Ros2 Robotics

ROS 2 Robotics Development

This skill covers building ROS 2 packages and nodes, including package structure, communication patterns (topics, services, actions), timing and frames, build/test workflows with colcon, and common pitfalls in robotics software.

Workflow for Building a ROS 2 Package

1. Scaffold the package — Run ros2 pkg create --build-type ament_python <name> (Python) or --build-type ament_cmake <name> (C++) inside a colcon workspace's src/ directory.
2. Define interfaces — Add custom .msg, .srv, or .action files under msg/, srv/, action/ only when a standard interface (std_msgs, geometry_msgs, sensor_msgs, nav_msgs) doesn't fit.
3. Implement nodes — Write focused, composable nodes; declare parameters explicitly; choose topics for streams, services for quick request/response, and actions for long-running goals with feedback and cancellation.
4. Write launch files — Compose nodes, parameters, and remappings in a launch/*.launch.py file using launch_ros.actions.Node.
5. Set QoS profiles intentionally — Match publisher/subscriber QoS (reliability, durability, history depth) for sensor data vs. command/control paths.
6. Build the workspace — Run colcon build --symlink-install from the workspace root, then source install/setup.bash.
7. Test — Add unit tests (pytest for Python, gtest/launch_testing for C++/integration) and run colcon test.
8. Run and inspect — Use ros2 run, ros2 launch, ros2 topic echo, ros2 node info, and ros2 doctor to verify runtime behavior.

Package Structure

• Keep each package focused on one robot capability or integration boundary (e.g., lidar_driver, path_planner, arm_controller) rather than a monolithic package.
• Use package.xml consistently with the build type declared in it, and either CMakeLists.txt (ament_cmake, C++) or setup.py/setup.cfg (ament_python).
• Organize files with launch/ for launch scripts, config/ for YAML parameter files, msg/ for message definitions, srv/ for services, action/ for actions, and urdf//xacro/ for robot descriptions.
• Use namespaces and topic remapping instead of hardcoded topic names whenever a node might run multiple times (e.g., multi-robot setups) or be reused across projects.
• Declare all dependencies explicitly in package.xml (<depend>, <exec_depend>, <build_depend>) so the workspace builds reproducibly.

Nodes and Interfaces

• Keep nodes small and composable — one node per logical responsibility, wired together via topics/services/actions rather than one giant node doing everything.
• Declare ROS 2 parameters explicitly with self.declare_parameter('rate_hz', 10.0) and read them with self.get_parameter('rate_hz').value; never rely on undeclared parameters.
• Prefer messages (topics) for continuous state streams, services for quick, blocking request/response operations, and actions for long-running goals that report feedback and support cancellation/preemption.
• Use standard message types (geometry_msgs/Twist, sensor_msgs/LaserScan, nav_msgs/Odometry) before creating custom interfaces — custom types fragment the ecosystem and lose compatibility with existing tools.
• Document topic, service, action, frame, and parameter contracts (name, type, units, frame_id, update rate) in the package README or node docstring.

Example: A Minimal rclpy Publisher Node

import rclpy
from rclpy.node import Node
from rclpy.qos import QoSProfile, ReliabilityPolicy, HistoryPolicy
from geometry_msgs.msg import Twist


class SafeVelocityPublisher(Node):
    """Publishes bounded velocity commands at a fixed rate."""

    def __init__(self) -> None:
        super().__init__("safe_velocity_publisher")

        self.declare_parameter("rate_hz", 10.0)
        self.declare_parameter("max_linear_speed", 0.5)
        rate_hz = self.get_parameter("rate_hz").value
        self._max_linear = self.get_parameter("max_linear_speed").value

        # Reliable QoS for a low-rate command topic; sensor streams would
        # typically use BEST_EFFORT with a shallow history instead.
        qos = QoSProfile(
            reliability=ReliabilityPolicy.RELIABLE,
            history=HistoryPolicy.KEEP_LAST,
            depth=10,
        )
        self._publisher = self.create_publisher(Twist, "cmd_vel", qos)
        self._timer = self.create_timer(1.0 / rate_hz, self._on_timer)
        self._target_linear = 0.0

    def _on_timer(self) -> None:
        msg = Twist()
        msg.linear.x = max(-self._max_linear, min(self._max_linear, self._target_linear))
        self._publisher.publish(msg)


def main(args: list | None = None) -> None:
    rclpy.init(args=args)
    node = SafeVelocityPublisher()
    try:
        rclpy.spin(node)
    except KeyboardInterrupt:
        pass
    finally:
        node.destroy_node()
        rclpy.shutdown()


if __name__ == "__main__":
    main()

Timing and Frames

• Use ROS time (self.get_clock().now(), and use_sim_time when a simulation clock is in play) instead of wall-clock time whenever simulation or bag replay matters.
• Use tf2 (tf2_ros.Buffer + TransformListener) for all frame transforms; never hand-roll coordinate transforms between named frames.
• Document frame names (base_link, odom, map, sensor frames) and their conventions (REP-103 for axis orientation, REP-105 for the map→odom→base_link tree).
• Avoid blocking callbacks in a node's executor thread — move long-running work to timers, a MultiThreadedExecutor, worker threads, or actions so the node keeps servicing other callbacks.
• Set QoS profiles intentionally: BEST_EFFORT/volatile for high-rate sensor data where dropped samples are acceptable, RELIABLE/TRANSIENT_LOCAL for latched-like configuration topics, and RELIABLE/volatile for command paths.

Build and Test

• Use colcon build (typically --symlink-install during development) and keep inter-package dependencies explicit in package.xml.
• Run the linters and formatters the workspace already uses — ament_flake8, ament_pep257 for Python; ament_cpplint, ament_uncrustify for C++ — via colcon test.
• Add launch_testing-based integration tests for multi-node behavior (e.g., "does node A receive what node B publishes within N seconds").
• Use simulation (Gazebo/Ignition), recorded rosbags, or fixture data for repeatable sensor scenarios instead of relying on live hardware for every test run.
• Explicitly test failure cases: missing/late transforms (tf2.LookupException), stale sensor data, and services or actions that are unavailable when called.

Common Mistakes

• Hardcoding absolute filesystem paths instead of resolving them via ament_index_python.packages.get_package_share_directory.
• Publishing velocity or actuator commands without validating frame_id, units, and timestamp freshness — a stale or wrong-frame command can move a robot unsafely.
• Creating a custom message/service/action type when an existing standard interface already models the data.
• Ignoring QoS incompatibility warnings between a publisher and subscriber (e.g., a BEST_EFFORT publisher with a RELIABLE subscriber) — the connection silently never delivers data.
• Doing significant work inside a subscription callback without offloading it, starving the executor and delaying other callbacks (including safety-critical ones).
• Forgetting use_sim_time:=true when running against a simulator, causing timestamps and tf2 lookups to disagree with the simulated clock.