inertia_wiki

Inertia

In physics and applied mathematics, the mass moment of inertia, usually denoted by I, measures the extent to which an object resists rotational acceleration about a particular axis, and is the rotational analogue to mass (1).

The inertial properties (2):

Origin

optional: defaults to identity if not specified

This is the pose of the inertial reference frame, relative to the link reference frame. The origin of the inertial reference frame needs to be at the center of gravity. The axes of the inertial reference frame do not need to be aligned with the principal axes of the inertia.

xyz (optional: defaults to zero vector)

    Represents the $$x,y,z$$ offset. 

rpy (optional: defaults to identity if not specified)

    Represents the fixed axis roll, pitch and yaw angles in radians. 

MASS

The mass of the link is represented by the value attribute of this element

INERTIA

The 3x3 rotational inertia matrix, represented in the inertia frame. Because the rotational inertia matrix is symmetric, only 6 above-diagonal elements of this matrix are specified here, using the attributes ixx, ixy, ixz, iyy, iyz, izz.

For more information how to setup the inertia values use the following link: http://gazebosim.org/tutorials?tut=inertia.

Calculate Inertia

The inertia can be also calculated using the following macro:

https://github.com/uos/uos_tools/blob/fuerte/uos_common_urdf/common.xacro

You can set the values relate to an object:

  <xacro:cylinder_inertial radius="0.35" length="0.74" mass="40"/> 
  <xacro:box_inertial x="0." y="0.4" z="0.74" mass="40"/>
  <xacro:sphere_inertial radius="0.08" mass="0.1"/> 

and then run the node using the following command

rosrun xacro xacro --inorder -o model.urdf inertia.xacro

to generate the SDF file containing all the inertia properties.