Adding friction to model wheels

asked 2020-03-03 08:22:50 -0500

John999991
37 ●16 ●24 ●26

updated 2020-03-03 10:16:27 -0500

Dear all, I have a 4-wheel small car URDF model which spawns correctly on Gazebo but when I issue a pub command on the velocity controller to move it, I have an unexpected slippery behavior. If for example I issue a velocity command on the front left wheel, I would expect the robot to perform an (almost) perfect circle around an axis but what I get is the robot to start rotate for 0.5sec, go straight for 0.4sec rotate again then straight again etc. This is showing a lost contact of the wheels with the ground due to low friction. How can I add rotational friction to the connection between wheel-ground?

Please note that I have already added a friction part on my URDF but I don't think it is working properly. Otherwise, the weights on my robot are quite realistic.

URDF:

<?xml version="1.0" ?>
<!-- =================================================================================== -->
<!-- |    This document was autogenerated by xacro from robot.xacro                    | -->
<!-- |    EDITING THIS FILE BY HAND IS NOT RECOMMENDED                                 | -->
<!-- =================================================================================== -->
<robot name="robot" xmlns:xacro="http://www.ros.org/wiki/xacro">
  <!--This URDF was automatically created by SolidWorks to URDF Exporter! Originally created by Stephen Brawner (brawner@gmail.com) 
     Commit Version: 1.5.1-0-g916b5db  Build Version: 1.5.7152.31018
     For more information, please see http://wiki.ros.org/sw_urdf_exporter -->
  <link name="dummy">
  </link>
  <link name="base_link">
    <inertial>
      <origin rpy="0 0 0" xyz="0.00390777224516517 -0.032446267219719 0.184169550820421"/>
      <mass value="4.20121630268732"/>
      <inertia ixx="0.0149000386129946" ixy="-4.66831001352174E-09" ixz="5.23920338795194E-08" iyy="0.0234359493013497" iyz="0.000771538751024883" izz="0.0286744535302635"/>
    </inertial>
    <visual>
      <origin rpy="0 0 0" xyz="0 0 0"/>
      <geometry>
        <mesh filename="package://robot/meshes/base_link.STL"/>
      </geometry>
      <material name="">
        <color rgba="0.529411764705882 0.549019607843137 0.549019607843137 1"/>
      </material>
    </visual>
    <collision>
      <origin rpy="0 0 0" xyz="0 0 0"/>
      <geometry>
        <mesh filename="package://robot/meshes/base_link.STL"/>
      </geometry>
    </collision>
  </link>
  <joint name="dummy_joint" type="fixed">
    <parent link="dummy"/>
    <child link="base_link"/>
  </joint>
  <link name="FL">
    <inertial>
      <origin rpy="0 0 0" xyz="-5.55111512312578E-17 9.10729824887824E-18 -6.93889390390723E-18"/>
      <mass value="0.0615219544751675"/>
      <inertia ixx="5.54433425808419E-05" ixy="-1.45453466603006E-20" ixz="-1.00225538664655E-21" iyy="3.00921775305435E-05" iyz="2.309188417276E-21" izz="3.00921775305435E-05"/>
    </inertial>
    <visual>
      <origin rpy="0 0 0" xyz="0 0 0"/>
      <geometry>
        <mesh filename="package://robot/meshes/FL.STL"/>
      </geometry>
      <material name="">
        <color rgba="0.298039215686275 0.298039215686275 0.298039215686275 1"/>
      </material>
    </visual>
    <collision>
      <origin rpy="0 0 0" xyz="0 0 0"/>
      <geometry>
        <mesh filename="package://robot/meshes/FL.STL"/>
      </geometry>
      <surface>
        <friction>
          <ode>
            <mu>100000.0</mu>
            <mu2>100000.0</mu2>
          </ode>
        </friction>
        <contact>
          <ode>
            <min_depth>0.003</min_depth>
          </ode>
        </contact>
      </surface>
    </collision>
  </link>
  <joint name="FLJ" type="continuous ...

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2 Answers

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answered 2020-03-03 10:19:05 -0500

bob-ROS

525 ●3 ●49 ●12

I would try to pull down the mu1 and mu2 values and increase min_depth to 0.01 or higher. I believe this behavior is caused by the robot being pushed off the ground and some wheels losing contact with the ground, simply increasing friction doesn't make it stick to the ground. Min_depth on the other hand allows the wheels to grip despite hovering slightly over the ground.

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Nop. That didn't do the trick. Behavior is the same. Turned mu and mu2 down to 1000 and raised min_depth to 0.5 but no change.

I have also edited the ground model of Gazebo and raised friction of the world ground plane but didn't work. Do you sir, have any other ideas?

John999991 ( 2020-03-03 10:49:50 -0500 )edit

I think the issue is that due to friction between the other 3 wheels and the ground, when spinning only 1 wheel, you get this type of not-cyclic movement which I don't think it is realistic... Also, when I turn velocity to zero for all 4 wheels, I get a parallel movement of the robot, without any wheels spinning, like the robot moving sideways with no effort...

John999991 ( 2020-03-03 14:31:19 -0500 )edit

I have a project here were we built a car that worked somewhat well in Gazebo.

https://github.com/bob-ROS/asc_project/blob/master/asc_description/urdf/macro_wheel.urdf.xacro https://github.com/bob-ROS/asc_project/blob/master/asc_description/urdf/asc.urdf.xacro

Video of it running

We did very little tinkering with the physics properties to make it work.

The hardware interface is a bit odd (such as static revolute steering joints) because we wanted to reuse a specific controller.