Research ArticlePerception-Based Tactile Soft Keyboard for theTouchscreen of Tablets

Kwangtaek Kim

Department of Information and Telecommunication Engineering, Incheon National University, Incheon, Republic of Korea

Correspondence should be addressed to Kwangtaek Kim; ktkim@inu.ac.kr

Received 11 July 2017; Revised 2 January 2018; Accepted 9 January 2018; Published 13 February 2018

Academic Editor: Maristella Matera

Copyright © 2018 Kwangtaek Kim. (is is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Most mobile devices equipped with touchscreens provide on-screen soft keyboard as an input method. However, many users areexperiencing discomfort due to lack of physical feedback that causes slow typing speed and error-prone typing, as compared to thephysical keyboard. To solve the problem, a platform-independent haptic soft keyboard suitable for tablet-sized touchscreens wasproposed and developed. (e platform-independent haptic soft keyboard was verified on both Android and Windows. Inaddition, a psychophysical experiment has been conducted to find an optimal strength of key click feedback on touchscreens, andthe perception result was applied for making uniform tactile forces on touchscreens. (e developed haptic soft keyboard can beeasily integrated with existing tablets by putting the least amount of effort. (e evaluation results confirm platform independency,fast tactile key click feedback, and uniform tactile force distribution on touchscreen with using only two piezoelectric actuators.(e proposed system was developed on a commercial tablet (Mu Pad) that has dual platforms (Android and Windows).

1. Introduction

With advancement in touchscreen technologies, users getused to various functions of mobile devices through touchinteractions. One of the most used functions is the softkeyboard input method which is very important for pro-ductive interaction on the touchscreen of a mobile device. Forthis reason, studies to design a better soft keyboard have beenactively conducted for the past years. One of the good ex-amples is to analyze and optimize keystroke patterns ontouchscreens in order to improve key typing productivity[1–3]. Nevertheless, most of the users are not satisfied evenwith a better-designed soft keyboard since lack of physical keypressing feedback is the most frustrating thing when typingon a touchscreen. To this end, a low-cost linear motor hasbeen widely used for mobile phones to create synchronizedvibrations when phone users type on the touchscreen.However, the vibration generated from a linear motor is faraway from a real-like key click effect. (erefore, there is theneed of developing a high-definition (HD) tactile feedbacktechnology of key click on the touchscreen of mobile devicesincluding tablets that become popular these days.

To provide real-like key click feedback, designing a newactuator that can mimic a real-like key click movement ona touchscreen is imperative. It is learned that the piezo-electric actuator is prominent for implementing a virtual keyclick effect on touchscreens since its response is not onlyvery fast, but also precisely controlled by applying inputvoltage. Despite the strength of piezoelectric actuators, thereis very little known about how to utilize them to key clicktactile feedback on tablet-sized touchscreens because pie-zoelectric actuators in general require high voltage (e.g., 100to 200 Vpp for a single-layer ceramic bender) to be operated.Besides, driving multiple piezoelectric actuators at the sametime on the touchscreen of a tablet is a challenging issue. Totackle the problem, Han and Kim developed the first pro-totype of the haptic soft keyboard for a tablet (MicrosoftSurface Pro) with four piezoelectric actuators for high-definition key click effects [4].

For the work, a soft keyboard module was implementedunder the Windows platform, and a haptic driver system thatdrives the four piezoelectric actuators at the same time wasalso developed as a portable prototype.(e key pressing eventwas synchronized with four actuators attached under the

HindawiMobile Information SystemsVolume 2018, Article ID 4237346, 9 pageshttps://doi.org/10.1155/2018/4237346

touchscreen.(e study showed how to use and drive multiplepiezoelectric actuators on a commercial tablet for the firsttime. It also demonstrated the effectiveness of precise key clicktactile feedback and the improved typing performance byconducting a user study with the developed prototype.However, the prototype had a significant time delay (over10ms) when driving the four piezoelectric actuators at thesame time, which resulted in unsynchronized tactile feedbackto fast typing. Another problem was nonuniform distributionof tactile feedback on the touchscreen, so a typist often feltunpleasantly strong tactile feedback near the actuators whilefelt weak at the center of the touchscreen. (e other was noverification on other platforms since demanding additionalwork was needed to test with another platform.

