Abstract:
The present invention relates to an ultrasonic motor comprising a stator formed by using an acoustic waveguide of helical coil shape or an acoustic waveguide of in-plane spiral coil shape, so that a rotor placed in close proximity to, or in close contact with the coil is rotated, traveled, or performed curved line movement by the propagation of the wave on the stator, thereby eliminating the provision of a preload spring and an absorber, simplifying the structure of the motor, reducing the size of the motor, and enabling to work in a liquid environment without water proofing and rotation of a hollow tube.

Description:
TECHNICAL FIELD  
       [0001]     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.  
       BACKGROUND ART  
       [0002]     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.    
 
         [0006]     Conventional motors can be evaluated as follows from the viewpoint of said requirements:  
         [0007]     Traveling wave type ultrasonic motors are widely used in robotics and in cameras because they have an advantage over conventional electromagnetic 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.  
         [0008]     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: 
    (a) Use of a spring for the application of the preload prevents a decrease of the size of the motor.     (b) This motor does not work in a liquid environment. In type B, water proofing is necessary for use in water.     (c) 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.    
 
         [0012]     In conclusion, the conventional traveling wave type ultrasonic motors do not satisfy the said requirements (See 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, November 2003)  
         [0013]     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.  
         [0014]     Furthermore, it is difficult to use a hollow tube as a rotor (See 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).  
       SUMMARY OF THE INVENTION  
       [0015]     The object of the present invention is to solve the following problems occurring in the conventional traveling wave type ultrasonic motor: 
    (a) The conventional traveling wave type ultrasonic motor cannot work without using a preload spring, thus preventing the further reduction in its size.     (b) The conventional traveling wave type ultrasonic motor cannot work in a liquid environment unless appropriate waterproofing is provided.     (c) A hollow tube cannot be used as a rotor inside a vessel.    
 
