Vehicular control apparatus

A vehicle control apparatus includes a PHEV-ECU containing a measuring unit which measures the amount of torsional stress on the front drive shafts in a parking lock state and a control unit which controls the torque of the front motor. When the parking lock state is released, the torque acting as a load on the rotation of the first motor upon the release of the torsional stress on the drive shaft, is determined based on the torsional stress measured. Further, the front motor is controlled to output the determined torque.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2013-153798, filed Jul. 24, 2013, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a control apparatus for vehicles which produce a driving force with a motor, and more specifically, a vehicular control apparatus with an improved parking lock release operation.

2. Description of the Related Art

In general, the parking lock units of automobiles are configured to lock the front reduction gear from rotating.

When an automobile is parked on a slope, the front wheels are still forced to rotate even in the parking lock state. Here, since the rotation of the front drive shaft is inhibited by the parking lock unit, the front drive shaft does not substantially rotate. However, a small torsional stress is exerted on the front shaft drive, and therefore actually, the automobile moves slightly when the parking lock is executed.

Then, when the parking lock is released, the torsion stress on the front drive shaft is released, which allows the front reduction gear and the front driving source to rotate abruptly. Because of such an abrupt rotation, vehicular passengers may feel a shock, producing a unpleasant feeling and discomfort.

Techniques for reducing shocks produced when releasing the parking lock such as above are disclosed in Publication 1 (Japanese Patent No. 4297135), Publication 2 (Jpn. Pat. Appln. KOKAI Publication No. 2009-143270) and the like.

Publication 1 teaches a technique of controlling the torque of an electric motor by estimating the torque due to torsion accumulated in a direct coupling range (the range maintained in a power transmission state to the driving wheels, which is on the automatic transmission or between the automatic transmission and driving wheels) while the motive power of the power source is being transmitted to the driving wheels, and controlling the torque of the electric motor based on the estimated torque due to torsion. With the technique of Publication 1, directed to the above-described effect, it is not possible to reduce the shock caused by the torsional stress on the front drive shaft in the parking state.

Publication 2 teaches a technique of producing a vibration controlling torque by detecting the degree of torsional vibration produced when the operation of the parking lock unit is complete, and producing a torque corresponding to the amplitude of the detected vibration. With the technique of Publication 2, directed to the above-described effect, it is not possible to inhibit the initial shock since a vibration controlling torque is not produced until the torsional vibration actually occurs.

BRIEF SUMMARY OF THE INVENTION

An object of the present invention is to provide a vehicular control apparatus configured to inhibit a shock from the initiation of the production thereof, the shock being caused by a torsional stress applied to the drive shaft during a stop state, as the stress is released when the parking lock is released.

The vehicular control apparatus according to the present invention comprises: a first motor configured to drive wheels of one of a front side and a rear side provided on a vehicle; a second motor configured to drive wheels of the other one of the front and rear sides provided on the vehicle; a drive shaft; a first transmission mechanism configured to transmit a driving force produced by the first motor to the wheels of the one of the front and rear sides; a second transmission mechanism configured to transmit a driving force produced by the second motor to the wheels of the other one of the front and rear sides; a parking lock unit configured to create a parking lock state which inhibits transfer of the driving force produced by the first motor; a lock control unit configured to control the parking lock unit to release the parking lock state in response to an operation which requests the release of the parking lock state; a measuring unit configured to measure a torsional stress on the drive shaft in the parking lock state based on a degree of rotation of each of the first motor and the second motor; a determination unit configured to determine a torque acting as a load on the rotation of the first motor, produced when the torsional stress on the drive shaft is released, based on the torsional stress measured by the measuring unit; and a motor control unit configured to control the first motor to output the torque determined by the determination unit.

With the present invention, when releasing the parking lock, the motor is controlled to output a torque to cancel the torsional stress of the drive shaft, based on the torsional stress on the drive shaft during the parking lock state, and thus the shock produced by the torsional stress on the drive shaft during the parking state can be inhibited from its initiation.

DETAILED DESCRIPTION OF THE INVENTION

A control apparatus according to an embodiment will now be described with reference toFIGS. 1 to 4.

It should be first noted that this embodiment will be described in connection with a plug-in hybrid automobile, but the present invention can be carried out similarly in other various types of automobiles as well, which produce a driving force by a motor. Further, the present invention is not necessarily limited to the control of automobiles, but can be applied to the control of various types of vehicles as long as they are of the types which produce a driving force by a motor.

