cmd_vel topic for velocity commands in ROS

Introduction

The cmd_vel topic is used to send movement commands to a robot. It tells the robot how fast to move and turn.

When you want to control a robot's speed and direction remotely.

When writing a program to make a robot move forward, backward, or turn.

When testing robot movement in simulation or real life.

When combining sensor data to adjust robot velocity automatically.

Syntax

ROS

rostopic pub /cmd_vel geometry_msgs/Twist '{linear: {x: 0.5, y: 0.0, z: 0.0}, angular: {x: 0.0, y: 0.0, z: 1.0}}'

The message type geometry_msgs/Twist contains linear and angular parts.

linear.x controls forward/backward speed, angular.z controls rotation speed.

Examples

Move the robot forward at speed 1.0 with no rotation.

ROS

rostopic pub /cmd_vel geometry_msgs/Twist '{linear: {x: 1.0, y: 0.0, z: 0.0}, angular: {x: 0.0, y: 0.0, z: 0.0}}'

Rotate the robot in place at speed 0.5 without moving forward.

ROS

rostopic pub /cmd_vel geometry_msgs/Twist '{linear: {x: 0.0, y: 0.0, z: 0.0}, angular: {x: 0.0, y: 0.0, z: 0.5}}'

Move the robot backward at speed 0.5 and rotate clockwise at speed 1.0.

ROS

rostopic pub /cmd_vel geometry_msgs/Twist '{linear: {x: -0.5, y: 0.0, z: 0.0}, angular: {x: 0.0, y: 0.0, z: -1.0}}'

Sample Program

This Python ROS node sends velocity commands to the /cmd_vel topic. It moves the robot forward while turning slightly. It sends the command 5 times, once per second.

ROS

import rospy
from geometry_msgs.msg import Twist

rospy.init_node('move_robot')
pub = rospy.Publisher('/cmd_vel', Twist, queue_size=10)
rate = rospy.Rate(1)  # 1 Hz

move_cmd = Twist()
move_cmd.linear.x = 0.5  # Move forward
move_cmd.angular.z = 0.2  # Slight turn

for _ in range(5):
    pub.publish(move_cmd)
    rospy.loginfo('Published velocity command')
    rate.sleep()

Important Notes

Always stop the robot by sending zero velocities to cmd_vel when done.

Check your robot's documentation for max speed limits to avoid damage.

You can use rostopic echo /cmd_vel to see velocity commands being sent.

Summary

cmd_vel is the main topic to control robot movement in ROS.

It uses geometry_msgs/Twist messages with linear and angular velocities.

Publishing to /cmd_vel moves and turns the robot as commanded.

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