Abstract:
An anti-noise method for the Direct Current Brushless motor System, which includes a startup circuit, phase detective circuit, motor phase commutation circuit, driving circuit, BEMF detective circuit, and frequency detector, utilizes the BEMF detective circuit to detect the BEMF induced from the coils of the outer motor, and utilizes the sampled voltage phase to determine rotation speed and phase of the external motor by the phase detection circuit and frequency detector. Further, the sampling voltage of the BEMF detection circuit is feedback controlled by the frequency detector, utilized to keep good BEMF to noise ratio, and avoids the BEMF sampling error from the system.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is related to an anti-noise method for the direct current brushless motor without using sensor device, and more particularly, related to a anti-noise method utilizing a Back Electromotive Force (BEMF) detection circuit to detect a BEMF induced from the coils of the outer motor and a gain control signal feedback from the phase detection circuit and frequency detector to determine rotation speed and phase of the external motor so as to guarantee that BEMF signal from the direct current brushless motor is able to determine rotation speed and the phase of the external motor. 
     2. Description of the Prior Art 
     The technique related to the DC brushless motor in prior art discloses a anti-noise method utilizing a startup circuit to output different startup frequencies and output different driving currents from the control circuit to pass through the driving coil of the external motor and feedback the external BEMF to detect by the detective circuit so as to determine the startup and the operation of the motor in order to guarantee the motor is working properly. 
     As shown in  FIG. 1A , it is a block diagram illustrating that a DC brushless motor system without sensor device. As shown in  FIG. 1A , the system includes an external motor  11 , a control circuit  12 , a output circuit  13 , a detective circuit  14 , a startup circuit  15 , and a switching circuit  17 . The startup circuit  15  outputs different driving frequencies square waves to the output circuit  13  and the corresponding output current is outputted to the driving coil of the external motor  11 . The driving coil of the external motor  11  will generate the Back Electromotive Force (BEMF) to feedback to the detective circuit  14  and the detective circuit  14  will determine the rotation speed and the phase of the external motor  11  in accordance with the BEMF so as to control the startup and the rotation speed of the motor. When the startup circuit  15  is activated to output the activated frequency signal to the control circuit  12 , and the signal is transformed to be a six steps driving control signal shown in  FIG. 1B , into the output circuit  13 . The current of the driving coil of the external motor  11  is accordance with the phase difference of the six steps driving control signal, and the rotation speed and the phase of the external motor  11  is determined by the current of the driving coil. 
       FIG. 2  is a view illustrating a conventional BEMF detector without sensor device. The detector includes a three-phase induction motor  11 , a BEMF sampler  26 , a normal phase comparator  202 , a reverse phase comparator  204 , a voltage reference (Vth), a normal phase switch  210 , a reverse phase switch  212  and a output switch  214 . When the motor system is activated, the six steps driving control signal is inputted into the output circuit  13  and three-phase current is outputted from the output circuit  13  to the three-phase motor coil  11  so as to generate the BEMF by the current of the three-phase motor coil  11 . 
     The BEMF is inputted in the positive end of the normal phase comparator  202  and the negative end of the reverse phase comparator  204 . The voltage reference (Vth)  206  is connected to the negative end of the normal phase comparator  202  and the positive end of the reverse phase comparator  202 . When the phase of the BEMF outputted from the BEMF sampler is between 0˜180 degree, the phase value is more than the positive voltage reference, the voltage outputted from the normal phase comparator  202  is high voltage reference. At final, the output of the normal phase comparator  202  and the output of the reverse phase comparator  204  will pass through the normal phase switch  210  and the reverse phase switch  212 , and then merge together to be a square wave signal in the output switch  214 . Therefore, the square wave outputted from the output switch  214  will inputs to the detective circuit  14  so as to detect the rotation speed and the phase of the external motor  11 . 
     According the description above, The BEMF generated at the three-phase motor coil is detected by the BEMF sampler  26 . The high voltage reference is sampled by the normal phase comparator  202  and the reverse phase comparator  204  and inputted into the detective circuit  14  for determining the rotation speed and the phase of the external motor  11 . However, the motor driving circuit system in initial rotation and high speed rotation will generate different noise. If the sample voltage of the BEMF is sampled in accordance with the same voltage in different rotation, the system would make a mistake to make a wrong decision so as to cause the malfunction of the system. 
     According to the motor driving circuit system described above, a different motor driving circuit is provided in the present invention for utilizing at different rotation mode to provide different sample voltage of the BEMF. Therefore, the noise ratio of the BEMF is better and the rotation speed and the phase of the external motor are detected properly so as to achieve a better stability of the system. 
     SUMMARY OF THE INVENTION 
     The present invention is to provide a direction current brushless motor system without sensor device having Back Electromotive Force (BEMF) detective circuit. The main object is to determine the rotation speed of the DC brushless motor system is sequentially achieved at the first startup frequency and the second startup frequency to activate the motor driving system. When the motor system has been activated, the BEMF generated by the motor driving system is configured to accurately detect the rotation speed of the motor. 
