EUREKA JUNE 2001 COVER FEATURE STORY

Friction drive steps to smaller sizes

A new, long life micro motor looks exceptionally amenable to low cost mass manufacture for a range of consumer applications. Tom Shelley reports

A flat piezo electric motor can be made by printed circuit board techniques yet exhibits good torque, reversibility, and zero backlash


Friction motors have come before, but none as cheap or compact as this.

Applications range from automotive, through mobile phone vibrators to cameras and watches. The motors could soon come to be totally ubiquitous

The Miniswys motor is the latest idea to come out of Creaholic, a Swiss company established in 1986 by Elmar Mock, one of the inventors of the Swatch watch. Previous developments include the WoodWelding process described in Eureka's May 2000 edition.

The latest idea is the brainchild of Bontko Witteveen, Harry Seiffert and Matthias Hell. Development began in 1998. It has already won the company its third successive Swiss Technology Award, the previous winning developments being a radio control system in 1999 and the WoodWelding in 2000.


Matthias Hell (left) and Harry Sieffert (right)

Its aim is to get over the cost manufacturing cost barriers which make magnetic motors with coils of wire increasingly unattractive to produce at sizes smaller than would fit into a 10mm cube. Piezoelectric friction motors offer an interesting alternative and various designs have come to market during the last 20 years, but while they work well (see box), none have yet made a major impact. Creaholic says that existing designs are over complicated, expensive to make, and tend to suffer from wear, giving them a short life span.

The breakthrough achieved by Creaholic is to come up with a motor of extreme simplicity which can be made by printed circuit board type technology.

It consists of two or three 'T' shaped 'resonators', each fitted with a small piezoelectric device to make it vibrate. The end of the top of the 'T' shape impinges on the surface of a rotor and pulls it round. The company describes it as having a 'feather spring' mechanism. Manufacturing does not require tight tolerances, since the end of the resonator moves down to engage on the surface of the rotor, wherever it happens to find it. There is no need for bearings because the resonators hold the rotor. A particularly cunning feature of the concept is that the resonator vibrates in different modes at different frequencies so that in one frequency band, it pulls the rotor round in one direction, while in another, it pushes the rotor round in the opposite direction. The effect is well known and has previously been employed in micro machined silicon pumps. Only one wire and an earth connection is required with very simple driving electronics.

An inherent feature of friction motors is that they have no backlash, since they never completely let go of the rotor they are driving. They exhibit no sticking friction for the same reason and require no gearboxes, since they run efficiently over a very wide speed range. The developers say that wear is minimal.

Two motors were available for inspection on the Swiss technology stand at the just held Hannover Fair. The larger, about 10mm across and 1.6mm thick was able to achieve a torque of 1 N-cm at 60 rpm.

The other motor was about 4mm across. The company has also made a motor with two resonators which rotates a ball. Eureka was told that supply voltage could be 1 to 24V as opposed to 50 to 100V for conventional piezoelectric motors. The prototypes have piezoelectric crystals glued onto resonators made of stamped out or etched out phosphor bronze. Other materials may turn out to be equally or even more suitable. The fundamental design is one which lends itself to etching out or silicon, and could be a very attractive solution for micro and perhaps nano scale electric motors, where magnetics are completely impractical, and the only competitive technology is electrostatic.

One of the target application areas is to drive vibrators in mobile phones, where the small size and weight make them particularly attractive compared with conventional motors. In the car industry, the motors can be used for motor adjustable wing mirrors and closing mechanisms. In the computer industries, potential applications include CD drives and mini printers. And for a business which started out from the Swiss watch, an obvious potential application area is in clocks and watches. However, if the motor makes feasible a future generation of mini, micro and perhaps even nano robots for inside component manufacturing and repair and invasive medical procedures, new markets could open up which dwarf those for conventional machines.

The anticipated market for Miniswys motors is 1,200 million per year. This could prove to be a major underestimate. More information is available at www.miniswys.com

Electric motors have nano motion

A good example of the present generation of practical piezoelectric motors are those made by the Israeli company, Nanomotion, and sold in the UK by Heason Technologies. They were originally revealed in Eureka's July 1998 edition.

The motors are made up of individual modules, 11mm long, 1.5mm thick and 3.3mm wide. By combining two separate wave forms, the crystals can be made to oscillate in a circular motion which may either rotate or move the given element. Typical applications include X-Y-Z stages for semiconductor processing and applications in harsh vacuum environments.

Heason says speed is dependent on the pre-load applied. Under normal circumstances, a pre-load of around 20N is applied, resulting in a velocity of over 200mm/s. For a single finger at 20N pre-load, dynamic holding force is 4N and static holding force, 6N. Models are available able to work in vacuums down to 10-10 Torr. The steps are so small, 5nm, and the stepping frequency so high, 40kHz, that there is no significant perceived vibration. More information may be found at www.heason.com

Other commercially available products include rotary Japanese developed rotary friction motors, mainly aimed at the camera market and inchworm actuators available from Burleigh Instruments in New York, more information at www.burleigh.com

.Design Pointers
Small and compact design, easy to achieve at very low cost. Typical diameter: 4 to 10mm. Height: less than 1.6mm. High torque and force at low speed. No gearbox required

Design requires no iron, no copper and no magnets. Volume and weight are about one fifth of those of electro magnetic motors at the same power output. Produces no electromagnetic interference

Only two connection leads. No backlash. No stick slip

wear is compensated for due to integrated spring in resonator. Voltage required: 1 to 24V. Reversal is achieved by changing applied frequency. Polarity remains the same

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