IROS2019国际学术会议论文集0603

Abstract We develop a novel cube-shaped finger-wearable haptic interface named as HaptiCube. With the aim of implementing the tactile device with improved functionality and wearability, we focus on designing the device to have two characteristics: multi-DOF force feedback displayability and large force capability with compact and lightweight structure. In designing the device, we mainly consider the type and configuration of actuator and driving mechanism, since both have a great effect on the size, weight, and output force of the device. As the actuator, we select the shape memory alloy exhibiting high energy density. And, as the driving mechanism, two different types of compliant mechanism are designed for two important functions: to convert contraction of the SMA to the desired motion as a motion guide and to apply bias-force to the SMA as a bias-spring. The device is designed as the miniature interface that can display 3-DOF pressure and 2-DOF shearing force to users fingerpad. And, it is implemented as the functional prototype with the total weight of 26 g including actuators and all mechanical components. The experimental results on working performances (e.g. stroke, output force, and bandwidth) demonstrate that the device has superiority in terms of multi-DOF displayability, compact-sizability and wearability. I. INTRODUCTION Tactile interfaces enable the user to dexterously and precisely manipulate the object placed in remote space or the object simulated in virtual reality (VR) environment when performing tele-operation or VR interaction 13. Based on the cutaneous information provided to users fingerpad by the tactile interfaces, the user can perform diverse manipulation tasks with feeling immersion and coexistence with the remote or VR environment 35. The tactile interfaces that display cutaneous feedback to users fingerpad could be categorized into three different types in terms of portability and/or wearability: grounded devises, handheld devices, and finger-wearable devices. In the case of the grounded devices 68, they have an advantage in that the devices can display multi-modal tactile feedback with a wide area of touch surface. Since there are no serious constraints on the size and weight of the device, it would be relatively straightforward to realize the device having the multi-modal tactile feedback functionality by embedding required electronic components and mechanical parts in the *This work was supported in part by the Korea Institute of Science and Technology Institutional Program under Grant KIST 2E29460, in prat by the Convergence Technology Development Program for Bionic Arm through the NRF funded by the MSIP under Grant 2014M3C1B2048419, and in part by the Global Frontier R&D Program on Human-centered Interaction for Coexistence funded by the National Research Foundation of Korea grant funded by the Korean Government (NRF-M1AXA003-2010- 0029748). The authors are with the Center for Intelligent and Interactive Robotics Research, Korea Institute of Science and Technology, Seoul 02792, South Korea (Corresponding authors e-mail: donghyunkist.re.kr). B. Lim is also with the Department of Mechanical Engineering, Korea University, Seoul 02841, South Korea. grounded platform. As compared to the grounded devices, the handheld devices have been implemented in a compact and lightweight tactile interface with focus on high portability 912. Thus it is relatively easy to carry the device and use it anywhere. Also, since the user just simply holds the device by hand to use it, they can move the arm freely without constraints on motion range in performing the task. These tactile interfaces, i.e. grounded and handheld devices, have been being practically utilized in various fields based on their unique functionality, suitability for specific applications, and uncomplicated and intuitive usage. However, these devices may not be suitable for performing sophisticated tasks such as the multi-fingered tele-manipulation and the VR interaction, because there are some serious limitations. Firstly, the user cannot make diverse finger-gestures freely while performing the tasks. When using the grounded or handheld interfaces, the user must be touching the screen of the grounded device with the fingertips or grasping the handheld device continually in order to receive the tactile information. In these cases, the movement of the fingers is inevitably limited. Also, this naturally causes the deterioration of manipulability with the poor hand-eye coordination (i.e. the synchronization of eye and hand movements) 13. Secondly, the grounded and handheld devices are not suitable for the function to selectively stimulate each fingertip of the user in general. Since the end-effectors of the both devices are not being designed for the function to independently display the tactile feedback to each finger, it would not be proper to use these kinds of devices for multi-fingered manipulation tasks. For these reasons, the finger-wearable tactile interfaces have been diversely developed in many previous researches 1428. While it is difficult to assure that these devices are absolutely superior as compared to the grounded and handheld devices, it is obvious that the finger-wearable interfaces have strong points in terms of multi-finger usability with naturalness of hand gesture, wearability with ease of donning and doffing, and portability. In addition to these merits, another advantage of the finger-wearable interfaces is that the HaptiCube: a Compact 5-DoF Finger-wearable Tactile Interface* Byeongkyu Lim, Keehoon Kim, Sang-Rok Oh, and Donghyun Hwang, Member, IEEE (a) (b) Fig. 1. A proposed HaptiCube: a compact 5-DoF finger-wearable tactile interface. (a) The device connected to a wrist-mounted wireless control unit. (b) The fabricated functional prototype. 