Abstract:
A method for reducing a torque ripple of a Switched Reluctance Motor for detecting a position of the rotor using a position detection sensor, designing a pulse width if a position detection signal as an optimum one and decreasing a torque ripple of the motor by adjusting a duty rate of the pulse width modulation signal comprises the steps of setting a pulse width of a position detection signal in accordance with a position detection result of a motor rotor, outputting a signal for controlling of each phase in synchronization with a rising and falling edge of the position detection signal, and varying and outputting a duty rate of a pulse width modulation signal from the moment that a falling edge of the position detection signal is detected thus to have advantage of decreasing more than fifty percents of the torque ripple.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for reducing a torque ripple of a Switched Reluctance Motor (SRM) and particularly, to a method for reducing a torque ripple of a SRM, which detects the position of a rotor of a motor using a position detection sensor and reduces the torque ripple of the motor by designing the optimum pulse width of the position detecting signal and adjusting the duty rate of the pulse width modulation signal. 
     2. Description of the Background Art 
     Generally, a position signal of a motor rotor is required to drive a SRM (hereinafter as motor) and the motor can be controlled in the normal/reverse direction by recognizing a normal/reverse turning point of rotation accurately with a minimum one position sensor. 
     FIG. 1 is a block diagram showing a motor in accordance with the conventional art. As shown in the drawing, a conventional motor is comprised of three position detection sensors  301 ,  302  and  303  for detecting position of a motor rotor, pulse width modulation signal  110 , a main controlling unit  100  for controlling three phase signals  120 ,  130  and  140  and the position detection sensors  301 ,  302  and  303 , a motor driving unit  50  for inputting the three phase electric current to the motor by the three phase signal inputted from the main controlling unit  100 , and a motor  200  driven by the three phase electric current inputted from the motor driving unit  50 . Reference numerals  115 ,  125  and  135  designate AND gates. 
     FIG. 2 shows respective wave form charts of signals outputted from FIG.  1  and FIG. 3 is a wave form chart of a torque ripple in accordance with the conventional art. 
     With reference to FIGS. 1,  2  and  3 , description of driving of the motor according to the conventional art is as follows. 
     The position detection sensors  301 ,  302  and  303  input the position detection signals  150 ,  160  and  170  to the main controlling unit  100  by detecting the rotor position of the motor. According to the detection result of the sensors the pulse width modulation signal  110  outputted from the main controlling unit  100  and the three phase signals  120 ,  130  and  140  are computed by a logical AND operation and then inputted to the motor driving unit  50 . The motor driving unit  50  inputs electric current into each phase according to the signals inputted from the main controlling unit  100 . 
     Wave forms of respective signals according to rotation of the motor  200  are shown in FIG.  2 . Firstly, in the wave forms of respective phases, the position detection signal is on for a certain time, for example, a time duration that the rotor of motor rotates fifteen degrees of a mechanical angle (hereinafter the mechanical angle will be omitted) whenever the respective sensor in a rising edge of respective detection signals and the respective signals are inputted to the motor driving unit after performing logical AND operation with the pulse width modulation signal  110 . The calculated signals controls the three phase electric current of the motor driving unit and electric current of respective phase of the motor driving unit  50  is inputted to the motor  200 . 
     Namely, if the first sensor  301  is turned on, A phase signal of  120  in the rising edge of the first position detection signal  150  becomes ON. The signal of phase A is inputted to the motor driving unit  50  after performing a logical AND operation with the pulse width modulation signal  110  and the motor driving unit  50  inputs the A phase electric current to the motor according to the control of the signal. At this time, the motor  200  starts to rotate and after a certain time duration (for example, a time that the motor rotor rotates fifteen degrees) and if the second sensor  302  is turned on, the B phase signal  130  is turned on in the rising edge of the second position detection signal  160 . The B phase signal  130  is inputted to the motor driving unit  50  after performing logical AND operation with the pulse width modulation signal  110  and the motor driving unit  50  inputs the B phase electric current to the motor  200  according to control of the signal. 
     Later, if the second sensor  302  is turned off, the A phase signal  120  becomes OFF in a falling edge of the second position detection signal and if the third sensor is turned on after a certain time period, a C phase signal  140  in the rising edge of the third position detection signal  170 . The signal is inputted to te motor driving unit after performing a logical AND operation with the pulse width modulation signal  110  and the motor driving unit  50  inputs the C phase electric current to the motor according to the control of the signal. 
     If the third sensor  303  is turned off, the B phase signal  130  becomes OFF in the falling edge of the third position detection signal. If the first sensor  301  is turned on after a certain time period, the A phase signal  120  becomes ON in the rising edge of the first position detection signal  150  and the C phase signal  140  becomes OFF in the falling edge of the first detection signal. 
     According to the above process, signals of respective phase signals and the pulse width modulation signal  110  are inputted to the motor driving unit  50  after performing logical AND operations respectively. In addition, the motor driving unit  50  inputs electric current of each phase to a stator coil and the motor  200  rotates in the above pattern. 
