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Uxrce Dds

Raffaele Rossi

uXRCE-DDS (PX4-ROS 2/DDS Bridge)

::: info uXRCE-DDS replaces the Fast-RTPS Bridge used in PX4 v1.13. If you were using the Fast-RTPS Bridge, please follow the migration guidelines. :::

PX4 uses uXRCE-DDS middleware to allow uORB messages to be published and subscribed on a companion computer as though they were ROS 2 topics. This provides a fast and reliable integration between PX4 and ROS 2, and makes it much easier for ROS 2 applications to get vehicle information and send commands.

PX4 uses an XRCE-DDS implementation that leverages eProsima Micro XRCE-DDS.

The following guide describes the architecture and various options for setting up the client and agent. In particular it covers the options that are most important to PX4 users.

Architecture

The uXRCE-DDS middleware consists of a client running on PX4 and an agent running on the companion computer, with bi-directional data exchange between them over a serial or UDP link. The agent acts as a proxy for the client, enabling it to publish and subscribe to topics in the global DDS data space.

Architecture uXRCE-DDS with ROS 2

In order for PX4 uORB topics to be shared on the DDS network you will need uXRCE-DDS client running on PX4, connected to the micro XRCE-DDS agent running on the companion.

The PX4 uxrce_dds_client publishes to/from a defined set of uORB topics to the global DDS data space.

The eProsima micro XRCE-DDS agent runs on the companion computer and acts as a proxy for the client in the DDS/ROS 2 network.

The agent itself has no dependency on client-side code and can be built and/or installed independent of PX4 or ROS.

Code that wants to subscribe/publish to PX4 does have a dependency on client-side code; it requires uORB message definitions that match those used to create the PX4 uXRCE-DDS client so that it can interpret the messages.

Code Generation

The PX4 uxrce_dds_client is generated at build time and included in PX4 firmare by default. The agent has no dependency on client code. It can be built standalone or in a ROS 2 workspace, or installed as a snap package on Ubuntu.

When PX4 is built, a code generator uses the uORB message definitions in the source tree (PX4-Autopilot/msg) to compile support for the subset of uORB topics in PX4-Autopilot/src/modules/uxrce_dds_client/dds_topics.yaml into uxrce_dds_client.

PX4 main or release builds automatically export the set of uORB messages definitions in the build to an associated branch in PX4/px4_msgs.

ROS 2 applications need to be built in a workspace that includes the same message definitions that were used to create the uXRCE-DDS client module in the PX4 Firmware. These can be included into a workspace by cloning the interface package PX4/px4_msgs into your ROS 2 workspace and switching to the appropriate branch. Note that all code generation associated with the messages is handled by ROS 2.

Micro XRCE-DDS Agent Installation

The Micro XRCE-DDS Agent can be installed on the companion computer using a binary package, built and installed from source, or built and run from within a ROS 2 workspace. All of these methods fetch all the dependencies needed to communicate with the client (such as FastCDR)

::: info The official (and more complete) installation guide is the Eprosima: micro XRCE-DDS Installation Guide. This section summarises the options that have been tested with PX4 during creation of these docs. :::

Install Standalone from Source

On Ubuntu you can build from source and install the Agent standalone using the following commands:

sh git clone https://github.com/eProsima/Micro-XRCE-DDS-Agent.git cd Micro-XRCE-DDS-Agent mkdir build cd build cmake .. make sudo make install sudo ldconfig /usr/local/lib/

::: info There are various build configuration options linked from the corresponding topic in the official guide, but these have not been tested. :::

To start the agent with settings for connecting to the uXRCE-DDS client running on the simulator:

sh MicroXRCEAgent udp4 -p 8888

Install from Snap Package

Install from a snap package on Ubuntu using the following command:

sh sudo snap install micro-xrce-dds-agent --edge

To start the agent with settings for connecting to the uXRCE-DDS client running on the simulator (note that the command name is different than if you build the agent locally):

sh micro-xrce-dds-agent udp4 -p 8888

::: info At time of writing the stable of version installed from snap connects to PX4 but reports errors creating topics. The development version, fetched using --edge above, does work. :::

Build/Run within ROS 2 Workspace

The agent can be built and launched within a ROS 2 workspace (or build standalone and launched from a workspace. You must already have installed ROS 2 following the instructions in: ROS 2 User Guide > Install ROS 2.

