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55 lines
3.1 KiB
Markdown
55 lines
3.1 KiB
Markdown
# Multi-Vehicle Simulation with ROS 2
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[XRCE-DDS](../middleware/uxrce_dds.md) allows for multiple clients to be connected to the same agent over UDP.
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This is particular useful in simulation as only one agent needs to be started.
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## Setup and Requirements
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The only requirements are
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- To be able to run [multi-vehicle simulation](../simulation/multi-vehicle-simulation.md) without ROS 2 with the desired simulator ([Gazebo](../sim_gazebo_gz/multi_vehicle_simulation.md), [Gazebo Classic](../sim_gazebo_classic/multi_vehicle_simulation.md#multiple-vehicle-with-gazebo-classic), [FlightGear](../sim_flightgear/multi_vehicle.md) and [JMAVSim](../sim_jmavsim/multi_vehicle.md)).
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- To be able to use [ROS 2](../ros2/user_guide.md) in a single vehicle simulation.
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## Principle of Operation
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In simulation each PX4 instance receives a unique `px4_instance` number starting from `0`.
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This value is used to assign a unique value to [UXRCE_DDS_KEY](../advanced_config/parameter_reference.md#UXRCE_DDS_KEY):
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```sh
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param set UXRCE_DDS_KEY $((px4_instance+1))
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```
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:::info
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By doing so, `UXRCE_DDS_KEY` will always coincide with [MAV_SYS_ID](../advanced_config/parameter_reference.md#MAV_SYS_ID).
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:::
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Moreover, when `px4_instance` is greater than zero, a unique ROS 2 [namespace prefix](../middleware/uxrce_dds.md#customizing-the-namespace) in the form `px4_$px4_instance` is added:
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```sh
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uxrce_dds_ns="-n px4_$px4_instance"
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```
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:::info
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The environment variable `PX4_UXRCE_DDS_NS`, if set, will override the namespace behavior described above.
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:::
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The first instance (`px4_instance=0`) does not have an additional namespace in order to be consistent with the default behavior of the `xrce-dds` client on a real vehicle.
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This mismatch can be fixed by manually using `PX4_UXRCE_DDS_NS` on the first instance or by starting adding vehicles from index `1` instead of `0` (this is the default behavior adopted by [sitl_multiple_run.sh](https://github.com/PX4/PX4-Autopilot/blob/main/Tools/simulation/gazebo-classic/sitl_multiple_run.sh) for Gazebo Classic).
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The default client configuration in simulation is summarized as follows:
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| `PX4_UXRCE_DDS_NS` | `px4_instance` | `UXRCE_DDS_KEY` | client namespace |
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| ------------------ | -------------- | ---------------- | --------------------- |
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| not provided | 0 | `px4_instance+1` | none |
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| provided | 0 | `px4_instance+1` | `PX4_UXRCE_DDS_NS` |
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| not provided | > 0 | `px4_instance+1` | `px4_${px4_instance}` |
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| provided | > 0 | `px4_instance+1` | `PX4_UXRCE_DDS_NS` |
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## Adjusting the `target_system` value
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PX4 accepts [VehicleCommand](../msg_docs/VehicleCommand.md) messages only if their `target_system` field is zero (broadcast communication) or coincides with `MAV_SYS_ID`.
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In all other situations, the messages are ignored.
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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.
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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.
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