Nav2 in ROS 2 overview - Mini Project: Build & Apply

Nav2 in ROS 2 Overview

📖 Scenario: You are working on a robot that needs to move around a room safely and efficiently. To do this, you will use the Navigation 2 (Nav2) framework in ROS 2, which helps robots plan paths and avoid obstacles.

🎯 Goal: Build a simple ROS 2 Nav2 setup that includes the main components: a map, a robot pose, a goal, and the Nav2 lifecycle nodes to enable navigation.

📋 What You'll Learn

Create a ROS 2 node that holds the initial map data

Set a configuration variable for the robot's starting position

Implement the Nav2 lifecycle node to manage navigation states

Add the final launch configuration to start the Nav2 stack

💡 Why This Matters

🌍 Real World

Robots use Nav2 in ROS 2 to move safely in homes, warehouses, and factories by planning paths and avoiding obstacles.

💼 Career

Understanding Nav2 lifecycle and configuration is essential for robotics engineers working on autonomous navigation systems.

1

Create the initial map data

Create a Python dictionary called map_data with these exact entries: 'resolution': 0.05, 'width': 100, 'height': 100, and 'origin': [0.0, 0.0, 0.0] to represent the map setup.

ROS

# Your code here

Hint

Think of the map as a grid. The resolution is how big each square is, width and height are the number of squares, and origin is where the map starts.

2

Set the robot's starting position

Create a variable called start_pose and set it to a dictionary with keys 'x', 'y', and 'theta' having values 1.0, 1.0, and 0.0 respectively to represent the robot's initial pose.

ROS

map_data = {
    'resolution': 0.05,
    'width': 100,
    'height': 100,
    'origin': [0.0, 0.0, 0.0]
}
# Your code here

Hint

The robot's pose includes its position (x, y) and orientation (theta in radians).

3

Implement the Nav2 lifecycle node

Write a Python class called Nav2LifecycleNode with an __init__ method that sets a state attribute to 'unconfigured'. Then add a method called configure() that changes state to 'inactive'.

ROS

map_data = {
    'resolution': 0.05,
    'width': 100,
    'height': 100,
    'origin': [0.0, 0.0, 0.0]
}

start_pose = {
    'x': 1.0,
    'y': 1.0,
    'theta': 0.0
}

# Your code here

Hint

Lifecycle nodes have states like 'unconfigured' and 'inactive'. The configure method moves the node to 'inactive'.

4

Add the final launch configuration

Create a dictionary called launch_config with keys 'map', 'start_pose', and 'nav2_node'. Set 'map' to map_data, 'start_pose' to start_pose, and 'nav2_node' to an instance of Nav2LifecycleNode.

ROS

map_data = {
    'resolution': 0.05,
    'width': 100,
    'height': 100,
    'origin': [0.0, 0.0, 0.0]
}

start_pose = {
    'x': 1.0,
    'y': 1.0,
    'theta': 0.0
}

class Nav2LifecycleNode:
    def __init__(self):
        self.state = 'unconfigured'

    def configure(self):
        self.state = 'inactive'

# Your code here

Hint

The launch configuration bundles all parts to start the Nav2 system together.

Practice

1. What is the primary purpose of Nav2 in ROS 2?

easy

A. To process sensor data for object recognition

B. To control robot arm movements

C. To manage robot battery levels

D. To help robots plan paths and navigate safely

Solution

1. Step 1: Understand Nav2's role in ROS 2

   Nav2 is designed to help robots move safely by planning paths and avoiding obstacles.

2. Step 2: Compare options with Nav2's function

   Only To help robots plan paths and navigate safely describes path planning and safe navigation, which matches Nav2's purpose.

3. Final Answer:

   To help robots plan paths and navigate safely -> Option D

4. Quick Check:

   Nav2 = path planning and navigation [OK]

Hint: Nav2 is about robot movement and path planning [OK]

Common Mistakes:

• Confusing Nav2 with sensor processing
• Thinking Nav2 controls robot arms
• Assuming Nav2 manages hardware like batteries

2. Which of the following is the correct way to send a navigation goal to Nav2 in ROS 2?

easy

A. ros2 service call /start_navigation std_srvs/Empty '{}'

B. ros2 topic pub /cmd_vel geometry_msgs/Twist '{linear: {x: 1.0}}'

C. ros2 action send_goal /navigate_to_pose nav2_msgs/action/NavigateToPose '{pose: {position: {x: 1.0, y: 2.0}}}'

D. ros2 param set /nav2_controller speed 1.0

Solution

1. Step 1: Identify the command to send a navigation goal

   The correct command uses ros2 action send_goal with the NavigateToPose action and a pose message.