     Another object of the present invention is to utilize different BEMF sampling voltage by the BEMF detective circuit to sample the BEMF of the external motor when the DC brushless motor is in different modes. The DC brushless motor is able to accurately detect the rotation speed and the phase of the external motor in accordance with the BEMF signal. 
     One another object of the present invention is to provide a DC brushless motor system. The sampling signal generated by the frequency detector to control the internal sampling voltage of the BEMF detective circuit to sample the BEMF of the external motor so as to generate a high level detective signal. The high level detective signal is inputted to the phase detective circuit to determine the rotation speed and the phase of the external motor. The frequency detector is feedback to determine the new BEMF detective voltage. The system is working in different modes to acquire the same anti-noise ration so as to guarantee that the DC brushless motor system detects the rotation speed and the phase of the external motor and the system stability in accordance with the BEMF signal. 
     Moreover, one more object of the present invention is to provide a DC brushless motor system to choice different gain control mode in accordance with the different rotation speeds by the control of the BEMF detective circuit so as to achieve a better anti-noise ration guarantee that the DC brushless motor system detects the rotation speed and the phase of the external motor and the system stability in accordance with the BEMF signal. 
     According to the objects described above, the present invention provides a direct current (DC) brushless motor system without sensor device having two steps startup function including a control device and one end of the control device is connected to an oscillation device, a switching device and one end of the switching device is connected to the control device and the other end is connected to a startup device, a detective device and one end of the detective device is connected to the startup device, a driving circuit and one end of the driving circuit is connected to the detective device and the other end is connected to an external motor and feedbacks to the other end of the detective circuit; when the DC brushless motor system has been activated, the rotation speed of the DC brushless motor system is sequentially achieved in a first predetermined startup rotation speed and a second predetermined startup rotation speed with different frequency so as to achieve a system predetermined rotation. 
     The present invention also provides a direct current (DC) brushless motor system without sensor device includes a control device, a switch device, a startup device, a detective device, a driving circuit, and a phase lock loop frequency device. One end of the control device is connected to an oscillation device. One end of the switching device is connected to the other end of the control device. One end of the startup device is connected to the other end of the switching device. One end of the detective device is connected to the other end of the startup device. One end of the driving circuit is connected to the other end of the detective device and the other end is connected to an external motor, and feedbacks to the other end of the detective circuit from a three-phase coil of the external motor. The phase lock loop frequency device is connected to the detective device, wherein the detective device includes a phase detective circuit, and one end of the phase detective device is connected to the startup device and the other end is connected to the frequency detector and the phase lock loop frequency device; a phase rotation circuit, and one end of the phase rotation circuit is connected to the other end of the phase detective circuit and the other end is connected to one end of the driving circuit; a BEMF detector, and one end of the BEMF detector is connected to the feedback of the three-phase coil of the external motor and the other end is connected to the phase detective circuit; a frequency detector, and one end of the frequency detector is connected to the phase detective circuit and the phase lock loop frequency device and the other end is connected to the BEMF detector; wherein the BEMF detector includes: a BEMF detective switch, and one of the BEMF detective switch is connected to the feedback of the three-phase coil of the external motor; a BEMF sample amplifier, wherein a first input end is connected to an output end of the BEMF switch, a second input end is connected to a level voltage of the three-phase coil, and a third input end is connected to the frequency detector and outputs a positive voltage sine wave and a negative voltage sine wave; a hysteresis comparator including a first hysteresis level, and an input end is connected to the positive voltage sine wave and the negative voltage sine wave and the other end is connected to the frequency detector and outputs a BEMF detective signal to the phase detective circuit. 
