前言：ROS机器人采用urdf文件描述运动学和动力学特征，视觉表示，碰撞模型等。
在URDF中，机器人模型由连接件（link）和连接件的关节(joint)、传感器(sensor)传动件(transmission)等部件组成。
1.连接件
在urdf支持连接件的标签为link，link标签用于描述具有惯性和视觉特征的刚体，也即机器人的物理部件。link标签通过name属性描述连接件的名称。
link标签可包含inertial(惯性特征),visual(视觉特征),collision(碰撞特征)等标签
(1)惯性特征
物体的惯性和物体的质量，形状，以及质量分布有关，出于计算复杂性和刚体特征考虑，通常默认质量是均匀分布的。

对于半径r和质量为m的实心球体，其选红钻惯性矩阵为:

球体:

    <!-- Macro for inertia matrix --><xacro:macro name="sphere_inertial_matrix" params="m r"><inertial><mass value="${m}" /><inertia ixx="${2*m*r*r/5}" ixy="0" ixz="0"iyy="${2*m*r*r/5}" iyz="0" izz="${2*m*r*r/5}" /></inertial></xacro:macro>

球体矩阵模块使用:

<sphere_inertial_matrix  m="${caster_mass}" r="${caster_radius}" />

对于宽度w、高h、长为d、质量为m的实心立方体，其旋转惯性矩阵为:

立方体惯性矩阵模块:

   <xacro:macro name="Box_inertial_matrix" params="m l w h"><inertial><mass value="${m}" /><inertia ixx="${m*(h*h + l*l)/12}" ixy = "0" ixz = "0"iyy="${m*(w*w + l*l)/12}" iyz= "0"izz="${m*(w*w + h*h)/12}" /></inertial></xacro:macro>

对于半径r、高为h、质量为m、的实心圆柱体，其旋转惯性矩阵为:

圆柱体定义:

    <xacro:macro name="cylinder_inertial_matrix" params="m r h"><inertial><mass value="${m}" /><inertia ixx="${m*(3*r*r+h*h)/12}" ixy = "0" ixz = "0"iyy="${m*(3*r*r+h*h)/12}" iyz = "0"izz="${m*r*r/2}" /> </inertial></xacro:macro>

圆柱体惯性矩阵使用:

<cylinder_inertial_matrix  m="${wheel_mass}" r="${wheel_radius}" h="${wheel_length}" />

(2)视觉特征
visual标签定义了连接件的视觉特征，可包含的子标签有origin、geometry、material、mesh等。其中origin标签定义了连接件的位姿；geometry定义了连接件的几何特征。
目前支持立方体(box)，圆柱体(cylinder)、球体(sphere)等。material定义了连接件的材料特征，支持颜色(color)，纹理(texture)等；mesh定义了导入三维建模文件，格式为dae,stl等

视觉特征定义使用RGB色表

    <!-- Defining the colors used in this robot --><material name="blue"><color rgba="0 0 0.8 1"/></material><material name="black"><color rgba="0 0 0 0.95"/></material><material name="green"><color rgba="1 0 0 1"/></material><material name="white"><color rgba="1 1 1 1"/></material>

URDF:

 <material name="blue" />

xacro:

        <gazebo reference="${prefix}_wheel_link"><material>Gazebo/Gray</material></gazebo>

(3)碰撞特征
collision标签定义了连接件的碰撞特征，可包含的子标签有origin,geometry等。其中origin标签定义了连接件的位姿检测区相对于link的偏移位置；geometry定义了连接件的几何特征，目前支持立方体(box)，圆柱体(cylinder)、球体(sphere)等。
需要注意的是,origin标签是利用空间笛卡尔直角坐标系和欧拉角来描述位姿，其中xyz属性描述了连接件X,Y,Z轴上的偏移；rpy属性描述了连接件X,Y,Z轴上的旋转角度，即欧拉角中的滚转角(Roll)、俯仰角(Pitch)，偏航角(Yaw)，顺时针为正，逆时针为负

<collision><origin xyz="0 0 0" rpy="${M_PI/2} 0 0" /><geometry><cylinder radius="${wheel_radius}" length = "${wheel_length}"/></geometry>
</collision>

