Two-dimensional Active Type Surface Acoustic Wave Tactile Display On a Computer Screen Masaya Takasaki*, Hiroyuki Kotani and Takeshi Mizuno
Takaaki Nara Graduate School of Interdisciplinary Information Studies, The University of Tokyo
Department of Mechanical Engineering, Saitama University * Interaction and Intelligence, PRESTO, JST
ABSTRACT We have already proposed a novel method to provide human tactile sensation using surface acoustic wave (SAW). A pulse modulated driving voltage excites temporal distribution of standing SAW. The distribution generates friction shift on the surface of a SAW substrate. When the surface with the burst SAW is explored through a slider by a finger, the friction shift generates a mechanical vibration similar to a stick-slip vibration on the finger. The vibration can be perceived as tactile sensation at mechanoreceptors in the finger skin. Controlling the burst frequency according to measured rubbing motion, reality of the displayed sensation can be enhanced. In this research, we install the tactile display with a transparent stator transducer on a computer screen. Two-dimensional rubbing motion is available for the display. An experimental apparatus with finger motion capture system is fabricated on trial and controlled according to the captured motion. In a demonstration, operators can rub on a solid mark under the display to recognize its shape through tactile sensation. 1
INTRODUCTION
Reproduction of human haptic sensation has lately attracted attention for various fields, such as virtual reality, remote control of robots, computer interfaces and so on. Physiologically, haptic sensation is divided into two parts. One is proprioception, which is sensation of weight, resistance, or the approximate shape of an object. The proprioception is perceived at muscles and joints of our bodies. The other is tactile sensation, which is a sense of roughness, friction, or the otherwise variegated texture of an object's surface. The tactile sensation is perceived at mechanoreceptors in our finger skins. Proprioception displays have been reported by a lot of groups and a few displays of them have been applied to actual device productions [1]. Compared with development of proprioception displays, few principles of tactile displays have been published. The principles used various actuators such as miniaturized loud speakers [2], pin arrays [3], pneumatic actuators [4], and ultrasonic vibrators [5]. These actuators need some volume to build in. Previously, surface acoustic wave (SAW) was focused for the actuator of the tactile display [6]. Properties of SAW are E-mail:
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Symposium on Haptic Interfaces for Virtual Environment and Teleoperator Systems 2006 March 25 - 26, Alexandria, Virginia, USA 1-4244-0226-3/06/$20.00 ©2006 IEEE
high operating frequency of more than a few MHz, thin structure, simple fabrication, easy installation of a transducer, high energy density and so on. A thin tactile display with high performance can be developed using SAW properties. To reproduce a tactile sensation of roughness, providing mechanical vibration to our finger skin is effective. Arranging frequency and strength of the vibration enhances reality of the reproduction. Methods to provide the mechanical vibration excited by SAW have been reported [6][7]. In these reports, two types of SAW tactile displays, “passive type [6]” and “active type [7][8],” were proposed. The passive type used progressive wave of SAW and principle of ultrasonic motors. The vibration could be generated directly on operator’s finger. On the other hand, the active type used standing wave of SAW and principle of friction shift. To generate the vibration, the mechanism needed relative motion (active motion) between a SAW transducer and the finger. Therefore, this type was called active type. For the active type SAW tactile display, feedback control according to rubbing velocity was effective [9][10]. Previously, the tactile display was controlled relative to rubbing velocity and slider position measured by a linear encoder. Pulse modulation signals were generated in a microprocessor (SH-2/7045) according to the velocity. Control parameters were provided by a host computer. By using the slider position, the controller could express different roughness. Experiments to discriminate the differed sensations were carried with a large number of subjects. In this research, we propose a transparent active type SAW tactile display. Characteristics of transparency can create various applications on the display. The piezoelectric material used for a transducer in these researches is transparent. The transparent and wide SAW transducer was fabricated and could work successfully. 2
PRINCIPLE
2.1 Reproduction of roughness sensation When we rub on a solid surface, microscopic, vibration is generated in our finger skin. The vibration is perceived at mechanoreceptors in the finger as a tactile sensation. Generation of the vibration in the finger is effective for the tactile display. To reproduce a tactile sensation when we rub on a rough surface, the vibration frequency controlled according to finger motion speed seems to enhance its realistic perception. As mentioned, artificial generation of the vibration in our finger skin is effective to reproduce the tactile sensation, especially roughness sensation. In our research, the vibration was generated by using the SAW. Generally, operating frequency of the SAW is too high (more than a few MHz order) for human to perceive directly. We applied bursting SAW to excite the artificial vibration.
