Problem with lidar, and doubt with free joint

[WeekendArchaeo]

Intro

Hi!

I am a associate professor at Universidad de Málaga, I use MuJoCo for my research on RL on mobile robots. My apologies if my questions are dumb, I've been working on them for a few weeks and reading all the documentation I have found, but still I cannot solve them.

My setup

I use Mujoco with Python on an Ubuntu 22.04 platform.

My robot is a turtlebot3 burger. The XML model has been adapted from the URDF/Xacro models for this robot that are available elsewhere on the web, and then tuned in order to add some elements like floor, obstacles, and actuators and sensors.

My question

I have tried to create a 360º lidar using the replicate tag. However, as you can see in the image, the lidar does not cover 360 degrees. I have checked the documentation and examples you provide, but unfortunatley, I have no idea of what I'm doing wrong.

I would also like to ask why the free joint I have added to the chassis body is not attached to the center of mass of the robot. I have also noticed that if I remove that free joint, the lidar is reduced to a front cone of about 20º and a rear beam of the same degrees. I do not understand why removing a joint should interfere with the lidar.

Minimal model and/or code that explain my question

turtlebot3burgermjmodel_diff.txt

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]

The missing rays are the ones that do not hit anything. If a ray does not hit anything the distance returned is -1 and the ray is not visualized.

[yuvaltassa]

If you want your freejoint at the CoM, try using align

[WeekendArchaeo]

Thanks a lot for your quick and useful reply :)!!