Innovative robot hand designs of reduced complexity for dexterous manipulation

This thesis investigates the mechanical design of robot hands to sensibly reduce the system complexity in terms of the number of actuators and sensors, and control needs for performing grasping and in-hand manipulations of unknown objects. Human hands are known to be the most complex, versatile, dexterous manipulators in nature, from being able to operate sophisticated surgery to carry out a wide variety of daily activity tasks (e.g. preparing food, changing cloths, playing instruments, to name some). However, the understanding of why human hands can perform such fascinating tasks still eludes complete comprehension. Since at least the end of the sixteenth century, scientists and engineers have tried to match the sensory and motor functions of the human hand. As a result, many contemporary humanoid and anthropomorphic robot hands have been developed to closely replicate the appearance and dexterity of human hands, in many cases using sophisticated designs that integrate multiple sensors and actuators---which make them prone to error and difficult to operate and control, particularly under uncertainty. In recent years, several simplification approaches and solutions have been proposed to develop more effective and reliable dexterous robot hands. These techniques, which have been based on using underactuated mechanical designs, kinematic synergies, or compliant materials, to name some, have opened up new ways to integrate hardware enhancements to facilitate grasping and dexterous manipulation control and improve reliability and robustness. Following this line of thought, this thesis studies four robot hand hardware aspects for enhancing grasping and manipulation, with a particular focus on dexterous in-hand manipulation. Namely: i) the use of passive soft fingertips; ii) the use of rigid and soft active surfaces in robot fingers; iii) the use of robot hand topologies to create particular in-hand manipulation trajectories; and iv) the decoupling of grasping and in-hand manipulation by introducing a reconfigurable palm. In summary, the findings from this thesis provide important notions for understanding the significance of mechanical and hardware elements in the performance and control of human manipulation. These findings show great potential in developing robust, easily programmable, and economically viable robot hands capable of performing dexterous manipulations under uncertainty, while exhibiting a valuable subset of functions of the human hand.Open Acces

