Pseudo-Haptic Feedback : Can Isometric Input Devices Simulate Force Feedback? Anatole L´ ecuyer Aerospatiale Matra CCR Sabine Coquillart INRIA Rocquencourt Abderrahmane Kheddar CEMIF-SC, UEVE Paul Richard LRP, CRIIF Philippe Coiffet CNRS, LRP Abstract This paper considers whether a passive isometric input device, such as a TM , used together with visual feedback, could provide the operator with a pseudo-haptic feedback. For this aim, two psychophysical experiments have been conducted. The first experiment consisted of a compliance discrimination, between two virtual springs hand-operated by means of the TM . In this experiment, the stiff- ness (or compliance) JND turned out to be 6%. The second experiment assessed stiffness discrimination between a vir- tual spring and the equivalent spring in reality. In this case, the stiffness (or compliance) JND was found to be 13.4%. These results are consistent with previous outcomes on manual discrimination of compliance. Consequently, this consistency reveals that the passive apparatus that was used can, to some extent, simulate haptic information. In addition, a final test indicated that the proprioceptive sense of the subjects was blurred by visual feedback. This gave them the illusion of using a non isometric device. 1. Introduction Isotonic or isometric 3D input devices 1 are clever de- vices for the purpose of 3D interactions and 3D manipula- tions of objects. They are compatible with almost all ma- Corresponding author: Anatole L´ ecuyer. Aerospatiale Matra, Centre Commun de Recherche Louis Bl´ eriot, 12 rue Pasteur, 92152 Suresnes, France. Email: [email protected]or ana- [email protected]Corresponding author: Sabine Coquillart. INRIA, Domaine de Voluceau, Rocquencourt - BP 105, 78153 Le Chesnay Cedex, France. Email: [email protected]1 Zhai classified the input devices into two categories : ISOMETRIC devices (they offer resistance and stay put while you exert force on them) and ISOTONIC devices (they offer no significant resistance and are used to track users as they move around the virtual world) [21]. jor CAD softwares available on the market. Their user- friendliness has shown their potential usability as a tool for off-line robot programming and teleoperation, or vir- tual prototyping. For instance, to teleprogram the Mars Pathfinder Sojourner robot, the operator uses a TM as an input tool along with a virtual reality interface [1]. The TM is an isometric input device with six de- grees of freedom (dof) which is now commercialized by the Spacetec company [3]. The Magellan TM [2] - another 6dof isometric device - was successfully used by the DLR - the German Space Agency - operators and astro- nauts to teleoperate a space robot within the context of the well-known ROTEX experiment [11]. Since isometric or isotonic input interfaces are com- pletely passive, they have never been regarded as being able to return forces. How the use of the properties of an isomet- ric input device, the Spacetec TM 2003C model, together with visual feedback to provide force information to the operator is the subject of the following paragraph. To begin with, there is to take advantage of the mechani- cal characteristics of the isometric device : its internal stiff- ness and its thrust. Those characteristics are combined with visual feedback to provide a kind of pseudo-force feedback. For example, let us assume that one manipulates a virtual cube in a 3D virtual environment (VE). The cube must be inserted inside a narrow duct. As the cube penetrates the duct, its speed is reduced. In other words, the TM output resolution, which controls the cube motion, is de- creased. Consequently, the user will instinctively increase its pressure on the ball which results in the feeding back of an increasing reaction force by the static device. The cou- pling between the slowing down of the object on the screen and the increasing reaction force coming from the device gives the user the illusion of a force feedback as if a friction force between the cube and the duct was directly applied to him. This “illusion” of force feedback was first qualitatively estimated with a group of 18 people during an experiment
Transcript
Pseudo-Haptic Feedback :Can Isometric Input Devices Simulate Force Feedback?
AnatoleLecuyer�
AerospatialeMatraCCR
SabineCoquillart�
INRIA Rocquencourt
AbderrahmaneKheddarCEMIF-SC,UEVE
Paul RichardLRP, CRIIF
PhilippeCoiffetCNRS,LRP
Abstract
This paperconsiders whethera passiveisometricinputdevice, such as a ��������� ����� TM, usedtogether with visualfeedback, couldprovide theoperator with a pseudo-hapticfeedback.