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Sliding Direction
h leig Ray e Wav
Interdigital Transducer (IDT)
Piezoelectric Substrate Figure 1. Excitation of Rayleigh wave.
Skin
{
Rubber film Steel balls SAW transducer Slider
Kinetic Friction (a) Without SAW
Friction Reduced by SAW (b) With SAW
Figure 4. Friction shift by switching the SAW.
Open Metal Strip Array A Operator's Finger
B
}
Rubber Film Steel Balls
Figure 2. Reflection of Rayleigh wave.
LiNbO 3 Substrate IDT Reflector Surface Particles Elastic Material
Surface
Figure 5. Basic structure of an active type SAW tactile display.
Particles Motion
Figure 3. Particles motion in the standing wave.
2.2 Surface acoustic wave Figure 1 indicates the excitation of Rayleigh wave, a kind of SAW. An interdigital transducer (IDT) is arranged on a piezoelectric substrate (Single crystals are usually used for SAW devices). The IDT consists of a metal strip array. When AC driving voltage is applied to the IDT, Rayleigh wave is excited and propagates on the substrate surface in the direction indicated by the arrows in the figure. The frequency of the driving voltage is decided according to the size of the IDT, particularly pitch of the IDT electrodes. The substrate is also used as a media on which Rayleigh wave propagates. In the case of Rayleigh wave, the vibration propagates with gathering its vibration energy of more than 99 % in the media surface of 2 wavelengths in depth. Therefore, the substrate can be fixed easily, for instance, by applying cement to the backside of the transducer. Standing wave of the Rayleigh wave can be generated easily by combination of two opposed progressive waves. To excite the standing wave, two opposed IDTs or two reflectors with one IDT are required on the piezoelectric substrate surface. The reflector is configured by an open metal strip array (OMSA), as shown in Figure 2. The IDTs and the reflectors can be formed simultaneously by using a photolithography process. In the standing Rayleigh wave, the surface particles vibrate horizontally on nodes and vertically on loops, as shown in Figure 3. 2.3 Friction control When a hard material like a steel ball is put on the surface with the wave, the time the material contacts the surface changes according to the very fast vibration. Averaged coefficient of
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Slider
friction between the material and the substrate surface reduced according to the fast vibration. As a result, the kinetic friction seems to be reduced and smaller than normal kinetic friction. If the steel balls are fixed on a tactile display operator’s finger, the friction shift can be applied to generate the vibration in the finger skin with rubbing motion. With the finger skin contacting directly to the surface, however, the friction shift is not available. The SAW vibration amplitude is around 10 nm, and vibration frequency is more than a few MHz. Such a very small and fast vibration is absorbed at finger skin surface due to viscosity and elasticity of cells. Such absorption prevents the friction shift. To apply such a minute vibration for the friction shift, hard materials like steel balls or metal film are required to be installed on the finger skin. In the case of the slider with the steel balls, the friction shift can be controlled by switching SAW excitation (On/Off), as shown in Figure 4. If the hard material is fixed on operator’s finger skin, shear force applied on the finger is equal to the friction during sliding motion. With repeating of the On/Off switching of the SAW at regular intervals (repeat (a) and (b) in Figure 4), the shear force fluctuates. The fluctuation is perceived as a vibration at the mechanoreceptors in our finger skins. The switching is much faster than human perception response due to SAW operating frequency of more than a few MHz. Therefore, frequency of around 1 kHz is possible to excite. Strength of the vibration can be arranged by changing the time of the On, namely duty ratio of the switching timing. If the control follows the vibration at rubbing on the rough solid surface, the substrate explored with the hard materials feels grain according to the frequency. The frequency control is based on finger motion velocity and roughness to be presented. The friction shift cannot be perceived by a direct touch with a finger, as
46 mm
2 mm
15 mm
Reflector
46 mm
43 mm
43 mm Reflector (Metal Strip Array)
IDT
Figure 6. Configuration of the SAW transducer. (The interdigital transducers and the reflectors.)
