Collision avoidance in planning in ROS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the ROS package for collision avoidance planning.

ROS

import [1]

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Ageometry_msgs

Bmoveit_commander

Csensor_msgs

Drospy

Attempts:

3 left

💡 Hint

Common Mistakes

Importing rospy instead of moveit_commander

Using sensor_msgs which is for sensors

The moveit_commander package provides interfaces for motion planning including collision avoidance.

2fill in blank

medium

Complete the code to initialize the MoveGroupCommander for the robot arm.

ROS

group = moveit_commander.[1]('arm')

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APlanningSceneInterface

BTrajectoryExecutionManager

CRobotCommander

DMoveGroupCommander

Attempts:

3 left

💡 Hint

Common Mistakes

Using PlanningSceneInterface which manages the environment

Using RobotCommander which represents the robot model

MoveGroupCommander is used to control a group of joints for planning and execution.

3fill in blank

hard

Fix the error in the code to set the target pose for collision-free planning.

ROS

pose_target = geometry_msgs.msg.Pose()
pose_target.position.x = 0.4
pose_target.position.y = 0.1
pose_target.position.z = 0.3
pose_target.orientation.w = [1]
group.set_pose_target(pose_target)

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A0.0

B0.5

C1.0

D-1.0

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0.0 which means no rotation is invalid

Using negative values which are not normalized

The orientation quaternion's w component should be 1.0 for no rotation.

4fill in blank

hard

Fill both blanks to add a collision object and attach it to the robot.

ROS

collision_object = moveit_msgs.msg.CollisionObject()
collision_object.id = ['box']
collision_object.header.frame_id = group.get_planning_frame()
box = shape_msgs.msg.SolidPrimitive()
box.type = shape_msgs.msg.SolidPrimitive.[1]
box.dimensions = [0.1, 0.1, 0.1]
collision_object.primitives = [box]
collision_object.[2] = [geometry_msgs.msg.Pose()]
planning_scene = moveit_commander.PlanningSceneInterface()
planning_scene.add_collision_object(collision_object)
group.attach_object(collision_object.id[0])

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ABOX

BCYLINDER

Cprimitive_poses

Dmesh_poses

Attempts:

3 left

💡 Hint

Common Mistakes

Using CYLINDER instead of BOX

Using mesh_poses which is for mesh shapes

The primitive type for a box is BOX. The poses of the primitives are set in the primitive_poses attribute.

5fill in blank

hard

Fill all three blanks to plan and execute a collision-free path.

ROS

plan = group.[1]()
if len(plan.[2]) > 0:
    group.[3](plan)

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Aplan

Bjoint_trajectory.points

Cexecute

Dcompute_cartesian_path

Attempts:

3 left

💡 Hint

Common Mistakes

Using compute_cartesian_path which returns a tuple

Checking plan length instead of trajectory points

plan() generates a plan. Checking len(joint_trajectory.points) ensures the plan has points. execute() runs the plan.

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