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Why my vehicle cant not go in a straight line?

I set the same force to "left_wheel_hinge"(joint) and "right_wheel_hinge"(joint) of my vehicle model at the same moment, it can not go in a straight line but still go in circles. The model's parameters are symmetrical.what 's wrong?Thanks!

The following is the vehicle model:

  <model name="my_newneuB">
<static>false</static>
<link name='chassis'>
        <pose>0 0 0.02575 0 0 0</pose>
        <inertial>
        <pose>-0.015 0 -0.015 0 0 0</pose>          
        <mass>1.0</mass>
          <inertia>
            <ixx>9.0e-4</ixx><!--8.71e-4 1.235e-3 1.096e-3-->
            <iyy>1.3e-3</iyy>
            <izz>1.1e-3</izz>
          </inertia>
    </inertial>
    <velocity_decay>
        <linear>0</linear>
        <angular>0.1</angular>
    </velocity_decay>

  <collision name='body_collision'>
    <pose>0 0 0 0 0 0</pose>
    <geometry>
      <box>
        <size>.075 .057 .03</size>
      </box>
    </geometry>
    <surface>
             <bounce>
                <restitution_coefficient> 0.5 </restitution_coefficient>
                <threshold> 0 </threshold>
             </bounce>
             <friction>
                <ode>
                 <mu> 0.1 </mu>
                </ode>
             </friction>
            </surface>
  </collision>
  <visual name='body_visual'>
    <pose>0 0 0 0 0 0</pose>
    <geometry>
      <box>
        <size>.075 .057 .045</size>
      </box>
    </geometry>
    <material>
      <script>
        <uri>file://media/materials/scripts/gazebo.material</uri>
        <name>Gazebo/Blue</name>
      </script>
    </material>
  </visual>


  <collision name='Fwheel_collision'> <!-- left wheel -->
    <pose>.03 0 -0.0225 0 0 0</pose>
    <geometry>
      <sphere>
        <radius>0.0015</radius>
      </sphere>
    </geometry>
    <surface>
                <bounce>
                    <restitution_coefficient> 0 </restitution_coefficient>
                    <threshold> 100000 </threshold>
                </bounce>
                <friction>
                    <ode>
                     <mu> 0 </mu>
                    </ode>
                </friction>
            </surface>
  </collision>
  <visual name='Fwheel_visual'>
    <pose>0.03 0 -0.0225 0 0 0</pose>
    <geometry>
      <sphere>
        <radius>0.0015</radius>
      </sphere>
    </geometry>
  </visual>

  <collision name='Bwheel_collision'>
    <pose>-0.03 0 -0.0225 0 0 0</pose>
    <geometry>
      <sphere>
        <radius>0.0015</radius>
      </sphere>
    </geometry>
    <surface>
                <bounce>
                    <restitution_coefficient> 0 </restitution_coefficient>
                    <threshold> 100000 </threshold>
                </bounce>
                <friction>
                    <ode>
                     <mu> 0 </mu>
                    </ode>
                </friction>
            </surface>        
  </collision>
  <visual name='Bwheel_visual'>
    <pose>-0.03 0 -0.0225 0 0 0</pose>
    <geometry>
      <sphere>
        <radius>0.0015</radius>
      </sphere>
    </geometry>
  </visual>

<link name="left_wheel">
  <pose>0 0.0335 .02 1.570796327 0 0</pose>
  <inertial>
          <mass>0.03</mass>
          <pose>0 0 0 0 0 0</pose>
          <inertia>
              <ixx>6.0e-5</ixx><!-- 3.16e-6 3.16e-6 6.0e-6 -->
              <iyy>6.0e-5</iyy>
              <izz>6.0e-5</izz>
          </inertia>
    </inertial>
  <collision name="collision">
    <pose>0 0 0 0 0 0</pose>
    <geometry>
      <cylinder>
        <radius>.02</radius>
        <length>.008</length>
      </cylinder>
    </geometry>
            <surface>
             <bounce>
                <restitution_coefficient> 0.1 </restitution_coefficient>
                <threshold> 100000 </threshold>
             </bounce>
             <friction>
                <ode>
                 <mu> 1.0 </mu>
                 <mu2> 1.0 </mu2>   
                 <fdir1>1 0 0</fdir1>
                 <slip1>0.0</slip1>
                 <slip2>0.1</slip2> <!---->     
                </ode>
             </friction>
            </surface>
  </collision>
  <visual name="visual">
  <pose>0 0 0 0 0 0</pose>
    <geometry>
      <cylinder>
        <radius>.02</radius>
        <length>.008</length>
      </cylinder>
    </geometry>
  </visual>
        <sensor name="left_wheel_sensor" type="imu">
        </sensor>
</link>

