An MMG-based Human-Assisting ManipulatorUsing Acceleration Sensors

Keisuke Shima and Toshio TsujiGraduate School of Engineering

Hiroshima University1-4-1 Kagamiyama, Higashi-hiroshima, Japan

{shima, tsuji}@bsys.hiroshima-u.ac.jp

Abstract—This paper proposes a control method for a human-assisting manipulator using acceleration sensors. The techniqueinvolves an arm control part (ACP) and a hand-and-wrist controlpart (HWCP); the ACP controls the manipulator’s shoulder andelbow joints using acceleration signals, while the HWCP controlsthe corresponding joints using mechanomyogram (MMG) signalsmeasured from the human operator. A distinctive feature ofthe proposed method is its estimation of information on forceand motion from measured acceleration signals using MMGprocessing and a probabilistic neural network. Experimentsdemonstrated that the MMG patterns seen during hand andwrist motion can be classified sufficiently (average rate: 94.3 %),and that a prosthetic manipulator can be controlled using theacceleration signals measured. Such manipulators are expectedto prove useful as assistive devices for people with physicaldisabilities.

Index Terms—Mechanomyogram, pattern classification, prob-abilistic neural network, human-machine interface

I. INTRODUCTION

Electromyogram (EMG) detection is the most commonlyutilized means of interaction between amputees and externallypowered prostheses [1]–[5]. Since EMG signals change de-pending on the level of muscular contraction, these externallypowered units can be controlled in a natural way.

Using EMG signals, Fukuda et al. [6] developed a human-assisting manipulator consisting of a forearm part and anupper-arm part. The forearm part can be controlled using EMGelectrodes attached to the skin surface, while the upper-armpart uses a 3D position sensor attached to the end point of thearm. Although this method allows the operator to control themanipulator at will, it requires the combination of two kindsof sensors. It is also quite difficult to use the manipulator forlong periods of time because EMG signals are easily affectedby changes in the skin’s impedance.

Meanwhile, some utilization of mechanomyogram (MMG)[7] signals has been explored [8], [9]. These signals expressthe mechanical vibration of muscles, and lie between EMGsignals – which express electrical activity in muscles – andthe resulting muscle force. Therefore, in the same way astheir EMG counterparts, MMG signals change according to thelevel of muscular contraction [10], [11]. It should be noted thatMMG signals are not affected by changes in skin impedancecaused by sweating [8]. Barry et al. [8] and Silva et al. [9]achieved on/off control of a prosthetic hand by ascertainingthe presence or absence of muscular contraction. However,

Human-assisting manipulator Operator (amputee)

Transmitter

Acceleration sensors

Forearmpart

Upper-armpart

Fig. 1. Example of human-assisting manipulator operation by an amputee

these studies cannot be extended to proportional control ofmultifunctional prostheses using MMG signals because classi-fication of multiple motions and estimation of muscular forcefrom the measured signals are necessary.

In this paper, we propose a novel control method for amultifunctional human-assisting manipulator based on motionclassification of MMG signals using a probabilistic neuralnetwork. First, from the signals measured by the accelerationsensors, two kinds of biological signals – MMG and motionacceleration (MAC) – are estimated using digital filters. Theoperator’s intended forearm/hand motion is then estimatedfrom the MMG signals, which are utilized to proportionallycontrol the forearm part of the manipulator. The MAC signalsare also used to control the upper-arm part of the manipulatorafter being integrated twice. As a result, it is possible to controlthe manipulator using acceleration sensors only.

This paper is organized as follows: in Section II, thedetails of the proposed method are described. The validity ofthe proposed system is examined in Section III. Finally, thepaper is concluded in Section IV, and future study plans arediscussed.

II. MANIPULATOR CONTROL USING ACCELERATIONSIGNALS

Figure 1 shows an example involving a forearm amputeeusing the proposed method to operate a human-assisting ma-nipulator. The manipulator [6] consists of a forearm part that

Proceedings of the 2009 IEEE International Conference on Systems, Man, and CyberneticsSan Antonio, TX, USA - October 2009

978-1-4244-2794-9/09/$25.00 ©2009 IEEE2507

Operator Human-assisting manipulator

Motion decisionPattern discrimination

LLGMNFea

ture

ex

trac

tio

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Force information

Sig

nal

sep

arat

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MMGsignals

End-pointdisplacement

MAC signals

Arm control

Hand and wrist control

Digital integrator

Motion

Signalmeasurement

Nonli

nea

rtr

ansf

orm

atio

n

Fig. 2. The proposed human-assisting manipulator control method

uses three ultrasonic motors with one degree of freedom (DOF)each, while the upper-arm part uses a four-DOF robot arm(Mitsubishi Electric Corporation). The forearm part can alsobe used as a prosthetic hand attached to the amputated part.

