04-02-2013, 10:18 AM
ULTRASONIC MOTORS
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Introduction
Ultrasonic motors, which have superior characteristics like high torque at low
speed, absence of magnetic interference, and compactness in size, are good candidates for
medical applications, automation, robotics, aerospace engineering and various other
fields. Many different types of ultrasonic motors have been proposed up to date [1]-[5].
The stator of an ultrasonic motor that is excited by piezoelectric elements in
ultrasonic frequency range develops different kinds of vibrations depending on its
structure. From the way of creating an elliptical motion on the stator, ultrasonic motors
were classified into two major groups, such as standing wave and traveling wave types.
Further classifications include mode-conversion, multi-mode [6,7,8] and mode-rotation
types of motors [9] that are suitable for miniaturization and can be manufactured less
costly.
Ultrasonic motors are of great interest due to the flexibility of miniaturization in
comparison with conventional electromagnetic motors whose efficiency decreases
significantly. Especially in information systems [4] and medical industry [10], compact
size of these motors makes them find wider applications.
Objective
The miniaturization of a mechatronic device depends on the size of the motor
equipped in the device to run it. Miniaturization of conventional electromagnetic motors
to several millimeters with high efficiency is difficult since a gear mechanism is required
to obtain a high torque from the electromagnetic motor.
Structures and Operating Principles of Motors
A square beam has two orthogonal bending modes whose resonance frequencies
are equal to each other. The first bending mode frequencies in any direction for circular
cylinders are also equal to each other. The stator of the motor combines the circular and
square cross-sections.
The outside surface of a hollow metal cylinder was flattened on two sides at 90-
degrees to each other and two uniformly electroded rectangular piezoelectric plates were
bonded onto these two flattened surfaces (Fig.1 (a)). The basic configuration of the motor
is shown in Fig.1 (b). Since the stator is symmetric with respect to the x'-axis, the area
moment of inertia about the principal axis is on the x'-axis. The area moment of inertia
about the other principal axis is on the y'-axis. This causes the stator to have two
degenerated orthogonal bending modes, whose resonance frequencies are close to each
other. The split of the bending mode frequencies is due to the partially square/partially
circular outside surface of the hollow cylinder. Driving one piezoelectric plate (while
short circuiting the other to ground) at a frequency between the two orthogonal bending
mode frequencies excites both modes, thus, causing the cylinder to wobble. When the
other piezoelectric plate is driven at the same frequency, the direction of wobble motion
is reversed.
Set-up for the Characterization of the Motors
The performance of the motors was measured using a transient characterization
method, which was initially proposed by Nakamura [11]. The principle of this method is
mounting a load (usually a disk whose moment of inertia is known) onto the motor,
at 71 kHz
at 74 kHz
running the motor, and, finally, analyzing the transient speed obtained as a function of
time. More explicitly, the angular acceleration of the motor is calculated from the speed
measurement by Newton's second law.
The transient torque is then calculated by multiplying the angular acceleration
with the moment of inertia of the load. Using this method, the starting transient response
of the motor gives the speed-torque relation. A load is mounted onto the stator and the
motor is then driven with an AC voltage. The position of the rotating disk is detected
through an optical encoder. The transient position data were then converted into voltage
signal using a frequency-to-voltage converter.
Conclusion
The design, fabrication and characterization of a piezoelectric ultrasonic
micromotor have been investigated.
The piezoelectric motor makes use of two orthogonal bending modes of a hollow
cylinder. The vibrating element, stator, of the motor consists of a brass tube and two
piezoelectric plates bonded to two flattened surfaces of the tube.
Three different motors were fabricated. In the first two designs, a solid stainless
steel was used as a shaft, a spring as a pre-stress and a ferrule to hold the rod and the
spring. Major improvement of the 2nd motor with respect to the 1st one was the reduction
in the dimensions of the stator. 3rd motor was the smallest in length of all and the shaft
was replaced with a spring. Consequently, miniaturization and simplification of the
proposed design were achieved.
All three motors were characterized individually. Admittance spectra of the free
stators, torque, power and efficiency values of each motor were presented.