In this study, a new haptic soft keyboard technology isintroduced as an extended version of the previous work byHanand Kim in that three issues of the previous work—platformdependency, delayed tactile feedback, and nonuniform tactilefeedback distribution on the touchscreen—have been resolvedby developing a standalone microprocessor-based tactilefeedback module that can be easily integrated with existingtablets. In addition, a perception study to find an optimumthreshold of key click feedback strength on a touchscreen hasbeen conducted by using a well-known psychophysicalmethod(two interval one-up one-down adaptive method), and theresult was adapted to the developed key click tactile feedbacksystem. (is study shows the first work employing perceptiondata to the haptic soft keyboard on tablets and confirming a fasttactile feedback response on both Android and Windows.Additionally, this study showed the possibility of makinguniform tactile force distribution on tablet-sized touchscreenswith only two piezoelectric actuators. (is paper is organizedas follows. Section 2 introduces related studies that deal withimproving usability of the soft keyboard by analyzing user’styping behaviors or adding haptic feedback. Section 3 presentsa new platform-independent haptic soft keyboard developedon both Android andWindows. A perception study conductedby using a psychophysical experiment is described in Section 4.Experimental results for the quantitative evaluation of thedeveloped haptic soft keyboard are reported in Section 5.Finally, discussions and conclusions are presented in Sections 5and 6, respectively.

2. Related Work

Most of the studies for the soft keyboard focused on analyzingthe position of typing fingers and key input patterns ontouchscreens to design an improved key input interface. Forexample, Findlater et al. designed a new QWERTY layout ofsoft keyboard by utilizing user’s typing patterns and behav-iors. As a result, typing speed with ten fingers was greatlyimproved by achieving eyes-free touchscreen keyboard typ-ing [1, 2]. Similarly, Sax et al. also showed that adaptivelyarranging the keys on the touchscreen to natural positionsand movements of user’s fingers can significantly improvetyping performance [3]. Another research by Goel et al. in-troduced a new text entry model that adapts user’s handposture information such as two thumbs, the left thumb,the right thumb, or the index thumb to improve typing

performance on a mobile touchscreen [5]. From a designperspective, these efforts were all good to provide betterusability for typing on touchscreens but physical key typingfeedback.

An early effort adding tactile feedback to the touchscreenof a mobile device was made by Poupyrev and Maruyama[6]. (ey designed and implemented a tactile interface ona PDA touchscreen and tested the usability with 10 par-ticipants under two conditions: audio feedback and tactilefeedback. (e result of their study confirmed that tactilefeedback is more effective than audio feedback on a smalltouchscreen. In a medical perspective, Rabin and Gordonconducted an experiment to investigate the role of tactilecues in typing. (eir study showed that tactile cues provideinformation of the start position of the typing fingers, whichis necessary to perform typing movements accurately [7].(ese studies initiated the need of tactile feedback tech-nologies on touchscreens of mobile devices.

Brewster et al. further studied the effect of tactile feedbackon a mobile device by attaching a commercial actuator to thebackside of the mobile device [8]. (eir study showed thattactile feedback improves typing speed and accuracy eventhough the feedback vibrates the backside of the device.Hoggan et al. did a similar study but examined with a specifictask: text typing on a touchscreen [9]. (e results also showedthat tactile feedback significantly improves typing perfor-mance on touchscreens. As a commercial application, Kos-kinen et al. compared pleasant feeling between piezoelectricactuators and linear motors [10]. (ey found that piezo ac-tuators are slightly more pleasant than the linear vibrationmotors on touchscreens with commercial mobile phones.Jansen et al. developed a system that can provide localizedtactile feedback on touching a surface [11]. Another interestingidea was to add tactile feedback to an error preventionmethodof existing word processors so that each key provides tactilefeedback to prevent errors during text entry [12].

