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‎docs/packages/diffbot_base/index.md‎

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@@ -58,8 +58,12 @@ The previous approach (high-level PID) is documented in [High-Level Approach](hi
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## Developing a low-level controller firmware and a high-level ROS Control hardware interface for a differential drive robot
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In the following two sections, the base controller, mentioned in the Navigation
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Stack, will be developed. For DiffBot/Remo, this platform-specific node is split
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into two software components.
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Stack, will be developed.
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![Navigation Stack]({{ asset_dir }}/navigation/navigation_stack.png)
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For DiffBot/Remo, this platform-specific node is split into two software
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components.
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The first component is the high-level `diffbot::DiffBotHWInterface` that
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inherits from `hardware_interface::RobotHW`, acting as an interface between
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An overview of ROS Control in simulation and the real world is given in the
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following figure (http://gazebosim.org/tutorials/?tut=ros_control):
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<figure>
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<a href="{{ asset_dir }}/packages/diffbot_base/roscontrol-sim-reality.svg"><img src="{{ asset_dir }}/packages/diffbot_base/roscontrol-sim-reality.svg"></a>
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<figcaption>ROS Control in Simulation and Reality</figcaption>
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<figure markdown>
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![ROS Control simulation and reality]({{ asset_dir }}/packages/diffbot_base/ros-control-simulation-and-reality.svg)
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<figcaption>ROS Control in Simulation and Reality</figcaption>
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</figure>
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The second component is the low-level base controller that measures angular
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wheel joint positions and velocities and applies the commands from the
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high-level interface to the wheel joints. The following figure shows the
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communication between the two components:
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<figure>
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<a href="{{ asset_dir }}/packages/diffbot_base/block-diagram-low-high-level.svg"><img src="{{ asset_dir }}/packages/diffbot_base/block-diagram-low-high-level.svg"></a>
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<figcaption>Block diagram of the low-level controller and the high-level hardware interface</figcaption>
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<figure markdown>
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![Block diagram of low-level controller and high-level hardware interface]({{ asset_dir }}/packages/diffbot_base/block-diagram-low-level-base_controller-high-level-hardware_interface.svg)
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<figcaption>Block diagram of the low-level controller and the high-level hardware interface (ROS
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Control)</figcaption>
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</figure>
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The low-level base controller uses two PID controllers to compute PWM signals
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for each motor based on the error between measured and target wheel velocities.
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`RobotHW` receives measured joint states (angular position (rad) and angular
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velocity (rad/s)) from which it updates its joint values. With these measured
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velocities and the desired command velocity (`geometry_msgs/Twist` message on the
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`cmd_vel` topic), from the Navigation Stack, the `diff_drive_controller` computes
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the target angular velocities for both wheel joints using the mathematical
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equations of a differential drive robot. This controller works with continuous
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wheel joints through a `VelocityJointInterface` class. The computed target
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commands are then published within the high-level hardware interface inside the
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robots `RobotHW::write` method. Additionally, the controller computes and
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publishes the odometry on the odom topic (`nav_msgs/Odometry`) and the transform
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from `odom` to `base_footprint`.
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velocities and the desired command velocity (`geometry_msgs/Twist` message on
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the `cmd_vel` topic), from the Navigation Stack, the `diff_drive_controller`
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computes the target angular velocities for both wheel joints using the
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mathematical equations of a differential drive robot. This controller works with
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continuous wheel joints through a `VelocityJointInterface` class. The computed
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target commands are then published within the high-level hardware interface
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inside the robot's `RobotHW::write` method. Additionally, the controller
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computes and publishes the odometry on the odom topic (`nav_msgs/Odometry`) and
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the transform from `odom` to `base_footprint`.
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Having explained the two components of the base
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controller, the low-level firmware is implemented first. The high-level hardware
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interface follows the next section.
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Having explained the two components of the base controller, the low-level
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firmware is implemented first. The high-level hardware interface follows the
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next section.
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But before this an introduction to the PID controllers are given in [PID Controllers](pid.md).
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But before this an introduction to the PID controllers are given in [PID
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Controllers](pid.md).
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### diffbot_base Package
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The `diffbot_base` package is created with `catkin-tools`:
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The `diffbot_base` package was created with `catkin-tools`:
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```console
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fjp@diffbot:/home/fjp/catkin_ws/src$ catkin create pkg diffbot_base --catkin-deps diff_drive_controller hardware_interface roscpp sensor_msgs rosparam_shortcuts

