Learning Motion of Dexterous Manipulation For A Pair of Multi‐DOF Fingers With Soft‐TIPS

This paper shows that a pair of dual multi‐DOF fingers with soft‐tips can learn iteratively a desired periodic motion of manipulation of an object if sensory feedback signals are designed adequately. It is shown that dynamics of the overall fingers and object system satisfy passivity but residual error dynamics for a given periodic posture of the object and a fixed value of contact force satisfy output‐dissipativity only in an approximate sense. Numerical simulation results are presented which show that the pair of fingers manipulating an object is capable of learning iteratively a variety of dexterous motions with a good performance.     

Intelligent control of multi-fingered hands

This paper is concerned with intelligent control for grasping and manipulation of an object by multi-fingered robot hands with rigid or soft hemispheric finger ends that induce rolling contacts with the object. Even in the case of 2D motion like pinching by means of a pair of multi-degrees of freedom robot fingers, there arises an interesting family of Lagrange’s equations of motion with many geometric constraints, which are under-actuated, redundant, and non-holonomic in some sense. Regardless of underactuation of dynamics, it is possible to find a class of sensory feedback signals that realize secure grasp of an object together with control of object orientation. In regard to the secure grasping, a problem of force/torque closure for 2D objects in a dynamic sense plays a crucial role. It is shown that proposed sensory feedback signals satisfying the dynamic force/torque closure can be constructed without knowing object kinematic parameters and location of the mass center. To prove the convergence of motion of the overall fingers–object system under the circumstance of redundancy of joints, new concepts called “stability on a manifold” and “asymptotic stability on a manifold” are introduced. Based on the results found for intelligent control of robotic hands, the last two sections attempt to discuss why human multi-fingered hands can become so dexterous at grasping and object manipulation.     

Dexterity and versatility in the pinching motion of robot fingers with multi-degrees of freedom

This paper focuses on dexterity and versatility in pinching a rectangular object by a pair of robot fingers based on sensory feedback. In the pinching motion of humans, it is possible to execute concurrent pinching and orientation control quickly and precisely by using only the thumb and index finger. However, it is not easy for robot fingers to perform such imposed tasks agilely and simultaneously. In the case of robotic grasping, to perform concurrently such plural tasks retards the convergence speed in the execution of the overall task. This means that in order to increase versatility by imposing additional tasks, dexterity in the execution of each task may deteriorate. In this paper it is shown that both dexterity and versatility in the execution of such imposed tasks can be enhanced remarkably, without any deterioration in dexterity in the execution of each task, by using a sensory feedback method based on the idea of role-sharing joint control which comes from observation of the functional role of each human finger joint.     

Dynamic force/torque balance of 2D polygonal objects by a pair of rolling contacts and sensory‐motor coordination

It is well known that three frictionless fingers suffice to immobilize any 2D object with triangular shape but four fingers are necessary for a parallelepiped. However, it has been recently shown that only two fingers are enough to realize secure grasp of a rigid object with parallel flat surfaces in a dynamic sense if finger ends have a hemispherical shape with appropriate radius and thereby rollings are induced between finger ends and object surfaces. This paper focuses on the two problems: (1) dynamic force/torque balance of 2D polygonal objects under the effect of gravity force by means of a pair of rolling contacts and (2) concurrent realization of dynamically secure grasp and orientation control of 2D polygonal objects by using a pair of multi‐fingered hands with hemispherical ends and sensory feedback signals without knowing object kinematics and mass center. It is shown that the force/torque balance can be attained by controlling both the contact positions and inducing adequate forces in both normal and tangential directions at each of contact points indirectly through finger joints without knowing object mass center and other kinematic parameters. © 2003 Wiley Periodicals, Inc.     

Slippage control in soft finger grasping and manipulation

Handling objects with robotic soft fingers without considering the odds of slippage are not realistic. Grasping and manipulation algorithms have to be tested under such conditions for evaluating their robustness. In this paper, a dynamic analysis of rigid object manipulation with slippage control is studied using a two-link finger with soft hemispherical tip. Dependency on contact forces applied by a soft finger while grasping a rigid object is examined experimentally. A power-law model combined with a linear viscous damper is used to model the elastic behavior and damping effect of the soft tip, respectively. In order to obtain precise dynamic equations governing the system, two second-order differential equations with variable coefficients have been designed to describe the different possible states of the contact forces accordingly. A controller is designed based on the rigid fingertip model using the concept of feedback linearization for each phase of the system dynamics. Numerical simulations are used to evaluate the performance of the controller. The results reveal that the designed controller shows acceptable performance for both soft and rigid finger manipulation in reducing and canceling slippage. Furthermore, simulations indicate that the applied force in the soft finger manipulation is considerably less than the rigid “one.”.     

