Vehicle drive system

This application is a continuation of U.S. application Ser. No. 13/723,962, filed on Dec. 21, 2012 which is based upon and claims priority/priorities from Japanese Patent Application Nos. 2011-281854 filed on Dec. 22, 2011 and 2012-020110 filed on Feb. 1, 2012, the entire contents of which are incorporated herein by reference.

The present invention relates to a vehicle drive system in which a left wheel drive system for driving a left wheel and a right wheel driving system for driving a right wheel are provided.

JP-3138799-B describes a vehicle drive system that includes a left-wheel drive unit having a first motor for driving a left wheel of a vehicle and a first epicyclic transmission that is provided on a power transmission line between the first motor and the left wheel and a right-wheel drive unit having a second motor for driving a right wheel of the vehicle and a second epicyclic transmission that is provided on a power transmission line between the second motor and the right wheel. In the first and second epicyclic transmissions, the first and second motors are individually connected to sun gears, and the left and right wheels are individually connected to planetary carriers, ring gears being coupled to each other. Brake devices are provided for controlling the rotation of the ring gears by bringing the coupled ring gears into disengagement from or engagement with each other.

In the above-described vehicle drive system, a start assist control is executed when the vehicle is started by applying the brake devices. Further, after the vehicle is started, by executing a left and right opposite torque control so that the first and second motors produce torque in opposite directions with the brake devices released, even when a yawing moment is applied to the vehicle by disturbance, a moment that is opposite to the yawing moment is produced so as to increase the straight line and turning vehicle stabilities.

In recent years, there are strong demands for energy saving and improved fuel economy. In the vehicle drive system of JP-3138799-B, there is still room for improvement.

One object thereof is to provide a vehicle drive system that enables energy saving and an improvement in fuel economy by controlling either of first and second motors to attain its target speed while producing a desired yawing moment when a left and right opposite torque control is executed.

The first aspect of the present invention defines a vehicle drive system, including:

a left-wheel drive unit having

a first motor for driving a left wheel of a vehicle and a first transmission that is provided on a power transmission line between the first motor and the left wheel;

a right-wheel drive unit having

a second motor for driving a right wheel of the vehicle and a second transmission that is provided on a power transmission line between the second motor and the right wheel; and a motor control unit for controlling the first motor and the second motor,

wherein the first and second transmissions each has a first to third rotational elements,

wherein the first motor is connected to the first rotational element of the first transmission,

wherein the second motor is connected to the first rotational element of the second transmission,

wherein the left wheel is connected to the second rotational element of the first transmission,

wherein the right wheel is connected to the second rotational element of the second transmission,

wherein the third rotational element of the first transmission and the third rotational element of the second transmission are coupled to each other, and

wherein the first and second transmissions each further has a fourth rotational element which are supported so as to revolve around by the second rotational element while meshing with the first rotational element and the third rotational element,

wherein the motor control unit is configured to:

to control the first motor based on a first target revolution state quantity of the first transmission which is induced so that a rotating direction of the fourth rotational element of the first transmission rotating in one direction or an other direction is not reversed, and/or to control the second motor based on a second target revolution state quantity of the second transmission which is induced so that a rotating direction the fourth rotational element of the second transmission rotating in one direction or an other direction is not reversed, to thereby control the first motor and the second motor such that the rotating direction of the fourth rotational element of the first transmission and/or the rotating direction of the fourth rotational element of the second transmission are/is not reversed.

The first aspect of the present invention further defines, the system

wherein there is further provided a connection/disconnection unit that can be released or applied and which slows a rotation of the third rotational elements by being applied,

wherein the motor control unit controls the first motor and the second motor further based on a target revolution state quantity of the coupled third rotational elements, and

wherein, when the connection/disconnection unit is released, the target rotation state quantity of the third rotational elements is set so that the third rotational elements are put in a substantially zero rotation state, and the connection/disconnection unit is applied when the third rotational elements are put in the substantially zero rotation state.

The first aspect of the present invention further defines, the system

wherein there is further provided a rotational-direction restriction unit that permits a rotation of the third rotational elements in one direction based on backward torque of the first and second motors when disengaged, and which restricts a rotation of the third rotational elements in an other direction based on forward torque of the first and second motors when engaged, and

wherein the motor control unit controls the first motor and the second motor so that the rotating direction of the fourth rotation element rotating in the one direction is not reversed as a result of non-engagement of the rotational-direction restriction unit.

The first aspect of the present invention further defines, the system

wherein the motor control unit controls the first motor and the second motor so that the third rotational elements are put in a substantially zero rotation state rotating in the one direction as a result of non-engagement of the rotational-direction restriction unit.

The first aspect of the present invention further defines, the system

wherein the first and second transmissions are planetary gear mechanisms, and

wherein the first rotational elements are sun gears, the second rotational elements are carriers, the third rotational elements are ring gears, and the fourth rotational elements are planetary gears.

The first aspect of the present invention further defines, the system

wherein the motor control unit is configured to:

to control the first motor based on the first target revolution state quantity of the first transmission which is induced so that the rotating direction of the fourth rotational element of the first transmission rotating in the one direction or the other direction is not reversed, to control the second motor based on the second target revolution state quantity of the second transmission which is induced so that the rotating direction the fourth rotational element of the second transmission rotating in the one direction or the other direction is not reversed, and to control the first motor and the second motor such that the rotating direction of the fourth rotational element of the first transmission and the rotating direction of the fourth rotational element of the second transmission are not reversed by satisfying either one of the first target revolution state quantity and the second target revolution state quantity if both of them cannot be satisfied.

According to the first aspect of the present invention, by executing the left and right opposite torque control in which the first motor and the second motor produce the torque in the opposite directions, even when the third rotational elements are not locked, left and right opposite torque can be produced in the left and right wheels so as to produce an initial yawing moment. Further, either of the first and second motors can be controlled to attain its target speed. By so doing, it is possible to realize energy saving and an improvement in fuel economy.

According to the first aspect of the invention, it is possible to reduce the consumed electric power by obtaining the target revolution state quantity of the one motor based on the efficiency of the one motor and/or the electric power supply unit. In other words, by making use of the advantage that an arbitrary speed can be attained, it is possible to realize a state where the consumed electric power becomes least.

According to the first aspect of the invention, since the efficiencies of the motor and the electric power converter occupy a large portion of the efficiency of the electric power supply system, it is possible to reduce the consumed electric power more by obtaining the target revolution state quantity of the one motor based on the efficiencies of both the motor and the electric power converter.

According to the first aspect of the invention, when obtaining efficiency in an experimental fashion, the preparation of an efficiency map is facilitated, and when obtaining efficiency through sequential detection and estimation, it is possible to reduce a control quantity.

According to the first aspect of the invention, a desired rotation state quantity can be produced in the third rotational elements, and it is possible to put the third rotational elements in a state where the rotational loss is small.

According to the first aspect of the invention, by slowing the third rotational elements by applying the connection/disconnection unit, it is possible to transmit the torque in the same direction of the first and second motors to the wheels.

According to the first aspect of the invention, by applying the connection/disconnection unit when the rotation speed of the third rotational elements is reduced to the substantially zero rotation state, it is possible to reduce the shock at the time of application of the connection/disconnection unit and the deterioration thereof.

According to the first aspect of the invention, by providing the rotational-direction restriction unit on the third rotational elements, it is possible to transmit mechanically the forward torque of the first and second motors to the wheels without consuming energy for application of the connection/disconnection unit.

According to the first aspect of the invention, with the rotational-direction restriction unit disengaged, it is possible to realize a state where the rotational loss of the third rotational elements is small.

According to the first aspect of the invention, since a shock is transmitted to the wheels when the rotational-direction restriction unit is engaged, the rotation in one way is maintained to thereby avoid the engagement of the rotational-direction restriction unit, thereby making it possible to suppress the shock produced when the rotational-direction restriction unit is engaged.

According to the first aspect of the invention, since the target revolution state quantity of the one motor is obtained based on the rotations of the second rotational elements or the wheels in addition to the target rotation state quantity of the third rotational elements, it is possible to control the rotation of the third rotational elements more accurately.

According to the first aspect of the invention, since the target torque state quantity of the other motor is obtained based on the target turning state quantity of the vehicle, it is possible to increase the turning performance of the vehicle.

According to the first aspect of the invention, by making the target torque state quantity half the target torque difference between the first motor and the second motor, it is possible to put the vehicle in its target turning state in an ensured fashion.

According to the first aspect of the invention, by controlling the absolute value of the torque of the one motor that is controlled based on the target revolution state quantity to be larger than the absolute value of the torque of the other motor, it is possible to transmit the torque of the other motor that is controlled based on the target torque state quantity to the wheels at all times, whereby the yawing moment of the vehicle is not changed, thereby making it possible to stabilize the behaviors of the vehicle.

According to the first aspect of the invention, the torque that increases the speeds of the motors is torque in the forward direction, and in the event that the forward torque is applied to the motor that is producing torque in the backward direction, the torque in the forward direction is cancelled by the torque in the backward direction or vise versa. Therefore, by adding the correction torque to the motor of the first and second motors that is producing the forward torque when the target revolution state quantity of the one motor is higher than the target revolution state quantity of the other motor, it is possible to suppress the cancellation of the torques.

According to the first aspect of the invention, it is possible to control the rotational state of the fourth rotational elements that mesh with the first and third rotational elements as required.

According to the first aspect of the invention, it is possible to prevent the generation of a backlash due to a reverse of the rotational direction of the fourth rotational elements, thereby making it possible to prevent a disturbance in torque produced in the wheels that would otherwise be generated due to the backlash.

According to the first aspect of the invention, by causing the prevention of generation of a backlash to take priority over the efficiency of the motor, it is possible to increase the comfortableness in the vehicle.

According to the first aspect of the invention, by causing the control of the rotating state of the third rotational elements to take priority over the prevention of generation of a backlash, it is possible to prevent the occurrence of a shock as when the connection/disconnection unit is applied or released in an ensured fashion, thereby making it possible to increase the stability of the vehicle.

According to the first aspect of the invention, by using the general-purpose planetary gear mechanisms, it is possible to reduce the costs.

FIG. 1 is a block diagram showing a schematic configuration of a hybrid vehicle in which a vehicle drive system according to embodiments can be mounted.

FIG. 2 is a vertical sectional view of a rear-wheel drive system.

