ROS进阶学习手记 10 - 搭建自己的TurtleBot（2）- Drive Base

ROS进阶学习手记 10 - 搭建自己的TurtleBot（2）- Drive Base

 

    要略过的一些内容：

参考《ROS by Example _ Hydro _ Vol1》书中所述，实现到“7.5  Sending Twist Messages to a Real Robot”

中间的过程就按书中来进行即可。涉及到的比如设备驱动、ROS网络配置 等内容可参考本博客前的相关文章。

    要在这里讨论的问题：

         在 roslaunch rbx1_bringup turtlebot_minimal_create.launch 这个命令运行时，PC（my_robot1）可以接收cmd_vel消息，并通过串口驱动 Create2® 。

         这里调用的顺序是什么？启动了哪些nodes? 参数是如何设置的？
         我们运行了-v 选项的 roslaunch 命令以后，得到如下的状态打印：一行行来分析

$ roslaunch -v rbx1_bringup turtlebot_minimal_create.launch
... logging to /home/exbot/.ros/log/d0c59c34-8dba-11e5-a599-a021b74eea2b/roslaunch-my_robot1-10296.log  //这是加载日志文件
Checking log directory for disk usage. This may take awhile.
Press Ctrl-C to interrupt
Done checking log file disk usage. Usage is <1GB.

... loading XML file [/opt/ros/hydro/etc/ros/roscore.xml]    //第一个加载的xml文件，是运行roscore, 略过。
... executing command param [rosversion roslaunch]
Added parameter [/rosversion]
... executing command param [rosversion -d]  
Added parameter [/rosdistro]                                 // 前面这几行都是获取 ROS 的版本 = hydro
Added core node of type [rosout/rosout] in namespace [/] 
... loading XML file [/home/exbot/catkin_ws/src/rbx1/rbx1_bringup/launch/turtlebot_minimal_create.launch]   //1.从这里开始加载launch文件了
Added parameter [/use_sim_time]       //上一个文件的第8行“<param name="/use_sim_time" value="$(arg simulation)"/>”，开始设置参数值。
... loading XML file [/opt/ros/hydro/share/turtlebot_bringup/launch/includes/robot.launch.xml]          // 2.加载robot.launch.xml 文件
... loading XML file [/opt/ros/hydro/share/turtlebot_bringup/launch/includes/description.launch.xml]    // 3.加载description.launch.xml文件
... executing command param [/opt/ros/hydro/share/xacro/xacro.py '/opt/ros/hydro/share/turtlebot_description/robots/create_circles_kinect.urdf.xacro']
Added parameter [/robot_description]
... done importing include file [/opt/ros/hydro/share/turtlebot_bringup/launch/includes/description.launch.xml]
Added parameter [/robot/name]     //来自：【loading XML file [/opt/ros/hydro/share/turtlebot_bringup/launch/includes/robot.launch.xml]】
Added parameter [/robot/type]     //来自：【loading XML file [/opt/ros/hydro/share/turtlebot_bringup/launch/includes/robot.launch.xml]】
Added parameter [/robot_state_publisher/publish_frequency]
Added node of type [robot_state_publisher/robot_state_publisher] in namespace [/]  //在【launch/includes/robot.launch.xml】文件中启动node:[robot_state_publisher]

Added parameter [/diagnostic_aggregator/analyzers/digital_io/path] //【载入pkg=diagnostic_aggregator nodename=diagnostic_aggregator,载入参数文件/opt/ros/hydro/share/turtlebot_bringup/param/create/diagnostics.yaml】
Added parameter [/diagnostic_aggregator/analyzers/digital_io/type]//1【参数文件中path,type,timeout,startswitch】
Added parameter [/diagnostic_aggregator/analyzers/digital_io/timeout]
Added parameter [/diagnostic_aggregator/analyzers/digital_io/startswith]

Added parameter [/diagnostic_aggregator/analyzers/sensors/path]   //2【参数文件中path,type,timeout,startswitch】
Added parameter [/diagnostic_aggregator/analyzers/sensors/type]
Added parameter [/diagnostic_aggregator/analyzers/sensors/timeout]
Added parameter [/diagnostic_aggregator/analyzers/sensors/startswith]