     At this time, a torque which is a sum of respective torques is generated by the three phase electric current inputted to the motor and a torque ripple as shown in FIG. 3 is generated. In FIG. 3, a horizontal axis designates a rotational angle and a vertical axis designates a size of the torque ripple. 
     The driving method of the motor in the conventional art has a disadvantage that much amount of noise is generated due to much amount of the torque ripple. 
     SUMMARY OF THE INVENTION 
     Therefore, an object of the present invention is to provide a method for reducing a torque ripple of an SRM for detecting a position of the rotor using a position detection sensor, designing a pulse width if a position detection signal as an optimum one and decreasing the torque ripple of the motor by adjusting a duty rate of the pulse width modulation signal. 
     To achieve these and other advantages and in accordance with the purpose of the present invention, the present invention comprises the steps of setting a pulse width of a position detection signal in accordance with a position detection result of a motor rotor, outputting a signal for controlling of each phase in synchronization with a rising and falling edge of the position detection signal, and varying and outputting a duty rate of a pulse width modulation signal from the moment that a falling edge of the position detection signal is detected. 
     The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. 
     In the drawings: 
     FIG. 1 is a block diagram showing a motor in accordance with the conventional art; 
     FIG. 2 shows respective wave form charts of three phases driving, sensor and pulse width modulation of the motor in accordance with the conventional art; 
     FIG. 3 is a wave form chart of a torque ripple in accordance with the conventional art; 
     FIG. 4 is a block diagram of a motor in accordance with the present invention; 
     FIG. 5 is a wave form chart illustrating three phases driving, sensor and pulse width modulation of the motor in accordance with the present invention; 
     FIG. 6 is a flow chart to decrease a torque ripple of the motor in accordance with the present invention; 
     FIG. 7 is a wave form chart showing the torque ripple generated before controlling a duty rate of a pulse width modulation signal in the optimum condition of the pulse width of a position detection signal in accordance with the present invention; and 
     FIG. 8 is a wave form chart showing the torque ripple generated after controlling a duty rate of a pulse width modulation signal in the optimum condition of the pulse width of a position detection signal in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. 
     FIG. 4 is a block diagram of a motor in accordance with the present invention. As shown in the drawing, a motor in accordance with the present invention is comprised of a position detection sensor  310  for detecting position of a motor rotor, pulse width modulation signal  110 , a main controlling unit  300  for controlling a pulse width modulation signal and three phase signal, a pulse duty rate controlling unit  400  for controlling a duty rate of a pulse width modulation signal in accordance with a position detection signal detected in the position detection sensor, a motor driving unit  50  for inputting three phases electric current to the motor by the three phase signal inputted from the main controlling unit  300  and a motor  200  driven by the three phase electric current inputted from the motor driving unit  50 . Reference numerals  115 ,  125  and  135  designate end gates. 
     FIG. 5 is a wave form chart of respective signals outputted in FIG.  4 . 
     FIG. 6 is a flow chart to decrease a torque ripple of the motor in accordance with the present invention. With reference to FIGS. 4,  5  and  6 , a method for decreasing a torque ripple of the motor will be described. The position detection sensor  310  detects rotor position of a motor and inputs a position detection signal  350  to the main controlling unit  300  S 1 . 
     According to the result of detection the rotor position of the motor, the main controlling unit  300  defines the pulse width of the position detection signal to be minimum value of the torque ripple S 2 . 
     At this time, an optimum result of the pulse width is generated when a rotational angle of a rotor is about six degrees. On the basis of the pulse width of the position detection sensor determined above, the main controlling unit  300  inputs the three phase signals  120 ,  130  and  140  to the motor driving unit in synchronization with a rising edge or falling edge of the position detection signal and the motor driving unit  50  inputs the three phase electric current to the motor  200  according to control of the signals. 
     Also, when the sensor  310  is turned on on the basis of the pulse width determined above, the rotor rotates a certain angle (six degrees) and the sensor  310  is turned off again. Later, the sensor  310  is turned on again after rotating a certain angle (nine degrees) more. With the above method, the turn on and off of the sensor is determined S 3 . 
     In addition, the pulse duty controlling unit  400  varies duty rates of the pulse width modulation signal in respective falling edges of the position detection signal  350 , then performs a logical AND operation of the signals in each phase and the pulse width modulation signal  410  and inputs the result to the motor driving unit  50  S 4 . 
     Namely, when the sensor  310  is turned on at the first time, the A phase signal  120  becomes ON and the signal is inputted to the motor driving unit after performing a logical AND operation. The motor driving unit  50  inputs the A phase electric current to the motor  200  in accordance with control of the A phase signal  120  inputted in the main controlling unit  300  and accordingly, the sensor  310  rotates. At this time, the rotor rotates a certain angle (six degrees) more and the sensor is turned off. However, the motor  200  rotates continuously since the A phase signal  120  is still ON. 
