A Wearable Rehabilitation Robotic Hand Driven by PM-TS Actuators

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1 A Wearable Rehabilitation Robotic Hand Driven by PM-TS Actuators Jun Wu, Jian Huang, Yongji Wang, and Kexin Xing Key Laboratory of Image Processing and Intelligent Control, Department of Control Science and Engineering, Huazhong University of Science and Technology, , Wuhan, China Abstract. Robotic-assisted therapy is of great benefit to the recovery of motor function for the patients survived from stroke. However there have been few emphases on the patients hand training/exercise during the rehabilitation process. The goal of this research is to develop a novel wearable device for robotic assisted hand therapy. Unlike the traditional agonist/antagonist PM actuator, we propose a new PM-TS actuator comprising a Pneumatic Muscle (PM) and a Torsion Spring (TS) for joint drive. Based on the proposed PM-TS actuator, we design a robotic hand which is wearable and provides assistive forces required for finger training. The robotic hand has two distinct degrees of freedom at the metacarpophalangeal (MP) and proximal interphalangeal (PIP) joints. The variable integral PID (VIPID) controller was designed to make the joint angle of robotic hand can accurately track a given trajectory. The results show that the VIPID controller has better performance than the conventional PID controller. The proposed rehabilitation robotic hand is potentially of providing supplemental at-home therapy in addition to the clinic treatment. Keywords: Rehabilitation Robotic Hand, Pneumatic Muscle (PM), Torsion Spring (TS), PM-TS Actuator. 1 Introduction Adult disability is a common result of stroke in many countries, and usually involves deficits of motor functions. In the case of stroke victims, it is widely accepted that spontaneous recovery accounts for the motor and functional restoration taking place within the first months after the stroke incident. The findings have shown that intense practice of repetitive movements can help to improve the strength and functional recovery of the affected arm/hand. However, traditional approach of rehabilitation training is very labor intensive and lack of consistency and objective assessment. Many kinds of assist devices for hand therapy have been developed to offer many patients the intensive training, such as the Rutgers Hand Master II-ND Force- Feedback Glove [1], the HWARD robotic hand-therapy device [2], the finger therapy robot at the Rehabilitation Institute of Chicago [3], the GENTLE/G hand robotic therapy system developed at the University of Reading [4], and the cabledriven finger therapy robots [5] which have been successful in providing grasp assistance and hand H. Liu et al. (Eds.): ICIRA 2010, Part II, LNAI 6425, pp , Springer-Verlag Berlin Heidelberg 2010

2 A Wearable Rehabilitation Robotic Hand Driven by PM-TS Actuators 441 opening. Unfortunately, it should be noted that many robotic systems are developed for research purpose, these robotic hands/devices are too heavy, complex, bulky and unwearable, or too expensive for home use by individuals. Therefore, there is a need for affordable, safe and practical devices to assist therapy for practical use. In robotic control systems, most researchers select motors as driving sources for rehabilitation training [6][7]. Recently, the compliant pneumatic muscle actuators (PMA) have found many applications in the robot-based rehabilitation therapy. Aseven degree-of-freedom upper arm training/rehabilitation (exoskeleton) system driven by PMA was designed [8]. Compared with the electric motor actuator, which is lack of the necessary compliance between the actuator and the limb being moved in exoskeleton applications, PMA is soft and exhibits many properties of human muscle. It is more important that PMA is at a very low level of risk of injury. This is not the case for hydraulic and/or electric motor actuators, which produce a far greater risk of injury to the user adjacent to the device when failing. PMA possesses many unique advantages (e.g. cheapness, light weight, compliance). At the same time it has very high power/weight and power/volume ratios [9][10]. Therefore, the hand therapy devices with PMAs can interact with the patients in a safer and more natural way. A challenge for the application of PMA is that it can be only operated in the contractile direction. Hence, PMs have to be used in antagonistic pairs to achieve bidirectional motion for a single joint. Antagonistic pairs of actuators are usually an acceptable design solution for a simple flexion and extension joint, like the arm or ankle [8][11]. In many cases, we hope to use a compact structure to realize the bidirectional motion of joint driven by the PM. If the PMs are used in antagonistic pairs, it will not only increase the complexity of mechanical design and control, but also make the device/system bulk, weight and even the cost greatly increased. An available way to meet the requirement is to minimize the number of PMs in the actuators. For the above reasons, we proposed a new actuator structure comprising a PM and a torsion spring (TS), called PM-TS actuator, and designed a novel wearable robotic hand for assisted repetitive driven by the new PM-TS actuator. In the conventional PID control, the fixed PID parameters cannot guarantee a satisfactory performance in the entire control process. Some studies demonstrate that better control performance may be attained when using a PID control strategy with variable coefficient other than a conventional PID method. Thus, in this study we designed a variable integral PID (VPID) controller to control the proposed PM-TS actuators. Experimental results found that the VPID performs better in our application. In summary, the contributions of this study include 1): proposing a new PM-TS actuator for joint drive, 2): designing a wearable robotic hand for rehabilitation training, 3): implementing the control of the rehabilitation robotic hand by using VPID, 4): integrating the robotic hand with the sensor system with virtual reality (VR). This paper is organized as follows. Section 2 proposes a new PM-TS actuator and describes the mechanics of the wearable rehabilitation robotic hand. Section 3 analyzes the control structure of the entire rehabilitation system. In section 4, the experimental results of the trajectory tracking control of the proposed PM-TS driven rehabilitation robotic hand are presented. In the last section, the conclusion of the current work and the possible future improvements are described.

