Abstract:
A synchronous motor control system includes a motor including a stator having a plurality of sets of stator teeth and phase coils, a rotor having a plurality of rotor teeth disposed opposite the stator teeth and a stopper for restricting the rotation angle of the rotor relative to the stator and a control unit for sequentially controlling electric supply to the phase coils to rotate the rotor to a prescribed rotation position. The motor is controlled by the control unit to operate through the following steps of supplying current to a specific group of the phase coils, sequentially supplying current to the phase coils to rotate the rotor in one direction at a speed not to step out of synchronization to rotate the rotor until it is stopped by the means for restricting, and sequentially supplying current to the phase coils to rotate the rotor in the other direction at a speed not to step out of synchronization until the number of times of current supply becomes a prescribed number.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   The present application is based on and claims priority from Japanese Patent Application 2004-105441, filed Mar. 31, 2004, the contents of which are incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a control system of a synchronous motor. 
   2. Description of the Related Art 
   In a reluctance type synchronous motor, electric supply to a plurality of phase coils is sequentially controlled to induce magnetomotive force in a plurality of stator teeth to pull rotor teeth of a rotor, thereby rotating the rotor. Usually, an encoder is employed to control the rotation angle and the number of rotations of the motor. Such an encoder generates pulse signals that correspond to the rotation of the motor. The pulse signals are counted by a control unit to detect a rotation angle of the motor, so that electric current is sequentially supplied to the phase coils. 
   The encoder has a permanent magnet or other parts that are fixed to the rotor and Hall effect ICs or other parts that are fixed to the stator. Because the control unit has to count many pulse signals to calculate the rotation angle or position, it has to have a large computing capacity. 
   SUMMARY OF THE INVENTION 
   Therefore, an object of the invention is to provide an improved synchronous motor control system. 
   A further object of the invention is to provide a synchronous motor control system in which an encoder is omitted. 
   According to a feature of the invention, a synchronous motor control system includes a motor having a stator with a plurality of phase coils and a rotor having a plurality of stator teeth and a control unit having a counter for counting the number of times of electric supply to the phase coils. The control operation of the control unit includes a first step of supplying current to a specific group of the stator phase coils to position or synchronize the rotor relative to the phase coils and a second step of sequentially supplying electric current to rotate the rotor until the counter counts a prescribed number of times of supplying current to the phase coils. Preferably, all the phase coils are concurrently supplied with current after current is supplied to the specific group of the phase coil to accurately position the rotor relative to the stator. The control operation may further have a third step of concurrently supplying holding current to all the phase coils when the rotor stops after the third step to hold the rotor relative to the stator. In this case, an amount of holding current in the third step is preferably less than current to rotate the rotor in the second step. 
   According to another feature of the invention, a synchronous motor control system including a synchronous motor having a stator with a plurality of sets of stator teeth and phase coils, a rotor with a plurality of rotor teeth disposed opposite the stator teeth and means for restricting the rotation angle of the rotor relative to the stator, and a control unit for sequentially controlling electric supply to the phase coils to rotate the rotor to a prescribed rotation position. With the above arrangement, the control unit includes a control program that is composed of a step of supplying current to a specific group of the phase coils, a step of sequentially supplying current to the phase coils to rotate the rotor in one direction at a speed not to step out of synchronization to rotate the rotor until it is stopped by the means for restricting and a step of sequentially supplying current to the phase coils to rotate the rotor in the other direction at a speed not to step out of synchronization until the number of times of current supply becomes a prescribed number. Thus, an initial rotation position can be set accurately without providing an encoder. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, features and characteristics of the present invention as well as the functions of related parts of the present invention will become clear from a study of the following detailed description, the appended claims and the drawings. In the drawings: 
       FIGS. 1A and 1B  are schematic diagram illustrating a control operation of a synchronous motor control system according to a preferred embodiment of the invention; 
       FIG. 2  is a cross-sectional side view of a rotary actuator to which the synchronous motor control system is applied; 
       FIG. 3  is a block diagram illustrating a shift range changing system to which the synchronous motor control system is applied; 
       FIG. 4  is a perspective view of the shift range changing system; 
       FIG. 5  is a schematic diagram of a brushless synchronous motor; 
       FIG. 6  is a perspective rear view of a speed reduction unit that is mounted in the rotary actuator shown in  FIG. 2 ; 
       FIG. 7  is a perspective front view of the speed reduction unit; and 
       FIG. 8  is a perspective exploded view of the speed reduction unit. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A brushless synchronous motor according to a preferred embodiment of the present invention will be described with reference to  FIGS. 1-8 . 
