Vehicle braking / driving force control device

Abstract

When a braking / driving force for each wheel is used for vehicle motion control, a distribution adjustment part is provided to reduce noise generated by a zero-crossing of engine torque acting on a vehicle, wherein the vehicle is constructed such that a conversion rate of converting a braking / driving force on the wheel into a force in a vertical direction of a body by a suspension differs between a front wheel side and a rear wheel side. The distribution adjustment part sets distributions of a driver-requested driving force to front / rear wheels so that it is larger for the wheels coupled to the suspensions whose conversion rate is lower than for the wheels coupled to the suspensions whose conversion rate is higher.As a result, a zero crossing of a motor torque is restricted, resulting in a reduction in the generation of noise caused by kickback of speed reduction gears.

Description

BACKGROUND OF THE INVENTION1. Field of the InventionThe present invention relates to a vehicle braking / driving force control device for independently controlling a driving force and a braking force of each of front left and right and rear left and right wheels of a vehicle.2. Explanation of Prior ArtThere is already known a vehicle braking / driving force control apparatus for independently controlling a driving force and a braking force (collectively referred to as braking / driving force) for front left and right and rear left and right wheels of a vehicle. For example, a vehicle having an in-wheel motor is known as one form of an electric vehicle in which a motor is disposed inside or near the wheel of the vehicle, and the wheel is directly driven by the motor. In the in-wheel motor type vehicle, each motor is individually controlled to achieve driving performance or regeneration, and a driving torque or a braking torque acting on each wheel is thus individually controlled, thereby generating a braking / driving force for each wheel.Each wheel is suspended from a vehicle body via a suspension. Generally, as illustrated in FIGS. 2A and 2B, a current center of rotation Cf of a suspension for coupling a front wheel 10 fto a vehicle body B is positioned behind and above the front wheel 10 f, and a current center of rotation Cr of a suspension for coupling a rear wheel 10 rto the vehicle body B is positioned in front of and above the rear wheel 10 r. Therefore, a force Ff1in a traveling direction of the vehicle acts forward at a ground contact point of the front wheel 10 fwhen a driving torque acts on the front wheel 10 f, and a perpendicular force Fzf1is generated by the force Ff1at the ground contact point of the front wheel 10 fwhich biases the vehicle body B downward. Thus, as a result of driving the front wheel 10 f, the force acts in the direction of lowering the vehicle body B. On the other hand, as illustrated in FIG. 2B, when a braking torque is applied to the front wheel 10 f, a force Ff 2 acts rearward in the traveling direction of the vehicle at the ground contact point of the front wheel 10 f, and a perpendicular force Fzf 2 that biases the vehicle body B upward is generated by the force Ff 2 at the ground contact point of the front wheel 10 f. Thus, as a result of braking the front wheel 10f, the force acts in the direction of raising the vehicle body B.On the other hand, the generation direction of the vertical force to the rear wheel 10r is opposite to that to the front wheel 10f. In other words, a force Fr1in the vehicle's direction of travel acts forward at a ground contact point of the rear wheel 10 r, and a perpendicular force Fzr1which biases the vehicle body B upward via the suspension is generated by the force Fr1at the ground contact point of the rear wheel 10 rwhen a driving torque acts on the rear wheel 10 r. Thus, the force acts in the direction of raising the vehicle body B as a result of driving the rear wheel 10r. On the other hand, a force Fr 2 in the traveling direction of the vehicle acts rearward at the ground contact point of the rear wheel 10 rwhen a braking torque acts on the rear wheel 10 r, and a perpendicular force Fzr 2 biasing the vehicle body B downward via the suspension is generated by the force Fr 2 at the ground contact point of the rear wheel 10 r. Thus, as a result of braking the rear wheel 10r, the force acts in the direction of lowering the vehicle body B.In this way, a part of the longitudinal braking / driving force on the wheel is converted by the suspension into the force in the vertical direction of the body B. Thus, motion control (including body posture control such as restraint of roll motion, pitch motion, and lift motion) for the vehicle can be executed by controlling braking / driving force on each wheel. Therefore, a target braking / driving force for each wheel is calculated as a resultant force from a distributed driving force required by a driver. The distributed driving force required by the driver is obtained by distributing a driving force required by the driver, which is set based on an operation amount by a driver, and a control braking / driving force for executing the motion control for the vehicle to the individual wheels.On this basis, for example, a vehicle travel control device proposed in Japanese Patent Application Laid-Open No. 2009-143310, JP 2009-143310 A controls a braking / driving force of each in-wheel motor to restrict a roll motion of the vehicle.Incidentally, the direction of the target braking / driving force may be inverted depending on the state of movement of the vehicle when the movement control for the vehicle is executed by means of the braking / driving forces at the wheels. For example, the direction of the target braking / driving force is inverted when a relationship between the magnitudes of the two forces changes while the control braking / driving force is opposite to the direction of the distributed driving force required by the driver. In this case, the engine is switched from a state (the power supply) in which the driving force is generated at the wheel to a state (the regeneration) in which the braking force is generated at the wheel, or is switched from the state (the regeneration) in which the braking force is generated at the wheel to the state (the power supply) in which the driving force is generated at the wheel. Thus, inversion of the motor torque (so-called zero crossing of the motor torque) occurs. Therefore, noise is generated when the motor torque is transmitted to the wheel through reduction gears if a backlash is caused by the reversal of the motor torque.JP 2007 161 032 A discloses a vehicle control device according to the preamble of claim 1.BRIEF EXPLANATION OF THE INVENTIONThe present invention has been made in view of the above problem, and thus solves the object of restraining the generation of noise by a kickback.In order to achieve the above-described object, a feature of an embodiment of the present invention includes a vehicle braking / driving force control device including: a motor (30) provided on each of a front left wheel, a front right wheel, a rear left wheel, and a rear right wheel, which is capable of transmitting torque to each of the wheels via gears, thereby generating a braking / driving force on the respective one of the wheels; a suspension (20) for coupling the respective one of the front left wheel, the front right wheel, the rear left wheel, and the rear right wheel to a vehicle body, and converting the braking / driving force on the respective one of the wheels into a force in a vertical direction of the vehicle body; a target braking / driving force calculating means (50, S11 to S15) for calculating a target braking / driving force for each of the wheels, the target braking / driving force comprising: a distributed driving force required by the driver obtained by distributing a driving force required by the driver, which is set based on an operation amount by a driver, to each of the wheels; and a control braking / driving force for each of the wheels required for vehicle motion control; and a motor control means (50, S16) for controlling an operation of the motor by following the target braking / driving force, wherein the suspension is configured so that a conversion rate (tan(θf), tan(θr)) for converting the braking / driving force at each of the wheels into the force in the vertical direction of the body is different between a front wheel side and a rear wheel side, wherein the means for calculating the target braking / driving force includes distribution setting means (50, S13) for setting a distribution of the driver required driving force to the front and rear wheels so that a larger part of the driver required driving force is distributed to the wheels coupled to the suspension on a side whose conversion rate is smaller than to the wheels coupled to the suspension on a side on which the conversion rate is larger.In the present embodiment, the front / rear left / right wheels are coupled to the vehicle body via the suspensions. The motor is provided for each wheel. The motor transmits the torque to the wheel via the gears, thereby generating the braking / driving force (the force representing a driving force and a braking force) on the wheel. The braking / driving force on the wheel is converted into the force in the vertical direction of the body by the suspension. Vehicle movements can be controlled by controlling the vertical forces. For example, control of restraining a roll, a pitch, a lift, and the like of the vehicle may be performed.The target braking / driving force calculating means calculates the target braking / driving force for each of the wheels including the distributed driving force required by the