Pneumatic Multi-Pocket Elastomer Actuators for Metacarpophalangeal Joint Flexion and Abduction ...

22 Pages·2017·14.63 MB·English

actuators
Article
Pneumatic Multi-Pocket Elastomer Actuators for
Metacarpophalangeal Joint Flexion and
Abduction-Adduction
TapioVeliJuhaniTarvainen1andWenweiYu1,2,*
1 DepartmentofMedicalEngineering,GraduateschoolofEngineering,ChibaUniversity,Inage-ku,
Yayoi-cho,1-33,Chiba263-8522,Japan;[email protected]
2 CenterforFrontierMedicalEngineering,ChibaUniversity,Inage-ku,Yayoi-cho,1-33,Chiba263-8522,Japan
* Correspondence:[email protected];Tel.:[phone]
Received:22July2017;Accepted:6September2017;Published:19September2017
Abstract: Duringrecentyears,interesthasbeenrisingtowardsdevelopingfluidicfiber-reinforced
elastomeractuatorsforwearablesoftroboticsusedinhandrehabilitationandpower-assist. However,
theydonotenablefingerabduction-adduction,whichplaysanimportantroleinactivitiesofdaily
living,whengraspinglargerobjects. Furthermore,thedevelopedglovesoftendonothaveseparate
controlofjoints,whichisimportantfordoingvariouscommonrehabilitationmotions. Themain
obstacle for the development of a fully-assisting glove is moving a joint with multiple degrees
of freedom. If the functions are built into the same structure, they are naturally coupled and
affecteachother,whichmakesthemmoredifficulttodesignandcomplextocontrolthanasimple
flexion-extensionactuator. Inthisstudy, weexploredthekeydesignelementsandfabricationof
pneumatic multi-pocket elastomer actuators for a soft rehabilitation glove. The goal was to gain
morecontroloverthemetacarpophalangealjoint’sresponsebyincreasingthedegreeofactuation.
Three main functional designs were tested for achieving both flexion and abduction-adduction.
Five prototypes, with four different actuator geometries and four different reinforcement types,
were designed and fabricated. They were evaluated by recording their free motion with motion
captureandmeasuringtheirtorqueoutputusingadummyfinger. Resultsshowedthestrengthsand
weaknessesofeachdesigninseparatingthecontrolofthetwofunctions. Wediscussthedifferent
improvementsthatareneededinordertomakeeachdesignplausiblefordevelopinganactuator
that meets the requirements for full assist of the hand’s motions. In conclusion, we show that it
is possible to produce multi-pocket actuators for assisting MCP joint motion in both flexion and
abduction-adduction, although coupling between the separate functions is still problematic and
shouldbeconsideredfurther.
Keywords:softrobotics;fluidicelastomeractuators;fiber-reinforced;multi-pocket;handrehabilitation;
fingerabduction-adduction
1. Introduction
1.1. Background
The probability of stroke, physical trauma and disease-related disabilities increases with age,
asthebodybecomesmorefragile. Thus,asthepopulationagesindevelopedcountries,demandfor
rehabilitationtherapyincreasestosupportthepeopleinremainingasfunctioningmembersofsociety.
This has led to increased interest towards developing devices that could be used for supporting,
protectingandrecoveringthefunctionsoftheaginghumanbody[1–4].
Previousstudieshaveshownthatusingroboticsystemsmayimprovethequalityandoutcomeof
rehabilitationandeasetheincreasingworkloadoftherapists[4]. However,traditionally,thesedevices
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havebeenbasedonrigidmechanics,whichrequiresprecisionengineeringtoensuresafeforcesand
torquesonthejoints[5]. Thissafetyconcernleadstohighcost,whichisoneofthemainobstaclesfor
introducingnewclinicalproducts[3]. Furthermore,effectiverehabilitationoftenrequiresexercisesto
bedoneathome. Thisleadstotheneedforeasy-to-useportablelow-costdevices[3]. Thus,thereis
roomfordevelopingcompletelydifferentapproaches.
