Plotting uORB Topic Data in Real Time using PlotJuggler

This topic shows how you can graph the "live" values of uORB topics (in real time) using PlotJuggler and the uXRCE-DDS Agent.

This technique uses PX4 uXRCE-DDS middleware to export uORB topics as ROS2 topics, which can then be read and plotted by PlotJuggler as they change (PlotJuggler cannot directly read uORB topics, but the values of the corresponding ROS 2 topics are the same).

The video below demonstrates this for a simulated vehicle — the approach works equally well on real hardware.

Prerequisites

Follow the ROS 2 Installation & Setup instructions in the ROS2 user guide to install:

• ROS 2

• Micro XRCE-DDS Agent

• PX4/px4_msgs: PX4/ROS2 shared message definitions.

• PX4 source code and build the simulator.

  TIP

  If you're using real hardware instead of the simulator, you will only need PX4 source code if you need to change the set of topics that are published to ROS 2 (only a subset of uORB topics are published by default).

You will also need to install:

• PlotJuggler for ROS2

  TIP

  Use the Debian packages (the snap files are not supported).

Usage

First we need to build a ROS 2 workspace that includes the px4_msgs that correspond to the PX4 build to be monitored, and then launch PlotJuggler from within that workspace. This allows ROS 2 and PlotJuggler to interpret the messages. If you're using unmodified PX4, the definitions from PX4/px4_msgs can be used.

INFO

This is the same process as covered in Build ROS 2 Workspace in ROS 2 Installation & Setup.

Assuming your ROS 2 workspace is named ~/ros2_ws/, fetch and build the px4_msgs package in a terminal as shown:

cd ~/ros2_ws/src/
git clone https://github.com/PX4/px4_msgs.git
cd ..
colcon build
source install/setup.bash

Then run PlotJuggler by entering the following commands in a terminal:

ros2 run plotjuggler plotjuggler

To start sending ROS 2 topics from PX4, the uXRCE-DDS client has to be running on PX4, and the MicroXRCEAgent has to be running on the same computer as PlotJuggler.

PX4 Simulator

Next we'll start the Gazebo simulator for a quadcopter. Because we're using a PX4 simulator the client is started automatically, but we will still need to start the agent and connect to the client.

First open another terminal. Then navigate to the root of the PX4 source code and start the simulator using the following commands:

cd ~/PX4-Autopilot
make px4_sitl gz_x500

Open another terminal and start the MicroXRCEAgent to connect to the the simulator:

MicroXRCEAgent udp4 -p 8888; exec bash

That's all that should be needed for connecting to the simulator.

PX4 on Hardware

If you're working with real hardware you'll need to explicitly start the client on PX4 and your agent connection command will be slightly different. Using flight controller hardware in the ROS 2 User Guide provides links to setup information.

Unavailable/New Messages

All PX4 message definitions from main are exported to the PX4/px4_msgs repository. These must be imported into your ROS 2 workspace, allowing PlotJuggler to interpret messages from PX4.

INFO

Exporting the messages allows ROS 2 and the uXRCE-DDS agent to be independent of PX4, which is why you only need the PX4 source code if you need to build the simulator or modify the messages.

While px4_msgs has messages for all uORB topics in PX4, not all messages in px4_msgs are available to ROS 2/PlotJuggler by default. The set that are available must be built into the client running on PX4. These are defined in dds_topics.yaml.

The instructions below explain the changes needed to monitor topics that are not available by default.

Missing Topics

If a normal uORB topic is not available in PlotJuggler you will need to modify the dds_topics.yaml to include the topic and rebuild PX4.

If working with real hardware you will need to build and install custom firmware after changing the YAML file.

Modified Messages

If you have modified any uORB messages you must update the ROS2 messages used by PlotJuggler.

You will need to rebuild PX4 with your new messages, and replace the px4_msgs (from the repository) in your workspace with the new ones.

Assuming that you have already built PX4 in the directory ~/PX4-Autopilot/, and that ~/ros2_ws is your ROS2 workspace, enter the following commands to copy the messages across and rebuild your workspace:

rm -f ~/ros2_ws/src/px4_msgs/msg/*.msg
cp ~/PX4-Autopilot/msg/*.msg ~/ros2_ws/src/px4_msgs/msg/
cd ~/ros2_ws/ && colcon build

Custom Topics

After defining the topic, follow the instructions above to add the topic to dds_topics.yaml, and export the new message into your ROS 2 workspace.

See also

ROS 2 User Guide