driver, the control braking / driving force required for the vehicle motion control for each of the wheels by distributing the driving force required by the driver (the driving force required by the driver), the control braking / driving force required for the vehicle motion control based on the operation amount (the operation amount for controlling the travel of the vehicle). The motor controller controls the operation of the motors by following the desired braking / driving forces.The suspensions are configured such that the conversion rate in converting the braking / driving force into the force in the vertical direction of the body differs between the front wheel side and the rear wheel side. For example, the conversion rate is a value that matches a magnitude of an angle between a line connecting a ground contact point of a wheel and a current rotation center of the suspension for coupling the wheel to the body and a horizontal ground surface in a side view of the vehicle. Thus, the angle between the line connecting the ground contact point of the front wheel to the present rotation center of the suspension and the horizontal ground surface, and the angle between the line connecting the ground contact point of the rear wheel to the present rotation center of the suspension and the ground contact surface are not equal.Therefore, the wheel coupled to the suspension on the side whose conversion rate is smaller in converting the braking / driving force into the perpendicular force to the vehicle body needs to change the braking / driving force more than the wheel coupled to the suspension on the side whose conversion rate is larger when the motion control for the vehicle is performed via the braking / driving forces of the wheel. As a result, the torque of the motor for driving the wheel coupled to the suspension on the side whose conversion rate is smaller tends to be inverted. In other words, at the motor for the wheel coupled to the suspension on the side whose conversion rate is smaller, the zero crossing of the motor torque is more likely to occur. When the zero crossing of the engine torque occurs, noise is generated when a backlash of the gears is to be absorbed.Thus, in the present invention, the distribution setting device sets the distributions of the driver's required driving force to the front / rear wheels so that the distributions are larger for the wheels coupled to the suspensions on the side whose conversion rate is smaller than for the wheels coupled to the suspensions on the side whose conversion rate is larger. Thus, the distribution of the driver's required driving force to the smaller conversion rate side wheels can be increased, thereby restraining a frequency of zero crossing of the engine torque at the smaller conversion rate side wheels. As a result, the frequency of zero-crossing of the engine torque for the four wheels can be restricted as a whole although the driver's required driving force is obtained. Therefore, the noise generated by the rebound is restricted.An aspect of the present invention is that the distribution setting means sets, with respect to the distribution of the driver required driving force to the front wheels and the rear wheels, a ratio of the distribution to the wheels coupled to the suspensions on the side whose conversion rate is smaller to a constant set distribution ratio (tan θr / (tan θf+tan θr)) that is greater than 0.5 when the driver required driving force is less than a set driving force, and changes the ratio of the distribution (Fmay / Fd) such that the distribution ratio decreases within a range of the set distribution ratio or less when the driver required driving force increases if the driver required driving force is equal to or greater than the set driving force.In the present invention, the distribution setting ratio sets the ratio of the distribution at the wheels coupled to the suspension on the side whose conversion rate is smaller to the constantly set distribution ratio that is larger than 0.5 when the driver's required driving force is smaller than the set driving force. As a result, the frequency of zero-crossing of the motor torque at the wheels on the side whose conversion rate is smaller may be limited. When the driver required driving force is large and the set distribution ratio is maintained after the driver required distributed driving force reaches a driving limit value of the motor for the wheel coupled to the suspension on the side whose conversion rate is smaller, the driver required driving force is no longer distributed to the front / rear wheels. Thus, the distribution setting means changes the distribution ratio for the wheels coupled to the suspensions on the side whose conversion rate is smaller so that the distribution ratio decreases within the range of the set distribution ratio or smaller as the driver required driving force increases when the driver required driving force is equal to or greater than the set driving force. As a result, the driver's required driving force can be properly distributed to the front / rear wheels while restricting the frequency of zero-crossing of the engine torque at the wheels on the side whose conversion rate is smaller.An aspect of the present invention is that the distribution setting means uses, as the set distribution ratio, a value calculated when a sum (tanθf+tanθr) of the conversion rate of the suspensions for the front wheels and the conversion rate of the suspensions for the rear wheels is set as a denominator and the conversion rate (+tanθr) of the suspensions on the side whose conversion rate is larger is set as a numerator.In the present invention, the set distribution ratio is set to the value calculated when the sum of the conversion rate of the suspensions for the front wheels and the conversion rate of the suspensions for the rear wheels is set as the denominator and the conversion rate of the suspensions on the side whose conversion rate is larger is set as the numerator. Therefore, due to a reason described later, a distance to a zero crossing of the torque of the motor for the front wheel and a distance to a zero crossing of the torque of the motor for the rear wheel may be equal to each other. As a result, the driver's required driving force is appropriately transmitted to the front / rear wheels, and the frequency of zero crossing of the engine torque can thus be more appropriately limited for the four wheels as a whole.An aspect of the present invention is that a vehicle braking / driving force control device further includes vehicle speed acquisition means for acquiring a vehicle speed (S 131), and the distribution setting means sets, with respect to the distribution of the driver's required driving force to the front wheels and the rear wheels, a distribution ratio to the wheels coupled to the suspensions on the side whose conversion rate is less to a value greater than 0.5 (S 133) when the vehicle speed is less than a first set vehicle speed (V 1), and sets the distribution ratio to a value equal to or less than 0.5 (S 134) when the vehicle speed is equal to or greater than a second set vehicle speed (V 2) equal to or greater than the first set vehicle speed.In general, background sounds (such as engine sound, wind sound, and tire sound) are quiet during travel at a low vehicle speed, and therefore the driver tends to hear the sound caused by the backlash of the speed reduction gears, and an input from the road surface is often a large input with a low frequency. An amplitude of the control braking / driving force for the input from the road surface therefore increases, and the zero crossing of the motor torque tends to occur. In addition, the driver's required driving force is small and the motor torque itself is small, and the distance to the zero crossing is thus small. On the other hand, the background noise is loud during a high vehicle speed running, and the noise caused by the backlash of the speed reduction gears is therefore hardly heard by the driver. In addition, the input from the road surface is often a small input at high frequency, and the amplitude of the control braking / driving force for the road surface input therefore decreases. In addition, a torque that matches a large running resistance acts as the driver's required driving force, and the motor torque itself is thus large, and the distance to the zero crossing is large. Conversely, however, a distance in the driving direction of the motor torque decreases, and the motor torque tends to reach the driving limit first on the wheel on the side whose conversion rate by the vehicle motion control is smaller.Therefore, in the present invention, when the vehicle speed is less than the first set vehicle speed, the distribution setting means sets the ratio of distribution to the wheels coupled to the suspensions on the side whose conversion rate is less at the vehicle speed less than the first set vehicle speed to a value greater than 0.5. Thus, the zero crossing of the engine torque that may occur during low vehicle speed running is restricted, resulting in a reduction in noise caused by the kickback.On the other hand, during high vehicle speed running, the engine torques at the wheels on the side whose conversion rate is smaller tend to reach the drive limit earlier than the engine torques at the wheels on the side whose conversion rate is larger, but in the present invention, the distribution setting means sets the distribution ratio for the wheels coupled to the suspensions on the side whose conversion rate is smaller to a value equal to or smaller than 0.5 when the vehicle speed is equal to or larger than the second set vehicle speed equal to or larger than the first set vehicle speed. Therefore, such an error that the motor torques on the side whose conversion rate is smaller first reach the drive limit can be reduced, resulting in an increased vehicle movement control range.In the above description, reference numerals used in the embodiments are enclosed in parentheses and assigned to each configuration of the invention as appropriate to the embodiments in order to facilitate understanding of the invention, but each configuration requirement of the invention is not limited to the embodiments described by the reference numerals.BRIEF EXPLANATION OF THE FIGURESFIG. 