ROS node concept - Mini Project: Build & Apply

ROS Node Concept

📖 Scenario: You are building a simple ROS (Robot Operating System) node that will publish a greeting message to a topic. This is a common starting point for learning how ROS nodes communicate.

🎯 Goal: Create a ROS node named greeting_publisher that publishes the message "Hello, ROS!" to the topic /greetings once every second.

📋 What You'll Learn

Create a ROS node named greeting_publisher

Publish a string message "Hello, ROS!" to the topic /greetings

Set the publishing rate to 1 Hz (once per second)

Use the std_msgs/String message type

💡 Why This Matters

🌍 Real World

ROS nodes are the building blocks of robot software. Publishing messages lets different parts of a robot communicate, like sensors sending data or commands controlling motors.

💼 Career

Understanding how to create and manage ROS nodes is essential for robotics software engineers and developers working with autonomous systems.

1

Create the ROS node and publisher

Write code to initialize a ROS node named greeting_publisher and create a publisher that publishes std_msgs.msg.String messages to the topic /greetings.

ROS

# Initialize the ROS node and create the publisher
# Your code here

Hint

Use rclpy.init() to start ROS, then create a Node with the name greeting_publisher. Use create_publisher on the node to publish String messages to /greetings.

2

Set the publishing rate

Create a variable called rate that sets the publishing frequency to 1 Hz using node.create_rate(1).

ROS

import rclpy
from rclpy.node import Node
from std_msgs.msg import String

rclpy.init()
node = Node('greeting_publisher')
publisher = node.create_publisher(String, '/greetings', 10)

# Create a rate object to publish at 1 Hz
# Your code here

Hint

Use node.create_rate(1) to create a rate object that controls the loop frequency at 1 Hz.

3

Publish the greeting message in a loop

Write a while rclpy.ok() loop that creates a String message with data "Hello, ROS!", publishes it using publisher.publish(), and calls rate.sleep() to maintain the 1 Hz rate.

ROS

import rclpy
from rclpy.node import Node
from std_msgs.msg import String

rclpy.init()
node = Node('greeting_publisher')
publisher = node.create_publisher(String, '/greetings', 10)
rate = node.create_rate(1)

# Publish the message in a loop
# Your code here

Hint

Use a while rclpy.ok() loop to keep publishing. Create a String message, set its data to 'Hello, ROS!', publish it, then sleep to keep the rate.

4

Shutdown the ROS node cleanly

After the loop, call node.destroy_node() and rclpy.shutdown() to cleanly close the ROS node.

ROS

import rclpy
from rclpy.node import Node
from std_msgs.msg import String

rclpy.init()
node = Node('greeting_publisher')
publisher = node.create_publisher(String, '/greetings', 10)
rate = node.create_rate(1)

while rclpy.ok():
    msg = String()
    msg.data = 'Hello, ROS!'
    publisher.publish(msg)
    rate.sleep()

# Cleanly shutdown the node
# Your code here

Hint

After the publishing loop ends, call node.destroy_node() to clean up the node, then rclpy.shutdown() to stop ROS.

Practice

1. What is a ROS node in simple terms?

easy

A. A type of sensor used in robots

B. A hardware part of the robot

C. A small program that runs in the robot system

D. A communication cable between robots

Solution

1. Step 1: Understand the role of a ROS node

   A ROS node is a software program that runs to perform tasks in a robot system.

2. Step 2: Differentiate from hardware and other parts

   It is not hardware or cables but a small program that communicates with others.

3. Final Answer:

   A small program that runs in the robot system -> Option C

4. Quick Check:

   ROS node = small program [OK]

Hint: Remember: node = small program in ROS [OK]

Common Mistakes:

• Thinking a node is hardware
• Confusing nodes with sensors
• Believing nodes are cables

2. Which of the following is the correct way to initialize a ROS node in Python?

easy

A. rospy.init_node('node_name')

B. ros.init('node_name')

C. node.start('node_name')

D. init.node('node_name')

Solution

1. Step 1: Recall ROS Python syntax for node initialization

   The correct function to start a node is rospy.init_node with the node name as argument.

2. Step 2: Check other options for correctness

   Other options use incorrect function names or syntax not used in ROS Python.

3. Final Answer:

   rospy.init_node('node_name') -> Option A

4. Quick Check:

   Initialize node with rospy.init_node() [OK]

Hint: Use rospy.init_node('name') to start a node [OK]

Common Mistakes:

• Using ros.init instead of rospy.init_node
• Calling node.start which doesn't exist
• Wrong function order or names

3. Consider this Python ROS node snippet:

import rospy
rospy.init_node('talker')
rospy.loginfo('Node started')

What will this code do when run?

medium

A. Cause a syntax error due to missing parentheses

B. Start a node named 'talker' and print 'Node started' info message

C. Start a node but no message will be printed

D. Start a node named 'listener' and print 'Node started'

Solution

1. Step 1: Analyze node initialization

   The code calls rospy.init_node with 'talker', so it starts a node named 'talker'.

2. Step 2: Check logging output

   rospy.loginfo prints the message 'Node started' to the console or log.

3. Final Answer:

   Start a node named 'talker' and print 'Node started' info message -> Option B

4. Quick Check:

   Node 'talker' starts and logs info [OK]

Hint: rospy.loginfo prints messages after node starts [OK]

Common Mistakes:

• Confusing node name with 'listener'
• Thinking no message prints
• Assuming syntax error without checking code

4. This ROS Python node code has an error:

import rospy
rospy.init_node('listener')
rospy.spin

What is the error?

medium

A. rospy.init_node should be called after rospy.spin

B. Wrong node name 'listener'

C. Missing import of std_msgs

D. Missing parentheses after rospy.spin

Solution

1. Step 1: Check usage of rospy.spin

   rospy.spin is a function and must be called with parentheses to keep the node alive.

2. Step 2: Verify other parts

   Node name 'listener' is valid, std_msgs import is not required here, and init_node must be called before spin.

3. Final Answer:

   Missing parentheses after rospy.spin -> Option D

4. Quick Check:

   Call rospy.spin() with parentheses [OK]

Hint: Always call rospy.spin() with parentheses [OK]

Common Mistakes:

• Forgetting parentheses on rospy.spin
• Thinking node name causes error
• Misordering init_node and spin

5. You want to create two ROS nodes that communicate by sending messages. Which is the best way to organize your code?

hard

A. Write two separate node programs, each with its own init_node call

B. Write one program with two init_node calls for both nodes

C. Use one node and switch its name dynamically

D. Create one node and run it twice without changes

Solution

1. Step 1: Understand ROS node design

   Each node is a separate program with its own init_node call to run independently.

2. Step 2: Evaluate other options

   One program cannot have two init_node calls; switching names dynamically or running one node twice without changes won't create two communicating nodes properly.

3. Final Answer:

   Write two separate node programs, each with its own init_node call -> Option A

4. Quick Check:

   Separate programs = separate nodes [OK]

Hint: One node per program with init_node call [OK]

Common Mistakes:

• Trying to call init_node twice in one program
• Reusing one node for two roles
• Not separating node logic into programs