For this aim, two psychophysicalexperimentshavebeenconducted.Thefirst experimentconsistedof a compliancediscrimination,betweentwo virtual springshand-operatedby meansof the ��������� ����� TM. In this experiment,the stiff-ness(or compliance)JNDturnedout to be6%. Thesecondexperimentassessedstiffnessdiscriminationbetweena vir-tual springandtheequivalentspringin reality. In thiscase,thestiffness(or compliance)JND wasfoundto be13.4%.
In addition,a final testindicatedthat theproprioceptivesenseof the subjectswasblurredby visual feedback. Thisgavethemtheillusion of usinga nonisometricdevice.
1. Introduction
Isotonic or isometric3D input devices1 are clever de-vicesfor thepurposeof 3D interactionsand3D manipula-tions of objects. They arecompatiblewith almostall ma-
Correspondingauthor: Sabine Coquillart. INRIA, Domaine deVoluceau,Rocquencourt- BP 105, 78153 Le ChesnayCedex, France.Email: [email protected]
1Zhai classifiedthe input devices into two categories: ISOMETRICdevices(they offer resistanceandstayput while you exert forceon them)andISOTONIC devices(they offer no significantresistanceandareusedto trackusersasthey movearoundthevirtual world) [21].
jor CAD softwaresavailable on the market. Their user-friendlinesshas shown their potential usability as a toolfor off-line robot programmingand teleoperation,or vir-tual prototyping. For instance,to teleprogramthe MarsPathfinderSojournerrobot,theoperatorusesa �������������� � TM
as an input tool along with a virtual reality interface[1].The �����!�������� � TM is an isometricinput device with six de-greesof freedom(dof) which is now commercializedby theSpaceteccompany [3]. The Magellan �����!�"�"#%$'&�()� TM [2]- another6dof isometricdevice - wassuccessfullyusedbytheDLR - theGermanSpaceAgency - operatorsandastro-nautsto teleoperatea spacerobotwithin thecontext of thewell-known ROTEX experiment[11].
Since isometric or isotonic input interfacesare com-pletelypassive,they haveneverbeenregardedasbeingableto returnforces.How theuseof thepropertiesof anisomet-ric input device, the Spacetec�����!�"�*���!�+� TM 2003Cmodel,togetherwith visual feedbackto provide force informationto theoperatoris thesubjectof thefollowing paragraph.
To begin with, thereis to takeadvantageof themechani-cal characteristicsof theisometricdevice : its internalstiff-nessandits thrust.Thosecharacteristicsarecombinedwithvisualfeedbackto provideakind of pseudo-forcefeedback.For example,let us assumethat onemanipulatesa virtualcubein a 3D virtual environment(VE). The cubemustbeinsertedinsidea narrow duct. As the cubepenetratestheduct, its speedis reduced.In otherwords,the �������������� � TM
output resolution,which controlsthe cubemotion, is de-creased.Consequently, the userwill instinctively increaseits pressureon theball which resultsin thefeedingbackofan increasingreactionforceby thestaticdevice. Thecou-pling betweentheslowing down of theobjecton thescreenand the increasingreactionforce coming from the devicegivestheusertheillusion of a forcefeedbackasif a frictionforcebetweenthecubeandtheductwasdirectly appliedtohim.
This “illusion” of force feedbackwasfirst qualitativelyestimatedwith a groupof 18 peopleduringan experiment
calledthe swamp. The subjectsweretold to manipulateavirtual cube,displayedon thehorizontalplane,andto crosssquareareas(seeFigure 1). When over theseareas,thespeedof the cubewaseitheracceleratedor slowed down.Thesubjectsweretold to describetheirsensationswhenthecubewascrossingtheareas,andto comparethesensationsthey felt whenusingeither the �����!�������� � TM or a classical2D mouse.While usingthe �����!�"�*���!�+� TM, their accountsre-
Figure 1. Swamp experiment: cube crossinga slo wing down area.
vealedthatthesubjectsfelt “something”asthecubecrossedtheseareas.Most subjectsexperienceda senseof friction,gravity or viscositywhenthecube’smovementwassloweddown. They foundthat forcesweremuchmoreperceptiblewith the �����!�������� � TM than with the 2D mouse. The per-ceptionof forceswasa bit lesssharpwhen the cubewasaccelerated.This is probablydueto the fact that the reac-tive force from the �������������� � TM is moreperceptibleduringcompressionphases.
