02-07-2013, 02:53 PM
Piezoelectric ultrasonic motors: overview
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Abstract
This paper reviews recent developments of ultrasonic motors using
piezoelectric resonant vibrations. Following the historical background, ultrasonic
motors using standing and traveling waves are introduced. Driving principles and
motor characteristics are explained in comparison with conventional
electromagnetic motors. After a brief discussion on speed and thrust calculation,
finally, reliability issues of ultrasonic motors are described.
Introduction
In office equipment such as printers and floppy disk drives,
market research indicates that tiny motors smaller than
1 cm3 would be in large demand over the next ten
years. However, using the conventional electromagnetic
motor structure, it is rather difficult to produce a motor
with sufficient energy efficiency. Piezoelectric ultrasonic
motors, whose efficiency is insensitive to size, are superior
in the mm-size motor area.
In general, piezoelectric and electrostrictive actuators
are classified into two categories, based on the type of
driving voltage applied to the device and the nature of
the strain induced by the voltage: (1) rigid displacement
devices for which the strain is induced unidirectionally
along an applied dc field, and (2) resonating displacement
devices for which the alternating strain is excited by an
ac field at the mechanical resonance frequency (ultrasonic
motors). The first category can be further divided into
two types: servo displacement transducers (positioners)
controlled by a feedback system through a positiondetection
signal, and pulse-drive motors operated in a
simple on/off switching mode, exemplified by dot-matrix
printers.
Classification of ultrasonic motors
Historical background
Electromagnetic motors were invented more than a hundred
years ago. While these motors still dominate the industry,
a drastic improvement cannot be expected except through
new discoveries in magnetic or superconducting materials.
Regarding conventional electromagnetic motors, tiny
motors smaller than 1 cm3 are rather difficult to produce
with sufficient energy efficiency. Therefore, a new class
of motors using high power ultrasonic energy—ultrasonic
motors—is gaining widespread attention.
Standing-wave type motors
Rotary motors
T Sashida developed a rotary type motor similar to the
fundamental structure [4]. Four vibratory pieces were
installed on the edge face of a cylindrical vibrator, and
pressed onto the rotor. This is one of the prototypes
which triggered the present development fever on ultrasonic
motors. A rotation speed of 1500 rpm, torque of 0.08 N m
and output of 12 W (efficiency 40%) were obtained under
an input of 30 W at 35 kHz. This type of ultrasonic
motor can provide a speed much higher than the inchworm
types, because of high frequency and an amplified vibration
displacement at the resonance frequency.
Linear motors
K Uchino et al invented a -shaped linear motor [9]. This
linear motor is equipped with a multilayer piezoelectric
actuator and fork-shaped metallic legs as shown in
figure 10. Since there is a slight difference in the
mechanical resonance frequency between the two legs, the
phase difference between the bending vibrations of both
legs can be controlled by changing the drive frequency.
The walking slider moves in a way similar to a horse
using its fore and hind legs when trotting. A test motor
20205 mm3 in dimension exhibited a maximum speed
of 20 cm s−1 and a maximum thrust of 0.2 kgf with a
maximum efficiency of 20%, when driven at 98 kHz at
6 V (actual power = 0.7 W). Figure 11 shows the motor
characteristics of the linear motor. This motor has been
employed in a precision X–Y stage.
Rotary motors
When we deform the rod discussed in the previous section
to make a ring by connecting the two ends topologically,
we can make a rotary type motor using a bending vibration.
Two types of ‘ring’ motor design are possible: (a) bending
mode and (b) extensional mode [14]. Though the principle
is similar to the linear type, more sophisticated structures
are employed in the ceramic poling and in the mechanical
support mechanism.
Speed/thrust calculation
We will introduce the speed and thrust calculation for
ultrasonic motors roughly in this section [27]. These
calculations depend on the type of motor as well as
the contact conditions. The intermittent drive must be
considered for the vibratory coupler type motors, while
the surface wave type provides the continuous drive in the
calculation.
Heat generation
The largest problem in ultrasonic motors is heat generation,
which sometimes drives temperatures up to 120 C and
causes a serious degradation of the motor characteristics
through depoling of the piezoceramic. Therefore, the
ultrasonic motor requires a very hard type piezoelectric
with a high mechanical quality factor Q, leading to the
suppression of heat generation. It is also notable that
the actual mechanical vibration amplitude at the resonance
frequency is directly proportional to this Q value.
Figure 28 shows mechanical Q versus basic composition
x at effective vibration velocity v0 D 0:05 m s−1
and 0.5 m s−1 for Pb(ZrxTi1−x)O3 doped with 2.1 at.% of
Fe [28]. The decrease in mechanical Q with an increase
of vibration level is minimum around the rhombohedral–
tetragonal morphotropic phase boundary (52/48). In other
words, the worst material at a small vibration level becomes
the best at a large vibration level, and the data obtained by
a conventional impedance analyser are not relevant to high
power materials.
Frictional coating and lifetime
Figure 31 plots the efficiency and maximum output
of various friction materials [31]. High ranking
materials include PTFE (polytetrafluoroethylene, Teflon),
PPS (Ryton), PBT (polybutyl terephthalate) and PEEK
(polyethylethylketone). In practical motors, Econol
(Sumitomo Chemical), carbon fiber reinforced plastic
(Japan Carbon), PPS (Sumitomo Bakelite) and polyimide
have been popularly used. Figure 32 shows the wear
and driving period for CFRP, which indicates that the
0.5 mm thick coat corresponds to 6000–8000 hours life
[32]. Although the lifetime of the ultrasonic motor is
limited by the characteristics of the friction material, this
problem has been nearly solved in practice for some cases.