Decentralised Compliant Control for Hexapod Robots: a Stick Insect Based Walking Model
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Decentralised Compliant Control for Hexapod Robots: A Stick Insect Based Walking Model Hugo Leonardo Rosano-Matchain I V N E R U S E I T H Y T O H F G E R D I N B U Doctor of Philosophy Institute of Perception, Action and Behaviour School of Informatics University of Edinburgh 2007 Abstract This thesis aims to transfer knowledge from insect biology into a hexapod walking robot. The similarity of the robot model to the biological target allows the testing of hypotheses regarding control and behavioural strategies in the insect. Therefore, this thesis supports biorobotic research by demonstrating that robotic implementations are improved by using biological strategies and these models can be used to understand biological systems. Specifically, this thesis addresses two central problems in hexapod walking control: the single leg control mechanism and its control variables; and the different roles of the front, middle and hind legs that allow a decentralised architecture to co-ordinate complex behavioural tasks. To investigate these problems, behavioural studies on insect curve walking were combined with quantitative simulations. Behavioural experiments were designed to explore the control of turns of freely walking stick insects, Carausius morosus, toward a visual target. A program for insect tracking and kinematic analysis of observed motion was developed. The re- sults demonstrate that the front legs are responsible for most of the body trajectory. Nonetheless, to replicate insect walking behaviour it is necessary for all legs to con- tribute with specific roles. Additionally, statistics on leg stepping show that middle and hind legs continuously influence each other. This cannot be explained by previous models that heavily depend on positive feedback controllers. After careful analysis, it was found that the hind legs could actively rotate the body while the middle legs move to the inside of the curve, tangentially to the body axis. The single leg controller is known to be independent from other legs but still capa- ble of mechanical synchronisation. To explain this behaviour positive feedback con- trollers have been proposed. This mechanism works for the closed kinematic chain problem, but has complications when implemented in a dynamic model. Furthermore, neurophysiological data indicate that legs always respond to disturbances as a negative feedback controller. Additional experimental data presented herein indicates that legs continuously oppose forces created by other legs. This thesis proposes a model that has a velocity positive feedback control modulated via a subordination variable in cascade with a position negative feedback mechanism as the core controller. This allows legs to oppose external and internal forces without compromising inter-leg collaboration for walking. The single leg controller is implemented using a distributed artificial neu- ral network. This network was trained with a wider range of movement to that so far found in the simulation model. The controller implemented with a plausible biological i model further increase the connection with the real insect. Further similarities with the stick insect in support of this controller are presented. The control hypotheses and behavioural results were incorporated into a 3D dy- namic robot simulation. The simulation can replicate the turns made by the stick insect more precisely than any previous model. Results demonstrate that the single leg con- troller can operate in a dynamic system by opposing external forces. Simultaneously, the controller can be integrated in a decentralised architecture and still co-operate with other leg controllers. The robot simulation was tested at various surface inclinations and with variations in weight and size. Evidence is presented that indicates the feasi- bility of implementing this model in a real robot. ii Acknowledgements First, I would like to thank my supervisor Dr. Barbara Webb for her experienced guidance, suggestions and patience during this research; she has invaluably improved this research. Additionally, I have to thank her for her help on non-academic matters as well, which helped me focus on my work. My research could not have been possible without the financial support of CONA- CyT M´exico, which continuously sponsored this thesis for almost four years. Many thanks to my second supervisor Mark Van Rosum, for our discussions at various stages of this thesis. To Russell Smith, primary author of ODE, I thank him for sharing and supporting the physics dynamic libraries used herein. Specials thanks must go to the Biological Cybernetics Institute at the University of Bielefeld. To Prof. Dr. Holk Cruse, Prof. Dr. Josef Schmitz, Dr. Volker