These and other generalized concepts about microactuators formed a basis for comparing microactuation efforts in Japan and the United States. Table 4.1 summarizes some impressions gained by the JTEC panel during its review of actuation activities within the MEMS area.
Microactuation Activity in Japan
An illustration of the broad range of microactuation techniques under study in Japan is provided by the MITI Micromachine Technology Project. Table 4.2 is a map of the elemental technologies for MITI's Mother Ship and Microcapsule project, which has provided a focus for twenty-four laboratories in Japan and three foreign laboratories. As seen in Table 4.2, the map shows a total of eleven actuation mechanisms being considered for the MITI project alone. Only a few of these mechanisms are associated directly with lithography-based manufacture, especially with manufacture that employs IC-derived processes and materials.
One of the laboratories in Japan that specializes in microactuation using lithography-based processing operates within the Institute of Industrial Science (IIS) at the University of Tokyo (Roppongi Campus). There a Research Group of Excellence in Micromechatronics has been formed under the leadership of Professor Hiroyuki Fujita. This group joins the research of seven professors "to integrate micromachines and microelectronics into a complete mechatronics system which works in the microworld." At IIS there are substantial ties with industrial laboratories such as IBM Research in Tokyo, with which the Fujita group has joined to study the production of needle bearings that are able to support micromotor rotors (of near hair's breadth dimensions) that turn at more than 10,000 rpm under electrostatic drive (Hirano, Furuhata, and Fujita 1993). Other impressive Fujita-lab actuator work has been the development of thermobimorph actuators fabricated in large arrays so that the 500-micron bimorphs are able to move small objects placed upon them by ciliary motion (much as objects are moved within the animal throat by cilia) (Ataka, Omodaka, and Fujita 1993). Fujita's group is looking very broadly at IC-based technologies to develop a wide scope of actuation means. An example that demonstrates the scope is an actuator system in which an electrostatically operated polyimide valve is used to direct a microstream of air in order to push an object in a specified direction on a surface (Konishi and Fujita 1993).
Elemental Technology Map
Also at the University of Tokyo, but not within the Institute of Industrial Science, Professor Hirofumi Miura is leading a group with actuation interests. The presentation by this group of a mechanical ant, which is energized by selective resonant coupling to tuned limbs that provide a walking motion, was very well received at the 1993 International Conference on Sensors and Actuators, held in June in Yokohama (Yasuda, Shimoyama, and Miura 1993). This novel concept is shown in Figure 4.1.
At the University of Nagoya, an active group is focusing on microactuation with a particular interest in the construction of microrobotic systems. Professor Toshio Fukuda and colleagues have shown tiny (millimeter-sized) objects in which electromagnetic resonators carried onboard and powered with small wires cause motion by the action of bent legs that move selectively in one preferred direction (1992). This investigation is a step forward in a project that Fukuda and his associates believe will produce self-organizing robotic systems. Presently, the results are confined to elements and systems that are measured in millimeter dimensions, although the long-term goal is for such robotic systems to be shrunk sufficiently for activity within the body, eventually inside a living heart.
Figure 4.1. Mechanical ant actuated from a vibrating platform.
In the micron-scaled world which is much more adaptably approached via lithographic fabrication technologies, electrostatic actuation has been a very major focus. For surface-micromachined structures, polycrystalline silicon is the major building material at present both in Japan and the United States. Actuation using a stepping motion similar to that of the University of Nagoya's electromagnetic resonator microrobotic devices can also be accomplished via electrostatic drive, as is shown in the moving slider in Figure 4.2, which is described by T. Akiyama and K. Shono from Sophia University (Akiyama and Shono 1993).
A resonant electrostatic-drive mechanism is also employed to power an optical chopper of micron dimensions that was made at Toyota Central Research Laboratories by O. Tabata and associates (1993). The resonant-drive structure in this latter case is made by SOI (silicon-over-insulator) technology and is covered with aluminum as an absorber for the infrared signal that is to be chopped.
