The Arched Flexure VSA: A Compact Variable Stiffness Actuator with Large Stiffness Range

The high stiffness of conventional robots is beneficial in attaining highly accurate positioning in free space. High stiffness, however, limits a robot\u27s ability to perform constrained manipulation. Because of the high stiffness, geometric conflict between the robot and task constraints during constrained manipulation can lead to excessive forces and task failure. Variable stiffness actuators can be used to adjust the stiffness of robot joints to allow high stiffness in unconstrained directions and low stiffness in constrained directions. Two important design criteria for variable stiffness actuation are a large range of stiffness and a compact size. A new design, the Arched Flexure VSA, uses a cantilevered beam flexure of variable cross-section and a controllable load location. It allows the joint to have continuously variable stiffness within a finite stiffness range, have zero stiffness for a small range of joint motion, and allow rapid adjustment of stiffness. Using finite element analysis, flexure geometry was optimized to achieve high stiffness in a compact size. A proof-of-concept prototype demonstrated continuously variable stiffness with a ratio of high stiffness to low stiffness of 55

Learning-based Position and Stiffness Feedforward Control of Antagonistic Soft Pneumatic Actuators using Gaussian Processes

Variable stiffness actuator (VSA) designs are manifold. Conventional model-based control of these nonlinear systems is associated with high effort and design-dependent assumptions. In contrast, machine learning offers a promising alternative as models are trained on real measured data and nonlinearities are inherently taken into account. Our work presents a universal, learning-based approach for position and stiffness control of soft actuators. After introducing a soft pneumatic VSA, the model is learned with input-output data. For this purpose, a test bench was set up which enables automated measurement of the variable joint stiffness. During control, Gaussian processes are used to predict pressures for achieving desired position and stiffness. The feedforward error is on average 11.5% of the total pressure range and is compensated by feedback control. Experiments with the soft actuator show that the learning-based approach allows continuous adjustment of position and stiffness without model knowledge.© 2023 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works

Design and Testing of a Prosthetic Foot Prototype With Interchangeable Custom Rotational Springs to Adjust Ankle Stiffness for Evaluating Lower Leg Trajectory Error, an Optimization Metric for Prosthetic Feet

A prosthetic foot prototype intended for evaluating a novel design objective for passive prosthetic feet, the Lower Leg Trajectory Error (LLTE), is presented. This metric enables the optimization of prosthetic feet by modeling the trajectory of the lower leg segment throughout a step for a given prosthetic foot and selecting design variables to minimize the error between this trajectory and target physiological lower leg kinematics. Thus far, previous work on the LLTE has mainly focused on optimizing conceptual foot architectures. To further study this metric, extensive clinical testing on prototypes optimized using this method has to be performed. Initial prototypes replicating the LLTE-optimal designs in previous work were optimized and built, but at 1.3 to 2.1 kg they proved too heavy and bulky to be considered for testing. A new, fully-characterized foot design reducing the weight of the final prototype while enabling ankle stiffness to be varied is presented and optimized for LLTE. The novel merits of this foot are that it can replicate a similar quasi-stiffness and range of motion of a physiological ankle, and be tested with variable ankle stiffnesses to test their effect on LLTE. The foot consists of a rotational ankle joint with interchangeable U-shaped constant stiffness springs ranging from 1.5 Nm/deg to 16 Nm/deg, a rigid structure extending 0.093 m from the ankle-knee axis, and a cantilever beam forefoot with a bending stiffness of 16 Nm2. The prototype was built using machined acetal resin for the rigid structure, custom nylon springs for the ankle, and a nylon beam forefoot. In preliminary testing, this design performed as predicted and its modularity allowed us to rapidly change the springs to vary the ankle stiffness of the foot. Qualitative feedback from preliminary testing showed that this design is ready to be used in larger-scale studies. In future work, extensive clinical studies with testing different ankle stiffnesses will be conducted to validate the optimization method using the LLTE as a design objective.Massachusetts Institute of Technology. Tata Center for Technology and DesignMassachusetts Institute of Technology. Department of Mechanical Engineerin

