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Proceedings of the 2000 I E E E Internatlorial Mol khhopon Robot and Human Intrracttve CommuriicatmliOsaka. Japan - September 27-29 2000

A W earable Robotic Arm with High Force-Reflection C apabilityYounkoo Jeong, Dongjoon Lee, Kyunghwan Kim, and JongO h Park

Korea Institute of Science & TechnologyP.O.Box 131, Cheongryang, Seoul, Korea, 130-650

AbstractMany exoskeletal robotic arms have been developedfo r teleoperation having for ce reflection. They can measureoperators arm motion and apply reactive forc e to theoperator as well. The previous research has emphasizedcontrol performance of m otion tracking and force reflectionmainly but ignored how comfortable an operator feels, thatis, issues of human wearability. Most robotic arms aredesigned heavily in weight and give an operator excessfatig ue during the teleoperation. In this paper, w e proposea new robotic arm that satisfies high w earability and highforce-reflection simultaneously. In order to accomplishthese two objectives, the proposed arm has parallelmechanism, one fo r a forearm and the other fo r a brachium.The parallel m echanism has an analogy to hum an muscularstructure in which many extensors and flexors interact witheach other and generate torque. The prismatic joints in theparallel mechanism act as virtual human muscles anddetermine join t torque by contraction and relaxation. Theconfiguration of the prismatic joints enhances humanwearability because all the joint s are placed on thecircumference of cylinders. Consequent&, this circularconfiguration distributes internal force over an arm andthus contributes to reduce human fatig ue during operation.In addition, the robotic arm does n ot make any singularityin its kinematics. Pneumatic actuators are used to be incompliance with human motion smooth&. The maximumpower of each actuator is high enough to resist humanmuscular strength. The kinematic parameters in the roboticarm are selected to maximize operators workspace whileconsidering performance of for ce reflection.

1. IntroductionA wearable robotic arm is an exo skeletal robotic armto follow human arm motion and to provide human armwith force feedback if needed. It can be used for human-power amplification, medical rehabilitation and VRexperience. Master arms used in teleoperation can be usedto embody concept of the wearable robotic arm.Previous researches have emphasized controlperformance of motion trac king and force reflectio n mainlybut ignored how comfortable an operator feels, that is,

issues of human wearability [1,2]. Because most roboticarms are heavy in w eight and fixed at a structure such as awall or a pole, operators arm m otion is limited in a spaceand the operator is apt to feel excess fatigue. This restrictsits range of applications, especially, in the area of medicalrehabilitation or VR experience where issues of humanwearability are very important.We already developed two wearable robotic arms thathave h igh human wearability [3,4]. The robo tic arms haveparallel mechanism by analogy with human muscles andthe robotic arms are attached to operators arm. Operatorwearing these robotic arms can move freely duringoperation and this reduces operators fatigue. Andisadvantage of the previous versions is computationalcomplexity in kinematics and low force-reflectioncapability [5] .The proposed wearable robotic arm is designed tohave no t only high wearability, but also simple kinematicsand high force-reflection capability. To simplify itskinematics without degrading wearability, rotationalmotions of human arm are decoupled from the parallelmechanism. In ad dition, Pneumatic actuators are used to bein compliance with human motion smoothly and to improveforce-reflection capability. A new proportional controlvalve is developed for the pneum atic control system.2. Design Co ncepts and Kinem atics Analysis2.1 Design Concepts2.1.1 Anatomic Analysis of Human ArmMovement

Human arm shown in Fig.1 has 7 degrees of freedomwhen translation of shoulder joint is neglected. Shoulderjoint has three degrees of freedom, that is, flexiodextension,abductiodadduction and medial rotatiodlateral rotation.Elbow joint has two degrees of freedom, that is,flexiodex tension and pronatiodsupination. Wrist joint has2 degrees of freedom, that is, flexiodextension andabductiodadduction [ 6 ] .All these motions originate frommuscular movement in which many extensors and flexorsaround bones interact with each other and generate torque.The parallel mechanism has an analogy to this humanmuscular movement. The prismatic joints in the parallel

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mechanism may act as virtual human muscles and affectoperator's muscular strength by contraction and relaxation.The co nfiguration of the prismatic joints in parallelmechanism enhances human wearability because all thejoints are placed on the surface of human arm.Consequently, this distributes internal force over an armand thus contributes to reduce human fatigue duringoperation.

F = A(n - j - 1 ) + C J ; (1),Here,2 : DOF of the space in which a mechanism is intended toiz : numberof links in a mechanism , including the fixed base.j : number of joints in a mechanism, assuming that all the jointsA. : degree of relative motion permitted by joint

function.are binary.

