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published_or_final_version / Electrical and Electronic Engineering / Master / Master of Philosophy

by Wong Hon-chuen. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1999. / Includes bibliographical references (leaves 83-87). / Abstracts in English and Chinese. / Acknowledgements --- p.i / List of Figures --- p.v / List of Table --- p.viii / Abstract --- p.ix / Contents / Chapter 1 --- Introduction --- p.1 / Chapter 2 --- Algorithm Outline --- p.7 / Chapter 2.1 --- Assumptions --- p.7 / Chapter 2.2 --- Algorithm Outline --- p.8 / Chapter 3 --- Obstacle Detection --- p.11 / Chapter 3.1 --- Introduction --- p.11 / Chapter 3.2 --- Image Processing --- p.14 / Chapter 3.3 --- Coordinate Transformation --- p.14 / Chapter 3.4 --- Example --- p.20 / Chapter 4 --- Real-time Construction of Configuration Space --- p.22 / Chapter 4.1 --- Introduction --- p.22 / Chapter 4.2 --- Configuration Space --- p.23 / Chapter 4.3 --- Type-A Contact --- p.26 / Chapter 4.4 --- Type-B Contact --- p.27 / Chapter 4.5 --- Inverse Mapping Method --- p.29 / Chapter 4.6 --- Simulation --- p.31 / Chapter 5 --- Motion Planning and Re-Construction of C-space --- p.34 / Chapter 5.1 --- Introduction --- p.34 / Chapter 5.2 --- Path Planning --- p.36 / Chapter 5.3 --- Update of C-space --- p.41 / Chapter 5.4 --- Re-planning of Robot Path --- p.44 / Chapter 6 --- Implementation and Experiments --- p.55 / Chapter 6.1 --- Introduction --- p.55 / Chapter 6.2 --- Architecture of the Mobile Robot System --- p.55 / Chapter 6.3 --- Algorithm Implementation --- p.56 / Chapter 6.4 --- Experiment --- p.58 / Chapter 6.4.1 --- Experiment on a Fixed Unknown Environment --- p.58 / Chapter 6.4.2 --- Experiment on a Dynamic Unknown Environment --- p.70 / Chapter 7 --- Conclusions --- p.81 / References --- p.83

Additionally, we suggest a new formulation for the perspective camera projection matrix. In particular, regarding how the 3 x 3 rotation matrix, R, of the camera should be incorporated into the 3 x 4 camera projection matrix, P. We show that the incorporated rotation should neither be the camera rotation R nor its transpose, but a reversed (left-handed) version of it. The fundamental matrix between a pair of stereo cameras is reformulated more accurately accordingly. This is extremely useful when we want to calculate the fundamental matrix accurately from the stereo camera matrices. It is especially true when the feature correspondences are too few for robust methods, such as RANSAC, to operate. We expect that this new model would have an impact on various applications. / Furthermore, the process of estimating the rotation and translation parameters between a stereo pair from the essential matrix is investigated. This is an essential step for our multi-camera pose estimation method. We show that there are 16 solutions based on the singular value decomposition (not four or eight as previously thought). We also suggest a checking step to ascertain that the proposed algorithm will come up with accurate results. The checking step ensures the accuracy of the fundamental matrix calculated using the pose obtained. This provides a speedy way to calibrate a stereo rig. Our proposed theories are supported by the real and synthetic data experiments reported in this thesis. / In this thesis, we solve the pose estimation problem for robot motion by placing multiple cameras on the robot. In particular, we use four cameras arranged as two back-to-back stereo pairs combined with the Extended Kalman Filter (EKF). The EKF is used to provide a frame by frame recursive solution suitable for the real-time application at hand. The reason for using multiple cameras is that the pose estimation problem is more constrained for multiple cameras than for a single camera. Their use is further justified by the drop in price which is accompanied by the remarkable increase in accuracy. Back-to-back cameras are used since they are likely to have a larger field of view, provide more information, and capture more features. In this way, they are more likely to disambiguate the pose translation and rotation parameters. Stereo information is used in self-initialization and outlier rejection. Simple yet efficient methods have been proposed to tackle the problem of long-sequence drift. Our approaches have been compared, under different motion patterns, to other methods in the literature which use a single camera. Both the simulations and the real experiments show that our approaches are the most robust and accurate among them all as well as fast enough to realize the real-time requirement of robot navigation. / Mohammad Ehab Mohammad Ragab. / "April 2008." / Adviser: K. H. Wong. / Source: Dissertation Abstracts International, Volume: 70-03, Section: B, page: 1763. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2008. / Includes bibliographical references (p. 138-148) and index. / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Electronic reproduction. [Ann Arbor, MI] : ProQuest Information and Learning, [200-] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstracts in English and Chinese. / School code: 1307.

