IROS2019国际学术会议论文集2367

Abstract Gravity compensated arms are widely used to support mechanical hand-held devices such as laser therapy machines and screw tapping machines. Particularly for heavy devices that require repetitive motion, it is necessary to apply correct gravity compensation mechanisms and also minimize friction to move them lightly. Therefore, we developed a statically balanced passive tool handler that allows users to handle a heavy automatic hand-held hair implanting device with minimal effort. To allow balancing of all movements of 6 degrees of freedom, this balancing arm adopted a spherical gimbal for orientating at the end of a positioning part utilizing noncircular pulleys, while providing multiple strands of electric wires for controlling the attached distal device. The proposed design is quantitatively evaluated by measuring required joints torque for movement using a practical prototype. Experimental results show that the tool handler has an excellent balancing ability and it is expected to be applied to support various heavy devices in the future. I. INTRODUCTION With the increase in human life expectancy, a correlated interest to maintain ones appearance has also increased in the developed world; especially in men who suffer hair loss. Hair transplantation surgery is one of few proven solutions to the hair loss problem by transplanting hair follicular units from a donor site or an occipital region to a recipient site such as a frontal or top region. However, since this task just relocates a limited number of hairs locally, the total number of hairs does not increase through the surgery. In addition, this surgery is a very repetitive task in which hair follicular units should be individually extracted and planted. The current operation is performed using manual devices such as hair implanters or small forceps. For example, when using KNU hair implanters, the extracted follicular units (FU) must be loaded one by one into the distal needle of the implanters to be implanted. In general, since thousands of FUs are transplanted for a surgery, the conventional methods are not effective in terms of time and operators fatigue. Therefore, research and development to automate the hair transplantation surgery have been recently activated 1, 2. Restoration Robotics Inc., at the forefront of automating the hair transplantation surgery, commercialized ARTAS robot for collecting hair follicle units using Follicular Unit Extraction (FUE) method, and recently released a new version with a function of implanting hairs. A hand-held automatic hair implanting device developed by Electronics This research was supported by the Electronics and Telecommunications Research Institute (ETRI) grant funded by the Korean government No. 19ZD1100. J. Suh is with Kyungpook National University (KNU), Daegu 41566, South Korea (corresponding author to provide e-mail: jwsuhknu.ac.kr). E. Choi is with the ETRI, Daegu 42994, South Korea (e-mail: ecchoietri.re.kr). and Telecommunications Research Institute (ETRI) can decrease the frequency of replacements of the implanting devices or storage magazines compared to the conventional manual devices, thereby allowing continuous implantation procedure and consequently shortening the operation time. However, due to its heavy weight, it is difficult to use the automatic implanter for the whole procedure. Therefore, a statically balanced tool handler is required that can support its weight during repetitive movement of the heavy hand-held device as shown in Figure 1. The automatic hair implanter should be able to move repeatedly to any position and orientation with less effort by using the gravity compensated tool hander. And a passive supporting mechanism is preferable to controlled robots considering the system cost. To achieve similar goals, Kang et al. developed a stackable mechanism capable of balancing a distal device using counter-weights 3. Endo et al. proposed a balancing mechanism for handling a dummy end- effector utilizing noncircular pulleys and springs, and Kim et al. also developed another balancing mechanism for applying to exoskeletons in a similar way 4, 5. However, they have a limitation that the counter-balancing mechanisms are not available to counteract the orientation change of the distal device. Kuo et al. developed a special orientating mechanism using two symmetric five-bar open chains to overcome this limitation and applied it to a laparoscope handler, but its usefulness has not yet been fully verified 6. Chung et al. developed a rotational 2-DOF balancer utilizing a scotch yoke mechanism with a double parallelogram, but its relatively complex structure seems to have limited application 7. Therefore, this study proposes a new tool handler to support the heavy weight of the hand-held automatic hair implant. The requirements of the system to be satisfied are as follows. 1) 6-DOF handling: 3-DOF for positioning and the other 3-DOF for orientating 2) Passive balancing for easy manufacture and maintenance 3) Supporting a 1.5 kg load of the distal device 4) Enough workspace for patients full head 5) Positioning brakes to fix posture 6) Electric wiring for power supply and signal transmission to the distal device In the remainder of this paper, a design of the proposed tool handler is described including kinematic models for positioning and orientating parts. Then, the gravity compensation functions are verified utilizing a manufactured prototype. Moreover, the experimental results of the proposed tool handler are discussed with its design. Design and Verification of a Gravity Compensated Tool Handler for Supporting an Automatic Hair Implanting Device Jung-wook Suh, Member, IEEE, and Eun-chang Choi IEEE Robotics and Automation Letters (RAL) paper presented at the 2019 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS) Macau, China, November 