3 442 J. Wu et al. 2 Mechanics of Wearable Rehabilitation Robotic Hand The design goal is to develop a therapeutic robotic hand that provides finger training. The robotic hand needs to be compact for portability, relatively easy to use for don and doff, capable of interacting with a personal computer based visual biofeedback system to capture the interest of the user, objective to evaluate the functional performance and has the potential to be inexpensive and amenable for home use. 2.1 Mechanic Structure The Mckibben pneumatic actuators used in the joint drive provide compliant actuation and thus augment the safety compared to the traditional rigid drivers. PMA is made from inexpensive materials, such as natural latex rubber for the inner bladder covered by a polymer based braid (Fig. 1). The maximum contraction ratios of PMAs which are made in our Lab are about 25%. In order to transform the translation into the rotation, a pulley mechanism is designed and arranged above the joints. In traditional way, PMs have to be used in antagonistic pairs to achieve bi-directional movement for a single joint as shown in Fig. 2(b). This arrangement requires the control of at least two actuators simultaneously, and it will increase the complexity of mechanical design and control, the device s bulk, weight and cost. An available way to meet the requirement is to minimize the number of PMs in the actuators. For these reasons, we proposed a new actuator structure comprising a PM and a torsion spring, called PM-TS actuator. The PM is arranged in the suitable place to provide the pulling moment and the torsion spring provides opposing torque, it is shown in Fig. 2(a). Fig. 1. Materials of pneumatic muscle actuator and pneumatic muscle actuator made in lab (a) (b) Fig. 2. (a) 1-DOF joint driven by PM-TS actuator. (b) 1-DOF joint driven by a pair of PMs.

4 A Wearable Rehabilitation Robotic Hand Driven by PM-TS Actuators 443 Based on the proposed PM-TS actuator, we designed a rehabilitation robotic hand for hand therapy. Fig. 3 shows the 3D model of the wearable robotic hand, which is designed in Solidworks. The major design goals are summarized as follows: 1) There are 2 DOFs (Degrees of Freedom) for the fingers, which makes the movement of fingers more suitable and flexible. 2) The rotation scope of joint angle is limited by the mechanism for safety. 3) There is a bi-directional movement for each DOF. 4) The robotic hand can be worn comfortably and easily. Fig. 3. The 3D model of the wearable robotic hand. 1-pneumatic muscle, 2-steel wire, 3- wire rack, 4-wire conduits, 5-angle sensor, 6-press plate, 7-finger splint link, 8-finger splint, 9-glide mechanism, 10-torsion spring, 11-pulley, 12-forearm attachment, 13-forearm hoop. Fig. 4. The prototype of the wearable rehabilitation robotic hand Fig. 4 shows the prototype of the wearable rehabilitation robotic hand worn on the user s left hand. The PMs are arranged on the top of the forearm attachment and the torsion springs are mounted on the joints. The steel wires are used to connect the PMs and the pulleys. The steel wires driven by PM-TS actuators lead to the rotation of the pulleys, which results in the rotation of finger joints. The wire conduit can avoid the movement coupling of the DOFs. A press plate is placed above the proximal phalanges to ensure that the proximal phalanges are parallel to the corresponding mechanical link. A glide mechanism is assembled on the press plate to adapt the relative movement between exoskeleton and human fingers. There are two Hall-effect encoders mounted to the axes of each joint, the Hall-effect encoders are used to measure the angles of the joints.