Added parameter [/diagnostic_aggregator/analyzers/nodes/path]    //3【参数文件中path,type,timeout,CONTAINS】
Added parameter [/diagnostic_aggregator/analyzers/nodes/contains]
Added parameter [/diagnostic_aggregator/analyzers/nodes/type]
Added parameter [/diagnostic_aggregator/analyzers/nodes/timeout]

Added parameter [/diagnostic_aggregator/analyzers/power/path]    //4【参数文件中path,type,timeout,startswitch】
Added parameter [/diagnostic_aggregator/analyzers/power/type]
Added parameter [/diagnostic_aggregator/analyzers/power/timeout]
Added parameter [/diagnostic_aggregator/analyzers/power/startswith]

Added parameter [/diagnostic_aggregator/analyzers/mode/path]     //5【参数文件中path,type,timeout,startswitch】
Added parameter [/diagnostic_aggregator/analyzers/mode/type]
Added parameter [/diagnostic_aggregator/analyzers/mode/timeout]
Added parameter [/diagnostic_aggregator/analyzers/mode/startswith]

Added parameter [/diagnostic_aggregator/pub_rate]                //6【参数文件中pub_rate,base_path,timeout,CONTAINS】
Added parameter [/diagnostic_aggregator/base_path]

Added node of type [diagnostic_aggregator/aggregator_node] in namespace [/] //相关参数载入完成，在【launch/includes/robot.launch.xml】文件中启动node:[diagnostic_aggregator]
... done importing include file [/opt/ros/hydro/share/turtlebot_bringup/launch/includes/robot.launch.xml]  //完成：【loading XML file [/opt/ros/hydro/share/turtlebot_bringup/launch/includes/robot.launch.xml]】完成载入

//进入：【loading XML file [/home/exbot/catkin_ws/src/rbx1/rbx1_bringup/launch/includes/tb_create_mobile_base.launch.xml]】
... loading XML file [/home/exbot/catkin_ws/src/rbx1/rbx1_bringup/launch/includes/tb_create_mobile_base.launch.xml]
Added parameter [/turtlebot_node/bonus]
Added parameter [/turtlebot_node/update_rate]
Added parameter [/turtlebot_node/port]
Added node of type [create_node/turtlebot_node.py] in namespace [/]  //相关参数载入完成，在【tb_create_mobile_base.launch.xml】文件中启动node:[create_node/turtlebot_node.py]
Added node of type [create_node/kinect_breaker_enabler.py] in namespace [/]  // 载入 node: kinect_breaker_enabler.py

Added parameter [/robot_pose_ekf/freq]
Added parameter [/robot_pose_ekf/sensor_timeout]
Added parameter [/robot_pose_ekf/publish_tf]
Added parameter [/robot_pose_ekf/odom_used]
Added parameter [/robot_pose_ekf/imu_used]
Added parameter [/robot_pose_ekf/vo_used]
Added parameter [/robot_pose_ekf/output_frame]
Added node of type [robot_pose_ekf/robot_pose_ekf] in namespace [/] //【载入node】: pkg="robot_pose_ekf" type="robot_pose_ekf" name="robot_pose_ekf"

Added node of type [nodelet/nodelet] in namespace [/] //【载入node】//node pkg="nodelet" type="nodelet" name="mobile_base_nodelet_manager" args="manager"

Added parameter [/cmd_vel_mux/yaml_cfg_file]   //[param name="yaml_cfg_file" value="$(find turtlebot_bringup)/param/mux.yaml"]
//# Configuration for subscribers to multiple cmd_vel sources.  //filepath: /opt/ros/hydro/share/turtlebot_bringup/param/mux.yaml
Added node of type [nodelet/nodelet] in namespace [/] //【载入node】pkg="nodelet" type="nodelet" name="cmd_vel_mux"

... done importing include file [/home/exbot/catkin_ws/src/rbx1/rbx1_bringup/launch/includes/tb_create_mobile_base.launch.xml]