   Such a brushless motor is applied to a gear shift range changing system. The shift range changing system includes a rotary actuator  1  shown in  FIG. 2 , an automatic transmission unit  2  shown in  FIG. 3  and a shift range changing unit  3 , which includes a parking mechanism  4 , as shown in  FIG. 4 . 
   The rotary actuator  1  operates the shift range changing unit  3 . The rotary actuator  1  includes a brushless synchronous motor  5  and a speed reduction unit  6 . The brushless synchronous motor  5  is a switched reluctance motor (SR motor) that is not equipped with a permanent magnet. 
   The motor  5  includes a rotor  11  and a stator  12 , which is disposed to be coaxial with the rotor  11 . The rotor  5  includes a rotary shaft  13  and a rotor core  14 . The rotary shaft  13  is supported by a pair of roller bearings  15 ,  16  at the front (right in  FIG. 2 ) and rear (left in  FIG. 2 ) ends thereof. 
   The speed reduction unit  6  has an output shaft  17 , and the front bearing  15  is fitted to the center hole of the output shaft  17 . The output shaft  17  is rotatably supported by a metal bearing  19 , which is fixed to the inner periphery of a front housing  18 . That is, the front end of the rotary shaft  13  is supported by the front housing  18  via the metal bearing  19 , which is supported by the output shaft  17  via the roller bearing  15 . The metal bearing  19  is located to overlap the front roller bearing  15  in the axial direction, so that the rotary shaft  13  can be prevented from bending due to the reaction force of the speed reduction unit  6 , which may be caused when a sun gear  26  engages with a ring gear  27 . The rear roller bearing  16  is press-fitted to the rear end of the rotary shaft  13  and supported by the rear housing  20 . That is, the outer race of the bearing  16  is fitted to an inner wall of the rear housing  20 , and the inner race thereof is fitted to the outer surface of the rotary shaft  13  at its rear end. 
   The stator  12  includes a stator core  21  and a plurality of phase coils  22  (i.e.  22 U,  22 U′,  22 V,  22 V′,  22 W, and  22 W′), as shown in  FIG. 5 . The phase coils  22 U,  22 U′ correspond to phase U, the phase coils  22 V,  22 V′ correspond to phase V, and the phase coils  22 W,  22 W′ correspond to phase W. The stator core  21  is a laminar member of thin plates of magnetic material, which is fixed to the rear housing  20 . The stator core  21  has twelve stator teeth  23  that project radially inward at intervals of 30 degrees in mechanical angle. Each of the phase coils  22  is wound around one of the stator teeth  23 . 
   The rotor core  14  is a laminar member of thin plates of magnetic material, which is force-fitted to the rotary shaft at the center thereof. The rotor core  14  has eight rotor teeth  24  that project radially outward at intervals of 45 degrees in mechanical angle. 
   When electric current supply is sequentially changed from the U-phase coils to the V-phase coils and from the V-phase coils to the W-phase coils, the rotor  11  rotates clockwise by 45 degrees in mechanical angle. On the other hand, the rotor rotates counterclockwise by 45 degrees in mechanical angle when electric current supply is sequentially changed from the W-phase coils to the V-phase coils and from the V-phase coils to the U-phase coils. 
   The speed reduction unit  6  has a planetary gear type or a cycloid type speed reduction mechanism, as shown in  FIGS. 6-8 . 
