INVESTIGATION OF SURFACE ACOUSTIC WAVE MOTOR’S MOTION RESOLUTION DETERMINANT

A surface acoustic wave (SAW) motor exhibits subnanometer motion resolution, over centimeter-level long stroke. The determinant of the motion resolution was experimentally investigated. The experimental results indicated that amplitude of Rayleigh wave, which is utilized for friction drive, determined the motion resolution. The stepping drives by 20-nm amplitude Rayleigh wave limited the resolution to 1 nm. However, by using 9-nm amplitude Rayleigh wave, 0.5 nm stepping drive was achieved. The smaller amplitude achieved the higher resolution stepping motion. Introduction Since ultrasonic motors (USMs) have high-resolution, high-holding torque/force and ability of direct drive characteristics, they are recently regarded as suitable actuators for precision equipments or reasonable alternatives to electromagnetic motors in the marketplace. A motion resolution of an USM seems to be dependent on amplitude of a stator vibration. That is recognized as the reason why a SAW motor exhibits higher resolution motion, ranging down to sub-nanometer, than conventional USMs do [1][2]. However, a motion resolution determinant or relations between the motion resolution and driving parameters, e.g. vibration amplitude or a preload, of an USM has not yet discussed. We discuss the motion resolution’s determinant and relations to the driving parameters of the SAW motor in this paper. Principle A schematic view of a SAW motor is illustrated in Fig. 1. A SAW device (stator) is made of 128° y-rotated x-propagating LiNbO3 substrate. Both end of the stator, interdigital transducers (IDTs) are located. RF electrical power (9.6 MHz) is transduced to Rayleigh wave at the IDT. The propagating Rayleigh wave energy is transferred to a mechanical slider motion through frictional force [3]. Surface particles of the stator move in elliptical motion when the Rayleigh wave propagates. The rotating Pre-Load SAW Device (Stator) Electrode (IDT) Slider Rayleigh Wave RF Power Figure 1: Schema of a surface acoustic wave motor. Experimental Setup An experimental setup is shown in Fig. 3. A stator was fixed on a glass substrate, which was connected to a base. A linear guide rail was fixed to a steel block that could only vertically move in the figure. The stator and the rail were installed in parallel. A washer was glued onto a linear guide stage. A steel hemisphere, which was glued to the slider, was put in the washer. The hemisphere-washer connection yielded a self-alignment structure to maintain the slider and stator surfaces’ parallel contact condition. The stator was 60x15x1mm3. At each end of the stator, IDTs were fabricated. The dimensions of the IDT were 0.4 mm in pitch, 0.1 mm in electrode strip width and 9 mm in aperture. The IDT was composed of 20 strip electrode pairs and the resonance frequency for Rayleigh wave generation was 9.6 MHz. A silicon slider was 4x4x0.6 mm3. The stroke of the slider motion was 25 mm. A laser beam of the laser Doppler interferometer was incident surface particles contact the slider surface at its making wave crests. The particle motion, then, drives the slider through frictional force. The Rayleigh wave amplitude on the stator is several tens of nanometers at most. To control the contact condition in order to utilize the small amplitude vibration and to obtain output power efficiently, a surface microfabricated silicon substrate element has been used as the slider. Many cylindrical projections were fabricated on the surface as depicted in Fig. 2. A diameter and/or a pitch of the projections extremely change a motor performance [4]. Pre-load to the slider enlarges the output force due to the enlarged frictional force. WCU 2003, Paris, september 7-10, 2003