Several drones

In this tutorial you start three drones and fly them from one script. They take off together, line up at three heights and land together.

Before you start

You've done A mission in Python. This tutorial builds on DroneInterface and wait=False.

Every drone runs its own copy of the stack, the platform, state estimator, motion controller and behaviors, under its own namespace, like drone0, drone1 and drone2. Their topics and actions never clash, so the drones share one ROS 2 network without knowing about each other.

They do share one frame. Every drone's position is given in earth, so a point means the same place for all of them. Each drone starts where the world file puts it. The Frames page explains how earth relates to each drone's own frames.

Start three drones

With -m, the Quickstart project reads config/world_swarm.yaml, which places three drones a few metres apart:

config/world_swarm.yaml (excerpt)

drone0:
  platform:
    ros__parameters:
      vehicle_initial_pose:
        x: -2.0
        y: 0.0
        z: 0.0

drone1 starts at x = 2 and drone2 at the origin. Start them, and the ground station for all three:

./launch_as2.bash -m
./launch_ground_station.bash -m

Each drone gets a tmux session named after it, with the same windows as a single drone, and your terminal attaches to drone0's. RViz shows all three.

Connect to every drone

One DroneInterface per drone, each with its namespace:

NAMESPACES = ["drone0", "drone1", "drone2"]
drones = [DroneInterface(drone_id=ns) for ns in NAMESPACES]

Calling drone.takeoff() on each in turn would work, but each call waits for its behavior, so the drones would take off one after another. To fly them together, start every behavior with wait=False, then wait for all of them. A behavior's wait_to_result() waits until it ends and returns whether it succeeded:

def wait_all(behaviors) -> bool:
    """Wait until every behavior ends. True if all of them succeeded."""
    results = [behavior.wait_to_result() for behavior in behaviors]
    return all(results)

The behaviors run at the same time, so waiting for them one by one takes as long as the slowest.

Fly together

Create swarm.py in the project folder:

swarm.py

import rclpy
from as2_python_api.drone_interface import DroneInterface

NAMESPACES = ["drone0", "drone1", "drone2"]


def wait_all(behaviors) -> bool:
    """Wait until every behavior ends. True if all of them succeeded."""
    results = [behavior.wait_to_result() for behavior in behaviors]
    return all(results)


rclpy.init()
drones = [DroneInterface(drone_id=ns) for ns in NAMESPACES]

try:
    for drone in drones:
        drone.arm()
        drone.offboard()

    # Start every take off, then wait for all of them
    for drone in drones:
        drone.takeoff(height=1.5, speed=0.5, wait=False)
    print("Take off:", wait_all([drone.takeoff for drone in drones]))

    # Move to a line along y, each drone at a different height
    for i, drone in enumerate(drones):
        drone.go_to(0.0, 2.0 * (i - 1), 1.0 + 0.5 * i, speed=1.0, wait=False)
    print("Line:", wait_all([drone.go_to for drone in drones]))

    for drone in drones:
        print(drone.drone_id, [round(v, 2) for v in drone.position])
finally:
    for drone in drones:
        drone.land(speed=0.5, wait=False)
    print("Land:", wait_all([drone.land for drone in drones]))
    for drone in drones:
        drone.manual()
        drone.shutdown()
    rclpy.shutdown()

Run it:

python3 swarm.py

The three drones take off together, fly to a line along y, at 1, 1.5 and 2 m, and land together:

Take off: True
Line: True
drone0 [-0.03, -1.97, 1.01]
drone1 [0.04, -0.0, 1.5]
drone2 [0.0, 1.87, 1.96]
Land: True

The positions are all in earth. drone0 started at x = -2 and ended at y = -2, the place the script sent it to, wherever it started from.

Stop everything

./stop.bash

Going further

Choreographies. The project's mission_swarm.py flies the three drones through a sequence of formations, built the same way.
One script per drone. Each drone can also run its own mission, from its own script or its own behavior tree. Nothing ties a drone to the script that started it.
Keep them apart. Nothing stops two drones from flying into each other. The swarm flocking behavior flies a group in formation around a moving centroid.

Before you start

One namespace per drone

Start three drones

Connect to every drone

Fly together

Stop everything

Going further

Next steps

Move to Gazebo

Frames

On this page