Collision avoidance in planning in ROS - Step-by-Step Execution

Concept Flow - Collision avoidance in planning

Start Planning Request

↓

Generate Candidate Path

↓

Check for Collisions

↓

Accept Path

↓

Execute Path

↓

End

The planner generates a path, checks for collisions, and either accepts it or modifies it to avoid obstacles before execution.

Execution Sample

ROS

path = planner.plan(start, goal)
if planner.check_collision(path):
    path = planner.modify_path(path)
execute(path)

This code plans a path, checks for collisions, modifies the path if needed, then executes it.

Execution Table

Step	Action	Path State	Collision Check	Result
1	Generate initial path	Path from start to goal	Not checked yet	Path ready
2	Check collision on path	Path from start to goal	Collision detected at obstacle region	Collision found
3	Modify path to avoid collision	Adjusted path around obstacle	Not checked yet	Path modified
4	Check collision on modified path	Adjusted path around obstacle	No collision detected	Path safe
5	Execute path	Safe path	No collision	Execution started

💡 Execution stops after safe path is found and executed

Variable Tracker

Variable	Start	After Step 1	After Step 3	After Step 4	Final
path	None	Initial path planned	Modified path avoiding collision	Modified path confirmed safe	Path executed

Key Moments - 3 Insights

Why do we check for collisions after planning the path?

Because the initial path might intersect obstacles; checking ensures safety before execution as shown in step 2 of the execution table.

What happens if a collision is detected?

The path is modified to avoid obstacles, as seen in step 3, ensuring the robot does not collide during execution.

Why do we check collisions again after modifying the path?

To confirm the new path is safe and free of collisions before executing, as shown in step 4.

Visual Quiz - 3 Questions

Look at the execution table, what is the state of 'path' after step 3?

A Modified path avoiding collision

B Initial path from start to goal

C Path execution started

D Collision detected on path

💡 Hint

Refer to the 'Path State' column in row for step 3

At which step does the planner confirm the path is safe to execute?

A Step 2

B Step 4

C Step 3

D Step 5

💡 Hint

Check the 'Result' column for collision check results

If no collision was detected at step 2, what would happen next?

A Modify the path

B Replan the path

C Execute the path immediately

D Stop execution

💡 Hint

Look at the flow from collision check 'No' branch in concept_flow

Concept Snapshot

Collision avoidance in planning:
- Plan a path from start to goal
- Check path for collisions
- If collision detected, modify path
- Re-check modified path
- Execute safe path
Ensures robot moves safely avoiding obstacles.

Full Transcript

Collision avoidance in planning means the robot plans a path, checks if it hits anything, changes the path if needed, checks again, then moves. The process starts by generating a path. Then the system checks for collisions. If it finds any, it changes the path to go around obstacles. It checks the new path again to be sure it is safe. Finally, it executes the safe path. This keeps the robot from crashing into things.

Practice

1. What is the main purpose of collision avoidance in ROS planning?

easy

A. To make the robot ignore obstacles and move freely

B. To prevent the robot from hitting obstacles while moving

C. To speed up the robot's movement regardless of obstacles

D. To stop the robot from moving at all

Solution

1. Step 1: Understand collision avoidance goal

   Collision avoidance means the robot should not bump into objects while moving.

2. Step 2: Identify correct purpose

   The main goal is to keep the robot safe by avoiding obstacles during planning.

3. Final Answer:

   To prevent the robot from hitting obstacles while moving -> Option B

4. Quick Check:

   Collision avoidance = prevent hitting obstacles [OK]

Hint: Collision avoidance means safety, not speed or stopping [OK]

Common Mistakes:

• Thinking collision avoidance speeds up movement
• Believing robot ignores obstacles
• Assuming robot should stop moving

2. Which ROS component is used to add obstacles for collision avoidance during planning?

easy

A. Planning Scene

B. Move Group Interface

C. Robot State Publisher

D. Joint State Listener

Solution

1. Step 1: Identify obstacle representation in ROS planning

   Obstacles are added to the planning scene to inform the planner about their presence.

2. Step 2: Match component to function

   The Planning Scene holds information about obstacles and environment for collision checking.

3. Final Answer:

   Planning Scene -> Option A

4. Quick Check:

   Obstacles added via Planning Scene [OK]

Hint: Obstacles go into the Planning Scene, not robot state [OK]

Common Mistakes:

• Confusing Move Group Interface with obstacle storage
• Thinking Robot State Publisher manages obstacles
• Assuming Joint State Listener adds obstacles

3. Given this ROS planning code snippet, what will happen if the planner finds no valid path?

move_group.setStartStateToCurrentState()
move_group.setPoseTarget(target_pose)
plan = move_group.plan()
if plan:
    move_group.execute(plan)
else:
    print("No valid path found")

medium

A. The robot moves to the target pose anyway

B. The robot moves to a random position

C. The robot prints "No valid path found" and does not move

D. The planner crashes with an error

Solution

1. Step 1: Analyze the plan check condition

   The code checks if plan is valid (truthy). If not, it prints a message.

2. Step 2: Understand behavior when no path found

   If no valid path, the else block runs, printing "No valid path found" and skipping execution.

3. Final Answer:

   The robot prints "No valid path found" and does not move -> Option C

4. Quick Check:

   No path means print message, no movement [OK]

Hint: Check plan validity before execute to avoid errors [OK]

Common Mistakes:

• Assuming robot moves without a valid plan
• Expecting a crash instead of safe handling
• Thinking robot moves randomly

4. Identify the error in this ROS planning snippet that prevents collision avoidance:

planning_scene = PlanningSceneInterface()
obstacle = CollisionObject()
obstacle.id = "box"
obstacle.header.frame_id = "world"
# Missing shape and pose assignment here
planning_scene.addCollisionObjects([obstacle])

medium

A. The obstacle has no shape or pose defined

B. The planning scene is not initialized

C. The obstacle ID is missing

D. The frame_id should be "base_link"

Solution

1. Step 1: Check obstacle definition

   The obstacle lacks shape and pose, so it has no physical presence.

2. Step 2: Understand impact on collision avoidance

   Without shape and pose, the planner cannot detect or avoid the obstacle.

3. Final Answer:

   The obstacle has no shape or pose defined -> Option A

4. Quick Check:

   Obstacle needs shape and pose to exist [OK]

Hint: Always define shape and pose for obstacles [OK]

Common Mistakes:

• Ignoring shape and pose assignment
• Thinking frame_id must be base_link always
• Assuming missing ID causes failure

5. You want to ensure your robot avoids a newly added obstacle during planning. Which sequence correctly updates the planning scene and checks for a valid path before moving?

hard

A. Only call plan() without adding obstacle

B. Call execute() first, then add obstacle, then plan()

C. Add obstacle, execute plan without checking validity

D. Add obstacle to Planning Scene, call plan(), check if plan valid, then execute plan

Solution

1. Step 1: Add obstacle to Planning Scene

   Adding the obstacle updates the environment for collision checking.

2. Step 2: Plan and check validity before execution

   Call plan() to find a path avoiding obstacles, then verify plan is valid before executing.

3. Step 3: Avoid unsafe execution

   Executing without a valid plan or before adding obstacles risks collisions or errors.

4. Final Answer:

   Add obstacle to Planning Scene, call plan(), check if plan valid, then execute plan -> Option D

5. Quick Check:

   Update scene, plan, validate, then execute [OK]

Hint: Always add obstacles before planning and check plan validity [OK]

Common Mistakes:

• Executing before planning
• Not adding obstacles before planning
• Skipping plan validity check