     The present invention also provides a rotation speed and phase detective method for a direct current (DC) brushless motor system without sensor device, and the DC brushless motor system includes a control device and one end of the control device is connected to an oscillation device, a switching device and one end of the switching device is connected to the control device and the other end is connected to a startup device, a detective device and one end of the detective device is connected to the startup device, a driving circuit and one end of the driving circuit is connected to the detective device and the other end is connected to an external motor and feedbacks to the other end of the detective circuit from a three-phase coil of the external motor; a phase lock loop frequency device connected to the detective device and wherein the detective device includes a phase detective circuit, and one end of the phase detective device is connected to the startup device and the other end is connected to the frequency detector and the phase lock loop frequency device; a phase rotation circuit, and one end of the phase rotation circuit is connected to the other end of the phase detective circuit and the other end is connected to one end of the driving circuit; a BEMF detector, and one end of the BEMF detector is connected to the feedback of the three-phase coil of the external motor and the other end is connected to the phase detective circuit; a frequency detector, and one end of the frequency detector is connected to the phase detective circuit and the phase lock loop frequency device and the other end is connected to the BEMF detector; wherein the BEMF detector includes: a BEMF detective switch, and one of the BEMF detective switch is connected to the feedback of the three-phase coil of the external motor; a BEMF sample amplifier, wherein a first input end is connected to an output end of the BEMF switch, a second input end is connected to a level voltage of the three-phase coil, and a third input end is connected to the frequency detector and outputs a positive voltage sine wave and a negative voltage sine wave; a hysteresis comparator including a first hysteresis level, and an input end is connected to the positive voltage sine wave and the negative voltage sine wave and the other end is connected to the frequency detector and outputs a BEMF detective signal to the phase detective circuit, wherein when the DC brushless motor system has been activated, the rotation speed and the phase detective method including the following steps: the step of providing a BEMF generated by three-phase current sequence on the three-phase coil passing the BEMF detective switch; the step of providing a level voltage provided by the three-phase coil; the step of provide a pair of sine wave voltages generated in accordance with comparing the BEMF and the level voltage by the BEMF amplifier, and the pair of sine wave voltages includes a positive voltage sine wave and a negative voltage sine wave; the step of providing a BEMF detective signal generated in accordance with the sine wave voltages by the hysteresis comparator, and the BEMF detective signal is connected to the phase detective circuit to detect the present rotation speed and the phase of the external motor; and the step of providing a gain control signal, and the gain control signal is outputted in accordance with the preset rotation speed of the phase lock loop frequency device and the present rotation speed of the external motor by the frequency detector. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1A  is a block diagram illustrating a direct current brushless motor system without sensor device in prior art; 
         FIG. 1B  is a view illustrating a six steps motor driving method circuit in prior art; 
         FIG. 2  is a view illustrating a Back Electromotive Force (BEMF) structure in prior art; 
         FIG. 3  is a structure view illustrating a motor driving circuit in the present invention; 
         FIG. 4  is a view illustrating a startup mode in the present invention; 
         FIG. 5  is a waveform diagram illustrating the startup mode in the present invention; 
         FIG. 6  is a structure view illustrating the BEMF detector in the present invention; 
         FIG. 7A  is a waveform diagram illustrating the BEMF detector sampling level in the present invention; and 
         FIG. 7B  is a waveform diagram illustrating the BEMF detector sampling level in another embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     First of all, please referring to  FIG. 3 , it is a block diagram illustrating a DC brushless motor system without sensor device. As shown in  FIG. 3 , the DC brushless motor system includes a control device  30  and the input end thereof is connected to the oscillation device  31 . The input end of the switch device  32  is connected to the control device  30  and the output end thereof is connected to the startup device  33 . The startup device  33  includes a startup circuit  331  and the normal rotation circuit  333 . The detective device  34  includes a phase detective circuit  341 , a phase rotation circuit  343 , a Back Electromotive Force (BEMF) detector  345  and a frequency detector  347 . The input end of the driving circuit  35  is connected to the phase rotation circuit  343  of the detective circuit  34  and the output end thereof is connected to the external motor  36 . The phase detective circuit  341  of the detective device  34  is connected to the startup device  33 , the phase lock frequency device  37  and the phase rotation circuit  343 . The BEMF detective circuit  345  is connected to the external motor  36 , the phase lock frequency device  37  and the phase rotation circuit  343 . The BEMF detective circuit  345  is connected to the external motor  36 , the phase detective circuit  341  and the frequency detective circuit  347 . The frequency detective circuit  347  is connected to the phase lock frequency device  37 . 
     When the DC brushless motor system is activated, the control device will control the oscillation device  31  to output an oscillation signal. The startup circuit  331  will activate the rotation speed in accordance with the oscillation signal generated from the oscillation device  31  and the rotation speed will input to the phase detective circuit  341 . The phase detective circuit  341  will output the corresponding three-phase driving voltage in accordance with the rotation speed and the rotation speed is transformed into the corresponding six steps driving voltage to the motor driving circuit  35  so as to drive the external motor  36 . When the external motor  36  is activated, the BEMF is generated by the current difference in the motor coil and inputted to the BEMF detector  345 . The BEMF detector  345  will detect the rotation speed and the phase of the external motor  36  in accordance with the sampled BEMF. At this moment, when the startup rotation speed of the motor is not more than the first predetermined startup rotation speed, the control device  30  will request the oscillation device  31  to transmit the signal continually so as to force the motor rotating. When the rotation speed of the motor is more than the first predetermined startup rotation speed (for example the first determined startup rotation speed is 30 rpm), the control device  30  will activate the switch device  32  to switch the startup device  33  to be the normal rotation circuit  333  and the driving motor will start rotating in accordance with the signal provided by the normal rotation circuit  333 . 
