Generation of virtual sliding resistance on a glassy panel using vibration

Transcription

1 PROCEEDINGS of the 22 nd International Congress on Acoustics Structural Acoustics and Vibration (others): 182 Generation of virtual sliding resistance on a glassy panel using vibration Da-Young Kim, Jeong-Guon Ih Center for Noise and Vibration Control (NoViC), Dept. of Mechanical Engineering KAIST, Daejeon, Korea, d.y.kim@kaist.ac.kr; J.G.Ih@kaist.ac.kr Abstract The problem of writing or drawing using the electronic pen as an input device on smart devices is the slip on the display panel due to the low sliding resistance between panel and stylus. The writing perception and the script style on the smart device are different from those using a pencil on the paper. This study is about a method for generating a virtual resistance on the glass panel to be identical with that using a pencil on the paper. It is thought that the dynamic friction can compensate the friction deficit by appending the normal vibration, in addition to the friction due to material characteristics of pen and glass. However, the question about the effective dynamic friction value of the oscillatory excitation, being thought as zero, remains. In this work, the answer to this query is investigated through the experiments. A prototype stylus having a stack of piezo actuators behind the tip is developed. Experiments are conducted and the measured data are used as the reference: the normal force range in writing, the range of writing speeds, the friction coefficients at two sets of contact conditions, i.e., pencil-paper and plastic pen-tipglass panel, and the usual spectrum in writing scripts by a pencil on a paper. The effective frequency range is measured as Hz. For the stylus and glass set, the normal force amplitude that should be compensated for realizing the same writing feeling with the pencil and paper set is at least 0.11 N for the sliding velocity of mm/s. The vibration signal having the peak envelope of 0.13 N is fed to the actuator in the stylus to generate the virtual resistance feeling. Subjective test with 10 subjects reveal that they can clearly perceive the increased sliding resistance. Keywords: sliding resistance, virtual feeling, resistance perception, IT device, friction generation, vibration excitation

2 Generation of virtual sliding resistance on a glassy panel using vibration 1 Introduction In using the handheld smart devices like mobile phone, tablet PC, etc., the electronic pen is often used as an input device. Many users of such devices do not feel happy about the slipping of the pen-tip in writing or drawing on a display panel due to little friction in between the pen tip and the glass panel. As a result, the writing perception and the script style on the smart device are different from those in using a pencil on the paper. This study is about suggesting a method for generating a realistic virtual resistance on the glass panel to be identical with that using a graphite pencil on the usual paper. The vibration, accompanied with tool-mediated scanning situation, has been used for rendering the artificial and realistic virtual material texture in the haptic field. Lots of research works have been conducted to modulate the roughness on the display surface, thus making people s perception on surface texture. However, it has been still remained overlooked to increase the sliding resistance to prevent the slip of a scripting tool on the display [1-4]. Although the dynamic friction at a rough surface fluctuates due to the oscillating normal force, the force oscillation is small at a smooth surface, which results a very small sliding resistance. This study aims at controlling extra normal force using the vibration excitation, and verifying the friction deficit on the glassy panel can be compensated by using the additional vibration envelope in the normal direction, thus creating the virtual resistance feeling during writing or drawing on such panels. 2 Theoretical background The major difference between pencil-paper and plastic-glass plate comes from the roughness and topography of each surface as well as material property. Whereas the paper consists of woven and compressed fibers, associated with rough surface profile [5], plastic and glass panel both have relatively smooth and slippery surface. As an example, Figure 1 exhibits, pencil- Figure 1: A comparison of measured friction forces for two pen-surface sets; paper;, plastic-glass. 2

