URDF models inertia value seems so big !

asked 2018-01-25 15:45:07 -0500

Nurul Karim
1 ●2 ●3 ●3

updated 2018-01-27 12:35:12 -0500

Dear altruists, I have a robot model and its main body is mentioned here. Body size is a hollow cover of (0.90.90.5) cubic meters. weight nearly 14 kilo, is inertia got from meshlab here is reasonable? and approximately how much effort is required for lifting five times this wight (1459.8 N)?

<link name="body">        
    <inertial>
        <origin xyz="0 0 0" rpy="0 0 0"/>
        <mass value="14.1"/>  
 <inertia ixx="-2685.745" ixy="47.8420869" ixz="-13.070708" iyy="4702.4455" iyz="864.4434" izz="1845.634138"/>
    </inertial>
    <visual>
        <material name="Yellow"/>
        <origin xyz="0 0 0" rpy="0 0 0"/>
        <geometry>
            <mesh filename="package://tt_tutorial/meshes/body.stl" scale="0.001 0.001 0.001"/>
        </geometry>
    </visual>
    <collision>
        <material name="Red"/>
        <origin xyz="0.0 0.0 0.0" rpy="0 0 0"/>
        <geometry>               
            <mesh filename="package://tt_tutorial/meshes/body.stl" scale="0.001 0.001 0.001"/>
        </geometry>
    </collision>
     <turnGravityOff>true</turnGravityOff>
  <gazebo reference="base_link">  
  </gazebo>
</link>

answered 2018-01-30 00:49:30 -0500

Francisco
143 ●1 ●4 ●7

Yes, those inertia values are huge. To give you a sense of magnitude, these are the inertia values of a cylinder with a radius=0.45 meters and height=0.5 meters and a mass=14.1 Kilograms.

<inertial>
  <origin xyz="0 0 0.25" rpy="0 0 0" />
  <mass value="14.1" />
  <inertia  ixx="1.0075625" ixy="0" ixz="0"
            iyy="1.0075625" iyz="0"
            izz="1.427625" />
</inertial>

From the visual and collision fields, it's clear that the units of your mesh (body.stl) are in millimeters, so, you may want to scale it to meters before computing the inertia with meshlab.

Another option is to approximate the body to a simpler geometry (I normally use a cylinder) for which computing the inertia is straight forward: https://en.wikipedia.org/wiki/List_of...

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@Francisco Thanks for your reply, I later rescaled mesh and computed inertia, and then divided it with volume computed by Meshlab as it was suggested in forums. Is it correct way?

Nurul Karim ( 2018-01-31 16:18:05 -0500 )edit

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URDF models inertia value seems so big !

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