Biomechatronic

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The objective of the work describe in this paper is to develop an artificial hand aimed  at replicating the appearance and performance of the natural hand  the ultimate goal of this research is to obtain a complete functional substitution of the natural hand. This means that the artificial hand should be felt by the user as the part of his/her own body (extended physiological proprioception(EPP) ) and it should provide the user with the same functions  of natural hand: tactile exploration, grasping , and manipulation (“cybernetic” prosthesis). Commercially available prosthetic devices, as well as multifunctional hand designs have good (sometimes excellent) reliability and robustness, but their grasping capabilities can be improved. It has been demonstrated the methodologies and knowledge developed for robotic hands can be apologies and knowledge developed for robotic hands can be applied to the domain of prosthetics to augment final performance. The first significant example of an artificial hand designed according to a robotic approach is the Belgrade/USC Hand. Afterwards, several robotic grippers and articulated hands have been developed, for example the Stanford/JPL hand and the Utah/MIT hand which have achieved excellent results. An accurate description and a comparative analysis of state of the art of artificial hands can be found in. These hands have achieved good performance in mimicking human capabilities, but they are complex devices requiring large controllers and their mass and size are not compatible with the strict requirements of prosthetic hands.
In fact, the artificial hands for prosthetics applications pose challenging specifications and problems, as is usually the case for devices to be used for functional replacement in clinical practice. These problems have forced the development of simple, robust, and reliable commercial prosthetic hands, as the Otto Brock Sensor Hand prostheses which is widely implanted and appreciated by users. The Otto Bock hand has only one degree of freedom(DOF), it can move the fingers at proportional speed from 15-130 mm/s and  can generate grip force up to 100 N.
According to analysis of the state of art, the main problems to be solved in order to improve the performance of prosthetic hands are
1)                     lack of sensory information gives to the amputee;
2)                     lack of “natural” command interface;
3)                     limited grasping capabilities;
4)                     Unnatural movements of fingers during grasping.
In order to solve these problems, we are developing a biomechatronic hand, designed according to mechatronic concepts and intended to replicate as much as possible the architecture and the functional principles of the natural hand.
The first and second problems can be addressed by developing a “natural” interface between the peripheral nervous system (PNS) and the artificial device (i.e., a “natural” neural interface (NI) to record and stimulate the PNS in a selective way. The neural interface is the enabling technology for achieving ENG-based control of the prostheses, i.e., for providing the sensory connection between the artificial hand and the amputee. Sensory feedback can be restored by stimulating in an appropriate way user’s afferent nerves after characterization of afferent PNS signals in response to mechanical and proprioceptive stimuli. 

The research described in this paper is focused on the third and the fourth points. In general, cosmetics requirements force to incorporate the entire device in a glove and to keep size and mass of the entire device comparable to that of the human hand. It turns out that the combination of robust design goals, cosmetics, and limitation of available components, can be matched only with a drastic reduction of DOF’s, as compared to those of the natural hand. In fact, in prosthetic hands active bending of joints is restricted only to two or three joints (metacarpo-pha-langeal joints of the thumb, of the index and of the middle finger), while the other joints are fixed. Due to the lack of DOF’s prostheses are characterized by low grasping functionality and, thus they do not allow adequate encirclement of objects in comparison to the human hand ; low flexibility and low adaptivity of artificial fingers leads to instability of the grasp in presence of an external perturbation, as illustrated in. In conclusion, commercial prostheses have been designed to be simple, robust and low cost, at the expense of their grasping ability.
This paper presents a novel multi-DOF hand several active joints, which is designed to obtain better grasping performance and natural fingers movements. The hand is designed according to a biomechatronic approach: miniature actuators and Hall-effect position sensors are embedded in the hand structure in order to enable the control of available DOF’s. This paper describes a prototype of the artificial hand which has been designed, fabricated, and tested in vitro, in order to assess the feasibility of the proposed approach.

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