这样一个简单的mobile robot模型

 首先写xacro文件，创建 link joint transmission

<?xml version="1.0"?>
<robot xmlns:xacro="http://www.ros.org/wiki/xacro" name="whill_modelc" ><xacro:property name="PI" value="3.1415926"/><link name="base_link"><visual><geometry><sphere radius="0.001" /></geometry></visual></link><link name="base_floor"><visual><geometry><box size="0.600 0.400 0.050"/></geometry><material name="grey"><color rgba="0.5 0.5 0.5 1.0"/></material></visual><collision><geometry><box size="0.600 0.400 0.050"/></geometry></collision><inertial><origin xyz="0 0 0" rpy="0 0 0" /><mass value="40" /><inertia ixx="1.0"  ixy="0.0"   ixz="0.0"iyy="1.0"   iyz="0.0"izz="1.0" /></inertial></link><joint name="base_body_joint" type="fixed"><parent link="base_link" /><child link="base_floor" /><origin xyz="0.3 0 0.0" rpy="0 0 0"/></joint><link name="footrest_link"><collision><geometry><box size="0.250 0.260 0.050"/></geometry></collision><visual><geometry><box size="0.250 0.260 0.050"/></geometry><material name="grey"><color rgba="0.5 0.5 0.5 1.0"/></material></visual><inertial><origin xyz="0 0 0" rpy="0 0 0" /><mass value="1" /><inertia ixx="1.0"  ixy="0.0"   ixz="0.0"iyy="1.0"   iyz="0.0"izz="1.0" /></inertial></link><joint name="footrest_joint" type="fixed"><parent link="base_floor" /><child link="footrest_link" /><origin xyz="0.320 0 0" rpy="0 0 0"/></joint><link name="left_wheel_link"><visual><geometry><cylinder length="0.05" radius="0.1325"/></geometry><origin xyz="0.0 0.0 0.0" rpy="0.0 0.0 0.0" /><material name="black" /></visual><collision><geometry><cylinder length="0.05" radius="0.1325"/></geometry><origin xyz="0.0 0.0 0.0" rpy="0.0 0.0 0.0" /></collision> <inertial><mass value="0.500"/><inertia ixx="0.0013541667" ixy="0" ixz="0" iyy="0.0013541667" iyz="0" izz="0.0025"/></inertial></link><joint name="leftWheel" type="continuous"><parent link="base_floor" /><child link="left_wheel_link" /><origin xyz="-0.3 0.245 0" rpy="${-PI / 2} 0.0 0.0" /><axis xyz="0 0 1" /></joint><link name="right_wheel_link"><visual><geometry><cylinder length="0.05" radius="0.1325"/></geometry><origin xyz="0 0 0" rpy="0 0 0" /><material name="black" /></visual><collision><geometry><cylinder length="0.05" radius="0.1325"/></geometry><origin xyz="0 0 0" rpy="0.0 0.0 0.0" /></collision><inertial><mass value="0.500"/><inertia ixx="0.0013541667" ixy="0" ixz="0" iyy="0.0013541667" iyz="0" izz="0.0025"/></inertial></link><joint name="rightWheel" type="continuous"><parent link="base_floor" /><child link="right_wheel_link" /><origin xyz="-0.3 -0.245 0" rpy="${-PI / 2} 0.0 0.0" /><axis xyz="0 0 1" /></joint><link name="frontwheel_left"><collision><geometry><cylinder length="0.05" radius="0.1325"/></geometry></collision><visual><geometry><cylinder length="0.05" radius="0.1325"/></geometry><material name="black" /></visual><inertial><origin xyz="0 0 0" rpy="0 0 0" /><mass value="1.0" /><inertia ixx="1.0"  ixy="0.0"   ixz="0.0"iyy="1.0"   iyz="0.0"izz="1.0" /></inertial></link><joint name="frontwheel_left_joint" type="fixed"><parent link="base_floor" /><child link="frontwheel_left" /><origin xyz="0.3 -0.245 0" rpy="1.57 0 0" /><axis xyz="0 0 1" /></joint><link name="frontwheel_right"><collision><geometry><cylinder length="0.05" radius="0.1325"/></geometry></collision><visual><geometry><cylinder length="0.05" radius="0.1325"/></geometry><material name="black" /></visual><inertial><origin xyz="0 0 0" rpy="0 0 0" /><mass value="1.0" /><inertia ixx="1.0"  ixy="0.0"   ixz="0.0"iyy="1.0"   iyz="0.0"izz="1.0" /></inertial>    </link><joint name="frontwheel_right_joint" type="fixed"><parent link="base_floor" /><child link="frontwheel_right"/><origin xyz="0.3 0.245 0" rpy="1.57 0 0" /><axis xyz="0 0 -1" /></joint><!-- ===============  Transmission =============== --><transmission name="right_wheel_trans"><type>transmission_interface/SimpleTransmission</type><joint name="rightWheel"><hardwareInterface>hardware_interface/VelocityJointInterface</hardwareInterface></joint><actuator name="right_wheel_motor"><hardwareInterface>hardware_interface/VelocityJointInterface</hardwareInterface><mechanicalReduction>30</mechanicalReduction></actuator></transmission><transmission name="left_wheel_trans"><type>transmission_interface/SimpleTransmission</type><joint name="leftWheel"><hardwareInterface>hardware_interface/VelocityJointInterface</hardwareInterface></joint><actuator name="left_wheel_motor"><hardwareInterface>hardware_interface/VelocityJointInterface</hardwareInterface><mechanicalReduction>30</mechanicalReduction></actuator></transmission><gazebo reference="base_floor"><selfCollide>true</selfCollide><mu1 value="0.05" /><mu2 value="0.05" /></gazebo><gazebo reference="sensor_arm_link"><selfCollide>true</selfCollide><mu1 value="0.05" /><mu2 value="0.05" /></gazebo><gazebo reference="rearwheel_right"><selfCollide>true</selfCollide><mu1 value="1.0" /><mu2 value="1.0" /></gazebo><gazebo reference="rearwheel_left"><selfCollide>true</selfCollide><mu1 value="1.0" /><mu2 value="1.0" /></gazebo><gazebo reference="frontwheel_right"><selfCollide>true</selfCollide><mu1 value="0.0" /><mu2 value="0.0" /></gazebo><gazebo reference="frontwheel_left"><selfCollide>true</selfCollide><mu1 value="0.0" /><mu2 value="0.0" /></gazebo></robot>

