EMG BASED WRIST REHABILITATIONUSING PNEUMATIC PARALLEL MANIPULATOR

Masahiro TAKAIWA* , Toshiro NORITSUGU*, Daisuke SASAKI*

* The Graduate School of Natural Science and TechnologyOkayama University

3-1-1 Tsusimanaka Kitaku Okayama, 700-8530 Japan(E-mail:takaiwa@sys.okayama-u.ac.jp)

ABSTRACT

To support wrist rehabilitation mechanically, we propose introducing a pneumatic parallel manipulator thatprovides enough degrees of freedom (D.O.F.)corresponding to wrist motion and is inherent by compliant thanksto air compressibility. We also propose electromyographically based training in which a payload set between thejoint angle and the muscle to be trained strengthens this muscle directly. We further propose detecting musclefatigue based on EMG frequency analysis. The effectiveness of our proposals is confirmed in experiments.

KEY WORDSWrist Rehabilitation, Pneumatic Parallel Manipulator,EMG,MPF

INTRODUCTION

Although 2008 physical therapy white paper[1] statesthat about 12,000 of rehabilitation facilities and35,000 of physiotherapist (P.T.) are currently avail-able in Japan, at least 80,000 P.T. are needed toserve all rehabilitation facilities equally. The use ofrobot technology is expected to cope with the short-ages of nursing labors in a medical/welfare fields andsome mechanical rehabilitation devices have been de-veloped up to now[2][3][4][5][6].In our focus on mechanical wrist rehabilitation de-

vices motion of human wrist joint and aim at de-veloping a mechanical device to support a rehabili-tation training instead of P.T. A pneumatic parallelmanipulator[7] is introduced from a view that it has 6D.O.F. sufficient to correspond to complex wrist mo-tion and has back-drivability resulted from air com-pressibility, which works as safe function.In generally, wrist rehabilitation with mechanical

system is implemented by giving a payload betweenjoint torque and joint angle/joint angular velocity.We introduce a surface electromyogram (hereaftercalled EMG) signal instead of a joint torque into thewrist rehabilitation[9][10]. Owing to a rehabilitation

based on an EMG, we can select a corresponding mus-cle and give it a training intensively. EMG signal isalso well known to be used as an index of a fatigue,which means a possibility of a rehabilitation with fa-tigue consideration.

In Japan, medical services under health insurancewere limited to 180 days. Using a mechanical de-vice with the proposed scheme, patients can continuethe training even in their home. The validity of theproposed system are confirmed through some experi-ments.

DEVELOPED WRIST REHABILITATIONDEVICE

A pneumatic parallel manipulator shown in Figure 1(a) is introduced as a wrist rehabilitation device[11]from view that its multiple D.O.F. can correspond tothe motion of human wrist joint[12][13]. 6 low frictiontype pneumatic cylinders(Airpel Co. Ltd., 9.3 mmin internal diameter, 150 mm in rod stroke) are em-ployed as driving actuators to form so called Stewarttype platform[14]. Figure 1(b) shows the pneumaticdriving circuit. The pressure in each cylinder’s cham-ber, p1, p2 are detected by a pressure sensor and the

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(a)overview

u

ℓ

i

p1 A1

p2 A2

fℓ

converter

Computer

Pressurelink

Pneumatic

Servo valve

encoder m

counter

Pressure

ps

pa

A/D

sensor

sensorcylinder6 axisforce/momentsensor

D/Aconverter

(b)pneumatic driving circuit

(c)schematic diagram

supinationpronation

extensionflexion

radial flexion

ulnar flexion

(d)wrist motion

Figure 1: Wrist rehabilitation device

displacement of a piston rod ` is measured by a wiretype rotary encoder(0.025mm in resolution). A pres-sure in chambers are regulated by a flow control typeservo valve(FESTO MPYE-5), where a supply pres-sure is set to 500 kPa. Control algorithm is imple-

Table 1: Control parameters

Tp,Tpn Time constant of pressure responseKp,Kpn Steady gain of pressure response

Kv Steady gain between piston velocityand pressure

m,mn Equivalent mass for one cylinderb, bn Viscous coefficientfh Force/moment applied by patientfpt Force/moment applied by P.T.fe Force equivalently applied on a linkfs Force/moment measured by a sensor

A1,A2 cross sectional area of head/piston sidep1,p2 air pressure in head/piston side

` displacement of piston rodJ Jacobi matrix

Tq,Tpq Time constant of filteru control input (input voltage of valve)

mented on RTAI a real-time extension of Linux withsampling interval of 5 ms.

Figure 1 (c) shows the schematic diagram of themanipulator. The position/orientation of the upperplatform is expressed by a hand coordinate frameh = [x, y, z, φ, θ, ψ]T using roll-pitch-yaw angle no-tation. The origin of hand coordinate frame h is setat an above of a center point of an upper platformof a manipulator, which is the same with that of thewrist joint. A patient put its forearm above the upperplatform along with x axis of manipulator and receiverehabilitation exercise by holding a jig mechanicallyattached with a 6-axis force/moment sensor equippedon an upper platform.

