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THE SCOOP ON ULTRASONIC MOTORS

IN JAPAN

I was invited to Japan by Prof. Kenji Uchino of Sophia University (and more
recently of Pennsylvania State) to give a talk at the Japanese Material Research
Society's Solid State Actuator Symposium. My research in the Mobile Robot
Group at the Massachusetts Institute of Technology (MIT) Artificial
Intelligence Laboratory involves using ferroelectric thin films to make
piezoelectric ultrasonic micromotors. This is a joint project with the
Pennsylvania State University Materials Research Laboratory and the Lincoln
Laboratory Solid State Division. This article describes my visit to Japan and
what I learned of the state of the art in ultrasonic motors there.

BACKGROUND

While we have been building robots for many years in our own group, one problem we face today is that while sensing and control for our robots can fit in small inexpensive packages, motors and batteries tend to be the largest, most costly items. Consequently, I am doing my Ph.D. research on ways to make very tiny, mass-produced robots and better actuators for micromachines. I am focusing on piezoelectric ultrasonic micromotors using ferroelectric thin films of PZT (lead zirconate titanate) in a joint project with the Pennsylvania State Materials Research Laboratory and Massachusetts Institute of Technology (MIT) Lincoln Laboratory. That is the reason I was invited to give the symposium talk at Kanagawa Science Park on 14 December 1991.

Piezoelectric ultrasonic motors are fairly new but widely popular in Japan, while almost unheard of in the United States. They were invented in 1981 in Japan by Toshi Sashida and there are now over 90 U.S. patents (all of Japanese authorship). The interesting part of my

by Anita Flynn

trip was that I was able to visit many companies that developed these motors, met the inventors, and learned of yet other newer companies that are incorporating these types of actuators into products. I visited Ricoh, NEC, Canon, and Panasonic. I also met many of the academic founders of the field at the conference and spent a day at Sophia University.

While my research focuses on using ferroelectric thin films to pattern ultrasonic micromotors (and minimotors) on silicon wafers, typically ultrasonic motors are macroscopic (1 to 3 inches in diameter) and formed from bulk ceramic PZT, on the order of 200 μm thick as compared to our 0.30-um-thick films, which are made at Penn State in a sol-gel process. While ferroelectric thin films have been heavily supported by the Defense Advanced Research Projects Agency (DARPA) in the United States in the past few years for nonvolatile memories, our contribution has been to recognize that we can use solgel processed films to take advantage of their piezoelectric characteristics and actually pattern motors.

The idea behind a piezoelectric ultrasonic motor is to generate a traveling wave of bending in a metallic ring. The bending wave is excited by voltages applied to piezoelectric elements attached to the ring, and the excitation at the resonance frequency of the ring is typically in the ultrasonic range. A traveling wave in a beam causes a particle on the surface to generate elliptic motion. This forms the stator of the motor. A rotor or carriage pressed against the vibrating stator is then propelled along through frictional coupling.

Of course, other types of piezoelectric actuators have been around for years, such as multilayer stack actuators and inchworm motors for precision X-Y stages and hydrophone transducers for Navy sonar. The interesting thing about ultrasonic motors is that they deliver continuous macroscopic motion at inherently high torques and low speeds without the need for gears. Consequently, they can be made very thin and compact. They are also very quiet since there are no gears. In addition, they have quick response time. and large holding torque even when

power is removed. Efficiency has reached 45% to this point.

I was surprised to see a company move so quickly! They didn't say specifically what their target application was, but generally they were investigating the area for possible uses in printheads.

With all that as background, let me relate what I learned of ultrasonic motors in Japan in my travels. Prof. Uchino works in the area of piezoelectric materials and actuators and has contact with NEC many researchers in companies. He set up meetings for me at Ricoh, NEC, and Canon. I set up the meeting at Panasonic as we have collaborative work with them in the "Mobot" Laboratory.

RICOH

On 9 December we visited Ricoh Central Research and Development (R&D) Laboratories where approximately 350 people are employed. In their showroom of products we saw soon-to-be-introduced products in the areas of speech recognition boards for PCs, rewritable magneto-optical disks, conducting polymers for electronic displays, credit card thin batteries, and neural network chips for handwriting recognition. Then we heard talks by two researchers in the areas of micromachining and sol-gel PZT films for microactuators. The first talk described a parallelogram silicon electrostatic microactuator that had been a collaborative project between Ricoh, Prof. Fujita at the University of Tokyo, and visiting scientist Dr. Ken Gabriel, of AT&T Bell Laboratories and now of the Naval Research Laboratory (NRL), I believe. The second talk was by a young Japanese woman, a chemist, who was making thin film PZT using sol-gel processing. I was surprised that it was so similar to our work (their goal was also actuators, as opposed to memories, which is what most effort is focused on), but then the head of Ricoh R&D, Dr. Mario Onoe, had visited the Mobot Laboratory last year and also we had delivered papers on our work both at the Ultrasonics Symposium in Hawaii last December and the MEMS (Micro Electro Mechanical Systems) conference in Nara, Japan, last January. Still,

Neither Ricoh nor NEC seemed to have a clear picture of where micromotors might lead them, but it was amazing to see the amount of resources applied to this realm (especially in light of the fact that Bell Laboratories has now discontinued all activity in the area, while they were the initial impetus for much of this field).

