Patent Publication Number: US-2007110410-A1

Title: Stationary position detection circuit and motor drive circuit

Description:
This application is a continuation of U.S. patent application Ser. No. 11/476,052 filed Jun. 28, 2006. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a stationary position detection circuit and a Hall sensorless motor drive circuit capable of detecting the position of a motor rotor.  
      2. Description of the Background Art  
      In driving a motor having a rotor such as a small three-phase DC brushless motor, the motor drive circuit is required to be kicked at the time of starting. In the process, unless the rotor position can be detected properly, the proper starting is impossible.  
      The rotor position can be detected by arranging a Hall sensor configured of a Hall element in the neighborhood of the motor rotor. The use of the Hall sensor, however, leads to an increased cost and a bulkiness. Currently, therefore, vigorous efforts are made to develop what is called a Hall sensorless motor using no Hall sensor.  
      In the Hall sensorless motor, no induction voltage (counter electromotive voltage) is generated as long as the motor is stationary, so that the position of the rotor cannot be detected. As described in Japanese Patent Application Laid-Open Nos. 2002-345286, 2002-335691 and 2002-315385, therefore, a method has been developed in which the length of the kickback time for turn-off operation is detected by a stationary position detection circuit thereby to detect the position of the rotor of a motor in stationary mode.  
      Japanese Patent Application Laid-Open No. 2003-47280 is also available as another patent document related to the present patent application.  
      In the case where the kickback voltage at the time of turning off is measured as in the stationary position detection circuit described in Japanese Patent Application Laid-Open Nos. 2002-345286, 2002-335691 and 2002-315385, the difference in the kickback voltage value with a minuscule inductance difference depending on the rotor position while the motor is stationary makes it possible to detect the rotor position of a stationary motor by detecting the length of the kickback time.  
      In the detection method using the kickback voltage, however, a large kickback voltage is required to be generated by supplying a large kickback current (for example, about 1 A) to detect the minuscule inductance difference due to the difference of rotor position. This is by reason of the fact that a large kickback voltage is required to sufficiently recognize the difference in the length of the kickback time. The large kickback current is a cause of vibrations.  
      Also, in the detection method using the kickback voltage, the information indicating the inductance difference can be obtained only for the very short period during which the kickback occurs, and therefore the information may not be sufficiently detected.  
     SUMMARY OF THE INVENTION  
      An object of the present invention is to provide a stationary position detection circuit and a motor drive circuit capable of detecting the rotor position more properly.  
      According to a first aspect of the present invention, a stationary position detection circuit for a motor including a rotor and at least one-phase load detects the position of the rotor in stationary mode, and includes a current amount detector, a time counter, a time difference amplifier and a position determinator.  
      The current amount detector operates in such a manner that an alternating current flowing alternately in a first direction and a second direction opposite to the first direction is rendered to flow through the load by a control circuit for controlling an inverter circuit for driving the motor, the fact that the alternating current flowing in the first direction has reached a value a is detected, after which the alternating current is rendered to flow in the second direction by gradually decreasing amount of the alternating current through the control circuit, and the fact that the alternating current flowing in the second direction has reached a value β equal to and opposite in sign to the value α is detected, after which the alternating current is rendered to flow in the first direction again by gradually decreasing amount of the alternative current through the control circuit, the detection of the values α and β and the control of the alternating current by the control circuit being subsequently repeated a predetermined number of times.  
      The time counter counts the first time for which the alternating current changes from α to β and the second time for which the alternating current changes from β to α.  
      The time difference amplifier converts the counted first time and second time into electrical signals and amplifies the electrical signals in accordance with the accumulation of the first time and second time by the predetermined number of times, and  
      The position determinator determines the position of the rotor in stationary mode in accordance with the value of the electrical signals.  
      The time counter counts the first and second time, the time difference amplifier converts the first and second time to electrical signals and amplifies the electrical signals in accordance with a predetermined number of accumulations of the first and second time. The use of the alternating current, unlike the kickback voltage, makes it possible to amplify the electrical signals with an increased number of alternations for a higher detection accuracy. Also, in view of the fact that an increased number of alternations makes it possible to amplify the electrical signals without increasing the alternating current values α and β, the alternating current of a large value is not required unlike the kickback voltage. As a result, the alternating current can be reduced to a small value (about 0.1 A, for example) and the vibrations can be suppressed. Thus, a stationary position detection circuit capable of detecting the rotor position more properly is realized.  
      According to a second aspect of the present invention, a motor drive circuit includes the stationary position detection circuit according to the first aspect, the inverter and the control circuit.  
      In view of the fact that the motor drive circuit includes the stationary position detection circuit according to the first aspect, a motor drive circuit capable of detecting the rotor position more properly is realized.  
      These 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  
       FIG. 1  is a diagram showing a motor drive circuit and a motor according to a first embodiment;  
       FIG. 2  is a diagram showing the principle of the present invention;  
       FIG. 3  is a diagram showing a detailed configuration of the stationary position detection circuit according to the first embodiment;  
       FIG. 4  is a diagram showing a detailed configuration of the current amount detector in the stationary position detection circuit;  
       FIG. 5  is a diagram showing a detailed configuration of the time counter, the time difference amplifier and the rotor position determinator in the stationary position detection circuit;  
       FIG. 6  is a timing chart while the stationary position detection circuit detects the stationary rotor position before kicking at the time of starting;  
       FIG. 7  is a partly enlarged timing chart for an alternating current generation period between U and V phases;  
       FIG. 8  is a circuit diagram for considering the transient phenomenon during the period TA with a DC voltage applied to a load having a resistance and an inductance;  
       FIG. 9  is a circuit diagram for considering the transient phenomenon during the period TB;  
       FIG. 10  is a diagram showing the operation of the inverter circuit;  
       FIG. 11  is a diagram showing the operation of the inverter circuit;  
       FIG. 12  is a diagram showing the operation of the inverter circuit;  
       FIG. 13  is a diagram showing the operation of the inverter circuit;  
       FIG. 14  is a diagram for explaining the relation of correspondence between the result of generating the alternating current between the phases and determining the rotor position on the one hand and the rotor position on the other hand;  
       FIG. 15  is a diagram for explaining the relation of correspondence between the result of generating the alternating current between the phases and determining the rotor position on the one hand and the rotor position on the other hand;  
       FIG. 16  is a diagram showing the stationary position detection circuit according to a second embodiment; and  
       FIG. 17  is a diagram showing the stationary position detection circuit according to a third embodiment. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     First Embodiment  
      According to the first embodiment, there are provided a stationary position detection circuit and a motor drive circuit in which an alternating current is supplied to a motor load and the time during which the current flows in a first direction and the time during which the current flows in a second direction opposite to the first direction are converted into electrical signals, which are amplified to determine the position of the motor rotor in stationary mode according to the value of the electrical signals.  