FIG. 1is a diagram showing a structure of an automobile100. Note that the automobile100comprises a number of elements similar to those of other conventional hybrid automobiles, but only some of the elements are actually shown inFIG. 1.

The automobile100comprises a main body1, front wheels2aand2b, rear wheels3aand3b, front drive shafts4aand4b, rear drive shafts5aand5b, a front reduction gear6, a rear reduction gear7, an internal combustion engine8, a front motor9, a rear motor10, a generator11, a battery12, inverters13,14and15, electromagnetic contactors16a,16b,16c,17a,17band17c, an external power supply plug18, a charging device19, a parking unit (to be referred to as a P-lock unit, hereinafter)20, a power switch21, a multi-information display (MID)22, a vehicle speed sensor23, a shift unit24, a parking lock switch (to be referred to as a P-lock switch, hereinafter)25, an accelerator sensor26, a pitch angle sensor27, a parking-lock electronic control unit (P-lock ECU)28, a one-touch start system electronic control unit (OSS-ECU)29, an electronic time and alarm control system (ETACS-ECU)30, an engine electronic control unit (engine-ECU)31and a plug-in hybrid electric vehicle electronic control unit (PHEV-ECU)32.

The main body1comprises a chassis and a car body, which hold and support all the other elements and create a space (compartment) for passengers to ride in.

The front wheels2aand2bare secured to ends of the front drive shafts4aand4b, respectively, and the rear wheels3aand3bare secured to ends of the rear drive shafts5aand5b, respectively. The front wheels2aand2band the rear wheels3aand3bsupport the main body1as they touch the ground, and they rotate to move the main body1.

The front drive shafts4aand4bare configured to support the positions of the front wheels2aand2bon the main body1in a predetermined state with relative to each other and also to transmit a torque transmitted from the front reduction gear6to the front wheels2aand2b. The rear drive shafts5aand5bare configured to support the positions of the rear wheels3aand3bto the main body1in a predetermined state relative to each other and also to transmit a torque transmitted from the rear reduction gear7to the rear wheels3aand3b.

The front reduction gear6is configured to support the front drive shafts4aand4bto be individually rotatable. The front reduction gear6is coupled to a rotating shaft8aof the internal combustion engine8, a rotating shaft9aof the front motor9and a rotating shaft11aof the generator11, individually.

The front reduction gear6consists of various types of gears including a differential gear, shafts, clutches and the like, as they assembled together as conventionally known. The front reduction gear6selectively takes the form of the state in which, the rotating shaft8ais coupled to the front drive shafts4aand4b, the rotating shaft8ais coupled to the rotating shaft11a, the torque of the rotating shaft8ais transmitted distributively to the front drive shaft4aand4band the rotating shaft11a, the rotating shaft9ais coupled to the front drive shafts4aand4b, the rotating shaft11ais coupled to the front drive shafts4aand4bor the front drive shafts4aand4bare allowed to rotate freely. Further, the front reduction gear6is configured to reduce the rate of rotation of the engine8and front motor9and transmit the reduced rotation to the front drive shafts4aand4b.

The rear reduction gear7is configured to support the rear drive shafts5aand5bto be individually rotatable. The rear reduction gear7is coupled with a rotating shaft10aof the rear motor10. The rear reduction gear7consists of various types of gears including a differential gear, shafts, clutches and the like, as they assembled together as conventionally known. The rear reduction gear7selectively takes the form of the state in which, the rotating shaft10ais coupled to the rear drive shafts5aand5bor the rear drive shafts5aand5bare allowed to rotate freely. Further, the rear reduction gear7is configured to reduce the rate of rotation of the rear motor10and transmit the reduced rotation to the rear drive shafts5aand5b.

The internal combustion engine8is configured to produce the torque from the combustion of fuel to rotate the rotating shaft8a. As a typical case, the internal combustion engine8uses gasoline as the fuel. But the fuel is not limited to this, and the engine may be of another type which uses a fuel other than gasoline, for instance, another fuel oil such as light oil, or liquefied petroleum gas (LPG). When the front reduction gear6couples the rotating shaft8aand the front drive shafts4aand4bto each other, the internal combustion engine8turns the front wheels2aand2b.

The front motor9and the rear motor10produce torques from electrical energy, to turn the rotating shafts9aand10a. When the front reduction gear6couples the rotating shaft8aand the front drive shaft4aand4bto each other, the front motor9turns the front wheels2aand2b. When the rear reduction gear7couples the rotating shaft10aand the rear drive shafts5aand5bto each other, the rear motor10turns the rear wheels3aand3b. In this manner, the front motor9and rear motor10function as the first and second motors, respectively. The front motor9and rear motor10are equipped with rotational angle sensors9band10b. The rotation angle sensors9band10bare configured to detect rotational phases of the front motor9and rear motor10.