   The speed reduction unit  6  includes an eccentric portion  25  of the rotary shaft  13 , a sun gear or an inner gear  26 , a ring gear or an outer gear  27  and a carrier member  28 . The sun gear  26  is rotatably supported by the eccentric portion  25  via a bearing  31  to rotate eccentrically relative to the center axis of the rotary shaft and is in mesh with the ring gear  27 , which is fixed to the front housing  18  shown in  FIG. 2 . The carrier member  28  includes a flange  33  that has a plurality of inner pin-holes  34  and as many inner pins  35  that project in the axial direction from the front surface of the sun gear  26  as the inner pin holes  34 . The flange  33  is fixed to the rear end of the output shaft  17  to rotate together. The carrier member  28  transmits the rotation of the sun gear  26  to the output shaft  17  via the inner pins  35  in engagement with the inner-pin holes  34 . 
   When the rotary shaft  13  rotates, the sun gear  26  rotates about the eccentric portion  25  at a speed lower than the rotary shaft  13 . This rotation is transmitted to the output shaft  17 , which is connected to a control rod  45  of the shift range changing unit  3 . 
   Incidentally, the sun gear  26  may have the inner-pin holes  34  instead of the inner pins  35  if the flange  33  has the inner pins  35  instead of the inner-pin holes  34 . 
   The shift range of the automatic transmission unit  2 , which usually includes ranges P, R, N, D, is changed when a manual spool valve  42  of an oil pressure control box  41  is operated. Locking or unlocking of the parking mechanism  4  is carried out when a projection  44   a  of a parking pole  44  engages with or disengages from a recess  43   a  of a parking gear  43 . The parking gear  43  is linked, via a differential gear, with the output shaft of the automatic transmission unit  2 . Therefore, vehicle wheels are locked when the parking gear  43  is locked. 
   A fan-shaped detent plate  46  is fixed to the control rod  45  of the shift range changing unit  3  by means of a spring pin or the like. The detent plate  46  has a plurality of recesses  46   a  at the arc-shaped peripheral portion. A spring plate  47  is fixed to the pressure control box  41  and engages one of the recesses  46   a  to hold one of the shift ranges. The detent plate  46  has a pin  48  to drive the manual spool valve  42 . The pin  48  engages an annular groove  49  formed on the manual spool valve  42 . When the detent plate  46  moves the control rod rotates  45 , the pin  48  moves in an arc, so that the manual spool  42  moves straight in the pressure control box  41 . 
   When the control rod  45  rotates clockwise viewed from position A in  FIG. 4 , the pin  48  pushes the manual spool valve  42  via the detent plate  46  into the inside of the oil pressure control box  41 . Therefore, the oil passages in the oil pressure control box  41  are changed in a direction P-R-N-D of the shift range of the automatic transmission unit  2 . When the control rod  45  rotates counter-clockwise, the oil passages in the oil pressure control box  41  are changed in the other direction, that is D-N-R-P. 
   A park rod  51  is also fixed to the detent plate  46  to drive the parking pole  44 . The park rod  51  has a conical member  52  at its one end. The conical member  52  is disposed between the parking pole  44  and a projection  53  that projects from the housing of the automatic transmission unit  2 . 
   When the control rod  45  turns clockwise, the park rod  51  is moved by the detent plate  46  in the direction indicated by an arrow B, so that the conical member  52  lifts the parking pole  44 . Consequently, the parking pole  44  rotates about its axis  44   b  in the direction indicated by an arrow C, so that the projection  44   a  of the parking pole  44  engages the recess  43   a  of the parking gear  43  to lock the parking mechanism  4 . 
   When the control rod  45  turns counterclockwise, the park rod  51  is moved by the detent plate  46  opposite the direction indicated by an arrow B, so that the parking pole  44  is not lifted by the conical member  52 . Consequently, the parking pole  44  is rotated by a coil spring (not shown) about its axis  44   b  opposite the direction indicated by the arrow C, so that the projection  44   a  of the parking pole  44  disengages from the recess  43   a  of the parking gear  43  to unlock the parking mechanism  4 . 
