Abstract:
In an apparatus for detecting a rotation by utilizing a residual magnetism remaining in a core, when a field current attenuates gradually through a current return diode during a deceleration period for the stop of rotation, there occurs an AC demagnetizing action which causes the field current to attenuate, so that also during the deceleration period, the supply of electric power to a control circuit and field current to a field coil are continued.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is based on and incorporates herein by reference Japanese Patent Application No. 2000-205147 filed on Jul. 6, 2000. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a voltage control unit used for a vehicular AC generator. 
     2. Description of Related Art 
     JP-A-55-127849 and JP-A-6-284598 disclose to detect, during rotation of a revolving-field type alternator, an AC voltage (also called armature coil induced voltage with residual magnetism) which is induced in a field coil as a result of a magnetic flux being modulated with rotation of field poles, the magnetic flux remaining in a core and interlinked with the armature coil, and thereby detecting the rotation of the alternator, i.e., start-up of an engine. 
     JP-A-3-215200 discloses to detect a potential difference between two-phase voltages out of polyphase AC voltages which are induced by interlinkage of a magnetic flux with a polyphase armature core, the magnetic flux remaining in rotating magnetic poles as constituents of a rotating-field type alternator, thereby detecting the start of rotation. 
     According to the above conventional power generation detecting prior arts, IG cable is omitted and in the case where an engine does not start despite turning ON of an IG switch, the alternator is not energized. 
     However, in the above rotation detection based on the armature coil induced voltage with residual magnetism, the residual magnetism of core often becomes small and hence the armature coil induced voltage with residual magnetism is small, so that it is desired to improve the detection reliability. 
     Similarly, the field coil induced voltage with residual 10 magnetism, which is cased by residual magnetism of the armature core, is influenced by magnetism decline. This is more remarkable. 
     Generally, since the inductance of a field coil is large, a current return diode is connected in parallel with the field coil to inhibit the occurrence of overvoltage. This means that even when a switching transistor connected in series with a field coil is turned ON, allowing a field current to flow through the field coil, and is thereafter turned OFF, the field current continues to flow in the field coil through the current return diode for as long as several hundred seconds while attenuating. Further, even when fuel injection for an engine is stopped upon turning OFF of the switching transistor and hence upon stop of external power supply to the field coil, the engine and the vehicular AC generator continue to rotate for a predetermined period of time by the force of inertia while slowing down. As a result, various portions of the armature core are magnetized alternately by field poles of opposite polarities of which magnetic field intensity attenuate gradually with attenuation of the field current. Under an AC demagnetizing action which is known well in the field of magnetic recording, the residual magnetism disappears, as shown in FIG.  6 . 
     It has experimentally turned out that when external power supply to a field wirding is stopped under the conditions of time constant τ of the field coil: 200 msec, the number of magnetic poles: 16 and the number of rotation: 2000 rpm, a field current continues to flow while attenuating exponentially for subsequent 200 msec or so. At this time, an attenuated alternating field Hs of 266.66 Hz is applied to the armature core, so that the residual magnetism of the armature core remarkably decreases. 
     In the detection of an armature coil induced voltage with residual magnetism, residual magnetism of a field core is important. But, since it is influenced by residual magnetism of an armature core and a current field of the armature core, it so much decreases, thereby giving rise to the problem of a lowering in residual magnetism like the armature core. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a voltage control unit for a vehicular AC generator preventing the attenuation of residual core magnetism in the vehicular AC generator, thereby increasing an induced voltage with residual magnetism. 
     According to the present invention, stop of external power supply to a field coil caused by the stop of power supply to a control circuit is done at least after the stop of a rotor, whereby it is possible to prevent a field current from being attenuated by the foregoing AC demagnetizing action which is attributable to interruption of the power supply to the control circuit before stop of the rotor. As a result, it is possible to prevent the occurrence of the AC demagnetization phenomenon, thereby preventing the decrease of residual magnetism of a core, especially the armature core. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Additional objects and advantages of the present invention will be more readily apparent from the following detailed description of preferred embodiments thereof when taken together with the accompanying drawings in which: 
     FIG. 1 is a schematic view showing a constitution of the present invention; 
     FIG. 2 is a schematic view showing a circuit of power supply hold means (first embodiment); 
     FIG. 3 is a timing chart showing states of various portions (first embodiment); 
     FIG. 4 is a schematic view showing a circuit of power supply hold means (modification); 
     FIG. 5 is a schematic view showing a circuit of power supply hold (second embodiment); 
     FIG. 6 is a characteristic diagram showing an AC demagnetization process of an armature core, and 
     FIG. 7 is a schematic view showing a control circuit of a transistor for turning ON-OFF of field current (third embodiment). 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     First Embodiment 
     FIG. 1 is a block diagram showing a circuit configuration of a vehicular AC generator in the first embodiment. 
