An Energy Circulation Driving Surface Acoustic Wave Motor

Transcription

1 An Energy Circulation Driving Surface Acoustic Wave Motor Minoru K. Kurosawa Tokyo Institute of Technology Yokohama, Japan Purevdagva Nayanbuu Tokyo Institute of Technology Yokohama, Japan Katsuhiko Asai Matsushita Electric Indust. Co., Ltd. Koyoto, Japan ABSTRACT Low power driving method, namely, energy circulation drive of surface acoustic wave (SAW) motor is reported here. The maximum no-load speed and the maximum output force were. m/s and 3. N when the driving power was W. The driving power was only a half of non-circulation drive for same performances. Mechanical output power of the actuator was estimated to be. W. KEYWORDS actuator, linear motor, ultrasonic motor, MEMS, friction drive INTRODUCTION We have proposed to utilize HF frequency band (3-3 MHz) surface acoustic wave (SAW) devices for ultrasonic motors and demonstrated its possibility [1]. A multi contact type slider using a lot of steel balls [] and a silicon slider [3] have been proposed. They were succeeded in gaining large output force. The driving frequency of SAW device was enhanced up to 7 MHz for miniaturization []. For saving the driving electric power, energy circulation driving methods were also investigated [5]. A simple simulation model was proposed to explain excessive pre-load causes lower output force []. Diameter and density of projections fabricated on silicon sliders were examined experimentally [7]. Merits of SAW device as an actuator transducer are huge power density operation and high precision surface micro machining process. The power density of SAW device is 1 W/cm or higher. This value is several orders higher than low frequency PZT transducers. For mass production process management, surface micro machining technology has much higher reliability and reproducibility comparing with the fabrication technology of conventional ultrasonic motors. An interesting point of SAW motor is very tiny vibration amplitude of elastic wave. It s only about nm or less, whereas usual ultrasonic motor s vibration amplitude is around 1 mm. Due to large contact pressure at slider surface, elastic deformation is brought about. This elastic deformation value is almost same as vibration amplitude in case of SAW motor. Therefore, investigation of the friction drive with elastic contact condition is significantly important matter to understand the operation of the motor. In case of single point contact slider, it was successfully explained that the excessive pre-load causes lower output force due to elastic deformation of thrust direction []. For multi point contact slider, simulation model to explain the friction drive including elastic deformation has not been established yet. Hence, we investigated the relation between the static elastic deformation due to pre-load and the driving performances of the SAW motor in experiment. PRINCIPLE OF MOTOR OPERATION A schematic view of a surface acoustic wave (SAW) motor is illustrated in Fig. 1. A surface acoustic wave device is a stator to drive a slider in linear motion. RF electrical power is transduced into elastic wave motion by piezo electric effect of the substrate for the transducer. The Rayleigh wave is excited at an interdigital transducer (IDT). The Rayleigh wave propagating beneath the slider transmit driving force through the frictional force. The slider is pressed to the slider to obtain large thrust. For motor operation, traveling wave is required. By traveling wave propagation, the surface particles of the SAW device move in elliptical motion as illustrated in Fig.. Between the crests of the wave and the pre-loaded slider,friction RF power Rayleigh wave pre-load Slider SAW device (Stator transducer) Electrode (IDT) Figure 1. Schematic view of a surface acoustic wave motor.

2 conventional ultrasonic motor. force is at work. The particles at the crest have peak vibration velocity component in horizontal direction. Therefore, linearly driving force act to the slider in opposite direction of the wave propagation. TRANSDUCER FOR ENERGY CIRCULATION DRIVE A schematic view of the energy circulation SAW device is illustrated in Fig. 3(a). Two RF electrical power sources are used to excite elastic wave. Namely, one way traveling Rayleigh wave is excited with two interdigital transducers (IDTs) with orthogonal electrical power sources. The Rayleigh wave arrived at a unidirectional IDT is converted to RF electrical power without reflection. Then, the electrical Since the amplitude of the elliptical motion is tens nm order, the contact condition of the slider is very critical []. To control the contact condition, actually, the elastic deformation of the slider and the stator surface in nano meter order, for example, a lot of projections are fabricated on the slider surface. The contact point diameter is from several micron to tens of micron. The surface waves have large energy density only around the surface of the propagating material. Therefor, they are called as surface wave. In the case of the Rayleigh wave, the distribution of the particle motion is shown in Fig. 3. This distribution of the wave motion is convenient for the motor. Because the effective motion of the particles is only the driving surface and the opposite surface motion is not good for rigid fix of the stator. Hence, the Rayleigh wave is superior to the flexural wave which is commonly used for (a) E=Ecos E1=Esin t t Unidirectional IDT Stator Transducer Reflector (b) pre-load friction drive Slider 1 nm particle motion wave Elastic material Figure. Schematic view of an energy circulation surface acoustic wave motor, (a) its IDT arrangement and (b) fabricated device. µm Figure. Principle of a surface acoustic wave motor using Rayleigh wave. 35 Circulation power [W] forward 3 backward Driving power [W] Figure 3. Distribution of particle vibration motion of the Rayleigh wave; traveling wave propagation. Figure 5. Circulation power of the fabricated transducer; reflectors are IDT. 13