The front drive shafts4aand4band the front reduction gear6described above constitute a first transmission mechanism configured to transmit a driving force to the front wheels2aand2bof one of the two pairs of wheels. Further, the front drive shafts4aand4bfunction as first drive shafts and the front reduction gear6functions as a first reduction gear. On the other hand, the rear drive shafts5aand5b, which function as second drive shafts, and the rear reduction gear7, which function as second reduction gear, constitute a second transmission mechanism configured to transmit a driving force to the rear wheels3aand3bof the other one of the two pairs of wheels. It should be noted here that the drive shafts referred to in connection with the present invention are equivalent to, in this embodiment, the first drive shafts, that is, the front drive shafts4aand4b.

The generator11is configured to utilize the rotation of the rotating shaft11ato produce electricity by electromagnetic induction. When the front reduction gear6couples the rotating shaft8aand the rotating shaft11ato each other, the generator11produces electricity by utilizing the torque produced by the internal combustion engine8. When the front reduction gear6couples the front drive shafts4aand4bwith the generator11to each other, the generator11produces electricity by utilizing the torques of the front drive shafts4aand4b.

The battery12applies a direct current.

The inverters13and14are configured to convert the direct current output from the battery12into an alternating current. The inverters13and14may be of a conventional structure which contains switching elements including insulated gate bipolar transistor (IGBT). The inverter13is configured to supply electrical energy to the front motor9by applying the alternating current to the front motor9. The inverter14is configured to drive the rear motor10by supplying the alternating current to the rear motor10. The inverters13and14are configured to change a switching frequency of a switching element, an output current and a frequency (output frequency) thereof, under the control of PHEV-ECU32.

The inverter15is configured to convert an alternating current produced by the generator11into a direct current. The direct current obtained by the converter15is supplied to the battery12.

The contactors16a,16band16care inserted between the positive electrode of the battery12and the inverters13,14and15, respectively. The contactors16a,16band16care configured to electrically connect or disconnect (turn on/off) the contact between the positive electrode of the battery12and the inverters13,14and15, respectively, under the control of PHEV-ECU32.

The contactors17a,17band17care inserted between the negative electrode of the battery12and the inverters13,14and15, respectively. The contactors17a,17band17care configured to electrically connect or disconnect (turn on/off) the contact between the negative electrode of the battery12and the inverters13,14and15, respectively, under the control of PHEV-ECU32.

The external power supply plug18is configured to receive a cable to be connected thereto for power supply from an external power source, if necessary. When a cable is connected thereto, the external power supply plug18serves to electrically connect the cable and the charge device19.

The charge device19is configured to charge the battery12by the power supplied from the external power source via the cable connected to the external power supply plug18.

The P-lock unit20is configured to selectively constitute a lock state in which the front drive shafts4aand4bare locked by mechanical engagement not to rotate or an unlock state in which the lock is released. The lock state is equivalent to the parking lock state in which the transfer of the driving force by the first reduction gear is inhibited.

The power switch21is configured to be operated by a user to instruct a start and stop of the automobile100.

The multi-information display22is mounted in, for example, a meter panel equipped in the main body1, and configured to indicate various types of information including shift positions.

The vehicle speed sensor23is configured to detect the running speed of the automobile100based on the rate of rotation of, for example, the rear drive shaft5b.

The shift unit24includes a shift lever and a group of sensors to detect the position of the shift lever. The shift unit24is configured to enter an instruction from the driver regarding the changing of a running mode (shift position) in response to the operation of the shift lever by the driver.

The P-lock switch25is configured to enter an instruction from the driver regarding the switching between the lock state and unlock state of the P-lock unit25in response to the operation of the shift lever by the driver.

The accelerator sensor26is configured to detect the degree of opening of the throttle as the degree of depression of the accelerator pedal, not shown in the figure.

The pitch angle sensor27is configured to detect the angle of pitch of the main body1.

The P-lock ECU28is configured to control the P-lock unit20.

The OSS-ECU29is configured to perform power control of each member after authentication by communications when the power switch21is operated by the user.

The ETACS-ECU30is configured to control various types of electrical equipment mounted in the automobile100. The electrical equipment subjected to the control of the ETACS-ECU30is, for example, a multi-information display22, and also a headlight, a door mirror, a windshield wiper, a door lock mechanism, interior lighting equipment, a security alarm, which are omitted fromFIG. 1, and the like.

