Control device for four-wheel drive vehicle

Abstract

A control device for a four-wheel drive vehicle applied to a four-wheel drive vehicle (10), comprising: a drive unit (20) which generates a driving force; a center differential device (31) which transmits the driving force to a front axle (32) and a rear axle (33) and allows differential operation between the front axle and the rear axle; a differential limiting device (34) which varies a degree of differential limitation between the front axle and the rear axle; a braking device (40) which applies a braking force to each of the plurality of wheels of the vehicle (10), wherein a braking force distribution ratio of the rear wheels to the front wheels is set to be lower than an ideal distribution ratio based on a ground contact load ratio of the rear wheels to the front wheels when the differential limiting degree is set to a first limiting degree by the differential limiting device, wherein the control device comprises: Wheel speed sensors (82) each detecting a wheel speed which is a rotational speed of each of the plurality of wheels, a yaw rate sensor (84) for detecting a yaw rate of the vehicle, and an electronic control unit (50, 60 and 70) which is equipped: varying the braking force applied to each of the wheels in response to a braking action performed by the driver of the vehicle (10); estimate a speed of a body of the vehicle as an estimated vehicle body speed using a maximum wheel speed from a plurality of detected wheel speeds; calculate a brake slip ratio of each of the wheels based on the estimated vehicle body speed and the respective wheel speeds; to perform an anti-lock control for adjusting the braking force applied to the respective wheels based on the brake slip ratio of the respective wheels; and setting the differential limiting degree by the differential limiting device (34) to a second limiting degree which is greater than the first limiting degree when the braking operation is carried out, characterized in that the control unit (50, 60 and 70) is set up: to determine whether a specific condition has occurred in which the detected yaw rate is lower than a predetermined yaw rate threshold and an amount of magnitude decrease in the maximum wheel speed of a plurality of detected wheel speeds for a predetermined time is greater than a magnitude decrease threshold when the braking operation is performed; setting the differential limiting degree by the differential limiting device (34) to the second limiting degree when it is determined that the specific condition has not occurred while the braking operation is being performed; and setting the differential limiting degree by the differential limiting device (34) to a third limiting degree which is smaller than the second limiting degree when it is determined that the specific condition has occurred while the braking operation is being performed.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention

[0001] The present invention relates to a control device for a four-wheel drive vehicle equipped with various differential limiting devices for limiting a differential between a front axle and a rear axle. 2. Description of the state of the art

[0002] Generally, a center differential device of a four-wheel drive vehicle is equipped with a differential limit control device that controls the degree of differential limitation between the front and rear wheels. Previously, a differential limit control device (hereinafter referred to as a "conventional device") is known that is configured to reduce the degree of differential limitation between the front and rear wheels when braking control with anti-skid control is performed on a road with a low friction coefficient of the road surface (hereinafter also referred to as a "low-μ road") with a high degree of differential limitation between the front and rear wheels (a strong tendency of a four-wheel drive mode) (see, for example, Japanese Patent Application Laid-Open (Kokai) JP H08-53058 A).

[0003] In a state where the four-wheel drive mode tendency is strong, the front wheel speed and the rear wheel speed converge. For example, in a state where the four-wheel drive mode tendency is strong, when braking control is performed while the vehicle is traveling on a low-μ road where the wheels tend to slip, the wheel speed of either the front or rear wheels decreases, and the wheel speeds of all four wheels tend to decrease. In anti-lock control, a highest wheel speed among the wheel speeds of all wheels is generally selected and used to estimate the vehicle body speed.Consequently, as the wheel speeds of all four wheels decrease, the estimated vehicle body speed approaches the speeds of all four wheels and thus becomes lower than the actual vehicle body speed. Therefore, a brake slip ratio obtained as the ratio of a difference (deviation) between the vehicle body speed and the wheel speed to the vehicle body speed becomes lower than an actual brake slip ratio (obtained based on the actual vehicle body speed).

[0004] As a result, even if the actual brake slip ratio exceeds the appropriate brake slip ratio and therefore becomes a brake slip ratio at which anti-lock control should be executed, it is possible that anti-lock control (in other words, control that reduces the brake slip ratio so that the brake slip ratio is within an appropriate range) is not executed or is delayed. That is, when the vehicle is traveling on a low-μ road in a state where the tendency of the four-wheel drive mode is strong, an operation delay of the anti-lock control may occur. Therefore, the conventional device is configured to reduce the degree of differential limitation between the front wheels and the rear wheels by making the traveling mode closer to a two-wheel drive mode.

[0005] Meanwhile, the ground contact load (ground contact pressure) of the rear wheels is generally smaller than the ground contact load of the front wheels. That is, the friction circle of each rear wheel is smaller than the friction circle of each front wheel. Therefore, when the braking force of the rear wheels becomes disproportionately high, the lateral force of the rear wheels becomes insufficient, causing the vehicle to easily skid.

[0006] In view of the above, a distribution ratio of the braking force of the rear wheels to the front wheels is usually set lower than an ideal distribution ratio based on the ratio of the ground contact load of the rear wheels to the ground contact load of the front wheels. For example, when the vehicle is cornering on a low-μ road, if the braking control is executed in a state where the differential limiting degree between the front wheels and the rear wheels is low, the braking force of the front wheels becomes disproportionately large, so that the lateral force on the front wheels becomes disproportionately small. Consequently, a problem arises that the understeer tendency of the vehicle becomes increased / disproportionately strong.

[0007] As described above, the conventional device reduces the degree of differential limitation between the front and rear wheels while performing braking control on the low-μ road, thus preventing the anti-lock brake control from operating indefinitely. However, according to the conventional device, the understeer tendency may become excessive when the vehicle corners / turns.

[0008] DE 698 29 449 T2 discloses a control device for a four-wheel drive vehicle, comprising a drive unit, a center differential device, a differential limiting device, a braking device, wheel speed sensors, a yaw rate sensor, and a control unit. The control device weakens the differential limit (to zero, i.e., to a two-wheel drive state) only when it is determined that ABS is required. The control device determines that ABS is required when a braking force generated by a driver's braking action becomes greater than a reference braking force, i.e., a braking force slightly smaller than a braking force that causes the wheels to lock. This reference braking force can be varied depending on the friction coefficient of the road surface, the gradient of the road surface, or the lateral acceleration, which can be interpolated from the yaw rate and the vehicle speed.Regardless of whether the differential limitation is weakened depending on the comparison result between the braking force and the reference braking force (to zero, i.e. to a two-wheel drive state), the differential limitation is also weakened when the deceleration of the wheel speed is greater than a predetermined magnitude decrease threshold in order to better retrieve the wheel from the locked state during ABS.

[0009] From DE 60 2004 010 635 T2 it can be seen that a degree of differential limitation between the right drive wheel and the left drive wheel (but not between the front and rear wheels) is changed depending on the lateral acceleration.

[0010] For the state of the art, reference is also made to JP 2011 - 131 618 A and DE 195 29 491 A1. SUMMARY OF THE INVENTION

[0011] The present invention is designed to solve the above-mentioned problem. That is, one of the objects of the present invention is to provide a control device for a four-wheel drive vehicle that can prevent an increase in the understeer tendency during braking control when the vehicle is cornering / turning, and that can prevent the operation delay of the vehicle's anti-lock brake control during braking control while the vehicle is traveling straight ahead.

