Tactile Feedback Systems

Michael Fritschi, Martin BussInstitute of Automatic Control Engineering (LSR)

Technische Universitat MunchenD-80290 Munchen, Germany

Email: michael.fritschi@ei.tum.dewww.lsr.ei.tum.de

Knut Drewing, Regine Zopf, Marc O. ErnstMax Planck Institute for Biological Cybernetics

Spemannstr. 38D-72076 Tuebingen, Germany

Email: marc.ernst@tuebingen.mpg.dewww.kyb.mpg.de

Abstract— Tactile feedback is among haptics one of themore recent modalities for human-system interaction. Re-search in tactile feedback using pin-array type actuators hasbeen going on during the past years or so. A survey abouttechnological achievements, human sensing capabilities, andpsychophysical evaluation in this area is presented. Then thefocus is on novel approaches in actuator technology and tactilefeedback systems providing shear force (tangential force tothe finger-tip).

I. INTRODUCTION

The human tactile perception is a complex combinationin sensing various physical information from our envi-ronment. While manipulating an object with our hand,we can perceive reaction forces, pressure, temperature orpain using different receptors/nerve-endings beneath ourskin. Research in tactile feedback systems in general tryto recreate these environment in a technical way either inform of a telepresence scenario [1] or based on virtualreality [2].

Most approaches in technical realization of tactile dis-plays followed the idea to display the shape of an object,that has to be explored as a kind of spacial ’picture’.Similar to a Braille generator, displaying script informationfor blind people in a tactile way to the human finger pulp,wherefore in many cases an array of movable pins is used.Each of these pins must independently be able to fulfillmovement normal to the human skin (mostly toward thefinger-tip of the index finger). To reach a realistic impres-sion of the generated object, the density and performanceof the pins have to be adapted to the performance of the hu-man tactile perception system (mechanoreceptors). Basedon this principle of so called normal force displays, severaltactile displays have been build [3]–[7]. More recentstudies pointed out, that considering the forces tangentialto the human skin could be also important. This forcesarise for instance while sliding with the finger pulp over asurface. Moreover, this stimulation of shear force seems tohave more influence to the tactile perception as expected.Meanwhile there are some Displays providing shear forcein different ways, like pin based displays [8], [9], displaysthat are designated to display slip at the fingertip [10], [11]or displays specialized to provide stimuli of object contactand edges [12]. Most of this devices are in a prototypestate or used to explore the influence of shear force to thehuman perception.

A common problem on these existing tactile displaysfrom the technical point of view is the demand on highpin density at a concurrently large excursion and highfrequencies of pin movement.

However, there are still some unanswered questions onthe perceptual side. The discrimination performance ofhumans to distinguish between different direction of shearforce provided to the skin and the perceptual integration(based on the hypothesis of [13]) of multi-pin movement,are only two of those questions, such as will be discussedin this paper.

To approach these problems in a rambling way, ourresearch in tactile feedback systems can be split into twomain directions:

Advancement in actuator technologies. In this issuewe explore new appendages to solve the problem ofproviding the necessary functionality of tactile stimuliin a technical way, by combining common actuatortechnology and micro-mechanical solutions to a com-bined actuator system adapted to the psychophysicalcharacteristic of the human tactile perception.Illusion based tactile displays At this direction weaspire novel concepts in tactile displays by an iterativeprocess between technical design and psychophysicalevaluation. In a first approach we build a tactile shearforce prototype device for psychophysical evaluationof discrimination performance regarding the humantactile perception of shear force and do some prelim-inary studies exploring the perceptual integration ofmulti-pin movement tangential to the skin.

After an introduction of the human cutaneous sense insection II the short overview of common used actuatortechnologies for tactile displays is given. Thereby thefocus is only on available actuators that are commonlyin use for tactile displays. Regarding the research in newactuator designs, this paper presents the first concept ofcombined pin actuator, using two piezo crystal actuators,that are coupled by an mechanical lever system describedin chapter IV. On our direction to find illusion based tactilesystems, the design of a novel tactile shear force prototypedevice, as well as first evaluations regarding the humandiscrimination in shear force direction and the integrationof multi stimulus in the human brain is presented inchapter V.

TABLE I

FUNCTIONAL FEATURES OF THE FOUR-TYPES OF MECHANORECEPTORS BENEATH THE HUMAN HAND. ADAPTED FROM [17] [18] [19]

Meissner Pacinian Merkel’s RuffiniFeatures Corpuscles Corpuscles Disks Corpuscles

Superficial Dermis and Basal Dermis andLocation dermis subcutaneous epidermis subcutaneousRate of adaptation Rapid (RA-I) Rapid (RA-II) Slow (SA-I) Slow (SA-II)Spatial resolution Poor Very poor Good Fair

Mean receptive area mm mm mm mmSensory units % % % %

Response frequency range Hz Hz Hz HzMin. threshold frequency Hz Hz Hz Hz

Velocity, local Vibration, slip, Local shape, Skin stretch,Physical parameter sensed shape, slip acceleration pressure locale forceForce thresholds mN mN mN mN

II. HUMAN CUTANEOUS SENSE

The skin is the largest receptor surface of any of thehuman senses. It gives us the possibility to perceive variouskinds of environmental impressions as touch (pressure),temperature, cold, pain, tickle, and itch. This differentperceptual qualities are described by the general termcutaneous senses. Fig. 1 shows the schematic structure and

Fig. 1. A cross section of the skin, showing the layers and the receptors.Adapted from [14]

position of the four types of touch receptors within thehuman glamorous skin, which are called the mechanore-ceptors1. They can be seen as specialized nervous cellsor neurons for transmission of tactile information intofibers. This information in form of electrical pulses con-verge via second-order neurons into the central nervesystem [15]. The mechanoreceptors are generally dividedin two classes, rapidly adapting (RA)2 and slowly adapt-

1The receptors which are responsible for perception of temperature andpain are not discussed in detail here.

2In some publications the equal notation FA (fast adapting) is used.

ing (SA) mechanoreceptors. RAs in general response ontemporal changes in force indentation e.g. at the phase ofcontact or release of the skin with an object. The SAs re-spond also on constant force exertion to the skin [14] [16].The two main classes of mechanoreceptors (RA, SA) aredivided into two sub types characterized by numbers.The Meissner corpuscles, located in the lower part of theepidermis (Fig. 1), respond in a frequency range of

Hz and optimal to vibrations at frequencies of aroundHz they are assigned to the RA-I mechanoreceptor type.

Pacinian corpuscles, located between the epidermis and thedermis are associated to the RA-II receptor type. Theyrespond on a frequency spectrum of Hz with anoptimal response in the frequency band of Hz.Both SA mechanoreceptor types are located in deeperlayers of the dermis and respond on frequencies up to

Hz whereas Merkel’s disks are assigned to the SA-Iand Ruffini corpuscles to the SA-II receptor type [17] [18].Tab. I shows a comparison of the described bio-physicalfeatures collected from different investigations. The ob-tained results about the mechanoreceptor density are relatedto the human hand. The human hand contains aboutmechanoreceptive units innervating the glabrous skin [20].Its most sensitive area is located in the finger pulp, whichhas with % of SA-I units, % of SA-II units, %of RA-I units, and % of RA-II units one of the highestreceptor density regarding the human skin.