3 power is transduced into elastic wave at the other unidirectional IDT again. If we change the phase of one power source, the wave propagation direction is alternated. With this mechanism, the traveling Rayleigh wave is excited efficiently in both direction. A fabricated SAW device is shown in Fig. 3(b). The SAW device is the stator of the motor; the dimensions were x1x1 mm 3 and the material was LiNb 3 1-degree y-rotated x-propagation. The electrodes were newly designed to obtain less standing wave at 1.3 MHz. The circulation power is plotted against the driving power of both driving IDTs in Fig 5. The circulation power is almost same in both direction, forward and backward. It was almost three times of the driving power. For example, the driving power of 1 W was required for 3 W traveling wave power. On the other hand, in the case of one electrical source and one normal IDT as indicated in Fig. 1, we need W for 3 W one way traveling wave power. Because the wave propagates in both direction. This means that the energy circulation driving was six times efficient. We tested another electrode design as shown in Fig.. This transducer had different type reflector. The reflectors were short metal strip array (SMSA). This type of reflector has slightly low reflection coefficient than IDT type reflector shown in Fig.. The other demerit of the SMSA is bad space factor, due to the low reflection coefficient. But SMSA has wide band width. Therefor the SMSA is robust against the miss tuning of the resonance of the refrector. The circulation power against the driving power is shown in Fig. 7, in case of SMSA reflector. The circulation power was about five times of the driving power. For example, at the driving power of 1 W, the circulation power was about 5 W in both direction. This transducer was more efficient than the previous IDT reflector transducer as a result. This seems to be miss tuning of the IDT reflector. It seemed that the improvement of the power circulation efficiency would have possibility. If we carry out more precise design of the electrode, the efficiency of the circulation power would be improved. EXPERIMENTAL SETUP FOR SAW MOTOR The performance of the motor was measured by using the IDT reflector transducer. An experimental setup is shown in Fig.. The silicon slider was attached to a linear ball bearing guide. A semi-sphere ball was glued to the silicon slider to maintain parallel between the slider and the stator as shown in enhanced photograph. On the linear slider, a base which had a receptacle hall was glued. The stator was holed on a stainless bar with two holder plates which had electrical contacts to the electrodes of the Figure. Photograph of a stator device which has short metal strip array reflectors (SMSA) for energy circulation electrodes. 5 Circulation power [W] 3 1 SMSA backward SMSA forward IDT 1 1 Driving Power [W] Figure 7. Circulation power of the transducer shown in Fig.. Figure. Photograph of the experimental setup for the energy circulation driving surface acoustic wave motor.

