Proc. of 3rd Int. Heinz Nixdorf Symp., pp. 113-118, Paderborn, Germany, May, 1999 Surface Acoustic Wave Linear Motor Minoru Kuribayashi Kurosawa and Toshiro Higuchi Dept. of Precision Machinery Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan mkur@pe.u-tokyo.ac.jp Abstract. This paper describes an application of surface acoustic wave (SAW) device to an ultrasonic linea motor. A merit of SAW device is high energy density and small size. But driving frequency is around 1 MHz or higher. In spite of the difficulty of high frequency, the high energy density is attractive for actuator application. The SAW linear motor's no-load speed and maximum output force were 1.1 m/sec and 3.5 N using a silicon slider. The silicon slider dimensions were 4x4x.3 mm 3. We made a lot of 3 micron diameter projections on the silicon surface. The acceleration was 1 m/sec 2 and response frequency was 13 khz or more. The 4 nm step motion was achieved. The SAW motor is expected to be high speed, quick response, and high resolution micro actuator much more. The most significant problem was the high voltage of the driving. For the previous experiments, we required more than 1 V, due to the primitive design of the driving electrode. Our new designed electrode proved that the driving voltage was reduced less than 1 V to excite the traveling wave. 1 Introduction Surface acoustic wave devices are widely used in communication instruments. These devices are operated at high energy density conditions in order to save materials and space. As is well known, the energy density of piezoelectric devices is roughly in proportion to the operating frequency. Thus the use of surface acoustic wave devices as actuators at high frequency operation conditions can be expected to produce good results. Previously, the realization of a friction drive motor with a surface acoustic wave at high frequency -- e.g. 1 MHz -- was considered to be almost impossible due to the small vibration amplitude around 1 nm. However, the difficulty of the friction drive has been overcome by control of the contact pressure between a slider and a stator. Kurosawa et al. have proposed an X-Y linear motor that operates at 1 MHz in two dimensions and uses a LiNbO 3 wafer three inches in diameter (Kurosawa, Takahashi et al., 1994). We have already demonstrated the operation of an HF band (3 to 3 MHz) ultrasonic motor at 1 MHz (Kurosawa, Takahashi et al., 1996a) and 2 MHz (Kurosawa, Takahashi et al., 1996b) (Kurosawa Takahashi et al., 1997a), which was made possible by operating the Rayleigh wave, a kind of surface acoustic wave was applied to the ultrasonic motors. At high frequency operation, vibration amplitude is very small. Hence, high contact pressure of, for example, several hundred MPa is required for the friction drive to avoid the influence of squeeze film of the air. The operation conditions and basic performance of ultrasonic motors under high-frequency operation have been investigated experimentally (Takahashi, Kurosawa et al., 1995) (Kurosawa, Takahashi et al., 1996c) (Kurosawa, Takahashi et al., 1998). The most significant result of these research was the discovery of the high output force density of the friction drive. The output force density was 5 N/mm 2. However, the actual output force was 1 mn because the tested slider was one steel ball. The maximum speed and maximum acceleration were.8 m/sec and 9 m/sec 2. For a small size linear actuator, the surface acoustic wave motor has numerous advantages, including high output force, high speed, long stroke up to centimeter order, high energy density, easy holding, and high resolution positioning. And while the advantages of a linear motor have been well demonstrated, the motor itself remains to be developed.