FIG. 3 is a partially enlarged view of the rear-wheel drive system shown in FIG. 2 .

FIG. 4 is a table depicting a relation between a front-wheel drive system and the rear-wheel drive system in various vehicle states together with operating states of motors.

FIG. 5 is a speed collinear diagram of the rear-wheel drive system when the vehicle is stopped.

FIG. 6 is a speed collinear diagram of the rear-wheel drive system when the vehicle is traveling forwards at low vehicle speeds.

FIG. 7 is a speed collinear diagram of the rear-wheel drive system when the vehicle is traveling forwards at intermediate vehicle speeds.

FIG. 8 is a speed collinear diagram of the rear-wheel drive system when the vehicle is decelerated for regeneration.

FIG. 9 is a speed collinear diagram of the rear-wheel drive system when the vehicle is traveling forwards at high vehicle speeds.

FIG. 10 is a speed collinear diagram of the rear-wheel drive system when the vehicle is reversed.

FIG. 11 is a timing chart while the vehicle is being driven.

FIG. 12 is a speed collinear diagram of the rear-wheel drive system when a left and right opposite torque control according to a first embodiment is executed (before a target revolution is attained).

FIG. 13 is a speed collinear diagram of the rear-wheel drive system that depicts a balance between a first motor torque and a balancing torque of a second motor torque in FIG. 12 .

FIG. 14 is a speed collinear diagram of the rear-wheel drive system when the left and right opposite torque control according to the first embodiment is executed (after the target revolution is reached).

FIG. 15 is a diagram depicting four patterns of the left and right opposite torque control of the first embodiment.

FIG. 16 is a flowchart depicting a flow of the left and right opposite torque control of the first embodiment.

FIG. 17 is a chart depicting yawing moment command signs and motor speed control direction signs.

FIG. 18 shows speed collinear diagrams of a left and right opposite torque control of the rear-wheel drive system according to a second embodiment in a time-series fashion, in which (a) is a speed collinear diagram depicting a balanced state, (b) is a speed collinear diagram depicting a state where a revolution matching is executed, and (c) is a speed collinear diagram depicting a state where hydraulic brakes are applied.

FIG. 19 is a diagram depicting two patterns of the left and right opposite torque control according to the second embodiment.

FIG. 20 is a flowchart depicting a flow of the left and right opposite torque control according to the second embodiment.

FIG. 21 shows speed collinear diagrams of a left and right opposite torque control of the rear-wheel drive system according to a third embodiment in a time-series fashion, in which (a) is a speed collinear diagram depicting a state where the hydraulic brakes are applied, (b) is a speed collinear diagram depicting a state where a revolution matching is executed, and (c) is a speed collinear diagram depicting a balanced state.

FIG. 22 is a diagram depicting a speed difference in a first motor and a speed difference in a second motor.

FIG. 23 is a flowchart depicting a flow of the left and right opposite torque control according to the third embodiment.

Firstly, one embodiment of a vehicle drive system will be described based on FIGS. 1 to 3 .

The vehicle drive system according to the embodiment is such as to use motors as drive sources for driving axles and is used in a vehicle that incorporates, for example, a drive system as shown in FIG. 1 . In the following description, the vehicle drive system will be described as being used as a rear-wheel drive system. However, the vehicle drive system may be used for a front-wheel drive system.

A vehicle 3 shown in FIG. 1 is a hybrid vehicle having a drive system 6 (hereinafter, referred to as a front-wheel drive system) in which an internal combustion engine 4 and a motor 5 are connected in series at a front part of the vehicle. Power of this front-wheel drive system 6 is transmitted to front wheels Wf via a transmission 7 , while power of a drive system 1 (hereinafter, referred to as a rear-wheel drive system) that is provided at a rear part of the vehicle separately from the front-wheel drive system 6 is designed to be transmitted to rear wheels Wr (RWr, LWr). The motor 5 of the front-wheel drive system 6 and first and second motors 2 A, 2 B of the rear-wheel drive system 1 on a rear wheel Wr side are connected to a battery 9 , so that an electric power supply from the battery 9 and energy regeneration to the battery 9 are enabled. Reference numeral 8 is a control unit for controlling variously the whole of the vehicle.

FIG. 2 is an overall vertical sectional view of the rear-wheel drive system 1 . In the same figure, reference numerals 10 A, 10 B denote left and right axles of the rear wheels Wr of the vehicle 3 , and the left and right axles are disposed coaxially in a width direction of the vehicle. A reduction gear case 11 of the rear-wheel drive system 1 is formed into a substantially cylindrical shape in whole. The first and second axle driving motors 2 A, 2 B and first and second epicyclic reduction gears 12 A, 12 B that decrease the speed of driving revolutions of the first and second motors 2 A, 2 B are disposed concentrically with the axles 10 A, 10 B in an interior of the reduction gear case 11 . The first motor 2 A and the first epicyclic reduction gear 12 A function as a left-wheel drive unit for driving a left rear wheel LWr, and the second motor 2 B and the second epicyclic reduction gear 12 B function as a right-wheel drive unit for driving a right rear wheel RWr. The first motor 2 A and the first epicyclic reduction gear 12 A and the second motor 2 B and the second epicyclic reduction gear 12 B are disposed symmetric laterally in the width direction within the reduction gear case 11 .

Stators 14 A, 14 B of the first and second motors 2 A, 2 B are fixed to insides of both left and right end portions of the reduction gear case 11 , and annular rotors 15 A, 15 B are disposed rotatably on inner circumferential sides of the stators 14 A, 14 B. Cylindrical shafts 16 A, 16 B that surround outer circumferences of the axles 10 A, 10 B are connected to inner circumferential portions of the rotors 15 A, 15 B. These cylindrical shafts 16 A, 16 B are supported on end walls 17 A, 17 B and intermediate walls 18 A, 18 B of the reduction gear case 11 via bearings 19 A, 19 B so as to rotate relative to and concentric with the axles 10 A, 10 B. Resolvers 20 A, 20 B that feed information on rotational positions of the rotors 15 A, 15 B back to a controller (not shown) for controlling the first and second motors 2 A, 2 B are provided on outer circumferences of one end portions of the cylindrical shafts 16 A, 16 B and on the end walls 17 A, 17 B of the reduction gear case 11 .

The first and second epicyclic reduction gears 12 A, 12 B include sun gears 21 A, 21 B, plural planetary gears 22 A, 22 B that are caused to mesh with the sun gears 21 , planetary carriers 23 A, 23 B that support these planetary gears 22 A, 22 B and ring gears 24 A, 24 B that are caused to mesh with outer circumferential sides of the planetary gears 22 A, 22 B. Driving forces of the first and second motors 2 A, 2 B are inputted into the first and second epicyclic reduction gears 12 A, 12 B from the sun gears 21 A, 21 B and the decelerated driving forces are outputted therefrom through the planetary carriers 23 A, 23 B.

The sun gears 21 A, 21 B are formed integrally with the cylindrical shafts 16 A, 16 B. For example, as shown in FIG. 3 , the planetary gears 22 A, 22 B are double pinions that have first pinions 26 A, 26 B that are larger in diameter and which are caused to mesh directly with the sun gears 21 A, 21 B and second pinions 27 A, 27 B that are smaller in diameter than the first pinions 26 A, 26 B, and the first pinions 26 A, 26 B and the second pinions 27 A, 27 B are formed integrally in such a manner that the first and second pinions are concentric and are offset in an axial direction. The planetary gears 22 A, 22 B are supported by the planetary carriers 23 A, 23 B. Axially inner end portions of the planetary carriers 23 A, 23 B extend radially inwards and spline fit on the axles 10 A, 10 B, whereby the planetary carriers 23 A, 23 B are supported on the axles 10 A, 10 B so as to rotate together therewith. The planetary carriers 23 A, 23 B are also supported on the intermediate walls 18 A, 18 B via bearings 33 A, 33 B.

The intermediate walls 18 A, 18 B divide motor accommodating spaces where the first and second motors 2 A, 2 B are accommodated and reduction gear spaces where the first and second epicyclic reduction gears 12 A, 12 B are accommodated and are bent so that an axial space defined therebetween expands from a radially outer side to a radially inner side. Then, the bearings 33 A, 33 B that support the planetary carriers 23 A, 23 B are disposed on radially inner sides of the intermediate walls 18 A, 18 B and on sides thereof which face the first and second epicyclic reduction gears 12 A, 12 B, and bus rings 41 A, 41 B for the stators 14 A, 14 B are disposed on radially outer sides of the intermediate walls 18 A, 18 B and sides thereof which face the first and second motors 2 A, 2 B (refer to FIG. 2 ).

The ring gears 24 A, 24 B include gear portions 28 A, 28 B that mesh with the second pinions 27 A, 27 B which are smaller in diameter on inner circumferential surfaces thereof, small-diameter portions 29 A, 29 B that are smaller in diameter than the gear portions 28 A, 28 B and which are disposed so as to face oppositely each other in an intermediate position of the reduction gear case 11 and connecting portions 30 A, 30 B that connect axially inner end portions of the gear portions 28 A, 28 B with axially outer end portions of the small-diameter portions 29 A, 29 B in a radial direction. In the case of this embodiment, maximum radii of the ring gears 24 A, 24 B are set so as to be smaller than maximum distances of the first pinions 26 A, 26 B from centers of the axles 10 A, 10 B. Both the small-diameter portions 29 A, 29 B spline fit on an inner race 51 of a one-way clutch 50 , which will be described later, and the ring gears 24 A, 24 B rotates together with the inner race 51 of the one-way clutch 50 .