Modeling and design of energy efficient variable stiffness actuators

In this paper, we provide a port-based mathematical framework for analyzing and modeling variable stiffness actuators. The framework provides important insights in the energy requirements and, therefore, it is an important tool for the design of energy efficient variable stiffness actuators. Based on new insights gained from this approach, a novel conceptual actuator is presented. Simulations show that the apparent output stiffness of this actuator can be dynamically changed in an energy efficient way

Energy Efficient Actuation with Variable Stiffness Actuators

Research effort in the field of variable stiffness actuators is steadily increasing, due to their wide range of possible applications and their advantages. In literature, var- ious control methods have been proposed, solving particular problems in human-robot and robot-environment interaction applications, in which the mechanical compliance introduced by variable stiffness actuators has been shown to be beneficial. In this work, we focus on achieving energy efficient actuation of robotic systems using variable stiffness actuators. In particular, we aim to exploit the energy storing properties of the internal elastic elements

A three-dimensional forward dynamic model of the golf swing optimized for ball carry distance

The final publication is available at Springer via http://dx.doi.org/10.1007/s12283-016-0197-7A 3D predictive golfer model can be a valuable tool for investigating the golf swing and designing new clubs. A forward dynamic model, which includes a four degree of freedom golfer model, a flexible shaft based on Rayleigh beam theory, an impulse-momentum impact model and a spin rate dependent aerodynamic ball model, is presented. The input torques for the golfer model are provided by parameterized joint torque generators that have been designed to mimic muscle torque production. These joint torques are optimized to create swings and launch conditions that maximize carry distance. The flexible shaft model allows for continuous bending in the transverse directions, axial twisting of the club and variable shaft stiffness as a function of the length. The completed four-part model with the default parameters is used to estimate the ball carry of a golf swing using a particular club. This model will be useful for experimenting with club design parameters to predict their effect on the ball trajectory and carry distance.Natural Sciences and Engineering Research Council of Canad

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

Impedence Control for Variable Stiffness Mechanisms with Nonlinear Joint Coupling

The current discussion on physical human robot interaction and the related safety aspects, but also the interest of neuro-scientists to validate their hypotheses on human motor skills with bio-mimetic robots, led to a recent revival of tendondriven robots. In this paper, the modeling of tendon-driven elastic systems with nonlinear couplings is recapitulated. A control law is developed that takes the desired joint position and stiffness as input. Therefore, desired motor positions are determined that are commanded to an impedance controller. We give a physical interpretation of the controller. More importantly, a static decoupling of the joint motion and the stiffness variation is given. The combination of active (controller) and passive (mechanical) stiffness is investigated. The controller stiffness is designed according to the desired overall stiffness. A damping design of the impedance controller is included in these considerations. The controller performance is evaluated in simulation

Automatic Analyzer for Iterative Design

The Office of Naval Research Department Of The Navy Contract Nonr 1834 (03) Project NR-064-18

Compliance Analysis of an Under-Actuated Robotic Finger

Under-actuated robotic hands have multiple applications fields, like prosthetics and service robots. They are interesting for their versatility, simple control and minimal component usage. However, when external forces are applied on the finger-tip, the mechanical structure of the finger might not be able to resist them. In particular, only a subset of disturbance forces will meet finite compliance, while forces in other directions impose null-space motions (infinite compliance). Motivated by the observation that infinite compliance (i.e. zero stiffness) can occur due to under-actuation, this paper presents a geometric analysis of the finger-tip compliance of an under-actuated robotic finger. The analysis also provides an evaluation of the finger design, which determines the set of disturbances that is resisted by finite compliance. The analysis relies on the definition of proper metrics for the joint-configuration space. Trivially, without damping, the mass matrix is used as a metric. However, in the case of damping (power losses), the physical meaningful metric to be used is found to be the damping matrix. Simulation experiments confirm the theoretical results