Fig. 2 Configuration of the proposed robotic arm.Fig. 1 Muscles of the thorax and right upper extremity.2.1.2 Mecha nism Design

It is possible to embody 3 degrees of freedom atshoulder joint using a parallel mechanism based on thestewart platform. The rotational motion along the arm(medial rotatiodlateral rotation), however, increases thecomplexity of parallel mechanism as w ell as its kinematics.As the complexity of kinematics increases, required time tosolve it increases and singularity problem beco mes serious.To avoid this, a revolute joint is attached at moving plate ofparallel mechanism that has only 2 degrees of freedom forflexiodextension and abductiodadduction. The parallelmechanism for shoulder joint is compo sed of 4 R-P-S(Revolute, Prismatic and Spherical) joints as sho wn in Fig.2. The parallel mechanism for wrist joint is com posed of 3R-P-S joints and it has 2 degrees of freedom forflexiodextension and abductiodadduction of wrist joint.For pronatiodsupination of elbow joint, a revolute joint isembedded between the elbow joint (revolute joint) and baseplate of the parallel mechanism for wrist part.2.2 Kinem atics Analysis

Total degrees of freedom for the propo sed robotic armis shown in Fig. 3an d two parallel mechanisms are shownin Fig. 4.Degrees of freedom of m oving plate for parallelmechanism can be calculated with Gr bler's criterion (1).Both degrees of freedom of moving plate for shoulderparallel mechanism and wrist parallel mechanism arecalculated as two degrees of freedom like eq. (2).

!

Fig. 3 Kinematic model of the proposed robotic arm.

(a) @)Fig. 4 Simplified kinematic model for upper and lower arms. (a)4 RPS (shoulder). @) 3 RPS (wrist).2.2.1 Inverse kinematics

The rotation matrix from plate A to B can beexpressed in terms of the direction cosines of U, v andw as equation (3).U X v x wx

(3)

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2

*"4 B"I(c)Fig. 5 Kinematic model for an upper arm. (a) coordinate system.(b) lower plate. (c) upper plate.

Position vectors of points 4.an d Bi in the coordinatesystems A and B can be expressed as equations (4) and (5).

a,, = [g,O,OITa,, = [h cos@,h sin e,O]'8,s = [-h , 0, 01' (4)au4= [ g cos(n +e ) , g s in ( r +e),olT

(5)

From equations (4), (5) and (6), The position vectorq i of B ja nd length vector of prismatic joints can berepresented as equations (6 ) and (7) respectively.

From the vector (7), length o f prismatic joints can beobtained as equations (8)-(1 1):d:, = < P i + P f y + P f ; ) + m , 2 ( u i + u ~ y + u f z )

+2m, (Pur% + P , U y + Pur% 1-2m,guux-2guP, +g u 2

+2mu(P,vx+ PUYVY + P U V , )dt , = ( p ~ + p : y + p ~ ) + m Z ( v ~ + v : + v ~ )

-2h,m,(v, cosO+vy s ine )-2h, (p, c o s O + p u y s i n 8 ) +h,'

df , = ( p i + pfy +p i ) + mu 2(U,'+us +u: )-2% (P IU%+ PuyUy + Pur% 1-2m,h,u, -2h,pu +h,'

di d= ( p k + p iy+ p i )+mu2(v i + v: +vf )-2mu (PIUVX + PuyVy + P m V z )-2g,m, (v, co s8+vysin 0)- 2g , ( p , c os 8 + puysin e )+gu2

(1 1)

2.2.2 Forward kinematicsSince the diagonal of the plate B is mu and the

position vector of Bi is qi ,equation ( 12) can be derivedfrom Pitagoras' theory.

T[ q u i - q u i + l ] [ q u i - q u i + , ] ='mu2 (12)If ang les between prismatic joints and plate A are

defined as vector, #i , equation (13) is derived fromtrigono metric identities.

e,,,,cos #ui co s #ui+I -k eu2i sinhi sin @ d + l (13)+e,,,,cos @ui +e,,4icos#ui+l +eusi=0

(14)1-ti ' 2ti1+ t ; 1 + ti'cos#i =- sin#i =-When [t sub I] is defined as equation (14), equation

(13)can be represented by [t sub I] as equation (15).~, ,~~t ,z t i2 ,~+ +~,~~t,?+,+ Eu4ititi+l+ = O (15)

