Publish chassis-level commands

What you will need

• A Duckietown robot turned ON and visible on dts fleet discover

What you will get

• Learn how to control the Duckiebot’s chassis linear and angular velocities using a ROS Publisher

Topic and message type of interest

The Duckiebot subscribes to chassis commands on the topic:

/ROBOT_NAME/car_cmd_switch_node/cmd

The message type is duckietown_msgs/Twist2DStamped, with fields:

std_msgs/Header header
float32 v
float32 omega

• header: standard ROS header

• v: linear velocity in m/s (forward positive)

• omega: angular velocity in rad/s (counter‑clockwise positive)

Note

Effective v and omega commands require proper kinematic calibration.

Create Publisher ROS Node

Create twist_control_node.py in the src/ directory:

#!/usr/bin/env python3


import os
import rospy
from duckietown.dtros import DTROS, NodeType
from duckietown_msgs.msg import Twist2DStamped


# Twist command parameters
VELOCITY = 0.3  # linear m/s, forward (+)
OMEGA    = 4.0  # angular rad/s, CCW (+)


class TwistControlNode(DTROS):


   def __init__(self, node_name):
       super(TwistControlNode, self).__init__(node_name=node_name, node_type=NodeType.GENERIC)
       vehicle_name = os.environ['VEHICLE_NAME']
       twist_topic  = f"/{vehicle_name}/car_cmd_switch_node/cmd"
       self._v     = VELOCITY
       self._omega = OMEGA
       self._publisher = rospy.Publisher(twist_topic, Twist2DStamped, queue_size=1)


   def run(self):
       rate = rospy.Rate(10)
       message = Twist2DStamped(v=self._v, omega=self._omega)
       while not rospy.is_shutdown():
           self._publisher.publish(message)
           rate.sleep()


   def on_shutdown(self):
       stop = Twist2DStamped(v=0.0, omega=0.0)
       self._publisher.publish(stop)


if __name__ == '__main__':
   node = TwistControlNode(node_name='twist_control_node')
   node.run()
   rospy.spin()

Make it executable:

chmod +x ./packages/my_package/src/twist_control_node.py

Define launcher

Create launchers/twist-control.sh:

#!/bin/bash


source /environment.sh


dt-launchfile-init
rosrun my_package twist_control_node.py
dt-launchfile-join

Rebuild the image:

dts devel build -f

Launch the node

Danger

The robot’s wheels will start moving immediately. Ensure adequate clear space to avoid accidents.

Run the node:

dts devel run -R ROBOT_NAME -L twist-control

Stop with Ctrl+C; on_shutdown() will halt movement.

Congratulations 🎉

You just built and ran a ROS node that controls the Duckiebot chassis using Twist commands.