... loading XML file [/opt/ros/hydro/share/turtlebot_bringup/launch/includes/netbook.launch.xml] //【进入[netbook.launch.xml]文件】
Added parameter [/turtlebot_laptop_battery/acpi_path]
Added node of type [linux_hardware/laptop_battery.py] in namespace [/]  //【载入node】pkg="linux_hardware" type="laptop_battery.py" name="turtlebot_laptop_battery"
... done importing include file [/opt/ros/hydro/share/turtlebot_bringup/launch/includes/netbook.launch.xml]

started roslaunch server http://my_robot1:58659/

SUMMARY
========

PARAMETERS
 * /cmd_vel_mux/yaml_cfg_file
 * /diagnostic_aggregator/analyzers/digital_io/path
 * /diagnostic_aggregator/analyzers/digital_io/startswith
 * /diagnostic_aggregator/analyzers/digital_io/timeout
 * /diagnostic_aggregator/analyzers/digital_io/type
 * /diagnostic_aggregator/analyzers/mode/path
 * /diagnostic_aggregator/analyzers/mode/startswith
 * /diagnostic_aggregator/analyzers/mode/timeout
 * /diagnostic_aggregator/analyzers/mode/type
 * /diagnostic_aggregator/analyzers/nodes/contains
 * /diagnostic_aggregator/analyzers/nodes/path
 * /diagnostic_aggregator/analyzers/nodes/timeout
 * /diagnostic_aggregator/analyzers/nodes/type
 * /diagnostic_aggregator/analyzers/power/path
 * /diagnostic_aggregator/analyzers/power/startswith
 * /diagnostic_aggregator/analyzers/power/timeout
 * /diagnostic_aggregator/analyzers/power/type
 * /diagnostic_aggregator/analyzers/sensors/path
 * /diagnostic_aggregator/analyzers/sensors/startswith
 * /diagnostic_aggregator/analyzers/sensors/timeout
 * /diagnostic_aggregator/analyzers/sensors/type
 * /diagnostic_aggregator/base_path
 * /diagnostic_aggregator/pub_rate
 * /robot/name
 * /robot/type
 * /robot_description
 * /robot_pose_ekf/freq
 * /robot_pose_ekf/imu_used
 * /robot_pose_ekf/odom_used
 * /robot_pose_ekf/output_frame
 * /robot_pose_ekf/publish_tf
 * /robot_pose_ekf/sensor_timeout
 * /robot_pose_ekf/vo_used
 * /robot_state_publisher/publish_frequency
 * /rosdistro
 * /rosversion
 * /turtlebot_laptop_battery/acpi_path
 * /turtlebot_node/bonus
 * /turtlebot_node/port
 * /turtlebot_node/update_rate
 * /use_sim_time

NODES
  /
    cmd_vel_mux (nodelet/nodelet)
    diagnostic_aggregator (diagnostic_aggregator/aggregator_node)
    kinect_breaker_enabler (create_node/kinect_breaker_enabler.py)
    mobile_base_nodelet_manager (nodelet/nodelet)
    robot_pose_ekf (robot_pose_ekf/robot_pose_ekf)
    robot_state_publisher (robot_state_publisher/robot_state_publisher)
    turtlebot_laptop_battery (linux_hardware/laptop_battery.py)
    turtlebot_node (create_node/turtlebot_node.py)

auto-starting new master
process[master]: started with pid [10313]
ROS_MASTER_URI=http://my_robot1:11311

setting /run_id to d0c59c34-8dba-11e5-a599-a021b74eea2b
process[rosout-1]: started with pid [10326]
started core service [/rosout]
process[robot_state_publisher-2]: started with pid [10339]
process[diagnostic_aggregator-3]: started with pid [10360]
process[turtlebot_node-4]: started with pid [10379]
process[kinect_breaker_enabler-5]: started with pid [10443]
process[robot_pose_ekf-6]: started with pid [10447]
process[mobile_base_nodelet_manager-7]: started with pid [10487]
process[cmd_vel_mux-8]: started with pid [10508]
process[turtlebot_laptop_battery-9]: started with pid [10541]

