200 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 6, NO. 3, SEPTEMBER 1997 Micro Impact Drive Mechanisms Using Optically Excited Thermal Expansion Osamu Ohmichi, Yutaka Yamagata, Associate Member, IEEE, and Toshiro Higuchi Abstract—The physical phenomenon of thermal expansion of solid materials is useful for microdisplacement actuators because of the scale effect. The response speed of thermally excited actuators is directly coupled with the thermal emission speed, and its value is quite low for mechanisms of the macroscale. However, this speed becomes considerably higher as the actuator becomes smaller. Various methods exist to supply thermal energy to the actuator in order to accomplish thermal expansion. Among them, thermal expansion by means of optical excitation proves to be a good candidate. Major advantages of optical excitation is that it enables noncontact thermal energy supply and remote operation in a special environment such as vacuum and at high environmental temperatures. The structure of the microactuator is simple, and it can be made from various materials using easy fabrication processes. Supported by these advantages, thermally driven micro impact drive mechanisms were developed and fabricated. The global size of the realized micromechanisms is approximately 1.7 0.6 0.4 mm . All are made of aluminum alloy by precision-cutting tech- niques, which is suitable for the fabrication of three-dimensional (3-D) shapes. The maximum travel speed and minimum displace- ment of the developed mechanisms are about 30 mm/s and 1 m, respectively. [239] Index Terms—Optical excitation, microactuator, microfabrica- tion, thermal expansion, ultrahigh precision machining. I. INTRODUCTION T HE AUTHORS have previously succeeded in developing “impact drive mechanisms” in the scale range of millime- ters–centimeters, which can move with small displacement by using impulsive forces and friction. The impulsive force is generated either electromagnetically or piezoelectrically. The piezoelectric mechanism achieves a steady movement, a mini- mum displacements of nanometer order, and a maximum speed of 9 mm/s [1], [2]. However, it requires wires for electrical energy supply, which prevent the miniaturization of these mechanisms. Moreover, at high temperatures heating problems occur, caused by the adhesive, solder, and piezoelectric ele- ments. As an alternative to the piezoelectric effect, thermal expansion can be considered for electromechanical energy Manuscript received October 18, 1996; revised May 23, 1997. Subject Editor, R. O. Warrington. O. Ohmichi was with the Department of Precision Machinery Engineering, University of Tokyo. He is now with the East Japan Railway Company. Y. Yamagata is with the Institute of Physical and Chemical Research (RIKEN), Wako City, Saitama Prefecture, Japan, and the Kanagawa Academy of Science and Technology, Kawasaki City, Kanagawa, Prefecture, Japan (e- mail: yamagata@intellect.pe.u-tokyo.ac.jp). T. Higuchi is with the Department of Precision Machinery Engineering, University of Tokyo, and the Kanagawa Academy of Science and Technology, Kawasaki-City, Kanagawa, Prefecture, Japan. Publisher Item Identifier S 1057-7157(97)06323-3. conversion. A removal of the above mentioned constraints can be achieved by supplying the thermal energy photothermally by using laser pulses, which enables noncontact motion con- trol. For macro-sized mechanisms, actuation based on thermal expansion is rather impractical for high-speed drive because of its slow response. However, for micromachines the scale effect results in a fast response. As a result, photothermally driven micro impact drive mechanisms have the advantages of simple structure, noncontact operation compared to the previous piezoelectric or electromagnetic type and as a result, the photothermal effect has recently gained more attention in the field of micromachines [3], [4]. Generally, micromachining is performed using the basic silicon microfabrication processes such as selective etching and photolithography. These conventional processes make it difficult to construct complex three-dimensional (3-D) geome- tries, forcing most micromachines to be planar in character and to use flexural vibrations. Precision-cutting and grind- ing techniques can overcome such disadvantages as shown by the successful creation of 3-D structures of micrometer order by the authors [5], [6]. Since the “micro impact drive mechanisms” consist of 3-D structures, the precision-cutting technique has been adopted for their fabrication. II. PRINCIPLE OF MOTION The driving principle of the impact drive mechanism is il- lustrated in Fig. 1. The mechanism consists of three distinctive parts, which are called: 1) the main body; 2) the actuation part; and 3) the weight. Static friction keeps the main body in a stationary position on a guide surface. When a rapid extension of the actuation part is photothermally generated, the main body and the weight move away mutually. The instantaneous displacement of the mechanism is obtained from (1), assuming that the impulsive force is much larger than the static frictional force and that the mass of the actuation part is small enough compared to that of the main body and the weight (1) where mass of the main body; mass of the weight; maximum extension of the actuation part. As a result, the mechanism gains an initial velocity and begins to move. Subsequently, the kinetic friction slows the mecha- nism down until it comes to a standstill. The actuation part 1057–7157/97$10.00  1997 IEEE