# ROS 2 User Guide (PX4-ROS 2 Bridge)

This topic explains how to setup and use ROS 2 with PX4.

It provides an overview of the ROS2-PX4 bridge architecture and application pipeline, along with instructions on how to install all the needed software and build ROS 2 applications.

Note

The Fast DDS interface in the PX4 Autopilot can be leveraged by any applications running and linked in DDS domains (including ROS nodes).

For information about using the microRTPS bridge without ROS 2, see the RTPS/DDS Interface section.

Note

For a more detailed and visual explanation on how to use PX4 with ROS 2 see these presentations from the PX4 maintainers:

  1. ROS World 2020 - Getting started with ROS 2 and PX4 (opens new window)
  2. PX4 Dev Summit 2019 - "ROS 2 Powered PX4" (opens new window)

# Overview

The application pipeline for ROS 2 is very straightforward, thanks to the native communications middleware (DDS/RTPS). The microRTPS Bridge consists of a client running on PX4 and an agent running on the Mission/Companion Computer, which communicate to provide bi-directional data exchange and message translation between UORB and ROS 2 message formats. This allows you to create ROS 2 subscribers or publisher nodes that interface directly with PX4 UORB topics! This is shown in the diagram below.

Architecture with ROS 2

ROS 2 uses the px4_msgs (opens new window) and px4_ros_com (opens new window) packages to ensure that matching message definitions are used for creating both the client and the agent code (this is important), and also to remove the requirement for PX4 to be present when building ROS code.

  • px4_msgs contains PX4 ROS message definitions. When this project is built it generates the corresponding ROS 2-compatible typesupport, used by ROS 2 nodes, and IDL files, used by fastddsgen to generate the microRTPS agent code.
  • px4_ros_com contains the microRTPS agent code templates for the agent publishers and subscribers. The build process runs a fastddsgen instance to generate the code for the micrortps_agent, which compiles into a single executable.

The PX4 Autopilot project automatically updates px4_msgs (opens new window) with new message definitions whenever they are changed (in the master branch).

Note

The subset of uORB topics that will be accessible to ROS applications can be found in the bridge configuration yaml file (opens new window).

PX4 firmware contains the microRTPS client based on its build-time message definitions.

Note

Astute readers will note that the generated agent might not have been built with that same set of definitions (unless they were both built of the same 'master' commit).

Right now this is not a problem because the PX4 message set/definitions are relatively stable and the updated/new messages get automatically deployed to px4_msgs.

In the near future the intention is to:

  1. Create also a branch per release in both px4_ros_com and px4_msgs, so both the message definitions and agent code match the ones present on the PX4/client side by the time of the release.
  2. Have an initial message exchange of the bridge configuration, using the messages structs MD5SUMs to verify if the messages definitions are the same, and if not, disable their stream and warn the user.

WARNING

You cannot use an agent generated as part of a "normal" PX4 build with ROS 2 (e.g. if the user uses BUILD_MICRORTPS_AGENT=1 make px4_sitl_rtps). While microRTPS client is the same, the IDL files used by ROS 2 are slightly different from the custom ones with generate in PX4. The other detail is that the "normal" PX4 build doesn't use fastddsgen with typesupport for ROS 2 networks - and that's also one of the main reasons we have a separate microRTPS agent in px4_ros_com, which is completely compatible with ROS 2 networks. We use the px4_msg to generate appropriate IDL files for the micrortps_agent in px4_ros_com.

# Installation & Setup

To setup ROS 2 for use with PX4 you will need to:

# Install Fast DDS

Follow the Fast DDS Installation Guide to install Fast RTPS(DDS) 2.0.2 (or later) and Fast-RTPS-Gen 1.0.4 (not later!) and their dependencies.

WARNING

Check the guide to confirm the latest dependencies! You won't be able to continue with this guide until the correct versions of Fast RTPS(DDS) and Fast-RTPS-Gen have been installed.

# Install ROS 2

Note

This install and build guide covers ROS 2 Foxy in Ubuntu 20.04.