5 444 J. Wu et al. 2.2 Fundamental Motion The extensions of the metacarpophalangeal (MP) and proximal interphalangeal (PIP) joints of fingers in able-bodied volunteers with the wearable robotic hand are done as the fundamental experiment. Table 1 shows the joint angles are measured when the robotic hand works, where θ 0 means the initial angle, δθ represents the increased angle and θ is the current angle. θ 0, δθ and θ are illustrated respectively in Fig. 5. Table 1. The ranges of motion for the robotic hand PIP[rad./deg.] MP[rad./deg.] θ / /110 Δθ / /70 Θ / /180 (a) (b) Fig. 5. (a) Disposition of joints. (b) Initial angle and increased angle for the fingers. θ is the joint angle. θ 0 means the initial angle, and δθ represents the increased angle. 2.3 Safety For a rehabilitation robotic hand, the safety is dominant within all performances. The affected hand needs to be absolutely safe when interacting with the robotic hand. Firstly, the risk to patients is minimized by utilizing the compliant PMAs. All assisted movements will be made slow to protect against significant increases in muscle tone. Secondly, the mechanical design allows the device physically to limit range of motion of individual joints. Moreover, the safety envelope in the control algorithm will further guarantee the safety. If the calculated control pressure for the PM exceeds the preset limit, the real control pressure will be set to the limit value. In addition, a panic button is provided for easy access by the unaffected hand to exhaust the PMAs and shut the device down if the patients are feeling any discomfort, fatigue or anxiety. 3 Control System Task-oriented repetitive movements can improve muscular strength and movement coordination in patients with impairments due to neurological problems. A typical repetitive hand functional movement consists of grasping (flexion) and release (extension). Fig. 6 shows the block diagram of control system for the robotic hand. The final goal of control scheme is to provide controllable, quantifiable assistance specific to some particular patients by adapting the level of the assistance provided.

6 A Wearable Rehabilitation Robotic Hand Driven by PM-TS Actuators 445 The sensors can feedback angle, pressure and force information for adaptation of the assistive force and quantitative evaluation of task performance. The angle sensors are used to measure the joint flexion and extension. The robotic hand with the strain gauge compression force sensors involves the patients in their hand rehabilitation by encouraging self-powered motion by the fingers, and assisting movement when necessary. In addition, there is also a pressure sensor integrated in the electric proportional valve to provide the local feedback. The real-time data acquisition card is responsible for sampling all sensory data. The data is exchanged via a USB connection between the data acquisition card and the host computer. The analog output of the card is connected to the electric proportional valve to control the PMAs. The air transmission unit is an air-conditioning circuit which contains a filter valve and an electric proportional valve. The host computer runs the control algorithms and a virtual reality game. It can build a task-oriented interactive game environment to deliver an engaging and interesting training process. The evaluation of task performance is fed back to patients by intuitive cues integrated with the VR games. Fig. 6. Block diagram of control system for the rehabilitation robotic hand To realize the angle tracking, an incremental PID controller is firstly implemented in the host computer. The incremental PID control algorithm is described by Δ uk ( ) = k[ ek ( ) ek ( 1)] + kek ( ) + k[ ek ( ) 2 ek ( 1) + ek ( 2)] (1) p i d where Δu(k) is the control increment at the k-th sampling time. e(k) is the deviation of the system at the k-th sampling time. k p is the proportional gain. k i = k p *T/T i is the integral coefficient, and k d = k p *T d /T is the derivative coefficient. In the incremental PID control algorithm, the integral coefficient k i is a constant. If the integral coefficient k i is too small while the deviation is large, the incremental control variable may not be enough at some inflection points. On the other hand, if k i is too large, the control process of the system may be unstable. Thus, it is important to