Softroboticsresearchconcentratesondevelopinghighlycompliantstructuresforawidevariety
ofapplications[6]. Recentadvancesinthefieldhavebroughtnewmethodsfordevelopinglightand
safewearabledevicesthatcanbeusedforpower-assistandrehabilitation[7–10]. Thesestructurescan
beusedforcouplingtheforcesfromactuatorstothehumanbodysafelyandefficiently,becausethey
conformtoitsshapeandfollowitsnaturaljointtrajectories,althoughwithsomesafetyprecautions[5].
Furthermore, the materials and methods for making soft robotics are often much lighter and less
expensivethanthoseusedformakingequivalentrigidmechanisms. Especiallyhandrehabilitationisa
fieldwheresoftwearabledevicescouldproveuseful,astheyhavethepotentialtosupportthehand
motions,withoutblockingorotherwisehinderingthem,whilebeinglightandcomfortabletowear.
Hand rehabilitation exercises are done to recover lost functions by mobilizing and activating
asmanyofthejointsandmusclesofthehandaspossible. Thisreducesmuscleandjointstiffness,
minimizes loss of muscle and bone mass and prevents the possible formation of scar tissue [11].
These hand exercises are often referred to as six-pack, meaning six simple mobilizing exercises,
whichinclude(withsomevariationdependingonthetherapist):
1. Flexion-extensionoffingerMetacarpophalangeal(MCP)andthumbcarpometacarpaljoints
2. Flexion-extensionofInterphalangeal(IP)jointstoformahook
3. Flexion-extensionofMCPandproximalIPjoints
4. Formingafist
5. Fingerandthumbabduction-adduction
6. Thumboppositiontoeachfingerandslidingthumbfromthetipoflittlefingertoitsbase
1.2. RelatedResearch
Severalgroupsaroundtheworldhavestartedtodevelopsofthandrehabilitationandpower-assist
systems[12–14]. Thesesystemsarebasedonsoftfluidicfiber-reinforcedactuators,whichconsistof
apneumatically-orhydraulically-pressurizedpocket,orchamber,enclosedinapolymer,whichis
reinforcedwithsoftstrainlimitingmaterials.
Therehavebeenseveralstudiesondifferentdesignsforthistypeofactuator. Gerbonietal.[15]
developedmodularsoftmanipulatorswithmulti-pocketstructuresforminimally-invasivesurgery;
Park et al. [9] prototyped a power-assist device for the knee; Deimel and Brock [16] used them as
fingersfortheirrobotichand;andfinally,Noritsuguetal.[12],Polygerinosetal.[13]andYapetal.[14]
eachdevelopedtheirownversionsofaportablepower-assist/rehabilitationglove.
Thecurrentsoftfluidicactuatorsforrehabilitationandpower-assistglovesystemscontrolonly
oneaxisofmotionforeachjoint[12–14]. ThismeansthatthemotionoftheMCPjointcanbesupported
onlyinonedegreeoffreedom(DoF),althoughithastwo,flexion-extensionandabduction-adduction.
Sofar,thefinalprototypesofthedevelopedsystemshaveassistedeachfingeronlyinflexion-extension
and have not included the abduction-adduction motion [12–14], which is an important function
in activities of daily living, as it enables grasping of larger objects. Therefore, the current designs
do not achieve a full set of six-pack exercise motions. Furthermore, as the actuators have only
one input, these systems cannot control the joints separately. Instead, several actuators need to
be attached [12], which leads to cluttering of input tubing, or the actuators need to be switched
according to the exercise [17], which makes the device less convenient to use, for example in an
at-homerehabilitationscenario.
Amajorobstacleforimplementingafull2-DoFsupportfortheMCPjointmaybethecouplingof
theDoFs,whenthistypeofactuatorused. Thetwodesignedfunctionsmayconflictandaffecteach
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other, whichmakesitdifficulttocontrolmotiononlyinonedirection. Inotherwords, combining
flexionandabduction-adductionfunctionsinoneactuatormay,e.g.,causeittoflexforward,whenonly
abductionisintended.
1.3. GoalsofThisStudy
Ourfinalgoalistomakeapower-assistglovethatcanbeusedforassistingallsix-packexercises.
