Abstract:
An ultrasonic motor includes a rotor and a rotor accommodated in a housing. The stator includes a piezoelectric element and the housing is secured to a base by screws. The rotor contacts the stator. The piezoelectric element vibrates the stator to rotate the rotor. An insulation plate is located between the stator and the base. An insulation washer is located between the stator and each screw. A rotary shaft is rotatably supported by the housing. The rotary shaft is coupled to the rotor with an insulation collar in between. Therefore, the stator and the rotor are electrically insulated from the housing and the rotary shaft. This arrangement reduces electromagnetic noise, which interferes with other electric devices, such as radios.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an ultrasonic motor employed in vehicles. More particularly, the present invention pertains to an ultrasonic motor that lowers electromagnetic noise. 
     FIG. 8 illustrates a typical ultrasonic motor  50 . The motor  50  has a metal base  54  and a metal cover  58 . The base  54  and the cover  58  form a motor housing. The base  54  is secured, for example, to a vehicle body frame. The motor housing houses a stator  52  made of iron and a rotor  53  made of aluminum. The rotor  53  is pressed against the stator  52 . A rotary shaft  55  is supported by the base  54 . A ball bearing  56  is located between the shaft  55  and the base  54  to allow the shaft  55  to rotate relative to the base  54 . The rotor  53  is secured to the shaft  55  to integrally rotate with the shaft  55 . 
     The stator  52  includes a disk-shaped diaphragm  45 , a stationary plate  46  fixed to the inner bottom surface of the diaphragm  45  and a ring-shaped piezoelectric element  51  secured to the outer bottom surface of the diaphragm  45 . The stationary plate  46  is secured to the base  54  by screws. The diaphragm  45  has radially extending teeth  45   a  along its the circumference. The distal end  45   b  of each tooth  45   a  contacts the bottom surface of the rotor  53 . 
     The piezoelectric element  51  is polarized and has an A-phase region  51   a,  a B-phase region  51   b  and a vibration detecting region  51   c  as shown in FIG.  9 . Each of the regions  51   a,    51   b,    51   c  is connected to a lead wire  57   a,    57   b,    57   c  by an electrode, respectively. Further, the regions  51   a - 51   c  are connected to a common grounding lead wire  57   d.  As illustrated in FIG. 8, the lead wires  57   a - 57   d  are connected to a connector  59  located outside of the cover  58 . 
     As illustrated in FIG. 9, the connector  59  is connected to an electronic control unit (ECU)  60  by a shielded line  61 . The ECU  60  is located far from the ultrasonic motor  50 . The shielded line  61  includes an A-phase power supply wire  61   a,  a B-phase power supply wire  61   b,  a feedback signal wire  61   c  and a grounding wire  61   d.  The ECU  60  applies high-frequency voltage to the A-phase region  51   a  through the A-phase power supply wire  61   a,  the connector  59  and the lead wire  57   a.  The ECU  60  also applies high-frequency voltage to the B-phase region  51   b  through the B-phase power supply wire  61   b,  the connector  59  and the lead wire  57   b.  There is a ninety-degree phase difference between the voltage sent to the A-phase region  51   a  and the voltage sent to the B-phase region  51   b.  The high frequency voltages vibrate the A-phase region  51   a  and the B-phase region  51   b.  The vibration vibrates the distal ends  45   b  of the teeth  45   a  with respect to the stator  52 . The vibration of the distal ends  45   b  generates a progressive wave. The progressive wave rotates the rotor  53 , which is pressed against the distal ends  45   b.  The rotation of the rotor  53  is transmitted to the rotary shaft  55 . 
     The feedback signal wire  61   c  is connected to the vibration detecting region  51   c  by the connector  59  and the lead wire  57   c.  The region  51   c  generates voltage in accordance with vibration of the A-phase region and the B-phase region  51   b  and outputs the generated voltage to the ECU  60 . The ECU  60  feedback controls the high frequency voltage applied to the A-phase region  51   a  and the B-phase region  51   b  based on this voltage. The grounding wire  61   d  is connected to the grounding lead wire  57   b  by the connector  59 . The regions  51   a - 51   c  of the piezoelectric element  51  are grounded to the vehicle body frame through the ECU  60 . 
