cmd_vel topic for velocity commands in ROS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to publish a velocity command on the cmd_vel topic.

ROS

pub = rospy.Publisher('cmd_vel', [1], queue_size=10)

Drag options to blanks, or click blank then click option'

AString

BTwist

CInt32

DPose

Attempts:

3 left

💡 Hint

Common Mistakes

Using a message type like String or Int32 instead of Twist.

Not specifying the correct message type for cmd_vel.

The cmd_vel topic expects messages of type Twist which contain linear and angular velocity commands.

2fill in blank

medium

Complete the code to set the linear x velocity to 0.5 m/s.

ROS

vel_msg = Twist()
vel_msg.linear.[1] = 0.5

Drag options to blanks, or click blank then click option'

Ax

By

Cz

Dangular

Attempts:

3 left

💡 Hint

Common Mistakes

Setting velocity on the y or z axis instead of x.

Confusing linear and angular velocity fields.

The linear velocity along the x-axis controls forward/backward movement.

3fill in blank

hard

Fix the error in publishing the velocity message.

ROS

pub.publish([1])

Drag options to blanks, or click blank then click option'

Avel_msg

Bvelocity

Ccmd_vel

DTwist

Attempts:

3 left

💡 Hint

Common Mistakes

Publishing the topic name string instead of the message object.

Publishing the message type class instead of an instance.

You must publish the message object, which is vel_msg in this case.

4fill in blank

hard

Fill both blanks to create a velocity message with angular z velocity 1.0 and linear x velocity 0.2.

ROS

vel_msg = Twist()
vel_msg.linear.[1] = 0.2
vel_msg.angular.[2] = 1.0

Drag options to blanks, or click blank then click option'

Ax

By

Cz

Dangular

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up x, y, and z components.

Using angular as a component instead of a field.

Linear velocity forward is on the x-axis, angular velocity rotation is around the z-axis.

5fill in blank

hard

Fill all three blanks to create a dictionary of velocity commands with keys 'linear_x', 'linear_y', and 'angular_z'.

ROS

vel_dict = {
  '[1]': vel_msg.linear.x,
  '[2]': vel_msg.linear.y,
  '[3]': vel_msg.angular.z
}

Drag options to blanks, or click blank then click option'

Alinear_x

Blinear_y

Cangular_z

Dlinear_z

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect keys that don't match velocity components.

Confusing linear and angular keys.

The dictionary keys should match the velocity components they represent.

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