Local planner (DWA, TEB) in ROS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main purpose of a local planner in ROS navigation?

A local planner generates short-term velocity commands to safely guide the robot around obstacles and towards the goal within the robot's immediate surroundings.

beginner

What does DWA stand for in the context of ROS local planners?

DWA stands for Dynamic Window Approach, a local planner that samples possible velocities and selects the best one based on robot dynamics and obstacle avoidance.

intermediate

How does the TEB local planner differ from DWA in ROS?

TEB (Timed Elastic Band) optimizes the robot's trajectory as a sequence of poses over time, considering kinematic constraints and dynamic obstacles, while DWA samples velocities without explicit trajectory optimization.

intermediate

Which local planner is better for dynamic environments with moving obstacles: DWA or TEB?

TEB is generally better for dynamic environments because it optimizes the trajectory over time and can better handle moving obstacles and complex maneuvers.

beginner

What inputs do local planners like DWA and TEB require to compute velocity commands?

They require the robot's current pose, the global plan or goal, the local costmap showing obstacles, and the robot's velocity and kinematic constraints.

What does the DWA local planner primarily sample to decide the next move?

A Possible velocity commands

B Global paths

C Sensor data

D Map landmarks

DWA samples possible velocity commands within the robot's dynamic limits to select the best safe velocity.

Which local planner uses trajectory optimization over time?

A DWA

B TEB

C A*

D RRT

TEB optimizes a timed sequence of poses, refining the trajectory considering time and kinematics.

In ROS, what is the local costmap used for by local planners?

A To plan long-distance routes

B To store the global map

C To detect obstacles near the robot

D To control robot speed

The local costmap shows obstacles around the robot to help local planners avoid collisions.

Which local planner is more suitable for robots with complex kinematic constraints?

A TEB

B DWA

C Global planner

D None

TEB explicitly considers kinematic constraints during trajectory optimization.

What is a key advantage of using TEB over DWA?

A Faster computation

B Works without a map

C Simpler implementation

D Better handling of dynamic obstacles and smoother trajectories

TEB produces smoother, time-optimized trajectories and adapts better to moving obstacles.

Explain how the Dynamic Window Approach (DWA) local planner works in ROS.

Think about how the planner tests different speeds and picks the safest and most efficient one.

Describe the main differences between the DWA and TEB local planners and when you might choose each.

Compare how each planner plans movement and their strengths.

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