Inourpresentstudy,wehaveconcentratedonexploringthedesignofsoftactuatorsforachievingfull
assistoftheMCPjointmotions,includingbothabduction-adductionandflexion-extension. Wehave
excluded the distal joints of the fingers from this consideration, as they have only a single DoF,
andachievingtheflexion-extensionmotionhasbeenstudiedextensivelybefore[12–14,18].
Byaddingseparately-controlledpocketstotheactuators,i.e.,increasingthenumberofdegrees
ofactuation(DoA),itcouldbepossibletocontrolthemotionofeachjointseparately,alsoincluding
theabduction-adductionmotion,andachievingallsix-packexercises. However,weneedtosolvethe
problemofcouplingbetweenthetwoDoFs,whilemeetingthefunctionalrequirementsforeachof
them,tomakethisapproachplausible.
Themainfocusofthismanuscriptistodescribeandevaluatethefeasibilityofthreefunctional
designsformulti-pocketactuatorsfor2-DoFMCPjointmotionassist. Thedescriptionofourstudyis
dividedintotwoparts. Inthefirstpart,weexplainthedesignprinciplesanddesignofouractuators,
andinthesecond,wedescribethefabricationandevaluationoftheprototypes.
2. ActuatorPrototypeDesign
Inthissection,wedescribethebasicdesignprinciplesweusedforouractuators,theirfunctional
requirementsandthedifferentprototypedesignsthatwefabricatedandevaluated.
2.1. DesignPrinciples
Thefunctionofafluidicelastomeractuatorisdefinedbytheexpansionpropertiesofitsstructure,
when its internal pressure is changed. In other words, it is a balloon that has been restricted from
expanding in specific parts, which makes it exert forces and achieve motions and functions in
desireddirections.
Thedirectionandstrengthoftheactuator’sexpansionarecontrolledbyfourprinciples:
1. Geometryoftheelastomerstructure;
2. Heterogeneityofelastomermaterialsused;
3. Materialpropertiesandgeometryofembeddedreinforcingandstrain-limitingmaterials;
4. Couplingbetweentheactuatorandthemovedobjectorstructure.
Controlling the expansion by adjusting the geometry of an otherwise homogeneous material
is perhaps the simplest approach. The basic principle is that thinner walls will deform more than
thickerones, whentheactuatorispressurized. Thisisduetotherelativedifferenceinstiffnessof
thewalls. Furthermore,thealignmentofseparately-expandingpocketsrelativetoeachotherhasa
major influence on the actuator’s response, as the passive pockets still affect the expansion of the
activepocketsthroughthesurroundingstructures. Forsimplegeometries,itispossibletoestimate
thedeformationbyusingbasicbeamequations[16,19]. However,thefiniteelementmethodisoften
usedinsimulationstudiesforestimatingtheresponseofmorecomplexgeometries[20]. Theeffects
ofchangingdifferentstructuralparameters,suchaswallthickness,havebeenconsideredinseveral
previousstudies,e.g.,byDeimelandBrock[16],Polygerinosetal.[20]andUdupaetal.[21].
Thesecondprincipleistohavedifferenttypesofelastomersarrangedintheactuatorbodyso
thattheoneswithhigherstiffnessworkasstrainlimitersforthemoreelasticones. Again,thestiffness
parameterdefineswhichpartsoftheactuatordeformthemostrelativetotheotherparts. Anexample
ofthisisthestudyonfinger-likeactuatorsbyConnollyetal.[22]. However,inourstudy,weuseonly
onetypeofsiliconeelastomertoreducethenumberofvariablesaffectingtheactuators’response.
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Insteadofusingheterogeneouselastomers,itisalsopossibletochangetherelativestiffnessby
embedding,orlaminating,pre-madestructuresintotheelastomerthatformsthemainbodyofthe
actuator. Thesestructurescanbeforexampleintheformofasheetornetthatpartiallyorcompletely
limits the strain of the side of the actuator on which it is placed [16,20]. Another commonly-used
structureisareinforcementfiberthatiswoundaroundtheactuatorinspecificpatternsandangles,
limitingtheradialexpansionand,forexample,causingtheactuatortotwistarounditslongitudinal
axis[23]. Combiningactuatorsinparallelwithdifferentpatternsofreinforcementfibershasalsobeen
testedbyBishop-Moseretal.[24].
