The present invention relates to an ultrasonic motor that
uses a stator in the form of a coil for use in the fields of medicine and industry.
Ultrasonic motors or actuators find many applications in
the field of mechanical engineering such as in robotics, electric motors, in the
field of medicine such as diagnosis and therapy, and in the field of measurements
that must be free from electro magnetic interference. The motors for use in medicine
such as in intravascular ultrasound (IVUS) and thrombectomy require the following
characteristics, because the motors are used inside a vessel:
- (a) The motor must be as small as possible, typically 1 mm in diameter and 5
mm in length.
- (b) The motor should work in a liquid environment.
- (c) The rotor should be a hollow tube.
Conventional motors can be evaluated as follows from the
viewpoint of said requirements:
- Traveling wave type ultrasonic motors are widely used in robotics and in cameras
because they have an advantage over conventional electro-magnetic motors because
of excellent characteristics such as large torque for their size and low speed.
The basic principle of the traveling wave type ultrasonic motor is as follows. When
the ultrasonic wave (the Lamb wave) propagates along a slab (stator), surface particles
of the slab move elliptically. Therefore, if the mover or rotor is pressed against
the slab using a spring, the mover is driven in the opposite direction of the wave
direction due to friction.
In the conventional traveling wave type ultrasonic motors,
there are two types of configurations. In type A, a plate is used as an acoustic
waveguide, and in type B, a ring in a plane is used as a waveguide. In these motors,
the ultrasonic wave propagates in a plane. Therefore, the contact region between
the stator and the rotor is short. Therefore, a spring is used to apply sufficient
preload. Therefore, the conventional traveling wave type ultrasonic motors have
the following drawbacks:
- (1) Use of a spring for the application of the preload prevents a decrease of
the size of the motor.
- (2) This motor does not work in a liquid environment. In type B, water proofing
is necessary for use in water.
- (3) In type B, the rotor can be a hollow tube. However, an increase of the thickness
in the radial direction prevents its use inside a vessel. In type A, a hollow tube
cannot be used as a rotor.
In conclusion, the conventional traveling wave type ultrasonic
motors do not satisfy the said requirements (
J. L. Jones, H. Rodriguez, R. Ceres, and L. Calderon, "Novel Modeling Technique
for the Stator of Traveling Wave Ultrasonic Motors, " IEEE Transactions of Ultrasonics,
Ferroelectrics, and Frequency Control,Vol. 50, No.11,pp. 1429-1435,Nov. 2003
Apart from the traveling wave type ultrasonic motor, there
are some types of ultrasonic motors that can be miniaturized. For example, a motor
that uses vibration at the top of a fiber can be miniaturized, because it does not
use a spring. However, it cannot be used in water because it is difficult to vibrate
the fiber in water at a high speed. Furthermore, it is difficult to use a hollow
tube as a rotor (
R. Carotenuto, A. Lula, and G. Galiano, " Flexible Piezoelectric Motor based
on Acoustic Fiber," Applied Physics Letters, Vol.27, No.12, pp. 1905-1907, 2000
DISCLOSURE OF THE INVENTION
The object of the present invention is to solve the following
problems occurring in the conventional traveling wave type ultrasonic motor.
- 1. The conventional traveling wave type ultrasonic motor cannot work without
using a preload spring, thus preventing the further reduction in its size.
- 2. The conventional traveling wave type ultrasonic motor cannot work in a liquid
environment unless appropriate waterproofing is provided.
- 3. A hollow tube cannot be used as a rotor inside a vessel.
The invented motor operates as a transmission type ultrasonic
motor. The motor uses a coiled waveguide in the shape of a helical coil as a stator.
The use of a coiled stator enables driving the rotor in a wide range of interfaces
between the rotor and the stator. Therefore, the rotor can be driven with a small
preload, making a preload spring unnecessary.
The motor has the following features.