创建xcaro模型时，下面几个地方要注意：

（1）

车轮是一个cylinder link，

<link name="right_wheel_link"><visual><geometry><cylinder length="0.05" radius="0.1325"/></geometry><origin xyz="0 0 0" rpy="0 0 0" /><material name="black" /></visual><collision><geometry><cylinder length="0.05" radius="0.1325"/></geometry><origin xyz="0 0 0" rpy="0.0 0.0 0.0" /></collision>

link的局部坐标系原点在这个物体的形心，右手坐标系。

<joint name="rightWheel" type="continuous"><parent link="base_floor" /><child link="right_wheel_link" /><origin xyz="-0.3 -0.245 0" rpy="${-PI / 2} 0.0 0.0" /><axis xyz="0 0 1" /></joint>

车轮在跟父link连接的时候，放在父link局部坐标系中-0.3 -0.245 0这个位置，然后绕child link自己的X轴转-90度（rpy=-pi/2 0 0），也就是这样转下来，原来的z轴转到原来的y轴位置

<axis xyz="0 0 1" />

这句话的意思是，这个物体绕自己的z轴旋转（注意z轴已经有刚才的 rpy命令移动了） 。

注意左右车轮的z轴正向要是一致的，否则会出现，你给往前走的指令，robot旋转，你给旋转的指令，robot反而向前走直线！

transmission是给车轮绑定一个gazebo中的电机

然后在transmission下面加入一个双轮差速控制器

<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>leftWheel</leftJoint> <!-- 左轮 --><rightJoint>rightWheel</rightJoint> <!-- 右轮 --><wheelSeparation>0.49</wheelSeparation> <!-- 车轮间距 --><wheelDiameter>0.265</wheelDiameter> <!-- 车轮直径 --><broadcastTF>false</broadcastTF><wheelTorque>30</wheelTorque><wheelAcceleration>1.8</wheelAcceleration><commandTopic>cmd_vel</commandTopic> <!-- 运动控制话题 --><odometryFrame>odom</odometryFrame> <odometryTopic>odom</odometryTopic> <!-- 里程计话题 --><robotBaseFrame>base_link</robotBaseFrame> <!-- 根坐标系 --></plugin></gazebo>

在给双轮差速器发送控制指令的时候，topic是robotNamespace+commandTopic，具体来说，robotNamespace是 / ，commandTopic是 cmd_vel，那就要发送 /cmd_vel，如果robotNamespace是 /robot，那就要发送 /robot/cmd_vel

下一步写luanch文件

<launch><!-- these are the arguments you can pass this launch file, for example paused:=true --><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"/><!-- We resume the logic in empty_world.launch --><include file="$(find gazebo_ros)/launch/empty_world.launch"><arg name="debug" value="$(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><!-- Load the URDF into the ROS Parameter Server --><param name="robot_description" command="$(find xacro)/xacro '$(find ros_whill)/xacro/modelc.xacro'" /> <!-- Run a python script to the send a service call to gazebo_ros to spawn a URDF robot --><node name="urdf_spawner" pkg="gazebo_ros" type="spawn_model" respawn="false" output="screen"args="-urdf -model whill_modelc -param robot_description"/> <!-- 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" /><!-- 运行robot_state_publisher节点，发布tf  --><node name="robot_state_publisher" pkg="robot_state_publisher" type="robot_state_publisher"><remap from="/joint_states" to="/whill_modelc/joint_states" /></node></launch>

用rqt发送控制指令，rviz可视化，rviz可视化的时候，注意 Fixed Frame是odom

不然就会看到，robot在rviz中不动，odom坐标系在往后退