Local planner (DWA, TEB) in ROS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

Understanding DWA Local Planner Velocity Commands

In ROS, the DWA local planner computes velocity commands for the robot. Given a robot moving towards a goal, which option best describes the velocity command output by the DWA planner when an obstacle is detected directly in front?

A The planner outputs zero angular velocity and maximum linear velocity to push through the obstacle.

B The planner increases linear velocity to quickly pass the obstacle.

C The planner outputs a zero linear velocity and a positive angular velocity to rotate the robot away from the obstacle.

D The planner stops all motion commands immediately without rotation.

💡 Hint

Think about how the robot avoids obstacles by turning rather than pushing forward.

The DWA planner generates velocity commands that steer the robot safely. When an obstacle is detected ahead, it reduces forward speed and commands a turn to avoid collision.

📝 Syntax

intermediate

Correct ROS Parameter for TEB Planner Configuration

Which of the following ROS parameter YAML snippets correctly sets the TEB local planner's optimization weight for obstacle avoidance to 5.0?

A obstacle_avoidance_weight: 5.0

B obstacle_avoidance_weight = 5.0

C obstacle_avoidance_weight: "5.0"

D obstacle_avoidance_weight: 'five'

💡 Hint

YAML uses colon and space to assign values, and numeric values should not be quoted.

In ROS YAML configuration, parameters are set using key: value pairs. Numeric values should be unquoted for correct parsing.

❓ state_output

advanced

TEB Planner Behavior with Dynamic Obstacles

When the TEB local planner is running and a dynamic obstacle suddenly appears in the robot's path, what is the expected behavior of the planner's trajectory output?

A The planner stops the robot instantly without any new trajectory.

B The planner immediately replans a new trajectory that avoids the dynamic obstacle while maintaining smoothness.

C The planner ignores dynamic obstacles and continues on the original path.

D The planner reverses the robot along the previous path.

💡 Hint

Consider how TEB optimizes trajectories in real-time for moving obstacles.

The TEB planner continuously optimizes the robot's trajectory considering dynamic obstacles, ensuring smooth and safe navigation.

🔧 Debug

advanced

Diagnosing DWA Planner Stuck Behavior

A robot using the DWA local planner gets stuck and does not move even though the goal is reachable. Which of the following is the most likely cause?

A The robot's maximum velocity parameters are set to zero, preventing movement.

B The global planner is not publishing a path.

C The robot's laser scanner is turned off.

D The robot's battery is fully charged.

💡 Hint

Check the velocity limits in the planner configuration.

If maximum velocities are zero, the DWA planner cannot generate movement commands, causing the robot to stay still.

🧠 Conceptual

expert

Comparing DWA and TEB Local Planners

Which statement best describes a key difference between the DWA and TEB local planners in ROS navigation?

A TEB cannot handle dynamic obstacles, but DWA can.

B DWA uses graph optimization for trajectories, while TEB samples velocity commands randomly.

C Both planners use the exact same algorithm but differ only in parameter names.

D DWA samples velocity commands and evaluates them, while TEB optimizes the entire trajectory using a graph-based approach.

💡 Hint

Think about how each planner generates and refines robot paths.

DWA samples possible velocity commands and scores them to pick the best, while TEB formulates trajectory planning as an optimization problem solved with graph-based methods.

Practice

1. What is the main purpose of a local planner like DWA or TEB in ROS?

easy

A. To control the robot's battery usage

B. To map the entire environment for the robot

C. To help the robot navigate safely around obstacles nearby

D. To communicate with other robots

Solution

1. Step 1: Understand the role of local planners

   Local planners focus on immediate surroundings to avoid obstacles and plan short paths.

2. Step 2: Compare options

   Mapping the environment is done by global planners or mapping nodes, not local planners.

3. Final Answer:

   To help the robot navigate safely around obstacles nearby -> Option C

4. Quick Check:

   Local planner = safe nearby navigation [OK]

Hint: Local planners handle nearby obstacles, not mapping or communication [OK]

Common Mistakes:

• Confusing local planner with global planner
• Thinking local planner maps environment
• Assuming local planner manages robot hardware

2. Which of the following is the correct way to enable the DWA local planner in a ROS navigation stack configuration file?

easy

A. base_local_planner: "dwa_local_planner/DWAPlannerROS"

B. base_local_planner: "teb_local_planner/TEBPlanner"

C. planner: "global_planner/GlobalPlanner"

D. planner: "dwa_local_planner/DWAPlannerROS"

Solution

1. Step 1: Identify the correct parameter name for local planner

   The parameter to specify the local planner plugin is usually 'base_local_planner'.