[0012] A control device for a four-wheel drive vehicle (hereinafter referred to as “the present inventive device”) is applied to a four-wheel drive vehicle having: a drive unit which generates a driving force; a center differential device that transmits drive power to a front axle and a rear axle and allows differential operation between the front axle and the rear axle; a differential limiting device that adjusts a degree of differential limitation between the front axle and the rear axle; and a braking device which applies a braking force to each of the plurality of wheels of the vehicle, wherein a braking force distribution ratio from rear wheels to front wheels is set to be lower than an ideal distribution ratio based on a ground contact load ratio of the rear wheels to the front wheels when the differential limiting degree is set to a first limiting degree by the differential limiting device (for example, Tcu = 0).

[0013] The present inventive device comprises wheel speed sensors, a yaw rate sensor, and a control unit. Each wheel speed sensor detects a wheel speed, which is a rotational speed of each of the plurality of wheels. The yaw rate sensor detects a yaw rate of the vehicle.

[0014] The control unit: varies the braking force applied to each of the wheels in response to a braking action performed by the driver of the vehicle; estimates a speed of a body of the vehicle as an estimated vehicle body speed using a maximum wheel speed from a plurality of detected wheel speeds; calculates a brake slip ratio of each of the wheels based on the estimated vehicle body speed and the respective wheel speeds; and performs anti-lock control to adjust the braking force applied to each wheel based on the brake slip ratio of each wheel.

[0015] Further, the control unit sets the differential limiting degree when the braking operation is performed by the differential limiting device to a second limiting degree (for example, Tcu = Tcumax) which is larger than the first limiting degree.

[0016] According to the above configuration, when braking, the differential limitation degree is set to the second limitation degree, which is greater than the first limitation degree. Therefore, a difference between the wheel speed of the front wheel and the wheel speed of the rear wheel becomes small, and therefore the brake slip ratio of the front wheels is adjusted to the brake slip ratio of the rear wheel. Therefore, the braking force distribution ratio between the front and rear wheels approaches the ideal distribution ratio based on the ground contact load ratio between the front and rear wheels. For example, the first limitation degree is realized by setting the clutch torque to 0, so that there is no limitation. The second limitation degree is realized by setting the clutch torque to the maximum value of the clutch torque.In this example, the difference between the front wheel speed and the rear wheel speed becomes zero, and the front wheel brake slip ratio becomes equal to the rear wheel brake slip ratio. Therefore, the front wheel brake force decreases and the rear wheel brake force increases compared to a case where the differential limiting degree is set to the first limiting degree. Consequently, the understeer tendency during braking control when the vehicle is cornering / turning becomes weak, as the lateral force of the front wheels increases and the lateral force of the rear wheels decreases.

[0017] Meanwhile, while the vehicle is traveling straight or substantially straight (in other words, when the vehicle's yaw rate is lower than a predetermined yaw rate threshold), a left wheel speed and a right wheel speed approach each other. For example, while the vehicle is traveling straight, the left wheel speed and the right wheel speed are substantially the same. In this case, if the differential limitation degree between the front wheels and the rear wheels is set to the second limitation degree, the front wheel speed and the rear wheel speed approach each other, and thus the wheel speeds of all wheels approach each other. Therefore, as the front wheel speeds (two of the rear wheels) decrease, the wheel speeds of the rear wheels (two of the front wheels) decrease.As described above, when the wheel speeds of all four wheels decrease simultaneously, the vehicle body speed becomes close to the wheel speeds of all four wheels and lower than the actual vehicle body speed. Therefore, the calculated brake slip ratio becomes smaller than the actual brake slip ratio. Therefore, if the differential limitation degree between the front and rear wheels is set to the second limitation degree, the start timing of the anti-lock brake control can be delayed.

[0018] To solve the above-mentioned problem, the control unit is set up in view of the above: to determine whether a specific condition has occurred in which the detected yaw rate is lower than a predetermined yaw rate threshold and an amount of magnitude decrease in the maximum wheel speed from a plurality of detected wheel speeds for a predetermined time is greater than a magnitude decrease threshold when the braking operation is performed; setting the differential limiting degree by the differential limiting device to the second limiting degree when it is determined that the specific condition has not occurred while the braking operation is being performed; and setting the differential limiting degree by the differential limiting device to a third limiting degree which is smaller than the second limiting degree when it is determined that the specific condition has occurred while the braking operation is being performed.

[0019] According to the device of the present invention, the maximum wheel speed of the plurality of detected wheel speeds is selected as the estimated vehicle body speed. However, as described above, if the wheel speed of the rear wheels decreases along with the decreasing wheel speed of the front wheels during the execution of the braking operation, the specific state may occur in which a magnitude of the estimated vehicle body speed is larger than a magnitude decrease threshold for the predetermined time. In this case, the device of the present invention sets a differential limitation degree to the third limitation degree, which is lower than the second limitation degree. Accordingly, the wheel speed of the rear wheels is adjusted according to the braking force of the rear wheels without being affected by the wheel speed of the front wheels.Therefore, the wheel speed of the rear wheels increases, approaching the speed at which the rear wheels were originally rotating and the actual vehicle body speed. Accordingly, the estimated vehicle body speed approximates the actual vehicle body speed, an operating condition for the anti-lock brake control is determined earlier, and a functional delay of the anti-lock brake control can be prevented. The third limitation level may be the same as the first limitation level, and the third limitation level may, for example, be a level realized when the clutch torque can be a value of zero.

[0020] Therefore, according to the above configuration, it is possible to prevent the understeer tendency of the vehicle from being increased in the braking control during cornering / turning of the vehicle, and it is possible to prevent the operation delay of the anti-lock control of the vehicle in the braking control when the vehicle is traveling straight.

[0021] When cornering / turning and decelerating the vehicle, the inside wheels are more likely to slip than the outside wheels. Therefore, the wheel speed of the turning inside wheel tends to decrease sooner than the wheel speed of the turning outside wheel when the vehicle is cornering / turning and decelerating. Accordingly, it is unlikely that the wheel speeds of all four wheels will decrease simultaneously, even if the differential limitation degree between the front and rear axles is set to a degree close to the maximum limitation degree. Therefore, it is prudent to lower / weaken the differential limitation degree between the front and rear axles when the vehicle's yaw rate is lower than the specified yaw rate threshold.

[0022] In one of the embodiments of the control device for a four-wheel drive vehicle according to the invention, the control unit is configured to set the differential limitation degree to the third limitation degree by removing / releasing the differential limitation given by the differential limitation device.

[0023] According to this embodiment, when it is determined that the specific condition has arrived where the detected yaw rate of the vehicle is lower than the predetermined yaw rate threshold and the magnitude of a decrease in the maximum wheel speed among a plurality of detected wheel speeds for the predetermined time is greater than the magnitude decrease threshold, the differential limiting degree between the front axle and the rear axle is released. That is, the differential limiting degree is lowered to a "zero" degree. In other words, the clutch torque by the differential limiting device is set to "zero." As a result, the estimated vehicle body speed, which was about to decrease, can be converged sooner to the actual vehicle body speed.Therefore, the operational delay of the vehicle's anti-lock control during braking control when the vehicle is traveling straight ahead can be prevented earlier.

