JPH09249109A - Antiskid control device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To effectively control a center differential gear even when it is set to the constrained state. SOLUTION: When a center differential gear is set to the constrained state (step 160), a vehicle is not traveled on a μ-split road (step 170), and a phenomenon peculiar to a four-wheel drip is detected (step 180), then a judgment flag is set on (step 190). When the judgment flag is set on and the control mode for the low-speed side wheel of front wheels is set to the decompression mode, the control mode for rear wheels is forcibly set to the decompression mode (steps 260-280), and otherwise normal antiskid control is applied (steps 240, 250). The brake oil pressure of the rear wheels is limited only when control is really required, so that the brake oil pressure of the rear wheels is prevented from unnecessarily reducing and resulting in extending the braking distance except in four-wheel drop state.

Description

Detailed Description of the Invention

[0001]

The present invention relates to a method for braking a vehicle,
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an anti-skid control device that prevents wheels from slipping excessively on a road surface, and more particularly to an anti-skid control device that is mounted on a four-wheel drive vehicle that has a restraint mechanism that restrains a differential motion of a center differential gear. .

[0002]

2. Description of the Related Art A center differential gear (hereinafter,
In a four-wheel drive vehicle equipped with a restraint mechanism for restraining a differential motion, the four wheels will interfere with each other if a brake operation is performed while the center differential is in a restrained state. A so-called four-wheel drop occurs in which the thigh falls below the actual vehicle speed.

When anti-skid (ABS) control is performed with the four wheels dropped, the brake oil pressure of the rear wheels tends to become excessive, and the brake torque of the rear wheels spills over to the front wheels, causing the wheels of the front wheels to fall. It works to reduce the speed. As a result, even if the brake oil pressure of the front wheels is reduced, the rotation of the front wheels cannot be restored to the wheel speed at which the slip ratio is such that the braking force works sufficiently.
Not only the S control cannot be performed, but also the four wheels are locked because the four wheels are deteriorated.

As a means for preventing such a four-wheel drop, for example, in Japanese Unexamined Patent Publication No. Sho 62-105753, the locked state of the center differential is detected by a differential lock detection switch, and when the center differential is in the locked state. Discloses an ABS control device that controls so that the brake hydraulic pressure of the rear wheels does not increase.

[0005]

However, in this system, the brake hydraulic pressure of the rear wheels is unconditionally limited when the center differential is restrained regardless of whether or not the vehicle is in the four-wheel drop state. Therefore, when the four wheels are not in the depressed state, there is a problem that the slip of the rear wheels becomes shallower than necessary and the braking distance is extended.

The present invention has been made to solve the above problems.
An object of the present invention is to provide an anti-skid control device capable of detecting a condition of four wheels falling down, which requires control for suppressing brake hydraulic pressure of rear wheels, and capable of performing effective control even when a center differential is in a restrained state.

[0007]

In the anti-skid control device according to claim 1 of the present invention made to achieve the above object, the wheel speed detecting means detects the wheel speeds of the front and rear wheels, and the detection thereof is performed. Based on the result, the vehicle body speed estimating means estimates the vehicle body speed from the wheel speed of the fastest wheel, and based on the vehicle body speed estimated by the vehicle body speed estimating means and the wheel speeds of the front and rear wheels detected by the wheel speed detecting means. The braking force control means adjusts the brake pressures of the front and rear wheels during vehicle braking, and performs ABS control so that excessive slip does not occur between the front and rear wheels and the road surface.

At this time, when the restraint state detecting means detects the restraint state of the central differential mechanism, the four-wheel drop determining means
Based on the detection result of the wheel speed detection means, it is determined whether or not the wheel speeds of the front and rear wheels are all lower than the actual vehicle body speed, and it is determined that the four wheels are in the depressed state. The limit control means limits the brake pressure on the rear wheels.

As a result, the brake pressure of the rear wheels, which tends to become excessive, is suppressed during the ABS control in the four-wheel drop state,
Since the sneaking of the rear wheel brake torque to the front wheels is suppressed, the rotation of the front wheels can be returned to the wheel speed at which a sufficient braking force is exerted, and good ABS control is performed.

As described above, according to the anti-skid control device of the present invention, since the brake pressure of the rear wheels is limited only when the central differential mechanism is in the restrained state and the four wheels are in the depressed state, the four wheels are in the depressed state. In some cases, it is possible to quickly return to a state in which good ABS control can be performed, and when the four-wheel drop state is not achieved, the slip will not become unnecessarily shallow and the braking distance will not be extended unlike the conventional device. Therefore, effective ABS control can be performed according to the state of the vehicle.

