gazebo velocity controller

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I tried to simulate my car model in the gazebo, I built the front and rear wheel of the car, I tried to use the velocity controller in the ros_control to control my car front wheel and rear wheels respectively. had been loaded out of Wheel1_velocity_controller and Wheel2_velocity_controller：

Test "rostopic pub-1/rrbot/wheel1_velocity_controller/command std_msgs/float64 data:1.5" "Rostopic pub-1/rrbot/wheel1_velocity_controller/command std_msgs/float64 data:1.0"

The car can be movement, theoretically should be driven in a straight line, and the result of the simulation is turning movement, why?

Supplementary questions: when i run

rostopic pub-1/rrbot/wheel1_velocity_controller/command std_msgs/float64  data:1.0
rostopic pub-1/rrbot/wheel1_velocity_controller/command std_msgs/float64  data:1.0

The car is going to swerve, isn't it supposed to be a straight motion?

this is my urdf file:

<?xml version="1.0"?>
<robot name="rrbot" xmlns:xacro="http://www.ros.org/wiki/xacro">
<xacro:property name="length_wheel" value="0.05" />
<xacro:property name="radius_wheel" value="0.05" />
<xacro:property name="camera_link" value="0.05" /> 
<xacro:macro name="default_inertial" params="mass">
  <inertial>
    <mass value="${mass}" />
    <inertia ixx="1.0" ixy="0.0" ixz="0.0"
      iyy="1.0" iyz="0.0"
      izz="1.0" />
    </inertial>
  </xacro:macro>

  <xacro:include filename="$(find rrbot_description)/urdf/mytest.gazebo" />
  <xacro:include filename="$(find rrbot_description)/urdf/materials.xacro" />

  <link name="base_link">
    <visual>
      <geometry>
        <box size="0.2 .3 .1"/>
      </geometry>
      <origin rpy="0 0 1.54" xyz="0 0 0.05"/>
      <material name="orange">
        <color rgba="1 1 1 1"/>
      </material>
    </visual>
    <collision>
      <geometry>
        <box size="0.2 .3 0.1"/>
      </geometry>
    </collision>
    <xacro:default_inertial mass="10"/>
  </link>

  <link name="wheel_1">
    <visual>
      <geometry>
        <cylinder length="${length_wheel}" radius="${radius_wheel}"/>
      </geometry>
      <!-- <origin rpy="0 1.5 0" xyz="0.1 0.1 0"/> -->
      <origin rpy="0 0 0" xyz="0 0 0"/>
      <material name="black">
        <color rgba="0 0 0 1"/>
      </material>
    </visual>
    <collision>
      <geometry>
        <cylinder length="${length_wheel}" radius="${radius_wheel}"/>
      </geometry>
    </collision>
    <xacro:default_inertial mass="1"/>
  </link>

  <link name="wheel_2">
    <visual>
      <geometry>
        <cylinder length="${length_wheel}" radius="${radius_wheel}"/>
      </geometry>
      <!-- <origin rpy="0 1.5 0" xyz="-0.1 0.1 0"/> -->
      <origin rpy="0 0 0" xyz="0 0 0"/>
      <material name="black"/>
    </visual>
    <collision>
      <geometry>
        <cylinder length="${length_wheel}" radius="${radius_wheel}"/>
      </geometry>
    </collision>
    <xacro:default_inertial mass="1"/>
  </link>

  <joint name="wheel1_joint" type="continuous">
    <parent link="base_link"/>
    <child link="wheel_1"/>
    <origin rpy="-1.5707 0 0" xyz="0.1 0 0"/>
    <axis xyz="0 0 1" />
  </joint>

  <joint name="wheel2_joint" type="continuous">
    <axis xyz="0 0 1" />
    <anchor xyz="0 0 0" />
    <limit effort="100" velocity="100" />
    <parent link="base_link"/>
    <child link="wheel_2"/>
    <origin rpy="-1.5707 0 0" xyz="-0.1 0 0"/>
  </joint>

  <joint name="camera_joint" type="fixed">
    <axis xyz="0 1 0" />
    <origin xyz="0.125 0 0.125" rpy="0 0 0"/>
    <parent link="base_link"/>
    <child link="camera_link"/>
  </joint>

  <link name="camera_link">
    <collision>
      <origin xyz="0 0 0" rpy="0 0 0"/>
      <geometry>
        <box size="0.05 0.05 0.05"/>
      </geometry>
    </collision>

    <visual>
      <origin xyz="0 ...