BRIEF DESCRIPTION OF THE DRAWINGS
- 1. The motor can be extremely miniaturized. Even the use of a carbon nano-coil
may be considered.
- 2. The motor can be used in a liquid environment.
- 3. A hollow tube can be used as a rotor.
- 4. The rotor can be linearly displaced if the helical groove is provided on
- 5. Any axisymmetric object such as a sphere, a cone , or an ellipse can be rotated.
- 6. A rod and a hollow tube can be moved along the axis of the mover
THE PREFEED EMBODIMENT OF THE INVENTION
[The principle of operation of the conventional traveling wave
type ultrasonic motor]
- Fig.1 is a diagram describing the principle of the conventional traveling wave
type ultrasonic motor.
- Fig.2 is a diagram describing the basic system of the invented ultrasonic motor
that uses a coiled acoustic waveguide as a stator.
- Fi.g.3 is a diagram describing the direction of the vibration of the Lamb wave
traveling along the coiled acoustic waveguide.
- Fig.4 is a diagram describing the working principle of the invented ultrasonic
motor where the particles at the points P, Q, R, S drive the rotor sequentially
- Fig.5 is a diagram describing the principle of the example 1 of the invented
motor that uses the acoustic waveguide in the form of a helical coil as a stator
placed outside of the rotor.
- Fig.6 is a diagram describing the paired acoustic waveguide that transmits the
Lamb wave from the transducer to the invented ultrasonic motor.
- Fig.7 is a diagram describing the structure of the invented motor shown in example
2 that uses the stator placed outside of the rotor where the two waveguides are
used to allow forward and backward rotation.
- Fig.8 is a diagram describing another example of the invented motor that uses
the stator placed inside of the rotor.
- Fig.9 is a diagram describing the structure of the motor that uses the paired
waveguide and the stator placed outside of the rotor.
- Fig. 10 is a diagram describing the invented motor that uses a sphere or a hollow
tube as a rotor.
- Fig.11 is a diagram describing the structure of the encoder.
- Fig. 12 is a diagram describing the encoder shown in Fig. 11 counting a pulse.
- Fig. 13 is a graph that shows the output of the encoder shown in Fig.11.
- Fig. 14 is a diagram describing the application of the invented motor to ultrasonic
- Fig. 15 is a diagram describing the operation of example 6 that enables the
mover to move along an axis.
- Fig. 16 is a diagram describing an acoustic feed back loop system that enables
enhancement of the torque of the motor using a transducer arrangement.
- Fig. 17 is a diagram describing a method for generating an ultrasonic wave in
a specific direction by activating the transducers with the waveform as shown.
- Fig. 18 is a graph showing the group velocity of the flexural wave propagating
along the coiled waveguide as a function of the inner diameter.
- Fig. 19 is a diagram describing how to carry out a series connection of the
- Fig.20 is a diagram describing how to carry out a parallel connection of the
- Fig. 21 is a diagram describing the structure of example 10 in an elevational
view (a) and in a top view (b).
First, since the principle of operation of the motor of
this invention is the same as that of the conventional traveling wave type ultrasonic
motor, the principle of operation of it is described. Here, attention is first paid
to the displacement at each point on the surface when a flexural wave traveling
along a slab.
Fig. 1 (a) expresses the temporal progression of the deformation
of the slab as the flexural wave travels from left to right. The initial form of
the slab at time t0 is shown in the figure; then, the slab changes its form as time
progress as shown. In the figure, t1, t2, t3 correspond to the form of the slab
at the time of &pgr;/2, &pgr;, 3&pgr;/2 seconds later.
When attention is paid to the point on the central line
of an elastic body (for example, the point q), it turns out that point q is displaced
to the vertical direction only. Next, attention is paid to the surface point (for
example, the point p). It turns out that point p is displaced not only vertically
but also tangentially. The locus of the displacement of point p is shown Fig. 1
(b). Thus the surface elements of the slab move elliptically.
