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# Difference between <dynamics damping="0.7" friction="100.0" /> and <mu1>1</mu1>

Hi,

Could anyone give me some explanation the difference between dynamics damping, and mu1, mu2 in the urdf ?

As I understand it, mu1 and mu2 are tangential friction forces applied between body and surface, so that the tangential friction force applied to the body as the body weight when mu=1.

<dynamics damping="0.7" friction="100.0"/> is the damping force which is proportional to the velocity of the moving body.

What should I do in the urdf to applying the damping force which is proportional to the velocity in the simple sliding box? Below is the sliding box urdf code:

<robot name="sliding_box"
xmlns:body="http://playerstage.sourceforge.net/gazebo/xmlschema/#body"
xmlns:controller="http://playerstage.sourceforge.net/gazebo/xmlschema/#controller"
xmlns:interface="http://playerstage.sourceforge.net/gazebo/xmlschema/#interface" >

<inertial>
<mass value="1"/>
<origin xyz="0 0 0"/>
<inertia ixx="0.05" ixy="0" ixz="0" iyy="0.05" iyz="0" izz="0.05"/>
</inertial>
<visual>
<origin rpy="0 0 0" xyz="0 0 0.0"/>
<geometry>
<box size="1.0 1.0 1.0"/>
</geometry>
</visual>
<collision>
<origin rpy="0 0 0" xyz="0 0 0.0"/>
<geometry>
<box size="1.0 1.0 1.0"/>
</geometry>
</collision>

<mu1>0</mu1>
<mu2>0</mu2>
<fdir1 value = "1 0 0"/>
<dynamics damping="0.7" friction="100.0" />
<material>Gazebo/Black</material>
<turnGravityOff>false</turnGravityOff>
</gazebo>

<gazebo>
<controller:gazebo_ros_force name="box_force_controller" plugin="libgazebo_ros_force.so">
<alwaysOn>true</alwaysOn>
<updateRate>15.0</updateRate>
<topicName>box_force</topicName>
</controller:gazebo_ros_force>
<controller:gazebo_ros_p3d name="p3d_base_controller" plugin="libgazebo_ros_p3d.so">
<alwaysOn>true</alwaysOn>
<updateRate>15.0</updateRate>
<topicName>base_pose_ground_truth</topicName>
<gaussianNoise>0</gaussianNoise>
<frameName>map</frameName>
<xyzOffsets>0 0 0</xyzOffsets>
<rpyOffsets>0 0 0</rpyOffsets>
<interface:position name="p3d_base_position"/>
</controller:gazebo_ros_p3d>
</gazebo>

</robot>

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The "dynamics" element is part of the URDF description, and applies to the damping and friction in a joint. See the URDF page for more details.

The "mu1" and "mu2" elements are part of a Gazebo description, and apply to the material properties of a body.

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2

The current link is http://wiki.ros.org/urdf/XML/joint (J needs to be lower case). http://gazebosim.org/tutorials?tut=ro... has more info on the gazebo surface friction.

( 2014-11-14 17:24:30 -0500 )edit

+1 Link in the post updated

( 2014-11-14 19:12:58 -0500 )edit

<dynamics> (optional)

An element specifying physical properties of the joint. These values are used to specify modeling properties of the joint, particularly useful for simulation. damping (optional, defaults to 0)

The physical damping value of the joint ($$\frac{N \cdot s}{m}$$ for prismatic joints, $$\frac{N \cdot m \cdot s}{rad}$$ for revolute joints).

friction (optional, defaults to 0)

The physical static friction value of the joint ($$N$$ for prismatic joints, $$N \cdot m$$ for revolute joints).

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