Nav2 in ROS 2 overview - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize the Navigation2 stack node in ROS 2.

ROS

auto nav2_node = std::make_shared<[1]>();

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

Anav2::Planner

Bnav2::Controller

Cnav2::Navigator

Dnav2::LifecycleNode

Attempts:

3 left

💡 Hint

Common Mistakes

Using a regular node instead of a LifecycleNode.

Confusing controller or planner classes with the main node.

The Navigation2 stack uses a LifecycleNode to manage its states and transitions.

2fill in blank

medium

Complete the code to send a navigation goal using the Nav2 action client.

ROS

auto goal_msg = nav2_msgs::action::NavigateToPose::Goal();
goal_msg.pose = target_pose;
action_client->[1](goal_msg);

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

Asend_goal_async

BsendGoal

Csend_goal

DsendGoalAsync

Attempts:

3 left

💡 Hint

Common Mistakes

Using synchronous send_goal which does not exist.

Incorrect casing or method name.

The Nav2 action client uses send_goal_async to send goals asynchronously.

3fill in blank

hard

Fix the error in the lifecycle transition call to activate the Nav2 node.

ROS

nav2_node->[1](nav2_lifecycle_msgs::msg::Transition::TRANSITION_ACTIVATE);

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

Atrigger_transition

Bset_state

Cactivate

Dchange_state

Attempts:

3 left

💡 Hint

Common Mistakes

Using non-existent methods like activate or set_state.

Confusing lifecycle transition methods.

The correct method to trigger lifecycle transitions is trigger_transition.

4fill in blank

hard

Fill both blanks to create a Behavior Tree node that checks if the robot has reached its goal.

ROS

class IsGoalReached : public BT::[1]Node {
public:
  IsGoalReached(const std::string & name, const BT::NodeConfiguration & config) : BT::[2]Node(name, config) {}
};

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

ACondition

BAction

CDecorator

DControl

Attempts:

3 left

💡 Hint

Common Mistakes

Using ActionNode instead of ConditionNode.

Mixing different node types in inheritance.

Behavior Tree condition nodes check states like goal reached, so both blanks use Condition.

5fill in blank

hard

Fill all three blanks to define a ROS 2 parameter for the Nav2 planner plugin name with a default value.

ROS

rclcpp::NodeOptions options;
options.[1]("planner_plugin", "nav2_navfn_planner/NavfnPlanner");
auto planner_node = std::make_shared<nav2::Planner>(options);
planner_node->declare_parameter("planner_plugin", [2]);
std::string plugin_name = planner_node->get_parameter("planner_plugin").[3]();

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

Adeclare_parameter

Bget_parameter

Cas_string

Dset_parameter

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing declare_parameter and set_parameter methods.

Forgetting to convert parameter to string with as_string().

Use set_parameter on options to set default, declare_parameter on node, and as_string() to get the parameter value as string.

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