Abstract:
A vehicle drive device comprises an actuator located between an electric motor and axles for driving the vehicle, a built-in power source for supplying electric power to the actuator, and a control means for controlling the drive of the actuator. The built-in power source can be consisted of a battery, and the actuator can be consisted of an electromagnetic clutch. Here, the vehicle drive device further comprises a boost means, such as a DC-DC converter, for boosting voltage of the built-in power source such as the battery. The output voltage of the boost means is supplied electric power to the actuator such as the electromagnetic clutch. The output voltage of the boost means can be also supplied the electric power to the field coil of the electric motor.

Description:
CLAIM OF PRIORITY 
     The present application claims priority from Japanese application serial no. 2004-52687, filed on Feb. 27, 2004, the content of which is hereby incorporated by reference into this application. 
     FIELD OF THE INVENTION 
     The present invention relates to a vehicle drive device and specifically to a vehicle drive device suitable for driving an actuator used for controlling a vehicle. 
     BACKGROUND OF THE INVENTION 
     Conventionally, as shown in an electric four-wheel drive vehicle, a well-known vehicle drive device in which front wheels are driven by an internal combustion engine and rear wheels are driven by an electric motor is commonly equipped with an electromagnetic clutch as an actuator for switching the transfer and non-transfer of the drive force between rear wheels and the electric motor for driving the rear wheels. The vehicle drive device is, for example, described in Japanese Patent Laid-open No. 2003-326997. Generally, a battery supplies electric power to the coil located in the electromagnetic clutch. 
     On the other hand, there is a well-known conventional vehicle drive device as described in Japanese Patent Laid-open No. 2003-079004. In the vehicle drive device, when a generator&#39;s output voltage is less than a prescribed voltage equivalent to a storage battery&#39;s output voltage, the electric power outputted from the generator&#39;s rectification circuit is boosted to a prescribed voltage by using a boost circuit, such as a DC/DC converter, and the amount of current generated at the low-speed start when the number of motor revolutions is few is increased, thereby high motor torque is obtained. 
     However, in the vehicle drive device described in Japanese Patent Laid-open No. 2003-079004, the voltage outputted by the generator is boosted, thereby causing the input voltage of the boost means to increase up to the maximum output voltage of the generator and also causing output voltage of the generator to change due to fluctuations of the load of the motor. As a result, there are problems such as increase in conversion noise, power loss and the size of the parts. Therefore, it is necessary to use a boost means which has variable input and output and can withstand high pressure. 
     Another well-known vehicle drive device is described in FIG. 4 of Japanese Patent Laid-open No. 2001-352795. In the vehicle drive device, the output of a power source such as a battery is boosted by a DC/DC converter and supplied to a field coil of the electric DC motor. This configuration keeps battery voltage almost constant, which eliminates the above problems and also makes it possible to increase the motor&#39;s output torque. 
     SUMMARY OF THE INVENTION 
     However, when an electric motor&#39;s output torque is increased by the method described in Japanese Patent Laid-open No. 2001-352795, if a battery is used as a power source and the output is supplied to the coil of the electromagnetic clutch located between the electric motor and the rear wheels, as described in Japanese Patent Laid-open No. 2003-326997, a problem arises. It was found that the electromagnetic clutch&#39;s engagement force is not sufficient and the electric motor&#39;s output torque is not sufficiently transferred to the wheels. As a result, the force to transfer torque is decreased. 
     Similarly, an electromagnetic brake installed in a vehicle drive device or an actuator, such as an electromagnetic limited-slip differential gear, functions as a driven body driven by battery voltage. However, it was found that if a battery is used as a power source to supply electric power to those driven components, performance of those components will be less than optimal. 
     The object of the present invention is to provide a vehicle drive device equipped with a high-performance actuator. 
     (1) To achieve the above object, a vehicle drive device according to the present invention is equipped with an actuator used for driving the vehicle, built-in power source for supplying electric power to the actuator, and a control means for controlling the drive of the actuator, wherein a boost means for boosting the output voltage of the built-in power source is provided and the electric power boosted by the boost means can be supplied to the actuator. 
     This configuration makes it possible to reduce the size of the actuator as well as heat generation, thereby enabling high-performance. 
     (2) In the above item 1, preferably, the actuator is an electromagnetic clutch. 