An outline of the proposed control method for human-assisting manipulators is shown in Fig 2. The techniqueconsists of three parts: signal measurement, hand and wristcontrol and arm control. In the signal measurement part, themeasured signals are separated into MMG and MAC types.The operator’s intended motion is estimated from the MMGsignals for the hand and wrist control part (HWCP), andthe forearm part of the manipulator (the prosthetic hand) isthen controlled using the estimated results. The arm controlpart (ACP) computes the three-dimensional motion of theoperator’s arm from the MAC signals, and the upper arm of themanipulator is then controlled to track the desired trajectory.The details of each control part are outlined in the followingsubsections.

A. Signal measurement part

First, acceleration signals are measured from L channelsof acceleration sensors attached to the muscle surface on theforearm. Although the measured signals contain informationon motion acceleration (MAC) in addition to the MMG, thefrequency components of the MMG are higher [11] than thoseof MAC. Accordingly, the two components can be separatedin the frequency region using a set of digital filters withappropriate properties.

In this paper, the two types of digital band-pass filter usedfor signal separation are a low-band filter (fAl Hz to fAh Hz)for MAC signals and a high-band filter (fMl Hz to fMh Hz)for the MMG signals. After signal separation, the tth samplesof MAC and MMG signals are defined as MACl(t) (l =1, 2, . . . , L) and MMGl(t) at a given time t.

Since MMG and MAC signals reflect muscle activation andinformation on physical movement, respectively, these data areused to control each part of the manipulator.

B. Hand and wrist control part (HWCP)

In the HWCP, the operator’s intended motion and forceinformation are estimated from the MMG signals separated

from the acceleration signals, and the forearm part of themanipulator is controlled in line with the estimated results.

1) Feature extraction: The MMG signals are rectified andsmoothed through a second-order low-pass Butterworth filter(cutoff frequency: fcut Hz). The filtered signals are then nor-malized to make the sum of L channels equal to 1, and are de-fined as the feature vector x(t) = [x1(t), x2(t), · · · , xL(t)]T ∈�L at a given time t. Here, each element of the feature vectorxi(t)(i = 1, · · · , L) is defined as

xi(t) =MMG i(t) − MMGst

i

L∑i′=1

(MMG i′ (t) − MMGsti′ )

, (1)

where MMGsti is the mean value of MMG i(t), which is mea-

sured while the muscles are relaxed. The feature vector x(t)is input into the probabilistic neural network for estimation ofthe operator’s intended motion.

Since the amplitude level of MMG signals is known tochange in proportion to muscle force, the operator’s musclecontraction level can be estimated from these signals formanipulator control. First, the root mean square (RMS) of theMMG signals is computed, and then the value is normalizedusing the root mean square MMG with maximum voluntarycontraction (MVC). The average of L normalized signals isalso used to compute force information FMMG(t) as follows:

FMMG(t) =1L

L∑i=1

RMS i(t) − RMS sti

RMSmaxi − RMS st

i

, (2)

RMS i(t) =

√√√√ 1n

n−1∑τ=0

MMG i(t − τ)2 , (3)

where n is the number of RMS values, RMSmaxi is the mean

value of RMS i(t) at the MVC of the muscle, and RMS sti is the

mean value of RMS i(t), which is measured while the musclesare relaxed. The motors of the forearm part are operated in bythe level of force information FMMG(t) using the impedancecontrol method [6].

2) MMG pattern classification: The feature vector ex-tracted from the MMG signals is classified by a probabilisticneural network (PNN), and the operator’s intended motion isestimated. In the proposed method, the log-linearized Gaussianmixture network (LLGMN) [12] proposed by Tsuji et al. isused as the PNN for the discrimination of MMG patterns.The LLGMN is based on a Gaussian mixture model (GMM),and can estimate the probability density function (pdf) fromthe input vector through learning. The LLGMN has beenutilized as a classification method for various biological signalssuch as EMGs and electroencephalograms (EEGs) with a highclassification rate [6], [12].

