Mechatronics 23 (2013) 318–325

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Mechatronics

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Robotic index finger prosthesis using stackable double 4-BAR mechanisms

Giho Jang, Chulwoo Lee, Hoyul Lee, Youngjin Choi ⇑Department of Electronic Systems Engineering, Hanyang University, Ansan 426-791, South Korea

a r t i c l e i n f o

Article history:Received 20 January 2012Accepted 20 January 2013Available online 6 March 2013

Keywords:Stackable double 4-bar mechanismElectromyography (EMG)Robotic index finger prosthesis

0957-4158/$ - see front matter � 2013 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.mechatronics.2013.01.006

⇑ Corresponding author.E-mail address: cyj@hanyang.ac.kr (Y. Choi).

a b s t r a c t

This paper suggests a robotic index finger prosthesis realized to be one degree-of-freedom by using stack-able double 4-bar mechanisms. Also its control method makes use of two electromyographic (EMG) sig-nals measured on skin surfaces of flexor digitorum superficialis (FDS) and extensor indicis (EI) in a lowerarm. In this paper, we assume that EMG signals have some relations with velocity of muscle movementby neglecting finger dynamics due to its negligible small mass. In order to obtain desired position andvelocity of robotic index finger, the measured raw EMG signals are processed by sequential proceduressuch as root mean squaring, applying threshold operation to extract the initial burst part, subtractingantagonistic EMG signal, and integrating by every 2 millisecond. Finally the effectiveness of the suggestedmechanism and control method is verified through experiments.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

An amputation deteriorates not only physical abilities due tomuscle retraction, but also social mental problem due to the factthat he/she is different from others. Researches on robotic prosthe-ses are necessary to encourage the amputee in the social activities.Furthermore prosthetic fingers have been developed as a part ofhand although there exist more amputees who lose their single fin-ger than hand. Also, finger prosthesis has been mainly made by askilled artist using his/her own techniques in sculpting and experi-ence to imitate living finger in viewpoint of appearance. Most pros-thetic fingers have not considered a functional mobility to pursuecooperative motion with living fingers. In this paper we are to de-velop a robotic index finger controlled by EMG signals of musclesin the lower arm in real-time fashion.

Almost all robotic hands have been developed in such a way tograsp something well with two or more fingers in several decades[1–15]. In detail, early robotic hands and fingers have used tendon-driven mechanism actuated by two active motors for both exten-sion and flexion motions [1–7]. The tendon-driven was consideredas one of the muscle-like mechanisms but it was not easy to con-trol because of nonlinearities of wire-pulling system itself. For thisreason, some researchers have studied new mechanism to makerobotic finger with high performance. For example, DLR hand,which has one degree-of-freedom finger mechanism with geartransmission for 3-phalanx motion, has been developed in [8–10]. Also, the 3-phalanx finger mechanism without using the geartransmission and the wire-driven method has been released in

ll rights reserved.

[11]. It was able to be fabricated relatively strong and lightweightbecause it made use of parallel structure and under-actuatedmechanism, possible to make human-like motion and grasp, andeasy to calculate position of the finger tip. Another type of ten-don-driven mechanism with an active actuator and a passivespring has been also proposed recently in [12–15]. Though it isnot about robotic finger, a stackable 4-bar mechanism has beensuggested for fully actuated surgical manipulators in [16]. In thispaper, an under-actuated mechanism realized by one actuator isproposed both by stacking double 4-bar linkage mechanisms andby adopting slider-crank mechanism for driving input.

The EMG signal processing methods can be classified into time-domain approach, frequency-domain approach, and EMG activa-tion classification approach according to reference [17]. Time-do-main methods contain Wilson amplitude method (WAMP) asmuscle contraction indicator [18], cepstrum method based on theautoregressive coefficients (AR) [19], wavelet method [20], andthe PCA method to estimate continuous joint angles from EMG sig-nals [21]. Differently from the time-domain approach, the fre-quency-domain approach can be utilized as an indicator of motorunit action potential (MUAP) that is a fundamental element ofEMG signal. The frequency-domain approach includes the random-ness-based spectral analysis [22], the median frequency method[23] and so on. The EMG activation classification approach includesthe linear discriminant analysis (LDA), hidden Markov model(HMM) [17], Gaussian classifiers [24], and neural networks [25].