Similarly a link vector is defined as ` = [`a, ...., `f ]T

with an element of a displacement of each piston rod.Force/moment vector at an origin of h is defined as

fh = [fheT |τhe

T ]T = [fx, fy, fz, |τφ, τθ, τψ]T , whichis obtained through coordinate transformation basedon the measured force/moment with a force/momentsensor. Also, fh acts on a piston rod as an externalforce fe, which satisfy the following relation from aprinciple of a virtual work.

fh = JTfe (1)

, where J is a Jacobi matrix which forms the nextrelation.

d`

dt= J

dh

dt(2)

Figure 1(d) shows a wrist motion, where prona-tion/supination, radial flexion/ulnar flexion and flex-ion/extension motion correspond to ψ, θ and φ, re-spectively.

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Figure 3: Aspect of rehabilitation based on EMG

WRIST REHABILITATION BASED ONEMG

In generally, rehabilitation is implemented basedon the relation between an applied torque and a jointangle/angular velocity.In this study, we introduce an EMG for a wrist

rehabilitation. The EMG based rehabilitation allowus to train a muscle we want to train and evaluatea property of a muscle directly, which is said to beeffective in the early period of rehabilitation after astroke.Figure 2 (a) shows a control system for an EMG

based rehabilitation. The position based impedancecontrol system is employed and the input signal to theimpedance model Imp is an integral of EMG shown inEq.(3) as the index of muscle force. Figure (b) showsa force generation control system included in figure(a).

iEMG(t) =

∫ t

t−T|EMG(p)|dp (3)

where an integral time T is set to be 1.0 s. The controlparameters are described in Table 1.Figure 3 shows an aspect of wrist rehabilitation

based on EMG, where a patient with EMG sensorput their palm into a mechanical jig attached on themanipulator. Figure 4 shows the align of muscles ina forearm, which are explained in Table 2.

EXPERIMENTAL RESULTS

Stiffness as payload

Figure 5 shows experimental results for setting stiff-ness (Imp = K) between extension/flexion (φ) an-gle and the antagonist muscle (extensor carpi radialislongus and flexor carpi ulnaris), which are dominantmuscles for that direction. Figure (a) (b) correspondto the extention/flexion joint angle and iEMG of theboth muscles, respectively. In figure (b), we can seethe antagonist muscles are activated in tern. In fig-ure (a), red line shows the desired angle calculated

(a)Muscle for Flexion

(b)Muscle for Extension

Figure 4: Muscle for Flexion/Extension

by dividing iEMG with stiffness K. An actual angleshown by blue line agrees with its desired one, whichshows the reference stiffness is well realized.

Figure 6 shows the results of the same experimentwith Figure 5, except for the motion direction ischanged to radial flexion/ulnar flexion. The proposedtraining method correspond to the various directionin wrist motion.

EFFECTIVENESS FOR AVOIDING OFTRICK MOTION

A trick motion sometimes becomes matter in a re-habilitation training. A trick motion means a motionwhich is implemented not by correct muscle but byanother muscle. For example, wrist extension motionis implemented by a motion of whole forearm with-out using a corresponding muscle. This kind of trickmotion occurs sometimes due to the fatigue and itis not easy to avoid under the general torque basedrehabilitation.

Figure 7 shows the same experiment with Figure 5except that the input of impedance model is not aniEMG but a wrist joint torque. In the Figure 7, wecan confirm the proper iEMG since trick motion isnot occurred in this case.

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Generation ForceControl System

Pk(s)Plant

Q(s)1

(1 + Tqs)2

Filter

L(s)H(s)

Hr(s)

J

J−T

P−1n (s)Nominal Model

Convergence Calculation(Newton-Raphson Method)

Fd(s)

Kp + Kds

sL(s)

sLd(s)

PD Controller

mns2 + bns

1

ms + b

1

s

applied by patient

Fg(s)+

−

+

−

+

−

+

−

Fh(s)

F^

l(s)

Hd(s)

+

Fl(s)

I -1 mp

−

bn

iEMG

(a) EMG based rehabilitation control system

1

KpnA1

Fd(s) U (s) Pressure transferpart Eq.(2.1)

sL(s)

A1

P1(s)

A2

P2(s)

Fg(s)

+ +

+

-

- -

Kv

Kp

Kp2+ Ki1s

PI Controller

(b) generation force control system

Figure 2: EMG based rehabilitation control system

Table 2: Muscles in forearm

motion dominant muscle symbol support muscle symbol

Flexion Flexor carpi ulnaris A Flexor digitorum superficialisFlexor carpi radialis etc. B Flexor pollicis longus etc.

Extention Extensor carpi radialis longus C Extensor digittorum EExtensor carpi ulnaris etc. D Extensor pollicis longus etc.

Radial flexion Extensor carpi radialis longus C Flexor carpi radialis BExtensor carpi rasialis brevis Flexor pollicis longus etc.

Ulnar flexion Flexor carpi ulnaris AExtensor carpi ulnaris

In the mean while, Figure 8 shows the case of trickmotion, where iEMG is low since the muscle is notactivated in spite that angle and torque are largevalue. Apparently it seems to be a correct rehabilita-tion motion, but actually there is little effectiveness.Therefore it is not easy to avoid trick motion if thetorque based control system is employed.