3 the measured friction force for each surface-slider set, and significant differences can be observed between two surface-slider sets. Recalling Coulomb s friction law, which is one of simple friction models when describing glassy surface like the tablet PC display [6], the friction force, F, is equal to the multiplication of kinetic friction coefficient,, and normal force, N, as F N. For the tool-mediated scanning condition, surface profile is actually the source of vibration [7]. Because the normal and friction force are transient in general, each force can be decomposed as the time-invariant and timevariant components. Therefore, the friction force representation can be written as: F t F f t N n t (1) 0 i 0 t, where F 0 denotes the time invariant component, f(t) the time-variant component, N 0 the steady normal force, i the friction coefficient for time-invariant normal force, n(t) the transient normal force, and t the friction coefficient for time-variant normal force, which is assumed to be constant. We postulate that the envelope of peak normal vibration at high frequencies, which is hard to be felt by the human hand as vibration [8], can increase the time-variant normal force. Based on the fact that the normal force is the main motive forming the resistance, the friction can be also compensated with the increase of time-varying normal force in addition to the inherent kinetic friction from the material characteristics of pen and glass combination. Consequently, if a proper amount of dynamic force, F d, is exerted to generate an extra timevarying normal force in using a plastic-tip stylus on a glassy surface, the friction deficiency can be overcome as p g p g F t F t F t ( ) N n t n' t. (2) d p g i i 0 t t Here, subscripts d, p, g, represents the dynamic force, the pencil-paper set, and the plastic-glass set, respectively, which are also used in the superscripts, and n(t), n (t) mean the time-varying normal force at each contact condition, respectively. 3 Measurement of friction force 3.1 Measurement set-up and data processing condition The friction feeling in writing or drawing is associated with the resistance in between the relatively moving pen and pad. In this study, the resistance feeling in using the graphite pencil on a normal paper is set as the desired resistance to be achieved. An experiment is conducted to figure out the target amount of compensation, by investigating the magnitude and frequency range which are the characteristics of each pen-surface set. Overall procedure complies with the ISO standard for friction measurement of materials [9]. Figure 2 shows main part of the experimental setup. Because there is a need to maintain the normal force as constant during the measurement, a dead weight of 1 kg is attached to the fixture for the pen. In the preliminary measurement, it is found that the usual hand writing speed ranges from 22 to 28 mm/s. 3

4 Figure 2: Experimental setup: test for a plastic tip on glass, measurement setup. To simulate the pen sliding at such speed condition, the relative movement speed is controlled by the test rig using a step motor, which yields the sliding velocity of v=12-33 mm/s as indicated in Table 1. A piezo type quasi-static tri-axial force transducer (Kistler 9067) connected to a charge amplifier (B&K 2635) is mounted in between the two acrylic plates, 30 mm in thickness, which are fixed on an anti-vibration bed. Two contact conditions are chosen in the test: a HBgrade pencil sliding on the wood-free paper (80 g/m 2 ), and a plastic pen on the glass screen having 9H surface hardness. A data acquisition device (NI 9234) is used with a sampling rate of 25.6 khz, and the obtained data is truncated and analyzed with 1% error considering the discharge time of piezoelectric transducer. For each movement of forward (+x direction) or backward direction, 10 data samples are collected to minimize the error depending on the pad location or movement direction at each sliding speed. Table 1: Relative pen speeds in the measurement. Cases v1 v2 v3 v4 v5 v6 Velocity (mm/s) Experimental results Figure 3 depicts the kinetic friction coefficient varying the contact materials and sliding speeds. For all cases, mean value of the kinetic friction coefficient is 0.26 (standard deviation, =0.03) for paper-pencil set, and 0.09 ( =0.01) for plastic-glass set, respectively. These results indicate that the vibration force of 0.17 N, or at least 0.11 N considering 1 -range variation, is required to compensate the discrepancy in sliding resistances of two sets. One can find that a Figure 3: Kinetic friction coefficient varying the speed: pencil-paper, plastic-glass. 4

5 larger fluctuation in friction force happens if the pencil-paper set is used than using the plasticglass set, which is due to the random and anisotropic surface property of the paper. Meanwhile, the 1/3-octave band spectrum of time-varying friction force is analyzed to decide the characteristic of the vibration input signal. By taking the average for 20 times, the effective frequency range, which might be related to the surface roughness, is revealed as khz as shown in Figure 4. It is expected that the vibration excitation in this frequency range would be most effective in enabling the user to perceive the writing resistance which is the equivalent feeling to the pencil-paper situation. Figure 4: The 1/3-oct. band spectrum of oscillating friction force: pencil-paper, plasticglass. The sliding velocities, v, are:, 12 mm/s;, 16 mm/s;, 20 mm/s;, 25 mm/s;, 28 mm/s;, 33 mm/s. 4 Performance evaluation of prototype pen A prototype electronic pen is developed using a stack of piezo actuators (5x5x20 mm 3 ) behind the pen tip as displayed in Figure 5. To test the validity of the suggested method, an experiment is conducted using this pen on the Gorilla glass. All measurement setup and data processing conditions are same with the previous experiment except that the prototype itself can exert a vibratory normal force. A band filtered white noise of Hz is used for the excitation signal to concentrate the vibration energy within the effective frequency zone. Figure 5: Detail view of the developed prototype pen. In Figure 6, the time signals and RMS values are shown, which is related to the tactile perception according to ref. [10]. When a vibration with peak envelope of 0.13 N is fed to the actuator in the stylus, the envelope of sliding resistance becomes similar to that of paper-pencil 5