     FIG. 1 shows a vehicular AC generator (alternator)  1 , a vehicular battery  2 , a three-phase armature coil  3 , a full-wave rectifier circuit  4  connected to each phase output terminal of the armature coil  3 , a field coil  6  wound around a filed core (not illustrated) having field poles, and a voltage control unit  7  which adjusts a field current and controls the output voltage of the alternator  1  within a predetermined range. 
     The voltage control unit  7  includes a transistor  71  connected in series with the field coil  6  and turning ON and OFF of field current, a flywheel diode  72  for causing the field current to return when the transistor  71  is made OFF, a control circuit  73  which monitors an output voltage of the full-wave rectifier circuit  4  and produces a signal for driving the power transistor  71  so that the output voltage is within a predetermined range, a power supply circuit  74  for supplying electric power to keep ON of the voltage control circuit  73 , and a power supply drive circuit  75  which detects the rotation of a rotor and which produces a signal for driving the power supply circuit  74 . 
     The power supply circuit  74  is a conventional circuit for supplying an electric voltage into the control circuit  73 . The power supply circuit  74  may be constituted by a voltage regulator circuit. A supply voltage input to an IG terminal of the power supply circuit  74  may be applied to the control circuit  73 . The control circuit  73  includes a comparator which compares a battery voltage with a predetermined regulated voltage and which controls to turn ON and OFF of the transistor  71  in accordance with the result of the comparison. The power supply circuit  74  and the control circuit  73  are the same in the prior art. 
     FIG. 2 shows the power supply drive circuit  75 . 
     As shown in FIG. 2, the power supply drive circuit  75  includes a first comparator  751 , a resistor group  752 , a second comparator  753 , a third comparator  754 , an RS flip-flop  755 , a CR integrating circuit  757 , and a transistor  759 . The first comparator  751  compares a one-end voltage of the field coil with a predetermined constant voltage (ground potential in the present embodiment). The resistor group  752  divides a supply voltage Vcc. Each resistor forming the resistor group  752  is required to have same resistance. The second comparator  753  compares two-thirds of the supply voltage Vcc with an output of the CR integrating circuit  757 . The third comparator  754  compares an output voltage of the first comparator  751  with one-third of the supply voltage Vcc. The RS flip-flop  755  receives an output of the second comparator  753  at a reset terminal thereof and receives an output of the third comparator  754  at a set terminal thereof. The CR integrating circuit  757  comprises a capacitor C 1  and a resistor R 2  connected in series with each other. The transistor  759  receives an inverted Q output from the flip-flop  755  through a base resistor Rb and causes the capacitor C 1  to discharge. 
     The power supply drive circuit  75  further includes a resistance voltage divider circuit  761 , a condenser  763 , a comparator  764 , an OR gate  765 , a voltage regulator circuit  766 , and an analog switch  760 . 
     The resistance voltage divider circuit  761  comprises resistors R 3  and R 4  connected in series with each other and outputs a divided voltage V 2  of the DC output voltage from the alternator  1 . The condenser  763  is connected in parallel with the resistor R 4  to eliminate a high-frequency component from the divided voltage V 2 . The comparator  764  compares the divided voltage V 2  with a predetermined voltage Vref. The OR gate  765  attains an OR of a Q output of the flip-flop  755  and an output of the comparator  764 . The voltage regulator circuit  766  reduces the DC output voltage of the alternator  1  and forms a constant supply voltage Vcc. The analog switch  760  is actuated with the output of the OR gate  765  and turns ON and OFF of the electric power to be fed to the IG terminal of the main power supply circuit  74 . 
     A high-frequency bypass condenser Cb is connected between a pair of input terminals of the comparator  751 . The bypass capacitor Cb prevents malfunctioning of the comparator  751  caused by mixing of a high-frequency noise voltage in the ground potential. Although the field coil induced voltage generated by residual magnetism upon starting of rotation of the vehicular AC generator is an AC voltage, the attenuation effect by the high-frequency bypass condenser Cb is small because the induced voltage has a low frequency. 