To build the agent within ROS:

  1. Create a workspace directory for the agent:

sh mkdir -p ~/px4_ros_uxrce_dds_ws/src

  1. Clone the source code for the eProsima Micro-XRCE-DDS-Agent to the /src directory (the main branch is cloned by default):

sh cd ~/px4_ros_uxrce_dds_ws/src git clone https://github.com/eProsima/Micro-XRCE-DDS-Agent.git

  1. Source the ROS 2 development environment, and compile the workspace using colcon:

:::: tabs

::: tab humble

sh source /opt/ros/humble/setup.bash colcon build

:::

::: tab foxy

sh source /opt/ros/foxy/setup.bash colcon build

:::

::::

This builds all the folders under /src using the sourced toolchain.

To run the micro XRCE-DDS agent in the workspace:

  1. Source the local_setup.bash to make the executables available in the terminal (also setup.bash if using a new terminal).

:::: tabs

::: tab humble

sh source /opt/ros/humble/setup.bash source install/local_setup.bash

:::

::: tab foxy

sh source /opt/ros/foxy/setup.bash source install/local_setup.bash

:::

::::

1) Start the agent with settings for connecting to the uXRCE-DDS client running on the simulator:

sh MicroXRCEAgent udp4 -p 8888

Starting Agent and Client

Starting the Agent

The agent is used to connect to the client over a particular channel, such as UDP or a serial connection. The channel settings are specified when the agent is started, using command line options. These are documented in the eProsima user guide: Micro XRCE-DDS Agent > Agent CLI. Note that the agent supports many channel options, but PX4 only supports UDP and serial connections.

::: info You should create a single instance of the agent for each channel over which you need to connect. :::

For example, the PX4 simulator runs the uXRCE-DDS client over UDP on port 8888, so to connect to the simulator you would start the agent with the command:

sh MicroXRCEAgent udp4 -p 8888

When working with real hardware, the setup depends on the hardware, OS, and channel. For example, if you're using the RasPi UART0 serial port, you might connect using this command (based on the information in Raspberry Pi Documentation > Configuring UARTS):

sh sudo MicroXRCEAgent serial --dev /dev/AMA0 -b 921600

::: info For more information about setting up communications channels see Pixhawk + Companion Setup > Serial Port setup, and sub-documents. :::

Starting the Client

The uXRCE-DDS client module (uxrce_dds_client) is included by default in all firmware and the simulator. This must be started with appropriate settings for the communication channel that you wish to use to communicate with the agent.

::: info The simulator automatically starts the client on localhost UDP port 8888 using the default uxrce-dds namespace. :::

The configuration can be done using the UXRCE-DDS parameters:

  • UXRCE_DDS_CFG: Set the port to connect on, such as TELEM2, Ethernet, or Wifi.

  • If using an Ethernet connection:

  • UXRCE_DDS_PRT: Use this to specify the agent UDP listening port. The default value is 8888.

  • UXRCE_DDS_AG_IP: Use this to specify the IP address of the agent. The IP address must be provided in int32 format as PX4 does not support string parameters. The default value is 2130706433 which corresponds to the localhost 127.0.0.1.

    You can use Tools/convert_ip.py to convert between the formats:

    • To obtain the int32 version of an IP in decimal dot notation the command is:

    sh python3 ./PX4-Autopilot/Tools/convert_ip.py <the IP address in decimal dot notation>

    • To get the IP address in decimal dot notation from the int32 version:

    sh python3 ./PX4-Autopilot/Tools/convert_ip.py -r <the IP address in int32 notation>

  • If using a serial connection:

  • SER_TEL2_BAUD, SER_URT6_BAUD (and so on): Use the _BAUD parameter associated with the serial port to set the baud rate. For example, you'd set a value for SER_TEL2_BAUD if you are connecting to the companion using TELEM2. For more information see Serial port configuration.

  • Some setups might also need these parameters to be set:

  • UXRCE_DDS_KEY: The uXRCE-DDS key. If you're working in a multi-client, single agent configuration, each client should have a unique non-zero key. This is primarily important for multi-vehicle simulations, where all clients are connected in UDP to the same agent. (See the official eprosima documentation , uxr_init_session.)