这里我逐层来分析launch 文件：

【turtlebot_minimal_create.launch】
  <arg name="base"       default="create"/>  <!-- create, rhoomba -->
  <arg name="battery"    default="$(optenv TURTLEBOT_BATTERY /proc/acpi/battery/BAT0)"/>  <!-- /proc/acpi/battery/BAT0 -->
  <arg name="stacks"     default="circles"/>  <!-- circles, hexagons -->
  <arg name="3d_sensor"  default="kinect"/>  <!-- kinect, asus_xtion_pro -->
  <arg name="simulation" default="$(optenv TURTLEBOT_SIMULATION false)"/>
  <param name="/use_sim_time" value="$(arg simulation)"/>

  <include file="$(find turtlebot_bringup)/launch/includes/robot.launch.xml">
    <arg name="base" value="$(arg base)" />
    <arg name="stacks" value="$(arg stacks)" />
    <arg name="3d_sensor" value="$(arg 3d_sensor)" />
  </include>

    【loading XML file [/opt/ros/hydro/share/turtlebot_bringup/launch/includes/robot.launch.xml]】
     <arg name="base"/>
     <arg name="stacks"/>
     <arg name="3d_sensor"/>
  
     <include file="$(find turtlebot_bringup)/launch/includes/description.launch.xml">
       <arg name="base" value="$(arg base)" />
       <arg name="stacks" value="$(arg stacks)" />
       <arg name="3d_sensor" value="$(arg 3d_sensor)" />
     </include>

       【loading XML file [/opt/ros/hydro/share/turtlebot_bringup/launch/includes/description.launch.xml]】
          <arg name="base"/>
          <arg name="stacks"/>
          <arg name="3d_sensor"/>
          
          <arg name="urdf_file" default="$(find xacro)/xacro.py '$(find turtlebot_description)/robots/$(arg base)_$(arg stacks)_$(arg 3d_sensor).urdf.xacro'" />
          <param name="robot_description" command="$(arg urdf_file)" />
          【[/opt/ros/hydro/share/xacro/xacro.py '/opt/ros/hydro/share/turtlebot_description/robots/create_circles_kinect.urdf.xacro']】
	   用Xacro 来清空 urdf文件。  Using Xacro to Clean Up a URDF File - ROS ...
	   
            【/opt/ros/hydro/share/xacro/xacro.py】

            【/opt/ros/hydro/share/turtlebot_description/robots/create_circles_kinect.urdf.xacro】
            <?xml version="1.0"?>
            <!--
                - Base      : create
                - Stacks    : circles
                - 3d Sensor : kinect
            -->    
            <robot name="turtlebot" xmlns:xacro="http://ros.org/wiki/xacro">
            
              <xacro:include filename="$(find turtlebot_description)/urdf/turtlebot_library.urdf.xacro" />
            
              <create/>
              <stack_circles parent="base_link"/>
              <sensor_kinect  parent="base_link"/>
            </robot>

              【/opt/ros/hydro/share/turtlebot_description/urdf/turtlebot_library.urdf.xacro】
              <?xml version="1.0"?>
              <!--
                The complete turtlebot library of xacros for easy reference
               -->
              <robot xmlns:xacro="http://ros.org/wiki/xacro">
                <!-- General -->
                <xacro:include filename="$(find turtlebot_description)/urdf/common_properties.urdf.xacro"/>
                <xacro:include filename="$(find turtlebot_description)/urdf/turtlebot_properties.urdf.xacro"/>
                <!-- Bases -->
                <xacro:include filename="$(find create_description)/urdf/create.urdf.xacro"/>
                <xacro:include filename="$(find kobuki_description)/urdf/kobuki.urdf.xacro" />
                <!-- Stacks -->
                <xacro:include filename="$(find turtlebot_description)/urdf/stacks/circles.urdf.xacro"/>
                <xacro:include filename="$(find turtlebot_description)/urdf/stacks/hexagons.urdf.xacro"/>
                <!-- 3D Sensors -->
                <xacro:include filename="$(find turtlebot_description)/urdf/sensors/kinect.urdf.xacro"/>
                <xacro:include filename="$(find turtlebot_description)/urdf/sensors/asus_xtion_pro.urdf.xacro"/>
              </robot>
              这样走下去发现越来越深了，参考这个教程：http://wiki.ros.org/urdf/Tutorials/Using%20Xacro%20to%20Clean%20Up%20a%20URDF%20File
              里面说，xacro就是拿来生成urdf的方便工具。
              这里我挑一个urdf.xacro出来，看看是神马东东：
                【/opt/ros/hydro/share/turtlebot_description/urdf/sensors/kinect.urdf.xacro】
                 里面出现了大量的joint link inertial标签。我觉得可以终止了。
       
