ROS学习初探之自建小车模型并进行仿真（二）

其他 2020-09-08 10:40:52 阅读次数: 0

roslaunch mbot_teleop mbot_teleop.launch # URDF文件转xacro文件，并加入控制插件

一、URDF文件

我们都知道URDF是ROS中机器人模型的描述格式，包含对机器人刚体外观、物理属性、关节类型等方面的描述。我们先看看从SolidWorks中导出的URDF文件。我们打开tianbot_mini_description.urdf:

<?xml version="1.0" encoding="utf-8"?>
<!-- This URDF was automatically created by SolidWorks to URDF Exporter! Originally created by Stephen Brawner ([email protected]) 
     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 -->
<robot
  name="tianbot_mini_description">
  <link
    name="base_link">
    <inertial>
      <origin
        xyz="0.025566 -0.0012293 0.024367"
        rpy="0 0 0" />
      <mass
        value="0.15009" />
      <inertia
        ixx="7.0347E-05"
        ixy="-2.608E-07"
        ixz="1.5444E-05"
        iyy="7.3091E-05"
        iyz="-1.2084E-06"
        izz="0.00010959" />
    </inertial>
    <visual>
      <origin
        xyz="0 0 0"
        rpy="0 0 0" />
      <geometry>
        <mesh
          filename="package://tianbot_mini_description/meshes/base_link.STL" />
      </geometry>
      <material
        name="">
        <color
          rgba="1 1 1 1" />
      </material>
    </visual>
    <collision>
      <origin
        xyz="0 0 0"
        rpy="0 0 0" />
      <geometry>
        <mesh
          filename="package://tianbot_mini_description/meshes/base_link.STL" />
      </geometry>
    </collision>
  </link>
  <link
    name="left_wheel_link">
    <inertial>
      <origin
        xyz="-8.8919E-05 -0.0038269 -8.1628E-05"
        rpy="0 0 0" />
      <mass
        value="0.014111" />
      <inertia
        ixx="2.4055E-06"
        ixy="5.333E-08"
        ixz="1.1993E-09"
        iyy="4.2136E-06"
        iyz="3.7603E-08"
        izz="2.4044E-06" />
    </inertial>
    <visual>
      <origin
        xyz="0 0 0"
        rpy="0 0 0" />
      <geometry>
        <mesh
          filename="package://tianbot_mini_description/meshes/left_wheel_link.STL" />
      </geometry>
      <material
        name="">
        <color
          rgba="0.50196 0.50196 0.50196 1" />
      </material>
    </visual>
    <collision>
      <origin
        xyz="0 0 0"
        rpy="0 0 0" />
      <geometry>
        <mesh
          filename="package://tianbot_mini_description/meshes/left_wheel_link.STL" />
      </geometry>
    </collision>
  </link>
  <joint
    name="left_wheel_joint"
    type="continuous">
    <origin
      xyz="0.0015549 0.053866 0.0011204"
      rpy="0 0 0" />
    <parent
      link="base_link" />
    <child
      link="left_wheel_link" />
    <axis
      xyz="0.028847 0.99937 0.020786" />
  </joint>
  <link
    name="right_wheel_link">
    <inertial>
      <origin
        xyz="8.944E-05 0.0038269 7.9359E-05"
        rpy="0 0 0" />
      <mass
        value="0.014111" />
      <inertia
        ixx="2.4056E-06"
        ixy="5.3313E-08"
        ixz="1.0756E-09"
        iyy="4.2136E-06"
        iyz="3.7601E-08"
        izz="2.4044E-06" />
    </inertial>
    <visual>
      <origin
        xyz="0 0 0"
        rpy="0 0 0" />
      <geometry>
        <mesh
          filename="package://tianbot_mini_description/meshes/right_wheel_link.STL" />
      </geometry>
      <material
        name="">
        <color
          rgba="0.50196 0.50196 0.50196 1" />
      </material>
    </visual>
    <collision>
      <origin
        xyz="0 0 0"
        rpy="0 0 0" />
      <geometry>
        <mesh
          filename="package://tianbot_mini_description/meshes/right_wheel_link.STL" />
      </geometry>
    </collision>
  </link>
  <joint
    name="right_wheel_joint"
    type="continuous">
    <origin
      xyz="-0.0015549 -0.053866 -0.0011204"
      rpy="0 0 0" />
    <parent
      link="base_link" />
    <child
