Surface Acoustic Wave Linear Motor

Keyword: Surface Acoustic Wave, Ultrasonic Motor, Linear Motor, Rayleigh Wave, Piezoelectric Transducer, Silicon Slider

Abstract

We have already demonstrated an extremely high performance surface acoustic wave (SAW) linear motor operating at 10 MHz [1][2]. The SAW motor has a lot of merits such as high output force of 3.5 N [1], high speed more than 1 m/s [2], fine resolution positioning of 10 nm order [1][2], long stroke of centimeter, high energy density, easy holding and suitability for miniaturization. For transducer the miniaturization, we should use high frequency. If the operating frequency is 50 MHz, the width and the thickness of the transducer will be a fifth approximately.

Principle

When the Rayleigh wave propagates on elastic material surface, particles on the surface move along an elliptical locus as shown in Fig. 1. A slider arranged on the elastic substrate is driven by frictional force. The slider is pre-loaded so that friction force is enough to drive.
Surface particles movement
Fig 1. Surface particles movement

A piezoelectric material of 128o Y-cut LiNbO3 was used for the SAW motor. When high frequency voltage is input to an interdigital transducer (IDT) on the piezoelectric substrate, the Rayleigh wave is generated and propagates, as shown in Fig. 2.
Rayleigh wave generation
Fig 2. Rayleigh wave generation

The slider made of silicon has many projections on its surface in order to control contact conditions. Figure 3 is a photograph of the slider surface. The diameter of the projections is 10 to 50 µm.
The projections on the silicon slider surface
Fig 3. The projections on the silicon slider surface

10 MHz Motor

The size of the transducer was 15 x 60 x 1 mm3. The operating frequency was 9.6 MHz. The dimension of the silicon slider was 4 x 4 x 0.3 mm3. Figure 4 shows experimental setup of the 10 MHz linear motor. The slider was pre-loaded by leaf spring and the pre-load was about 30 N.
10 MHz motor experimental setup
Fig 4. 10 MHz motor experimental setup

Figure 5 shows the transient response of the 10 MHz SAW linear motor. When the driving voltage was 170 V0-p, the 10 MHz motor worked at more than 1.1 m/s. The maximum output force was 3.5. The response was more than 130 kHz.
10 MHz motor transient response with change of driving voltage
Fig 5. 10 MHz motor transient response

We also succeeded in the step driving. 80 V0-p 30 cycle of the operating frequency was driven to the IDT. The slider motion was observed by using the laser Doppler displacement meter. 40 nm steps can be seen in Fig. 6.
Step driving
Fig 6. Step driving

Miniaturization

Limits of the stator transducer dimensions such as the width and the thickness described in Fig. 7 are proportional to wavelength of the SAW. Therefore, the transducer can be miniaturized by using higher operating frequency. If we use 5 times higher frequency, the limits of the width and the thickness are a fifth. In the case of 50 MHz operation frequency, the limits of the width and the thickness of the stator transducer are 3 mm and 0.2 mm approximately. A problem may occur with miniaturizing the SAW motor. When tangential vibrating velocity is 1 m/s, vibration amplitude is about 20 nm at 10 MHz operating frequency. By using 5 times higher frequency, the amplitude becomes a fifth, 4 nm. However, surface-roughness of the stator transducer is (Ra =) 5 nm and is same as the vibration amplitude. It is wondered that the miniaturized SAW motor can actually work.
Dimensions of the stator transducer
Fig 7. Dimensions of the stator transducer

Miniaturized SAW Motor

Figure 8 is a photograph of the miniaturized stator transducer which was fabricated on trial. The size of the transducer was 5 x 50 x 0.5 mm3. The volume became a seventh of the previous one. Resonance frequency of the IDT, the operating frequency, was 49.76 MHz.
The miniaturized stator transducer
Fig 8. The miniaturized stator transducer

Figure 9 shows the schematic view of the miniaturized SAW linear motor. The stator transducer was arranged on a iron plate. Dimension of the silicon slider was 2 x 2 x 0.3 mm3. The diameter of the projections was 20 mm. A magnet whose dimension was 4 x 4 x 3 mm3 was arranged on the silicon slider.
Schematic view of the miniaturized SAW linear motor
Fig 9. Miniaturized SAW linear motor

Figure 10 is a photograph of the silicon slider combined with the magnet. The slider total weight was 0.18 g. The pre-load, magnetic force added to the gravity force, was 0.13 N.
The silicon slider with the magnet
Fig 10. The silicon slider with the magnet

Estimation

Performance of the miniaturized SAW motor was estimated. If the driving voltage is 60 V0-p, the tangential vibrating velocity is 1 m/s. This estimation was curried out by using the relation between the driving voltage and the velocity at the operating frequency of 10 MHz, because the voltage-velocity relation at 50 MHz can not be observed due to the frequency height. When the vibrating speed was 1.0 m/s, 10 MHz motor speed was 0.8 m/s, namely, 80 % of the vibrating velocity. Therefore, we can estimate the 50 MHz motor speed of 0.8 m/s. When the pre-load is 0.13 N, the novel motor output force will be 0.022 N, for output force of previous 10 MHz motor was about 17 % of the pre-load. The 50 MHz motor has no linear guide. Therefore, the output force will be more than 17 %.

Experiment

The miniaturized SAW linear motor was driven for 8 msec. Driving voltage was varied from 40 to 60 V0-p. The slider motion was observed by using a laser Doppler displacement meter. Figure 11 shows the transient response with the change of driving voltage. The maximum traverse speed of 0.7 m/s was obtained when the driving voltage was 60 V0-p. This is 70 % of the tangential vibration velocity. The maximum acceleration of 200 m/s2 was observed from the figure. The maximum output force of 0.036 N was calculated from the acceleration and the slider weight. This force was 28 % of the pre-load. The 50 MHz motor has no guide. It seems that the increase of the ratio is due to the linear guide.
The transient response with the change of driving voltage
Fig 11. Transient response

Conclusion

An outline of the SAW linear motor was described and the first trial and the first success of the miniaturized SAW linear motor driven at 50 MHz was demonstrated. The stator transducer could be miniaturized into a seventh. The maximum traverse speed was 0.7 m/s. The maximum output force was 0.036 N.

Future

The miniaturized SAW linear motor with much higher operating frequency will be fabricated for further miniaturization. We will demonstrate possibility of a "micro linear motor". The pre-load and the projections condition will be optimized for higher performance. Moreover, our miniaturized linear motor will be combined with an energy circulation method which has been already reported [3], a micro linear motor driven at lower input voltage such as a few volts will be developed.

Reference

[1] N. Osakabe, M. Kurosawa, T. Higuchi and O. Shinoura, "Surface acoustic wave linear motor using silicon slider", Proc. of IEEE Workshop on Micro Electro Mechanical Systems, Heidelberg, Jan. 25-29, 1998.
[2] M. K. Kurosawa, N. Osakabe, K. Tojo, M. Takasaki and T. Higuchi, "Surface Acoustic Wave Linear Motor with a Silicon Slider", Technical Report of IEICE, US98-33, pp.55-62, 1998 (in Japanese).
[3] K. Tojo, M. K. Kurosawa and T. Higuchi, "Circulated Energy Surface Acoustic Wave Motor", The 10th Symposium on Electromagnetics and Dynamics, pp.505-508, 1998 (in Japanese).
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