This thesis presents a model of legged locomotion in which position and velocity of body are directly controlled by positions and velocities of feet. One central relationship between foot acceleration, leg stroke and body velocity is developed. Procedures for determining all parameters of a step sequence including periods of constant body velocity (steady state) and constant linear acceleration of body (transient state) are presented. The following assumptions are used. Symmetrical trapezoidal velocity profiles are used for body and feet. Transient period is longer than or equal to one step time and a multiple of half step time. Step time and duty factor are constant during each locomotion stage. Stepping movements of a pair of legs are 180° out of phase and successive prints of one foot are symmetrically placed relative to the other foot. Starting and stopping occur with feet on a line perpendicular to the direction of body motion. Locomotion starts by lifting one foot and ends with one foot on the ground and the other being placed. When analyzing walking, designing a walking machine or designing a stepping sequence for an existing walking machine, it is important to understand constraints placed on body motion by motion of a single leg. Two dimensionless numbers which describe foot velocity profile are developed. Two additional dimensionless numbers result from constraint of leg workspace by foot acceleration and body velocity during steady state. These numbers provide useful relationships for design procedures. Defining a walking sequence requires transformation of objectives from global to body coordinates and continuously accounting for the relationship between these two systems. The technique described does this when body acceleration is non-zero as well as when body velocity is constant. Relationship between body and global coordinates is tracked for one leg pair using two diagrams: 1) position of feet relative to body versus time; 2) distances moved by feet and body in the global frame. A closed form inverse kinematic solution and an algorithm to find workspace for general three-revolute manipulator are presented. / Graduation date: 1991

Biped walking remains a difficult problem and robot models can greatly {facilitate} our understanding of the underlying biomechanical principles as well as their neuronal control. The goal of this study is to specifically demonstrate that stable biped walking can be achieved by combining the physical properties of the walking robot with a small, reflex-based neuronal network, which is governed mainly by local sensor signals. This study shows that human-like gaits emerge without {specific} position or trajectory control and that the walker is able to compensate small disturbances through its own dynamical properties. The reflexive controller used here has the following characteristics, which are different from earlier approaches: (1) Control is mainly local. Hence, it uses only two signals (AEA=Anterior Extreme Angle and GC=Ground Contact) which operate at the inter-joint level. All other signals operate only at single joints. (2) Neither position control nor trajectory tracking control is used. Instead, the approximate nature of the local reflexes on each joint allows the robot mechanics itself (e.g., its passive dynamics) to contribute substantially to the overall gait trajectory computation. (3) The motor control scheme used in the local reflexes of our robot is more straightforward and has more biological plausibility than that of other robots, because the outputs of the motorneurons in our reflexive controller are directly driving the motors of the joints, rather than working as references for position or velocity control. As a consequence, the neural controller and the robot mechanics are closely coupled as a neuro-mechanical system and this study emphasises that dynamically stable biped walking gaits emerge from the coupling between neural computation and physical computation. This is demonstrated by different walking experiments using two real robot as well as by a Poincar\' map analysis applied on a model of the robot in order to assess its stability. In addition, this neuronal control structure allows the use of a policy gradient reinforcement learning algorithm to tune the parameters of the neurons in real-time, during walking. This way the robot can reach a record-breaking walking speed of 3.5 leg-lengths per second after only a few minutes of online learning, which is even comparable to the fastest relative speed of human walking.

published_or_final_version / Electrical and Electronic Engineering / Master / Master of Philosophy