       FIG. 1  is a diagram showing a motor drive circuit and a motor according to this embodiment. As shown in  FIG. 1 , the motor  1  is, for example, a three-phase DC brushless Hall sensorless motor including a rotor  10  of a permanent magnet and a stator  11  configured of a three-phase load with an armature coil wound on a field core. The load of the stator  11  exists for each of the phases U, V, W and coupled to each other at a center tap (CT).  
      The motor drive circuit, on the other hand, includes an inverter circuit  2  for driving the motor  1  with an output signal  2   a  thereof, an output transistor control circuit  3  for controlling the inverter circuit  2  with a signal  3   a  thereof, a stationary position detection circuit  4  for detecting the position of the rotor  10  in stationary mode, a position detection comparator  5  for detecting the position of the rotor  10  in operation, a position detection mask circuit  6  for masking a part of the output signal  5   a  of the position detection comparator  5 , a sensorless drive operation circuit  7  for performing the arithmetic operation for driving in response to the output signal  6   a  of the position detection mask circuit  6 , and a signal select circuit  8  for supplying the output transistor control circuit  3  with, as an output signal  8   a , either the output signal  7   a  of the sensorless drive operation circuit  7  or the output signal  4   a  of the stationary position detection circuit  4 . The stationary position detection circuit  4  works while the motor is stationary, and the sensorless drive operation circuit  7  functions while the motor is rotating. The signal  4   b  between the two circuits is a shake-hand signal for the operation of the two circuits.  
      The inverter circuit  2  is a three-phase inverter circuit having transistors Q 1  to Q 6 , in which the transistors Q 1 , Q 2  connected in series make up a first arm, the transistors Q 3 , Q 4  connected in series make up a second arm, and the transistors Q 5 , Q 6  connected in series make up a third arm. The junction between the transistors Q 1 , Q 2  is connected to the U-phase load, the junction between the transistors Q 3 , Q 4  is connected to the V-phase load, and the junction between the transistors Q 5 , Q 6  is connected to the W-phase load. An end of each arm is applied with a power supply voltage VCC, and the other end of each arm supplied with a grounding voltage GND through a resistor  21  for detecting the current amount.  
       FIG. 2  is a diagram showing the principle of the present invention. In this example, an alternating current is supplied between the U-phase load and the V-phase load of the stator  11 . The direction from the U-phase load to the V-phase load is defmed as a positive direction and the other direction as a negative direction. Also, the U-phase load and the V-phase load are collectively defined as a load  11   a , and the effect that the magnetic lines of force from the rotor  10  have on the load  11   a  is indicated by a magnet  10   a  as a simulation.  
      Currents flow in both positive and negative directions in the load  11   a  while the motor is rotating. The value of the resistor R of the load  11   a  remains constant regardless of the physical position of the rotor  10  and the direction of the current flowing in the load  11   a . The value of the inductance L of the load  11   a , however, varies with the physical position of the rotor  10  and the direction of the current flowing in the load  11   a . This is by reason of the fact that the strength of the magnetic lines of force of the magnet  10   a  and the strength of the magnetic lines of force generated by the current flowing in the load  11   a  affect the value of the inductance L of the load  11   a.    
      The value of the inductance L of the load  11   a  changes with the physical position of the rotor  10  in stationary mode and the direction of the current flowing in the load  11   a  not only while the motor is rotating but also while the rotor  10  is stationary. The magnitude of the value of the inductance L of the load  11   a  corresponds to the physical position of the rotor  10  and the direction of the current flowing in the load  11   a.    
      Specifically, by defining the aforementioned relation of correspondence in advance, the physical position of the rotor  10  can be determined by detecting the value of the inductance L of the load  11   a . This is described in detail later with reference to  FIGS. 14, 15 . According to the present invention, the position of the rotor  10  in stationary mode is detected by the stationary position detection circuit  4  before kick at the time of starting.  
       FIG. 3  is a diagram showing a detail configuration of the stationary position detection circuit  4  according to this embodiment. As shown in  FIG. 3 , the stationary position detection circuit  4  includes a current amount detector  40 , a time counter  41 , a time difference amplifier  42  and a rotor position determinator  43 .  
      The current amount detector  40  receives a signal  2   b  of the voltage generated by a resistor  21  in the inverter circuit  2 , and based on this signal, generates a detection signal  40   a . Based on the detection signal  40   a , the time counter  41  counts the time during which the alternating current flows in the first direction under each phase load of the stator  11  and the time during which the alternating current flows under each phase load of the stator  11  in the second direction opposite to the first direction, and outputs a signal S 4  as a current signal.  
      Also, the time difference amplifier  42  converts the signal S 4  constituting a current signal into a voltage signal S 3 , and outputs by amplifying the voltage signal S 3  corresponding to the accumulation, by the number of alternations of the alternating current, of the time during which the alternating current flows in the first and second direction under each phase load of the stator  11 . The rotor position determinator  43  determines the position of the rotor in stationary mode in accordance with the value of the voltage signal S 3 .  
       FIG. 4  is a diagram showing a detailed configuration of the current amount detector  40 . The current amount detector  40  includes a power supply  400 , a comparator  401 , an AND gate circuit  402 , a D-flip-flop  403  and a mask signal generating circuit  404 . In the comparator  401 , the voltage drop Vr across the resistor  21  generated by the alternating current in the inverter circuit  2  is compared with a predetermined voltage V 1  generated by the power supply  400 , and in the case where the voltage drop Vr is larger than the voltage V 1 , the output of the comparator  401  is activated.  