      link="right_wheel_link" />
    <axis
      xyz="0.028847 0.99937 0.020786" />
  </joint>
  <link
    name="caster_wheel_link">
    <inertial>
      <origin
        xyz="-0.0021044 -0.0014298 -0.014923"
        rpy="0 0 0" />
      <mass
        value="0.014141" />
      <inertia
        ixx="2.1919E-06"
        ixy="3.8453E-07"
        ixz="8.1679E-08"
        iyy="2.2221E-06"
        iyz="6.9274E-09"
        izz="3.4078E-06" />
    </inertial>
    <visual>
      <origin
        xyz="0 0 0"
        rpy="0 0 0" />
      <geometry>
        <mesh
          filename="package://tianbot_mini_description/meshes/caster_wheel_link.STL" />
      </geometry>
      <material
        name="">
        <color
          rgba="0.75294 0.75294 0.75294 1" />
      </material>
    </visual>
    <collision>
      <origin
        xyz="0 0 0"
        rpy="0 0 0" />
      <geometry>
        <mesh
          filename="package://tianbot_mini_description/meshes/caster_wheel_link.STL" />
      </geometry>
    </collision>
  </link>
  <joint
    name="caster_wheel_joint"
    type="continuous">
    <origin
      xyz="0.067196 -0.0022154 0.011383"
      rpy="0 0 0" />
    <parent
      link="base_link" />
    <child
      link="caster_wheel_link" />
    <axis
      xyz="-0.0264 0.021537 -0.99942" />
  </joint>
  <link
    name="casterball_link">
    <inertial>
      <origin
        xyz="7.0778E-06 -6.9887E-06 -3.6348E-07"
        rpy="0 0 0" />
      <mass
        value="0.0091741" />
      <inertia
        ixx="6.5492E-07"
        ixy="-5.0039E-09"
        ixz="-2.5621E-10"
        iyy="6.5468E-07"
        iyz="2.1952E-10"
        izz="6.4987E-07" />
    </inertial>
    <visual>
      <origin
        xyz="0 0 0"
        rpy="0 0 0" />
      <geometry>
        <mesh
          filename="package://tianbot_mini_description/meshes/casterball_link.STL" />
      </geometry>
      <material
        name="">
        <color
          rgba="0.55686 0.55686 0.55686 1" />
      </material>
    </visual>
    <collision>
      <origin
        xyz="0 0 0"
        rpy="0 0 0" />
      <geometry>
        <mesh
          filename="package://tianbot_mini_description/meshes/casterball_link.STL" />
      </geometry>
    </collision>
  </link>
  <joint
    name="casterball_joint"
    type="revolute">
    <origin
      xyz="0.0063432 0.007688 -0.022315"
      rpy="0 0 0" />
    <parent
      link="caster_wheel_link" />
    <child
      link="casterball_link" />
    <axis
      xyz="0.71538 -0.69791 -0.033936" />
    <limit
      lower="0"
      upper="0"
      effort="0"
      velocity="0" />
  </joint>
  <link
    name="lidar_Link">
    <inertial>
      <origin
        xyz="0.0047077 0.0002084 -0.0069054"
        rpy="0 0 0" />
      <mass
        value="0.073208" />
      <inertia
        ixx="2.1832E-05"
        ixy="4.6598E-07"
        ixz="8.4247E-06"
        iyy="4.1492E-05"
        iyz="-3.6568E-07"
        izz="4.7274E-05" />
    </inertial>
    <visual>
      <origin
        xyz="0 0 0"
        rpy="0 0 0" />
      <geometry>
        <mesh
          filename="package://tianbot_mini_description/meshes/lidar_Link.STL" />
      </geometry>
      <material
        name="">
        <color
          rgba="0.75294 0.75294 0.75294 1" />
      </material>
    </visual>
    <collision>
      <origin
        xyz="0 0 0"
        rpy="0 0 0" />
      <geometry>
        <mesh
          filename="package://tianbot_mini_description/meshes/lidar_Link.STL" />
      </geometry>
    </collision>
  </link>
  <joint
    name="lidar_joint"
    type="fixed">
    <origin
      xyz="0.028391 -0.0024308 0.066849"
      rpy="0 0 0" />
    <parent
      link="base_link" />
    <child
      link="lidar_Link" />
    <axis
      xyz="0 0 0" />
  </joint>

