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Control

DiffBot Control Package

As described in the ROS Integration and Gazebo Simulation sections, DiffBot makes use of ROS Control repositories. Specifically the diff_drive_controller package from the ros_controllers meta package. To leverage ROS Control for the simulation with Gazebo the robot description and the controller configuration (usually a MYROBOT_control.yaml file) is required. For the real hardware its required to implement a class derived from hardware_interface::RobotHW.

The convention to control a robot (in simulation and in the real world) is to have a package named MYROBOT_control. In case of DiffBot its called diffbot_control and created with catkin create pkg PKG_NAME [--catkin-deps [DEP [DEP ...]]]:

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catkin create pkg diffbot_control --catkin-deps diff_drive_controller roscpp sensor_msgs 
Creating package "diffbot_control" in "/home/fjp/git/diffbot/ros/src"...
Created file diffbot_control/CMakeLists.txt
Created file diffbot_control/package.xml
Created folder diffbot_control/include/diffbot_control
Created folder diffbot_control/src
Successfully created package files in /home/fjp/git/diffbot/ros/src/diffbot_control.

To work with this package the specified dependencies must be installed either using the available Ubuntu/Debian packages for ROS Noetic or have to be built from source first. The following table lists the dependencies that we have to install because they are not already part of the ROS Noetic desktop full installation. Refer to the section ROS Noetic Setup for how this was done.

Dependency Source Ubuntu/Debian Package
diff_drive_controller https://github.com/ros-controls/ros_controllers ros-noetic-ros-controllers

To install a package from source clone (using git) or download the source files from where they are located (commonly hosted on GitHub) into the src folder of a ros catkin workspace and execute the catkin build command. Also make sure to source the workspace after building new packages with source devel/setup.bash.

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cd /home/fjp/git/diffbot/ros/  # Navigate to the workspace
catkin build              # Build all the packages in the workspace
ls build                  # Show the resulting build space
ls devel                  # Show the resulting devel space

Make sure to clone/download the source files suitable for the ROS distribtion you are using. If the sources are not available for the distribution you are working with, it is worth to try building anyway. Chances are that the package you want to use is suitable for multiple ROS distros. For example if a package states in its docs, that it is only available for kinetic it is possible that it will work with a ROS noetic install.

ROS Control in Gazebo

Two great resources to get the diff_drive_controller working inside Gazebo is the Gazebo ROS Control Tutorial of rrbot and the R2D2 ROS URDF Tutorial, especially the last section, The Wheels on the Droid Go Round and Round.

To spawn DiffBot inside Gazebo, RViz and control it with the rqt_robot_steering plugin, launch the diffbot.launch inside the diffbot_control package:

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roslaunch diffbot_control diffbot.launch

This launch file makes use of diffbot_gazebo/launch/diffbot.launch, diffbot_control/launch/diffbot_control.launch to run gazebo and the diff_drive_controller. It also opens RViz with the configuration stored in diffbot_control/rviz/diffbot.rviz. The following video shows the result of launching. Note the video may be outdated when you read this and the model has improved.

ROS Control on the Real Hardware

As mentioned above the its required to implement a class derived from hardware_interface::RobotHW. Let's call it DiffBotHW and create it inside the diffbot_control/src folder.

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