Robot Lab: Robot Path Planning

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Robot Lab:Robot Path Planning

William RegliDepartment of Computer Science

(and Departments of ECE and MEM)Drexel University

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Introduction to Motion Planning

• Applications• Overview of the Problem• Basics – Planning for Point Robot

– Visibility Graphs– Roadmap– Cell Decomposition– Potential Field

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Goals• Compute motion strategies, e.g.,

– Geometric paths – Time-parameterized trajectories– Sequence of sensor-based motion commands

• Achieve high-level goals, e.g.,– Go to the door and do not collide with

obstacles– Assemble/disassemble the engine– Build a map of the hallway– Find and track the target (an intruder, a

missing pet, etc.)

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Fundamental Question

Are two given points connected by a path?

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Basic Problem• Problem statement: Compute a collision-free path for a rigid or articulated

moving object among static obstacles.• Input

– Geometry of a moving object (a robot, a digital actor, or a molecule) and obstacles

– How does the robot move?– Kinematics of the robot (degrees of freedom)– Initial and goal robot configurations (positions &

orientations)

• Output Continuous sequence of collision-free robot

configurations connecting the initial and goal configurations

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Example: Rigid Objects

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Example: Articulated Robot

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Is it easy?

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Hardness Results

• Several variants of the path planning problem have been proven to be PSPACE-hard.

• A complete algorithm may take exponential time.– A complete algorithm finds a path if one exists and

reports no path exists otherwise.

• Examples– Planar linkages [Hopcroft et al., 1984]– Multiple rectangles [Hopcroft et al., 1984]

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Tool: Configuration Space

Difficulty– Number of degrees of freedom (dimension of

configuration space)– Geometric complexity

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Extensions of the Basic Problem

• More complex robots– Multiple robots– Movable objects– Nonholonomic & dynamic constraints– Physical models and deformable objects– Sensorless motions (exploiting task

mechanics)– Uncertainty in control

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Extensions of the Basic Problem• More complex environments

– Moving obstacles– Uncertainty in sensing

• More complex objectives– Optimal motion planning– Integration of planning and control– Assembly planning– Sensing the environment

• Model building• Target finding, tracking

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Practical Algorithms

• A complete motion planner always returns a solution when one exists and indicates that no such solution exists otherwise.

• Most motion planning problems are hard, meaning that complete planners take exponential time in the number of degrees of freedom, moving objects, etc.

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Practical Algorithms• Theoretical algorithms strive for completeness

and low worst-case complexity– Difficult to implement– Not robust

• Heuristic algorithms strive for efficiency in commonly encountered situations. – No performance guarantee

• Practical algorithms with performance guarantees– Weaker forms of completeness– Simplifying assumptions on the space: “exponential

time” algorithms that work in practice

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Problem Formulation for Point Robot• Input

– Robot represented as a point in the plane

– Obstacles represented as polygons

– Initial and goal positions

• Output– A collision-free

path between the initial and goal positions

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Framework

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Visibility Graph Method• Observation: If there is

a collision-free path between two points, then there is a polygonal path that bends only at the obstacles vertices.

• Why? – Any collision-free path

can be transformed into a polygonal path that bends only at the obstacle vertices.

• A polygonal path is a piecewise linear curve.

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Visibility Graph

• A visibility graphis a graph such that– Nodes: qinit, qgoal, or an obstacle vertex.– Edges: An edge exists between nodes u and v if

the line segment between u and v is an obstacle edge or it does not intersect the obstacles.

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A Simple Algorithm for Building Visibility Graphs

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Computational Efficiency

• Simple algorithm O(n3) time• More efficient algorithms

– Rotational sweep O(n2log n) time– Optimal algorithm O(n2) time– Output sensitive algorithms

• O(n2) space

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Framework

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Breadth-First Search

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Other Search Algorithms

• Depth-First Search

• Best-First Search, A*

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Framework

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Summary• Discretize the space by constructing

visibility graph

• Search the visibility graph with breadth-first search

Q: How to perform the intersection test?