In the proposed method, the system first measures M typesof MMG patterns during motion conducted by an operator,such as hand grasping and opening. The feature vectorscalculated from these MMG signals are then input to theLLGMN as teacher vectors, and the LLGMN is trained toestimate the a posteriori probabilities of each motion. After

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learning, the a posteriori probability of each learned motionY m(t)(m = 1, · · · , M ) can be output for each input vector.According to the learning ability of the LLGMN, it is possiblefor it to correspond to changes in MMG signals stemming fromdifferences among individuals.

For motion classification, the force information FMMG(t)is evaluated using the determination threshold Fth. WhenFMMG(t) exceeds Fth, the system judges the occurrence ofoperator motion, and the feature vector is input to the LLGMN.At the same time, in order to prevent discrimination errors, wecalculate the entropy H(t), defined as

H(t) = −M∑

m=1

Y m(t) log2 Y m(t) . (4)

If H(t) is smaller than the discrimination determination thresh-old Hth, the motion with the highest a posteriori prob-ability becomes the result of discrimination. Otherwise, ifH(t) exceeds Hth, discrimination is suspended as obscuremotion since large entropy means that the LLGMN output isambiguous. When this happens, the LLGMN outputs the sameresult as one sample before to the manipulator. In addition,when and only when the same discrimination results are givenT istb times repeatedly, the manipulator is controlled in order toprevent manipulator malfunction. Further, the driving speed ofthe HWCP is controlled in proportion to the force informationFMMG(t).

C. Arm control part (ACP)

In the ACP, the MAC signals separated from the accelera-tion signals are integrated twice, and hand displacement is thenestimated from these signals. The manipulator’s end-point isthen moved to the target position computed. In order to removethe influence of the measurement bias of the accelerationsensors, the value of a single integrated MAC signal is definedas Vint, and the value of velocity V (t) is calculated using thefollowing equation:

V (t) = F [Vint(t) − Vfil(t − tdel)] , (5)

where Vfil(t) is the smoothed Vint(t) through a second-orderlow-pass filter (cutoff frequency: fint Hz), and tdel describesthe phase-lag element of the filter. F [x] represents a functionwith the dead zone expressed as follows:

F [x] =

⎧⎪⎨⎪⎩

x − Vth (x > Vth)0 (|x| ≤ Vth)x + Vth (x < −Vth)

. (6)

V (t) is then integrated twice using trapezoidal integration, andthe displacement P (t) is calculated to control the end-pointdisplacement of the manipulator.

The desired values for the joint angles of the manipulator’supper arm are then determined according to the end-pointdisplacement calculated, and the corresponding joints are con-trolled using a proportional-integral-derivative (PID) controlmethod.

Frequency (Hz)

(x10−5)

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m2/s

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2001000 50 150

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m2/s

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(b): Hand grasping(a): Rest

(c): Upper-arm movements

Frequency (Hz)

Pow

er (

m2/s

4) 2.0

1.0

0.0

(x10−5)

(d): Hand grasping

with upper-arm movements

2001000 50 150

Fig. 3. Power spectra of the measured acceleration signals (subject A)

III. EXPERIMENTS

To demonstrate and verify the validity of the proposedmethod, we conducted experiments with four normal subjects(male; A, B: age 24; C, D: age 23) and a forearm amputee(male; E: age 49). One triaxial acceleration sensor (NihonKohden Corporation, TA-513 G: rectangular parallelepiped,longitudinal 20.0 mm, transversal 15.0 mm, thickness 12.0mm, weight 12.5 g) and three uniaxial acceleration sensors(Nihon Kohden Corporation MT-3 T: wheel, diameter 3.0 mm,thickness 6.5 mm, weight 3.0 g) were attached to right forearmof the subjects. The sampling frequency for measuring acceler-ation signals was 1 kHz. Muscular activity was measured froma total of four channels consisting of the z-axis of the triaxialacceleration sensor and the three uniaxial acceleration sensors.Elements of upper arm movement were measured from the x-, y-, and z-axes of the triaxial acceleration sensor. Informedconsent was obtained from all subjects.

A. Preliminary experiments

First, we attempted to separate MAC and MMG signalsfrom the measured signal using the acceleration sensor. Auniaxial acceleration sensor was attached to the skin surface ofthe right forearm (near the flexor carpi radialis), and the signalsof four states – (a) rest, (b) hand grasping, (c) upper-armmovement and (d) hand grasping with upper-arm movement– were measured. We conducted frequency analysis using theFFT (periodogram); the number of data was 2,048, with 512overlap data. The experimental results are shown in Fig. 3.