[0024] In one of the embodiments of the control device for a four-wheel drive vehicle according to the invention, the control unit is arranged: to select the maximum wheel speed from a plurality of wheel speeds whenever a given calculation period expires; and if a lower limit of a protection value is greater than or equal to the selected maximum wheel speed, setting the estimated vehicle body speed at a current time to the lower limit of the protection value, wherein the lower limit of the protection value is calculated by subtracting a constant value from the estimated vehicle body speed estimated at a time of a predetermined calculation period before the current time; and to determine that the specific condition has occurred when a case in which the lower limit of the protection value is set as the estimated vehicle body speed continues to exist for a predetermined number of times.

[0025] In the above configuration, the lower limit of the protection value is a limit value in the equation of the estimated vehicle body speed and is determined, for example, by the estimated vehicle body speed previously estimated during the calculation period, the predetermined lower limit of the protection slope, and the calculation period. The above-described device of one of the embodiments according to the present invention determines that the specific state has been established when the case where the lower limit of a protection value is set as the estimated vehicle body speed continues a predetermined number of times. Further, based on the determination result, the device sets the differential limitation degree between the front axle and the rear axle to the third limitation degree, which is smaller than the second limitation degree.Therefore, the anti-lock brake control can be started at a more appropriate time without delay.

[0026] Further, in the control device of a four-wheel drive vehicle according to an embodiment of the present invention, the control unit is configured to set the yaw rate threshold to a value corresponding to a yaw rate occurring when the vehicle is judged to be traveling straight.

[0027] As described above, when the yaw rate exceeds a certain level, it is unlikely that the wheel speeds of all four wheels will decrease simultaneously because a difference will arise between the wheel speed of the rotating outer wheel and the wheel speed of the rotating inner wheel. Therefore, the yaw rate threshold for setting the differential limiting degree to the third limiting degree can be set to a value corresponding to the yaw rate (a value close to zero) that occurs when the vehicle is judged to be traveling straight.

[0028] In the above description, to facilitate understanding of the present invention, designations and / or reference numerals used in an embodiment of the present invention described below have been enclosed in parentheses and assigned to elements of the invention corresponding to the embodiment. However, the respective elements of the present invention are not limited to the embodiment defined by the designations and / or reference numerals. Other objects, other features, and attendant advantages of the present invention will become clear from the description of the embodiment of the present invention given with reference to the following drawings. Short description of the drawings Fig. 1 is a schematic diagram showing a control device for a four-wheel drive vehicle according to an embodiment of the present invention. Fig. 2 is a diagram for describing a relationship between a braking force of a front wheel and a braking force of a rear wheel of a Fig. 1 shown braking device. Fig. 3 is a diagram for describing a relationship between a brake slip ratio and a braking force of the Fig. 1 shown braking device. Fig. 4 is a diagram for describing a braking force and a lateral force when a differential limiter between a front axle and a rear axle is released when the Fig. 1 vehicle turns. Fig. 5 is a diagram describing the braking force and lateral force when the differential between the front axle and the rear axle is limited when the Fig. 1 vehicle turns. Fig. 6 is a diagram for describing a method for calculating an estimated vehicle body speed which is used by the Fig. 1 shown braking device is used. Fig. 7 is a graph showing a time change of each wheel speed when the differential limit between the front axle and the rear axle is released after a lower limit of a protection consecutive selection time has elapsed while the vehicle is traveling straight on a low-µ road. Fig. 8 is a graph showing a temporal change of each wheel speed when the differential limit between the front axle and the rear axle is not released after the lower limit of a protection consecutive selection time has elapsed while the vehicle is traveling straight on a low-µ road. Fig. 9 is a flowchart illustrating a “clutch torque reduction control program” executed by a CPU of a brake ECU incorporated in Fig. 1 is executed. DESCRIPTION OF THE EMBODIMENTS (Configuration)

[0029] A control device for a four-wheel drive vehicle according to an embodiment of the present invention (hereinafter referred to as “the present control device”) is applied to a vehicle as shown in Fig. 1 illustrated four-wheel drive vehicle 10 (hereinafter referred to simply as “vehicle”).

[0030] The vehicle 10 includes a drive device 20, a drive transmission mechanism 30, a brake device 40, a drive ECU 50, a four-wheel drive (4WD) ECU 60, a brake ECU 70, and the like. These ECUs correspond to a control device of the present invention. Note that two or more of these ECUs may be integrated into a single ECU.

[0031] ECU is an abbreviation for "Electronic Control Unit" and is an electronic control circuit that includes a microcomputer as its main component, which includes a CPU, a ROM, a RAM, a backup RAM (or non-volatile memory), an interface, and the like. The CPU is configured to execute instructions (routines) stored in the memory (ROM) to implement various functions, which will be described later.

[0032] The drive device 20 generates a drive force for driving the wheels of the vehicle 10 (the left front wheel WFL, the right front wheel WFR, the left rear wheel WRL, and the right rear wheel WRR) via the drive force transmission mechanism 30. The drive device 20 may be any type of drive device for a vehicle known in the art, such as a combination of an internal combustion engine and a transmission of a conventional vehicle, a combination of an electric motor and a transmission, a hybrid system that is a combination of an internal combustion engine, an electric motor, and a transmission, or the like.

[0033] The drive force transmission mechanism 30 includes a center differential device 31, a front axle 32, a rear axle 33, a differential limiting device 34, a front wheel differential gear 35, a left front wheel axle 36L, a right front wheel axle 36R, a rear wheel differential gear 37, a left rear wheel axle 38L, a right rear wheel axle 38R, and the like.

[0034] The center differential device 31 is configured to transmit the drive force from the drive device 20 to the front axle 32 and the rear axle (propeller shaft) (33) and to allow a different rotational speed between the front axle 32 and the rear axle 33. In this embodiment, the center differential device 31 includes an electrically controlled differential limiting device 34.

[0035] The differential limiting device 34 has a function of controlling a mutual locking force between the front axle 32 and the rear axle 33 by means of a center clutch so as to vary a degree of differential limitation between the front axle 32 and the rear axle 33. The mutual locking force between the front axle 32 and the rear axle 33, that is, a clutch torque Tcu of the center clutch, is controlled by the four-wheel drive ECU 60.

[0036] The drive power of the front axle 32 is transmitted to the left front wheel axle 36L and the right front wheel axle 36R via the front wheel differential gear 35, whereby the left front wheel WFL and the right front wheel WFR are rotationally driven. Similarly, the drive power of the rear axle 33 is transmitted to the left rear wheel axle 38L and the right rear wheel axle 38R via the rear wheel differential gear 37, whereby the left rear wheel WRL and the right rear wheel WRR are rotationally driven.

[0037] The brake device 40 includes a brake pedal 41, a master cylinder 42, a hydraulic circuit 43, a wheel cylinder 44 (44FL, 44FR, 44RL and 44RR) and the like.