Primitive Object Grasping for Finger Motion Synthesis

We developed a new framework to generate hand and finger grasping motions. The proposed framework provides online adaptation to the position and orientation of objects and can generate grasping motions even when the object shape differs from that used during motion capture. This is achieved by using a mesh model, which we call primitive object grasping (POG), to represent the object grasping motion. The POG model uses a mesh deformation algorithm that keeps the original shape of the mesh while adapting to varying constraints. These characteristics are beneficial for finger grasping motion synthesis that satisfies constraints for mimicking the motion capture sequence and the grasping points reflecting the shape of the object. We verify the adaptability of the proposed motion synthesizer according to its position/orientation and shape variations of different objects by using motion capture sequences for grasping primitive objects, namely, a sphere, a cylinder, and a box. In addition, a different grasp strategy called a three‐finger grasp is synthesized to validate the generality of the POG‐based synthesis framework.     

Kinematics of grasping and manipulation of a B‐spline surface object by a multifingered robot hand

Kinematics of grasping and manipulation by a multifingered robotic hand where multifinger surfaces are in contact with an object is solved. The surface of the object was represented by B‐spline surfaces to model objects of various shapes. The fingers were modeled by cylindrical links and a half ellipsoid fingertip. Geometric contact equations have been solved for all possible contact combinations between the finger surface and the object. The simulation system calculated joint displacements and contact locations for a given trajectory of the object. Since there are no closed form solutions for contact or intersection between these surfaces, kinematics of grasping was solved by recursive numerical calculation. The initial estimate of the contact point was obtained by approximating the B‐spline surface by a polyhedron. As for the simulation of manipulation, exact contact locations were updated by solving the contact equations according to the given contact conditions such as pure rolling, twist‐rolling, or slide‐twist‐rolling. Several examples of simulation of grasping and manipulation are presented. ©1999 John Wiley & Sons, Inc.     

A differential-geometric approach for 2D and 3D object grasping and manipulation

This article presents an expository work on a differential-geometric treatment of fundamental problems of 2D and 3D object grasping and manipulation by a pair of robot fingers with multi-joints under holonomic or nonholonomic constraints. First, Lagrange’s equation of motion of a fingers-object system whose motion is confined to a vertical plane is derived under holonomic constraints when rolling contacts between finger-ends and object surfaces are permitted. Then, a class of control signals called “blind grasping” and constructed without knowing the object kinematics or using any external sensing like vision or tactile sensation is shown to realize stable object grasping in a dynamic sense. Stability of motion and its convergence to an equibrium manifold are treated on the basis of differential geometry of solution trajectories of the closed-loop dynamics on the constraint manifolds. Second, a mathematical model of 3D object grasping and manipulation by a pair of multi-joint robot fingers is derived under the assumption that spinning motion of rotation around the opposing axis between contact points does no more arise. It is shown that, differently from the 2D case, the instantaneous axis of rotation of the object is time-varying, which induces a nonholonomic constraint expressed as a linear differential equation of rotational motion of the pinched object. It is shown that there is a class of control signals constructed without knowing the object kinematics or using external sensings that can realize “blind grasping” in a dynamic sense. Finally, it is shown that the proposed differential geometric treatment of stability can naturally cope with redundancy resolution problems of surplus degrees-of-freedom (d.f.) of the overall fingers-object system, which is closely related to Bernstein’s d.f. problem.     

A novel coupled and self-adaptive under-actuated multi-fingered hand with gear–rack–slider mechanism

Aiming to overcome the serious disadvantages of two kinds of under-actuated fingers: coupled finger and self-adaptive finger, this paper proposed a novel grasping mode, called Coupled and Self-Adaptive (COSA) grasping mode, which includes two stages: first coupled and self-adaptive grasping. A 2-joint COSA finger with a double gear–rack–slider mechanism (called COSA-GRS finger), is developed based on the COSA grasping mode: at the beginning, the 2-joint finger bends with coupled mode, two joints of the finger rotate simultaneously with a fixed ratio until the proximal phalanx touches the grasped object, then the finger will automatically decouple and rotate with self-adaptive mode, the distal phalanx quickly rotates until it touches the object. The new finger unit has the advantages of coupled fingers and self-adaptive fingers. The finger is not only able to rotate all joints simultaneously to pre-shape before grasping objects, but also able to self-adapt different sizes and shapes of objects. Using the same mechanism as the 2-joint finger, a 3-joint COSA finger is designed. Force analyses and a structure optimization rule of the new finger are given and discussed. The simulation results show that the finger unit is effective: it can successfully realize coupling and decoupling and it can stably grasp objects. An under-actuated humanoid robot hand is developed, called the COSA-GRS Hand. The hand has 5 fingers, 15 joints and 6 motors. All fingers of the hand are COSA fingers. The hand is more similar to human hand in appearance and actions, able to grasp different objects more dexterously and stably than traditional coupled or self-adaptive under-actuated hands.     

