Abstract:
The invention pertains to a method for determining whether a rotor is good in magnetic induction by measuring the electromotive force (emf) of a motor. A standard stator of the motor is prepared as the standard of measurement. A set of induction coil is wound upon the standard stator so that when the rotor is combined with the standard stator and is subject to running by a driver, the induction coil can detect the back-emf signal generated by the rotor, by which the rotor quality can be determined. Since the measuring method disclosed in the invention is performed within the closed system composed of the rotor and the stator, the result is not only close to a real motor in rotation, the detection is simple and free from the problem of axis alignment. Thus, this method can increase the production efficiency of the product line.

Description:
This application is a continuation-in-part of 09/642,465 filed Aug. 17, 2000 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates to a method for determining the quality of a motor and, in particular, to a method for discerning the quality of a rotor by winding an induction coil on a stator and measuring the electromotive force of the motor. 
     2. Related Art 
     Currently, development in information recording media aims at high-density data storage density, data transmission speed has to become faster too. Correspondingly, the rotational speed of the main-axis motor on the storage device such as an optical disk drive or DVD has to be able to satisfy such a requirement. Therefore, the quality and characteristic of the main-axis motor are very important factors. 
     One factor that has great influence on the motor characteristic is the back-emf constant Ke of the motor. In the MKS system, it is numerically identical to the torque constant Kt. Since the back-emf Ke is a load for the external voltage applied to the motor, the external voltage V has to be greater than the back-emf so that the motor can function in the form of a motor. Otherwise, it would be running like an alternator. One then knows that only the difference between the external voltage and back-emf can provide a current on the motor coil and have a torque output. Furthermore, one intuitively thinks that a motor with a large Kt value can obtain a constant torque with an extremely small current. However, the Ke value also increases. That is, a tiny rotational speed can make the back-emf greater than the external voltage. Under such a condition, the achieved rotational speed is certainly low and unsatisfactory. Thus, the value of Ke actually determines the rotational speed and the torque character of the motor. 
     There are two major methods of measuring the value of Ke. Referring to FIG. 1, the first method is to use an active motor  11  to drive a test motor  12  into rotation so as to measure the back-emf of the stator in the motor  12  (Eb in the drawing) and thus the Ke value. Although this method can have fairly accurate results, the driving of the active motor  11  on the motor  12  is mediated through a connection axis. Thus, there is the problem of axis alignment. If improperly manipulated, the connection axis will affect the measurement and do harm to the rotational axis of the motor  12 . Moreover, the industry can not test the motor rotor immediately after it is made. The rotor can be tested only after a set of motors  12  are assembled. If there is any problem, the whole set of motors  12  has to be thrown away and invested product line equipment has to be adjusted. Since an active motor  11  is employed to drive test motors  12 , the testing costs more time. 
     Please refer to FIG.  2 . Another conventional method is to measure a single test motor rotor  22  using a Gauss meter  21  to obtain the magnetic flux density B. Yet this method is not measuring the result of a rotor and stator system under operation, the obtained result will be quite different from the actual situation. That is, it is impossible to measure the back-emf generated due to the magnetic force line crossing between the rotor and the stator. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, it is an object of the invention to provide a method for determining the magnetic induction of a rotor along, whose result is not different from directly measuring the motor under actual operation. This method can control the stability of the motor quality. 
     Pursuant to the above object, the invention provides a method for discerning whether a rotor is good in magnetic induction by measuring the electromotive force of the motor. According to the disclosed technology, a motor standard stator is manufactured to be the standard of all test rotors. In addition to driving coils, the standard stator is further coiled with a set of induction coil on its teeth. When a user wants to test the quality of a rotor, he only needs to combine the test rotor with the standard stator. A driving voltage is provided to the driving coil through a driver to rotate the rotor. Through the induction of the induction coil, the back-emf signal generated by the test rotor is given out. The back-emf signal is also retrieved and converted into a physical signal related to the motor rotational speed. By computing the back-emf signal, the physical signal and the ratio of coil rounds on the driving coil and the induction coil, the back-emf constant of the motor can be obtained. Therefore, the disclosed method can discern the quality of motors. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow illustration only, and thus are not limitative of the present invention, and wherein: 
     FIG. 1 shows a schematic view of the first method for measuring the back-emf of motors; 
     FIG. 2 shows a schematic view of the second method for measuring the back-emf of motors; 
     FIG. 3 shows a schematic view of measuring the back-emf of motors according to the invention; 
     FIG. 4 shows a schematic view of signal transmission in FIG. 3; 
     FIG. 5A shows a schematic view of a first coiling method of the induction coil in the invention; 
     FIG. 5B shows a schematic view of a second coiling method of the induction coil in the invention; 
     FIG. 5C shows a schematic view of a third coiling method of the induction coil in the invention; 
     FIG. 5D shows a schematic view of a fourth coiling method of the induction coil in the invention; and 
     FIG. 6 shows a schematic functional block of the invention applied to a system. 