Our visit to Canon on 11 December was slightly different. This was not central research but rather the Lens Development Group, where they have developed and productized ultrasonic motors as the autofocus mechanism in their EOS series single-lens reflex (SLR) cameras. They produce 50,000 of these cameras each month and the motors are truly beautiful. Canon began work in 1983 and holds over one-third of the U.S. patents on these vibration wave

We visited NEC Central R&D Laboratories on 10 December. NEC is a huge company, and over 1,600 people CANON worked in that research facility. We also had two researchers there present their work to us. The first was on a new type of ultrasonic motor they had developed. (This work was presented at the 1989 Ultrasonics Symposium in Montreal). It was a spark plug shaped device that combined longitudinal and torsional vibrations to achieve elliptic motion and hence rotary output. They brought in a working prototype and demonstrated it for us. This motor can deliver high torque and they are working with Olympus to investigate possibilities for using it as the film-winding actuator in cameras. They have worked on this project for 4 years. They also have another flat, thin type ultrasonic motor for paper pushing in a very thin printer.

The second talk at NEC described their work in silicon micromechanics. I was surprised at how many different types of sensors and actuators they had fabricated. They started a few years ago, and their first project was a silicon pressure sensor with piezo resistors and temperature compensation circuitry. They had also made silicon accelerometers targeted for the automobile industry. More interestingly, they had made linear arrays of ultrasonic proximity sensors; a single-crystal, waferbonded, variable capacitance micromotor; a linear comb-type micromotor; and an electrostatic wobble motor that was fairly thick, about 30μm. Basically, they have duplicated just about every type of device that has been discussed in recent MEMS conferences.

motors.

Ring-type ultrasonic motors fit an autofocus application nicely because the motors are essentially hollow-perfect for passing wires through without a slip ring, or even for allowing light to be transmitted to the film from the lens. Since no gears are needed, the motor fits in the periphery of the lens barrel cylinder, and its quick response time and quiet sound make for an elegant camera. Also, it is interesting to note that one current problem with ultrasonic motors is that they tend to wear out because of friction and material fatigue. Consequently, an application such as fan motor might not be amenable, but autofocusing pictures is just fine.

Three engineers from the Canon group presented their work to us, gave us the specification sheets on three varieties of motors, and handed out copies of papers they had written. One talk explained the general fundamentals of the motor, one talk discussed how they overcame an early problem with squeal noise, and the third talk

nicely described finite element analysis of the rotor-stator slip-stick interaction. It was all quite impressive and they answered many questions I had formed after taking apart one of their motors back home and reading through their patents.

After the presentations, they walked us around their laboratories and showed us equipment they used in developing their motors, such as optical microscopes for measuring micron-scale deflections on the surface of the vibrat

firm called Piezotech set up to sell the
technology.

Dr. Kumada also had the informa-
tion on the new Toshiba electromag-
netic (EM) motor and the 10-mm Seiko
ultrasonic motor:

Toshiba EM motor:

3 mm diameter x 5 mm thick

Dimensions
No-load speed .. 200,000 rpm
Stall torque
10-5 Nm

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Dimensions 10 mm diameter x 5 mm thick
No-load speed.. 6,000 rpm
Stall torque.....

1 g f-cm

ing stators. Canon has 25 engineers Seiko ultrasonic motor:
working on ultrasonic motors. They
also gave me some catalogs for the
smallest electromagnetic motors com-
mercially available today--Namiki
motors, which are 7 mm in diameter.
They mentioned that they had heard
that Seiko-Densi had made a 10-mm
ultrasonic motor, but they did not know
any more details. I asked them about
the Toshiba announcement of an elec-
tromagnetic motor, supposedly 1 mm
in diameter, which had earned a two-
paragraph article in the New York Times
on 5 November, but they hadn't heard
about it. The Times article gave few
details.