McAdam et al. confirmed improved typing performanceon tabletop computers in terms of speed and accuracy butwith a different setup attaching tactile actuators to the user’sbody [13]. After that, Chen et al. investigated the frequency ofa real-like key click signal on a touchscreen, which was foundto be 500Hz on the touchscreen of a smart phone [14]. Mostrecently, Han and Kim developed a prototype of haptic softkeyboard by embedding piezoelectric actuators under themobile touchscreen which provides high-resolution key clickeffects on a Windows smart phone and a tablet [4, 15]. (eexperimental results demonstrated that key click tactilefeedback underneath the touchscreen provides better per-formance in terms of typing speed, accuracy, and efficiency.(ey also reported that there was a tactile feedback delay(about 11ms) due to the complexity of the prototype, whichneeds to be improved.

3. Development of Platform-IndependentHaptic Soft Keyboard for Tablets

TA platform-independent haptic soft keyboard that can beintegrable to existing tablets was designed as seen in Figure 1.(e key idea is how to easily integrate the additional tactile

2 Mobile Information Systems

feedback module into existing tablet touchscreens in min-imizing additional work. With the proposed scheme, all hasto do is just to get a trigger signal (key press down) from thetouchscreen (or digitizer). �e trigger signal can be obtainedfrom a key press event of the soft keyboard module or di-rectly from the digitizer driver, regardless of the types ofmobile operating systems. �e trigger signal initiates gen-erating a tactile pulse (e.g., key click, button down, and slidebar) that becomes an input to tactile feedback actuators. �egenerated tactile signal is then trimmed by a tactile noise�lter. �e role of this �lter is to remove potential noisecomponents of the generated tactile signal. In many cases,the noise components turn out to be an annoying jittersound that is formed when piezoelectric actuators are vi-brated on the touchscreen. �e �ltered signal is then am-pli�ed by a signal ampli�er so that the ampli�ed signal canhave su�cient amount of energy to vibrate multiple pie-zoelectric actuators. All of these steps are synchronized bythe clock of the microprocessor. �e last thing is to mountactuators onto the touchscreen. In the proposed scheme, theway of mounting is �exible up to the need. For instance, thepiezoelectric actuators can be integrated under the touchscreenfor an embedded haptic soft keyboard [15] or attached on thetouchscreen as a form of a portable cover. In the following, thedetail of the development is described.

3.1. Development of Platform-Independent Tactile FeedbackModule. A platform-independent tactile feedback module(often called haptic driver) has been developed with a low-cost 8-bit microcontroller (Arduino micro) as shown inFigure 2. �is module consists of four sequential blocks:tactile pulse generation, tactile signal �lter, tactile signalampli�er, and tactile feedback actuators (piezoelectric ac-tuators). �e tactile pulse generation forms a key click pulseby using the pulse width modulation technique. �ree pa-rameters, frequency, duty cycle and duration, are taken asthe input, and a square waveform is then generated. In thedevelopment, the three parameters were �xed to 500Hz,50%, and 2ms for a tactile key click pulse by referring Hanand Kim’s study [4]. �e second step, a tactile signal �lter,

which removes undesirable sound noise (jitters) when vi-brating actuators, was implemented by using a digitalsmooth �lter function of the Arduino library. However,there is a trade-o� between reducing the high-frequencycomponents and maximizing the signal energy that is thesource of tactile feedback strength on the touchscreen.Finding an ideal trade-o� value is another research topic thatneeds to be studied further in the future.

A tactile signal ampli�er plays an important role tovibrate multiple piezoelectric actuators since high voltage isrequired for a piezoelectric actuator to produce real-like keyclick feedback on the touchscreen. Based on a pilot study, theminimum voltage that provides a perceptible tactile feelingon the touchscreen was around 80 Vpp. �e minimumvoltage ampli�cation was achieved by using an acousticsignal ampli�er (PDU 100) that ampli�es up to 100 Vpp. Forprototyping, an ampli�er was used to drive two multilayerpiezoelectric actuators (L×W×H: 32× 7.8× 0.7mm man-ufactured by Noliac) on the touchscreen. �e multilayeractuator generates a bending mode and produces a stroke upto ±475 μm when a 200 Vpp square wave is applied.