To install ROS 2 and its dependencies:

  1. Install ROS 2 Foxy (opens new window)

  2. The install process should also install the colcon build tools, but in case that doesn't happen, you can install the tools manually:

    sudo apt install python3-colcon-common-extensions
    
  3. eigen3_cmake_module is also required, since Eigen3 is used on the transforms library:

    sudo apt install ros-foxy-eigen3-cmake-module
    
  4. Some Python dependencies must also be installed (using pip or apt):

    sudo pip3 install -U empy pyros-genmsg setuptools
    

# Build ROS 2 Workspace

This section shows how create a ROS 2 workspace hosted in your home directory (modify the commands as needed to put the source code elsewhere). The px4_ros_com and px4_msg packages are cloned to a workspace folder, and then a script is used to build the workspace.

Note

The build process will open new tabs on the console, corresponding to different stages of the build process that need to have different environment configurations sourced.

To create and build the workspace:

  1. Create a workspace directory using:
    $ mkdir -p ~/px4_ros_com_ros2/src
    
  2. Clone the ROS 2 bridge packages px4_ros_com and px4_msgs to the /src directory (the master branch is cloned by default):
    $ git clone https://github.com/PX4/px4_ros_com.git ~/px4_ros_com_ros2/src/px4_ros_com
    $ git clone https://github.com/PX4/px4_msgs.git ~/px4_ros_com_ros2/src/px4_msgs
    
  3. Use the build_ros2_workspace.bash script to build the ROS 2 workspace (including px4_ros_com and px4_msgs).
    $ cd ~/px4_ros_com_ros2/src/px4_ros_com/scripts
    $ source build_ros2_workspace.bash
    

TIP

All script options can be listed by calling it with the --help argument. In particular the --verbose argument shows the full colcon build output.

Note

The px4_ros_com/scripts directory contains multiple scripts for building different kinds of workspaces.

# Sanity Check the Installation

One way to check that the installation/setup succeeded is to test that the bridge can communicate with PX4. We can do this by running the bridge against PX4 running in the simulator.

  1. Setup your PX4 Ubuntu Linux development environment - the default instructions get the latest version of PX4 source and install all the needed tools.

  2. Open a new terminal in the root of the PX4 Autopilot project, and then start a PX4 Gazebo simulation using:

    make px4_sitl_rtps gazebo
    

    Once PX4 has fully started the terminal will display the NuttShell/System Console. Note also that PX4 SITL will automatically start the micrortps_client connected to UDP ports 2019 and 2020.

  3. On a new terminal, source the ROS 2 workspace and then start the micrortps_agent daemon with UDP as the transport protocol:

    $ source ~/px4_ros_com_ros2/install/setup.bash
    $ micrortps_agent -t UDP
    
  4. Open a new terminal and start a "listener" using the provided launch file:

    $ source ~/px4_ros_com_ros2/install/setup.bash
    $ ros2 launch px4_ros_com sensor_combined_listener.launch.py
    

    If the bridge is working correctly you will be able to see the data being printed on the terminal/console where you launched the ROS listener:

    RECEIVED DATA FROM SENSOR COMBINED
    ================================
    ts: 870938190
    gyro_rad[0]: 0.00341645
    gyro_rad[1]: 0.00626475
    gyro_rad[2]: -0.000515705
    gyro_integral_dt: 4739
    accelerometer_timestamp_relative: 0
    accelerometer_m_s2[0]: -0.273381
    accelerometer_m_s2[1]: 0.0949186
    accelerometer_m_s2[2]: -9.76044
    accelerometer_integral_dt: 4739
    

You can also verify the rate of the message using ros2 topic hz. E.g. in the case of sensor_combined use ros2 topic hz /fmu/sensor_combined/out:

average rate: 248.187
	min: 0.000s max: 0.012s std dev: 0.00147s window: 2724
average rate: 248.006
	min: 0.000s max: 0.012s std dev: 0.00147s window: 2972
average rate: 247.330
	min: 0.000s max: 0.012s std dev: 0.00148s window: 3212
average rate: 247.497
	min: 0.000s max: 0.012s std dev: 0.00149s window: 3464
average rate: 247.458
	min: 0.000s max: 0.012s std dev: 0.00149s window: 3712
average rate: 247.485
	min: 0.000s max: 0.012s std dev: 0.00148s window: 3960

# ROS 2 Example Applications

# Creating a ROS 2 listener

With the px4_ros_com built successfully, one can now take advantage of the generated microRTPS agent app and also from the generated sources and headers of the ROS 2 msgs from px4_msgs, which represent a one-to-one matching with the uORB counterparts.

To create a listener node on ROS 2, lets take as an example the sensor_combined_listener.cpp node under px4_ros_com/src/examples/listeners.

