pnichat/aws-deepracer-servo-pkg

The DeepRacer Servo ROS package creates the servo_node that is responsible to map the input servo throttle and steering angle messages to raw PWM values to move the car.

AWS DeepRacer servo package

Overview

The AWS DeepRacer servo ROS package creates the servo_node, which is part of the core AWS DeepRacer application and launches from the deepracer_launcher. For more information about the application and the components, see the aws-deepracer-launcher repository.

This node is responsible for mapping the input servo throttle and steering angle ratios to raw PWM values and writing that to the buffer so that car moves accordingly. It also sets and gets the [min, mid, max, polarity] calibration values for steering and throttle and the tail light LED PWM values.

License

The source code is released under Apache 2.0.

Installation

Follow these steps to install the AWS DeepRacer servo package.

Prerequisites

The AWS DeepRacer device comes with all the prerequisite packages and libraries installed to run the servo_pkg. For more information about the preinstalled set of packages and libraries on the DeepRacer and about installing the required build systems, see Getting started with AWS DeepRacer OpenSource.

The servo_pkg specifically depends on the following ROS 2 packages as build and run dependencies:

• deepracer_interfaces_pkg: This package contains the custom message and service type definitions used across the AWS DeepRacer core application.

Downloading and building

Open a terminal on the AWS DeepRacer device and run the following commands as the root user.

1. Switch to the root user before you source the ROS 2 installation:

    sudo su
   

2. Source the ROS 2 Foxy setup bash script:

    source /opt/ros/foxy/setup.bash 
   

3. Create a workspace directory for the package:

    mkdir -p ~/deepracer_ws
    cd ~/deepracer_ws
   

4. Clone the servo_pkg on the DeepRacer device:

    git clone https://github.com/aws-deepracer/aws-deepracer-servo-pkg.git
   

5. Fetch unreleased dependencies:

    cd ~/deepracer_ws/aws-deepracer-servo-pkg
    rosws update
   

6. Resolve the servo_pkg dependencies:

    cd ~/deepracer_ws/aws-deepracer-servo-pkg && rosdep install -i --from-path . --rosdistro foxy -y
   

7. Build the servo_pkg and deepracer_interfaces_pkg:

    cd ~/deepracer_ws/aws-deepracer-servo-pkg && colcon build --packages-select servo_pkg deepracer_interfaces_pkg
   

Usage

The servo_node provides the core functionality to map the input servo throttle and steering angle ratios to raw PWM values and writing that to the buffer so that car moves accordingly. Although the node is built to work with the AWS DeepRacer application, you can run it independently for development, testing, and debugging purposes.

Run the node

To launch the built servo_node as the root user on the AWS DeepRacer device, open another terminal on the AWS DeepRacer device and run the following commands as the root user.

1. Switch to the root user before you source the ROS 2 installation:

    sudo su
   

2. Source the ROS 2 Foxy setup bash script:

    source /opt/ros/foxy/setup.bash 
   

3. Source the setup script for the installed packages:

    source ~/deepracer_ws/aws-deepracer-servo-pkg/install/setup.bash
   

4. Launch the servo_node using the launch script:

    ros2 launch servo_pkg servo_pkg_launch.py
   

Launch files

The servo_pkg_launch.py, included in this package, provides an example demonstrating how to launch the servo_node.

from launch import LaunchDescription
from launch_ros.actions import Node

def generate_launch_description():
    return LaunchDescription([
        Node(
            package='servo_pkg',
            namespace='servo_pkg',
            executable='servo_node',
            name='servo_node'
        )
    ])

Node details

servo_node

Subscribed topics

Topic name	Message type	Description
/ctrl_pkg/servo_msg	ServoCtrlMsg	This message holds the scaled throttle and angle values in the range of [-1, 1] that are translated based on the calibration to raw PWM values.
/ctrl_pkg/raw_pwm	ServoCtrlMsg	This message holds the raw PWM throttle and angle values to be set on the motor and servo.

Services

Service name	Service type	Description
servo_gpio	ServoGPIOSrv	A service that is called enable or disable the servo GPIO pin.
set_calibration	SetCalibrationSrv	A service that is called to set the [min, mid, max, polarity] PWM values for the speed and steering calibration and store them in a file.
get_calibration	GetCalibrationSrv	A service that is called to get the [min, mid, max, polarity] PWM values for the speed and steering calibration from the saved file.
set_led_state	SetLedCtrlSrv	A service that is called to set the [red, green, blue] channel PWM values for the tail light LEDs and store them in a file.
get_led_state	GetLedCtrlSrv	A service that is called to get the [red, green, blue] channel PWM values for the tail light LEDs from the saved file.

Resources

• Getting started with AWS DeepRacer OpenSource