Dynamics and Stability of Blind Grasping of a 3-Dimensional Object under Non-holonomic Constraints

A mathematical model expressing the motion of a pair of multi-DOF robot fingers with hemi-spherical ends, grasping a 3-D rigid object with parallel flat surfaces, is derived, together with non-holonomic constraints. By referring to the fact that humans grasp an object in the form of precision prehension, dynamically and stably by opposable forces, between the thumb and another finger (index or middle finger), a simple control signal constructed from finger-thumb opposition is proposed, and shown to realize stable grasping in a dynamic sense without using object information or external sensing (this is called "blind grasp" in this paper). The stability of grasping with force/torque balance under non-holonomic constraints is analyzed on the basis of a new concept named "stability on a manifold". Preliminary simulation results are shown to verify the validity of the theoretical results.     

Design and development of a pneumatic anthropomorphic hand

This work deals with a pneumatically actuated hand composed of four fingers and an opposable thumb. Mechanisms to convert actuator motion into phalanx rotation were studied to make each finger as similar as possible to the human specimen. Force tactile sensors are disposed along the phalanxes to allow a closed‐loop force control, while the thumb position is sensorized by a potentiometer and hence position controlled. Each component is controlled by fuzzy logic. This solution allowed the existing strong nonlinearities to be easily managed. A fuzzy supervisor applies a grasping strategy whose target is an approximate identification of the shape and size of an object to grasp it most efficiently with the disposable fingers. ©2000 John Wiley & Sons, Inc.     

Control of twisting manipulation using a multi-fingered robotic hand with a common rotational axis

The main purpose of the present study is to prove the usability of a mechanism with a common rotational axis during twisting manipulation using a multi-fingered robotic hand where two fingers and two other fingers can independently rotate in inner and outer circles with a dual turning mechanism. Although various types of conventional multi-fingered hands have potential capability to achieve twisting manipulations such as opening a bottle cap from within a hand, it is well-known that such tasks are difficult to execute quickly due to limited working space of the fingers and complexity of control. The proposed hand with a common rotational axis is effective in rotational manipulation around a particular axis, where each joint role assignment is completely decoupled into internal force control for grasping an object and velocity control around the axis for rotating the object. We prove the usability of this mechanism with a common rotational axis through the use of a control scheme, and show experimental results involving manipulation tasks where twisting manipulation is dominant.

     

The importance of the number of degrees of freedom for rotation of objects

In an experiment input methods for object rotation with differing degrees of freedom were assessed. The results are relevant for human- computer interfacing, not only for the finger tip controlled interface proposed in this paper but also for evaluation of existing approaches to rotation. When designing an interface with finger tip controlled rotation of virtual objects, for technical reasons the number of finger tips to be registered should be minimized. Performance of subjects who rotated real objects with different numbers of finger tips was assessed. Subjects rotated a transparent sphere encasing an object according to their personal preference, with three, two or one finger, and restricted to three orthogonal axes. The latter reflects rotation in much current 3D software, whereby only one rotational degree of freedom (DOF) is accessible at a time. Performance in the three and two finger conditions did not differ significantly from the free condition, whilst performance with one finger and orthogonally restricted was significantly lower. However, only the three finger condition was rated as comfortable as the free condition, whilst the two finger, one finger and orthogonally restricted conditions were rated as less comfortable. It is argued that the number of DOFS which can be accessed simultaneously is an important design consideration when quick and intuitive rotation is to be achieved.     

虚拟手抓持力觉生成算法真实性的评价

目的 虽然许多学者研发了多种虚拟手交互触力觉生成算法,但是如何评价虚拟手交互触力觉生成算法的真实性是一个富有挑战性的新问题,值得深入研究.方法 构建手指抓持力测量平台,设计3种抓持姿态下指尖静力抓持球体实验内容,测得各指尖作用力的实测值;通过虚拟手静力抓持力觉生成算法,求得这3种抓持姿态下各手指作用力的理论值;对实测值进行统计和分析,并与理论值进行对比和讨论;结果 日常抓持经验和实测值是完全相符的,实测值和理论值很接近且偏差均在可接受范围之内.单个手指作用力或多个手指合力的实测值与理论值的偏差均在1%6%.结论 本文实现了一种基于物理的实验方法,评价和分析了虚拟手静力抓持力觉生成算法的真实性,证实此算法可以逼真地生成虚拟手抓持力,可应用于具有力反馈的自然的虚拟手交互.     