<link name="right_wheel">
  <pose>0 -0.0335 .02  1.570796327 0 0</pose>
    <inertial>
            <mass>0.03</mass>
            <pose>0 0 0 0 0 0</pose>
            <inertia>
              <ixx>6.0e-5</ixx><!-- 3.16e-6 3.16e-6 6.0e-6 -->
              <iyy>6.0e-5</iyy>
              <izz>6.0e-5</izz>
            </inertia>
        </inertial>  
        <collision name="collision">
    <pose>0 0 0 0 0 0</pose>
    <geometry>
      <cylinder>
        <radius>.02</radius>
        <length>.008</length>
      </cylinder>
    </geometry>
            <surface>
             <bounce>
                <restitution_coefficient> 0.1 </restitution_coefficient>
                <threshold> 100000 </threshold>
             </bounce>
             <friction>
                <ode>
                 <mu> 1.0 </mu>
                 <mu2> 1.0 </mu2>
                 <fdir1>1 0 0</fdir1>
                 <slip1>0.0</slip1>
                 <slip2>0.1</slip2> <!---->                  
                </ode>
             </friction>
            </surface>        
  </collision>
  <visual name="visual">
  <pose>0 0 0 0 0 0</pose>
    <geometry>
      <cylinder>
        <radius>.02</radius>
        <length>.008</length>
      </cylinder>
    </geometry>
  </visual>
  <sensor name="right_wheel_sensor" type="imu">
        </sensor>
</link>

<joint type="revolute" name="left_wheel_hinge">
  <pose>0 0 0 0 0 0</pose>
  <child>left_wheel</child>
  <parent>chassis</parent>
  <axis>
    <xyz>0 1 0</xyz>
    <dynamics>
      <damping>0.02</damping>
      <friction>0</friction>
    </dynamics>
  </axis>
  <physics>
    <ode>
      <limit>
        <cfm>0.5</cfm>
        <erp>0.3</erp>
      </limit>
    </ode>
  </physics>
</joint>

<joint type="revolute" name="right_wheel_hinge">
  <pose>0 0 0 0 0 0</pose>
  <child>right_wheel</child>
  <parent>chassis</parent>
  <axis>
    <xyz>0 1 0</xyz>
    <dynamics>
      <damping>0.02</damping>
      <friction>0</friction>
    </dynamics>
  </axis>
  <physics>
    <ode>
      <limit>
        <cfm>0.5</cfm>
        <erp>0.3</erp>
      </limit>
     </ode>
  </physics>
</joint>

</model>

Why my vehicle cant not go in a straight line?

I set the same force to "left_wheel_hinge"(joint) "left__wheel__hinge"(joint) and "right_wheel_hinge"(joint) "right__wheel__hinge"(joint) of my vehicle model at the same moment, it can not go in a straight line but still go in circles. The model's parameters are symmetrical.what 's wrong?Thanks!

The following is the vehicle model:

  <model name="my_newneuB">
<static>false</static>
<link name='chassis'>
        <pose>0 0 0.02575 0 0 0</pose>
        <inertial>
        <pose>-0.015 0 -0.015 0 0 0</pose>          
        <mass>1.0</mass>
          <inertia>
            <ixx>9.0e-4</ixx><!--8.71e-4 1.235e-3 1.096e-3-->
            <iyy>1.3e-3</iyy>
            <izz>1.1e-3</izz>
          </inertia>
    </inertial>
    <velocity_decay>
        <linear>0</linear>
        <angular>0.1</angular>
    </velocity_decay>

  <collision name='body_collision'>
    <pose>0 0 0 0 0 0</pose>
    <geometry>
      <box>
        <size>.075 .057 .03</size>
      </box>
    </geometry>
    <surface>
             <bounce>
                <restitution_coefficient> 0.5 </restitution_coefficient>
                <threshold> 0 </threshold>
             </bounce>
             <friction>
                <ode>
                 <mu> 0.1 </mu>
                </ode>
             </friction>
            </surface>
  </collision>
  <visual name='body_visual'>
    <pose>0 0 0 0 0 0</pose>
    <geometry>
      <box>
        <size>.075 .057 .045</size>
      </box>
    </geometry>
    <material>
      <script>
        <uri>file://media/materials/scripts/gazebo.material</uri>
        <name>Gazebo/Blue</name>
      </script>
    </material>
  </visual>