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Summary

• Represent the connectivity of the configuration space in the visibility graph

• Running time O(n3)– Compute the visibility graph– Search the graph– An optimal O(n2) time algorithm exists.

• Space O(n2)

Can we do better?

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Classic Path Planning Approaches

• Roadmap – Represent the connectivity of the free space by a network of 1-D curves

• Cell decomposition – Decompose the free space into simple cells and represent the connectivity of the free space by the adjacency graph of these cells

• Potential field – Define a potential function over the free space that has a global minimum at the goal and follow the steepest descent of the potential function

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Roadmap• Visibility graph Shakey Project, SRI

[Nilsson, 1969]

• Voronoi Diagram Introduced by

computational geometry researchers. Generate paths that maximizes clearance. Applicable mostly to 2-D configuration spaces.

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Voronoi Diagram

• Space O(n)

• Run time O(n log n)

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Other Roadmap Methods

• Silhouette

First complete general method that applies to spaces of any dimensions and is singly exponential in the number of dimensions [Canny 1987]

• Probabilistic roadmaps

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Cell-decomposition Methods

• Exact cell decomposition

The free space F is represented by a collection of non-overlapping simple cells whose union is exactly F

• Examples of cells: trapezoids, triangles

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Trapezoidal Decomposition

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Computational Efficiency

• Running time O(n log n) by planar sweep

• Space O(n)

• Mostly for 2-D configuration spaces

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Adjacency Graph

• Nodes: cells• Edges: There is an edge between every pair of

nodes whose corresponding cells are adjacent.

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Summary

• Discretize the space by constructing an adjacency graph of the cells

• Search the adjacency graph

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Cell-decomposition Methods

• Exact cell decomposition• Approximate cell decomposition

– F is represented by a collection of non-overlapping cells whose union is contained in F.

– Cells usually have simple, regular shapes, e.g., rectangles, squares.

– Facilitate hierarchical space decomposition

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Quadtree Decomposition

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Octree Decomposition

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Algorithm Outline

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Potential Fields• Initially proposed for real-time collision avoidance

[Khatib 1986]. Hundreds of papers published.• A potential field is a scalar function over the free

space.• To navigate, the robot applies a force proportional

to the negated gradient of the potential field.• A navigation function is an ideal potential field that

– has global minimum at the goal– has no local minima– grows to infinity near obstacles– is smooth

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Attractive & Repulsive Fields

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How Does It Work?

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Algorithm Outline

• Place a regular grid G over the configuration space

• Compute the potential field over G

• Search G using a best-first algorithm with potential field as the heuristic function

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Local Minima

• What can we do?– Escape from local minima by taking

random walks– Build an ideal potential field –

navigation function – that does not have local minima

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Question

• Can such an ideal potential field be constructed efficiently in general?

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Completeness

• A complete motion planner always returns a solution when one exists and indicates that no such solution exists otherwise.– Is the visibility graph algorithm

complete? Yes.– How about the exact cell decomposition

algorithm and the potential field algorithm?

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Why Complete Motion Planning?

• Probabilistic roadmap motion planning– Efficient– Work for complex problems

with many DOF

– Difficult for narrow passages

– May not terminate when no path exists

• Complete motion planning– Always terminate

– Not efficient– Not robust even for

low DOF

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Path Non-existence Problem

ObstacleObstacle

GoalInitial

Robot

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Main Challenge

Obstacle

GoalInitial

Robot

• Exponential complexity: nDOF

– Degree of freedom: DOF– Geometric complexity: n

• More difficult than finding a path– To check all possible paths

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Approaches• Exact Motion Planning

– Based on exact representation of free space

• Approximation Cell Decomposition (ACD)

• A Hybrid planner

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Configuration Space: 2D Translation

Workspace Configuration Space

xyRobot

Start

Goal

Free

Obstacle C-obstacle

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Configuration Space Computation• [Varadhan et al, ICRA 2006]• 2 Translation + 1 Rotation• 215 seconds

Obstacle

Robot

x

y

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Exact Motion Planning• Approaches

– Exact cell decomposition [Schwartz et al. 83]– Roadmap [Canny 88]– Criticality based method [Latombe 99]– Voronoi Diagram– Star-shaped roadmap [Varadhan et al. 06]

