Finger Force Precision for Computer Pointing

Ted Selker Joseph D. Rutledge

IBM TJ.Watson Research Center, Yorktown N.Y. 10598

ABSTRACTThis paper addresses motor control constraints whichaffect analog pointing devices wsed with computer in-terfaces, we have investigated th« accuracy and pre-cision with which subjects can apply force to a smallisometric joystick with a fingertip or finger andthumb, for osc and two dimensional force speciUca*tion.

We find the icfonnation contest of A force applicationto be in the range of 4 to 6 bits per dimension withvisual feedback, and without time constraint, Theforce application task in two dimensions is no moredifficult than in one dimension and gives twice theinformation content. Us? of an opposing thumb andfinger g?v«$ no improvement over * smg!e finger.Both inaccuracy «fil< imprecision are concentratedalong the direction of the specified force ^subjectstend to be both more accurate and more precise in thedirection of force than in its magnitude.

Implications for aoatejj pub bag device} we discussed.

KSYWORDSMouse, Joystick* Dexterity. Force Precision

SWTRODUCTIONRecent work [5, 7} baa demonstrated that pointingperformance can be significantly improved when hu-man motor perceptual limitations ftre taken into ac-count.

This reopens questions of strategies for physical con-trol posed at the time of radio knob desiga, etc,[2-4, 8] What ere the perceptual motor constraintsof physical control design? now should these con-straints affect the relationship between the physicalcoatro! end a machine's response to its movement?By understanding the relationships between these in-formation channels we can improve design interfaces,

Motivated by work in designing a finger pressurecontrolled pointer, we wanted to understand the con-trol channels available to vinous analog motor taeke.We report here a stud,y of the fmear.presstire channel.We have considered visual, auditory, tactile and. pro*prioccptfve feedback. Some tactile feftd back is inevi-tably present {barring anesthesia). Preliminaryexperiments indicated that auditory feedback alone umuch less effective tbao visual, even in the onc-di-mensional ease, wkik tactile feedback aiono appearsto allow less than 3 bits in each dimension. Thisstudy focuses on finger pressure with tactile and visualfeedback, without tune constraint. The subject may

take several seconds to apply the specified force, andthe force Is then measured as the mean of instantane-ous forces »mp!«d over a 2.4 second integrating pe-riod. Even under these conditions accuracy ar»dprecision we surprisingly low.

The force range which we nave investigated Is thatappropriate for one or two fingprs qn a small sccsor(a 3 mm by 8 mm cylinder), 0 - 225

While there are many studies of complex tasks suchas pointing and tracking which use the finger or handforce channel, there seem to be few if any which ad-dress the accuracy and precision available in theehaiind itwlf. We have found only [6], which studieswhole ann moveovsnt* *t much greater forces. Hisstudies found sybiecis could apply a arm force towithin JOpercciit ol a attempted target force,

METHOD

ApparatusSubjects were scaled at a standard office desk, m achair adjusted to comfortable height by the subject.On the desk were a CRT display »nd a J0!»kcy IBMPS/2 keyboard, plooed about 10 em bade from theedge of the desk. La the center of the keyboard, be-tween the G and H keys, was an isometric joystick(the same sensor used in the Pointing Stick, [5])topped by a dished 3*5 mm diameter linger rest, 4nun above the level of the ksy caps Figure I. Thejoystick top moves an undetsctable .13 mm at max-imum force.

1. Fincertip grip oT an bometnc

An !BM 8514 b VGA graphics mode .4 mm square pixels on a 480 by 640 screen

3 0 0 ' 3 9 W d mtJw w a r 35 -B 15

For the fingertip grip condition the subject placed afinger tip on the finger rest. For pen grip experimems,adjoining key caps wen; rcraovec exposing an 8 mmlong 3 mm dierneter "per.', groped b*tw»*n thumb«nd forefinger. Figure 2

tmt of an isometric joystick

Experiments were also run using a ainsJc finger orthumb pressure on the side of the joystick to under-stand the value of the opposing dint b the pen gripcondition. This is called the me, gup.

Strain gauge signals from the sensor were processedto produce signals on an IBM PS/2 pointing deviceinterface such that the resulting cursor position re-presented the horizontal force Ibeiag applied to thewtaa*, within the limits of the display screen. Strain

Eige sensing and signal orocesstna was (performeda separate IBM PC{XT with & Scientific Systemsbmastei data acquisition board which communi-

cated to the PS/2 through its mouse port.

A progrwis running on a IBM PS/2 model 80 pre-sented stimuli, provided feedback, and recorded data.