Key clicksound play

8-bit MCU

Trigger

Touch input

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Tactile signalamplifier

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Touch inputmodule

Touchscreen(digitizer)

Tactilefeedback

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Tablet

Figure 1: Block diagram of a proposed haptic soft keyboard scheme.

Figure 2: A prototype of the platform-independent haptic driver(40× 40mm) capable of driving two multilayer piezoelectric ac-tuators at the same time.

Mobile Information Systems 3

�e proposed tactile feedback module can be integratedinto either the touch input module or the soft keyboardmodule of any type of operation systems. For the proposedtactile feedback, a key press down event is used for sendinga trigger signal to the tactile feedback module. In developinga prototype, the proposed tactile feedback module was in-tegrated into the touch input module of a tablet (Mu Pad II,1.8 GHz CPU, 2GB RAM) that has dual operating systems,Android and Windows. Besides, visual and aural feedbackmodules were also implemented as seen in Figure 3. For thevisual feedback, each key color is changed to show which keyis pressed or released while the aural feedback plays a keyclick sound. For the haptic feedback, actuators attached tothe touchscreen are simultaneously bent to make the entiretouchscreen be vibrated for a real-like key click e�ect.

3.2. Implementation of a Soft Keyboard Module on DualPlatforms (Windows andAndroid). A platform-independentsoft keyboard module was designed and implemented ona table device (Mu Pad) that has dual platforms, Windowsand Android. For the QWERTY soft keyboard design in-terfacing with the tactile feedback module, the QWERTYdesign provided in the mobile with Windows platform(Figure 4) was used for the implementation on both plat-forms,Windows and Android. With implementing the samesoft keyboard scheme on the di�erent platforms, the de-veloped tactile feedback module was able to objectively becompared in terms of performance and expendability. �eimplemented soft keyboard module consists of getting a keypress input from the digitizer (touchscreen) and displayingfeedback signals (vision, touch, and/or sound) to user’styping actions.

To reduce mistyping on the keyboard, key-pressableareas were de�ned as shown in Figure 5, and those areaswere synchronized with feedback modules so that feedbackfor key press con�rmation can be provided only when user’s

�nger touches the de�ned areas. �e feedback signals for thethree modalities, vision, touch, and audio, were designeddi�erently as seen in Figure 6. For the visual feedback, thecolor of a pressed key is changed to white while tactile andaural feedback signals are used for the same acoustic signal,a 500Hz square wave (one cycle), by referring a prior study[4], but the tactile signal is generated by pulse widthmodulation (PWM) of an 8-bit microprocessor (Arduinomicro), and the generated tactile signal is automatically sentto the developed haptic driver that drives multiple piezo-electric actuators on the touchscreen of the tablet (Mu Pad).

4. Towards Perceptible Tactile Feedback fora Key Click Effect on Touchscreens

A psychophysical experiment was designed and conductedto measure a detection threshold—barely perceptiblemagnitude—of key click tactile feedback on touchscreen ofa commercial touchscreen. Twelve volunteers (7 males and5 females: average age 23.75 years old) took part in theexperiment. All of the participants were right handed by self-report. None of the participants had experience on haptic-assisted mobile devices or similar experiments. For theperception study, a commercial 10.1 inch touchscreen ofSamsung Galaxy tab was used for the experiment. Prior tothe psychophysical experiment, key click tactile feedbackstrength on the touchscreen driven by a multilayer piezo-electric actuator (manufactured by Noliac) was calibrated byan accelerometer to quantify haptic perception levels fora key click e�ect (Figure 7). �e piezoelectric actuator wastightly mounted on the touchscreen, and an accelerometer(PCB 352A24, Sensitivity 10.2mV/(m/s2)) was positioned5mm to the edge of the piezoelectric actuator. Accelerationwas then measured with incrementing 20 Vpp as the input(a 500Hz square wave), and the measured values are plottedin Figure 7. �e plot shows that the acceleration ontouchscreenmonotonically increases to 160Vpp, which is theperformance of the piezoelectric actuator used for this study.

