ethod for Controlling Tactile Sensation of Surface Roughness Using Ultrasonic Vibration

Transcription

1 ethod for Controlling Tactile Sensation of Surface Roughness Using Ultrasonic Vibration Toshio Watanabe and Shigehisa Fukui NTT nterdisciplinary Research Laboratories , Midori-cho, Musashino-shi, Tokyo 180, Japan A method is proposed for controlling the tactile sensation of surface roughness. This method creates a smoother feeling on a surface by applying ultrasonic vibration, with a few micrometers amplitude, to the surface. The sensation can thus be controlled without altering the actual surface profile. Experiments using five to ten subjects showed the properties of the proposed method, such as the effect of ultrasonic vibration on tactile sensation, the positive relationship between the generation of a smooth feeling and vibration amplitude, and the dependence on vibration frequency. The typical perceived feeling with this method, air smoothness, is postulated to be due to the squeeze air film effect between the finger and the surface. The calculated squeeze force qualitatively agreed well with our experimental results. The proposed method also enables the generation of resistant impressions, such as the surface is rougher/inore sticky and a virtual protrusion exists on the surface, when the duration of the ultrasonic vibration is short enough. 1 ntroduction n teleoperation or virtual reality systems, the operators need tactile feedback in order to improve their ability to manipulate objects[ ], i21 because the tactile sense is the only one that represents the dyimrnica.1 effects involved with contact. Several works have investigated this area[31-[71. While we are usually not conscious of our tactile sense, we use it in various situations. For example, based on an unconscious analysis of the tactile information received while holding - an object, grasping force becomes a little stronger than that needed to keep the held object from slipping. n spite of the importance of tactile information, there are very difficult mechanical and methodological restrictions on tactile feedback because tactile sensing requires mechanical contact. A better understanding is therefore needed of the basic and elemental techniques of transmitting tactile information. n our work we have focused on the sensations involved with surface roughness, such as rough and smooth, that are considered to be important in dexterous manipulation. Human tactile sensation can be affected by even a slight difference in texture on the order of a micrometer. Conventional methods transfer tactile information by reconstructing the shape of the object mechanically. This approach is limited by the ability to miniaturize the mechanical elements used to form shapes. To transfer the tactile sensations of a surface configuration on the order of a micrometer, we have developed a method for controlling tactile sensation that uses ultrasonic vibration. 2 Coiitrolliiig Tactile Sensation Using Ultrasoiiic Vibration The proposed control method of tactile sensation uses ultrasonic vibration to add a smooth feeling to a rough surface(fig. 1). Rough surface i ultrasonic vibrations Figure 1: Concept of proposed control method. EEE nternational Conference on Robotics and Autornation /95 S EEE

2 - finger motion V ultrasonic vibration Figure 4: A feeling of air smoothness Catches finger 1 Figure 2: Experimental apparatus. 9 Vibration Plate Z Mirror Laser Doppler vibrometer protmiion Figure 5: Perceived surface configuration Figure 3: Standing wave vibration The key feature of this method is that we never alter the actua.1 surface configuration, but only change the sensation. That is, we create a. kind of virtual unreality. By controlling the amplitude of the ultrasonic vibration, we can vary the ta.ctile sensation from the normal rough feeling to a smooth feeling. 3 Basic Properties of Proposed Method 3.1 Experiment Primarily using the apparatus shown in Fig. 2, we clarified the basic properties of the proposed method. Depending on their availability, between five and ten subjects traced a display surface under various vibration conditions and evaluated their tactile impression. The subjects were not given any directions about moving their finger, how much pressure to use, and so on. They touched and traced the surface as they norinaliy would. The apparatus uses Langevin-type vibrators with a resonance frequency of 77 khz. The longitudinal vibration, carried through the horn, generates an ultrasonic bending vibration of a standing wave on the display surface (see Fig. 3). This type of apparatus is often used in experiments on ultrasonic linear motors. While the apparatus has two vibrators to generate a progressive wave, only one is necessary for a standing wave. Our experiments were performed using one loop area of the standing wave, which can be regarded as translational vibration. The amplitude was simultaneously mea,sured at an other loop area with a laser Doppler vibrometer with a resolution of 8 nm. The maximum amplitude of the vibration was about 2 pm. The mode of vibration of the plate was designed to correspond to the resonance frequency of the vibrator. The actual resonance frequency of the apparatus, however, shifted to about 75 khz depending on the environmental conditions and errors in design and production. Therefore, we tuned the vibrating frequency to resonmce for each experiment. The apparatus also produced vibrations at other frequencies: around 36.5, 67.5, and 89.3 khz. 3.2 Tactile Transformation with Ultrasonic Vibration We first examined the tactile transformation caused by ultrasonic vibration under three surface conditions: i) a, well-finished flat surface, ii) a rusty rough surface (the surface had micro, high-density protrusions), and iii) a, surface with single protrusions made of an adhesive agent or by machining. The surface material was steel, the vibration amplitude was about 2 pm, and the frequency was 75.6 khz. The subject,s were asked to describe the tactile feeling of the surface with and without ultrasonic vibration. While the comments of the subjects included some differences in nuance, they