     (3) In the above item 2, preferably, a vehicle drive device according to the present invention is equipped with a high-output driving generator driven by an internal combustion engine and an electric motor driven by the output voltage supplied by the high-output driving generator, wherein the electromagnetic clutch is located between the electric motor and axles, and the electric power boosted by the boost means is supplied to the field coil of the high-output driving generator and the field coil of the electric motor. 
     (4) In the above item 1, preferably, the actuator is an electromagnetic brake. 
     (5) In the above item 1, preferably, the actuator is an electromagnetic limited-slip differential gear. 
     (6) To achieve the above object, a vehicle drive device according to the present invention is equipped with an actuator used for driving the vehicle, built-in power source for supplying electric power to the actuator, and a control means for controlling the drive of the actuator, wherein a boost means for boosting output voltage of the built-in power source, a high-output driving generator driven by an internal combustion engine, and an electric alternating current (AC) motor driven by the output voltage supplied by the high-output driving generator are provided, and the electric power boosted by the boost means is supplied to the field coil of the high-output driving generator and the field coil of the electric AC motor. 
     According to the present invention, it is possible to provide a vehicle drive device that is equipped with a high-performance actuator. 
     Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic diagram showing the configuration of an electric four-wheel drive vehicle that uses a vehicle drive device according to an embodiment of the present invention. 
         FIG. 2  is a schematic diagram showing the configuration of a vehicle drive device according to an embodiment of the present invention. 
         FIG. 3  is a flow chart showing the control mechanism of the 4WDCU  60  incorporated in the vehicle drive device according to an embodiment of the present invention. 
         FIG. 4  is a characteristic diagram of a high-output generator used for the vehicle drive device according to an embodiment of the present invention. 
         FIG. 5  is a circuit diagram showing the configuration of a DC/DC converter used for the vehicle drive device according to an embodiment of the present invention. 
         FIG. 6  is a flow chart showing the control mechanism of the vehicle drive device according to an embodiment of the present invention. 
         FIG. 7  is a schematic diagram showing the configuration of a vehicle drive device according to another embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereafter, with reference to  FIGS. 1 through 5 , configuration and operations of a vehicle drive device according to an embodiment of the present invention will be explained. 
     First, by referring to  FIG. 1 , configuration of an electric four-wheel drive vehicle which uses a vehicle drive device according to this embodiment. 
       FIG. 1  is a schematic diagram showing the configuration of an electric four-wheel drive vehicle which uses a vehicle drive device according to an embodiment of the present invention. 
     A four-wheel drive vehicle  10  is equipped with an engine  20  and an electric motor  30 . The drive force of an engine  20  is transferred to front wheels  26 A and  26 B, for example, via a transmission  22  and first axles  24 A and  24 B, thereby driving the front wheels  26 A and  26 B. A drive force of an electric motor  30  is transferred to rear wheels  36 A and  369 , for example, via an electromagnetic clutch  32 , electromagnetic limited-slip differential gear (LSD)  33  and second axles  34 A and  343 , thereby driving the rear wheels  36 A and  363 . When the electromagnetic limited-slip differential gear  33  is engaged with the electromagnetic clutch  32 , a rotational force of the electric motor  30  is transferred to the rear-wheel axles  34 A and  349  via the electromagnetic clutch  32  and the electromagnetic limited-slip differential gear  33 , thereby driving the rear wheels  36 A and  36 B. The electromagnetic clutch  32  is capable of controlling an engagement force of the clutch by controlling the amount of current flowing through the electromagnetic clutch coil  32 A. When the electromagnetic clutch  32  is disengaged, the electric motor  30  is mechanically separated from the rear wheels  36 A and  36 B, and accordingly, the rear wheels  36 A and  36 B do not convey the drive force to the road. The electric motor  30  incorporates an electric motor field coil  31 . For example, an electric AC motor which switches efficiently and easily between forward and reverse rotations is used. Furthermore, an inverter, not shown, is disposed between the generator  44  and the electric motor  30  to convert current outputted from a high-output driving generator  44  into alternating current. 
     In the above explanation, a four-wheel drive vehicle in which front wheels  26 A and  26 B are driven by an engine  20  and rear wheels  36 A and  36 B are driven by an electric motor  30  is described as an example. However, front wheels can be driven by an electric motor and rear wheels can be driven by an engine. Furthermore, a vehicle with six wheels or more, such as a truck, or a tractor vehicle, such as a trailer, is also applicable. 
