INTRODUCTION
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” prothesis). 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 prothestics 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 achived excellent results. An accurate description and a comparative analysis of state of the art of artificial hands can be found in . These hands have achived 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 SensorHand prothesis 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 generatea 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 prothetic 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 th 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 prothesis, 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 “biomechatronic” design process described above is illustrated in the scheme depicted in Fig.1.
The reseach 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 charcterised by low grasping functionality and, thus they do not allowe adequate encirclement of objects in comparison to the humand hand ; low flexibility and low adaptivity of aritifical fingers leads to instability of the grasp in presence of an external perturbation, as illustrared in. In conclusion, commercial protheses 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 acuatours 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.
DESIGN OF THE BIOMECHATRONIC HAND
A. 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 synthesized with the term: “biomechatronic” design.
B. Architecture of the Biomechatronic Hand
The design goal of the biomechatronic 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 “micromotors” allows to augment functionality in grasping objects by means of “human-like” compliant movements of fingers. This result addresses the very basic requirements of “cosmetic” apperarance of the hand in static and dynamic conditions.
The biomechatronic hand has three fingers to provide atripodgrasp:two identical fingers(index and middle fingers) and the thumb(see Fig.2)
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 microactuators which drive the meta carpophalengal (MP) and the proximal interphalengal (PIP) joint.the thumb actuation system is based on microactuators 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 phasse two, the second actuator system provides thumb opposition for grasping.
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.
The approach we propose to invert the previous”loop” by using microactuators and by exploiting the advantage of increasing DOF’s.
According to the design philosophy, an artificial hand actuated by a plurality of microdrives would have enhanced mobility and, thus, larger contact areas between phalanges and grasped object. Therefore, a reduction of power actuation could be accepted and compensated by increasing contact areas in order to augment grasp stability. In fact according to a hand with independently movable fingers and multiple p[halanges can encircle the object much better than a hand with rigid fingers. In addition, the contact area between an object and the finger can be larger and, thus, grasping stability is enhanced.
DESIGN OF HAND PROTOTYPE.
In order to demonstrate the feasibility of the described biomechatronic approach, we have developed a three fingered hand prototype with two identical fingers (index and middle) and thumb. Actuators, position sensors and a 2-D force sensor are integrated in the hand structure.
The index/middle finger has been designed by reproducing, as closely as possible, the size and kinematics of a human finger. Each finger consists of three phalanges and a palm needed to house the proximal actuator.
A. Actuator System Architecture
In order to match the size of a human finger, two micromotors have been integrated within the palm housing and the proximal phalange of each finger.
The selected micromotors are Smoovy (RMB, eckweg, CH) microdrivers (5mm diameter) high precision linear actuators, based on the dc brushless motors with planetary gears. The rotary motion of the shaft is converted to linear motion using lead screw transmission.
The main mechanical characteristics of the linear actuators are listed below (see table I).
The selected actuator fulfills almost all the specifications for application in the prosthetic finger: small size and low weight. The main problem encountered is related to noise which turns out to be relatively high, atleast in the current implementation. Despite of his limitation, we decided to proceed with the application of the linear actuator in order to investigate integration problems and global performance.
The shell housing provides mechanical resistance of the shaft to both axial and radial loads system . this is veery important diring grasping task, when the forces generated from the thumb opposition act on the whole finger structure
B. Kinematics architecture
The kinematics of each finger joint is described in the follwing subsections
1. mp joint: the proximal actoator is integrated in the palm and transmits the movement through a slider- crank mechanism to the proximal phalanx, thus , prviding flexion/extension movement. The slider is driven by the lead screw transmission mounted directly on the motor shaft.
2. pip joint : the same mechanism used for mp moves the pip joint. Only the geometricl features in order that the size of the mechanism fits within the space available according to the strict specification of the biomechatronic hand.
3. dip joint: a four bars link has been adopted for the dip joint and its geometrical features have been designed in order to reproduce as closely as possible the natural dip joint flexion. The mechanism has been synthesized according to the three prescribed postion method.
Due to the high transmission rate(planetary gears and lead screw transmission) friction is high and, thus, the joints are not back-drivable. This causes problem in controlling accurately in hand. However the positive side effects of the friction is that the grasping forces can be exerted even when power supply is off, a very important function for hand procestheses.
C. THUMB DESIGN
The thumb has been designed to perform grasping task by thumb opposition. The thumb has been obtained by simly removing the distal phalanx from the index/middle finger .
HAND FABRICATIONS
The hand protected comprises the three fingers (index middle and thumb), each with two-DOF’s actuated by micro motors and sensorised by hall-effect position sensors and by strain gage-based force sensors. The characteristics of the position sensors and of the force sensors are illustrated in following sections.