Now, a movable body (mover) is in contact with the slab,
and the flexural wave is propagated along the slab, as shown in Fig. 1 (c). The
movement of the surface particle is transmitted to the mover through frictional
forces. The direction of the movement of the mover is opposite to the direction
of propagation of the flexural wave.
[The principle of operation of the ultrasonic motor of this
The fundamental composition and the principle of operation
of a traveling wave type ultrasonic wave motor of this invention are explained.
As shown in Fig. 2 (a), the motor M of this invention consists
of a rotor 2 with a circular cross section, and stator 1 which is an acoustic waveguide
in the shape of a helical coil wound around the rotor 2. A spirally coiled acoustic
waveguide constructed on a plane can also be used as a stator in some configurations.
Usually, in any traveling wave type ultrasonic wave motor,
it is necessary to move surface particle of the stator elliptically. The coiled
waveguide has a finite thickness. Therefore, when the flexural wave progresses along
the coiled waveguide, the surface elements of the coiled waveguide move elliptically.
Here, as shown in Fig. 2, the point P, Q, R, S are separated at a 30-degree interval
as shown in Fig. 2(b). The arbitrary point near the surface of the acoustic waveguide
1 (any one point of P, Q, R, S) performs an elliptical movement in concert (what
I wanted to express!!) with the progress of the flexural wave. Note that the main
direction of the displacement of the flexural wave is radial, as shown in Fig. 3.
When there is no propagation of a flexural wave, the rotor
2 and the acoustic waveguide 1 are placed in non-contact close proximity. When the
flexural wave propagates in the axial direction (from P to Q), the particle at P
moves in a clockwise direction as shown in Fig.3. When the amplitude of the flexural
wave is small, the surface element traces the elliptic locus
1. When the amplitude is large, the element traces the line
2 from A to B in Fig.3, because the element touches the surface of the rotor.
Then the element drives the rotor in the counter clockwise direction.
Assume that the wavelength of the flexural wave is equal
to the pitch of one turn of the helical coil. The surface elements at P, Q, R, S
touch the rotor consecutively as the flexural wave propagates, and cause the rotor
to rotate, as shown in Fig. 4. If the element at P and the element at the axisymmetric
point drive the rotor at the same time, the rotor is driven without any preload
spring. Since the element on the helical coil moves in the axial direction, the
rotor can also be driven in the axial direction.
Thus the invented motor is constructed without using the
spring and the absorber, making the motor simple and robust compared to the conventional
traveling wave type ultrasonic motor. If the stator is constructed using a coil
in a plane, it is necessary to place the coil between two disks.
Any object that has an axial-symmetry such as a rod, a
disk, a tube, a cone, or a sphere can be used as a rotor. If a groove is constructed
on the surface of the rod, linear displacements of the mover can also be made. The
rotor can be made of any solid such as metal or ceramic. However, the choice of
material must take wear into account.
The flexural wave considered here is known as the Lamb
wave. Use of a thick slab enhances the driving force. Furthermore, the part of the
wire must be flattened to enhance the driving force, because the wider contact area
enhances the driving force. Any kind of acoustic wave other than the Lamb wave,
such as a surface wave, can be used for the coiled stator.
Fig.5 shows the principle of example 1 of the invented
traveling wave type ultrasonic motor that uses a coiled waveguide as a stator. The
motor consists of transducers 3 and 4, waveguides 5a and 5b, a rotor 2, and a coiled
acoustic waveguide 1a wound around the rotor 2.
When an electric voltage is applied to the transducer 3,
the flexural wave is excited, and is propagated along the waveguide 5a to the coiled
acoustic waveguide 1a. The propagation of the flexural wave along the coiled acoustic
waveguide causes an elliptical orbit of the point on the contact surface of the
waveguide. The elliptical motion of the points causes the rotor to rotate in the
direction opposite to that of the propagation of the flexural wave via the frictional
force between the coiled acoustic waveguide and the rotor. Here every point on the
waveguide in the contact region causes a driving force in the same direction. The
flexural wave is attenuated as it propagates along the coiled waveguide. Although
the driving force per length that is provided by the coiled waveguide is small,
the long contact region allows transmission of all the power sent by the waveguide
to the rotor 2 less the transmission loss. When the transducer 4 is excited, the
rotor 2 rotates in the other direction.
