Multi-Vehicle Simulation with ROS 2

XRCE-DDS allows for multiple clients to be connected to the same agent over UDP. This is particular useful in simulation as only one agent needs to be started.

Setup and Requirements

The only requirements are

• To be able to run multi-vehicle simulation without ROS 2 with the desired simulator (Gazebo, Gazebo Classic, FlightGear and JMAVSim).
• To be able to use ROS 2 in a single vehicle simulation.

Principle of Operation

In simulation each PX4 instance receives a unique px4_instance number starting from 0. This value is used to assign a unique value to UXRCE_DDS_KEY:

param set UXRCE_DDS_KEY $((px4_instance+1))

INFO

By doing so, UXRCE_DDS_KEY will always coincide with MAV_SYS_ID.

Moreover, when px4_instance is greater than zero, a unique ROS 2 namespace prefix in the form px4_$px4_instance is added:

uxrce_dds_ns="-n px4_$px4_instance"

INFO

The environment variable PX4_UXRCE_DDS_NS, if set, will override the namespace behavior described above.

The first instance (px4_instance=0) does not have an additional namespace in order to be consistent with the default behavior of the xrce-dds client on a real vehicle. This mismatch can be fixed by manually using PX4_UXRCE_DDS_NS on the first instance or by starting adding vehicles from index 1 instead of 0 (this is the default behavior adopted by sitl_multiple_run.sh for Gazebo Classic).

The default client configuration in simulation is summarized as follows:

PX4_UXRCE_DDS_NS	px4_instance	UXRCE_DDS_KEY	client namespace
not provided	0	px4_instance+1	none
provided	0	px4_instance+1	PX4_UXRCE_DDS_NS
not provided	>0	px4_instance+1	px4_${px4_instance}
provided	>0	px4_instance+1	PX4_UXRCE_DDS_NS

Adjusting the target_system value

PX4 accepts VehicleCommand messages only if their target_system field is zero (broadcast communication) or coincides with MAV_SYS_ID. In all other situations, the messages are ignored. Therefore, when ROS 2 nodes want to send VehicleCommand to PX4, they must ensure that the messages are filled with the appropriate target_system value.

For example, if you want to send a command to your third vehicle, which has px4_instance=2, you need to set target_system=3 in all your VehicleCommand messages.