The ETACS-ECU30is configured to obtain necessary information from the OSS-ECU29, engine ECU31and PHEV-ECU32as needed, by telecommunications, and control this electrical equipment to realize predetermined operations. For example, the ETACS-ECU30automatically extends the door mirror from a retracted position, if so, when the vehicle speed is greater than or equal to a predetermined value. Further, the ETACS-ECU30controls the multi-information display22to display a shift position corresponding to a running mode of the automobile100.

The engine ECU31is configured to control the operation of the internal combustion engine8. The engine ECU31is also configured to obtain from the ETACS-ECU30and the PHEV-ECU32information necessary to carry out various types of control, as needed, by telecommunications.

The PHEV-ECU32is configured to perform various types of control for the running of the automobile100. For example, the PHEV-ECU32controls the states of the front reduction gear6and rear reduction gear7in accordance with the running state of the automobile100. Further, the PHEV-ECU32controls the states of the inverters13and14and the contactors16a,16b,16c,17a,17band17c. For example, in an electric vehicle (EV) mode running state, the PHEV-ECU32sets the front reduction gear6in a state to couple the rotating shaft9aof the front motor9and the front drive shafts4aand4bwith each other. Further, the PHEV-ECU32sets the rear reduction gear7in a state to couple the rotating shaft10aof the rear motor10and the rear drive shafts5aand5bwith each other. At the same time, the PHEV-ECU32sets all of the contactors16a,16b,16c,17a,17band17cto be on.

With this state, the PHEV-ECU32calculates a required running power according to the degree of opening of the accelerator detected by the accelerator sensor26. Then, the PHEV-ECU32controls the outputs of the inverters13and14and thus drive the front motor9and the rear motor10so as to obtain the calculated running power. Furthermore, the PHEV-ECU32controls the states of the front reduction gear6, the rear reduction gear7, the inverters13and14and the contactors16a,16b,16c,17a,17band17cso as to establish various types of operation states realized for other conventional hybrid cars. The PHEV-ECU32obtains information necessary information for various types of controls from the P-lock ECU28, the ETACS-ECU30and the engine ECU31by telecommunications, as needed.

Incidentally, the PHEV-ECU32is equipped with a computer comprising a central processing unit (CPU), a read-only memory (ROM), a random-access memory (RAM) and a non-volatile memory. The PHEV-ECU32realizes out the above-described controls by carrying out processing according to programs stored in the ROM while the CPU accessing the RAM and non-volatile memory as needed. The ROM or non-volatile memory is configured to store a pattern data table.

FIG. 2is a diagram schematically showing an example of the pattern data table.

As shown inFIG. 2, the pattern data table indicates the torque difference D and four torque request patterns while they are associated with each other. Each torque request patterns indicate a change in the torque to be requested along with time.

Next, the operation of the automobile100having the above-described structure will be described.

While a current is being supplied thereto, the PHEV-ECU32executes the parking lock control process shown inFIG. 3. It should be noted here that the contents of the process described below are only one example, and various types of processes which enable to obtain similar results can be used as needed.

In step Sa1, the PHEV-ECU32determines whether or not the parking lock unit20is in the lock state. When it is determined as NO, that is, the parking lock unit20is in the unlock state, the PHEV-ECU32proceeds to step Sa2.

In step Sa2, the PHEV-ECU32stands by for the P-lock switch25to be turned on. When the P-lock switch25is turned on by the operation of the driver, the PHEV-ECU32determine as YES and proceeds to step Sa3.

In step Sa3, the PHEV-ECU32substitutes the rotational phases of the front motor9and the rear motor10detected respectively by the rotational angle sensors9band10bat this point, into variables Pf1and Pr1, respectively.

In step Sa4, the PHEV-ECU32instructs the P-lock-ECU28to set the P-lock unit20in the lock state. Upon this instruction, the P-lock-ECU28sets the P-lock unit20in the lock state.

FIG. 4is a timing chart showing the change in the state of the automobile100in connection with the parking lock controlling process.

In general, the movement of the automobile100is controlled with a footbrake, not shown in the figure. When the automobile100is stopped, the P-lock switch25is turned on while the footbrake is still in the braking state. Then, after the P-lock unit20is set in the lock state, the footbrake is released. Thus, the braking with the footbrake is carried out in a period PA inFIG. 4, the state is shifted to the parking lock state at a time point Ta in the middle of the period. But, there may be also such a case where the footbrake is released beforehand, and the P-lock switch25is turned on afterwards.