1 is a schematic configuration diagram of a vehicle to which a vehicle brake / driving force control device according to an embodiment of the present invention is installed. FIGS. 2A and 2B are diagrams illustrating a relationship between a braking / driving force and a vertical force. FIG. 3 is a flowchart illustrating an engine control program according to a first embodiment of the present invention. FIG. 4 is a diagram illustrating target braking / driving torques during roll control according to the first embodiment of the present invention. FIG. 5 is a diagram showing one of target braking / driving forces during roll control according to a related art example. FIG. 6 is a diagram showing a characteristic of a driving force at a front wheel. FIG. 7 is a graph showing a characteristic of a driving force at a rear wheel. FIG. 8 is a graph showing a characteristic of a driving force distribution coefficient for a zero-crossing distance oriented distribution. FIG. 9 is a graph showing a characteristic of the driving force distribution coefficient for the zero-cross distance oriented distribution. FIG. 10 is a graph showing a characteristic of the driving force distribution coefficient for distribution oriented to a reserve force for vertical force generation according to a second embodiment of the present invention. FIG. 11 is a flowchart illustrating a program for setting a driving force distribution coefficient according to the second embodiment of the present invention.DETAILED EXPLANATION OF THE PREFERRED EMBODIMENTSNow, a detailed description will be given of embodiments of the present invention with reference to the figures. FIG. 1 schematically illustrates a configuration of a vehicle 1 in which a vehicle brake / driving force control device according to this embodiment is mounted.The vehicle 1 includes a front left wheel 10 fl, a front right wheel 10 fr, a rear left wheel 10 rl, and a rear right wheel 10rr. The front left wheel 10 fl, the front right wheel 10 fr, the rear left wheel 10 rl, and the rear right wheel 10rr are respectively suspended from a vehicle body via independent suspensions 20 fl, 20 fr, 20 rl, and 20rr.The suspensions 20 fl, 20 fr, 20 rl, and 20rr are coupling structures for coupling the vehicle body B to the wheels 10 fl, 10 fr, 10 rl, and 10rr with each other, and include link mechanisms 21 fl, 21 fr, 21 rl, and 21rr configured of suspension arms and the like, suspension springs 22 fl, 22 fr, 22 rl, and 22rr for supporting loads in the perpendicular direction and absorbing shocks, and shock absorbers 23 fl, 23 fr, 23 rl, and 23rr for attenuating vibration of a sprung mass (a vehicle body B). A well-known four-wheel independent suspension such as a suspension of the suspension of the suspension arm type and a suspension of the suspension strut type can be used as the suspension 20 fl, 20 fr, 20 rl, and 20 rr.Motors 30 fl, 30 fr, 30 rl, and 30rr are respectively installed inside the front left wheel 10 fl, the front right wheel 10 fr, the rear left wheel 10 rl, and the rear right wheel 10rr. The motors 30 fl, 30 fr, 30 rl, and 30rr, which are so-called in-wheel motors, are respectively disposed at unsprung locations of the vehicle 1 together with the front left wheel 10 fl, the front right wheel 10 fr, the rear left wheel 10 rl, and the rear right wheel 10rr, and are coupled to the front left wheel 10 fl, the front right wheel 10 fr, the rear left wheel 10 rl, and the rear right wheel 10rr, so that motor torques are transmittable to the respective wheels via speed reduction gears 31. In the vehicle 1, rotations of the respective motors 30 fl, 30 fr, 30 rl, and 30rr can be independently controlled to independently control the driving forces and the braking forces generated at the front left wheel 10 fl, the front right wheel 10 fr, the rear left wheel 10 rl, and the rear right wheel 10rr.Hereinafter, the respective wheels 10 fl, 10 fr, 10 rl, and 10rr, the suspensions 20 fl, 20 fr, 20 rl, and 20rr, the link mechanisms 21 fl, 21 fr, 21 rl, and 21rr, the suspension springs 22 fl, 22 fr, 22 rl, and 22rr, the shock absorbers 23 fl, 23 fr, 23 rl, and 23rr, and the motors 30 fl, 30 fr, 30 rl, and 30rr are generally referred to as the wheel 10, the suspension 20, the link mechanism 21, the suspension spring 22, the shock absorber 23, and the motor 30 unless a specific one needs to be identified. In addition, the front wheels 10 fland 10 frare referred to as a front wheel 10 f, and the rear wheels 10 rland 10rrare referred to as a rear wheel 10 rwhen the front wheels 10 fland 10 frand the rear wheels 10 rland 10rrhave to be distinguished from each other and identified among the wheels 10 fl, 10 fr, 10 rland 10rr. Similarly, the suspension 20, the link mechanism 21, the suspension spring 22, the shock absorber 23, and the motor 30 are referred to as front wheel suspension 20 f, front wheel link mechanism 21 f, front wheel suspension spring 22 f, front wheel shock absorber 23 f, and front wheel motor 30 fwhen identified on the front wheel side, and if identified on the rear wheel side, are referred to as rear wheel suspension 20 r, rear wheel link mechanism 21 r, rear wheel suspension spring 22 r, rear wheel shock absorber 23 r, and rear wheel motor 30 r.For example, brushless motors are used as the respective motors 30. The respective motors 30 are connected to a motor driver 35. The motor driver 35 includes inverters, for example, and four sets of inverters are provided for the respective motors 30. The motor driver 35 converts direct current supplied from a battery 60 into alternating current power, and supplies the alternating current power to the respective motors 30 one by one. As a result, the driving of the respective motors 30 is controlled to generate torques to apply the driving forces to the respective wheels. Such a state that the electric power is supplied to the motor 30 to generate the driving torque is referred to as power running.In addition, each motor 30 also functions as an electric power generator to generate electric power from rotational energy of each wheel 10, and to charge the battery 60 with the generated electric power via the motor driver 35. The braking torque generated by the electric power generation of the motor 30 applies the braking force to the wheel 10. A brake device is provided for each wheel 10, but is not directly related to the present invention, and description and illustration thereof will therefore be omitted.The motor driver 35 is connected to an electronic control unit 50 for motor control. The electronic control unit 50 for engine control (hereinafter referred to as engine ECU 50) includes a microcomputer that is constituted by a CPU, a ROM, and a RAM as main components and executes various programs to independently control the operation of the individual motors 30. The engine ECU 50 is connected to an operation state detector 40 for detecting an operation state of operations performed by the driver to guide the vehicle and a motion state detector 45 for detecting a motion state of the vehicle, and receives detection signals output from the detectors 40 and 45.The operation state detection device 40 is configured by an accelerator pedal sensor for detecting an accelerator operation amount by the driver based on a depression amount (or an angle or a pressure) of an accelerator pedal, a brake sensor for detecting a brake operation amount by the driver based on a depression amount (or an angle or a pressure) for a brake pedal, a steering angle sensor for detecting a steering operation amount of an operation of a steering wheel by the driver, and the like. The motion state detection device 45 is configured by an appropriate combination of a vehicle speed sensor for detecting a traveling speed of the vehicle body B, a yaw rate sensor for detecting a yaw rate of the vehicle body B, a sprung mass acceleration sensor for detecting an acceleration of the vehicle body B (the sprung mass) in the perpendicular direction at each wheel position, a lateral acceleration sensor for detecting a lateral acceleration in the lateral direction of the vehicle body B, a pitch rate sensor for detecting a pitch rate of the vehicle body B, a roll rate sensor for detecting a roll rate of the vehicle body B, a stroke sensor for detecting a stroke amount of the respective suspensions 20, and the like, an unsprung mass acceleration sensor for detecting acceleration of an unsprung mass of each wheel 10 in the vertical direction, and the like. A direction of a sensor value including a direction element is detected depending on the sign thereof.As illustrated in FIGS. 2A and 2B, the suspension 20 for suspending each wheel 10 is configured such that, in the side view of the vehicle, a present center of rotation Cf (a present center for the front wheel 10 fwith respect to the vehicle body B and a present pole, respectively) of the front wheel suspension 20 fis positioned behind and above the front wheel 10 f, and a present center of rotation Cr (a present center for the rear wheel 10 rwith respect to the vehicle body B) of the rear wheel suspension 20 ris positioned in front of and above the rear wheel 10 r. In addition, a