# URDF link mass inertia properties

Hi

I'm building a URDF for rviz, and eventually Gazebo. I'm a bit confused about the best values to put in for the Inertia Origin and Moment of Inertia matrix.

Below you should see a screenshot of a robot shoulder in SolidWorks. (Right-click and open to see it in full size)

The origin is the tri-color axis, which I put there because it matches the link origin.

This means I should be able to specify the mesh visual origin as 0,0,0.

The pink axis is the Center of Mass found by SolidWorks.

The URDF XML doc: http://ros.informatik.uni-freiburg.de/roswiki/urdf(2f)XML(2f)inertial.html says the Inertial Origin should be the "Link Center of Mass".

So I could use the SolidWorks Center of Mass x,y,z co-ordinates as the Inertial Origin.

But I don't know which Ix,Px, Lxx, or Ixx values from SolidWorks to use?

Or, why can't I just set the Inertial Origin to 0,0,0 and use the very last set of values from SolidWorks: Ixx, Ixy, Ixz, etc.

Will Gazebo/ODE get confused because the Inertial Origin is *not* the Center of Mass of the link?

These two posts provided some info, but no complete explanation:

http://answers.ros.org/question/11407/what-is-the-origin-of-the-inertia-tensor-in-urdf/

http://answers.ros.org/question/30539/choosing-the-right-coefficients-for-gazebo/

Thanks.