-e,, = e,,-e,,-e,, + e,,,Where,Z 2 , = -eli -e3,+e,, + e,,,Z33i= -e,, + e,,-e,, +e,,,Z,, = 4eZi,Z,, = e,,+e,, +e,, +e,,

Equation (15) can be solved by Sylver dialyticelimin ation method[7], a nd as the result, tilting angle vectorof the prismatic joints ( (bUu2,#u3, #,,4 ) is obta ined. Fromthis vector, the position of upper plate B can be calculatedsuccessively.For wrist parallel mechanism, equations of forwardkinematics and inverse kinematics can be derived andsolved using the same m ethod.By solving equations of forward kinematics forparallel mechanisms, a l , a2, a6an d a7are obtained(Fig. 3) . By sensing angles of revolute joints at shoulder,elbow and wrist, a, ,a 4 ,as are obtained. With thesevalues and dimension of the proposed robo tic arm, the total

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2.3 Parameter Design of Parallel mechanisms

Shoulder(stroke)4 20"Cylinder (90")

Spec. 4 16"(65")Joint space -44 -44degMax..torque 17.8"

In parametric design of parallel mechanisms, diagonalof upper plate (mu), diagonal ratio of base plate (g, / h, )and distance between two plates (P, ) are selected as designparameters. To define the performance index, maximumjoint range, maximum joint torque and uniformity factor (afactor related to position of maximum torque) are used.Since maximum joint range affects the dexterity of roboticarm, the performance index is defined to give high weightfactor to maximum joint range. The performance index(P1)is defined as equation (19) and the maximum value of PIwas 1.Where, PI = JR*W l+ JT*W 2 +UF*W3 (19)

JRJTUF :normalized uniform factorW 1W2W3

: normalized maximum joint range: normalized maximum joint torque: weight factor of JR = 0.5:weight factor of JT = 0.3: weight factor of UF = 0.2

Elbow wrist(stroke) (stroke)4 16" 9 10"(33") (50")0-97deg -30-30deg7.1" 4"

In consideration of total weight and wearability,limitation of diameter of pneumatic cylinder is set as below20" for shoulder part, 16 mm for elbow part and 10 mmfor wrist part. Fig. 6 shows the maximum joint range andmaximum joint torque for shoulder part at various diagonalratios. According to PI, diagonal ratio (g, / h, ) is selectedas 2.1. A t that value, maximum joint range is as about 97degree and the maximum joint torque is about 17.8 Nm.

............

...... ...... ........ . . . . .. . . . . . .1.8 I 7 18 1.9 2 2 1 2 2 2.3 2.4 25&AFig. 6 Maximum workspace & torque for shoulder4RPSWith similar method, parameters of parallelmechanism for forearm can be designed. Fig. 7 shows thejoint range, joint torque and lengths of prismatic joints atoptimized PI value.

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3. Experimental Results3.1 P neumatic Control ValveThe proposed robotic arm is totally manufactured withengineering plastic except connection part to reduce weight.

The total weight of proposed robotic arm is about 4 kgf. Adummy w earing the proposed robotic arm is shown in Fig.9.A small size proportional control valve that has linearcharacte ristic related to differential pressure is developed tocontrol thrust forces of pneumatic cylinders effectively.This control valve uses two pressure sensors embedded inthe valve body to provide differential pressure signalbetween two ports and this signal is used to maintain thelinear differential pressure characteristic according to inputvoltage. Since the size of the control valve is small (about20*30*10 mm) and its weight is low, it is possible to attachit near to pneumatic cylinder on the robotic arm. Thismakes the length of pneumatic tube between valve andcylinder shorten and as the result of it, the time delay duringthe movem ent of pneumatic cylinder reduces.

VIfiuaIRobot, Slave Robot, Operator Model .._1 ?

- 1 4Fig. 11 Schematic diagram of control system

(1) When the con trol system stops suddenly, the mainpneumatic pressure supply line is cut off automatically byclosing the main cut-off valve.(2) In the shoulder part parallel mechanism, homeposition of one pair pneumatic cylinders of mechanism isdesigned to be opposite to the other. This prevents the mostdangerous posture of a parallel mechanism, which mayoccur when all the length of pneumatic cylinder becomesmaximized.(3) The d esigned joint ranges of robotic arm are withinallowable joint range of hum an arm. So , joint angles ofrobotic arm do not ex ceed reasonable values.3.3 Exp erimental Results

Fig. 9 A dummy wearing the robotic arm

Fig. 10413 way proportional control valve3.2 C ontrol System Design and Fail-safe Method

The schematic diagram for the total control system ofthe proposed robotic arm is shown in Fig. 11. At motiondetection mode, the control system tries to keep d ifferentialpressure of pneumatic cylinders as zero value. This allow soperator to m ove freely during operation with low effort. Atforce reflection mode, the control system calculatesreference thrust forces for each pneumatic cylinder andprovides them to sub-controller to control the differentialpressure of pneumatic cylinder to trace the reference forcevalue.Since the maximum thrust force of pneumatic cylinder isgreat(about 13ON),any failure of control system may cau sedangerous situation. To protect the operator at failure state,the system has several fail-safe methods.