The EMG signals have been utilized to control the robotic devicesince Wiener had proposed the concept of an EMG-controlled pros-thetic hand in [26]. There were several pioneering works for EMG-controlled robots such as Waseda hand in [27], Boston arm in [28],and Utah artificial arm in [29]. Recent works are mainly involvedwith neural networks to handle nonlinearities such as time-vary-

G. Jang et al. / Mechatronics 23 (2013) 318–325 319

ing, sensor location-varying, and environment-varying propertiesin [30–36]. The current state of the art about arm prosthesis wouldbe a DARPA’s prosthetic arm controlled neuronally with 22 de-grees-of-freedom in [37]. In this paper, we are focusing on real-time digital processing of EMG signals for the cooperative motionwith living fingers.

This paper is organized as follows: Section 2 suggests both de-sign and kinematics for robotic index finger prosthesis developedby stacking double 4-bar mechanisms; Section 3 proposes bothpreprocessing and real-time processing methods with respect tothe measured EMG signals; Section 4 discusses about experimentalresults and Section 5 draws the conclusion of the paper.

2. Prosthetic index finger mechanism

Stackable double 4-bar linkages are utilized to develop one de-gree-of-freedom prosthetic index finger as shown in Fig. 1. In de-tail, the Fig. 1 illustrates three link layers, in which the first linklayer of Fig. 1a consists of one slider-crank input linkage, the sec-ond link layer of Fig. 1b consists of one input linkage and double4-bar mechanisms, and the third link layer of Fig. 1c is composedof one input linkage and two double 4-bar mechanisms. Also, alllink layers are stacked sequentially and then common bottomjoints such as A, B, D and G in Fig. 1 are joined each other and fixedtogether. Here, we should notice that one linear actuator throughthree slider-crank linkages drives all link layers at the same time.The robotic finger proposed in this paper has four advantages asfollows; First, it is able to separate the electrical components suchas the electric motor, wiring and sensors from working mechanicalfinger, thus the suggested finger prosthesis can be fabricated onlywith selective materials. Second, since the suggested finger con-sists of 4-bar closed-chain mechanisms, it is structurally strong,even though it is made thin. Third, it may be implanted as a pros-thetic finger because it can be fabricated as a small size. Fourth, thegear transmission is unnecessary because it makes use of linear

(a)

(b)

(c)

Fig. 1. Kinematic model of 1-DOF prosthetic finger mechanisms, l02B , and l02D0 , denote linkl03B; l

03D0 l

03D and l03G0 , denote BC03, D0E03;DF 03 and G0H03 in the third link layer, respectively. No

actuator type. In total the finger suggested in Fig. 1 consists of 20links and 28 joints and its mobility is one.

Though the suggested finger consists of three slider-cranks andthree double 4-bar mechanisms, one common input linear actuatoroperates it through three slider-cranks. For this, locations of jointsO and B are earthed at the ground. Also the last links of each layerare triangular shapes and the bottom links of each layer are tied uptogether. Since the ranges of motion of each joint of human indexfinger become approximately 0 to 90 degrees, the joint angle rangeof l1 in Fig. 1a should be 0 to 90 [deg] while the translational inputd is moving 0 to 10 [mm]. The angle ranges of other links, l2 and l3,also should be 0 to 180 [deg] and 0 to 270 [deg], respectively, forgiven input range of d. Here notice that we make use of double4-bar mechanisms differently from the suggested in [13,16] so thatsmall input can provide large range of motion at an output link. Inaddition, we are to suggest the comparison results with the previ-ous ones as shown in Table 1 in viewpoint of mechanism structuresin order to show the properties of the proposed mechanism. Alsowe should notice that the proposed mechanism is driven by the sli-der-crank input while most other mechanisms are driven by thetendon-type rotary input.

2.1. Kinematics of Prosthetic Finger

To explain how to solve the kinematics easily, we choose onedyad from first 4-bar of the Fig. 1b as shown in Fig. 2. The Fig. 2 de-picts a core kinematic component useful in analyzing slider-crankand 4-bar linkage. Positions of C02 and D0 can be obtained from pre-vious inputs with coordinates ðxC02

; yC02Þ and ðxD0 ; yD0 Þ respectively.