Thosequalitative indicationsrevealedthe potentialitiesof this concept,but they did not measureor identify thecharacteristicsof suchanillusion. It wasnecessaryto eval-uatethe feedbackmorequantitatively. A compliancedis-crimination task betweena real and a virtual spring waschosenas a simple evaluationtask. The real spring wastestedfirst in the real environment(RE), then the virtualspringwastestedin theVE. Thevirtual springwasgraph-ically displayedon a computerscreenand was dynami-cally animatedwhenpushingthe �������������� � TM ball. If the�����!�"�*���!�+� TM, usedtogetherwith visualfeedback,allowsoneto discriminatea virtual spring stiffnessfrom a real one,thenthewholesystemmaythusbefit for feedinghapticin-formationwhich wassupposedlydifficult to provide with-outa forcefeedbackinterface.
First,anoverview of previouswork in thefield of 3D in-put interfacesevaluationandsensoryillusionswill be pre-sented. It will be followed by a descriptionof the exper-imentalsystemwhich wassetup for the evaluationof thestiffnessof the real and virtual springs. Previous worksconcerningcomplianceor stiffnessdiscriminationandJND(JustNoticeableDifference)will also be mentioned. Thetwo following sectionsdescribetwo psychophysicalexper-
iments:thecompliancediscriminationbetweentwo virtualsprings,andthe compliancediscriminationbetweena vir-tual spring and a real one. The first experimentis a VEevaluation;while thesecondexperimentis themainsubjectof thestudy. Thepaperendswith a generalconclusionandareferenceto furtherwork.
2 Previous work
Force/tactileinterfaceshave beendevelopedin recentyears[6] in orderto provide force/touchfeedbackto users.They receive motor actionsfrom the userandsendhapticimagesto him. Theseinterfacesareusedto simulateawiderangeof object dynamicssuchas hardnessandelasticity.Yet, today, they arestill expensiveandcomplex.
Theperceptionof realor virtual environmentsis not re-strictedto the intra-sensoryinterpretationcues. Cuessentby differentsensesaresomehow interpretedtogether. Forinstance,manipulatingobjectscombinestactile,kinestheticsensationsandoften vision [7]. Given thesecomplexities,it would seemmoreappropriateto investigatethe “plural-istic” natureof sensoryperception,ratherthanoneisolatedsense.Aldridge [4] observedthat thevisual representationof a virtual objecthassomeeffect on the integrationof thetouchfeedback.He statedthat furtherexperimentsneededto becarriedout in orderto explore theextentof such“vi-sualdominance”.
Previous work on visual dominanceshowed that multi-ple cuesoffer a high level of redundancy andcanimprovesignal-to-noiseratios. For instance,it hasbeenshown thatlip-readingmodifiestheauditorycortex, andenhancesaudi-tory perception[14]. Oneinterestingissueis how thesedif-ferentsourcesof informationareall combinedto form whatmight becalledholistic perceptions.A famousexampleofvisualdominanceis theVentriloquist’seffect [19]. Diderot(1749), offeredearly supportfor the existenceof sensorydominance[13]. Several researchershave demonstratedadominanceof visionovertaction[7]. LeeandLishmanpro-vided evidencethat vision playsan integral role in humanstancecontrol (balance).This “visual proprioceptive con-trol” isshown to dominateovernon-visualinformation.LeeandLishmandescribedalsothetuningrole thatvisualpro-prioceptionplaysin learninga new stance(i.e. ankle-footproprioception).Thissuggeststhatvisionplaysamajorrolein makingthingsfeel theway they do.