• Not practical– Due to free space computation

– Limit for special and simple objects • Ladders, sphere, convex shapes• 3DOF

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• A Hybrid Planner Combing ACD and PRM

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Approximation Cell Decomposition (ACD)

• Not compute the free space exactly at once• But compute it incrementally

• Relatively easy to implement– [Lozano-Pérez 83]– [Zhu et al. 91]– [Latombe 91]– [Zhang et al. 06]

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full mixed

empty

Approximation Cell Decomposition

• Full cell

• Empty cell

• Mixed cell– Mixed– Uncertain

Configuration Space

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Connectivity GraphGf : Free Connectivity Graph G: Connectivity Graph

Gf is a subgraph of G

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Finding a Path by ACD

Goal

Initial Gf : Free Connectivity Graph

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Finding a Path by ACDL: Guiding Path• First Graph Cut Algorithm

– Guiding path in connectivity graph G

– Only subdivide along this path

– Update the graphs G and Gf

Described in Latombe’s book

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First Graph Cut Algorithm

Only subdivide along L

L

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Finding a Path by ACD

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ACD for Path Non-existence

C-space

Goal

Initial

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Connectivity Graph Guiding Path


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Connectivity graph is not connected

No path!

Sufficient condition for deciding path non-existence

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Two-gear Example

no path!

Cells in C-obstacle

Initial

Goal

Roadmap in F

Video 3.356s

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Cell Labeling

• Free Cell Query– Whether a cell

completely lies in free space?

• C-obstacle Cell Query– Whether a cell

completely lies in C-obstacle?

full mixed

empty

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Free Cell Query A Collision Detection Problem

•Does the cell lie inside free space?

• Do robot and obstacle separate at all configurations?

Obstacle

WorkspaceConfiguration space

?

Robot

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Clearance

• Separation distance– A well studied geometric problem

• Determine a volume in C-space which are completely free

d

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C-obstacle QueryAnother Collision Detection Problem

•Does the cell lie inside C-obstacle?

• Do robot and obstacle intersect at all configurations?

Obstacle

WorkspaceConfiguration space

?Robot

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‘Forbiddance’

• ‘Forbiddance’: dual to clearance• Penetration Depth

– A geometric computation problem less investigated

• [Zhang et al. ACM SPM 2006]

PD

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Limitation of ACD

• Combinatorial complexity of cell decomposition

• Limited for low DOF problem– 3-DOF robots

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• A Hybrid Planner Combing ACD and PRM

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Hybrid Planning• Probabilistic roadmap

motion planning+ Efficient+ Many DOFs

- Narrow passages- Path non-existence

• Complete Motion Planning+ Complete

- Not efficient

Can we combine them together?

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Hybrid Approach for Complete Motion Planning • Use Probabilistic Roadmap

(PRM): – Capture the connectivity for

mixed cells– Avoid substantial subdivision

• Use Approximation Cell Decomposition (ACD)– Completeness – Improve the sampling on

narrow passages

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Connectivity GraphGf : Free Connectivity Graph G: Connectivity Graph

Gf is a subgraph of G

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Pseudo-free edges

Pseudo free edge for two adjacent cells

Goal

Initial

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Pseudo-free Connectivity Graph:

Gsf

Goal

Initial

Gsf = Gf + Pseudo-edges

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Algorithm

• Gf

• Gsf

• G

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Results of Hybrid Planning

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Results of Hybrid Planning

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Results of Hybrid Planning• 2.5 - 10 times speedup

3 DOF 4 DOF 4 DOF

timing cells # timing cells # timing cells #

Hybrid 34s 50K 16s 48K 102s 164K

ACD 85s 168K ? ? ? ?

Speedup 2.5 3.3 ≥10 ? ≥10 ?

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Summary • Difficult for Exact Motion Planning

– Due to the difficulty of free space configuration computation

• ACD is more practical– Explore the free space incrementally

• Hybrid Planning– Combine the completeness of ACD and

efficiency of PRM

Documents

Robot Lab: Robot Path Planning