   An ECU  60  controls the rotation speed and the rotation angle of the motor  5  according to a range operating unit (not shown). As shown in  FIG. 3 , the ECU  60  is powered by a battery  61 . Reference numeral  62  represents a display that indicates a current shift range and the operational state of the rotary actuator  1 . The display may include a warning lamp or a buzzer. The ECU  60  connects to a current supply circuit  63 , which supplies electric current to the motor  1 . Reference numeral  64  is a vehicle speed sensor, and reference numeral  65  represents various sensors such as a gear position sensor and a break switch sensor. 
   As shown in  FIG. 3 , the current supply circuits  63  is connected between the ECU  60  and the motor  5 . The phase coils  22 U,  22 V and  22 W are connected in the star arrangement, and the phase coils  22 U′,  22 V′ and  22 W′ are also connected in the star arrangement. 
   When the motor  5  is not powered, the rotation position of the motor  5  is held by the detent plate  46  and the spring plate  47  (detent mechanism). However, there is no assurance that the rotor  11  is not moved. Even if a motor switch is turned off at P position of the shift range, accurate position of the rotor  11  can not be detected because the P position of the shift range covers ±2 degrees in mechanical angle, which corresponds to ±154 degrees of the rotation angle. 
   The ECU  60  includes a learning control program which supplies current to a specific phase coils  22 U,  22 U′ (e.g. U-phase and U′-phase coils). When only the phase coils  22 U,  22 U′ are supplied with current, rotor teeth  24  that are located near the phase coils  22 U,  22 U′ are pulled thereto so that initial synchronization of the rotor  11  relative to the stator  12  can be almost settled. After a prescribed time in which rotor vibration completely stops, the learning program commands to supply current to the phase coils  22 V,  22 V′,  22 W and  22 W′, so that the rotor teeth are pulled by the phase coils  22 V,  22 V′,  22 W and  22 W′ to be completely settled, as indicated by reference character α in  FIG. 1B . Thus, the initial synchronization is completed. 
   The output shaft  17  has a pair of stopper walls (e.g. a P range side stopper and a D range side stopper) for restricting the rotation angle (e.g. 40 degrees) thereof. 
   The ECU  60  sequentially changes the current supply to the phase coils  22  at an open-control speed not to step out of synchronization to rotate the output shaft in one direction (e.g. toward the P range side stopper) until it is stopped by one of the stoppers (e.g. the P range side stopper). 
   When the output shaft  17  is stopped, the ECU  60  resets a counter, and sequentially supplies current to the respective phase coils at an open-control speed not to step out of synchronization to rotate in the other direction and makes the counter count the number of times of current supply to the respective phase coils  22  until the number becomes a predetermined number. Thus, the initial rotation position of the output shaft  17  is set. 
   When a range changing command is given by the range operating unit to the ECU  60 , it calculates a target count number that is necessary to rotate the output shaft  17  from the initial rotation position based on a difference between the initial rotation position of the output shaft  17  and a target rotation position. The ECU  60  rotates the output shaft  17  until the target count number is counted at the open-control speed not to step out of synchronization by use of the learning program, thereby rotating the output shaft  17  to the target rotation position. 
   The ECU  60  is programmed to concurrently supply current to all the phase coils  22  to position the stator teeth  23  and the rotor teeth  24  correctly when the output shaft  17  and the rotor  11  have rotated to the target rotation position and stop in the same manner as the initial rotation position setting described above. 
   When the rotor  11  (with the output shaft  17 ) stops, the ECU  60  supplies a less amount of holding current to the respective coils  22  than the current supplied when it rotates the rotor  11 . The less the holding current, the more power saving. 
   Incidentally, the switched reluctance motor (SR motor) may be replaced by another motor such as a synchronous reluctance motor, or a synchronous motor having permanent magnets (SPM or IPM). The cycloid type speed reduction unit may be replaced by a planetary gear type speed reduction unit, or by a speed increasing unit. The rotary actuator may be replaced by a different type rotary actuator for changing the phase angle of a cam shaft. 
   In the foregoing description of the present invention, the invention has been disclosed with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made to the specific embodiments of the present invention without departing from the scope of the invention as set forth in the appended claims. Accordingly, the description of the present invention is to be regarded in an illustrative, rather than a restrictive, sense.