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Hardware Bringup

DiffBot Bring Up Package

The bringup package is used to initialize the real hardware of DiffBot and to actually drive the robot around. First the package is created using catkin-tools:

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fjp@diffbot:~/git/diffbot/ros/src$ catkin create pkg diffbot_bringup
Creating package "diffbot_bringup" in "/home/fjp/git/diffbot/ros/src"...
Created file diffbot_bringup/package.xml
Created file diffbot_bringup/CMakeLists.txt
Successfully created package files in /home/fjp/git/diffbot/ros/src/diffbot_bringup.

The package provides a launch folder which includes minimal.launch and bringup.launch.

The minimal.launch is used to load DiffBot's robot descripton and the controller configuration onto the ROS parameter server using the launch file from the diffbot_base package. It will also setup the ROS controller manager with DiffBot's hardware interface.

For the motor driver the node motor_driver.py from the grove_motor_driver package is started. And for the encoders rosserial communicates with the Teensy microcontroller to publish the encoder ticks. 

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<launch>
    <!-- Including the following launch file from diffbot_base package will -->
    <!-- Load the robot description onto the parameter server -->
    <!-- Run the controller manager with DiffBot's hardware interface -->
    <!-- Load the controller config onto the parameter server -->
    <include file="$(find diffbot_base)/launch/diffbot.launch">
        <!-- arg name="model" value="$(arg model)" /-->
    </include>


    <!-- Motors -->
    <!-- -->
    <node name="motor_driver" pkg="grove_motor_driver" type="motor_driver.py" respawn="false"
        output="screen" ns="diffbot" />

    <!-- Encoders -->
    <!-- Run rosserial to connect with the Teensy 3.2 board connected to the motor encoders -->
    <node name="rosserial_teensy" pkg="rosserial_python" type="serial_node.py" respawn="false"
        output="screen" ns="diffbot" args="_port:=/dev/ttyACM0
                                            _baud:=115200"/>
</launch>

As mentioned, the ROS controller used for DiffBot is the diff_drive_controller. This controller publishes a transform message (see its published topics), via the /tf topic, between the odom frame and the frame configured in the controller's configuration specified by the base_frame_id. In the case of DiffBot this is the base_footprint, a conventional link, defined in REP-120, for mobile robots that specifies the robot's footprint.

Because this is the only transform published by diff_drive_controller another node is needed to publish rest of the link transformations. It is the well known robot_state_publisher, which uses the joint states published by the ROS controller joint_state_controller (not to be confused with joint_state_publisher - it is not used here, see this answer for the difference) to create the transforms between the links.

To do this the bringup.launch includes the minimal.launch and then runs the robot_state_publisher:

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<launch>
    <include file="$(find diffbot_bringup)/launch/minimal.launch">
        <!-- arg name="model" value="$(arg model)" /-->
    </include>

    <!-- Starting robot state publish which will publish tf -->
    <!-- This is needed to publish transforms between all links -->
    <!-- diff_drive_controller publishes only a single transfrom between odom and base_footprint -->
    <!-- The robot_state_publisher reads the joint states published by ros control's joint_state_controller -->
    <node name="robot_state_publisher" pkg="robot_state_publisher" type="robot_state_publisher"
        output="screen" ns="diffbot" />
</launch>