  <collision name='Fwheel_collision'> <!-- left wheel -->
    <pose>.03 0 -0.0225 0 0 0</pose>
    <geometry>
      <sphere>
        <radius>0.0015</radius>
      </sphere>
    </geometry>
    <surface>
                <bounce>
                    <restitution_coefficient> 0 </restitution_coefficient>
                    <threshold> 100000 </threshold>
                </bounce>
                <friction>
                    <ode>
                     <mu> 0 </mu>
                    </ode>
                </friction>
            </surface>
  </collision>
  <visual name='Fwheel_visual'>
    <pose>0.03 0 -0.0225 0 0 0</pose>
    <geometry>
      <sphere>
        <radius>0.0015</radius>
      </sphere>
    </geometry>
  </visual>

  <collision name='Bwheel_collision'>
    <pose>-0.03 0 -0.0225 0 0 0</pose>
    <geometry>
      <sphere>
        <radius>0.0015</radius>
      </sphere>
    </geometry>
    <surface>
                <bounce>
                    <restitution_coefficient> 0 </restitution_coefficient>
                    <threshold> 100000 </threshold>
                </bounce>
                <friction>
                    <ode>
                     <mu> 0 </mu>
                    </ode>
                </friction>
            </surface>        
  </collision>
  <visual name='Bwheel_visual'>
    <pose>-0.03 0 -0.0225 0 0 0</pose>
    <geometry>
      <sphere>
        <radius>0.0015</radius>
      </sphere>
    </geometry>
  </visual>

<link name="left_wheel">
  <pose>0 0.0335 .02 1.570796327 0 0</pose>
  <inertial>
          <mass>0.03</mass>
          <pose>0 0 0 0 0 0</pose>
          <inertia>
              <ixx>6.0e-5</ixx><!-- 3.16e-6 3.16e-6 6.0e-6 -->
              <iyy>6.0e-5</iyy>
              <izz>6.0e-5</izz>
          </inertia>
    </inertial>
  <collision name="collision">
    <pose>0 0 0 0 0 0</pose>
    <geometry>
      <cylinder>
        <radius>.02</radius>
        <length>.008</length>
      </cylinder>
    </geometry>
            <surface>
             <bounce>
                <restitution_coefficient> 0.1 </restitution_coefficient>
                <threshold> 100000 </threshold>
             </bounce>
             <friction>
                <ode>
                 <mu> 1.0 </mu>
                 <mu2> 1.0 </mu2>   
                 <fdir1>1 0 0</fdir1>
                 <slip1>0.0</slip1>
                 <slip2>0.1</slip2> <!---->     
                </ode>
             </friction>
            </surface>
  </collision>
  <visual name="visual">
  <pose>0 0 0 0 0 0</pose>
    <geometry>
      <cylinder>
        <radius>.02</radius>
        <length>.008</length>
      </cylinder>
    </geometry>
  </visual>
        <sensor name="left_wheel_sensor" type="imu">
        </sensor>
</link>

<link name="right_wheel">
  <pose>0 -0.0335 .02  1.570796327 0 0</pose>
    <inertial>
            <mass>0.03</mass>
            <pose>0 0 0 0 0 0</pose>
            <inertia>
              <ixx>6.0e-5</ixx><!-- 3.16e-6 3.16e-6 6.0e-6 -->
              <iyy>6.0e-5</iyy>
              <izz>6.0e-5</izz>
            </inertia>
        </inertial>  
        <collision name="collision">
    <pose>0 0 0 0 0 0</pose>
    <geometry>
      <cylinder>
        <radius>.02</radius>
        <length>.008</length>
      </cylinder>
    </geometry>
            <surface>
             <bounce>
                <restitution_coefficient> 0.1 </restitution_coefficient>
                <threshold> 100000 </threshold>
             </bounce>
             <friction>
                <ode>
                 <mu> 1.0 </mu>
                 <mu2> 1.0 </mu2>
                 <fdir1>1 0 0</fdir1>
                 <slip1>0.0</slip1>
                 <slip2>0.1</slip2> <!---->                  
                </ode>
             </friction>
            </surface>        
  </collision>
  <visual name="visual">
  <pose>0 0 0 0 0 0</pose>
    <geometry>
      <cylinder>
        <radius>.02</radius>
        <length>.008</length>
      </cylinder>
    </geometry>
  </visual>
  <sensor name="right_wheel_sensor" type="imu">
        </sensor>
</link>