      回到【loading XML file [/opt/ros/hydro/share/turtlebot_bringup/launch/includes/robot.launch.xml]】    
      <!-- important generally, but specifically utilised by the current app manager -->  
      <param name="robot/name" value="$(optenv ROBOT turtlebot)"/>
      <param name="robot/type" value="turtlebot"/>
  
      <node pkg="robot_state_publisher" type="robot_state_publisher" name="robot_state_publisher">
        <param name="publish_frequency" type="double" value="5.0" />
      </node>
      <node pkg="diagnostic_aggregator" type="aggregator_node" name="diagnostic_aggregator" >
        <rosparam command="load" file="$(find turtlebot_bringup)/param/$(arg base)/diagnostics.yaml" />
        【loading yaml file:[ /opt/ros/hydro/share/turtlebot_bringup/param/create/diagnostics.yaml]】

        pub_rate: 1.0 # Optional
        path: '' # Optional, prepended to all diagnostic output
        analyzers:
          nodes: 
            type: diagnostic_aggregator/GenericAnalyzer
            path: 'Nodes'
            timeout: 5.0
            contains: ['Node']
          mode:
            type: diagnostic_aggregator/GenericAnalyzer
            path: 'Mode'
            timeout: 5.0
            startswith: ['Operating Mode']
          sensors: 
            type: GenericAnalyzer
            path: 'Sensors'
            timeout: 5.0
            startswith: ['Cliff Sensor', 'Wall Sensor', 'Gyro Sensor']
          power:
            type: diagnostic_aggregator/GenericAnalyzer
            path: 'Power System'
            timeout: 5.0
            startswith: ['Battery', 'Charging Sources', 'Laptop Battery']
          digital_io:
            type: diagnostic_aggregator/GenericAnalyzer
            path: 'Digital IO'
            timeout: 5.0
            startswith: ['Digital Outputs']


      </node> 【至此$(find turtlebot_bringup)/launch/includes/robot.launch.xml  文件结束】


  回到【turtlebot_minimal_create.launch 文件】
  <include file="$(find rbx1_bringup)/launch/includes/tb_create_mobile_base.launch.xml" />
    进入【[ /home/exbot/catkin_ws/src/rbx1/rbx1_bringup/launch/includes/tb_create_mobile_base.launch.xml]】
    <launch>
      <!-- Turtlebot Driver 驱动文件-->
      <node pkg="create_node" type="turtlebot_node.py" name="turtlebot_node" respawn="true" args="--respawnable">
         <param name="bonus" value="false" />
         <param name="update_rate" value="30.0" />
         <param name="port" value="/dev/ttyUSB0" />
         <remap from="cmd_vel" to="mobile_base/commands/velocity" />
         <remap from="turtlebot_node/sensor_state" to="mobile_base/sensors/core" />
         <remap from="imu/data" to="mobile_base/sensors/imu_data" />
         <remap from="imu/raw" to="mobile_base/sensors/imu_data_raw" />
       </node>
     
       <!-- Enable breaker 1 for the kinect -->
       <node pkg="create_node" type="kinect_breaker_enabler.py" name="kinect_breaker_enabler"/>

       <!-- The odometry estimator -->
       <node pkg="robot_pose_ekf" type="robot_pose_ekf" name="robot_pose_ekf">
         <remap from="imu_data" to="mobile_base/sensors/imu_data"/>
         <remap from="robot_pose_ekf/odom" to="odom_combined"/>
         <param name="freq" value="10.0"/>
         <param name="sensor_timeout" value="1.0"/>
         <param name="publish_tf" value="true"/>
         <param name="odom_used" value="true"/>
         <param name="imu_used" value="true"/>
         <param name="vo_used" value="false"/>
         <param name="output_frame" value="odom"/>
       </node>
     