<gazebo reference="base_link"> 
     <material>Gazebo/Yellow </material>
    </gazebo>
 <gazebo reference="right_wheel_link"> 
     <material>Gazebo/Black </material>
    </gazebo>
 <gazebo reference="left_wheel_link"> 
     <material>Gazebo/Black </material>
    </gazebo>
 <gazebo reference="caster_wheel_link"> 
     <material>Gazebo/Blue </material>
    </gazebo>
  <gazebo reference="lidar_Link"> 
     <material>Gazebo/Red </material>
    </gazebo>

</robot>

我们再看看URDF中各个link的关系图:
在这里插入图片描述
我们可以看到从SolidWorks导出的URDF文件，已经配置好了物理属性和碰撞属性，可以在gazebo中打开。并且我配置了一下各个link 的颜色，用gazebo打开后就能看到颜色变了。

二、配置xacro文件

我们知道xacro文件是针对URDF模型产生的另一种精简版、可复用、模块化的描述形式。首先创建一个tianbot_mini.urdf.xacro文件，我们在模型< robot>标签中加入xacro的声明:

<?xml version="1.0" encoding="utf-8"?>
<robot name="tianbot_mini_description" xmlns:xacro="http://www.ros.org/wiki/xacro">

接着把上面的URDF文件除去最上面两行全部放进去。

添加传动装置

我们的机器人模型可看作一个两轮差速驱动的机器人，通过调节两个轮子的速度比例，完成前进、转向、倒退等动作。此时需要加入驱动机器人运动的动力源，为使用ROS控制驱动机器人，需要在模型中加入transmission元素，将传动装置与joint绑定。

 <!-- Transmission is important to link the joints and the controller -->
 <xacro:macro name="wheel" params="prefix reflect">
        <transmission name="${prefix}_wheel_joint_trans">
            <type>transmission_interface/SimpleTransmission</type>
            <joint name="${prefix}_wheel_joint" >
                <hardwareInterface>hardware_interface/VelocityJointInterface</hardwareInterface>
            </joint>
            <actuator name="${prefix}_wheel_joint_motor">
                <hardwareInterface>hardware_interface/VelocityJointInterface</hardwareInterface>
                <mechanicalReduction>1</mechanicalReduction>
            </actuator>
        </transmission>
    </xacro:macro>

 <wheel prefix="left"  reflect="1"/>
  <wheel prefix="right" reflect="-1"/>

添加gazebo控制器插件

我们直接加入gazebo提供的libgazebo_ros_diff_drive.so两轮差速插件，可以将其应用到现有的机器人模型上。

<!-- controller -->
        <gazebo>
            <plugin name="differential_drive_controller" 
                    filename="libgazebo_ros_diff_drive.so">
                <rosDebugLevel>Debug</rosDebugLevel>
                <publishWheelTF>true</publishWheelTF>
                <robotNamespace>/</robotNamespace>
                <publishTf>1</publishTf>
                <publishWheelJointState>true</publishWheelJointState>
                <alwaysOn>true</alwaysOn>
                <updateRate>100.0</updateRate>
                <legacyMode>true</legacyMode>
                <leftJoint>left_wheel_joint</leftJoint>
                <rightJoint>right_wheel_joint</rightJoint>
                <wheelSeparation>0.1</wheelSeparation>
                <wheelDiameter>0.043</wheelDiameter>
                <broadcastTF>1</broadcastTF>
                <wheelTorque>30</wheelTorque>
                <wheelAcceleration>1.8</wheelAcceleration>
                <commandTopic>cmd_vel</commandTopic>
                <odometryFrame>odom</odometryFrame> 
                <odometryTopic>odom</odometryTopic> 
                <robotBaseFrame>base_footprint</robotBaseFrame>
            </plugin>
        </gazebo>