<joint type="revolute" name="left_wheel_hinge">
  <pose>0 0 0 0 0 0</pose>
  <child>left_wheel</child>
  <parent>chassis</parent>
  <axis>
    <xyz>0 1 0</xyz>
    <dynamics>
      <damping>0.02</damping>
      <friction>0</friction>
    </dynamics>
  </axis>
  <physics>
    <ode>
      <limit>
        <cfm>0.5</cfm>
        <erp>0.3</erp>
      </limit>
    </ode>
  </physics>
</joint>

<joint type="revolute" name="right_wheel_hinge">
  <pose>0 0 0 0 0 0</pose>
  <child>right_wheel</child>
  <parent>chassis</parent>
  <axis>
    <xyz>0 1 0</xyz>
    <dynamics>
      <damping>0.02</damping>
      <friction>0</friction>
    </dynamics>
  </axis>
  <physics>
    <ode>
      <limit>
        <cfm>0.5</cfm>
        <erp>0.3</erp>
      </limit>
     </ode>
  </physics>
</joint>

</model>

Why my vehicle cant not go in a straight line?

I set the same force to "left__wheel__hinge"(joint) "left_wheel_hinge"(joint) and "right__wheel__hinge"(joint) "right_wheel_hinge"(joint) of my vehicle model at the same moment, it can not go in a straight line but still go in circles. The model's parameters are symmetrical.what 's wrong?Thanks!

The following is the vehicle model:

  <model name="my_newneuB">
<static>false</static>
<link name='chassis'>
        <pose>0 0 0.02575 0 0 0</pose>
        <inertial>
        <pose>-0.015 0 -0.015 0 0 0</pose>          
        <mass>1.0</mass>
          <inertia>
            <ixx>9.0e-4</ixx><!--8.71e-4 1.235e-3 1.096e-3-->
            <iyy>1.3e-3</iyy>
            <izz>1.1e-3</izz>
          </inertia>
    </inertial>
    <velocity_decay>
        <linear>0</linear>
        <angular>0.1</angular>
    </velocity_decay>

  <collision name='body_collision'>
    <pose>0 0 0 0 0 0</pose>
    <geometry>
      <box>
        <size>.075 .057 .03</size>
      </box>
    </geometry>
    <surface>
             <bounce>
                <restitution_coefficient> 0.5 </restitution_coefficient>
                <threshold> 0 </threshold>
             </bounce>
             <friction>
                <ode>
                 <mu> 0.1 </mu>
                </ode>
             </friction>
            </surface>
  </collision>
  <visual name='body_visual'>
    <pose>0 0 0 0 0 0</pose>
    <geometry>
      <box>
        <size>.075 .057 .045</size>
      </box>
    </geometry>
    <material>
      <script>
        <uri>file://media/materials/scripts/gazebo.material</uri>
        <name>Gazebo/Blue</name>
      </script>
    </material>
  </visual>


  <collision name='Fwheel_collision'> <!-- left wheel -->
    <pose>.03 0 -0.0225 0 0 0</pose>
    <geometry>
      <sphere>
        <radius>0.0015</radius>
      </sphere>
    </geometry>
    <surface>
                <bounce>
                    <restitution_coefficient> 0 </restitution_coefficient>
                    <threshold> 100000 </threshold>
                </bounce>
                <friction>
                    <ode>
                     <mu> 0 </mu>
                    </ode>
                </friction>
            </surface>
  </collision>
  <visual name='Fwheel_visual'>
    <pose>0.03 0 -0.0225 0 0 0</pose>
    <geometry>
      <sphere>
        <radius>0.0015</radius>
      </sphere>
    </geometry>
  </visual>

  <collision name='Bwheel_collision'>
    <pose>-0.03 0 -0.0225 0 0 0</pose>
    <geometry>
      <sphere>
        <radius>0.0015</radius>
      </sphere>
    </geometry>
    <surface>
                <bounce>
                    <restitution_coefficient> 0 </restitution_coefficient>
                    <threshold> 100000 </threshold>
                </bounce>
                <friction>
                    <ode>
                     <mu> 0 </mu>
                    </ode>
                </friction>
            </surface>        
  </collision>
  <visual name='Bwheel_visual'>
    <pose>-0.03 0 -0.0225 0 0 0</pose>
    <geometry>
      <sphere>
        <radius>0.0015</radius>
      </sphere>
    </geometry>
  </visual>