If one transducer is used, the rotor rotates in one direction
only. If two transducers are excited at the same time with an appropriate phase
difference, or a frequency difference, a standing wave is formed along the coiled
waveguide. If the frequency or phase is swept, the standing wave can be moved along
the waveguide. Therefore, the operation as a standing wave type ultrasonic motor
Any materials such as metal, ceramics, sapphire, or fused
quartz can be used as a helical coil. A slab, a rod, or a partly flattened rod made
of one of these materials may be used as a coiled waveguide.
A motor based on Fig.5 was manufactured. The flexural wave
at a frequency of 50 kHz was propagated along the acoustic waveguides 5a, and 5b.
The acoustic waveguide was constructed using a 0.05 mm thin, 1 mm wide nickel plate
. The flexural wave progressed along the acoustic waveguides 5a or 5b, and the helical
coil activated the rotor. The rotor was constructed using a 2 mm diameter, 20 mm
long aluminum rod.The rotor could be equipped with a fan.
A second motor based on the Fig. 5 was manufactured. In
this case only one waveguide was manufactured using a 0.2 mm diameter piano wire..
The inner diameter, number of windings, and pitch of the coil were 2.1 mm, 10 turns,
and 0.4 mm. Output torque was measured to be 0.1 µN·m by applying 80 V
to the transducer (a Langevin type transducer) at the frequency of 8.8 kHz.
Fig.6 shows a diagram describing a paired waveguide used
for the invented motor.
An example of the invented motor is shown in Fig. 7. The
motor M consists of paired waveguides 5a and 5b, a rotor 2, a stator 1a placed inside
the rotor 2, transducers 3 and 4. In this example, an electric voltage is applied
to the transducer 3, and the flexural wave propagates along the waveguide 5a to
the stator 1a. The elliptical movement of the surface element of the stator 1a causes
the rotor to rotate as described in Example 1. Fig. 8 shows the enlarged arrangement
of the stator 1a and the rotor 2.
The stator 1 can also be placed outside the rotor 2 as
shown in Fig. 9.
The rotation speed and the torque of the motor depend on
the diameters of the acoustic waveguide and the rotor, the amplitude of the flexural
wave, and the duration of the pulse if the flexural wave is pulse modulated. Furthermore,
a groove on the surface of the rotor modifies the torque and the rotation speed.
A motor based on the Fig. 8 was constructed. In this case,
only one waveguide 5a made of a 0.2 mm diameter piano wire was constructed. The
dimensions of the rotor are 0.7 mm outer diameter, 0.1 mm pitch, and 15 turns of
the windings. The dimensions of the rotor are 0.8 mm inner diameter, 1.0 mm outer
diameter, and 5 mm length. The motor worked in water.
Thus, the motor could be used in vessel.
Fig. 10(a) is a diagram describing how an axial symmetric
body such as a sphere can be rotated by using a coiled waveguide wound around it
as shown. Fig. 10(b) is a diagram showing that a hollow tube 2 placed outside the
coil 1 can be moved linearly along the axis of the coil.
An encoder suited to the invented motor is described using
Fig. 11 through Fig. 13. In Fig. 11, conductive regions 7a and 7b (shown by bold
lines), and non-conductive region 8 (shown by thin lines) are constructed on the
surface of the rotor 2. The conductive regions 7a and 7b rotate with the rotor 2.
Conductive contacts 9 are connected to an electric circuit comprising a resistor
and a battery. The conductive regions 7a and 7b are connected electrically.