  • UXRCE_DDS_DOM_ID: The DDS domain ID. This provides a logical separation between DDS networks, and can be used to separate clients on different networks. By default, ROS 2 operates on ID 0.
  • UXRCE_DDS_PTCFG: uXRCE-DDS participant configuration. It allows to restrict the visibility of the DDS topics to the localhost only and to use user-customized participant configuration files stored on the agent side.
  • UXRCE_DDS_SYNCT: Bridge time synchronization enable. The uXRCE-DDS client module can synchronize the timestamp of the messages exchanged over the bridge. This is the default configuration. In certain situations, for example during simulations, this feature may be disabled.

::: info Many ports are already have a default configuration. To use these ports you must first disable the existing configuration:

Once set, you may need to reboot PX4 for the parameters to take effect. They will then persist through subsequent reboots.

You can also start the uxrce_dds_client using a command line. This can be called as part of System Startup or through the MAVLink Shell (or a system console). This method is useful when you need to set a custom client namespace, as no parameter is provided for this purpose. For example, the following command can be used to connect via Ethernet to a remote host at 192.168.0.100:8888 and to set the client namespace to /drone/.

sh uxrce_dds_client start -t udp -p 8888 -h 192.168.0.100 -n drone

Options -p or -h are used to bypass UXRCE_DDS_PRT and UXRCE_DDS_AG_IP.

Starting the Client in Simulation

The simulator startup logic (init.d-posix/rcS) uses the client startup commands for single and multi vehicle simulations, enabling the setting of appropriate instance ids and DDS namespaces. By default the client is started on localhost UDP port 8888 with no additional namespace.

Environment variables are provided that override some UXRCE-DDS parameters. These allow users to create custom startup files for their simulations:

For example, the following command can be used to start a Gazebo simulation with che client operating on the DDS domain 3, port 9999 and topic namespace drone.

sh ROS_DOMAIN_ID=3 PX4_UXRCE_DDS_PORT=9999 PX4_UXRCE_DDS_NS=drone make px4_sitl gz_x500

Supported uORB Messages

The set of PX4 uORB topics that are exposed through the client are set in dds_topics.yaml.

The topics are release specific (support is compiled into uxrce_dds_client at build time). While most releases should support a very similar set of messages, to be certain you would need to check the yaml file for your particular release.

Note that ROS 2/DDS needs to have the same message definitions that were used to create the uXRCE-DDS client module in the PX4 Firmware in order to interpret the messages. The message definitions are stored in the ROS 2 interface package PX4/px4_msgs, and they are automatically synchronized by CI on the main and release branches. Note that all the messages from PX4 source code are present in the repository, but only those listed in dds_topics.yaml will be available as ROS 2 topics. Therefore,

  • If you're using a main or release version of PX4 you can get the message definitions by cloning the interface package PX4/px4_msgs into your workspace.
  • If you're creating or modifying uORB messages you must manually update the messages in your workspace from your PX4 source tree. Generally this means that you would update dds_topics.yaml, clone the interface package, and then manually synchronize it by copying the new/modified message definitions from PX4-Autopilot/msg to its msg folders. Assuming that PX4-Autopilot is in your home directory ~, while px4_msgs is in ~/px4_ros_com/src/, then the command might be:

sh rm ~/px4_ros_com/src/px4_msgs/msg/*.msg cp ~/PX4-Autopilot/mgs/*.msg ~/px4_ros_com/src/px4_msgs/msg/

::: info Technically, dds_topics.yaml completely defines the relationship between PX4 uORB topics and ROS 2 messages. For more information see DDS Topics YAML below. :::

Customizing the Topic Namespace

Custom topic namespaces can be applied at build time (changing dds_topics.yaml) or at runtime (which is useful for multi vehicle operations):

  • One possibility is to use the -n option when starting the uxrce_dds_client from command line. This technique can be used both in simulation and real vehicles.
  • A custom namespace can be provided for simulations (only) by setting the environment variable PX4_UXRCE_DDS_NS before starting the simulation.

::: info Changing the namespace at runtime will append the desired namespace as a prefix to all topic fields in dds_topics.yaml. Therefore, commands like:

sh uxrce_dds_client start -n uav_1

or

sh PX4_UXRCE_DDS_NS=uav_1 make px4_sitl gz_x500

will generate topics under the namespaces:

sh /uav_1/fmu/in/ # for subscribers /uav_1/fmu/out/ # for publishers

:::

PX4 ROS 2 QoS Settings

PX4 QoS settings for publishers are incompatible with the default QoS settings for ROS 2 subscribers. So if ROS 2 code needs to subscribe to a uORB topic, it will need to use compatible QoS settings. One example of which is shown in ROS 2 User Guide > ROS 2 Subscriber QoS Settings.