But vision may sometimesmake things feel differentthan they are. Katz [7] observed that different materials(paper, rubber, leather, etc..) caneasilybe interpreteddif-ferentlyby blindfoldedsubjects.Srinivasan[16] foundthatvisioncouldalsomisleadsomeoneduringacompliancedis-crimination taskbetweentwo springs. The displacementof thespringswasvisually observedon a computerscreen,while springswere pressedmanuallyby meansof a me-
chanicalapparatus.Srinivasanobserved that an inapropri-atevision feedbackcantotally invert the stiffnesspercep-tion and the result of the discrimination,which ushersintheillusion concept.
Illusion playsa centralrole in a VE perception.Illusionis a non veridicalperception,a mistake madeby the brainandnotby oursenses.Well-known opticalillusionssuchastheMuller-Lyer illusion areextensively describedin scien-tific works[10]. Somehapticillusionsmayalsoberevealedby simpleexperiments.For example,Weberfirst observedthat the temperatureof anobjectinfluencesthe hapticper-ceptionof its weight: a cold coin seemsheavier than thesamecoinwhenwarmer[15]. Anotherhapticillusion is thesize-weightillusion: a largeradiusball seemsheavier thana ball of the sameweight, but with a smallerradius. Re-cently, Ellis andLederman[8] establishedthe size-weightillusion asa primarily hapticphenomenon,despiteits hav-ing beenmoretraditionallyconsideredanexampleof visioninfluencinghapticprocessing.Theresortto intra andinter-sensoryillusionsanddominancecanberelevantwhenusedin VR applications. The following paperconcentratesontheuseof thesepotentialities.
3. Experimental set-up
As alreadystated,the aim of the experimentalsystemdescribedis to measurethecapacityto feedbackhapticin-formationby meansof a passive isometricinputdeviceandvision feedback,which is the key issueof the scheme.Itis coupledwith the force appliedon the �����!�"�*���!�+� TM, andany changein the visual feedbackgeneratesa differencein force perception.The perceptionof the stiffnessof thespringinvolvesamulti-modalcombinationof forceanddis-placement.Thus,it seemedto be an appropriatemodeltodemonstratethegeneralconcept.
An experimentof compliancediscriminationof springswasthenchosenbecauseof the simplicity of the modelofstiffness, relevant previous works on manualdiscrimina-tion of compliance,andthefact thatspringsarea classicaland fundamentalelementin computergraphicsand com-puter hapticsmodeling. The stiffnessdiscriminationliesbetweena virtual spring anda real one. In the VE case,thespringdisplacementis visuallydisplayedonacomputerscreen.The force informationof the springstiffnessis in-herentto the reactive force from one’s interactionwith the�����!�"�*���!�+� TM . Thosetwo independentsensorycues(virtualdisplacementmotionandreactiveforceof the �����!�������� � TM)shouldallow the userto discriminatethe stiffnessof a vir-tualspringfrom thatof arealone.If thisworksout,thecon-ceptis aptto providehapticinformationwhich wasa priorinot to besimulatedwithoutanactualforcefeedbackdevice.Realspringsaretestedin RE conditions.Eachspringlookslikeatrumpetpiston(seeFigure2). Threerealspringswere
Figure 2. Real spring embed ded in a piston
usedwith differentdegreesof stiffness:249,363 and544N/m. Their stiffnesswas empirically derived by measur-ing springdisplacementwhenfixed weightswereappliedoneachof them.Frictioneffectsinsidetherealpistonwerenearlycanceledby directlyapplyinglubricanton thespringandinsidethestaticpartof thepiston.
The visual displayof the virtual springis fundamental:spatial referencemust be the sameone when comparingthe virtual spring motionsand the real ones. The virtualspringis thusvisually displayedon a monoscopicworksta-tion screen,assimilaraspossibleto therealone(seeFigure3), andof thesamesizeastherealspring.Specialattentionwasgivento many graphicalfeatures(color, texturing,etc.)in orderto recreatethe virtual pistonwith thehighestpos-siblerealism.A �������������� � TM ball wasalsorenderedon theleft sideof thescreento facilitatethecomprehensionof thescalefactorbetweenVE andRE.