Figure 8 shows the experimental setup used for conductinga psychophysical experiment that measures a detectionthreshold on the index �nger. Participants took a �ve-minute

So� keyboardGUI

Visual feedback module

Touch input module

Audiooutput

Auditory feedback module

Piezoelectricactuators

Tactile feedback module

Touchscreen

User action: Key press

Key click color change Key click sound Key click signal

Key click event Key click event Key click event

Figure 3: Feedback scheme for the developed haptic soft keyboard.

Figure 4: QWERTY soft keyboard design of the Windowsplatform.

Figure 5: Design of key typing areas (green) for key press feedback.

4 Mobile Information Systems

training session to understand the experiment procedureincluding the �nger positioning on the touchscreen beforestarting the main experiment. During the experiment, thewaveform (one cycle of a square wave pulse at 500Hz) inFigure 6 was repeatedly sent to the piezoelectric actuatorthough the developed haptic driver and tactile feedbackmodule. Participants wore a headphone and listened to whitenoise so that they can focus on the provided tactile cue duringthe experiment. A well-known psychophysical method calleda two interval one-up one-down method (2I1U1D [16]) thatadaptively measures participants’ perception was employed tomeasure a detection threshold of tactile key click e�ect ontouchscreen from the six participants. On each trial, theparticipant was asked to respond whether the presentedstimulus on the touchscreen has a key click tactile feedbacksignal—one has a tactile feedback signal and the other has no

tactile signal. �e participant had to respond yes if he/she feltthe key click feedback on the touchscreen.

By the rule of the 2I1U1D method, the magnitude ofinput voltage (Vpp) was increased after each incorrect re-sponse and decreased after each correct response. For eachseries of trials, the initial value was set to 200Vpp and thenchanged by 4 dB during the �rst 4 reversals and thenchanged by 1 dB for 12 reversals. Note that the initial largerstep size (4 dB) allows �nding the convergence level quicklywhile the following smaller step size (1 dB) plays a role toimprove the resolution of the �nal perception level. Eachparticipant repeated three trials, and the �nal detectionthreshold was estimated by taking the average of the threetrials. It took each participant 20 to 30 minutes to completeall trials including the training session. From the psycho-physical experiment, the estimated detection threshold ofthe key click tactile feedback was 1.17 ± 0.22 G (m/s2).�e detection threshold provides a guideline of perceptiblekey click tactile feedback on touchscreens. With the ob-tained perception data, a modi�ed tactile soft keyboard wasdesigned as shown in Figure 9. From the new design, thetactile force adjustment computes globally equalized tactileforce levels on the entire touchscreen by referring theperception detection threshold. Without this function,a typist feels a stronger tactile force near the piezoelectricactuator (near the bezel of the touchscreen) but a weak forceat the center of the touchscreen. �e perception databaseprovides an ideal tactile force level that can be adaptively setto personal preferences. Finally, a prototype was developed

(a) (b)

Figure 6: Visual feedback (a) and a 500Hz square wave for tactile and aural feedback (b).

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0

2

4

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20 40 60 80 100 120 140 160 180 200

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Figure 7: A commercial touchscreen with an attached piezoelectric actuator: tactile feedback calibration on touchscreen with anaccelerometer (a) and the measured acceleration values for increasing input voltage on the actuator (b).

Actuator

Figure 8: Experimental setup of a perception study on key clicktactile feedback.

Mobile Information Systems 5

on a tablet (Mu Pad), and two piezoelectric actuators weremounted next to the home button on the bezel of thetouchscreen to generate tactile key click feedback.