2. Step 2: Check other options for correctness

   ros2 topic pub /cmd_vel geometry_msgs/Twist '{linear: {x: 1.0}}' publishes velocity commands, not goals. ros2 service call /start_navigation std_srvs/Empty '{}' calls a service unrelated to sending goals. ros2 param set /nav2_controller speed 1.0 sets parameters, not goals.

3. Final Answer:

   ros2 action send_goal /navigate_to_pose nav2_msgs/action/NavigateToPose '{pose: {position: {x: 1.0, y: 2.0}}}' -> Option C

4. Quick Check:

   Send goal = ros2 action send_goal [OK]

Hint: Use ros2 action send_goal for Nav2 goals [OK]

Common Mistakes:

• Using topic publish instead of action send_goal
• Calling unrelated services
• Setting parameters instead of sending goals

3. Given the following Nav2 action client code snippet in ROS 2, what will be the output if the robot successfully reaches the goal?

client.send_goal(goal_msg)
result = client.get_result()
print(result.status)

medium

A. An integer status code indicating success

B. A string message describing the error

C. A boolean True if goal reached, False otherwise

D. No output, the code will raise an exception

Solution

1. Step 1: Understand Nav2 action client result

   The get_result() method returns a result object with a status code as an integer.

2. Step 2: Interpret the printed status

   Printing result.status outputs an integer code representing success or failure, not a string or boolean.

3. Final Answer:

   An integer status code indicating success -> Option A

4. Quick Check:

   result.status = integer code [OK]

Hint: Nav2 action results use integer status codes [OK]

Common Mistakes:

• Expecting a string message instead of code
• Thinking result.status is boolean
• Assuming code raises exception on success

4. You wrote a Nav2 client node but it never moves the robot after sending a goal. Which of the following is the most likely cause?

medium

A. The robot's wheels are physically blocked

B. The Nav2 stack is not running or not properly launched

C. The robot's battery is low

D. The goal message has an incorrect frame_id

Solution

1. Step 1: Check common Nav2 goal issues

   If the Nav2 stack is not running or not properly launched, the robot will not move because the navigation system is inactive.

2. Step 2: Evaluate other options

   While an incorrect frame_id can cause issues, the most common cause when the robot never moves after sending a goal is that the Nav2 stack is not running. Hardware issues like blocked wheels or low battery are less likely in software debugging context.

3. Final Answer:

   The Nav2 stack is not running or not properly launched -> Option B

4. Quick Check:

   Nav2 stack running = robot moves [OK]

Hint: Ensure Nav2 stack is running if robot doesn't move [OK]

Common Mistakes:

• Assuming Nav2 stack is always running
• Ignoring coordinate frame mismatches
• Blaming hardware before software checks

5. You want your robot to navigate through a dynamic environment using Nav2 in ROS 2. Which combination of Nav2 components should you configure to ensure the robot replans paths when obstacles move?

hard

A. Use the planner_server with a global planner and enable the controller_server with obstacle avoidance

B. Use only the behavior_tree without planners or controllers

C. Disable the controller_server and rely on manual velocity commands

D. Use the map_server alone to update the robot's position

Solution

1. Step 1: Identify components for path planning and obstacle avoidance

   The planner_server creates global paths, and the controller_server handles local obstacle avoidance and path following.

2. Step 2: Understand dynamic replanning needs

   Enabling both planner and controller servers allows Nav2 to replan paths when obstacles move, ensuring safe navigation.

3. Final Answer:

   Use the planner_server with a global planner and enable the controller_server with obstacle avoidance -> Option A

4. Quick Check:

   Planner + Controller = dynamic replanning [OK]

Hint: Planner and controller servers enable dynamic path replanning [OK]

Common Mistakes:

• Ignoring controller_server role in obstacle avoidance
• Thinking behavior_tree alone handles planning
• Relying only on map_server for navigation