Youlun Xiong

On the Dynamic Stability of Grasping

Stability is one of the important properties that a robot hand grasp must possess to be able to perform tasks similar to those performed by human hands. This paper discusses the dynamic stability of a grasped object. To analyze the stability of grasps, we build the model of the dynamics of the grasped object in response to the small perturbances. Furthermore, we determine the conditions associated with the dynamic stability and discuss the effects of various factors on the grasp stability. A quantitative measure for evaluating grasps is then presented. Finally, the effectiveness of the proposed theory is verified via examples.

Stability index and contact configuration planning for multifingered grasp

It is necessary to plan the contact configuration to guarantee a stable grasp. This article discusses the grasping stability of multifingered robot hands. The fingers are assumed to be point contacts with friction. A stability index for evaluating a grasp, which is proportional to the ellipsoidal volume in the grasping task space, is proposed. The invariance of the index is proved under an object linear coordinate transformation and under a change of the torque origin. The similar invariance of the index is also proved under a change of the dimensional unit. The optimal grasping of an object by a multifingered robot hand can be obtained using the stability index to plan the grasp configurations. The index is applicable to plan adaptable fixtures as well. A nonlinear programming method to plan configurations is addressed. Several examples are given using the index to evaluate a grasp, in which the obtained optimal grasping is consistent with what human beings expect. The sensibility of the optimal grasping is analyzed in these examples.xa0© 1998 John Wiley & Sons, Inc.

Qualitative analysis and quantitative evaluation of fixturing

Abstract Form-closure is considered as a purely geometric property of a set of unilateral contact constraints such as those applied on a workpiece by a mechanical fixture. This paper provided qualitative analysis of form-closure fixturing. The necessary and sufficient condition for form-closure fixturing is derived. Some fundamental problems related to form-closure are solved such as minimum number of frictionless contact points and the way to arrange them to achieve form-closure. On the basis of qualitative analysis, the quantitative evaluation of form-closure is investigated. To assess quantitatively the form-closure fixturing, two quantitative indices, one to minimize the sum of all normal contact forces and the other to minimize the maximum normal contact force, are presented. Finally, the given example verifies the analysis method and evaluating indices.

Kinematic analysis and dexterity evaluation of upper extremity in activities of daily living

The redundant kinematic structure of upper extremity (UE) provides it with increased dexterity in activities of daily living (ADL). Most functional tasks during ADL can be implemented in various positions; however, only some routine trajectories are employed in healthy humans. Exploring motion ability and analyzing dexterity are clinically helpful in understanding the motion principles of the UE during ADL. The aim of this work was to develop a qualitative and quantitative evaluation method for unconstrained movement analysis of the UE. Ten healthy male volunteers with no UE pathology were tested based on a kinematic model proposed in this paper which follows the ISB recommendations. The dexterity measure (DM) and manipulability ellipsoid (ME) were used to evaluate the dexterity distribution and motion ability in space. Representative dexterity trajectories and corresponding ellipsoids were determined for the ADL tasks. The results of the study showed that there was a most dexterous posture for each functional task. The UE movement followed an approximately optimal kinematic performance trajectory when the hand moved in front of the thorax during ADL.

Grasp capability analysis of multifingered robot hands

Abstract This paper addresses the problem of grasp capability analysis of multifingered robot hands. The aim of the grasp capability analysis is to find the maximum external wrench that the multifingered robot hands can withstand, which is an important criterion in the evaluation of robotic systems. The study of grasp capability provides a basis for the task planning of force control of multifingered robot hands. For a given multifingered hand geometry, the grasp capability depends on the joint driving torque limits, grasp configuration, contact model and so on. A systematic method of the grasp capability analysis, which is in fact a constrained optimization algorithm, is presented. In this optimization, the optimality criterion is the maximum external wrench, and the constraints include the equality constraints and the inequality constraints. The equality constraints are for the grasp to balance the given external wrench, and the inequality constraints are to prevent the slippage of fingertips, the overload of joint actuators, the excessive forces over the physical limits of the object, etc. The advantages of this method are the ability to accomodate diverse areas such as multiple robot arms, intelligent fixtures and so on. The effectiveness of the proposed method is confirmed with a numerical example of a trifingered grasp.