     An auxiliary generator (ALT 1 )  40  and auxiliary battery  42  which make up of a normal charging and generating system are disposed in an engine room, and the output from the auxiliary generator  40  driven by an engine  20  is stored in the auxiliary battery  42 . A high-output driving generator (ALT 2 )  44  driven by an engine  20  via the belt is disposed in the vicinity of the auxiliary generator  40 . The output from the high-output driving generator  44  drives the electric motor  30 . The auxiliary generator  40  is, for example, an ordinary 12-V and 2-kW generator, and the high-output driving generator  44  is a generator, such as a 36-V and 6-kW generator, which enables higher outputs than the auxiliary generator  40 . 
     The output of the engine  20  is controlled by an electronically-controlled throttle  52  driven by the instructions of the engine control unit (ECU)  50 . The electronically-controlled throttle  52  has an accelerator opening sensor  54  which detects the degree of opening of the accelerator. When a mechanically-linked accelerator pedal and throttle are used instead of an electronically-controlled throttle, the accelerator pedal can be equipped with an accelerator opening sensor. The ECU  50  controls the amount of fuel injection supplied to the engine  20  and the engine ignition timing. 
     The ECU  50  also controls the transmission  22 . The transmission  22  is an automatic transmission which is automatically controlled by the select lever  23  so as to become a selected gear ratio. The position of the select lever  23  is detected by a gear position detection sensor  25 . As a transmission  22 , a manual transmission (MT) or a continuously variable transmission (CVT) can be used. 
     Each of the front wheels  26 A and  26 B and rear wheels  36 A and  36 B has a rotary sensor  56 A,  56 B,  58 A and  58 B which detects rotation speed and rotational direction. The rotary sensor  56 A,  56 B,  58 A and  58 B is provided for each wheel, however, it can be provided for either a front-wheel axle or rear-wheel axle, or both. 
     An ABS control unit  55  detects the lock condition of the wheels based on the number of revolutions of each wheel  26 A,  26 B,  36 A and  36 B detected by each rotary sensor  56 A,  56 B,  58 A and  58 B. When a wheel is locked, the ABS control unit  55  issues control commands to electromagnetic solenoids  29 A,  29 B,  39 A and  39 B, thereby controlling the brake force generated at the electromagnetic brakes  28 A,  28 B,  38 A and  38 B. 
     The foul-wheel drive control circuit (4WDCU)  60  calculates vehicle speed based on the rotation speed of the wheels  26 A and  26 B,  36 A and  36 B detected by rotary sensors  56 A,  56 B,  58 A and  58 B, and controls the high-output driving generator  44  and electric motor  30  based on the calculated vehicle speed. The 4EDCU  60  detects rear wheel slipping according to the difference between the front wheel speed detected by rotary sensors  56 A and  56 B of the front wheels  26 A and  26 B and the rear wheel speed detected by rotary sensors  58 A and  58 B of the rear wheels  36 A and  365 . And, in the case of rear wheel slipping, the 4WDCU  60  controls field current of the high-output driving generator  44  and the electric motor  30 , thereby controlling drive torque outputted from the electric motor  30 . Details of the control executed by the foul-wheel drive control circuit (4WDCU)  60  will be explained later with reference to  FIGS. 3 and 4 . 
     The DC/DC converter  70  boosts terminal voltage of the battery  42  and supplies electric power to a field coil  47  of the high-output driving generator  44 , field coil  31  of the electric motor  30 , coil  32 A of the electromagnetic clutch  32 , coil  33 A of the electromagnetic limited-slip differential gear  33 , and coils of the brakes  28 A,  285 ,  38 A and  38 B. 
     Next, with reference to  FIG. 2 , the configuration of the vehicle drive device according to this embodiment will be explained. 
       FIG. 2  is a schematic diagram showing the configuration of a vehicle drive device according to an embodiment of the present invention. Items in  FIG.2  that have identical numbers to items in  FIG. 1  are identical. 