       <!-- velocity commands multiplexer -->
       <node pkg="nodelet" type="nodelet" name="mobile_base_nodelet_manager" args="manager"/>

       <node pkg="nodelet" type="nodelet" name="cmd_vel_mux" args="load yocs_cmd_vel_mux/CmdVelMuxNodelet mobile_base_nodelet_manager">
         <param name="yaml_cfg_file" value="$(find turtlebot_bringup)/param/mux.yaml"/>

         <remap from="cmd_vel_mux/output" to="mobile_base/commands/velocity"/>
         <remap from="cmd_vel_mux/input/navi" to="cmd_vel"/>
       </node>
    </launch>



  【回到】【turtlebot_minimal_create.launch 文件】
  <include file="$(find turtlebot_bringup)/launch/includes/netbook.launch.xml">
    <arg name="battery" value="$(arg battery)" />
    【进入[/opt/ros/hydro/share/turtlebot_bringup/launch/includes/netbook.launch.xml]文件】
    <launch>
      <arg name="battery"/>
      <node pkg="linux_hardware" type="laptop_battery.py" name="turtlebot_laptop_battery">
            <param name="acpi_path" type="string" value="$(arg battery)" />
      </node>
    </launch>
    【离开[/opt/ros/hydro/share/turtlebot_bringup/launch/includes/netbook.launch.xml]文件】
  </include>
</launch>
【至此，我们就分析完了turtlebot_minimal_create.launch 文件，下面分别分解里面的功能。】

从roslaunch -v 的状态打印来看，系统主要做了两方面的工作：

1.   Added parameter. Generated by <param name="" value=""/>
2.   executing command param. Generated by <param name="" command="$(arg arg_name)"/>
3.   loading XML file
4.   added node of type [xx] in namespace

先看Add parameter, 这是由.launch文件或者.xml 文件中 <param name="" value="" /> tag attributed.
We also see <arg name="" default="" /> Tag.
Q:What's the difference between parameter and argrement?

Let's see the Stackflow, it says:
=============
     A Parameter is a variable in the declaration of a function:

    functionName(parameter) {
    // do something
    }


    An Argument is the actual value of this variable that gets passed to the function:

    functionName(argument);
=============
I modified the turtlebot_minimal_create.launch file and saved it as turtlebot_minimal_create_me.launch
Let's see the turtlebot_minimal_create_me.launch File line by line below:

<launch>
  <arg name="simulation" default="$(optenv TURTLEBOT_SIMULATION false)"/><!-- For param "/use_sim_time" -->
  <!--Next three lines are for .../launch/includes/robot.launch.xml-->
  <arg name="base"       default="roomba"/>   <!-- create, roomba, kobuki -->
  <arg name="stacks"     default="circles"/>  <!-- circles, hexagons -->
  <arg name="3d_sensor"  default="asus_xtion_pro"/>  <!-- kinect, asus_xtion_pro -->
  <!--Next line is for .../launch/includes/netbook.launch.xml-->
  <arg name="battery"    default="$(optenv TURTLEBOT_BATTERY /proc/acpi/battery/BAT0)"/>  <!-- /proc/acpi/battery/BAT0 -->
	
  <param name="/use_sim_time" value="$(arg simulation)"/> <!--Added parameter /use_sim_time-->
  
  <include file="$(find turtlebot_bringup)/launch/includes/robot.launch.xml">
    <arg name="base" value="$(arg base)" />       <!--Plugged in three arguments:base; stacks; 3d_sensor-->
    <arg name="stacks" value="$(arg stacks)" />
    <arg name="3d_sensor" value="$(arg 3d_sensor)" />
  </include>
  
  <!--Below: Turtlebot Driver/ breaker1for kinect/ odometry estimator/ vel_com_multiplexer-->
  <include file="$(find rbx1_bringup)/launch/includes/tb_create_mobile_base.launch.xml" />

  <!--Netbook battery monitor-->
  <include file="$(find turtlebot_bringup)/launch/includes/netbook.launch.xml">
    <arg name="battery" value="$(arg battery)" />
  </include>
</launch>