3 Case i) ii) Tactile mpression without vibration with vibration flat but sticky slippery as with an.. air layer rough smooth less slippery were in basic agreement. Regardless of the surface conditions, the tactile impressions were transformed into smoother ones by vibrating the surface. n particular, the following characteristics commonly appeared: A feeling of air smoothness, like that of the surface of an air hockey table, was obtained (Fig. 4). 9 Any configurations, such as protrusions or uneveness in the surface, were perceived to become duller (Fig. 5). The perceived tactile change, however, depended on the surface conditions (see Table 1). The strength of air smoothness for case ii) was considerably smaller than for case i). The tactile impression for cae ii) seemed to be dry compared with that for case i). n general, surface roughness decreased the feeling of air smoothness. The experimental results for the various tested materials (steel, acrylic resin, and wood) were all identical. That is, while the absolute tactile sensations were different, in all cmes a smoother tactile impression was obtained by vibrating the surface. 3.3 Dependence on Vibration Amplitude and Surface Configuration To evaluate the dependence of tactile sensation on surface configuration and Vibration amplitude, the surfaces were roughened with four grades of abrasive paper (JS #8OO, #looo, #l2oo, and #1500). The profiles of the surfaces prepared with #SO0 and #l500 paper, as measured by a surface profiler, are shown in Fig. 6. Even with #1500 paper, the unevenness was larger than the vibration amplitude (about 2 pm). Using four steps from the sensation is still rough to the sensation changes into adequately smooth, the subjects were asked to identify the boundaries between each step. The reason we used a relative evaluation was that it is difficult to compare tactile sensations objectively under different conditions with only one - 1- rl! kw7-b i- t b) Figure 6 Profiles of surfaces prepared by (a)#800 and (b)#1500 abrasive paper vague index ( smoothness ). While the results varied by subject, the tendencies were the same (Fig. 7). 1. As the vibration amplitude became larger, the tactile feeling became smoother. 2. The rougher the surface profile, the larger the vibration amplitude needed to generate a smooth feeling. These results shows that the tactile sensation of surface roughness can be changed by controlling the vibration amplitude and that the control range is determined by the maximum vibration of the surface. 3.4 Dependence on Vibration Frequency The relationship between the generation of a smooth feeling and vibration frequency is roughly illustrated in Fig The tactile sensation of the surface did not become smoother when the vibration frequency was lower than fo. 2. The generation of smoothness became saturated when the vibration frequency went above fl. For vibration frequencies of 36.5, 67.5, 75.6, and 89.3 khz, a difference in the effect of the vibration could not be found. n the experiment with a smooth acrylic plate vibrated by a stacked piezoelectric actuator, the frequencies fo and fl were approximately 10 and 20 khz, respectively. Although their absolute values need further verification, this result confirms that