6 set. In Figure 6, the short time average RMS value of the friction force is compared, and one can find that the prototype pen can compensate the friction force by about 0.09 N. Figure 6: A comparison of friction forces related to the three pen-pad conditions. Time history:, pencil-paper;, plastic-glass,, plastic-glass with additional vibration input. RMS value for a short time span:, pencil-paper;, plastic-glass;, plastic-glass with additional vibration input. The subjective test is also conducted to check the actual feeling of the users whether they can recognize the increase of resistance due to additional vibration excitation. Total 10 subjects (7 males, 3 females, in the average age of 24.7) have participated in the test, and each person is provided with sufficient time to try and handle the prototype in both the vibration on/off conditions. Questionnaires are about the personal perception for the surface roughness during movement of the pen, pleasantness in grasping the pencil, and recognition of increased friction in using prototype pen compared with the switch-off condition. The responses are statistically dealt with, and the result is summarized in Figure 7. The results reveal that, with switch-on condition, the user feels the surface as rough one, and the resistance is increased virtually, while the pleasantness is not very harmed, in comparison with the switch-off condition. However, the wide distribution of response in distinguishing between vibration and friction suggests a further study is needed on selecting the appropriate frequency range and amplitude setting as a future work. Figure 7: Result of subjective test. 5 Conclusions In this study, a vibration control technique is suggested for generating a virtual resistance on the slippery panel to be identical with that using a pencil on the paper. The required writing condition is limited as inclined angle of 0 and usual speed for writing a line, mm/s. From 6

7 measurement of friction coefficient for a graphite pencil-paper set, the effective frequency range that will invoke the real pencil-paper feeling is found to be khz. It is also found that, for the stylus and glass set, the amount of normal force that should be compensated for providing the same sliding resistance feeling with the pencil and paper set should be at least 0.11 N for a range of pen sliding velocity of mm/s. The vibration signal having peak envelope of about 0.13 N is fed to the actuator in the stylus to generate the virtual resistance feeling. A subjective test is conducted with 10 subjects, and all response data, after statistical processing, report that the increase of sliding resistance can be felt clearly, and the letters can be written in the same feeling for using a pencil on a paper. Although the task of selecting the input frequency range and amplitude remains as a further work yet, it is expected that the present method has a good potential to be applied to various electronic pens being used in the IT devices like tablet PCs or mobile phones. Acknowledgments This work has been partially supported by BK21 project and NRF ( and 2016K2A9A2A ). References [1] Kuchenbecker, K. J.; Culbertson, H. Haptic Rendering of Textures. IEEE Haptics Symposium, Houston, Texas, USA, Feb. 22, 2014, pp [2] McMahan, W.; Kuchenbecker, K. J. Haptic display of realistic tool contact via dynamically compensated control of a dedicated actuator. IEEE/RSJ International Conference on Intelligent Robots and Systems, St. Louis, USA, Oct , 2009, pp [3] McDonald, C. G.; Kuchenbecker, K. J. Dynamic simulation of tool-mediated texture interaction. World Haptics Conference, Daejeon, Korea, April 14-17, 2013, pp [4] Bau, O.; Poupyrev, I.; Israr, A.; Harrison, C. TeslaTouch: electrovibration for touch surfaces. Proceedings of the 23nd annual ACM symposium on User interface software and technology, New York, USA, Oct. 3-6, 2010, pp [5] Blau, P. J.; Gardner Jr, J. K. Tribological characteristics of graded pencil cores on paper. Wear, Vol 197 (1), 1996, pp [6] Richard, C.; Cutkosky, M. R.; MacLean, K. Friction identification for haptic display. Proc. of Haptic Interfaces for Virtual Environments and Teleoperator Systems (HAPTICS). Nashville, TN, USA, Nov , [7] Bolanowski Jr,; Stanley J.; et al. Four channels mediate the mechanical aspects of touch. The Journal of the Acoustical society of America, Vol 84 (5), 1988, pp [8] Klatzky, R. L.; et al. Perceiving roughness via a rigid probe: Effects of exploration speed. Proceedings of the ASME Dynamic Systems and Control Division. Vol pp [9] ASTM D : Standard Test Method for Static and Kinetic Coefficients of Friction of Plastic Film and Sheeting, ASTM International, West Conshohocken, PA, 2014 [10] Smith, A. M.; Chapman, C. E.; Deslandes, M.; Langlais, J. S.; Thibodeau, M. P. Role of friction and tangential force variation in the subjective scaling of tactile roughness, Experimental Brain Research, Vol 144 (2), 2002, pp