7 446 J. Wu et al. improve the system quality by changing the integral coefficient k i according to the value of deviation. To attain better control performance, variable integral PID (VIPID) controller in an incremental way was applied. The control structure of the proposed rehabilitation robotic hand is depicted by Fig. 7. The relationship of integral coefficient f[e(k)] and error e(k) for VIPID can be designed as follows: ki _ Min e( k) B ( ki _ Max ki _ Min) f[()] e k = ki _ Min+ ( e() k B) B < e() k A+ B A ki _ Max e( k) > A+ B (2) Fig. 7. The control structure of rehabilitation robotic hand with trajectory plan Then the VIPID control algorithm is given by Δ uk ( ) = k[ ek ( ) ek ( 1)] + f[ ek ( )] ek ( ) + k[ ek ( ) 2 ek ( 1) + ek ( 2)] (3) p d 4 Experiment 4.1 Experimental Condition Fig. 4 and Fig. 6 show the experimental apparatus and schematic diagram for the control of the wearable rehabilitation robotic hand. The experimental apparatus includes pneumatic muscles, the electromagnetic proportional valve, pressure regulating valve, Hall-effect encoder, and the acquisition cards. Table 2 and 3 show the equipments and physical parameters of pneumatic muscles. The parameters of conventional PID controller are: k p =0.02, k i =0.005 and k d =0.02. The parameters of VIPID controller are: k p =0.02, k d =0.02, k i _Min=0.005, k i _Max=0.02, A=0.05 and B= The two control algorithms were respectively implemented in the control system. Two control loops run independently for the two PM-TS actuators which drive the MP and the PIP joints to rotate. The controlled variable for each control loop is the measured angle at each joint. The software running on the PC was developed using Visual C Experimental Results In order to further assess the performance of the system and the realization of the ultimate clinical application, we conducted preliminary clinical trials, as shown in

8 A Wearable Rehabilitation Robotic Hand Driven by PM-TS Actuators 447 Table 2. The main performance parameters of the equipments Name Model Performance Manufacturer Mute air compressor FB-0.017/7 Rating exhaust pressure: 0.7Mpa Taiwan JAGUAR Co., Ltd Electromagnetic ITV BS Input: 0-5V SMC Co., Ltd proportion valve Output: Mpa Pressure regulating AW20-02BCG Range: Mpa SMC Co., Ltd valve Data acquisition card USB channel DI, 8 channel DO 16 channel AI, 2 channel AO Advantech Co., Ltd Switching power supply Hall-effect encoder (Angle sensor) Strain Gauge Force Sensor Q-120DE ED-18-SB V-P FSR400 Sampling rate: 200 ks/s Input: AC 220V±10% 50Hz Output: ±5V, ±12V, ±24V Degree range: Output: DC 0-5V Bushing mounting torque: 10 in-lb max Sensing area: 0.2 circle Force sensitive rang: 0 to 10kg Taiwan MEAN WELL Measurement Specialties, Inc. Interlink Electronics Others Joints connector, air pipe, etc SMC Co., Ltd Table 3. The parameters of pneumatic muscles for MP and PIP Joint MP PIP Initial length L 0 211mm 232mm Initial diameter D mm 12.26mm Initial angle of mesh grid θ 0 22º 22º Thickness of rubber sleeve t k 1.64mm 1.64mm Fig. 8. Some experiments were performed on a healthy subject, the performance of PID and VIPID controllers were evaluated using sine and ramp waves as the desired joint angle trajectory that should be tracked for the extension/flexion of the MP joint and PIP joint. The user was passive and not working with or against the motion. Fig. 8. The sequence of a clinical rehabilitation trial

