cmd_vel 坐标系

### cmd_vel Topic Coordinate System in ROS for Robot Motion Control In the context of robot motion control within ROS, particularly when configuring a two-wheeled differential drive controller as described for FishBot[^1], understanding the `cmd_vel` topic's coordinate system is crucial. The message type associated with this topic is `geometry_msgs/msg/Twist`, which contains linear and angular velocities. The standard convention for the `cmd_vel` topic assumes that: - **Linear Velocity (`linear.x`)**: Represents forward movement along the X-axis of the base_link frame. Positive values indicate moving forward; negative values mean backward. - **Angular Velocity (`angular.z`)**: Indicates rotation around the Z-axis (vertical axis). A positive value causes counterclockwise rotation while a negative one results in clockwise rotation. This setup ensures commands sent to the robot are relative to its own orientation rather than an external reference point like world coordinates or map frames. This approach simplifies local navigation tasks where only immediate surroundings matter without needing global positioning information. For accurate operation, especially during simulation using Gazebo alongside ROS2, it’s important to ensure proper configuration parameters such as wheelbase width and diameter are correctly set up under plugins like `gazebo_ros_diff_drive`. These settings influence how input from `cmd_vel` translates into actual movements by adjusting motor speeds accordingly based on kinematic equations specific to wheeled robots[^3]. Additionally, tools mentioned elsewhere can help visualize these transformations between different sensor inputs and outputs used across various nodes communicating over topics including but not limited to `/image_raw/compressed` and `/cv_bridge_image` through scripts implementing functionalities similar to those provided by packages like `cv_bridge_test.py`[^2]. However, direct relevance here pertains more specifically towards ensuring correct interpretation and application of velocity commands via `cmd_vel`. ```python import rclpy from geometry_msgs.msg import Twist def publish_velocity_command(publisher): msg = Twist() # Example command: Move straight at speed 0.5 m/s msg.linear.x = 0.5 # Turn left slowly (-ve would turn right) msg.angular.z = 0.1 publisher.publish(msg) if __name__ == '__main__': node = rclpy.create_node('velocity_publisher') pub = node.create_publisher(Twist, 'cmd_vel', 10) try: publish_velocity_command(pub) finally: node.destroy_node() ```