# 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 $((px4_instance+1))
Note
By doing so, UXRCE_DDS_KEY will always coincide with MAV_SYS_ID.
Moreover, when px4_instance is greater than zero, a unique ROS 2 namespace prefix in the form px4_$px4_instance is added:
uxrce_dds_ns="-n px4_$px4_instance"
Note
The environment variable PX4_UXRCE_DDS_NS, if set, will override the namespace behavior described above.
The first instance (px4_instance=0) does not have an additional namespace in order to be consistent with the default behavior of the xrce-dds client on a real vehicle. This mismatch can be fixed by manually using PX4_UXRCE_DDS_NS on the first instance or by starting adding vehicles from index 1 instead of 0 (this is the default behavior adopted by sitl_multiple_run.sh (opens new window) for Gazebo Classic).
The default client configuration in simulation is summarized as follows:
PX4_UXRCE_DDS_NS | px4_instance | UXRCE_DDS_KEY | client namespace |
|---|---|---|---|
| not provided | 0 | px4_instance+1 | none |
| provided | 0 | px4_instance+1 | PX4_UXRCE_DDS_NS |
| not provided | >0 | px4_instance+1 | px4_${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.