     Similarly, after the control device  30  switch the startup device  33  to the normal rotation circuit  333 , the phase detective circuit  341  will detect the normal rotation speed of the motor. Therefore, the phase detective circuit  341  will output the corresponding three-phase driving circuit in accordance with the normal rotation speed of the motor. At the time, the external motor  36  will generate the BEMF in accordance with the current different of the motor coil and input it to the BEMF detector  345 . The BEMF detector  345  will detect the rotation speed and the phase of the external motor in accordance with the sampled BEMF. It should be noted that the control device will further detect if the rotation speed of the motor is in the second predetermined startup rotation speed in the preferred embodiment of the present invention (for example the second predetermined startup rotation speed is 180 rpm). When the BEMF detector  345  detects the rotation speed of the motor is not at 180 rpm, it means that the motor has not been activated. Therefore, the control device  30  will drive the switch device  33  to switch the startup device  33  to the startup circuit  331  and request the motor to rotate according to the signal provided by the startup circuit  331 . When the phase detective  341  determines that the rotation speed of the motor is in the first predetermined rotation speed, the control device  30  will drive the switch device  32  to switch the startup device  33  to be the normal rotation circuit  302 . When the rotation speed of the motor detected by the BEMF detective circuit  341  is more than the second predetermined startup rotation speed (the speed is 180 rpm), it means that the motor is rotated properly in accordance with the signal of the normal rotation circuit  333 . At this moment, the control device  30  will determine that the motor has been activated and the startup device  22  is connected to the normal rotation circuit  333 . Finally, the BEMF detective circuit  345  will detect the current rotation speed and the phase of the external motor  36  and output the signal to the phase detective circuit  341  to confirm that the driving frequency of the output of the external motor  36  is equal to the output of the phase detective circuit  341 . 
     Now, please referring to  FIG. 4 , it is a flow chart illustrating the activated steps of the DC brushless motor system without sensor device. First, at step  401 , in the first activated step, the control device  30  of the DC brushless motor system without sensor device will request the oscillation device to output an oscillation signal to the startup circuit  331 . The motor  36  will be activated to generate a startup rotation speed. The startup rotation speed of the motor  36  will pass through the BEMF detector  345  to the phase detective circuit  341 . A step  402 , the control device  30  will continually detect the startup rotation speed of the motor  36 . When the startup rotation speed of the motor  36  is in the first predetermined rotation speed, such as the first predetermined rotation speed is 30 rpm, the control device will be in step  403 . At step  403 , the startup rotation speed of the motor  36  will be in the first predetermined rotation speed, the control device  30  will drive the switch device  32  to switch the startup device  33  to be the normal operative mode and the motor  36  will be rotate properly. Then, at step  404 , the control device  30  will continuingly detect the rotation speed of the motor  36 . When the rotation speed of the motor  36  will be in the second predetermined rotation speed, such as the second predetermined rotation speed is 180 rpm, the motor  36  has been activated. At step  405 , the control device  30  will terminate the startup procedure and the motor  36  is still in the normal operative mode so as to be the predetermined operative rotation speed in the DC brushless motor system without sensor device. On the other hand, when the control device  30  activates the motors  36  and the rotation speed is not in the first predetermined rotation speed, the control device  30  will stay at step  402  and the control device will force the oscillation  31  of the motor system to continually output the oscillation signal to speed up the rotation speed of the motor till the rotation speed is in the first predetermined rotation speed. 
     The control device  30  will drive the startup device  33  to switch the startup device  33  to be the normal operative mode to the motor keep outputting the rotation speed. When the DC brushless motor system without sensor device is not able to be in the second predetermined rotation speed by the phase detective circuit  341 , the motor is not activated properly, as shown in step  404 . At this moment, the control device  30  will go back to step  402 , the control device  30  will drive the startup device to switch back to the startup circuit  331  to force the motor system to keep outputting the oscillation signal so as to make sure when the motor is in the first predetermined rotation speed and the second activated frequency, the system will go to step  405  and the motion of the system startup is done. 