     As illustrated in FIG. 8, the stationary plate  46  of the stator  52  is fastened to the base  54 . The rotor  53  is pressed against the diaphragm  45  of the stator  52  and is coupled to the rotary shaft  55 . The shaft  55  is supported by the bearing  56 . That is, the stator  52  is directly and electrically connected the base  54  and is indirectly and electrically connected to the base  54  via the rotor  53 , the rotary shaft  55  and the bearing  56 . 
     Therefore, the regions  51   a,    51   b,    51   c  of the piezoelectric element  51  are grounded not only through the ECU  60  but also through the base  54 . In other words, the ultrasonic motor  50  is directly grounded to the vehicle body frame. The impedance of the grounding wire  61   d  between the ultrasonic motor  50  and the ECU  60  is higher than the impedance of the body frame. Therefore, when the ECU  60  applies high frequency voltage to the motor  50 , grounded current does not flow through the grounding wire  61   d  but flows to the ECU  60  through the vehicle body frame. The motor  50 , the shielded line  61   d  and the body frame form a closed loop circuit. The closed loop circuit functions as a loop antenna, which generates electromagnetic noise. The farther from the motor  50  the ECU  60  is located, the larger the area surrounded by the closed loop circuit becomes. A larger area surrounded by the closed loop circuit increases the electromagnetic noise. The electromagnetic noise generates radio noise, which interferes with the sound of the car radio. The electromagnetic noise also adversely affects other communications devices. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an objective of the present invention to provide an ultrasonic motor that reduces electromagnetic noise. 
     To achieve the foregoing and other objectives and in accordance with the purpose of the present invention, an ultrasonic motor is provided. The motor includes a stator, a movable body and a housing. The stator has a piezoelectric element. The movable body contacts the stator. The piezoelectric element vibrates the stator to move the movable body. The housing supports the stator. The stator is electrically insulated from the housing. 
     Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings. 
     FIG. 1 is a cross-sectional view illustrating an ultrasonic motor according to a first embodiment of the present invention; 
     FIG. 2 is an exploded perspective view illustrating the ultrasonic motor of FIG. 1; 
     FIG. 3 is an enlarged partial cross-sectional view showing the ultrasonic motor of FIG. 1; 
     FIG. 4 is a circuit diagram illustrating the ultrasonic motor and the ECU illustrated in FIG. 1; 
     FIG. 5 is an exploded perspective view illustrating an ultrasonic motor according to a second embodiment; 
     FIG. 6 is an enlarged partial cross-sectional view illustrating an ultrasonic motor according to a third embodiment of the present invention; 
     FIG. 7 is a front view, with a part cut away, illustrating an ultrasonic motor according another embodiment of the present invention; 
     FIG. 8 is a cross-sectional view illustrating a prior art ultrasonic motor; and 
     FIG. 9 is a circuit diagram illustrating the ultrasonic motor and the ECU illustrated in FIG.  8 . 
     FIG. 10 is a schematic drawing of a vehicle having a motor of the invention associated therewith. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An ultrasonic motor  1  according to a first embodiment of the present invention will now be described with reference to FIGS. 1 to  4 . The motor  1  is used in a telescopically adjustable steering column. 
     As illustrated in FIG. 1, a housing  2  of the motor  1  includes a base  3  and a cover  4 . The base  3  is metal and includes a support  3   a,  a boss  3   b,  a rim  3   c  and a hole  3   d.  The support  3   a  is fastened to a bracket of a steering device, which is a part of a vehicle body frame. The boss  3   b  supports a ball bearing  5 . The rim  3   c  extends radially inward from the upper end of the boss  3   b  and defines the hole  3   d.  The hole  3   d  receives a rotary shaft  6 . The diameter of the hole  3   d  is larger than that of the rotary shaft  6 . The rim  3   c  has threaded holes  3   e  (see FIG.  2 ). 
     The cover  4  is formed by pressing a metal plate and has a boss  4   a  for supporting a bearing  7 . A through hole  4   b  is formed in the side wall of the cover  4 . The rotary shaft  6  is supported by the bearings  5 ,  7  to be rotatable relative to the housing  2 . 
     As illustrated in FIGS. 1 and 2, a ring-shaped insulation plate  8  is located on the top surface of the rim  3   c  of the base  3 . The plate  8  has a hole  8   a  in its center to receive the rotary shaft  6 . The diameter of the hole  8   a  is equal to that of the hole  3   d  of the base  3 . The plate  8  also has holes  8   b,  each of which corresponds to one of the threaded holes  3   e  of the rim  3   c.  A stator  23  is located on the top of the plate  8 . 
