Development of a 6DOF Exoskeleton Robot for Human Upper-Limb Motion Assist

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1 Development of a 6DOF Exoskeleton Robot for Human Upper-Limb Motion Assist R. A. R. C. Gopura + and Kazuo Kiguchi ++ Department of Advanced Systems Control Engineering Saga University, Saga, Japan gopura@ieee.org ++ kiguchi@ieee.org Telephone: (+81) , Fax: (+81) Abstract The upper-limb motions are essential to perform the daily activities. This paper proposes a six degree of freedom (6DOF) upper-limb motion power assist exoskeleton robot for shoulder vertical and horizontal motion, elbow motion, forearm supination/pronation motion, wrist motion and wrist radial/ulnar deviation motion. The axes offset of wrist joint and the moving center of rotation (CR) of shoulder joint are applied in the hardware design of the exoskeleton robot. The robot is named as SUEFUL-6 to indicate 6DOF Saga University Exoskeleton For Upper-Limb. The paper describes the anatomy details of upper-limb towards the development of the exoskeleton robot and the hardware design of the exoskeleton robot. Experiments have been performed to evaluate the effectiveness of the hardware design of the proposed exoskeleton robot. I. INTRODUCTION EVERAL countries of the world are facing the problem Sof aged society. The birthrate is decreasing since the fertility rate is declining and the people enjoy longer life since the increasing in life expectancy. Therefore, such society is an aged society (i.e., more percentage of population is aged). In this society, the working population is decreasing and taking care of physically weak individuals (aged injured or disabled people) are becoming a problem. In this situation, robotic technology is supposed to play an important role in the field of welfare and medicine. Several exoskeleton robots and their control methods [1]-[12] have been studied to assist the daily activity and/or rehabilitation of physically weak individuals. Since the upper-limb motions are very important to perform daily activities, many upper-limb exoskeleton robots and their control methods [1]-[10] have been proposed to assist the upper-limb motions. This paper proposes a six DOF upper-limb power-assist exoskeleton robot (SUEFUL-6) for assisting shoulder vertical and horizontal motion, elbow motion, forearm supination/pronation motion, wrist flexion/ extension motion and wrist radial/ulnar deviation motion of physically weak individuals. The axes offset of wrist joint and the moving center of rotation (CR) of shoulder joint are considered in the hardware design of the exoskeleton robot. In addition, the hand-robot interface has been designed not to disturb the finger motions. The paper describes the details of upper-limb anatomy towards the development and the hardware design of the exoskeleton robot. Experiments have been performed to evaluate the effectiveness of the proposed SUEFUL-6. Development of the SUEFUL-6 has been carried out through three stages. In the first stage of development of the SUEFUL-6, the 3DOF upper-limb exoskeleton robot for shoulder vertical and horizontal motion and elbow motion was developed and its control method was studied [2]. The other 3DOF exoskeleton robot (W-EXOS) for forearm supination/pronation motion, wrist motion and wrist radial/ulnar deviation motion was developed and its control methods were studied [3], [4] in the second stage. In the third stage, the 3DOF exoskeleton robot designed in the first stage and the W-EXOS designed in the second stage were combined together to form 6DOF SUEFUL-6 exoskeleton robot. II. UPPER-LIMB ANATOMY TOWARD THE EXOSKELETON ROBOT Human upper-limb mainly consists of shoulder complex, elbow complex and wrist joint. In addition, fingers have several joints. Shoulder complex consists of three bones, the clavicle, scapula and humerus, and four articulations, the glenohumeral, acromioclavicular, sternoclavicular, and scapulothoracic, with the thorax as a stable base (Fig. 1). The sternoclavicular joint is the only joint that connects the shoulder complex to the axial skeleton. Basically, Shoulder joint can be modeled as a ball and socket