      The AND gate circuit  402  calculates the logic product of the mask signal  6   b  output from the mask signal generating circuit  404  and the output of the comparator  401 , and outputs a signal Sr. The D-flip-flop  403  outputs the output Q as a detection signal  40   a . Also, the inverted output/Q is an inverted signal of the output Q and applied to the input D of the D-flip-flop  403 . The signal Sr is applied to the clock input T of the D-flip-flop  403 . The mask signal generating circuit  404  generates a mask signal  6   b.    
       FIG. 5  is a diagram showing a detailed configuration of the time counter  41 , the time difference amplifier  42  and the rotor position determinator  43  in the stationary position detection circuit  4 . The time counter  41  includes a current source  410 , a first switch  411  for selectively outputting a current I 1  from the current source  410  when the logic value of the detection signal  40   a  is Low, and a second switch  412  for selectively outputting the current I 1  from the current source  410  when the logic value of the detection signal  40   a  is Hi. The output from the first switch  411  is a signal S 4   a  constituting one part of the signal S 4 , and the output from the second switch  412  is a signal S 4   b  constituting the other part of the signal S 4 .  
      The time difference amplifier  42  includes a first capacitor  423  of a predetermined capacitance charged by the output of the first switch  411  and a second capacitor  421  having the same capacitance as the first capacitor  423  and charged by the output of the second switch  412 . One end of the first capacitor  423  is connected to the first switch  411 , and the other end thereof is supplied with the grounding potential GND. One end of the second capacitor  421  is connected to the second switch  412 , and the other end thereof applied with the grounding potential GND. The potential at one end of the first capacitor  423  constitutes a signal S 3   a  making up one part of the voltage signal S 3 , and the potential at one end of the second capacitor  421  constitutes a signal S 3   b  making up the other part of the voltage signal S 3 .  
      Also, the time difference amplifier  42  includes a transistor  422  to discharge the first capacitor  423  by applying the grounding potential GND to one end of the first capacitor  423  during the activation of a reset signal S 2  and a transistor  420  to discharge the second capacitor  421  by applying the grounding potential GND to one end of the second capacitor  421  during the activation of the reset signal S 2 .  
      The rotor position determinator  43  includes a comparator  430  having the positive and negative terminals thereof supplied with the signal S 3   b  making the other part of the voltage signal S 3  and the signal S 3   a  making up the one part of the voltage signal S 3 , respectively, so that the output logic value functions as a determination signal  4   a  for the position of the rotor in stationary mode.  
      Next, the operation of the stationary position detection circuit  4  according to this embodiment is explained.  FIG. 6  is a timing chart for the stationary position detection circuit  4  to detect the position of the rotor  10  in stationary mode before kick at the time of starting.  
      As shown in  FIG. 6 , according to the present invention, the alternating current is supplied between U and V phases, between V and W phases and between W and U phases before determining the kick position. Specifically, during the period Tul, the alternating current is supplied between U and V phases to detect the position of the rotor  10 , and during the subsequent period Tu 2 , the information on the detection result between U and V phases is stored in the output transistor control circuit  3 . Incidentally, the U-phase current and the V-phase current have complementary waveforms due to the fact that the current applied to the U-phase load (or the V-phase load) is set as positive, and the current flowing out of the U-phase load (or the V-phase load) as negative.  
      In a similar fashion, the alternating current is supplied between V and W phases during the period Tv 1 , and during the subsequent period Tv 2 , the information on the detection result between V and W phases is stored in the output transistor control circuit  3 . Also, the alternating current is supplied between W and U phases during the period Tw 1 , and the information on the detection result between W and U phases is stored in the output transistor control circuit  3  during the subsequent period Tw 2 .  
       FIG. 7  is an enlarged timing chart for one part U 1  of the alternating current generation period between U and V phases in  FIG. 6 . In the motor drive circuit according to the present invention, the time of transient response of the current flowing in each phase load of the stator  11 , which is affected by the magnetic field of the rotor  10 , is counted thereby to determine the magnitude of the inductance value, and based on the result thereof, the physical position of the rotor  10  is determined.  
      First, as indicated by the period TA in  FIG. 7 , the stationary position detection circuit  4 , through the output transistor control circuit  3  for controlling the inverter circuit  2 , generates a current flowing in the first direction from U to V phase, which current is increased to a value α (for example, an absolute value 0.1 A). Once the current flowing in the first direction from U to V phase has reached the value α, the stationary position detection circuit  4  reduces and returns the current value back to 0 A through the output transistor control circuit  3  as shown by the period TB in  FIG. 7 . Then, a current flowing from V to U phase in the second direction opposite to the first direction is generated, and increased to reach a value β of opposite sign to the value α (i.e., the absolute value 0.1 A like the value α, for example).  
      Once the current flowing in the second direction from V to U phase has reached the value β, the stationary position detection circuit  4  reduces the current value back to 0 A through the output transistor control circuit  3  as shown by the period TC in  FIG. 7 . Then, a current flowing in the first direction from U to V phase is generated again, and increased to reach the value α. Subsequently, the stationary position detection circuit  4 , through the output transistor control circuit  3 , generates the current alternating between values α and β by the number of times equal to the number of alternations of the alternating current.  
      A circuit equation is formulated with the load  11   a  of  FIG. 2  as a model for each of the periods TA to TC. First,  FIG. 8  is a circuit diagram for considering the transient phenomenon during the period TA with a DC voltage E applied to the load  11   a  having the resistance R and the inductance L. In the circuit diagram of  FIG. 8 , the initial value of the current flowing in the load  11   a  is set to io.  
      In this circuit diagram, the current i(t) changing with time t is given as follows.  
               i   ⁡     (   t   )       =         E   R     ⁢     {     1   -     ⅇ       -     R   L       ⁢   t         }       +       i   0     ⁢     ⅇ       -     R   L       ⁢   t                   (   1   )             
 
      As shown in  FIG. 7 , no current flows in the beginning of the period TA, and therefore the second term of Equation 1 is considered 0. In other words, the current i(t) during the period TA is expressed as follows.  
               i   ⁡     (   t   )       =       E   R     ⁢     {     1   -     ⅇ       -     R   L       ⁢   t         }               (   2   )             
 
      Equation 2 is modified as follows.  
             t   =         -     L   R       ·   ln     ⁢     {     1   -     α     E   R         }               (   3   )             
 
      This equation represents time t.  