4 transducers on the stator. The stainless bar was pushed up by a coil spring which gave the pre-load for the motor. The pre-load was controlled with a micro meter head by measurement of the shrunk length of the spring. The motion of the moving part was measured with a laser Doppler velocity meter. A slider of the motor is also very important component for friction drive. A photograph of the slider and enhanced image are shown in Fig. 9. The material of the slider is silicon. The surface of the slider was micro machined to form a lot of projections for friction drive. This are was mm by mm. The projection diameter and the interval were 5 micron and 15 micron in this experiment. The other slider which had 1 micron diameter and 5 micron interval was also used. The height of the projection was.5 micron. MOTOR OPERATION Speed and output force of the motor was measured by changing the driving power and the pre-load; pushing force of the slider against the stator. The results of the 5 micron diameter slider and the 1 micron diameter slider are shown in Figs. 1 to 13. The no-load speed of the motor were changed by the preload as shown in Figs. 1 and 1. The speed increased with the driving power, because the vibration amplitude increases with the power. The no-load speed was maximum around the pre-load of 3 to N. The reason of this phenomenon is not sure. Usually, the no-load speed decreases with increase of the pre-load. The maximum speed was about. m/s by 5 and 15 micron slider with a pre-load of N. On the other hand, the maximum speed of the 1 and 5 micron slider was about. m/s with the pre-load of N. The output force of the motor depended on the driving power and the pre-load as shown in Figs. 11 and 13. These curves of the output force were similar to the non-energy circulation driving condition. The maximum output force was around 7 N in both slider case. The energy circulation drive enabled continuous alternation operation as shown in Fig. 1. The slider alternated at 1 Hz. We confirmed two hours continuous operation under the cycle of second alternation and second rest. After that, we found no scratch on the driving surface of the stator. CONCLUSION The energy circulation drive of surface acoustic wave (SAW) No-load Speed [m/s] W 1 Figure 1. No-load speed against the pre-load as a function of the two IDT s driving power; 5 mm diameter and 15 mm interval projections slider. Output force [N] 1 W 1 Figure 9. Photographs of the slider and the enhanced surface texture by SEM images of the surface acoustic wave motor. Figure 11. Output force against the pre-load as a function of the two IDT s driving power; 5 mm diameter and 15 mm interval projections slider.

5 motor is shown. The driving power without the slider was reduced a sixth of the single driving source. The maximum no-load speed and the maximum output force were. m/ s and 7 N when the driving power was W. The driving power was only a half of non-circulation drive for same performances. ACKNOWLEDGMENTS This investigation has been supported by the Grant-in-Aid for the Creation of Innovations through Business-Academic- Public Sector Cooperation of the Ministry of Education, Culture, Science Sports and Technology of Japan.. No-load speed [m/s] REFERENCES [1] M. Kurosawa, M. Takahashi and T. Higuchi, Ultrasonic linear motor using surface acoustic wave, IEEE Ultrasonics, Ferroelectrics and Frequency Control, vol. 3, no. 5, 199, pp [] M. Kurosawa, M. Chiba, and T. Higuchi, Multi Contact Points Slider for A Surface Acoustic Wave Motor, Trans of Inst. Electrical Engineers of Jpn. E, vol.117e, no., 1997, pp [3] N. Osakabe, M. Kurosawa, T. Higuchi and O. Shinoura, Surface acoustic wave linear motor using silicon slider, MEMS 9, 199, pp [] M. Takasaki, M. K. Kurosawa and T. Higuchi, Optimum silicon slider design for 5 MHz SAW linear motor, Transducer 99, 1999, pp [5] K. Asai, M. K. Kurosawa and T. Higuchi, Novel power circulation methods for a surface acoustic wave motor, Proc. of IEEE Ultrason. Symp. 99, 1999, pp [] K. Asai, M. K. Kurosawa and T. Higuchi, Evaluation of the driving performance of a surface acoustic wave linear motor, Proc. of IEEE Ultrason. Symp., 3C-. [7] M. K. Kurosawa, H. Itoh, K. Asai, M. Takasaki and T. Higuchi, Optimization of slider contact face geometry for surface acoustic wave motor, Proc. of Micro Electoro Mechanical Systems 1, Jan. 1-5, 1, pp [] M. K. Kurosawa, M. Takahashi and T. Higuchi, Elastic Contact Conditions to Optimize Friction Drive of Surface Acoustic Wave Motor, IEEE Trans. Ferroelectrics and Frequency Control, vol. 5, no. 5, Sept., 199, pp Figure 1. No-load speed against the pre-load as a function of the two IDT s driving power; 1 mm diameter and 5 mm interval projections slider...3 Output Force [N] 1 1 Speed [m/s] Time [s] Figure 13. Output force against the pre-load as a function of the two IDT s driving power; 1 mm diameter and 5 mm interval projections slider. Figure 1. Continuous cyclic motion of the slider by the energy circulation drive; every 5 ms interval driving phase alternation at the driving power of 7 W and pre-load of.9 N.