introduction To motion Planning J urek Czyzowicz UQO

1. The problem of the shortest path The classic Dijkstra algorithm solution to this problem The adaptation of this solution to the problem of robot motion planning Point robot Polygonal robot Other robot motion related problems introduction To motion PlanningJurek CzyzowiczUQO University of Liverpool, February 24th 2011

2. Algorithm Algorithm data Example of an algorithm: searching in a sequence Sorting Algorithm efficiency Algorithmics University of Liverpool, February 24th 2011

3. Compute the shortest mileage between Ottawa and San Francisco along North American roads Determine the earliest time of plane arrival to Liverpool departing from Ottawa after 4 p.m. today Determine if it is possible to reach a goal by a robot, moving in the plane among obstacles (starting from a given initial position) Find a path taking the smallest amount of time for a soldier traversing a terrain composed of regions of various maximal speeds Examples of the shortest path problems University of Liverpool, February 24th 2011

4. Modelising the shortestpathproblem Montréal • The « crossings » of the path are represented by the vertices of the graph • The « traversals » between the crossings are represented by the arcs of the graph • The distances are represented by the weights of the arcs 200 Ottawa 550 Boston 660 320 New York University of Liverpool, February 24th 2011

5. The Dijkstra Algorithm intuitively A 0 Dijkstra (G, w, s) Initialize (G,s) Reds ¬Æ Greens ¬ S While Greens ≠ Ædo X ¬ vertex from Greens with minimal d(X) Move vertex X from Greens to Reds Release all arcs outgoing from X 2 2 C A B 1 4 Initialize (G,s): d(A) ¬ O for each other vertex X: d(X) ¬ ∞ C 1 7 5 5 3 6 6 7 D Release (X,Y): if d(Y) < d(X) + weight (X,Y) then d(Y) ¬ d(X) + weight (X,Y) 8 E 5 4 B 2 D E F Animation of the Dijkstra algorithm F 7 9 University of Liverpool, February 24th 2011

6. Motion Planning for point robots A 25 I 1. Consider the obstacles’ sides 2. Determine the tangents to pairs of obstacles (visibility segments) 85 67 175 C B 3. Determine the tangents to obstacles and initial et final robot positions 145 D 93 128 83 4. Compute the lengths of all segments (arcs) from points 1 to 3 E 92 F 5. For the weighted graph obtained above (visibility graph) execute the Dijkstra algorithm having the robot initial position as the source vertex University of Liverpool, February 24th 2011

7. Motion planning for polygonal robot in the plane (moving by translation only) 1. Fixe a reference point of the robot 2. Determine the positions of the robot’s reference point while the robot is in contact with the obstacle 3. The curve traced by the reference point of the robot contouring the obstacle (staying in contact with it) forms an enlarged obstacle – the Minkowski sum of the robot and the obstacle 4. The movement of the polygonal robot among the obstacles is equivalent to the movement of its reference point de amidst the enlarged obstacles University of Liverpool, February 24th 2011

8. the motion planning algorithm for a polygonal robot moving by translation in the polygonal environment 1. Fix a reference point of the robot. 2. Enlarge each obstacle of the environment (i.e. obtain the Minkowski sum of the obstacle and the robot). 3. Solve the motion planning problem for the point robot moving among the enlarged obstacles. An animation of the motion planning of polygonal robot University of Liverpool, February 24th 2011

9. Other problems concerning robot motion • General movement of the robot (admitting possibility of rotation) • Motion in 3-dim • Environment evolution • Motion admitting bounded curvature • Motion admitting robot morphism • Exploration and searching • Visibility full or restraint • Searching for an intruder • Collaborative work • Other problems for mobile robots • Rendezvous • Pattern formation Cégep de University of Liverpool, February 24th 2011