Incidentally, a cylindrical space portion is secured between the reduction gear case 11 and the ring gears 24 A, 24 B. Then, hydraulic brakes 60 A, 60 B, which are configured as brake units for the ring gears 24 A, 24 B, are disposed in the space portion so as to overlap the first pinions 26 A, 26 B in the radial direction and overlap the second pinions 27 A, 27 B in the axial direction. In the hydraulic brakes 60 A, 60 B, plural fixed plates 35 A, 35 B that spline fit on an inner circumferential surface of a cylindrical, radially outside support portion 34 that extends in the axial direction on a radially inner side of the reduction gear case 11 and plural rotational plates 36 A, 36 B that spline fit on outer circumferential surfaces of the ring gears 24 A, 24 B are disposed alternately in the axial direction, and these plates 35 A, 35 B, 36 A, 36 B are operated to be engaged with and disengaged from each other by annular pistons 37 A, 37 B. The pistons 37 A, 37 B are accommodated in a reciprocating fashion in annular cylinder compartments 38 A, 38 B that are formed between a horizontally dividing wall 39 that extends radially inwards from the intermediate position of the reduction gear case 11 so as to divide horizontally the interior of the reduction gear case 11 into left and right portions, the radially outside support portion 34 and a radially inside support portion 40 which are connected with each other by the horizontally dividing wall 39 . The pistons 37 A, 37 B are caused to advance by introducing highly pressurized oil into the cylinder compartments 38 A, 38 B, while the pistons 37 A, 37 B are withdrawn by discharging the oil from the cylinder compartments 38 A, 38 B. The hydraulic brakes 60 A, 60 B are connected to an electric oil pump 70 (refer to FIG. 1 ).

More specifically, the pistons 37 A, 37 B have first piston walls 63 A, 63 B and second piston walls 64 A, 64 B which are disposed forward and rearward of each other in the axial direction. These piston walls 63 A, 63 B, 64 A, 64 B are connected together by cylindrical inner circumferential walls 65 A, 65 B. Consequently, annular spaces that are opened radially outwards are formed between the first piston walls 63 A, 63 B and the second piston walls 64 A, 64 B, and these annular spaces are partitioned axially horizontally by partition members 66 A, 66 B that are fixed to inner circumferential surfaces of outer walls of the cylinder compartments 38 A, 38 B. Spaces defined between the horizontally dividing wall 39 of the reduction gear case 11 and the second piston walls 64 A, 64 B are configured as first hydraulic chambers S 1 into which highly pressurized oil is introduced directly, and spaces defined between the partition members 66 A, 66 B and the first piston walls 63 A, 63 B are configured as second hydraulic chambers S 2 that communicate with the first hydraulic chambers S 1 by way of through holes formed in the inner circumferential walls 65 A, 65 B. Spaces defined between the second piston walls 64 A, 64 B and the partition members 66 A, 66 B communicate with the atmospheric pressure.

In these hydraulic brakes 60 A, 60 B, oil is introduced into the first hydraulic chambers S 1 and the second hydraulic chambers S 2 from a hydraulic circuit 71 , which will be described later, and the fixed plates 35 A, 35 B and the rotational plates 36 A, 36 B can be pressed against each other by virtue of the pressure of the oil that acts on the first piston walls 63 A, 63 B and the second piston walls 64 A, 64 B. Consequently, a large pressure bearing surface area can be obtained by the first and second piston walls 63 A, 63 B, 64 A, 64 B that are disposed forwards and rearwards of each other in the axial direction. Therefore, it is possible to obtain a large pressing force to be applied to the fixed plates 35 A, 35 B and the rotational plates 36 A, 36 B while suppressing an increase in radial surface area of the pistons 37 A, 37 B.

In the case of the hydraulic brakes 60 A, 60 B, the fixed plates 35 A, 35 B are supported on the radially outside support portion 34 that extends from the reduction gear case 11 , while the rotational plates 36 A, 36 B are supported on the ring gears 24 A, 24 B. Therefore, when both the plates 35 A, 35 B, 36 A, 36 B are pressed against each other by the pistons 37 A, 37 B, a braking force is applied to the ring gears 24 A, 24 B to fix (lock) them in place by virtue of a frictional engagement attained between both the plates 35 A, 35 B and 36 A, 36 B. Then, when the engagement of the plates attained by the pistons 37 A, 37 B is released from that state, the ring gears 24 A, 24 B are permitted to rotate freely.

A space portion is secured between the connecting portions 30 A, 30 B of the ring gears 24 A, 24 B that face oppositely each other in the axial direction, and the one-way clutch 50 is disposed in the space portion, the one-way clutch 50 being adapted to transmit only power acting in one direction on the ring gears 24 A, 24 B and to cut off power acting in the other direction. The one-way clutch 50 is such that a number of sprags 53 are interposed between the inner race 51 and an outer race 52 , and the inner race 51 spline fits on the small-diameter portions 29 A, 29 B of the ring gears 24 A, 24 B so as to rotate together therewith. The outer race 52 is positioned by the radially inside support portion 40 while being restricted from rotation. The one-way clutch 50 is brought into engagement when the vehicle 3 travels forwards based on the power of the first and second motors 2 A, 2 B so as to lock the rotation of the ring gears 24 A, 24 B. To describe this more specifically, the one-way clutch 50 is put in an engaged state when torque in a forward direction (a rotational direction when the vehicle 3 travels forwards) at the first and second motors 2 A, 2 B is inputted to the rear wheels Wr, while the one-way clutch 50 is put in a disengaged state when torque in a backward direction at first and second the motors 2 A, 2 B is inputted into the rear wheels Wr. The one-way clutch 50 is put in the disengaged state when forward torque at the rear wheels Wr is inputted into the first and second motors 2 A, 2 B, while the one-way clutch 50 is put in the engaged state when backward torque at the rear wheels Wr is inputted into the first and second motors 2 A, 2 B. In other words, when disengaged, the one-way clutch 50 permits a rotation of the ring gears 24 A, 24 B in one direction based on the backward torque of the first and second motors 2 A, 2 B, whereas when engaged, the one-way clutch 50 restricts a rotation of the ring gears 24 A, 24 B in the other or opposite direction based on the forward torque of the first and second motors 2 A, 2 B.

In this way, in the rear-wheel drive system 1 of this embodiment, the one-way clutch 50 and the hydraulic brakes 60 A, 60 B are provided in parallel on a power transmission line between the first and second motors 2 A, 2 B and the rear wheels Wr. The two hydraulic brakes 60 A, 60 B do not have to be provided, and therefore, a hydraulic brake is provided only for one of the first and second epicyclic reduction gears 12 A, 12 B and the remaining space may be used as a breather chamber.

Here, the control unit 8 (refer to FIG. 1 ) is a control unit for executing various controls in the whole vehicle. Vehicle speed, steering angle, accelerator pedal opening AP, gear position, SOC and oil temperature are inputted into the control unit 8 , while outputted from the control unit 8 are a signal that controls the internal combustion engine 4 , a signal that controls the first and second motors 2 A, 2 B, signals indicating a generating state, charging state and discharging state of the battery 9 , and a control signal that controls the electric oil pump 70 .

Namely, the control unit 8 includes at least a function as a motor control unit for controlling the first and second motors 2 A, 2 B.

FIG. 4 is a table depicting a relation between the front-wheel drive system 6 and the rear-wheel drive system 1 in various vehicle states together with operating states of the first and second motors 2 A, 2 B. In the figure, a front unit denotes the front-wheel drive system 6 and a rear unit denotes the rear-wheel drive system 1 . Rear motors denote the first and second motors 2 A, 2 B. OWC denotes the one-way clutch 50 , and BRK denotes the hydraulic brakes 60 A, 60 B. FIGS. 5 to 10 , FIGS. 12 to 15 , FIG. 18 , FIG. 19 , FIG. 21 and FIG. 22 depict speed collinear diagrams of the rear-wheel drive system 1 in the various states. In each of the figures, LMOT denotes the first motor 2 A and RMOT denotes the second motor 2 B. S, C and PG on a left-hand side denote the sun gear 21 A of the first epicyclic reduction gear 12 A coupled to the first motor 2 A, the planetary carrier 23 A coupled to the axle 10 A, and the planetary gear 22 A, respectively. S, C and PG on a right-hand side denote the sun gear 21 B of the second epicyclic reduction gear 12 B coupled to the second motor 2 B, the planetary carrier 23 B coupled to the axle 10 B, and the planetary gear 22 B, respectively. R denotes the ring gears 24 A, 24 B, BRK denotes the hydraulic brakes 60 A, 60 B and OWC denotes the one-way clutch 50 . In the following description, the rotating direction of the sun gears 21 A, 21 B that are rotated by the first and second motors 2 A, 2 B when the vehicle travels forwards is referred to as a forward direction. In the figures, a portion above a line denoting a state where the vehicle is stopped denotes a forward rotation, whereas a portion below the line denotes a backward rotation. Arrows directed upwards denote forward torque, whereas arrows directed downwards denote backward torque.

While the vehicle is stopped, neither the front-wheel drive system 6 nor the rear-wheel drive system 1 is driven. Consequently, as shown in FIG. 5 , the first and second motors 2 A, 2 B of the rear-wheel drive system 1 are stopped, and the axles 10 A, 10 B are also stopped. Therefore, no torque acts on any of the elements. As this occurs, the hydraulic brakes 60 A, 60 B are released (OFF). Since the first and second motors 2 A, 2 B are not driven, the one-way clutch 50 is not engaged (OFF).

Then, while the vehicle is traveling forwards at low vehicle speeds by EV start and EV cruise with good motor efficiency after the key position is switched to an ON position, the vehicle is driven through rear-wheel drive by the rear-wheel drive system 1 . As shown in FIG. 6 , when the first and second motors 2 A, 2 B are power driven so as to rotate in the forward direction, forward torque is added to the sun gears 21 A, 21 B. As this occurs, as described above, the one-way clutch 50 is engaged, and the ring gears 24 A, 24 B are locked. By so doing, the planetary carriers 23 A, 23 B rotate in the forward direction, whereby the vehicle is allowed to travel forwards. A running resistance from the axles 10 A, 10 B act on the planetary carries 23 A, 23 B in the backward direction. In this way, when the vehicle 3 is started, the key position is switched to the ON position and the torque at the first and second motors 2 A, 2 B is increased, whereby the one-way clutch 50 is engaged mechanically, and the ring gears 24 A, 24 B are locked.