     In the various drawings, the same references relate to the same elements. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 3, the disclosed technology takes a standard stator  22  as the standard for testing many test rotors  21 . The standard stator  22  comprises a plurality of stator grooves  221  and the same number of stator teeth  222  for separating the plurality of stator grooves. In addition to driving coils  23  on the plurality of stator teeth  222 , a set of induction coil  24  is also wound thereon. When a driver  30  provides a driving voltage to the standard stator  22 , the electromagnetic action of the driving voltage generates different electromagnetic poles on the driving coil  23  wound stator teeth  222 . The electromagnetic poles of the stator teeth  222  will interact with the magnetic poles of the magnets of the test rotor  21 , pushing the test rotor  21  into rotation in the standard stator  22 . In rotation, the relative motion of the test rotor  21  to the standard stator  22  results in magnetic force line crossing, which generates a back-emf signal (Eb′ in the drawing). The invention measures the back-emf signal using the induction coil  24  and outputs the result. 
     The above measurement method can be applicable to all sorts of motors. According to the product types, they can be the main-axis motors of DVD or CD-ROMs. According to the phases of the driving voltages, they can be motors of a single phase, dual phases, or multiple phases. In other words, as long as the test motor product contains a rotor and a stator, the disclosed method can be used to test the quality during the production. 
     In practice, the signal transmission relation between the driver  30  and the motor  20  can be explained using the following three-phase motor  20  as an example. With reference to FIG. 4, three-phase Hall device signals Hu  86 , Hv  87  and Hw  88  are continuously fed back to the driver  30  during the operation of the motor  20 . The driver  30  determines where the test rotor  21  rotates to, whose result is used to distribute the driving voltage  80  to a U phase voltage  81 , a V phase voltage  82  or a W phase voltage  83  through three terminals so that the driving voltage  80  can keep the motor  20  running all the time in the change of three phases. Therefore, the induction coil  24  can sense the back-emf signal generated by the motor rotation. The rotational speed of the motor  20  can be adjusted using a speed control signal  90  that is input to the driver  20 . 
     In short, the Hall device signals  86 ,  87 ,  88  are used for positioning the rotor  21 . Aside from the above-mentioned positioning method, an encoder can be added into the motor and a small hole can be inscribed on the wall of the rotor  21 . By emitting light or electric force lines from the encoder to the rotor  21 , one can also achieve positioning with the interaction between the small hole and the light or the electric force lines. 
     The winding method of the induction coil  24  in the invention can vary in accordance with the motor type or practical situations. A three-phase motor  20  is again taken as an example to describe some of the preferred embodiments of the invention. FIG. 5A shows a three-phase motor  20  with nine stator grooves  221  and stator teeth  222 . Driving coils  23  of the three phases are intertwined on the nine stator teeth  222  so that the corresponding stator teeth  222  generate electromagnetic poles with the U, V or W phase. Therefore, the induction coil  24  can only wound on one of the stator teeth  222 . For example, it can be wound on a U-phase stator tooth  222 . Alternatively, it can be simultaneously wound on three U-phase stator teeth  222 , as shown in FIG.  5 B. It can be wound on two neighboring U-phase and V-phase stator teeth  222 , as shown in FIG.  5 C. Analogously, it can be simultaneously wound on three pairs of neighboring U-phase and V-phase stator teeth  222 . 
     The described embodiments include only a small fraction of all possible winding methods of the invention. In other words, one can wind the induction coil  24  on one, two or three W-phase stator teeth  222  or simultaneously on one pair, two pairs or three pairs of V-phase and W-phase stator teeth  222 . In any case, the invention can be properly implemented as long as the winding of the induction coil  24  does not cancel the measured back-emf. For instance, if the induction coil  24  is wound on a set of U-phase, V-phase and W-phase stator teeth, the back-emf measured would be zero. That is to say the winding method of the induction coil  24  in the invention can be, in terms of the number of phases in the motor, single phase single tooth, single phase multiple teeth, dual phases dual teeth, dual phases multiple teeth or even multiple phase single tooth and multiple phase multiple teeth. 
     FIG. 6 shows a schematic functional block of the invention applied to a system. When a user wants to test whether the produced motor rotor satisfies required specifications, he only needs to put the test rotor on the standard stator  22  to form a motor  20 . The user then presses a test key  451  on an operation interface  45  to enter command signals to a control unit  40 . The command signals are converted by a digital-to-analogue signal converter (DAC)  51  from digital ones to analogue ones, which are then output to the driver  30 . The driver  30  provides the three-phase driving voltages  81 ,  82 ,  83  to the motor  20  according to the signals (it is assumed that the tested motor is also a three-phase motor  20 ). As described hereinbefore, the motor  20  will feed back three-phase Hall device signals  86 ,  87 ,  88  to the driver  30  to keep the motor  20  rotating or to control its speed. When the motor  20  rotates for a period of time, the induction coil  24  will detect a back-emf signal and send the signal to a detector  60 . On one hand, the back-emf signal passes through an analogue-to-digital signal converter (ADC)  51  so as to be converted from analogue signals into digital ones, which are output to the control unit  40 . On the other hand, an operational amplifier  70  simultaneously extract the back-emf signal and converts it into a physical quantity that is proportional to the motor rotational speed and is output to the control unit  40 . The control unit  40  integrates the obtained back-emf signal, the physical quantity that is proportional to the motor rotational speed, the winding ratio of the driving coil  23  and the induction coil  24  to compute the back-emf constant Ke for the motor  20  in actual operation. It is then output to a display  452  on the operation interface  45 . The user can determine whether the test rotor  21  is good in its magnetic property by observing the monitor  452 . 
     More specifically, in the present invention the coil the back-emf constant Ke is calculated according to the following formula: 
     