     If the rotor rotates a certain angle (nine degrees) more, the sensor  310  is turned on again. In the moment that the position detection signal  350  is a rising edge, the B phase signal  130  becomes ON. The B phase signal is inputted to the motor driving unit  50  after performing a logical AND operation with the pulse width modulation signal  410 . The motor driving unit  50  inputs the B phase electrical current according to control of the B phase signal  130  inputted in the main controlling unit  300  thus to drive the motor. 
     If the rotor rotates a certain angle (six degrees) again, The sensor  310  is turned off. In the moment that the position detection signal  350  is a falling edge, the A phase signal  120  becomes OFF. The A phase signal  120  is inputted to the motor driving unit  50  after performing a logical AND operation with a pulse width modulation signal  410 . The motor driving unit  50  stops the A phase electric current inputted to the motor  200  by the signal. However, the motor rotor rotates continuously since the B phase signal is still ON. 
     Also, if the rotor rotates a certain angle (nine degrees) more, the sensor is turned on. In the moment that the position signal  350  is a rising edge, the C phase signal  140  becomes ON. The signal is inputted to the motor driving unit  50  after performing a logical AND operation with the pulse width modulation signal  410 . Then the motor driving unit  50  inputs the C phase electric current to the motor  200 . 
     Also, if the rotor rotates a certain angle (six degrees) more, the sensor  310  is turned off. In the moment that the B phase signal  130  is inputted to the motor driving unit after a logical AND operation with the pulse width modulation signal  410 . By control of the signal, the motor driving unit  50  stops input of the B phase electric current inputted to the motor  200 . 
     Also, if the rotor rotates a certain angle (nine degrees), the sensor  310  is turned on. In the moment that the position detection signal is a rising edge, the A phase signal  120  becomes ON. The A phase signal  120  is inputted to the motor driving unit  50  after a logical AND operation with the pulse width modulation signal  410  and the motor driving unit  50  inputs the A phase electric current  120  to the motor. 
     Later, if the rotor rotates a certain angle (six degrees), the sensor  310  is turned off and at the moment that the position detection signal  350  is a falling edge, the C phase signal  140  becomes OFF. The C phase signal  140  is inputted to the motor driving unit  50  after performing a logical AND operation with the pulse width modulation signal  410 . By control of the signal, the motor driving unit  50  stops the input of the C phase electric current inputted to the motor. 
     With the above method, respective phases electric currents  120 ,  130  and  140  inputted to the motor  200  can be controlled using one position detection sensor  310 . 
     In the main controlling unit  300 , a Bipolar Junction Transistor (BJT) is used to adjust the turn on and off but another switching devices (for example, Metal Oxide Semiconductor Field Effect Transistor (MOSFET) and MOS gate Bipolar Transistor (MBT)) can be used. 
     The duty rate of the pulse width modulation signal changes the duty rate if the position detection signal  350  is detected as a falling edge decreasing the duty rate from the moment for several seconds and then increasing gradually again. Namely, if the pulse width of the pulse width modulation signal  410  is reduced to the half, for about 600 μs after the detection of the falling edge of the sensor detection signal. Also, after 600 μs, the pulse width becomes again as usual. 
     Then, a torque which is a sum of torques of each phase, which is generated by electric current inputted to respective phases and a torque ripple is generated. 
     FIG. 7 is a wave form chart showing the torque ripple generated before controlling a duty rate of a pulse width modulation signal in the optimum condition of the pulse width of a position detection signal in accordance with the present invention. Here, the horizontal axis designates a rotational angle and the vertical axis designates strength of the torque. As shown in the drawing, the torque ripple in the present invention has little difference with a conventional torque ripple. 
     FIG. 8 is a wave form chart showing the torque ripple generated after controlling a duty rate of a pulse width modulation signal in the optimum condition of the pulse width of a position detection signal in accordance with the present invention. Here, the horizontal axis designates a rotational angle and the vertical axis designates strength of the torque. As shown in the drawing, by setting an optimum machine angle and controlling the duty rate, about fifty percents of torque ripple is decreased. 
     As shown above, by designing the width of the position detection signal of the rotor detected using one position detection sensor  310  as an optimum one, the duty rate of the pulse width modulation signal for driving the switch device is changed for a certain time from the moment that the position detection signal  350  is a falling edge thus to reduce more than fifty percent of the torque ripple. 
     As the present invention may be embodied in several forms without departing from the spirit or essential characteristics thereof, it should also be understood that the above-described embodiments are not limited by any of the details of the foregoing description, unless otherwise specified, but rather should be construed broadly within its spirit and scope as defined in the appended claims, and therefore all changes and modifications that fall within the meets and bounds of the claims, or equivalence of such meets and bounds are therefore intended to be embraced by the appended claims.