joint. Therefore, shoulder joint has mainly 3DOF: shoulder horizontal and vertical and interior/exterior rotation. However, center of rotation (CR) of shoulder joint itself is moving with the shoulder motions. The elbow complex includes the elbow joint and the radioulnar joints (Fig. 2). The elbow complex is a compound joint consisting of two kinds of joints: the humeroradial, between the capitulum and radial head, and the humeroulnar, between the trochlea and the trochlear notch of the ulnar. The humeroradial joint is a ball and socket joint. However, its close association with humeroulnar and superior radioulnar /08/$ IEEE ICIAFS08

2 performed around an instantaneous center. The path of the centrode is small, however, the displacement of the instantaneous center of rotation is ignored and the rotation axes for the and ulnar/radial deviation are considered fixed customarily. The axes pass through the capitate, a carpal bone articulating with the third metacarpal. The rotation axes of the and the ulnar/radial deviation are offset about 5 mm [13]. III. HARDWARE DESIGN Fig. 1. Shoulder joint-right side (front view) Fig. 2. Elbow joint A. Requirement of an Upper-Limb Exoskeleton Robot An upper-limb exoskeleton robot should be adaptable to the human upper-limb in terms of segmental lengths and locations of center of rotation. It should have the ability to follow moving center of rotation of the shoulder joint if the exoskeleton assists both forearm and upper-arm. The robot should provide the axes deviation of wrist axis and wrist radial/ulnar axis. Furthermore, the exoskeleton robot should be able to be attached easily to the human upper-limb. The links, cables, pulleys, other hardware components and motors of the exoskeleton robot should be located not to disturb the motion of the robot and the user. The exoskeleton robot should not disturb the motion of the fingers also. As the robot is in directly contact with the human user, the safety requirement is paramount. Moreover, motions of the exoskeleton robot have to be sufficiently flexible and smooth, since the robot is supposed to be used for daily activities of the user. In addition, it should be comfortable and easy to wear. Fig. 3. Wrist joint joint restricts the joint motion from three to two DOFs. The humeroulnar joint is a hinge joint. As a whole, the elbow joint complex allows 2DOF: and pronation/ supination. The wrist joint anatomy is shown in Fig. 3. The wrist, or carpus, is a deformable anatomic entity that connects the hand to the forearm. This is a collection of eight carpal bones and the surrounding soft tissue structure. The wrist contains several joints, including the radiocarpal joint, several intercarpal joints, and five carpometacarpal joints. The radiocarpal joint is a condyloid joint formed by the end of the radius and the three carpal bones: the scaphoid, the lunate, and the triquetrum. The wrist joint possesses 2DOF, flexion/ extension and ulnar/radial deviation. Wrist motions are B. Hardware Design of SUEFUL-6 The proposed SUEFUL-6 is shown in Fig. 4(c). It can be installed on a wheel chair of physically weak persons. Therefore, the user does not feel the weight of the robot. The proposed SUEFUL-6 mainly consists of shoulder motion support part, elbow motion support part, forearm motion support part and wrist motion support part. The robot was developed by assembling the 3DOF exoskeleton robot [Fig. 4(a)] and the W-EXOS [Fig. 4(b)]. The forearm part of the 3DOF exoskeleton robot was replaced with the W-EXOS. Some modifications also have done for previous hardware of the W-EXOS. Thrust bearings are added in between the surfaces of revolute joints of wrist joints of the W-EXOS as shown in Fig. 4(d). The shoulder motion support part of the SUEFUL-6 consists of an upper arm link, driven pulleys for shoulder horizontal motion and shoulder vertical motion, potentiometers, an arm holder, a slider and the mechanism for moving center of rotation (moving CR) [1]. In order to generate 2DOF shoulder motion (shoulder vertical and horizontal flexion/ extension motion), motor pulleys act as driver pulleys and pulleys connected to the shoulder joint act as driven pulleys. The motors for the shoulder motions have been fixed in the separate locations of the frame of the robot as shown in Fig. 6. Motor-1 and motor-3 are the motors for the shoulder

