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I've been stuck on this for a while. Most resources online point to complex equations even for the simplest of shapes. Surely there is some sort of software in which I can just build my robot and have it generate inertial values?
I've found the following link to be very helpful: http://gazebosim.org/tutorials?tut=in...
And I actually just make my inertial values the same as a box the same size as my collision shapes so the inertial values are just calculated using the formula lists for cuboids on this wikipedia page using the values from my collision boxes. Like that gazebo tutorial mentions, if you temporarily set mass = 1, you can view the inertial cube in gazebo and it should be the same as your collision cube.
This is close enough for most applications.
I implemented a python script reading an 3D file like stl or dae and outputting the full URDF XML inertial tag based on the gazebosim tutorial.
At first the script calculates the center of mass of the object. After that it scales the mesh by a factor of 100 to increase the numerical accuracy. You could manually change that, if you need to scale it more up. Now the convex hull of your object will be calculated. For the hull the geometric properties will be calculated, scaled back down and outputted as URDF XML inertial tag like that (the precision could also be changed in the script)
<inertial>
<origin xyz="0.00000002 -0.00000001 0.00000000"/>
<mass value="5.00000000"/>
<inertia ixx="7.90866056" ixy="0.00000006" ixz="0.00000000" iyy="7.90866060" iyz="0.00000000" izz="2.48398783"/>
</inertial>
Check it out here: https://github.com/vonunwerth/MeshLab...
Check out the MyBot Tutorial and specifically its xacro file :
<macro name="cylinder_inertia" params="m r h">
<inertia ixx="${m*(3*r*r+h*h)/12}" ixy = "0" ixz = "0"
iyy="${m*(3*r*r+h*h)/12}" iyz = "0"
izz="${m*r*r/2}" />
</macro>
<macro name="box_inertia" params="m x y z">
<inertia ixx="${m*(y*y+z*z)/12}" ixy = "0" ixz = "0"
iyy="${m*(x*x+z*z)/12}" iyz = "0"
izz="${m*(x*x+z*z)/12}" />
</macro>
<macro name="sphere_inertia" params="m r">
<inertia ixx="${2*m*r*r/5}" ixy = "0" ixz = "0"
iyy="${2*m*r*r/5}" iyz = "0"
izz="${2*m*r*r/5}" />
</macro>
These macroses are calculations for basic shapes and could be easily pulled into your project. But if you need inertias for some custom shapes, then you should look into some 3D modeling SW, they have capability to calculate necessary matrices for you.
Asked: 2017-04-28 03:45:07 -0500
Seen: 3,849 times
Last updated: Jun 28 '22
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I don't know of any software that would "just" do this for you unfortunately. But to make sure: if you are not planning on using any sort of dynamic simulator (Gazebo, V-REP, etc), you don't need to specify any inertial properties.