As a first step in development of an actual linear motor, newly designed multi-contact-point sliders (Chiba, Takahashi, et al., 1997) (Kurosawa, Chiba et al., 1998) were examined to determine if they could provide sufficient thrust for practical use. A simplified simulation model (Kurosawa, Takahashi et al., 1997c) (Kurosawa, Chiba et al., 1998) was used to estimate the available thrust. The result showed that a surface acoustic wave motor has significantly high potential to serve as small linear motor. Recently, we demonstrated that 7 MHz operation frequency is possible. 2 Principle Basic construction of the surface acoustic wave motor is shown in Fig. 1. The stator transducer is a surface acoustic wave device which is made of litium niobate. At the each end, interdigital transducers are deposited. The Rayleigh wave is excited by the interedigital transducer with a high frequency electrical power source. The driving frequency is the resonance which is fixed by the electrode pitch of the electrode. The driving frequency is about 1 to 1 MHz now. There is an ability of much higher frequency application. The particle motion of the traveling Rayleigh wave is illustrated in Fig. 2. The surface particles move in elliptical. This elliptical motion is applicable to traveling wave type ultrasonic motor as show in Fig. 3. The basic principle is same as traveling wave type ultrasonic motor, however, the Rayleigh Wave N ; Pre-load Slider Interdigital Transducer (IDT) λ propagation Surface Acoustic Wave Device (LiNbO 3 ) Fig. 1 Principle of the surface acoustic wave motor. λ/2 pre-load friction drive slider Fig. 2 Particle motion of the Rayleigh wave. 1nm particle motion wave 4µm Elastice body Fig. 3 Principle of the surface acoustic wave motor.
vibration displacement of the wave is extremely different. Due to the high driving frequency, the displacement is the order of 2 nm or much smaller. This value is the same as the roughness of the stator transducer surface. We have already examined that the most important point of the success is the control of the contact condition between the stator and the slider. 3 Construction of silicon slider motor Photograph of a surface acoustic wave linear motor is shown in Fig. 4. The silicon slider is pushed by a spring which is fixed to a linear guide slider. The dimension of the piezoelectric substrate for a stator transducer was 6x15x1 mm 3. The piezo material was 128 degree y-rotated LiNbO 3. Traveling wave of the Rayleigh wave at 9.6 MHz was generated by interdigital transducers. A slider fabricated with a silicon has a lot of micro contact points on the surface in order to control contact conditions to the stator transducer as shown in Figs. 5 and 6. We tried several Linear guide SAW device Silicon slider Fig. 4 Photograph of the surface acoustic wave linear motor. 4 mm Fig. 5 Photograph of the slicon slider; driving surface. Fig. 6 SEM view of the silicon slider surface.
diameters of the contact points from 1 to 5 µm. The dimensions of the slider is 4x4x1.5 mm 3. The weight of the moving part of the motor was about 3.1 g. 4 Performance of 1 MHz motor We measured the transient response, step motion and the high frequency response up to 13 khz. The silicon slider had 2 µm diameter projections of which center interval was 3 µm. The pre-load was 3 N. The transient responses of the slider is shown in Fig. 7 at several driving voltages. The stating acceleration was larger than 1 m/sec 2. From this value, the output force was 3 N or more. The no-load speed was more than 1 m/sec. Step motion by 4 V driving voltage and 3 waves is shown in Fig. 8. The distance of each step was about 4 nm. This result is open loop control. If we use fine linear scale and closed loop control method, we will obtain nm or sub nm order positioning system. Velocity [m/s] 1.4 1.2 1.8.6.4 Driving Voltage : 17V -p 14V -p 1V -p Displacement [µm].5.4.3.2.1 4 nm.2-2 2 4 6 8 1 12 Time [ms] Fig. 7 Transient response of the surface acoustic wave motor. 13 khz Displacement Slider Stator transducer Fig. 9 Wave trains of the frequency response measurement. Displacement [nm].2.4.6.8 1 1.2 Time [ms] Fig. 8 Step motion of the slider by 4 V and 3 waves 7 6 5 4 3 2 1-1 13 khz 2 4 6 8 Time [µs] Fig. 1 Vibration response of the slider at 13 khz.
