J Intell Robot Syst (2013) 72:343–355DOI 10.1007/s10846-012-9799-x

Improvement of Violinist Robot usinga Passive Damper Device

Byung-Cheol Min · Eric T. Matson ·Jinung An · Donghan Kim

Received: 2 March 2012 / Accepted: 11 December 2012 / Published online: 11 January 2013© Springer Science+Business Media Dordrecht 2013

Abstract The aim of this study was to determinehow the violinist robot could produce a good qual-ity of violin sounds. We began our study with thebasic physics of producing sound with a violin. Wefound three parameters that influenced the qualityof the sound produced by the violin; the bowingforce, the bowing velocity and the sounding point.In particular, the bowing force was found to bethe most important parameter in producing goodsounds. Furthermore, to produce such sounds, asame amount of the bowing force must be appliedon the contact point between a bow and a string.However, it is hard to keep a same amount of the

B. C. Min · E. T. MatsonM2M Lab., Department of Computer and InformationTechnology, Purdue University, West Lafayette, IN47907, USA

B. C. Mine-mail: minb@purdue.edu

E. T. Matsone-mail: ematson@purdue.edu

E. T. Matson · D. KimDepartment of Electronics and Radio Engineering,Kyung Hee University, Yongin 446-701, Korea

D. Kime-mail: donghani@khu.ac.kr

J. An (B)Pragmatic Applied Robot Institute, DGIST, Daegu771-873, Koreae-mail: robot@dgist.ac.kr

bowing force on the contact point due to inherentcharacteristics of a bow. Thus, we primarily fo-cused on the bowing force by considering bowinga string as a spring-mass system. Then, we deviseda passive damper device to offset variables in thespring-mass system that may result in changing thebowing force on the contact point. We then vali-dated our methodology with the violinist robot, ahuman-like torso robot.

Keywords Physics of violin · Violinist robot ·Entertainment robot · Bowing ·Passive damper device

1 Introduction

Up to now, industrial robots are mainly covered inthe field of Robotics. Although it has been proventhat industrial robots increase product quality andimprove productivity in the factory [1], robotscannot provide human beings with interactionsthat seem to occur as expressions of emotions.To make up for such shortcoming, a new genreof the robots has been recently introduced, whichis called entertainment robots. The entertainmentrobot is a one of the type, which is a humanintimate type robot or is built for fun and amuse-ment [2].

Entertainment robots can be seen in many ar-eas of our life and culture. The human styled

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robots called ‘Hubo’ [3],‘Asimo’ [4], and ‘HRP’[5]. Furthermore, Sony’s Qrio and Aibo [6],Aldebaran’s Nao [7], and ROBOTIS’s DARwin-OP [8] are designed to perform some taskscapable of entertaining people by interacting withthem through communication or motion. The vio-lin playing robot that we have built, which will bediscussed in this paper, is also a type of entertain-ment robot.

The violin playing robot is classified as a mu-sical performance robot that is operated by au-tomatic mechanism. Several attempts to build ananthropomorphic musical robot have been madefor decades, and different outstanding performingrobots have been built with advanced technolo-gies. Recently, the Waseda Saxophonist Robothas been developed at Waseda Unversity [9], asa follow-up model of Flutist Robot [10]. WasedaSaxophonist Robot is the human-like robot thatis able to play a saxophone with expressing emo-tions. Also, the superb violin-playing robot, whichis equipped with highly advanced control tech-nology, exhibited at the press conference hasbeen built by Toyota [11]. In addition, anotherviolin playing robot producing expressive soundsby considering sensibility has been developed atRyukoku University [12].

Even though the violin has been the most stud-ied of all the classical instruments, it is still adifficult instrument, as it has one of the mostdifficult physical interfaces of any musical instru-ment. Compared with a piano, played with similarleft and right hand techniques, a violin is playedwith different demanding techniques required ofthe left and right hand. For this reason, when a be-ginner tries to play a violin, he or she typically hasdifficulty in regularly placing his or her left fingerson a particular point of the fingerboard and bow-ing on the specific string. These difficulties mightresult in producing a musical note at the desiredpitch, but might also result in an undesirablesound, such as a screech or raucous noise. Toovercome these difficulties, the violinist has topractice playing the violin frequently until theyare producing quality sounds. Moreover, perceiv-ing the principles of the basic physics of producinga violin’s sound should be conducted at the sametime as practicing playing the violin.