      Next, consider the transient phenomenon during the period TB.  FIG. 9  is a circuit diagram for this purpose. In the beginning of the period TB, a current of initial value α flows and therefore, in  FIG. 9 , the current α is indicated in the same direction as io of  FIG. 8 . At the end of the period TB, on the other hand, a current of value β inverted in sign from the value α flows, and therefore, in  FIG. 9 , the current β opposite in direction to the current α is also indicated. Also, a voltage opposite in direction to the voltage for the period TA is applied to the load  11   a  under the control of the inverter  2 , and therefore, the direction of the DC voltage E in  FIG. 9  is opposite to the direction of the DC voltage E of  FIG. 8 .  
      In this circuit diagram, an equation for the time point when the current of value β flows is considered on the basis of Equation 1 as follows.  
             β   =         E   R     ⁢     {     1   -     ⅇ       -     R   L       ⁢   t         }       -     αⅇ       -     R   L       ⁢   t                 (   4   )             
 
      Equation 4 is modified as follows.  
               ⅇ       -     R   L       ⁢   t       =         E   R     -   β         E   R     +   α               (   5   )             
 
      Equation 5 can be further modified as follows.  
             t   =       -     L   R       ·     ln   ⁡     [         E   R     -   β         E   R     +   α       ]                 (   6   )             
 
      This equation represents time t. In  FIG. 7 , the period TB is expressed as time t 1  of the detection signal  40   a , and therefore, t 1  is employed in place of time t. Also, assuming that the inductance L of the load  11   a  for the period TB is L 1 , Equation 6 can be expressed as follows.  
               t   ⁢           ⁢   1     =       -       L   ⁢           ⁢   1     R       ·     ln   ⁡     [         E   R     -   β         E   R     +   α       ]                 (   7   )             
 
      Next, consider the transient phenomenon for the period TC. In this case, as understood from  FIG. 7 , the only difference is that the initial value is β and the end value is α, and the other points are similar to those for the period TB. In  FIG. 9  and Equation 6, therefore, the values α and β for the period TB are replaced with each other. Thus, the circuit equation for the period TC is given as follows.  
             t   =       -     L   R       ·     ln   ⁡     [         E   R     -   α         E   R     +   β       ]                 (   8   )             
 
      In  FIG. 7 , the period TC is expressed by time t 2  of the detection signal  40   a . By employing t 2  in place of time t and setting the inductance of the load  11   a  for the period TC to L 2 , therefore, Equation 8 can be expressed as follows.  
               t   ⁢           ⁢   2     =       -       L   ⁢           ⁢   2     R       ·     ln   ⁡     [         E   R     -   α         E   R     +   β       ]                 (   9   )             
 
      Comparison between Equations 7 and 9 shows that as long as α and β are equal to each other in absolute value, the ratio between time t 1  and time t 2  is expressed as follows.
 
t1:t2=L1:L2  (10)
 
      As understood from Equation 10, the ratio between the inductances L 1  and L 2  coincides with the ratio between the time t 1  for which the alternating current changes from α to β in value and the time t 2  for which the alternating current changes from β to α in value. By counting the times t 1  and t 2  and specifying the relative magnitudes thereof, therefore, the relation between the U-V load  11  and the position of the rotor  10  can be defined.  
      Incidentally, FIGS.  10  to  13  are diagrams showing the operation of the inverter circuit  2  for the periods TA to TC.  FIG. 10  shows a case in which the alternating current flows increasingly in the first direction from U phase to V phase (the portion of the periods TA and TC in  FIG. 7  during which the current value is larger than 0 A). In this case, the transistors Q 1 , Q 4  in the inverter circuit  2  turn on, while the other transistors remain off.  
       FIG. 11  shows a case in which the alternating current flowing in the first direction is attenuated (the portion of the period TB in  FIG. 7  during which the current value is larger than 0 A). In this case, the transistors Q 2 , Q 3  in the inverter circuit  2  turn on, while the other transistors remain off.  FIG. 12  shows a case in which the alternating current flows increasingly in the second direction from V phase to U phase (the portion of the period TB in  FIG. 7  during which the current value is smaller than 0 A). In this case, the transistors Q 2 , Q 3  in the inverter circuit  2  turn on, while the other transistors remain off.  FIG. 13  shows a case in which the alternating current flowing in the second direction is attenuated (the portion of the period TC in  FIG. 7  during which the current value is smaller than 0 A). In this case, the transistors Q 1 , Q 4  in the inverter circuit  2  turn on, while the other transistors remain off.  
      The operation of each circuit for generating the alternating current and determining the position of the rotor  10  is explained. First, the value of the voltage V 1  generated by the power supply  400  in the current amount detector  40  shown in  FIG. 4  is set to a value slightly lower than the maximum value of the voltage drop Vr across the resistor  21  in the inverter circuit  2  for each period of TB, TC, etc.  
      The value of the voltage drop Vr across the resistor  21  is the product of the current α and the resistance value of the resistor  21  in the beginning of the period TB. With the lapse of the period TB, the current value decreases, and therefore the value of the voltage drop Vr also decreases along a waveform similar to that of the current value.  
      During the decrease in the current value, a spike SP 1  appears in the voltage drop Vr. A current in the direction opposite to the direction from the power supply Vcc toward the grounding potential GND is flowing in the transistors Q 2 , Q 3  (dashed arrow in  FIG. 11 ), and a voltage of an inverse bias is applied between drain and source thereof. At about a point where the current changes from the first to the second direction (at about a point where  FIG. 11  changes to  FIG. 12 ), therefore, the charge accumulated in the drain-source capacitance flows sharply through the resistor  21  in addition to the current in the direction from the power supply Vcc toward the grounding potential GND (solid arrow in  FIG. 12 ). It is for this reason that the voltage drop Vr develops the spike SP 1 .  
      The mask signal  6   b  is for masking the spike SP 1  not to be detected, and output from the mask signal generating circuit  404  for a predetermined length of time (say, 2 μsec) from the time point when the current reaches the value α or β. The mask signal generating circuit  404  detects the signal  3   a  to detect the turn on/off time of the transistors Q 1  to Q 6  and outputs a mask signal  6   b  for a predetermined length of time from the time of turn on/off. The mask signal  6   b , as shown in  FIG. 7 , is a Low active signal. During the mask period, the mask signal  6   b  is Low and therefore an AND gate circuit  402  continues to output a Low signal regardless of the output of the comparator  401 .  
