Title: On Biomechanics of Human Hand Motion in Grasping: A Mechanical Model
Author: prof. Alberto Rovetta N. AA-4
References:

- Proc. Symp. on Theory and Practice of Robots and Manipulators since 1973
- A.K.Beiczy, Allocation of control between man and computer in remote manipulation, Ro.man.sy, 1976


Objectives

A mechanical model of the human hand in grasping. The analogies with articular and muscular mechanics of human limbs are pointed out. The system, composed of two fingers and a palm, allows a specific stability in prehension process. A mathematical model, with a computer program, aids the study of kinematics and dynamics of the mechanical hand. Experimental tests are developed to analyze similar behavjour of mechanical and human extremities.

Contents
This mechanical hand is formed by three elements: two linked fingers and a palm. This design solution is proposed and executed with the aim to obtain a system which has the muscular characteristics of the human upper extremities in the grasping process. Many authors, with theoretical and experimental researches, have examined and developed the mechanical study of the hand and of the fingers. Every finger is connected with the next by means of a joint and a spring, and is moved by a wire. The joint works like the articulation of the hand and has a planar motion. The motion of the finger is controlled by a stressed wire and a spring; the wire ( moved by an electric a.c. motor) represents the action of flexor digitorum tendons, and acts on every finger; the spring is a flexural spring, located around the joint and designed with mechanical characteristics so that it may reproduce the physical action of extensor digitorum tendons. The mechanical design of the elements is developed to obtain functions which are quite similar to the human ones. The palm is constituted by a plate leaning on the object, taken by the hand; it exerts the main function of ensuring the prehension stability. The palm is one of the main aspects of this mechanical hand; its function is confirmed by experimental tests. The motion of the two wires is controlled by a system of two coaxial motors: one motor lifts he fingers and the grasped object, the other one lowers them. The wire is connected with the proximal phalanx by means of a special material; a technological research proposes the choice of suitable materials with high tensile strength, good ftexibility, considerable endurance. The phalanges of the prototype are constructed in aluminium. The motion is transmitted by the joint and the wire: the sequence of the fingers motion depends on the mechanical characteristics of the links, of the springs and on the wire tension. In order to reproduce the human motion, the springs are preloaded; the sequence of the motion is a function of the preloads and of the springs elastic stiffness. The design of prototype is developed in order to obtain optimal angles.

Results
A computerized program is completed for the exam of the behaviour of the mechanical hand, in order to calculate the forces acting in the system and specifically in the wire (corresponding to tendon), and to know the configuration of the objects and of the mechanical hand. The theoretical analysis is developed by means of the fundamental equatìons of mechanics. The system is automatized for an automatic prehension process. When the object is inside fingers and palm for the effect of an optical device, the motor automatically runs. The object reaches the required position, since the palm is sensitive to displacements. The grasp is ensured also by the specific properties of the mechanical extremities. More sophisticated control systems, also with micro-processors, may be connected with mechanical hand. The plate moves upwards and compresses an helical spring with a restoring mechanical effect: the spring elastic energy is accumulated and the restoring force consents a prehension process with three contact points on the object. Mechanics of handling process shows that in a plane it is possible to obtain a grasping action with the aid of three right contact points. The fingers and the palm of the mechanical hand exert a prehension, which may be considered quite similar to the prehension of the human hand.

Future possible developments
A new model of a prototype of human hand is presented. It is composed of mechanical elements reproducing the action of the tendons, of the palm, and the hyperextesion of the proximal phalanx. Experimental tests present some likeness with the prehension process of human grasping.