In this sense, the violinist robot we have de-signed is in a similar situation as a novice humanbeing. Accordingly, we have focused on studyingthe principles of the violin and how to produce agood violin’s sound, with the violinist robot in thefirst stage of developing the accomplished violinistrobot.

The sound quality of the violin is primarilyinfluenced by three parameters; the bowing forceon the string, the bowing velocity, and the sound-ing point (it is also called the bow-bridge distancefrom the bridge to the bow) [13]. In particular,the bowing force was found to be the most impor-tant parameter in producing the violin’s sounds.Furthermore, to produce a good quality sound, asame amount of the bowing force must be appliedon the contact point between a bow and a string.However, it is hard to keep a same amount of thebowing force on the contact point due to inher-ent characteristics of a bow. Thus, we primarilyfocus on the bowing force by considering bowinga string as a spring-mass system. Then, we devisea passive damper device to offset variables in thespring-mass system that may result in changingthe bowing force on the contact point. Duringthis process, we confirm the close relationshipsbetween the sound quality of the violin and thebowing force.

The rest of the paper is organized as follows:Section 2 introduces basic physics of the violinto understand the mechanism of the violin soundgeneration, and describes application methods forthe violinist robot to produce the violin’s sound.Section 3 presents a description of the violinistrobot we have designed, and Section 4 shows thesummary of results of the experiments having therobot plays the violin. Finally, Section 5 summa-rizes our conclusions and future scope of work toimprove the capabilities of the violinist robot.

2 The Physics of the Violin

2.1 Bowed String of the Violin

The ultimate aim of developing the violinist robotis to mimic an accomplished violinist expressingthe range of expression in standard repertoire. To

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become a professional violinist, the robot mustfirst be able to control a violin’s pitch, timbre, andloudness; Pitch represents the perceived funda-mental frequency of a sound. It is often referred toas “the height of a note”. Timbre is the particularquality or acoustic of a sound produced by aninstrument or a voice. It is sometimes referred toas “ sound color” or “tone color”. Lastly, loudnessis defined as the perceived strength of a pieceof audio. In particular, timbre is the largest fac-tor that affects the sound quality, and it is, to agreat extent, connected with the bowing velocity,the bowing force, and the bowing distance. Inthis sense, this section introduces the methodswe approached to obtain a good violin’s soundproduced by the violinist robot.

There are three techniques used to producesound on a violin: plucking, bowing, and strik-ing [14]; Plucking is pulling the strings with thefingers. Bowing is pulling the strings with a bow.Lastly, striking is hitting the strings with fingers.The most common technique, and consequentlythe one we focus on in this paper, is bowing onstrings. When bowing a violin, the parameterscontrol the string vibrations: (1) V, the velocityof a bow relative to the violin, (2) F, the forceto the bow from the violin (bowing force or “bowpressure”), and, (3) p, the sounding point that isthe position where the bow touches the strings

(bow-bridge distance). These three primary bow-ing parameters are defined in Fig. 1a.

Of the three parameters, the most importantfactor used to determine the sound quality ofthe violin is the bowing force [16]. In Schelleng’sstudy, the author verified that the bowing forceis primarily important as the catalytic agent thatmakes possible a correct reaction between bow-ing velocity and sounding point. Furthermore, heverified that there are the upper limit to the bow-ing force, called the “maximum bowing force”,and the lower limit, called “minimum bowingforce”, depicted in Fig. 1b. If F is increased pastthe maximum bow force, the musical note is lost,and the bow will produce noise called “raucous”.Conversely, if F is too low, the bow will producewhat is known as “surface sound”. Once these lim-its have been defined, the range of normal forcecan be analyzed from the following equations:

Fmax = 2Z Vp(us − ud)

(1)

Fmin = Z 2V2p2 R(us − ud)

(2)

where us and ud are respectively the coefficientsof sticking friction and sliding friction. The othernotation follows [16]: V is the bow velocity, Z isthe characteristic wave resistance of the string, R