2. Step 2: Match the correct plugin name for DWA

   DWA local planner plugin is 'dwa_local_planner/DWAPlannerROS'. base_local_planner: "dwa_local_planner/DWAPlannerROS" matches this exactly.

3. Final Answer:

   base_local_planner: "dwa_local_planner/DWAPlannerROS" -> Option A

4. Quick Check:

   Correct plugin name and parameter = base_local_planner: "dwa_local_planner/DWAPlannerROS" [OK]

Hint: Look for 'base_local_planner' key with DWA plugin name [OK]

Common Mistakes:

• Using 'planner' instead of 'base_local_planner'
• Confusing DWA and TEB plugin names
• Using global planner plugin name for local planner

3. Given the following ROS parameter snippet for TEB local planner:

teb_local_planner:
  max_vel_x: 0.5
  max_vel_theta: 1.0
  min_turning_radius: 0.2

What will happen if the robot tries to turn with an angular velocity of 1.5 rad/s?

medium

A. The planner will allow the turn at 1.5 rad/s

B. The planner will limit the angular velocity to 1.0 rad/s

C. The planner will stop the robot immediately

D. The planner will ignore the max_vel_theta parameter

Solution

1. Step 1: Understand max_vel_theta parameter

   This parameter sets the maximum allowed angular velocity for the robot.

2. Step 2: Compare requested angular velocity with max_vel_theta

   The requested 1.5 rad/s exceeds the max of 1.0 rad/s, so the planner limits it to 1.0 rad/s.

3. Final Answer:

   The planner will limit the angular velocity to 1.0 rad/s -> Option B

4. Quick Check:

   Angular velocity capped by max_vel_theta = The planner will limit the angular velocity to 1.0 rad/s [OK]

Hint: Planner caps velocities at max limits set in parameters [OK]

Common Mistakes:

• Assuming planner allows velocities above max
• Thinking planner stops robot on limit breach
• Ignoring max_vel_theta parameter

4. You configured the DWA local planner but the robot keeps colliding with obstacles. Which of the following is the most likely cause?

medium

A. The obstacle distance parameter is set too low, causing late avoidance

B. The global planner is disabled

C. The robot's battery is low

D. The robot's wheels are not calibrated

Solution

1. Step 1: Identify parameters affecting obstacle avoidance

   DWA uses parameters like obstacle distance to decide when to start avoiding obstacles.

2. Step 2: Analyze effect of low obstacle distance

   If set too low, the planner reacts too late, causing collisions.

3. Final Answer:

   The obstacle distance parameter is set too low, causing late avoidance -> Option A

4. Quick Check:

   Late avoidance due to low obstacle distance = The obstacle distance parameter is set too low, causing late avoidance [OK]

Hint: Check obstacle distance parameter if collisions occur [OK]

Common Mistakes:

• Blaming global planner for local planner issues
• Ignoring parameter tuning for obstacle avoidance
• Assuming hardware issues cause collisions without evidence

5. You want your robot to navigate a narrow corridor smoothly using the TEB local planner. Which combination of parameters should you adjust to improve smooth turning and obstacle clearance?

hard

A. Increase min_turning_radius and decrease obstacle_avoidance_force

B. Increase max_vel_x and decrease max_vel_theta

C. Decrease max_vel_x and disable obstacle avoidance

D. Decrease min_turning_radius and increase obstacle_avoidance_force

Solution

1. Step 1: Understand min_turning_radius effect

   Decreasing min_turning_radius allows tighter turns, useful in narrow corridors.

2. Step 2: Understand obstacle_avoidance_force effect

   Increasing obstacle_avoidance_force makes the planner keep safer distance from obstacles, improving clearance.

3. Step 3: Combine effects for smooth navigation

   Tight turns plus stronger obstacle avoidance helps smooth, safe navigation in tight spaces.

4. Final Answer:

   Decrease min_turning_radius and increase obstacle_avoidance_force -> Option D

5. Quick Check:

   Tight turns + strong avoidance = Decrease min_turning_radius and increase obstacle_avoidance_force [OK]

Hint: Tighten turning radius and boost avoidance force for narrow spaces [OK]

Common Mistakes:

• Increasing turning radius makes turns wider, not tighter
• Disabling obstacle avoidance causes collisions
• Confusing velocity limits with turning parameters