     The stator  23  includes a diaphragm  10 , a metal ring plate  9  brazed to the radially inner portion of the diaphragm&#39;s bottom surface, a base ring  11  brazed to the radially outer portion of the diaphragm&#39;s bottom surface and a ring shaped piezoelectric element  12  adhered to the bottom of the base ring  11 . The diaphragm  10  is made of rolled steel such as cold rolled steel. The diaphragm  10  has teeth  10   b  extending radially outward. The distal end of each tooth  10   b  is formed like a crank and includes a contact  10   d.  The ring plate  9  is located on the insulation plate  8 . The diameter of the ring plate  9  is smaller than the diameter of the insulation plate  8 . 
     The ring plate  9  and the diaphragm  10  each have a hole  9   a,    10   a  to receive the rotary shaft  6 . The diameter of the holes  9   a,    10   a  is equal to the diameter of the holes  3   d,    8   a.  Thus, the diameter of the holes  9   a,    10   a  is larger than the diameter of the rotary shaft  6 . The ring plate  9  and the diaphragm  10  each have holes  9   b,    10   c,  which correspond to one of the threaded holes  3   e  formed in the rim  3   c.    
     A screw  13  having an insulation washer  14  is inserted into each set of the holes  10   c,    9   b,    8   b  of the diaphragm  10 , the ring plate  9  and the insulation plate  8 . The screw  13  is then screwed to the corresponding threaded hole  3   e  of the rim  3   c.  The screws  13  fasten the stator  23  to the base  3  with the insulation plate  8  in between. As illustrated in FIG. 3, the diameter of each washer  14  is equal to or slightly smaller than the diameter of the hole  10   c  formed in the diaphragm  10 . The diameter of the hole  10   c  is larger than the diameter of the screw head  13   a.  The diameter of the hole  9   b  is larger than the diameter of the threaded portion  13   b  of the screw  13 . Therefore, when the stator  23  is fastened to the base  3  by the screws  13 , the ring plate  9  and the diaphragm  10  do not contact the screws  13 . In other words, the stator  23  is electrically insulated from the base  3 . 
     As illustrated in FIGS. 1 and 2, a movable body, or rotor  16 , is located on the stator  23 . The rotor  16  is made of stainless steel or aluminum alloy. A lining member  15  is secured to the lower circumferential surface of the rotor  16 . The contacts  10   d  of the diaphragm  10  contact the lining member  15 . 
     The rotor  16  has a hole  16   a  in its center. An insulation collar  17  is fitted in the hole  16   a.  Recesses  16   b  are formed in the wall of the hole  16   a.  The recesses  16   a  are spaced at equal angular intervals. The insulation collar  17  includes a hub  17   a  and a flange  17   b,  which is formed on one end of the hub  17   a.  The hub  17   a  has projections  17   c  formed on its circumferential surface. Each projection  17   c  corresponds to and is engaged with one of the recesses  16   a  on the rotor  16 . The engagement secures the collar  17  to the rotor  16 . The collar  17  also has a hole  17   d,  which has a pair of parallel flat walls. The rotary shaft  6  is formed to correspond to the shape of the hole  17   d  and is fitted in the hole  17   d.  The collar  17  secures the rotor  16  to the rotary shaft  6  such that the shaft  6  rotates integrally with the rotor  16 . 
     As illustrated in FIG. 1, the lower surface of the flange  17   b  contacts the upper surface of the rotor  16 , and the upper surface of the flange  17   b  is pressed by a pressing member  18 . The pressing member  18  includes a disk spring  18   a  and a plate  18   b.  The plate  18   b  is engaged with a snap ring  5   a  fitted to the rotary shaft  6 . The snap ring  5   a  prevents the plate  18   b  from moving upward. Thus, the pressing member  18  presses the rotor  16  against the stator  23 . The pressing member  18  rotates integrally with the rotor  16  and the shaft  6 . The collar  17  electrically insulates the rotor  16  from the pressing member  18  and the shaft  6 . 
     The base  3  is secured to the bracket of a vehicle steering device (not shown). In other words, the base  3  fixes the ultrasonic motor  1  to the body frame. The motor  1  is controlled by an electronic control unit (ECU)  19 , which is independent from the motor  1  and is fixed to the vehicle body frame. 
