Abstract:
A step motor includes a rotor and two stators. The two stators are installed around the rotor, having magnetic poles in different numbers to generate different step angles.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The invention relates to a step motor, and more particularly, to a step motor with multiple stepping angles to simultaneously achieve the goal of high rotational speed and high precision.  
           [0003]    2. Description of the Prior Art  
           [0004]    A motor is an indispensable power-transformation device in industry and the information society that is capable of transforming electrical power into kinetic energy. Motors that are in common use include DC-motors, AC-motors, and step motors. The former two are generally adopted in devices in which high precision is not required, such as in electric fans. Step motors are utilized in devices in which high precision control is required since their angular displacement and rotational speed can be managed with electric power by a controlling system. Therefore, step motors are utilized in devices demanding high-precision control, like a step motor driving the movement of a scanner module in a scanner. As digital information products are rapidly developing, step motors are commonly used in control systems of digital products to achieve positions and speeds required of digital appliances.  
           [0005]    Variable reluctance motors and permanent magnet motors are the two most common types of step motors. Since permanent magnet motors are efficient in periods of discrete operation, the operating principles of common motors exemplified by a permanent magnet motor are described below.  
           [0006]    Please refer to FIG. 1, which is a schematic diagram of a prior art step motor  10 . The step motor  10  includes a rotor  12  and a stator  14 . The stator  14  surrounds and is fixed outside the rotor  12 , whereas the rotor  12  can rotate on a spindle. The rotor  12  is a permanent magnetic, comprising 6 magnetic north poles R 1  to R 6  equally distributed within the rotor  12  with each pole separated by 60 degrees from the adjacent poles. The stator  14  comprises 8 magnetic poles L 1  to L 8  formed by coils A and B coiled around electromagnets M 1  to M 8 , and each magnetic pole is separated by 45 degrees from the adjacent poles. The polarity of each magnetic pole is decided by the rotation direction of the coil A or B, and the voltage polarity in the coil A or B.  
           [0007]    The step motor  10  in this example comprises two sets of coils A and B. Coil A is wrapped around magnetic poles L 1 , L 3 , L 5 , and L 7 . Each magnetic pole L 1 , L 3 , L 5 , and L 7  is different from each other in their coiling directions in order to generate different magnetic poles while applying electric charges. Similarly, coil B is wrapped around magnetic poles L 2 , L 4 , L 6 , and L 8  in a manner comparable to the magnetic poles wrapped by coil A. A controller  16  is included in the step motor  10  and is electrically connected to coils A and B to control the current flowing through coils A and B and implicitly control the rotational speed of the rotor  12  in the step motor  10 .  
           [0008]    During a certain period of time when the coil A is conducting but the coil B is not conducting, one end of the electromagnets M 1 , M 5  facing the rotor  12  become magnetic south poles. The L 1 , L 5  poles of the stator  14  attract the R 1 , R 4  poles of the rotor  12  respectively, and make the L 1 , L 5  polar coils face the R 1 , R 4  poles. Now the R 2 , L 2  and the R 5 , L 6  are both 15° counter clockwise separated from each other and the R 3 , L 4  and the R 6 , L 8  are both 15° clockwise separated from each other. The R 6 , L 7  are 30° counter clockwise separated from each other.  
           [0009]    In another period of time when the coil B is conducting but the coil A is not conducting, the ends of the electromagnets M 2 , M 6  facing the rotor become magnetic south poles. The L 2  pole and the L 6  pole of the stator  14  attract the R 2  and R 5  poles of the rotor  12  which makes the rotor  12  counter rotate clockwise 15° and makes the L 2  and L 6  poles directly face the R 2  and R 5  poles respectively.  
           [0010]    If the coil A is conducting once again, the ends of the electromagnets M 3 , M 7  become magnetic south. The electromagnets M 3 , M 7  of the stator  14  attract the R 3 , R 6  poles of the rotor  12 , the rotor  12  rotates counter clockwise 15°, and makes the electromagnets M 3 , M 7  directly face the R 3 , R 6  poles of the rotor  12 .  
           [0011]    It follows, if sequentially making the polarities of the ends facing the rotor  12  of the electromagnets M 4 , M 8 , the electromagnets M 1 , M 5  and the electromagnets M 2 , M 6  appear to be magnetic south poles, then the rotor  12  rotates counter clockwise with a stepping angle of 15°. The faster the current in the coils A and B changes, the faster the rotor  12  rotates. Notice that the stepping angle does not change, and a reversal of rotational direction can be achieved merely by altering the current direction.  
           [0012]    The above mentioned is the operational principle of each step that the step motor  10  rotates. Of course, the step motor  10  can rotate a half step or a quarter step too, and the principles are mentioned below.  