0.01 0.02

0.001

0.010

0.100

1.000

0.04 0.06 0.10 0.20

Relative Sounding Point, p

Rel

ativ

e B

owin

g Fo

rce,

F

RAUCOUS

Helmholtz Regime

SURFACE SOUND NORMALp

F

VF = Bowing ForceV = Bowing Velocityp = Sounding Point

(a) (b)

Fig. 1 a Definition of the bowing parameters V, F, and p, from [15]. b Principal relation between maximum and minimumforce and bow-bridge distance for a given bow velocity, from [14] and [16]

346 J Intell Robot Syst (2013) 72:343–355

indicates the equivalent of the rate of energy lossinto the violin body, and p specifies the soundingpoint.

Equations 1, 2, and Fig. 1b illustrate a closerelationship between the bowing force and thesounding point. For a given string, the sound pres-sure is proportional to V/p. The same volume istherefore maintained when the change in bow-ing velocity is proportional to the change in thedistance of the bow from the bridge. We thenobtained Eq. 3 by diving Eq. 2 by Eq. 1,

Fmax

Fmin= 2pR

Z. (3)

Equation 3 reveals that the shorter the bow-bridge distance, the narrower is the allowed rangein bowing force. In other words, the closer thebow is to the bridge, the steadier is bowing forceneeded to preserve acceptable tone, as illustratedin Fig. 1b.

For all these reasons, it is quite difficult forbeginners to determine the proper bowing forceaffecting mostly the quality of violin’s sound.Hence, we will introduce the method to deter-mine the proper force necessary to produce goodsounds in the following sections.

2.2 Spring-Mass System

Bowing is the most fundamental and predominanttechnic to produce a violin sound. The best wayto master bowing is that one bows a string atthe same bowing speed as slow as possible whilekeeping producing a good quality of sound. Someof the professional violinists even practice thismethod in order to produce a good quality sound.If we take a look at violinists who can producea good sound, we can simply recognize that theygradually vary the bowing force according to thevariation of a contact point between the bow andthe string. When the contact point approaches toa part of the frog of the bow, they apply less forceto the bow. Conversely, when the contract pointapproaches to a part of tip of the bow, they applymore force to the bow.

ybF

bk

bm

contact pointbF

Fig. 2 Spring-mass system

If we assume that bowing a string is a spring-mass system, we can draw its system as shown inFig. 2, and describe it as follows.

∑Fb = mb

d2 ydt2

+ kb y (4)

where mb means a mass of bow measured on thecontact point and kb is a spring coefficient thatcorresponds to tension of bow hair in this system.

Because the bow stick is not stiff and bow hair isdangling from the tip and frog of the bow, tensionsof bow hair are different according to the bowposition. Tensions of the bow hair connected atthe ends of the bow are high and their middleof the bow is low, depicted in Fig. 3b. Clearlyincreases in tensions cause the increase of kb . Inaddition, because the weight of the part of thefrog is much heavier than that of the tip, thisalso results in the increase of mb , depicted in Fig.3c. As a result, if we assume that accelerationof y is constant in Eq. 4, the bowing force Fb

would be forced to increase as the contact pointbetween the bow and the string approaches to thepart of the frog depicted in Fig. 3d. Note thatfrom the contact point of view denoted with thedotted line in Fig. 2, while bowing either upwardor downward, the values of bowing force appliedto the string must be same no matter where thepoint approaches to the frog or it approaches tothe tip. Thus, as bowing with the bow hair of thepart of the frog, the bowing force applied to thestring would transgress “maximum bowing force”depicted in Fig. 1b. These inherent characteristicsof the bow compel violinists to pay particularattention to the contact point and to vary thebowing force according to it.

