Universal Robots Forum

Correct mass and center or mass?

By comparing the urdf model of UR5e with the mass properties in this article, I noticed that there were some differences, especially in mass. Which data source is closest to the true values? Is there a more official spec somewhere?

For reference, UR5e link mass comparisons (urdf on the right):
Link1: 3.71 vs 3.7
Link2: 8.058 vs 8.393
Link3: 2.846 vs 2.275
Link4: 1.37 vs 1.219
Link5: 1.3 vs 1.219
Link6: 0.365 vs 0.1879

We are using the urdf model in a simulation, so it is important that the mass properties are correct.

We are currently fixing this on the melodic-devel-staging branch. See also this PR related to this. However, I think I indeed missed to update the masses there, as well.

Thanks mauch!

Currently trying to use the updated values in our project.

Some things I noticed in physical_parameters.yaml (UR5e) of the PR:

  • upper_arm_cog.x is set to -0.17 but comment says that model.x - upper_arm_length, which with values inserted leads to x = 0.2125 - (--0.425) = -0.2125, i.e. not -0.17. Is this a mistake or is there a reason for it being set to -0.17? From what values should ‘upper_arm_length’ and ‘forearm_length’ be read?

  • Almost all dh_parameters are slightly different from UR website and default_kinematics.yaml. Very small difference though. Should these in theory be set the same?

Somewhat out of scope of this forum, but:

  • In our project we need to use .urdf files. I previously converted the UR5e root xacro file (of the kinetic branch) to urdf using the following command:

    rosrun xacro xacro -o "Path\ur5e_robot.urdf" "Path\ur5e_robot.urdf.xacro"

    …but in the melodic branch the file structure has changed quite a bit and the root file for UR5e seems to now be load_ur5e.launch. Do you have any hint on how to generate an UR5e urdf file from this? Or do I have to create my own UR5e xacro root file manually first? (I’m actually currently doing this on Windows)

Noticed that the center of mass from UR for link6 on UR5e is at the very end of the link ( [0, 0, -0.001159] ). This seems strange, and even physically impossible, I would assume it to be more “inside” the actual link.

I’ll come back to that… However, best we discuss this further on GH.