JP7636138B2 - Vehicle Control Systems - Google Patents

Description

The present invention relates to a vehicle control system for a vehicle with a drive wheel switching function, and in particular to a control technique for when all wheels are in a slipping state in an all-wheel drive state.

There are vehicles that can be switched between an all-wheel drive state, in which all of the wheels are driven, and a partial-wheel drive state, in which only some of the wheels are driven. For example, there are four-wheel vehicles that can be switched between a four-wheel drive state and a two-wheel drive state.

Related prior art includes the following patent documents 1-4.

JP 2009-166706 A JP 2004-131007 A JP 2007-22161 A Japanese Patent Application Publication No. 4-95532

Generally, it is believed that driving in all-wheel drive mode improves the vehicle's ability to traverse rough terrain such as snowy roads, but depending on the condition of the road and the manner in which the vehicle is driven, it is possible for all wheels to slip even in all-wheel drive mode.

Some vehicles estimate the vehicle body speed based on the wheel rotation speed detected for each wheel. Some all-wheel drive vehicles, such as four-wheel drive vehicles, that can drive all wheels, control the output torque of the front and rear wheels based on the vehicle body speed estimated from the wheel rotation speed in this way.

In a vehicle in which output torque control of the front and rear wheels is performed based on the vehicle speed estimated from the wheel rotation speed as described above, if all of the wheels enter a slipping state as described above, the accuracy of the estimation of the vehicle speed decreases, and as a result, the accuracy of the output torque control also decreases.
A decrease in the accuracy of output torque control will result in the actual amount of torque down being either too much or too little compared to the required amount. If the amount of torque down is excessive, the vehicle will not accelerate, and if the amount of torque down is too small, problems such as the vehicle not recovering from a slip may occur, which may result in a deterioration in off-road performance.

The present invention was made in consideration of the above circumstances, and aims to improve the vehicle's driving performance in driving environments where all wheels are in a slipping state.

The vehicle control system of the present invention is a vehicle control system for a vehicle having a plurality of wheels and capable of switching between an all-wheel drive state in which all of the wheels are driven, and a partial-wheel drive state in which only some of the wheels are driven, and is equipped with a wheel rotational speed detection unit that detects the wheel rotational speed of each of the wheels, and a control unit that performs control to switch from the all-wheel drive state to the partial-wheel drive state when it is determined that all of the wheels are in a slipping state in the all-wheel drive state.
When all of the wheels are in a slipping state, some of the wheels are switched to a driven state, and the wheels that have been switched to a non-driven state, i.e., the wheels that have been switched to a driven wheel, are released from the slipping state.

In the vehicle control system of the present invention described above, when the control unit determines that all of the wheels are in a slipping state in the all-wheel drive state, and when it determines that the vehicle is traveling on a curved road or is about to enter a curved road, the control unit can be configured not to perform control to switch to a partial-wheel drive state.
When switching from all-wheel drive to partial-wheel drive while traveling on a curved road, the vehicle behavior may change and the vehicle may fall into an oversteer or understeer state. For this reason, when it is determined that the vehicle is traveling on a curved road or is about to enter a curved road as described above, the system does not switch to partial-wheel drive.

In the vehicle control system of the present invention described above, the vehicle is a vehicle that can be switched to a front-wheel drive state or a rear-wheel drive state as a switch from an all-wheel drive state to a partial-wheel drive state, and the control unit can be configured not to perform control to switch to the front-wheel drive state or the rear-wheel drive state when it determines that the vehicle is traveling on a curved road or is about to enter a curved road.
When switching from all-wheel drive to front-wheel drive or rear-wheel drive while traveling on a curved road, the vehicle is likely to fall into an oversteer or understeer state. For this reason, when it is determined that the vehicle is traveling on a curved road or is about to enter a curved road, the vehicle is not allowed to switch to front-wheel drive or rear-wheel drive.

In the vehicle control system of the present invention described above, the vehicle can be configured to be a vehicle that can be switched to a front-wheel drive state or a rear-wheel drive state as a switch from an all-wheel drive state to a partial-wheel drive state, and when the control unit determines that all of the wheels are in a slip state in the all-wheel drive state, it determines whether the current position corresponds to the oversteer occurrence location or the understeer occurrence location based on under/oversteer history information in which information indicating the understeer occurrence location, which is a location including the position where understeer has occurred, and information indicating the oversteer occurrence location, which is a location including the position where oversteer has occurred, is stored, and if it is determined that the current position corresponds to the understeer occurrence location, it performs control to switch from the all-wheel drive state to the rear-wheel drive state, and if it is determined that the current position corresponds to the oversteer occurrence location, it performs control to switch from the all-wheel drive state to the front-wheel drive state.
In other words, if it is determined based on historical information related to the occurrence of understeer or oversteer that the current position corresponds to a location where understeer is occurring, a switch to rear-wheel drive is performed to counteract the tendency toward understeer, and if it is determined that the current position corresponds to a location where oversteer is occurring, a switch to front-wheel drive is performed to counteract the tendency toward oversteer.

In the vehicle control system according to the present invention described above, in the under/oversteer history information, the information indicating the location where understeer occurred is associated with identification information indicating whether or not understeer occurred in a rear-wheel-biased drive state, and the information indicating the location where oversteer occurred is associated with identification information indicating whether or not oversteer occurred in a front-wheel-biased drive state, and the control unit can be configured such that if it determines that the current position is a location where understeer occurred in a rear-wheel-biased drive state, it does not perform control to switch from the all-wheel drive state to the rear-wheel drive state, and if it determines that the current position is a location where oversteer occurred in a front-wheel-biased drive state, it does not perform control to switch from the all-wheel drive state to the front-wheel drive state.
This makes it possible to prevent inappropriate control such as intensifying understeer or oversteer tendencies rather than mitigating them, which would occur if the vehicle were to switch to rear-wheel drive or front-wheel drive when the cause of understeer or oversteer is unknown.

In the vehicle control system of the present invention described above, the vehicle is a vehicle that can be switched to a front-wheel drive state or a rear-wheel drive state as a switch from an all-wheel drive state to a partial wheel drive state, and the control unit can be configured to be configured such that when it determines that all of the wheels are in a slipping state in the all-wheel drive state, and when it determines that the vehicle is about to enter a curved road, it determines whether the vehicle is accelerating or decelerating, and if it determines that the vehicle is accelerating, it performs control to switch from the all-wheel drive state to the rear-wheel drive state, and if it determines that the vehicle is decelerating, it performs control to switch from the all-wheel drive state to the front-wheel drive state.
During acceleration, the weight is biased toward the rear wheels, so if the vehicle attempts to enter a curved road in front-wheel drive mode, the front wheels are likely to skid and the vehicle may not be able to turn the curve (i.e., the vehicle may tend to understeer). For this reason, the vehicle switches to rear-wheel drive mode when accelerating as described above. During deceleration, the weight is biased toward the front wheels, so if the vehicle attempts to enter a curved road in rear-wheel drive mode, the rear wheels are likely to skid and the vehicle may turn too far in relation to the steering angle (i.e., the vehicle may tend to oversteer). For this reason, the vehicle switches to front-wheel drive mode when decelerating as described above.