Appropriate lengths between phalanges of multijointed fingers for stable grasping

An appropriate arrangement of finger joints is very important since the stability of grasping an object greatly depends on that arrangement. Multijointed fingers can grasp an object with many points of contact each of which is pressed against the object as if wrapping up that object. The amount of the wrapped up area and the form of the finger when an object is grasped are therefore important factors for determining the stability of grasping. We propose the wrapping factor to be used for the evaluation of the stability of grasping by using these factors. We consider 28 models for the finger having three joints, and perform a simulation of their ability to grasp various shapes stably. Based on the simulation results, an appropriate arrangement of lengths between phalanges for a multijointed finger is presented.     

Three-dimensional finger joint angles by hand posture and object properties

The objective of this study was to identify three-dimensional finger joint angles for various hand postures and object properties. Finger joint angles were measured using a VICON system for 10 participants while they pinched objects with two, three, four and five fingers and grasped them with five fingers. The objects were cylinders and square pillars with diameters of 2, 4, 6 and 8 cm and weights of 400, 800, 1400 and 1800 g. Hand posture and object size more significantly affected the joint flexion angles than did object shape and weight. Object shape affected only the metacarpophalangeal (MCP) joint angle of the index finger and the flexion angle of the MCP joint of the little finger. Larger flexion angles resulted when the hand posture was grasping with five fingers. The joint angle increased linearly as the object size decreased. This report provides fundamental information about the specific joint angles of the thumb and fingers.

Practitioner Summary: Three-dimensional finger joint angles are of special interest in ergonomics because of their importance in handheld devices and musculoskeletal hand disorders. In this study, the finger joint angles corresponding to various hand postures and objects with different properties were determined.     

The importance of the number of degrees of freedom for rotation of objects

In an experiment input methods for object rotation with differing degrees of freedom were assessed. The results are relevant for human- computer interfacing, not only for the finger tip controlled interface proposed in this paper but also for evaluation of existing approaches to rotation. When designing an interface with finger tip controlled rotation of virtual objects, for technical reasons the number of finger tips to be registered should be minimized. Performance of subjects who rotated real objects with different numbers of finger tips was assessed. Subjects rotated a transparent sphere encasing an object according to their personal preference, with three, two or one finger, and restricted to three orthogonal axes. The latter reflects rotation in much current 3D software, whereby only one rotational degree of freedom (DOF) is accessible at a time. Performance in the three and two finger conditions did not differ significantly from the free condition, whilst performance with one finger and orthogonally restricted was significantly lower. However, only the three finger condition was rated as comfortable as the free condition, whilst the two finger, one finger and orthogonally restricted conditions were rated as less comfortable. It is argued that the number of DOFS which can be accessed simultaneously is an important design consideration when quick and intuitive rotation is to be achieved.     

SMA丝驱动的手指康复机器人设计及实验研究

为了研制一种轻便、可穿戴的仿生手指康复机器人,在分析手指肌肉骨骼生物参数及致动机理的基础上,设计了一种形状记忆合金丝(Shape Memory Alloy, SMA)驱动的软体柔性手指康复机器人,建立了其运动学和力学模型。以手套为结构设计的原型,通过控制SMA丝的收缩来模拟手指肌肉肌腱的收缩,从而实现辅助手指屈曲伸展运动的功能。试验研究了手指康复机器人的运动性能和抓握性能。试验结果表明,手指柔性康复机器人最大弯曲角度接近正常人手关节角度,最大指尖力可达18N,能完成日常所需的屈曲伸展以及抓握功能。     

Virtual grasp release method and evaluation

We address a “sticking object” problem for the release of whole-hand virtual grasps. The problem occurs when grasping techniques require fingers to be moved outside an object's boundaries after a user's (real) fingers interpenetrate virtual objects due to a lack of physical motion constraints. This may be especially distracting for grasp techniques that introduce mismatches between tracked and visual hand configurations to visually prevent interpenetration. Our method includes heuristic analysis of finger motion and a transient incremental motion metaphor to manage a virtual hand during grasp release. We integrate the method into a spring model for whole-hand virtual grasping to maintain the physically-based pickup and manipulation behavior of such models. We show that the new spring model improves release speed and accuracy based on pick-and-drop, targeted ball-drop, and cube-alignment experiments. In contrast to a standard spring-based grasping method, measured release quality does not depend notably on object size. Users subjectively prefer the new approach and it can be tuned to avoid potential side effects such as increased drops or visual distractions. We further investigated a convergence speed parameter to find the subjectively good range and to better understand tradeoffs in subjective artifacts on the continuum between pure incremental motion and rubber-band-like convergence behavior.