In state (1) in Fig. 12, an electric circuit is closed
(ON-state), and an electric voltage is produced across the register. State (2) shows
the end of the ON-state. State (3) shows the OFF-state, and state (4) shows the
end of the OFF state.
An electric pulse as shown in Fig. 13 can be obtained by
measuring the voltage across the register, which allows counting the number of rotation
of the invented motor.
Fig. 14 is a diagram describing the structure of the intravascular
ultrasound (IVUS) system that uses a micro-motor. A mechanically rotating IVUS system
uses a long flexas (a flexible wire that transmit torque) between the ultrasound
element and the motor driving it. Use of a hard catheter is needed for safety, but
it hinders the flexibility that is needed in order to scan inside a small vessel.
An IVUS system using a micro-motor attached at the tip of the catheter may avoid
An IVUS using the invented motor can be built as shown
in Fig. 14. A reflection mirror 12 for imaging is attached to the rotor. An electrical
cable 10 can be placed inside the rotor. The ultrasonic beam emitted from the transducer
is steered by the rotation of the mirror 12. Since the motor is small and the rigid
region is limited at the top where motor is installed, this IVUS system can be very
A motor M shown in Fig. 15 can also be used to drive a
lens of a small camera. While based on the same principle, in this configuration,
the mover is a tube and the coiled stator is an acoustic waveguide that is wound
along the groove of the mover. The coil was made of a 0.5 mm diameter iron wire.
The diameter of the tube was 6 mm. When the flexural wave propagates along the waveguide,
the elliptical motion of the surface element of the waveguide drives the tube in
an axial direction, and the tube is moved in the linear direction. If the flexural
wave propagates in the opposite direction, the tube moves in the opposite direction.
The experiment was conducted at 50 kHz, with an electric power of 10 W applied to
Fig. 16 is a diagram describing a method to enhance the
efficiency of the invented motor with a small number of windings. In Fig 16, an
arrayed ultrasonic transducer 13 generates a flexural wave that propagates only
in one direction via an acoustic waveguide 5a. A part of the flexural wave that
is not converted into the rotational motion of the rotor travels back to the arrayed
transducer, and is added to the flexural wave that is generated by the transducer
in phase. This mechanism enhances the conversion efficiency from the flexural wave
power to the rotational power of the motor.
The arrayed transducer 13 consists of small transducers
as shown by A, B, C separated appropriately. By applying electric voltages to the
transducers as shown, a flexural wave that propagates one direction is excited.
The phase of the phase equalizer 14, which consists of
a coiled waveguide, is controlled by the width and the length of the waveguide.
Fig. 18 shows a group velocity for coils made of stainless steel wires (0.1 mm,
0.2 mm, and 0.3 mm diameters) as a function of the inner diameter of the coil.
Fig. 19 shows a diagram describing the series connection
of the invented motors to e nhance output torque. Flexural waves excited by transducers
13-1, 13-2, and 13-3 drive the rotor to rotate in the same direction with the aid
of phase equalizers 14-1, 14-2, an d 14-3.
Fig. 20 is a diagram showing the parallel connection of
the invented motors. Flexural waves excited by transducer 13 propagate to the motors
15-1, 15-2, and 15-3 via waveguides 15a-1, 15a-2, and 15a-3.
A disk can be rotated by using a spiral coil in a plane
as shown in Fig. 21. In thi s case, the coil is placed between the disks like a
yo-yo. A prototype motor based on Fig. 21 was constructed using a disk with 10-mm
diameter, and a coil made of 0.3-mm diameter brass wire. The motor operated at 40
As mentioned above, the invented motors or actuators may
find many applications in the field of mechanical engineering such as robotics and
electric motors, in the field of medicine such as diagnosis and therapy, and in
the field of measurement that must be free from electro magnetic interference. In
particular, it can be applied in instruments for use in vessels such as an intravascular
ultrasound (IVUS) system, or a thrombus removal or prevention device.