relationship is satisfied that θr is larger than θf (θf<θr) when an angle (smaller and acute angle) formed by a ground horizontal surface and a line connecting a ground contact point of the front wheel 10 fwith the present rotation center Cf is denoted by θf, and an angle (smaller angle) formed between the ground horizontal surface and a line connecting a ground contact point of the rear wheel 10 rwith the present rotation center Cr is denoted by θr. Hereinafter, θf is referred to as an instantaneous rotation angle θf, and θr is referred to as an instantaneous rotation angle θr.In this structure (this geometry) of the suspension 20, a force Ff 1 in a vehicle traveling direction acts forward at the ground contact point of the front wheel 10 f, and a perpendicular force Fzf 1 biasing the vehicle body B downward is generated by the force Ff 1 at the ground contact point of the front wheel 10 fwhen a driving torque acts on the front wheel 10 f. Thus, as a result of driving the front wheel 10 f, the force acts in the direction of lowering the vehicle body B. On the other hand, a force Ff 2 in the vehicle traveling direction acts rearward at the ground contact point of the front wheel 10 f, and a vertical force Fzf 2 biasing the vehicle body B upward is generated by the force Fzf 2 at the ground contact point of the front wheel 10 fwhen a braking torque acts on the front wheel 10 f, as illustrated in FIG. 2. Thus, as a result of braking the front wheel 10 f, the force acts in the direction of raising the vehicle body B. In addition, as illustrated in FIG. 2A, a force Fr 1 acts forward in the traveling direction of the vehicle at a ground contact point of the rear wheel 10 rwhen a driving torque acts on the rear wheel 10 r, and a perpendicular force Fzr 1 biasing the vehicle body B upward is generated by the force Fr 1 at the ground contact point of the rear wheel 10 r. Thus, as a result of driving the rear wheel 10 r, the force acts in the direction of raising the vehicle body B. On the other hand, as illustrated in FIG. 2B, a force Fr 2 acts rearward in the traveling direction of the vehicle at the ground contact point of the rear wheel 10 rwhen a braking torque acts on the rear wheel 10 r, and a perpendicular force Fzr 2 biasing the vehicle body B downward is generated by the force Fr 2 at the ground contact point of the rear wheel 10 r. Thus, as a result of braking the rear wheel 10 r, the force acts in the direction of lowering the vehicle body B. The suspension 20 thus converts the driving force and the braking force on the respective wheel 10 into the force in the perpendicular direction of the vehicle body B.Thus, the force in the perpendicular direction can act on the vehicle body B by controlling the driving force or the braking force (braking / driving force) on the wheel 10, resulting in control of the moving state of the vehicle. Hereinafter, an absolute value of the magnitude will be used when a magnitude of the braking / driving force is discussed.The engine ECU 50 calculates a driver required driving force Freq based on the accelerator operation amount detected by the operation state detection device 40, and calculates a control braking / driving force Fcx independently for each of the wheels 10 based on the vehicle movement state detected by the movement state detection device 45. Then, as described later, the engine ECU 50 distributes the driver required driving force Freq to the four wheels to calculate a driver required distributed driving force Fdx for each wheel 10. In addition, the engine ECU 50 sets a sum of the distributed driving force Fdx required by the driver and the control braking / driving force Fcx as a target braking / driving force Fx for each of the wheels 10. The motor ECU 50 controls the motor driver 35 to generate output torque corresponding to the target braking / driving force Fx for the respective motor 30. Note that the control braking / driving force Fcx, the driver required distributed driving force Fdx, and the target braking / driving force Fx respectively refer to the control braking / driving forces, the driver required distributed driving forces, and the target braking / driving forces at the respective wheels 10.A magnitude of the vertical force acting on the vehicle body B is a value obtained by multiplying the braking / driving force Ff (Ff 1 or Ff 2) by tan(θf) for the front wheel 10 fside, and is a value obtained by multiplying the braking / driving force Fr (Fr 1 or Fr 2) by tan(θr) for the rear wheel 10 rside. These terms of tan(θf) and tan(θr) each represent a conversion rate for converting the braking / driving force into the perpendicular force on the vehicle body B (which will be referred to as "vertical force conversion rate" hereinafter). The function of converting the braking / driving force into the perpendicular force to the body B is provided mainly by the link mechanism 21. However, the final vertical force conversion rate is determined by the positions of the instantaneous centers of rotation Cf and Cr, and the instantaneous centers of rotation Cf and Cr also depend on the suspension springs 22 and shock absorbers 23, and the entire suspension 20 can thus be regarded as a part for converting the braking / driving force into the vertical force on the body B.In the vehicle according to this embodiment, the suspension 20 ffor the front wheel 10 fhas a lower conversion rate for the vertical force than the suspension 20 rfor the rear wheel 10 r. Therefore, the amount of change in the braking / driving force for the front wheel 10f is larger than that for the rear wheel 10r when vertical forces are generated at the same level at the front / rear wheels 10f and 10r for the motion control of the vehicle.For example, such a case is considered that the engine 30 is operated to generate the yaw motion of the vehicle. When the yaw motion is generated, the control braking / driving force Fcx in the forward direction is applied to an outer wheel turning on a curve, and the control braking / driving force Fcx having the same magnitude and opposite to the direction of the control braking / driving force Fcx acting on the outer wheel turning on an inner wheel turning on a curve. Therefore, a roll moment (hereinafter referred to as a front wheel side roll moment) generated by the driving forces Ff1 on the front wheels 10f and a roll moment (hereinafter referred to as a rear wheel side roll moment) generated by the driving forces Fr1 on the rear wheels 10r act on the body B. The front wheel side roll moment and the rear wheel side roll moment are opposite to each other in the direction. In this case, the vertical force conversion rate for the rear wheels 10r is greater than the vertical force conversion rate for the front wheels 10f, and the rear wheel side roll moment is greater than the front wheel side roll moment. Therefore, the control braking / driving force Fcx for the front wheel 10 fmust be larger than that for the rear wheel 10 rin order to balance the front wheel-side roll moment and thus the rear wheel-side roll moment with each other. To meet this need, the control braking / driving force Fcx for the front wheels 10f is larger than for the rear wheels 10r when a lateral yaw motion is generated on the vehicle while the roll control is being executed.FIG. 5 shows transitions of target braking / driving forces Ffl and Ffr to the front wheels 10 fl and 10 fr and target braking / driving forces Frl and Frr to the rear wheels 10 rl and 10rr when the vehicle is controlled to generate the lateral yaw motion while the roll control is being executed. This graph shows such an example that the driver required driving force Freq is distributed uniformly to the front / rear wheels 10 fand 10 r(in other words, distributed uniformly to the four wheels) as in the related-art apparatus. When a request for the yaw motion occurs at a time point t 1, the control braking / driving force Fcx for the yaw motion is added to the distributed driving force Freq / 4 required by the driver, which is distributed to the four wheels. In this case, the control braking / driving force Fcx for the front wheel 10 fis set to a larger value than for the rear wheel 10 rto balance the roll moment on the front wheel side and the roll moment on the rear wheel side with each other, in other words, to generate the perpendicular forces of the same magnitude on the front wheel 10 fside and on the rear wheel 10 rside. Then, the direction of the control braking / driving force Fcx changes each time the direction of the yaw motion changes. Thus, as shown in FIG. 5, the variation width of the control braking / driving force Fcx of the front wheel 10 fis larger than that of the rear wheel 10 r. As a result, signs (positive or negative) of the target braking / driving forces Ffl and Ffr for the front wheels 10 fl and 10 fr are more likely to be inverted. On the other hand, signs (positive or negative) of the target braking / driving forces Frl and Frr for the rear wheels 10 rl and 10rr are less likely to be inverted. Therefore, an operation of the engine 30 fon the front wheel 10 fshifts from the power supply for regeneration or from the regeneration for power supply during the roll motion control. In other words, the zero crossing of the engine torque occurs at the front wheels 10 f. Therefore, the noise is generated at every zero crossing of the engine torque when a kickback of the speed reduction gears 31 is received.Therefore, in this embodiment, the distribution of the driver required driving force Freq to the wheels 10 on the side whose vertical force conversion rate is smaller is increased to increase a distance for reversing the sign of the target braking / driving forces Fx, in other words, to increase