The present invention can improve vehicle performance in driving environments where all wheels are in a slipping state.

1 is a diagram showing an outline of the configuration of a drive system for wheels in a vehicle equipped with a vehicle control system according to an embodiment of the present invention; 1 is a block diagram showing an outline of the configuration of a vehicle control system according to an embodiment; 5 is a flowchart showing an example of a specific processing procedure to be executed to realize drive wheel switching control according to the first embodiment. FIG. 2 is an explanatory diagram of locations where understeer and oversteer occur. FIG. 11 is an explanatory diagram of drive wheel switching control according to a second embodiment. 4 is a flowchart of a process related to the construction of understeer/oversteer history information. 10 is a flowchart showing a drive wheel switching control process according to a second embodiment. FIG. 11 is an explanatory diagram of drive wheel switching control according to a third embodiment. 10 is a flowchart showing an example of a specific process procedure to be executed to realize drive wheel switching control according to a third embodiment.

1. First embodiment
[1-1. Configuration of wheel drive system and vehicle control system]
FIG. 1 shows an outline of the configuration of a drive system for wheels in a vehicle equipped with a vehicle control system 1 according to an embodiment of the present invention.
In this embodiment, the present invention is applied to a four-wheel vehicle having two wheels W as front wheels and two wheels as rear wheels.
As shown in the figure, the vehicle in this embodiment is provided with a wheel W1 as a left front wheel, a wheel W2 as a right front wheel, a wheel W3 as a left rear wheel, and a wheel W4 as a right rear wheel, a drive source 50, and a center differential 51. Wheels W1, W2, W3, and W4 are provided with wheel rotation speed sensors 11a-1, 11a-2, 11a-3, and 11a-4 that detect the wheel rotation speed, which is the rotation speed of the wheels W.

The drive source 50 is a drive source for the wheels W, and can be constituted by, for example, an engine (internal combustion engine) or a motor.
Driving force from a driving source 50 can be transmitted via a center differential 51 to wheels W1, W2 as front wheels and wheels W3, W4 as rear wheels.
The center differential 51 is, for example, a hydraulically controlled center differential, and torque distribution to the front wheels and torque distribution to the rear wheels are changed by increasing/decreasing the hydraulic pressure at a specific internal location.
The vehicle of this example is capable of switching, by means of this center differential 51, between an all-wheel drive state in which all of the wheels W1, W2, W3, and W4 are driven, and a partial-wheel drive state in which only some of the wheels W are driven. Specifically, in this example, switching to the partial-wheel drive state can be performed by switching to a front-wheel drive state in which only the front wheels are driven, and a rear-wheel drive state in which only the rear wheels are driven.

Figure 2 is a block diagram showing an outline of the configuration of a vehicle control system 1 as an embodiment mounted on the vehicle shown in Figure 1. Note that Figure 1 shows only the main components of the vehicle control system 1 that are relevant to the present invention. In the embodiment, an example of the configuration of the vehicle control system 1 corresponding to the case where the vehicle shown in Figure 1 is a hybrid vehicle (Hybrid Electric Vehicle: HEV) is shown. In the case of a hybrid vehicle, an engine and a motor (motor generator) are provided as the drive source 50.

As shown in FIG. 2, the vehicle control system 1 includes a driving assistance control unit 2, an HEV control unit 3, an engine control unit 4, a motor control unit 5, a brake control unit 6, an engine-related actuator 7, a motor drive unit 8, a brake-related actuator 9, a differential drive unit 10, sensors and controls 11, and a bus 12.

The driving assistance control unit 2 has an imaging section 21, an image processing section 22, and a control section 23, and executes various control processes for driving assistance (hereinafter referred to as "driving assistance control processes").
The imaging unit 21 obtains image data obtained by imaging the traveling direction of the vehicle (forward in this example). The imaging unit 21 in this example is provided with two camera units, each of which is provided with a camera optical system and an imaging element such as a CCD (Charge Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor). In each camera unit, a subject image is formed on the imaging surface of the imaging element by the camera optical system, and an electrical signal corresponding to the amount of received light is obtained in pixel units. Each camera unit is installed so as to enable distance measurement by a so-called stereo imaging method. The electrical signal obtained by each camera unit is subjected to A/D conversion and a predetermined correction process, and is supplied to the image processing unit 22 as a digital image signal (image data) representing a luminance value in a predetermined gradation in pixel units.

The image processing unit 22 is configured with, for example, a microcomputer equipped with a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), etc., or a DSP (Digital Signal Processor), and performs predetermined image processing related to recognition of the outside environment of the vehicle based on the captured image data obtained by the imaging unit 21.
Specifically, the image processing unit 22 executes various types of image processing based on each captured image data obtained by stereo imaging, recognizes forward information such as three-dimensional object data and white line data ahead of the host vehicle, and estimates the host vehicle's driving path based on this recognized information, etc. Furthermore, the image processing unit 22 detects a preceding vehicle on the host vehicle's driving path based on the recognized three-dimensional object data, etc.

Specifically, the image processing unit 22 performs the following processing based on each stereo image data. First, for each pair of images as the image data, distance information is generated by the principle of triangulation from the displacement (parallax) of the corresponding positions. Then, a known grouping process is performed on the distance information, and the grouped distance information is compared with three-dimensional road shape data and solid object data stored in advance to extract white line data, side wall data such as guard rails and curbs existing along the road, and solid object data such as vehicles. Furthermore, the image processing unit 22 estimates the vehicle's running path based on the white line data and side wall data, and extracts (detects) a solid object existing on the vehicle's running path that moves in approximately the same direction as the vehicle at a predetermined speed (for example, 0 Km/h or more) as a preceding vehicle. Then, when a preceding vehicle is detected, the following vehicle information is calculated: the inter-vehicle distance cd (= the inter-vehicle distance from the vehicle), the relative speed ds (= the rate of change in the inter-vehicle distance cd), the preceding vehicle speed ss (relative speed ds + vehicle speed js), and the like. The host vehicle speed js corresponds to the "vehicle speed" described later.
Furthermore, the image processor 22 recognizes preceding vehicles whose preceding vehicle speed ss is equal to or lower than a predetermined value (eg, 4 km/h or lower) and which are not accelerating as preceding vehicles that are substantially stopped.
The image processor 22 calculates the preceding vehicle information for each frame of the captured image data, for example, and sequentially stores the calculated preceding vehicle information. The image processor 22 also sequentially stores information on the vehicle's travel path estimated based on the white line data, sidewall data, etc., as described above.

The control unit 23 is configured with a microcomputer equipped with, for example, a CPU, ROM, RAM, etc., and executes driving assistance control processing based on the results of image processing by the image processing unit 22, detection information obtained by the sensors and operators 11, operation input information, etc.

The control unit 23 is connected to each of the HEV control unit 3, engine control unit 4, motor control unit 5, and brake control unit 6, which are also configured with a microcomputer, via a bus 12, and is capable of mutual data communication with each of these control units. The control unit 23 issues instructions to the necessary control units among the above control units to execute operations related to driving assistance.