Most of this mechanoreceptor type specific values orig-inate from neurological measurements, done by directrecording the single afferent units with so called turgstenneedle electrodes, which are percutaneously inserted in aregion about cm above the elbow [21]. With his methodit is possible to locate and record the response of nearlyevery mechanoreceptor.

The tactile stimulus provided to the human skin cannotbe directly assigned to a type of mechanoreceptor. Onereason for instance is the overlapping in response frequency(Tab. I). This values let assume, that the quality of cuta-neous perception is caused by the correlation of inputs fromdifferent receptor types. Thus frequencies in the range of

Hz are responded by mostly all receptors withrising sensitivity and perceived as vibration. Frequenciesabove Hz can only be acquired by RA-II receptorsand due to their poor resolution it is difficult to localizethe source of vibration. Investigations referenced in [18]note that the receptive areas and frequency response rateindicate that a single vibratory stimulus at the fingertip canbe used to present vibration information for frequenciesabove Hz. This psychophysical characteristic could playan important role on design of a tactile device, because ofreducing actuator performance.

The force thresholds of the mechanoreceptors, shownin Tab. I are mainly related to indentation forces normalto the skin. An exception from the bio-physical point ofview are the Ruffini corpuscles. Their long axes are usuallyoriented parallel to the skin surface, as shown in Fig. 1.This let assume, that they are able to transmit a neuralimage of skin stretch. The high force thresholds of theseSA-II receptors to punctuate stimulation normal to the skinmay be caused by these properties [19].

TABLE II

TACTILE SENSATION AND MECHANICAL IMPEDANCE OF THE HUMAN

FINGER. ADAPTED FROM [22] [23]

Indentation Threshold ( mN/mm )

Static mHz mHz m

Feature DetectionTexture m (lat.)

Single dot mDetecting features (NF) N

Temporal SolutionSuccessive tap time ms

Bandwidth sense HzReaction time ms to ms

Spatial ResolutionLateral localization mm

Lateral 2-point limen mm

Mechanical Impedance (static)

Range of finger pat indentation mmInitial contact mm N mm

Further indentation mm N mmFurther indentation mm N mm

indentation mm N mmLateral indentation N mm

Tab. II shows some general thresholds related to theskin of the human index finger pulp, based on differentpsychophysical investigations. As expected by the allo-cation and characteristics of the mechanoreceptors, theindentation threshold normal to the skin surface decreasesby an increase of stimulation frequency. Additionally therehas been found a coherence between the sensibility ofindentation and the cross section of the used contactor [24].Thus, lowering the cross section from cm to cmcauses an increase of indentation sensibility by about

times. The feature detection resolution of a surfaceis very high while strokeing the finger pulp about anobject, whereas the spatial resolution in the static case is

comparatively bad. This is also caused by the fact thatthe vibrations caused by friction on sliding the finger atan object can sensed very well by nearly all types ofmechanoreceptors. Investigations in mechanical impedanceof the skin at the finger pulp has been result a nonlinearincrease of impedance on deeper ranges of indentation untila maximum indentation of approx. mm, constraint by thecompression of the tissue between contact surface and boneof the finger. Studies of lateral indentation thresholds [23]identified a much higher but mostly constant mechanicalimpedance of approximately N mm. However, thetangential displacement thresholds of the finger pad is only

times as high as the reference normal displacement.In order to produce a realistic tactile impression of

the environment it is probably as important to provideforces lateral to the human skin, so called shear forces.This is particularly reasonable when considering percep-tions evoked by movements of the skin relative to theenvironment, e.g. when stroking with the finger acrossa surface. The perception of movement across the skinhas been demonstrated to rely upon skin stretch probablymediated via SA receptors as well as upon the translationof a stimulation probably mediated by FA receptors.Human perception here seems to be particularly sensitiveto the stretch component [25]. To give another example,shear forces do also play an important role in applying aprecision grip. When gripping and lifting a fragile objectlike an egg, grip forces should be sufficiently large toavoid slip of the object, but also sufficiently small to avoiddamage to the object. FA receptors in the skin have beendemonstrated to immediately feed back slip and, thus toallow for a precise force control [26]. Note that currenttactile devices are most likely to address other types ofreceptors in the skin, namely SA (shape devices) andFA (rapid vibrotactile devices) receptors. An impressivemovement illusion evoked by shear forces can be easilygenerated attaching a comb to the finger and sliding apencil along its teeth [27]. This illusion further corroboratesthe assumption that shear force plays an important role forhuman tactile perception.

The attempt on technical side is on the other sideconstrained on the present actuator technology, while thereis no actuator available that can satisfy all this demands onperformance at present. One approach to compensate thiseffort is finding a way to cheat our tactile perception in away to reduce actuator capacity. Results in this directiondemonstrate that the display quality of tactile displaysdepends strongly on the demands of the application [28].Furthermore recent psychophysical integration hypothe-sis [13] may indicate new possibilities for illusion basedtactile displays.

III. ACTUATOR TECHNOLOGIES FOR TACTILE DEVICES

The choice of actuator is an essential decision forthe design of a tactile display. As the findings in theprevious chapter let assume, there is a necessity of a veryhigh power density on the mechanical side, to provide arealistic stimulation of the cutaneous sense. For instance

an ideal display which exerts normal forces to the fingertiprequires N cm peak pressure, mm stoke, and Hzbandwidth; that is a power density of W cm with anactuator density of 1 mm [29]3. This ideal is far fromwhat can be achieved with current actuator technologies.

In this section a brief survey of actuators for tactiledisplays which are available or under development is given.A more detailed summary about new materials, that arepredestined for design of tactile displays but currently indevelopment can be found in [30].

A. Electromagnetic actuators

In robotics the by far most commonly used DC (directcurrent) motor is related to the type of electromagneticactuators. It uses a combination of permanent magnet, coilsand a commutator to produces a rotating electromagneticfield, that produce a rotatory motion at the output shaft ofthe motor. Attached reduction gears are generally used toincrease the torque but decrease the speed. DC motors incombination with encoders for position measurement anda position control, so called servo motors, are available indifferent sizes and provide a good position accuracy. Onedisadvantage of DC motors in general is that the producedrotary motion must be transformed in the necessary linearmotion by rocker arms or combinations of screws and nuts.

TACTACT is an example for a tactile normal forcedisplay which uses gear-less DC motors [31]. Therebythe rotatory motion of the position controlled motors istransformed into linear motion by excentric discs to whosecirculation edges guided the pins in linear direction. Eachpin of the pin-array display can reach a maximumstroke of mm, continuously controllable in position. Itprovides forces up to N, and has a mechanical band-width of Hz. Irrespective of the very large pin spacingof mm, it provides a highly realistic stimulus for someapplications, like the palpation of the human skin accordingto detect some medical objects (e.g. tumors).

Another example using DC servo motors for tactiledisplays is presented in [32]. This display consists ofpins in an order of a matrix with a pin spacing of

mm. It provides tactile force normal to the fingertip,with continuously controllable pin deflections of mm.To actuate the pins, commercial radio-controlled (RC)servo motors have been used. Those motors come witha embedded position sensor and a closed-loop positioncontroller. This device provide frequencies up to Hzfor the maximum pin deflection of mm and up to Hzfor amplitudes less then mm.