Let us assume that link lengths of l2C0 and l2D0 are known and R inFig. 2 is a length between C02 and D0. (We will deal with how to de-sign both link lengths of l2C0 and l2D0 in next section.) The secondlaw of cosines is utilized to get both b1 and b2 as follows:

cos b1 ¼l22C0 � l2

2D0 þ R2

2l2C0Rand cos b2 ¼

l22C0 � l2

2D0 � R2

2l2D0Rð1Þ

lengths of line segments BC02, and D0E02 in the second link layer, respectively. Also,te that the lengths of line segments BD0 and DG0 are l1/2 and l2/2, respectively.

Table 1Comparison of finger/hand mechanisms.

Driving type Kinematic type Stack-ability Active/passive DOF

LARM hand [11,12] Rotary 4-Bar No 1/2Gosselin hand [13] Tendon-driven 4-Bar Yes 1/2TBM hand [14] Tendon-driven Parallel Yes 1/2Rutgers hand [38] Tendon-driven Serial No 3/0The proposed Slider-crank Double 4-bar Yes 1/2

Fig. 2. Three revolute joints dyad example.

320 G. Jang et al. / Mechatronics 23 (2013) 318–325

Also the angle, a, is obtained as

a ¼ tan�1ðV=UÞ ð2Þ

where V ¼ yD0 � yC02, U ¼ xD0 � xC02

. Angles from the global coordi-nates in Fig. 2 are obtained as w2C0 ¼ aþ b1 andw2D0 ¼ aþ b2because a is a negative value in Fig. 2. In the sameway, every dyadic angle, wim, in Fig. 1 can be obtained for i = 1, 2,3 and m = A, B, D0, D, G0, G. The phalanx angle, h1, between line seg-ments OB (it is the same as the global coordinate X0 in Fig. 1) andBD, h2between BD and DG, h3between DG and GJ, are calculated byusing the dyadic angles as follows:

h1 ¼ w1B � 0w1B

h2 ¼ w2D � 0w2D � h1

h3 ¼ w3G � 0w3G � h1 � h2

ð3Þ

where 0w1B, 0w2D, 0w3G denote the initial constant 1st, 2nd, 3rd-phalanx joint angles represented from the global coordinate X0 inFig. 1, respectively, when the phalanxes are completely extended.

Fig. 3. Prototype of robotic index finger (its total mas

In other words, they express the postures represented from the glo-bal coordinate X0 when every hi are zeros as shown in Fig. 3.

The chain rule is used to figure out the velocity relation be-tween input and output. For notation ease, sinusoidal functionsare expressed as short form such as cx = coswx, sx = sinwx. On thefirst dyad of each link layer in Fig. 1, position of Ci for i = 1, 2, 3can be calculated from both point A and B:

xiA þ liAciA ¼ xiB þ liBciB; ð4ÞyiA þ liAsiA ¼ yiB þ liBsiB; ð5ÞxiB � xiA ¼ d; ð6ÞyiB � yiA ¼ 0: ð7Þ

After differentiating Eqs. (4)–(7) and rearranging them, we can getthe following relations:

_xiB � _xiA

_yiB � _yiA

� �¼�liAsiA liBsiB

liAciA �liBciB

� � _wiA

_wiB

" #ð8Þ

_xiB � _xiA

_yiB � _yiA

� �¼

1 00 1

� � _d

0

" #ð9Þ

Here, let us define matrices as follows:

Gi1 ¼�liAsiA liBsiB

liAciA �liBciB

� �and Ki1 ¼

1 00 1

� �

and if we rearrange Eqs. (8) and (9) about a dyad output, _wiB, thenwe have

_wiB ¼ G�1i1

h i½Ki1�

h i2;

_d0

" #for i ¼ 1;2;3 ð10Þ

where 2; denotes the second row of corresponding matrix. From Eq.(3), we know that _w1B ¼ _h1, _w2D ¼ _h1 þ _h2, and _w3G ¼ _h1 þ _h2 þ _h3,here _h1 is an angular velocity when i = 1 in Eq. (10). After applying

s is 80 [g] including linear actuator mass 19 [g]).

Anchor

Actuator

ProstheticFinger

Fig. 4. Conceptual design to connect the developed finger prosthesis to the end offinger root bone.