[0038] Brake pressures on the wheel cylinders 44FL, 44FR, 44RL, and 44RR are controlled via the hydraulic circuit 43, thereby controlling respective braking forces of the left front wheel WFL, the right front wheel WFR, the left rear wheel WRL, and the right rear wheel WRR. The hydraulic circuit 43 includes an accumulator, an oil pump, various valve devices, and the like (not shown). The hydraulic circuit 43 functions as a brake actuator. As described later, the brake pressure of each wheel cylinder 44 is normally controlled by the brake ECU 70 based on the pressure (hereinafter also referred to as "master cylinder pressure") Pm of the master cylinder 42, which is operated according to the driver's depression of the brake pedal 41, or the brake pressure of each wheel cylinder 44 is individually controlled as needed.

[0039] As described later, the drive ECU 50 is connected to the four-wheel drive ECU 60 and the brake ECU 70 so that they are capable of exchanging information via CAN (Controller Area Network). The drive ECU 50 is electrically connected to a pedal opening sensor 81 and the like. The drive ECU 50 is configured to receive output signals from these sensors. The pedal opening sensor 81 is configured to generate an output signal corresponding to an operation amount AP of the accelerator pedal 81a operated by the driver.

[0040] The four-wheel drive ECU 60 is electrically connected to the wheel speed sensors 82 (82FL, 82FR, 82RL, and 82RR) and the like. The four-wheel drive ECU 60 is configured to receive output signals from these sensors. The wheel speed sensors 82FL, 82FR, 82RL, and 82RR are configured to generate output signals representing the wheel speeds Vwfl, Vwfr, Vwrl, and Vwrr of the left front wheel WFL, the right front wheel WFR, the left rear wheel WRL, and the right rear wheel WRR, respectively.

[0041] The brake ECU 70 is electrically connected to a steering angle sensor 83, a yaw rate sensor 84, a master cylinder pressure sensor 85, and the like. The brake ECU 70 is configured to receive output signals from these sensors. The steering angle sensor 83 is configured to generate an output signal representing a steering angle St of the steering wheel 83a operated by the driver. The yaw rate sensor 84 is configured to generate an output signal representing a yaw rate Yr of the vehicle 10. The master cylinder pressure sensor 85 is configured to generate an output signal representing the master cylinder pressure Pm. The steering angle sensor 83 and the yaw rate sensor 84 are configured to detect the steering angle St and the yaw rate Yr, respectively, in such a manner that both the steering angle Sr and the yaw rate Yr are positive when the vehicle steers to the left.

[0042] The brake ECU 70 calculates a target braking force Fbflt, Fbfrt, Fbrlt, and Fbrrt of the left front wheel WFL, the right front wheel WFR, the left rear wheel WRL, and the right rear wheel WRR based on the master cylinder pressure Pm. The brake ECU 70 controls the braking forces of the wheel cylinders 44FL, 44FR, 44RL, and 44RR so that the respective braking forces equalize the corresponding target braking force. A braking force distribution ratio of the rear wheel to the front wheel is shown by a solid line L1 in Fig. 2 when the differential limitation degree between the front axle 32 and the rear axle 33 is set small, in other words, when the differential limitation degree is set to a first limitation degree. As can be seen from Fig. 2, the braking force distribution ratio is set lower than the ratio of the ideal distribution line (dashed line L2) based on the ground contact load ratio of the left front wheel WFL, the right front wheel WFR, the left rear wheel WRL, and the right rear wheel WRR, in view of manufacturing tolerances of the parts constituting the braking device 40.

[0043] That is, the target braking force Fbflt of the left front wheel WFL and the target braking force Fbfrt of the right front wheel WFR are controlled to be higher than the braking force of the front wheels determined based on the ideal distribution ratio of the braking forces of the front and rear wheels, and to be higher than the target braking force Fbrlt of the left rear wheel WRL and the target braking force Fbrrt of the right rear wheel WRR.

[0044] After Fig. 3, the braking force of the wheel increases with increasing brake slip ratio SL when the brake slip ratio SL is less than or equal to a predetermined brake slip ratio (hereinafter also referred to as "ideal slip ratio") SLi, which is determined by characteristics of a tire and the like, and the braking force of the wheel decreases with increasing brake slip ratio SL when the brake slip ratio SL is higher than the predetermined brake slip ratio SLi. Therefore, the brake ECU 70 calculates the brake slip ratio SL of each of the wheels based on the wheel speeds Vwfl, Vwfr, Vwrl, and Vwrr to execute the anti-lock control (hereinafter also referred to as "ABS control") known in the art.

[0045] The four-wheel drive ECU 60 controls the clutch torque Tcu of the center clutch of the differential limiting device 34. The differential limiting device 34 allows relative rotation between the front axle 32 and the rear axle 33 when the clutch torque Tcu is zero, and prohibits relative rotation between the front axle 32 and the rear axle 33 when the clutch torque Tcu is the maximum value Tcumax. Furthermore, the differential limiting device 34 gradually increases the degree of differential limitation between the front axle 32 and the rear axle 33 with increasing clutch torque Tcu when the clutch torque Tcu is a value between zero and the maximum value Tcumax.

[0046] Therefore, the clutch torque Tcu is an index value indicating the differential limiting degree of the differential limiting device 34, and a target clutch torque Tcut, which will be described later, is an index value indicating a target differential limiting degree of the differential limiting device 34. (Operation)

[0047] Next, the operation of the control device will be described. First, a behavior of the vehicle 10 while braking and cornering / turning will be described. As shown in Fig. 4, when the differential limiting degree is set to the first limiting degree (for example, the clutch torque Tcu is controlled to be "zero" or a value close to "zero"), while the vehicle 10 is decelerating and cornering / turning, the braking forces Fbf of the left front wheel WFL and the right front wheel WFR become high, and the braking forces Fbr of the left rear wheel WRL and the right rear wheel WRR become low because the braking force distribution ratio of the rear wheels is set low. Therefore, a lateral force Fyf of the front wheels becomes small, and a lateral force Fyr of the rear wheels becomes large. Therefore, the behavior of the vehicle 10 tends to be an understeering behavior.

[0048] On the other hand, as in Fig. As shown in Figure 5, when the differential limiting degree is set to the second limiting degree that is greater than the first limiting degree (for example, the clutch torque Tcu is controlled to be the maximum value Tcumax or a value close to the maximum value Tcumax) while the vehicle 10 is decelerated and cornering / turning, the wheel speeds Vwfl, Vwfr, Vwrl, and Vwrr of the respective wheels substantially coincide with each other (equalize). Therefore, the braking forces of the front wheels and the rear wheels become braking forces along the distribution line indicated by the dashed line L3 near the ideal distribution line indicated by the dashed line L2 in Fig. 2 is displayed because the brake slip ratio of the front wheels and the rear wheels are essentially the same.