a distance for zero crossing of the engine torque, thereby restraining generation of noise due to rebound.FIG. 3 illustrates an engine control program for solving the problem. The engine ECU 50 repeats the engine control program in a short cycle set in advance. When this program starts, the engine ECU 50 first detects the driver operation state and the vehicle movement state in step S 11. In this case, the engine ECU 50 acquires the accelerator operation amount, the brake operation amount, and the steering operation amount, which are acquired from sensor values of the operation state detection device 40, and acquires motion state amounts representing degrees of the vehicle speed and motion states (the yaw motion, the roll motion, the pitch motion, and the lift motion) of the vehicle body acquired from the sensor values acquired from the motion state detection device 45.Then, in step S 12, the engine ECU 50 calculates the driver required driving force Freq based on the accelerator operation amount. The driver required driving force Freq is a driving force in a front-rear direction of the vehicle, which generation is required by the driver for the entire vehicle, namely, a driving force for traveling (a force having such a direction as to increase the rotation speed of the wheels). The engine ECU 50 stores link data such as a map for deriving the driver required driving force Freq from the accelerator operation amount, and uses the link data to calculate the driver required driving force Freq. For example, the driver required driving force Freq is set to such a value as to increase as an accelerator operation amount (such as an accelerator opening degree) increases. In this case, the vehicle speed may be considered such that the driver required driving force Freq is corrected to decrease as the vehicle speed increases.Then, in step S 13, the engine ECU 50 calculates a driving force distribution coefficient α for the front / rear wheels 10 fand 10 r. On this occasion, the driving force distribution coefficient α represents a ratio of the driver's required driving force Freq distributed to the front wheels 10 f. Thus, the ratio of distribution to the rear wheels 10 ris represented as (1-α). The driving force distribution coefficient α calculated in step S 13 is set so that the distribution of the driver required driving force Freq is larger for the front wheels 10 fthan for the rear wheels 10 r. In other words, the driving force distribution coefficient α is set to a value of more than 0.5 and equal to or less than 1.Now, a description will be given of the calculation of the driving force distribution coefficient α between the front wheels 10f and the rear wheels 10r. In this embodiment, the distribution ratio of the driver required driving force Freq to the front / rear wheels 10 fand 10 ris set so that the distance to the zero crossing of the torque is the same for the front and rear wheels 10 fand 10 r. In other words, the distribution of the driver required driving force Freq to the front wheels 10 fon the side whose vertical force conversion rate is smaller is increased so that the distance to the zero crossing of the motor torque is equal for the front / rear wheels 10 fand 10 r. A description will now be given of a specific idea for this distribution.1. Vertical forceWhen the driving force to the front wheel 10 fis denoted by Ff and the driving force to the rear wheel is denoted by Fr, the vertical force Fzf generated from the driving force Ff to the front wheel 10 fand the vertical force Fzr generated from the driving force Fr to the rear wheel 10 rare represented by the following equations (defining: θf=tanθf) (defining: θr=tanθr)2. Driving forces on the front and rear wheels after distributionWhen the driver's required driving force Freq is represented as Fd and the driving force distribution coefficient α for the driving force distribution ratio for the front wheel 10 fis represented as α (0≤α≤1), the driving force distribution ratio for the rear wheel 10 ris represented by (1-α). The driving force Ff for the front wheel 10 fand the driving force Fr for the rear wheel 10 rmay be represented by the following equations using the driving force distribution coefficient α.When α is 0, 100% of the driver required driving force Freq is distributed to the rear wheel 10 r, and when α is 1, 100% of the driver required driving force Freq is distributed to the front wheel 10 f.3. Vertical Force After DistributionThe vertical force Fzf generated by the driving force Ff to the front wheel 10 fand the vertical force Fzr generated by the driving force Fr to the rear wheel 10 rafter distributing the driver's required driving force Freq to the front and rear wheels 10 fand 10 rmay be represented by the following equations.4. Driving force distribution ratio α for canceling zero crossing distance for the front / rear wheelsWhen the driver's required driving force Freq is distributed to the front / rear wheels 10f and 10r and the driving force distribution ratio is determined so that the perpendicular forces are generated in the same magnitude for the front / rear wheels 10f and 10r, it is assumed that the distances to the zero crossing of the engine torque can be made equal for the front / rear wheels 10f and 10r. Thus, α is selected to satisfy the following relationship.As described above, the relationships Fzf=Ff×Θf and Fzr=Fr×Θr hold, and the relationship Fzf=Fr is thus rewritten into the following equation.Thus, a relationship represented by the following equation is obtained.This relational equation is represented using the driving force distribution coefficient α as the following equation.Thus, the driving force distribution coefficient α can be represented as the following equation. θr and θf are constant values, and the driving force distribution coefficient α is thus also a constant value.5. Driving force distribution ratio α after the front / rear driving forces come into the saturation regionWhen the driving force distribution coefficient α is set as described above, the driving force on the front wheel 10 f, which is the wheel on the side whose vertical force conversion rate is smaller, is saturated earlier than the driving force on the rear wheel 10 r. In other words, the driving force reaches the driving limit of the motor 30, on this occasion, the driving force Ff at the front wheels 10 fis represented as Ff=α·Ff before saturation when the maximum driving force, which is the maximum value of a sum of the driving forces on the left and right two wheels, is represented as Fmax. Moreover, Fd is represented by the following equation using the relationship of Fmax=fd·α when the driving force Ff on the front wheels 10f comes into the saturation region (Ff=F).Thus, as shown in FIG. 6, the driving force Ff on the front wheels 10 ftakes a value of α·Fd in the range where Fd is less than Fmax / α, and is maintained at a value of the constant maximum driving force Fmax when Fd is equal to or greater than Fmax / α.On the other hand, the driving force Fr to the rear wheels 10r is set to a value depending on Fd as shown in FIG. 7. In other words, the driving force Fr to the rear wheels 10 ris represented as Fr=(1-α)·Fd before the driving force Ff to the front wheels 10 ftakes saturation. Moreover, the driving force Fr to the rear wheels 10r is represented as the following equation when the driving force Ff to the front wheels 10f saturates.Moreover, a total increase in the driver's required driving force Freq is added to the driving force Fr for the rear wheels 10 rafter the driving force Ff at the front wheels 10 ftakes saturation. Thus, the driving force Fr at the rear wheels 10 ris represented as the following equation after the driving force 10 fat the front wheels 10 fgoes into saturation.Thus, the following relational equation can be acquired using the driving force distribution coefficient α after the driving force Ff to the front wheels 10 fis saturated.This equation is solved in the following equation by the driving force distribution coefficient α.FIG. 8 shows the driving force distribution coefficient α set depending on Fd as described above. The driving force distribution coefficient α is set to a constant value of (tan θr) / (tan θf+tan θr) when Fd is set to be less than Fmax·(tanθf+tanθr) / (tanθr), and is set to Fmax / Fd when Fd is equal to or greater than Fmax·(tanθf+tanθr) / (tanθr).On this occasion, the driving force distribution ratio α can also be reproduced as follows using the maximum driving force ratio A (0≤A≤1) when the value of Fd / 2Max is set as a maximum driving force ratio A after the driving force Ff on the front wheels 10 fgoes saturation.A state in which the maximum driving force ratio A is 0 (A=0) means that the driver required torque Freq is 0 (Freq=0) and a state in which the maximum driving force ratio A is 1 (A=1) means that the driver required torque Freq becomes the maximum value.FIG. 9 is a graph showing the driving force distribution coefficient α while the maximum driving force ratio A is associated with the horizontal axis.The engine ECU 50 thus sets the driving force distribution coefficient α based on the driver required driving force Freq (=Fd). Alternatively, the accelerator opening degree may be used to set the driving force distribution coefficient α when the accelerator opening degree is regarded as the maximum driving force ratio A.After the engine ECU 50 sets the driving force distribution coefficient α in step S 13, the engine ECU 50 proceeds to step S 14 in the process. In step S 14, the motor ECU 50 calculates the control braking / driving force Fcx for each wheel 10, namely, the control braking / driving force Fcfl for the front left wheel 10 fl, the control braking / driving force Fcfr for the front right wheel 10 fr, the control braking / driving force Fcrl for the rear left wheel 10 rl, and the control braking / driving force Fcrr for the rear right wheel 10 rr. The vehicle motion control is performed in a case where the difference between the ideal yaw rate and the actual yaw rate detected by the yaw rate sensor exceeds an allowable value, or in a case where at least one of the roll state amount, the pitch state amount, and the lift state amount exceeds an allowable value. Therefore, the control brake / drive force Fcx is set to zero when the vehicle movement control does not need to be performed.For example, the control braking / driving force Fcx for each wheel 10 is calculated using a target roll moment Mx for suppressing a roll motion of the vehicle body about a front-rear axis (roll axis) passing through the center of gravity Cg of the vehicle, a target pitch moment My for suppressing a pitch motion of the vehicle body about a lateral axis (pitch axis) passing through the center of gravity Cg of the vehicle, a target yaw moment Mz for rotating the vehicle body about a vertical axis (yaw axis) passing through the center of gravity Cg of the vehicle, and a target lift force Fz for suppressing a lift motion (a jackknife) that is a vertical motion at the position of the center of gravity Cg of the vehicle. Various well-known calculation methods may be used to calculate these target values. For example, the engine ECU 50 uses the sensor values detected by the stroke sensors and the sprung mass vertical acceleration sensors to detect the positions, the speeds, and the accelerations in the vertical direction on the four wheels, thereby detecting the roll state amount, the pitch state amount, and the lift state amount, and calculates the target roll torque Mx, the target roll torque My, and the target lift force Fz having predetermined relationships with these detected state amounts to compensate for these movements. Moreover, based on the difference between the ideal yaw rate set based on the steering angle and the vehicle speed and the actual yaw rate detected by the yaw rate sensor, the motor ECU 50 calculates the target yaw moment Mz set to eliminate the difference.The engine ECU 50 calculates the control braking / driving forces Fcfl, Fcfr, Fcrl, and Fcrr using the following equation, for example.Here, the symbol tf represents a track width between the front left and right wheels 10f, and the symbol tr represents a track width between the rear left and right wheels 10r. The symbol Lf represents a horizontal distance in the front-rear direction between the center of gravity Cg of the vehicle and a center of the front left or right wheel 10 f, and the symbol 10 rrepresents a horizontal distance in the front-rear direction between the center of gravity Cg of the vehicle and a center of the rear left or right wheel 10 r.In this case, the engine ECU 50 selects three from among the target roll torque Mx, the target pitch torque My, the target yaw torque Mz, and the target lift force Fz to calculate the target braking / driving forces Fcfl, Fcfr, Fcrl, and Fcrr. This is because the braking / driving forces to be finally generated at the respective wheels 10 are determined by the driver required driving force Freq, in other words, there is such a limitation that a sum of the control braking / driving forces Fcfl, Fcfr, Fcrl, and Fcrr is set to zero, and thus the four target values cannot be simultaneously used for the calculation. In this case, the engine ECU 50 preferably selects the target yaw moment Mz and the target roll moment Mx when the yaw motion control is necessary, and uses these two target values Mz and Mx and one of the remaining target pitch moment My and the target lift force Fz for the calculation.Then, in step S 15, the engine ECU 50 calculates a final target braking / driving force Fx for each wheel 10, namely, a target braking / driving force Ffl for the front left wheel 10 fl, a target braking / driving force Ffr for the front right wheel 10 fr, a target braking / driving force Frl for the rear left wheel 10 rl, and a target braking / driving force Frr for the rear right wheel 10rr, by the following equations. A common driving force distribution coefficient (α, (1-α)) is used for the left and right wheels.As described above, α is set as follows.Before the driving force Ff at the front wheels 10f saturates:After the driving force Ff at the front wheels 10f saturates:Then, in step S 16, the motor ECU 50 converts the target braking / driving force Fx into a target motor torque Tx for driving the motor 30, and outputs a braking / driving force command signal corresponding to the target motor torque Tx to the motor driver 35. When the target motor torque Tx represents a driving torque, a current flows from the motor driver 35 to the motor 30, and when the target motor torque Tx represents a braking torque, a current flows from the motor 30 to the battery 60 via the motor driver 35, The power running control or the regenerative control will act on the motors 30 in this manner, resulting in generation of the target braking / driving force Fx at each wheel 10.The motor ECU 50 outputs the braking / driving force command signal to the motor driver 35, and then once terminates the motor control program. Then, the engine ECU 50 repeats the engine control program in the short cycle set in advance.The vehicle braking / driving force control device according to this embodiment, as described above, sets the ratio of the distribution of the driver required driving force Freq to the front / rear wheels 10 fand 10 rsuch that the distance to the zero crossing of the motor torque is the same for the front and rear wheels 10 fand 10 r. With this, the distribution of the driver's required driving force Freq to the front wheels 10 fthat are the wheels on the side whose vertical force conversion rate is smaller increases. As a result, the number of times of zero-crossing of the engine torque for the four wheels can be restricted as a whole while obtaining the driver's required driving force Freq. For example, when the yaw motion of the vehicle is generated and the control braking / driving forces Fcfl and Fcfr on the front wheels 10 fare larger than the control braking / driving forces Fcrl and Fcrr on the rear wheels 10 rto balance (make equal) the front wheel-side roll moment and the rear wheel-side roll moment with each other, as shown in FIG. 4, the zero crossing distances a for the front / rear wheels 10 fand 10 rare approximately equal to each other, and thus the motor torque is prevented from making the zero crossing at all four wheels. Thus, a generation frequency and a generation level of the noise and vibration caused by the kickback can be restricted. Moreover, the driving force distribution coefficient α is decreased as the driver required driving force Freq increases so that the total increase of the driver required driving force Freq is distributed to the driving forces for the rear wheels 10 rwhen the driving force distribution coefficient α is set as shown in FIG. 8 after the driving forces on the front wheels 10 fachtain the driving limit, and the distribution of the driver required driving force Freq can thus be appropriately distributed to the front / rear wheels 10 fand 10 rwhile the frequency of zero-crossing of the motor torque at the front wheel 10 fis limited.[Second Embodiment]A description will now be given of a second embodiment of the present invention. The above-described embodiment will be referred to as a first embodiment below. In the first embodiment, the distribution ratio of the driver required driving force Freq to the front / rear wheels 10 fand 10 ris set so that the distance to the zero crossing of the motor torque is the same for the front and rear wheels 10 fand 10 r. In this case, it can be suppressed that noise and vibration caused by the kickback are generated, but the target braking / driving forces Ffl and Ffr on the front wheels 10 f, which are the wheels on the side whose vertical force conversion rate is smaller, first reach the driving limit of the motors 30. For example, as described above, the maximum value of the control braking / driving forces Fcfl and Fcfr for the front wheels 10 fis larger than the maximum value of the control braking / driving forces Fcrl and Fcrr for the rear wheels when the roll control is executed to balance the front wheel-side roll moment and the rear wheel-side roll moment with each other. Therefore, the target braking / driving force Ffl or Ffr for the front wheel 10f during turning (the outside-turning wheel) can first leave the control range of the engine 30 depending on the running state of the vehicle. In this case, the control range of the vehicle movement control decreases.Therefore, in the second embodiment, distribution (hereinafter referred to as a vertical force generation reserve force distribution) of the driver required driving force is performed for the driver required driving force instead of the zero crossing distance distribution depending on the running state of the vehicle in which the vertical force generation reserve forces for the front / rear wheels 10f and 10r are equal, while distribution (hereinafter referred to as a zero crossing distance distribution) of the driver required driving force is performed as in the first embodiment in which the zero crossing distances of the motor torque for the front / rear wheels 10f and 10r are equal.First, a description will be given of the distribution oriented to the reserve force for generating the vertical force. In the distribution oriented to the reserve force for generating the vertical force, the driver required driving force Freq is