      With the attenuation of the alternating current flowing in the first direction while the alternating current increasingly flows in the second direction, the value of the voltage drop Vr across the resistor  21  approaches the value of the product of the current β and the resistance value of the resistor  21  in the last half of the period TB. As a result, the value of the voltage drop Vr also increases with a waveform similar to that of the current value.  
      Once the value of the voltage drop Vr increases beyond the value of the voltage V 1  generated by the power supply  400 , the comparator  401  activates the output thereof to Hi level. At this time point, the mask period is already ended and the mask signal  6   b  assumes a Hi level. Thus, the AND gate circuit  402  outputs a signal Sr as an activated output from the comparator  401 . It is for this reason that the signal Sr is generated in pulses in  FIG. 7 .  
      With the start of the period TC, the AND gate circuit  402  receives the mask signal  6   b  activated to Low again, and outputs a Low signal without regard to the output of the comparator  401 . Upon the lapse of the mask period in the period TC, the AND gate circuit  402  outputs the signal Sr from the comparator  401  activated when the value of the voltage drop Vr increases beyond the value of the voltage V 1 . During the subsequent periods, the AND gate circuit  402  similarly outputs a pulse-like signal Sr. In this way, the AND gate circuit  402  functions as a logic gate circuit for passing the. output of the comparator  401  only in each last half of the time t 1  and t 2 .  
      The D-flip-flop  403  has the inverted output/Q thereof applied to the input D thereof. With the activation of the clock input T, therefore, the output Q thereof alternates between Hi and Low states. The D-flip-flop  403 , with the signal Sr applied to the clock input T, functions to invert the logic value of the output with the activation of the output of the AND gate circuit as a motive.  
      The inverted output of the D-flip-flop  403  constitutes a detection signal  40   a  for the values α and β and a control signal  40   a  for the alternating current. Specifically, by the control signal  40   a  shown in  FIG. 3 , the current amount detector  40 , through the output transistor control circuit  3  for controlling the inverter circuit  2  for driving the motor  1 , so operates that the alternating currents flowing alternately in the first direction from U to V phases and in the second direction from V to U phases are rendered to flow through the load  11   a  between U and V phases, and after detection that the alternating current flowing in the first direction has reached the value α, the alternating current is reduced gradually through the output transistor control circuit  3  to flow in the second direction. Also, after detection that the alternating current flowing in the second direction has reached the value β, the alternating current is gradually reduced through the output transistor control circuit  3  to flow again in the first direction. Subsequently, the detection of the values α and β and the control of the alternating current through the output transistor control circuit  3  are repeated by the number of times equal to the number of alternations.  
      Incidentally, the reset signal S 1  shown in  FIG. 4  is activated after the flow of the alternating current between U and V phases before the alternating current next begins to flow between V and W phases. In similar fashion, the reset signal S 1  is activated after the flow of the alternating current between V and W phases before it next begins to flow between W and U phases, so that the result of detection between the phases may have no effect on the next detection between the phases.  
      The time counter  41  shown in  FIGS. 3, 5 , in accordance with the detection signal  40   a , so functions as to count the time t 1  (the period TB in  FIG. 7 ) for which the alternating current changes from value α to β and the time t 2  (the period TC in  FIG. 7 ) for which the alternating current changes from value β to α and output the current signal S 4  (S 4   a , S 4   b ) for the time t 1 , t 2  thus counted.  
      Specifically, the first switch  411  in the time counter  41 , based on the detection signal  40   a , counts the time t 1  by selectively outputting the current I 1  from the current source  410  during the period (the Low period of the detection signal  40   a ) after detection of the value α to the detection of the value β by the current amount detector  40 . Similarly, the second switch  412  in the time counter  41 , based on the detection signal  40   a , counts the time t 2  by selectively outputting the current I 1  from the current source  410  during the period (the Hi period of the detection signal  40   a ) from the detection of the value β to the detection of the value α by the current amount detector  40 .  
      The time difference amplifier  42  shown in  FIGS. 3, 5  converts the signal S 4  constituting a current signal into a voltage signal S 3  and amplifies the voltage signal S 3  (S 3   a , S 3   b ) in accordance with the accumulation of the time t 1 , t 2  by the number of alternations. Specifically, the first capacitor  423  accumulates the charge each time the current signal S 4   a  is input from the first switch  411  turned on during the time t 1 , and increases the accumulated charge in accordance with the accumulation of time t 1  by the number of alternations thereby to amplify the signal S 3   a . In similar manner, the second capacitor  421  accumulates the charge each time the current signal S 4   b  is input from the second switch  412  turned on during the time t 2 , and increases the accumulated charge in accordance with the accumulation of time t 2  by the number of alternations thereby to amplify the signal S 3   b.    
      The first capacitor  423  and the second capacitor  421  have the same capacitance value and are supplied with the same current I 1 . Assuming that time t 1  and t 2  are the same, therefore, the signals S 3   a  and S 3   b  take the same value. In the case where time t 1  and t 2  are different in value, however, the difference between time t 1  and t 2  is emphasized in output in view of the fact that the signals S 3   a , S 3   b  are amplified by the number of alternations.  
      Incidentally, the reset signal S 2  shown in  FIG. 5  is activated after the alternating current flows between U and V phases before next beginning to flow between U and W phases, and similarly activated after the alternating current flows between V and W phases before next beginning to flow between W and U phases. This signal is for preventing the detection result between the phases (the charge amount of the first capacitor  423  and the second capacitor  421 ) from affecting the next detection between the phases.  
      The comparator  430  of the rotor position determinator  43  shown in  FIG. 5  compares the magnitude of the signal S 3   a  with that of the signal S 3   b  and outputs a Hi logic value in the case where the signal S 3   b  is larger than the signal S 3   a , and outputs a Low logic value in the case where the signal S 3   a  is larger than the signal S 3   b . The output  4   a  of this comparator  430  finctions as a determination signal for the position of the rotor  11  in stationary mode.  