~/catkin_ws/src/mbot_gazebo/launch/view
_mbot_gazebo_camera_room.launch

<launch><!-- 设置launch文件的参数 --><arg name="world_name" value="$(find mbot_gazebo)/worlds/playground.world"/><arg name="paused" default="false"/><arg name="use_sim_time" default="true"/><arg name="gui" default="true"/><arg name="headless" default="false"/><arg name="debug" default="false"/><!-- 运行gazebo仿真环境 --><include file="$(find gazebo_ros)/launch/empty_world.launch"><arg name="world_name" value="$(arg world_name)" /><arg name="debug" value="$(arg debug)" /><arg name="gui" value="$(arg gui)" /><arg name="paused" value="$(arg paused)"/><arg name="use_sim_time" value="$(arg use_sim_time)"/><arg name="headless" value="$(arg headless)"/></include><!-- 加载机器人模型描述参数 --><param name="robot_description" command="$(find xacro)/xacro --inorder '$(find mbot_description)/urdf/xacro/gazebo/mbot_with_laserandcamera_gazebo.xacro'" /> <!-- 运行joint_state_publisher节点，发布机器人的关节状态  --><node name="joint_state_publisher" pkg="joint_state_publisher" type="joint_state_publisher" ></node> <!-- 运行robot_state_publisher节点，发布tf  --><node name="robot_state_publisher" pkg="robot_state_publisher" type="robot_state_publisher"  output="screen" ><param name="publish_frequency" type="double" value="50.0" /></node><!-- 在gazebo中加载机器人模型--><node name="urdf_spawner" pkg="gazebo_ros" type="spawn_model" respawn="false" output="screen"args="-urdf -model mbot -param robot_description"/> <!-- 运行rviz可视化界面 --><node name="rviz" pkg="rviz" type="rviz" args="-d $(find mbot_description)/config/mbot_urdf.rviz" required="true" /></launch>

~/catkin_ws/src/mbot_description/urdf/xacro/gazebo/mbot_with_laserandcamera_gaze.xacro

<?xml version="1.0"?>
<robot name="arm" xmlns:xacro="http://www.ros.org/wiki/xacro"><xacro:include filename="$(find mbot_description)/urdf/xacro/gazebo/mbot_base_gazebo.xacro" /><xacro:include filename="$(find mbot_description)/urdf/xacro/sensors/lidar_gazebo.xacro" /><xacro:include filename="$(find mbot_description)/urdf/xacro/sensors/camera_gazebo.xacro" /><xacro:property name="lidar_offset_x" value="0" /><xacro:property name="lidar_offset_y" value="0" /><xacro:property name="lidar_offset_z" value="0.0825" /><xacro:property name="camera_offset_x" value="0.17" /><xacro:property name="camera_offset_y" value="0" /><xacro:property name="camera_offset_z" value="0.0775" /><!-- lidar --><joint name="lidar_joint" type="fixed"><origin xyz="${lidar_offset_x} ${lidar_offset_y} ${lidar_offset_z}" rpy="0 0 0" /><parent link="base_link"/><child link="laser_link"/></joint><xacro:rplidar prefix="laser"/><gazebo reference="laser_link"><material>Gazebo/Black</material></gazebo><!-- Camera --><joint name="camera_joint" type="fixed"><origin xyz="${camera_offset_x} ${camera_offset_y} ${camera_offset_z}" rpy="0 0 0" /><parent link="base_link"/><child link="camera_link"/></joint><xacro:usb_camera prefix="camera"/><gazebo reference="camera_link"><material>Gazebo/Green</material></gazebo><mbot_base_gazebo/></robot>

~/catkin/src/mbot_description/urdf/xacro/sensors/lidar.xacro

<?xml version="1.0"?>
<robot xmlns:xacro="http://www.ros.org/wiki/xacro" name="laser"><xacro:macro name="rplidar" params="prefix:=laser"><link name="${prefix}_link"><visual><origin xyz=" 0 0 0 " rpy="0 0 0" /><geometry><cylinder length="0.05" radius="0.05"/></geometry><material name="red"/></visual></link></xacro:macro></robot>

~/catkin/src/mbot_description/urdf/xacro/sensors/camera.xacro

<?xml version="1.0"?>
<robot xmlns:xacro="http://www.ros.org/wiki/xacro" name="camera"><material name="green"><color rgba="1 0 0 1"/></material><xacro:macro name="usb_camera" params="prefix:=camera"><link name="${prefix}_link"><visual><origin xyz=" 0 0 0 " rpy="0 0 0" /><geometry><box size="0.01 0.04 0.04" /></geometry><material name="green"/></visual></link></xacro:macro></robot>