IDT
90
45
Acoustic Port Electric Port
0.2
Open
0.15
e Figure 7. Equivalent circuit model.
Table 1. Dimensions of the electrodes.
Metal strip width
Pitch
IDT
63 Pm
127Pm
10 finger pairs
Reflector
100Pm
130Pm
120 strips
0.1
Resonance frequency
0.05 0
0.1
-0.05
0.05 0 14.95
15
15.05 15.1 Frequency [MHz]
15.15
Susceptance [S]
Reflector 2
Figure 8. A transparent SAW transducer.
Z0
Conductance [S]
i
Line 3
IDT 2
Line 2
IDT 1
Z0
Line 1
Reflector 1
0
-0.1 15.2
Figure 9. Frequency characteristics of the fabricated transducer.
mentioned above. Previously, we installed a slider with distribution of steel balls as hard materials. Recently, a metal film seems to have sufficient effect on the friction control in the authors’ experiences. 2.4 Basic structure The basic structure of the active type tactile display is drawn in Figure 5. In this research, a wide SAW transducer was employed in order to enable finger motion in direction B drawn in the figure. (For conventional SAW tactile display, rubbing motion was realized only in direction A.) To excite the standing wave, two IDTs were formed on a piezoelectric substrate. Reflectors behind the IDTs were arranged to increase energy efficiency. The standing wave was generated in the center of the substrate. Steel balls as hard materials were fixed on a rubber film on which the operator felt the vibration. The slider consisted of the thin rubber film and the steel balls. A metal film as the slider is also available. The operator rubbed on the substrate surface through the slider. Additionally, measurement system was required to detect the operator’s finger motion. With bursting SAW by pulse modulated driving voltages, friction between the substrate surface and the slider can be controlled. Therefore, the vibration can be produced in the similar manner to a stick-slip vibration. To recognize the controlled friction, the operator must rub the substrate surface with his/her finger through the slider.
3
FABRICATION
3.1 SAW transducer A LiNbO3 128° Y-cut X-prop wafer had been employed for the SAW transducer. For normal wafers of the material, only the obverse surface was polished and the reverse surface was grained. Since the SAW material is typically used for signal processing for communication devices, the grain is formed to reduce noise due to scattering of waves on the reverse side. The LiNbO3 is originally transparent material. Commercially available wafers of the material, however, look like white opaque material. In this research the wafer with both surface polished (special-ordered wafer) was used as the SAW transducer. To excite standing Rayleigh waves, two IDTs were arranged on the surface, since combination of two opposed progressive waves builds a standing wave. Reflectors, which consist of an open metal strip array, were located behind the IDTs to generate the Rayleigh standing wave efficiently. Figure 6 shows a configuration of the electrodes. An IDT has 10 finger pairs and a reflector consists of 120 metal strips. The operating frequency of 15 MHz was decided so that the all electrodes could be aligned on a 4-inch LiNbO3 wafer, because electrodes area would be reduced with higher operating frequency. We optimized dimension of each component and their alignment. The optimized device has high Q factor at resonance frequency, for the configuration of the electrodes is a resonant
51
100
Camera Velocity [mm/s]
80
PC screen
60 40 20 0 Time [10ms/Div]
Slider SAW transducer
Figure 10. Slider velocity in direction 0°.
100 Figure 13. An experiment apparatus.
Velocity [mm/s]
80 60
Retro-reflective marker
40 20 0 Time [10ms/Div] Figure 11. Slider velocity in direction 45°.
Aluminum film Figure 14. A slider with retro-reflective markers.