Finally,themethodofcouplingtheactuatortotheobjectitmoves,e.g.,afinger,definesthelast
restrictionsforitsexpansion. Thedifferentstrapsandsleevesthatareusedfortheconnectionrestrict
theactuator’sexpansion[25],whiletheshapeandmobilityofthefingeraddfurtherconstraintsto
itsmotion.
2.2. FunctionalRequirements
Inthisstudy,weconcentratedonlyonthemotionassistofthefingerMCPjoint,astheflexionof
thedistaljointshasbeenstudiedindepthinpreviousstudies,andwehavealreadydevelopedour
methodsforincludingthosejointsinafullfingeractuator[18].
Ourmaingoalwastodevelopactuatorstocontrolthetwo-directionalmotionoftheMCPjoint
easilyandefficiently. Thus,themainrequirementwastheabilityofactuatorstocontrolthesetwo
DoFsseparately,orinotherwords,minimizingthedetrimentaleffectsbetweenthetwofunctionsbuilt
intothesamestructure.
InordertohavefullmotionassistanceoftheMCPjoint,theactuatorsneedtohaveawideenough
RangeofMotion(RoM).Asareferenceforthis,weusedthefullactiveRoMsforthefingerMCPjoints.
Theyareonaverage: −45◦–100◦ forflexion-extensionand−20◦–20◦ forabduction-adduction[26,27].
Wedidnotconsiderextensioninthisstudy,buthaditonlyasapassivefunctionthroughmaterial
stiffness. Thus, we set the target flexion-extension RoM to be 0◦–100◦. The actuators also need to
expandinlengthinordertocompensateforthelengthdifferencebetweenthetopofastraightand
bentjoint. Intheprototypedesign,thispropertywasonlyapproximated,asitwasnotthemainfocus
ofourstudy.
Further,theactuatorsneedtotransferenoughtorquetothejointtomakeitmove. Thetorque
requirementsarenaturallydifferentforarelaxedhandandahandthatissufferingfromjointstiffness
caused by either scar tissues, or possible muscle spasticity related to neurological impairments.
We approximated this requirement based on acceptable torques for assisting the index finger
MCP joint motions. They are approximately 24–30 Ncm for flexion-extension and 17 Ncm for
abduction-adduction,accordingtoapreviousassessmentbyKawasakietal.[28].
Finally, the actuators need to be reliable and safe [5]. Thus, they should have a precise and
accurateenoughoutput,whichshouldnotchangeradicallyduringuse. Furthermore,theyshouldbe
robusttonotgetdamagedeasilyandhavealowpressuretotorqueoutputratio,tomitigatedangers,
iftheydogetdamaged.
Considering these requirements and applying the four design principles, we produced the
followingthreemainfunctionaldesigns.
2.3. GeneralPrototypeDesignSpecifications
Allofthedesignedactuatorshadthesamebasicdimensions(Figure1),approximatedtomatch
thesizeofanaverageindex,middleorringfinger,basedonmeasurementsbyBuryanovetal.[29].
Therootparthadaconnectorgrooveforconnectingtheactuatorstoatestbenchorasupporting
orthotic structure on the hand. All pressure input tubing was inserted through the root part into
the pockets. Each prototype had ready guide grooves around it for a radially-reinforcing string.
Thesegroovesweresettobeat2-mmintervalsalongtheactuator.
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Overall, four different internal geometries (Simple1, Simple2, Opposing and Parallel) and
fourdifferentreinforcementtypes(A–D)weredesignedforfiveprototypes(Simple1-A,Simple2-A,
Simple2-B,Opposing-C,andParallel-D).Detailsofthedifferencesintheprototypes’geometryand
reinforcementsareshowninFigures1and2,respectively. Thesedetailsaredescribedfurtherinthe
followingsections.