SOLID STATE ACTUATOR
SYMPOSIUM

The first day of the Solid State Actuator Symposium was 12 December. One speaker was Akio Kumada, who holds several patents on some very unique ultrasonic motors. He gave a talk on his latest, called a Revolving Center of Gravity Resonator, which vibrates radially, much like a hula hoop. His vision is to build a new generation of milliwatt motors that run from complementary metal oxide semiconductor (CMOS) chips and button batteries. He brought along an impressive working prototype that fit inside a watch face and rotated via an on-stator PC board and a 1.5-V lithium battery. His motor draws 15 mA. He has a small

While at the conference, I talked to some young engineers at various car companies and found out that Toyota companies and found out that Toyota now has a ring-type ultrasonic motor in the headrest of the new Crown Majesta. The new top-of-the-line cars have over 60 actuators in the seats, mirrors, windows, antennas, etc. Nissan is also working on another, different type of ultrasonic motor. Essentially, the automotive companies are interested in these motors for their luxury cars because they are compact, very thin, and quiet.

On 13 December I gave my talk and showed videotapes of small ultrasonic stators patterned on silicon wafers that spun glass lenses 1.5 mm in diameter. Afterwards, a young engineer from Olympus that I was sitting next to said that he also was working on an ultrasonic micromotor. Prof. Tomikawa, from Yamagata University, who advises 20 students in this area, said he was affected by the talk and wanted to try to build a similar micromotor.

One person noticeably absent from the conference was Sashida, the original inventor of ultrasonic motors. Apparently, many companies now commercially producing ultrasonic motors have licensed the technology from him. He runs a small development company called Shinsei-Kogyo.

One company they have licensed to, which manufactures and sells the motors, is Fukoko--a company that Toyota works closely with as a supplier of rubber for windshield wipers.

MATSUSHITA

I made a trip down to Osaka on 16 December and visited Central R&D at Matsushita Electric (Panasonic) and met with the ultrasonic motor group. At each company I visited, I gave my talk and explained our goal of building cheap, mass-producible small robots using ultrasonic motors. The group at Matsushita had been working on these motors for 8 years and showed me four or five working models: disk type, ring type, and linear models. They were extremely helpful and gave me insights from their experience to help me in designing better motors with higher output. They also offered to collaborate in the future and to answer any questions I might write to them about later.

Matsushita does not have any of their motors in products at the moment because they typically sell low-cost consumer electronics and the ceramic materials now are too expensive. For instance, one motor that they had built and which I saw spinning had the following specifications:

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Separately from the ultrasonic motor work, Matsushita has another research group in Tokyo that is involved in the Ministry of International Trade and Industry's (MITI) large 5-year program in microrobotics. Matsushita is teamed with Mitsubishi and Murata to build small, pipe-crawling microrobots. They will be 10 mm or so in diameter and will be formed in modules and interconnected like a train. Sensors, such as ultrasonic imagers, will be actuated so that they can turn and inspect the walls of tubes.

SOPHIA UNIVERSITY

Finally, on 17 December, I spent the morning with Prof. Uchino at his laboratory at Sophia University. Four of his students presented their work to me on new high strain materials using antiferroelectric-ferroelectric phase switching, sputtered barium titanate thin films, fatigue analysis, and novel actuators such as "moonie" actuators, which incorporate the best features of both bimorph benders and multilayer stack actuators.

CONCLUDING REMARKS

All in all, it was a productive trip and I learned many things that will be helpful in designing our next generation of piezoelectric micromotors.

Anita Flynn received the B.S. and M.S. degrees in electrical engineering from the Massachusetts Institute of Technology, Cambridge, in 1983 and 1985, respectively. Subsequent to that, she spent 5 years as a research scientist at the MIT Artificial Intelligence Laboratory in the Mobile Robotics Group working on sensing and control problems in autonomous robots. Since 1990 she has been a Ph.D. student at the Artificial Intelligence Laboratory researching piezoelectric motors for miniature robots.

TELE-EXISTENCE WORK AT THE
RESEARCH CENTER FOR ADVANCED
SCIENCE AND TECHNOLOGY AT THE
UNIVERSITY OF TOKYO

Tele-existence work at S. Tachi's University of Tokyo laboratory is described.

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Real time interactive computer simulation (computer simulation, real time computer simulation, etc.)

Communication with a sensation of presence (telephone, teleconference, etc.)

• Tele-existence/telepresence (teleoperation, telerobotics, etc.)

Research into some of these topics began as early as the 1960s, for example, with Ivan Sutherland's computer graphics projects. The early 1980s saw rapid growth due to work by Furness, Kruger, Sheridan, and others. Currently in the United States, centers of excellence are at the University of Washington, MIT, and the University of North Carolina in the academic world; the National Aeronautics and Space Administration (NASA) and the Naval Ocean Systems Center (NOSC) within the U.S. Government; and at several small companies that are marketing products. (This list is meant to be suggestive, not exhaustive.)

Tachi's work was also mentioned in my Scientific Information Bulletin article on virtual reality ["Virtual reality," 16(4), 43-45 (1991)]. That report references a conference held July 1991 on

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