4 Abrasive paper, #1500 (8.0*0.6*) #12oo (9.5t0.8*) #loo0 ( * #800 ( O* *JS abrasive grain distribution cumulative frequency 50% grain diameter (pm) amplitude (* m) Fixed disk B (S=nr2) P / - - a Squeeze force w AA p v vy \ 2Ah Vibration plate A!(Vibration amplitude) Figure 9: Squeeze air film effect. Table 2: Surface type and equivalent gap. Figure 7: Dependence of tactile sensation on vibration amplitude and surface configuration. Surface h,, (um) (A) (B) (C) (D) flat #1500* #1000* #600* * type of abrasive paper coefficient of viscosity of air) is larger than 10, this force depends almost entirely on excursion ratio E(= Ah,/h)@, which is calculated as shown in Eq. (l)[9]. This equation means that the squeeze force becomes infinitely large as the amplitude, Ah, becomes close to the mean clearance. h. Jo J1 Vibration frequency Figure 8: quency. Dependence of tactile sensation on fre- the effect of the proposed method is independent of the frequency within the range of ultrasonic vibration. 4 Theoretical Considerations We theoretically considered the typical effects of ultrasonic vibration that cause a feeling of air lubrication by focusing on the squeeze air film effect among several possible factors. This phenomenon matches very well with the experimental results, such as the dependence on frequency. The squeeze effect is briefly explained as follows (see Fig. 9). When the mean clearance, h, between plate A and disk B is sufficiently smaller than the size of disk B and the vibration frequency of plate A is high enough, as in the case shown in Fig. 9, the air in the gap is almost enclosed; consequently, its mean pressure, P, becomes higher than the ambient pressure, Pa. As a result, disk B is acted upon by an upward floating force, w, which is called squeeze force. n the case that the squeeze number c = (12p~/Pa)(r/h)2(p: We simulated this effect for the four cases shown in Table 2 by replacing plate A and disk B with a display surface and a finger, respectively. Case (D) is where the tactile impression is barely made smoother by a vibration with an amplitude of about 2 pm. The mean clearance, h, is the sum of the gap created when the finger skin cannot follow the ultrasonic vibration of the plate at all, which equals vibration amplitude, Ah, plus the equivalent gap for the finger skin unevenness, hf, and for the surface unevenness, h,. n this estimation, we assumed that hf was 2 pm and that h, was as shown in Table 2, which was determined from the surface profile data. The squeeze forces calculated using Eq.(l) are shown in Fig. 10; the area of contact, S, was 1 cm2 and the ambient pressure, Pa, was 1.0 x lo5 Pa. When the frequency, f ( = w/2n), was 20 khz, the squeeze number, U, even with surface type #600 and an amplitude of 2.5 pm, was 11.5 (>lo). The shaded area in the figure indicates the normal contact force for a person tracing the surface. t is about N according to our measurements. When squeeze force occurs, the actual contact force becomes smaller than the applied force, N, by the

5 i Fall time Rise time1 1 (C)hp= lop Vibration amplitude Ah (pm) (D) h, = 20 p Figure 10: Calculated squeeze forces. Figure 12: Vibration amplitude and input voltage. _ *- -. _ t \ j ~ ss _ i,!... i \i.... i,... i Vibration amplitude Ah (pm) Figure 11: Calculated relative friction coefficients. squeeze force, N,. f we let the apparent coefficient of friction, as judged by a person, and the actual coefficient of friction be p and /, respectively, Eq.(2) holds for the friction, F. Hence, the relative coefficient of friction, p /,u, is expressed in terms of Eq.(3). The calculated results when N = 0.3 N are shown in Fig. 11. F = p(n - N,) = p N. (2) (3) While friction loes not always match the tactile surface-roughness sensation, it is closely related to such sensations as rough and smooth. From this viewpoint, we assumed that the reduction of friction by ultrasonic vibration described the generation of a smooth feeling. The calculated results quantitatively agree to some extent with the experimental results, such as the relationship between tlie generated smoothness and the vibration amplitude shown in Fig. 7. This implies that the squeeze effect does occur and that our assumption is valid. 5 Texture Representation with Short- Duration Ultrasonic Vibration f the duration of the vibration is short enough, less than about 50 msec, the ultrasonic vibration, on the contrary, can cause more resistant sensations, such as the surface is rougher/more sticky and a virtual protrusion exists on the surface at the moment of the vibration. We examined the effect of such momentary ultrasonic vibrations. n our experiments, a short vibration was generated by tlie amplitude modulation of the input voltage to the vibrator (Fig. 12). The vibration amplitude was about 1.6,mi; the frequency was 74.7 khz. Although it included the rise time and fall time of the vibration, the duration was about 10 msec. The subjects were not informed about either the occurrence of the vibration or its timing, but just asked their impression after the vibration. The experiments showed that: A rougher/more sticky feeling was clear when the difference between the sensation of the surface with and without vibration was large. The virtual protrusion was sharp and clear when the surface was flat, the contact force was light, and the contact area of the finger on the surface was small. The strength of the feelings of rougher/more sticky and virtual protrusion seemed to be related to the slew rate of rise-up and fall-down of the vibration, respectively. By repeatedly applying such momentary ultrasonic vibrations, the tactile sensation for the whole surface can be transformed into a more resistant one. According to the subjects, their impressions while tracing the surface were tne resistance that they did not