12 20 12 16 12 10 5 5
8
16
Simple1-A Simple2-A/B
(a) (b)
40
3
4
Opposing-C Parallel-D
(c) (d)
Figure 1. Transverse (left) and longitudinal (right) cross-sections of the four internal geometries
usedinthefabricatedprototypes,showinginsertionroutesfortheinputtubing.(a)Simple1,simple
side-pocketswithoutsupportinginternalwalls;(b)Simple2,simpleside-pocketswithtwosupporting
internalwalls;(c)Opposing,opposingside-pockets;(d)Parallel,fourparallelpockets.
bottom top bottom top bottom top bottom top
Simple1/2-A Simple2-B Opposing-C Parallel-D
(a) (b) (c) (d)
Figure2.Thefourdifferentreinforcementlayoutsusedinthefabricatedprototypes.(a)A,two-directional
thread winding, crossing itself on the bottom and top in the middle and on the sides otherwise;
(b) B, two-directional winding, crossing in the opposite manner from A; (c) C, two-directional
winding,crossingitselfmainlyonthebottomandonceonthetopinthemiddle;(d)D,singleloops.
A,B,andChadastrain-limitinglayerunderthem,excludedfrommiddle,whileDhadonlytheradial
reinforcementthread.
2.4. SimpleSide-Pockets
Thefirstdesign,Simple1,hadsimpleside-pocketsforabduction-adductiononthesidesofabigger
central pocket for flexion (Figure 1a). For this design, we made a variation, Simple2, which had
supportinginternalwallsintheabduction-adductionpockets(Figure1b). Thegoalwastotestwhether
thesupportswouldaffecttheactuators’response.Withoutsupportingwalls,thepocketswereexpected
toballoonfreely,leadingtoarelativelylargesidewaysexpansion,includingtowardstheinsideofthe
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actuator. Thisexpansionwasexpectedtobelimitedbythesupportingwalls,thusdirectingitmore
towardstheactuator’stipandincreasingtheforceoutputinthewanteddirection.
Thereinforcementhelixwaswoundaroundtheprototypesfromtheroottothetipandbackagain.
Thus,itcrosseditselftwiceeveryround. PrototypesSimple1-AandSimple2-Ahadthehelixcrossing
itselfontopandbottomoftheactuatorinthecenter16-mmpart(Figure2a). Thisway,thethreadwas
alignedstraightandevenlyonthesides. Ontheotherpartsoftheactuators,thehelixcrosseditselfon
thesides. ForprototypeSimple2-B,thehelixwaswoundintheoppositefashion(Figure2b).
A strain-limiting layer made of rayon net was embedded inside the silicone wall under the
actuators (Figure 2a,b). The middle region was left free to let the actuators expand in length
around the curvature of the MCP joint and to reduce the effect of flexion structures on the
abduction-adductionfunction.
2.5. OpposingSide-Pockets
The second design (Figure 1c) had two opposing pockets on each side of the flexion pocket.
This design of the side-pockets, sometimes referred to as bellows-type [30] or pleated type [31],
relied on opposing ballooning structures that press against each other to cause a bending motion.
Theside-pocketshadtheirthinnestwall(1mm)onthesideonwhichthedeformationneededtobe
largest,i.e.,wherethetwowallsweretopressagainsteachother.
We considered the main advantages of the opposing pocket structure to be that it would
concentratethedeformationtoalimitedarea,leadingtoamorepivot-likeresponseforabduction-
adduction,andseparateitfromtheflexionfunction.
FortheprototypeOpposing-C,thereinforcementhelixwaswoundsothatitwasalwayscrossing
itselfonthesides, exceptinthecenter, whereitcrosseditselfundertheactuator, andonceontop,
asit passedthrough thegapsbetween theopposingpockets (Figure2c). The strain-limiting layer
was embedded under the actuator in the same way as with the Simple prototypes, leaving the
abduction-adductionpartofthebottomfree.
2.6. ParallelPockets
The third prototype design (Figure 1d) was based on the idea of using multiple long pockets
laidoutinasymmetricalpatternalongthelengthoftheactuator. Differentialinflationofthepockets
wouldprovidemorecontroloverthedeformation,sothatthefingerjointmotionscouldbeadjustedto
matchthetargetmotions.
With a three-pocket design, it would be possible to achieve a full 360◦ directional control of
an actuator’s free bending, as shown by Gerboni et al. [15]. However, in the case of hand motion
assist,themajordifferenceisthatthefingerpresentsanextraconstraintfortheactuator. Furthermore,
the MCP joint has two perpendicular DoFs that we want to control separately. These aspects add
specificrestrictionsforactuatorgeometry,whichneedtobeconsideredinordertoachieveandcontrol
thewantedmotions.
