Joystick control with joy package in ROS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the ROS message type for joystick data.

ROS

from sensor_msgs.msg import [1]

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

ATwist

BLaserScan

CJoy

DImage

Attempts:

3 left

💡 Hint

Common Mistakes

Importing the wrong message type like Twist or LaserScan.

Forgetting to import the Joy message.

The Joy message type is used to receive joystick data from the joy package.

2fill in blank

medium

Complete the code to subscribe to the joystick topic named '/joy'.

ROS

sub = rospy.Subscriber('/joy', [1], joy_callback)

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

ATwist

BJoy

CString

DPose

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong message type for the subscriber.

Incorrect topic name.

The subscriber must use the Joy message type to receive joystick data.

3fill in blank

hard

Fix the error in the callback function to access the first joystick axis value.

ROS

def joy_callback(msg):
    axis_value = msg.axes[[1]]
    rospy.loginfo(f'Axis 0 value: {axis_value}')

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

A0

B1

C-1

D2

Attempts:

3 left

💡 Hint

Common Mistakes

Using index 1 instead of 0.

Using a negative index or out-of-range index.

The first axis is at index 0 in the axes list.

4fill in blank

hard

Fill both blanks to publish a Twist message with linear x velocity from joystick axis 1.

ROS

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

cmd = [2]()
cmd.linear.x = msg.axes[1]

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

ATwist

BString

DJoy

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong message type for publisher or command.

Not creating an instance of the message class.

The publisher must use the Twist message type, and the command variable must be an instance of Twist.

5fill in blank

hard

Fill all three blanks to complete the ROS node that subscribes to '/joy', processes joystick input, and publishes velocity commands.

ROS

import rospy
from sensor_msgs.msg import [1]
from geometry_msgs.msg import [2]

rospy.init_node('joy_to_cmd_vel')
pub = rospy.Publisher('/cmd_vel', [3], queue_size=10)

def joy_callback(msg):
    cmd = Twist()
    cmd.linear.x = msg.axes[1]
    pub.publish(cmd)

rospy.Subscriber('/joy', Joy, joy_callback)
rospy.spin()

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

AJoy

BTwist

DPose

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up message types in imports or publisher.

Forgetting to create the publisher before subscribing.

The node imports Joy and Twist message types, publishes Twist messages, and subscribes to the /joy topic with Joy messages.

Practice

1. What is the main purpose of the joy package in ROS?

easy

A. To read joystick inputs and publish them as ROS messages

B. To control robot motors directly

C. To visualize sensor data in RViz

D. To simulate robot movements in Gazebo

Solution

1. Step 1: Understand the role of the joy package

   The joy package reads input from a joystick device and converts it into ROS messages.

2. Step 2: Identify what the package publishes

   It publishes joystick data on the /joy topic for other nodes to use.

3. Final Answer:

   To read joystick inputs and publish them as ROS messages -> Option A

4. Quick Check:

   joy package = joystick input publisher [OK]

Hint: Remember joy package publishes joystick data on /joy topic [OK]

Common Mistakes:

• Confusing joy package with motor control packages
• Thinking joy package visualizes data
• Assuming joy package simulates robots

2. Which ROS topic should you subscribe to in order to receive joystick data from the joy package?

easy

A. /scan

B. /cmd_vel

C. /odom

D. /joy

Solution

1. Step 1: Identify the topic published by joy package

   The joy package publishes joystick messages on the /joy topic.

2. Step 2: Match the topic to subscription

   To get joystick data, subscribe to /joy, not other topics like /cmd_vel or /odom.

3. Final Answer:

   /joy -> Option D

4. Quick Check:

   Subscribe to /joy for joystick data [OK]

Hint: Joystick data is always on /joy topic [OK]

Common Mistakes:

• Subscribing to /cmd_vel instead of /joy
• Confusing sensor topics like /scan with joystick
• Using /odom which is for odometry

3. Given the following Python callback for joystick data, what will be printed if the joystick's first axis value is 0.5 and the first button is pressed (value 1)?

def joy_callback(data):
    print(f"Axis 0: {data.axes[0]}")
    print(f"Button 0: {data.buttons[0]}")

medium

A. Axis 0: 1
Button 0: 0.5

B. Axis 0: 0.5
Button 0: 1

C. Axis 0: 0
Button 0: 0

D. Axis 0: 1
Button 0: 1

Solution

1. Step 1: Understand data.axes and data.buttons arrays

   The axes array holds float values for joystick axes; buttons holds integers for button states.

2. Step 2: Match given values to print statements

   Given axis 0 is 0.5 and button 0 is pressed (1), the print outputs match exactly those values.

3. Final Answer:

   Axis 0: 0.5 Button 0: 1 -> Option B

4. Quick Check:

   axes[0]=0.5, buttons[0]=1 [OK]

Hint: Axes are floats, buttons are integers in joystick messages [OK]

Common Mistakes:

• Mixing axes and buttons values
• Assuming buttons are floats
• Confusing index positions

4. What is wrong with this ROS Python subscriber code for joystick data?

import rospy
from sensor_msgs.msg import Joy

def callback(data):
    print(data.axes[0])

rospy.init_node('joy_listener')
rospy.Subscriber('/joy', Joy, callback)
rospy.spin()

medium

A. The subscriber topic name should be '/joy_data' not '/joy'

B. The callback function is missing the 'self' parameter

C. The code is correct and will print the first axis value

D. The message type should be Twist, not Joy

Solution

1. Step 1: Check topic and message type correctness

   The joy package publishes on '/joy' topic with message type Joy, which matches the subscriber.

2. Step 2: Verify callback function signature and usage

   The callback takes one argument (data) and prints the first axis value correctly.

3. Final Answer:

   The code is correct and will print the first axis value -> Option C

4. Quick Check:

   Subscriber to /joy with Joy message and callback is correct [OK]

Hint: Callback for subscriber needs one argument, no self unless in class [OK]

Common Mistakes:

• Using wrong topic name
• Adding 'self' in non-class callback
• Using wrong message type

5. You want to control a robot's forward speed using the joystick's vertical axis (axis 1). Which of these code snippets correctly converts the joystick input to a velocity command published on /cmd_vel topic using the joy package data?

hard

A. velocity.linear.x = data.axes[1]; pub.publish(velocity)

B. velocity.linear.x = data.buttons[1]; pub.publish(velocity)

C. velocity.angular.z = data.axes[1]; pub.publish(velocity)

D. velocity.linear.y = data.axes[0]; pub.publish(velocity)

Solution

1. Step 1: Identify joystick axis for forward speed

   Vertical axis is usually axis 1, which controls forward/backward movement.

2. Step 2: Assign axis value to linear.x velocity

   Forward speed is controlled by linear.x in Twist messages, so assign data.axes[1] to velocity.linear.x.

3. Final Answer:

   velocity.linear.x = data.axes[1]; pub.publish(velocity) -> Option A

4. Quick Check:

   Forward speed = linear.x = axes[1] [OK]

Hint: Forward speed maps to linear.x and vertical joystick axis [OK]

Common Mistakes:

• Using buttons instead of axes for speed
• Assigning to angular.z instead of linear.x
• Using wrong axis index