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The key to have realistic simulations is to provide a correct section <inertias> ... </inertias>

A proper section should look like this:

<inertia> <!-- interias are tricky to compute -->
      <!-- http://answers.gazebosim.org/question/4372/the-inertia-matrix-explained/ -->
      <ixx>0.000033719</ixx>       <!-- for a box: ixx = 0.083 * mass * (y*y + z*z) -->
      <ixy>0.0</ixy>         <!-- for a box: ixy = 0 -->
      <ixz>0.0</ixz>         <!-- for a box: ixz = 0 -->
      <iyy>0.001305175</iyy>       <!-- for a box: iyy = 0.083 * mass * (x*x + z*z) -->
      <iyz>0.0</iyz>         <!-- for a box: iyz = 0 -->
      <izz>0.001322294</izz>       <!-- for a box: izz = 0.083 * mass * (x*x + y*y) -->
    </inertia>
  </inertial>

Steps to follow:

0- Use Blender to automatically set the center of mass and arrange the axis of the object at that center (this will make later steps easier) 1- Use the tutorial chapulina has suggested to calculate the inertial parameters and set the model. Don't forget to set a correct mass value (the one you have now seems unrealistic) 2- Use Gazebo's option View->Center of mass/Inertia to observe if your inertias are correct (see figures attached for a wrong one an a correct one, for a spoon) 3- If they do not show correct, iterate

INCORRECT INERTIAS image description

CORRECT INERTIAS image description