       
         
           Ke=Eb/ω 
         
       
     
     where Eb is the back-emf signal of the moter and ω is the rotational speed of the motor. 
     Furthermore, since Eb=N*dλ/dt, and ω=dθ/dt(λ is the magnetic flux of the motor, θ is the shift angle of the motor, and t is time), the following derivations can be made: 
     
       
         
           N*dλ/dt=Eb 
         
       
     
     
       
         
           N*dλ/dθ*dθ/dt=Eb 
         
       
     
     
       
         
           N*dλ/dθ*ω=Eb 
         
       
     
     
       
         
           Ke=Eb/ω=N*dλ/dθ 
         
       
     
     Thus, 
       dλ/dθ=Ke/N   
     Therefore, in the present invention, the quality of the motor rotor dλ/dθ can be calculated by dividing the back-emf constant Ke by the winding ratio between the driving coil and the induction coil, N. 
     Similarly, FIG. 6 only depicts one embodiment of the invention. Since the driver  30  can rectify rectangular waves, the back-emf signal can be directly output to the driver  30  and the needed physical quantity can be output to the control unit  40  through the driver  30 . The rectangular wave rectifying effects of the driver  30  are nevertheless not so good. It is preferable to process the rectangular wave rectification individually. 
     There is a small defect in the above method of winding an additional induction coil  24  on the stator teeth  222 . That is the induction coil  24  does only measure the back-emf generated by the rotating test rotor  21 , but also includes the current-excited magnetism on the driving coil  23  itself. Thus, the result is slightly different from the pure back-emf signal. However, the influence of the current-excited magnetism on the induction coil  24  is not large. It is experimentally tested to contribute about one tenth of the total result. Also, the object of the invention is not to measure a very accurate back-emf constant but to ensure the stability of the quality of produced motors  20 . Accordingly, if the computed back-emf constant is within a standard range in practice, the motor is considered as a good product. 
     Effects of the Invention 
     In the disclosed method, s standard stator is provided and a set of induction coil is included in addition to the original driving coil. This method is featured in that: 
     The invention is implemented in a closed system composed of a rotor and a stator. The measured back-emf constant is therefore closer to the result of a real motor in rotation. 
     The invention can test individual rotors. It is convenient and does not have such problem of axis alignment. Using this method can save the time and facilitate mass production. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.