<link name="left_wheel">
  <pose>0 0.0335 .02 1.570796327 0 0</pose>
  <inertial>
          <mass>0.03</mass>
          <pose>0 0 0 0 0 0</pose>
          <inertia>
              <ixx>6.0e-5</ixx><!-- 3.16e-6 3.16e-6 6.0e-6 -->
              <iyy>6.0e-5</iyy>
              <izz>6.0e-5</izz>
          </inertia>
    </inertial>
  <collision name="collision">
    <pose>0 0 0 0 0 0</pose>
    <geometry>
      <cylinder>
        <radius>.02</radius>
        <length>.008</length>
      </cylinder>
    </geometry>
            <surface>
             <bounce>
                <restitution_coefficient> 0.1 </restitution_coefficient>
                <threshold> 100000 </threshold>
             </bounce>
             <friction>
                <ode>
                 <mu> 1.0 </mu>
                 <mu2> 1.0 </mu2>   
                 <fdir1>1 0 0</fdir1>
                 <slip1>0.0</slip1>
                 <slip2>0.1</slip2> <!---->     
                </ode>
             </friction>
            </surface>
  </collision>
  <visual name="visual">
  <pose>0 0 0 0 0 0</pose>
    <geometry>
      <cylinder>
        <radius>.02</radius>
        <length>.008</length>
      </cylinder>
    </geometry>
  </visual>
        <sensor name="left_wheel_sensor" type="imu">
        </sensor>
</link>

<link name="right_wheel">
  <pose>0 -0.0335 .02  1.570796327 0 0</pose>
    <inertial>
            <mass>0.03</mass>
            <pose>0 0 0 0 0 0</pose>
            <inertia>
              <ixx>6.0e-5</ixx><!-- 3.16e-6 3.16e-6 6.0e-6 -->
              <iyy>6.0e-5</iyy>
              <izz>6.0e-5</izz>
            </inertia>
        </inertial>  
        <collision name="collision">
    <pose>0 0 0 0 0 0</pose>
    <geometry>
      <cylinder>
        <radius>.02</radius>
        <length>.008</length>
      </cylinder>
    </geometry>
            <surface>
             <bounce>
                <restitution_coefficient> 0.1 </restitution_coefficient>
                <threshold> 100000 </threshold>
             </bounce>
             <friction>
                <ode>
                 <mu> 1.0 </mu>
                 <mu2> 1.0 </mu2>
                 <fdir1>1 0 0</fdir1>
                 <slip1>0.0</slip1>
                 <slip2>0.1</slip2> <!---->                  
                </ode>
             </friction>
            </surface>        
  </collision>
  <visual name="visual">
  <pose>0 0 0 0 0 0</pose>
    <geometry>
      <cylinder>
        <radius>.02</radius>
        <length>.008</length>
      </cylinder>
    </geometry>
  </visual>
  <sensor name="right_wheel_sensor" type="imu">
        </sensor>
</link>

<joint type="revolute" name="left_wheel_hinge">
  <pose>0 0 0 0 0 0</pose>
  <child>left_wheel</child>
  <parent>chassis</parent>
  <axis>
    <xyz>0 1 0</xyz>
    <dynamics>
      <damping>0.02</damping>
      <friction>0</friction>
    </dynamics>
  </axis>
  <physics>
    <ode>
      <limit>
        <cfm>0.5</cfm>
        <erp>0.3</erp>
      </limit>
    </ode>
  </physics>
</joint>

<joint type="revolute" name="right_wheel_hinge">
  <pose>0 0 0 0 0 0</pose>
  <child>right_wheel</child>
  <parent>chassis</parent>
  <axis>
    <xyz>0 1 0</xyz>
    <dynamics>
      <damping>0.02</damping>
      <friction>0</friction>
    </dynamics>
  </axis>
  <physics>
    <ode>
      <limit>
        <cfm>0.5</cfm>
        <erp>0.3</erp>
      </limit>
     </ode>
  </physics>
</joint>

</model>

Why my vehicle cant not go in a straight line?

I set the same force to "left_wheel_hinge"(joint) and "right_wheel_hinge"(joint) of my vehicle model at the same moment, it can not go in a straight line but still go in circles. The model's parameters are symmetrical.what 's wrong?Thanks!