G. Jang et al. / Mechatronics 23 (2013) 318–325 321

similar procedures, we can also get _h2 when j = 2 from the followingequation:

_wjD ¼ G�1j3

h iKj3� �h i

2;

_h1

_wjD0

" #for j ¼ 2;3 ð11Þ

where

Gj3 ¼�ljE0sjE0 ljDsjD

ljE0cjE0 �ljDcjD

" #and Kj3 ¼

�l1sh1=2 l0jD0 sjD0

l1ch1=2 �l0jD0cjD0

" #

We finally can get _h3 from the following equation:

_w3G ¼ G�135

h i½K35�

h i2;

_h1 þ _h2

_w3G0

" #; ð12Þ

where

G35 ¼�l3E0s3E0 l3Ds3D

l3E0c3E0 �l3Dc3D

� �and K35 ¼

�l2sh12=2 l03D0s3D0

l2ch12=2 �l03D0c3D0

" #

Now, we know every phalanx angles and angular velocities.Therefore the kinematic Jacobian for end-point J of stackable fingermechanism can be obtained as if it is a serial-type planar manipu-lator having three revolute joints:

_E ¼ J _H; ð13Þ

where

_E ¼ _xJ _yJ� �T and _H ¼ _h1

_h2_h3

� �T; ð14Þ

J ¼�l1sh1 � l2sh12 � l3sh123 �l2sh12 � l3sh123 �l3sh123

l1ch1 þ l2ch12 þ l3ch123 l2ch12 þ l3ch123 l3ch123

� �

Till now, we have suggested the kinematics of prosthetic index fin-ger. The following section discusses about how to design the linklengths.

2.2. Link length design for dyads

According to the Korean standard length and thickness of pha-lanxes in [39], we are to determine the thickness and length ofeach phalanx in the robotic finger; in order words, the left side linklengths of each 4-bar such as l02B0 , l02D0 , l03B0 , l03D0 , l03D and l03G0 in Fig. 1 aredetermined from the standard thickness of phalanxes, also, thebottom link lengths such as l1, l2, and l3 are determined from thestandard length of phalanxes in advance. And then the remaininglink lengths of 4-bar linkages are determined from the range ofmotion of each joint. Let us consider one dyad as shown inFig. 2; we can get the following relations:

l2C0c2C0 ¼ xD0 � xC02þ l2D0c2D0 ; ð15Þ

l2C0 s2C0 ¼ yD0 � yC02þ l2D0 s2D0 ; ð16Þ

where c2C0 ¼ cos w2C0 , s2C0 ¼ sin w2C0 , c2D0 ¼ cos w2D0 , ands2D0 ¼ sin w2D0 . After squaring above two equations, respectively,and adding them, we have

l22C0 ¼ R2 þ 2l2D0 ðU � c2D0 þ V � s2D0 Þ þ l2

2D0 : ð17Þ

Now we apply the minimum and maximum values from the desiredrange of motion mw2D0 6 w2D0 6

Mw2D0 to above Eq. (17), respectively,as follows:

l22C0 ¼ mR2 þ 2l2D0

mU � mc2D0 þ mmV � mms2D0ð Þ þ l22D0 whenw2D0

¼ mmw2D0

l22C0 ¼ MR2 þ 2l2D0

MU � Mc2D0 þ MV � Ms2D0� �

þ l22D0 when w2D0 ¼ Mw2D0

After subtracting above equations each other, we can get the lengthof l2D0 as follow:

l2D0 ¼mR2 � MR2

2 MU � Mc2D0 þ MV � Ms2D0 � mU � mc2D0 � mV � ms2D0ð Þ ð18Þ

Also the remaining link length l2C0 is automatically determined if thelink length l2D0 determined from Eq. (18) is applied to one of bothminimum and maximum equations. In other words, l2C0 is deter-mined from Eq. (17). Link lengths of every nine dyads can be alsodetermined in the same way. Fig. 3 shows a prototype of robotic fin-ger prosthesis designed by the proposed method. The following sec-tion discusses about how to connect the proposed prosthetic fingermechanism to the living hand.