Thedisplacementof thevirtualspring , virtual isdeducedfrom the force appliedby the user - user using the well-known equation1, in which . virtual is the virtual springstiffness.Theforceappliedby theuseron theball is mon-itored by internal ��������*����� � TM sensors. The �������������� � TM
(force appliedby user)/(sensorsoutput)profile wasman-ually identified with a dynamometer. A maximum 10%uncertaintyin the outputdatawasobserved. A maximumpushinglimit is indicatedon the virtual display by a redmarkonthemoving partof thepiston.It correspondsto thesensinglimit of the �����!�"�*���!�+� TM’s forcesensorsin thecaseof thestiffestvirtual spring.Theredmarkis alsoprintedontherealspringto keepthesamevisualaspect.
, virtual / - user0 . virtual (1)
In orderto obtainsimilar tactileandgraspingsensationsintherealandvirtual cases,thesamemovingpartof thepistonwasfixedon the �������������� � TM, by meansof two plasticlinks(seeFigure4). The graspingof the virtual spring is thus
similar to the real one,thanksto the plasticupperlink onthe �������������� � TM on which thesubjectcanput his forefingerandhis middlefinger(seeFigure4).
Figure 3. Visual displa y of a vir tual spring
Finally, for the testingof the realspring,subjectsgraspthe real pistonasshown in Figure2, andpushthe movingpartof the pistonwith an active motion of the thumb. Forthe testingof thevirtual spring,the subjectappliesa forceonthe �����!�"�*���!�+� TM by pushingthemovingpartof thepistonfixedon the �����!�"�*���!�+� TM’s basewith thethumb(seeFigure4). He/shelooksat thescreensoasto seethedisplacementof thevirtual springresultingfrom his/heractions.
Figure 4. “Modified” isometric device
4. Previous work on compliance discrimination
In theRE case,thecompliancediscriminationhasbeenwidely studied. The JustNoticeableDifference(JND) isthejustdetectableincrement(or decrement)of intensityfora specificstimulus. JonesandHunter [12] found that the
compliancediscriminationJNDin forearmswas23%of theintensityof the stimulus. Tan studiedthe manualdiscrim-ination of the compliancewith active motion of the fingerby usingan electromechanicalapparatuscalledthe ‘lineargrasper’.Shefirst founda JND of 8%in thecaseof a fixedsqueezingdistance[18]. But after reducingterminalforcecuesby usingaroving squeezingdistance,theJNDreachedup to 22%[18]. Whenthemechanicalwork cueswerealsoeliminatedthroughan equal-work-force-displacementpro-file, theJNDwasfoundto varybetween15%and99%[17].
In theVE case,Tzafestasfounda JND of 44%with thehelpof theLRPdextroushandmasterwhichis anexoskele-tonglovewith 14activedof [20]. Thevirtual discriminationwas madebetweentwo virtual balls displayedon a com-puterscreen.Theballswerepressedalternatively with thethumbandtheforefingerof themasterglove.
As far asthe authorsof this paperknow, no studiesoncompliancediscriminationhaveeverbeencarriedoutusingsimultaneouslya realspringanda virtual one.
5. Compliance discrimination between two vir-tual springs
It seemsat first that the experimentalset-uppresentsmany uncertaintieslinked to the identification of the��������*����� � TM ’s force/outputprofile, graphical approxima-tions, manual evaluation in RE of the stiffness of thesprings, small differencesin graspingsand frictions be-tweenREandVE. Thereforethereliability of theVE hastobe taken into accountfirst. In orderto evaluatethe virtualmodel of the spring, a compliancediscriminationexperi-mentbetweentwo virtual springsis first carriedout.
The psychophysicmethodusedwas a constantstimulimethodwith a forcedchoiceand(+,-) paradigm(see[9] fora descriptionof the method).During eachtrial thesubjecthadto choosebetweentwo virtual springsdisplayedon thesamecomputerscreenandto saywhich oneof thetwo wasthestiffer.