           [0013]    To control the step motor  10  to rotate a half or a quarter step, electric charges can be applied to the coils A and B, and make the sides of two adjacent magnets facing the rotor  12  become magnetic south. For example, applying electric charges to coils A and B simultaneously makes the ends of the electromagnets M 2 , M 3  and the electromagnetic M 6 , M 7  become magnetic south. However, the currents applied into coils A and B does not need to be the same. For example, if coil B is applied with a larger current, then the magnetic south poles of the electromagnets M 2 , M 6  have a stronger polarity then those of the electromagnets M 3 , M 7 , and the rotor  12  rotates counter clockwise slightly. Therefore, adjusting the current differences between two coils can control the step motor  10  to rotate a half, a quarter, or other micro steps.  
           [0014]    Please refer to FIG. 2, which is a lateral view of the step motor  10  in FIG. 1. The step motor  10  comprises a transmission shaft  18  fixed between the rotor  12  and an external gear wheel, which is for transmitting angular kinetic energy from the rotor  12  to the gear wheel  20 . The wheel gear  20  is connected to an external output device to drive the external output device.  
           [0015]    Although the step motor  10  can rotate a half or a quarter step by controlling the input current, it is still not possible to precisely control the rotor  12  by magnetic force. Deviation in several factors like the weight of the rotor, the weight of the output device, the precision of the controlling current, and any other related deviations all deteriorate the precision of the whole system. For example, a normal step motor has a 7% deviation when rotating a step, a 30% deviation when rotating half a step, and even worse when rotating a quarter step. Therefore, step motor  10  is not suitable when precise control is required.  
           [0016]    Certainly, increasing the quantity of N-poles and S-poles of the rotor  12  and the stator  14  in the step motor  10  can reduce the rotating deviation because the step motor  10  can rotate a full step instead of a half or a micro step. However, a drawback of not being able to reach a high rotational speed then occurs since the rotation of the rotor  12  is controlled by current in the coils A and B, and the faster the current changes, the faster the rotor  12  rotates. Due to the limitation of the response time between the magnetic poles and the electromagnets, it is hard for the current in the coils A and B change rapidly. Therefore, if the stepping angle of the motor  10  is too small, the step motor  10  cannot reach high rotating speeds.  
           [0017]    As above mentioned, the prior art step motor  10  can not meet both demands of high precision and high rotating speed simultaneously. In general devices, motive systems usually require both the characteristics of high precision and high rotating speed. However, the prior art step motor  10  can only supply one or the other.  
         SUMMARY OF THE INVENTION  
         [0018]    It is therefore a primary objective of the claimed invention to provide a step motor with multiple stepping angles to meet both demands of high precision and high rotating speed simultaneously.  
           [0019]    The claimed step motor includes a rotor and two stators. The two stators are installed around the rotor, having magnetic poles in different numbers to generate different step angles. The stators can be operated individually or in unison to provide a wide variety of rotational speeds and precision.  
           [0020]    A major advantage of the claimed invention is the ability to use one of the stators with a larger number of magnetic poles to achieve precise rotational angles or to use the other stator with a smaller number of magnetic poles to achieve high rotational speed within a single step motor. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0021]    [0021]FIG. 1 is a schematic diagram of a step motor according to prior art.  
         [0022]    [0022]FIG. 2 is a lateral view of the step motor in FIG. 1.  
         [0023]    [0023]FIG. 3 is a schematic diagram of a step motor according to the present invention.  
         [0024]    [0024]FIG. 4 is a front view of a first stator of the step motor in FIG. 3.  
         [0025]    [0025]FIG. 5 is a front view of a second stator of the step motor in FIG. 3.  
         [0026]    [0026]FIG. 6 is a schematic diagram of a scanner according to the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0027]    Please refer to FIG. 3, which is a lateral view of the step motor  30  according to the present invention. The step motor  30  is a permanent magnetic motor comprising a rotor  32 , a first stator  34 , a second stator  36 , a first controller  38 , and a second controller  40 . The rotor  32  can rotate with a fixed spindle; the first stator  34  and the second stator  36  are fixed outside the rotor  32 . The first controller  38  is for controlling the current in the coil of the first stator  34  to rotate the rotor  32 . The second controller  40  is for controlling the current in the coil of the second stator  36  to rotate the rotor  32 . The rotor  32  can therefore be rotationally driven by both the first stator  34  and the second stator  36 .  
         [0028]    The step motor  30  further comprises a spindle  42  connected between the rotor  32  and a gear wheel  44 , to transmit the angular kinetic energy from the rotor  32  to the gear wheel  44 . The gear wheel  44  is connected to an output device to drive the output device. A metal sheet is utilized between the first stator  34  and the second stator  36  to separate the magnetic line generated by the first stator  34  and the second stator  36  in order to avoid magnetic induction between the first stator  34  and the second stator  36 .  