As this occurs, the hydraulic brakes 60 A, 60 B are controlled to be put in a weakly applied state. The weak application means a state where although power can be transmitted, the hydraulic brakes 60 A, 60 B are applied with a weaker application force than an application force with which the hydraulic brakes 60 A, 60 B are applied normally. When forward torque at the first and second motors 2 A, 2 B is inputted into the rear wheels Wr, the one-way clutch 50 is put in an engaged state, and the power transmission is enabled only by the one-way clutch 50 . However, by keeping the hydraulic brakes 60 A, 60 B that are provided in parallel with the one-way clutch 50 in the weakly applied state and also keeping the first and second motors 2 A, 2 B and the rear wheels Wr in the connected state, even when the input of the forward torque from the first and second motors 2 A, 2 B is temporarily decreased, thereby putting the one-way clutch 50 in a disengaged state, it is possible to restrain the power transmission between the first and second motors 2 A, 2 B and the rear wheels Wr from being interrupted. A motor speed control is made unnecessary that would otherwise have to be carried out to put the first and second motors 2 A, 2 B and the rear wheels Wr in the connected state when the vehicle is shifted to a regenerative deceleration, which will be described later. The consumed energy that is consumed when the hydraulic brakes 60 A, 60 B are applied is reduced by making the application force of the hydraulic brakes 60 A, 60 B when the one-way clutch 50 is in the engaged state weaker than the application force of the hydraulic brakes 60 A, 60 B when the one-way clutch 50 is in the disengaged state.

When the vehicle speed is increased from the state where the vehicle is traveling forwards at low vehicle speeds, causing the vehicle to travel forwards at middle vehicle speeds where the engine efficiency is improved, the driving of the vehicle is shifted from the rear-wheel drive by the rear-wheel drive system 1 to the front-wheel drive by the front-wheel drive system 6 . As shown in FIG. 7 , when the power driving of the first and second motors 2 A, 2 B is stopped, forward torque attempting to drive the vehicle forwards is applied to the planetary carriers 23 A, 23 B from the axles 10 A, 10 B, and therefore, as described above, the one-way clutch 50 is put in the disengaged state. In this case, too, the hydraulic brakes 60 A, 60 B are controlled to be put in the weakly applied state.

When the first and second motors 2 A, 2 B are attempted to be driven for regeneration from the state depicted in FIG. 6 or 7 , as shown in FIG. 8 , the forward torque attempting to keep the vehicle traveling forwards is applied to the planetary carriers 23 A, 23 B from the axles 10 A, 10 B. Therefore, as described above, the one-way clutch 50 is put in the disengaged state. As this occurs, the hydraulic brakes 60 A, 60 B are controlled to be put in the applied state (ON). Consequently, the ring gears 24 A, 24 B are fixed, and backward regenerative braking torque is applied to the first and second motors 2 A, 2 B, whereby a regenerative deceleration is executed by the first and second motors 2 A, 2 B. In this way, when the forward torque at the rear wheels Wr is inputted into the first and second motors 2 A, 2 B, the one-way clutch 50 is put in the disengage state, and it is not possible to transmit the power only by the one-way clutch 50 . However, by causing the hydraulic brakes 60 A, 60 B that are provided in parallel with the one-way clutch 50 to be applied and keeping the first and second motors 2 A, 2 B and the rear wheels Wr in the connected state, a power transmissive state can be maintained between the first and second motors 2 A, 2 B and the rear wheels Wr. Then, by controlling the first and second motors 2 A, 2 B to be put in a regenerative drive state in this state, the energy of the vehicle 3 can be regenerated.

Next, when the vehicle is accelerated, the vehicle is driven through four-wheel drive involving the front-wheel drive system 6 and the rear-wheel drive system 1 . Then, the rear-wheel drive system 1 is in the same state as the state depicted in FIG. 6 which results when the vehicle is traveling forwards at low vehicle speeds.

When the vehicle travels forwards at high vehicle speeds, the vehicle is driven through front-wheel drive by the front-wheel drive system 6 . As this occurs, the control of the rear-wheel drive system 1 differs whether or not a request for a yawing moment is made. When no request for a yawing moment is made, the first and second motors 2 A, 2 B are stopped. On the other hand, when a request for a yawing moment is made, a left and right opposite torque control is executed in which the first and second motors 2 A, 2 B generate torque in opposite directions. The left and right opposite torque control will be described later, and here, a case will be described where no request for a yawing moment is made.

When there is made no request for a yawing moment, the first and second motors 2 A, 2 B are stopped. As shown in FIG. 9 , when the power driving of the first and second motors 2 A, 2 B is stopped, the forward torque attempting to drive the vehicle forwards is applied to the planetary carriers 23 A, 23 B from the axles 10 A, 10 B. Therefore, as described above, the one-way clutch 50 is put in the disengaged state. As this occurs, rotational losses of the sun gears 21 A, 21 B and the first and second motors 2 A, 2 B are inputted into the sun gears 21 A, 21 B as resistance, and rotational losses of the ring gears 24 A, 24 B are produced in the ring gears 24 A, 24 B.

As this occurs, the hydraulic brakes 60 A, 60 B are controlled to be put in the released state (OFF). Consequently, the entrained rotation of the first and second motors 2 A, 2 B is prevented, whereby the over speed or revolution of the first and second motors 2 A, 2 b is prevented when the vehicle travels forwards at high vehicle speeds by the front-wheel drive system 6 .

When the vehicle is reversed, as shown in FIG. 10 , in the event that the first and second motors 2 A, 2 B are power driven backwards, backward torque is given to the sun gears 21 A, 21 B. As this occurs, the one-way clutch 50 is put in the disengaged state as described above.

As this occurs, the hydraulic brakes 60 A, 60 B are controlled to be put in the applied state. Consequently, the ring gears 24 A, 24 B are fixed in place, and the planetary carriers 23 A, 23 B are rotated backwards, whereby the vehicle is reversed. The running resistance is applied in the forward direction to the planetary carriers 23 A, 23 B from the axles 10 A, 10 B. In this way, when the backward torque at the first and second motors 2 A, 2 B is inputted into the rear wheels Wr, the one-way clutch 50 is put in the disengaged state, and it is not possible to transmit the power only by the one-way clutch 50 . However, by causing the hydraulic brakes 60 A, 60 B that are provided in parallel with the one-way clutch 50 to be applied and keeping the first and second motors 2 A, 2 B and the rear wheels Wr in the connected state, a power transmissive state can be maintained between the first and second motors 2 A, 2 B and the rear wheels Wr, whereby the vehicle 3 can be reversed by the torque of the first and second motors 2 A, 2 B.

In this way, in the rear-wheel drive system 1 , the application and release of the hydraulic motors 60 A, 60 B is controlled according to the driving states of the vehicle, in other words, according to in which direction the first and second motors 2 A, 2 B rotate; in the forward or backward direction, and from which the power is inputted; from the first and second motors 2 A, 2 B or the rear wheels Wr. Further, even when the hydraulic brakes 60 A, 60 B are in the applied state, the application force is adjusted.

FIG. 11 is a timing chart of the electric oil pump 70 (EOP), the one-way clutch 50 (OWC), and the hydraulic brakes 60 A, 60 B (BRK) from the time when the vehicle starts from a stopped state to the time when the vehicle stops again through events of EV start→EV acceleration→ENG acceleration→regenerative deceleration→middle-speed ENG cruising→ENG+EV acceleration→high-speed ENG cruising→regenerative deceleration→stop→reversing.

Firstly, the one-way clutch 50 is kept in the disengaged state (OFF) and the hydraulic brakes 60 A, 60 B are kept in the released state (OFF) until the key position is switched to the ON position, the gear is then shifted from the P range to the D range and the accelerator pedal is depressed. When the accelerator pedal is depressed in that state, EV start and EV acceleration are executed through rear-wheel drive (RWD) by the rear-wheel drive system 1 . As this occurs, the one-way clutch 50 is engaged (ON), and the hydraulic brakes 60 A, 60 B are put in the weakly applied state. Then, when the vehicle speed is increased to reach a middle vehicle speed zone from a low vehicle speed zone and the driving of the vehicle is switched from rear wheel drive to front wheel drive, an ENG driving (FWD) by the internal combustion engine 4 is executed. As this occurs, the one-way clutch 50 is disengaged (OFF), while the hydraulic brakes 60 A, 60 B are kept in the same states as before (in the weakly applied state). Then, when a regenerative deceleration occurs in the vehicle with the brake pedal depressed, the hydraulic brakes 60 A, 60 B are applied (ON) while the one-way clutch 50 is kept disengaged (OFF). The same state as when the ENG driving is executed results while the middle speed cruising by the internal combustion engine 4 is executed. Next, when the accelerator pedal is depressed further to switch the driving of the vehicle from front wheel drive to four or all wheel drive (AWD), the one-way clutch 50 is engaged (ON) again. Then, when the vehicle speed reaches a high vehicle speed zone from the middle vehicle speed zone, the ENG driving (FWD) by the internal combustion engine 4 is executed again. As this occurs, the one-way clutch 50 is disengaged (OFF), and the hydraulic brakes 60 A, 60 B are released (OFF). As this occurs, when there is made no request for a yawing moment, the first and second motors 2 A, 2 B are stopped, whereas when there is made a request for a yawing moment, a left and right opposite torque control, which will be described later, is executed. Then, when the vehicle is decelerated for regeneration, the same state results as the state that resulted when the vehicle was decelerated for regeneration as described. Then, when the vehicle is stopped, the one-way clutch 50 is disengaged (OFF), and the hydraulic brakes 60 A, 60 B are released (OFF).

Next, when the vehicle is reversed, the hydraulic brakes 60 A, 60 B are applied (ON) while the one-way clutch 50 is kept disengaged (OFF). Then, when the vehicle is stopped, the one-way clutch 50 is disengaged (OFF), and the hydraulic brakes 60 A, 60 B are put in the released state (OFF).

Nets, the left and right opposite torque control will be described.

The left and right opposite torque control is a control in which one motor of the first motor 2 A and the second motor 2 B is controlled so as to produce a forward torque or backward torque and the other motor to produce a backward torque or forward torque that is opposite in direction to the torque produced by the one motor, and thus, the first motor 2 A and the second motor 2 B are controlled so as to produce the torque in the opposite directions. By so doing, it is possible to keep a yawing moment produced even under conditions in which the one-way clutch 50 is not engaged and the hydraulic brakes 60 A, 60 B are controlled to be kept in the released state, that is, even while the ring gears 24 A, 24 B are rotating. In the embodiments, a revolution matching is executed in this state by a manner (A) in which one motor of the first motor 2 A and the second motor 2 B is controlled based on a target speed of the one motor and the other motor is controlled based on a target torque of the other motor or a manner (B) in which both motors of the first motor 2 A and the second motor 2 B are controlled based on target torques of both the motors and a correction torque is added only to the one motor of the first motor 2 A and the second motor 2 B to cause the one motor to attain its target speed. Backward torque denotes torque applied in a direction in which the rotation in the backward direction is increased or torque applied in a direction in which the rotation in the forward direction is decreased.