   Conclusion: This launch file defined 1 argument "simulation", 3 args "base","stacks","3d_sensor" for robot description, 1 arg "battery" for netbook.xml as battery monitor.(I didn't use netbook.) , Called three .launch.xml files to finish work:

• /opt/ros/hydro/share/turtlebot_bringup/launch/includes/robot.launch.xml
• /home/exbot/catkin_ws/src/rbx1/rbx1_bringup/launch/includes/tb_create_mobile_base.launch.xml
• /opt/ros/hydro/share/turtlebot_bringup/launch/includes/netbook.launch.xml
  

   I'll first look into the 2nd, tb_create_mobile_base.launch.xml to set the parameters as my turtlebot's real situation. And see how it works.

tb_create_mobile_base.launch.xml

 

<launch>
  <!-- Turtlebot Driver -->
  <node pkg="create_node" type="turtlebot_node.py" name="turtlebot_node" respawn="true" args="--respawnable">
    <param name="bonus" value="false" />
    <param name="update_rate" value="30.0" />
    <param name="port" value="/dev/ttyUSB0" />
    <remap from="cmd_vel" to="mobile_base/commands/velocity" />
    <remap from="turtlebot_node/sensor_state" to="mobile_base/sensors/core" />
    <remap from="imu/data" to="mobile_base/sensors/imu_data" />
    <remap from="imu/raw" to="mobile_base/sensors/imu_data_raw" />
  </node>

  <!-- Enable breaker 1 for the kinect -->
  <node pkg="create_node" type="kinect_breaker_enabler.py" name="kinect_breaker_enabler"/>

  <!-- The odometry estimator -->
  <node pkg="robot_pose_ekf" type="robot_pose_ekf" name="robot_pose_ekf">
    <remap from="imu_data" to="mobile_base/sensors/imu_data"/>
    <remap from="robot_pose_ekf/odom" to="odom_combined"/>
    <param name="freq" value="10.0"/>
    <param name="sensor_timeout" value="1.0"/>
    <param name="publish_tf" value="true"/>
    <param name="odom_used" value="true"/>
    <param name="imu_used" value="true"/>
    <param name="vo_used" value="false"/>
    <param name="output_frame" value="odom"/>
  </node>

  <!-- velocity commands multiplexer -->
  <node pkg="nodelet" type="nodelet" name="mobile_base_nodelet_manager" args="manager"/>
  <node pkg="nodelet" type="nodelet" name="cmd_vel_mux" args="load yocs_cmd_vel_mux/CmdVelMuxNodelet mobile_base_nodelet_manager">
    <param name="yaml_cfg_file" value="$(find turtlebot_bringup)/param/mux.yaml"/>
    <remap from="cmd_vel_mux/output" to="mobile_base/commands/velocity"/>
    <remap from="cmd_vel_mux/input/navi" to="cmd_vel"/>
  </node>
</launch>

第一部分：

  <!-- Turtlebot Driver -->
  <node pkg="create_node" type="turtlebot_node.py" name="turtlebot_node" respawn="true" args="--respawnable">
    <param name="bonus" value="false" />
    <param name="update_rate" value="30.0" />
    <param name="port" value="/dev/ttyUSB0" />
    <remap from="cmd_vel" to="mobile_base/commands/velocity" />
    <remap from="turtlebot_node/sensor_state" to="mobile_base/sensors/core" />
    <remap from="imu/data" to="mobile_base/sensors/imu_data" />
    <remap from="imu/raw" to="mobile_base/sensors/imu_data_raw" />
  </node>

这里先为node = turtlebot_node.py 定义了三个parameter: bonus/ update_rate/ port , 然后 remap 了4个消息名。最后启动 turtlebot_node.py ，实际是个底盘驱动。

第二部分：

  <!-- Enable breaker 1 for the kinect -->
  <node pkg="create_node" type="kinect_breaker_enabler.py" name="kinect_breaker_enabler"/>

 查一下 kinect_breaker_enabler.py 是个什么东东？

    进入到 kinect_breaker_enabler.py 代码里面发现是 Set_to_full_Mode ，看来是个设置底板模式的功能。别的涉及到proxy, server啥的，看不懂。~_~