     The startup steps in  FIG. 4  and the block diagram in  FIG. 3  will be further discussed at the following description. First, as shown in step  401 , the control device  30  of the motor system will output an oscillation signal so as to drive the switch device  32  to switch the startup circuit  32  to be the startup circuit  331 . The startup circuit  331  will transform the oscillation frequency outputted from the oscillation device  31  and output a startup rotation speed to the phase detective circuit  341 . The system will go to step  402 , the motor system is in the first activation mode and the startup rotation speed of the motor will be continually detected. The phase detective circuit will output the corresponding three-phase driving control signal in accordance with the activated rotation speed outputted by the startup circuit  331  to the phase rotation circuit  343 . The phase rotation circuit  343  will convert the three-phase driving control signal to be the six steps driving voltage to the motor driving circuit  35 . The motor driving circuit  35  determines the three-phase current in accordance with the six steps driving voltage and the three-phase current will output to the driving coil of the external motor  36  to force the external motor  36  rotating. The phase switching of the three-phase current is determined according to the phase difference of the six steps driving voltage of the motor driving circuit  35 . Because the six steps driving voltage is a three-phase switching driving voltage, there is only one phase in Stop State at the same time. At the Stop State, the external motor  36  will stop providing the current on the driving coil. Because of the electromagnetic effect, when the current on the driving coil is stopped, a BEMF will be generated. By utilizing the phase and the frequency of the BEMF, the BEMF is inputted to the BEMF detector  345  to detect the anti-noise ration of the BEMF and the rotation speed and the rotor phase of the external motor  36 . However, when the rotation speed of the external motor  36  is not at the first startup rotation speed (such as 30 rpm), the amplifier of the BEMF on the driving coil of the external motor  36  is not large enough and the detective ability of the BEMF detector  345  on the anti-noise of the BEMF is weak. Therefore, The PWM signal outputted from the BEMF detector  345  is easily affected by the noise so as to have an uncertain output value. When the PWM rotation control signal outputted from the BEMF detector  345  is inputted to the phase detective circuit  341 , the phase detective circuit  341  will determine the rotation speed of the external motor  36  in accordance with the PWM rotation control signal. When the rotation speed of the external motor  36  is in the first activated rotation speed (ex: 30 rpm), the system will go to step  403 . The control device  30  will drive the switch device  32  to switch the startup circuit  331  to the normal rotation circuit  333  and the DC brushless motor is in the normal operative mode. However, the external motor  36  starts to work but it is not in completely start condition. When the rotation speed of the external motor  36  is in the first predetermined startup rotation speed (30 rpm), the system will switch the detective frequency of the detective circuit  341  in the second predetermined startup rotation speed (180 rpm). In the present embodiment, the second predetermined startup rotation speed (180 rpm) is the multiple of the first predetermined startup rotation speed (30 rpm). The motor system will enter the second startup mode from the first startup mode and continually detect the rotation speed of the external motor  36  at step  404 . The normal rotation circuit  333  in the startup device  33  will generate the startup rotation speed, which will continually speed up, to the phase detective circuit  341 . The phase detective circuit  341  generates three-phase driving control signal in accordance with the startup rotation speed and the three-phase driving control signal is inputted to the phase rotation circuit  343 . By converting in the internal circuit of the phase rotation circuit  343 , the phase rotation circuit  343  will output a six steps driving voltage to the motor driving circuit  35 . The six steps driving voltage will convert to be three-phase current by the motor driving circuit  35  and transmit to the driving coil of the external motor  36 . As the description above, the driving current supply on the driving coil of the external motor  36  is corresponding to the six steps driving voltage of the phase rotation circuit  343  and there is only one coil phase stayed in Stop State at the same time. According to the electromagnetic effect, a larger BEMF is generated and transmitted to the BEMF detective circuit  345 . Because the BEMF is larger, the ability of the anti-noise is better. The BEMF detective circuit  345  is able to detect the phase of the BEMF properly so as to output the corresponding PWM rotation speed control signal and feed back to the phase detective circuit  341 . When the PWM rotation speed control signal is in the second predetermined startup rotation speed (the multiple of 30, such as 180 rpm=30 rpm×6), the startup device  33  will go to step  405  to finish the complete startup steps. 
     According to the description above, when the DC brushless motor system is in step  402  (the first startup step), the motor system will continually detect the rotation speed of the external motor  36 . When the startup rotation speed of the external motor  36  is not in the first predetermined startup rotation speed (30 rpm), the motor system will keep staying in step  402 . The BEMF generated by the driving coil of the external motor  36  is directly proportional to the driving voltage on the driving coil of the external motor  36 , which is in low rotation speed state. The anti-noise ratio of the BEMF detected by the BEMF detective circuit  345  is weak and the PWM rotation control signal detected by the phase detective circuit  341  is showing that the rotation speed is not going to represent that the motor system is being started normally. Therefore, when the external motor  36  is in the first predetermined startup rotation speed (30 rpm), at step  403 , it is going to the normal step. The control device  30  drive the switch device  32  to switch to the normal rotation circuit  333  in the second startup mode from the first startup mode. The BEMF generated by the driving coil of the external motor  306  is large enough to detect the rotation speed and the phase of the external motor  36  for the BEMF detector  345 . The motor system will go to step  404  and continually detect the startup rotation speed of the external motor  36 . If the PWM rotation control signal detected by the phase detective circuit  341  is not larger than the first predetermined startup rotation speed or the control signal is not in the second predetermined startup rotation speed during a predetermined period, the control device  30  will determine that the startup of the external motor is failure and the motor system will go back to step  402  and the previous step will be repeated till the phase detective circuit  341  detects the rotation speed of the motor is in the first predetermined startup rotation speed (30 rpm) and the second rotation speed (180 rpm). On the other hand, if the phase detective circuit  341  detects the PWM rotation speed control signal is in the second predetermined rotation speed (180 rpm), the motor system finishes the startup procedure. The control device  30  will keep working to speed up the rotation speed to the high speed rotation mode. 