Usingafour-pocketdesign,withpocketsinasquarepattern(Figure1d),itwouldbepossibleto
controltheflexion-extensionandabduction-adductionmotionsbyapplyingpressuretotwopockets
atatimeandadjustingtheirrelativepressures. Forexample,actuatingthetoppocketswouldmake
the top of the actuator expand, while the passive, relatively thicker bottom layer would limit the
expansion,whichwouldleadtoaflexionmotion. Similarly,whenoneortwoleftsidepocketsare
inflated,theactuatorwouldbendtotheright,leadingtoabduction-adductionmotion.
TheprototypeParallel-Dhadonlythereinforcementfibersaroundit,insingleloopsforsymmetry
(Figure2d),torestricttheradialexpansion. Wedidnotincludeastrain-limitinglayer,astheactuator’s
operatingprinciplewasbasedonstructuralsymmetryanddifferencesinrelativesiliconethicknesson
theactuatedandnon-actuatedsides.
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3. PrototypeFabricationandEvaluation
Thefunctionofthedescribedactuatordesignswasvalidatedbyfabricatingandtestingthefive
prototypes. Inthissection,wewilldescribethefabricationmethodsformakingtheprototypesand
theevaluationmethods(trajectoryandtorquemeasurements)fortestingandcomparingthem.
3.1. Fabrication
Theactuatorswerefabricatedoutofatwo-partplatinumcuringsilicone,DragonSkin10Slow,
byusing3D-printedmoldsandembeddedreinforcingmaterials. Siliconeglue,Sil-Poxy,wasused
for input tubing attachment, fixing possible delaminations, etc. Rayon net with approximately a
1mm×2mmmeshwasusedforthestrainlimitinglayer,and0.7mm-thickcottonstringwasusedfor
thereinforcementfiber. Siliconetubingwasusedforconnectionsintheactuatorsandforconnectingto
thepressuresources. ThemoldswereprintedoutofPolylacticAcid(PLA).
Weusedtwodifferentcastingworkflowsformakingthetestedprototypes’mainbody,i.e.,thepocket
structure.Thefirstwasusedfortheprototypeswithsimpleside-pocketsandopposingpocketsandthe
otherfortheparallelpocketprototype.
Thefirstworkflowhadtwomolds. Thefirstmoldwasusedforcastingthetoppartofthesilicone
pockets. Insertionholesfortheinputtubingwerecreatedbyattaching2mmdiametersteelsticksto
themold. Aftercastingthetopoftheactuatorstructure,theinputtubingwasplacedintotheholes
andgluedinplacewithsiliconeglue. Thesecondmoldwasthenusedforclosingthepockets. Inthis
step,thestrainlimitinglayerwaslaidoutinthemoldsothatitgotembeddedinsidethe2mm-thick
enclosure. Therayonnetwaslaidoutinthreelayers,whichwerealignedina45◦ anglerelativeto
eachother. Thisarrangementmadethecompositelayerbecomepracticallyinextensibleandstrong
againstdelaminatingforces,butstillhighlyflexible.
The parallel pocket prototype had only one mold. Furthermore, input tubing was placed in
themoldbeforecasting,whichreducedtherequiredwork. Thetippartsofthepocketswereopen
aftercastingthemainbodyandwereclosedbygluingasiliconeglue-filled2cm-longsiliconetube
intothem.
Thelaststepinmakingtheprototypeswasthewindingofthereinforcementhelixaroundthem
byhand. Thecottonstringwasfixedinplacebypouringalayerofsiliconeonit. Thesiliconewas
allowed to cure by hanging the actuators freely tip down from the input tubing. This ensured an
evendistributionofsiliconeonallsides,reducingresponsechangescausedbyunevenwallthickness.
ThefinalprototypesareshowninFigure3.
(a) (b)
Figure3.Thefiveprototypesshownfromthe(a)bottomand(b)right.Fromlefttoright:Simple1-A,
Simple2-A,Simple2-B,Opposing-C,Parallel-D.