PX4 uses the following QoS settings for publishers:

cpp uxrQoS_t qos = { .durability = UXR_DURABILITY_TRANSIENT_LOCAL, .reliability = UXR_RELIABILITY_BEST_EFFORT, .history = UXR_HISTORY_KEEP_LAST, .depth = 0, };

PX4 uses the following QoS settings for subscribers:

cpp uxrQoS_t qos = { .durability = UXR_DURABILITY_VOLATILE, .reliability = UXR_RELIABILITY_BEST_EFFORT, .history = UXR_HISTORY_KEEP_LAST, .depth = queue_depth, };

ROS 2 uses the following QoS settings (by default) for publishers and subscriptions: "keep last" for history with a queue size of 10, "reliable" for reliability, "volatile" for durability, and "system default" for liveliness. Deadline, lifespan, and lease durations are also all set to "default".

DDS Topics YAML

The PX4 yaml file dds_topics.yaml defines the set of PX4 uORB topics that are built into firmware and published. More precisely, it completely defines the relationship/pairing between PX4 uORB and ROS 2 messages.

The file is structured as follows:

```yaml publications:

  • topic: /fmu/out/collision_constraints type: px4_msgs::msg::CollisionConstraints

...

  • topic: /fmu/out/vehicle_odometry type: px4_msgs::msg::VehicleOdometry

  • topic: /fmu/out/vehicle_status type: px4_msgs::msg::VehicleStatus

  • topic: /fmu/out/vehicle_trajectory_waypoint_desired type: px4_msgs::msg::VehicleTrajectoryWaypoint

subscriptions:

  • topic: /fmu/in/offboard_control_mode type: px4_msgs::msg::OffboardControlMode

...

  • topic: /fmu/in/vehicle_trajectory_waypoint type: px4_msgs::msg::VehicleTrajectoryWaypoint

subscriptions_multi:

  • topic: /fmu/in/vehicle_optical_flow_vel type: px4_msgs::msg::VehicleOpticalFlowVel

...

```

Each (topic,type) pairs defines:

  1. A new publication, subscription, or subscriptions_multi, depending on the list to which it is added.
  2. The topic base name, which must coincide with the desired uORB topic name that you want to publish/subscribe. It is identified by the last token in topic: that starts with / and does not contains any / in it. vehicle_odometry, vehicle_status and offboard_control_mode are examples of base names.
  3. The topic namespace. By default it is set to:
  4. /fmu/out/ for topics that are published by PX4.
  5. /fmu/in/ for topics that are subscribed by PX4.
  6. The message type (VehicleOdometry, VehicleStatus, OffboardControlMode, etc.) and the ROS 2 package (px4_msgs) that is expected to provide the message definition.

subscriptions and subscriptions_multi allow us to choose the uORB topic instance that ROS 2 topics are routed to: either a shared instance that may also be getting updates from internal PX4 uORB publishers, or a separate instance that is reserved for ROS2 publications, respectively. Without this mechanism all ROS 2 messages would be routed to the same uORB topic instance (because ROS 2 does not have the concept of multiple topic instances), and it would not be possible for PX4 subscribers to differentiate between streams from ROS 2 or PX4 publishers.

Add a topic to the subscriptions section to:

  • Create a unidirectional route going from the ROS2 topic to the default instance (instance 0) of the associated uORB topic. For example, it creates a ROS2 subscriber of /fmu/in/vehicle_odometry and a uORB publisher of vehicle_odometry.
  • If other (internal) PX4 modules are already publishing on the same uORB topic instance as the ROS2 publisher, the instance's subscribers will receive all streams of messages. The uORB subscriber will not be able to determine if an incoming message was published by PX4 or by ROS2.
  • This is the desired behavior when the ROS2 publisher is expected to be the sole publisher on the topic instance (for example, replacing an internal publisher to the topic during offboard control), or when the source of multiple publishing streams does not matter.