     An operation of the power supply drive circuit (power supply hold means in the present invention)  75  will be described with reference to FIG.  3 . 
     Magnetism remains in each core of the alternator  1  as a result of the previous power generation. In the field coil  6 , with rotation of the rotor, a combined residual magnetism of the residual magnetism in the field core and the residual magnetism in the armature core, fluctuates periodically with respect to the field poles, so that a residual magnetic flux formed by the combined residual magnetism is interlinked with the field winding, fluctuates periodically, whereby an AC voltage is induced. 
     That is, in a space distribution of the residual magnetism in the armature core, N and S poles are alternately created circumferentially on an inner peripheral surface of the armature core in accordance with standstill positions of pawl-like field poles of the rotor and in a total number matching the total number of field poles. As a result, due to rotation of the field poles, the magnetic flux formed by the residual magnetism of the armature core and interlinked with the field winding  6  undergoes a periodic change, whereby an AC voltage is generated in the field coil  6 . At this time, the switch  71  is assumed to be OFF. For example, the amplitude of the AC voltage is about 0.2-0.4V and the frequency thereof is P1□N/60 [Hz], assuming that the number of rotating magnetic poles is 2P1 and the number of rotations is N [rpm]. 
     The comparator  751  compares the AC voltage with a constant voltage (the ground potential in the present embodiment) and outputs a rectangular pulse voltage “in” with a duty ratio of 50% and a frequency of P1□N/60. The rectangular pulse voltage “in” is input to the third comparator  754 , in which it is compared with a divided voltage Vcc/3 and of which output is received by the set terminal of the flip-flop  755 . here, by seeing the reference voltage to a voltage other than the ground potential, it is possible form a rectangular pulse of a desired duty ratio. 
     The output of the CR circuit  757  (the first timer) is input to the second comparator  753  and is compared with a divided voltage 2□Vcc/3. Thus, when the output of the CR circuit  757  has reached the divided voltage 2□Vcc/3, the second comparator  753  outputs a Hi level signal to reset the flip-flop  755 . 
     While the output of the second comparator  753  is Lo, that is, while the output of the CR circuit is 2□Vcc/3 or less, the Q output of the flip-flop  755  is Hi, the inverted Q output thereof is Lo, the transistor  759  turns OFF, and the condenser C 1  is charged. When the condenser C 1  is charged and the potential Vc reaches 2□Vcc/3, the flip-flop  755  is reset to turn the transistor  759  ON and the condenser C 1  is discharged. After all, the flip-flop  755  outputs Hi level signals while the condenser  757  is charged, that is, during a certain period substantially equivalent to the time constant of the CR circuit  757 . When the output of the flip-flop  755  is at Hi level, the switch  760  which supplies electric power to the IG terminal of the power supply circuit  74  is kept ON, thereby allowing the power supply circuit  74  to operate. 
     When the number of rotations is small, the CR time constant of the CR circuit  757  is shorter than the set input cycle of the flip-flop  755 , so that the set input is Lo when the flip-flop  755  is reset, that is, when the potential of the condenser C 1  becomes 2□Vcc/3. Next, until the set input becomes Hi, the flip-flop  755  maintains Lo output and an output Out 1  of the power supply drive circuit  75  is kept at Lo. 
     When the number of rotation exceeds a predetermined number, the set input cycle of the flip-flop  755  becomes shorter than the CR time constant, so that when the flip-flop  755  is reset, that is, when the potential of the condenser C 1  becomes 2□Vcc/3, the set input is also Hi. The flip-flop  755  continues to output Hi level signals and the output Out 1  of the power supply drive circuit  75  is held Hi. That is, as the number of rotation increases, the frequency of the voltage induced in the field coil  6  becomes high and the OFF period of the output Out 1  is shortened gradually and finally turns ON continuously. Thus, the power supply circuit  74  is held ON continuously, with intermittent operations being performed in a region of low rotation and with a shift being made to a continuous operation at a certain number of rotation or more. 
     For example, In an alternator having twelve poles (6-pole pairs), when R 2  is set at 100 kΩ and C 1  at 0.1 μF, a continuous operation can be done at about 1000 [rpm]. Generally, when an alternator having 2□P1 poles is to be operated continuously at N1 [rpm], it can be done by setting the time constant of the CR circuit  757  to 60/(P1□N1) [sec]. 
