Hyun-Yong Han

Dynamics and control of a set of dual fingers with soft tips

This paper attempts firstly to derive a mathematical model of the dynamics of a set of dual fingers with soft and deformable tips which grasps and manipulates a rigid object with some dexterity. To gain a physical insight into the problem, consideration is restricted to the case that the motion of the whole system is confined to a horizontal plane. Secondly on the basis of the derived model it is shown that the rotation of the object can be indirectly controlled by the change of positions of the center points of both contact areas on the object. Then, each of the center points of contact areas can be positioned by inclining the last link of each corresponding finger against the object. It is further shown that, when both forces of pressing the object becomes almost equal, the equation of motion of the object in terms of rotational angles assumes the form of a harmonic oscillator with a forcing term, which can be regulated coordinately by the relative angle between the two last links contacting with the object. It is also shown that dynamics of this system satisfy passivity. Finally, design problems of control for dynamic stable grasping and enhancing dexterity in manipulating things are discussed on the basis of passivity analysis.

Analysis of friction on human fingers and design of artificial fingers

In many cases of human hand tasks, grasping and manipulation of objects are the most basic motions. However, the influence of physical characteristics of human finger on grasping motions have not yet been sufficiently clarified. For example, friction which is an important characteristic of grasping motions has hardly been investigated. In this study, we pay attention, to friction between a finger and an object. First, we develop a system to measure the friction on fingers. Next, based on the experimental results obtained from the measurement system, we propose a new model for the maximum static friction force on human fingers. Also, we find that the friction forces are changed depending on the directions of tangential forces. Finally, we realize a finger-tip which is made of silicone and whose characteristics are similar to those of human fingers.

Extension of impedance matching to nonlinear dynamics of robotic tasks

The concept of impedance matching for linear electric circuits is extended to nonlinear position-dependent circuits that express nonlinear dynamics of robotic tasks such as holding an object of soft material and handling a rigid object with soft fingers. At the first step, impedance control is realized by negative-feedback connection of two passive (hyper-stable) blocks, one is in the forward path expressing position control of the tool endpoint and the other is in the feedback path expressing force control of pressing the object. This negative-feedback framework is naturally introduced owing to the situation that both the tool mass and the nonlinear characteristics of reproducing force of the soft material are unknown. Extension of the concept of impedance matching to such nonlinear circuits is fulfilled by optimizing the regulation of impedance control and subsequently choosing optimal parameters from the viewpoint of both the transient and stead-state responses. The relations of this extension with the well-known theorem of maximum power supply and the H-infinity tuning for disturbance attenuation are also presented.

Analysis of stiffness of human fingertip and comparison with artificial fingers

Human fingers have mechanical characteristics which enable human hands to manipulate objects dexterously in complex environments. Research on such characteristics will help in the design of robotic fingers with capability of dexterous manipulation. In this work, the stiffness of human fingertips tissue is investigated for loading and unloading situations at various contact angles and forces on an acrylic block. The stiffness shows strong nonlinearity for each condition. Based on experimental results, we propose two models to express the stiffness of soft fingertips tissue. Also the contact area of human fingertips exhibits a proportional relation with the deformation of fingertips tissue. Subsequently, the stiffness of fingertips can be expressed to vary linearly with the mean pressure which is calculated from the force and the contact area. It is revealed how the natural frequency in peg-in-hole tasks changes depending on the nonlinear stiffness of fingertips tissue and the mass of object. These stiffness properties have an important role in understanding the degree of smoothness and skill in execution of the task by human hands. Finally, these properties obtained from human fingertips are compared with those of artificial fingertips, made of silicone rubber and silicone gel.

Grasping and position control for multi-fingered robot hands with uncertain Jacobian matrices

Most research on multifingered robot control has assumed that the Jacobian matrices from joint space to task space is exactly known. This implies that the locations of contact points, geometry of the object, kinematics of the multifingered robot hands must be exactly known. In this paper, a task-space feedback control problem of multifingered robot hands with uncertain Jacobian matrices is formulated and solved. The stability and robustness of the proposed controllers to the uncertainties in Jacobian matrices are analyzed.