J Intell Robot Syst (2013) 72:343–355 347

Fig. 3 a The componentsof a typical bow, denotedwith the five bowpositions and the tensionlevels. b Variations of thebow hair’s tensionaccording to the bowpositions. c Variations ofthe bow’s mass accordingto the bow positions.d Variation of the bowingforce would applied onthe string according tothe bow positions

10 9 8 7 6 5 5 6 7 8 9 10

FrogTip Hair Stick

1 23 4 5 6 7 8 9 10 11

12

Tens

ion

Lev

elM

ass

Lev

el

1 2 3 4 5 Bow position (Bp)

1 2 3 4 5 Bow position (Bp)

1 2 3 4 5 Bow position (Bp)

12 14 16 18 2022

Bow

ing

Forc

e L

evel

1 2 3 4 5 Bow position (Bp)

111111111111 11

(a)

(b)

(c)

(d)

As mentioned above, maintaining the samebowing force at the contact point between thebow and the bow hair is of paramount importancewhen it comes to producing a good violin sound.However, because the values of mb and kb in theEq. 4 are continually changed due to the inherentcharacteristics of the bow, it is almost impossibleto retain the same force without considering analternative. Therefore, we will introduce a passivedamper system later that will offset a bowing forceif it is applied too much on the string.

3 Violinist Robot

3.1 Introduction to Violinist Robot

The violinist robot we designed is a human-liketorso robot of 0.5 m in height and 6 kg in weight(Fig. 4b). The robot body consists of 18 joints, so

there are a total of 18 degrees of freedom (DOF).In particular, the right-hand arm is given a greatdeal of weight in the playing violin, as shown inTable 1. Accordingly, the robot’s right arm canbe implemented by 6 DOFs. On the other hand,6 DOFs for the left arm is used only to hold theviolin. As a result, it is not necessary for the leftarm to be equipped with motors. There remains 6DOFs for fingers, designed for the robot to adjustits fingers to push the appropriate note.

The proposed robot is composed of severalprimary modules: two personal computers (In-tel Core2 Duo CPU 2.40 GHz each) and sixmicroprocessors (DSP Texas 320F2811) are in-corporated in the operation of this system. Oneof the computers, called the “control PC”, isin charge of storing the motion data, includingpath information generated by RecurDyn [18],and transmitting control input to the robot. Theother computer, called the “measurement PC”, is

348 J Intell Robot Syst (2013) 72:343–355

(a) (b)

Fig. 4 a A model of the violinist robot simulated in SolidWorks. b The external appearance of the developed violin playingrobot named “Violinist Robot”

used to evaluate recorded sound from a micro-phone (MIC). This MIC is attached to the violin’sbody.

3.2 Passive Damper Device

To generate the paths that the violinist robot’sright arm would track to play the violin, we em-ployed motion analyzer commercial software, Re-curDyn. First we modeled the violinist robot inSolidworks [19] as shown in Fig. 4a and then,the modeled information was sent to RecurDyn.Using the software we generated a number ofstraight paths and stored them in the control PC.Each path is for the violinist robot to bow onestring either upward or downward for about 300mm in length.

Table 1 The specification of the robot’s right arm

DOF 6DOFActuator DC servo + incremental

encoderArm length 244 mm + 295 mm + 196 mmRepeatability 0.3 mmSpeed 0.60 m/sPayload/Weight 2 kg / 6 kgCommunication CAN 3.0Power 24Vdc

However, if the robot’s right arm holding thebow tracks a straight path upward to bow a violin,denoted with a solid line in Fig. 5, the bowingforce Fb will not be kept at the same value andbe changed in proportion to the variations of mb

and kb as stated in Section 2.2 (see Fig. 3c) due tothe mb and kb that are continually changed withrespect to a bow position.

On the other hand, if the robot’s arm tracksa curved path like a dashed line in Fig. 5, this

Upward

straight path

curved path

Δy

Fig. 5 A solid straight line cannot compensate the vari-ations of mb and kb causing the different bowing force.On the other hand, a dashed curved line can generate �y,displacement of y, and finally would compensate thosevariations

J Intell Robot Syst (2013) 72:343–355 349

Fig. 6 A passive damperdevice

Bowing position (Bp)

Violin string

dF

dkdb

dm

y

bF

θ

Δy

alteration can generate the displacement of y withrespect to a bow position, and it can finally com-pensate the increase of mb and kb . i.e, as thecontact point approaches to the part of the frog(around Bp 3 to 5, see Fig. 3), the value of y isgradually decreased in Eq. 4, and it finally resultsin the same value on the left side of the equation.(Note that decreasing values of y is made possibleby defining a direction of y downward as shown inFig. 2.) Thus, the bowing force can be maintainedat the same value.