3 (a) First stage - 3DOF exoskeleton robot (b) Second stage - W-EXOS (c) Final stage - 6DOF upper-limb motion power assist (d) Modified part of W-EXOS (top view). X, Y and Z are the exoskeleton robot: SUEFUL-6 direction of forward-backward, up-down and left-right forces Fig. 4. Development stages of the SUEFUL-6 horizontal and vertical motions, respectively. The rotational motions generated in the motors are transferred to the shoulder driven pulleys of the SUEFUL-6 through cable drives. The arm holder is made of thin flexible plastic with magic tape ribbon to hold the user s upper arm. The distance between the arm holder and the CR of the shoulder joint of the exoskeleton is moderately adjusted in accordance with the shoulder motion, in order to cancel out the ill effects caused by the position difference between the CR of the robot shoulder and the human shoulder [1]. The mechanism of CR makes the CR of the robot shoulder joint move behind (farther position from the arm holder) in accordance with the shoulder vertical flexion angle in the case of vertical flexion motion, and move inward (closer position to the arm holder) in accordance with the shoulder horizontal extension angle in the case of horizontal extension motion. Details of mechanism of CR and activation of slider and upper-arm links can be referred in [1].The 1DOF elbow motion assist part of the exoskeleton robot consists of the forearm link-1, pulleys and a potentiometer. The motor pulley acts as driven pulley, to generate elbow motion. The motor (motor-2) for the elbow motion have been fixed in the separate location of the frame of the robot as shown in Fig. 6. The rotational motion generated in the motor-2 is transferred to the elbow driven pulleys of the SUEFUL-6 through a cable drive. The forearm motion support part consists of the forearm cover, the forearm link-2, a DC motor and forearm force sensor, a wrist holder, and torque sensors (strain gauges). The wrist motion support part consists of a link attachment, two DC motors, two drive and driven bevel gear pairs, a palm holder, a wrist force sensor and a wrist link which connects the palm holder and the link attachment as shown in Fig. 4(c). The motor for supination/pronation motion (motor-4) is attached in the outer housing of the forearm cover. The rotational motion generated from the motor is transferred to the inner hollow cylinder of the forearm cover through a spur gear pair (gear ratio-1:3), shown in Fig. 4(b). The outer housing of the forearm cover and the inner hollow cylinder are assembled through two

4 (a) Shoulder vertical motion (c) Elbow motion (b) Shoulder horizontal motion (d) Forearm motion Motion TABLE 1 MOVABLE RANGES Movable range of exoskeleton Average Movable range of Human [Deg] [Deg] Shoulder vertical extension 0 60 Shoulder vertical flexion Shoulder horizon extension 0 30 Shoulder horizontal flexion Elbow extension 0-5 Elbow flexion Forearm pronation Forearm supination Wrist extension Wrist flexion Wrist ulnar deviation Wrist radial deviation Axis Shoulder vertical Shoulder horizontal Elbow Forearm pronation/supination Wrist Wrist ulnar/radial deviation TABLE II ACTUATION OF THE AXES. Cont. Torque (Max.Torque) Motor Type 35(98) Nm Harmonic drive RHS E100DO 8.8(28.4) Nm Harmonic drive RHS T 8.8(28.4) Nm Harmonic drive RHS T 4.2 (8.1) Nm Harmonic drive, super mini RH-8D-6006-E0360DO 0.58(1.38)Nm Harmonic drive, super mini 0.58(1.38)Nm RH-5A AO Harmonic drive, super mini RH-5A AO (e) Wrist (f) Wrist ulnar/radial deviation Fig. 5. Movable range of the exoskeleton robot. bearings such a way that the forearm can be inserted to the hole of the hollow cylinder. Therefore, the inner hollow cylinder of the forearm cover can be rotated with respect to the fixed outer housing to generate forearm pronation/supination motion. The forearm link-1 is attached to the outer housing of the forearm cover through the forearm force sensor as shown in Fig. 4(c). The forearm