Computational Geometry Piyush Kumar (Lecture 10: Robot Motion Planning) Welcome to CIS5930
Reading Chapter 13 in Textbook David Mount’s Lecture notes. Slides Sources Lecture notes from Dr. Bayazit Dr. Spletzer, Dr. Latombe. Pictures from the web and David Mount’s notes.
Robots Real WorldFiction
Robot Motion Planning Free space Obstacles
Robot Motion Planning Work Space : Environment in which robot operates Obstacles : Already occupied spaces of the world. Free Space : Unoccupied space of the world.
Configuration Space OR C-space Helps in determining where a robot can go. Modelling a robot Configuration: Values which specify the position of a robot Geometric shape description
Motion Planning Given a robot, find a sequence of valid configurations that moves the robot from the source to destination. start goalobstacles
Configuration Space Configuration: Specification of the robot position relative to a fixed coordinate system. Usually a set of values expressed as a vector of positions/orientations. Configuration Space: is the space of all possible robot configurations.
Configuration Space Example reference point x y robot reference direction workspace – 3-parameter representation: q = (x,y, ) – In 3D? 6-parameters - (x,y,z, )
Configuration Space X Y A robot which can translate in the plane X Y A robot which can translate and rotate in the plane x Y C- space: 2-D (x, y) 3-D (x, y, ) Euclidean space: Courtesy J.Xiao
C-Space q1q1q1q1 q2q2q2q2 q = (q 1,q 2,…,q 10 )
Configuration Space /Obstacles Circular Robot
C-Obstacles Convex polygonal robot
Minkowski Sum A B = { a+b | a A, b B }
Minkowski Sums 3D Minkowski sum difficult to compute Many Applications Configuration Space Computation Offset Morphing Packing and Layout Friction model
Configuration Obstacle Only for robots in 2d that can translate. CP = { p | R(p) ∩ P ≠ Null }
C-Obstacles Lemma : CP = P (-R) Lemma : CP = P (-R) Proof: Show that R(q) intersects P iff q є P (-R). q є P (-R) iff there exists p є P and (-r) є (-R) such that q = p – r R(q) intersects P iff there exists r є R and p є P such that r+q = p. Equivalent
An illustration
Computing Minkowski sum For a given convex polygonal obstacle (with n vertices) and a convex footprint robot (with m vertices), how fast can we compute the CP? O(m + n) Idea: Walk.
Complexity of Minkowski Sums? Can we bound the complexity of the minkowski sum of disjoint convex obstacles with n vertices in the plane? Naïve bound? Triangulate the obstacles : O(n) edges. Minkowski sum of R with triangles = O(nm) Complexity of the union? O((nm) 2 )?
Pseudodisks: Defn. A set of convex objects {o1,o2,…,on} is called a collection of pseudodisks if for any two distinct objects oi and oj both of the set theoretic differences oi\oj and oj\oi are connected.
Lemma 1 Given a set of convex objects {T 1, T 2,…, T n,} with disjoint interiors and convex R, the set {T i R | i = 1..n } is a collection of psedodisks. Proof: On the chalkboard
Lemma 2 Given a collection of pseudodisks with n vertices, the complexity of their union is O(n). Why?
Planning approaches in C-space Roadmap Approach: Visibility Graph Methods Cell Decomposition Approach Potential Fields Many other Algorithms…
Visibility Graph in C-space start goal Each path in c-space from s to t represents a viable move from s to t of the Robot in the original space.
Visibility Graph in C-space start goal Each path in c-space from s to t represents a viable move from s to t of the Robot in the original space. Computation time?
Vis Graph in higher dimensions? Will it work?
Cell Decomp: Trapezoidal Decomp. GOAL START ) Decompose Region Into Cells
Cell Decomp: Trapezoidal Decomp. 1) Decompose Region Into Cells GOAL START ) Construct Adjacency Graph
Cell Decomp: Trapezoidal Decomp. GOAL START
Cell Decomposition: Other approaches UniformQuadtree
Potential field approach The field is modeled by a potential function U(x,y) over C Motion policy control law is akin to gradient descent on the potential function
Next Class Final Review: Q&A session.