     The power generated by an auxiliary generator  40  is stored in an auxiliary battery  42 . The output voltage is, for example, 12 V. The DC/DC converter  70  boosts terminal voltage of the battery  42  and supplies the power to the field coil  47  of the high-output driving generator  44 . The boosted voltage is, for example, 24 V. Detailed configuration of the DC/DC converter  70  will be described later by referring to  FIG. 5 . The high-output driving generator  44  incorporates an armature coil  45 , diodes  46 A and  46 B, a field coil  47 , and a field coil voltage regulator  48 . Output voltage of the DC/DC converter  70  is supplied to the field coil  47  via the diode  46 A. 
     As  FIG. 1  shows, the high-output driving generator  44  is driven by an engine. The output from the armature coil is converted into AC voltage by means of an inverter circuit (INV)  35  and is supplied to an electric motor  30 , and is also supplied to the field coil  47  of the generator  44  via a diode  46 A. As a voltage inputted into the field coil  47 , diodes  46 A and  46 B automatically select either output voltage of the DC/DC converter  70  or that of the generator armature coil  45  whichever is higher. The voltage regulator  48  is driven by the MPU  63  of the 4WDC/U  60  and controls voltage inputted into the field coil. By controlling voltage of the field coil, it is possible to change the field current (amount of field magnetic flux), thereby controlling current supplied to the electric motor  30 . 
     The voltage outputted by the DC/DC converter  70  is supplied to the field coil  31  of the electric motor  30  via an H-bridge circuit  66  of the 4WDCU  60 . The H-bridge circuit  66  consists of four bridge-connected MOSFETS (MOS 1 , MOS 2 , MOS 3 , MOS 4 ). 
     The 4WDCU  60  is equipped with an I/O circuit  61 , CAN circuit  62 , MPU  63 , and voltage regulators  64  and  67 . The MPU  63  controls field current that flows through the field coil  31  of the electric motor  30  by means of a voltage regulator  64  so that the torque generated by an electric motor  30  conforms to the required value. To reverse a vehicle, it is possible to reverse the rotational direction by shifting the phase by means of an inverter  35 . 
     Furthermore, the voltage outputted by the DC/DC converter  70  is supplied to a coil  32 A of the electromagnetic clutch  32 . Voltage that flows through the coil  32 A is regulated by a voltage regulator  67 . 
     Herein, operations of the 4WDCU  60  will be explained. Gear position information detected by a gear position detection sensor  25  is fetched by the MPU  63  via an I/O circuit  61 . Information of rotation speed and direction of the wheels  26 A and  26 B,  36 A and  36 B detected by rotary sensors  56 A,  56 B,  58 A and  589  as well as information of accelerator opening detected by an accelerator opening sensor  54  are first fetched by an engine control unit (ECU)  50  and calculated, and then fetched by the MPU  63  via the CAN circuit  62 . 
     The MPU  63  has the CPU and a memory for storing the program and data for controlling the electric motor. Based on the inputted information, the MPU  63  calculates vehicle speed and electric power outputted by the high-output driving generator  44 , and calculates generator field voltage that satisfies the power output of the generator. The calculated generator field voltage is inputted into the voltage regulator  48  as a generator field voltage command. The voltage regulator  48  controls the field current which is supplied to the field coil  47  of the high-output driving generator  44  based on the generator field voltage command. Thereby the input voltage of the electric motor  30  is controlled. Furthermore, MPU  63  calculates the electric motor field voltage and output it to the voltage regulator  64  so that the characteristics of the electric motor  30  conform to the required value. The voltage regulator  64  regulates field current that flows through the field coil  31  of the electric motor  30 . Furthermore, the MPU  63  creates the engagement force control command of the electromagnetic clutch  32 , and controls current which is supplied to the electromagnetic coil  32 A of the electromagnetic clutch  32 . Moreover, the MPU  63  also controls DC-AC power conversion in the inverter circuit  35 . 