     That is, in the present embodiment, since whether electric power is to be supplied or not is determined by comparing the cycle of a field coil induced voltage with residual magnetism (a frequency-related signal voltage, a rotation induced electric signal) with the time constant of the CR circuit (the first timer)  757 , it is possible to determine that the number of rotation has reached a predetermined number or more and hence possible to supply electric power stably to the control circuit. 
     In case of using the RS flip-flop  755 , when both set and reset inputs are Hi, the flip-flop output becomes uncertain. Therefore, an output voltage of the alternator  1  is detected by the resistance voltage divider circuit  761 , and is compared with a reference voltage higher than an open terminal voltage of the battery by means of the comparator  764 , then a Hi level signal is outputted when the output voltage of the alternator  1  is higher than the reference voltage, Vref, and the gate of the switch  760  is enabled with an OR signal of the signal and the output Out 1 , whereby the operation can be made still more stable. 
     By passing the result of the comparison between the output voltage of the alternator  1  and the reference voltage higher than the open terminal voltage of the battery through the OR gate, it is possible to further attain the following advantage. 
     Consideration is now given to the process of stopping the vehicular engine at the end of vehicular running. Usually, since the battery is maintained in a substantially fully charged state, its terminal voltage, i.e., an input voltage of the comparator  764  is of a value corresponding approximately to 14.5V and the output of the comparator  764  is Hi. When the driver of the vehicle stops the engine, the alternator  1  also turns OFF immediately. But the main power supply of the voltage control unit in the alternator  1  is still active with Hi signal received from the comparator  764 . Therefore, an exciting current is continued to be supplied to the field coil  6  in the alternator  1  at an approximate duty ratio. The battery terminal voltage finally drops to a no-load voltage of approximately 12.8V due to disappearance of charging polarization for example, so that the output of the comparator  764  is inverted to disable the power supply circuit  74 . 
     Since the conventional vehicular battery utilizes a chemical reaction, several ten seconds are required from the time when the engine stops and so does the power generation of the alternator  1  until when the power supply circuit  74  in the voltage control unit turns OFF and the exciting current flowing in the field coil  6  becomes extinct completely. 
     In the present embodiment, therefore, the phenomenon that the attenuated exciting current of the field coil  6  demagnetizes the armature core at the time of stopping the rotation is realized by delaying the stop of power supply to the control circuit on the basis of the battery voltage, i.e., the DC output voltage of the alternator  1 . Thus, also at the next time of engine start-up, the voltage control unit in the alternator  1  can be restored positively to its stand-by state. Here, delaying the stop of power supply to the control circuit may be realized not by a change of the DC output voltage but by a change in frequency thereof or by a change in generated voltage or frequency of the armature coil. 
     In the present embodiment, there is shown an example of having realized the timer function by an analog signal processing which utilizes the time constant of the CR circuit. The same advantage can be obtained also by a digital signal processing which utilizes any of various digital counters. FIG. 3 shows potential states of various portions. 
     (Modification) 
     FIG. 4 shows a modification of the first embodiment. 
     In the above-described first embodiment, an induced AC voltage in the field coil  6  is detected and used as a trigger signal for triggering the power supply circuit. Alternatively, in this modification, a one-phase output voltage from the armature coil  3  is detected and used as a trigger signal. The operation is the same as in the first embodiment. 
     Second Embodiment 
     The second embodiment of the present invention will be described with reference to FIG.  5 . 
     A power supply drive circuit  75  used in the second embodiment is characterized by using in parallel both a first power supply drive circuit portion  75   a  having a function equal to that of the first power supply drive circuit  75  shown in FIG. 2, and which operates with the field coil voltage and a second power supply drive circuit portion  75   b  having a function almost equal to that of the power supply drive circuit  75  shown in FIG. 4, and which operates with the armature coil voltage. Here, a duration time Hi output from the comparator  753  and is compared with a divided voltage 2□Vcc/3. Thus, when the output of second power supply drive circuit  75   b  (a time constant of a CR circuit (the second timer) constituted by a series circuit of both condenser C 3  and resistor R 5 ) C 3 R 5  is set fully long in comparison with the duration time C 1 R 2  of Hi output from the first power supply drive circuit  75   a.    
