Abstract
An “ideal” artificial hand should match the requirements of prosthetics and humanoid robotics. It can be wearable by the user which means that it can be perceived as part of the natural body and should replicate sensory-motor capabilities of the natural hand. However, such an ideal bionic prosthesis is still far from reality. This paper describes the design and fabrication of a novel artificial hand based on a “biomechatronic” and cybernetic approach. Our approach is aimed at providing “natural” sensory-motor co-ordination, by integrating biomimetic mechanisms, force and position sensors, actuators and control, and by interfacing the hand with the peripheral nervous system. Keywords Artificial hand, Biomechatronics, Prosthetics
1. INTRODUCTION
The objective of the work described in this paper is to develop an artificial hand which can be used for functional substitution of the natural hand (prosthetics) and for humanoid robotics applications. The artificial hand is designed for replicating sensory-motor capabilities of human hand. The work described in this paper is started from the analysis of state of the art artificial hands designed either for prosthetics or for robotics applications. Commercially available prosthetic devices, such as Otto Bock SensorHand™, as well as multifunctional hand designs [ 1 , 2 , 3 , 4 , 5 , 6 , 7 ] are far from providing the grasping capabilities of the human hand [8]. In prosthetic hands active bending is restricted to two or three joints, which are actuated by a single motor drive acting simultaneously on the metacarpo-phalangeal (MP) joints of the thumb, of the index and of the middle finger, while other joints can bend only passively. This limitation in dexterity is mainly due to the very basic requirement of limited size and weight necessary for prosthetic applications. On the other hand robotics hands have achieved high level performance in grasping and manipulation, but they make use of large controllers which are not applicable in prosthetics or humanoid robotics where it is necessary to provide the user with a wearable artificial hand. In this paper we propose a novel design methodology, for design, development and fabrication of artificial hands, aimed at balancing the two opposite requirements of high dexterity and of natural size, weight and appearance. This process can be indicated with the word biomechatronic design, which indicates concurrent mechatronic design with a biomimetic approach, aimed at replicating as much as possible working principles and behavior of living systems. Starting from the considerations outlined above, we can summarize the basic specifications of the wearable artificial hand described in this paper: 1. natural grasping capability; 2. natural appearance (in prosthetics this is called cosmetic appearance); 3. secure grasping and sensory feedback; 4. ”natural” command interface. The first and second requirements can be addressed by optimizing the actuator system of the artificial hand. The actuator system, the hand structure and the controller module have been conceived with a biomechatronic approach. This integrated design process carries to increase the number of active and passive DOFs, and to integrate them with an appropriate mechanism aimed at replicating the movement ability of the human hand. Furthermore, the cosmetic appearance of the natural skin has to be replicated by means of a sort artificial skin which should incorporate tactile microsensors. Anyway, at present the main problems of prosthetic hands in clinical practice are related to the size, and to the kinematic architecture which are not able to provide natural appearance and natural movements. For this reason we decided to address the problems of designing novel actuation system and mechanisms inspired to obtain a “natural” movement with human-like performance during grasping tasks. These choice is also motivated by the consideration that many amputees still prefer to use passive artificial hands with cosmetic gloves which provide a natural appearance with no functionality. A possible explanation of this choice is related to the poor functionality of commercially available prosthetic hands. Artificial skin is certainly the further step towards the development of a good artificial hand really “wearable” for amputees and also for humanoid robots. This paper is focused on the actuation system, higher number of actuators has been incorporated in the hand structure and coupled joints have been designed in order to replicate natural grasping.


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