Three valuesof virtual referencestiffnesswere used:249,363and544N/m. Eachspringwascomparedwith fivepossiblestiffer springswhosestiffnessvariedfrom theref-erencestiffnessby +0, +5, +10,+15and+20percent.Eachsubjecttestedall the possiblepairs. For eachsubjecteachpairappeared25timesin randomorder. Thetotalamountoftrials wasthen100a pair andthetotal amountof trials was1500. For eachtrial the referencestiffnesswasrandomly
associatedeitherwith thetopspringor with thebottomoneon the screen. During eachtrial the subjecthad the pos-sibility to testeachspringasmany timesashe wantedto,but hewasaskedto answerasfastaspossible.No responsefeedbackwasgivenaftereachtrial. Whentestinga virtualspring subjectswere told not to go beyond the red markprintedon themoving partof thepiston.
5.2. Results and discussion
The analysisof the resultsusedto determinethe differ-ential thresholdcan be found in [9] and [5]. The Weberfraction is a commonparameterusedto evaluatethe per-formanceof the discrimination. The Weberfraction is theJND dividedby stimulusintensity. It is sometimesassim-ilatedto JND in literature([17], [12]). TheaverageWeberfraction found for compliancediscriminationbetweentwovirtual springswith oursystemis of 6%(seeFigure5). The
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Figure 5. Weber fraction for compliance dis-crimination of vir tual springs
red mark drawn on the piston to representthe maximumsqueezinglimit is considereda terminalforcecuefor sub-jects.Theconditionsof theexperimentaresimilar to Tan’sfirst experiment–i.e. with a constantsqueezingdistance–when she found a JND of 8% [18]. The JND discrimi-nationbetweenvirtual springsseemsthenconsistentwithpreviousworksaddressingmanualcompliancediscrimina-tion, althoughthe discriminationhasbeenmadewithin aVE with uncoupledsensoryinformation. Furthermoreallsubjectsfoundthatthemodelof thespringwasvisuallyandhapticallyrealistic.
The resultof this experimentindicatesthat the config-urationof this systemcansimulatethe modelof a virtualspringrealistically. However, a compliancediscriminationin a VE canbeperformedwith mostof otherinput devices,without forcefeedback,but by usingsoundor visualsubsti-
tutionsasforcecues.It is thereforenecessaryto studytheconceptwith a taskwhich couldnot beperformed,or so itseems,without usinga forcefeedbackdevice.
6. Compliance discrimination between a realspring and a virtual one
The test that waschosenis still a compliancediscrim-ination task, but betweena real spring and a virtual onein this case. Since isotonic input devices do not returnreal force information, this task shouldnot be achievablewith isotonic2D mice,or joystickswithout forcefeedback.With the adoptedapparatus,the discriminationtaskis stillnot achievableif onesimply pressesthe �������������� � TM withclosedeyes.Obviously this is dueto thefactthattherewillbe no feedbackof displacementinformation. Neither is itpossiblewhen one stopstouching the �����!�"�*���!�+� TM, sincehe/sheno longerreceivesforceinformation.
6.1. Experimental procedure
Figure 6. Overview of the experimental set up
27 people,from the ageof 21 to 63, took part in thisexperiment. There were 20 men and 7 women with noknownperceptiondisorders.Amongthem,3 menwereleft-handed.All the subjectsusedtheir dominanthandto per-form thegraspingtask.Thepsychophysicmethodusedwasalsoamethodof constantstimuli with aforcedchoiceanda(+,-) paradigm.Duringeachtrial thesubjectfirst hadto testthe stiffnessof a real spring (the referencestimulus),andthen, to test the stiffnessof a virtual spring(the compari-sonstimulus). The testingandgraspingof realandvirtual
springswereperformedasdescribedin Section3. After in-vestigatingthestiffnessstimuli of both thevirtual andrealsprings,the subjecthad to tell whetherthe virtual springwasmoreor lessstiff thantherealspringof reference(seeFigure6).
Threereal springswereusedwith stiffnessof 249,363and544N/m. Eachrealspringwascomparedwith twelvepossiblevirtual springswhosestiffnessvariedfrom theref-erenceREstiffnessby: -40,-30,-20,-10,0, +10,+20,+30,+40, +50, +60 or +70 percent.Eachsubjecttestedall thepossiblepairs. For eachsubjecteachpair appearedonlyoncein randomorder. The total amountof trials wasthen27apairandthetotalamountof trialswas972.Duringeachtrial, the subjecthadthe possibility to go from onespringto theotherwithout time limit. No responsefeedbackwasgiven after eachtrial Whentestingthe virtual springsub-jectswereaskednot to go beyondthe redmarkprintedonthemoving partof thepiston. But in the RE casesubjectshadnorestrictionsconcerningthepushingof thepistonandtheir goingbeyondtheredmark.