Velocity [mm/s]
100 80 60 40 20 0 Time [10ms/Div] Figure 12. Slider velocity in direction 90°.
structure. To optimize the dimensions and the alignment, we analyzed electric characteristics by using an equivalent circuit model shown in Figure 7. Each component was described by an F matrix. Each matrix indicated relationship among voltage and current in the acoustic port and electric port of both arms of each component. Therefore, there were 4 x 4 elements in each matrix. By multiplying the matrices, a matrix describing the whole device was calculated. With the equivalent circuit model, the alignment was arranged so that conductance (real part of i / e) of the electric port in the figure was higher. Parameters of the electrodes were decided in this manner. The decided dimensions are described in Table 1. The designed electrodes were formed on a LiNbO3 4-inch wafer (thickness = 0.9 mm) by aluminum evaporation and
52
photolithography. The wafer was cut into 53 mm x 98 mm. Figure 8 shows the fabricated SAW transducer. The LiNbO3 substrate is transparent. (In the figure, background can be seen.) Two IDTs on the surface were connected in parallel. Frequency characteristics were plotted in Figure 9. The conductance curve had some local peaks. The peak indicates resonance frequency. In the figure, three peaks can be seen at 14.9973 MHz, 15.0785 MHz and 15.1606 MHz. Q factors at the peaks, which were calculated from frequencies of conductance local maximum, susceptance local maximum and minimum, were 2617, 2037 and 1883, respectively. Operating frequency was decided at 14.9973 MHz. We measured vibration of the slider while being controlled by the previous control system [10] by using a laser Doppler vibrometer. The slider velocity along 0°, 45° and 90° directions described in Figure 8 were plotted with control signals in Figure 10, Figure 11 and Figure 12, respectively. It can be seen that the velocities were increased/decreased according to the control pulse signal in each direction. This result indicated realization of twodimensional tactile display. With rubbing motion in any direction, the velocity fluctuation can be perceived as vibration in operator’s finger skin. 3.2 Experiment apparatus Figure 13 shows the schematic view of experimental apparatus. The transparent SAW transducer was fixed on a PC screen. Rubbing area between the IDTs was 46 x 43 mm. To realize twodimensional motion, the slide was not guided. A camera was installed to capture the slider position. The slider consisted of
Skin Slider
Video Capture
SAW Transducer (a) Steel balls and rubber film
Slider Position
Aluminum film
Roughness Database
Camera
(b) Aluminum film
Figure 15. An aluminum film slider.
SH-2/7045F
Slider Tactile Display
Driving Signal
RF Amplifier Camera
vm
Pulse Frequency Calculation Host Computer d f
Slider Velocity
Modulation Signal Generation Pulse Signals
Synthesizer 15 MHz
Figure 18. A tactile display control system.
Light Half mirror
Retro-reflective marker Slider
Screen Figure 16. Principle of slider position capture.
Figure 19. Demonstration.
Tactile Display
Figure 17. A sample image of binarizing result.
retro-reflective markers and an aluminum film instead of steel balls used previously [10], as shown in Figure 14. The aluminum film had enough stiffness to transmit the friction between the film and the transducer surface as shown in Figure 15. The slider with the steel balls was replaced by the aluminum film. The markers were illuminated by an LED and a half mirror as illustrated in Figure 16. The illumination enhanced contrast of captured image to be binarized. Figure 17 indicates binarization result. The slider position as the center of the markers was calculated from the result. For this apparatus, a picture was projected on the PC screen during the experiment. Operators could see the picture under the SAW transducer. Therefore, the picture could be explored by the slider directly. Roughness sensation was indicated while the exploration simultaneously. 3.3 Control system The tactile display was controlled by the control system described in Figure 18. Position of the slider was calculated from the captured image in a host computer. Duty ratio d, which
PC Screen
Figure 20. Future application.