~/catkin_ws/src/mbot_description/urdf/xacro/gazebo/mbot_base_gaze.xacro

<?xml version="1.0"?>
<robot name="mbot" xmlns:xacro="http://www.ros.org/wiki/xacro"><!-- PROPERTY LIST --><xacro:property name="M_PI" value="3.1415926"/><xacro:property name="base_mass"   value="20" /> <xacro:property name="base_radius" value="0.20"/><xacro:property name="base_length" value="0.16"/><xacro:property name="base_link_length" value="0.35"/><xacro:property name="base_link_width" value="0.22"/><xacro:property name="base_link_high" value="0.115"/><xacro:property name="wheel_mass"   value="2" /><xacro:property name="wheel_radius" value="0.08"/><xacro:property name="wheel_length" value="0.04"/><xacro:property name="wheel_joint_y" value="0.21"/><xacro:property name="wheel_joint_z" value="0.07"/><xacro:property name="caster_mass"    value="0.5" /> <xacro:property name="caster_radius"  value="0.045"/> <!-- wheel_radius - ( base_length/2 - wheel_joint_z) --><xacro:property name="caster_joint_x" value="0.18"/><!-- Defining the colors used in this robot --><material name="blue"><color rgba="0 0 0.8 1"/></material><material name="black"><color rgba="0 0 0 0.95"/></material><material name="green"><color rgba="1 0 0 1"/></material><material name="white"><color rgba="1 1 1 1"/></material><!-- Macro for inertia matrix --><xacro:macro name="sphere_inertial_matrix" params="m r"><inertial><mass value="${m}" /><inertia ixx="${2*m*r*r/5}" ixy="0" ixz="0"iyy="${2*m*r*r/5}" iyz="0" izz="${2*m*r*r/5}" /></inertial></xacro:macro><xacro:macro name="cylinder_inertial_matrix" params="m r h"><inertial><mass value="${m}" /><inertia ixx="${m*(3*r*r+h*h)/12}" ixy = "0" ixz = "0"iyy="${m*(3*r*r+h*h)/12}" iyz = "0"izz="${m*r*r/2}" /> </inertial></xacro:macro><xacro:macro name="Box_inertial_matrix" params="m l w h"><inertial><mass value="${m}" /><inertia ixx="${m*(h*h + l*l)/12}" ixy = "0" ixz = "0"iyy="${m*(w*w + l*l)/12}" iyz= "0"izz="${m*(w*w + h*h)/12}" /></inertial></xacro:macro><!-- Macro for robot wheel --><xacro:macro name="wheel" params="prefix reflect"><joint name="${prefix}_wheel_joint" type="continuous"><origin xyz="0 ${reflect*(base_link_width+wheel_length)/2} ${-wheel_joint_z}" rpy="0 0 0"/><parent link="base_link"/><child link="${prefix}_wheel_link"/><axis xyz="0 1 0"/></joint><link name="${prefix}_wheel_link"><visual><origin xyz="0 0 0" rpy="${M_PI/2} 0 0" /><geometry><cylinder radius="${wheel_radius}" length = "${wheel_length}"/></geometry><material name="white" /></visual><collision><origin xyz="0 0 0" rpy="${M_PI/2} 0 0" /><geometry><cylinder radius="${wheel_radius}" length = "${wheel_length}"/></geometry></collision><cylinder_inertial_matrix  m="${wheel_mass}" r="${wheel_radius}" h="${wheel_length}" /></link><gazebo reference="${prefix}_wheel_link"><material>Gazebo/Gray</material></gazebo><!-- Transmission is important to link the joints and the controller --><transmission name="${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><!-- Macro for robot caster --><xacro:macro name="caster" params="prefix reflect"><joint name="${prefix}_caster_joint" type="continuous"><origin xyz="${reflect*caster_joint_x} 0 ${-(base_link_high/2 + caster_radius)}" rpy="0 0 0"/><parent link="base_link"/><child link="${prefix}_caster_link"/><axis xyz="0 1 0"/></joint><link name="${prefix}_caster_link"><visual><origin xyz="0 0 0" rpy="0 0 0"/><geometry><sphere radius="${caster_radius}" /></geometry><material name="black" /></visual><collision><origin xyz="0 0 0" rpy="0 0 0"/><geometry><sphere radius="${caster_radius}" /></geometry></collision>      <sphere_inertial_matrix  m="${caster_mass}" r="${caster_radius}" /></link><gazebo reference="${prefix}_caster_link"><material>Gazebo/Black</material></gazebo></xacro:macro><xacro:macro name="mbot_base_gazebo"><link name="base_footprint"><visual><origin xyz="0 0 0" rpy="0 0 0" /><geometry><box size="0.001 0.001 0.001" /></geometry></visual></link><gazebo reference="base_footprint"><turnGravityOff>false</turnGravityOff></gazebo><joint name="base_footprint_joint" type="fixed"><origin xyz="0 0 ${base_link_high/2 + caster_radius*2}" rpy="0 0 0" />        <parent link="base_footprint"/><child link="base_link" /></joint><link name="base_link"><visual><origin xyz=" 0 0 0" rpy="0 0 0" /><geometry><box size="${base_link_length} ${base_link_width} ${base_link_high}"/></geometry><material name="blue" /></visual><collision><origin xyz=" 0 0 0" rpy="0 0 0" /><geometry><box size="${base_link_length} ${base_link_width} ${base_link_high}"/></geometry></collision>   <Box_inertial_matrix m="${base_mass}" l="${base_link_length}" w="${base_link_width}" h="${base_link_high}" /></link><gazebo reference="base_link"><material>Gazebo/Blue</material></gazebo><wheel prefix="left"  reflect="-1"/><wheel prefix="right" reflect="1"/><caster prefix="front" reflect="-1"/><caster prefix="back"  reflect="1"/><!-- controller --><gazebo><plugin name="differential_drive_controller" filename="libgazebo_ros_diff_drive.so"><rosDebugLevel>Debug</rosDebugLevel><publishWheelTF>true</publishWheelTF><robotNamespace>/</robotNamespace><publishTf>1</publishTf><publishWheelJointState>true</publishWheelJointState><alwaysOn>true</alwaysOn><updateRate>100.0</updateRate><legacyMode>true</legacyMode><leftJoint>left_wheel_joint</leftJoint><rightJoint>right_wheel_joint</rightJoint><wheelSeparation>${wheel_joint_y*2}</wheelSeparation><wheelDiameter>${2*wheel_radius}</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> </xacro:macro></robot>