Multi-Segmented Finger Design of an Experimental Prosthetic Hand

Jungov seminar o Ničeovom Zaratustritoliko poplavila da ne možete preći preko i morate čekati neđeljama;možete čak i umrijeti od gladi ako ste uhvaćeni izmeđudvije rijeke. I nije rijeka opasna samo zbog poplava već i zboggaženja preko vode ili njenog premošćivanja, gotovo ste sigurnida upadate u neugodnu situaciju. Svakako, taj strah ovđe nemasmisla uopšte za duže vrijeme, ali je tada bio mnogo značajan.Sasvim neočekivano, recimo, uđete u rijeku četrdeset ili pedesetjardi široku, nasipi su prilično strmi, ona vrvi od krokodila takoda nema plivanja; nosite sav teret preko i dođavola, nepokretniste. Vjerovatno lutate duž obala satima i satima da pronađete gazđe ćete manje ili više bezbjedno preći. Ili je možda palo drvo ilije pośečeno od urođenika tako da je palo preko rijeke i ako jevrijeme vedro možete biti sposobni da puzite preko vrlo tankogdrveta, prvo preko korijena i debla, a zatim kroz grane i onda sečudite kako sav teret možete prenijeti preko, iako je po kišnomvremenu ono svakako pakleno klizavo. Ali, bez najneznatnijihočekivanje nađete se u poziciji u kojoj ste bolje postupili negošto ste željeli. To je savršeno šašavo, čovjek je bio u nekojsasvim ugodnoj situaciji prije i onda se iznenada nađe u opasnostida se pokliza s drveta. I niko vas ne može zadržati jer nepostoji soba, prelazite preko ako možete, a petnaest ili dvadesetstopa niže su krokodili koji čekaju svoj doručak.Dakle, to je arhetipska situacija koja se pojavila nebrojenoputa, ako to baš nijesu krokodili, postoje neprijatelji koji čekajuda vas uhvate kad ste u vodi savršeno bespomoćni. Dakle, pretpostavljase za gazove, teške prelaze i takva mjesta da su naseljenazmajevima i zmijama, postoje čudovišta u dubokim vodama,neprijatelji u šumama, iza stijena i tako dalje. Zato je gaženjepreko rijeke tipična situacija koja izražava vrstu ćorsokakatako da je arhetip baš formulisan kad je čovjek u nekoj opasnojteškoći, i zato mnogi ljudi postaju sasvim nepotrebno arhetipskiuplašeni, uhvtio ih je najneopravdaniji strah. Čovjek može rećida ne postoji opasnost - zašto vam đavo ne daje da idete naprijed?Ali, oni su uplašeni da pređu čak i mali potok. Ili to može192 MATICA, zima 2009. www. maticacrnogorska.me

Proceedings of the Sixth National Applied Mechanisms & Robotics Conference December 1999phalanx link and the medial phalanx link. It isattached at a point on the distal phalanx link. Tooperate the finger, the cable is drawn back via anactuator, through the tube rigidly linked to the palm,causing the finger to flex closed. When grasping alarge object in a power grip, this design worksreasonably well.However, there are some major problems withthis design. The first is that this finger design is notappropriate for precision grasps involving finger tipand thumb tip contact (bi-digital pinch), because thefinger becomes unstable. Working Model 2Dsimulations of the bi-digital pinch confirmed theinstability. Upon formation of the bi-digital pinch,the medial interphalangeal joint would buckleinward, as shown in Figure 6, and cause the pinch tocollapse. In the process of this collapse, the objectwithin the grasp would be ejected outwards.Figure 7. Cable Finger Tip TrajectoryObserve that the cable is attached to the distalphalanx, but only slides through the medial andproximal phalanges via the tubes. When tension isapplied to the cable, a torque is developed aroundjoint A and the distal phalanx rotates, however, thecable does not transmit any force to the medial orproximal phalanges. Only when the distal phalanxfully flexes, can the cable create a torque around jointB, and later still around joint C.Finally, a minor disadvantage is that the cabledesign needs springs to return the fingers to the openposition when the cable tension is relieved.Therefore, the cable finger design was abandoneddue to its disadvantages.Figure 6.Cable Finger Collapse During PinchThe second problem with this design is that thefingers would not close in a trajectory suitable forgrasping objects. This trajectory was also very noncosmeticin dynamic appearance. During fingerflexion, first the distal phalanx link would flex fully,then the medial phalanx link would flex somewhat,finally followed by the proximal phalanx