D¨urr and all the PhD students that with their comments and suggestions, put this thesis on the right track. In addition, I would like to thank the Neural Control of Locomotion Group at the University of Cologne, to all the staff and students; particularly Andreas B¨uschges for his orientation in the early stages of this research. Thanks to the Edinburgh Butterfly and Insect World, for providing the first stick insects that would eventually father all the insects used for this thesis. Particularly, I would like to thank Kevin for sharing his expertise on stick insect care and breeding. I am grateful with those that help me with coding, proofreads and invaluable feed- back. Thanks to Adrian Haith, Mikel Kochenderfer, Tim Lukins, Michael Mangan, Mark Payne, Darren Smith, Matt Szenher, Matt Whitaker and Jan Wessnitzer. Many more should be included and I apologise for not having enough space. I would also like to thank all the people at the IPAB, the badminton squad, the IAR team and the ‘ipub’ members for making this a brilliant experience. I would like to thank my parents for providing me with the education, support and values that allowed me to pursue this degree. Thanks also to all my family and friends that were constantly monitoring my progress and encouraging me to do my best. Last, but by no means least, I owe immense gratitude to my wife, Fatme; more than I could ever express. She has always been there to help and support me in good and bad times. Specially, I have to thank her for her enormous patience during all the weekends and holidays I had to work. But most importantly, for all this time I have been without a proper job. iii Declaration I declare that this thesis was composed by myself, that the work contained herein is my own except where explicitly stated otherwise in the text, and that this work has not been submitted for any other degree or professional qualification except as specified. (Hugo Leonardo Rosano-Matchain) iv Table of Contents 1 Introduction 1 1.1 Legs vsWheels ............................. 1 1.2 UsingLegs ............................... 5 1.3 Insect Based Robots ........................... 6 1.4 Complications .............................. 8 1.5 Thesis Contribution ........................... 10 1.5.1 Organization .......................... 11 2 Biological Overview and Related Work 13 2.1 Overview ................................ 14 2.2 Morphology ............................... 15 2.2.1 Sensors ............................. 18 2.2.2 Summary ............................ 19 2.3 Behaviour ................................ 19 2.3.1 SingleLeg ........................... 20 2.3.2 Leg Coordination ........................ 22 2.3.3 Thoracic Differences ...................... 25 2.3.4 Turning ............................. 25 2.3.5 Summary ............................ 27 2.4 Neurophysiology ............................ 28 2.4.1 Relevance ............................ 31 2.5 Biorobotics ............................... 32 2.5.1 Cockroach-Based Robots .................... 33 2.5.2 Stick Insect Inspired Robots .................. 34 2.5.3 Walknet ............................. 36 2.5.4 Discussion on Biorobots .................... 38 2.6 Discussion ................................ 39 v 2.6.1 Spiking and non-spiking neurons ............... 39 2.6.2 Independent and compliant ................... 40 2.6.3 Solution? ............................ 40 2.7 Problem and Approach ......................... 41 3 Behavioural Experiments on Stick Insect Turning 43 3.1 Review of Turning ........................... 44 3.2 Parameters of Turning .......................... 45 3.3 Tracking Algorithm Overview ..................... 46 3.3.1 Camera Movement Compensation ............... 48 3.3.2 Cumulative plot of the IAR. .................. 50 3.3.3 Additional Tools ........................ 53 3.4 Experimental Setting .......................... 54 3.4.1 Methodology .......................... 55 3.5 Results for Intact Insect ......................... 58 3.6 Experiments With Front Tarsi Blocked ................. 61 3.7 Analysis and interpretation of Turning ................. 66 3.7.1 Body Trajectory Analysis ................... 68 3.7.2 Body Rotation Analysis .................... 71 3.8 Discussion ................................ 73 3.8.1 Single Leg Controller Implications ............... 74 3.8.2 Thoracic Differentiation .................... 75 3.8.3 Open questions ......................... 76 3.8.4 Conclusions ........................... 76 4 Subordinated Single Leg Controller for Walking 78 4.1 Swing Controller ............................ 79 4.1.1 Swing direction (AEP) ..................... 81 4.1.2 Heuristic Swing Controller ................... 81 4.2 Stance Controller ............................ 82 4.2.1 Single Leg Controller ...................... 83 4.2.2 Subordination Study: 2D Limb Simulation