Sub|«ct*Over several month periods one subject performedexperiment* to calibrate and develop data collectiontechniques.

We report here on results from four subjects, hiredthrough an agency as office temporaries. They allnormally work in secretarial ana clerical jobs, fre-quently using word processor, and bid sE jjit famili-arity with puce, but ao prior experience with Othercomputer pointing devices. All were women, between25 and *5CV. who reported tywnE speeds between50 to 80 words per minute. Two played or hadplayed a musical instrument; no other lugh-maaua}-dSKterity hobbies were reported. Subjects participatedin these experiments aa part of & two-day sequenceof experiment* on pointing behavior, using thePointing Stick in its normal mode and a mouse inaddition to the present apparatus.

Proetaur*The cxpcrimtptaj paradigm is as follows; Subject ini-tiates oacb. trial with • keypress. A target tone itpresented u a position on the screen. The subject

attempts to apply the specified force, by bringing thecursor to the target and holding it there.

An initial movement towards the target is ended whenthe subject's precision limit is inevitably reached andthe cursor moves away from the target. During thefollowing "hold" phase the cursor is held as stably »spossible for wmc 2,4 seconds- The meen appliedforce during the Tiolo" phase will be called the trial'seffective force and its standard deviation is taker, asthe trial's imprecision or dither. The miss vector is thedifference between the target vector and the effectiveforce vector. The miss angle is the direction of thetnlss vector.

Figure 3. DlspU) Showing subtlitjr andof force 4pplic»tion JB a trial.

Feedback information u added to the target displayat the end of e4ch trial, Th« computer displays thetrack of the cursor's movement, A cross on the trackmarks the beginning of the hold phase and an ellipseis centered at the mean hit position with shape re-presenting the variation in applied force during theho54 phase.

Separating tubjeett target selection* ffotn target holdphke is exemplified in Figure 3- hi this figure it isnoticeable that after ft subject attempts to select atarget, stability at holding the requested force can becharacterized as a cloud of dither.

After 10 such trials the computer displayed numbersindicating feUtive accuracy and relative dither andstandard deviations for the group of trials. A menuselection is selected to ruo ten groups of these.

The subjects performed ihe« the trial type doing twodimensional (circle) targets^ verttc*! one dimensional,end horizontal on* dimensional targets, AU of theseconditions were investigated for the Sneer tip as wellas pen crip conditions. A nominal 300 irsais werecoliectec for each two dimensional condition and anominal 100 were collected for each one dimensionalcondition. For two subjects an additional ipinica!100 trials were collected tor the vide grip condition.

Target forces were pseudo-random, with uniformdistribution over the rang* corresponding to the dis-

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play screen., excluding an 1 cm »*rgb•no « disk of diameter 3 ceo to tins wrt'.«r.

An experuxtenter femaued in th* room to observerod to demonstrate protocols. Written iasiruclionsexplained each phase of the experiment, and directedthe sequence of phases; the sequence was varied insom* cases to maintain subject motivation. Toe ex-perimental protocol w*s otherwise Administered and«*«Ks recorded by the computer.

RESULTSTarget force, mean applied force, dither, and timewere recorded for each trial. Initial sets of trials werediscarded , as were a lew trials invalidated by ints*-ruptlona or equipment problems. Table I ave* over.all average! of mis* Mid dither for the three targettypes, and for the two grips. Figures 4-12 look atthe data in more detail,, examining dependencies ontarget force,

Figure 4 plot* error «.gs«n$t target fore* for the twopip conditions. The figure shows that whik absoluteaccuracy of force decreases with requested force, rela-tive accuracy increases with force. The large range iaaccuracy inoicated by the quite tall cloud shows thatrange in performance is wide- Note that the JWD pipcondition adds * alight advantage over the finger tipcondition.

Figure 5. Tb« relationship between force win!

Figure 5 plots dither against force for the two gripcondition*. Notke the blank area at the bottom ofthe asreec Thi« represeDtt a Emit tc steadiness. Thewidth of this band sliows that no one was abb to holdforce steady to within 2 grams. The average instabil-ity was 8 grams varying in individual from 5 to 12grams. Like the error, other also increased somewhatwith forge.

Figure €. Dbecti&n oT miss i-erttis tarjd direction

show* th« relatloiuHip between the targetdirection and the mkz angle. Note that th« miss tnjletendb ic be ISO degrees from lb« target angls showmethat subjects tend to undershoot the target forct. Ishows that swbjeeU wer? more accurate In the dirtc*lion tban ic magatumte of force applicatiQa. Tcm dataKOuid be «cd to help dtaiga velowty traasfcr fuac-tioaa Another OM ol this KUg^«t,s that menut shouldbe made *d«tp* in the dlrectton of most likely ap-proach. Edge menus, which are effectively infinitelydeep, are instances of this.