link-2 is attached to the inner hollow cylinder of the forearm cover. The wrist holder is also attached to the forearm link-2. The wrist holder can be worn to the forearm of the user is shown in the Fig. 4(c). Since the forearm link-2 is attached to the inner hollow cylinder, the forearm link-2 and wrist holder rotates with the rotating inner hollow cylinder. There by the forearm is rotated to generate pronation/supination motion. The motor for wrist motion (motor-5) is fixed on the forearm link-2. The rotational motion generated in the motor-5 is transferred to the wrist axis through the bevel gear pair-1 (gear ratio-1:2) as shown in Fig. 4(b). The forearm link-2 is attached with an L-shaped link attachment using a thrust bearing, a ball bearing and a stepped shaft to form a revolute joint. The link attachment rotates with respect to the forearm link-2. The rotation of link attachment generates the wrist motion and which is transferred to the users hand from the palm holder. The link attachment holds the motor (motor-6) for the wrist radial/ulnar deviation. The rotational motion generated in the motor-6 is transferred to the wrist radial/ulnar axis through the bevel gear pair-2 (gear ratio-1:2) as shown in Fig. 4(b). The wrist link is attached to the link attachment using a thrust bearing, a ball bearing and a stepped shaft to form a revolute joint. The wrist link rotates with respect to the link attachment. The rotation of wrist link generates the wrist radial/ulnar deviation motion and which is transferred to the users hand from the palm holder. The palm holder and the wrist force sensor are attached to the wrist link. The link attachment has been designed to provide the axes deviation of the ulnar/radial axis and the [3]. Considering the safety of the user and the minimally required motion in daily activities the limitation of the movable ranges of the SUEFUL-6 is decided as shown in TABLE 1. The movable ranges of the exoskeleton robot are shown in Fig. 5. Details of the actuation of six axes are shown in TABLE II. IV. EXPERIMENTS The experiments have been performed with a healthy young male subject (29 years old) to confirm that the SUEFUL-6 is able to generate the motions of upper-limb (shoulder vertical

5 (a) Shoulder horizontal flexion (b) Shoulder vertical flexion Fig. 6. Experimental set-up and horizontal motion, elbow flexion/ extension motion, forearm supination/pronation motion, wrist motion and wrist radial/ulnar deviation motion). Experimental set-up is shown in Fig. 6. It consists of the SUEFUL-6, a personal computer with two interface cards [RIF and JIF-171-1, Justware] and motor drivers. In the first experiment, on-off control method has been applied to make sure that the SUEFUL-6 can generate the motions throughout the movable ranges of each individual motion which was limited by mechanical stoppers. In this method, step inputs of desired torques were supplied as torque commands throughout the full movable ranges of each DOF (full movable range of forearm supination, wrist extension and radial deviation are 140 0, and 50 0, respectively), limited by mechanical stoppers. When the robot reached to its mechanical limits of movable ranges, the motion was automatically stopped by the software. Movable ranges obtained for 6DOFs are shown in Fig. 7. The movable ranges obtained from the experiment are similar to that of the movable ranges set from the mechanical stoppers. In the control program, the potentiometers and the force sensors signals have been filtered with second order Butterworth filter with cut off frequency of 8 Hz. In the second experiment, force sensor signal-based control was carried out to generate the upper-limb motions using the forearm and wrist force sensors. The desired forearm and wrist (hand) forces are zero in the force sensor signal-based control. The force control law is written as: u = K ( f f ) (1) f f df f T f J fu f τ = (2) u = K [ f ( f f )] (3) w w dw w f T w J wuw τ = (4) where u f, u w are forearm and wrist force control command vectors, respectively, K f, K w are gain for forearm and wrist force errors, respectively, f df, f dw are forearm and wrist desired (c) Elbow flexion (e) Wrist extension (d) Forearm supination (f) Wrist radial deviation (g)shoulder-wrist combined motion (h) Shoulder-forearm combined motion Fig. 7. Experimental results of on-off control force vectors, respectively and f f, f w are the generated forearm and wrist force vectors respectively measured by forearm and wrist force sensors, τ f, τ w are forearm and wrist joints torque command vectors and J f, J w are forearm and wrist Jacobian matrices, respectively. J f is calculated considering elbow and shoulder joints and J w is calculated considering wrist joint. K f and K w are decided by considering the frictional effect of each joint. The experimental results for some individual motions and combined motions are shown in Fig. 8 and Fig. 9

6 vertical and horizontal motion, elbow motion, wrist motion and wrist radial/ulnar deviation motion) using the force sensors signals. (a) Shoulder horizontal motion (b) Wrist radial deviation IV. CONCLUSION A 6DOF upper-limb exoskeleton robot is proposed to assist the upper-limb motions (shoulder vertical and horizontal, elbow, forearm supination/pronation, wrist and wrist radial/ulnar deviation) of physically weak individuals. Experimental results confirmed that the exoskeleton robot is able to generate the individual and combined motions of the 6DOFs within the designed movable ranges. ACKNOWLEDGMENT The authors gratefully acknowledge the support partially provided for this research by the Japan Society for the Promotion of Science (JSPS) through a Grant-in-Aid for Scientific Research (C) (c) Wrist Fig. 8. Experimental results of force control for individual motions (a) Shoulder motions (b) Shoulder & wrist motions Fig. 9. Experimental results of force control of combined motions respectively. The experimental results show that the SUEFUL-6 can be generated upper-limb motions (shoulder REFERENCES [1] K. Kiguchi, K. Iwami, M. Yasuda, K. Watanabe, and T. Fukuda, An Exoskeletal Robot for Human Shoulder Joint Motion Assist, IEEE/ASME Trans. on Mechatronics, vol.8, no.1, pp , [2] K. Kiguchi, M. H. Rahman, M. Sasaki and K. Teramoto, Development of a 3DOF Mobile Exoskeleton Robot for Human Upper Limb Motion Assist, Robotics and Autonomous Syst., vol.56, no.8, pp , [3] R. A. R. C. Gopura and K. Kiguchi, EMG-Based Control of an Exoskeleton Robot for Human Forearm and Wrist Motion Assist, Proc. of IEEE Int. Conf. on Robotics and Automat., pp , [4] R. A. R. C. Gopura and K. Kiguchi, A Human Forearm and Wrist Motion Assist Exoskeleton Robot with EMG-Based Fuzzy-Neuro Control, Proc. of IEEE RAS/EMBS Int. Conf. on Biomed. Robotics and Biomechatronics, pp , [5] J. C. Perry and J. Rosen, Upper-Limb Powered Exoskeleton Design, IEEE/ASME Trans. on Mechatronics, vol.12, no.4, pp , [6] A. Frisoli, F. Rocchi, S. Marcheschi, A. Dettori, F. Salsedo and M. Bergamasco A new force-feedback arm exoskeleton for haptic interaction in Virtual Environments, Proc. of Eurohaptics Conf. and Sym. on Haptic Interfaces for Virtual Environment and Teleoperator Systems, pp , [7] D. Sasaki, T. Noritsugu and M. Takaiwa, Development of Active Support Splint Driven by Pneumatic Soft Actuator (ASSIST), J. Robotics and Mechatronics, vol.16, pp , [8] T. Nef and R. Riener, ARMin-Design of a Novel Arm Rehabilitation Robot, Proc. of Int. Conf. on Rehabilitation Robotics, pp , [9] J. Rosen, N. Brand, M. B. Fuchs and M. Arcan, A Myosignal -Based Powered Exoskeleton System, IEEE Trans. on Syst., Man Cybern., A, vol.31, no.3, pp , [10] N. G. Tsagarakis and D. C. Caldwell, Development and Control of a Soft-Actuated Exoskeleton for Use in Physiotherapy and Training, J. Autonomous Robots, vol.15, pp.21-33, [11] S. Lee and Y. Sankai, Power Assist Control for Walking Aid with HAL-3 Based on EMG and Impedance Adjustment around Knee Joint, Proc. of IEEE/RSJ Int. Conf. Intell. Robots Syst., pp , [12] H. He and K. Kiguchi, A study on EMG-based Control of Exoskeleton Robot for Human Lower-Limb Motion Assist, Proc. of Int. Special Topic Conf. on Inform. Tech. Applications in Biomed., pp , [13] J. G. Andrews and Y. Youm, A biomedical investigation of wrist kinematics, J. Biomechanics, vol.12, pp.83-93, 1979.