2.3. Idea about how to attach prosthetic finger to living hand

For the index finger prosthesis development, we have gatheredmuch information from medical doctors with rehabilitation major;one of them is that many index finger-amputated persons haverevisited the hospital in order to remove the remaining finger partsfor cosmetic reason. Thus any anchor device can be attached to theend of finger root bone when the medical doctors perform theremoving operation of remaining finger. Ultimately the linear actu-ator part will be located on the palm part connected to the fingerroot bone (metacarpal bone or carpal bone) as shown in Fig. 4. Inreality, when the linear actuator is attached, a few safety issuessuch as malfunction by external shock and implant material selec-tion should be considered for clinical test later. The utilized linearactuator model is PQ-12 (manufactured by Figelli TechnologiesInc.). It has the following specifications; maximum stroke: 20[mm], maximum force: 18 [N] at 6 [mm/s], no-load speed: 12[mm/s], weight: 19 [g], dimension: 45 � 21.5 � 15 [mm], and max-imum power consumption: 1.25 [W]. We hopefully presume that itmight be inserted in the palm part without difficulties. The follow-ing section suggests a real-time EMG signal processing method toextract the desired finger motion from the muscle activations.

3. EMG signal processing and control

Surface electromyography (sEMG) is measured from flexor dig-itorum superficialis (FDS) and extensor indicis (EI) in the lowerarm with sEMG sensor. For this, sEMG sensor was developed usingOP-AMPs (AD620 for separation of body ground from referenceground). Since it is known that EMG signal has a bandwidth of10 to 500 [Hz], the developed sEMG sensor has sequential func-tions such as 250 times amplification, bandpass filtering of 7.23to 477 [Hz], one more 10 times amplification, and then 1.8 [V] levelshifting for analog-to-digital conversion. Also the developed EMGsensor has the following characteristics; two channels to acquireflexion and extension information of the index finger, small input

0 2 4 6 8 10Time [s]

sEMG ofFlexor digitorumsuperficialis

sEMG ofExtensor indicis

Effect of flexion of index finger

Effect of Extension of index finger

Fig. 5. Comparison of sEMG signals measured from flexor digitorum superficialis(FDS) and extensor indicis (EI) muscles while an index finger is flexing andextending repeatedly.

322 G. Jang et al. / Mechatronics 23 (2013) 318–325

voltage noise; 9 [nV=ffiffiffiffiffiffiHzp

] at 1 [kHz], dimension: 57 � 39 [mm],and input voltage: 3.3 [V].

The acquired sEMG signal can be divided into an initial burstpart generated before and during actual motion and a long plateaupart appeared during no-motion. From anatomical references in[40,41], it is known that FDS and EI muscles have charge of flexionand extension motions of index finger, respectively. As a matter offact, the motion intention is sequentially transferred and realizedfrom the brain to the movement through the corresponding neuraland muscle activities, the time at which a human feels the urge tomove precedes the time of his/her actual movement by a few doz-ens to hundreds milliseconds [42]. In other words, the initial burstpart of EMG precedes the onset of actual movement by dozens tohundreds milliseconds [43]. Thus, the initial burst part becomesmore important to make the cooperative motion of prosthetic fin-ger with living fingers. In this paper, the initial burst is utilized fordesired finger motion generation and the long plateau is removedby introducing the threshold.

Fig. 5 shows raw sEMG acquired by using the developed sensorwhile an index finger is repetitively flexing and extending. Themeasured sEMG signals are processed every 2 millisecond usingthe method suggested in Fig. 6. Aforementioned we first measuresEMG signals at rest for 4 [s] and then we set the threshold valuesto remove the long plateau parts of sEMG. This is referred to as pre-processing in this paper. For a preprocessing in the Fig. 6, N sam-ples (here, N = 2000 because it is measured for 4 [s] with 0.002

Fig. 6. Block diagram of sE

[s] sampling time) are measured at rest. The threshold is set tobe a little bit larger than an average of M samples root meansquared value (namely, an average of outputs from M samplesRMS window) as following form:

Threshold ¼ at

NffiffiffiffiffiMp

XN�1

n¼0

XM�1

k¼0

x2ðn� kÞ" #1

2

; ð19Þ

where x(n) implies the measured sEMG signal and at denotes a scal-ing factor for the threshold.

