Abstract :
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.
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 prosthesis 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 generates 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
lack of
sensory information gives to the amputee;
lack of
“natural” command interface;
limited
grasping capabilities;
Unnatural
movements of fingers during grasping.
In order to
solve these problems, we are developing a bionic 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 prosthesis, i.e., for providing the sensory connection between
the artificial hand and the amputee.
Bio
mechatronic Design
The main
requirements to be considered since the very beginning of prosthetic hand
design are the following: cosmetics, controllability, noiselessness, lightness,
and low energy consumption. These requirements can be fulfilled by an
integrated design approach aimed at embedding different functions within a
housing closely replicating the shape, size and appearance of human hand. This
approach can synthesize with the term: “bionic” design.
Architecture
of the Bionic Hand
The design
goal of the bionic hand is to improve to some extent one of the most important
limitations of current prosthetic hands (no dexterity and no adaptability),
while preserving the main advantages of such hands, that is lightness and
simplicity. This objective has been pursued by using small actuators(two of
each finger) instead of one single large actuator( as in most current
prosthetic hands) And by designing a kinematic architecture able to provide
better adaptation to object shape during
grasping. It turns out that the use of “micro motors” allows augmenting
functionality in grasping objects by means of “human-like” compliant movements
of fingers. This result addresses the very basic requirements of “cosmetic”
appearance of the hand in static and dynamic conditions.
The bionic
hand has three fingers to provide a tripod grasp : two identical fingers(index
and middle fingers) and the thumb(see Fig.)
In fact, as
explained in , at least three fingers (non rolling and non sliding contact) are
necessary to completely restrain an object.
The hand
performs two grasping tasks:
1)
Cylindrical grasp
2) Tripod
grasp
The finger
actuation system is based on two micro actuators which drive the meta carpophalengal
(MP) and the proximal interphalengal (PIP) joint. The thumb actuation system is
based on micro actuators and has two active DOF’s at the MP and the
interphalengeal (IP) joint, respectively.
The
grasping task performed by the hand compromises two subsequent phases:
Reaching
and shape-adapting phase
Grasping
phase with thumb opposition.
In phase
one, the first actuator system allows the finger to adapt to the morphological
characteristics of the grasped object. In phase two, the second actuator system
provides thumb opposition for grasping.
In section
III, the basic criteria for designing the actuation system according to bionic
approach are described.
C.
Actuation System
The
adoption of bulk and heavy actuators in the design of commercial upper limb
prostheses, leads to an extreme reduction of DOF’s. The goal is to achieve
stable grasp by means of high grip forces. This design philosophy can be represented
as a loop. The above schematization shows how this approach leads to design
hands with a maximum of two DOF’s and able to obtain stable grasps using high
pinch force (about 100N). To summarize, mechanical grippers such as state of
art prosthetic hands, can generate large grasping forces and are simple to
implement and control, but they are not adaptable and may cause problems of low
grasping stability.
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