         [0029]    Please refer to FIG. 4 and FIG. 5. FIG. 4 is a front view of the first stator  34  in FIG. 3 according to the present invention. FIG. 5 is a front view of the second stator  36  in FIG. 3. The structures of the first stator  34  and the second stator  36  are similar to a prior art stator. The first stator  34  comprises a plurality of magnetic poles D 1  to D 8  formed by electromagnets C 1  to C 8  that are encircled by coils  48 A and  48 B. The first controller  38  for controlling the rotor  32  adjusts current flowing in coils  48 A and  48 B. The second stator  36  comprises a plurality of magnetic poles F 1  to F 16  formed by electromagnets E 1  to E 16  encircled by coils  50 A and  50 B. The second controller  40  adjusts current flowing in coils  50 A and  50 B to control the rotor  32 . The second stator  36  has more magnetic poles than the first stator  32  does, and the ratio of magnetic poles owned by the second stator  36  to the first stator  34  is an integer; therefore, the first stator  34  has a larger stepping angle than the second stator  36  when driving the rotor  32 .  
         [0030]    The rotor  32  is driven by the first stator  34  and the second stator  36  and the stepping angle that the first stator  34  drives the rotor  32  is larger than the second stator  36 . Therefore, the first stator  34  operates when the step motor  30  needs to rotate at a high speed, and on the other hand, the second stator  36  operates when the step motor  30  needs to rotate precisely.  
         [0031]    As for deciding the quantity of the electromagnets C i  of the first stator  34  and E i  of the second stator  36 , the demanding stepping angle should be taken into consideration. If the external gear wheel  44  of the step motor  30  has a separation angle of 7.5° for each gear tooth, then the stepping angle of the first stator  34  can be a double of 7.5°, which is 15°. The stepping angle for the second stator  36  can be half of 7.5°, which is 3.75°. Therefore, to rotate the step motor  30  half a gear tooth (3.75°), it can be achieved by driving the rotor  32  with the second stator  36  rotating one full step instead of the half step required in the prior art. The full step of the present invention results is less deviation than the half step of the prior art and is therefore preferable for precision control.  
         [0032]    A conventional step motor  30  has a deviation of 7% when rotating a full step and 30% when rotating a half step which implies that by utilizing the step motor  30 , the deviation can be decreased from 30% to 7%, greatly increasing the precision of the gear wheel  44 . Similarly, if a gear wheel needs to rotate at a high speed, the step motor  30  can drive the rotor  32  with the first stator  34  of which the stepping angle is 15°. When the current in coils  48 A and  48 B of the first stator  34  changes direction once, the rotor  32  can rotate 15°. The prior art stator  14  with a stepping angle of 7.5° needs to change the current direction twice for the rotor  12  to rotate 15°. Therefore, if the response time of current changing direction of the present art equals that of the prior art, then the present invention rotor  32  can rotate at double the speed of the prior art. Therefore, the step motor  30  according to the present invention can achieve both high rotational speed and high precision by controlling the rotor  32  with the first stator  34  and the second stator  36 .  
         [0033]    Please refer to FIG. 6, which is a schematic diagram of a scanner according to the present invention. Take a scanner module  54  in a scanner  52  as an example. The scanner module  54  is utilized in the scanner  52  for scanning the document  56  by moving forward and backward, and the step motor  30  drives the scanner module. The document  56  is placed on a scan area  60  of a scanner platform  58 , and a transitional area  62  needs to be passed before the scanner module  54  reaches the scan area  60 .  
         [0034]    The step motor  30  can drive the rotor  32  with the first stator  34  to make the scanner module  54  pass the transitional area  62  at high speed to the scan area  60 , in order to save time wasted in passing the transitional area. When the scanner module enters the scan area  60 , the step motor  30  then drives the rotor  32  with the second stator  36  to make the rotor  32  scan the document  56  at smaller stepping angles, which results in high precision and a higher resolution. The step motor  30  according to the present invention can be applied into other electronic devices as well, like a printer; the step motor  30  according to the present invention can control the motion of the printhead of the printer.  
         [0035]    Besides, the phases of the first stator  34  and the second stator  36  do not need to be coupled, since the step motor  30  can take advantage of only the first controller  38  or the second controller  40  to control the rotor  32 . Certainly, the quantity of the electromagnets C i  of the first stator  34  can equal that of the second stator  36 ; such design is needed in devices where a high torque is required. In this design, the corresponding currents in the magnetic poles of the first stator  34  and the second stator  36  are conducted simultaneously to drive the rotor  32  at the same time, which results in a higher torque.  
         [0036]    In contrast to the prior art, the present invention calibration method comprises at least two stators  34  and  36  to drive the rotor  32 . The stepping angle of the first stator  34  is larger, which is for driving the rotor  32  at a high speed; and the stepping angle of the second stator  36  is smaller, which is for driving the rotor  32  with high precision. Therefore, the step motor  30  according to the present invention can simultaneously meet the demands of high-speed rotation and high precision. Of course, the number of the stators is not restricted to two, it can be added conditionally.  
         [0037]    Those skilled in the art will readily observe that numerous modifications and alterations of the device may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.