The thresholds obtained from the preprocessing are subtractedfrom the RMS signals, and then all signals less than zero are elim-inated as follow:

EMGðnÞ ¼ r � dðnÞ � ½RMSðnÞ � Threshold�; ð20Þ

where

dðnÞ ¼1; RMSðnÞ > Threshold

0; else

in which RMSðnÞ ¼ 1=ffiffiffiffiffiMp PM�1

k¼0 x2ðn� kÞh i1=2

; dðnÞ is a switchingfunction to eliminate the signal less than the given threshold, andr is a scaling factor to adjust intensity of signal because the inten-sity of EMG signal becomes different according to subjects and sen-sor locations attached to FDS and EI muscles, in the Fig. 6, rf for FDSand re for EI.

As a next step of main processing in Fig. 6, since the FDS muscleacts as flexor of index finger and the EI muscle as extensor, we de-fine the synergistic EMG signal by subtracting the EI signal fromthe FDS signal as follows:

EMGsynðnÞ ¼ EMGf ðnÞ � EMGeðnÞ ð21Þ

where subscripts f and e denote FDS and EI muscles, respectively.Now we assume that the synergistic EMG signal of Eq. (21) hassome relation with velocity of finger movement by neglecting fingerdynamics due to its negligible small mass. In other words, we as-sume that the synergistic EMG signal is proportional to the inputvelocity of the linear actuator. For practical use, we introduce thesaturation function to limit both maximum forward and backwardspeeds of the linear actuator. It is expressed as

_ddesðnÞ ¼Vmax; EMGsynðnÞ > Vmax

Vmin; EMGsynðnÞ < Vmin

EMGsynðnÞ; else

8><>: ð22Þ

where Vmax and Vmin denote maximum forward and backwardspeeds of the linear actuator, respectively.

MG signal processing.

Fig. 7. Block diagram of entire control scheme, where dr is the actual displacement of the linear actuator.

0 2 4 6

-0.02

-0.01

0

0.01

0.02

Time [s]

0 2 4 6

0.01

0.02

0.03

Time [s]

0 2 4 60

0.01

0.02

0.03

Time [s]

RMS Signal of FDSA

RMS Signal of EI

Threshold

Threshold

B

DC The synergetic EMG signal

E

0 2 4 60

0.01

0.02

Time [s]

0 2 4 60

0.01

0.02

Time [s]

0 2 4 6-0.02

-0.01

0

0.01

0.02

0.03

Time [s]

1 2 3 4 5 62

4

6

8

Time [s]

Dis

plac

emen

t [m

m]

Actual Desired

Fig. 8. Experimental results; A. finding the thresholds of FDS and EI (Eq. (19)), B. obtaining the initial burst part of FDS and EI (Eq. (20)), C. synergistic EMG (Eq. (21)), D.desired velocity of linear actuator (Eq. (22)), E. control performance; where desired displacement is obtained by Eq. (23) and actual displacement is measured from the built-in potentiometer.

G. Jang et al. / Mechatronics 23 (2013) 318–325 323

The desired displacement of the linear actuator is obtainedby integrating the desired velocity such as ddesðnÞ ¼ddesðn� 1Þ þ T � _ddesðnÞ with sampling time T. Since above displace-

ment is also constrained to the range of motion of linearactuator dmin 6 d 6 dmax, we modify the desired displacement asfollows:

0 1 2 3 4 5 6 70.5

1

1.5

2

Time [s]

Angl

e [ra

d]

θ1+θ2+θ3θ1+θ2θ1

Fig. 9. Actual 3-phalax motions resolved by using the kinematic resolution methodin Section 3.

324 G. Jang et al. / Mechatronics 23 (2013) 318–325

ddesðnÞ ¼dmax; ddesðnÞ > dmax

dmin; ddesðnÞ < dmin

ddesðnÞ; else

8><>: ð23Þ

where dmax and dmin denote maximum and minimum displacementof the linear actuator, respectively.

Till now we have obtained the desired displacement ddes(n)through the real-time EMG signal processing procedures suggestedin Fig. 6. On the other hand, actual displacement is measured bybuilt-in potentiometer of linear actuator. The conventional PIDcontroller is applied to the error between desired and actual dis-placements. A dead zone caused by friction, however, exists in realactuator system. To remove the dead zone, the following compen-sation is used:

V ¼ uþ a � signðuÞ ð24Þ

where u is the output of PID controller, a is the dead-zone constantand V denotes control input for linear actuator. Finally Fig. 7 showsan entire control scheme for the prosthetic index finger. The follow-ing section suggests the experimental results to verify the effective-ness of the proposed method.

