ROS Navigation stack architecture - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main purpose of the ROS Navigation stack?

The ROS Navigation stack helps a robot move safely from one place to another by planning paths, avoiding obstacles, and controlling its movement.

beginner

Name the core components of the ROS Navigation stack.

The core components are:

• Global Planner: plans a path from start to goal.
• Local Planner: creates short-term movement commands.
• Costmap: represents obstacles around the robot.
• Controller: sends velocity commands to the robot.

beginner

What is the role of the Costmap in the ROS Navigation stack?

The Costmap shows obstacles and free space around the robot. It helps planners avoid collisions by marking unsafe areas.

intermediate

How does the Global Planner differ from the Local Planner?

The Global Planner finds a complete path from the robot's current position to the goal using a map. The Local Planner focuses on immediate movements to follow that path safely, reacting to nearby obstacles.

beginner

Why is sensor data important in the ROS Navigation stack?

Sensor data updates the Costmap with real-time obstacle information. This helps the robot avoid new or moving obstacles while navigating.

Which component in the ROS Navigation stack plans the full path from start to goal?

A Global Planner

B Local Planner

C Costmap

D Controller

The Global Planner calculates the entire route from the robot's current position to the destination.

What does the Costmap represent?

A Robot's battery level

B Obstacles and free space around the robot

C Robot's speed

D Goal location

The Costmap shows where obstacles are so the robot can avoid them.

Which planner reacts to immediate obstacles and adjusts the robot's movement?

A Global Planner

B Map Server

C Local Planner

D Sensor Manager

The Local Planner creates short-term commands to safely follow the path and avoid obstacles.

Why is real-time sensor data important for navigation?

A To plan the global path

B To recharge the robot

C To control the robot's speed

D To update the Costmap with current obstacles

Sensors provide live information about obstacles, helping the robot avoid collisions.

What does the Controller component do in the ROS Navigation stack?

A Sends velocity commands to the robot

B Updates the Costmap

C Plans the global path

D Processes sensor data

The Controller sends commands to move the robot based on planner outputs.

Explain the roles of the Global Planner, Local Planner, and Costmap in the ROS Navigation stack.

Think about how the robot plans and moves safely.

Describe how sensor data interacts with the ROS Navigation stack to help a robot avoid obstacles.

Consider the flow from sensors to movement.

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