The control unit 23 performs, for example, auto-cruise control as one of the driving assistance control processes. That is, the control unit 23 controls the speed of the vehicle so as to satisfy the specified driving conditions. In particular, the control unit 23 in this example performs a process for realizing ACC (Adaptive Cruise Control: Cruise control with distance control) as the auto-cruise control.
In the ACC, the target vehicle speed St and the target inter-vehicle distance Dt are set based on an operation input from a predetermined operator provided on the sensor/operator group 11. In this example, the driver can select an arbitrary inter-vehicle distance mode from three inter-vehicle distance modes, for example, "long,""medium," and "short," by operation, and the control unit 23 sets a different target inter-vehicle distance Dt for each selected mode, for example, according to the vehicle speed js.
In the following description, the "target vehicle speed St" will be referred to as the "set vehicle speed St."

During ACC, if no preceding vehicle is detected, the control unit 23 performs constant speed driving control to converge the host vehicle speed js to the set vehicle speed St. In addition, if the control unit 23 recognizes a preceding vehicle during constant speed driving control, it performs following driving control to converge the inter-vehicle distance cd to the preceding vehicle to the target inter-vehicle distance Dt.

During ACC, the control unit 23 calculates a target driving force for realizing the constant speed cruise control and follow-up cruise control as described above. In addition, during a state other than ACC (a state in which the acceleration/deceleration of the vehicle is controlled based on the accelerator operation or brake operation of the driver), the control unit 23 calculates the target driving force based on the accelerator operation or brake operation of the driver.
Here, the target driving force is calculated as a value with different polarity depending on whether it is the acceleration side or the deceleration side, for example, as a positive value on the acceleration side or a negative value on the deceleration side.

The control unit 23 in this example calculates a required driving force and a required brake hydraulic pressure based on the calculated target driving force. The required driving force is the driving force of the vehicle required to realize the target driving force, and in the case of the vehicle in this example having an engine and a motor as the driving source of the wheels, the required driving force is calculated as the total driving force of the engine and the motor.
The required brake fluid pressure is the brake fluid pressure required to achieve the target driving force.

Here, in the case of HEVs, in addition to brake mechanisms such as disc brakes, there is also a regenerative brake that uses regeneration from a motor (motor generator) as a vehicle braking means. Therefore, when the vehicle decelerates, not only the required brake hydraulic pressure but also the required driving force is calculated so that a deceleration state according to the calculated target driving force is achieved.

The sensors and operators 11 collectively represent various sensors and operators provided on the vehicle. The wheel rotation speed sensor unit 11a of the sensors and operators 11 collectively represents the wheel rotation speed sensors 11a-1 to 11a-4 shown in Fig. 1. The sensors of the sensors and operators 11 include an accelerator opening sensor 11b that detects an accelerator opening from the depression amount of the accelerator pedal, a brake operation amount sensor 11c that detects the operation amount (depression amount) of the brake pedal, a motion sensor 11d that has, for example, an acceleration sensor or an angular velocity sensor and detects the motion of the vehicle, a position sensor 11e that is a sensor that detects the current position such as a GPS sensor, and a steering angle sensor 11f that detects the steering angle of the steering wheels (front wheels in this example).
In this embodiment, a three-axis acceleration sensor and a yaw rate sensor are provided as the motion sensor 11d.
In addition, although not shown in the figure, the sensors and operators 11 also include other sensors such as an engine speed sensor that detects the engine speed, an intake air volume sensor that detects the amount of intake air into the engine, a throttle opening sensor that is installed in the intake passage and detects the opening of a throttle valve that adjusts the amount of intake air supplied to each cylinder of the engine, a water temperature sensor that detects the coolant temperature that indicates the engine temperature, an outside air temperature sensor that detects the air temperature outside the vehicle, and a gradient sensor that detects the gradient of the road on which the vehicle is traveling.
The sensors and operators 11 include, as operators, a start switch for instructing start/stop of the vehicle control system 1, and operators for carrying out the above-mentioned ACC-related operations.

Here, although an explanation using illustrations will be omitted, in the vehicle control system 1 of this example, the "wheel speed" and the "vehicle speed" are calculated based on the wheel rotation speed of each wheel W detected by the wheel rotation speed sensor unit 11a.
Both the wheel speed and the vehicle speed are index values that indicate the vehicle speed, and the "wheel speed" is a value calculated based on the wheel rotation speed and the outer diameter of the wheel W. When the wheel W is in a slipping state (idling state), this wheel speed has a relatively large difference from the actual vehicle speed. In other words, when the wheel W is not in a slipping state, this value correctly indicates the actual vehicle speed.
The "vehicle speed" is a value calculated using the wheel rotation speed in the same manner, but is calculated by using various information other than the wheel rotation speed, such as acceleration information, accelerator opening information, gear ratio information, steering angle information, etc., in combination so as to reduce the difference between the actual vehicle speed when the wheels W are in a slipping state. Note that various methods have been proposed for calculating the vehicle speed using such various information, and a known method may be used. Furthermore, the various information used to calculate the vehicle speed is not limited to the information exemplified above, and only a part of the exemplified information may be used, or information not exemplified may be used in combination.

The wheel speed and vehicle speed may be calculated by any one of the driving assistance control unit 2, the HEV control unit 3, the engine control unit 4, the motor control unit 5, and the brake control unit 6, or may be calculated by a control unit other than these control units (not shown).

The HEV control unit 3 issues instructions to the engine control unit 4 and the motor control unit 5 based on the required driving force calculated by the control unit 23 in the driving assistance control unit 2 to control the operation of the vehicle.
Based on the required driving force input from the control unit 23, the HEV control unit 3 calculates the engine required driving force, which is the driving force required of the engine, and the motor required driving force, which is the driving force required of the motor, and instructs the engine required driving force to the engine control unit 4 and the motor required driving force to the motor control unit 5, respectively.

Furthermore, the HEV control unit 3 of this example issues instructions to the differential drive section 10 to control torque distribution to the front and rear wheels by the center differential 51. The differential drive section 10 is configured to include various actuators for hydraulic control of the center differential 51, and the HEV control unit 3 controls torque distribution by controlling the drive of the actuators.
By controlling torque distribution in this manner, the HEV control unit 3 in this embodiment is capable of switching the drive state of the wheels W among three states: all-wheel drive, front-wheel drive, and rear-wheel drive. The drive wheel switching control in this embodiment will be described later.

The engine control unit 4 controls various actuators provided as engine-related actuators 7 based on the engine required driving force instructed by the HEV control unit 3. The engine-related actuators 7 include various actuators related to engine drive, such as a throttle actuator that drives a throttle valve and an injector that injects fuel.
The engine control unit 4 controls the engine output by controlling the fuel injection timing, the fuel injection pulse width, the throttle opening, etc. based on the engine required driving force. The engine control unit 4 is also capable of controlling the start/stop of the engine.

The motor control unit 5 controls the operation of the MG 10 by controlling the motor drive unit 8 based on the required motor driving force instructed by the HEV control unit 3. The motor drive unit 8 is configured as an electric circuit unit having a drive circuit for the MG 10.
Based on the motor required driving force, the motor control unit 5 issues instructions to the motor drive unit 8 to cause the MG 10 to rotate in powered rotation when the MG 10 should be rotated in powered rotation, and issues instructions to the motor drive unit 8 to cause the MG 10 to rotate in regenerative rotation when the MG 10 should be rotated in regenerative rotation.

The brake control unit 6 controls various actuators provided as brake-related actuators 9 based on detection signals from predetermined sensors provided in the sensors and operators 11 and operation input information from the operators. The brake-related actuators 9 include various brake-related actuators, such as hydraulic control actuators for controlling the output hydraulic pressure from the brake booster to the master cylinder and the hydraulic pressure in the brake hydraulic piping. The brake control unit 6 realizes so-called ABS (Antilock Brake System) control by increasing or decreasing the hydraulic pressure using the hydraulic control actuators based on the slip ratio of the wheel W calculated based on the wheel speed and vehicle speed described above. The brake control unit 6 also controls the hydraulic control actuators to control the brakes based on the required brake hydraulic pressure instructed by the control unit 23 of the driving assistance control unit 2.

Just to clarify, the above slip ratio is a value calculated as "(vehicle speed - wheel speed) / vehicle speed x 100%", where a slip ratio of 0 indicates that the wheel W is not slipping at all, and a slip ratio of 100 indicates that the wheel W is locked. When the slip ratio is a negative value, this means that the wheel speed is greater than the vehicle speed, and the wheel W is in a spinning state (slipping state).

[1-2. Drive wheel switching control as a first embodiment]
Here, as described above, when all the wheels slip in the all-wheel drive state, the accuracy of estimating the vehicle speed based on the wheel rotation speed decreases, which reduces the accuracy of the output torque control of the wheels W. This reduction in accuracy of the output torque control results in an excess or deficiency of the actual torque down amount relative to the required torque down amount. If the torque down amount is excessive, the vehicle will not accelerate, and if the torque down amount is too small, problems such as the vehicle not recovering from slipping may occur, which may lead to a deterioration in running performance.

Therefore, in this embodiment, in order to improve the vehicle's running performance in a vehicle running environment in which all the wheels are in a slipping state, the following drive wheel switching control is performed.
That is, when it is determined that all the wheels W are in a slipping state in the all-wheel drive state, control is performed to switch from the all-wheel drive state to a partial wheel drive state. Specifically, in this example, control is performed to switch from the all-wheel drive state to either the front wheel drive state or the rear wheel drive state.

When all the wheels W are in a slipping state, by switching some of the wheels W to a driven state, the wheels W switched to a non-driven state, i.e., the wheels W switched to driven wheels, are released from the slipping state.
Therefore, in a vehicle driving environment where all of the wheels W are in a slipping state, it is possible to improve the accuracy of estimating the vehicle speed and improve the accuracy of controlling the output torque of the wheels W. As a result, in a vehicle driving environment where all of the wheels W are in a slipping state, it is possible to improve the drivability.

In this embodiment, the above-mentioned switching from the all-wheel drive state to the partial-wheel drive state is not performed in all cases where it is determined that all of the wheels are in a slipping state, but rather, if a predetermined condition is met, the switching to the partial-wheel drive state is not performed. Specifically, if it is determined that all of the wheels are in a slipping state, and the vehicle is traveling on a curved road or is about to enter a curved road, the switching to the partial-wheel drive state is not performed.
Here, "immediately before entering a curved road" means a state in which the distance from the vehicle's current position to the start of the curved road is within a specified distance, or a state in which the predicted time to reach the start of the curved road is within a specified time.

When switching from all-wheel drive to partial-wheel drive while traveling on a curved road, the vehicle behavior may change and the vehicle may fall into an oversteer or understeer state. For this reason, when it is determined that the vehicle is traveling on a curved road or is about to enter a curved road as described above, the system does not switch to partial-wheel drive.
This makes it possible to prevent the vehicle from going into an oversteer or understeer state when traveling on a curved road due to switching from an all-wheel drive state to a partial wheel drive state.

[1-3. Processing procedure]
FIG. 3 is a flowchart showing an example of a specific processing procedure to be executed to realize the drive wheel switching control according to the first embodiment described above.
In this embodiment, the driving wheel switching control is mainly executed by the HEV control unit 3. The HEV control unit 3 executes the process shown in Fig. 3 based on a program stored in a storage device such as an internal ROM.
The process shown in FIG. 3 is executed repeatedly, for example, at a predetermined interval in the all-wheel drive state.

First, in step S101, the HEV control unit 3 determines whether or not all the wheels are slipping, that is, whether or not all the wheels W are in a slipping state.
The determination as to whether the wheel W is in a slip state is made based on the above-mentioned slip ratio. Specifically, for example, a determination result that the wheel W is in a slip state is obtained when the slip ratio falls below a predetermined threshold value on the negative polarity side. For example, such slip determination based on the slip ratio is made for each wheel W, and when it is determined that all the wheels W are in a slip state, a determination result that all the wheels are in a slip state is obtained.
If it is determined in step S101 that all the wheels are not slipping, the HEV control unit 3 ends the series of processes shown in Fig. 3. That is, in this case, the all-wheel drive state is not switched to the partial-wheel drive state.

On the other hand, if it is determined in step S101 that all wheels are slipping, the HEV control unit 3 determines in step S102 whether or not the vehicle is traveling on a curved road. Whether or not the vehicle is traveling on a curved road can be determined, for example, based on the yaw rate or lateral acceleration detected by the motion sensor 11d. Alternatively, whether or not the vehicle is traveling on a curved road can be determined based on the recognition results of the external environment by the image processing unit 22 (for example, information on the vehicle's travel path, etc., as described above), or based on the current position detected by the position sensor 11e and map information.

If it is determined in step S102 that the vehicle is traveling on a curved road, the HEV control unit 3 ends the series of processes shown in FIG. 3. As a result, even if all the wheels are in a slipping state, if the vehicle is traveling on a curved road, the all-wheel drive state is not switched to the partial-wheel drive state.

If it is determined in step S102 that the vehicle is not traveling on a curved road, the HEV control unit 3 determines in step S103 whether the vehicle is about to enter a curved road. The determination of whether the vehicle is about to enter a curved road can be made based on, for example, the recognition result of the external environment by the image processing unit 22, particularly the information on the vehicle's travel path described above. Alternatively, the determination can be made based on the current position detected by the position sensor 11e and map information.
If it is determined in step S103 that the vehicle is about to enter a curved road, the HEV control unit 3 ends the series of processes shown in FIG. 3 (that is, switching to the partial wheel drive state is not performed).

On the other hand, if it is determined in step S103 that the vehicle is not about to enter a curved road, the HEV control unit 3 proceeds to step S104, performs control to switch from the all-wheel drive state to the partial wheel drive state, and then ends the series of processes shown in FIG.
As described above, in this example, it is possible to switch to either a front-wheel drive state or a rear-wheel drive state as the partial wheel drive state, but in the first embodiment, it is sufficient to switch to any of these front-wheel drive states or rear-wheel drive states.

In the above, the condition for excluding switching to the partial wheel drive state, i.e., the condition for not switching to the partial wheel drive state even if it is determined that all wheels are in a slipping state, is set to be the condition of traveling on a curved road or the stage immediately before entering a curved road. However, the condition for excluding switching may be only one of the conditions of traveling on a curved road or the stage immediately before entering a curved road.

Here, in this embodiment, after all the wheels are in a slip state and the all-wheel drive state is switched to the partial-wheel drive state, the partial-wheel drive state is switched back to the all-wheel drive state on the condition that the slip state of all the wheels is resolved. However, at this time, a discrepancy occurs in the time until torque output between the drive wheels and the driven wheels.
In order to deal with this problem, control is performed to output torque to the wheels W as driven wheels in the partial wheel drive state. Specifically, for example, after switching to the partial wheel drive state, control is performed to output a torque that is sufficiently low (torque equal to or less than a predetermined value) to the wheels W as driven wheels in response to the establishment of a predetermined condition, such as the passage of a predetermined time.
As a result, in the all-wheel drive state after the slip state of all wheels is eliminated, the time until the torque of the wheel W, which was the driven wheel, reaches the expected torque can be shortened, thereby improving responsiveness to driving operations.

<2. Second embodiment>
Next, a second embodiment will be described.
In the second embodiment, a selection is made as to whether to switch to a front-wheel drive state or a rear-wheel drive state, based on historical information relating to the location where understeer or oversteer has occurred.
In the second embodiment, the configuration of the vehicle drive system and the hardware configuration of the vehicle control system 1 are similar to those in the first embodiment, so duplicated explanations will be avoided.

In the second embodiment, when all the wheels are in a slip state, the drive wheel switching control uses under/oversteer history information Is, which contains information indicating where understeer has occurred and information indicating where oversteer has occurred.

FIG. 4 is an explanatory diagram of locations where understeer and oversteer occur.
The understeer occurrence location is a location including the position where understeer occurs, and the oversteer occurrence location is a location including the position where oversteer occurs.
In this example, the location where understeer or oversteer occurs is assumed to be a location defined as an area Ae having a radius r centered on the occurrence position Pe, as shown in the figure.

In the vehicle control system 1 of the second embodiment, each time understeer or oversteer occurs, information on the location of understeer or oversteer occurrence based on the position of occurrence is stored in a prescribed storage device, and understeer/oversteer history information Is is constructed in which the information on the location of understeer or oversteer occurrence is stored as a history.
In this example, the process of constructing such understeer/oversteer history information Is is performed by, for example, the HEV control unit 3.

Specifically, the HEV control unit 3 performs a process to determine whether understeer or oversteer has occurred. Whether understeer or oversteer has occurred can be determined, for example, based on the steering angle and yaw rate. If the yaw rate is small relative to the steering angle (e.g., below a predetermined value), it can be determined that an understeer state exists, and if the yaw rate is large relative to the steering angle (e.g., above a predetermined value), it can be determined that an oversteer state exists.

When it is determined that understeer has occurred, the HEV control unit 3 performs a process of adding information about the location where understeer has occurred, with the current position set as the occurrence position Pe, to the understeer/oversteer history information Is. On the other hand, when it is determined that oversteer has occurred, the HEV control unit 3 performs a process of adding information about the location where oversteer has occurred, with the current position set as the occurrence position Pe, to the understeer/oversteer history information Is.

Based on the understeer/oversteer history information Is thus constructed, a selection is made as to whether to switch to the front-wheel drive state or the rear-wheel drive state.
Specifically, in the second embodiment, when the HEV control unit 3 determines that all wheels are slipping, and determines that the current position (represented by "Pn" in the figure) corresponds to the location where understeer is occurring (represented by "Ae" in the figure), as illustrated in FIG. 5A, the HEV control unit 3 performs control to switch from the all-wheel drive state to the rear-wheel drive state.
On the other hand, if the HEV control unit 3 determines that all wheels are slipping and that the current position (Pn) corresponds to the location (Ae) where oversteer is occurring, as illustrated in FIG. 5B, the HEV control unit 3 performs control to switch from the all-wheel drive state to the front-wheel drive state.

In this way, if it is determined that the current position corresponds to a location where understeer occurs, the vehicle is switched to rear-wheel drive in order to counteract the understeer tendency, whereas if it is determined that the current position corresponds to a location where oversteer occurs, the vehicle is switched to front-wheel drive in order to counteract the oversteer tendency.
This makes it possible to improve the accuracy of estimating vehicle speed by switching to a partial wheel drive state when all wheels are in a slipping state, while preventing the vehicle from entering an oversteer or understeer state when traveling on a curved road.

Here, in the second embodiment, even if the current position corresponds to a location where understeer has occurred, if the location where understeer has occurred is a location where understeer has occurred in a rear-wheel biased drive state, no switching from all-wheel drive state to rear-wheel drive state is performed, and also, even if the current position corresponds to a location where oversteer has occurred, if the location where oversteer has occurred is a location where oversteer has occurred in a front-wheel biased drive state, no switching from all-wheel drive state to front-wheel drive state is performed.
A rear-wheel biased drive state refers to a drive state in which the torque distribution between the front and rear wheels is biased toward the rear wheels, and a front-wheel biased drive state refers to a drive state in which the torque distribution between the front and rear wheels is biased toward the front wheels.

In this case, the information on the location of understeer occurrence and the location of oversteer occurrence in the understeer/oversteer history information Is are provided with identification information indicating whether the location is understeer occurrence in a rear-wheel biased drive state or not, and whether the location is oversteer occurrence in a front-wheel biased drive state or not. That is, when constructing the understeer/oversteer history information Is, if understeer occurs, the HEV control unit 3 determines whether the wheel drive state at that time is a rear-wheel biased drive state or not, and if it is a rear-wheel biased drive state, adds the understeer occurrence location information with flag information indicating that to the understeer/oversteer history information Is. On the other hand, if oversteer occurs, the HEV control unit 3 determines whether the wheel drive state at that time is a front-wheel biased drive state or not, and if it is a front-wheel biased drive state, adds the oversteer occurrence location information with flag information indicating that to the understeer/oversteer history information Is.

In this manner, in the under/oversteer history information Is of this example, flag information indicating that the location where understeer occurred is a location where the rear wheels are unbalanced in drive is attached as the understeer occurrence location information, and flag information indicating that the location where oversteer occurred is a location where the front wheels are unbalanced in drive is attached as the oversteer occurrence location information.
Therefore, in this case, when an all-wheel slip state occurs and the current position corresponds to an understeer occurrence location, the HEV control unit 3 determines whether or not a flag for the corresponding understeer occurrence location information is present, and if a flag is present, it determines that this is a location where understeer is occurring in a rear-wheel biased drive state, and does not perform control to switch from all-wheel drive to rear-wheel drive.
Furthermore, when the current position corresponds to a location where oversteer has occurred, the HEV control unit 3 determines whether or not a flag for the corresponding oversteer occurrence location information is present, and when the flag is present, it determines that the location is one where oversteer has occurred in a front-wheel biased drive state, and does not perform switching control from the all-wheel drive state to the front-wheel drive state.

In the first embodiment, even if it was determined that all the wheels were slipping, the vehicle would not switch to a partial wheel drive state if the vehicle was traveling on a curved road. Similarly, in the second embodiment, the vehicle will not switch to a partial wheel drive state if the vehicle is traveling on a curved road.

An example of a specific processing procedure for implementing the drive wheel switching control according to the second embodiment described above will be described with reference to the flowcharts of FIGS.
In the following description, processes similar to those already described will be denoted by the same reference numerals and description thereof will be omitted.

FIG. 6 is a flowchart showing a process for constructing the understeer/oversteer history information Is.
First, in step S201, the HEV control unit 3 performs a process of waiting for a slip state. That is, the process is a process of waiting for the wheels W to enter a slip state. Note that the process of step S201 may be a process of waiting for all the wheels W to enter a slip state, or a process of waiting for some of the wheels W to enter a slip state.
Here, the process of step S201 may be omitted.

If it is determined in step S201 that a slip state exists, the HEV control unit 3 determines in step S202 whether or not oversteer has been experienced (detected), and if it is determined that oversteer has not been experienced, it determines in step S203 whether or not understeer has been experienced. If it is determined in step S203 that understeer has not been experienced, the HEV control unit 3 ends the series of processes shown in FIG. 6. In other words, if neither oversteer nor understeer has occurred, no additional processing of oversteer occurrence location information or understeer occurrence location information is performed.

If it is determined in step S202 that oversteer has occurred, the HEV control unit 3 proceeds to step S204 and determines whether or not the vehicle is in a front-wheel biased drive state.
If the vehicle is not in a front-wheel-biased drive state, the HEV control unit 3 proceeds to step S205 to add oversteer occurrence location information, that is, to add the information to the understeer/oversteer history information Is, and then ends the series of processes shown in FIG.
On the other hand, if the vehicle is in a front-wheel-biased drive state, the HEV control unit 3 proceeds to step S206, performs a process of adding flagged oversteer occurrence location information, and ends the series of processes shown in FIG.

Furthermore, if the HEV control unit 3 determines in step S203 that understeer has occurred, the process proceeds to step S207 to determine whether or not the vehicle is in a rear-wheel biased drive state.
If the vehicle is not in a rear-wheel-heavy drive state, the HEV control unit 3 proceeds to step S208 to perform a process of adding information on the location where understeer has occurred, and then ends the series of processes shown in FIG.
On the other hand, if the vehicle is in a rear-wheel biased drive state, the HEV control unit 3 proceeds to step S209, performs a process of adding flagged understeer occurrence location information, and ends the series of processes shown in FIG.

FIG. 7 is a flowchart showing a drive wheel switching control process according to the second embodiment.
First, the processes in steps S101 and S102 in the figure are the same as those in FIG. 3, so the description thereof will be omitted.
In this case, in response to determining in step S102 that the vehicle is not traveling on a curved road, the HEV control unit 3 determines in step S301 whether the current position corresponds to a location where understeer or oversteer has occurred. This determination process is performed based on the understeer/oversteer history information Is, and if the current position is included in the occurrence location Ae of the understeer occurrence location information or oversteer occurrence location information stored in the understeer/oversteer history information Is, the determination result indicates that the current position corresponds to a location where understeer or oversteer has occurred.

If it is determined in step S301 that the current position does not correspond to a location where understeer or oversteer is occurring, the HEV control unit 3 proceeds to step S302, performs processing to switch to front-wheel or rear-wheel drive, and ends the series of processing shown in FIG.
In other words, when all the wheels are slipping, if the vehicle is not traveling on a curved road and understeer or oversteer is not predicted to occur, the drive state can be switched to either the front-wheel drive state or the rear-wheel drive state.

On the other hand, if it is determined in step S301 that the current position corresponds to a location where understeer or oversteer has occurred, the HEV control unit 3 proceeds to step S303 and determines whether or not the current position corresponds to a location where understeer has occurred. In other words, it determines whether or not the occurrence location information that was determined in step S301 as the occurrence location Ae to include the current position is understeer occurrence location information.

If it is determined that the location is an understeer occurrence location, the HEV control unit 3 proceeds to step S304 to determine whether or not the location is a flagged understeer occurrence location. That is, it is determined whether or not the understeer occurrence location information, the occurrence location Ae of which is determined in step S301 to include the current position, is a flagged understeer occurrence location information.
If the location is not a flagged understeer occurrence location, i.e., if the location is not a location where understeer is occurring in a rear-wheel-biased drive state, the HEV control unit 3 proceeds to step S305, performs processing to switch to a rear-wheel drive state, and then ends the series of processing shown in FIG. 7.

On the other hand, if the location is a flagged understeer occurrence location in step S304, the HEV control unit 3 ends the series of processes shown in FIG.
As a result, even if the current position corresponds to a location where understeer occurs, if the location where understeer occurs is a location where understeer occurs in a rear-wheel biased drive state, switching to the rear-wheel drive state is not performed.

Furthermore, if it is determined in step S303 that the location is not an understeer occurrence location (that is, the current location is an oversteer occurrence location), the HEV control unit 3 determines in step S306 whether the location is a flagged oversteer occurrence location.
If the location is not a location where flagged understeer has occurred, i.e., if the location is not a location where oversteer has occurred in a front-wheel-biased drive state, the HEV control unit 3 proceeds to step S307, performs processing to switch to a front-wheel drive state, and then ends the series of processing steps shown in FIG. 7.

On the other hand, if the location is a flagged oversteer occurrence location in step S306, the HEV control unit 3 ends the series of processes shown in FIG.
As a result, even if the current position corresponds to a location where oversteer has occurred, if the location where oversteer has occurred is a location where oversteer has occurred in a front-wheel biased drive state, switching to the front-wheel drive state is not performed.

In the above description, the case has been exemplified where the understeer/oversteer history information Is is information that accumulates only the experience information of the own vehicle (i.e., only the understeer/oversteer information experienced by the own vehicle). However, the understeer/oversteer history information Is can also be information that includes experience information of other vehicles.
In this case, the understeer/oversteer history information Is is constructed by uploading information on the location where understeer or oversteer has occurred to a specified server device in response to the occurrence of understeer or oversteer in the subject vehicle and each of the other vehicles. When it is determined that all wheels are slipping, the understeer/oversteer history information Is thus constructed in the server device is referenced in determining whether the current position corresponds to the location where understeer or oversteer has occurred. Alternatively, it is also possible to obtain the understeer/oversteer history information Is from the server device in advance and reference the obtained understeer/oversteer history information Is.

<3. Third embodiment>
In the third embodiment, when it is determined that all wheels are slipping in the all-wheel drive state, if the vehicle is about to enter a curved road, the system selects whether to switch to rear-wheel drive or front-wheel drive depending on whether the vehicle is accelerating or decelerating.

Consider a situation in which a vehicle is turning a curved road while accelerating in front-wheel drive mode, as shown in Fig. 8A. During acceleration, the weight is biased toward the rear wheels, making the front wheels more likely to skid. If the front wheels skid, there is a risk that the vehicle will not be able to complete the turn (i.e., there will be a tendency for the vehicle to understeer).
On the other hand, when considering a situation in which the vehicle is turning a curve while decelerating in rear-wheel drive mode as shown in FIG. 8B, the weight is biased toward the front wheels during deceleration, which makes the rear wheels more likely to skid and may result in the vehicle turning too far in relation to the steering angle (i.e., there is a tendency for the vehicle to oversteer).

Taking these points into consideration, in the third embodiment, when all wheels are in a slip state and the vehicle is about to enter a curved road, if the vehicle is accelerating, the drive state is switched to rear-wheel drive, and if the vehicle is decelerating, the drive state is switched to front-wheel drive.
This makes it possible to prevent the vehicle from switching to front-wheel drive mode when accelerating just before entering a curved road, resulting in understeer as shown in Fig. 8A, and also makes it possible to prevent the vehicle from switching to rear-wheel drive mode when decelerating just before entering a curved road, resulting in oversteer as shown in Fig. 8B.

In the third embodiment, as in the first and second embodiments, even if it is determined that all wheels are slipping, the vehicle will not switch to a partial wheel drive state when traveling on a curved road.

FIG. 9 is a flowchart showing an example of a specific processing procedure for implementing the drive wheel switching control according to the third embodiment.
In the figure, the processes in steps S101, S102, and S103 are the same as those in FIG. 3, and therefore the description thereof will be omitted.

In this case, if the HEV control unit 3 determines in step S103 that the vehicle is not about to enter a curved road, the process proceeds to step S401 to switch to front-wheel or rear-wheel drive, and the series of processes shown in FIG. 9 is completed. In other words, if all wheels are slipping, the vehicle is not traveling on a curved road, and the vehicle is not about to enter a curved road, the drive state can be switched to either the front-wheel drive state or the rear-wheel drive state.

On the other hand, if it is determined in step S103 that the vehicle is about to enter a curved road, the HEV control unit 3 proceeds to step S402 to determine whether or not the vehicle is accelerating. If the vehicle is accelerating, the HEV control unit 3 proceeds to step S403 to switch to rear-wheel drive, and then ends the series of processes shown in FIG. 9.

If the HEV control unit 3 determines in step S402 that the vehicle is not accelerating, it determines in step S404 whether the vehicle is decelerating. If it determines that the vehicle is not decelerating, the HEV control unit 3 proceeds to step S401. In other words, if the vehicle is not accelerating or decelerating, the vehicle may be switched to either the front-wheel drive state or the rear-wheel drive state.

On the other hand, if it is determined in step S404 that deceleration is occurring, the HEV control unit 3 proceeds to step S405 to switch to front-wheel drive, and then ends the series of processes shown in FIG. 9.

In the third embodiment, the choice of switching to rear-wheel drive or front-wheel drive when the vehicle is about to enter a curved road can be made not only based on the determination information of whether the vehicle is accelerating or decelerating, but also on the relationship with the steering angle, yaw rate, and lateral acceleration.
For example, during acceleration, if the yaw rate or lateral acceleration is not large relative to the steering angle, it is expected that there will be a tendency for understeer, so the vehicle will switch to rear-wheel drive mode. Conversely, during deceleration, if the yaw rate or lateral acceleration is too large relative to the steering angle, it is expected that there will be a tendency for oversteer, so the vehicle will switch to front-wheel drive mode.

4. Modifications
It should be noted that the embodiment is not limited to the specific examples given above, and various modifications are possible.
For example, in the above, a vehicle in which torque is distributed to the front and rear wheels by a torque distribution mechanism such as center differential 51 was given as an example of a vehicle that can be switched from an all-wheel drive state to a front-wheel drive state or a rear-wheel drive state, but the present invention can also be suitably applied to vehicles in which switching from an all-wheel drive state to a front-wheel drive state or switching from an all-wheel drive state to a rear-wheel drive state is performed by a disconnect mechanism.
The present invention can also be suitably applied to an EV vehicle that includes a front motor for driving the front wheels and a rear motor for driving the rear wheels.
Furthermore, the present invention can also be suitably applied to an in-wheel motor vehicle equipped with a motor on every wheel W. In this case, the partial wheel driving state can include a state in which only one or an odd number of wheels W, such as three, are driven.

In the above, the HEV control unit 3 executes the processes related to drive wheel switching illustrated in Figures 3, 6, 7, 9, etc., but some or all of these processes may be executed by a control unit other than the HEV control unit 3. It is also possible to configure the processes to be shared and executed by multiple control units.

In addition, while the above example illustrates the application of the present invention to an HEV, the present invention can also be suitably applied to EVs and engine vehicles (vehicles equipped only with an engine as a drive source for the wheels W).

5. Summary of the embodiment
As described above, the vehicle control system (1) as an embodiment is a vehicle control system for a vehicle having a plurality of wheels (W) and capable of switching between an all-wheel drive state in which all of the wheels are driven and a partial-wheel drive state in which only some of the wheels are driven, and is equipped with a wheel rotation speed detection unit (wheel rotation speed sensor unit 11a) that detects the wheel rotation speed for each wheel, and a control unit (HEV control unit 3) that performs control to switch from the all-wheel drive state to the partial-wheel drive state when it is determined that all of the wheels are in a slipping state in the all-wheel drive state.
When all of the wheels are in a slipping state, some of the wheels are switched to a driven state, and the wheels that have been switched to a non-driven state, i.e., the wheels that have been switched to a driven wheel, are released from the slipping state.
Therefore, in a vehicle driving environment in which all wheels are in a slipping state, the accuracy of estimating vehicle speed can be improved, and the accuracy of wheel output torque control can be improved, thereby improving off-road performance.

In addition, in the vehicle control system of the embodiment, when the control unit determines that all of the wheels are in a slipping state in the all-wheel drive state, and determines that the vehicle is traveling on a curved road or is about to enter a curved road, the control unit does not perform control to switch to a partial-wheel drive state.
When switching from all-wheel drive to partial-wheel drive while traveling on a curved road, the vehicle behavior may change and the vehicle may fall into an oversteer or understeer state. For this reason, when it is determined that the vehicle is traveling on a curved road or is about to enter a curved road as described above, the system does not switch to partial-wheel drive.
This makes it possible to prevent the vehicle from going into an oversteer or understeer state when traveling on a curved road due to switching from an all-wheel drive state to a partial wheel drive state.

Furthermore, in the vehicle control system of the embodiment, the vehicle is capable of switching from an all-wheel drive state to a front-wheel drive state or a rear-wheel drive state as a switch to a partial-wheel drive state, and when the control unit determines that the vehicle is traveling on a curved road or is about to enter a curved road, it does not perform control to switch to the front-wheel drive state or the rear-wheel drive state.
When switching from all-wheel drive to front-wheel drive or rear-wheel drive while traveling on a curved road, the vehicle is likely to fall into an oversteer or understeer state. For this reason, when it is determined that the vehicle is traveling on a curved road or is about to enter a curved road, the vehicle is not allowed to switch to front-wheel drive or rear-wheel drive.
This makes it possible to prevent the vehicle from oversteering or understeering when traveling on a curved road due to switching from the all-wheel drive state to the front-wheel drive state or the rear-wheel drive state.