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Figure 7.

«•»!•

StaMHy in direction of target » wonc thusin ailhoBoitt} direction _

A striking result cmer$es by looking at the sbe.pe ofth« data aloud of subject hold data.. Figure 7. Theellipse that represents the mean of the cither doudhas approximately a 2 to 1 axis ratio in the directionof the target. We have collected data for pressure to-wards targets above below to the right and to the left.Our data collection procedure for angular forces docsnot distinguish this at other angies.

i -

»t-

f Hi t* M» t.M *.«*»•»»»•

. • ..V.-A.&-** J v " " it*'

Figure 8. Demonstration that pen grip » not fccttothan pushing with a side grip.

Experiments comparing the side grip to the pen griptest the role of opposing grip for pressure accuracyand stability Figure 8. Note that in the data the sidegrip is not d&iagujsbed from the pen grip. Thissuggests that the slight advantage of the pen gripcondition over the tip grip k due to improvement iacontrol by pushing on the side of the post rather thanto any additional stability provided by another finger.

Samples Error Dstber

Tip 1567

HIS

7.77.8

Pen<? 6.1

1 Dttaessional - Horizoaui BarsTip 389 4.5

* 4.7P«a 260 13

a 4.3

8.86.08.35.2

6.04.04.84.2

Horizontal Projectioa of 2 Duneorioaal DataTip 1510 6,2

4.26.24.1

5.65.25.64,4

5.35.7

Pen 1074 4,7a 4.9

! Dimensional - Vertical BarsTip 390 5.1

o 7.0Pen 397 4.3

<r 4.9Vertical Projection of 2 Dimensional DataTip 1519 4.6 6.0„ 6.4 S.O

Pen 1079 3,9 5.4v 4.7 3.9

9. A Table of average error and dither ingrams showing tfis inherent low accu-racy and instability in force applica-tion, the relation between 1 and 2dimensional trials and the improve-meets afforded by the pea grip. _

Subject 1

Subject 2Subject 3Subject 4

Samples Error293 3.0587 4.5330 15.Q357 9,9

Dither5.86.910.612.7

Figure 1C- A Table showing the range of indi-vidual differences in Average srrorand fiitber fores between subsets forthe iwo dimensional tip grip eosdi*lion

Figure 9 pves the average error and dither under thevarious condhioos. This table shows, that even whenthe data is averaged over all individuals and targetforces its major Teamra are still visible. The ex-tremely large standard deviations jwfiact severs] fea-tures of the data; first that subjects are unable to hold» constant force, second tbe large variation in accu-racy between inoivSdciaJb Figuit: 10, and third the factthat error and dither wicnsaae with ore«.

Tb? maximum fo;rce tisat ntbjecti teem comfortableapplying with one £cgsr is aoout S ounces or 225grams. Taking this as the working ncgt for fingerpressure* average short-term precision can be calcu-lated from Figure 9 The smallest interval of error foribe 2 dkwsttitsoal Up «uc ia S grantt. Adding the

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deviations to error and dither for the 2 di-mensional tip case gives a value of (7.7 + 7.8) + (8.84 6.01™ 3Ujparr» « representing aj> upper range forerror. This gives a precision of J to 12 percent, al-lowing 8 to 50 choices in each dimension Figure 9oo page 4. Taking the log base two of these numbersrave* an information coa**fii ranging iicfs 1 to 4.9bits for each trial. Averaged over & lone bold time(2.4 seconds) subject accuracy came to a precision of3 to 15 grams, giving 3.9 to 6.2 bits, for each dimen-sion, Taking a long careful attempt, then, only al-lowed o'or best subject to be ai>Jc to accurately selectOne of 70 force value* in each dkctjwson. Compiledtnto two dimeaaons this gives a total informationcontent of 7.S to 12.2 bits.

The general leve! of information content reported bySchmidt et.al. for arm force [6] (Standard deviationabout 10 percent of applied force) is consistent with

Tat I dimmtiooal projection of &e 2 duneasianaJhorizontal data results from setting aJQ horizontalcomponents to 0 fa the data from 1 dimensional tri-ali. Note that it agrees closely vrith the data for theone dimensional targets, Daia from trials with verti-cal targets and the corresponding projectioo of the 2dimensional trial data are quite ttniulw, with the samedose correspondence. The differences it error be*tween 1 and 2 dimensional trials is completely ac-counted for by the dimension of the trail. Totsubjects appear to be able to add a second dtaensteato their task without any interference giving twice theinformation content without additional effort.

learning Curve

Figure 1i, Dejnonrtrttkss of th« experience lubjeclihad relative to asymptotic improve-ffleat with use of pointing device.