Figure 3(a) shows that the elements of the sensor’s noise andphysiological tremor have low frequency, and (b) indicates thatthe power spectrum of the MMG signals generated by musclecontraction with hand grasping is included in the range ofabout 20 to 160 Hz. Fig. 3(c) also indicates that the frequencycomponent of the MAC signals is contained in the range of

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m/s

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0

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m/s

2]

(b) MMG signals

(a) Acceleration signals

0 4 8Time [s]

0

0.4

0.2

Dis

pla

cem

ent

[m]

(c) Hand displacement

0 4 8Time [s]

0 4 8Time [s]

Fig. 4. Measured and extracted signals (raw acceleration, MMG, andestimated hand displacement)

about 0 to 15 Hz. In addition, Fig. 3(d) shows that the twofrequency elements of the MAC and MMG signals can beobserved in the low- and high-frequency ranges at the sametime. Using a second-order band-pass Butterworth filter, theseparated signals are next shown in Fig. 4, in which figures(a) to (c) describe the MAC signals, the MMG signals and theestimated hand displacement, respectively. The parameters ofthe band-pass filter are set as fMl = 30 Hz and fMh = 150 Hzfor the separation of the MMG signals. The gray area indicatesthe time during which the hand is grasping. In the figure,the MAC and MMG signals are generated when the hand isbeing physically moved and the hand is grasping, respectively,and these signals can be separated using the band-pass filter.Furthermore, hand displacement can be estimated from theMAC signals; we therefore concluded that MMG signalsand hand displacement could be measured and extracted asmutually independent.

Moreover, to confirm the MMG characteristics for muscleactivation, comparison experiments for EMG and MMG sig-nals were conducted. For the measurement of EMG signals,an EMG electrode was attached to the skin surface of the rightforearm (near the flexor carpi radialis, as close as possible tothe measurement position of the acceleration signals). Exam-ples of the measured signals are shown in Fig. 5. The grayareas indicate the period during motion (hand grasping). Fromthe figure, it is observed that the MMG signals change witha tendency similar to that of EMG signals. The coefficientof correlation between the smoothed and rectified EMG andthe smoothed and rectified MMG is 0.945. These resultsindicate that the muscle activation level can be estimated byMMG signals through separation from acceleration signals

−1

−0.5

0

0.5

1

−1

−0.5

0

0.5

1

(a) EMG signals

Time [s]

Time [s]

(b) MMG signals

0 0.5 1.0 1.5 2.0

0 0.5 1.0 2.01.5

Fig. 5. EMG and MMG signals measured during hand grasping motion

1: Hand opening 2: Hand grasping 3: Wrist flexion 4: Wrist extension

Fig. 6. Forearm motions used in the experiments

with almost the same accuracy as EMG measurement.

B. Estimation of motion

We conducted experiments to demonstrate that it is possibleto estimate the subjects’ intended motion with MMG signalsand ascertain the direction of hand movement using MACsignals. In these experiments, each subject performed thefour motions (M = 4; hand opening, grasping, flexion andextension) shown in Fig. 5. The parameters of the band-passfilter for MMG signal separation were set as fMl = 30 Hzand fMh = 150 Hz, and the MMG signals were smoothedwith a low-pass filter cut-off at fcut = 0.5 Hz for extractionof the feature vector. In the learning process of the LLGMN,200 patterns were randomly selected from MMG signals foreach motion, and a total of M × 200 patterns were used asteacher vectors. In addition, the threshold Hth was set as 0.4,Fth as 0.25, and T istb as 400. In the ACP, the cut-off fint

was set as 0.7, the phase-lag element tdel as 0.5 s, and thevelocity threshold Vth as 0.2 m/s. The subjects were asked toconduct linear reciprocating movement with the upper arm andto move along the y-axis as precisely as possible.