The three fingers have been fabricated using the fused seposition modelling (FDN) process. This process allows obtaining 3-E complex shapes from CAD models easily, quickly and cheaply. The main limitation of the FDM process resides in poor mechanical characteristics of the material that must be used, which is acrylonitrile/butadiene/styrene (ABS).how ever; this is acceptable for a prototype.
POSITION AND FORCE SENSORS
Position and force sensors have been incorporated in the fingers in order to increase the controllability of the hand
A. position sensors
A position sensor, based on the Hall Effect sensor is mounted at each active joint of the hand. The main advantages the Hall Effect sensors are there small sizes and there contact less working principle. In each finger, the hall sensors are fixed, respectively, to the palm and to the proximal phalanxes, where as the magnets are mounted directly on the sliders of each active joint.
In this configuration the sensor measures the linear movement of the slider, which is related to the angular position of the joint. In each MP joint, the linear range of the sensor is 5.2mm, where as in the PIP joint the linear range is 8mm.
Using a micrometric translator stage we found to optimal configurations for the position sensors. In the first optimal configurations two magnets are used at a distance of 3.5mm.this configuration has a working range of 5.4mm with a linearity of5.34%.
The second optimal configuration (suitable for MP joints) has six magnets and a working range of 8.4mm with a linearity of 3.81%.
A finger prototype with integrated position sensors is showed in fig 8.
B.2D FORCE SENSORS
A 2-D force sensor, based on strain gages technology, has been developed in order to sensorize the digital phalanx of index and middle fingers. The force sensor measures both normal and tangential forces.
The sensor design has been optimized using the pro-mechanica structure software.
D. SENSORS CHARACTERIZATION
1. Characterization of position sensors: we found that the best simplest way to characterize these sensors is use an optical method. Used a Nikon Coolpix 950 digital camera mounted on a tripod in order to record the movement of the finger. The movement of each Smoovy actuator was driven by a CCS00001 controller (RMB, CH).each controller has a power supply of 11V,.while each sensor was supplied with 6V.
For each active joint 100 different frames, 50 for flexion and 50 for extension movements where acquired. For each frame the output value of the sensor was measured with a digital multimeter and recorded, where as the position of the joint was measured using the module measures to Adobe Photoshop 5.5 with a precision of 0.1o .
Results are presented in fig 10. for the sensor in MP joints and in the PIP joints ,respectively. the flexion phase is indicated with a small dark circles,while the extension is indicated small light squares.
It is important to point out the both curves for both sensors generally present low hysterieses. Tehe differerce between the flexion and the extension curves is mainely due to the mechanical clearance of the sensorised slider.
2. characterization of 2-D 4 sensor: the force sensor was characterize using an INSTRON 4464 testing machine.
A traction-compression loading cycle (0N-10N-0-N)was performed for each direction.
Results are presented in fig 11, for the normal loading direction and the tangential loading direction respectively. Diagram show a linear behaviour of the 2-D foce sensor.
FINGERED TRIP FORCE ANALYSIS
a first set of experimental test has been perfrmed in order to ec\valuate the force that the index/middle finger is able to exert on an external object . to this aim we are measured the force resulting when the finger is pressing directly on the high accuracy piezoelectric load cell.corresponding to different configuration of the joints.
To “pressing “ task were identify in order to evaluate seperateli and independenly the force generated by actuators of the fingers .
TASK1: the pshing action is exerted only by the distal actuater
TASK2: the pushing action is exerted only by the proximal actuator
CONCLUSION
In this paper, novel approach to the design and fabrication of prosthetic hands, called biomechatronic design, has been presented. The biomechatronic design consists of integrating multiple DOF’s finger mechanisms, multi sensing capabilities and distributed control in order to obtain human like appearance, simple and direct controllability and low mass. The biomechatronic design approach can lead to the development of hand and prostheses, when combined with other important factors, such as low energy consumption for adequate autonomy( at least eight hours between recharges ), noiseless operation fore not disrupting social interactions, cost suitable for support by the health insurance system and above all sensory feedback to the amputee through interfaces. A biomechatronic hand prototype with three fingers and a total of six independent DOF’s has been designed and fabricated. This paper is focused particularly on the analysis of the actuation system, which is based on miniature electromagnetic motors.
Current work in our lab is directed to improve the limitations of the prosthesis is presented in this paper. First of all, a new design has been devised aimed at increasing the grasping the force of the hand while retaining the main positive characteristics of previous design. The new hand architecture is based on under actuated mechanisms, comprising a total of two dc motors. The hand has nine non independent DOF’s (so it can still grasp rather effectively objects of complex shape) and can generate grasping force of about 30 N.
A second important objective that we are pursing is to implement a neural of the hand by means of interfaces implanted at peripheral nerves of the amputee. This very challenging goal could ultimately lead to the development of a truly cybernetic hand, controlled and received by the amputee almost at his /her own lost hand and, therefore, a real potential alternative to hand transplantation.
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