In the following description, a case where either of the first and second motors 2 A, 2 B is controlled to attain an arbitrary target speed (hereinafter, referred to as a motor target speed) will be referred to as a first embodiment, a case where either of the first and second motors 2 A, 2 B is controlled to attain the motor target speed so that the ring gears 24 A, 24 B attain a ring gear target speed will be referred to as a second embodiment, and a case where either of the first and second motors 2 A, 2 B is controlled to attain the motor target speed based on the speed of the planetary gears 22 A, 22 B will be referred to as a third embodiment. In the first to third embodiments, while the motor speed (r/min) is used as a revolution state quantity, the revolution state quantity is not limited to the motor speed (r/min), and hence, other revolution state quantities such as an angular velocity (rad/s) may be used. Similarly, in the first to third embodiments, while the motor torque (N·m) is used as a torque state quantity, other torque state quantities such as a motor current (A) that is correlated with the motor torque may be used.

First Embodiment

FIG. 12 is a speed collinear diagram of the rear-wheel drive system when a left and right opposite torque control is executed (before a target revolution is attained) while the vehicle is traveling forwards at high vehicle speeds. In the figure, vectors depicting losses that are normally generated in the individual rotational elements are omitted. While the vehicle is traveling forwards at high vehicle speeds, the vehicle is driven through front-wheel drive by the front-wheel drive system 6 as described above, and therefore, the one-way clutch 50 is disengaged. As this occurs, the hydraulic brakes 60 A, 60 B are controlled to be put in the released state. Consequently, the coupled ring gears 24 A, 24 B are allowed to rotate without being locked.

In this state, a torque control is executed on the first motor 2 A based on the target torque so that a first motor torque TM 1 in the forward direction is produced, and a speed control is executed on the second motor 2 B based on the motor target speed so that a second motor torque TM 2 in the backward direction is produced. An absolute value of the second motor torque TM 2 is set to a larger value than an absolute value of the first motor torque TM 1 .

By executing the torque control on the first motor 2 A based on the target torque so that the first motor torque TM 1 in the forward direction is produced, the first motor torque TM 1 in the forward direction acts on the sun gear 24 A. As this occurs, as has happed in FIG. 9 , forward torque (not shown) that attempts to drive the vehicle forwards is applied to the planetary carriers 23 A, 23 B from the axles 10 A, 10 B. Consequently, in the first epicyclic reduction gear 12 A, the planetary gear 23 A functions as a fulcrum, and the first motor torque M 1 in the forward direction is applied to the sun gear 21 A which functions as a point of application, whereby a first motor torque distributed force TM 1 ′ in the backward direction acts on the ring gears 24 A, 24 B which function as a point of action (refer to FIG. 13 ). In the second epicyclic reduction gear 12 B, the planetary carrier 23 B functions as a fulcrum, and the first motor torque distributed force TM 1 ′ in the backward direction is applied to the ring gears 24 A, 24 B which function as the point of application, whereby a first motor torque distributed force TM 1 ″ in the forward direction acts on the sun gear 21 B which functions as a point of action (refer to FIG. 13 ).

On the other hand, by executing the speed control on the second motor 2 B based on the motor target speed so that a speed control torque in the backward direction is produced, the second motor torque TM 2 in the backward direction acts on the sun gear 21 B. The second motor torque TM 2 can be divided into a balancing torque Tb whose absolute value equals the absolute value of the first motor torque TM 1 and which acts in the backward direction and a speed controlling torque Tnc in the backward direction which is the remaining of the second motor torque TM 2 . Consequently, in the second epicyclic reduction gear 12 B, the planetary carrier 23 B functions as a fulcrum, and the balancing torque Tb in the backward direction and the speed controlling torque Trio in the backward direction are applied to the sun gear 21 B which functions as a point of application, whereby a balancing torque distributed force Tb′ in the forward direction and a speed controlling torque Tnc′ in the forward direction act on the ring gears 24 A, 24 B which function as a point of action (refer to FIGS. 12, 13 ). In the first epicyclic reduction gear 12 A, the planetary carrier 23 A functions as a fulcrum, and the balancing torque distributed force Tb′ in the forward direction and the speed controlling torque Tnc′ in the forward direction are applied to the ring gears 24 A, 24 B which function as a point of application, whereby a balancing torque distributed force Tb″ in the backward direction and a speed control torque distributed force Tnc″ in the backward direction act on the sun gear 21 A which functions as a point of action (refer to FIGS. 12, 13 ).

Here, the first motor torque TM 1 and the balancing torque Tb are opposite in direction and equal in magnitude (absolute values), and therefore, as shown in FIG. 13 , the first motor torque TM 1 in the forward direction and the balancing torque distributed force Tb″ in the backward direction which both act on the sun gear 21 A cancel each other, and the first motor torque distributed force TM 1 ′ in the backward direction and the balancing torque distributed force Tb′ in the forward direction which both act on the ring gears 24 A, 24 B cancel each other, the first motor torque distributed force TM 1 ″ in the forward direction and the balancing torque Tb in the backward direction which both act on the sun gear 21 B canceling each other. Consequently, the sun gears 21 A, 21 B and the ring gears 24 A, 24 B are allowed to balance so that their rotating states are maintained by the first motor torque TM 1 and the balancing torque Tb. As this occurs, a carrier torque TT 1 in the forward direction that is obtained by multiplying the first motor torque TM 1 by a reduction ratio of the first epicyclic reduction gear 12 A acts on the planetary carrier 23 A, and a carrier torque TT 2 in the backward direction that is obtained by multiplying the balancing torque Tb by a reduction ratio of the second epicyclic reduction gear 12 B acts on the planetary carrier 23 B.

Since the reduction ratios of the first and second epicyclic reduction gears 12 A, 12 B are equal, a clockwise yawing moment M is produced in a stable fashion by the carrier torques TT 1 , TT 2 .

On the other hand, due to there being basically no torque that balances them, the speed controlling torque Tnc and the speed controlling torque distributed force Tnc′ and the speed controlling torque distributed force Tnc″, which are distributed force of the speed controlling torque Tnc, are not outputted to the planetary carriers 23 A, 23 B and are consumed to change the speeds of the sun gears 21 A, 21 B and the ring gears 24 A, 24 B. In association with this, the speed controlling torque Tnc in the backward direction of the second motor torque TM 2 that acts on the sun gear 21 B and the speed controlling torque distributed force Tnc″ in the backward direction that is the distributed force of the speed controlling torque Tnc and which acts on the sun gear 21 A each decrease the speeds of the sun gears 21 A, 21 B, that is, the speeds of the first and second motors 2 A, 2 B, and the speed controlling torque distributed force Tnc′ in the forward direction that acts on the ring gears 24 A, 24 B increases the speeds of the ring gears 24 A, 24 B. In the event that the speed controlling torque Tnc changes too drastically, a resistance force, not shown, that equals the inertia of the ring gears 24 A, 24 B functions to resist a reaction force and is outputted to the planetary carrier 23 B, and therefore, the speed controlling torque Tnc is to be controlled so as not to change drastically.

In this way, in the left and right opposite torque control while the ring gears 24 A, 24 B are rotating, by executing the torque control on the first motor 2 A based on the target torque so that the first motor torque TM 1 in the forward direction is produced and executing the speed control on the second motor 2 B based on the motor target speed so that the second motor torque TM 2 in the backward direction is produced, the first motor torque TM 1 and the balancing torque Tb of the second motor torque TM 2 contribute to the production of a yawing moment M in the vehicle 3 , and the speed controlling torque Tnc of the second motor torque TM 2 contributes in allowing the speed of the second motor 2 B to reach the motor target speed.

When the second motor 2 B reaches the motor target speed, the second motor torque TM 2 is made up only of the balancing torque Tb whose absolute value equals that of the first motor torque TM 1 and which acts in the backward direction, which reduces the speed controlling torque Tnc. FIG. 14 is a speed collinear diagram of the rear-wheel drive system when the left and right opposite torque control according to the first embodiment is executed (after the target revolution is reached). By executing the left and right opposite torque control, the first motor torque TM 1 balances the second motor Torque TM 2 in such a state that the second motor 2 B holds the motor target speed, and the clockwise yawing moment so produced is maintained.

In this left and right opposite torque control, in place of executing the torque control on the first motor 2 A based on the target torque so that the first motor torque TM 1 in the forward direction is produced and executing the speed control on the second motor 2 B based on the motor target speed so that the second motor torque TM 2 in the backward direction is produced, even in the event that both of the first motor 2 A and the second motor 2 B are controlled based on a target torque and a correction torque is added only to the second motor 2 B to allow the second motor 2 B to attain a target revolution state quantity, the same function can be attained. The following second and third embodiments will be described based on the latter controlling approach.

It is preferable to obtain a target torque in torque controlling the first motor 2 A based on a target yawing moment of the vehicle 3 . How to obtain the target torque will be described using the following expressions.

When a left rear wheel target torque of the left rear wheel LWr is referred to as WTT 1 , a right rear wheel target torque of the right rear wheel RWr is referred to as WTT 2 , a total target torque of the left and right rear wheels LWr, RWr (a sum of a left rear wheel torque and a right rear wheel torque) is referred to TRT, and a target torque difference of the left and right rear wheels LWr, RWr (a difference between the left rear wheel torque and the right rear wheel torque) is referred to as ΔTT, the following expressions (1), (2) are established.

WTT1+WTT2= TRT   (1)

WTT1−WTT2=ΔTT  (2)

When a target yawing moment (a clockwise moment is referred to as positive) is referred to as YMT, a wheel radius is referred to as r, and a tread width (a distance between the left and right rear wheels LWr, RWr) is referred to as Tr, ΔTT is expressed by the following expression (3).

ΔTT=2 ·r·YMT/Tr   (3)

Here, the torque produced in the same direction by the first and second motors 2 A, 2 B is not transmitted in a longitudinal direction of the vehicle 3 while the ring gears 24 A, 24 B are rotating, and therefore, the total target torque TRT of the left and right rear wheels LWr, RWr is zero. Consequently, the target torques WTT 1 , WTT 2 of the left and right rear wheels LWr, RWr are determined primarily from the expressions (1), (2) above.