The following is the vehicle model:

  <model name="my_newneuB">
<static>false</static>
<link name='chassis'>
        <pose>0 0 0.02575 0 0 0</pose>
        <inertial>
        <pose>-0.015 0 -0.015 0 0 0</pose>          
        <mass>1.0</mass>
          <inertia>
            <ixx>9.0e-4</ixx><!--8.71e-4 1.235e-3 1.096e-3-->
            <iyy>1.3e-3</iyy>
            <izz>1.1e-3</izz>
          </inertia>
    </inertial>
    <velocity_decay>
        <linear>0</linear>
        <angular>0.1</angular>
    </velocity_decay>

  <collision name='body_collision'>
    <pose>0 0 0 0 0 0</pose>
    <geometry>
      <box>
        <size>.075 .057 .03</size>
      </box>
    </geometry>
    <surface>
             <bounce>
                <restitution_coefficient> 0.5 </restitution_coefficient>
                <threshold> 0 </threshold>
             </bounce>
             <friction>
                <ode>
                 <mu> 0.1 </mu>
                </ode>
             </friction>
            </surface>
  </collision>
  <visual name='body_visual'>
    <pose>0 0 0 0 0 0</pose>
    <geometry>
      <box>
        <size>.075 .057 .045</size>
      </box>
    </geometry>
    <material>
      <script>
        <uri>file://media/materials/scripts/gazebo.material</uri>
        <name>Gazebo/Blue</name>
      </script>
    </material>
  </visual>


  <collision name='Fwheel_collision'> <!-- left wheel -->
    <pose>.03 0 -0.0225 0 0 0</pose>
    <geometry>
      <sphere>
        <radius>0.0015</radius>
      </sphere>
    </geometry>
    <surface>
                <bounce>
                    <restitution_coefficient> 0 </restitution_coefficient>
                    <threshold> 100000 </threshold>
                </bounce>
                <friction>
                    <ode>
                     <mu> 0 </mu>
                    </ode>
                </friction>
            </surface>
  </collision>
  <visual name='Fwheel_visual'>
    <pose>0.03 0 -0.0225 0 0 0</pose>
    <geometry>
      <sphere>
        <radius>0.0015</radius>
      </sphere>
    </geometry>
  </visual>

  <collision name='Bwheel_collision'>
    <pose>-0.03 0 -0.0225 0 0 0</pose>
    <geometry>
      <sphere>
        <radius>0.0015</radius>
      </sphere>
    </geometry>
    <surface>
                <bounce>
                    <restitution_coefficient> 0 </restitution_coefficient>
                    <threshold> 100000 </threshold>
                </bounce>
                <friction>
                    <ode>
                     <mu> 0 </mu>
                    </ode>
                </friction>
            </surface>        
  </collision>
  <visual name='Bwheel_visual'>
    <pose>-0.03 0 -0.0225 0 0 0</pose>
    <geometry>
      <sphere>
        <radius>0.0015</radius>
      </sphere>
    </geometry>
  </visual>

<link name="left_wheel">
  <pose>0 0.0335 .02 1.570796327 0 0</pose>
  <inertial>
          <mass>0.03</mass>
          <pose>0 0 0 0 0 0</pose>
          <inertia>
              <ixx>6.0e-5</ixx><!-- 3.16e-6 3.16e-6 6.0e-6 -->
              <iyy>6.0e-5</iyy>
              <izz>6.0e-5</izz>
          </inertia>
    </inertial>
  <collision name="collision">
    <pose>0 0 0 0 0 0</pose>
    <geometry>
      <cylinder>
        <radius>.02</radius>
        <length>.008</length>
      </cylinder>
    </geometry>
            <surface>
             <bounce>
                <restitution_coefficient> 0.1 </restitution_coefficient>
                <threshold> 100000 </threshold>
             </bounce>
             <friction>
                <ode>
                 <mu> 1.0 </mu>
                 <mu2> 1.0 </mu2>   
                 <fdir1>1 0 0</fdir1>
                 <slip1>0.0</slip1>
                 <slip2>0.1</slip2> <!---->     
                </ode>
             </friction>
            </surface>
  </collision>
  <visual name="visual">
  <pose>0 0 0 0 0 0</pose>
    <geometry>
      <cylinder>
        <radius>.02</radius>
        <length>.008</length>
      </cylinder>
    </geometry>
  </visual>
        <sensor name="left_wheel_sensor" type="imu">
        </sensor>
</link>