‎docs/packages/index.md‎

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# ROS Software Packages
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After having verified that the hardware requirements for the Navigation Stack are met, an
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overview of Remo's software follows.
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After having verified that the hardware requirements for the Navigation Stack
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are met, an overview of Remo's software follows.
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## Software requirements for the ROS Navigation Stack
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The [`diffbot`](https://github.com/ros-mobile-robots/diffbot/) and
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[`remo_description`](https://github.com/ros-mobile-robots/remo_description) repositories
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contain the following ROS packages:
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[`remo_description`](https://github.com/ros-mobile-robots/remo_description)
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repositories contain the following ROS packages:
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- `diffbot_base`: This package contains the platform-specific code for the base
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controller component required by the ROS Navigation Stack. It consists of the
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firmware based on rosserial for the Teensy MCU and the C++ node running
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on the SBC that instantiates the ROS Control hardware interface including the
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`controller_manager` control loop for the real robot. The low-level `base_controller`
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component reads the encoder ticks from the hardware, calculates
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angular joint positions and velocities, and publishes them to the ROS Control
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hardware interface. Using this interface makes it possible to use the `diff_drive_controller`
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package from [ROS Control](http://wiki.ros.org/diff_drive_controller).
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It provides a controller (`DiffDriveController`) for a
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differential drive mobile base that computes target joint velocities from commands
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received by either a teleop node or the ROS Navigation Stack. The computed
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target joint velocities are forwarded to the low-level base controller, where they are
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compared to the measured velocities to compute suitable motor PWM signals using
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two separate PID controllers, one for each motor.
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firmware based on rosserial for the Teensy MCU and the C++ node running on the
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SBC that instantiates the ROS Control hardware interface including the
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`controller_manager` control loop for the real robot. The low-level
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`base_controller` component reads the encoder ticks from the hardware,
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calculates angular joint positions and velocities, and publishes them to the ROS
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Control hardware interface. Using this interface makes it possible to use the
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`diff_drive_controller` package from [ROS
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Control](http://wiki.ros.org/diff_drive_controller). It provides a controller
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(`DiffDriveController`) for a differential drive mobile base that computes
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target joint velocities from commands received by either a teleop node or the
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ROS Navigation Stack. The computed target joint velocities are forwarded to the
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low-level base controller, where they are compared to the measured velocities to
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compute suitable motor PWM signals using two separate PID controllers, one for
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each motor.
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- `diffbot_bringup`: Launch files to bring up the hardware driver nodes (camera,
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lidar, microcontroller, and so on) as well as the C++ nodes from the `diffbot_base`
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package for the real robot.
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lidar, microcontroller, and so on) as well as the C++ nodes from the
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`diffbot_base` package for the real robot.
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- `diffbot_control`: Configurations for `DiffDriveController` and
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`JointStateController` of ROS Control used in the Gazebo simulation and the
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real robot. The parameter configurations are loaded onto the parameter server with
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`JointStateController` of ROS Control used in the Gazebo simulation and the real
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robot. The parameter configurations are loaded onto the parameter server with
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the help of the launch files inside this package.
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- `remo_description`: This package contains the URDF description of Remo
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including its sensors. It allows you to pass arguments to visualize different
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camera and SBC types. It also defines the `gazebo_ros_control` plugin.
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Remo's description is based on the description at https://github.com/ros-mobile-robots/mobile_robot_description,
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which provides a modular URDF structure that makes it easier to model your own differential drive robot.
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camera and SBC types. It also defines the `gazebo_ros_control` plugin. Remo's
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description is based on the description at
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https://github.com/ros-mobile-robots/mobile_robot_description, which provides a
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modular URDF structure that makes it easier to model your own differential drive
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robot.
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- `diffbot_gazebo`: Simulation-specific launch and configuration files for Remo
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and Diffbot, to be used in the Gazebo simulator.
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- `diffbot_msgs`: Message definitions specific to Remo/Diffbot, for example, the
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- `diffbot_slam`: Configurations for simultaneous localization and mapping using
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implementations such as gmapping to create a map of the environment.
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After this overview of the ROS packages of a differential robot that fulfill the requirements
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of the Navigation Stack, the next section implements the base controller component.
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## Developing a low-level controller and a highlevel ROS Control hardware interface for a differential drive robot
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In the following two sections, the base controller, mentioned in the Navigation Stack, will be developed.
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![Navigation Stack]({{ asset_dir }}/navigation/navigation_stack.png)
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For Remo, this platform-specific node is split into two software components.
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The first component is the high-level `diffbot::DiffBotHWInterface` that
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inherits from `hardware_interface::RobotHW`, acting as an interface between
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robot hardware and the packages of ROS Control that communicate with the Navigation
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Stack and provide [`diff_drive_controller`](http://wiki.ros.org/diff_drive_controller) –
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one of many available controllers from ROS Control. With the
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`gazebo_ros_control` plugin, the same controller including its configuration can be
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used in the simulation and the real robot. An overview of ROS Control in a simulation
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and the real world is given in the following figure (http://gazebosim.org/tutorials/?tut=ros_control):
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<figure markdown>
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![ROS Control simulation and reality]({{ asset_dir }}/packages/ros-control-simulation-and-reality.svg)
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<figcaption>ROS Control in simulation and reality</figcaption>
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</figure>
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The second component is the low-level base controller that measures angular wheel
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joint positions and velocities and applies the commands from the high-level interface
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to the wheel joints. The following figure shows the communication between the two
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components:
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<figure markdown>
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![Block diagram of low-level controller and high-level hardware interface]({{ asset_dir }}/packages/low-level-base_controller-high-level-hardware_interface.svg)
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<figcaption>Block diagram of the low-level controller and the high-level hardware interface (ROS
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Control)</figcaption>
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</figure>
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The low-level base controller uses two PID controllers to compute PWM signals for each
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motor based on the error between measured and target wheel velocities.
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`RobotHW` receives measured joint states (angular position (rad) and angular velocity
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(rad/s)) from which it updates its joint values. With these measured velocities and the
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desired command velocity (`geometry_msgs/Twist` message on the `cmd_vel`
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topic), from the Navigation Stack, the `diff_drive_controller` computes the
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target angular velocities for both wheel joints using the mathematical equations of a
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differential drive robot. This controller works with continuous wheel joints through a
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`VelocityJointInterface` class. The computed target commands are then published
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within the high-level hardware interface inside the robot's `RobotHW::write` method.
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Additionally, the controller computes and publishes the odometry on the odom topic
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(`nav_msgs/Odometry`) and the transform from `odom` to `base_footprint`.
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Having explained the two components of the base controller, the low-level firmware is
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implemented first. The high-level hardware interface follows the next section.
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!!! note "TODO"
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TODO Details about implementation will follow (see code for now)
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After this overview of the ROS packages of a differential robot that fulfill the
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requirements of the Navigation Stack, the next pages explain those packages in
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more detail.

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