     The torque generated by the electric motor  30  is controlled in three ways: control of the field current of the high-output driving generator  44 , control of the field current of the electric motor  30 , and the phase control of three-phase alternating current in the inverter  35 . For example, when necessary motor speed is low and necessary torque is high in the case as the start of a vehicle, it is possible to make the motor speed low and make the output torque high by reducing voltage outputted by the high-output driving generator  44  while increasing the amount of field current flowing through the field coil  31  of the electric motor  30  so as to increase the output current. When a vehicle is traveling, high speed and low torque is required for the electric motor. This condition can be achieved by increasing voltage outputted by the high-output driving generator  44  and reducing the output current. Furthermore, by reducing the field current of the electric motor  30 , it is possible to increase motor speed while improving the responsiveness of the vehicle during traveling. Moreover, when a required torque distribution value for the front wheel  26  is higher than that for the rear wheel  36 , it is possible to make torque distribution of the front wheel  26  and rear wheel  36  variable by reducing the field current of the high-output driving generator  44 . Furthermore, by controlling the inverter  35  to control the phase of the three-phase alternating current according to the motor&#39;s rotation position, that is, by executing the field weakening control for controlling the phase of the armature current so as to control field magnetic flux, it is possible to increase accuracy of the field control specifically in the high rotation range where the amount of field magnetic flux should be low. As a result, it is possible to accurately and effectively control torque over a wide range. Furthermore, the use of an electric AC motor is more efficient than the use of an electric DC motor because there is no brush loss in an AC electric motor, and the efficiency can be further increased by advancing the phase according to the motor speed. Moreover, the above explanation describes a separately-excited AC motor that can use both the magnetic field and the armature for control. However, an AC motor excited by an interior permanent magnet can be used because the field weakening control is possible in a high rotation range by simply controlling the phase of the armature current in response to the position signal. 
     Next, with reference to  FIGS. 3 and 4 , operations of a vehicle drive device according to this embodiment will be explained. 
       FIG. 3  is a flow chart showing the control mechanism of the 4WDCU  60  incorporated in the vehicle drive device according to an embodiment of the present invention.  FIG. 4  is a characteristic diagram of a high-output generator used for a vehicle drive device according to an embodiment of the present invention. 
     In step s 10  in  FIG. 3 , the 4WDCU  60  calculates vehicle speed by determining a low speed as a vehicle speed, for example, based on rotation speed information of front and rear axles inputted by rotary sensors  56 A,  56 B,  58 A and  58 B. 
     Next, in step s 20 , the 4WDCU  60  calculates motor drive torque required in response to the traveling circumstances which have been determined in step s 10 . 
     In step s 30 , the 4WDCU  60  calculates the voltage value commanded for the driving generator  44  so as to obtain the calculated motor drive torque, and outputs the value to the driving generator  44 . The driving generator  44  internally executes feedback control so that the output voltage becomes the command value, and the generator outputs the voltage V to the electric motor  30 . This voltage V causes actual torque of the electric motor  30  to be inputted into the rear wheel  36  to output actual wheel speed, thereby executing feedback control of the entire system. 
     Next, with reference to  FIG. 4 , characteristics of the high-output generator will be explained. In  FIG. 4 , the output voltage is outputted from the high-output driving generator  44 , and it is considered as an input voltage of the electric motor  30  excluding the wiring resistance. In  FIG. 4 , solid line X 1  shows the output voltage—output current characteristics during the self excitation in which voltage outputted by the high-output driving generator  44  is directly supplied to the field coil. Dotted line X 2  shows the output voltage—output current characteristics when the field coil  47  of the high-output driving generator is separately excited (separate excitation by power source) by the voltage V 1  of the constant-voltage power source  49 . Dashed line X 3  shows the output voltage—output current characteristics when output voltage of the constant-voltage power source is boosted (separate excitation by boosted power source) by a boost circuit, such as a DC/DC converter  70 , to the nearly equivalent to the output voltage V 2  which causes maximum output current  12  in the self excitation condition, and the field coil  47  of the high-output driving generator is separately excited by the voltage. Herein, if output voltage of the high-output driving generator  44  exceeds V 1  in the case of separate excitation by the power source or exceeds V 2  in the case of separate excitation by the boosted power source, diodes  46 A and  46 B select the voltage outputted by the high-output driving generator  44  as the voltage that is inputted into the field coil  47  of the high-output driving generator. As a consequence, the generator  44  enters the self-excitation condition. 
     When a vehicle is being driven or getting out of rut, high torque is required. However, when vehicle speed is low, the number of revolutions of the electric motor  30  also decreases, causing induction voltage of the electric motor  30  to decrease. At that time, engine speed is also low, and therefore, voltage outputted by the driving generator  44  is also low, nearly V 1  or below V 1 . It is indicated that the amount of output current when the power source is boosted is greatly higher than that of output current when output voltage is low. (Ex. I 1 &lt;&lt;I 1 ′: when output voltage is V 1 ). Magnitude of the motor torque is according to the amount of flowing current. Accordingly, higher torque can be outputted when the power source is boosted and the coil is separately excited by the voltage. 