The experimental results obtained from Subject E are shownin Fig. 7, which outlines the MAC signals of the triaxial accel-eration sensor, the MMG signals of the uniaxial accelerationsensors and triaxial acceleration sensor (z-axis), force infor-

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8.0 24.020.016.012.00.0

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]

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5

0.0

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Discriminated Misdiscriminated

SUS

SUS: Suspended motion

Ch. 4

4.0

Fig. 7. Classification results using MMG patterns (Subject E)

mation FMMG(t), entropy H(t), the discrimination results,and the estimated hand displacement by the MAC signals.The gray areas indicate that there was no motion because theforce information FMMG(t) was below the threshold Fth. TheMMG patterns seen during each motion are also shown inFig. 8. The solid lines and shaded areas describe the meanvalue and standard deviation of the MMG patterns. From thesefigures, it is observed that different patterns can be measuredaccording to each motion, and these patterns can be classifiedusing the proposed method. Furthermore, Fig. 7 indicates thathand displacement and the hand movement direction can beestimated from the acceleration signals with little time lagduring control of the manipulator’s upper-arm part.

The average discrimination rates and standard deviationfor all subjects’ motions are shown in Table I. The numberof trials for each motion was five. Here, Subject A hadextensive experience in MMG control, while Subjects B –E no experience. The table indicates that the discriminationrates were high for each subject, although the beginners’ rateswere lower than that of Subject A. The average discriminationrate for all the subjects was 94.3 %. The experimental resultsindicate that MMG patterns can be classified correctly andused to estimate operator motion.

0

0.2

0.4x

x

x

x

1

2

3

4

0

0.2

0.4

0

0.2

0.4

0

0.2

0.4

1: Hand opening

2: Hand grasping

3: Wrist flexion

4: Wrist extension

Fig. 8. MMG patterns during forearm motions (subject E)

TABLE IMOTION DISCRIMINATION RATES

Subject Subject

A

B

D

E

TotalC

94.04 3.42

99.20 1.23

96.30 2.61

89.64 2.50

92.38 7.34

94.31 3.42

(Ave SD %)Disc. rates

(Ave SD %)Disc. rates

C. Examples of the manipulator control

To ascertain the effectiveness of the proposed controlmethod, we used it in manipulator control experiments. Figure9 shows examples of motion pictures taken during manipulatorcontrol. Figures 9 (a) and (b) represent control of the forearmpart using the MMG signals of an amputee and the upper-arm part using the MAC signals of a normal subject. Theplotted circles track the trajectories of the triaxial accelerationsensor attached to the subject’s forearm and the manipulator’send-point with time interval of 0.1 s. From Fig. 9(a), itis confirmed that the forearm part (prosthetic hand) can becontrolled by classifying the amputee’s intended motion usingMMG signals. Furthermore, it is observed that the upper-arm part of the manipulator is moved in response to the armmovement conducted by the subject (Fig. 9(b)). These resultslead us to conclude that MMG patterns could be used toestimate the amputee’s intended motion, and that MAC signalscould be used to determine the large movement of the upper-arm conducted by the operator for control of the manipulator’supper-arm part.

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a−1

b−4

(a) Forearm contol by an amputee

(b) Upper-arm control by a normal subject

1: Opening

Accelerationsensor

a−2

2: Grasping

a−3

3: Flexion

a−4

4: Extension

b−3b−2b−1

Fig. 9. Examples of the motion pictures during manipulator control

IV. CONCLUSION

This paper proposes a method whereby MMG and MACsignals can be measured at the same time and used as abase control method for human-assisting manipulators. In theexperiments performed, an appropriate filter was set for signalsmeasured from acceleration sensors, and it was confirmed thatMMG signals (which reflect muscle activity) and MAC signals(which reflect physical movement) could be separated. Bydiscriminating the separated MMG signals using an LLGMN,we also confirmed that the operator’s intended motion couldbe estimated, and showed that the hand displacement of theoperator could be estimated from MAC signals measured usingacceleration sensors only. Moreover, the acceleration sensorsdo not have to be attached directly to the skin surface [6]; thismeans that the measured signals are not affected by changesin skin impedance, and the user can operate the manipulatoreasily with less stress for long periods.

MMG signal research is a relatively new field, and thereare few reports on studies involving interfaces such as the oneexamined in this paper. In future work, we would like to createan algorithm to enable adaptation to pattern changes resultingfrom fatigue and long periods of discrimination in order todevelop a rehabilitation system using this technique, and toapply the method to other computer interfaces.

ACKNOWLEDGMENT

The authors would like to express their gratitude to Mr.Tomoyuki Ichiza for his efforts in the experiments. This studywas partially supported by the 21st Century COE Programof JSPS (Japan Society for the Promotion of Science) on“Hyper-Human Technology toward the 21st Century IndustrialRevolution”.

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