     As shown in FIG. 4, the piezoelectric element  12  is polarized and has an A-phase region  12   a,  a B-phase region  12   b  and a vibration detecting region  12   c.  Each of the regions  12   a,    12   b,    12   c  is connected to a lead wire  20   a,    20   b,    20   c  by an electrode. Further, the regions  12   a - 12   c  are connected to a common grounding lead wire  20   d.  The lead wires  20   a - 20   d  are connected to a connector  21  located outside the cover  4  through the hole  4   b  formed in the cover  4 . 
     As illustrated in FIG. 4, the connector  21  is connected to the ECU  19  by a shielded line  22 . The shielded line  22  includes an A-phase power supply wire  22   a,  a B-phase power supply wire  22   b,  a feedback signal wire  22   c  and a grounding wire  22   d.  The ECU  19  applies high-frequency voltage to the A-phase region  12   a  through the A-phase power supply wire  22   a,  the connector  21  and the lead wire  20   a.  The ECU  19  also applies high-frequency voltage to the B-phase region  12   b  through the B-phase power supply wire  22   b,  the connector  21  and the lead wire  20   b.  There is a ninety-degree phase difference between the voltage sent to the A-phase region  12   a  and the voltage sent to the B-phase region  12   b.  The high frequency voltages vibrate the A-phase region  12   a  and the B-phase region  12   b.  The vibration of the regions  12   a,    12   b  vibrates the contact  10   d  of each tooth  10   b  of the diaphragm  10  in the stator  23 . The vibration of the distal ends  10   d  generates a progressive wave. The progressive wave rotates the rotor  16 , which is pressed against the distal ends  10   d.  The rotation is then transmitted to the rotary shaft  6 . 
     The ultrasonic motor  1  has the following advantages. 
     The insulation plate  8  is located between the stator  23  and the base  3  of the housing  2 . Further, the insulation washers  14  insulate the screws  13 , which fasten the stator  23  to the base  3 . Thus, the stator  23  and the rotor  16  are electrically insulated from the housing and the rotary shaft  6 . When the ECU  19  applies high-frequency voltage to the stator  23 , a grounding current does not flow to the vehicle body frame through the housing  2 . Instead, a grounding current flows to the ECU  19  through the grounding wire  22   d.  Therefore, the ECU  19 , the motor  1  and the body frame do not form a closed loop circuit. In other words, a loop antenna, which generates electromagnetic noise, is not formed. Thus, electromagnetic noise is not generated. 
     The direction of the current flowing in the grounding wire  22   d  is opposite to the direction of the current flowing in the power supply wires  22   a,    22   b.  Thus, electromagnetic noise generated by the current in the grounding wire  22   d  and electromagnetic noise generated by the current in the supply wires  22   a,    22   b  cancel each other. This lowers the electromagnetic noise generated by the shielded line  22 . 
     It should be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. Particularly, it should be understood that the invention may be embodied in the following forms. 
     The stator  23  may be fixed to the base  3  by means other than the screws  13 . FIG. 5 shows an example. In FIG. 5, the metal ring plate  9  and the diaphragm  10  have radially extending recesses  23   a,  the number of which is six in the embodiment of FIG.  5 . The recesses  23  are formed adjacent to the holes  9   a  and  10   a.  Also, the rim  3   c  of the base  3  includes six radially extending recesses  3   f,  which are formed adjacent to the hole  3   d.  The insulation plate  8 , which is located between the base  3  and the stator  10 , is made of high-strength rigid resin. The plate  8  has first protrusions  81  that are engaged with the recesses  23   a  and second protrusions  82  that are engaged with the recesses  3   f.  The protrusions  81 ,  82  are integrally formed with the insulation plate  8 . The stator  23  and the insulation plate  8  are fixed to each other by mating the first protrusions  81  with the recesses  23   a.  The protrusions  81  and the recesses  23   a  prevent the stator  23  and the plate  8  from moving relative to each other in the radial and angular directions. The base  3  and the insulation plate  8  are fixed to each other by mating the second protrusions  82  with the recesses  3   f.  The protrusions  82  and the recesses  3   f  prevent the base  3  and the plate  8  from moving relative to each other in the radial and angular directions. Consequently, the stator  23  is prevented from moving radially relative to the base  3 . The stator  23  and the insulation plate  8  are assembled to the base  3  by simply mating the protrusions  81 ,  82  with the recesses  23   a,    3   f.  The assembly of the motor  1  is facilitated, accordingly. 