      The foregoing is the description of the generation of the alternating current between U and V phases and the determination of the rotor position in  FIG. 6 . After that, the alternating current is generated between V and W phases and between W and U phases and the rotor position determined similarly.  
      Specifically, the current amount detector  40  detects the values α and β between V and W phases of the load  11   a  and controls the alternating current through the output transistor control circuit  3 . The time counter  41  counts the time t 1 , t 2  between V and W phases of the load  11   a , and the time difference amplifier  42  amplifies by converting the load  11   a  between V and W phases into a voltage signal S 3 . The rotor position determinator  43 , on the other hand, makes the determination of the load  11   a  between V and W phases in response to the voltage signal S 3 . After that, the current amount detector  40  detects the values α and β between W and U phases of the load  11   a  and controls the alternating current through the output transistor control circuit  3 . The time counter  41  counts the time t 1 , t 2  between W and U phases of the load  11   a , and the time difference amplifier  42  amplifies by converting the load  11   a  between W and U phases to the voltage signal S 3 . The rotor position determinator  43  is supplied with the voltage signal S 3  and makes the determination of the load  11   a  between W and U phases.  
       FIGS. 14, 15  are diagrams for explaining the relation of correspondence between the result of generation of the alternating current between the phases and determination of the rotor position on the one hand and the position of the rotor  10  on the other hand. Take the alternating current between U and V phases as an example. In the case where the time t 1  during which the alternating current flows in the first direction from U to V phases is longer than the time t 2  during which the alternating current flows in the second direction from V to U phases, as indicated by circles  1 ,  5 ,  6  in  FIG. 14 , the difference of voltages of the signals S 3   a , S 3   b  is negative and the output  4   a  of the comparator  430  is Low.  
      In the process, the relative positions of the rotor  10  and the stator  11  can be considered such that they are at any of the positions indicated by the circles  1 ,  5 ,  6  in  FIG. 15 . Especially, in the case of the circle  6  in  FIG. 15 , the U-phase load of the stator  11  is opposed squarely to the S pole of the rotor  10  and the V-phase load of the stator  11  is squarely opposed to the N pole of the rotor  10 , and therefore the difference between the inductances L 1 , L 2  becomes most conspicuous.  
      In the case where the time t 1  during which the alternating current flows in the first direction from U to V phases is shorter than the time t 2  during which the alternating current flows in the second direction from V to U phases, on the other hand, as indicated by circles  2 ,  3 ,  4  in  FIG. 14 , the difference of voltages between the signals S 3   a , S 3   b  assumes a positive value, and the output  4   a  of the comparator  430  is Hi.  
      In the process, the relative positions of the rotor  10  and the stator  11  can be considered such that they are at any of the positions indicated by the circles  2 ,  3 ,  4  in  FIG. 15 . Especially, in the case of the circle  3  in  FIG. 15 , the U-phase load of the stator  11  is opposed squarely to the N pole of the rotor  10  and the V-phase load of the stator  11  is squarely opposed to the S pole of the rotor  10 , and therefore the difference between the inductances L 1 , L 2  becomes most conspicuous.  
      In similar fashion, between V and W phases and between W and U phases, the rotor position is determined by the output  4   a  of the comparator  430 , and therefore, the position of the rotor  10  in stationary mode is more accurately determined based on the combination of the determination results for the respective phases of the load  11   a . Specifically, as shown in  FIGS. 14, 15 , the rotor position is determined as circle  1  in the case where the determination result between U and V phases is Low, the determination result between V and W phases is Low and the determination result between W and U phases is Hi, as circle  2  in the case where the determination result between U and V phases is Hi, the determination result between V and W phases is Low and the determination result between W and U phases is Hi, as circle  3  in the case where the determination result between U and V phases is Hi, the determination result between V and W phases is Low and the determination result between W and U phases is Low, as circle  4  in the case where the determination result between U and V phases is Hi, the determination result between V and W phases is Hi and the determination result between W and U phases is Low, as circle  5  in the case where the determination result between U and V phases is Low, the determination result between V and W phases is Hi and the determination result between W and U phases is Low, and as circle  6  in the case where the determination result between U and V phases is Low, the determination result between V and W phases is Hi and the determination result between W and U phases is Hi.  
      These determination results indicate the position of the motor rotor in stationary mode and is referred to by the output transistor control circuit  3  at the time of kick operation.  
      In the stationary position detection circuit and the motor drive circuit according to this embodiment, the time counter  41  counts the time t 1 , t 2 , and the time difference amplifier converts the time t 1 , t 2  to the voltage signal S 3  and amplifies the voltage signal S 3  in accordance with the accumulation of the time t 1 , t 2  by the number of alternations of the alternating current. Since the alternating current is used, unlike in the case where the kickback voltage is used, the voltage signal S 3  can be amplified with an increased number of alternations for a higher detection accuracy. Also, an increased number of alternations makes it possible to amplify the voltage signal S 3  without increasing the values α, β of the alternating current, and therefore, unlike in the case where the kickback voltage is used, the alternating current of a large value is not required to be supplied (about 0.1 A, for example). As a result, the alternating current can be reduced to a small value, and the vibration can be suppressed. In this way, a stationary position detection circuit and a motor drive circuit capable of detecting the position of the rotor  10  more appropriately can be realized.  
      Also, in the stationary position detection circuit and the motor drive circuit according to this embodiment, the rotor position determinator  43  determines the rotor position based on the voltage signal S 3  for each phase of the load on the one hand and determines the position of the rotor  10  in stationary mode also based on the combination of the determination results for the respective phases of the load on the other hand. In view of the fact that the position of the rotor  10  is varied with the combination of the determination results for the respective phases of the load, the position of the rotor  10  can be detected more accurately.  
      Further, in the stationary position detection circuit and the motor drive circuit according to this embodiment, the current amount detector  40  includes the comparator  401 , the AND gate circuit  402  and the D-flip-flop  403  and operates in such a manner that the inversion of the output of the D-flip-flop  403  constitutes the detection signal  40   a  of the values α and β and the control signal  40   a  of the alternating current. Thus, the current amount detector  40  can be configured of a simplified circuit.  