사람 근골격 특성을 반영한 로봇 손가락 설계

학위논문(박사) -- 서울대학교대학원 : 융합과학기술대학원 융합과학부(지능형융합시스템전공), 2023. 2. 박재흥.What the manipulator can perform is determined by what the end-effectors, including the robotic hand, can do because it is the gateway that directly interacts with the surrounding environment or objects. In order for robots to have human-level task performance in a human-centered environment, the robotic hand with human-hand-level capabilities is essential. Here, the human-hand-level capabilities include not only force-speed, and dexterity, but also size and weight. However, to our knowledge, no robotic hand exists that simultaneously realizes the weight, size, force, and dexterity of the human hand and continues to remain a challenge. In this thesis, to improve the performance of the robotic hand, the modular robotic finger design with three novel mechanisms based on the musculoskeletal characteristics of the human hand was proposed. First, the tendon-driven robotic finger with intrinsic/extrinsic actuator arrangement like the muscle arrangement of the human hand was proposed and analyzed. The robotic finger consists of five different tendons and ligaments. By analyzing the fingertip speed while a human is performing various object grasping motions, the actuators of the robotic finger were separated into intrinsic actuators responsible for slow motion and an extrinsic actuator that performs the motions requiring both large force and high speed. Second, elastomeric continuously variable transmission (ElaCVT), a new concept relating to continuously variable transmission (CVT), was designed to improve the performance of the electric motors remaining weight and size and applied as an extrinsic actuator of the robotic finger. The primary purpose of ElaCVT is to expand the operating region of a twisted string actuator (TSA) and duplicate the force-velocity curve of the muscles by passively changing the reduction ratio according to the external load applied to the end of the TSA. A combination of ElaCVT and TSA (ElaCVT-TSA) is proposed as a linear actuator. With ElaCVT-TSA, an expansion of the operating region of electric motors to the operating region of the muscles was experimentally demonstrated. Finally, as the flexion/extension joints of the robotic finger, anthropomorphic rolling contact joint, which mimicked the structures of the human finger joint like tongue-and-groove, and collateral ligaments, was proposed. As compliant joints not only compensate for the lack of actuated degrees of freedom of an under-actuated system and improve grasp stability but also prevent system failure from unexpected contacts, various types of compliant joints have been applied to end-effectors. Although joint compliance increases the success rate of power grasping, when the finger wraps around large objects, it can reduce the grasping success rate in pinch gripping when dealing with small objects using the fingertips. To overcome this drawback, anthropomorphic rolling contact joint is designed to passively adjust the torsional stiffness according to the joint angle without additional weight and space. With the anthropomorphic rolling contact joint, the stability of pinch grasping improved.엔드이팩터는 로봇과 주변 환경이 상호작용하는 통로로 매니퓰레이터가 수행할 수 있는 작업은 엔드이펙터의 성능에 제한된다. 사람 중심의 환경에 로봇이 적용되어 사람 수준의 다양한 작업을 수행하기 위해서는 사람 손 수준의 성능을 갖는 로봇 손이 필수적이며 사람 손 수준의 성능은 단순히 힘-속도, 자유도만을 포함하는 것이 아닌 크기와 무게 그리고 물체 조작에 도움을 주는 여러 손 특성도 포함한다. 그러나 현재까지 사람 손 수준의 무게, 크기, 힘 그리고 자유도를 모두 만족시키는 로봇 손은 개발되지 않았으며 여전히 도전적인 과제로 남아있다. 본 논문에서는 로봇 손가락의 성능을 향상하기 위하여 사람의 근골격 특성을 반영한 세 가지의 새로운 메커니즘을 제안하고 이를 통합한 모듈형 로봇 손가락 구조를 보인다. 첫 번째로, 사람의 손 근육 배치와 유사한 내재/외재 구동기 배치를 적용한 힘줄 구동 로봇 손가락 구조를 제안하고 분석한다. 로봇 손가락은 다섯 개의 서로 다른 힘줄과 인대로 구성된다. 사람 손동작 분석에 기반하여 로봇 손가락의 구동기는 느린 속도를 담당하는 내재 구동기와 빠르고 큰 힘이 모두 요구되는 외재 구동기로 구분된다. 두 번째로, 구동기의 크기와 무게를 유지하며 성능을 향상하기 위하여 새로운 개념의 무단 변속기 Elastomeric Continuously Variable Transmission (ElaCVT) 을 제안하고 이를 로봇 손가락의 외재 구동기에 적용하였다. ElaCVT는 선형 구동기의 작동 영역을 확장하고 출력단에 가해지는 외부 하중에 따라 감속비를 수동적으로 변경하여 근육의 힘-속도 곡선을 모사할 수 있다. 본 연구에서는 근육의 특성을 모사하기 위해 선형 액추에이터로 ElaCVT에 줄 꼬임 메커니즘을 적용한 ElaCVT-TSA를 제안, 근육의 동작 영역을 모사할 수 있음을 보였다. 마지막으로 로봇 손가락의 모든 굽힘/펼침 관절에 적용된 사람의 관절구조를 모사한 유연 구름 접촉 관절 (Anthropomorphic Rolling Contact joint)을 제안한다. Anthropomorphic rolling contact joint는 사람 관절의 tongue-and-groove 형상과 collateral ligament를 모사하여 관절의 안정성을 향상시켰다. 기존의 유연 관절과 달리 관절이 펴진 상태에서는 유연한 상태를 유지하며 굽혀진 상태에서는 강성이 증가한다는 특징을 갖는다. 특히, 강성 변화에 별도의 구동기가 요구되지 않아 기존의 관절에서 무게, 크기 증가 없이 해당 특징 구현이 가능하다. 이는 로봇 손가락에 적용되어 손가락을 펴고 물체를 탐색하는 과정에서는 충격을 흡수하여 안정적인 접촉을 구현할 수 있으며 물체를 파지하는 과정에서는 손가락을 굽혀 강인하게 물체를 파지할 수 있게 한다. Anthropomorphic rolling contact joint를 적용한 그립퍼를 이용하여 제안하는 가변 강성 유연 관절이 pinch grasping의 파지 안정성을 높임을 보였다.1 INTRODUCTION 1 1.1 MOTIVATION: ROBOTIC HANDS 1 1.2 CONTRIBUTIONS OF THESIS 10 1.2.1 Intrinsic/Extrinsic Actuator arrangement 11 1.2.2 Linear actuator mimicking human muscle properties 11 1.2.3 Flexible rolling contact joint 12 2 ROBOTIC FINGER STRUCTURE WITH HUMAN-LIKE ACTUATOR ARRANGEMENT 13 2.1 ANALYSIS OF HUMAN FINGERTIP VELOCITY 14 2.2 THE ROBOTIC FINGER WITH INTRINSIC/EXTRINSIC ACTUATORS 18 2.2.1 The structure of proposed robotic finger 18 2.2.2 Kinematics of the robotic finger 20 2.2.3 Tendons and Ligaments of the proposed robotic finger 26 2.2.4 Decoupled fingertip motion in the sagittal plane 28 3 ELASTOMERIC CONTINUOUSLY VARIABLE TRANSMISSION COMBINED WITH TWISTED STRING ACTUATOR 35 3.1 BACKGROUND & RELATED WORKS 35 3.2 COMPARISON OF OPERATING REGIONS 40 3.3 DESIGN OF THE ELASTOMERIC CONTINUOUSLY VARIABLE TRANSMISSION 42 3.3.1 Structure of ElaCVT 42 3.3.2 Design of Elastomer and Lateral Disc 43 3.3.3 Advantages of ElaCVT 48 3.4 PERFORMANCE EVALUATION 50 3.4.1 Experimental Setup 50 3.4.2 Contraction with Fixed external load 50 3.4.3 Contraction with Variable external load 55 3.4.4 Performance variation of ElaCVT over long term usage 55 3.4.5 Specifications and Limitations of ElaCVT-TSA 59 4 ANTHROPOMORPHIC ROLLING CONTACT JOINT 61 4.1 INTRODUCTION: COMPLIANT JOINT 61 4.2 RELATED WORKS: ROLLING CONTACT JOINT 65 4.3 ANTHROPOMORPHIC ROLLING CONTACT JOINT 67 4.3.1 Fundamental Components of ARC joint 69 4.3.2 Advantages of ARC joint 73 4.4 TORSIONAL STIFFNESS EVALUATION 75 4.4.1 Experimental Setup 75 4.4.2 Design and Manufacturing of ARC joints 77 4.4.3 Torsional Stiffness Change according to Joint Angle and Twist Angle 79 4.5 TORSIONAL STIFFNESS WITH JOINT COMPRESSION FORCE DUE TO TNESION OF TENDONS 80 4.6 TORSIONAL STIFFNESS WITH LUBRICATION STRUCTURE 82 4.7 GRASPING PERFORMANCE COMPARISON OF GRIPPERS WITH DIFFERENT ARC JOINTS 86 5 CONCLUSIONS 92 Abstract (In Korean) 107박

A finger mechanism for adaptive end effectors

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