[0049] In this case, the friction circle of the front wheel becomes relatively small and the friction circle of the rear wheel becomes relatively large. Therefore, if, for example, it is assumed that the braking force Fbf of the front wheel and the braking force Fbr of the rear wheel are set to zero or close to zero during the clutch torque Tcu control, the values ​​from the point P in Fig. 2, for example, the braking force Fbf of the front wheels and the braking force Fbr of the rear wheels change to values ​​determined by the point Q in Fig. 2. In this way, the braking force Fbf of the left front wheel WFL and the right front wheel WFR decreases, and the braking force Fbr of the left rear wheel WRL and the right rear wheel WRR increases. Consequently, the lateral force of the front wheel Fyf increases, and the lateral force of the rear wheel Fyr decreases, thus reducing the degree of understeer behavior of the vehicle 10. <antiblockiersteuerung>

[0050] Anti-lock brake control is a brake control system that assists a main brake control system to control the braking force of the wheels based on the driver's braking action. In anti-lock brake control (ABS), the brake slip ratio SL of each wheel is calculated according to formula (1) using the wheel speed Vw of each wheel and the vehicle body speed Vb. The brake slip ratio SL is hereinafter referred to simply as the "slip ratio SL." SL=(Vb−Vw) / Vb

[0051] Generally, the vehicle body speed Vb cannot be detected. Therefore, an estimated vehicle body speed Vx, which is estimated based on the wheel speed Vw, is used to calculate the slip ratio SL instead of the vehicle body speed Vb. In the anti-lock control, the hydraulic pressure of the brake device 40 is controlled so that the slip ratio SL approaches the ideal slip ratio SLi (see Fig. 3) when the slip ratio SL of each wheel becomes greater than or equal to the specified slip ratio threshold SLth.

[0052] The estimated vehicle body speed Vx(n) (n is an integer) used for anti-lock control is calculated by the following equation. Vx(n)=MID(Vx(n−1)+αup⋅ΔT,Vwsel,Vx(n−1)−αdw⋅ΔT)

[0053] Vx(n-1) represents a previously calculated value of the estimated vehicle body speed Vx(n). Vwsel represents a selected wheel speed. αup represents an upper limit of a guard slope (positive value) of the estimated vehicle body speed. αdw represents a lower limit of a guard slope (positive value) of the estimated vehicle body speed. ΔT represents a calculation period. MID represents a function for selecting a median value. The highest wheel speed among the wheel speeds of the left front wheel, the right front wheel, the left rear wheel, and the right rear wheel of the vehicle is selected as the selected wheel speed Vwsel for each calculation time.

[0054] According to equation (1), the estimated vehicle body speed Vx(n) becomes an intermediate value (median value) among the following values ​​(A) to (C): (A) a value obtained by adding the product of the upper limit of the protection slope αup and the calculation period ΔT to the estimated vehicle body speed Vx(n-1) calculated at the previous calculation time Tn-1, that is, the upper limit of the protection value of the estimated vehicle body speed Vx, (B) the selected wheel speed Vwsel, (C) a value obtained by subtracting a product of the lower limit of the protection slope αdw and the calculation period ΔT from the estimated vehicle body speed Vx(n-1), that is, the lower limit of the protection value of the estimated vehicle body speed Vx.

[0055] The following describes a method for calculating the estimated vehicle body speed Vx with reference to Fig. 6. The selected wheel speed Vwsel is shown by a solid line in Fig. 6. The horizontal axis represents time, and times T0 to T5 represent the calculation times of the estimated vehicle body speed Vx(n), respectively. An interval between the adjacent calculation times is the calculation period ΔT.

[0056] It is assumed that the selected wheel speed Vwsel at calculation time T0 is selected as the median value. That is, it is assumed that the estimated vehicle body speed Vx (Vx0) is equal to the selected wheel speed Vwsel at calculation time T0. In this case, at the next calculation time T1, the median value is selected from the value Vx0 + αup·ΔT, the selected wheel speed Vwsel at calculation time T1, and the value Vx0 - αdw·ΔT at time T1. The value (Vx0 + αup·ΔT) is a value on a line extending from the estimated vehicle body speed Vx0 at calculation time T0 with / at the upper limit of the guard slope αup at calculation time T1. The value (Vx0 - αdw·ΔT) is a value on a line extending from the estimated vehicle body speed Vx0 at the calculation time T0 with / at the lower limit of the protective slope αdw at the calculation time T1.In the example shown here, the median value at the calculation time T1 (i.e., the estimated vehicle body speed Vx1) is the selected wheel speed Vwsel.

[0057] At calculation time T2, the median value is selected from the value Vx1 + αup·ΔT, the selected wheel speed Vwsel at calculation time T2, and the value Vx1 - αdw·ΔT. The value (Vx1 + αup·ΔT) is a value on a line extending from the estimated vehicle body speed Vx1 at calculation time T1 with / at the upper limit of the protection slope αup to calculation time T2. The value (Vx1 - αdw·ΔT) is a value on a line extending from the estimated vehicle body speed Vx1 at calculation time T1 with / at the lower limit of the protection slope αdw to calculation time T2. In the present example shown, the median value at calculation time T2 (i.e., the estimated vehicle body speed Vx2) is the lower limit of the protection value Vx1 - αdw·ΔT.

[0058] Furthermore, at calculation time T3, the median value is selected from the value Vx2 + αup·ΔT, the selected wheel speed Vwsel at calculation time T2, and the value Vx2 - αdw·ΔT. The value (Vx2 + αup·ΔT) is a value on a line extending from the lower limit of the protection value Vx1 - αdw·ΔT (= Vx2) at calculation time T2 with / at the upper limit of the protection gradient αup at calculation time T3. The value (Vx2 - αdw·ΔT) is a value on a line extending from the lower limit of a protection value Vx1 - αdw·ΔT (= Vx2) at calculation time T2 with / at the lower limit of the protection gradient αdw at calculation time T3. In the present example shown, the median value at calculation time T3 (i.e., the estimated vehicle body speed Vx3) is the upper limit of the protection value Vx2 + αup·ΔT.

[0059] Similarly, the median value at calculation time T4 (i.e., the estimated vehicle body speed Vx4) is the selected wheel speed Vwsel. The estimated vehicle body speed Vx(n) calculated in this way is indicated by a thick dashed line. Here, the frequency with which the upper limit of the protection value is consecutively selected is defined as the upper limit of a protection consecutive selection number Nup, and the frequency with which the lower limit of the protection value is consecutively selected is defined as the lower limit of a protection consecutive selection number Ndw. In the example shown here, neither the upper limit of the protection value nor the lower limit of the protection value is consecutively selected. Therefore, the upper limit of the protection consecutive selection number Nup is one, and the lower limit of the protection consecutive selection number Ndw is one.

[0060] The estimated vehicle body speed (or estimated wheel speed Vwsel) never drops sharply when at least one of the four wheels' wheel speeds is close to the actual vehicle body speed Vbr. In this case, the lower limit of the protection value is unlikely to be consecutively selected. In other words, the lower limit of the protection consecutive selection number Ndw is unlikely to be two or more. That is, it is inferred that the wheel speed of the left front wheel, the wheel speed of the right front wheel, the wheel speed of the left rear wheel, and the wheel speed of the right rear wheel are all lower than the actual vehicle body speed Vbr when the lower limit of the protection consecutive selection number Ndw is two or more.

[0061] That is, the control device is configured to select the maximum wheel speed Vwsel from a plurality of wheel speeds Vw each time a predetermined calculation period ΔT elapses, and to set the estimated vehicle body speed Vx(n) at the current time to the lower limit of the protection value Vx(n-1) - αdw ΔT when the lower limit of the protection value Vx(n-1) - αdw ΔT is greater than or equal to the selected maximum wheel speed Vwsel. The lower limit of the protection value Vx(n-1) - αdw ΔT is calculated by subtracting the constant value αdw ΔT from the estimated vehicle body speed Vx(n-1), which is the estimated vehicle body speed Vx(n) at the time of the predetermined calculation period ΔT before the current time.