Control methods for exoskeleton rehabilitation robot driven with pneumatic muscles

Purpose – The purpose of this paper is to present the control methods of the exoskeleton robotic arm for stroke rehabilitation.Design/methodology/approach – The robotic arm is driven by the pneumatic muscle actuators. The control system provides independent control for the robot. The joint axes of the robotic arm are arranged to mimic the natural upper limb workspace.Findings – Findings are the classification of training modes and control methods of rehabilitation training, and the characters of both the instant spasm and the sustaining one.Research limitations/implications – This paper is a preliminary step in the control system and the kinematical characteristics should be analyzed to achieve high precision of movement.Originality/value – Based on a hierarchical structure, the control system allows the execution of sequence of switching control methods: position, force, force/position and impedance. Patient‐active‐robot‐passive and patient‐passive‐robot‐active (PPRA) training modes are also presented in...

Neural-network based force planning for multifingered grasp

The real-time control of multifingered grasp involves the problem of the force distribution which is usually underdetermined. It is known that the results of the force distribution are used to provide force or torque setpoints to the actuators, so they must be obtained in real-time. The objective of this paper is to develop a fast and efficient force planning method to obtain the desired joint torques which will allow multifingered hands to firmly grasp an object with arbitrary shape. In this paper, the force distribution problem in a multifingered hand is treated as a nonlinear mapping from the object size to joint torques. We represent the nonlinear mapping using artificial neural networks (ANNs), which allow us to deal with the complicated force planning strategy. A nonlinear programming method, which optimizes the contact forces of fingertips under the friction constraints, unisense force constraints and joint torque constraints, is presented to train the ANNs. The ANNs used for this research are based on the functional link (FL) network and the popular back-propagation (BP) network. It is found that the FL-network converges more quickly to the smaller error by comparing the training process of the two networks. The results obtained by simulation show that the FL-network is able to learn the complex nonlinear mapping to an acceptable level of accuracy and can be used as a real-time grasp planner.

On the prediction of passive contact forces of workpiece-fixture systems

Abstract The prediction of passive forces in a frictional workpiece-fixture system is an important problem, since the contact forces have a strong influence on clamp design and on workpiece accuracy during machining. This paper presents a general method for the computation of passive contact forces. Firstly, an indeterminate system of static equilibrium is defined, in which the passive, frictional contact forces cannot be determined arbitrarily as in an actively controlled robotic multifinger grasp. Then, a locally elastic contact model is used to describe the non-linear coupling between the contact forces and elastic deformations at the contact point. This model captures the essence of the passive contact. Further, a set of ‘compatibility’ equations is given so that the relationship can be developed between the elastic deformations at all contacts and the displacement of the workpiece. Finally, combining the force equilibrium, the locally elastic contact model and the ‘compatibility’ conditions, the passive force computation problem is transformed into a determinate system of non-linear equations governing all of the elastic deformations at all of the passive contacts. By solving the resulting non-linear equations, all passive contact forces can be accurately predicted in the frictional workpiece-fixture system. This method is illustrated with example cases. The method presented here may also have an application to other passive, indeterminate problems such as power grasps in robotics.

An integrated localization system for robots in underground environments

Purpose – The purpose of this paper is to design an integrated localization system for mobile robots in underground environments for exploring and rescuing tasks after incidents and detection of hazard gas in tunnels before ingress.Design/methodology/approach – An integrated localization system mainly based on a strap‐down inertial measurement unit and a digital compass is designed for exploring and rescuing task in coal mines and tunnels. After a system model was founded, a filtering algorithm combining a wavelet‐based pre‐filter with unscented Kalman filters was developed for reckoning tracks of robots and localizing it.Findings – Based on this research, an integrated localization system for robots in underground environments can be developed to explore some regions and rescue people. Although errors of localization exist, performance of the integrated system should be improved if some sensors and landmarks or maps of tunnels are introduced.Originality/value – What is proposed in this paper is an integr...