4. Experimental result

Now we are to suggest the experimental parameters requiredto implement the proposed algorithm in the Section 3. First thesampling time is set as T = 0.002 [s] for acquiring both the sEMGsignals (sEMGf and sEMGe) and actual displacement of the linearactuator (dr). Second N = 2000 (for sEMG 4 [s] data), M = 25 (forRMS operation) and at = 1.005 (as a scaling factor) are used todetermine the threshold values as suggested in Eq. (19). As

Fig. 10. Snapshots of exp

shown in Fig. 8.A, two threshold values were determined as0.013 for FDS and 0.008 for EI, respectively. Third two scalingfactors rf = 0.9 for flexion and re = 0.7 for extension were usedin Eq. (20) to extract the initial burst part appeared during in-motion. The Fig. 8.B shows the initial burst parts of FDS and EIobtained by applying Eq. (20), respectively. Fourth the synergis-tic EMG is obtained by using Eq. (21) as shown in Fig. 8C. Fifththe forward and backward speeds of the linear actuator are lim-ited by using Vmax = 0.02 [m/s] and Vmin = �0.02 [m/s] in Eq. (22).The Fig. 8D shows the result obtained by applying Eq. (22). Sixththe maximum and minimum displacement of the linear actuatorare also limited by setting dmax = 0.009 [m] and dmin = 0.0025 [m]in Eq. (23). The solid line of Fig. 8E shows the desired displace-ment of the linear actuator obtained by applying Eq. (23). Final-ly, the performance of the tracking controller suggested in theFig. 7 is shown in the Fig. 8E. We can know that the actual dis-placement tracks the desired one well in spite of the disadvan-tage of the dead-zone effect in the linear actuator. Also weshould notice that the total time delay for the proposed real-time signal processing is M samples (M = 25 for RMS operation).In other words, since the sampling time is 0.002 [s], the pro-posed algorithm has 0.05 [s] time delay during the real-timeprocessing.

The Fig. 8 shows the sequential graphs obtained by applying Eq.(20)–(24) to the measured sEMG signals. In addition, the actualdisplacement can be resolved into 3-phalanx motion by using thekinematic resolution method in the Section 2. A as shown inFig. 9. Also, Fig. 10 shows a few snapshots of experimental videoclip.

5. Conclusion and future works

The mechanism for robotic index finger prosthesis was pro-posed by using stackable double 4-bar mechanisms and the sli-der crank mechanism. The proposed mechanism has severaladvantages; first electrical components such as actuator and sen-sor could be separated from mechanical components; second itcould be fabricated structurally strong by stacking the 4-barmechanism; third it could be designed to be implanted in theliving hand. In the case of finger, since its mass is very small,the initial burst part of corresponding sEMG becomes more dom-inant than the long plateau part affected by gravity while an in-dex finger is moving. Thus, we have introduced the thresholdoperation in the proposed EMG signal processing, ultimately, toproduce the desired velocity/displacement of the linear actuator.Finally, we have shown the effectiveness of the entire controlscheme including the signal processing method through experi-ments. Still we have many future works; first robustnessimprovement of EMG signal processing against noises, secondportability improvement by miniaturizing the sensor and the sig-nal-processing modules, and third clinical test for prosthesisapproval.

eriment movie clip.

G. Jang et al. / Mechatronics 23 (2013) 318–325 325

Acknowledgements

This work was supported in part by the Global Frontier R&DProgram on ’’Human-centered Interaction for Coexistence’’ fundedby the National Research Foundation of Korea grant funded by theKorean Government (MEST) (2012M3A6A305), in part by the Min-istry of Knowledge Economy (MKE) and Korea Institute forAdvancement in Technology (KIAT) through the Workforce Devel-opment Program in Strategic Technology, and in part by the MKEunder the Human Resources Development Program for Conver-gence Robot Specialists support program supervised by the NIPA(National IT Industry Promotion Agency) (NIPA-2012-H1502-12-1002), Republic of Korea.

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