Namely, WWT1=−WTT2=ΔTT/2  (4)

When the target torque of the first motor 2 A that is coupled to the left rear wheel LWr is referred to as TTM 1 , the target torque TTM 1 of the first motor 2 A is calculated from the following expression (5).

TT M 1=(1/Ratio)·WTT1  (5)

where Ratio is the reduction ratio of the first and second epicyclic reduction gears 12 A, 12 B.

The target torque TTM 1 of the first motor 2 A is expressed by the following expression (6) from the expressions (4), (5).

TT M 1=(1/Ratio)·ΔTT/2  (6)

Consequently, the target torque difference ΔTT between the left and right rear wheels LWr, RWr is obtained based on the target yawing moment YMT of the vehicle 3 , and a value obtained by dividing the torque that is half the target torque difference ΔTT by the reduction ratio of the first epicyclic reduction rear 12 A is referred to as the target torque TTM 1 of the first motor 2 A on which the torque control is executed, whereby a desired yawing moment can be produced.

It is preferable that the motor target speed used in executing the speed control is obtained based on at least one of the efficiency of the second motor 2 B and the efficiency of an electric power supply unit that supplies electric power to the second motor 2 B. In such a state that the ring gears 24 A, 24 B are locked by the hydraulic brakes 60 A, 60 B and/or the one-way clutch 50 , the speed of the first and second motors 2 A, 2 B is associated with the rotation of the planetary carriers 23 A, 23 B and becomes a predetermined speed that corresponds to the reduction ratio of the first and second epicyclic reduction gears 12 A, 12 B. However, in such a state that the ring gears 24 A, 24 B are not locked, that is, when the ring gears 24 A, 24 B are rotating, the speed of the first and second motors 2 A, 2 B is not associated with the rotation of the planetary carriers 23 A, 23 B and can be an arbitrary speed. The electric power supply unit is made up of a PDU, not shown, which includes an inverter or a three-phase wire and is mainly made up of a PDU. By obtaining the motor target speed based on the efficiency of the second motor 2 B and the efficiency of the PDU which occupy a large portion of the efficiency of an electric power supply system, it is possible to reduce the consumed electric power further. On the other hand, the motor target speed may also be obtained based only on the efficiency of the second motor 2 B. When the motor target speed is obtained in an experimental fashion, the preparation of an efficiency map is facilitated, and when the motor target speed is obtained through sequential detection and estimation, it is possible to reduce the control quantity.

In the embodiment, the pattern is described in which the motor target speed of the second motor 2 B is lower than an actual speed thereof (hereinafter, referred to as a motor actual speed) and the clockwise yawing moment is produced. However, in reality, the electric motor on which the torque control is to be executed and the electric motor on which the speed control is to be executed are determined based on whether the motor target speed is higher or lower than the motor actual speed and whether the target yawing moment is clockwise or counterclockwise.

As shown at (a) in FIG. 15 , when the target yawing moment is clockwise, and the motor actual speed is smaller than the motor target speed (motor actual speed<motor target speed) and hence, the motor actual speed is increased (motor speed controlling direction: upward), the first motor 2 A is speed controlled so as to produce forward torque, and the second motor 2 B is torque controlled so as to produce backward torque. As shown at (b), when the target yawing moment is clockwise, and the motor actual speed is larger than the motor target speed (motor actual speed>motor target speed) and hence, the motor actual speed is decreased (motor speed controlling direction: downward), the second motor 2 B is speed controlled so as to produce backward torque and the first motor 2 A is torque controlled so as to produce forward torque (the pattern of the embodiment).

On the other hand, as shown at (c), when the target yawing moment is counterclockwise, and the motor actual speed is smaller than the target motor speed (motor actual supped<motor target speed) and hence, the motor actual speed is increased (motor speed controlling direction: upward), the second motor 2 B is speed controlled so as to produce forward torque and the first motor 2 A is torque controlled so as to produce backward torque. As shown at (d), when the target yawing moment is counterclockwise, and the motor actual speed is larger than the motor target speed (motor actual speed>motor target speed) and hence, the motor actual speed is decreased (motor speed controlling direction: downward), the first motor 2 A is speed controlled so as to produce backward torque so as to produce and the second motor 2 B is torque controlled so as to produce forward torque.

Next, a flow of the left and right opposite torque control according to the first embodiment will be described by reference to FIG. 16 .

Firstly, when a command to execute the left and right opposite control is issued, commands to obtain a motor target speed, a motor actual speed and a target yawing moment are acquired (S 11 ). Next, the target yawing moment command is coded based on the target yawing moment obtained, and the motor speed controlling direction is coded based on the motor target speed and the motor actual speed that are so obtained (S 12 ).

In coding, as shown in FIG. 17 , if the target yawing moment is clockwise, the target yawing moment command is signed as “positive” and if the target yawing moment is counterclockwise, the target yawing moment command is signed as “negative.” If the motor actual speed is smaller than the motor target speed (motor actual speed>motor target speed) and hence, the motor actual speed is increased, the motor speed controlling direction is signed as “positive” and if the motor actual speed is larger than the motor target speed (motor actual speed>motor target speed) and hence, the motor actual speed is decreased, the motor speed controlling direction is signed as “negative.”

Next, the coded target yawing moment command sign is multiplied by the coded motor speed controlling direction sign to obtain a control selection sign (S 13 ). A control selection sign obtained by multiplying “positive” by “positive” or multiplying “negative” by “negative” is referred to as “positive,” whereas a control selection sign obtained by multiplying “positive” by “negative” and multiplying “negative” by “positive,” is referred to as “negative.”

As a result, it is detected whether or not the control selection sign is “positive” (S 14 ), and if the control selection sign is “positive,” the first motor 2 A is speed controlled (S 15 ), and the second motor 2 B is torque controlled (S 16 ). On the other hand, if the control selection sign is “negative,” the first motor 2 A is torque controlled (S 17 ), whereas the second motor 2 B is speed controlled (S 18 ).

Thus, as described above, when the left and right opposite torque control is executed while the coupled ring gears 24 A, 24 B are rotating, one motor of the first motor 2 A and the second motor 2 B is controlled based on the target speed of the one motor, and the other motor is controlled based on the target torque of the other motor, whereby it is possible to control the one motor to attain the motor target speed while satisfying the target yawing moment even in a state where the ring gears 24 A, 24 B are allowed to rotate freely without being fixed by the hydraulic brakes 60 A, 60 B and the one-way clutch 50 . By setting the motor target speed based on the efficiency of the one motor, it is possible to realize energy saving and an improvement in fuel economy.

Second Embodiment

Next, a left and right opposite torque control according to a second embodiment will be described.

The left and right opposite torque control of this embodiment is a control in which the first and second motors 2 A, 2 B are controlled so that the ring gears 24 A, 24 B attain their ring gear target speed. In the following description, a configuration will be described as an example in which a correction torque is added to the second motor 2 B so that the ring gears 24 A, 24 B are put in a zero rotation state for application of the hydraulic brakes 60 A, 60 B, whereby the first and second motors 2 A, 2 B are allowed to attain the motor target speed.

As described in FIG. 9 , when the vehicle travels forwards at high vehicle speeds, the power driving of the first and second motors 2 A, 2 B is stopped, whereby forward torque that attempts to drive the vehicle forwards is applied to the planetary carriers 23 A, 23 B from the axles 10 A, 10 B, and the one-way clutch 50 is put in the disengaged state. When the target yawing moment command in the backward direction is inputted in this state, as shown in FIG. 18( a ) , the torque control is executed on the first motor 2 A based on the target torque so that a first motor torque TM 1 in the backward direction is produced, and the torque control is executed on the second motor 2 B so that a second motor torque TM 2 in the forward torque is produced that is equal in magnitude to and which is opposite in direction to the first motor torque TM 1 .

Consequently, as described in FIG. 13 , there results a state where the first motor torque TM 1 balances the second motor torque TM 2 . As this occurs, a carrier torque TT 1 in the background direction that is obtained by multiplying the first motor torque TM 1 in the backward direction by the reduction ratio of the first epicyclic reduction gear 12 A is applied to the planetary carrier 23 A, while a carrier torque TT 2 in the forward direction that is obtained by multiplying the second motor torque TM 2 in the forward direction by the reduction ratio of the second epicyclic reduction gear 12 B is applied to the planetary carrier 23 B, whereby a counterclockwise yawing moment is produced by the carrier torques TT 1 , TT 2 .

When a command to apply the hydraulic brakes 60 A, 60 B is issued in this state, the ring gear target speed of the ring gears 24 A, 24 B is set to zero so as to apply the hydraulic brakes 60 A, 60 B, and in order to reduce the rotation of the ring gears 24 A, 24 B to zero, as shown in FIG. 18( b ) , a correction torque Tad in the forward direction is added further to the second motor 2 B that is producing the forward torque. As this occurs, in the second epicyclic reduction gear 12 B, the planetary carrier 23 B functions as a fulcrum, and the correction torque Tad in the forward direction is applied to the sun gear 21 B which functions as a point of application, whereby a correction torque distributed force Tad′ in the backward direction acts on the ring gears 24 A, 24 B which function as a point of action. In the first epicyclic reduction gear 12 A, the planetary carrier 23 A functions as a fulcrum, and the correction torque distributed force Tad′ in the backward direction is applied to the ring gears 24 A, 24 B which function as a point of application, whereby a correction torque distributed force Tad″ in the forward direction acts on the sun gear 21 A which functions as a point of action. Due to there being basically no torque that balances them, the correction torque Tad and the correction torque distributed force Tad′ and the correction torque distributed force Tad″, which are distributed force of the correction torque Tad, are not outputted to the planetary carriers 23 A, 23 B and are consumed to change the speeds of the sun gears 21 A, 21 B and the ring gears 24 A, 24 B. In association with this, the correction torque Tad and the correction torque distributed force Tad″ increase the speeds of the ring gears 24 A, 24 B, that is, the speeds of the first and second motors 2 A, 2 B, respectively, while the correction torque distributed force Tad′ in the backward direction which acts on the ring gears 24 A, 24 B decreases the speed of the ring gears 24 A, 24 B. By so doing, the speed of the second motor 2 B eventually becomes the motor target speed, and the speed of the ring gears 24 A, 24 B eventually becomes substantially zero which is the ring gear target speed.