     As stated above, output voltage of the DC/DC converter  70  is supplied to the field coil  32 A of the driving generator  44 , and therefore, the driving generator  44  can output a high voltage. In addition, by controlling field current of the driving generator  44  so as to control output voltage and output current of the generator, it is possible to increase output current as indicated by the dashed line X 3 . Accordingly, output torque of the electric motor  30  driven by the voltage outputted from the generator  44  can be increased. Furthermore, by controlling field current of the electric motor  30 , the electric motor  30  can rotate from low speed to high speed, thereby increasing the motor drive range. 
     Again in  FIG. 2 , in this embodiment, an electromagnetic clutch  32  can change the engagement force of the clutch by controlling current flowing through the electromagnetic clutch coil  32 A by means of the 4WDC/U  60 . The power supply line of the coil  32 A of the electromagnetic clutch  32  is connected to the output terminal of the DC/DC converter  70 . Therefore, voltage that is supplied to the coil  32 A of the electromagnetic clutch  32  can be increased, thereby the fastening power of magnetic clutch of  32  can be increased further than the case in which DC-DC converter  70  is not used. As stated above, when field currents of the driving generator  44  and electric motor  30  are controlled and output torque of the electric motor  30  becomes high, if the engagement force of the electromagnetic clutch  32  is weak, the clutch slips, which prevents torque of the electric motor  30  from being effectively transferred to the wheels and causes losses. However, by making the engagement force of the electromagnetic clutch  32  strong as shown in this embodiment, it is possible to reduce losses caused by the clutch slipping. Furthermore, when voltage that is applied to the coil  32 A of the electromagnetic clutch  32  is low because a DC/DC converter  70  is not used, it is possible to increase the engagement force of the electromagnetic clutch by increasing current flowing through the coil  32 A. However, in this case, a large current needs to be provided. Consequently, problems arise in that the size of the electromagnetic clutch increases and the amount of heat generated becomes high due to large current consumption. On the contrary, by using a DC/DC converter  70 , as shown in this embodiment, the engagement force can be increased and the size of the electromagnetic clutch can be reduced, thereby reducing the amount of heat generated. 
     Furthermore, by controlling the engagement force of the electromagnetic clutch  32  by means of a coil voltage regulator  67  without depending on the fluctuating power generated by the high-output driving generator  44 , it is possible to forcibly disengage the mechanical connection between rear wheels  36 A and  36 B and the electric motor  30  when the four-wheel drive function is not necessary. For example, when vehicle speed becomes 20 kilometer per hour, the electromagnetic clutch  32  is turned off and only front wheels are driven. By doing so, durability of the electric motor  30  can be increased in comparison with the system in which the electric motor is in operation during the entire range of the vehicle&#39;s speed. Furthermore, once the electromagnetic clutch  32  is disengaged, the electric motor  30  is not used. Therefore, it is possible to switch to the high-output driving generator  44  and uses it as a charging device or auxiliary power source. 
     As shown in  FIG. 2 , voltage boosted by the DC/DC converter  70  is supplied to the coil  33 A of the electromagnetic limited-slip differential gear (LSD)  33  via a voltage regulator  33 B. When voltage applied to the coil  33 A of the LSD  33  is low because a DC/DC converter  70  is not used, it is possible to operate the LSD  33  by increasing the current flowing through the coil  33 A. However, in this case, a large current needs to be provided. Consequently, problems arise in that the size of the LSD  33  increases and the amount of heat generated becomes high due to large current consumption. On the contrary, by using a DC/DC converter  70 , as shown in this embodiment, the size of the electromagnetic LSD can be reduced, thereby reducing the amount of heat generated. 