5. Experimental Results with the IntegratedTablet (Mu Pad)

5.1. Experimental Design. �ree experimental measurementswere performed to quantitatively test performance in terms ofthe similarity of tactile key click, force feedback distributionon a touchscreen, and delay time with the developed softkeyboard system (Windows and Android) on the Mu Padtablet integrated with the developed tactile feedback module.For the �rst and second experiments, an accelerometer wasused for measuring acceleration values that were recorded aswaveform pro�les for the similarity measured with a pre-recorded physical click waveform and as force values on thetouchscreen for ensuring the force distribution on a 10.1-inchtouchscreen.

5.2. Data Collection. An acceleration pro�le (waveform) wasmeasured with an accurate accelerometer (PCB 352A24,Sensitivity 10.2mV/(m/s2)) as a 500Hz square wave (onecycle), a haptic key click signal, was sent to two actuatorsattached to the bezel of the touchscreen. Formeasuring discretetactile feedback strength on the touchscreen, the same accel-erometer was placed at six di�erent positions that were equallyspaced and premarked on the touchscreen. Acceleration valueswere recorded �ve times for each point and then averagedwhen one cycle of 500Hz square wave (100Vpp) was applied tothe two piezoelectric actuators activated by a key press eventfrom both Windows and Android. �e last measurement wasconducted to test the delay time from pressing a key on thetouchscreen to driving the piezoelectric actuators through thedeveloped haptic driver. �e measurement was repeated tentimes both on Android and Windows, respectively, andmeasured values were averaged to be compared.

5.3. Results. �e �rst experiment was to verify whether thekey click tactile feedback on the touchscreen is a real-like key

click e�ect. For this, the measured acceleration waveform wascompared with the acceleration waveform recorded froma mechanical key pad by Chen et al. [14]. �e comparisonresult is shown in Figure 10. �e blue curve is the measuredwaveform in this study, and the black curve is the waveformrecorded from a mechanical key pad. It is obvious that thereare goodmatches in peaks and valleys. Note that the �rst peakis most important to mimic a real-like key click e�ect.

�e second experiment was to quantify tactile forcedistribution on the touchscreen of the integrated tablet sincevibrations generated by piezoelectric actuators on the bezelof a tablet are diminished in strength while traveling froma side to the center. So the goal of this experiment is toinvestigate the diminishing by visualizing tactile feedbackstrength on the touchscreen of the soft keyboard. As theresult, all averaged values are graphically visualized in Figure 11.�e stronger tactile feedback is colored in the redder. Fromthe distribution image, it is clear that tactile feedback is notequally distributed over the soft keyboard. �e reddest areaon the arrow keys, right next to a piezoelectric actuator,is too strong (larger than 2 G), whereas the areas of the

8-bit MCU

Perceptiondatabase

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Perception-based platform-independent tactile feedback module

Figure 9: Perception-based tactile soft keyboard of tablets.

Figure 10: Comparison of the measured acceleration waveform(blue) on the touchscreen of Mu Pad with the acceleration waveform(black) recorded from a mechanical key pad by Chen et al. [14].

6 Mobile Information Systems

backspace key and the “q” key are relatively weak (1.3 to 1.4G)though the strength is slightly over the estimated detectionthreshold (1.14 G). �e issue can be resolved by the tactileforce adjustment function in Figure 9. To achieve globallyequalized tactile feedback over the soft keyboard, the desiredtactile strength was set to 1.5 G by referring the estimateddetection threshold (1.14 G) since the estimated detectionthreshold is barely detectable. �e equalized distribution isseen in Figure 12. Overall, tactile forces are well distributedover the soft keyboard with an ignorable less force on thebackspace key.