Further, in the anti-skid control device of the present invention, the braking pressure applied to the rear wheels is limited by taking into consideration not only the restraint state of the differential mechanism but also the four-wheel drop state detected by using the wheel speed. Therefore, it is possible to easily avoid the following, which is a problem in the conventional device that limits the brake pressure applied to the rear wheels based only on the restrained state of the differential mechanism.

That is, in the conventional device, if the failure occurs while the restraint state detecting means of the differential mechanism remains ON, 2WD will occur.
Even in a state (for example, when there is no center differential restraint) or the like, there is a possibility that the rear wheel pressure increase may be limited, resulting in insufficient braking force. However, the antiskid control device of the present invention Then, such a possibility can be reduced as much as possible.

Next, in the anti-skid control device according to the second aspect, the four-wheel drop determination means determines that the four-wheel drop state is in the case where the wheel speed difference between the front wheels is equal to or more than a predetermined value. By the way, the relationship between the wheel speed VFR of the right front wheel, the wheel speed VFL of the left front wheel, and the wheel speed VFP of the front wheel drive shaft is given by the action of the front wheel differential mechanism as shown in equation (1),
As is clear from this relationship, when the wheel speed of the front wheel drive shaft is fixed and the wheel speed of one wheel increases, the wheel speed of the other wheel decreases. Note that K is the final ratio.

VFP = K · (VFR + VFL) / 2 (1) That is, the four-wheel drop state means that the front-wheel drive shaft and the rear-wheel drive shaft that are restrained by the restraint mechanism and rotate integrally (in this case, simply driven together). (Referred to as the shaft) occurs when the rotation speed is lower than the actual vehicle body speed. As shown in FIG. 6, the brake pressure is reduced by the ABS control so that the wheel gripped earlier has the actual vehicle body speed. There is a phenomenon in which the speed of the other wheel rapidly increases and the other wheel decelerates accordingly. Since the wheel speed difference between the left and right wheels that occurs at this time is greater as the state of the drive shaft being dropped is larger, the four-wheel drop state can be detected by examining the magnitude of the wheel speed difference between the left and right wheels.

As described above, according to the anti-skid control device of the present invention, a simple calculation is performed using the wheel speed that is normally used in the ABS control, without using a special detection device or the like. Therefore, it is possible to easily determine the four-wheel drop state. Next, in the anti-skid control device according to the third aspect, the braking force control means is a so-called select low control in which the control of the rear wheels is similarly controlled based on the wheels on the low speed side. When the wheel speed difference between the front wheels is equal to or greater than a predetermined value and the wheel speed difference between the rear wheels is equal to or less than the predetermined value, the four-wheel drop determination unit determines that the four-wheel drop state is established. .

Therefore, according to the anti-skid control device of the present invention, when the wheel speed difference between the front wheels becomes a predetermined value or more due to the road surface condition regardless of the four-wheel drop condition, the limit control means is used. It is possible to prevent the brake pressure of the rear wheels from being unnecessarily limited, and to improve the efficiency of the AB
S control can be performed.

That is, while traveling on the μ-split road (the left front wheel and the right front wheel of the vehicle, one is traveling on a high μ road surface having a large friction coefficient μ and the other is traveling on a low μ road surface having a small friction coefficient μ),
When the brake is operated, the wheel speed of the wheels on the low μ road surface side, which are prone to slip, becomes larger than the wheel speed of the wheels on the high μ road surface side, and the wheel speed difference between the front wheels is irrelevant regardless of the four-wheel drop state. It may be greater than or equal to a predetermined value. However, in this case, the brake pressure on the rear wheel on the high μ road surface side is reduced more than necessary by the select low control, so that the wheel grips reliably and the wheel speed substantially matches the actual vehicle speed.
As a result, the vehicle body speed is correctly obtained and the ABS control is normally performed, so that it is not necessary to limit the brake pressure of the rear wheels.

As described above, on the μ-split road, the rear wheels on the high μ road surface hardly slip and the wheels on the low μ road surface are controlled to slip to some extent by the ABS control. There is a certain difference in wheel speed between the wheels. On the other hand, on a road surface where the friction coefficient μ of the left and right wheels is uniform, the rear wheels are controlled uniformly by the select low control and therefore have substantially the same wheel speed.

Therefore, even if the wheel speed difference between the front wheels is larger than a predetermined value, if the wheel speed difference between the rear wheels is larger than the predetermined value, it can be determined that the vehicle is traveling on the μ-split road. is there. Next, according to the anti-skid control device of the fourth aspect, the restraint state detecting means determines the difference between the wheel speed sum between the front wheels and the wheel speed sum between the rear wheels based on the detection result of the wheel speed detecting means. Is calculated, and the case where the calculated value is equal to or less than a predetermined value is detected as the restrained state of the central differential mechanism.