     Moreover, as shown in  FIG. 2 , voltage boosted by a DC/DC converter  70  is supplied to the coil  29 B of the electromagnetic brake  28 B via a voltage regulator  27 B. A vehicle, shown in  FIG. 1  (not shown in  FIG. 2 ), is equipped with four electromagnetic brakes  28 A,  28 B,  38 A and  38 B, and voltage boosted by a DC/DC converter  70  is supplied to each coil of each electromagnetic brake  28 A,  38 A and  38 B via a voltage regulator  27 B. When slipping occurs, the ABSCU  55  regulates the brake force applied to each of the four wheels according to each wheel&#39;s speed detected by rotary sensors  56 A,  56 B,  58 A and  58 B, thereby creating control to prevent the vehicle from slipping. When voltage that is applied to the coil  29 B of the electromagnetic brake  28 B is low because a DC/DC converter  70  is not used, it is possible to increase the force of the electromagnetic brake  28 B by increasing current flowing through the coil  29 B. However, in this case, a large current needs to be provided. Consequently, problems arise in that the size of the electromagnetic brake  28 B increases and the amount of heat generated becomes high due to large current consumption. On the contrary, by using a DC/DC converter  70 , as shown in this embodiment, the size of the electromagnetic brake can be reduced, thereby reducing the amount of heat generated. 
     Next, with reference to  FIG. 5 , operations of a DC/DC converter  70  used for a vehicle drive device according to this embodiment will be explained. 
       FIG. 5  is a circuit diagram showing the configuration of the DC/DC converter used for a vehicle drive device according to an embodiment of the present invention. Items in  FIG. 5  that have identical numbers to items in  FIG. 1  are identical. 
     The power source  49  consists of an auxiliary generator  40  and an auxiliary battery  42 , and is made up of a general charge and discharge system among electric loads on the 12-V power source. The DC/DC converter  70  is connected to the power source  49  consisting of a battery  42  and auxiliary generator  40 , and to the 4WDCU  60 . The DC/DC converter  70  is equipped with a coil  71 , transistor  72 , capacitor  73 , and diodes  75 A and  75 B. The coil  71  is connected to the input terminal of the DC/DC converter  70 . The transistor  72  and capacitor  73  are connected in parallel with the power source  49  and loads. Furthermore, a diode  75 A is connected between the transistor  72  and the positive end of the capacitor  73 , and a diode  75 B is connected in parallel with the transistor  72 . 
     When the transistor  72  is oscillated by the 4WDCU  60  by means of the pulse-width modulation (PWM), the electric power is stored in the coil  71  when the switch is turned on, and the stored power is discharged when the switch is turned off. As a result, boosted voltage (steady-state and no loss), which is calculated as shown below, can be obtained.
 
 V out=( T on+ T off)/ T off× V in  (1)
 
     Herein, Vout is a voltage outputted from the DC/DC converter, Ton is time duration when the transistor  72  is turned on, Toff is time duration when the transistor  72  is turned off, and Vin is a voltage inputted into the DC/DC converter. For example, if Ton=Toff, the output voltage is boosted 200%. 
     Furthermore, current outputted from the diode  75  is smoothed by a capacitor  73 , and current shown below flows when the voltage is steady and there is no power loss.
 
 I out= I in·( V in/ V out)  (2)
 
     Herein, Iout is an output current of the DC/DC converter, and Iin is an input current of the DC/DC converter. 
       FIG. 5  shows a non-insulative DC/DC converter, however, an insulative DC/DC converter can be used. A boost-type DC/DC converter that uses a transformer can also be used. 
     Furthermore, instead of using an electric motor  30 , it is possible to use an electric generator (motor/generator) in such a way that an electric generator is utilized as a generator during high-speed traveling or climbing hills, and the generator generates power and charges the battery, thereby obtaining a braking force such as regenerative braking and power generation braking force. 
     As stated above, according to this embodiment, output voltage boosted by a DC/DC converter operates an electromagnetic clutch, thereby making it possible to increase the engagement force of the electromagnetic clutch. Specifically, in the configuration in which output torque of the electric motor can be increased by controlling field voltage of the high-output generator and electric motor by means of output voltage boosted by a DC/DC converter, it is possible to reduce slipping of the electromagnetic clutch and effectively use output torque of the electric motor as drive torque. 
     Furthermore, by operating an electromagnetic limited-slip differential gear by means of output voltage boosted by a DC/DC converter, it is possible to reduce the size of the electromagnetic limited-slip differential gear and also reduce heat generation. 
     Moreover, by operating an electromagnetic brake by means of output voltage boosted by a DC/DC converter, it is possible to reduce the size of the electromagnetic brake and also reduce heat generation. 
     Next, with reference to  FIG. 6 , the control mechanism of the vehicle drive device according to this embodiment will be explained. 
       FIG. 6  is a flow chart showing the control mechanism of the vehicle drive device according to an embodiment of the present invention. 
