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FLEXIBLE OBJECT MANIPULATION a Thesis FLEXIBLE OBJECT MANIPULATION A Thesis Submitted to the Faculty in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Computer Science by Matthew Bell DARTMOUTH COLLEGE Hanover, New Hampshire February 2010 Dartmouth Computer Science Technical Report TR2010-663 Examining Committee: (chair) Devin Balkcom Scot Drysdale Tanzeem Choudhury Daniela Rus Brian W. Pogue, Ph.D. Dean of Graduate Studies Abstract Flexible objects are a challenge to manipulate. Their motions are hard to predict, and the high number of degrees of freedom makes sensing, control, and planning difficult. Additionally, they have more complex friction and contact issues than rigid bodies, and they may stretch and compress. In this thesis, I explore two major types of flexible materials: cloth and string. For rigid bodies, one of the most basic problems in manipulation is the development of immo- bilizing grasps. The same problem exists for flexible objects. I have shown that a simple polygonal piece of cloth can be fully immobilized by grasping all convex vertices and no more than one third of the concave vertices. I also explored simple manipulation methods that make use of gravity to reduce the number of fingers necessary for grasping. I have built a system for folding a T-shirt using a 4 DOF arm and a fixed-length iron bar which simulates two fingers. The main goal with string manipulation has been to tie knots without the use of any sensing. I have developed single-piece fixtures capable of tying knots in fishing line, solder, and wire, along with a more complex track-based system for autonomously tying a knot in steel wire. I have also developed a series of different fixtures that use compressed air to tie knots in string. Additionally, I have designed four-piece fixtures, which demonstrate a way to fully enclose a knot during the insertion process, while guaranteeing that extraction will always succeed. i ii Contents 1 Introduction 1 1.1 Related Work . 2 1.1.1 Rigid Body Grasping . 3 1.1.2 Caging . 3 1.1.3 Deformable Object Manipulation . 3 1.1.4 Cloth Simulation . 4 1.1.5 Cloth Manipulation . 5 1.1.6 Cloth Immobilization . 6 1.1.7 Cloth Sensing . 6 1.1.8 Fixturing . 7 1.1.9 String Models . 7 1.1.10 Knot Tying . 8 1.1.11 Snake Robots and Needle Steering . 9 1.1.12 Mold Parting Directions . 9 2 Cloth Grasping 11 2.1 Cloth Models and Definitions . 12 2.1.1 Support Graphs . 12 2.2 Immobilizing Trees, Graphs, and Polygons . 13 2.2.1 Immobilizing Non-stretchable Graphs . 13 2.2.2 Grasping Polygons . 15 2.3 When Convex Vertices Are Not Enough . 18 2.3.1 Determining Free Motions . 18 2.3.2 Pinwheels . 20 2.4 Grasping with Fewer Fingers . 22 2.4.1 Grasp Reduction . 22 2.4.2 Graphical Method for Analyzing Immobilization . 24 2.5 Conclusion . 26 3 Cloth Folding 29 3.1 Automating the two-finger, three-point method . 30 3.2 Behavior of Cloth in a Vector Field . 32 3.3 Folding With Two Fingers and Gravity . 35 3.4 Enumerating Valid Two Finger Grasps . 36 3.4.1 Edge-Edge . 37 3.4.2 Vertex-Edge . 37 3.4.3 Vertex-Vertex . 37 iii 4 Knot Tying 39 4.1 Overview of the knot tying process . 40 4.2 Construction . 40 4.3 Insertion . 42 4.3.1 Pushing vs. pulling . 42 4.3.2 Pushing wire: Track/slider . 43 4.3.3 Pushing string . 44 4.4 Extraction . 45 4.4.1 Slit . 45 4.4.2 Slit with cuts through tube . 46 4.4.3 Rubber-covered slit . 46 4.4.4 Retractable rollers . 47 4.4.5 Two-piece approach (removable fixture wall) . 47 4.4.6 Four-piece fixtures . 51 4.4.7 Scalability . 54 4.5 Case studies . 55 4.5.1 Single-piece fixtures . 55 4.5.2 Two-piece fixtures . 58 4.5.3 Four-piece fixtures . 58 4.5.4 Single-chamber air fixtures . 59 4.5.5 Double-chamber air fixtures . 60 4.5.6 Tubing with air accelerators . 61 4.5.7 Modular, enclosed, air-powered tubing . 62 4.5.8 Track-based fixtures . 64 4.6 Verification and Experiments . 66 4.7 Curvature and friction in knot tubes . 66 5 Lessons Learned 71 5.1 Knot Tying . 71 5.1.1 Insertion/extraction conflict . 71 5.1.2 Different fixture designs for string and wire . 72 5.1.3 Compressed air . 72 5.1.4 Minimal sensing systems can be effective . 72 5.1.5 Lack of accurate simulation drives simple designs . 72 5.1.6 String thickness can be a major challenge in extraction . 73 5.1.7 Precise tightening . 73 5.1.8 Tying around objects . 73 5.2 Cloth Folding . 74 5.2.1 Folding is easy, flattening is hard . 74 5.2.2 Not many fingers are needed for successful folding . 74 5.2.3 Components necessary for a laundry folding robot . 75 5.3 Cloth Grasping and Immobilization . 75 6 Future Work 77 6.1 Exact number of fingers necessary for immobilization . 77 6.2 Gravity-assisted manipulation . 77 6.3 Cloth sensing . 78 6.4 Cloth flattening . 79 iv 6.5 Reliable grasping . 79 6.6 Modifying cloth . 79 6.7 Simulation of string in fixtures . 80 6.8 New separable fixtures . 80 6.9 Adding more actuation to knot fixtures . 80 6.10 The future . 81 Acknowledgments 82 Bibliography 83 v vi List of Figures 1.1 Shirt folded with robot arm . 1 1.2 Polygon requiring extra fingers . 1 1.3 Track-based fixture for an overhand knot . 2 2.1 Three flat cloth shapes grasped by fingers. All but b) are immobilized. 11 2.2 Star grasped with three fingers. 12 2.3 Polygon that cannot be immobilized by pinning convex vertices (closed circles). 12 2.4 Example of a support graph in a cloth polygon. 13 2.5 Allowed motion of a non-positively-spanned vertex. 13 2.6 Restriction on allowed motions of u............................ 14 2.7 Base case (vertex v is pinned, as indicated by the closed circle). 14 2.8 Inductive step. 14 2.9 Two types of.
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