WeproducedthesingleloopsforD-typereinforcement(Figure2d)byopeningthewindingsof
asinglethreadintothreeseparatestrands,andwindingeachonetwicearounditselfontheactuator.
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Thus,wegotthreeseparatethree-strandloops. Theendsofthestrandswerelockedinplacewith
areefknot.
3.2. TrajectoryMeasurements
The goal of the free trajectory measurements was to evaluate the designed prototypes’ RoM,
possibledeviationsfromthewantedstraighttrajectoriesandthesymmetryoftheirmotions. Thisgave
usinformationonhowwellthedesignedstructurescouldgeneratemotionwithouthavingtheirtwo
functionsaffecteachother.
Theprototypes’3D-trajectorywasmeasuredwithathree-cameramotioncapturesystem,attaching
amarkercluster,showninFigure4,ontheactuatortip. Themeasurementsweredoneat0–225kPa
pressures,with50-kPaintervals. However,theopposingpocketprototypehadamaximumpressure
of175kPaforabduction-adduction,becauseofitspockets’largedeformation. Formeasurementswith
twopocketsinflatedatthesametime,thepressurewasinputfromthesamesourcetobothpockets,
sothattheirpressurewasequal.
z
y
x
Figure4.Motioncapturemarkersetupfortrajectorymeasurements.Thetwocentermarkersonthetip
oftheactuatorformeditstipvector.Othermarkerswereusedasareference.
Astheparallelpocketprototypewassymmetrical,thetrajectorymeasurementsforflexionwere
donetwice. Fromhereon,werefertothesetwomeasurementsastopupandbottomup.
3.3. TorqueMeasurements
The goal of the torque measurements was to evaluate how well the actuators could transfer
theirdeformationtoforcesinthewanteddirectionsandthusapplytherequiredtorquesonahuman
finger. Aswiththetrajectory,wewereespeciallyinterestedintheprototypes’outputtorquerangeand
itssymmetry.
Forpracticalandsafetyreasons,thetorquemeasurementsweredonebycouplingtheactuators
to a two-bone dummy finger (Figure 5), which was scaled to match the dimensions of a middle
finger[29]. The3DmodelsusedwereacquiredfromThingiverse(HumanHandBones-Thumbby
sideritsandBall-SocketJointbyBrainSTEM)[32]. Thecombinedfingerstructurewas3D-printedand
attached to a clamp that held the prototype in place. A steel wire connected to a force gauge was
attachedtothetipofthedistalbone.
Thelengthofthedummyfinger’sproximalphalanx,fromthejointcentertotheattachmentpoint,
wasr =49mm. Theforcegaugewasalwayskeptinarightangletothebone,andthejointanglewas
changedbytiltingtheclamp. Thus,thetorqueatthejointwas:
τ =rF. (1)
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The dummy joint was set to specific angles: 0◦, 30◦and 60◦ for the flexion measurements
and 0◦ and 20◦ for abduction-adduction. The finger was restrained from flexing in the abduction-
adductionmeasurementwithafixedPTFE(Polytetrafluoroethylene)block. Thisensuredthatthewire
stayedalwaysatarightangle.
(a) (b)
Figure5.TorquemeasurementsetupwiththedummyMCPjoint.Thejointanglewassetto0◦,30◦and
60◦forflexionand0◦and20◦forabduction-adduction,keepingthedistalboneperpendiculartothe
forcegauge(above).Jointtorquewasthencalculatedfromthemeasuredforce.(a)Parallel-Dflexion,
60◦,175kPa;(b)Parallel-Dabduction-adductiontotheright,0◦,0kPa.
Theforcesweremeasuredata0–225kPapressurerange,with25-kPaintervals. Themaximum
pressures for each prototype were defined by observing the actuator’s deformation, keeping the
pocketsfromexpandingtoomuchinordertominimizeplasticdeformation. Theminimumpressure
fortheangledmeasurementswasdifferentforeachprototype. Itwasdefinedasthepressureatwhich
theactuatorsreachedthetargetangle. Simultaneousinflationoftwopockets,foropposingandparallel
pocket prototypes, was done by connecting both to the same pressure source, as in the trajectory
measurements.