     The stator  23  and the insulation plate  8  are not locked to the base  3  in the axial direction. However, the pressing member  18  shown in FIG. 1 constantly presses the stator  23  against the base  3  thereby preventing axial movement of the stator  23 . The construction of FIG. 5 therefore requires no fasteners such as screws and thus simplifies the motor  1 . 
     The first protrusions  81  and the second protrusions  82  are axially aligned with each other. Thus, the recesses  23   a  receiving the first protrusions  81  are axially aligned with the recesses  3   f  receiving the second protrusions  82 . Therefore, axial load of the pressing member  18  acting on the stator  23  is received by the base  3  through the insulation plate  8 . The axial load is not received by the insulation plate  8 . This extends the life of the plate  8 . 
     If the stator  23  is fixed to the base  3  by screws, the axial load of the pressing member  18  is concentrated at the screws. The concentrated load may deform the plate  8  and the stator  23 . However, since the embodiment of FIG. 5 uses no screws, the load of the pressing member  18  acts uniformly on the entire plate  8  and does not deform the plate  8  and the stator  23 . This results in stable contact between the rotor  16  and the stator  23 . The torque of the motor  1  is stabilized, accordingly. 
     In FIG. 5, the recesses  23   a  and  3   f  communicate with the holes  9   a  and  3   d,  and are formed through the entire thickness of the stator  23  and the base  3 , respectively. However, the recesses  23   a  and  3   f  may be replaced with openings having any shape as long as they receive the projections  81 ,  82  on the insulation plate  8 . For example, the recesses  23   a  and  3   f  may be replaced with holes that are independent from the holes  9   a  and  3   d.  Alternatively, the projections  81 ,  82  may be shortened in the axial direction and received by recesses formed in the facing surfaces of the stator  23  and the base  3 . 
     The motor illustrated in FIG. 1 is a progressive wave type ultrasonic motor. However, the present invention may be employed in a stationary wave type ultrasonic motor. FIG. 6 is a partial cross-sectional view illustrating a stationary wave type ultrasonic motor. A stator  31  includes a first and second piezoelectric elements  35 ,  36 , first and second electrode plates  37 ,  38  and first and second metal blocks  33 ,  34 . The blocks  33 ,  34  sandwich the piezoelectric elements  35 ,  36  and the electrode plates  37 ,  38 . The piezoelectric elements  35 ,  36  and the electrode plates  37 ,  38  each have an aligned hole. A cylindrical insulation collar  40  is fitted in the aligned holes of the elements  35 ,  36  and the plates  37 ,  38 . A rivet  41  (or a bolt) fastens the elements  35 ,  36 , the plates  37 ,  38  and the blocks  33 ,  34  together. A rotor  32  is pressed against the stator  31 . An insulation plate  42  is located between the first metal block  33  and the first electrode plate  37 , and an insulation plate  43  is located between the second metal block  34  and the second piezoelectric element  36 . The piezoelectric elements  35 ,  36  are electrically insulated from the rivet  41  and the blocks  33 ,  34 . The elements  35 ,  36  are not grounded through the metal blocks  33 ,  34 . 
     The housing  2 , the rotary shaft  6  and the screws  13  of the embodiment illustrated in FIGS. 1-4 are made of metal. However, the housing  2 , the shaft  6  and the screws  13  may be made of any high-strength insulative material such as, for example, plastic. Further, the surface of the housing  2 , the shaft  6  and the screws  13  may be covered with insulative material. 
     As illustrated in FIG. 7, the base  3  of a motor similar to that shown in FIG. 1 may be made of insulative material such as synthetic resin. That is, the motor  1  may be insulated from the body frame by the base  3 . The insulation plate  8  and the insulation washer  14  may be omitted in this construction. Thus, the ultrasonic motor of FIG. 7, which has fewer parts, has the same advantages as the motor of FIGS. 1-4. 
     The lining member  15  may be made of insulative material. 
     In the illustrated embodiments, the present invention is embodied in rotary type ultrasonic motors having a rotary shaft rotated by a rotor. However, the present invention may be embodied in a linear type ultrasonic motor, which linearly moves a movable body. 
     In the illustrated embodiments, the present invention is embodied in a motor used in a telescopically adjustable steering column. However, the present invention may be embodied in an ultrasonic motor used in a device for tilting a steering column. Further, the present invention may be embodied in ultrasonic motors used in machines other than vehicles. 
     Therefore, the present examples and embodiments are to be considered as illustrative and not restrictive and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalence of the appended claims.