# Revision history [back]

Hi there,

I think, I found a way to make my model behave as desired, i.e. execute the commanded velocities (without "dropping" when velocities 0.0 are sent to the joints).

I'm now using a fake inertia in all our links. Therefore, I defined the following xacro macro and use it in all our links without caring about actual mass and distribution of mass of the corresponding link of the real hardware:

  <xacro:macro name="default_inertial">
<inertial>
<mass value="0.01" />
<origin xyz="0 0 0" />
<inertia ixx="0.001" ixy="0.0" ixz="0.0" iyy="0.001" iyz="0.0" izz="0.001" />
</inertial>
</xacro:macro>


This macro corresponds to a box-shaped body of mass 0.01 kg and a size of about 0.8 x 0.8 x 0.8 m³ (x-y-z). With this macro, the arm now moves the joints with the velocities commanded by the controller.

What makes me disappointed about this, is the fact that we put tremendous efforts in assessing the correct inertia values for our (non-primitive) links by assigning mass properties to the CAD data. We also used data from the technical specs sheets of the real hardware for specifying other properties. Even playing around with all the other properties that can be set in the URDF like, damping, effort limits, soft-limits, mechanicalReduction of transmission as well as all the gazebo-/sdf- tags (see also this thread and this thread) did not improve the behaviour.

Questions
Thus, I would like to know whether Gazebo is able to correctly model non-primitive inertias? Like inertias of asymmetric links that have values != 0.0 for the entries ixy, ìxz and iyz, i.e. values beyond the diagonal of the inertia matrix?

Is there any robot out there that uses a (URDF or SDF) model that actually corresponds to its real hardware properties? That I could use as a template? That uses a VelocityJointInterface as well and works well in simulation?

What other option do I have to further investigate why our model (with real inertias) is not able to execute the commanded velocities?

Am I using wrong units?

Hi there,

I think, I found a way to make my model behave as desired, i.e. execute the commanded velocities (without "dropping" when velocities 0.0 are sent to the joints).

I'm now using a fake inertia in all our links. Therefore, I defined the following xacro macro and use it in all our links without caring about actual mass and distribution of mass of the corresponding link of the real hardware:

  <xacro:macro name="default_inertial">
<inertial>
<mass value="0.01" />
<origin xyz="0 0 0" />
<inertia ixx="0.001" ixy="0.0" ixz="0.0" iyy="0.001" iyz="0.0" izz="0.001" />
</inertial>
</xacro:macro>


This macro corresponds to a box-shaped body of mass 0.01 kg and a size of about 0.8 x 0.8 x 0.8 m³ (x-y-z). With this macro, the arm now moves the joints with the velocities commanded by the controller.

What makes me disappointed about this, is the fact that we put tremendous efforts in assessing the correct inertia values for our (non-primitive) links by assigning mass properties to the CAD data. We also used data from the technical specs sheets of the real hardware for specifying other properties. Even playing around with all the other properties that can be set in the URDF like, damping, effort limits, soft-limits, mechanicalReduction of transmission as well as all the gazebo-/sdf- tags (see also this thread and this thread) did not improve the behaviour.

Questions
Thus, I would like to know whether Gazebo is able to correctly model non-primitive inertias? Like inertias of asymmetric links that have values != 0.0 for the entries ixy, ìxz and iyz, i.e. values beyond the diagonal of the inertia matrix?

Is there any robot out there that uses a (URDF or SDF) model that actually corresponds to its real hardware properties? That I could use as a template? That uses a VelocityJointInterface as well and works well in simulation?

What other option options do I have to further investigate why our model (with real inertias) is not able to execute the commanded velocities?

Am I using wrong units?