cmd_vel topic for velocity commands in ROS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the cmd_vel topic in ROS?

The cmd_vel topic is used to send velocity commands to a robot. It tells the robot how fast to move and in which direction by publishing messages with linear and angular velocity.

beginner

What type of message does the cmd_vel topic use?

The cmd_vel topic uses the geometry_msgs/Twist message type. This message contains two vectors: linear for forward/backward speed and angular for rotation speed.

intermediate

Explain the components of the geometry_msgs/Twist message used in cmd_vel.

The Twist message has two parts: linear and angular. Each is a vector with x, y, and z values. Usually, linear.x controls forward/backward speed, and angular.z controls rotation left/right.

beginner

How do you publish a velocity command to move a robot forward at 1 m/s using cmd_vel?

You publish a geometry_msgs/Twist message with linear.x = 1.0 and all other values zero. This tells the robot to move straight forward at 1 meter per second.

intermediate

Why is it important to regularly publish messages to cmd_vel?

Regularly publishing to cmd_vel ensures the robot keeps moving as expected. If messages stop, many robots will stop for safety to avoid uncontrolled motion.

What message type is used on the cmd_vel topic?

A geometry_msgs/Twist

B std_msgs/String

C sensor_msgs/Image

D nav_msgs/Odometry

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

Which field controls the robot's forward speed in cmd_vel?

A angular.z

B linear.y

C angular.x

D linear.x

linear.x controls forward and backward speed.

What happens if you stop publishing messages to cmd_vel on many robots?

A Robot speeds up

B Robot stops for safety

C Robot reverses direction

D Robot ignores it and keeps moving

Many robots stop moving if cmd_vel messages stop to avoid accidents.

To rotate a robot left, which cmd_vel field should you set?

A angular.z positive

B linear.x positive

C linear.y positive

D angular.x positive

Positive angular.z rotates the robot left (counterclockwise).

What is the main use of the cmd_vel topic?

A Send sensor data

B Send camera images

C Send velocity commands

D Send GPS coordinates

cmd_vel is used to send velocity commands to control robot movement.

Describe how the cmd_vel topic controls a robot's movement in ROS.

Think about the message type and what parts control speed and turning.

Explain why it is important to keep publishing messages to the cmd_vel topic.

Consider what happens if the robot stops getting velocity commands.

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