decided strength of vibration to be provided to operator’s finger, was selected according to the position (referencing the database drawn in the figure). The differential of the position was transformed into finger motion velocity vm. Frequency f to switch the SAW excitation was decided by
f
vm kr
(1)
53
where constant kr described roughness. kr is also selected according to the slider position (referencing the database). The parameters were updated every 33 ms, because sampling rate of the image was 30 f/s. This rate was enough to present roughness sensation. The updated parameters were transmitted to a microprocessor (SH-2/7045) through serial communication port. Pulse signals whose frequency was f with duty ratio d were generated in the microprocessor. The signal switched the oscillation of the SAW driving voltage in a function synthesizer. The driving voltage outputted from the synthesizer was amplified by an RF amplifier and applied to the SAW transducer. As a result, operators can perceive vibrations at the frequency of f. 4
DEMONSTRATION
Demonstration experiment was carried out using the experiment apparatus. The transparent SAW tactile display was rubbed by authors, as exemplified in Figure 19. In the example, a solid circle shape was projected under the display. The control parameters (d and kr) were provided so that the solid could be a rough surface. We could rub on the image directly through the slider and feel roughness sensation successfully. For objective evaluation of the novel tactile display, psychophysical experiments will be carried out in the next period. 5
CONCLUSION
A transparent SAW tactile display was installed on a computer screen. Two-dimensional rubbing motion was available in this setup. As a result, a computer interface with both visual information and tactile information was provided as virtual reality equipment. 6
FUTURE WORKS
In our research, A LiNbO3 wafer was used for the stator transducer. Commercially available size of the wafer is limited to 100 mm in diameter due to its fabrication method. If wave excitation is available on a non-piezoelectric material like a glass substrate, the size limitation problem will be solved. The tactile display can have freedom in size and shape for its design. The large SAW tactile display would be integrated with acoustic position sensing for touch panels and have a quite simple structure.
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The glass tactile display would be installed on a PC screen so that we could enjoy integration of visual information and tactile information, as shown in Figure 20. To realize the glass substrate tactile display, it is needed to acquire sufficient vibration amplitude on the glass substrate. Development of the ultrasonic transducer to generate such a vibration will be carried out in the next project. REFERENCE [1]
For example, “PHANToM,” http://www.sensable.com/ [2] Howe, R. D., Peine, W. J., Kontarinis, D. A. and Son, J. S., “Remote Palpation Technology,” IEEE Eng. In Medicine and Biology, Vol.14, No.3, 1995, pp. 318-323. [3] Ikei, Y., Wakamatsu, K. and Fukuda, S., “Vibratory Tactile Display of Image-Based Textures,” IEEE Computer Graphics and Applications, Vol.17, No.6, 1997, pp. 53-61. [4] Asamura, N., Yokoyama, N. and Shinoda, H., “Selectively Stimulating Skin Receptors for Tactile Display,” IEEE Computer Graphics and Applications, Vol. 18, No. 6, 1998, pp. 32-37. [5] Watanabe, T. and Fukui, S., “A Method for Controlling Tactile Sensation of Surface Roughness Using Ultrasonic Vibration,” Proc. IEEE Int’l Conf. Advanced Robotics (ICAR), IEEE Robotics and Automation Soc., 1995, pp. 1134-1139. [6] Takasaki, M., Nara, T., Tachi, S. and Higuchi, T., “A Tactile Display Using Surface Acoustic Wave”, IEEE International Workshop on Robot and Human Interactive Communication, 2000, pp. 364-367. [7] Nara, T., Takasaki, M., Maeda, T., Higuchi, T., Ando, S. and Tachi, S., “Surface Acoustic Wave Tactile Display,” IEEE Computer Graphics and Applications, Vol.21, No.6, 2001, pp. 56-63. [8] Takasaki, M., Nara, T., Tachi, S. and Higuchi, T., “A Tactile Display Using Surface Acoustic Wave With Friction Control,” IEEE International Workshop on Micro Electro Mechanical Systems, 2001, pp. 240-243. [9] Takasaki, M., Nara, T. and Mizuno, T., “Standing Surface Acoustic Wave Tactile Display With Frequency Control Related To Rubbing Speed,” 6th International Conference on Mechatronics Technology, 2002, pp. 112-116. [10] Takasaki, M., Nara, T. and Mizuno, T., “Control Parameters For An Active Type SAW Tactile Display,” 2004 IEEE/RSJ International Conference on Intelligent Robots and Systems, 2004, pp. 4044-4049.