Furthermore, in the vehicle control system according to the embodiment, the vehicle is capable of switching from an all-wheel drive state to a partial wheel drive state, and can be switched to a front-wheel drive state or a rear-wheel drive state. When the control unit determines that all of the wheels are in a slipping state in the all-wheel drive state, the control unit determines whether the current position corresponds to a location where oversteer has occurred or a location where understeer has occurred based on under/oversteer history information (Is) in which information indicating the location where understeer has occurred, which is a location including the location where understeer has occurred, and information indicating the location where oversteer has occurred, which is a location including the location where oversteer has occurred, and when the control unit determines that the current position corresponds to a location where understeer has occurred, the control unit performs control to switch from the all-wheel drive state to the rear-wheel drive state, and when the control unit determines that the current position corresponds to a location where oversteer has occurred, the control unit performs control to switch from the all-wheel drive state to the front-wheel drive state.
That is, as explained in the second embodiment, if it is determined that the current position corresponds to a location where understeer is occurring based on historical information related to the occurrence of understeer or oversteer, a switch to rear-wheel drive is performed to counteract the understeer tendency, and if it is determined that the current position corresponds to a location where oversteer is occurring, a switch to front-wheel drive is performed to counteract the oversteer tendency.
This makes it possible to improve the accuracy of estimating vehicle speed by switching to a partial wheel drive state when all wheels are in a slipping state, while preventing the vehicle from entering an oversteer or understeer state when traveling on a curved road.

Furthermore, in the vehicle control system of the embodiment, in the under/oversteer history information, information indicating the location where understeer occurred is associated with identification information indicating whether or not understeer occurred in a rear-wheel-biased drive state, and information indicating the location where oversteer occurred is associated with identification information indicating whether or not oversteer occurred in a front-wheel-biased drive state. If the control unit determines that the current position is a location where understeer occurred in a rear-wheel-biased drive state, it does not perform control to switch from the all-wheel drive state to the rear-wheel drive state, and if the control unit determines that the current position is a location where oversteer occurred in a front-wheel-biased drive state, it does not perform control to switch from the all-wheel drive state to the front-wheel drive state.
This makes it possible to prevent inappropriate control such as intensifying understeer or oversteer tendencies rather than mitigating them, which would occur if the vehicle were to switch to rear-wheel drive or front-wheel drive when the cause of understeer or oversteer is unknown.
Therefore, the reliability of the control can be improved.

Furthermore, in a vehicle control system as an embodiment, the vehicle is capable of switching from an all-wheel drive state to a front-wheel drive state or a rear-wheel drive state as a switch from an all-wheel drive state to a partial wheel drive state, and when the control unit determines that all of the wheels are in a slipping state in the all-wheel drive state, and when it determines that the vehicle is about to enter a curved road, it determines whether the vehicle is accelerating or decelerating, and if it determines that the vehicle is accelerating, it performs control to switch from the all-wheel drive state to the rear-wheel drive state, and if it determines that the vehicle is decelerating, it performs control to switch from the all-wheel drive state to the front-wheel drive state (see the third embodiment).
During acceleration, the weight is biased toward the rear wheels, so if the vehicle attempts to enter a curved road in front-wheel drive mode, the front wheels are likely to skid and the vehicle may not be able to turn the curve (i.e., the vehicle may tend to understeer). For this reason, the vehicle switches to rear-wheel drive mode when accelerating as described above. During deceleration, the weight is biased toward the front wheels, so if the vehicle attempts to enter a curved road in rear-wheel drive mode, the rear wheels are likely to skid and the vehicle may turn too far in relation to the steering angle (i.e., the vehicle may tend to oversteer). For this reason, the vehicle switches to front-wheel drive mode when decelerating as described above.
Therefore, the behavior of the vehicle can be appropriately controlled.

W1, W2, W3, W4 Wheels 50 Drive source 51 Center differential 1 Vehicle control system 2 Driving assistance control unit 21 Imaging unit 22 Image processing unit 23 Control unit 3 HEV control unit 4 Engine control unit 5 Motor control unit 6 Brake control unit 7 Engine-related actuator 8 Motor drive unit 9 Brake-related actuator 10 Differential drive unit 11 Sensors and operators 11a Wheel rotation speed sensor units 11a-1, 11a-2, 11a-3, 11a-4 Wheel rotation speed sensor 11b Accelerator opening sensor 11c Brake operation amount sensor 11d Movement sensor 11e Position sensor 11f Steering angle sensor 12 Bus

Claims (4)

A vehicle control system for a vehicle having a plurality of wheels, capable of switching between an all -wheel drive state in which all of the wheels are driven and a partial-wheel drive state in which only some of the wheels are driven, and capable of switching from the all-wheel drive state to a front-wheel drive state and a rear-wheel drive state as switching from the all-wheel drive state to the partial -wheel drive state, comprising:
a wheel rotation speed detection unit for detecting a wheel rotation speed for each of the wheels;
a control unit that performs control to switch from the all-wheel drive state to a partial-wheel drive state when it is determined that all of the wheels are in a slip state in the all-wheel drive state,
The control unit is
After switching from the all-wheel drive state to the partial-wheel drive state, control is performed to switch from the partial -wheel drive state to the all-wheel drive state on the condition that the slip state of all the wheels is eliminated, and
After switching from the all-wheel drive state to the partial wheel drive state, control is performed so that a pre-torque output is performed on the driven wheels during the period from switching from the partial wheel drive state to the all-wheel drive state;
Furthermore, the control unit
When it is determined that all of the wheels are in a slip state in an all-wheel drive state, based on understeer/oversteer history information in which information indicating an understeer occurrence location, which is a location including the location where understeer has occurred, and information indicating an oversteer occurrence location, which is a location including the location where oversteer has occurred, is stored, it is determined whether the current position corresponds to the oversteer occurrence location or the understeer occurrence location;
On the condition that the current position is determined to correspond to the location where understeer occurs, control is performed to switch from an all-wheel drive state to a rear-wheel drive state;
If the current position is determined to correspond to the location where oversteer has occurred, control is performed to switch from all-wheel drive to front-wheel drive.
Vehicle control system. The control unit is
2. The vehicle control system according to claim 1, wherein when it is determined that all of the wheels are in a slipping state in the all-wheel drive state, and when it is determined that the vehicle is traveling on a curved road or is about to enter a curved road, no control is performed to switch to the front-wheel drive state or the rear-wheel drive state . In the under/oversteer history information, the information indicating the location of understeer occurrence is associated with identification information indicating whether or not the understeer occurred in a rear-wheel biased drive state, and the information indicating the location of oversteer occurrence is associated with identification information indicating whether or not the oversteer occurred in a front-wheel biased drive state,
The control unit is
If it is determined that the current position is a location where understeer occurs in a rear-wheel-biased drive state, control to switch from an all-wheel drive state to a rear-wheel drive state is not performed,
2. The vehicle control system according to claim 1 , wherein when it is determined that the current position is a location where oversteer occurs in a front-wheel-biased drive state, control for switching from an all-wheel drive state to a front-wheel drive state is not performed. The control unit is
When it is determined that all the wheels are in a slip state in the all-wheel drive state, and when it is determined that the vehicle is about to enter a curved road, it is determined whether the vehicle is accelerating or decelerating;
If it is determined that the vehicle is accelerating, the system switches from all-wheel drive to rear-wheel drive.
The vehicle control system according to claim 1 , further comprising: a control for switching from an all-wheel drive state to a front-wheel drive state when it is determined that the vehicle is decelerating.

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