<link name="right_wheel">
  <pose>0 -0.0335 .02  1.570796327 0 0</pose>
    <inertial>
            <mass>0.03</mass>
            <pose>0 0 0 0 0 0</pose>
            <inertia>
              <ixx>6.0e-5</ixx><!-- 3.16e-6 3.16e-6 6.0e-6 -->
              <iyy>6.0e-5</iyy>
              <izz>6.0e-5</izz>
            </inertia>
        </inertial>  
        <collision name="collision">
    <pose>0 0 0 0 0 0</pose>
    <geometry>
      <cylinder>
        <radius>.02</radius>
        <length>.008</length>
      </cylinder>
    </geometry>
            <surface>
             <bounce>
                <restitution_coefficient> 0.1 </restitution_coefficient>
                <threshold> 100000 </threshold>
             </bounce>
             <friction>
                <ode>
                 <mu> 1.0 </mu>
                 <mu2> 1.0 </mu2>
                 <fdir1>1 0 0</fdir1>
                 <slip1>0.0</slip1>
                 <slip2>0.1</slip2> <!---->                  
                </ode>
             </friction>
            </surface>        
  </collision>
  <visual name="visual">
  <pose>0 0 0 0 0 0</pose>
    <geometry>
      <cylinder>
        <radius>.02</radius>
        <length>.008</length>
      </cylinder>
    </geometry>
  </visual>
  <sensor name="right_wheel_sensor" type="imu">
        </sensor>
</link>

<joint type="revolute" name="left_wheel_hinge">
  <pose>0 0 0 0 0 0</pose>
  <child>left_wheel</child>
  <parent>chassis</parent>
  <axis>
    <xyz>0 1 0</xyz>
    <dynamics>
      <damping>0.02</damping>
      <friction>0</friction>
    </dynamics>
  </axis>
  <physics>
    <ode>
      <limit>
        <cfm>0.5</cfm>
        <erp>0.3</erp>
      </limit>
    </ode>
  </physics>
</joint>

<joint type="revolute" name="right_wheel_hinge">
  <pose>0 0 0 0 0 0</pose>
  <child>right_wheel</child>
  <parent>chassis</parent>
  <axis>
    <xyz>0 1 0</xyz>
    <dynamics>
      <damping>0.02</damping>
      <friction>0</friction>
    </dynamics>
  </axis>
  <physics>
    <ode>
      <limit>
        <cfm>0.5</cfm>
        <erp>0.3</erp>
      </limit>
     </ode>
  </physics>
</joint>

</model>

Why my vehicle cant can not go in a straight line?

I set the same force to "left_wheel_hinge"(joint) and "right_wheel_hinge"(joint) of my vehicle model at the same moment, it can not go in a straight line but still go in circles. The model's parameters are symmetrical.what 's wrong?Thanks!

The following is the vehicle model:

  <model name="my_newneuB">
<static>false</static>
<link name='chassis'>
        <pose>0 0 0.02575 0 0 0</pose>
        <inertial>
        <pose>-0.015 0 -0.015 0 0 0</pose>          
        <mass>1.0</mass>
          <inertia>
            <ixx>9.0e-4</ixx><!--8.71e-4 1.235e-3 1.096e-3-->
            <iyy>1.3e-3</iyy>
            <izz>1.1e-3</izz>
          </inertia>
    </inertial>

  <collision name='body_collision'>
    <pose>0 0 0 0 0 0</pose>
    <geometry>
      <box>
        <size>.075 .057 .03</size>
      </box>
    </geometry>
    <surface>
             <bounce>
                <restitution_coefficient> 0.5 </restitution_coefficient>
                <threshold> 0 </threshold>
             </bounce>
             <friction>
                <ode>
                 <mu> 0.1 </mu>
                </ode>
             </friction>
            </surface>
  </collision>
  <visual name='body_visual'>
    <pose>0 0 0 0 0 0</pose>
    <geometry>
      <box>
        <size>.075 .057 .045</size>
      </box>
    </geometry>
    <material>
      <script>
        <uri>file://media/materials/scripts/gazebo.material</uri>
        <name>Gazebo/Blue</name>
      </script>
    </material>
  </visual>


  <collision name='Fwheel_collision'> <!-- left wheel -->
    <pose>.03 0 -0.0225 0 0 0</pose>
    <geometry>
      <sphere>
        <radius>0.0015</radius>
      </sphere>
    </geometry>
    <surface>
                <bounce>
                    <restitution_coefficient> 0 </restitution_coefficient>
                    <threshold> 100000 </threshold>
                </bounce>
                <friction>
                    <ode>
                     <mu> 0 </mu>
                    </ode>
                </friction>
            </surface>
  </collision>
  <visual name='Fwheel_visual'>
    <pose>0.03 0 -0.0225 0 0 0</pose>
    <geometry>
      <sphere>
        <radius>0.0015</radius>
      </sphere>
    </geometry>
  </visual>