[0062] Next, operations of brake control by the present control apparatus will be described based on an exemplary situation in which a decreasing slip ratio of each wheel occurs due to slip of the front wheels when the brake control is performed in a case where the vehicle 10 is traveling straight and the differential limiting degree between the front axle 32 and the rear axle 33 is set to the second limiting degree (Tcu = Tcumax).

[0063] Fig. Figure 7 shows the estimated vehicle body speed Vx, the vehicle body speed Vb, and the wheel speed Vw of each wheel from a braking start in the exemplary situation. In this example, it is assumed that braking starts at time t0, and therefore the actual vehicle body speed Vbr (shown by the dashed line in Fig. 7) decreases with the passage of time, and each of the wheel speeds substantially coincides with the actual vehicle body speed Vbr during the period from time t0 to time t1. Assuming that the front-left wheel speed Vwfl and the front-right wheel speed Vwfr decrease due to the slippage of the front wheels in a state where the differential limitation degree between the front axle 32 and the rear axle 33 is high, the rear-left wheel speed Vwrl and the rear-right wheel speed Vwrr also begin to decrease along with the decrease in the front-left wheel speed Vwfl and the front-right wheel speed Vwfr. A time point at which the slippage of each of the wheels occurs is time t1.In this example, the front left wheel speed Vwfl, the front right wheel speed Vwfr, the rear left wheel speed Vwrl, and the rear right wheel speed Vwrr satisfy a relationship of Vwfl < Vwfr < Vwrl < Vwrr, while the wheel speeds of the four wheels all decrease (over a period of time t1 to t2). Therefore, the rear right wheel speed Vwrr, which is the highest of the four wheel speeds, is selected as the selected wheel speed Vwsel when calculating the estimated vehicle body speed Vx.

[0064] At time t1, the drop (decreasing value) of the right rear wheel speed Vwrr is smaller than the lower limit of the protection slope αdw, and therefore, the estimated vehicle body speed Vx is calculated as the lower limit of a protection value Vx(n-1) - αdw·ΔT based on Equation (2). The estimated vehicle body speed Vx is calculated so that it is the lower limit of the protection value Vx(n-1) - αdw·ΔT every time a predetermined calculation time arrives from time t1.

[0065] The lower limit of the protection consecutive selection time Tαdws reaches a predetermined value T1th at time t2. Therefore, at time t2, it is determined that a temporal decrease rate of the estimated vehicle body speed Vx is further below a predetermined decrease rate. In other words, when the lower limit of the protection value Vx(n-1) - αdw·ΔT is consecutively selected as the estimated vehicle body speed Vx a predetermined number of times, it is determined that a "specific state has occurred in which the magnitude of the decrease in the maximum wheel speed of a plurality of the detected wheel speeds (Vwfl, Vwfr, Vwrl, and Vwrr) in a predetermined time is greater than the magnitude decrease threshold," and therefore, the differential limitation between the front axle 32 and the rear axle 33 is removed / released / eliminated.That is, the lower limit of the protective consecutive selection time Tαdws is a product Ndw·ΔT of the lower limit of the protective consecutive selection number Ndw and the calculation period ΔT.

[0066] At this moment, the wheel speeds Vwr of the rear wheels (the left rear wheel speed Vwrl and the right rear wheel speed Vwrr) begin to increase because the braking control is performed again with the original braking force of the rear wheels. Then, the estimated vehicle body speed Vx begins to increase along with the right rear wheel speed Vwrr from time t3, when the lower limit of the consecutively selected protection value Vx(n-1) - αdw·ΔT becomes equal to or less than the right rear wheel speed Vwrr, which is the highest of the four wheel speeds. From this time, the rear left wheel speed Vwrl also begins to increase along with the estimated vehicle body speed Vx and the rear right wheel speed Vwrr.Therefore, from this point onwards, the estimated vehicle body speed Vx approaches the actual vehicle body speed Vbr.

[0067] Subsequently, the slip ratio SL of each of the front wheels (WFL and WFR), which is calculated using equation (1), increases because the deviation between the estimated vehicle body speed Vx and each of the front-left wheel speeds Vwfl and the front-right wheel speeds Vwfr increases with increasing estimated vehicle body speed Vx. A process for reducing the slip ratio SL compared to the ideal slip ratio SLi for the left front wheel WFL (hereinafter referred to as "slip ratio reducing process") is started to be executed when the slip ratio SL of the left front wheel WFL is greater than or equal to the predetermined slip ratio threshold SLth at time t4. Therefore, the wheel speed Vwfl of the left front wheel WFL starts to increase from time t4.The slip ratio reducing operation is also executed for the right front wheel WFR because the slip ratio SL of the right front wheel WFR becomes equal to or greater than the predetermined slip ratio threshold SLth, so that the front right wheel speed Vwfr also starts to increase.

[0068] Thereafter, the wheel speed Vwrr of the right rear wheel WRR and the wheel speed Vwrl of the left rear wheel WRL are substantially consistent with the actual vehicle body speed Vbr at approximately time t5. The wheel speed Vwfr of the right front wheel WFR and the wheel speed Vwfl of the left front wheel WFL are substantially consistent with the actual vehicle body speed Vbr at approximately time t6.

[0069] The operation of the control device has been described above. Next, an operation of the conventional device will be described. The conventional device does not remove / release / eliminate the differential limitation degree even when the temporal rate of decrease of the estimated vehicle body speed Vx is below the predetermined rate of decrease.

[0070] As in Fig. As shown in Figure 8, the estimated vehicle body speed Vx continues to decrease at the lower limit of the guard slope αdw from time t1. In this case, a deviation between the estimated vehicle body speed Vx and each of the wheel speeds Vwfl, Vwfr, Vwrl, and Vwrr gradually / moderately increases. The slip ratio SL of each of the wheels is determined according to a relationship between the wheel speed Vw of each of the wheels and the estimated vehicle body speed Vx. Therefore, it takes a considerable time until the slip ratio SL of each of the wheels becomes equal to or greater than the predetermined slip ratio threshold SLth, while the deviation of the estimated vehicle body speed Vx and each of the wheel speeds Vwfl, Vwfr, Vwrl, and Vwrr gradually / moderately increase. In the Fig. In the example shown in Fig. 8, the slip ratio SL of the left front wheel WFL becomes greater than or equal to the predetermined slip ratio threshold SLth at time t7 (which is later than the time t2 shown in Fig. 7), and therefore, the slip ratio reduction process is executed. Consequently, the wheel speed Vwfl of the left front wheel WFL begins to increase from time t7. Subsequently, the slip ratio reduction process is started to be executed for each of the other wheels when the slip ratio SL of each of the other wheels (the right front wheel WFR, the left rear wheel WRL, and the right rear wheel WRR) becomes equal to or greater than the predetermined slip ratio threshold SLth. Consequently, the wheel speed of each of the wheels begins to increase.

[0071] Thereafter, the wheel speed Vwrr of the right rear wheel WRR, the wheel speed Vwrl of the left rear wheel WRL, the wheel speed Vwfr of the right front wheel WFR and the wheel speed Vwfl of the left front wheel WFL become substantially equal to the actual vehicle body speed Vbr at time t8.