9 448 J. Wu et al. The sine trajectory tracking results of MP and PIP using the conventional PID and VIPID control strategies are showed in Fig. 9. The ramp trajectory tracking results of PIP using the two control strategies are showed in Fig. 10. The self-tuning of VIPID parameters can be realized by using the error, it prevails over the PID in many performances, such as response time, flexibility, adaptability and control precision. But the jitter of MP is bigger than that of PIP during the extending. The main reason is that the friction force of MP has bigger influence than that of PIP, and the influence cannot be eliminated completely in the control system Reference PID VIPID Reference PID VIPID MP Joint (rad) PIP Joint (rad) Time (s) Time (s) Fig. 9. The sine trajectory tracking result of MP and PIP using the PID and VIPID Reference PID VIPID Reference PID VIPID MP Joint (rad) PIP Joint (rad) Time (s) Time (s) Fig. 10. The ramp trajectory tracking result of MP and PIP using the PID and VIPID 5 Discussion and Conclusion In this paper we proposed a new PM-TS actuator configuration that uses a PM and a torsion spring for bi-directional movement of joint. A wearable rehabilitation robotic hand actuated by the proposed PM-TS actuator is also developed for the stroke patients. It is designed with the intention of becoming a low cost and safe takehome supplemental therapy device for repetitive hand therapy. It is obviously more

10 A Wearable Rehabilitation Robotic Hand Driven by PM-TS Actuators 449 easily-realized and compact than traditional actuators composed of PMs. In this way, the number of components which are often used in air circuit such as the expensive electric proportional valve and the fussy windpipe, can be greatly decreased. This meets the requirement of most rehabilitation devices building lightweight, more compact, compliant and economical system. Further, this arrangement requires fewer actuators to be controlled. Finally, to realize the accurate tracking control of the designed rehabilitation robotic hand, the PID and VIPID controllers were designed to track some desired trajectories. The results show that the VIPID controller has better performance than the PID controller in trajectory tracking. The future work is the development of an intelligent adaptive controller to encourage the patients voluntary efforts during task-oriented rehabilitation training. More experiments will be implemented to improve the accuracy and stability of the angle/position control. Other control modes considering the measured force will be developed for more flexible training ways. The current robotic hand presented in the preliminary experiment can be further improved in mechanism and intelligence in the future. It is expected that this system will eventually: 1) empower patients voluntary for intention control; 2) support therapists in a labor intensive task; 3) reduce dependency on hospital resources; and 4) assist in re-integrating patients with hand impairment back into the community. Acknowledgments This work is partially supported by Hi-tech Research and Development Program of China under Grant 2007AA04Z204 and Grant 2008AA04Z207, and in part by the Natural Science Foundation of China under Grant and References 1. Bouzit, M., Burdea, G., Popescu, G., Boian, R.: The Rutgers Master II New design forcefeedback glove. IEEE ASME Trans. Mechatron. 7, (2002) 2. Takahashi, C.D., Der-Yeghiaian, L., Le, V.H., Cramer, S.C.: A robotic device for hand motor therapy after stroke. In: 9th IEEE International Conference on Rehabilitation Robotics Conference, pp IEEE Press, Chicago (2005) 3. Worsnopp, T.T., Peshkin, M.A., Colgate, J.E., Kamper, D.G.: An actuated finger exoskeleton for hand rehabilitation following stroke. In: 10th IEEE International Conference on Rehabilitation Robotics, pp IEEE Press, Noordwijk (2007) 4. Loureiro, R.C., Harwin, W.S.: Reach & grasp therapy: Design and control of a 9-DOF robotic neuro-rehabilitation system. In: 10th IEEE International Conference on Rehabilitation Robotics, pp IEEE Press, Noordwijk (2007) 5. Dovat, L., Lambercy, O., Johnson, V., Salman, B., Wong, S., Gassert, R., Burdet, E., Leong, T.C., Milner, T.: A cable driven robotic system to train finger function after stroke. In: 10th IEEE International Conference on Rehabilitation Robotics, pp IEEE Press, Noordwijk (2007) 6. Lambercy, O., Dovat, L., Gassert, R., Burdet, E., Chee, L.T., Milner, T.: A Haptic Knob for Rehabilitation of Hand Function. IEEE Trans. Neural Syst. Rehabil. Eng. 15, (2007)

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