      Furthermore, in the stationary position detection circuit and the motor drive circuit according to this embodiment, the time counter  41  includes a current source  410  and first and second switches  411 ,  412 , the time difference amplifier  42  includes first and second capacitors  421 ,  423 , and the rotor position determinator  43  includes a comparator  430 . Thus, the time counter  41 , the time difference amplifier  42  and the rotor position determinator  43  can be configured of a simple circuit.  
     Second Embodiment  
      This embodiment is a modification of the stationary position detection circuit and the motor drive circuit according to the first embodiment, and represents another example of the configuration including the time counter  41  and the time difference amplifier  42  according to the first embodiment.  
       FIG. 16  is a diagram showing a detailed configuration of the time counter  41   a  and the time difference amplifier  42   a  in the stationary position detection circuit  4  according to this embodiment. The time counter  41   a  includes first and second current sources  410 ,  413 , a first switch  411  for selectively outputting the current I 1  from the first current source  410  when the logic value of the detection signal  40   a  is Low, and a second switch  412  for selectively drawing the current I 1  to the second current source  413  when the logic value of the detection signal  40   a  is Hi. The current output from the first switch  411  and the current drawn by the second switch  412  constitute a current signal S 4   c  which is the signal S 4 .  
      The time difference amplifier  42  includes a capacitor  424  having a predetermined capacitance with the inter-electrode voltage functioning as a voltage signal S 3   a , which is charged by the output of the first switch  411  and discharged by the current drawn by the second switch  413 , a constant voltage source  426  adapted to provide the initial value of the inter-electrode voltage of the capacitor  424  and a switch  425 .  
      One end of the capacitor  424  is connected to the first switch  411  and the second switch  412 , and the other end thereof applied with the grounding potential GND. The positive terminal of the constant voltage source  426  is connected to one end of the capacitor  424  through the switch  425 , and the negative terminal thereof applied with the grounding potential GND. The potential S 4   d  at the positive terminal of the constant voltage source  426  represents the signal S 3   b  making up the other part of the voltage signal S 3 .  
      The rotor position determinator  43  includes a comparator  430  having negative and positive terminals supplied with the signal S 3   a  constituting one part of the voltage signal S 3  and the signal S 3   b  constituting the other part of the voltage signal S 3 , respectively, in which the output logic value functions as a determination signal  4   a  for the rotor position in stationary mode.  
      The time counter  41   a  shown in  FIG. 16  has the function of counting, in accordance with the detection signal  40   a , the time t 1  (the period TB in  FIG. 7 ) for which the alternating current changes from α to β and the time t 2  (the period TC in  FIG. 7 ) for which the alternating current changes from β to α, outputting the current signal S 4   c  for the counted time t 1 , and drawing the current signal S 4   c  for the counted time t 2 .  
      Specifically, the first switch  411  in the time counter  41   a , in accordance with the detection signal  40   a , selectively outputs the current I 1  from the first current source  410  during the period (the Low period of the detection signal  40   a ) from the detection of the value α to the detection of the value β by the current amount detector  40  thereby to count the time t 1 . On the other hand, the second switch  412  in the time counter  41   a , in accordance with the detection signal  40   a , selectively draws the current I 1  into the second current source  413  during the period (the Hi period of the detection signal  40   a ) from the detection of the value β to the detection of the value α by the current amount detector  40  thereby to count the time t 2 .  
      The time difference amplifier  42   a  shown in  FIG. 16  converts the signal S 4   c  constituting the current signal to the voltage signal S 3   a , and amplifies the voltage signal S 3   a  in accordance with the accumulation of the time t 1 , t 2  by the number of alternations. Specifically, the capacitor  424  is applied with an initial value as a voltage generated by the constant voltage source  426  by temporarily turning on the switch  425  through the reset signal S 2 , after which the switch  425  is turned off.  
      The capacitor  424  accumulates the charge each time the current signal S 4   c  is input from the first switch  411  turned on during the time t 1 , and increases the accumulated charge in accordance with the accumulation of the time t 1  by the number of alternations thereby to amplify the signal S 3   a . The capacitor  424 , on the other hand, releases the charge each time the current signal S 4   c  is drawn by the second switch  412  turned on during the time t 2 , and decreases the accumulated charge in accordance with the accumulation of the time t 2  by the number of alternations thereby to reduce the signal S 3   a.    
      Both the current value from the first switch  411  and the current value drawn by the second switch  413  are I 1 . As long as the time t 1  and t 2  have the same value, therefore, the influent current amount and the outgoing current amount have the same value. Thus, the signal S 3   a  constituting inter-electrode voltage of the capacitor  424  remains the same as the voltage (signal S 3   b ) generated as an initial value by the constant voltage source  426 . In the case where the time t 1  and t 2  have different values, however, the signal S 3   a , which is amplified by the number of times equal to the number of alternations, is output by emphasizing the difference between time t 1  and t 2 , and therefore considerably different from the signal S 3   b  assuming the initial value.  
      Incidentally, the reset signal S 2  shown in  FIG. 16 , after the alternating current flows between U and V phases, is activated before the alternating current next begins to flow between V and W phases, and similarly activated before the alternating current begins to flow between W and U phases after flowing between V and W phases. The reset signal S 2  thus prevents the detection result (the charge amount of the capacitor  424 ) between the respective phases from affecting the next detection between the phases.  
      The comparator  430  of the rotor position determinator  43  shown in  FIG. 16  compares the magnitude of the signals S 3   a  and S 3   b , and outputs a Hi logic value in the case where the signal S 3   b  is larger than the signal S 3   a , and a Low logic value in the case where the signal S 3   a  is larger than the signal S 3   b . The output  4   a  of this comparator  430  functions as a determination signal for the position of the rotor  11  in stationary mode.  
      The operation of the time counter  41   a , the time difference amplifier  42   a  and the rotor position determinator  43  shown in  FIG. 16  is explained above. The operation of the other circuits is similar to that of the stationary position detection circuit and the motor drive circuit according to the first embodiment and not explained.  
      In the stationary position detection circuit and the motor drive circuit according to this embodiment, the time counter  41   a  includes the first and second current sources  410 ,  413  and first and second switches  411 ,  412 , the time difference amplifier  42   a  includes the capacitor  424  and the constant voltage source  426 , and the rotor position determinator  43  includes the comparator  430 . Thus, the time counter  41   a , the time difference amplifier  42   a  and the rotor position determinator  43  can be configured as a simple circuit. Also, the capacitor  424  is the only capacitor included in the time difference amplifier  42   a , and therefore the increase in circuit size can be suppressed.  