distributed to the front / rear wheels 10 fand 10 rsuch that the reserve forces of the vertical forces that can be generated by the driving force Ff at the front wheels 10 fand the reserve forces of the vertical forces that can be generated by the driving force Fr at the rear wheels 10 rare equal to each other. In this specification, "1st vertical force", "2nd driving forces on the front and rear wheels after distribution" and "3rd vertical forces after distribution" described in the first embodiment are conventional, and therefore these conventional parts are omitted, and the following different parts will be described.4. Maximum vertical forceWhen the maximum driving force that can be generated by each of the front wheel 10 fand the rear wheel 10 ris represented by Fmax, the maximum force in the perpendicular line Fzfmax that can be generated by the driving force Ff at the front wheel 10 fand the maximum vertical force Fzrmax that can be generated by the driving force Fr at the rear wheel 10 rcan be represented by the following equations.5. Reserve Vertical ForceWhen the reserve vertical force that can be generated by the driving force Ff at the front wheel 10 fis represented by Fzfc (which is referred to as the front wheel vertical reserve force Fzfc), and the reserve vertical force that can be generated by the driving force Ff or Fr at the rear wheel 10 ris represented by Fzrc (which is referred to as the rear wheel reserve vertical force Fzrc), the front wheel reserve vertical force Fzfc and the rear wheel reserve vertical force Fzrc can be represented by the following equations.6. Driving force distribution ratio α when reserve vertical forces between the front and rear wheels are balancedWhen the front reserve vertical force Fzfc and the rear reserve vertical force Fzrc are balanced, only a particular wheel is prevented from reaching the drive limit earlier. In this case, the driving force distribution coefficient α may be set as follows.On this occasion, Fd / (2F)=A (the maximum driving force ratio A) and Θr / Θf=D (the front / rear vertical force conversion ratio) are used to represent the above-explained equation as follows.When the equation is solved for the driving force distribution coefficient α, the equation can be represented as follows.7. Consideration of Driving Force Distribution Coefficient αCase of A=0The driving force distribution coefficient α can be represented by the following equation.The vertical force conversion ratio D fore / aft is a value greater than 1, and (1-D) is thus a negative value. Therefore, the driving force distribution coefficient α is a negative infinite value (α=-∞).A possible range that the driving force distribution coefficient α can take is between 0 and 1 (0≤α≤1), and thus the driving force distribution coefficient α needs to be zero (α=0).Case of A=1When a value A=1 is substituted into the above-mentioned equation, the driving force distribution coefficient α is 0.5 (α=0.5).On this occasion, the maximum driving force ratio A satisfying α=0 is obtained.Therefore, as shown in FIG. 10, the driving force distribution coefficient α is set to zero when the maximum driving force ratio A is less than (D-1) / (2D), and is set to (1-D+2AD) / (2A(1+D)) when the maximum driving force ratio A is equal to or greater than (D-1) / (2D).When the driving force distribution coefficient α is set in this manner, only a specific wheel 10 can be prevented from reaching the driving limit earlier.In the vehicle braking / driving force control device according to the second embodiment, the motor ECU 50 switches the calculation form of the driving force distribution coefficient α depending on the vehicle speed as follows. FIG. 11 illustrates a program for setting the driving force distribution coefficient executed by the engine ECU. This driving force distribution coefficient setting program is executed instead of the processing of step S 13 according to the first embodiment. Thus, the second embodiment is different from the first embodiment only in such a point that this program for setting the driving force distribution coefficient is executed, and is the same as the first embodiment in other constructions.After completion of the calculation of the driver required driving force Freq in step S 12 of FIG. 3, the engine ECU 50 starts the driving force distribution coefficient setting program illustrated in FIG. 11. When the program for setting the driving force distribution coefficient starts, the engine ECU 50 first reads the vehicle speed V detected by the vehicle speed sensor in step S131. Then, in step S 132, the engine ECU 50 determines a vehicle speed range (low vehicle speed, medium vehicle speed, and high vehicle speed) using the read current vehicle speed V. From these vehicle speed ranges, a range of 0≤V<V1 is set as the low vehicle speed, a range of V1≤V<V2 is set as the medium vehicle speed, and a range of V2≤V is set as the high vehicle speed, and these ranges are set in advance.When the vehicle speed V is the low vehicle speed, the engine ECU 50 sets a driving force distribution coefficient setting mode to the zero crossing distance oriented distribution mode in step S133. In the zero crossing distance oriented distribution mode, the calculation methods for the driving force distribution coefficient α illustrated in FIGS. 8 and 9 are selected as in the first embodiment. Moreover, the engine ECU 50 sets the mode for setting the driving force distribution coefficient to the distribution mode oriented to the reserve force for generating the vertical force when the vehicle speed V is the high vehicle speed. In the distribution mode oriented to the reserve force for generation of the vertical force, the calculation method for the driving force distribution coefficient α is selected as described in "6" and "7" and illustrated in FIG. 10. In addition, the engine ECU 50 sets the driving force distribution coefficient setting mode to a medium distribution mode in step S 135 when the vehicle speed V is the medium vehicle speed. In the middle distribution mode, a calculation method is selected for the driving force distribution coefficient α, which is a middle mode between the zero crossing distance oriented distribution mode and the reserve force vertical force generation oriented distribution mode. In this example, the calculation method is set to a calculation method using an average (α1+α2) / 2 of the driving force distribution coefficient (denoted as α1) calculated by the zero crossing distance oriented distribution mode and the driving force distribution coefficient (denoted as α2) calculated using the vertical force generation reserve force oriented distribution mode.After setting the setting mode for the driving force distribution coefficient in any of steps S 133, S 134, and S 135, in subsequent step S 136, the engine ECU calculates the driving force distribution coefficient α using the calculation method set by the set mode. When the engine ECU completes the calculation of the driving force distribution coefficient α in step S 136, the engine ECU terminates this routine and starts the processing of step S 14 of FIG. 3.In the second embodiment, the distribution mode oriented at the distance from the zero crossing is selected in the case of the low vehicle speed, and the distribution mode oriented at the reserve force for generating the vertical force is selected in the case of the high vehicle speed. A reason for this selection will now be explained. Background noise (such as engine noise, wind noise, and tire noise) is small at the low vehicle speed, and therefore the driver tends to hear the noise caused by the backlash of the reduction gears 31. In addition, a road surface displacement is often a large displacement having a small frequency, and an amplitude of the control braking / driving force Fcx for the road surface displacement therefore increases, and the zero crossing of the motor torque tends to occur. Moreover, the motor torque itself is small, and therefore the distance to the zero crossing is small.On the other hand, the background noise is high during travel at the high vehicle speed, and therefore the driver can hardly hear the noise caused by the backlash of the speed reduction gears 31. Moreover, the input of the road surface is often a small input having a high frequency, and the amplitude of the control braking / driving force Fcx for the road surface input therefore decreases. In addition, a torque associated with a large running resistance also acts as the driver's required driving force, and therefore the motor torque itself is large, and the distance to the zero crossing is large, but in such a case, it is assumed that the reserve force in the driving direction is small.Therefore, in the second embodiment, the zero crossing distance oriented distribution mode at the low vehicle speed is selected to efficiently reduce the generation of the noise generated by the kickback. Moreover, the distribution mode oriented to the reserve force for generating the vertical force is selected at the high vehicle speed, thereby preventing occurrence of such a defect that the reserve force for generating the vertical force is lost to the front wheels 10f earlier than to the rear wheels 10r, resulting in a broader control range of the vehicle motion control. For example, even when the roll control is executed simultaneously with the yaw motion control in the high-speed traveling, the target braking / driving force Fx on the front wheel 10 flis less likely to reach the output limit value, resulting in appropriate roll control. In addition, the driving force distribution coefficient α in the middle mode between the zero crossing distance oriented distribution mode and the reserve force