In the parking lock state, the rotations of the front drive shafts4aand4bare inhibited with the P-lock unit20via the front reduction gear6. However, due to the play of the gear built in the front reduction gear6, and the like, the front motor9rotates in a period PB. During this period, the automobile100slightly moves to cause the rear wheels3aand3bto rotate. As a result, the rear motor10is rotated during the period PB.

When the front drive shafts4aand4bare completely locked so as not to rotate, the front motor9does not rotate any more as well. But, after this, a torsional stress is applied to the front drive shafts4aand4b, and thus the front wheels2aand2brotate in a period PC. Due to the rotation of the front wheels2aand2b, the automobile100further moves, and thus the rear wheels3aand3bfurther rotate as well. As a result, the rear motor10is rotated during the period PC.

As described above, the front motor9rotates during the period PB only, whereas the rear motor10rotates during the period PC as well in addition to the period PB. For this reason, the degree of rotation of the front motor9after being set in the parking lock state at the time point Ta is different from that of the rear motor10.

The degrees of rotation of the front motor9and the rear motor10due to the above-described phenomenon corresponds to the differences in rotational phase, ΔPf and ΔPr, of the front motor9and the rear motor10, respectively, before and after rotation.

In step Sa5, the PHEV-ECU32stands by for the standby time to elapse from the time when the parking lock state is set or the point when the footbrake is released, whichever the later. The standby time is predetermined as a longer period which can be resulted as the sum of the durations of the period PB and period PC. In other words, the PHEV-ECU32stands by for the timing which is considered that the above-described rotations of the front motor9and the rear motor10have been reliably stopped. Then, when the standby time elapses and it is determined as YES in step Sa5, the PHEV-ECU32proceeds to step Sa6. InFIG. 4, the time point Tb corresponds to this timing.

In step Sa6, the PHEV-ECU32substitutes the rotational phases of the front motor9and the rear motor10detected respectively by the rotational angle sensors9band10bat this point, into variables Pf2and Pr2, respectively.

In step Sa7, the PHEV-ECU32calculates the difference in rotation between the front wheels2aand2band the rear wheels3aand3b, and stores it in the non-volatile memory built in the PHEV-ECU32. More specifically, the difference can be obtained in the following manner. That is, the variables Pf1and Pr1represent the rotational phases before rotation, whereas the variables Pf2and Pr2represent the rotational phases after rotation. Therefore, ΔPf can be obtained by subtracting Pf2from Pf1. Further, ΔPr can be obtained by subtracting Pr1from Pr2. Then, the difference in rotation can be obtained by subtracting ΔPf from ΔPr. The difference in rotation calculated here represents the torsional stress in the front drive shafts4aand4b. Thus, the PHEV-ECU32functions as a measuring unit. After this, the operation of the PHEV-ECU32returns to step Sa1.

After the P-lock unit20is set in the lock state as described above, the PHEV-ECU32determines YES in step Sa1. In this case, the PHEV-ECU32proceeds to step Sa8.

In the meantime, in the example of the operation shown inFIG. 4, when the key ignition of the automobile100is turned off at the time Tc, which is later than the time Tb, the power supply to the PHEV-ECU32is stopped (OFF). Therefore, the PHEV-ECU32halts operation. But, it is preferable that the power supply to the PHEV-ECU32be continued until the PHEV-ECU32finishes the operation of step Sa7.

When the power supply is restarted at the time td, the PHEV-ECU32restarts the parking lock control process. In this case, since the P-lock unit20is in the lock state, the PHEV-ECU32determines YES in step Sa1, and proceeds to step Sa8.

In step Sa8, the PHEV-ECU32stands by for the shift lever to be operated. When the shift unit24detects that the shift lever is operated, the PHEV-ECU32determines as YES, and proceeds to step Sa9.

In step Sa9, the PHEV-ECU32reads the difference in rotation, stored in step Sa7from the non-volatile memory.

In step Sa10, the PHEV-ECU32determines a torque output pattern associated with the read difference in rotation in the pattern data table, as one to be used this time.

In step Sa11, the PHEV-ECU32starts a torque request to obtain a torque output according to the torque output pattern determined as above. In the example shown inFIG. 4, the shift lever is operated at the time Te, and the torque request is started in reply to this.

The torque request is passed to a drive control process executed in another task different from that of the parking lock control process executed by the PHEV-ECU32. With the drive control process, the front motor9is controlled to output a torque corresponding to the torque request. Thus, the PHEV-ECU32functions as a motor control unit.