     Now referring to  FIG. 5 , it is a view to show that the startup mode is switched in the DC brushless motor system. As shown in  FIG. 5 , the startup mode includes four sections, the first section is the initial section, the second section is the speedup section, the third section is the rotation section and the forth section is the stable rotation section. The first section is the low rotation speed mode, and the control device  30  will drive the motor driving system. The BEMF generated by the external motor  36  is not large enough to determine that the motor system is in normal startup mode. When the motor driving system enters the second section, the BEMF generated by the driving coil of the external motor  36  is large enough to detect the rotation speed and the rotor phase of the external motor  36  so as to confirm the motor system is successfully started. When the rotation speed of the external motor  36  is in the second predetermined startup rotation speed (180 rpm), it means that the motor is completely started in the third section. At this moment, the rotation speed of the external motor  36  keeps going up and the system is in the third section. The external motor  36  is in the predetermined high rotation speed state, the stable state in the forth section. 
     Now referring to  FIG. 6 , it is a view illustrating the structure of the BEMF detector of the DC brushless motor system. As shown in  FIG. 6 , the BEMF detector  345  of the DC brushless motor system includes a BEMF detective switch  612 , a BEMF amplifier  613 , and a hysteresis comparator  614 . The BEMF amplifier  613  and the hysteresis comparator  614  are respectively connected to the output of the frequency detector  347 . The BEMF detective switch  612  is connected to the three-phase coil  611  at outside of the DC brushless motor system. 
     As shown in  FIG. 6 , when the DC brushless motor system is activated, the three-phase current on the external motor  36  is going to the three-phase coil  611 , and the three-phase coil  611  will generate the BEMF signal in accordance with the time difference of the three-phase current. In addition, the BEMF detective switch  612  determines the sequence of the internal switching in accordance with the difference of the three-phase current on the three-phase coil  611 . Therefore, the BEMF signal generated by the three-phase coil  611  will can pass through the switching control by the BEMF detective switch  612 . The BEMF signal passed through the switching control will be transmitted to the BEMF amplifier  613  to compare with a voltage reference V N . The voltage reference V N  is one half of the system voltage (½ Vcc) and is the initial voltage of the three-phase coil  611  and the system voltage is the standard Vcc of the motor driving system. When the BEMF signal is larger than the voltage reference V N , the sine voltage OPP outputted by the BEMF amplifier  613  is the positive level sine voltage. On the other hand, when the BEMF signal is smaller than the voltage reference V N , the sine voltage OPN outputted by the BEMF amplifier  613  is the negative level sine voltage. Therefore, the BEMF amplifier  613  will transmit the signal of the sine voltage OPP and the sine voltage OPN to the input end of the hysteresis comparator  614 . The hysteresis comparator  614  will determine the high level and the low level of the signal of the hysteresis comparator  614  in accordance with the sine wave OPP, sine wave OPN and internal hysteresis voltage (±Vhys). For example, when the inputted sine voltage OPP is larger than the internal hysteresis voltage, the BEMF detective signal BEMF_Det outputted by the hysteresis comparator  614  is in high level. When the inputted sine voltage OPP is smaller than the internal hysteresis voltage, the BEMF detective signal BEMF_Det outputted by the hysteresis comparator  614  is in low level. 
     Subsequently, the BEMF detective signal BEMF_Det is transmitted to the phase detective circuit  341  (as shown in  FIG. 3 ), and the phase detective circuit  341  determines the rotation speed and the phase of the external motor  36  in accordance with the duration of the voltage level of the BEMF detective signal (BEMF_Det). When the motor  36  is in the startup duration, because the rotational speed is not fast (60 rpm), the BEMF signal generated on the three-phase coil  611  is not large. The BEMF signal detected by the BEMF detector  345  is not large enough to resist the noise generated by the system. Therefore, the rotation speed and the phase of the external motor  36  detected in accordance with the BEMF signal (BEMF_Det) by the phase detective circuit  341  is distortion. The motor  36  is activated when the phase detective circuit  341  detects the rotation speed of the external motor  36  in the first predetermined rotation speed and the second predetermined rotation speed in accordance with the BEMF signal (BEMF_Det), the rotation speed is fast enough (1000 rpm) and the BEMF signal detected by the three-phase coil  611  is large enough to resist the system noise. At this moment, the BEMF amplifier  613  of the BEMF detector  345  will determine the voltage gain and the hysteresis comparator  614  will also determine the internal hysteresis level in accordance with the gain control signal outputted by the frequency detector  347 . The gain control signal outputted by the frequency detector  347  is generated by comparing the rotation speed of the external motor  36  and the predetermined frequency outputted by the phase lock loop circuit  37 . 