�e last experiment was conducted to measure the timefrom pressing a key on the touchscreen to driving the pie-zoelectric actuators through the developed haptic driver. �eresults were 1.9ms and 2.1ms on Android and Windows,respectively, which shows that there is no big di�erencebetween Android and Windows when the proposed tactilesoft keyboard system was integrated. A further experimenthas been conducted to compare with the previous study byHan and Kim [4]. Han and Kim’s approach was to drivepiezoelectric actuators by using an audio play function witha prestored key click waveform on aWindows tablet.�e timereported in their studywas over 10ms. To objectively comparethe result, the same experimental condition (playing a pre-stored key click waveform) was implemented on the Win-dows platform of Mu Pad, and the result was compared withthe proposed tactile key click system (i.e., generating a keyclick waveform directly from the developed haptic driver).�e numerical result of Han and Kim approach was 3.9mswhich is slower than the new result (2.1ms) of the proposedapproach in this study. It con�rms that the proposed

approach in this study outperforms in terms of a fast responseof tactile feedback on touchscreens.�e fast response of tactilefeedback is imperative since a fast typist on a tablet may notget tactile feedback on time. �is study shows a promisingdirection of haptic soft keyboard technologies by improvingthe responsiveness.

6. Discussions

To the best of my knowledge, the detection threshold mea-sured in this study is the �rst result ever reported fortouchscreens. �e detection threshold was used as a lowerbound perception value to ensure a tactile key click e�ect. Bytaking this approach, people can surely feel the tactile feed-back on 10.1-inch touchscreens, and the use of energy (mobilebattery) drivingmultiple actuators can also be optimized.�isis important in that adding tactile key click feedback toexisting tablets should not be a burden due to battery con-sumption.�is study shows a practical example how to utilizehuman perception data though further studies are needed toapply for various form factors in terms of the size andmaterialof touchscreens.

When designing a tactile soft keyboard for mobile de-vices, one of the challenging issues is how to make the tactilefeeling realistic although the feeling itself could be relative.To quantify the tactile feeling, an acceleration pro�le(waveform) was measured and compared with the accel-eration waveform reported by [14]. Based on pilot studiesconducted in this study to �gure out primary features thatare most important to imitate a key click e�ect, the pattern ofpeaks of the acceleration waveform was a key to simulate

Actuator 2 Actuator 1

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1.3

0.88

0.5

Figure 11: Acceleration distributed on the touchscreen of the tablet (Mu Pad): the measured values were superimposed onto the softkeyboard image to visualize the tactile strength generated from two piezoelectric actuators attached on the bezel. Note that red meansstronger tactile feedback.

Actuator 2 Actuator 1

2

1.6

1.3

0.88

0.5

Figure 12: Equalized acceleration distributed on the touchscreen by the tactile force adjustment function that computes globally equalizedtactile forces concerning the distance between actuators and key locations.

Mobile Information Systems 7

a key click effect on a touchscreen. As seen in Figure 10, aslong as the largest and the second largest peaks are wellformed like the physical key click, the tactile feedback wasfelt as a real-like key click on a planar touchscreen surface.However, the acceleration waveform is determined by pi-ezoelectric actuators that provide accurate force bending themounted touchscreen. Actuators used in this study were allbest-performed multilayer actuators manufactured byNoliac. (erefore, making the similar effect with even lowerquality of piezoelectric actuators is another research topicthat will be done in the near future.

Equalizing force feedback on a 10.1-inch touchscreen isnot simple since the vibration force is diminished as thedistance from an actuator increases. Figure 11 shows thestronger forces near the attached actuators, which is notdesirable. (ere are two solutions to solve this issue. One is todirectly control the input power of the piezoelectric actuatorwith respect to the location where a key click effect should bedelivered. In general, the larger voltage is required to get thebigger movement from the actuator (Figure 7).(e other is tochange the amplitude of an input tactile signal (e.g., a squarewave) that will determine the movement of the actuator in theend. In this study, the first solution was used since it waslearned that the resolution of the tactile feedback force witha piezoelectric actuator is significantly low with the secondapproach. (is is because the mapping between the magni-tude of the input signal and the input voltage of the actuatorsis not linear.