Figure 9 shows the case of trick motion but the con-trol system is based on iEMG as proposed in Figure2. In the figure (b), large joint torque is applied butthe joint angle shown in the figure (a) is not producedsince the iEMG is low (muscle is not activated). Ba-sically, in the EMG based system, rehabilitation mo-tion is not executed even in case of trick motion.

EFFECTIVENESS FOR FATIGUEEVALUATION

In generally, a rehabilitation training is carried outfor a long time, therefore it is required for a controlsystem to consider a fatigue of a patient. It is wellknown that a mean power frequency (MPF) of EMGshifts to the low frequency range along with an in-crease of fatigue of a muscle[15].

Figure 10(a) shows a MPF of a EMG of the exten-sor digitorm muscle during continuous squeezing forabout 2 minute. A black line is the MPF of EMG cal-culated using a FFT function in a GSL (GNU ScienceLibrary), where the calculation is implemented ev-

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0 10 20

−0.5

0

0.5

1 Angle Reference angle

Time [s]

Ang

le [r

ad]

(a) extention/flexion joint angle

0 10 20

0

0.01

0.02

0.03iEMG of Extensor carpi radialis longusiEMG of Flexor carpi ulnaris

Time [s]

iEM

G

(b) iEMG of antagonist muscle

Figure 5: Rehabilitation with stiffness control (forflexion/extention: φ)

0 10 20

−0.4

0.1

0.6

Angle Reference angle

Time [s]

Ang

le [r

ad]

(a) radial flexion/ulnar flexion joint angle

0 10 200

0.01

0.02iEMG of Extensor carpi radialis longusiEMG of Flexor carpi ulnaris

Time [s]

iEM

G

(b) iEMG of antagonist muscle

Figure 6: Rehabilitation with stiffness control (forradial flexion/ulnar flexion: θ)

ery 200 sampling period(=1.0s). A GLS can be usedby calling from rehabilitation program implementedwith C code and owing to the multi-task function ofreal time Linux. A gray line shows the raw EMG

0 10

0

0.2

0.4

0.6

Time [s]

Ang

le [r

ad]

(a)Angle

0 10

0

1000

0

0.02

0.04

Time[s]

Tor

que

[Nm

m]

iEM

G

TorqueiEMG

(b)Torque and iEMG

Figure 7: Rehabilitation without trick motion (con-trol with torque)

0 10

0

0.2

0.4

0.6

Time [s]

Ang

le [r

ad]

(a)Angle

0 10

0

1000

0

0.02

0.04

Time [s]

Tor

que

[Nm

m]

iEM

G

TorqueiEMG

(b)Torque and iEMG

Figure 8: Rehabilitation with trick motion (controlwith torque)

data indicates that a muscle produce a force contin-uously for 2 min. We can confirm MPF deterioratesgradually to the low frequency range according to thetime.

This 2 min. trial is repeated 5 times with 1 min.interval. Figure 10(b) shows the comparison with theresult of 5th time. The slope of the line in the case of5th trial is larger than that of first one, which meansthe fatigue is increased according to the progress of

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0 10

0

0.2

0.4

Time [s]

Ang

le [r

ad]

(a)Angle

0 10

0

1000

2000

0

0.02

0.04

0.06

Time [s]

Tor

que

[Nm

m]

iEM

G

TorqueiEMG

(b)Torque and iEMG

Figure 9: Rehabilitation with trick motion (controlwith EMG)

0 100

−0.4

0.1

0.6

1.1

0

20

40

60

80

Time [s]

EM

G

MP

F[H

z]

EMG MPF

(a)EMG and MPF

0 1000

20

40

60

MPF (1st tryal)MPF (5th tryal)

Time [s]

MP

F[H

z]

(b)Variation of MPF

Figure 10: Fatigue index with MPF

training motion.

Using the proposed scheme, a P.T. and a patientcan confirm the current fatigue property withoutstopping the rehabilitation training quantitativelyand it shows the possibility of regulating a rehabil-itation motion based on a fatigue index.

CONCLUSION

In this study, a pneumatic parallel manipulator isintroduced as a rehabilitation equipment for humanwrist joint from a view point of its remarkable fea-tures, such as, multiple D.O.F. suitable for complexwrist motion and safety property due to the air com-pressibility.

We propose a rehabilitation system based on anEMG in order to give training to a muscle directly.Using the proposed EMG based rehabilitation, thefollowing effectiveness are obtained.

1. We can train a muscle selectively and intensivelyfor variable payload.

2. We can select the pair of a motion and a muscleowing to the multiple D.O.F. robotic mechanism,which allow us to train a muscle even for a mo-tion which the muscle is not dominant for.

3. We can avoid the trick motion since it is notbasically occurred in the proposed EMG basedrehabilitation.

4. We can evaluate the fatigue of a patient quanti-tatively in parallel with a rehabilitation trainingusing a real time frequency analysis of EMG.

A concrete evaluation of the proposed system at arehabilitation facility and the more improvement ofrehabilitation method based on EMG are the matterto be settled at present as future works.

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