cmd_vel topic for velocity commands in ROS - Mini Project: Build & Apply

Publishing Velocity Commands on ROS cmd_vel Topic

📖 Scenario: You are programming a simple robot that moves by sending velocity commands. The robot listens to a special channel called cmd_vel to know how fast to move forward or turn.In this project, you will create a small ROS node that sends velocity commands to the cmd_vel topic to control the robot's movement.

🎯 Goal: Build a ROS node in Python that publishes velocity commands to the cmd_vel topic. You will set up the message data, configure the publisher, send velocity commands, and complete the node so the robot can move.

📋 What You'll Learn

Create a Twist message with linear and angular velocity

Set up a ROS publisher for the cmd_vel topic

Publish the velocity commands using the publisher

Complete the ROS node with initialization and spinning

💡 Why This Matters

🌍 Real World

Robots often receive movement commands through the 'cmd_vel' topic. This project shows how to send those commands to make a robot move.

💼 Career

Understanding how to publish velocity commands on 'cmd_vel' is essential for robotics developers working with ROS to control robot motion.

1

Create a Twist message with velocity values

Create a variable called vel_msg as a new Twist() message. Set vel_msg.linear.x to 0.5 and vel_msg.angular.z to 0.1.

ROS

import rospy
from geometry_msgs.msg import Twist

# Create a Twist message with linear.x=0.5 and angular.z=0.1
# Your code here

Hint

Use vel_msg = Twist() to create the message. Then set vel_msg.linear.x and vel_msg.angular.z to the given numbers.

2

Set up a ROS publisher for the cmd_vel topic

Create a publisher called pub that publishes Twist messages to the cmd_vel topic with a queue size of 10.

ROS

import rospy
from geometry_msgs.msg import Twist

vel_msg = Twist()
vel_msg.linear.x = 0.5
vel_msg.angular.z = 0.1

# Create a publisher named pub for cmd_vel topic
# Your code here

Hint

Use rospy.Publisher with the topic name 'cmd_vel', message type Twist, and queue_size=10.

3

Publish the velocity message using the publisher

Use the publisher pub to send the vel_msg message by calling pub.publish(vel_msg).

ROS

import rospy
from geometry_msgs.msg import Twist

vel_msg = Twist()
vel_msg.linear.x = 0.5
vel_msg.angular.z = 0.1

pub = rospy.Publisher('cmd_vel', Twist, queue_size=10)

# Publish the velocity message
# Your code here

Hint

Call pub.publish(vel_msg) to send the velocity command to the robot.

4

Complete the ROS node with initialization and spin

Initialize the ROS node with rospy.init_node('velocity_publisher') and keep it alive with rospy.spin().

ROS

import rospy
from geometry_msgs.msg import Twist

vel_msg = Twist()
vel_msg.linear.x = 0.5
vel_msg.angular.z = 0.1

pub = rospy.Publisher('cmd_vel', Twist, queue_size=10)

pub.publish(vel_msg)

# Initialize the node and keep it running
# Your code here

Hint

Use rospy.init_node to start the node and rospy.spin() to keep it running.

Practice

1. What is the main purpose of the /cmd_vel topic in ROS?

easy

A. To configure robot hardware settings

B. To receive sensor data from the robot

C. To send velocity commands to control robot movement

D. To log robot status messages

Solution

1. Step 1: Understand the role of /cmd_vel topic

   The /cmd_vel topic is used to send velocity commands to the robot.

2. Step 2: Identify what velocity commands control

   Velocity commands control the robot's movement, including linear and angular velocities.

3. Final Answer:

   To send velocity commands to control robot movement -> Option C

4. Quick Check:

   /cmd_vel controls robot movement = A [OK]

Hint: Remember: cmd_vel means command velocity for robot movement [OK]

Common Mistakes:

• Confusing cmd_vel with sensor data topics
• Thinking cmd_vel configures hardware
• Assuming cmd_vel logs messages

2. Which message type is published on the /cmd_vel topic to control robot velocity?

easy

A. geometry_msgs/Twist

B. std_msgs/String

C. sensor_msgs/LaserScan

D. nav_msgs/Odometry

Solution

1. Step 1: Identify the message type for velocity commands

   The /cmd_vel topic uses geometry_msgs/Twist messages to send velocity commands.