link. Thisincorrect motion is shown in Figure 7. During theclose of a natural hand, all of the phalanges flexinward at the same time, with the proximal phalanxflexing through a greater angle during the process. Inthis way, the natural hand can either pinch an object,or grasp an object and continue to ‘wrap’ theremaining phalanges around the object. The reasonfor the incorrect trajectory of the cable finger designcan best be explained with the use of Figure 7.The cable design was originally pursued afterinvestigation of a plastic toy robot hand with flexingfingers. It is interesting to note that the cable fingerdesign will work under the special condition wherethe cable is semi-flexible, as was the case for this toy.The cable itself could be a thick nylon orpolypropylene tendon, such that when tension isapplied to it, the flexing of the tendon defines thefinger shape, not the phalanges surrounding the cable.Because the tendon has ‘memory’ it returns to itsoriginal shape after the tension is released,eliminating the need for springs. The only drawbackof this design is the great amount of energy that isneeded to constantly flex this semi-rigid tendon.Such designs are currently being pursued by a groupin California[19].TWO D.O.F. RIGID LINK FINGERSThe single degree of freedom fingers that havebeen designed for the prototype hand, as shown inAMR 99-033-005

Proceedings of the Sixth National Applied Mechanisms & Robotics Conference December 1999Figures 3 and 4, can be converted into two degree offreedom fingers quite easily. This was not done forthe prototype hand due to space requirements and theneed for a second actuator, but it is shown here todemonstrate the versatility of the rigid link design.One degree of freedom can control the flexion andextension of the finger as before, while the additionaldegree of freedom can control the ‘curl’ or trajectoryof the finger. This is achieved by greater control oflink 6 in the finger as depicted in Figure 8.THUMB DESIGNThe distal portion of the thumb is similar to thatof the fingers, however the thumb uses only fourlinks. The thumb is able to flex and extend only thetwo outer segments (distal phalanx, and medialphalanx). The base segment (carpometacarpalphalanx) cannot flex or extend, but is able to adductand abduct (rotate). Rotation of the thumb isachieved by having the operator of the prosthesis usehis opposite hand to position the thumb, or bypushing the thumb against a fixed object, such as atable, to move it to the desired position.The thumb is illustrated in Figure 9. Link 4T isthe drive link that actuates the thumb. The entireassembly is able to rotate about the dashed linelabelled as the thumb rotational axis. This isachieved by rotation of the thumb base about thepalm base shaft.Figure 8.Two Degree of Freedom DesignLink 6 in the prototype hand is connected atthree points. One of these points is the slot pin whichslides through the straight slot in the x-axis. If thelink 6 slot pin is pulled in the positive x-direction, asindicated in Figure 8, the finger will flex closed. If itis driven in the negative x-direction, the finger willextend open. In order to add a second degree offreedom to this finger design, a mechanism could becreated that could drive the link 6 slot pin in the y-direction, in addition to the x-direction. By drivingthe slot pin in the positive y-direction, the fingerwould curl inwards more tightly during flexion.Alternatively, by driving the slot pin in the negativey-direction, the finger would curl less tightly(straighten out more) during flexion. This curlinginward or straightening out motion would beindependent of the flexion or extension of the finger.Together, if controlled properly, these two degrees offreedom could add increased functionality anddynamic cosmesis to a hand design.To implement the two D.O.F. design, anadditional actuator would be needed, more spacewould be required, and a proper control system forfinger coordination would need to be developed.Figure 9. Diagram of Thumb AssemblyThe drive cable provides the mechanical actuationneeded to flex and extend the two distal segments(phalanges) of the thumb. Flexion is achieved byapplying tension to the drive cable, which is loopedover the thumb pulley, and connected to link 4T.When the tension in the drive cable is relieved, thethumb return spring exerts a force on link 4T toreturn the thumb to the extended position. Figure 10illustrates the adduction and abduction (rotational)range of motion of the thumb. The use of a cableprovides for a very compact thumb design. The keyto the design is to keep the drive cable coaxial withthe thumb rotational axis. In this way, no matterwhich angle the thumb assembly is rotatedAMR 99-033-006