Add a topic to the subscriptions_multi section to:

  • Create a unidirectional route going from the ROS2 topic to a new instance of the associated uORB topic. For example, if vehicle_odometry has already 2 instances, it creates a ROS2 subscriber of /fmu/in/vehicle_odometry and a uORB publisher on instance 3 of vehicle_odometry.
  • This ensures that no other internal PX4 module will publish on the same instance used by uXRCE-DDS. The subscribers will be able to subscribe to the desired instance and distinguish between publishers.
  • Note, however, that this guarantees separation between PX4 and ROS2 publishers, not among multiple ROS2 publishers. In that scenario, their messages will still be routed to the same instance.
  • This is the desired behavior, for example, when you want PX4 to log the readings of two equal sensors; they will both publish on the same topic, but one will use instance 0 and the other will use instance 1.

You can arbitrarily change the configuration. For example, you could use different default namespaces or use a custom package to store the message definitions.

Fast-RTPS to uXRCE-DDS Migration Guidelines

These guidelines explain how to migrate from using PX4 v1.13 Fast-RTPS middleware to PX4 v1.14 uXRCE-DDS middleware. These are useful if you have ROS 2 applications written for PX4 v1.13, or you have used Fast-RTPS to interface your applications to PX4 directly.

::: info This section contains migration-specific information. You should also read the rest of this page to properly understand uXRCE-DDS. :::

Dependencies do not need to be removed

uXRCE-DDS does not need the dependencies that were required for Fast-RTPS, such as those installed by following the topic Fast DDS Installation. You can keep them if you want, without affecting your uXRCE-DDS applications.

If you do choose to remove the dependencies, take care not to remove anything that is used by applications (for example, Java).

_rtps targets have been removed

Anywhere you previously used a build target with extension _rtps, such as px4_fmu-v5_rtps or px4_sitl_rtps, you can now use the equivalent default target (for these cases px4_fmu-v5_default and px4_sitl_default).

The make targets with extension _rtps were used to build firmware that included client side RTPS code. The uXRCE-DDS middleware is included by default in builds for most boards, so you no longer need a special firmware to work with ROS 2.

To check if your board has the middleware, look for CONFIG_MODULES_UXRCE_DDS_CLIENT=y in the .px4board file of your board. Those files are nested in PX4-Autopilot/boards.

If it is not present, or if it is set to n, then you have to clone the PX4 repo, modify the board configuration and manually compile the firmware.

New client module and new start parameters

As the client is implemented by a new PX4 module, you now have new parameters to start it. Take a look at the client startup section to learn how this is done.

New file for setting which topics are published

The list of topics that are published and subscribed for a particular firmware is now managed by the dds_topic.yaml configuration file, which replaces urtps_bridge_topics.yaml

See Supported uORB Messages and DDS Topics YAML sections for more information.

Topics no longer need to be synced between client and agent.

The list of bridged topics between agent and client no longer needs to be synced for ROS 2, so the update_px4_ros2_bridge.sh script is no longer needed.

Default topic naming convention has changed

The topic naming format changed:

  • Published topics: /fmu/topic-name/out (Fast-RTPS) to /fmu/out/topic-name (XRCE-DDS).
  • Subscribed topics: /fmu/topic-name/in(Fast-RTPS) to /fmu/in/topic-name (XRCE-DDS).

You should update your application to the new convention.

::: info You might also edit dds_topic.yaml to revert to the old convention. This is not recommended because it means that you would have to always use custom firmware. :::

XRCE-DDS-Agent

The XRCE-DDS agent is "generic" and independent of PX4: micro-xrce-dds-agent. There are many ways to install it on your PC / companion computer - for more information see the dedicated section.

Application-Specific Changes

If you where not using ROS 2 alongside the agent (Fast DDS Interface ROS-Independent), then you need to migrate to eProsima Fast DDS.

ROS 2 applications still need to compile alongside the PX4 messages, which you do by adding the px4_msgs package to your workspace. You can remove the px4_ros_com package as it is no longer needed, other than for example code.

In your ROS 2 nodes, you will need to:

  • Update the QoS of your publishers and subscribers as PX4 does not use the ROS 2 default settings.
  • Change the names of your topics, unless you edited dds_topic.yaml.
  • Remove everything related to time synchronization, as XRCE-DDS automatically takes care of agent/client time synchronization.

In C++ applications you can set the timestamp field of your messages like this:

cpp msg.timestamp = this->get_clock()->now().nanoseconds() / 1000;

In Python applications you can set the timestamp field of your messages like this:

python msg.timestamp = int(self.get_clock().now().nanoseconds / 1000)

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