for the generation of the vertical force oriented distribution mode is calculated at the middle vehicle speed, and the actions / effects of both modes can thus be achieved in a balanced manner.The vehicle brake / driving force control device according to this embodiment has been described above, but the present invention is not limited to the embodiments explained above. Various modifications may be made without departing from the spirit of the present invention.For example, in the zero crossing distance oriented distribution mode according to the first embodiment and the second embodiment, such a configuration is adopted that the driving force distribution coefficient α for the driver required driving force Freq is set so that the zero crossing distances of the motor torque at the front / rear wheels are equal, but the equalization is not always necessary. For example, such a configuration may be adopted that a distribution ratio to the wheel (front wheel) on the side whose vertical force conversion rate of the driver required driving force Freq is smaller is set to a constant ratio larger than that for the wheel (rear wheel) on the side whose vertical force conversion rate is larger.In addition, this embodiment is applied to a vehicle in which the vertical force conversion rate (tan(θr)) for the rear suspension 20 ris greater than the vertical force conversion rate (tan(θf)) for the front suspension 20 f, but this embodiment may be applied to such a vehicle in which the vertical force conversion rate (tan(θf)) for the front suspension 20 fis greater than the vertical force conversion rate (tan(θr)) for the rear suspension 20 r. In this case, the distributions of the driver required driving force Freq to the front / rear wheels need only be opposite to those of this embodiment. More specifically, the distributions are set so that the rear wheels 10r are more strongly considered in distribution than the front wheels 10f.In addition, in the second embodiment, such a configuration is provided that the vehicle speed is divided into the three stages, and a mode matching the vehicle speed is selected from the three modes for setting the driving force distribution coefficients (the distribution mode oriented at the distance to zero crossing, the medium distribution mode, and the distribution mode oriented at the reserve force for vertical force generation), but such a configuration that the vehicle speed is divided into two stages (a low vehicle speed and a high vehicle speed), and the medium distribution mode is omitted may be provided. In other words, it may be configured such that V 1 and V 2, which are threshold values for the vehicle speed V, are set to the same value (V 1=V 2), the processing of step S 133 is executed when the vehicle speed V satisfies a relationship of 0≤V<V 1, the processing of step S 134 is executed when the vehicle speed V satisfies a relationship of V 1≤V, and the processing of step S 135 is omitted. In addition, the driving force distribution coefficient α does not always need to be calculated so that the vertical force generation reserve forces for the front / rear wheels are equal at the high vehicle speed, and the driving force distribution coefficient α may be set to a constant value less than 0.5 (50%), for example.In addition, in the second embodiment, the driving force distribution coefficient α is set in the middle distribution mode in which the mean (α1+α2) / 2 is used from the driving force distribution coefficient α1 calculated using the zero crossing distance oriented distribution mode and the driving force distribution coefficient α2 calculated using the vertical force generation reserve force oriented distribution mode, but the setting need not be performed in this manner. For example, the driving force distribution coefficient α 1 calculated using the zero crossing distance oriented distribution mode may be weighted more, and the driving force distribution coefficient α 2 calculated using the reserve force for generating the vertical force may be weighted less as the vehicle speed V decreases. In addition, the driving force distribution coefficient α 1 calculated using the zero crossing distance oriented distribution mode may be less weighted, and the driving force distribution coefficient α 2 calculated using the reserve force for generating the vertical force may be more weighted as the vehicle speed V increases. In other words, the distribution setting device may be configured to set the ratio of distribution to the wheels 10 coupled to the suspensions 20 on the side whose conversion rate is smaller to a larger value as the vehicle speed V decreases. In addition, a set driving force distribution coefficient α may be set as 0.5 (50%) in the middle distribution mode.

Claims

A vehicle braking / driving force control device comprising: each motor (30fl, 30fr, 30rl, 30rr) provided for a front left wheel (10fl), a front right wheel (10fr), a rear left wheel (10rl), and a rear right wheel (10rr) capable of transmitting torque to each of the wheels (10fl, 10fr, 10rl, 10rr) via gears, thereby generating a braking / driving force to each of the wheels (10fl, 10fr, 10rl, 10rr); a suspension (20fl, 20fr, 20rl, 20rr) for coupling each of the front left wheel (10fl), the front right wheel (10fr), the rear left wheel (10rl), and the rear right wheel (10rr) to a vehicle body (B), and for converting the braking / driving force on each of the wheels (10fl, 10fr, 10rl, 10rr) into a force (Fzf, Fzr) in a vehicle body height direction; a target braking / driving force calculation device (50, S11 to S15) for calculating a target braking / driving force (Fx) for each of the wheels (10fl, 10fr, 10rl, 10rr), the target braking / driving force (Fx) comprising: a driver required distributed driving force (Fdx) received by distributing a driver required driving force (Freq) set for each of the wheels (10fl, 10fr, 10rl, 10rr) based on an operation amount by a driver; and a control braking / driving force (Fcx) for each of the wheels (10fl, 10fr, 10rl, 10rr) necessary for vehicle movement control; and motor control means (50) for controlling an operation of the motor (30fl, 30fr, 30rl, 30rr) to follow the target braking / driving force (Fx), wherein the suspension (20fl, 20fr, 20rl, 20rr) is constructed such that a conversion rate (tan(θr), tan(θf)) of converting the braking / driving force of each of the wheels (10fl, 10fr, 10rl, 10rr) into the force in the vertical direction of the body (B) differs between a front wheel side and a rear wheel side, characterized in that the target braking / driving force calculation means (50, S11 to S15) comprises distribution setting means (50, S13), to adjust a distribution of the driver required driving force (Freq) to the front wheels (10fl, 10fr) and the rear wheels (10rl, 10rr) so that a larger driver required driving force (Freq) is distributed to the wheels (10fl, 10fr, 10rl, 10rr) coupled to the suspension (20fl, 20fr, 20rl, 20rr) on a side whose conversion rate (tan(θr), tan(θf)) is smaller than the wheels (10fl, 10fr, 10rl, 10rr) coupled to the suspension (20fl, 20fr, 20rl, 20rr) on a side whose conversion rate (tan(θr), tan(θf)) is larger, so that the frequency of zero crossing of the motor torque at the wheels (10fl, 10fr, 10rl, 10rr) is limited on the side whose conversion rate (tan(θr), tan(θf)) is smaller.The vehicle brake / drive force control device according to claim 1, wherein the distribution setting means (50, S13) sets, with respect to the distribution of the driver required drive force (Freq) to the front wheels (10fl, 10fr) and the rear wheels (10rl, 10rr), a ratio of the distribution to the wheels (10fl, 10fr, 10rl, 10rr) coupled to the suspension (20fl, 20fr, 20rl, 20rr) on the side whose conversion rate (tan(θr), tan(θf)) is smaller to a constant set distribution ratio larger than 0.5 when the driver required drive force (Freq) is smaller than a set drive force, and the ratio of the distribution changes so that the ratio of the distribution decreases within a range up to or below the set distribution ratio, when the driver required driving force (Freq) increases if the driver required driving force (Freq) is equal to or greater than the set driving force.The vehicle brake / driving force control device according to claim 2, wherein the distribution setting means (50, S13) uses, as the set distribution ratio, a value calculated by substituting a sum of the conversion rate (tan(θf)) of the suspensions (20fl, 20fr) for the front wheels (10fl, 10fr) and the conversion rate (tan(θr)) of the suspensions (20rl, 20rr) for the rear wheels (10rl, 10rr) as a denominator, and substituting the conversion rate (tan(θr), tan(θf)) of the suspensions (20fl, 20fr, 20rl, 20rr) on the side whose conversion rate (tan(θr), tan(θf)) is larger as a numerator.The vehicle brake / drive force control apparatus according to any one of claims 1 to 3, further comprising vehicle speed acquisition means for acquiring a vehicle speed (V), wherein the distribution setting means (50, S13) sets, with respect to the distribution of the driver required drive force (Freq) to the front wheels (10fl, 10fr) and the rear wheels (10rl, 10rr), a distribution ratio to the wheels (10fl, 10fr, 10rl, 10rr) coupled to the suspensions (20fl, 20fr, 20rl, 20rr) on the side whose conversion rate (tan(θr), tan(θf)) is smaller to a value larger than 0.5, when the vehicle speed (V) is less than a first set vehicle speed (V 1) and sets the ratio of the distribution to a value equal to or less than 0.5 when the vehicle speed is equal to or greater than a second set vehicle speed (V 2) equal to or greater than the first set vehicle speed (V 1).

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