  <collision name='Bwheel_collision'>
    <pose>-0.03 0 -0.0225 0 0 0</pose>
    <geometry>
      <sphere>
        <radius>0.0015</radius>
      </sphere>
    </geometry>
    <surface>
                <bounce>
                    <restitution_coefficient> 0 </restitution_coefficient>
                    <threshold> 100000 </threshold>
                </bounce>
                <friction>
                    <ode>
                     <mu> 0 </mu>
                    </ode>
                </friction>
            </surface>        
  </collision>
  <visual name='Bwheel_visual'>
    <pose>-0.03 0 -0.0225 0 0 0</pose>
    <geometry>
      <sphere>
        <radius>0.0015</radius>
      </sphere>
    </geometry>
  </visual>

<link name="left_wheel">
  <pose>0 0.0335 .02 1.570796327 0 0</pose>
  <inertial>
          <mass>0.03</mass>
          <pose>0 0 0 0 0 0</pose>
          <inertia>
              <ixx>6.0e-5</ixx><!-- 3.16e-6 3.16e-6 6.0e-6 -->
              <iyy>6.0e-5</iyy>
              <izz>6.0e-5</izz>
          </inertia>
    </inertial>
  <collision name="collision">
    <pose>0 0 0 0 0 0</pose>
    <geometry>
      <cylinder>
        <radius>.02</radius>
        <length>.008</length>
      </cylinder>
    </geometry>
            <surface>
             <bounce>
                <restitution_coefficient> 0.1 </restitution_coefficient>
                <threshold> 100000 </threshold>
             </bounce>
             <friction>
                <ode>
                 <mu> 1.0 </mu>
                 <mu2> 1.0 </mu2>   
                 <fdir1>1 0 0</fdir1>
                 <slip1>0.0</slip1>
                 <slip2>0.1</slip2> <!---->     
                </ode>
             </friction>
            </surface>
  </collision>
  <visual name="visual">
  <pose>0 0 0 0 0 0</pose>
    <geometry>
      <cylinder>
        <radius>.02</radius>
        <length>.008</length>
      </cylinder>
    </geometry>
  </visual>
        <sensor name="left_wheel_sensor" type="imu">
        </sensor>
</link>

<link name="right_wheel">
  <pose>0 -0.0335 .02  1.570796327 0 0</pose>
    <inertial>
            <mass>0.03</mass>
            <pose>0 0 0 0 0 0</pose>
            <inertia>
              <ixx>6.0e-5</ixx><!-- 3.16e-6 3.16e-6 6.0e-6 -->
              <iyy>6.0e-5</iyy>
              <izz>6.0e-5</izz>
            </inertia>
        </inertial>  
        <collision name="collision">
    <pose>0 0 0 0 0 0</pose>
    <geometry>
      <cylinder>
        <radius>.02</radius>
        <length>.008</length>
      </cylinder>
    </geometry>
            <surface>
             <bounce>
                <restitution_coefficient> 0.1 </restitution_coefficient>
                <threshold> 100000 </threshold>
             </bounce>
             <friction>
                <ode>
                 <mu> 1.0 </mu>
                 <mu2> 1.0 </mu2>
                 <fdir1>1 0 0</fdir1>
                 <slip1>0.0</slip1>
                 <slip2>0.1</slip2> <!---->                  
                </ode>
             </friction>
            </surface>        
  </collision>
  <visual name="visual">
  <pose>0 0 0 0 0 0</pose>
    <geometry>
      <cylinder>
        <radius>.02</radius>
        <length>.008</length>
      </cylinder>
    </geometry>
  </visual>
  <sensor name="right_wheel_sensor" type="imu">
        </sensor>
</link>

<joint type="revolute" name="left_wheel_hinge">
  <pose>0 0 0 0 0 0</pose>
  <child>left_wheel</child>
  <parent>chassis</parent>
  <axis>
    <xyz>0 1 0</xyz>
    <dynamics>
      <damping>0.02</damping>
      <friction>0</friction>
    </dynamics>
  </axis>
  <physics>
    <ode>
      <limit>
        <cfm>0.5</cfm>
        <erp>0.3</erp>
      </limit>
    </ode>
  </physics>
</joint>

<joint type="revolute" name="right_wheel_hinge">
  <pose>0 0 0 0 0 0</pose>
  <child>right_wheel</child>
  <parent>chassis</parent>
  <axis>
    <xyz>0 1 0</xyz>
    <dynamics>
      <damping>0.02</damping>
      <friction>0</friction>
    </dynamics>
  </axis>
  <physics>
    <ode>
      <limit>
        <cfm>0.5</cfm>
        <erp>0.3</erp>
      </limit>
     </ode>
  </physics>
</joint>

</model>