[0072] In this way, the present control device executes the slip ratio reduction operation earlier than the conventional device, which does not remove / release / eliminate the differential limitation (or does not set the differential limitation degree to zero) when the temporal rate of decrease of the estimated vehicle body speed Vx is further below the predetermined rate. As a result, the operation delay of the anti-lock control is shortened (the anti-lock control can be performed without undue operation delay). (Specific operation)<Kupplungsmomentreduzierungssteuerung während dem Bremsen>

[0073] The following describes the actual operation of the control device with reference to Fig. 9. In the present example, the description is given assuming that an initial value of the clutch torque Tcu is set to the maximum value Tcumax (the second limitation level). The CPU of the brake ECU 70 is configured to execute a clutch torque reduction control program after each lapse of a constant time, which is represented by a flowchart in Fig. 9 is shown.

[0074] The CPU starts the process from step 900 at a predetermined timing to proceed to step 905, at which the CPU obtains the estimated vehicle body speed Vx based on the equation (2) and proceeds to step 910 to obtain the lower limit of the protection consecutive selection time Tαdws.

[0075] Subsequently, the CPU proceeds to step 915 to determine whether the braking operation is being performed by the driver. In other words, the CPU determines whether the master cylinder pressure Pm is equal to or greater than the master cylinder pressure threshold Pmth (positive value). If the master cylinder pressure Pm is equal to or greater than the master cylinder pressure threshold Pmth, the CPU makes a "Yes" determination in step 915 to proceed to step 920, where the CPU determines whether the braking force control by the ABS controller is not being executed on any of the wheels. Specifically, the CPU determines whether the process (slip ratio reducing process) for reducing the slip ratio to a value close to the ideal slip ratio SLi is not being executed, the ratio reducing process being executed when the slip ratio SL of any of the wheels exceeds the predetermined slip ratio threshold SLth.

[0076] If the braking force control of the ABS control is not being executed for either wheel, the CPU makes a "YES" determination in step 920 to proceed to step 925, at which the CPU determines whether the lower limit of the protective consecutive selection time Tαdws is "zero." If the lower limit of the protective consecutive selection time Tαdws is "zero," the CPU makes a "YES" determination in step 925 to proceed to step 930, at which the CPU stores the yaw rate Yr detected by the yaw rate sensor 84 as a "lower limit of a protective selection start yaw rate Yrm" in the RAM. Then, the CPU proceeds to step 935. On the other hand, if the lower limit of the protective consecutive selection time Tαdws is not "zero," the CPU makes a "NO" determination in step 925 to directly proceed to step 935.

[0077] Accordingly, the CPU determines whether the lower limit of the protection consecutive selection time Tαdws is equal to or greater than the predetermined time T1th in step 935. If the lower limit of the protection consecutive selection time Tαdws is equal to or greater than the predetermined time T1th, the CPU makes a "Yes" determination in step 935 to proceed to step 940, at which the CPU determines whether the absolute value |Yrm| of the lower limit of a protection selection start yaw rate Yrm is lower than a predetermined yaw rate threshold Yrth. The predetermined yaw rate threshold Yrth is set to a value close to "zero." Determining whether the absolute value |Yrm| of the lower limit of the protection selection start yaw rate Yrm is lower than the predetermined yaw rate threshold Yrth is equivalent to determining whether the vehicle is traveling straight.In other words, the predetermined yaw rate threshold Yrth is a value corresponding to the yaw rate generated when the vehicle 10 can be determined to be traveling straight (or when the vehicle 10 is found to be traveling straight).

[0078] In this way, the CPU makes the determination in step 940 based on the lower limit of the protection selection start yaw rate Yrm instead of the yaw rate at the current time (the time at which the lower limit of the protection consecutive selection time Tαdws becomes greater than or equal to the predetermined time T1th). The reasons are as follows: For example, when the vehicle 10 is skidding and changing position, the absolute value of the yaw rate may become high while the wheel speeds Vw of all four wheels are decreasing even when the vehicle 10 is traveling straight. In this case, should the CPU make the determination in step 940 based on the yaw rate of the current moment, the CPU makes a "no" determination in step 940. That is, it may be determined that the vehicle 10 is turning even when the vehicle 10 is traveling straight, so starting the ABS control may be delayed when the wheel speeds Vw of all four wheels are decreasing.

[0079] When the absolute value |Yrm| of the lower limit of the protection selection start yaw rate Yrm is lower than the yaw rate threshold Yrth (that is, when the vehicle is traveling straight), the CPU makes a "Yes" determination in step 940 to proceed to step 945. The CPU sets the clutch torque Tcu to "zero" in step 945. That is, the CPU removes / releases / eliminates the differential limitation degree (sets the differential limitation degree to zero) between the front axle 32 and the rear axle 33 (the differential limitation degree is set to the third limitation degree, which is smaller than the second limitation degree) and proceeds to step 995 to temporarily terminate the current program.

[0080] On the other hand, when the lower limit of the protective consecutive selection time Tαdws is shorter than the predetermined time T1th, the CPU makes a "No" determination in step 935 to proceed to step 950, at which the CPU sets the clutch torque Tcu to the clutch torque (hereinafter referred to as "actual clutch torque Tcuact") determined by a separately executed clutch torque control program. In other words, in this case, the CPU maintains the clutch torque Tcu at the current torque. According to the above assumption, the clutch torque Tcu at the current moment is the maximum value Tcumax (the second limitation degree). Then, the CPU proceeds to step 995 to temporarily terminate the current program.

[0081] Further, when the absolute value |Yrm| of the lower limit of the protection selection start yaw rate Yrm is equal to or greater than the yaw rate threshold Yrth, the CPU makes a "No" determination in step 940 to proceed to step 950, at which the CPU sets the clutch torque Tcu as the actual clutch torque Tcuact and proceeds to step 995 to temporarily terminate the current program.

[0082] As described above, when the lower limit of the protective consecutive selection time Tαdws is shorter than the predetermined time T1th, the CPU maintains the value of the clutch torque Tcu at the actual clutch torque Tcuact even when the vehicle is traveling straight (|Yrm| < Yrth). For example, when the vehicle 10 is traveling on a traveling road (high μ road) with a high surface friction coefficient, the wheel speeds of all four wheels are unlikely to deviate from the actual vehicle body speed Vbr and decrease simultaneously. Therefore, in the above case, it is not necessary to remove / release / eliminate the differential limiter between the front axle 32 and the rear axle 33.

[0083] Furthermore, as described above, when the vehicle 10 turns on a road (low-µ road) with a low surface friction coefficient, the wheel speed of the inside wheel drops faster than the wheel speed of the outside wheel, and therefore, the wheel speeds of all four wheels are unlikely to deviate from the actual vehicle body speed Vbr and decrease simultaneously. Therefore, in this case, it is also not necessary to remove / release / remove the differential limiter between the front axle 32 and the rear axle 33.

[0084] When the master cylinder pressure Pm is lower than the master cylinder pressure threshold Pmth (when the CPU makes a "No" determination in step 915), or when the ABS control is being executed (when the CPU makes a "No" determination in step 920), the CPU proceeds to step 950 to set the clutch torque Tcu to the actual clutch torque Tcuact and proceeds to step 995 to temporarily terminate the current program.