Fig. 12 shows an experimental result when a sinusoidalform of reference differential pressure is inputted andoperators movement is not allowed. This shows goodtracking performance.Fig. 13 shows another experimental result whenreference differential pressure is set as a step-wise formand operators movement is allowed. In this case,operators movement affects thrust force of pneumaticcylinder as disturbance. Even though operator movescontinuously, differential pressure of cylinder is tracingthe reference input with good performance. Whenreference differential pressure is set as zero, operator canmove freely with low effort.

(sinusoidal reference input)

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time(tec)Fig. 13Reflection force tracking test

(step-wise form and operators movement allowed)4. Conclus ionsIn the future, a wearable robotic arm will be used notonly in the area of teleoperation, but also in areas of amedical rehabilitation, human power amplification, andentertainment.In this research, a new wearable robotic arm wasintroduced, that is designed in consideration of not only thewearability but also force-reflection capability. As theresults, the possibility of a wearable robotic arm withenough workspace and force reflection capability wasverified. A development and verification of task algorithmfor complex tasks will be continued and discussedafterwards.AcknowledgementAuthor would like to thank Dr.Yehsun Hong and Mr.Sibouk Ryu for their precious help in m aking proportionalcontrol valve.References[13 Akito Nakai, etc., Developm ent of 7 DO F Ex oskeletonType Haptic Interface, Journal of Robotics Society ofJapan,vol 17,110. 8,pp.1126-1133, 1999.[2] N. Tsagarakis, and D.G. C aldwell, A 7DO F PneumaticMuscle Actuator @MA) Powered Exoskeleton, RO -MAN99, 1999.[3] Jonghyun choi, Jeungtea Kim, Dongshin, ChongwonLee, Jong-Oh Park, Jang-Hyun Park, Design andCharacteristic Analysis of 7 D OF Hybrid Master Arm wothHuman Kinematics, Proceeding of the ASME DynamicSystems and Control Division, DSC-Vol. 64, ppl95-205,1998.[4] Jong-Hyun Ch oi, Ji-Heuk Song, Jung-Tae Kim, hong-Won L ee, Jong-Oh Park, Jang-Hyun Park, Human Arm-like Hybrid Master Arm for d exterous teleoperation,Proceedings of the 13th KAC C, pp.1807 - 1810, 1998.[5] Jungtea Kim, etc, Singularity Analysis and itsAvoidence for KIST Hybrid Master Arm, 99 ICAR, pp.[ 6 ] Blandine Calais-Germain,Anatomy of Movement, 1985[7] Lung-Wen Tsai, Robot Analysis, Wiley-Interscience,1999.

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Nome nc l atur e4 :The fixed base of upper arm.B, :The moving platform of upper arm.A, :The fixed base of lower arm.Bl :The moving platform of lower arm.Ai :The position of i th joint in fixed base of upper arm.BUi:The position of i th joint in moving platform of upper arm.a, , :The position vector of i th joint in fixed base of upper arm.b,, :The position vector of i th joint in moving platform of upper arm.d u i:The vector of i th link in upper armJ,, :The direction vector of i th joint in fixed base of upper arm.P, :The position vector of mowing platform of upper ami.qui:The direction vector of i th joint in upper arm.h, :The length of diagonal line in fixed base of upper ami.g, :The length of diagonal line in fixed base of upper ann.m, :The length of diagonal line in moving platform of upper arm.4ui: The tilt angel of ith link in upper arm.ali :The position vector of i th joint in fixed base of lower arm.b,, :The position vector of i th joint in moving platform of lower arm.d,, :The vector of i th link.J,,:The direction vector of i th joint in fixed base of lower arm.PI:The position vector of mowing platform of lower arm.q, , :The direction vector of i th joint in lower arm.h, :The length of diagonal line in fixed base of lower arm.g, :The length of diagonal line in fixed base of upper arm.m,:The length of diagonal line in moving platform of upper arm.(bIi:The tilt angel of i th link in lower arm.8 :The angel with diagonal line in fixed platform of upper arm.

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