6.2. Results
The analysisof the resultsfollows the classicalmethoddescribedin [9] and[5]. Theproportion(or probability)of“thevirtual springis stiffer” answerswhenthevirtual springis comparedto therealoneis usedto tracethepsychometricfunction concerningthe threevaluesof stiffness(seeFig-ure7). Assumingthatthesefunctionscorrespondto normaldistributions, the z-scoretransformationof the probabilityof “stif fer” answerswill becomea linearfunction(seeFig-ure8). On Figure8, thebestfitting straightlinesarefoundthroughthe leastsquaremethod. A z-scoreequalto 0.67
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Figure 7. Psyc hometric functions
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Figure 8. Z-score transf ormations
(probabilityof 25%)is generallyassociatedwith the lowerDifferentialLimen (DLl). ThePointof SubjectiveEquality(PSE)representsthevalueof thecomparedstimulussubjec-tively perceivedasbeingequalto thestimulusof reference.In mostcases,the PSEdoesnot correspondexactly to thephysicalvalueof thestandardreferencestimulus.PSEsaregiven by a z-scoreequal to 0.0 (probability of 50%) andaredisplayedon Figure9. The Weberfraction is givenbyequation2. TheWeberfractionfor thethreevaluesof stiff-nessareshown on Figure10. TheresultingaverageWeberfraction 1 is equalto 13.4%.
1 /32 ,547698;:<�>=@? 0 :<�>= (2)
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Figure 10. Weber fraction for compliance dis-crimination between a vir tual spring and areal one
6.3. Discussion
At the end of the experiment,the subjectswere askedto commentupon their cognitive strategies when evaluat-ing thevirtual andtherealsprings,andto tell whethertheyusedthe red marksor not, as they sometimesusedthemandsometimesdid not payany attentionto them. Thean-swersmakeit difficult to know whethertheredmarkwasal-waysusedasa terminalforcecuefor thevirtual/realspringcomparisonor not. The 13.4%JND shouldthusbe com-paredwith 8% and22% JNDspreviously found by Tan infixedandroving squeezingdistanceexperiments[18]. Thehereinresult seemsconsistentwith previous studiescon-cerningmanualdiscriminationof compliance.Neverthelessall previousexperimentswerecarriedoutwithin asingleen-vironmentwhile in this experiment,the transitionbetweentheRE andtheVE is alsoto betakeninto accountandhasprobablyhada negative influenceon the resultof the per-formance.
The PSErepresentsthe subjective displacementof thereferencestimulusduring the bilateraltransitionsfrom theRE to theVE. ThePSEdifferencefrom thereferencestim-ulusis alwayspositiveandit decreasesmonotonicallywithstiffnessandthuswith maximumdisplacementof thethumb(seeFigure9). Consequentlythe subjectshave a tendencyto underestimatethe virtual spring. The lessstiff the ref-erencespringis, the moreunderestimatedthe comparisonvirtual springis. Many reasonscouldaccountfor this phe-nomenon:
– Somepsychologicaleffectstemmingfrom thebilateraltransitionsbetweenthe VE andthe RE anda lack of con-fidencein the virtual spring,or perhapsaswell asa prob-lem of sensorymemorypersistencegeneratinga negative
error uponthe referencestimuli during the transientphaseof adaptationto VE.
– Another explanationcould be the contradictionbe-tweentheproprioceptivefeelingof motionof thethumbandthevisual feedbackof thedisplacementof thespring. Thethumbis nearlystaticwhenpressingthe ��������*����� � TM whilethespringmovesextensivelyonthescreen,especiallyin thecaseof the lessstiff spring. This makesfor a phenomenonof sensorydominance:thevisualfeedbackreplacesthepro-prioceptivesenseto someextent.Thevisualfeedbackmustbeassimilatedby thebrainto assessthedisplacementof thespringand the mechanicalwork, which is anotherimpor-tant factor in complianceevaluation[17]. But the studieson the transferfrom RE to VE andon sensorysubstitution[16] [7] fail to concludewhethervisualfeedbackcanor can-notcompensatefor bothmechanicalwork anddisplacementcueswithin thecontext of this study.