         [0019]     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.  
         [0020]     The motor has the following features: 
    (a) The motor can be extremely miniaturized. Even the use of a carbon nano-coil may be considered.     (b) The motor can be used in a liquid environment.     (c) A hollow tube can be used as a rotor.     (d) The rotor can be linearly displaced if the helical groove is provided on the rotor.     (e) Any axisymmetric object such as a sphere, a cone, or an ellipse can be rotated.     (f) A rod and a hollow tube can be moved along the axis of the mover.   
 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]      FIG. 1  is a diagram describing the principle of the conventional traveling wave type ultrasonic motor.  
         [0028]      FIG. 2  is a diagram describing the basic system of the invented ultrasonic motor that uses a coiled acoustic waveguide as a stator.  
         [0029]      FIG. 3  is a diagram describing the direction of the vibration of the Lamb wave traveling along the coiled acoustic waveguide.  
         [0030]      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 over time.  
         [0031]      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.  
         [0032]      FIG. 6  is a diagram describing the paired acoustic waveguide that transmits the Lamb wave from the transducer to the invented ultrasonic motor.  
         [0033]      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.  
         [0034]      FIG. 8  is a diagram describing another example of the invented motor that uses the stator placed inside of the rotor.  
         [0035]      FIG. 9  is a diagram describing the structure of the motor that uses the paired waveguide and the stator placed outside of the rotor.  
         [0036]      FIG. 10  is a diagram describing the invented motor that uses a sphere or a hollow tube as a rotor.  
         [0037]      FIG. 11  is a diagram describing the structure of the encoder.  
         [0038]      FIG. 12  is a diagram describing the encoder shown in  FIG. 11  counting a pulse.  
         [0039]      FIG. 13  is a graph that shows the output of the encoder shown in  FIG. 11 .  
         [0040]      FIG. 14  is a diagram describing the application of the invented motor to ultrasonic endoscopy.  
         [0041]      FIG. 15  is a diagram describing the operation of example 6 that enables the mover to move along an axis.  
         [0042]      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.  
         [0043]      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.  
         [0044]      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.  
         [0045]      FIG. 19  is a diagram describing how to carry out a series connection of the invented motor.  
         [0046]      FIG. 20  is a diagram describing how to carry out a parallel connection of the invented motors.  
         [0047]      FIG. 21  is a diagram describing the structure of example 10 in an elevational view (a) and in a top view (b). 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     The Principle of Operation of the Conventional Traveling Wave Type Ultrasonic Motor  
       [0048]     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.  
         [0049]      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 t 0  is shown in the figure; then, the slab changes its form as time progress as shown. In the figure, t 1 , t 2 , t 3  correspond to the form of the slab at the time π/ 2 , π,  3  π/ 2  seconds later.  
         [0050]     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.  
         [0051]     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.  
         [0000]     [The Principle of Operation of the Ultrasonic Motor of this Invention] 
         [0052]     The fundamental composition and the principle of operation of a traveling wave type ultrasonic wave motor of this invention are explained.  
         [0053]     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.  
         [0054]     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 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 .  
         [0055]     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 l 1 . When the amplitude is large, the element traces the line l 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.  
         [0056]     Assuming 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.  
         [0057]     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.  
         [0058]     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.  
         [0059]     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.  
       EXAMPLE 1  
       [0060]      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  5   a  and  5   b,  a rotor  2 , and a coiled acoustic waveguide  1   a  wound around the rotor  2 .  
         [0061]     When an electric voltage is applied to the transducer  3 , the flexural wave is excited, and is propagated along the waveguide  5   a  to the coiled acoustic waveguide  1   a.  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.  
         [0062]     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 is possible.  
         [0063]     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.  
         [0064]     A motor based on  FIG. 5  was manufactured. The flexural wave at a frequency of 50 kHz was propagated along the acoustic waveguides  5   a,  and  5   b.  The acoustic waveguide was constructed using a 0.05mm thin, 1 mm wide nickel plate. The flexural wave progressed along the acoustic waveguides  5   a  or  5   b,  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.  
         [0065]     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 by applying 80 V to the transducer (a Langevin type transducer) at the frequency of 8.8 kHz.  
         [0066]      FIG. 6  shows a diagram describing a paired waveguide used for the invented motor.  
       EXAMPLE 2  
       [0067]     An example of the invented motor is shown in  FIG. 7 . The motor M consists of paired waveguides  5   a  and  5   b,  a rotor  2 , a stator la 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  5   a  to the stator  1   a.  The elliptical movement of the surface element of the stator  1   a  causes the rotor to rotate as described in Example 1.  FIG. 8  shows the enlarged arrangement of the stator  1   a  and the rotor  2 .  
         [0068]     The stator  1  can also be placed outside the rotor  2  as shown in  FIG. 9 .  
         [0069]     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.  
         [0070]     A motor based on the  FIG. 8  was constructed. In this case, only one waveguide  5   a  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.  
         [0071]     Thus, the motor could be used in vessel.  
       EXAMPLE 3  
       [0072]      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 in close external contact with the sphere as shown.  FIG. 10 ( b ) is a diagram showing that a hollow tube  2  placed in close internal contact with the coil  1  can be moved linearly along the axis of the coil.  
       EXAMPLE 4  
       [0073]     An encoder suited to the invented motor is described using  FIG. 11  through  FIG. 13 . In  FIG. 11 , conductive regions  7   a  and  7   b  (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  7   a  and  7   b  rotate with the rotor  2 . Conductive contacts  9  are connected to an electric circuit comprising a resistor and a battery. The conductive regions  7   a  and  7   b  are connected electrically.  
         [0074]     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.  
         [0075]     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.  
       EXAMPLE 5  
       [0076]      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 this problem.  
         [0077]     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 flexible.  
       EXAMPLE 6  
       [0078]     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 the transducer.  
       EXAMPLE 7  
       [0079]      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  5   a.  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.  
         [0080]     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.  
         [0081]     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.  
       EXAMPLE 8  
       [0082]      FIG. 19  shows a diagram describing the series connection of the invented motors to enhance 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 , and  14 - 3 .  
       EXAMPLE 9  
       [0083]      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  15   a - 1 ,  15   a - 2 , and  15   a - 3 .  
       EXAMPLE 10  
       [0084]     A disk can be rotated by using a spiral coil in a plane as shown in  FIG. 21 . In this 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 kHz.  
         [0085]     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.