The experiments described in this paper were includedin a two day exercise designed to iratn subjects to us-ing isometric pointing devices for movse-like selectionActivities, Aa with the mcuee £tj w» find that a jub-j«dt awpraves at. selection tasks over ft few thousandpointing selections. Figure Figure Hshows twopoints at which Subjects performed these force ex*penmenu as part of a longer set of mas designed togive them experience with isometric pointing.

By noting the points along the performance curve atwhich dale was gathered Tor force experimems, iheimpact of pointing experience on these results can beestn»ftte$. Figure Figure 12 thaw* that this data i$coBsisteat with all other data describing accuracy cffifcger force.

l .Wt.U

i

Figure 12. Fwce accuracy data taken at tStffwwttimes on t user's joystick pobtbglearning curve.

CONCLUSION

Limits of Digit Fore* Centre)Even under the optimal condition of long integrationtbne, accuracy is quite Uwited. If we take the workingrange for this kind of finger pressure as 0 - 225grn,average ihort*terra preciaon, for oar fubjects, ss inthe range of 9 to 25 jrams or 4 to 10 percent of fullrange in each dknenaon, eontspoDding to an infor-mation content qf 3.3 to 4,6 bits for each trial. Overtime (2.4 jeeonds) this b sjiegrAted to a {trccitioc of3 to IS grams, saying 3-9 to 6.2 bits, for each dimen-sion, for a total iniotinalioD content of 7.8 to 12.2bits. This compares with 18.22 bits represented bythe selection of a single pixel on a VGA (640x480)screen. A force-to-positipn joystick is dearlv not ad-equate as a pointing device. By mapping force intovelocity ia«t«*d of pocition^ efficient time ieiepra'.icaa&d afbhra-riiy precise pomtmg may be achieved,provided that the proper mapptng is used. The im-precision of finger pressure impiies that speed can beclosely controlled onJy when the mapping h&s a pla-teau,, where the desired speed is maintained over arange of pressure* larger than the 4 to 10 ptrcent un-certainty. In the Pointing stick transfer function suchplateaus are found at zero speed for a stopped cursor,t siow speed for accurate pixel and character posi-tioning, and at maximum eye tracking speed for fasttccurate rjaovements [5] - Figure id traosfe' un-known-

Over the range of target fare** t**t*<i subjects werenever *Me to keep steady forces under 2 grams. Aprojected average 5 grams of dither at zero force setsa minimum deadband which will allow a user to resta finger on a neasor without cursor motion.

Two Dinwnalona is No Harder than One.

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Subjects applying a force in two dimensions mademuch the same errors along each axis «« when theywere concentrating on that a*i$ alone. One mightexpect that it would be easier to apply and hold a$p*c&ed force to the right, for example, if one needcot contmt in the up or down direction., This it notthe case.

Rote of Grip in Fere* ControlIt might have been expected that the opposing thumband forefinger hold would greatly improve both ac-curacy s,ud ti*ftdifi«*s of force ttpplicahpa. Data fromthis Study shows that the pen grip improved per.fonnance very little (Figure 4 on page 3, Figure 9 onpage 4). This improvement, however, does not comefrom the opposing thumb as predicted; a anger placedOB the aide, of the joystick in the side grip conditionslightly outperform* the pen grip Figure 8 cm page4shows that the observed force accuracy advantageof the pen grip is due to the finger or thumb positionon the side of the sensor rather that) on the top, andoot to the opposing digits grip. Two unsteady fingersare just as unsteady as one unsteady finger,

Error and Otther mrm Align** in iho TargetApplied force leads to be aligned to the diieetidu oftarget force but undershot. For force to velocitytransfer function* this is an advantage; the Boise alongthe intended direction of movement changes the cur-sor speed but not its diroctbn.

A specific style of osenu that this tuggests u a m«oualong the edge of a screen (which acts as if infinitelydeep).

The result* of these experiments help describe behav-ioral motor issues which contribute to the perform-ance ucp-rovecaenii achieved by tb* Pointing Sticktransfer function,

This study shows that individual performance differby a large factor; could ibis be takes into account mpersonalized or adaptive transfer functions?

The noise in a persons movement U greater Ln the axisdirectly blersecting the target; could this analysis beutilized to to augment user control?

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