Energy circulation methods for surface acoustic wave motor

SUMMARY It is important to improve efficiency of surface acous-tic wave motors in order to achieve their commercialization.In the present research, the energy of the surface wave,which has previously not been used effectively for drivingthe slider, but been consumed in sound-absorbing materials,is now circulated by two techniques, so that the surfacewave is excited efficiently. Trial fabrication of surfaceacoustic wave motors using the energy circulation methodsis carried out. It is confirmed that the traveling wave canactually be excited. In addition, the effectiveness of theenergy circulation method is demonstrated by comparisonof the driving performance with that in the absence ofenergy circulation methods. © 2003 Wiley Periodicals,Inc. Electron Comm Jpn Pt 3, 87(2): 10–19, 2004; Pub-lished online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecjc.10132 Key words: surface acoustic wave; actuator; en-ergy circulation; higher efficiency; MEMS. 1. Introduction Surface acoustic wave motors are a type of actuatorsthat are superior in terms of driving force, speed, drivingprecision, and energy density. Further, since the stator ofthe surface acoustic wave device and the silicon-basedslider are fabricated by micromachine techniques cultivatedin the MEMS field, the reliability and reproducibility of thedevice ar e excellent. However, the efficiency of the surfaceacoustic wave motors realized by research up to this timeis essentially less than 1%. It is therefore important forcommercialization to improve the efficiency. The efficiencyof the surface acoustic wave motors can be considered fromtwo aspects. One is the efficiency of conversion from thevibration energy of Rayleigh waves to the output at theslider. The other is the efficiency of conversion of the inputelectric energy to the vibrating energy of Rayleigh wavesused for driving the slider. The first item depends on thecontact surface shape and the contact condition of the slider.The slider output can be improved by improving the contactsurface area between the stator and the stator substrate andoptimization of the preload [1, 2]. The second item dependson the material and design of the stator substrate. In thesurface acoustic wave motors studied to date, only a slightportion of the input energy is used for driving of the slider.The remaining energy is consumed in the absorber. In orderto drive the slider efficiently, it is necessary to circulate thishitherto wasted energy [3].A method of energy circulat ion for the surface acous-tic wave motor has been reported by Tojo and colleagues[4]. In that report, a method for circulating the energy withtwo driving IDTs and four unidirectional electrodes on thestator substrate is presented. By providing input signalswhose phase difference is 90° to two driving IDTs, excita-tion of a traveling wave with a standing wave ratio of 1.6 isconfirmed. However, no report has yet reported success indriving an energy circulation surface acoustic wave motoror discussed its design.© 2003 Wiley Periodicals, Inc.Electronics and Communications in Japan, Part 3, Vol. 87, No. 2, 2004