     When a driver of an electric vehicle turns on the ignition switch (SW) in step s 100  and the driver of the same electric vehicle turns on the manual 4WD switch (M 4WD SW) in step s 105 , the 4WDCU  60  starts controlling to reduce backlash of the clutch  32 . The manual 4WD switch, not shown in  FIG. 1 , is a switch for a driver at any time to switch between 2WD and 4WD. When a driver wants the electric four-wheel vehicle to operate in four-wheel drive, turning on the switch will operate the vehicle as an electric four-wheel drive vehicle. Turning the switch off will allow only an engine to drive the vehicle. In the initial condition of the clutch  32 , backlash has not been reduced, and therefore, if the vehicle starts in that condition, an impact may occur. Accordingly, backlash reduction control of the clutch  32  is executed in step s 110 . 
     When backlash reduction control of the clutch  32  starts, in step s 115 , the 4WDCU  60  turns on the power-source relay (M4WD RLY), not shown, of the manual 4WD control system. Subsequently, electric power is supplied to the alternator field and the motor field, thereby enabling the backlash reduction control. Then, in step s 120 , the 4WDCU  60  turns on the DC/DC converter  70 , and then the 4WDCU  60  turns on and off the transistor switch  72  of the DC/DC converter  70 , shown in  FIG. 5 , and starts DC/DC conversion. 
     In step s 125 , the 4WDCU  60  turns on the clutch  32 , and in step s 130 , the 4WDCU  60  turns on the field current of the electric motor  30 , and in step s 135 , the 4WDCU  60  turns on the 42-V relay (not shown), and in step s 140 , the 4WDCU  60  turns on the field current of the alternator  44 . Thus, in step s 145 , backlash reduction control of the clutch is completed. 
     Next, when the 4WDCU  60  detects that an accelerator is turned on in step s 150 , in step s 155 , the 4WDCU  60  starts to control the field current of the electric motor  30  and the field current of the alternator  44 . 
     When the 4WDCU  60  detects that an accelerator is turned off in step s 160 , in step s 115 , the 4WDCU  60  starts the stop sequence control of the manual 4WD control system. Subsequently, in step s 170 , the 4WDCU  60  turns off the clutch  32 , and in step s 175 , the 4WDCU  60  turns off the field current of the electric motor  30 , and in step s 180 , the 4WDCU  60  turns off the 42-V relay (not shown), and in step s 185 , the 4WDCU  60  turns off the field current of the alternator  44 . In step s 190 , the 4WDCU  60  turns off the power-source relay (M4WD RLY), not shown, of the manual 4WD control system, and in step s 195 , the 4WDCU  60  turns off the DC/DC converter  70 . Thus, in step s 200 , the stop sequence control of the manual 4WD control system is completed. 
     When a driver of an electric vehicle turns off the manual 4WD switch (M4WD SW) in step s 205 , and the driver of the same electric vehicle turns off the ignition switch (SW) of the electric vehicle in step s 210 , the control is completed. 
     Next, with reference to  FIG. 7 , the configuration and operations of a vehicle drive device according to another embodiment of the present invention will be explained. The configuration of an electric four-wheel drive vehicle that uses a vehicle drive device according to this embodiment is the same as that shown in  FIG. 1 . 
       FIG. 7  is a schematic diagram showing the configuration of a vehicle drive device according to another embodiment of the present invention. Items in  FIG. 7  that have identical numbers to items in  FIGS. 1 and 2  are identical. 
     The characteristic of this embodiment is to use an electric DC motor  30 A although a vehicle drive device shown in  FIG. 1  or  FIG. 2  uses an electric AC motor  30 . 
     The DC/DC converter  70  boosts output voltage of the power source  49  including an auxiliary battery and supplies electric power to the field coil  47  of the generator  44 , field coil  31  of the electric DC motor  30 A, and the coil  32 A of the electromagnetic clutch  32 . 
     Therefore, according to this embodiment, by operating an electromagnetic clutch by means of the output voltage boosted by a DC/DC converter, it is possible to increase the engagement force of an electromagnetic clutch. Specifically, in the configuration in which output torque of the electric motor can be increased by controlling the field voltage of the high-output generator and electric DC motor by means of the output voltage boosted by a DC/DC converter, it is possible to reduce the electromagnetic clutch slipping and effectively use output torque of the electric motor as drive torque. 
     The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof.