JP7382218B2 - Vehicle control device - Google Patents

Description

The present invention relates to a vehicle control device that controls a vehicle that has a steering device and a braking/driving force difference generation device that generates a braking/driving force difference between left and right wheels.

In vehicles such as four-wheeled cars, the driver steers the vehicle by giving a steering angle to the front wheels according to the amount of steering input from the steering wheel, etc., and generating a slip angle and cornering force on the tires. There is.
In addition, in driving support control such as lane keeping support control that has become popular in recent years, and automatic driving control in which vehicles drive autonomously, it is possible to recognize the environment around the vehicle (road shape, obstacles, etc.) and drive toward a target. In addition to setting a trajectory, an actuator such as an electric motor is used to automatically steer the vehicle so that it traces the target travel trajectory.

Conventionally, it has been proposed to improve the stability of a vehicle by controlling the steering angle of a steering device.
For example, Patent Document 1 describes controlling the steering angles of the left and right steered wheels so that the generated yaw rate is canceled out on a so-called split μ road surface where the friction coefficients of the road surfaces in contact with the left and right wheels are different.

Japanese Patent Application Publication No. 2005-349914

The lateral force (cornering force) generated by the front wheels, which are the steered wheels, increases as the slip angle increases until the vector sum of the longitudinal and lateral forces generated by the tires reaches a predetermined friction circle limit. When the friction circle limit is reached, the cornering force will not increase even if the steering angle is increased further, and it will no longer be possible to generate a sufficient yaw rate to trace the prescribed travel trajectory, causing the travel trajectory to shift from the front wheel side. This results in an understeer condition where the vehicle bulges outward.
When such an understeer condition occurs, the line tracing performance of the vehicle is impaired, so even in severe conditions where the force generated by the front tires has no margin for the friction circle limit, it is necessary to suppress the occurrence of understeer and improve turning performance. It is requested that this be ensured.
In view of the above-mentioned problems, it is an object of the present invention to provide a vehicle control device that suppresses the tire-generated force of the front wheels from reaching its limit during steering.

The present invention solves the above-mentioned problems by the following solving means.
The invention according to claim 1 is a vehicle control device for controlling a vehicle including a steering device that steers front wheels by an actuator, and a braking/driving force difference generating device that generates a braking/driving force difference between left and right wheels, a target steering angle setting unit that sets a target steering angle of the steering device; a front wheel condition determining unit that determines a front wheel severe condition in which the front tire generated force is likely to reach a limit; and a front wheel condition determining unit that determines the front wheel severe condition. In this case, the actual steering angle of the steering device is made smaller than the target steering angle set by the target steering angle setting unit, and the braking/driving force difference generating device is controlled to promote yawing in the turning direction of the vehicle. a control unit that generates a driving force difference, and the front wheel condition determining unit changes a determination condition such that determination of the front wheel severe condition is easily established in accordance with an increase in the target steering angle. It is a control device.
According to this, when the front tire generated force does not have enough margin with respect to the friction circle limit, the actual steering angle is made smaller than the target steering angle even if the target steering angle increases. It is possible to prevent the tire generated force from reaching the friction circle limit due to an excessive increase in cornering force.
In addition, by compensating for the decrease in the yaw rate due to the decrease in the steering angle with the yaw moment generated by the difference in braking/driving force between the left and right wheels, understeer can be suppressed and line tracing performance can be ensured.
Furthermore, understeer can be appropriately suppressed during large steering angles (when the curvature is large) where understeer conditions are likely to occur, such as in so-called tight corners.

The invention according to claim 2 includes a target travel trajectory setting section for setting a target travel trajectory of the vehicle, and the target steering angle setting section is configured to set a target steering angle of the steering device so that the host vehicle traces the target travel trajectory. 2. The vehicle control device according to claim 1, wherein:
According to this, it is possible to prevent understeer from occurring in a state where the driver is unable to recognize that the force generated by the front tires is approaching the friction circle limit due to auxiliary steering force, etc., and causing a sense of discomfort and anxiety to the occupants.

The invention according to claim 3 is characterized in that the braking/driving force difference generating device generates the braking/driving force difference only between the left and right rear wheels when the severe state of the front wheels is determined. Or a vehicle control device according to claim 2.
According to this, by generating the tire longitudinal force for generating a difference in braking/driving force only in the rear wheels, it is possible to prevent the tire generated force of the front wheels from reaching the friction circle limit.

In the invention according to claim 4, the braking/driving force difference generating device is configured to generate the braking/driving force difference between the left and right rear wheels to be larger than the braking/driving force difference between the left and right front wheels when the front wheel severe state is determined. The vehicle control device according to claim 1 or 2, wherein the vehicle control device is made larger.
According to this, by generating the front-to-back tire force for generating a difference in braking/driving force with a bias toward the rear wheels, it is possible to prevent the tire-generated force of the front wheels from reaching the friction circle limit.
In addition, even if the coefficient of friction of the road surface is extremely low, such as on an icy and snowy road, and it is difficult to generate sufficient yaw moment due to the difference in braking force between the left and right rear wheels, for example, if there is surplus power in the front wheel on the inner side of the turn. By generating a difference in braking and driving force between the left and right front wheels, it is possible to suppress the occurrence of understeer behavior and obtain good line tracing performance.

The invention according to claim 5 includes a friction coefficient estimating section that estimates a friction coefficient of a road surface on which the front wheels touch the ground, and the front wheel state determining section is configured to detect the friction coefficient according to a decrease in the friction coefficient estimated by the friction coefficient estimating section. The vehicle control device according to any one of claims 1 to 4, characterized in that a front wheel severe state is determined.
According to this, when driving on a road surface with a low coefficient of friction where the tire-generated force of the front wheels tends to reach the friction circle limit, the above-mentioned effects can be appropriately obtained.

The invention according to claim 6 includes a friction circle limit estimating section that estimates a friction circle limit of the front wheel, and a tire generated force estimating section that estimates a tire generated force of the front wheel, and the front wheel state determining section is configured to The vehicle control device according to any one of claims 1 to 4, wherein the severe state of the front wheel is determined in accordance with the approach of the tire generated force to the friction circle limit.
According to this, by estimating the friction circle limit of the front wheels and determining the severe state of the front wheels according to the degree of approach to the force generated by the front tires, it is possible to appropriately determine conditions where understeer is likely to occur and intervene in the control. By doing so, the above-mentioned effects can be reliably obtained.
Moreover, if the tire generated force has sufficient margin with respect to the friction circle limit, the control is not intervened, so that it is possible to prevent the occupant from feeling uncomfortable.

As described above, according to the present invention, it is possible to provide a vehicle control device that suppresses the tire-generated force of the front wheels from reaching its limit during steering.

1 is a diagram schematically showing the configuration of a vehicle having a first embodiment of a vehicle control device to which the present invention is applied. FIG. 1 is a diagram schematically showing the configuration of a steering device for a vehicle having a vehicle control device according to a first embodiment. 5 is a flowchart showing control during automatic steering in the vehicle control device of the first embodiment. FIG. 3 is a diagram schematically showing the relationship between tire generated force and friction circle limit. FIG. 3 is a diagram schematically showing a state in which understeer suppression control is performed in a vehicle having the vehicle control device of the first embodiment. FIG. 6 is a diagram schematically showing a state in which understeer suppression control is performed in a vehicle having a second embodiment of a vehicle control device to which the present invention is applied.

<First embodiment>
Hereinafter, a first embodiment of a vehicle control device to which the present invention is applied will be described.
The vehicle control device of the first embodiment is provided, for example, in a vehicle such as a four-wheeled passenger car that is all-wheel drive (AWD) and has front wheels as steered wheels.
FIG. 1 is a diagram schematically showing the configuration of a vehicle having a vehicle control device according to a first embodiment.

As shown in FIG. 1, the vehicle 1 includes an engine 10, a torque converter 20, a transmission mechanism 30, an AWD transfer 40, a front differential 50, a rear differential 60, and the like.

The engine 10 is a power source for driving the vehicle 1, and is, for example, an internal combustion engine such as a gasoline engine.
The torque converter 20 is a fluid coupling that transmits the output of the engine 10 to the transmission mechanism section 30, and has a function as a starting device that allows the vehicle to start from zero vehicle speed.
Torque converter 30 includes a lock-up clutch that directly connects an input side and an output side.
The transmission mechanism section 30 is, for example, a continuously variable transmission (CVT) having a variator consisting of a pair of variable pulleys, a chain, a belt, etc., a step AT having a plurality of rows of planetary gear sets, etc. It increases or decreases the input output of the engine 10. The transmission mechanism section 30 cooperates with the torque converter 20 to configure a transmission.

The AWD transfer 40 is a driving force transmission device that distributes and transmits the driving force input from the transmission mechanism section 30 to the front differential 50 and the rear differential 60.
The AWD transfer 40 includes a center differential 41, a transfer clutch 42, and the like.
The center differential 41 is configured with, for example, a composite planetary gear set, and is a driving force distribution mechanism that distributes torque to the front differential 50 and the rear differential 60 so that the torque distribution ratio is approximately 35:65, for example. . Furthermore, the center differential 41 also functions as a differential mechanism that absorbs differential rotation between the front differential 50 and the rear differential 60 due to, for example, a difference in the trajectory of the front and rear wheels during turning.

The transfer clutch 42 is a differential limiting mechanism that restricts the differential between the front and rear wheel side output portions of the center differential 41. Transfer clutch 42 includes, for example, a wet multi-disc clutch driven by hydraulic pressure or electromagnetic force, and its engagement force (clutch pressure force), that is, differential limiting torque, is controlled by transmission control unit 130, which will be described later.
The AWD transfer 40 can steplessly adjust the driving force distribution ratio between the front wheels and the rear wheels, from 35:65 to 50:50, for example, by adjusting the engagement force of the transfer clutch 42. It becomes.

The front differential 50 performs final deceleration of the front wheel drive force transmitted from the AWD transfer 40 and transmits it to the front right wheel 51 and the front left wheel 52. Further, the front differential 50 functions as a differential mechanism that absorbs the differential rotation between the right front wheel 51 and the left front wheel 52.

The rear differential 60 performs final deceleration of the rear wheel side driving force transmitted from the AWD transfer 40 and transmits it to the right rear wheel 61 and the left rear wheel 62. Further, the rear differential 60 functions as a differential mechanism that absorbs the differential rotation between the right rear wheel 61 and the left rear wheel 62.

The vehicle is also provided with a brake device 70 that is a braking device.
The brake device 70 includes a brake pedal 71, a master cylinder 72, a hydraulic control unit (HCU) 73, a brake FR74, a brake FL75, a brake RR76, a brake RL77, and the like.
The brake pedal 71 is an input section through which the driver performs a brake operation.
The master cylinder 72 is connected to the brake pedal 71 and pressurizes brake fluid in response to depression of the brake pedal 71. The master cylinder 72 is provided with a vacuum booster that uses the intake pipe negative pressure of the engine 10 to amplify the input from the brake pedal 71.

The hydraulic control unit 73 individually increases or decreases the hydraulic pressure of brake fluid supplied to the wheel cylinders of each wheel, for example, for anti-lock brake control, yaw control control, automatic brake control, etc.
The hydraulic control unit 73 includes an electric pump that pressurizes brake fluid, a control valve that individually adjusts the hydraulic pressure of each wheel cylinder, and the like.

Brake FR74, brake FL75, brake RR76, and brake RL77 are provided on right front wheel 51, left front wheel 52, right rear wheel 61, and left rear wheel 62, respectively. Each brake includes a disc-shaped rotor that rotates together with the wheel, and a caliper that brings a pad into pressurized contact with the rotor. The caliper includes a foil cylinder that presses a pad with the hydraulic pressure of brake fluid supplied from the hydraulic control unit 73.

The vehicle 1 includes an engine control unit 110, a transmission control unit 120, a steering control unit 130, a brake control unit 140, an automatic driving control unit 150, an environment recognition unit 160, and the like.
Each unit has, for example, an information processing section such as a CPU, a storage section such as a RAM or ROM, an input/output interface, a bus connecting these, and the like.
Further, each unit is communicably connected, for example, via an in-vehicle LAN such as a CAN communication system, or directly.

The engine control unit 110 controls the engine 10 and its auxiliary machinery in an integrated manner.
The engine control unit 110 has a function of adjusting the output of the engine 10.

The transmission control unit 120 performs gear change control in the transmission mechanism section 30, forward/reverse switching control, lock-up clutch engagement force (restraint force) control in the torque converter 20, and the like.
Furthermore, the transmission control unit 120 has a function of controlling the driving force distribution ratio between the front wheels and the rear wheels by changing the engagement force of the transfer clutch 42 of the AWD transfer 40.

The steering control unit 130 centrally controls a steer-by-wire type steering device 200, which will be described later, and controls the steering angle of the vehicle.
The functions of the steering control unit 130 will be explained in detail later.
The steering control unit 130 includes a front wheel severe state determination section 131.
The front wheel severe state determination unit 131 determines a front wheel severe state in which the tire generated force, which is the resultant force of longitudinal force and lateral force acting between the front wheel and the road surface, is likely to reach the friction circle limit.

The front wheel severe state determination unit 131 has, for example, a function of estimating the coefficient of friction of the road surface on which the front wheels touch the ground. The front wheel severe state determining section 131 also functions as a road surface friction coefficient estimating section and a friction circle limit estimating section.
The friction coefficient can be estimated, for example, based on the lateral acceleration, yaw rate, etc. that actually occur in the vehicle body with respect to the vehicle speed and steering angle.
If the coefficient of friction can be estimated, the limit of the friction circle of the front wheels can be estimated from the ground load of each wheel.
The ground load of the wheels can be calculated from the vehicle's tread, wheel base, total weight, center of gravity, and other specifications, as well as the acceleration acting on the vehicle body. Note that, for example, when the road surface has a low μ and large load movement does not occur, a known constant may be simply used as the ground load.

Further, the front and rear force components (driving force DF (see FIG. 4), braking force BF (see FIG. 5)) of the tire-generated forces of the front wheels 51 and 52 are, for example, the output of the engine 10 transmitted from the engine control unit 110. It can be determined from the difference between the driving force determined from the torque, the gear ratio of the transmission mechanism section transmitted from the transmission control unit 120, the front-rear torque distribution ratio, etc., and the braking force transmitted from the brake control unit 130.
The lateral force component (cornering force CF) can be determined from the correlation between vehicle speed, steering angle, lateral acceleration, and yaw rate.
The front wheel severe state determining unit 131 determines that the front wheel is in a severe state when the tire generated force approaches the estimated friction circle limit within a predetermined range.

The brake control unit 140 has a function of controlling the hydraulic control unit 73 and individually controlling the wheel cylinder hydraulic pressure (correlated with braking force) of the brake FR74, brake FL75, brake RR76, and brake RL77.
The braking control unit 140 performs, for example, anti-lock brake control that periodically reduces the wheel cylinder hydraulic pressure of the wheel to restore rotation when a wheel locks due to braking, and controls the left and right wheels when oversteer behavior or understeer behavior occurs. It has a function to perform behavior control that suppresses yaw moment in the direction of suppressing each behavior by generating a difference in braking force.
In order to perform these controls, the brake control unit 140 is connected to a vehicle speed sensor that individually detects the rotational speed of each wheel, a yaw rate sensor that detects the yaw rate and lateral acceleration of the vehicle body, an acceleration sensor, and the like.
Information regarding vehicle speed, yaw rate, acceleration, etc. is also provided to other units as appropriate.
The brake control unit 140 functions as a braking/driving force difference generating device in cooperation with the brake device 70.

The automatic driving control unit 150 performs automatic driving control by giving commands to the engine control unit 110, transmission control unit 120, steering control unit 130, braking control unit 140, etc. so that the vehicle 1 runs automatically and autonomously. It is.
The automatic driving control unit 150 includes a traveling trajectory setting section 151, a target steering angle setting section 152, and the like.
The travel trajectory setting unit 151 generates an automatic driving scenario including a target travel trajectory of the own vehicle, a target vehicle speed history, etc., in accordance with information provided from the environment recognition unit 160.
During execution of automatic driving control, the target steering angle setting unit 152 determines the position for the own vehicle to trace the target traveling trajectory based on the relative position of the own vehicle with respect to the target traveling trajectory, the speed of the own vehicle, the estimated friction coefficient of the road surface, etc. The target steering angle is sequentially calculated and instructed to the steering control unit 130.
The target steering angle is subjected to sequential feedback control depending on the deviation of the own vehicle from the target travel trajectory.

The environment recognition unit 160 uses various sensors to recognize the environment around the vehicle, and provides the automatic driving control unit 150 with information regarding lane shapes, obstacles, and the like.
The environment recognition unit 160 is connected to, for example, various sensors such as a stereo camera device, a millimeter wave radar device, a 3D laser scanner device (LIDAR), and a navigation device having 3D high-precision map data and a positioning device such as GPS. There is.

The vehicle includes a steering device 200, which will be described below, in order to steer the right front wheel 51 and the left front wheel 52, which are steered wheels.
The steering device 200 is an electric and steer-by-wire type that includes a steering wheel 210, a reaction force generator 220, a rack shaft 230, a rack housing 240, a tie rod 250, a housing 260, an actuator unit 270, and the like. .

The steering wheel 210 is an annular operating member (steering input section) that is rotated by the driver to input a steering operation.
The steering wheel 210 is disposed in the vehicle interior, facing the driver's seat.
The steering wheel 210 is provided with a steering shaft 211, a steering angle sensor 212, and a torque sensor 213.

The steering shaft 211 is a rotating shaft with one end attached to the steering wheel 210.
The steering angle sensor 212 is a steering operation amount detection section that is provided at an intermediate portion of the steering shaft 211 and detects the rotation angle position of the steering shaft 211.
The torque sensor 213 is provided in an intermediate portion of the steering shaft 211 in a region on the reaction force generation device 220 side with respect to the steering angle sensor 212, and is configured to measure torque applied to the steering shaft 211 (operation of the steering wheel 11 by the driver). force or holding force).
The outputs of the steering angle sensor 212 and torque sensor 213 are transmitted to the steering control unit 130.
The steering control unit 130 controls the output of the motor 271 of the actuator unit 270 according to the outputs of the steering angle sensor 212 and torque sensor 213.

The reaction force generator 220 includes an actuator that applies torque to the steering shaft 211 in response to a command from the steering control unit 130 to generate a pseudo self-aligning torque.
An output shaft portion of the reaction force generating device 220 is connected to an end portion of the steering shaft 211 on the side opposite to the steering wheel 210 side.

The rack shaft 230 is a columnar member arranged so that its longitudinal direction (axial direction) runs along the vehicle width direction.
The rack shaft 230 is supported so as to be translatable in the vehicle width direction relative to the vehicle body.
A rack gear 231 that meshes with a pinion gear of a pinion shaft 273 is formed in a part of the rack shaft 230.
A rack gear 231 is driven by a pinion gear in response to rotation of the steering shaft 220, and the rack shaft 230 translates (moves straight) along the vehicle width direction.

The rack housing 240 is a substantially cylindrical member that accommodates the rack shaft 230 while supporting it so as to be relatively movable along the vehicle width direction.
The rack housing 240 cooperates with the rack shaft 230, pinion shaft 273, etc. to configure a steering gear box.
Rack boots 241 are provided at both ends of the rack housing 240.
The rack boot 241 is a member that prevents foreign matter such as dust from entering into the rack housing 240 while allowing the tie rod 250 to be displaced relative to the rack housing 240.
The rack boot 241 is made of a resin material such as an elastomer and is formed into a flexible bellows tube shape.

The tie rod 250 is a shaft-shaped interlocking member that connects the end of the rack shaft 230 and the knuckle arm 261 of the housing 260 and rotates the housing 260 around the kingpin axis in conjunction with the translational movement of the rack shaft 230. be.
The inner end of the tie rod 250 in the vehicle width direction is swingably connected to the end of the tie rod 230 via a ball joint 251.
The outer end of the tie rod 250 in the vehicle width direction is connected to a knuckle arm 261 of a housing 260 via a ball joint 252.
A turnbuckle mechanism (not shown) for toe-in adjustment is provided at the connection between the tie rod 250 and the ball joint 252.

The housing (knuckle) 260 is a member that accommodates a hub bearing that rotatably supports a hub to which the front right wheel 51 and the front left wheel 52 are attached around the axle.
The housing 260 has a knuckle arm 261 formed to protrude forward with respect to the axle.
The housing 260 is rotatably supported around a kingpin axis that is a predetermined rotation center axis.
For example, if the front suspension of the vehicle is a MacPherson strut type, the kingpin axis is an imaginary axis connecting the bearing center of the strut top mount and the center of the ball joint that connects the lower part of the housing 260 and the lower arm. .
When the housing 260 is pushed and pulled in the vehicle width direction by the rack shaft 230 via the tie rod 250, the housing 260 rotates around the kingpin shaft and steers the right front wheel 51 and the left front wheel 52.

The actuator unit 270 is a drive device that rotates the pinion shaft 273 to generate a rack thrust and performs a steering operation.
The actuator unit 270 includes a motor 271, a gear box 272, a pinion shaft 273, and the like.
The motor 271 is an electric actuator that generates a driving force to be applied to the pinion shaft 273.
The rotation direction and output torque of the motor 271 are controlled by the steering control unit 130.
The output shaft angular position (rotation angle) of the motor 271 is configured to be fed back to the steering control unit 130.
The gearbox 272 includes a reduction gear train that reduces the rotational output of the motor 271 (torque amplification) and transmits it to the pinion shaft 273.
The pinion shaft 273 is a rotating shaft that is rotationally driven by the motor 271 via the gear box 272.
A pinion gear that meshes with the rack gear 231 of the rack shaft 30 to drive the rack shaft 230 is formed at the tip of the pinion shaft 273 .

The operation of the vehicle control device according to the first embodiment will be described below.
FIG. 3 is a flowchart showing control during automatic steering in the vehicle control device of the first embodiment.
Below, each step will be explained in order.

<Step S01: Determining whether or not there is a steering operation request>
The steering control unit 130 determines whether a new steering operation is requested based on the target steering angle instructed by the automatic operation control unit 150.
For example, when the target steering angle increases from a straight-ahead state or when the target steering angle changes during a turning state, it is determined that there is a request for a steering operation.
For example, the steering control unit 130 determines that there is a steering operation request when the time-differentiated rate of change of the target steering angle is greater than or equal to a predetermined threshold, and proceeds to step S02.
In other cases, the series of processing ends (returns).

<Step S02: Road surface friction coefficient estimation>
The front wheel severe state determination unit 131 of the steering control unit 130 estimates the coefficient of friction of the road surface that the right front wheel 51 and the left front wheel 52 are in contact with.
The friction coefficient can be estimated, for example, based on the correlation between the vehicle speed and steering angle, and the lateral acceleration and yaw rate generated in the vehicle body.
After that, the process advances to step S03.

<Step S03: Friction circle limit estimation>
The front wheel severe state determination unit 131 of the steering control unit 130 estimates the friction circle limits of the right front wheel 51 and the left front wheel 52.
FIG. 4 is a diagram schematically showing the relationship between the tire generated force and the friction circle limit.
The radius of the friction circle limit L is the resultant force (tire It shows the limit of the generated force).
The friction circle limit L mainly depends on the friction coefficient of the road surface and the ground load of the tire.
As described above, the ground contact load of a tire can be calculated from the vehicle's tread, wheel base, total weight, center of gravity, and other specifications, as well as the acceleration acting on the vehicle body.
After that, the process advances to step S04.

<Step S04: Estimation of tire generated force>
The front wheel severe state determination section 131 of the steering control unit 130 estimates the tire generated force TF of the front wheels 51 and 52.
As shown in FIG. 4, the tire generated force TF is a vector sum of a longitudinal force (driving force DF in the case shown in FIG. 4) and a lateral force (cornering force CF).
After that, the process advances to step S05.

<Step S05: Front wheel severe state determination>
The front wheel severe state determination unit 131 of the steering control unit 130 determines whether at least one of the right front wheel 51 and the left front wheel 52 is in the front wheel severe state.
The difference obtained by subtracting the magnitude of the tire generated force TF (the length of the arrow in FIG. 4) from the radius of the friction circle limit L of each wheel is compared with a predetermined threshold, and if the difference is less than the threshold, the tire It is determined that the front wheels are in a severe condition where the generated force is likely to reach the friction circle limit.
Note that the threshold value is set to become smaller (so that it becomes easier to determine that the front wheels are in a severe state) as the target steering angle increases (increase in corner curvature) set by the target steering angle setting unit 152. .
If it is determined that at least one of the right front wheel 51 and the left front wheel 52 is in a severe front wheel state, the process proceeds to step S06, and in other cases, the process proceeds to step S07.

<Step S06: Execute understeer suppression control>
The steering control unit 130 prevents the tire-generated force TF of the front wheels from reaching the friction circle limit L, and executes understeer suppression control to suppress understeer behavior.
Specifically, the steering control unit 130 makes the actual steering angle in the steering device 200 smaller than the target steering angle set by the target steering angle setting unit 152, and causes the vehicle to travel along the target travel trajectory. In order to obtain the necessary yaw rate, an instruction is given to the brake control unit 140 to generate a braking force on the rear wheel on the inner side of the turn, thereby creating a difference in braking force between the left and right rear wheels.
After that, the series of processing ends.

<Step S07: Normal control execution>
The steering control unit 130 performs normal control to match the target steering angle set by the target steering angle setting section 152 with the actual steering angles of the front right wheel 51 and the front left wheel 52 by the steering device 200.
After that, the series of processing ends.

FIG. 5 is a diagram schematically showing a state in which understeer suppression control is performed in a vehicle having the vehicle control device of the first embodiment.
FIG. 5 shows a case where the vehicle enters a left corner (curved road) by automatic operation (automatic steering).

When the vehicle 1 enters a left corner at position P1, the target steering angle setting unit 152 increases the target steering angle of the steering device 200 from the straight-ahead state to start turn-in.
At this time, it is assumed that the above-mentioned front wheel severe condition has occurred.
Here, if normal control is executed to increase the actual steering angle according to the target steering angle, the tire generated force TF of the front right wheel 51 and the front left wheel 52 will reach the friction circle limit L and the slip will increase. , the cornering force CF necessary to trace the target travel trajectory cannot be obtained, and the vehicle 1 exhibits understeer behavior as in the state at position P2.

On the other hand, in the first embodiment, as in the state at position P3, the above-described understeer suppression control changes the actual steering angles of the front right wheel 51 and the front left wheel 52 in the steering device 200 relative to the target steering angle. In addition to suppressing the tire generated force TF from reaching the friction circle limit L, the brake RL 67 generates a braking force BF on the left rear wheel 62 on the inner wheel side of the turn to generate a yaw moment M in the direction of promoting the turn. By obtaining this, it is possible to obtain good line traceability while preventing the occurrence of understeer behavior.

As explained above, according to the first embodiment, the following effects can be obtained.
(1) If the tire generated force TF of the front right wheel 51 and the front left wheel 52 has no margin with respect to the friction circle limit L, even if the target steering angle increases, the actual steering angle is set to the target steering angle. It is possible to prevent the tire generated force TF from reaching the friction circle limit L due to an excessive increase in the cornering force CF.
Furthermore, by compensating for the decrease in yaw rate due to a decrease in the steering angle with the yaw moment M generated by the difference in braking/driving force between the left and right wheels, understeer can be suppressed and line tracing performance can be ensured.
(2) By performing understeer suppression control when the steering angle increases during automatic driving, the driver recognizes that the tire generated force TF of the front right wheel 51 and the front left wheel 52 is approaching the friction circle limit L due to auxiliary steering force, etc. It is possible to prevent understeer from occurring in a situation where the vehicle is not able to operate, causing a sense of discomfort and anxiety to the occupants.
(3) By generating the braking force BF for generating the braking/driving difference that generates the yaw moment M only in the rear wheels 61, 62, the tire generated force TF of the front wheels 51, 52 reaches the friction circle limit L. This can be prevented.
(4) By estimating the friction circle limit L of the right front wheel 51 and the left front wheel 52 and determining the front wheel severe state according to the degree of approach to the tire generated force TF, the state where understeer is likely to occur can be appropriately determined. The above effects can be reliably obtained by intervening control.
Further, if the tire generated force TF has a sufficient margin with respect to the friction circle limit L, the control is not intervened, thereby preventing the occupant from feeling uncomfortable.
(5) By changing the threshold value used for discrimination so that it is easier to determine whether the front wheels are in a severe condition as the target steering angle increases, we can improve the Understeer can be appropriately suppressed at certain times.

<Second embodiment>
Next, a second embodiment of a vehicle control device to which the present invention is applied will be described.
In the second embodiment, parts common to the first embodiment described above are given the same reference numerals, and the explanation thereof will be omitted, and the differences will be mainly explained.
FIG. 6 is a diagram schematically showing a state in which understeer suppression control is performed in a vehicle having the vehicle control device of the first embodiment.

In the second embodiment, when understeer suppression control is executed, braking force BF is generated not only at the rear wheel on the inner wheel side of the turn but also at the front wheel on the inner wheel side of the turn.
At this time, in order to prevent the tire generated force of the front wheel on the inside wheel side of the turn from reaching the friction circle limit, the braking force BF of the front wheel is made smaller than the braking force BF of the rear wheel, and the braking force BF of the left and right front wheels is It is preferable to make the difference smaller than the difference in braking/driving force between the left and right rear wheels.

According to the second embodiment described above, even when the coefficient of friction of the road surface is extremely low, such as on an icy and snowy road, and it is difficult to generate a sufficient yaw moment due to the difference in braking force between the left and right rear wheels, If the front wheel on the side has surplus power, by generating a difference in braking force between the left and right front wheels, it is possible to obtain good line tracing performance while suppressing the occurrence of understeer behavior.

(Modified example)
The present invention is not limited to the embodiments described above, and various modifications and changes are possible, and these are also within the technical scope of the present invention.
(1) The configurations of the vehicle and vehicle control device are not limited to the embodiments described above, and can be modified as appropriate.
For example, in each of the embodiments, the steering device is of a so-called steer-by-wire type in which the steering operation input section and the steering gear box are not mechanically connected. The present invention can also be applied to a steering system that is coupled to a steering system and has an electric power steering system.
(2) In each embodiment, the coefficient of friction of the road surface is estimated from the lateral acceleration, yaw rate, etc. during turning, but the coefficient of friction of the road surface is estimated and determined by other methods, for example. Good too.
For example, when a road surface is imaged by an imaging device and a wet state, snowy state, frozen state, etc. of the road surface is detected based on image processing, it may be determined that the coefficient of friction is low.
Furthermore, information regarding the friction coefficient of the road surface may be acquired through road-to-vehicle communication or vehicle-to-vehicle communication.
Further, it may be determined that the coefficient of friction is low when the outside temperature is low or when the wiper is activated.
Furthermore, in a vehicle in which the control state and characteristics of the vehicle can be selected from a plurality of modes, if a mode suitable for driving on a rough road or a low friction road is selected, it may be determined that the friction coefficient is low.
(3) In each embodiment, a braking force difference is generated by applying a larger braking force to the wheels on the inner side of the turn than on the outer wheels of the turn, and a yaw moment is generated in a direction that promotes turning. The invention is not limited to this, and a driving force difference may be generated by applying a larger driving force to the wheel on the outer side of the turning than the wheel on the inner side of the turning.
Alternatively, the driving force may be applied to the wheels on the outer side of the turn, and the braking force may be applied to the wheels on the inner side of the turn.
Here, the method of applying a driving force difference between the left and right wheels is not particularly limited, and, for example, a configuration may be provided that includes a speed increasing mechanism that speeds up the wheel on the outer wheel side of the turn. Further, while the driving force is applied to the left and right wheels, a braking force smaller than the driving force may be applied to the wheel on the inner wheel side of the turn.
(4) In each of the embodiments, for example, the state of automatic driving in which the vehicle sets a target travel trajectory and automatically steers the vehicle has been described as an example, but the present invention also provides, for example, lane-keeping support control and lane-keeping control. It can also be applied to driving support control such as deviation prevention control.
Further, the present invention can also be applied to manual driving in which steering is performed in accordance with the driver's steering operation.
In this case, a steering operation input unit such as a steering wheel functions as a target steering angle setting unit.
(5) In each embodiment, the front wheel friction circle limit and the tire generated force are estimated, and the front wheel severe condition is determined from the relationship. However, simply speaking, when the friction coefficient of the road surface is low, the front wheel severe condition It may be determined that the state is the same.
According to this, the above-mentioned understeer suppressing effect can be obtained with a simple configuration.
(6) In each of the embodiments, the front wheel severe state discrimination threshold is changed based on the target steering angle set by the target steering angle setting unit, but the target steering angle may be recognized by other methods.
For example, if the curvature of the target travel trajectory set by the target trajectory setting section is large, it may be determined that the target steering angle is large.
Additionally, the shape of the lane in front of the vehicle is recognized using an environment recognition means such as a stereo camera device, a navigation device with map data and a positioning device, etc., and if the curvature is large, the target steering angle is determined to be large. May be determined.

1 Vehicle 10 Engine 20 Torque converter 30 Transmission mechanism section 40 AWD transfer 41 Center differential 42 Transfer clutch 50 Front differential 51 Right front wheel 52 Left front wheel 60 Rear differential 61 Right rear wheel 62 Left rear wheel 70 Brake device 71 Brake pedal 72 Master cylinder 73 Hydraulic control unit (HCU)
74 Brake FR 75 Brake FL
76 Brake RR 77 Brake RL
110 Engine control unit 120 Transmission control unit 130 Steering control unit 140 Brake control unit 150 Automatic driving control unit 151 Travel trajectory setting section 152 Target steering angle setting section 160 Environment recognition unit 200 Steering device 210 Steering wheel 211 Steering shaft 212 Steering angle sensor 213 Torque sensor 220 Reaction force generator 230 Rack shaft 231 Rack gear 240 Rack housing 241 Rack boot 250 Tie rod 251 Ball joint 252 Ball joint 260 Housing 270 Actuator unit 271 Motor 272 Gear box 273 Pinion shaft

Claims (6)

a steering device that steers the front wheels using an actuator;
A vehicle control device for controlling a vehicle, comprising: a braking/driving force difference generation device that generates a braking/driving force difference between left and right wheels,
a target steering angle setting unit that sets a target steering angle of the steering device;
a front wheel condition determination unit that determines a severe front wheel condition in which the front tire generation force is likely to reach its limit;
When the front wheel severe state is determined, the actual steering angle of the steering device is made smaller than the target steering angle set by the target steering angle setting unit, and the braking/driving force difference generating device is configured to control the turning of the vehicle. A control unit that generates a braking/driving force difference that promotes yawing in the direction ,
The front wheel condition determination unit changes a determination condition so that determination of the severe front wheel condition is easily established in accordance with an increase in the target steering angle.
A vehicle control device characterized by: Equipped with a target travel trajectory setting section for setting a target travel trajectory of the vehicle,
The vehicle control device according to claim 1, wherein the target steering angle setting unit sets a target steering angle of the steering device so that the host vehicle traces the target travel trajectory. The vehicle according to claim 1 or 2, wherein the braking/driving force difference generating device generates the braking/driving force difference only between the left and right rear wheels when the front wheel severe state is determined. Control device. The braking/driving force difference generating device is configured to make the braking/driving force difference between the left and right rear wheels larger than the braking/driving force difference between the left and right front wheels when the front wheel severe state is determined. The vehicle control device according to claim 1 or claim 2. comprising a friction coefficient estimation unit that estimates a friction coefficient of a road surface on which the front wheels touch the ground;
The front wheel condition determining unit determines the front wheel severe condition according to a decrease in the friction coefficient estimated by the friction coefficient estimating unit. Vehicle control device. a friction circle limit estimation unit that estimates a friction circle limit of the front wheel;
and a tire generated force estimating unit that estimates the tire generated force of the front wheel,
The front wheel condition determining unit determines the front wheel severe condition according to the approach of the tire generated force to the friction circle limit, according to any one of claims 1 to 4. Vehicle control device .

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