2019 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS) Macau, China, November 4-8, 2019 978-1-7281-4003-2/19/$31.00 2019 IEEE5094 devices could be relatively easily coupled to kinesthetic haptic interfaces as the end-effector. When two different types of haptic devices are combined into one system, it can provide tactile and kinesthetic information to the user at the same time. In this work, we propose a novel finger-wearable tactile interface shown in Figure 1 with the aim of realizing the device of high functionality: multi-DOF force feedback displayability and high force-to-weight ratio characteristic (i.e. large force capacity with compact and lightweight structure). Further, we also consider the wearability/portability related to the practical usability and applicability. The proposed device is implemented as a functional prototype, and then its working characteristics and performances are experimentally assessed. II. TECHNICAL TRENDS AND DEVELOPMENT GOAL A. Finger-wearable Tactile Interfaces In this technical review, we examine the specifications of the several finger-wearable devices developed in the past decade. As the main technical specifications, we firstly consider the type of tactile feedback and the number of DOF of the motion of tactor (i.e. the devices end-effector in contact with users fingerpad). These two specification would be regarded as the main functionality of the finger-wearable tactile interfaces in general. Also, the type of actuator and the total weight of the device are analyzed, because both strongly influence on the performance and the wearability of the finger-wearable interfaces. The specifications of the reviewed devices are tabulated in Table I. Firstly, it can be seen that most of the previous devices have been designed to display touch/pressure, shearing force, and/or vibration. The reason for this might be that these kinds of tactile feedback are preferentially required for the manipulation tasks as compared to other types of feedback such as temperature. Secondly, the analysis result shows that the number of DOF of the motion of tactor is mostly one or two. The reason why devices with relatively low DOF have been developed mainly is because the size and weight of the device inevitably increase by applying a larger number of components (e.g. actuators and transmissions) to the device in proportion to the number of DOF in general. Thirdly, the table shows that conventional electromagnetic motors have been widely utilized as the actuator. Considering that the actuator influence significantly on the functional and structural features of the device, we analyze correlation between the number and type of actuator, the number of DOF, and total weight of the device. Also, we investigate the force-to-weight ratio of each device. The analysis result is presented in Figure 2. B. Development Goal On the basis of the review results, we set quantitative and qualitative development goals. As marked in Figure 2, it is aimed to design the multi-DOF lightweight finger-wearable interface having a high force-to-weight ratio characteristic. As a first functional-priority, the number of DOF of the device is considered. With consideration for that the device having higher DOF might be more useful in general, the target number of DOF of the tactor is set as 5 including 3-DOF tip- tilt motion for pressure and 2-DOF in-plane translational motion for shearing force. Secondly, we attempt to design the device that can generate relatively high force with a small size and light weight not only for functionality but also for wearability. If the device is small and light, but cannot generate large forces, the device will have a drawback in terms of the force- feedback capacity. On the contrary, even if the output force capability of the device is high enough, if the device is bulky and heavy, it will be difficult to practically use the device. The quantitative target values are set as 40 g or less for the total weigh and 0.7 N/g or more for the force-to-weight ratio. In addition, we take some practical issues related to the wearability into consideration. Considering that the finger size, especially height, is different between each fingers on a hand and varies from person to person 27, we try to design the fingertip-fitting component that enables the device to be properly fitted to the users fingers. Lastly, we attempt to configure the overall structure suitable for quick and easy donning and doffing of the device. III. CONCEPT AND DESIGN To achieve the development goal, we take into account the type of actuator and the composition of driving mechanisms first. This section introduces the selection criteria of the two key components and explains the structural configuration and operating principle of the proposed device in detail. TABLE I SPEC. OF PREVIOUSLY DEVELOPED FINGER-WEARABLE TACTILE DEVICES Ref. Tactile Feedback DOF Type of actuator W* g 14 Touch/pressure 1 a DC motor 18 15 Press. and vib. 3 a pair of DC motors 10 16 Press. and shear 2 a pair of DC motors 45 17 Touch/pressure 1 SMA wires - 18 Touch/pressure 1 a DEA array - 19 Touch/pressure 1 an EM servo actuator 1.4 20 Touch/pressure 1 a voice coil motor 56 21 Shearing force 2 two EM servo motors 39 22 Touch/pressure 1 a flat BLDC motor 4.8 23 Vibration 1 a linear resonant motor 7.5 24 Pressure 3 three DC motors 30 25 Touch/pressure 1 a DEA - 26 softness and cue 2 3 EM servo motors 100 27 Press. and shear 1 three SMA wires 10 28 Vibration 1 a vibrator - *W indicates the total weight of the device installed to the fingertip. 0 0.2 0.4 0.6 0.8 1 020406080100120 Force-to-weight ratio N/g Weight of device g 14 21 22 27 19 23 24 20 26 1 degree-of-freedom 2 degree-of-freedom 3 degree-of-freedom Multi-DOF w/ high energy lightweight device density Development goal Fig. 2. Comparison analysis on the characteristics of the force-to-weight ratio of the previously developed finger-wearable tactile interfaces. 5095 A. Actuators and Driving Mechanisms The most important consideration in determining the type of actuator for the compact/lightweight multi-DOF wearable devices would be the force-to-weight ratio characteristic of the actuator. Because, if the actuator has the high force-to-weight ratio characteristic, it is advantageous to implement a small- sized light device having high force capacity. In this work, we select the shape memory alloy (SMA) actuator as the suitable actuator, considering its force-to-weight ratio characteristic as well as other merits such as large strain and clean/silent operation. Although the low operational frequency and narrow bandwidth can be pointed out as drawbacks of the SMA actuator, it may not be regarded as a serious problem in the application to display tactile force-feedback. When designing the actuator system with SMA, the component functioning to provide bias-force to the SMA is basically required. When the SMA actuator is in non-activated condition, the external bias-force should be applied to the SMA as a pre-load causing the pre-strain of the material, since the SMA actuator cannot generate motion repetitively without the bias-force. Another essential component is a motion guide. Intrinsically, the SMA actuator produces the motion, when the material is contracted by heating up. In order to generate the desired motion with the SMA, the motion guide that can guide and/or convert the simple contraction motion of the SMA actuator to the desired motion is required. In this work, considering that both the component that provides the bias-force and the motion guide are the key components, we scheme to design compliant mechanisms. As one type of elastic mechanism, since it exhibits elastic behavior like a mechanical spring, the compliant mechanism could be suitably utilized as the bias-spring. Also, because the various types of compliant mechanism have been already widely used as precision motion guides, we expect that it can also function to guide the motion of SMA to the desired motion. Consequently, due to the compliant mechanism in charge of both necessary functions at the same time, the proposed device can be implemented with more compact and light structure. B. Structural Configuration and Operating Principle The structural configuration and operating principle of the proposed device are schematically shown in Figure 4. The device consists of five main pars: a base, a tip-tilt spring, a XY spring, a tactor, and an upper cover. The assembly procedure of these components is as follows. On the center of the base, the center of the tip-tilt spring assembled with four half-moon shaped wire-guides is mounted. Four spacers, the planar XY spring, and the tactor are sequentially assembled on the tip-tilt spring. And, the upper cover is bolted on the four pillars of the base, so that the device is finally implemented as the cube-shaped structure as indicated in Figure 3 (a-1). In this assembled structure of the mechanical components, the SMA actuators are configured as shown in Figure 3 (b-1) and (b-2). Firstly, four straight SMA wires (i.e. SW(1), SW(2), SW(3), and SW(4) to generate the 3-DOF tip-tilt motion are applied to the device. The middle portion of each SMA wire is hooked around the half-moon shaped wire-guide, and the both ends of each SMA wire are fixed to the electrical terminals embedded in the upper cover by bolting. In this configuration, (c-2) Translational motion in Z-axis (c-3) Trans. motion in X- or Y-axis(a-1) X Y dtip Z X (c-1) Angular motion in X- or Y-axis Applied force Z X Applied force force dtilt (d-3) Pressure in Z-axis(d-1) Pressure in X-axis (d-2) Pressure in Y-axis (d-4) Shearing force in X-axis(d-5) Shearing force in Y-axis Pressure Shearing Shearing force force Z Y Z Y Z X Z X Z X (a-2) 8Electrical terminal 1 Base2 Tip-tilt spring3 Planar XY spring4 Upper cover5 Tactor 6 Wire-guide7 Spacer 1 2 3 4 5 6 7 8 (b-1) (b-2) Y X A Zoomed view of A B B View of B-B 9 10 9 SMA wire 10 SMA helix SW(1) SW(2) SW(3) SW(4) SH(1) SH(3) SH(2) SH(4) Living hinge dXY Spring-clip for finger-fitting Furrows guiding the SMA wires w/ angular torque Pressure w/ angular torque Pressure w/ linear force Output force Output torque Applied Output Fig. 3. Structural configuration and working principle of the proposed 5-DOF finger-wearable tactile device. (a-1) and (a-2) show the configuration of the assembled and exploded device, respectively. (b-1) and (b-2) indicate the 3-DOF tip-tilt spring and the 2-DOF XY spring configured with four SMA wires and four SMA helixes, respectively. (c-1) to (c-3) and (d-1) to (d-5) schematically present the elastic deformation of each spring and the principle of displaying 5-DOF tactile force-feedback to the fingerpad. 5096 when one of SMA wires is being thermally activated by applied electric power for Joule heating and so contracts, one side of the rectangular rim of the tip-tilt spring in which the contraction force of the SMA wire is applied is elastically lifted up (see Figure 3 (c-1). As a result, based on this elastic tilt motion, the pressure in X- or Y-axis is displayed to the users fingerpad via the tactor, as illustrated in Figure 3 (d-1) and (d-2). And then, when the SMA wire is being deactivated by cooling, the restoring force of the tip-tilt spring is applied to the SMA wire, and the pressure induced on the fingerpad is removed as a result. Similar to this tilt motion, the tip motion to display the normal pressure is also easily generated. When the four SMA wires are simultaneously activated, the rectangular rim of the tip-tilt spring is vertically lifted up as shown in Figure 3 (c-2). This Z-directional translational motion allows to display normal pressure to the fingerpad, as indicated in Figure 3 (d-3). The reason why the 3-DOF force feedback can be simply displayed with the four SMA wires is due to the tip-tilt spring. In the operation, this compliant mechanism called as a slit diaphragm fle