For this reason, we have devised a passivedamper device like tongs so that it can gener-

ate �y. In other words, this device was designedto allow the bow, which is supposed to track astraight path, to track a curved path by lifting atip of the bow according to a bowing force appliedon the string. This devised damper device is thenmodeled as follows,

Fd = mdd2 ydt2

+ b ddydt

+ kd y (5)

where Fd indicates the downward force that isgenerated by a bowing force Fb applied on thestring, and the right side of the equation includingconstants of md, b d, and kd corresponds to the

(c) Robot hand with the device(b) When Δy is not generated

(a) When Δy is generated

FSR closed

FSR opened

Wireless Device

Fig. 7 A built passive damper device attached to the robot

350 J Intell Robot Syst (2013) 72:343–355

tension of the damper device, which can be mod-ulated by fastening both sides of the device with arubber band.

If a bow is attached to one side of the device, asillustrated in Fig. 6, and the other side is attachedto the palm of robot’s hand, �y can be generatedwhen the downward force Fd becomes strongerthan the tension of this passive damper device.Since the one side of the device rotates on anaxis, �y depends on the angle θ between two sidesof tongs, and is related to the bow posion Bp asfollows.

�y = Bptanθ (6)

In order to measure the bow force on the stringwhile playing the violin, we designed an electronicdevice with a force sensing resistor (FSR) andattached it to the passive damper device, as de-picted in Fig. 7. FSRs are base on conductive inktechnology. They are very sensitive, and their re-sistance is changed by several orders of magnitudeas force is applied. Further, this sensor providesa range of output values, but usually requires ananalog-to-digital converter to convert their valuesinto a binary numbers, which a computer can un-derstand. Then, these converted data are directlytransmitted to the measurement PC through awireless device.

Figure 7a shows when �y is generated. In thiscase, as some amount of force are applied tothe FSR, the output value becomes higher, which

means this device offsets a bowing force appliedtoo much on the string. Figure 7b shows when�y is not generated. In this case, as no force isapplied to the FSR, the output value becomeslower. Figure 7c shows the device with a wirelesscommunication device attached on the robot’spalm.

4 Experimental Results

To improve upon the quality violinist robot sound,we developed the passive damper device allowingthe bow, which is supposed to track a straightpath, to track a curved path by lifting a tip ofthe bow according to a bowing force applied onthe string. Since this device is designed to varyits tension, we broke down it into three lev-els: Level 1 (relatively weaker tension), Level 2(relatively medium tension), Level 3 (relativelystronger tension). These three different settingswere selected according to the results of a pre-liminary parametric study. During these experi-ments, we had the robot track the given straightpaths orthogonal to the D string, which has afundamental frequency 294 Hz, upward for 3 sec.The snapshots of 8 sequences of this motion aredepicted in Fig. 8. Then, the sounds were recordedon the measurement PC and analyzed.

This first experiment was performed withLevel 3 (relatively stronger tension). This case

00:00 sec

02:00 sec

00:50 sec

02:50 sec

01:00 sec

03:00 sec

01:50 sec

03:50 sec

Fig. 8 The snapshots of eight sequences of bowing upward motion

J Intell Robot Syst (2013) 72:343–355 351

Fig. 9 Sound data whilethe violinist robot wasbowing the D string usingthe passive damper devicewith Level 3: almost nobenefit from the use ofthe damper device

0

0.5

mp

litu

de

Waveform of Level 3

0 1 2 3 4 5 6 7-0.5

Time(sec)

Am

hz) Spectrogram

1000

Fre

qu

en

cy (

0 1 2 3 4 5 60

500

1000

Time(sec)

ncy

(hz) Filtered Fundamendtal Frequency

400

Frame(n)

Fre

que

n

50 100 150 200 250 300 350 400 450 500200

300

was intentionally designed to rarely generate �y,which means there would be almost no benefitfrom the used of the passive damper device. Theresults of the first experiment are depicted inFig. 9. In this figure, the first graph shows thesound strength, while the second and third graphsshow the sound spectrogram. In the sound spec-trogram, the strength of each frequency is rep-resented by a color. The horizontal axes of both

graphs represent time. With the naked eye, asthe robot bowed upward, we could see that therewere no smooth waves and the uneven variation ofthe amplitude in Fig. 9. These undesirable resultscome from too much bowing force applied on thestring when the robot used the bow position (Bp)from 4 to 5 to bow.

