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
Return to list of stories
page