Title: Design and Control of Mechanical Hands for Robots
Author: prof. Alberto Rovetta N. AA-1
References:

I. Kato: Mechanical Hand, Tokyo, 1977.


Objectives

Researches on biomechanics and studies on robotics, with specific reference to systems for grasp of generically shaped objects. Several examples of grasping systems have been realized, following criteria of design and methodologies of functionality with large differences. Some of them are inspired on biomechanical systems. Here is presented a mechanical hand prototype, which tries to reproduce some elements of the functionality of the human hand. By means of a large requirement of biomechanical analysis of human and animal prehension and with the use of advanced design methods, it is possible to realize systems for grasping, even of high complexity and wide efficiency, as actually may be required in robot extremities.

Contents
1. Number of Contact Points
In a bidimensional case, the contact between prehension system and an object is stable, if the contact points are present ( the system is isostatic) and the directions of forces actions lineare not concurrent. In the tridimensional case the contact is stable: there are five contact points, without a convergence of the pressure force action lines. In the human hand, such conditions are performed by two fingers and the palm of the hand itself in the bidimensional case and by the five fingers in the spatial case.
2. Increasing of Elastic Energy during the Prehension
The mechanical hand has been designed with elements ensuring that the fingers present subsequent positions with a bending towards the center of the hand, while the palm lifts. A corresponding increase of elastic energy of the system, due to the spring inserted in the fingers joints and behind the palm, is obtained.
3. Geometric-Kinematic Sequentiality of the Fingers
The biomechanical analysis of the kinematics of the human hand indicates that the motion of phalanges is depending on the actuation of the tendons system, and is strictly connected with the phalanges morphology. Effectively the motion is sequential. With the aim of obtaining some analogy with the motion of the human hand, the mechanical system has been realized with three phalanges moved by a traction wire, which acts on the extreme phalanx, and with a system of springs, inserted in the joints of the phalanges. These springs are designed with stiffness able to provoke a succession of motions of the phalanges when the wire is exerting its force.
4. Self-Centering Principle
The centre of gravity position of the grasped object is determinant to ensure a good relative position between object and hand; a stable position corresponds to a condition of acceptable grasping.
5. Influence of the Hyperextension
The open kinematic chain, which constitutes a mechanical finger, admits only one degree of freedom, for the effect of the mechanical connections and of the elastic elements of the system. Furthermore the anatomic analysis of the human hand shows that when the shape of the object to be grasped or its disposition inside the mechanical hand is not stable, a settlement motion of the object and a consequent variation of the fingers disposition are generated.
6. Influence of the Palm
The mechanical palm, present in bidimensional and in three-dimensional models, represents an element for the stabilization of the prehension.

Results
For this principle, the mechanical system with five fingers will be able to represent a more complete solution for its functional characteristics, for prehension of a large class of objects.

Future possible developments
The choice of these design principles is connected with a scientific analysis of the natural gifts offered by the human hand. An increase in the capabilities of human beings in front of working and life conditions might be obtained also in a future by the employment of procedures of interdisciplinarity. Results For this principle, the mechanical system with five fingers will be able to represent a more complete solution for its functional characteristics, for prehension of a large class of objects. Future possible developments The choice of these design principles is connected with a scientific analysis of the natural gifts offered by the human hand. An increase in the capabilities of human beings in front of working and life conditions might be obtained also in a future by the employment of procedures of interdisciplinarity.