     Subsequently, when the rotation speed of the external motor  36  is not the same as the outputted frequency of the phase lock loop circuit  37  (ex: 2000 rotations or 3000 rotations), the gain control signal outputted by the frequency detector  347  is low voltage. The BEMF amplifier  613  of the BEMF detective circuit  345  will switch to the normal voltage gain mode (the first voltage gain mode). The BEMF amplifier  613  will sequentially output the sine wave voltage OPP and OPN of the BEMF signal to the hysteresis comparator  614  (please referring to  FIG. 7A , and the detail description is in the following chapter). On the other hand, when the gain control signal outputted by the frequency detector  347  is low voltage, the sine wave voltages OPP and OPN of the BEMF signal outputted by the BEMF amplifier  613  is the normal voltage gain mode (the first voltage gain mode). The hysteresis level of the hysteresis comparator  614  is in the first hysteresis level (+Vhys) to resist the initial noise of the system. Now, the BEMF amplifier  613  will compare the BEMF signal detected by the three-phase coil  611  and the level voltage V N  the signal is adjusted by the voltage of the frequency detector  347 . The sine wave voltages OPP and OPN are inputted to the hysteresis comparator  614 , the BEMF detective signal (BEMF_Det) outputted by the hysteresis comparator  614  is also inputted to the phase detective circuit  341  to determine the rotation speed and the phase of the external motor  36 . 
     When the rotation speed of the external motor  36  is in the second predetermined rotation speed and the rotation speed of the external motor  36  is increased. For example, the rotation speed of the external motor  36  is in the predetermined frequency (2000 rotations or 3000 rotation), the gain control signal outputted by the frequency detector  347  will switch to be high voltage. The control device  30  in  FIG. 3  will utilize two implemented methods to have good anti-noise ration. At first, the first implemented method is to control the BEMF detector  345  by the control frequency detector  347  of the phase detective circuit  341  and drive the BEMF amplifier  613  of the BEMF detector  345  to switch to the voltage suppressing mode (the second voltage gain mode) from the normal voltage gain mode. The sine wave voltages OPP and OPN outputted by the BEMF amplifier  613  will not be continually increased in accordance with the BEMF signal, and the sine wave voltage OPP and OPN is maintained in low speed state. On the other hand, in the present embodiment, when the gain control signal outputted by the frequency detector  347  is changed to be high voltage, the amplitude of the sine wave voltages OPP and OPN outputted by the BEMF amplifier  613  is suppressed as the same as the normal voltage gain mode (the first voltage gain mode). In addition, in the second implemented method, the frequency detector  347  controlled by the phase detective circuit  341  is configured to control the BEMF detector  345  and the internal hysteresis level of the hysteresis comparator  614  of the BEMF detector  345  is switched from the first hysteresis level (+Vhys) to be the second hysteresis level (+Vhys 2 ) to resist the noise of the high rotation speed of the system. Obviously, the voltage level of the second hysteresis level (+Vhys 2 ) is larger than the first hysteresis level (+Vhys). 
     According to the first implemented method described above, the BEMF amplifier  613  will suppress the voltage of the BEMF signal and compare to the level voltage V N . After the voltage suppressing in the frequency detector  347 , the sine wave voltages OPP and OPN outputted by the BEMF amplifier  613  at the first voltage gain mode and the second voltage gain mode include the same amplitude and are inputted to the hysteresis comparator  614 . The hysteresis comparator  614  will compare the sine wave voltages OPP and OPN with the first hysteresis level (+Vhys). When the sine wave voltage OPP is larger than the first hysteresis level, the BEMF detective signal (BEMF_Detc) outputted by the hysteresis comparator  614  will be low voltage level. The BEMF detective signal (BEMF_Detc) outputted by the hysteresis comparator  614  will be inputted to the phase detective circuit  341  to determine the rotation speed and the phase of the external motor  36  so as to accurately detect and sample the rotation speed and the phase of the external motor  36 . 