    2016年5月6日17:05:39补充一大牛的回复：orangeheadlxm：

kinect_breaker_enabler.py里是写了一个客户端，其对应的服务器写在了turtlebot_node.py里。它们的作用是当irobot通过SCI和上位机连接上时，把irobot设置为full模式，若未连接上则返回错误并且客户端继续向服务器请求。把response = service_proxy(3)中的3改为1或2还可以把irobot设置成其他模式。

第三部分：

  <!-- The odometry estimator -->
  <node pkg="robot_pose_ekf" type="robot_pose_ekf" name="robot_pose_ekf">
    <remap from="imu_data" to="mobile_base/sensors/imu_data"/>
    <remap from="robot_pose_ekf/odom" to="odom_combined"/>
    <param name="freq" value="10.0"/>
    <param name="sensor_timeout" value="1.0"/>
    <param name="publish_tf" value="true"/>
    <param name="odom_used" value="true"/>  <!-- Sonictl Set as false -->
    <param name="imu_used" value="true"/>   <!-- Sonictl Set as false -->
    <param name="vo_used" value="false"/>
    <param name="output_frame" value="odom"/>
  </node>

  这里要根据我的情况修改几个参数设置了。我没有IMU的 ekf 功能，没有搭载 accelerometer, 也不知道怎么搭载。所以就把某些值设为false.再保存到tb_create_mobile_base.launch_me.xml 中

第四部分：

  <!-- velocity commands multiplexer -->
  <node pkg="nodelet" type="nodelet" name="mobile_base_nodelet_manager" args="manager"/>
  <node pkg="nodelet" type="nodelet" name="cmd_vel_mux" args="load yocs_cmd_vel_mux/CmdVelMuxNodelet mobile_base_nodelet_manager">
    <param name="yaml_cfg_file" value="$(find turtlebot_bringup)/param/mux.yaml"/>
    <!-- filepath: /opt/ros/hydro/share/turtlebot_bringup/param/mux.yaml -->
    <remap from="cmd_vel_mux/output" to="mobile_base/commands/velocity"/>
    <remap from="cmd_vel_mux/input/navi" to="cmd_vel"/>
  </node>

  这一部分是 多cmd_vel消息来了时候，进行multiplexer多路转换器，的功能。 这里先设置了一个parameter: yaml_cfg_file配置文件，设置到【/opt/ros/hydro/share/turtlebot_bringup/param/mux.yaml】，然后开了俩node：nodelet/mobile_base_nodelet_manager  &  nodelet/cmd_vel_mux , 都分别带了参数args.
  最后是remap 了俩消息。
  来看看yaml的配置文件：

# Created on: Oct 29, 2012
#     Author: jorge
# Configuration for subscribers to multiple cmd_vel sources.
#
# Individual subscriber configuration:
#   name:           Source name
#   topic:          The topic that provides cmd_vel messages
#   timeout:        Time in seconds without incoming messages to consider this topic inactive
#   priority:       Priority: an UNIQUE unsigned integer from 0 (lowest) to MAX_INT 
#   short_desc:     Short description (optional)

subscribers:
  - name:        "Safe reactive controller"
    topic:       "input/safety_controller"
    timeout:     0.2
    priority:    10
  - name:        "Teleoperation"
    topic:       "input/teleop"
    timeout:     1.0
    priority:    7
  - name:        "Navigation"
    topic:       "input/navi"
    timeout:     1.0
    priority:    5

  可见是设置了三个源的优先级。文件开头也写了： Configuration for subscribers to multiple cmd_vel sources.

  至此，我们就细看完了 tb_create_mobile_base.launch.xml 这个文件，一个驱动、一个odometry estimator配置，一个velocity commands multiplexer选择器。

  完成了驱动，我们有几个问题没有解决：

1.  驱动速度是否按照命令输入的进行？ -- 速度驱动校准
2.  没有了 ekf 的位置估计，如何利用Create底盘马达上自带的码盘进行位置估计？
3.  square 驱动实验，看能否准确。