6 Static friction causes larger finger distortion. \ - Sticks Static friction Figure 13: Rougher/more sticky impression. F tial distribution and the squeeze effect is just one of several phenomena. Therefore, further investigation is necessary. The texture of a surface, such as smooth or rough, is indispensable for dexterous manipulation. The proposed method provides one solution for the feedback of such sensations. The mechanism used is so simple that it can easily be applied to various applications. We expect that it will help improve the performance of teleoperation/virtual reality systems. We plan to study the principle of the proposed method, the objective evaluation of tactile sensation, and the tactile feedback mechanism that allows relative motion between a finger and a display surface. Figure 14: Virtual protrusion. experience, rather than that, the surface simply became rougher/more sticky. However, the resistance was very large and clear. The finger met resistance in the direction in which it moved. The resistant inipressions were obtained only when the finger was moving, almost nothing was perceived without finger motion. These characteristics are remarkable because they exactly correspond to the actual tactile perceptions. The perceived surface configuration depends on the form of the amplitude modulation. Therefore, the proposed method allows the representation of various textures by varying the amplitude modulation. The perceived resistance while tracing the surface seems due to the occurrence of stick-slip caused by the repetition of the smooth and rough/sticky state of the surface (Fig. 13). The virtual-protrusion impression seems due to the distribution of the pressure caused by instant squeeze force (Fig. 14). 6 Conclusioii We proposed a method for controlling tactile sensations. t creates a feeling of smoothness by applying ultrasonic vibration, with a few micrometers amplitude, to a surface. The proposed method can also generate a more resistant feeling by repeating a shortduration ultrasonic vibration. Based on the experimental results obtained by having several subjects evaluate the effects of the proposed method, we simulated the effect of the squeeze force, which may be the dominant factor causing an impression of air lubrication. The simulated results indicate to some extent the validity of our model. However, tactile sensation is determined by various factors Ref e re iic e s [l] D.G. Hanger and J.G. Webster, Telepresence for Touch and Proprioception in Teleoperator Systems, EEE Trans. Systems Man and Cybernetics, Vol. 18, No. 6, pp , [a] 1C.B. Shimoga, A Survey of Perceptual Feedback ssues in Dexterous Telemanipulation: Part 11. Finger Touch Feedback, VRAS 93, pp , [3] D.G. Caldwell and C. Gosney, Enhanced Tactile Feedback (Tele-Taction) using a Multi-Function Sensory System, Proc. of EEE nt. Conf. on Robotics and Automation, pp , [4] <. Hirota and M. Hirose, Development of Surface Display, VRAS 93, pp , [5] D.A. Kontarinis and R.D. Howe, Tactile Display of Contact Shape in Dextrous Telemanipulation, DSC-Vol. 49, Advances in Robotics Mechatronics and Haptic nterfaces, ASME, pp , [6] C.J. Hasser and J.M. Weiseiiberger, Preliminary Evauation of a Shape-Memory Alloy Tactile Feedback Display, DSC-Vol. 49, Advances in Robotics Mechatronics and Haptic nterfaces, ASME, pp , [7] S. no et al., Proposal of a Tactile Display Method for Presenting Quality of Materials Based on the Temperature Change of Skin Surface while Touching the Materials, Trans. of the SCE, Vol. 30, No. 3, pp , 1994 (in Japanese). [8] E.O.J. Salbu, Compressible Squeeze Films and Squeeze Bearings, ASME J. Basic Engng, Vol. 86, pp , [9] C.H.T. Pan, On Asymptotic Analysis of Gaseous Squeeze - Film Bearings, Trans. ASME, Ser. F, Vol. 89, pp ,