4-8, 2019 Copyright 2019 IEEE hair implanter base patient doctor tool handler Figure 1. Conceptual design of the balanced tool hander to support the automatic hand-held hair implanting device II. DESIGN It is obvious that six or more physical joints are required for the hand-held device attached to the end of the tool handler to freely move without any kinematic constraint. If this tool handler has a same structure as an industrial serial robot, three of their joints are required for positioning, while the remaining three may be responsible for orientating the distal device. Since the hair implanting device at the distal end is not a body of rotation, motions with full 6-DOF is required for the tool handler. Figure 2 shows a mechanical example of the 6-DOF tool handler consisting of a positioning part using two parallelogram joints and orientating part of an open gimbal wrist using two spherical linkages. A. Positioning Part Parallelogram mechanisms with auxiliary linkages are commonly used for positioning parts of gravity compensated serial robots. Serially assembled parallelograms can be kinematically decoupled, because each of them always keeps its distal orientation constant. The easiest way for balancing this parallelogram joint is utilizing counter-weights. However, the application of counter-weights greatly increases the inertia of the system, and as the number of the joints increases, this problem becomes even more serious. Therefore, a different method of storing elastic potential energy using springs has been proposed and generalized 8, 9. In particular, Agrawal et al. developed a gravity compensating method for spatial manipulators using springs and auxiliary parallelograms 10. In addition to static balancing, Gosselin has established a dynamic balancing method for parallel mechanisms to preserve the total angular momentum of the moving links 11. Koser developed a gravity compensation mechanism using a cam and a follower, and Chu et al. improved it to develop a self-regulated gravity balancer that automatically responds to variable payload 12, 13. Meanwhile, Ulrich et al. developed a gravity compensation mechanism using noncircular pulleys for robot manipulators 14. Furthermore, Bijlsma et al. showed that noncircular gears can be applied to achieve static balancing over an unlimited range of motion 15. Utilizing a small idle pulleys as shown in Figure 3 (a), balancing the weight of the distal mass M can be accomplished almost completely 16-18. However, this method is valid only when the radius of the auxiliary idlers is negligible or zero-free-length springs are applied. As the radii of the idlers increase, the accuracy of the gravity compensation decreases dramatically. On the other hand, as shown in Figure 3 (b), it is also possible to generate a desired counter-torque depending on the joint angle by applying noncircular pulleys 14, 19. Unlike the above method, which cannot consider the idlers size, it is possible to overcome such minor factors using nonlinear components. Therefore, in this study, we apply specially designed noncircular pulleys to the two parallelogram joints of the positioning part to better compensate the gravity applied to the distal device and to handle it more lightly. For balancing a singular parallelogram joint using a noncircular pulley, a cable extending from a tension spring is wound around the noncircular pulley fixed to a base linkage as shown in Figure 4. When the rotation angle of the joint is , the total length of the spring and cable u() is increased as the joint angle becomes smaller. If the distal end of the spring-cable is fixed at a point with a distance L on the longitudinal linkage of length LA = LB, the following equivalent mass can simplify this gravity compensation. )2( 2 CBA B eq MMM L L M (1) x0 y0 z0 Figure 2. Kinematic structure of the 6-DOF passive tool handler M M idlernon-circular pulley (a) (b) Figure 3. Two mechanical methods for precise gravity compensation MC MA MB Lrm u x y LB P R Figure 4. Schematic diagram for the statically balanced parallelogram joint using a noncircular pulley (a) (b) Figure 5. Two designed noncircular pulleys for the 2nd joint for 0 90 and the 3rd joint for 90 0 Figure 6. The resultant balancing torque for 90 90 Figure 7. Profile shape differences of the noncircular pulleys for various k and with Meq = 5.4 kg, L = 500 mm and 90 90 And its weight is compensated by the spring tension and the moment arm L. Here, the distal mass MC can be considered as a point mass on a moment arm LB, while MA and MB constitute the mass of the two longitudinal linkages. Meanwhile, in accordance with the energy conservation, the sum of the gravitational potential energy Vm and the elastic potential energy Vk is always constant regardless of the joint angle : Vm() + Vk() = const. The gravitational potential energy is a simple trigonometric function of the joint angle as: Vm() = MeqgLsin. Therefore, the length of the spring- cable u satisfies the following relation with the spring constant k and gravitational acceleration g based on the upright posture u0 = u(90) where the tension becomes zero. )sin1 ()( 2 1 2 0 gLMuuk eq (2) On the other hand, the torque generated by the tension of the spring-cable is k = rmk(u-u0). And, from the moment equilibrium, the torque k must simultaneously satisfy: k() = MeqgLcos. From the above relation, the length of the moment arm rm that generates the torque with respect to the center of rotation satisfies the following. )( cos 0 uuk gLM r eq m (3) And the small angle , between the spring-cable and the longitudinal linkage, can be expressed as follows. L rm sin (4) Using these relations, the point P (xP(), yP() constituting the profile of the noncircular pulley can be obtained 5. 