Proceedings of the Sixth National Applied Mechanisms & Robotics Conference December 1999to, the drive cable will always be able to flex andextend the thumb without slipping off the thumbpulley. The drive cable is connected to the adaptivegrasp mechanism at the other end.Figure 10.Thumb Range of Motion (Front View)The thumb can be rotated, within the anglesshown, giving the hand the ability to grasp flatobjects such as a credit card or key with the thumbout to the side, or to carry objects with only thefingers, such as a suitcase.Results and Discussion:One prototype hand was built according to thedesigns described. Figure 11 shows the prototypehand from the dorsal side, with fingers in theextended position and the thumb abducted (rotatedout to the side position).The cosmetic appearance of the hand is verygood. It is proportioned more or less like a child’shand. The fingers are 66 mm long (from knuckle totip), 9 mm wide and 11 mm thick. The palm is 80mm long, 65 mm wide and 25 mm thick. The palm islonger than normal, but future design changes canshorten it by up to 12 mm. To improve cosmeticappearance, a conventional prosthesis glove washeated and stretched to fit over top the prototypehand. With the glove on, the dynamic cosmesis ofthe hand, that is, the appearance of the hand while itis in motion, is excellent.The mass of the prototype hand alone is 217grams without the glove. The mass of the handincluding the motor, gearbox and wrist structure is280 grams. The motor used with the hand is a 6 VoltMicroMo 1724 with a 22:1 gearbox. Aluminium(7075-T6) was used for the finger links. The palmand thumb base were made of Delrin plastic. Thecable is made of Kevlar and the pulleys on which itrotates over are made of aluminium. In comparison,the Montreal Hand, which is capable of similarmechanical function to the prototype hand, has amass of 540 grams [6].The hand is able to achieve a maximum pinchforce of 14 N. This is approximately one-third thepinch force of a conventional prosthesis, such as theVASI 7-11 hands. Although the adaptive grasp maysecure some objects better, precision pinches of itemssuch as a fork would still require a high pinch force.Therefore, the pinch force must be improved upon.Also, the time to fully close the hand and achieve afirm pinch is 4 to 5 seconds, which is too slow to bepractical. It should be in the range of 1 to 1.5seconds. Both of these deficiencies are interrelatedand can be overcome with the implementation of atwo speed automatic transmission. Such a device hasbeen designed for prosthetic hands [20]. The OttoBock 2000 prosthesis [1], is a conventional, massproduced prosthesis that also uses a two speedautomatic transmission. The transmission wouldserve to increase the pinch force (when geared low)while decreasing the closing and opening times(when geared high).Figure 11.Prototype Hand, Dorsal ViewPull-out tests were done to determine the amountof force required to pull objects out of the hand’sgrasp. The grasp stability was found to be greater forsmall objects, when the fingers were able to partiallyor completely enclose objects by curling and adaptingaround them. Even with a pinch force of only 14 N,AMR 99-033-007

