ROS Navigation stack architecture - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize the ROS node for navigation.

ROS

ros::init(argc, argv, [1]);

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

A"sensor_listener"

B"navigation_node"

C"robot_driver"

D"move_base"

Attempts:

3 left

💡 Hint

Common Mistakes

Using unrelated node names like sensor_listener.

Confusing the driver node with the navigation node.

The navigation stack commonly uses the node name move_base to handle path planning and movement.

2fill in blank

medium

Complete the code to subscribe to the robot's odometry topic.

ROS

ros::Subscriber odom_sub = nh.subscribe([1], 1000, odomCallback);

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

A"/odom"

B"/map"

C"/scan"

D"/cmd_vel"

Attempts:

3 left

💡 Hint

Common Mistakes

Subscribing to the laser scan topic instead of odometry.

Using the velocity command topic which is for sending commands, not receiving data.

The odometry data is published on the /odom topic, which provides the robot's position and velocity.

3fill in blank

hard

Fix the error in the costmap configuration parameter name.

ROS

costmap_ros->setParameter("[1]_layer", true);

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

Aobstacle

Bobstacel

Cobstcale

Dobstacel_layer

Attempts:

3 left

💡 Hint

Common Mistakes

Misspelling 'obstacle' causes the parameter to be ignored.

Adding '_layer' twice in the parameter name.

The correct parameter name is obstacle_layer but here only the prefix obstacle is needed before _layer.

4fill in blank

hard

Fill both blanks to create a simple global planner plugin class declaration.

ROS

class [1] : public [2] { };

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

ASimpleGlobalPlanner

BBaseGlobalPlanner

CCostmap2DROS

DLocalPlanner

Attempts:

3 left

💡 Hint

Common Mistakes

Using LocalPlanner as a base class for global planner.

Confusing Costmap2DROS with planner base classes.

The global planner plugin class should inherit from BaseGlobalPlanner and can be named SimpleGlobalPlanner.

5fill in blank

hard

Fill all three blanks to complete the move_base action client initialization.

ROS

actionlib::SimpleActionClient<[1]> ac("[2]", true);

if(!ac.[3](ros::Duration(5.0))) {
  ROS_ERROR("Failed to connect to action server");
}

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

Amove_base_msgs::MoveBaseAction

Bmove_base

CwaitForServer

Dmove_base_msgs::MoveBaseGoal

Attempts:

3 left

💡 Hint

Common Mistakes

Using MoveBaseGoal instead of MoveBaseAction as template.

Wrong server name or method name causes connection failure.

The action client uses move_base_msgs::MoveBaseAction as the template, connects to the move_base action server, and calls waitForServer to wait for the server to be ready.

Practice

1. Which component in the ROS Navigation stack is responsible for localizing the robot on a known map?

easy

A. amcl

B. move_base

C. map_server

D. costmap_2d

Solution

1. Step 1: Understand the role of amcl

   amcl stands for Adaptive Monte Carlo Localization and it helps the robot find its position on a known map.

2. Step 2: Differentiate from other components

   map_server provides the map, move_base plans and executes paths, and costmap_2d manages obstacle data.

3. Final Answer:

   amcl -> Option A

4. Quick Check:

   Localization = amcl [OK]

Hint: Localization uses amcl in ROS Navigation stack [OK]

Common Mistakes:

• Confusing map_server as localization
• Thinking move_base localizes robot
• Mixing costmap_2d with localization

2. Which of the following is the correct way to launch the ROS Navigation stack using move_base?

easy

A. rosrun map_server map_server

B. rosrun move_base move_base

C. roslaunch amcl amcl.launch

D. roslaunch move_base move_base.launch

Solution

1. Step 1: Identify the launch command for move_base

   The Navigation stack uses launch files to start nodes with configurations. The correct launch file for move_base is move_base.launch.

2. Step 2: Differentiate from other commands

   rosrun runs single nodes without launch files. amcl.launch and map_server are for other components.

3. Final Answer:

   roslaunch move_base move_base.launch -> Option D

4. Quick Check:

   Launch move_base with roslaunch [OK]

Hint: Use roslaunch move_base move_base.launch to start navigation [OK]

Common Mistakes:

• Using rosrun instead of roslaunch for move_base
• Launching wrong component like amcl for navigation
• Missing launch file name in command

3. Given the following ROS Navigation stack setup, what will happen if the costmap_2d detects an obstacle in front of the robot?

1. Robot receives sensor data.
2. costmap_2d updates obstacle position.
3. move_base plans path.

medium

A. Robot ignores obstacle and moves forward.

B. Robot stops and replans path to avoid obstacle.

C. Robot shuts down navigation stack.

D. Robot moves backward without replanning.

Solution

1. Step 1: Understand costmap_2d role in obstacle detection

   costmap_2d creates a map of obstacles around the robot using sensor data.

2. Step 2: Analyze move_base behavior on obstacle detection

   move_base uses the costmap to plan safe paths. When an obstacle is detected, it replans to avoid collision.

3. Final Answer:

   Robot stops and replans path to avoid obstacle. -> Option B

4. Quick Check:

   Obstacle detected = path replanned [OK]

Hint: Obstacle detected triggers path replanning in move_base [OK]

Common Mistakes:

• Assuming robot ignores obstacles
• Thinking navigation stack shuts down on obstacles
• Believing robot moves backward automatically

4. You launched the ROS Navigation stack but the robot does not move even though the goal is set. Which of the following is the most likely cause?

medium

A. The robot's battery is fully charged.

B. map_server is publishing the map correctly.

C. amcl is not publishing the robot's pose.

D. move_base is receiving goals and planning paths.

Solution

1. Step 1: Check localization status

   If amcl is not publishing the robot's pose, move_base cannot know where the robot is to plan a path.

2. Step 2: Evaluate other components

   map_server publishing map and move_base receiving goals are good signs. Battery charged is unrelated to navigation stack software.

3. Final Answer:

   amcl is not publishing the robot's pose. -> Option C

4. Quick Check:

   Localization failure = no movement [OK]

Hint: No movement? Check if amcl publishes pose [OK]

Common Mistakes:

• Ignoring localization as cause
• Assuming battery status affects navigation stack
• Thinking map_server alone controls movement

5. You want your robot to navigate in a dynamic environment where obstacles can appear suddenly. Which combination of ROS Navigation stack components should you configure to ensure safe and smooth navigation?

hard

A. Use map_server for static map, amcl for localization, move_base with local_costmap for obstacle avoidance.

B. Use only map_server and move_base without costmaps.

C. Use amcl and disable move_base to avoid replanning.

D. Use costmap_2d only without localization or map.

Solution

1. Step 1: Identify components for dynamic obstacle handling

   local_costmap updates obstacles in real-time for move_base to avoid them.

2. Step 2: Combine with localization and static map

   amcl localizes robot on the static map from map_server. Together they enable safe navigation.

3. Step 3: Evaluate other options

   Options without costmaps or localization will fail in dynamic environments.

4. Final Answer:

   Use map_server, amcl, move_base with local_costmap -> Option A

5. Quick Check:

   Static map + localization + local costmap = safe navigation [OK]

Hint: Combine static map, localization, and local costmap for dynamic obstacles [OK]

Common Mistakes:

• Ignoring local costmap for dynamic obstacles
• Disabling move_base disables path planning
• Using costmap without localization or map