

My question

I need to get the bodies position, linear velocity, orientation and angular velocity in the with regard to global frame for every body in a humanoid model (not the same but similar to the one that you can found in the MuJoCo environment in the gymnasium package).

I think that i can get the position using the global position using the xpos attribute, reading the documentation it seems that I should also been able to get the global lineal and angular velocity with mj_objectVelocity but I've doing some experiment with a simple model (specifically the reacher also from gymnaisum package) but when i use the mj_objectVelocity function it does not provide results that made sense for me, an example, the "body0" is at the same position than the joint, so, if I'm not wrong it shouldn't have any linear velocity, but if I execute the code that you can find below the linear velocity is not 0, I suppose that I'm misunderstanding something.

Minimal model and/or code that explain my question

import mujoco

model_xml = """







































"""

model = mujoco.MjModel.from_xml_string(model_xml)
data = mujoco.MjData(model)
model.opt.timestep = 0.001

data.ctrl[0] = 0.1

for step in range(421):
mujoco.mj_step(model, data)

vel_array = np.zeros(6)
body_id = get_body_id_by_name(model, "body0")
mujoco.mj_objectVelocity(model, data, mujoco.mjtObj.mjOBJ_BODY, body_id, vel_array, 0)

print("Linear velocity for body '{}':".format("body0"), vel_array)

Confirmations

• I searched the latest documentation thoroughly before posting.
• I searched previous Issues and Discussions, I am certain this has not been raised before.

[yuvaltassa]

You probably want mjOBJ_XBODY. "BODY" is the inertial frame of the body, "XBODY" is the traditional body frame. Not great naming, I know...

[Hardwarize]

This solve my doubt, thanks!