The code first imports the C++ libraries needed to interface with the ROS 2 middleware and the required message header file:

#include <rclcpp/rclcpp.hpp>
#include <px4_msgs/msg/sensor_combined.hpp>

Then it creates a SensorCombinedListener class that subclasses the generic rclcpp::Node base class.

/**
 * @brief Sensor Combined uORB topic data callback
 */
class SensorCombinedListener : public rclcpp::Node
{

This creates a callback function for when the sensor_combined uORB messages are received (now as RTPS/DDS messages), and outputs the content of the message fields each time the message is received.

public:
	explicit SensorCombinedListener() : Node("sensor_combined_listener") {
		subscription_ = this->create_subscription<px4_msgs::msg::SensorCombined>(
			"fmu/sensor_combined/out",
			10,
			[this](const px4_msgs::msg::SensorCombined::UniquePtr msg) {
			std::cout << "\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n";
			std::cout << "RECEIVED SENSOR COMBINED DATA"   << std::endl;
			std::cout << "============================="   << std::endl;
			std::cout << "ts: "          << msg->timestamp    << std::endl;
			std::cout << "gyro_rad[0]: " << msg->gyro_rad[0]  << std::endl;
			std::cout << "gyro_rad[1]: " << msg->gyro_rad[1]  << std::endl;
			std::cout << "gyro_rad[2]: " << msg->gyro_rad[2]  << std::endl;
			std::cout << "gyro_integral_dt: " << msg->gyro_integral_dt << std::endl;
			std::cout << "accelerometer_timestamp_relative: " << msg->accelerometer_timestamp_relative << std::endl;
			std::cout << "accelerometer_m_s2[0]: " << msg->accelerometer_m_s2[0] << std::endl;
			std::cout << "accelerometer_m_s2[1]: " << msg->accelerometer_m_s2[1] << std::endl;
			std::cout << "accelerometer_m_s2[2]: " << msg->accelerometer_m_s2[2] << std::endl;
			std::cout << "accelerometer_integral_dt: " << msg->accelerometer_integral_dt << std::endl;
		});
	}

The lines below create a publisher to the sensor_combined uORB topic, which can be matched with one or more compatible ROS2 subscribers to the fmu/sensor_combined/out ROS2 topic.

private:
	rclcpp::Subscription<px4_msgs::msg::SensorCombined>::SharedPtr subscription_;
};

The instantiation of the SensorCombinedListener class as a ROS node is done on the main function.

int main(int argc, char *argv[])
{
	std::cout << "Starting sensor_combined listener node..." << std::endl;
	setvbuf(stdout, NULL, _IONBF, BUFSIZ);
	rclcpp::init(argc, argv);
	rclcpp::spin(std::make_shared<SensorCombinedListener>());

	rclcpp::shutdown();
	return 0;
}

# Creating a ROS 2 advertiser

A ROS 2 advertiser node publishes data into the DDS/RTPS network (and hence to the PX4 Autopilot).

Taking as an example the debug_vect_advertiser.cpp under px4_ros_com/src/advertisers, first we import required headers, including the debug_vect msg header.

#include <chrono>
#include <rclcpp/rclcpp.hpp>
#include <px4_msgs/msg/debug_vect.hpp>

using namespace std::chrono_literals;

Then the code creates a DebugVectAdvertiser class that subclasses the generic rclcpp::Node base class.

class DebugVectAdvertiser : public rclcpp::Node
{

The code below creates a function for when messages are to be sent. The messages are sent based on a timed callback, which sends two messages per second based on a timer.

public:
	DebugVectAdvertiser() : Node("debug_vect_advertiser") {
		publisher_ = this->create_publisher<px4_msgs::msg::DebugVect>("fmu/debug_vect/in", 10);
		auto timer_callback =
		[this]()->void {
			auto debug_vect = px4_msgs::msg::DebugVect();
			debug_vect.timestamp = std::chrono::time_point_cast<std::chrono::microseconds>(std::chrono::steady_clock::now()).time_since_epoch().count();
			std::string name = "test";
			std::copy(name.begin(), name.end(), debug_vect.name.begin());
			debug_vect.x = 1.0;
			debug_vect.y = 2.0;
			debug_vect.z = 3.0;
			RCLCPP_INFO(this->get_logger(), "\033[97m Publishing debug_vect: time: %llu x: %f y: %f z: %f \033[0m",
                                debug_vect.timestamp, debug_vect.x, debug_vect.y, debug_vect.z);
			this->publisher_->publish(debug_vect);
		};
		timer_ = this->create_wall_timer(500ms, timer_callback);
	}

private:
	rclcpp::TimerBase::SharedPtr timer_;
	rclcpp::Publisher<px4_msgs::msg::DebugVect>::SharedPtr publisher_;
};