注意leftJoint和rightJoint标签中间要输入自己机器人的左右jointname，wheelSeparation和wheelDiameter标签中间要输入自己机器人的轮间距和轮直径。
到现在xacro文件已经配置好了，我们可以看一看总的

<?xml version="1.0" encoding="utf-8"?>
<robot name="tianbot_mini_description" xmlns:xacro="http://www.ros.org/wiki/xacro">
  <link
    name="base_link">
    <inertial>
      <origin
        xyz="0.025566 -0.0012293 0.024367"
        rpy="0 0 0" />
      <mass
        value="0.15009" />
   。
   。
   。
   中间太长，我给删了
   。
   。
   。
  <joint
    name="lidar_joint"
    type="fixed">
    <origin
      xyz="0.028391 -0.0024308 0.066849"
      rpy="0 0 0" />
    <parent
      link="base_link" />
    <child
      link="lidar_Link" />
    <axis
      xyz="0 0 0" />
  </joint>

<gazebo reference="base_link"> 
     <material>Gazebo/Red </material>
    </gazebo>
 <gazebo reference="right_wheel_link"> 
     <material>Gazebo/Black </material>
    </gazebo>
 <gazebo reference="left_wheel_link"> 
     <material>Gazebo/Black </material>
    </gazebo>
 <gazebo reference="caster_wheel_link"> 
     <material>Gazebo/Blue </material>
    </gazebo>


 <!-- Transmission is important to link the joints and the controller -->
 <xacro:macro name="wheel" params="prefix reflect">
        <transmission name="${prefix}_wheel_joint_trans">
            <type>transmission_interface/SimpleTransmission</type>
            <joint name="${prefix}_wheel_joint" >
                <hardwareInterface>hardware_interface/VelocityJointInterface</hardwareInterface>
            </joint>
            <actuator name="${prefix}_wheel_joint_motor">
                <hardwareInterface>hardware_interface/VelocityJointInterface</hardwareInterface>
                <mechanicalReduction>1</mechanicalReduction>
            </actuator>
        </transmission>
    </xacro:macro>

 <wheel prefix="left"  reflect="1"/>
  <wheel prefix="right" reflect="-1"/>
<!-- controller -->
        <gazebo>
            <plugin name="differential_drive_controller" 
                    filename="libgazebo_ros_diff_drive.so">
                <rosDebugLevel>Debug</rosDebugLevel>
                <publishWheelTF>true</publishWheelTF>
                <robotNamespace>tianbot_mini</robotNamespace>
                <publishTf>1</publishTf>
                <publishWheelJointState>true</publishWheelJointState>
                <alwaysOn>true</alwaysOn>
                <updateRate>100.0</updateRate>
                <legacyMode>true</legacyMode>
                <leftJoint>left_wheel_joint</leftJoint>
                <rightJoint>right_wheel_joint</rightJoint>
                <wheelSeparation>0.1</wheelSeparation>
                <wheelDiameter>0.043</wheelDiameter>
                <broadcastTF>1</broadcastTF>
                <wheelTorque>30</wheelTorque>
                <wheelAcceleration>1.8</wheelAcceleration>
                <commandTopic>cmd_vel</commandTopic>
                <odometryFrame>odom</odometryFrame> 
                <odometryTopic>odom</odometryTopic> 
                <robotBaseFrame>base_footprint</robotBaseFrame>
            </plugin>
        </gazebo> 


</robot>

现在的xacro文件可以通过launch文件打开，并且用键盘控制节点进行小车的控制，这些我们放到下一话再讲。

小结

本节我们简单了解了一下URDF文件和xacro文件，并且说明了如何配置二轮差速小车在gazebo环境中的控制器插件，也就是简单了解一下ros_control。
下一节，由我来说一说配置launch文件和yaml文件，并在gazebo环境中实现对小车的控制，并调用激光雷达插件，在rviz中看到点云数据。
接下去的内容会在古月居的论坛，tianbot_mini板块中同步更新，敬请期待。

参考资料

1.古月老师的<<ROS机器人开发实践>>

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