Another optical use for electrostatic actuation was demonstrated in the automatic focusing of a diaphragm mirror in a system built by K. Saeki and colleagues at the research laboratories of Nippondenso. This mirror-focus system capitalizes on IC-derived technology to produce economically a variable-thickness mirror so that optical aberration can be held to tolerable limits in a very small bar-code reading system (Saeki et al. 1992). A demonstration of this system at the Nippondenso Research Laboratory showed impressive performance.
At the Hitachi Central Research Laboratory, M. Shikada and associates have used electrostatic drive on a conductive-diaphragm shutter to open and close a microfabricated gas valve (1993). The valve is appropriate for applications to gas-delivery systems for microelectronics fabrication equipment. Microflow systems are a topic of research and development at numerous Japanese research laboratories and industrial companies.
Extremely small-sized, conventionally designed electromagnetic motors are being produced by several large Japanese corporations, such as Nippondenso, Yasakawa, Seiko, Toshiba, Matsushita, and others. For example, Y. Tsutsui and his co-workers at Yasakawa used 0.2 mm diameter winding wires and thin-film magnets to produce a two-phase motor with a rotor 2.5 mm in diameter (1992). At Nippondenso, a microcar with a shell body that compares to rice-grain dimensions was powered by a microstepmotor having a 1.0 mm permanent-magnet rotor and achieving a micronewtonmeter of torque from a 3 V drive at 20 mA (Teshigahara 1992). In the lithography-based micromechanical arena, electromagnetic motors have not been demonstrated in Japan; however, work on magnetic levitation of rotating micromotors is being done by the H. Bleuler group at the Institute of Industrial Research at the University of Tokyo (Bleuler 1992).
Figure 4.2. Electrostatically driven stepping microstructure.
A special area of microactuation needs is presented by the interest in Japan in steerable catheters and endoscopes. One project in the MITI micromachine initiative, the Intraluminal Diagnostic & Therapeutic System, is intended to develop catheters with both observation capabilities and tools on their ends to carry out remote-controlled diagnostics and therapies in blood vessels, in the alimentary canal, and in the pancreatic and bile ducts. The lead industrial laboratory for this research (which is being conducted by four companies) is that of Olympus Corporation, where similar research on the Olympus product line has comparable aims. The longer-range goal of the MITI program in this area is to produce catheter devices equipped with rotating ultrasound emitters and detection systems, vidicon camera units, fluid delivery and extraction systems, and manipulatable tools. At the present time, segmented shaped-memory alloy links made of TiNi alloys are being used to actuate catheter elements. Although details on performance are unavailable, problems with localized heating, speed of response, and reproducibility of the SMA actuators appear to be a research focus. Other approaches to actuation of the steerable catheter are still being considered and studied.
A major actuator trend in Japan is based upon piezoactuation. Several companies (including Canon, Seiko, NEC, Toto, Matsushita, Brother, Toyota, Mitsubishi, Hitachi, Nippondenso, Minolta, Fuji Electric, and others) are investigating both rotary and linear actuation using piezoactuators. Some of the application areas for piezoactuators are highlighted in a chart that was prepared by Professor K. Uchino at Pennsylvania State University. The chart, reproduced in Figure 4.3, shows the division of application areas of 550 piezoactuator patents issued in Japan between 1988 and 1990.
Figure 4.3. Distribution of patent topics for piezoactuators in Japan (1988 to 1990).
Exploitation of piezoactuators has been possible as a consequence of the controlled preparation of several highly active materials in sheet and thin-film form, most especially derivatives of PZT (lead zirconate titanate) and polyvinylidene fluoride. Properties of these materials are presented in Table 4.3, which was prepared by I. Seo of Mitsubishi Petrochemical Co.
Piezoactive materials are used extensively to drive ultrasonic motors. Early uses for the adjustment of camera lenses are well known. Recent improvements in materials have allowed them to be employed in window-closing and even seat-adjusting applications in the automotive field. The successful deposition of thin films allows piezoactuation to become an important candidate for microactuation in lithography-based fabrication. Recent results in this area at Toyota with PVDF are interesting and impressive.