This second experiment was performed withLevel 2 (relatively medium tension). In this case,

Fig. 10 Sound data whilethe violinist robot wasbowing the D string usingthe passive damper devicewith Level 2

0

0.5

mpl

itud

e

Waveform of Level 2

0 1 2 3 4 5 6 7-0.5

Time(sec)

Am

hz) Spectrogram

1000

Fre

qu

en

cy (

0 1 2 3 4 5 60

500

Time(sec)

ncy

(hz

) Filtered Fundamendtal Frequency

400

Frame(n)

Fre

qu

en

100 200 300 400 500 600200

300

352 J Intell Robot Syst (2013) 72:343–355

Fig. 11 Sound data whilethe violinist robot wasbowing the D string usingthe passive damper devicewith Level 1: fully benefitfrom the use of thedamper device

0

0.5

mpl

itud

e

Waveform of Level 1

0 1 2 3 4 5 6 7-0.5

Time(sec)

Am

hz) Spectrogram

1000

Fre

qu

en

cy (

0 1 2 3 4 5 60

500

1000

Time(sec)1

ncy

(hz) Filtered Fundamendtal Frequency

400

Frame(n)

Fre

que

n

50 100 150 200 250 300 350 400 450 500 550200

300

it was intended that �y would be somewhat gen-erated. The results of the second experiment aredepicted in Fig. 10. As shown in this figure, thereis a little improvement on smooth waves and theuneven variation of the amplitude, compared tothe previous experiment, although it seems thatthere is still too much bowing force applied on thestring when the robot used the Bp from 4 to 5 tobow.

This third experiment was performed withLevel 1 (relatively weaker tension). In this case,we intended that �y would be suitably generatedto offest too much force applied on the string.The results of the third experiment are depictedin Fig. 11. As shown in this figure, there is a dis-tinguishable improvement on smooth waves andthe uneven variation of the amplitude, comparedto the previous two experiments. Especially, thisshowed a well-kept sound past the Bp 4. Fur-thermore, pitch of sound, timbre, and volume arerelatively clear and constant, as the third figureshows.

Figure 12 shows a comparison of the measuredvalues with the FSR attached to the damper de-vice. This comparison also shows how much forcesare compensated on the string by each Level. Notethat an output format of the FSR is a voltageranging from 0 to 3.3 volt, so the nearer to zerothe value is, the weaker force is compensated andapplied to the FSR. As you can see in this figure,

as the Level decreases from 3 to 1, there are biggeroffsets produced, and this finally results in a betterquality of violin sounds.

To show a quantitative analysis on a quality ofviolin sounds with the passive damper device, werefered to the sound quality evaluation functionpreviously introduced in [10] and slightly modifiedit. The authors in [10] considered the relationamong the pitch, the loudness, and the harmonicstructure content of flute sound. Similarly, theauthors in [17] also analyzed the pitch and loud-ness based on the harmonic structure of violinsound for their sound evaluation. In this paper, we

1 4Measured FSR

1.0

1.2

.Level 1Level 2Level 3

0 4

0.6

0.8

Out

put (

Vol

t.)

0 0.5 1.0 1.5 2.0 2.5 3.00

0.2

.

Time (s)

Fig. 12 Output of the force sensing resistor (FSR) accord-ing to three different tensions of the passive damper device

J Intell Robot Syst (2013) 72:343–355 353

Fig. 13 Sound data whilethe skilled human wasbowing the D string

0

0.2

mpl

itud

e

Waveform of Human

0 1 2 3 4 5 6 7-0.2

Time(sec)

Am

hz) Spectrogram

1000

Fre

qu

en

cy (

h0 1 2 3 4 5 6

0

500

Time(sec)

F 1nc

y (h

z) Filtered Fundamendtal Frequency

400

Frame(n)

Fre

que

n

50 100 150 200 250 300 350 400 450 500 550

200

300

investgated the change of harmonic in the rangeof 200 to 450 Hz for the simplicity of the evalua-tion. Therefore, we summed up the variations ofamplitude in waveforms shown in the third row ofFigs. 9, 10, and 11 with the following evaluationfunction.

