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is inertia important?

asked 2021-05-31 12:11:27 -0500

harish556 gravatar image

Like do we need to add inertia to urdf, like when I just set it 0 it still works but when I set to some value it explodes.

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answered 2021-06-01 12:23:49 -0500

djchopp gravatar image

updated 2021-06-01 12:31:43 -0500

Inertia is mainly important in simulation. If you are just using a URDF for visualization in RVIZ then it really doesn't matter. If you use a URDF to drive simulation (such as using it in gazebo) then it does matter. inertia tags in a URDF are the 3d tensor moments of inertia. This can be calculated with most mainstream CAD packages for complex geometry. If the geometry is simple you could also calculate it yourself using the formulas from here.

EDIT: Some more info is available on the ros wiki as well

As stated the inertia is a 3d tensor so it should have multiple fields. See example below:

Consider a sphere with mass of 10.5kg, and radius of 0.5 meters.

The tensor for a sphere is:


So along the diagonal, the values should be 2/5 * m * r^2 => 2/5 * 10.5 * 0.5^2 = 1.05

The inertial tag for this would look like:

  <origin xyz="0 0 0" rpy="0 0 0"/>
  <mass value="10.5"/>
    ixx="1.05"  ixy="0.00"  ixz="0.00"
    iyx="0.00"  iyy="1.05"  iyz="0.00"
    izx="0.00"  izx="0.00"  izz="1.05"

Hope this helps!

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thanks alot

harish556 gravatar image harish556  ( 2021-06-02 23:06:41 -0500 )edit

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Asked: 2021-05-31 12:11:27 -0500

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Last updated: Jun 01 '21