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Calculating inertial matrix for Gazebo

asked 2016-06-18 04:24:53 -0500

intelharsh gravatar image

updated 2016-06-22 05:06:35 -0500

I am having issues following the tutorial to calculate the inertial matrix for my gripper. This is the behaviour I see. I am able to successfully create a config package with PR2 gripper, so I am pretty sure it is because of the inertial matrix.

In Meshlab, following are the values I get at no-scale:

Mesh Bounding Box Size 0.164416 0.063248 0.077500
Mesh Bounding Box Diag 0.192456 
Mesh Volume is 0.000214
Mesh Surface is 0.089074
Thin shell barycenter -0.040913 -0.000254 0.039031
Center of Mass is -0.041006 -0.000043 0.037458
Inertia Tensor is :
| 0.000000 -0.000000 0.000000 |
| -0.000000 0.000001 0.000000 |
| 0.000000 0.000000 0.000000 |
Principal axes are :
| 0.938629 0.011849 0.344725 |
| 0.004914 0.998849 -0.047710 |
| -0.344893 0.046476 0.937491 |
axis momenta are :
| 0.000000 0.000001 0.000001 |

For a scale of 100, following are the values I get :

Mesh Bounding Box Size 16.441637 6.324847 7.750000
Mesh Bounding Box Diag 19.245613 
Mesh Volume is 214.333633
Mesh Surface is 890.744385
Thin shell barycenter -4.091284 -0.025389 3.903062
Center of Mass is -4.100575 -0.004325 3.745764
Inertia Tensor is :
| 1597.194946 -17.950985 1307.758423 |
| -17.950985 5213.373047 9.511752 |
| 1307.758423 9.511752 4675.780273 |
Principal axes are :
| 0.938629 0.011848 0.344725 |
| 0.004914 0.998849 -0.047710 |
| -0.344893 0.046475 0.937491 |
axis momenta are :
| 1116.573364 5213.602539 5156.172363 |

How do I use these values? I didn't quite understand the scaling part of the tutorial. Also, ideally, shouldn't the simulation work when I consider the gripper to be almost weightless?

  <mass value="0.001" />
  <inertia ixx="0.001" ixy="0"  ixz="0"
           iyy="0.001" iyz="0"
           izz="0.001" />

A unity matrix or the above value as well gives the same behaviour as in the video.

Edit 1:

I have tested the macro mentioned by @Stefan Kohlbrecher. Also, I divided the inertia matrix given by Meshlab by volume which gives me :

    <mass value="1.0" />
    <origin xyz="-0.041006 -0.000043 0.037458" />
    <inertia ixx="0.000746353" ixy="0"  ixz="0"
               iyy="0.002436156" iyz="0"
               izz="0.002184945" />

Still I observe the same behaviour. Does Gazebo need the inertia matrix wrt origin or wrt center of mass?

Edit 2:

@MarioSelvaggio 's solution seems to work perfectly fine. The arm is stable and I am also able to plan and execute trajectories. But the solution only works when in my final urdf file I translate the gripper :

<joint name="gripper_joint" type="fixed">
    <parent link="ur5_arm_ee_link" />
    <child link = "gripper" />
    <origin xyz="0.04100575 0.00004325 -0.03745764" rpy="0.0 4.71 3.14" />

Observe that the sign of origin xyz has been inverted. Here's how it looks: For rest of the cases, the simulation remains unstable ... (more)

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2 Answers

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answered 2016-06-21 09:04:17 -0500

mrslvgg gravatar image

When your model is too small, inertia matrix can usually assume very small values. When you scale your model by s (scale factor) in Meshlab in order to increase the precision, you're actually scaling the dimensions of your object. This means that the center of mass is changed according to s, volume, and thus mass, is changed according to s^3, moment of inertia, being mass*length^2, is changed according to s^5.

According to the values you posted (s=100) after scaling back I expect to see something like:

    <mass value="0.000214" />
    <origin xyz="-0.04100575 -0.00004325 0.03745764" />
    <inertia ixx="1.597e-7" ixy="-1.795e-9"  ixz="1.307e-7"
             iyy="5.213e-7" iyz="9.5117e-10"

Note that the mass value is the same as the computed volume, this means that a default value of density rho=1 has been used. The density of the material of your gripper expressed in kg/m^3 has to be multiplied by the volume in order to get the mass. Example density is rho = 7850 Kg/m^3 (steel).

You can have a useful feedback in Gazebo regarding your computed values by visualizing masses and inertia. Go to the Gazebo menu and select View->Inertia. If you still have troubles post here.

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Your solution works! But there seems to be a problem with pose of the gripper wrt the end-effector. Please check Edit 2.

intelharsh gravatar imageintelharsh ( 2016-06-21 10:27:59 -0500 )edit

It is not possible to judge what is happening from just these information. It could be self-collision or other strange things. My suggestion is try to visualize collision meshes, centre of masses and inertia from the menu View in order to get more information. Otherwise you can post your URDF here.

mrslvgg gravatar imagemrslvgg ( 2016-06-21 11:08:53 -0500 )edit

answered 2016-06-18 17:52:21 -0500

Making values very small (i.e. making bodies weightless) can actually hurt, as the numerical stability of solvers can be worse when there are large differences between inertial properties of different bodies. I often base inertia tensors on the formulas for the inertia tensors of simple geometric shapes (see list of moments of inertia. These can be put into xacro macros and then it is fairly easy to use and modify them.

See and

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But if I have to use the values generated by Meshlab, how do I do that? And if I HAVE to simplify, I probably have to use cuboid?

intelharsh gravatar imageintelharsh ( 2016-06-18 18:10:43 -0500 )edit

Does Gazebo need the inertia matrix wrt origin or wrt center of mass?

intelharsh gravatar imageintelharsh ( 2016-06-21 06:46:11 -0500 )edit

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Asked: 2016-06-18 04:24:53 -0500

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Last updated: Jun 22 '16