By changing the driving IDT alternately as shown in Fig. 9, the response of the slider was measured. At the alternate frequency of 13 khz, the motion of the slider was observed as indicated in Fig. 1. 5 Energy circulation The surface acoustic wave motor shown in Fig. 4 required high driving voltage of 1 V or more and 1 W for high speed operation. This is because the transducer did not circulate the power in the device. Almost all the driving power was absorbed at the absorber. For high efficiency driving, we developed new IDT design as shown in Fig. 11. The new transducer requires two driving IDTs and 2 unidirectional IDTs for circulation. We need two electrical sources whose phase difference is 9 degrees. We have succeeded in exciting traveling wave as shown in Fig. 12. Using this driving method, the driving voltage has been reduced extremely as indicated in Fig. 13. The driving voltage will be reduced down to around 5 V. 7 Conclusion A high performance surface acoustic wave motor was demonstrated. The no-load speed was 1.1 m/sec, the acceleration was 1 m/sec2 and the output force was 3.5 N. A low voltage driving method was developed. With this method, battery drive will be possible. Acknowledgment. We thank Mr. N.Osakabe and Mr. K. Tojo for their experiment job. This work was supported by the Grant-in-aid for general scientific research of the Ministry of Education, Science, Sports and Culture, and Kanagawa Industrial Technology Research Institute. E 1 =E o sin ωt E 2 =E o cos ωt.1 W.1 W IDT L wave 5 W IDT L Fig. 11 Electrode desing for the energy circulation. i 2 V 2 Circulating Power: 49.8 W 7 Driving voltage 1.35V -p 6 Fig. 12 Vbration amplitude distribution of the energy circulatied SAW transducer. Amplitude [nm] 5 4 3 2 1 5 1 15 2 25 3 35 4 Position [µm]
2 Traveling Wave Amplitude[nm] 15 1 5 2 4 6 8 1 Driving Voltage[V -p ] Fig. 13 Driving voltage of the circulated energy SAW motor stator transducer. 8 References M.Kurosawa, M.Takahasi, and T.Higuchi (1994): An Ultrasonic X-Y Stage Using 1MHz Surface Acoustic Wave. Proc. of IEEE Ultrasonics Symp., Cannes, pp.535-538, 1994. M.Kurosawa, M.Takahashi, and T.Higuchi (1996a): Ultrasonic Linear Motor Using Surface Acoustic Waves. IEEE Trans. Ultrasonics, Ferroelectrics, and Frequency Control, Vol.43(5), pp.91-96, September 1996. M. Kurosawa, M. Takahashi and T. Higuchi (1996b): Operation condition and output force of surface acoustic wave motor. Technical Report of Inst. Elec. Info. and Com. Eng., US96-76, pp. 43 5, Dec., 1996 (in Japanese). M. Kurosawa, M. Takahashi, and T. Higuchi (1997a): A surface acoustic wave motor using V shape groove guide. Trans. of Inst. Electronics, Information and Communication Engineers A, vol. J8-A, no. 1, pp. 1711-1717, 1997 (in Japanese). M. Takahashi, M. Kurosawa, and T. Higuchi (1995): Direct frictional driven surface acoustic wave motor. Proc. of Int. Conf. on Solid-state Sensors & Actuators, Transducers 95, Stockholm, Sweden, pp. 41-44, 1995. M. Kurosawa, M. Takahashi, and T. Higuchi (1996c): Optimum Pre-Load of Surface Acoustic Wave Motor. Proc. of IEEE International Ultrasonics Symp. 96, San Antonio, Texas, pp.369-372 (Nov. 3-6, 1996). M. K. Kurosawa, M. Takahashi and T. Higuchi (1998): Elastic Contact Conditions to Optimize Friction Drive of Surface Acoustic Wave Motor. IEEE Trans. Ultrasonics, Ferroelectrics and Frequency Control, vol. 45, no. 5, pp.1229-1237, Sept., 1998. M. Chiba, M. Takahashi, M. Kurosawa, and T. Higuchi (1997): Evaluation of a surface acoustic wave motor output force. Proc. of IEEE Workshop on MEMS, Nagoya, Japan, pp.25-255(1997). M.Kurosawa, M.Chiba, and T.Higuchi (1998): Evaluation of a surface acoustic wave motor using multi-contact-point slider. Smart Materials and Structures Vol. 7 pp. 35-311, 1998. M. Kurosawa, M. Takahashi, and T. Higuchi (1997b): Contact and driving condition of a surface acoustic wave motor. Proc. of IFAC Conf. on Control of Industrial Systems, Belfort, France, pp. 396-41, May 2-22, 1997.