Here, note that ΔPr is always larger than ΔPr in absolute terms. This is because ΔPf corresponds only to the rotation due to the play of the gears, whereas ΔPr includes an amount equivalent to the rotation caused by the torsional stress acting on the front drive shafts4aand4bin addition to the rotation due to the play of the gears. Therefore, when the front motor9rotates in a forward direction as shown inFIG. 4, the difference in rotation takes a positive value. Here, when the difference in rotation amount takes a positive value, a torque request is made to output a torque in the backward direction as shown inFIG. 4.

In step Sa12, the PHEV-ECU32instructs the P-lock-ECU28to release the parking lock at a predetermined timing synchronized with the torque request. In accordance with this instruction, the P-lock-ECU28unlocks the P-lock unit20. Thus, the PHEV-ECU32functions as a lock control unit. After that, the PHEV-ECU32returns to step Sa1.

In other words, the parking lock is released at the time Tf, which is later than the time Te. Then, in the period PD which starts from the time Tf, the front motor9is rotated as the torsional stress acting on the front drive shafts4aand4bis released. In the example shown inFIG. 4, the rotation of the front motor9is in the forward direction. But, before the front motor9starts to rotate, a backward torque is output to the front motor9. This torque serves as a load on the above-described rotation of the front motor9, and thus the rotation of the front motor9is gentle.

That is, when no measures are taken, the front motor9rotates with the production of vigorous fluctuations represented by a broken line W inFIG. 4since the torsional stress acting on the front drive shafts4aand4bis abruptly released. However, according to the embodiment, the front motor9rotates only with the production of the small fluctuations represented by a solid line inFIG. 4. This result indicates that with this embodiment, the shock produced upon release of the parking lock can be reduced as compared to the case where no measures are taken.

According to this embodiment, the torsional stress on the front drive shafts4aand4bis calculated in advance as the difference in rotation between the front motor9and the rear motor10when the parking lock state is set. Then, before the parking lock is released, the output of the torque is started. In this manner, the rotation of the front motor9which occurs upon the release of the torsional stress on the front drive shafts4aand4bcan be suppressed from the beginning.

It should be noted here thatFIGS. 2 and 4schematically show an example of a roughly estimated tendency of the torque request pattern and thus the actual torque request pattern may differ from that is shown in these figures. For example, the difference in torque between patterns is exaggerated. In reality, experiments and simulations, etc., are carried out, and based on the actual operating characteristics of the automobile100, torque request patterns are determined. From the determined patterns, a pattern data table is formed. The number of torque request patterns in the pattern data table may be any of two or more. Each and every torque request pattern is determined to such a degree that the automobile100does not move.

When the front motor9and rear motor10rotate in the backward direction immediately after being set in the parking lock state, the difference in rotation takes a negative value. Therefore, as shown inFIG. 2, a torque request pattern for outputting a forward torque, which is reverse to the aforementioned case, is selected. Thus, a torque which serves as a load on the backward direction rotation of the front motor9upon release of the torsional stress on the front drive shafts4aand4bis output by the front motor9.

Therefore, regardless of whether the automobile100is in a forward-tilted or backward-tilted attitude, the shock caused upon the release of the parking lock can be reduced.

It should be noted here that the present invention is not limited to the embodiment described above, but modified into various versions as will now be described.

That is, the P-lock unit20may be provided in the rear reduction gear7. In this case, the torsional stress is applied to the rear drive shafts5aand5b, and therefore the rear motor10is controlled to output a torque.

The torsional stress on the drive shafts can be estimated based on the angle of pitch of the main body1as well. In other words, the pattern data table may be determined to write the angle of pitch and torque request pattern while being associated with each other, and also to use torque request patterns associated with angles of pitch detected with the pitch angle sensor27while in the parking lock state.

The torque request may be started substantially at the same timing as that of the release of the parking lock state.

The computer comprising the CPU, ROM, RAM and nonvolatile memory may be mounted on the inverter13.

The connection state between the rotating shaft8aand the front drive shafts4aand4b, the connection state between the rotating shaft8aand the rotating shaft11a, the connection state between the rotating shaft9aand the front drive shafts4aand4b, or the connection state between the rotating shaft11aand the front drive shafts4aand4bmay not be selective, but may be physically connected from the beginning.

Similarly, the connection state between the rotating shaft10aand the rear drive shafts5aand5bmay not be selective, but may be physically connected from the beginning.