Solenoids actuators belong to the class of electromag-netic actuators that can produce direct linear motion. Inprinciple they consist of a coil of wire, generating amagnetic field, and an iron component that moves towardsthe coil and thus can implement e.g. pin motion. Based onthe physics of the electrical field that occurs on energize

3This example is calculated to stimulate the SA-I mechanoreceptorswith the highest mechanoreceptor density ( cm ) at the finger.

the coil, the relation between movement and the attractingforce of the iron is highly non-linear. Due to this fact it isvery difficult to implement a position control-loop. At hisreason solenoids are mostly used for vibrotactile displays.

So, for instance in [33] [34] an one pin vibrotactile ac-tuator, mounted to the kinesthetic display (PHANToMTM)is described. This solenoid actuator is attached to theoperator’s thimble providing a planar tactile stimulus withfrequencies up to Hz. The stroke position of theactuator is controlled by a PI controller.

A modified principle of solenoid actuators is used atthe pin matrix based tactile display TACTACT [35]. Inopposite to the general principle, on these actuators the coilis attached to the moving part of the mechanism, so activa-tion of the coil causes an attraction of the electromagneticrocker lever to the fixed component. The display containsa tactile normal force actuator array. Since it has noposition/force control the actuator pins can only be drivenin binary mode. The pins can exert an initial forces of

N and continuous forces of approximately N at thefixed maximum stroke of mm. The pins can be drivenindependently and reach frequencies larger than Hz.

The voice coil actuators are based on the well knownprinciple used for loud speaker chassis. As the fixatedpart, a cylindric permanent magnet establishes a staticmagnetic field. The coil, circular wounded around thepermanent magnet with a small gap between magnet andcoil, represents the moving part of the actuator. A currentapplied to the coil, produces a change in the magneticfield, thus resulting a force on the coil, that causes linearmotion of the moving part [36]. The so generated forceis proportional to the magnetic field of the permanentmagnet whereby a simple force control loop is possible.The moving part (coil) mostly has a low mass and canin general provide larger forces than solenoids. Voicecoil actuators have no mechanical hysteresis, no force ortorque ripple, and no backlash. Because of the problemswith position control of various static excursions theymaybe used mainly for vibrotactile displays.

B. Pneumatic actuators

Pneumatic actuators use the pressure or flow of the airto drive pins or inflate air chambers. The benefit of pneu-matic actuators is that the compressor which generates thepressure can be placed at peripheral locations. Thereforethey can provide a good local power density by usingsimple components. Disadvantages are the non-linearitiesand thus difficulties in control of either pressure or flow.Further, aerodynamic effects cause limitations on actuatorbandwidth. In [37] a chamber pneumatic display isshown. A pin display driven by pneumatic cylindersis presented in [38].

C. Piezoelectric actuators

Piezoelectric actuators (PTZ) based on the phenomenonof the deformation of a quartz crystal caused by an electri-cal field. Two main types of piezo actuators for longitudinal

motion are available: high voltage PTZs requiring aboutVolt for full extension, and low voltage (multilayer)

PTZs requiring about Volt for full motion. Multilayerdesigns, constructed from to m layersin low volt-age piezos, allow achieving a relative expansion (strain)up to %, if both the regular and inverse electric fieldis used. Drift and hysteresis of piezo actuators can beeliminated using position feedback control so that a highposition accuracy can be achieved. Piezoelectric actuatorscan excite very high frequencies up to several kHz. Toenhance the displacement range two thin piezo ceramicstrips can be connected together so that one of themcontracts while the other expands. The so called bimorphdesign is similar to the bimetallic gauge.

Piezoelectric actuators are used in [6] for vibrotactiletexture exploration attached to PHANToMTM . Each pin ofthe array is driven by a bimorph-like piezoelectricactuator at the frequency of Hz and a maximumamplitude of m.

A piezoelectric actuator based tactile shear force displayis presented in [27]. A field of 64 upstanding piezoelectricactuators initialize a coupled swing motion of the 112 skincontactors above, thus a differential displacement of twoneighboring actuators determine a lateral displacement onthe top of the contactor.

D. Shape memory alloys

Wires or pins manufactured with Shape memory alloys(SMA) can easily be deformed when they are cold. Whileheating the alloy force the wire to return into prior form,so it is able to ”remembering” his original shape. The mostcommon used SMA is nickle titanium. As direct drivelinear actuator SMA can exert thrusts up to N/mm .The recovery strain is in the order of %, and thusvery large compared with piezoelectric actuators. AlthoughSMA actuators appear to have the highest force per unitarea and large displacement, several factors make them dif-ficult to use for tactile displays. The main disadvantages areslow speed of response, fatigue problems (short lifetime)and cooling problems.

A pin array based tactile normal force display with 64contactors is presented in [39]. An approach in control ofa SMA wires based tactile display is presented in [40].The contactor of this display reached a bandwidth of

Hz (on dB). Furthermore, there are some interestingapproaches in control and design for using SMA as anactuator technology [41] [42].

E. MEMS actuators

Micro-electromechanical systems (MEMS) technologyprovides highly integrated mechanical structures resp. ac-tuators. The mayor technology used for MEMS actuatorsis typically magnetic, electrostatic or thermal.

An approach of a MEMS technology based displayis described in [43]. This tactile normal force displayprovides with actuators cm a very high pin density.

Unfortunately no performance data are available becauseit is still under development 4.

IV. COMBINED PIN ACTUATOR

The chapter above shows, nearly every known tactile dis-play uses one actuator per pin to generate a tactile stimulusto the human skin. Moreover is indicated, that no currentactuator principle exists to provide the affordable powerdensity or mechanical compactness and thus could be usedto display the necessary psychophysical qualities for a highfidelity tactile stimulus. A way to improve this could be anadaption of the mechanical actuator performance to thesepsychophysical necessity. Our approach in this case is touse two single actuators, optimized to an individual rangeof tactile stimulation, and combine them in serial mannerto an adapted combined pin actuator. So, the subject ofthis chapter is the design and realization of this combinedpin actuator. After the target specification, derived from thepsychophysical data found in chapter II the design and themodules of the combined pin actuator are described andevaluated.

A. Target specification

According to the psychophysical data, listed in Tab. IIthe general target specification of the combined pin actuatorin terms of excursion, frequency, and force can be statedas followed:

Excursion: The overall excursion is physically lim-ited by the compression of the tissue between thesurface and the bone of the index finger. The upperlimit is approximately 3 mm.Frequency: The frequency bandwidth of the cuta-neous perception is about Hz. The sensitivityof indentation increases until a maximum at around230 Hz [17].Force: In the static case the maximum force isrequired at an indentation of mm. With animpedance of N mm a force of N is needed.

Towards the objective to build up a pin actuator with twoactuator technologies the major question is the segmenta-tion of tasks for the specific actuators. Fundamentally, thereare two qualities of stimulation of pin actuated systemsattached to the skin: amplitude and frequency of thepin. Investigations based on psychophysical observationsshowed that there is a threshold in detecting vibrations [15].The lower limit for the fuse of pulses into one continuoussensation (vibration) can be considered to be in the rangeof Hz. In consideration of the general findings inchapter II it has been found that a reasonable breakdownof actuator tasks can be made by splitting the frequencydomain per actuator. The resulting actuator modules andtheir functionality ranges can be stated as follows:

Low frequency actuator:Provide large excursion of up to mm in a frequencyrange of Hz.

4http://www.ri.cmu.edu/projects/project 470.html

High frequency actuator:Provide vibro-tactile stimulations with excursions upto m in the frequency range of Hz.

In the case of the low frequency actuator, the fact of rapiddecrease of threshold detection in the frequency domain( m at Hz) [15] has been used to reduce excursion.The estimated value in excursion of times higher thanthe threshold is based on experiments made in [44], wherea stimulus with an amplitude of times higher regardingthe threshold at a frequency of Hz can be associatedwith a hard brush.

B. Low frequency module

Referring on inquires of actuator principles stack piezocrystals seems to provide the best trade-off between per-formance and power density for the low frequency module.After investigations in off-the-shelf stack piezo actuators interms of size, force, and excursion we chose a stack piezofrom Piezomechanik GmbH.

TABLE III

STACK PIEZO DATA

Type PStMax. excursion mMax. force NStiffness N/ mResonance freq. kHzSupply voltage VEl. capacity nFSize (stack) mmWeight (stack) g

10 mm

Fig. 2. Stack piezo actuator

Tab. III shows the data of the selected stack piezo. Afeature of this stack piezo series is the large expansionratio of %. For better load transmission a version withhalf spherical ceramic caps on bottom and top of the stack(Fig. 2) is chosen, that increase the fitting size up to mm.

Piezo crystals have a linear force-excursion characteris-tic between maximum force on zero excursion and zeroforce on maximum excursion. Following calculations onmechanics are based on half the max. excursion ( m).In this range the piezo can provide forces up to N

To reach a pin excursion of mm, a mechanical systemof levers has to transmit the movement of the actuator witha ratio of . The simplest way to realize this is to use a

lever system shown in Fig. 3. Based on a minimum distanceof AB mm, predetermined from the deviations of thepiezo, the length of the lever AC rises up to cm. Thatwould dissent consequently the demand of compactness.

Piezo

CA B

Fig. 3. One lever transmission

AB

C

D E F

Piezo

Fig. 4. Cascaded two lever transmission

Fig. 4 shows the schematic structure of the resultingcascaded two lever transmission system. Revolute jointsare displayed as circles; the filled circles show joints withfixed axis. The first stage (A,B,C) is enhanced by a second(D,E,F), acting the opposite direction to reduce the overallsize of the lever and increase compactness of the module.The transmission ratio of the combined lever system canbe calculated to:

BF BC EF

AC

AB

DF

DE(1)

With the additional constraints of:

EF EF DE mm (2)

the lever lengths result to:

lower AC mm upper DF mm (3)

Based on this kinematic dimensioning a mechanical con-struction has been found. A major point is to find a solutionfor the construction of the joints A, C, D, E and F. After acomparison between different standard techniques in finemechanics [45] the method of solid state joints has beenselected. These joints are realized by elastic bending of itsthin paths at their pivot points. They need no assemblingor adjustment, have no abrasion, nearly no friction andzero backlash. The disadvantage of this technique, the highrestoring force, can be used to provide the necessary pre-load for the stack piezo. As a consequence of this design,the whole lever transmission mechanism has to be built outof one piece.

Fig. 5 shows the first layout of the cascaded two leversystem designed on the described principle. The picture

FS

FP

Fig. 5. First layout of combined lever system

displays the side view of a cut out plate with a thicknessof mm. The lever system must be designed this way,that the piezo (hinted pictured) fits itself with some initialload. At the endpoint of the second lever the pin can beattached to use the amplified motion in vertical direction.The force of the stack piezo FP and the load of the pinFS, caused by skin impedance, are displayed at their actionpoints.

On further steps to analyze the range of motion and thetension at the bending joints the finite elements method(FEM) is used to optimize shape and functionality of thecombined lever system. As simplification for the FEMcomputation only the stack piezo load in direction of thefirst lever has been considered. This can be made becauseon the bottom point of contact no deformation or criticalstress is expected.

Fig. 6 shows the results of the first FEM calculations.The left picture displays the deformation of the leversystem at maximum load generated by the stack piezo.Furthermore, the arrangement of the notes used for thecomputation can be seen. The colored picture on the rightside of Fig. 6 displays the stress fields with a table of thecolor related stress forced aside. For the first three designloops quality steel (C55) with a thickness of mm has beenused. This steal has an elastic modulus of E N

mmand a fatigue resistance of N

mm . With this raw materialvalues the deviations of the three bending joints has beenpre-calculated to mm and the cross-sectionalarea of the levers to mm (thickness of lever systemplate d mm). First calculation results an excursion of

m at the end of the second lever, activated by a piezoexcursion of m at a force of N. The reason for thisbad excursion are the accidental bending behavior of thelevers and the body. The stress field picture indicates lowtensions in the joints and as a consequence to less bendingin the joints. It displays low stress values at some placesto adapt the shape as a result of the stress field.

During some iterations in design regarding FEM calcula-tions, some basically chances of the combined lever systemhas been made:

Decrease of thickness of the base plate down tomm.

Redesign of shape, based on stress field results offurther designs.Decrease of cross-section area length of the bending

joints.Elongation of upper and lower lever, to adapt trans-mission ratio.

The resulting values in the area of the bending joints stressfield are shown in Fig. 7. The movement at the end ofthe second lever raises up to mm at a stack piezoexcursion of m. In this maximum position the leversystem provides preloaded force of N to the stackpiezo. The stress field picture in Fig. 7 shows, that themaximum stress areas at the bending joints are safely inthe non deformable range when using heat-treated steel(50CrNi4) with a fatigue resistance up to N

mm .The body of the low frequency module was manufac-

tured based on the final design loop by the method ofwire-electro discharge machining.

Fig. 8. Final combined lever system with stack piezo

Fig. 8 shows the body of the combined lever systemwith attached stack piezo. At the end of the second leverthe fixation point for the pin is indicated.

Position control

In consideration of the expected dynamic behavior ofthe described lever system body, as also the hysteresis inthe response behavior of the stack piezo actuator, there isa need for a closed-loop position control for the excursionof the pin. For the control-loop a sensor has been attachedto the end of the second lever. This self-made sensor isbased on a PSD (position sensitive diode) from IC-HausLtd. (http://www.ichaus.com). With this PSD theposition of an infra red light spot can be detected withinan area of mm. The light spot is generated byaperture with a diameter of mm in a flat aluminum stick,which is attached to the end of the lever system body. Thesensor is fixed to the ground plate and receives the movinglight spot caused by shading effects of the aperture. Forsource of light a pulsed infra red diode at the opposite ofthe sensor is attached.

Modeling

A model for the combined rocker lever system is neededfor analyzing the mechanic behavior in view of the designof a position control algorithm and as shown later asa reference model for the finally used observer basedcontrol strategy. Fig. 9 shows a schematic construction of

DEFORMATION STRESS FIELD

Fig. 6. First FEM - design loop of the combined lever system

DEFORMATION STRESS FIELD

Fig. 7. Fourth FEM - design loop of the combined lever system

SENSOR

msensor

piezoc

SF

AB

C

ED F

G

ϕD

ϕA

PF

Fig. 9. Mechanical model

the system with external acting forces. The force of thestack piezo ( P) represents the control variable whereas

S, caused by the impedance of the human skin, acts asdisturbance variable to the control system. Compared withFig. 4, used for calculating the kinematics, the connectionsat the points C and E are fixed like at the real system,because of modeling as bending beam. Dynamic relevant

components, like the sensor rod at point F and the pinrods at G, shown in Fig. 9, have been considered in thecomputation.

Thus, the equations of motion can be calculated bysolving the Lagrangian differential equations:

(4)

with the Lagrangian equation:

(5)

Where represents the kinetic energy, the potentialenergy of the considered mechanical system, containsdissipation energy (e.g. in consequence of damping) and

the external load to the mechanical system, related tothe corresponding generalized coordinate .

The generalized coordinates are defined by the anglesof the lower ( A) and upper ( D) lever to the vector as:

A

D

(6)

Regarding the generalized coordinates , the equationsof the kinetic energy , the potential energy and the

dissipation energy of our mechanical system in generalform are given to:

AC A DG D

sensor DF cos D D (7)

A A D D

CE A D piezo AB sin A

AC AC sin A AC cos A

DG DG sin D DG cos D

sensor DF sin D (8)

A A D D CE A D (9)

With the inertia coefficient, gravitational acceleration,stiffness coefficient, damping coefficient, and the

masses, referring to the schematic in Fig. 9. The values AC

and DG represent the vertical position shifts of the centersof gravity from the corresponding centers of rotation.

In the energy terms (7)-(9) the following simplificationshave been made:

The kinetic energy of the stack piezo and the joints(beams) are neglected in (7).Only the vertical velocity has been considered in (7)The potential energy of the piezo body has beenneglected in (8).Because of optimization with the FEM in the sectionabove, the lower and upper lever are supposed as rigidbodies, so their dissipation energy have been neglectedin (9).

The solution of the Lagrangian differential equationsEq. (4) including the external load, caused by the piezostack, of A P AB results in:

AC A A A CE A CE D (10)

piezo AB sin A cos A AC AC cos A

AC AC sin A A A CE A CE D P AB

DG D sensor DF cos D D D D

CE D CE A DG DG cos D (11)

DG DG sin D sensor DF cos D

D D CE D CE A

sensor DF sin D cos D D

Under assumption of linear behavior between the force,generated by the piezo P, and its supplied voltage therelation:

P P with PP (12)

results, where signified P and are the maximumrange of force and voltage, respectively In our case, with

CP

I Pi

0

Power supply Piezo

R

U

piezoU

Fig. 10. Electrical model of the piezo, attached to the power supply

a maximum force P N and a maximum voltage ofV, the gain P results to P .

Electrically, the stack piezo can be modeled in a sim-plified way as a capacitor with capacitance of P [46].Fig. 10 shows the circuit diagram of the piezo attack to thepower supply, with the source voltage and an internalresistance . The voltage applied to the piezo is labeledwith and the current with P. Using Kirchhoff’s lawthe following equation results:

P with P P (13)

Thus, the dynamic behavior of the source voltage andthe piezo voltage can be described by:

P P

(14)

For further system analysis and controller design, thedynamic equations of the lever system (10), (11), as wellas the electromechanical equations (12), (14) have beentransformed in state space form:

D D (15)

The state vector and the control input can be definedto:

A

A

D

D

(16)

Based on this the system matrix , the input vector andthe output vector calculate to:

P

P AB

AC

A CE piezo AB AC AC

AC

CE

DG sensor DF

A CE

AC

CE

AC

CE

AC

CE

DG sensor DF

D CE DG DG

DG sensor DF

D CE

DG sensor DF(17)

P

DG

(18)

The disturbance vector D D containing constant andnonlinear terms of (11) is:

D D

AC AC

AC

sensor DF sin D cos D D DG DG sensor DF

DG sensor DF(19)

On transformation of (10) and (11) into the state spaceform (15) approximations for small angles:

sin cos for (20)

have been made. Because of very small angles of A andD this is practicable.Below the parameters of the system matrix will be

specified mathematically. The stiffness coefficients of thejoints A and D, are calculated based on a homogeneouslystraight bending beam. They can be calculated going outfrom the deflection curve derived by the hypothesis ofBernoulli:

(21)

where is the beam deflection in relation to thedistance of his fixation , the maximum momentumat the fixation point, represents the shear modulus ofthe used steel and the geometrical momentum of inertia.For the maximum length of deflection line as well asthe constraint due to no bending at the fixationpoint, the integration of (21) over the length of the bendingbeam results in:

(22)

As represents the gradient of the beam deflection atits end point the relation to is given to:

tan for (23)

By inserting the specific values:

(24)

(23) can be transformed under assumption of a short beamto:

(25)

The potential energy of the beam caused by bending canbe calculated to, see [47]:

(26)Hence the stiffness of the bending beams in relation to thegeometrical form and the modulus of elasticity resultsto:

(27)

Tab. IV displays the the stiffness of the three bendingjoints, with N

mm . As listed in Tab. III, the

TABLE IV

STIFFNESS COEFFICIENTS

Joint mm mm mm NmAB

CE

stiffness coefficient of the stack piezo is given byN

mm .

The inertia of the lower and upper level could becomputed directly by the CAD program based on thegeometrical data and the density of the used steel to:

AC kg m DG kg m(28)

Additional values, that are important to compute the systemmatrix and vectors, are summarized in Tab. V

With these results, the coefficients of the state spacesystem ( 15) can be calculated, except for the dampingvalues A CE D. In a first approach these damping valueshave been reduced to one unknown damping value ,based on the geometrical coherences of the bending beams.This damping values can be defined in the followingvalidation of the calculated dynamic model.

To use the calculated dynamic system for the furthercontroller design, it has been validated with the real hard-ware setup. Initially the frequency response of the hardwaresystem has to be compared to the computed model.

Fig. 14 shows the open loop frequency response char-acteristics of the real system in within a range of

Hz kHz, caused by a swept sine signal and pro-cessed by a wave analyzer. The first resonance frequencyof the system can be identified clearly at the frequency of

Hz. After this resonance magnification, the amplificationof the system falls significantly with a rate of dB perdecade. A second resonance magnification can be seen atthe frequency of approx. kHz. The behavior in the upperfrequency range ( Hz) deviates from the expectedcharacteristics of the modeled -System, probablecaused by the nonlinearities summarized in the disturbanceterm of the state space model (19). In comparison theretothe calculated dynamic model deviates with its resonancefrequency of Hz by about from the real setup.The second frequency magnification differs by about .This deviation has been adjusted by tuning the coefficientsof the modeled system.

Due to the fact, that our mathematical model consists ofa -System, the damping value must be identifiedat the hardware setup. Therefore the captured step responseof the real system has been used to adapt the dampingbehavior of the calculated model by tuning the dampingvalue in an iteration loop. Finally, the coefficients ofthe system matrix in (18), for the state space system

TABLE V

DEVIATIONS AND MASSES OF THE COMBINED LEVER SYSTEM

1st lever 2nd lever Sensor PiezoAC mm DG mm DF mm AB mmAC g DG g g –AC mm DG mm – –

101

102

103

104

−30

−20

−10

0

10

20

30

40

Frequency [Hz]

Am

plitu

de [d

B]

101

102

103

104

−4

−2

0

2

4

Frequency [Hz]

Pha

se [r

ad]

Fig. 11. Open loop frequency response, measured by experiment

(15) results to:

(29)

Fig. 12 shows the step response at a range of m tom of the real setup in comparison with the computed

dynamic model. Thereby the computed model follows thereal system with a small phase shift. Also the decay ofthe amplitude is not exactly as fast as at the real system.But is clear that the characteristic of the real system issufficiently approximated by the computed model, makingit a solid basis for further control design.

Control concept

Based on the dynamic model of the combined leversystem, in this section the concept and parametrization ofthe controller is described.

Fig. 13 shows the overall control concept of the lowfrequency module. The input value ( ) contains the set-point of position that is followed by the pin, measured atthe endpoint of the lever system ( ). Basically, the entirecontrol structure can be divided in two control loops:

An output control loop at the outer control structureis to guaranty steady state control accuracy. It consistsof an integral action controller, that ensures a goodreference tracking of the control loop.The state control in the inner control loop is imple-mented to enhance the dynamic behavior of the plant.It is realized by an observer based control structure.

Below both control loops are described separately, startingwith the outer control loop.

Outer control loop: The integral action controller forthe outer control loop has been designed by an LQ-controlapproach [48]. Therefore the equation of the dynamicsystem in state space form (15) has been extended by an

1 1.05 1.1 1.15 1.2 1.25 1.3 1.35 1.4 1.45

300

500

700

Time [s]

Pin

pos

ition

[μm

]

Real systemSimulated model

Fig. 12. Step response of the real system compared to the model.

Plant

Output controlState control

Fig. 13. Control structure

integrator to:

(30)with the input value and the matrix and vectors displayedin (18). In the closed loop the regulation value has beenbuilt by the states of the system to:

with

(31)The coefficients of the gain vector have been found bysolving the optimization problem:

with (32)

where the matrix weights the states of the systemand the coefficient weights the control input . With thestability margin , the settling time can be determined. Toprevent the control input from saturation of the maximumsupply voltage of the stack piezo ( V) the control inputweight coefficient is set to , whereas theweighting matrix of the states is set to diag .

The standardized step response in Fig. 14 shows theresult of the LQ-controller design at the reference inputregarding different stability margins. In every case the

0.998 1 1.002 1.004 1.006 1.008 1.01 1.012 1.014

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Time [s]

Pin

def

lect

ion

[μm

]

σ =−550σ =−678σ =−857

Fig. 14. Standardized step response at different stability margins

0.998 1 1.002 1.004 1.006 1.008 1.01 1.012 1.014

−0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

Time [s]

Con

trol

inpu

t

σ =−550σ =−678σ =−857

Fig. 15. Standardized control input at different stability margins

signal has aperiodic response behavior. Fig. 15 displaysthe standardized characteristics of the control input duringthe step response in terms of the maximum set voltageof V. It can be seen, that stability margins lower than

bring the control input in saturation of the stackpiezo amplifier. For further calculations a stability marginof has been chosen. With this value we get anstep response time of ms and the gain coefficientsresult to:

(33)

Inner control loop: To enhance the dynamic behaviorof the plant and compensate the noise of the positionsensor, the inner control loop has been designed as aproportional state controller. Therefore the states of thecomputed mechanical system have been recovered from

Real System

Model

Observer

1s

Fig. 16. Kalman-Bucy-Filter block diagram [49]

the measured position signal. This has been realized witha Kalman-Bucy-Filter, schematically shown in Fig. 16. Itconsists of a parallel observer system ( ), based on thecomputed state space model, with the control input andthe by the vector weighted difference of the measuredrespective computed output ( ) of the plants as inputvalue. So, the estimated states of the system are givenby the state vector . Under assumption of compliancebetween the real plant and the computed model (

and ) the state space form of the observerresults to:

(34)

The Kalman gain vector has been determined on mini-mization of the performance index of the state vector error

between the observer (34) and thesystem model (15) by minimization of variance:

(35)

min.

With the equation for the Kalman gain :

(36)

The covariance matrix consists of the positive definitesolution as a result of solving the Matrix-Riccati-Equation:

(37)

The weighting matrix is determined by the processnoise, whereas the weighting value depends on themeasurement noise. They are specified by measurementson the real system in an iterative process to:diag The Kalman gain vector

results to:

(38)

Controller performance

The entire dynamic system of the low frequency actu-ator containing an output control loop at the outer anda Kalman-Bucy-Filter at the inner control loop can besummarized in state space form as:

(39)

Due to the fact of using the LQ-control approach in bothcases for controller design, the entire controller for the lowfrequency module is inherently stable. For measuring theperformance of the entire controller, a step response in bothdirections of the pin moving space has been performed.

Fig. 17 shows the measured pin position of a stepresponse from m down to m, as well as the pinposition, calculated by the observer. After a small overshot,the system reaches a nearly steady state after approximately

ms. In Fig. 18 the corresponding characteristic of thecontrol voltage for the stack piezo amplifier is shown.Thereby, proportional amplification coefficient of the stackpiezo amplifier amounts to .

The measured observer pin position on a step responsefrom m up to m is shown in Fig. 19. Comparedwith the step downward it has a longer setting time sup-posed by the countering influence of inertia. Other reasons

5.49 5.5 5.51 5.52 5.53 5.54

200

300

400

500

600

700

800

900

1000

1100

1200

Time [s]

Pin

def

lect

ion

[μm

]

Reference value wActual value yObserver value

Fig. 17. Closed loop step response from m to m

5.49 5.5 5.51 5.52 5.53 5.54−2

−1

0

1

2

3

4

5

6

7

Time [s]

Con

trol

val

ue [V

]

Fig. 18. Control input at a step from m to m

could be the neglected nonlinearities. The correspondingcontrol voltage is shown in Fig. 20.

Compared with the open loop step response in Fig. 12a clear improvement of the positioning accuracy has beenreached. Even if the specifications are not reached com-pletely, the resulting low frequency actuator represents apromising approach in actuator technology for tactile pinactuators. Fig. 21 shows the entire hardware setup of thelow frequency actuator with position sensor.

C. High frequency module

The high frequency module provides the frequency rangeabove Hz of stimulation in the combined actuator con-cept. The specifications therefore are: pin excursions upto m at frequencies up to kHz. These efforts can berealized by a standard stack piezo actuator.

Caused by their functional principle, piezo crystals showa nonlinear behavior in response characteristic e.g. hystere-sis of movement [50]. Due to this fact a control designis necessary, to compensate this effects and provide goodposition accuracy.

4.98 5 5.02 5.04 5.06 5.08 5.1 5.12

200

300

400

500

600

700

800

900

1000

1100

1200

Time [s]

Pin

exc

ursi

on [

μm]

Reference value wActual value yObserver value

Fig. 19. Closed loop step response from m to m

4.98 5 5.02 5.04 5.06 5.08 5.1 5.120

1

2

3

4

5

6

7

8

9

Time [s]

Con

trol

val

ue [V

]

Fig. 20. Control input at a step from m to m

Pin movement

Sensor circuit

Position sensor

IR−diode

Piezo

Multi lever system

Fig. 21. Complete setup of the low frequency module

For the high frequency module an off-the-shelf stackpiezo with an integrated position sensor has been used.A picture of this piezo is shown in Fig. 22 besides thetechnical data in Tab. VI. Furthermore, the piezo comespre-pressed. This is necessary to prevent the piezo from

TABLE VI

STACK PIEZO DATA

Type PStMax. stroke mMax. force NPrestress N

Stiffness Nm

Resonance freq. kHzSupply voltage VEl. capacity nFPos. sensitivity nmLength (L) mm

Fig. 22. High frequency stack piezo actuator

destruction caused by unloaded excursion. The range ofmotion is m, thus, in combination with the low fre-quency module it can be used directly as a part of thewhole pin actuator.

The integrated position sensor is based on strain gagesattached to the piezo stack. Electrically they build a Wheat-stone bridge circuit of resistors. To adapt the movementcaused by resistor changing at the bridge circuit, a highimpedance amplifier has been built.

Position control

Due to the fact of capacitive behavior of the piezosupplied by a power supply with internal resistance, shownin Fig. 10 the plant can be modeled as a – System(first order lag), with the transfer function:

excursionsupplied voltage

(40)

with the voltage gain of the amplifier , and the excursionin relation to the max. input voltage . The timeconstant can be determined from the piezo capacityand the internal resistance of the power supply to

. As control concept a proportional plusintegral controller has been chosen.

Fig. 23 shows the control structure of the high frequencymodule, with the assumption of an ideal sensor ( )and the controller coefficients and . For the demandedfrequency range of up to kHz a setting time of

ms is necessary. Thus, a stability margin ofresults. Based on the root locus of the control system andadjustment of the zero point, with repect to the stability

margins constraints the control gains have been appointedto: and .

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5 100

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

5.5

6

6.5

7

7.5

8

8.5

9

9.5

10

Reference position

Mea

sure

d po

sitio

nFig. 24. Hysteresis of the stack piezo

Controller performance: Fig. 24 shows the behavior ofthe stack piezo in its entire range of movement, where thecharacteristic hysteresis can be seen.

3.684 3.6845 3.685 3.6855 3.686 3.6865 3.687 3.6875 3.688

2

3

4

5

6

7

8

Time [s]

Mea

sure

d an

d pr

e−se

t pos

ition

Pre−setMeasured

Fig. 25. Pulse signal response at Hz

The results of the position controlled stack piezo, usinga PI-control algorithm, are displayed in Fig. 25 and Fig. 26.They show the sinus and pulse response at the samefrequency ( Hz). Thereby, the measured voltage in therange of to V is proportional to the pin excursionof to m. In comparison with measurements ofthe real system the computed model differ in terms ofbandwidth with Hz of about . However,the controlled high frequency actuator can be used as partof the multi actuated pin concept, as planned.

V. TACTILE SHEAR FORCE PROTOTYPE DEVICE

This section presents the prototype of a shear forcedisplay for the finger tip and a first psychophysical eval-uation. As seen in chapter III most tactile displays are

w ye u

z

Fig. 23. High frequency module control structure

3.339 3.34 3.341 3.342 3.343 3.344 3.345 3.346 3.347

2

3

4

5

6

7

8

Time [s]

Mea

sure

d an

d pr

e−se

t pos

ition

Pre−setMeasured

Fig. 26. Sinus wave response at Hz

designed with the aim to display tactile pictures, using pins(needles) arranged in a grid like arrangement, where thepins are movable independently, mostly variably adjustablein position, normal to the surface of the skin. However,from the psychophysical point of view the mechanismof tactile perception is considerably more complex andpartly unexplored. At this reason we build a prototype oftactile display, to explore some open questions regardingthe tactile shear force perception, whose findings vice versaenhance the development of shear force displays in furtherdesign loops.

For the first prototype, presented in this paper, the designis based on the known psychophysical findings and thresh-olds summarized in chapter II. In order to explore whetherthe stimuli produced by the display are appropriate for hu-man perception we studied discrimination performance ofhumans for distinguishing between different directions ofpin movement. In a second step we explored the perceptualintegration of multi-pin movements.

A. Technical realization of shear force display

The fundamental concept of the display is based ona quadratically arranged pin array. To exert shearforce to the area of the finger tip, each pin is movablelaterally to the skin. Thereby it is possible to move eachpin independently along both axes of their two-dimensionalhorizontal directions. Design parameters have been chosenbased on psychophysical thresholds. A pin diameter of

mm, a center-to-center pin spacing of mm (zero positionof lateral movement) and a lateral movement of mm alongeach axis and for each pin are realized [51].

X

Z

Human FingerElastic Rubber Layer

Stiff Iron Pin

Servo MotorUniversal−Joint Shaft

Tuning Screw

Reduction Rocker Arm

Side View

Base Plate

Fig. 27. Mechanical design of shear force display.

The mechanical design for one pin in one axis isschematically shown in Fig. 27. The four pins are in directcontact with the finger tip (optional filtered by an elasticrubber layer). Two rods orthogonal attached to the upperregion of each pin transmit two-dimensional movement ofthe pins. To allow for this movement the four pin bodiesare attached with universal-joint shafts that are screwed tothe ground plate of the chassis. Tuning screws betweenthe base plate and the joints enable a justification alongthe -axis in a small range to vary the indentation offsetnormal to the finger pulp. The rods are connected overreduction rocker arms to the servo motor levers. To reduce

y

x

Fig. 28. Pin-body design and arrangement

pin motion normal to the skin down to a tolerable value, thelength of the pin raised to mm. In case of a pin diameterof mm a realization of a pin with this dimensions wouldbe unusable because of bending. Furthermore it would bedifficult to to attach the two steering rods providing thetangential motion. To avoid this problem, the length of thepin has been reduced to mm and attached to a prismaticbody (Fig. 28 left). The right picture of Fig. 28 shows thetop view of the mechanical design. The pins are fitted tothe inner edge of the bodies (black points) to provide alarge motion workspace for the pins.

Fifo

Parallel Port(Real Time Task)PWM Generation

Application

Computer (RT Linux)

Power Supply

Fig. 29. Control system.

On the actuator side, we use of-the-shelf servo motors,selected on criteria like high performance, small deviations,and light weight. The servo motors come with a positioncontrol circuit, accessed by a pulse-width modulated sig-nal, which contains the commanded position information.Fig. 29 shows the schematic design of the computerinterface. A real-time task generates the required signals forthe eight servo motors at the printer-port of the computer,whose data lines are connected to the corresponding servomotors. In the application process the position informationfor the servo motors are calculated and communicated tothe real-time task. To provide sufficient power for the servomotors an external power supply is used.

The upper picture in Fig. 30 shows the display in usewith a close-up view of the pin area that is in contact withthe finger tip through a hole in the cover plate. A moredetailed view in the lower picture of Fig. 30 shows themechanical details of the display like the pin bodies, the

Fig. 30. Hardware setup of the shear force display.

TABLE VII

TECHNICAL DATA OF THE SHEAR FORCE DISPLAY.

Property ValuePositioning resolution mMax. pin force (per axis) NMax. pin velocity mm/sPin work space mmSize mmWeight g

control rods, and the servo motors. Tab. V-A summarizesthe technical data of the tactile shear force display.

B. Psychophysical evaluation

The psychophysical evaluation focused on the ques-tion of whether the stimuli produced by the display areappropriate for human perception. In Experiment westudied discrimination performance for different directionsof single-pin movement. To our knowledge there is justa single study on human direction discrimination on thefinger tip [53]. That study, however, investigated single pinmovement paths of a length that our multi-pin device hasnot been designed to display. In the second experiment,we studied integration of parallel movement of multiplepins. From well-established theories on sensory integrationin humans, we expected that discrimination performanceshould profit from multi- as compared to single-pin display

- at least if the brain processes movements displayed bythe single pins independently from another [13].

Experiment 1

Methods. right-handed participants ( females, agerange to years, average age ) took part for pay.None of them had any known tactile deficit. The partici-pants sat in a quiet room in front of a table with their leftelbow resting comfortably on a custom-made support. Byusing robust tape their left hand was attached to the shearforce display so that the the tip of their left index fingerwas reliably centered at the midpoint of the pin display.We used the shear force display with a single pin only (forthis experiment the other three pins were removed) and aparticular base plate (Fig. 31 right, upper) that allowed forindividual adaptation of gap size and, thus, maximal spreadof skin stretch. A blind prevented participants from seeingthe display and white noise displayed via headphonesmasked the sounds of the display during pin movement.A custom-made program on a PC controlled the stimuluspresentation and collected the responses.

Tactile stimuli were unidirectional single strokes ofmm length (velocity cm s) starting at the midpoint of

the display. For the strokes we defined eight standard direc-tions with respect to the finger (separated by , Fig. 31left). To each standard a set of comparison strokes waschosen, the directions of which were distributed insteps around the corresponding standard within an area of

(Fig. 31 right, lower).

Fig. 31. Setup in Experiment 1.

In each single trial the participants successively felta standard and a comparison stroke and, then, had todecide by a button press which of the two strokes hadbeen oriented more in clock-wise direction. Between thestrokes participants had to lift their finger to allow formoving the pin back to the midpoint without evoking tactilestimulation. Required finger movements were signaled bydifferent sounds on the headphones.

Each pair of standard and comparison stroke was pre-sented twelve times during the experiment. The order ofstandard and comparison was balanced across repetitions.The order of pairs was completely random. For eachsingle participant the experiment lasted about four hoursperformed within two sessions (including three breaks) ondifferent days.

Using the psignifit toolbox for Matlab [54] we fittedindividual psychometric functions (cumulative Gaussians;Maximum Likelihood procedure) to the proportion of trialsin which the comparison stroke was perceived as moreclockwise-oriented than the standard stroke - against thecomparison direction. From the fits, we obtained individual

-discrimination thresholds per standard.Results and Discussion. After the experiment all par-

ticipants reported that they had felt pin translation andmost that they had perceived also skin stretch. Figure 32depicts medians and quartiles of the individual discrimi-nation thresholds by standard direction. Median thresholdsranged from to , thresholds quartiles ranged froma maximal -percentile of down to a minimal

-percentile of . Further, the individual thresholdsper standard direction entered a Friedman test. The testreached significance, , indicating aperceptual anisotropy, i.e., participants discriminated betterbetween movements in up-direction as compared to theother directions.

Fig. 32. Medians and quartiles of -thresholds by standard direction.

Most importantly here, threshold magnitudes demon-strate that humans can well discriminate between thedirections of the movements displayed. It is also importantto note that the technical directional resolution of the shearforce device clearly exceeds the perceptual thresholds weobserved. Thus, the shear force display produces tactilestimuli that are appropriate for human perception, and shearforces seem to be usable to obtain a differentiated impres-sion of at least one environmental aspect. Consequently,the next experiment explored to what degree shear forcesdisplayed by more than one pin are integrated.

Experiment 2

Methods. Twelve paid right-handed participants withoutknown tactile deficits took part in Experiment ( females,age range to years, average age ). The samesetup, procedure and stimuli as in Experiment were used.

However, we defined just two standard directions withrespect to the finger (up and down) including corre-sponding comparison directions. Directions were displayedeither with one, two or four pins (other pins removed, cf.Fig. 33 for directions and pin configuration). In the multi-pin conditions all pins moved simultaneously and withidentical velocity and direction; the pins were separatedby 3 mm (center-to-center). Trials with one, two or fourpins were performed in three sessions on different days.The order of sessions was balanced between participantsaccording to a Latin square design. The whole experimentlasted about three hours.

Fig. 33. Setup in Experiment 2.

Results and Discussion. Fig. 34 depicts medians andquartiles of the individual discrimination thresholds bystandard direction and pin number. Median thresholdsranged from to , thresholds quartiles ranged from amaximal -percentile of down to a minimal -percentile of . The individual thresholds per standarddirection and pin number entered a Friedman test. Thetest clearly failed to reached significance, ,

, demonstrating that in this experiment participants’discrimination performance did neither differ between thetwo movement directions nor between the conditions withdifferent pin numbers.

Fig. 34. Medians and quartiles of -thresholds by standard directionand pin number.

In general, the magnitude of discrimination thresholdsin Experiment confirmed that humans well discriminatebetween the directions of the movements displayed andthat the perceptual resolution did not exceed the technicalresolution. Thresholds in this experiment tended to beslightly lower than in Experiment . There were, however,

substantially less different standard directions in this ascompared to the last experiment, so that memory effectsmay have played a role. Surprisingly, we were not ableto replicate the advantage of up-directions observed inExperiment . Most importantly, we did not find any effectof pin number on discrimination performance. Accordingto well-established models on human signal integration, thehuman brain integrates independent redundant signals onthe same physical property such that the integrated signalis more reliable than the single signals were [13]. The lackof an integration benefit in the present study may indicatethat the movements of the different pins of the shear forcedevice were not independently processed in the perceptualsystem.

VI. CONCLUSION

A. Combined pin actuator

Regarding our research on advancement in actuatortechnologies, the FEM based design of a cascaded twolever mechanical structure for a multi actuated tactile pinactuator is presented. The lever system forms the low fre-quency module of the combined pin actuator and reach anexcursion length of 1.7 mm. The position control conceptof the module is based on position measurement and fullstate feedback control using the 5-dimensional state vectorof the linear time invariant model. State feedback gainsare designed using a linear quadratic regulator approach,the state is estimated from position measurements using aKalman filter approach. Experimental results confirm thatthe linear model of the system is very close to the hardwareprototype. The control approach shows good performancein position control, which is validated with experimentalresults of the prototype hardware. The high frequency mod-ule consists of an of-the-shelf piezo crystal providing largebandwidth, but only for small excursions. Experimentalresults confirm the effectiveness of this second actuator inthe high frequency module of the multi actuated pin. Bothmodules apart have the expected functionality and goodperformance. In further steps they will be mechanicallycombined in a serial manner.

B. Tactile shear force prototype device

This paper presents the design and buildup of a noveltactile shear force display. In the experiments we analyzedwhether the shear force display produces tactile stimuli thatare appropriate for human perception. Both experimentsdemonstrated that humans can well discriminate betweenthe shear forces displayed, and also that their perceptualresolution does not exceed the already realized techni-cal resolution of the device. Further development of thedevice can take advantage of these facts. In Experiment

we further found evidence that the human brain doesnot process stimulation from different pins independentof another at least concerning movement direction. Onthe one hand, future evaluation of the present prototypewill have to explore to what degree non-simultaneous,differential multi-pin patterns of movement are able toevoke discriminable percepts. On the other hand, future

development of the shear force display will have to includemechanisms that allow for an increased pin distance.

ACKNOWLEDGMENTS

This work is part of the TOUCH-HapSys project finan-cially supported by the 5th Framework IST Programme ofthe European Union, action line IST-2002-6.1.1, contractnumber IST-2002-38040. For the content of this paper theauthors are solely responsible for, it does not necessarilyrepresent the opinion of the European Community.

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