1 1)sin(sincos d d d d xp (5) )cos( sin )tan( L Xy pp (6) As can be expected, identical profile shapes can be obtained when the ratios of Meq and k are same. Therefore, same noncircular pulleys can be applied if the ratio of the two spring constants can be matched to the ratio of the equivalent masses applied to the 2nd and 3rd joints. Figure 5 graphically shows that identical noncircular pulleys can be utilized for balancing different weights. The left and right profiles were plotted with Meq = 9.0 kg, k = 4,166.7 N/mm and Meq = 5.4 kg, k = 2,500 N/mm, respectively, with a common parameter of L = LB = 500 mm. The blue solid lines show the spring- cable tangential to the noncircular pulleys, while the green dotted lines describe the moment arms for them. By extending the two joint angles to 90 90, it can be seen that the resultant two profiles are completely same as the red dotted lines. u u0 = 0 and rm = 0 are satisfied for the vertical standing posture ( = 90) and the vertical hanging posture ( = 90), respectively, thus no torque is generated by the spring tension. Figure 6 shows balancing torque k that is produced by this pulley profile, which appears in the form of a complete cosine function. For reference, the size of the noncircular pulley for balancing a given design with Meq and L is determined by the selected spring constant k. As shown in Figure 7, the larger the spring constants, the smaller the size of the noncircular pulleys, thus a slender arm can be implemented. However, the small pulleys require heavy springs and cables, as well as increase the radial load applied to rotating axes of the joints, thereby resulting in reduced accuracy for gravity compensation. Additionally since these profiles are not resemblance shapes, it is desirable to design appropriate noncircular pulleys through accurate calculation. B. Orientating Part The parallelogram structure has a property of keeping the orientation of the distal linage constant, so it is effective for balancing the positioning part. However, it is not applicable to the orientating part which requires variable distal orientation. Figure 8 shows the applicable mechanisms for the orientating wrist. First, it is considerable using a combination of conventional rotary joints. However, this method causes the center of mass of the attached distal device to fall away from the rotational axes, making the gravity compensation difficult. To compensate for this, even if the rotating axes are positioned near the mass center of the device, there is a limit in reducing the distance. And it can also make the users grip uncomfortable for the structure of the linkages and the joints. A universal joint or ball joint is also mechanically identical to the combination of the rotary joints and is not free from this mass center problem. Additionally, it is also difficult to apply the balancing method by storing elastic potential energy, because the angle of the base linkages of the orientating joints is not kept constant. Remote center of motion (RCM) mechanisms that allow the end-effector to rotate about its center of mass can be effectively applied to these orientating joints 20. The RCM mechanisms has been generalized to manipulate long surgical instruments of laparoscopic surgical robots on the basis of incision points. Among them, a spherical open gimbal and double parallelogram mechanisms can be considered as the orientating joint mechanism 21-23. A double parallelogram applied to da Vinci surgical robot could achieve a stable RCM, but it requires zero-free-length springs or zero-radius idle pulley for balancing that are physically difficult to achieve 6. Moreover, the double parallelogram is not easy to supply multiple electric wires starting from the base part to the distal device through the relatively complicated joint structure. On the other hand, the gimbal is structurally simple and good for electric wiring. However, a closed gimbal is prone to physical collision with other objects. Therefore, an open gimbal mechanism consisting of spherical linkages and revolute joints can be easily applied to the orientating wrist part. In the case of the open gimbal mechanism, unlike the closed gimbal using symmetrical linkages, the center of gravity is unbalanced for the unsymmetrical spherical linkages. To resolve this problem physically, we can adopt the following three methods as shown in Figure 9. 1) Application of additional counter-weights: It is the most obvious way to decouple the gravity compensation problem between joints, but it increases the overall inertia. Shortening the moment arm using a heavy weight can save space. (a) (b) (c) Figure 8. Candidates for orientating mechanism: (a) conventional robot wrist, (b) double parallelogram, (c) spherical open gimbal counter-weight spring translational adjustment (a) (b) (c) Figure 9. Three mechanical balancing methods for the orientating spherical wrist joints 4thjoint 5thjoint 6thjoint m4 m5 m6 m4d 4 5 r4 r5 r6 d4d 4c 5c r5+6 m5+6 Figure 10. Proposed gravity compensation mechanism for the three joints of the spherical open gimbal 2) Generating counter torque using a spring: It is a common method for balancing large positioning joints of robots without adding a counter-weight. However, in the case of the wrist joint for orientation, it might be applicable to the first joint only. It also requires room for mechanical components such as a spring, tension controller, and pulley. 3) Longitudinal movement of the distal part along the rotation axis: Spherical gimbal joints can solve the counter-balancing problem by adjusting the assembly distance between components without adding a counter-weight because the rotation axes of the gimbal are not parallel. However, since the distal joint axes may be distorted, this method is applicable to a limited extent. Figure 10 shows the proposed 3-DOF orientating wrist mechanism for static balancing. The three joint axes of the gimbal, consisting of two spherical