     Third Embodiment  
      This embodiment is also a modification of the stationary position detection circuit and the motor drive circuit according to the first embodiment, and represents another example of the configuration of the time counter  41  and the time difference amplifier  42  according to the first embodiment.  
       FIG. 17  is a diagram showing a detailed configuration of a time counter  41   b  and a time difference amplifier  42   b  in the stationary position detection circuit  4  according to this embodiment. The time counter  41   b  includes a current source  410 , a first switch  411  for selectively outputting the current I 1  from the current source  410  when the logic value of the detection signal  40   a  is Low, a second switch  414  for applying a predetermined potential when the logic value of the detection signal  40   a  is Low, a third switch  415  for selectively outputting the current I 1  from the current source  410  when the logic value of the detection signal  40   a  is Hi, a fourth switch  412  grounded when the logic value of the detection signal  40   a  is Hi, and a voltage source  416  for generating a predetermined potential. The current output from the first switch  411  and the current drawn by the fourth switch  412  constitute a current signal S 4   f  as a signal S 4 .  
      The time difference amplifier  42  includes a capacitor  427  of a predetermined capacitance value having a first electrode connected to the first switch  411  and the fourth switch  412  and a second electrode connected to the second switch  414  and the third switch  415 , in which the voltage between the first and second electrodes finctions as a voltage signal S 3  (S 3   a , S 3   b ).  
      The rotor position determinator  43  includes a comparator  430  and is so configured that the second electrode of the capacitor  427  is connected to the positive input terminal of the comparator  430  and the first electrode of the capacitor  427  is connected to the negative input terminal of the comparator  430 . The potential at the first electrode of the capacitor  427  constitutes the signal S 3   a , and the potential at the second electrode of the capacitor  427  constitutes the signal S 3   b.    
      The time counter  41   b  shown in  FIG. 17 , in accordance with the detection signal  40   a , counts the time t 1  (the period TB in  FIG. 7 ) for which the alternating current changes from α to β and the time t 2  (the period TC in  FIG. 7 ) for which the alternating current changes from β to α, outputs the current signal S 4   f  for the counted time t 1 , and draws the current signal S 4   f  for the counted time t 2 .  
      Specifically, the first switch  411  and the second switch  414  in the time counter  41   b , in accordance with the detection signal  40   a , selectively output the current I 1  constituting the current signal S 4   f  from the current source  410  during the period (the Low period of the detection signal  40   a ) from the detection of the value α to the detection of the value β by the current amount detector  40  thereby to count the time t 1 . On the other hand, the fourth switch  412  and the third switch  415  in the time counter  41   b , in accordance with the detection signal  40   a , selectively draw the current I 1  to the grounding potential GND through the fourth switch  412  as the current signal S 4   f  during the period (the Hi period of the detection signal  40   a ) from the detection of the value β to the detection of the value α by the current amount detector  40  thereby to count the time t 2 .  
      The time difference amplifier  42   b  shown in  FIG. 17  converts the signal S 4   f  constituting a current signal into voltage signals S 3   a , S 3   b  and amplifies the voltage signals S 3   a , S 3   b  in accordance with the accumulation of the time t 1 , t 2  by the number of alternations. Specifically, the capacitor  427  accumulates the charge each time the current signal S 4   f  is input by the turning on of the first switch  411  and the second switch  414  during the time t 1 , and increases the accumulated charge in accordance with the accumulation of time t 1  by the number of alternations thereby to amplify the signal S 3   a . The capacitor  427  releases the charge, on the other hand, each time the current signal S 4   f  is drawn by the turning on of the fourth switch  412  and the third switch  415  during the time t 2 , and decreases the accumulated charge in accordance with the accumulation of time t 2  by the number of alternations thereby to reduce the signal S 3   a.    
      The current value from the first switch  411  is I 1 , and the current value drawn by the fourth switch  412  is also I 1 . Assuming that the time t 1  and t 2  have the same value, therefore, the influent current amount and the outgoing current amount have the same value. Thus, the signals S 3   a , S 3   b  constituting inter-electrode voltages of the capacitor  427  develop no potential difference. In the case where the time t 1  and t 2  have different values, however, the signals S 3   a , S 3   b , which are amplified by the number of alternations, are output by emphasizing the difference between time t 1  and t 2 , and the signals S 3   a , S 3   b  develop a difference in magnitude in accordance with the difference between time t 1  and t 2 .  
      In order to reset the circuit of  FIG. 17  after detection between the respective phases, the first switch  411 , the second switch  414 , the third switch  415  and the fourth switch  412  are all turned on. As a result, the charge accumulated in the capacitor  427  is released.  
      The comparator  430  of the rotor position determinator  43  shown in  FIG. 17  compares the signals S 3   a  and S 3   b  in magnitude, and in the case where the signal S 3   b  is larger than the signal S 3   a , outputs a Hi logic value, while in the case where the signal S 3   a  is larger than the signal S 3   b , a Low logic value is output. The output  4   a  of this comparator  430  finctions as a determination signal for the position of the rotor  11  in stationary mode.  
      The foregoing is the description of the operation of the time counter  41   b , the time difference amplifier  42   b  and the rotor position determinator  43  shown in  FIG. 17 . The operation of the other circuits is similar to that of the stationary position detection circuit and the motor drive circuit according to the first embodiment, and therefore is not described again.  
      In the stationary position detection circuit and the motor drive circuit according to this embodiment, the time counter  41   b  includes a current source  410  and first to fourth switches  411 ,  414 ,  415 ,  412 , the time difference amplifier  41   b  includes a capacitor  427 , and the rotor position determinator  43  includes a comparator  430 . As a result, the time counter  41   b , the time difference amplifier  42   b  and the rotor position determinator  43  can be configured as a simple circuit. Also, the capacitor  427  is the only capacitor included in the time difference amplifier  42   b , and therefore the circuit size increase is suppressed.  
      While the invention has been shown and described in detail, the foregoing description is in all aspects illustrative and not restrictive. It is therefore understood that numerous modifications and variations can be devised without departing from the scope of the invention.