In addition, the performance of key click tactile feedbackon touchscreens is greatly influenced by the characteristics ofactuators to be mounted. (e actuators should be able toprovide high-definition tactile forces that make various typesof virtual touch feelings on touchscreens. Piezoelectric ac-tuators are in general precision actuators that convert electricenergy into linear motions with high speed and resolution.Due to this reason, piezoelectric actuators are suitable forimplementing a virtual key click effect on touchscreens ofmobile devices. However, high voltage (e.g., 200 Vpp fora single-layer piezoelectric plate) is typically required for apiezoelectric actuator to be sufficiently vibrated on atouchscreen to imitate a mechanical key click effect. Due tothe limitation of input voltage with a tablet device, handlingmultiple piezoelectric actuators is a challenging problem. Hanand Kim developed a most advanced tactile key click feedbackwith multiple piezoelectric actuators on a Microsoft SurfacePro tablet [4]. However, there was over 10ms delay time to gettactile feedback from pressing a key and so tactile feeling wasnot synchronized well. (e prototype of a haptic driver wasalso a bit bulky to be integrated with existing commercialtablets. (e last problem was that the developed haptic softkeyboard module was not completely independent from theWindows platform.(e present study can be considered as animproved version of the Han and Kim’s work by resolvingthose issues of the prior study.

(e present study focused on developing a platform-independent haptic soft keyboard system so that the tactilefeedback module can be integrated into existing commercialtablets (or mobile devices) by putting the least amount ofeffort. For this reason, a dual platform tablet with a 10.1-inch

touchscreen, widely used in these days, was chosen as a testdevice. As explained earlier, the developed tactile key clickfeedback module can be simply integrated by using a pulse-generating function of each platform that sends a triggersignal to the haptic driver generating a key click signal. (eplatform independency was proved by results of delay timethat are almost same between Android andWindows.(is isbecause a tactile signal is created from the separated hapticdriver but not from the operating system like Han and Kim’swork. Further, the proposed scheme outperforms the priorstudy in terms of response time, which also resulted frombeing less dependent on the operating system. One of theimportant contributions in the present study is to employhuman perception on key click tactile feedback on a com-mercial touchscreen. According to a prior study, Han andKim [4], one cycle of square waveform at 500Hz was usedfor a real-like key click tactile effect, and the effect wasquantitatively proved by analyzing the acceleration profile.

7. Conclusions

In the present study, a platform-independent haptic softkeyboard module that has the least dependency to mobileoperating systems has been designed and developed. (eproposed haptic soft keyboard module consists of three parts:soft keyboard with feedback, a mini haptic driver, and pie-zoelectric actuators. (e soft keyboard with feedback (visual,tactile, and aural feedback) was implemented on a dualplatform tablet to prove the platform independency. A minihaptic driver that can generate tactile key click pulses andremove noise was developed with an 8-bit microprocessor(Arduino) so that it can be simply integrated into existingtablets. (e haptic driver was specially designed to drivemultiple piezoelectric actuators that produce sufficient tactileforces on the touchscreen. In addition, a psychophysicalexperiment has been conducted to estimate the humanperception (detection threshold) on key click tactile feedback.By applying the obtained perception data to the haptic softkeyboard module, perceivable and uniformly distributedtactile feedback was implemented on the touchscreen. Ex-perimental results confirm that the proposed haptic softkeyboard outperforms the previous study by Han and Kim interms of platform independency, uniform tactile feedback,and synchronized tactile feedback (delay time). (e knowl-edge learned through this study can be an informativeguideline to engineers, researchers, and designers who areactively involved in design or development of soft keyboardon mobile devices. Tactile key click on touchscreens must bea promising technology that greatly improves the usability ofmobile input methods or multimedia-related interaction.However, there are still many open questions that needfurther research. One of them is localized tactile feedback withless number of piezoelectric actuators. As future work,a further study will be conducted to investigate a feasiblesolution on the topic.

Conflicts of Interest

(e author declares that there are no conflicts of interest.

8 Mobile Information Systems

Acknowledgments

(is research was supported by Basic Science ResearchProgram through the National Research Foundation ofKorea (NRF) funded by the Ministry of Education(2015R1D1A1A01060715).

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