Eachflexionmeasurementwasrepeated10times,andeachabduction-adductionmeasurement
15times,foreachpressure. Aswiththetrajectorymeasurements,theparallelpocketprototypewas
measuredtwiceintheflexiondirectiontodeterminethesymmetryofitstorqueoutput. Theprototype
wasturnedaround,andtheoppositesidepocketswereinflated. Asbefore,topupandbottomupare
usedtodistinguishthetwomeasurements.
4. Results
Inthissection,wepresenttheresultsofthetrajectoryandtorquemeasurements. Thefullsetof
numericaldataandmorephotosoftheexperimentsareavailableassupplementarymaterials.
4.1. Trajectory,Flexion
TheflexiontrajectorymeasurementresultsarepresentedinTable1andFigure6.
Table1.Maximumactuatortipanglesforflexiontrajectorymeasurements.
Parallel-D
Simple1-A Simple2-A Simple2-B Opposing-C
TopUp/BottomUp
FlexionRoM 160◦ 184◦ 166◦ 208◦ 107◦/103◦
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Looking at Table 1, we can see that all designs fulfilled the required RoM, bending past 100◦.
Opposing-ChadthelargestflexionRoMofalloftheprototypes,whileParallel-Dhadthesmallest.
Ofthesimpleside-pocketprototypes,Simple2-Awasflexingslightlymore(approximately20◦)than
theothers. Notably,thedifferencebetweentheseprototypeswastheinternalstructureforSimple1-A
andthereinforcementhelixlayoutforSimple2-B.
Simple1-A Simple2-A Simple2-B
10 10 10
0 kPa50 kPa 0 kPa 50 kPa 0 kPa50 kPa
8 8 8
6 100 kPa 6 6 100 kPa
ry 100 kPa
to m) 4 4 4
ajec Z (c 2 150 kPa 2 2 150 kPa
Tr 200 kPa 150 kPa
0 0 0 200 kPa
225 kPa 200 kPa 225 kPa
−2 −2 225 kPa −2
−4 −4 −4
n −1 −1 −1
o
tim)
ac 0 0 0
eviX (
D 1 1 1
0 5 10 0 5 10 0 5 10
Opposing-C Parallel-D Y (cm)
10 0 kPa 50 kPa 100 kPa50 kPa 100 kPa
8 8
y 6 6 150 kPa
r
to m) 4 100 kPa 4 225kPa
ajec Z (c 2 2 200 kPa
Tr
0 0
−2 150 kPa −2 TBooptt oump up
−4 225 kPa 200 kPa −4
n −1 −1
o
tim)
ac 0 0
eviX (
D 1 1
0 5 10 0 5 10
Y (cm) Y (cm)
Figure 6. Flexion trajectory results, showing the motion of the actuator tip vector formed by
twomarkers.OnlytheSimple2-AandParallel-Dprototypesstayedinarelativelystraighttrajectory.
Theotherstendedtoturntotheleft.Lengthwiseextensionisvisibleasthetipmarker’sdeviationfrom
aroundtrajectory.ForParallel-D,itispossibletoseeanendoftheflexionmotion,afterwhichonly
lengthwiseextensionoccurred.
Ofalltheprototypes,Simple2-AandParallel-Dhadthestraightestflexiontrajectories,whilethe
othersshowedmoredeviationfromthecenterline(Figure6).ThisdeviationwasstrongestforSimple2-B
andOpposing-C,whichdeviatedapproximately1cmtotheleftfromtheirstartingpoint.
Alloftheactuatorshadasimilarnonlinearpatternintheirbending(Figure6). Thefirst50kPa
ofpressurehadonlyasmalleffect,butalargedeformationhappenedbetween50kPaand200kPa.
The actuators also had a limit to their bending at approximately 200 kPa. After this, they mainly
continued to extend in the lengthwise direction, instead of increasing the flexion angle further.
ThiscanbeseenmostclearlyforprototypeParallel-D,whichhasthetipvectormakingabigjump
between100kPaand150kPaandthenmovingawayfromthecenter(z-axis),insteadofgoingdown,
between200kPaand225kPa. Thelengthwiseextensioncanbeseenalsofortheotherprototypesasa
deviationfromaroundtrajectory. ThisdeviationwassmallestforOpposing-C.