2. Step 2: Confirm other message types are unrelated

   sensor_msgs/LaserScan is for laser data, std_msgs/String is generic text, and nav_msgs/Odometry is for position data.

3. Final Answer:

   geometry_msgs/Twist -> Option A

4. Quick Check:

   Velocity commands use Twist messages = C [OK]

Hint: Twist means linear + angular velocity message [OK]

Common Mistakes:

• Choosing sensor or odometry messages instead of Twist
• Confusing std_msgs/String as velocity message

3. Given this Python ROS publisher code snippet, what linear x velocity will the robot move at?

import rospy
from geometry_msgs.msg import Twist

rospy.init_node('move_robot')
pub = rospy.Publisher('/cmd_vel', Twist, queue_size=10)
msg = Twist()
msg.linear.x = 0.5
msg.angular.z = 0.0
pub.publish(msg)

medium

A. 0.0 m/s

B. Undefined velocity

C. 1.0 m/s

D. 0.5 m/s

Solution

1. Step 1: Check the linear velocity set in the message

   The code sets msg.linear.x = 0.5, which means forward velocity is 0.5 meters per second.

2. Step 2: Confirm angular velocity does not affect linear speed

   msg.angular.z = 0.0 means no rotation, so linear speed remains 0.5 m/s.

3. Final Answer:

   0.5 m/s -> Option D

4. Quick Check:

   linear.x = 0.5 means speed 0.5 m/s [OK]

Hint: Look for msg.linear.x value for forward speed [OK]

Common Mistakes:

• Confusing angular.z with linear.x velocity
• Assuming default velocity if not set
• Ignoring the published message values

4. What is wrong with this ROS Python code snippet that publishes velocity commands?

import rospy
from geometry_msgs.msg import Twist

rospy.init_node('move_robot')
pub = rospy.Publisher('/cmd_vel', Twist)
msg = Twist()
msg.linear.x = 1.0
pub.publish(msg)

medium

A. Twist message fields are incorrectly assigned

B. Missing queue_size parameter in Publisher initialization

C. Node initialization is missing

D. Publisher topic name is incorrect

Solution

1. Step 1: Check Publisher initialization parameters

   The rospy.Publisher requires a queue_size parameter to avoid runtime warnings or errors.

2. Step 2: Verify other parts of the code

   Node initialization is present, message fields are correctly assigned, and topic name /cmd_vel is correct.

3. Final Answer:

   Missing queue_size parameter in Publisher initialization -> Option B

4. Quick Check:

   Publisher needs queue_size argument = A [OK]

Hint: Always add queue_size when creating a Publisher [OK]

Common Mistakes:

• Omitting queue_size causes errors
• Thinking node initialization is missing
• Assuming topic name is wrong

5. You want your robot to move forward at 0.3 m/s and rotate at 0.5 rad/s simultaneously using /cmd_vel. Which code snippet correctly publishes this combined velocity command in Python?

hard

A. msg.linear.x = 0.3
msg.angular.z = 0.5
pub.publish(msg)

B. msg.linear.y = 0.3
msg.angular.x = 0.5
pub.publish(msg)

C. msg.linear.x = 0.5
msg.angular.z = 0.3
pub.publish(msg)

D. msg.linear.z = 0.3
msg.angular.y = 0.5
pub.publish(msg)

Solution

1. Step 1: Identify correct fields for forward and rotational velocity

   Forward movement uses linear.x, and rotation uses angular.z in geometry_msgs/Twist.

2. Step 2: Match values to correct fields

   Set linear.x = 0.3 for forward speed and angular.z = 0.5 for rotation speed.

3. Step 3: Verify other options use incorrect axes

   Options A, B, and C assign values to wrong axes or swap linear and angular values.

4. Final Answer:

   msg.linear.x = 0.3 msg.angular.z = 0.5 pub.publish(msg) -> Option A

5. Quick Check:

   Forward = linear.x, rotate = angular.z = D [OK]

Hint: Forward speed = linear.x, rotation = angular.z [OK]

Common Mistakes:

• Using wrong linear or angular axes
• Swapping linear and angular values
• Setting velocities on unused axes