Robotic pinching by means of a pair of soft fingers with sensory feedback

This paper proposes a pair of single or multi-DOF robot fingers with soft and deformable tips that can pinch an object stably in a dynamic sense with the aid of real-time sensory feedback. To realize dynamic stable pinching, a practical method of using optical devices is proposed for measuring both the maximum displacement of finger-tip deformation and the relative angle between the object surface and each of finger links. It is shown that the overall closed-loop system of a pair of two single-DOF fingers with soft tips with real-time sensory feedback of the difference between the centers of two area-contacts at both sides of the object becomes asymptotically stable. This means that the pair achieves dynamic stable grasping (pinching). In the case of a pair of 1-DOF and 2-DOF fingers with soft tips, it is shown that the proposed method of closed-loop feedback of the difference between the centers of two area-contacts and the rotational angle of the object can establish not only dynamic stable grasping but also regulation of the posture of the object.

Iterative learning of impedance control

This paper proposes an iterative learning control scheme for impedance control of robotic tasks when the tool endpoint covered by soft and deformable material presses a rigid object or environment at a prescribed periodic force pattern. To this end, an iterative learning control scheme for a class of linear dynamical systems with a negative feedback structure is analyzed and convergence of the proposed learning update law after a sufficient number of repetitions is proved. It is shown that this convergence realizes impedance matching in a sense of electric circuit theory of the feedback system can be expressed as a lumped-parameter electric circuit. The iterative learning control scheme is then applied for a case of impedance control of robotic tasks when the characteristics of reproducing force of the deformable material is nonlinear in its displacement and unknown and the tool mass is uncertain. Simulation results are also presented, which show effectiveness of the proposed learning control scheme.

Stable Grasping and Posture Control for a Pair of Robot Fingers with Soft Tips

This paper derives and analyzes non-linear dynamics of grasping a rigid object by a pair of robot, fingers with soft and deformable tips. A method of synthesizing a feedback control signal for stable grasping and posture control of the object is presented, which is based on passivity analysis of the dynamics including extra terms of Lagranges multipliers arising from holonomic constraints of tight area contacts between soft finger-tips and surfaces of the object. Finally, usefulness of the proposed control method is discussed from the practical viewpoint. Simulation results suggest that a linear superposition of feedback signals realizes stable grasping and posture control of the object at, the same time. Nevertheless, challenging a design problem of sensory feedback for realizing dexterous motions of multi-fingered hands is quite important, because even at the present stage only open-loop control schemes are used for controlling dexterous hands.

Impedance matching for evaluation of dexterity in execution of robot tasks

This paper aims to generalize the concept of impedance matching to cope with nonlinear dynamics that govern robotic systems with or without constraints. In the case of single degree of freedom dynamics the impedance matching is first introduced by the concept of balancing between load impedance and internal impedance. Generalization of this concept to the case of multiple degrees of freedom robots can be fulfilled implicitly via a coordinates transformation by constructing a negative feedback connection of two hyper-stable blocks (one is a parameter estimator and positioning and the other is a position and force regulator).

Generalization of Impedance Matching to Nonlinear Dynamics of Robot Tasks

Abstract This paper aims at generalizing the concept of impedance matching to nonlinear circuits that express nonlinear dynamics of robot tasks, where a tool endpoint (or fingertip) with soft material presses a rigid and fixed object or environment. Under such a situation that characteristics of reproducing force of the elastic material with respect to displacement is nonlinear and unknown, impedance control is established by constructing a negative feedback connection of two hyperstable blocks. Then, the minimum upper-bound of the average power of force error under the existence of measurement noises of both displacement and force is derived in a sense of H-infinity disturbance attenuation, which corresponds to a generalization of the well-known theorem of maximum power transfer for impedance matching of linear electric circuits. Choice of design parameters is discussed from the viewpoint of better matching of two passive blocks to attain optimal transient response and attenuation of the effect of disturbances.