7. Conclusion
At the beginning of the experimentdescribedabove,thereweremany uncertaintiesconcerningthefeasibility ofa comparisonbetweena virtual anda real physicalmodelbecauseof thedifficulty of thebilateraltransitionbetweenRE and VE and problemsrelatedto the verisimilitude ofthe virtual springmodel. But with the apparatusthat wassetup, subjectswereableto discriminatesuccessfullybe-tweena virtual springanda realonewith a JND of 13.4%.This JND valueis consistentwith previousworkson man-ual compliancediscrimination[18] [17] [12]. It shows thepossibilityto feedbackhapticinformationto theuserwith-out usinga force feedbackdevice but simply by combin-ing apassive isometricinput devicewith a visualfeedback.This may remainimpossiblewhenusingan isotonic inputinterface.
The experimentis basedon the coupling of the visualfeedbackandtheinternalisometricdeviceresistancewhichnaturallyreactsto theuser’sappliedforce.Theoverall sys-temreturnsaforcecuecalledpseudo-hapticfeedback. Thispseudo-hapticfeedbackwill probablynot replaceanactualhapticonebut canbe useful for somesimplesimulations,makingfull useof the6 possibledegreesof freedomof iso-metricinputdevices.
Themostsurprisingresultis thesubstitutionof thepro-prioceptive informationat thelevel of thethumbby thevi-sual feedback. All subjectswereableto discriminatebe-tweena real springanda virtual onewithout the proprio-ceptive informationfrom thethumbsincethedisplacementduring the virtual spring evaluationwas very small. Fur-thermore,at the end of the experiment, the last 10 sub-jectswereasked to draw a line segmentcorrespondingtothe maximumdisplacementof their thumbwhenpressinga virtual spring. The result (seeFigure 11) indicatesan
overestimationof thedisplacementof their thumbsvaryingfrom 2 to 8 timestheactualdisplacement,with anaverageoverestimationof 5 times,which meansthat theillusion oftheirproprioceptivesenseis strong–asif they perceivedthe�����!�"�*���!�+� TM asa nonisometricdevice.
The experimentrelies on the “illusion” conceptwhichcould be usedin many moreVR applications.The visualdominancewasusedto influencetheperceptionof thedis-placementof avirtual spring.Indeed,peoplefoundthatthevirtual springmodelwasrealistic. This virtual springcon-stitutesa multi-modalvirtual objectwhoseperceptive cuesareuncoupledandprovidedby differentsensorymodalitieson thehapticandvisualmodes.
10mm
Spaceball maximummaxmin
displacement
5mm
displacement perceivedMaximum thumb
average
40mm
27mm
AB B BC C CD D D DE E E EF F F F F F F F FG G G G G G G G G
Figure 11. Thumb displacement perceived
Futurework dealswith the evaluationof pseudo-hapticuse of isometric interfaces. First, further evaluation ofthe pseudo-hapticfeedbackthrough different perceptualtasksshould be performedsuch as discriminationof theweight of objects. Secondly, a comparisonwith forcefeedbackdevicesis alsonecessaryto positionthepotentialof pseudo-hapticfeedback.Finally, applicationsof pseudo-haptic feedbackon industrial tasksremain to be defined,particularly when resorting to the 6 degreesof freedomof the �������������� � TM. A performanceexperimentwill beset up to measureif this illusion can be usedto improveperformancein anindustrialtask.
AcknowledgementsThe authorswould like to thankthe Spaceteccompany
for providing freelytheSpaceballmodelof thisexperiment.They wouldalsolike to thankall thesubjectswhotookpartin theseveraltestsfor their kindnessandtheir patience.Fi-nally, they would like to thank Mr P.R. Persiauxand MsN.M. Saint-Jeanfor their valuableremarks.
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