Q = 1

γ

∑Tn

n=1X [n] − X [n − 1] (7)

where X[.] is the sum of amplitudes in each wave-form frame n, Tn is the total number of frame, γ

is a positive constant, and Q is the sequence ofthe sound evaluation function, indicating a scoreof the recorded sound.

From this analysis, we are able to analyze thequality of the recorded sound. For example, ifthe value of Q is near zero, then there is lesschange of harmonic in the recorded sound, andthe pitch, timbre and volume of the sound arekept in a decent level to produce good sound.Conversely, if the value of Q is far to zero, thenit means that there are more change of harmonicin the recorded sound up and down, and alsomeans pitch, timbre and volume are not kept inthe decent level. Furthermore, in this case, we maylisten to raucous or surface sound. In short, thenearer to zero the value of the Q is, the better isthe quality of the sound.

Additionally, we conducted an experiment ina skilled human to verify the sound evaluation

function. In this experiment, the skilled humanbowed upward for 3 sec, and sound was recordedwith a microphone. The results of this experimentare depicted in Fig. 13, and the sound evaluationfunction provided a score as 1.1412, which areclose to zero. Therefore, this result validates thatthe good sound illustrates a small difference inthe amplitude of each waveform frame, as we ex-pected. Next, we analyzed further on sound datapreviously produced by the violinist robot to eval-uate its performances with the sound evaluationfunction. As a result, sound evaluation functionscored 8.672, 7.0317, and 2.9433, respectively, andthe results of these quantitative analysis are de-picted in Fig. 14.

8.672

7.0317

10

2.9433

5

Q S

core

1.1412

0Level3 Level2 Level1 Human

Fig. 14 Q scores showing the quality of the sounds. Notethat the nearer to zero the value of the score is, the betteris the sound quality of the violin

354 J Intell Robot Syst (2013) 72:343–355

As shown in Fig. 14, the stronger the tensionof the passive damper system was, the worse thesound quality was. When the violinist robot bowedthe D string with the Level 3, the scores showedthe largest value among the three levels. More-over, when the violinist robot bowed the D stringwith the Level 2 and Level 1, all scores exhibitedmuch smaller values than those of the Level 3.Consequently, it can be said that the weaker thetension of the passive damper system was, thebetter the sound quality was. Furthermore, thisnumerical result proves that the passive damperdevice could contribute to the improvement of theoverall qualities of the sound.

5 Conclusions

In the first stage of the development of the vi-olinist robot having an ability to produce goodviolin sounds, we observed the close relationshipbetween the bowing force and the sound qualityof the violin. On top of that, keeping the sameamount of bowing force at the contact point be-tween the bow and the bow hair was very impor-tant in terms of producing a good quality of violinsound. It was however not easy without consid-ering an alternative due to the inherent charac-teristics of the bow such as different tensions ofbow hair according to the bowing position anddifferent weight of the bow at the point. Thus,we devised the passive damper device with thepurpose of maintaining the same bowing force.This device was designed to be attached to thepalm of robot’s hand, and a bow was attachedto one side of the device. Using this device, therobot could maintain the same bowing force bygenerating a displacement of path. As a result,we confirmed whether the violinist robot couldreproduce a distinguishable sound when we usedthe passive damper device.

In this paper, we only varied a bowing forcewith a bowing velocity and the sounding pointremained constant to observe the relationship be-tween the bowing force and the sound qualityof the violin. However, a bowing velocity anda sounding point may both contribute to soundquality. Hence, in future studies, we will vary boththe bowing velocity and the sounding point to

determine their relationship as well as the soundquality.

Acknowledgements This work was supported by a grantfrom the Basic Science Research Program through theNational Research Foundation of Korea (NRF) fundedby the Ministry of Education, Science and Technology(2012R1A1A2043822), the Technology Innovation Pro-gram of the Knowledge economy (No. 10041834) fundedby the Ministry of Knowledge Economy (MKE, Korea),and the DGIST R&D Program of the Ministry of Educa-tion, Science and Technology of Korea (12-RS-01).

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