Then, when the speed of the ring gears 24 A, 24 B becomes substantially zero, as shown in FIG. 18( c ) , the hydraulic brakes 60 A, 60 B are applied, and the correction torque Tad is removed. By so doing, the absolute values of the first motor torque TM 1 and the second motor torque TM 2 become equal, whereby the counterclockwise yawing moment M that is being produced is maintained. Further, by applying the hydraulic brakes 60 A, 60 B to slow the ring gears 24 A, 24 B, it becomes possible to transmit the torque produced in the same direction by the first and second motors 2 A, 2 B to the wheels.

It is preferable that the target torque used in executing the torque control is obtained based on the target yawing moment. How to obtain this target torque is similar to that described in the first embodiment. When the target torque of the first motor 2 A which is connected to the left rear wheel LWr is referred to as TTM 1 ′ and the target torque of the second motor 2 B which is connected to the right rear wheel RWr is referred to as TTM 2 ′, the target torques TTM 1 ′, TTM 2 ′ of the first and second motors 2 A, 2 B which are disposed left and right are calculated from the following expressions (5)′, (7).

TT M 1′=(1/Ratio)·WTT1  (5)′

TT M 2′=(1/Ratio)·WTT2  (7)

where, Ratio is the reduction ratio of the first and second epicyclic reduction gears 12 A, 12 B.

The first target motor torque TTM 1 ′ and the second target motor torque TTM 2 ′ are expressed by the following expressions (6)′, (8) from the expressions (4), (5)′, (7).

TT M 1′=(1/Ratio)·ΔTT/2  (6)′

TT M 2′=−(1/Ratio)·ΔTT/2  (8)

Consequently, a target torque difference ΔTT between the first motor 2 A and the second motor 2 B is obtained based on the target yawing moment YMT of the vehicle 3 , and values obtained by dividing a torque that is half the target torque difference ΔTT by the reduction radio of the first and second epicyclic reduction gears 12 A, 12 B are controlled as the target torques of the first motor 2 A and the second motor 2 B, whereby it is possible to produce a desired yawing moment.

In the embodiment, the pattern ( FIG. 19( b ) ) to produce the counterclockwise yawing moment is described. However, in order to produce a clockwise yawing moment, as shown in FIG. 19( a ) , the torque control is executed on the first motor 2 A based on the target torque so that a first motor torque TM 1 in the forward direction is produced, and the torque control is executed on the second motor 2 B so that a second motor torque TM 2 in the backward direction is produced that is equal in magnitude to and which is opposite in direction to the first motor torque TM 1 . A correction torque Tad that contributes to the speed control is added to the first motor 2 A. Thus, the production of the clockwise yawing moment is realized. Namely, in order to decrease the speed of the ring gears 24 A, 24 B, the speed of the motors needs to be increased, and therefore, the correction torque needs to be added to the motor that is producing the forward torque. By so doing, it becomes possible to transmit the target torque to the rear wheels Wr at all times, whereby the yawing moment of the vehicle 3 remains unchanged, the behaviors of the vehicle 3 being thereby stabilized.

In the embodiment, the case is described where the target speed of the ring gears 24 A, 24 B is set to zero for application of the hydraulic brakes 60 A, 60 B. However, a similar control is also possible when the ring gear target speed of the ring gears 24 A, 24 B is set to zero for engagement of the one-way clutch 50 . Further, when both a command to apply the hydraulic brakes 60 A, 60 B and a command to engage the one-way clutch 50 are not issued, the ring gear target speed of the ring gears 24 A, 24 B may be set so that the ring gears 24 A, 24 B rotate in the forward direction and are put in a substantially zero state. By adopting this configuration, it is possible to put the ring gears 24 A, 24 B in a rotating state where the rotational loss of the ring gears 24 A, 24 B is small with the one-way clutch 50 disengaged. By setting the one-way clutch 50 so as not to be engaged, it is possible to avoid a risk of a shock being transmitted to the wheels Wr as a result of the engagement of the one-way clutch 50 , thereby making it possible to suppress the shock.

Next, a flow of the left and right opposite torque control of the second embodiment will be described by reference to FIG. 20 .

By executing the left and right opposite torque control while the ring gears 24 A, 24 B are rotating, in such a state that the yawing moment is continuously produced ( FIG. 18( a ) ), firstly, it is detected whether or not there exists a command to apply the hydraulic brakes 60 A, 60 B (S 21 ). If it is determined as a result of the detection that there exists no such application command, the controlling process ends there.

If it is determined that there exists such application command, a motor target speed is calculated from a wheel speed (S 22 ). This is intended to detect a motor target speed of the first motor 2 A and the second motor 2 B at which the rotation of the ring gears 24 A, 24 B becomes zero in anticipation of a case where the hydraulic brakes 60 A, 60 B are applied. Next, a correction torque is calculated according to a rotation difference between the motor target speed and an actual motor speed (S 23 ). Next, it is determined whether or not the torque of the first motor 2 A is larger than the torque of the second motor 2 B, that is, whether or not the first motor torque TM 1 is a torque in the forward direction (S 24 ). While the left and right opposite torque control is being executed, in the event that one motor is producing forward torque, the other motor is producing backward torque. Thus, in order to decrease the rotation of the ring gears 24 A, 24 B while maintaining the yawing moment M, the correction torque needs to be added to the motor that is producing the forward torque.

If it is determined as a result of the detection that the first motor torque TM 1 is larger than the second motor torque TM 2 , that is, the first motor torque TM 1 is the forward torque, the first motor 2 A is selected (S 25 ). On the other hand, if it is determined that the first motor torque TM 1 is smaller than the second motor torque TM 2 , that is, the second motor torque TM 2 is the forward torque, the second motor 2 B is selected (S 26 ). Next, the correction torque is added to either of the motors that was selected in S 25 or S 26 (S 27 ).

By so doing, the hydraulic brakes 60 A, 60 B can be applied in such a state that the speed of the ring gears 24 A, 24 B is decreased to substantially zero. In this way, by applying the hydraulic brakes 60 A, 60 B in such a state that the speed of the ring gears 24 A, 24 B is decreased to substantially zero, it is possible to reduce the deterioration of the hydraulic brakes 60 A, 60 B. Then, even before the ring gears 24 A, 24 B are locked, the first motor torque TM 1 balances the second motor torque TM 2 , thereby it is possible to produce a yawing moment.

The target speed of the first and second motors 2 A, 2 B may be obtained based on the speed of the planetary carriers 23 A, 23 B or the speed of the left wheel LWr or the right wheel RWr in addition to the ring gear target speed of the ring gears 24 A, 24 B. By so doing, it is possible to control the rotation of the ring gears 24 A, 24 B with greater accuracy.

Thus, as described above, when the left and right opposite torque control is executed while the coupled ring gears 24 A, 24 B are rotating, both the first motor 2 A and the second motor 2 B are controlled based on the target torque of both the motors, and the motor target speed of the first motor 2 A and the second motor 2 B is obtained based on the ring gear target speed. Then, the correction torque to allow the motors to attain the motor target speed is added only to one motor of the motors. Thus, it is possible to control the one motor to attain the motor target speed while satisfying the target yawing moment even in the state where the ring gears 24 A, 24 B are allowed to rotate freely without being fixed by the hydraulic brakes 60 A, 60 B and the one-way clutch 50 . Then, the motor target speed is set so that the rotation of the ring gear target speed becomes substantially zero, whereby the hydraulic brakes 60 A, 60 B are allowed to be applied when the rotation of the ring gears 24 A, 24 B becomes substantially zero, thereby making it possible to reduce the shock produced when the hydraulic brakes 60 A, 60 B are applied and the deterioration thereof.

Third Embodiment

Next, a left and right opposite torque control according to a third embodiment will be described.

The left and right opposite torque control of this embodiment is a control in which the first and second motors 2 A, 2 B are controlled so that one of the planetary gears 22 A, 22 B is allowed to attain a planetary gear target speed. In the following description, a configuration will be described as an example in which a correction torque is added to the first motor 2 A so that the rotating direction of the planetary gears 22 A, 22 B is not reversed so as to prevent a backlash that would otherwise be produced by the reversing of the rotating direction of the planetary gears 22 A, 22 B.

FIG. 21( a ) is a speed collinear diagram of the rear-wheel drive system while the vehicle 3 is turning to the left. In the rear-wheel drive system 1 , in order to assist the vehicle 3 in turning to the left, the torque control is executed on the first motor 2 A based on the target torque so that a first motor torque TM 1 in the backward direction is produced, and the torque control is executed on the second motor 2 B so that a second motor torque TM 2 in the forward direction is produced that is equal in magnitude to and which is opposite in direction to the first motor torque TM 1 .

As this occurs, a carrier torque TT 1 in the backward direction that is obtained by multiplying the first motor torque TM 1 in the backward direction by the reduction ratio of the first epicyclic reduction gear 12 A is applied to the planetary carrier 23 A, while a carrier torque TTM 2 in the forward direction that is obtained by multiplying the second motor torque TM 2 in the forward direction by the reduction ratio of the second epicyclic reduction gear 12 B is applied to the planetary carrier 23 B, whereby a counterclockwise yawing moment M is produced by the carrier torques TT 1 , TT 2 . Since the vehicle 3 is turning to the left, the speeds of the sun gear 21 B and the planetary carrier 23 B of the second epicyclic reduction gear 12 B becomes larger those of the sun gear 21 A and the planetary carrier 23 A of the first epicyclic reduction gear 12 A according to a difference in rotation between the left rear wheel LWr and the right rear wheel RWr.

In FIG. 21( a ) , a point (A 1 ) on an extension that is extended further from a collinear diagram that connects together the sun gear 21 A (S), the planetary carrier 23 A (C), and the ring gear 24 A (R) of the first epicyclic reduction gear 12 A denotes the speed (of rotation on its own axis) of the planetary gear 22 A (PG), and a point (B 1 ) on an extension that is extended further from a collinear diagram that connects together the sun gear 21 B (S), the planetary carrier 23 B (C), and the ring gear 24 B (R) denotes the speed (of rotation on its own axis) of the planetary gear 22 B (PG).