The instantiation of the DebugVectAdvertiser class as a ROS node is done on the main function.

int main(int argc, char *argv[])
{
	std::cout << "Starting debug_vect advertiser node..." << std::endl;
	setvbuf(stdout, NULL, _IONBF, BUFSIZ);
	rclcpp::init(argc, argv);
	rclcpp::spin(std::make_shared<DebugVectAdvertiser>());

	rclcpp::shutdown();
	return 0;
}

# Offboard Control

For a complete reference example on how to use Offboard control with PX4, see: ROS 2 Offboard control example.

# Manual Workspace Setup with a ROS1 compatible workspace (FYI Only)

Note

This is provided to help you better understand the build process and how to include the ROS1 workspace. It is not needed to build or use ROS 2. It additionally includes instructions for building the ros1_bridge package, which is used in ROS (1) via ROS 2 Bridge.

This section describes the process to manually setup your workspace and build the px4_ros_com, px4_msgs, and ros1_bridge package. The topic effectively explains the operation of the build_ros2_workspace.bash script in the installation instructions).

To build the ROS 2 workspace only:

  1. cd into px4_ros_com_ros2 dir and source the ROS 2 environment. Don't mind if it tells you that a previous workspace was set before:

    cd ~/px4_ros_com_ros2
    source /opt/ros/foxy/setup.bash
    
  2. Build the workspace:

    colcon build --symlink-install --event-handlers console_direct+
    

To build both ROS 2 and ROS (1) workspaces (replacing the previous steps):

  1. cd into px4_ros_com_ros2 dir and source the ROS 2 environment. Don't mind if it tells you that a previous workspace was set before:

    source /opt/ros/foxy/setup.bash
    
  2. Clone the ros1_bridge package so it can be built on the ROS 2 workspace:

    git clone https://github.com/ros2/ros1_bridge.git -b dashing ~/px4_ros_com_ros2/src/ros1_bridge
    
  3. Build the px4_ros_com and px4_msgs packages, excluding the ros1_bridge package:

    colcon build --symlink-install --packages-skip ros1_bridge --event-handlers console_direct+
    

    Note

    --event-handlers console_direct+ only serve the purpose of adding verbosity to the colcon build process, and can be removed if one wants a more "quiet" build.

  4. Then build the ROS(1) packages side. First open a new terminal window and source the ROS(1) environment that was installed on the system:

    source /opt/ros/melodic/setup.bash
    
  5. Build the px4_ros_com and px4_msgs packages on the ROS end (using the terminal opened in the previous step):

    cd ~/px4_ros_com_ros1 && colcon build --symlink-install --event-handlers console_direct+
    
  6. Open another new terminal and then source the environments and workspaces in the order listed below:

    source ~/px4_ros_com_ros1/install/setup.bash
    source ~/px4_ros_com_ros2/install/setup.bash
    
  7. Finally, build the ros1_bridge:

    cd ~/px4_ros_com_ros2 && colcon build --symlink-install --packages-select ros1_bridge --cmake-force-configure --event-handlers console_direct+
    

    Note

    The build process may consume a lot of memory resources. On a resource limited machine, reduce the number of jobs being processed in parallel (e.g. set environment variable MAKEFLAGS=-j1). For more details on the build process, see the build instructions on the ros1_bridge (opens new window) package page.

# Cleaning the workspaces

After building the workspaces there are many files that must be deleted before you can do a clean/fresh build (for example, after you have changed some code and want to rebuild).

Unfortunately colcon does not currently have a way of cleaning the generated build, install and log directories, so these directories must be deleted manually.

The clean_all.bash script (in px4_ros_com/scripts) is provided to ease this cleaning process, this script can be used to clean all of the workspace options listed above (ROS 2, ROS 1, and Both)

The most common way of using it is by passing it the ROS (1) workspace directory path (since it's usually not on the default path):

$ source clean_all.bash --ros1_ws_dir <path/to/px4_ros_com_ros1/ws>

TIP

Like the build scripts, the clean_all.bash script also has a --help guide.

# Additional information