     Further, the power supply drive circuit  75  in the second embodiment includes detector means  767  for detecting a frequency component of the output voltage from the armature coil  3 , e.g., a known F/V converter, a comparator  768  which compares an output of the detector means  767  with a predetermined constant voltage, an AND gate  769  which calculates the AND of a Q output from the second power supply drive circuit portion  75   b  and an output from the comparator  768 , and an OR gate  770  which ORs an output of the AND gate  769  with an output of the first power supply drive circuit portion  75   a.    
     An operation of the second embodiment will be explained. 
     As to the operation of the first power supply drive circuit portion  75   a , an explanation thereof will here be omitted because it is the same as in the first embodiment. 
     The second power supply drive circuit  75   b  is the same as in the modification of the first embodiment. Here, in the case where the output of the frequency detector  767  is larger than the predetermined frequency Vref, the comparator  768  outputs a Lo level signal. As a result, in the normal state of power generation in which the number of rotation is large, the output of the second power supply drive circuit portion  75   b  is rendered invalid. 
     When the engine has entered a stopping process, with drop in the number of rotation of the alternator  1 , and the frequency of the AC voltage in the armature coil has lowered to a frequency corresponding Vref  2  or less, the comparator  768  outputs a Hi level signal and the output of the second power supply drive circuit  75   b  is input to the OR gate  770 . At this time, since a Hi output period T 2  while a Hi level signal is output from the circuit  75   b  is sufficiently longer than a Hi output period T 1  while a Hi level signal is outputted from the circuit  75   a , the OR gate  770  continues to output Hi level signals for a certain period even after decrease in the number of rotation and complete stop of the engine, so that the power supply circuit  74  is kept ON. As long as the power supply circuit  74  is kept ON, an exciting current continues to flow in the field coil  6 , so that a magnetic field of a direct current continues to be applied to the armature core, thereby allowing power generation to be started easily when the engine is next to be started. 
     That is, in the second embodiment, the supply of electric power to the control circuit  3  is continued for a short period of time even after complete stop of rotation based on the generated voltage in the armature coil to prevent an AC demagnetization. 
     Third Embodiment 
     The third embodiment will be described with reference to FIG.  7 . 
     As shown in FIG. 7, the third embodiment includes a comparator  1000 , an OR gate  1001 , a diode  1002 , resistors  1003  and  1005 , a condenser  1004 , and a comparator  1006 . 
     The comparator  1000  compares a regulated voltage Vth with a battery voltage Vb. An output of the comparator  1000  controls the transistor  71  through the OR gate  1001 , and a DC output voltage is controlled for convergence to the regulated voltage Vth. The diode  1002  half-wave rectifies one-phase armature coil voltage (generated voltage) Vp. 
     The one-phase armature coil voltage (generated voltage) Vp is half-wave rectified by the diode  1002  and charges the condenser  1004  through the resistor  1003 , and the condenser  1004  is discharged through the resistor  1005 . 
     In a normal power generation stage in which the rectified voltage of the onephase armature coil voltage (generated voltage) Vp exceeds a predetermined value Vrefx, the comparator  1006  outputs a Lo level signal, while in an initial stage of rotation in which the number of rotation is smaller than a predetermined number and during and after the stop of rotation, the comparator  1006  outputs a Hi level signal. As a result, in the initial stage of rotation and during and after the stop of rotation, the OR gate  1001  is supplied with a field current. 
     By this, when the number of rotation has decreased below the predetermined value due to the stop of rotation, the field current flows sufficiently through the field coil  6  to prevent an AC demagnetization. Thereafter, the supply of line voltage to the control circuit  73  is cut off to prevent wasteful supply of the field current and power consumption in the control circuit  73 . Similarly, in the first and second embodiments, the supply of electric power to the control circuit  73  is not started yet in the initial stage of rotation, whereby wasteful supply of the field current is prevented. 
     Thus, irrespective of the value of the battery voltage Vb, a sufficient supply of the field current is ensured even after the stop of rotation is completed, so that AC magnetization does not occur. 
     (Modification) 
     In the above-described embodiments, the amplitude of the generated voltage Vp was used. Alternatively, the frequency thereof may be used, or the DC output voltage or the amplitude thereof may be used. 
     In the above-described embodiments, a forced supply of the field current was started during the deceleration period for the stop of rotation. Alternatively, it may be done after the completion of rotation stop and before the end of power supply. For example, in the circuit of FIG. 7, Vrefx may be set at 0V.