Proceedings of the Sixth National Applied Mechanisms & Robotics Conference December 1999these objects could not be easily removed from thehand in any direction. This enclosure of small objectsby the fingers is not possible with a conventionalprosthesis.The size and weight of the prototype hand arecomparable to a conventional child prosthesis, butcurrently the energy use, the closing/opening timesand the pinch force are not able to match aconventional prosthesis. It is expected that with theaddition of a transmission, the pinch force and timedeficiencies will be solved, but this will be at the costof increased weight and energy use.Conclusion:A child sized prosthesis has been designedwhich incorporates a passive adaptive graspmechanism. The adaptive grasp of this hand isachieved in three ways. Firstly, the fingers are able tocurl as they flex, and secondly an internal springmechanism allows the fingers to flex inwardsindependently of one another. The finger design hasbeen achieved with the use of multi-segmentedfingers that are restricted to a single degree offreedom. Although limited in some respects, thisfinger design greatly increases hand adaptability, withrelatively little increase in complexity. Thirdly, thethumb can be passively rotated, providing moregrasping configurations. These features have beenbuilt into a working prototype that is significantlysmaller than, and almost half the weight of any otherexperimental hand in its class.References:[1] Otto Bock Orthopaedic Industry GmbH, MYOBOCK-Arm Components 1997/98, Otto Bock(1997).[2] Variety Ability Systems Inc., Small and LightweightElectric Hands for Children, VASI, Toronto(1996).[3] Ramdial, S.;Wierzba, S., Personal Communication,Myoelectrics Service, Bloorview MacMillan Centre,Toronto.[4] Lozac’h, Y.; Drouin, G.; Vinet, R.; Beaudry, N., “Anew multifunctional hand prosthesis”, Proceedings of theInternational Conference of the Association for theAdvancement of Rehabilitation Technology, Montreal, pp86-87, 1988[5] Lozac’h, Y.; Drouin, G.; Vinet, R.; Chagnon, M.;Pelletier, M., “Specification for a New MultifunctionalHand Prosthesis”, Proceedings of RESNA 1986,Minneapolis, Minnesota, pp 117-119, 1986[6] Lozac’h, Y.; Madon, S., “Clinical Evaluation of aMultifunctional Hand Prosthesis”, Proceedings of RESNAInternational 1992, Toronto, 1992[7] Kyberd, P.J.; Chappell, P.H., “The Southampton Hand:An intelligent myoelectric prosthesis”, Journal ofRehabilitation Research and Development, Vol.31, No. 4,pp 326-334, 1994.[8] Chappell, P.H.; Kyberd, P.J., “Prehensile control of ahand prosthesis by a microcontroller”, Journal ofBiomedical Engineering, Vol. 13, pp363 -369, 1991[9] Bekey, G.A.; Tomovic, R.; Zeljkovic, I., “ControlArchitecture for the Belgrade/USC Hand”, pp 136-149. In:Venkataraman, S.T.; Iberall, T, Dextrous Robot Hands,Springer-Verlag New York Inc, New York, 1990[10] Rakic, Miodrag, “Multifingered Robot Hand WithSelfadaptability”, Robotics & Computer-IntegratedManufacturing, Vol.5, No. 2/3, pp.269-276, 1989[11] Jacobsen, S.C.; Wood, J.E.; Knutti, D.F.; Biggers,K.B., “The Utah/M.I.T. Dextrous Hand: Work inProgress”, The International Journal of Robotics Research,Vol. 3, No. 4, pp 21-50, 1984[12] McCammon, I.D.; Jacobsen, S.C., “Tactile Sensingand Control for the Utah/MIT Hand”, pp 239-266. In:Venkataraman, S.T.; Iberall, T, Dextrous Robot Hands,Springer-Verlag New York Inc, New York, 1990[13] Atkins, D.J.; Heard, D.C.Y.; Donovan, W.H.,“Epidemiologic Overview of Individuals with Upper-LimbLoss and Their Reported Research Priorities”, Journal ofProsthetics and Orthotics, Volume 8, Number 1, pp 2-11,1996[14] Dechev, N, Design of a Multi-Fingered, PassiveAdaptive Grasp Prosthetic Hand: Better Function andCosmesis, M.A.Sc. Thesis, Department of Mechanical andIndustrial Engineering, University of Toronto, 1998[15] Dechev, N.; Cleghorn, W. L.; Naumann, S., “MultipleFinger, Passive Adaptive Grasp Prosthetic Hand”,Mechanism and Machine Theory, Submitted forPublication, June 99.[16] Taylor, C.L., “Patterns of Hand Prehension inCommon Activities”, Engineering Prosthesis ResearchNo.3, Department of Engineering, University of CaliforniaLos Angeles, 1948[17] Knowledge Revolution, Working Model 2D version4.0, 1997, Knowledge Revolution, 66 Bovet Road, Suite200, San Mateo, CA.[18] Structural Dynamics Research Corporation, I-DEAS5.1, 1997, Structural Dynamics Research Corporation,2000 Eastman Dr., Milford, Ohio.[19] LeBlanc, M.; Setoguchi, Y.; Bowen, W.D.; Milner,C.; Burkholder, F.; Chen, S., “Design Concepts for anEndoskeletal Child-Sized Hand”, Journal of Prostheticsand Orthotics, Vol. 9, Num. 3, pp. 123-126, 1997[20] Guo, G.; Quian, X; Gruver, W.A., “A Single-DOFMulti-Functional Prosthetic Hand Mechanism with anAutomatically Variable Speed Transmission”, pp 149 -154.In: Robotics, Spatial Mechanisms, and MechanicalSystems, ASME, New York, DE-Vol.45, 1992AMR 99-033-008