     Subsequently, the signal wave diagram of the operation in the present invention is further described herein. Please referring to  FIG. 7A  and  FIG. 7B , it is a signal wave diagram illustrating the rotation speed of the DC brushless motor and the BEMF detector in the present invention. As shown in  FIG. 7A , the BEMF signal generated by the current sequential difference on the three-phase coil  611  of the external motor  36  is inputted to the BEMF detector  345  and the BEMF signal waveform is a sine wave voltage waveform. The BEMF amplifier  613  within the BEMF detector  345  will generate the normal voltage gain mode and the voltage suppressing mode in accordance the control signal of the frequency detector  347 . When the motor is in the first startup mode (the gain control signal outputted by the frequency detector  347  is the low voltage), the BEMF amplifier will compare the level voltage V N  and the BEMF signal. As shown in  FIG. 7A , when it is in the first voltage gain mode, the BEMF signal is larger than the level voltage V N , the sine wave voltage OPP outputted by the BEMF amplifier  613  is in the positive voltage sine wave, and the sine wave OPN is in the negative sine wave. The second level is in the startup mode, but it is not achieved to the predetermined frequency outputted by the phase lock loop circuit (2000 rotation or 3000 rotation). The gain control signal outputted by the frequency detector  347  is in the low voltage state. And the BEMF amplifier  613  will keep comparing the level voltage V N  and the BEMF signal and output the sine wave voltage OPP and OPN. The sine wave voltage OPP and OPN will be inputted to the hysteresis comparator  614 . Now, the hysteresis comparator  614  will compare the sine wave voltage OPP and OPN with the first hysteresis lever (+Vhys). When the sine wave voltage OPP is larger than the first hysteresis level (+Vhys), the BEMF detective signal (BEMF_Detc) of the hysteresis comparator  614  is in high voltage level. When the sine wave voltage OPP is less than the first hysteresis level, the BEMF detective signal of the hysteresis comparator  614  is in the low voltage level. Obviously, the first hysteresis lever (+Vhys) is configured to resist the low noise when the motor driving system is in the low rotation speed. When the rotation speed of the external motor  36  is in the predetermined frequency (2000 rotation or 3000 rotation), the motor is in high rotation speed. The gain control signal outputted by the frequency detector  347  is changed to be high voltage and the noise of the gain control signal is also become large. The BEMF amplifier  613  will do the voltage suppressing action (the second voltage gain mode) in accordance with the control signal of the frequency detector  347 . According to  FIG. 7 , at the second voltage gain mode, the larger BEMF signal will be suppressed and the suppressed BEMF signal is closed the BEMF signal in low rotation speed (the first voltage gain mode). The system noise of the motor driving system will also be suppressed and the suppressed BEMF signal will be inputted to the hysteresis comparator  614  and compared with the first hysteresis level (+Vhys). Obviously, the period of the sine wave voltage OPP and OPN in the second voltage gain mode is faster than in the first voltage gain mode. When the sine wave voltage OPP is lower than the first hysteresis level, the BEMF detective signal outputted by the hysteresis comparator  614  will be in the low level. The BEMF detective signal (BEMF_Dect) can avoid the distortion caused by the noise so as to achieve the goal of suppressing the noise. 
     Now, as shown in  FIG. 7B , it is a wave diagram illustrating the BEMF amplifier and the hysteresis comparator in another embodiment. As shown in  FIG. 7B , when the motor is in the first startup mode (the gain control signal outputted by the frequency detector  347  is low voltage), the BEMF amplifier  613  will compare the level voltage V N  with the BEMF signal. In  FIG. 7B , at the first voltage gain mode, the BEMF signal is larger than the level voltage VN, the sine wave voltage OPP of the BEMF amplifier is in positive voltage sine wave and the sine wave voltage OPN is in negative voltage sine wave. When the second mode is in the startup mode but it is not in the predetermine frequency (2000 rotation or 3000 rotation), the gain control signal of the frequency detector  347  will maintain in the low voltage state. The BEMF amplifier  613  will compare the level voltage V N  with the BEMF signal and output the sine wave voltage OPP and OPN. The sine wave OPP and OPN will input to the hysteresis comparator  614 , and the hysteresis comparator  614  will compare the sine wave voltage OPP and OPN with the first hysteresis level (+Vhys). When the sine wave voltage OPP is larger than the first hysteresis level, the BEMF detective signal (BEMF_Detc) outputted by the hysteresis comparator  614  is in the high voltage level. When the sine wave voltage OPP is less than the first hysteresis level, the BEMF detective signal (BEMF_Detc) outputted by the hysteresis comparator  614  is in the low voltage level. The first hysteresis level is configured to resist the low noise when the motor driving system is in low rotation speed. When the external motor  36  is in high rotation speed (ex: 2000 rotation or 3000 rotation), the BEMF signal is increased and the noise is also increased. The system noise generated by the motor driving system is also enhanced. The hysteresis level of the hysteresis comparator  614  of the BEMF detector  345  is switched to the second hysteresis level (+Vhys 2 ) from the first hysteresis level (+Vhys). The BEMF amplifier  613  will output the sine wave voltage OPP and OPN to the hysteresis comparator  614 . The hysteresis comparator  614  will compare the sine wave voltage OPP and OPN with the second hysteresis level (+Vhys) and output the BEMF detective signal (BEMF_Detec). When the sine wave voltage OPP is larger than the second hysteresis level, the BEMF detective signal (BEMF_Detc) outputted by the hysteresis comparator  614  is in the high voltage level. When the sine wave voltage OPP is less than the second hysteresis level, the BEMF detective signal (BEMF_Detc) outputted by the hysteresis comparator  614  is in the low voltage level. Therefore, the signal distortion caused by the noise is able to be avoided.