Abstract :
Ultrasonic
motor construction tends to be simpler than EM type motors. Fewer assembly
parts mean fewer moving parts and consequently less wear. The number of
components required to construct an USM is small thereby minimizing the number
of potential failure points.
As the
ultrasonic motor uses ultrasonic vibrations as its driving force,it comprises a
stator which is a piezoceramic material with an elastic body attached to it,and
a rotor to generate ultrasonic vibrations. It therefore does not use magnets or
coils. Therefore there is no problem of magnetic field and interference as in
the case of electric motors. In ultrasonic motors, piezoelectric effect is used
and therefore generates little or no magnetic interference.
Constructionally,a
stator and a rotor are coupled to form an ultrasonic motor. On one side of the
metal stator, the piezoceramic is glued on.It is an element that generates
ultrasonic vibrations when a high frequency voltage is applied. Ultrasonic
motors make use of a ring shaped piezoelectric crystal with segments having
sequentially reversed polarity.On the other side of the metal stator,comb tooth
grooves are set up.The rotor (dynamic body) is pressed tightly against this
side of the stator metal surface so that they are adhered together closely.
PIEZOELECTRIC
EFFECT
Many
polymers, ceramics and molecules are permanently polarized; that is some parts
of the molecules are positively charged, while other parts are negatively
charged. When an electric field is applied to these materials, these polarized
molecules will align themselves with the electric field, resulting in induced
dipoles within the molecular or crystal structure of the material. Further more
a permanently polarized material such as Quartz(SiO2) or Barium
Titanate(BaTiO3) will produce an electric field when the material changes
dimensions as a result of an imposed mechanical force.
These materials are
piezoelectric and this phenomenon is known as Piezoelectric effect. Conversely,
an applied electric field can cause a piezoelectric material to change dimensions.
This is known as Electrostriction or Reverse piezoelectric effect. Current
ultrasonic motor design works from this principle, only in reverse.
When a
voltage having a resonance frequency of more than 20KHz is applied to the
piezoelectric element of an elastic body (a stator),the piezoelectric element
expands and contracts. If voltage is applied, the material curls. The direction
of the curl depends on the polarity of the applied voltage and the amount of
curl is determined by how many volts are applied.
Eg:Quartz,Rochelle
salt,Tourmaline,Lead Zirconium Titanate
It
therefore does not make use of coils or magnets. It is a motor with a new
concept that does not use magnetic force as its driving force. It also
overcomes the principles of conventional motors. The working principle is based
on a traveling wave as the driving force. The wave drives the comb of the
piezoelectric ring. When applied, the piezoelectric combs will expand or
contract corresponding to the traveling wave form and the rotor ring which is
pressed against these combs start rotating.
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