When the hydraulic brakes 60 A, 60 B are released from this state, it is possible to allow the sun gears 21 A, 21 B (S) and the planetary gears 22 A, 22 B (PG) and the ring gears 24 A, 24 B (R) other than the planetary carriers 23 A, 23 B (C) that are connected to the left rear wheel LWr and the right rear wheel RWr, respectively, to rotate in an arbitrary fashion. Here, when a command to release the hydraulic brakes 60 A, 60 B is issued, a planetary gear target speed A 2 is set near to the zero rotation so that the rotating direction of the planetary gear 22 A that is rotating backwards is not reversed and that the speed (the absolute value) is reduced to a small level, and a speed difference DA between a motor actual speed MA 1 of the first motor 2 A and a motor target speed MA 2 of the first motor 2 A that is obtained based on the planetary gear target speed A 2 and the speed of the planetary carrier 23 A is calculated (refer to FIG. 22 ). Similarly, a planetary gear target speed B 2 is set near to the zero rotation so that the rotating direction of the planetary gear 22 B that is rotating backwards is not reversed and that the speed (the absolute value) is reduced to a small level, and a speed difference DB between a motor actual speed MB 1 of the second motor 2 B and a motor target speed MB 2 of the second motor 2 B that is obtained based on the planetary gear target speed B 2 and the speed of the planetary carrier 23 B is calculated (refer to FIG. 22 ). The speed difference DA at the first motor 2 A is compared with the speed difference DB at the second motor 2 B, and the first motor 2 A whose speed difference is small is determined as the motor to which the correction torque is added, that is, as the motor having the motor target speed.

Next, as shown in FIG. 21( b ) , the hydraulic brakes 60 A, 60 B are released, and a correction torque Tad in the backward direction is added further to the first motor 2 A that is determined as the motor having the motor target speed. As this occurs, in the first epicyclic reduction gear 12 A, the planetary carrier 23 A functions as a fulcrum, and the correction torque Tad in the backward direction is applied to the sun gear 21 A which functions as a point of application, whereby a correction torque distributed force Tad′ acts on the ring gears 24 A, 24 B which function as a point of action. In the second epicyclic reduction gear 12 B, the planetary carrier 23 B functions as a fulcrum, and the correction torque Tad′ in the forward direction is applied to the ring gears 24 A, 24 B which function as a point of application, whereby a correction torque distributed force Tad″ in the backward direction acts on the sun gear 21 B which functions as a point of action. Due to there being basically no torque that balances them, the correction torque Tad and the correction torque distributed force Tad′ and the correction torque distributed force Tad″, which are distributed force of the correction torque Tad, are not outputted to the planetary carriers 23 A, 23 B and are consumed to change the speeds of the sun gears 21 A, 21 B and the ring gears 24 A, 24 B. In association with this, the correction torque Tad and the correction torque distributed force Tad″ decrease the speeds of the sun gears 21 A, 21 B, that is, the speeds of the first and second motors 2 A, 2 B, respectively, and the correction torque distributed force Tad′ in the forward direction which acts on the ring gears 24 A, 24 B increases the speeds of the ring gears 24 A, 24 B and the planetary gear 22 A. By so doing, the speed of the first motor 2 A eventually becomes the motor target speed MA 2 , and the speed of the planetary gear 22 A eventually becomes the planetary gear target speed A 2 .

The speed of the sun gear 22 B, that is, the speed of the second motor 2 B is determined primarily by the speed of the planetary carrier 23 B which is coupled to the right rear wheel RWr and the speed of the ring gears 24 A, 24 B. When the speed difference DA at the first motor 2 A differs from the speed difference DB at the second motor 2 B, the speed of the second motor 2 B does not constitute the motor target speed MB 2 .

Then, when the speed of the planetary gear 22 A reaches the target planetary speed A 2 , as shown in FIG. 21( c ) , the correction torque Tad is removed. By so doing, the absolute values of the first motor torque TM 1 and the second motor torque TM 2 become equal to each other again, whereby the counterclockwise yawing moment M that is being produced is maintained.

It is preferable that the target torque used in executing the torque control is obtained based on the target yawing moment, and how to obtain this target torque is similar to that of the second embodiment, and therefore, the description thereof will be omitted here.

Next, a flow of the left and right opposite torque control according to the third embodiment will be described by reference to FIG. 23 .

By executing the left and right opposite torque control in such a state that the hydraulic brakes 60 A, 60 B are applied, firstly, it is detected in such a state that the yawing moment is produced continuously ( FIG. 21( a ) ) whether or not there exists a command to release the hydraulic brakes 60 A, 60 B (S 31 ). If it is determined as a result of the detection that there exists no such release command, the controlling process ends there.

If it is determined that there exists such a release command, the motor actual speed MA 1 of the first motor 2 A and the motor actual speed MB 1 of the second motor 2 B are obtained (S 32 ). Next, the planetary gear target speed A 2 is set near to the zero rotation so that the rotating direction of the planetary gear 22 A that is rotating is not reversed, and a motor target speed MA 2 of the first motor 2 A then is calculated. Similarly, the planetary gear target speed B 2 is set near to the zero rotation so that the rotating direction of the planetary gear 22 B that is rotating is not reversed, and a motor target speed MB 2 of the second motor 2 B then is calculated (S 33 ).

A speed difference DA between the motor actual speed MA 1 of the first motor 2 A and the motor target speed MA 2 of the first motor 2 A that were detected or calculated in S 32 and S 33 is calculated. Similarly, a speed difference DB between the motor actual speed MB 1 of the second motor 2 B and the motor target speed MB 2 of the second motor 2 B is calculated (S 34 ).

Next, it is detected whether or not the speed difference DA of the first motor 2 A is smaller than the speed difference DB of the second motor 2 B (S 35 ). If it is determined in step S 35 that the speed difference DA of the first motor 2 A is smaller than the speed difference DB of the second motor 2 B, the first motor 2 A is selected (S 36 ), whereas if it is determined in step S 35 that the speed difference DA of the first motor 2 A is larger than the speed difference DB of the second motor 2 B, the second motor 2 B is selected (S 37 ). In this way, by selecting the motor having the smaller speed difference as the motor to which the correction torque is to be added, that is, as the motor having the motor target speed, it is possible to restrain the motor to which the correction torque is not added, that is, the motor that does not have the motor target speed from being controlled excessively. Should the second motor 2 B which is the motor having the larger speed difference be selected as the motor to which the correction torque is to be added, that is, the motor having the motor target speed, the rotating direction of the planetary gear 22 A of the first epicyclic reduction gear 12 A is reversed, whereby the planetary gear 22 A is caused to rotate forwards.

As described above, by adding the correction torque to the motor having the smaller speed difference, there occurs no such situation that the rotating directions of the planetary gears 22 A, 22 B of the first and second epicyclic reduction gears 12 A, 12 B are reversed, whereby it is possible to prevent the occurrence of a backlash, thereby making it possible to prevent, in turn, a disturbance in torque that would otherwise be produced in the rear wheels Wr by the backlash produced.

The calculation of the motor target speed in the third embodiment may be executed in parallel with the calculation of the motor target speed in the first embodiment. Namely, the calculation of the motor target speed based on the planetary gear target speed may be executed in parallel with the calculation of the motor target speed based on the efficiencies of the motors and the electric power supply unit that supplies electric power to the motors. By adopting this approach, the consumed electric power can be reduced while preventing the occurrence of a backlash. However, in case there exists no motor speed that satisfies at the same time the motor target speed that is obtained based on the planetary gear target speed and the motor target speed that is obtained based on the efficiencies of the motors and the electric power supply unit that supplies electric power to the motors, it is preferable that the motor target speed that is obtained based on the planetary gear target speed should take priority over the motor target speed that is obtained based on the efficiencies of the motors and the electric power supply unit that supplies electric power to the motors. By so doing, the prevention of the occurrence of a backlash is allowed to take priority over the efficiencies of the motors, thereby making it possible to improve the comfortableness in the vehicle.

The calculation of the motor target speed in the third embodiment may be executed in parallel with the calculation of the motor target speed in the second embodiment. Namely, the calculation of the motor target speed based on the planetary gear target speed may be executed in parallel with the calculation of the motor target speed based on the ring gear target speed. By adopting this approach, it is possible to prevent a shock that would be produced when the hydraulic brakes 60 A, 60 B are applied or released while preventing the occurrence of a backlash. However, in case there exists no motor speed that satisfies at the same time the motor target speed that is obtained based on the planetary gear target speed and the motor target speed that is obtained based on the ring gear target speed, it is preferable that the motor target speed that is obtained based on the ring gear target speed should take priority over the motor target speed that is obtained based on the planetary gear target speed. By so doing, the prevention of the shock that would be produced when the hydraulic brakes 60 A, 60 B are applied or released is allowed to take priority over the occurrence of a backlash, thereby making it possible to increase the stability of the vehicle.

Thus, as described above, when the left and right opposite torque control is executed, both the motors of the first motor 2 A and the second motor 2 B are controlled based on the target torque of both the motors, and the motor target speed of the one motor of the first motor 2 A and the second motor 2 B is obtained based on the target speed of the planetary gears 22 A, 22 B. Then, the correction torque to allow the motors to attain the motor target speed is added only to the one motor. Thus, it is possible to control the one motor to attain the motor target speed while satisfying the target yawing moment even in the state where the ring gears 24 A, 24 B are allowed to rotate freely without being fixed by the hydraulic brakes 60 A, 60 B and the one-way clutch 50 . Then, the motor target speed is set so that the rotating direction of the planetary gears 22 A, 22 B that are rotating is not reversed, thereby making it possible to prevent a disturbance in torque that would otherwise be produced in the rear wheels Wr by the backlash produced.

The above embodiments do not limit but just exemplify the invention. The embodiments can be variously modified or improved.

For example, the hydraulic brakes 60 A, 60 B do not have to be provided individually for the ring gears 24 A, 24 B, and hence, at least one hydraulic brake and at least one one-way clutch should be provided on the coupled ring gears 24 A, 24 B. Either or both of the hydraulic brake and the one-way clutch may be omitted.

Although the hydraulic brakes are exemplified as the connection/disconnection unit, mechanical or electromagnetic brakes can also be selected, for example.

Although the first and second motors 2 A, 2 B are connected to the sun gears 21 A, 21 B, respectively, and the ring gears are coupled together, in the embodiments, the sun gears may be coupled together, and the first and second motors may be connected to the ring gears.

The front-wheel drive system may be such that the motor is used as a single drive source without using the internal combustion engine.