[0085] As described above, the control device comprises the control unit which: changes the braking force applied to each of the wheels in response to the braking action performed by the driver of the vehicle 10; estimates the speed of the vehicle body as an estimated vehicle body speed Vx using the maximum wheel speed from a plurality of detected wheel speeds Vw; the brake slip ratio SL of each of the wheels is calculated based on the estimated vehicle body speed Vx and the respective wheel speeds Vw; executes the anti-lock control to adjust the braking force applied to the respective wheels based on the brake slip ratios SL of the respective wheels; and further sets the differential limiting degree by the differential limiting device 34 to the second limiting degree (Tcu = Tcumax) which is larger than the first limiting degree (Tcu = 0) when the braking operation is performed.

[0086] The control unit is configured to determine whether the specific state occurs in which the detected yaw rate Yr is lower than the predetermined yaw rate threshold Yrth and the magnitude of the decreasing amount of the maximum wheel speed Vwsel of a plurality of detected wheel speeds Vw for the predetermined time is larger than the magnitude decrease threshold when the braking operation is performed, and to set the differential limiting degree by the differential limiting device to the third limiting degree (Tcu = 0) which is smaller than the second limiting degree (Tcu = Tcumax) when it is determined that the specific state occurs.

[0087] The control device thus configured is capable of preventing an increase in the understeer tendency of the vehicle when the vehicle turns during the braking control and preventing the operational delay of the ABS control of the vehicle when the vehicle travels straight ahead during the braking control. <abwandlung>

[0088] The present invention is not limited to the above embodiment, and various modifications can be made as described below within the scope of the present invention.

[0089] In the above embodiment, whether the vehicle 10 is decelerated is determined based on whether the master cylinder pressure Pm is greater than or equal to the master cylinder pressure threshold Pmth. However, it can also be determined based on whether the brake pedal pressure value BP is greater than or equal to the predetermined pressure value BPth.

[0090] In the above embodiment, when the vehicle 10 traveling straight is decelerated and the lower limit of the protection value for the predetermined time is consecutively selected in the calculation of the estimated vehicle body speed Vx, the differential limiting degree (the third limiting degree) of the differential limiting device 34 is set to "0," that is, the differential limiting is removed. On the other hand, in the above situation, the differential limiting degree (the third limiting degree) of the differential limiting device 34 may be set to a predetermined value greater than "0" and smaller than Tcu0.

[0091] In the above embodiment, whether the temporal decrease rate of the estimated vehicle body speed Vx is less than the predetermined decrease rate (that is, whether the specific condition has been established) is determined based on the magnitude of the lower limit of the protective consecutive selection time Tαdws. However, whether the temporal decrease rate of the estimated vehicle body speed Vx is less than the predetermined decrease rate may be determined based on whether an inclination of the selected wheel speed Vwsel is smaller than the lower limit of the protective inclination αdw.

[0092] Specifically, according to the above modification, a deviation Vwsele-Vwsels between the selected wheel speed Vwsel=Vwsels at a start time of the calculation period ΔT and the selected wheel speed Vwsel=Vwsele at an end time of the calculation period ΔT is calculated. Then, the above deviation determines that the gradient of the selected wheel speed Vwsel is lower than the lower limit of the guard gradient αdw when the deviation (Vwsele-Vwsels) is negative (i.e., Vwsele-Vwsels<0) and an absolute value |Vwsele-Vwsels| of the deviation is smaller than an absolute value |αdw·ΔT| of the product of the lower limit of the guard gradient αdw and the calculation period ΔT. It should be noted that the specific condition can be determined to have occurred when the increase of the selected wheel speed Vwsel over a given frequency is consecutively lower than the lower limit of the protection slope αdw.

[0093] In the above embodiment, the CPU of the brake ECU 70 executes the Fig. 9. However, the CPU of the 4WD ECU 60 may execute the above program, or a CPU of a single ECU formed by integrating the 4WD ECU 60, the brake ECU 70, and the like may execute the above program instead of the CPU of the brake ECU 70.< / abwandlung> < / antiblockiersteuerung>

Claims

[1] Control device for a four-wheeled vehicle, which is applied to a four-wheeled vehicle (10), comprising: a drive unit (20) which generates a driving force; a center differential device (31) which transmits the driving force to a front axle (32) and a rear axle (33) and allows differential operation between the front axle and the rear axle; a differential limiting device (34) which varies the degree of differential limitation between the front axle and the rear axle; a braking device (40) which applies a braking force to each of the plurality of wheels of the vehicle (10), wherein a braking force distribution ratio of the rear wheels to the front wheels is set such that it is lower than an ideal distribution ratio which is based on a ground contact load ratio of the rear wheels to the front wheels when the differential limiting degree is set to a first limiting degree by the differential limiting device, the control device includes: Wheel speed sensors (82) each detecting a wheel speed which is a rotational speed of each of the plurality of wheels, a yaw rate sensor (84) for detecting the yaw rate of the vehicle, and an electronic control unit (50, 60 and 70) that is set up: to vary the braking force applied to each of the wheels in response to a braking action performed by the driver of the vehicle (10); to estimate the speed of a vehicle body as the estimated vehicle body speed using a maximum wheel speed from a plurality of recorded wheel speeds; to calculate a brake slip ratio for each of the wheels based on the estimated vehicle body speed and the respective wheel speeds; to implement an anti-lock braking system to adjust the braking force based on the brake slip ratio of the respective wheels, which is applied to the respective wheels; and to adjust the differential limiting degree by the differential limiting device (34) to a second limiting degree which is greater than the first limiting degree when the braking process is carried out, characterized by , that the control unit (50, 60 and 70) is set up: to determine whether a specific condition has occurred in which the detected yaw rate is lower than a predetermined yaw rate threshold and the magnitude of a decrease in the maximum wheel speed of a plurality of detected wheel speeds for a predetermined time is greater than a decrease threshold when the braking process is carried out; to adjust the differential limiting degree by the differential limiting device (34) to the second limiting degree if it is determined that the specific condition has not occurred while the braking process is being carried out; and to adjust the differential limiting degree by the differential limiting device (34) to a third limiting degree which is smaller than the second limiting degree when it is determined that the specific condition has occurred while the braking process is being carried out. [2] Control device for a four-wheel-drive vehicle according to claim 1, wherein the control unit is configured to adjust the differential limiting degree to the third limiting degree by removing the differential limit specified by the differential limiting device. [3] Control device for a four-wheel-drive vehicle according to claim 1 or 2, wherein the control unit is configured as follows: to select the maximum wheel speed from a plurality of wheel speeds whenever a predetermined calculation period expires; and to set the estimated vehicle body speed at a current time to a lower limit of a protection value if the lower limit of the protection value is greater than or equal to the selected maximum wheel speed, wherein the lower limit of the protection value is calculated by subtracting a constant value from the estimated vehicle body speed, which was estimated at a time in a given calculation period prior to the current time; and to determine that the specific condition has occurred when a case in which the lower limit of the protection value is set as the estimated vehicle body speed continues to exist for a predetermined number of times. [4] Control device for a four-wheel-drive vehicle according to any one of claims 1 to 3, wherein the control unit is configured to set the yaw rate threshold to a value corresponding to a yaw rate that occurs when the vehicle is found to be traveling straight ahead.

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