That is, due to the action of the rear wheel differential mechanism, the relationship between the wheel speed VRR of the right rear wheel, the wheel speed VRL of the left rear wheel, and the rotational speed VRP of the rear wheel drive shaft is the same as in the equation (1). Holds,
Further, when the central differential mechanism is in the restrained state, the rotation speed VFP of the front wheel drive shaft and the rotation speed VRP of the rear wheel drive shaft become equal (VFP = VRP). As a result, the relationship of the expression (2) is established, and as is clear from the expression (2), when the central differential mechanism is in the restrained state, the sum of wheel speeds between the front wheels (VFL + VFR).
And the sum of wheel speeds between the rear wheels (VRR + VRL) match, so that if the difference between these values is less than or equal to a predetermined value, it can be considered that the central differential mechanism is in a restrained state.

K (VFR + VFL) / 2 = VFP = VRP = K (VRR + VRL) / 2 (2) As described above, according to the antiskid control device of the present embodiment, the restrained state of the central differential mechanism is Since it is detected from the wheel speed used in the control of a normal anti-skid device without providing a mechanically operated switch or the like, the device can be configured simply.

Next, in the anti-skid control device according to a fifth aspect of the present invention, the restriction control means reduces the brake pressure of the rear wheels by reducing the brake pressure of the rear wheels at the same time as reducing the brake pressure of the low speed side wheels. Limit wheel brake pressure. That is, when the brake pressure of the low-speed side wheels of the front wheels is reduced, the brake torque of the rear wheels circulates, so that no matter how much the brake pressure of the front wheels is reduced, the slip ratio will be sufficient to generate a sufficient braking force. If the wheel speed cannot be restored to normal, ABS control cannot be performed normally. Therefore, in order to prevent such a situation, the brake pressure on the rear wheels is reduced to prevent the brake torque from wrapping around. There is a need to. However, when the brake pressure on the low-speed side wheels of the front wheels is maintained or increased, the wheels are returning to a wheel speed at which the slip rate at which sufficient braking force is generated or at a speed higher than that. Then, the ABS control can be normally performed, and it is not necessary to limit the brake pressure of the rear wheels. Therefore, only when the brake pressure of the low speed side wheels of the front wheels is reduced as described above, The brake pressure should also be reduced.

Therefore, according to the anti-skid control device of the present invention, even when the four-wheel drop state is detected,
When the front wheel brake pressure is controlled normally,
Without limiting the brake pressure of the rear wheels, by performing normal control according to the wheel speed of the wheels, the brake pressure of the rear wheels will not be reduced more than necessary,
Good ABS control can be realized.

[0024]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic configuration diagram of an anti-skid control device that is an embodiment of the present invention.

The vehicle to which the anti-skid control device of this embodiment is applied is a front right (FR) wheel 1, a front left (FL) wheel 2,
A right rear (RR) wheel 3 and a left rear (RL) wheel 4 are provided, and the driving force of the internal combustion engine 6 is a center differential gear (hereinafter, simply referred to as a center differential) 8 as a central differential mechanism and a front wheel drive shaft. The left and right front wheels 1, via a propeller shaft 10, a front differential gear (hereinafter simply referred to as front differential) 11 as a front wheel differential mechanism, and a front drive shaft 44 connected to the front differential 11.
The center differential 8, the propeller shaft 12 as the rear wheel drive shaft, and the rear differential gear as the rear wheel differential mechanism (hereinafter simply referred to as the rear differential)
13 is a four-wheel drive vehicle that is transmitted to the left and right rear wheels 3 and 4 via a rear drive shaft 46 connected to the rear differential 13. The center differential 8 is composed of a limited slip differential (LSD) provided with a differential limiting device as a restraining mechanism that limits the differential operation of the wheel when it is spinning.

Then, the wheels 1 to 4 are respectively attached to the wheels 1 to 4.
Hydraulic brakes 18, 19, 20, which generate braking force on
21 are provided respectively, and an electromagnetic pickup type wheel speed sensor 1 for detecting the rotation speeds of the wheels 1 to 4 is provided.
4, 15, 16 and 17 are attached respectively.

In the hydraulic brakes 18 to 21, master cylinders 24, which generate brake hydraulic pressure in response to depression of the brake pedal 22, are provided with control valves 26, 27, 28, 2 respectively.
9, a system for supplying pressure oil from the master cylinder 24 to the hydraulic brakes 18, 21 of the right front wheel 1 and the left rear wheel 4, and the hydraulic brakes 19, 20 of the left front wheel 2 and the right rear wheel 3. It is constituted by two hydraulic systems of a system for supplying to.

Each of the control valves 26 to 29 has the same structure and has a pressure increasing position for connecting the master cylinder 24 and the hydraulic brakes 18 to 21, and a holding position for blocking the master cylinder 24 and the hydraulic brakes 18 to 21. , Hydraulic brakes 18 to 21 and a reservoir 3 provided for each hydraulic system
It is a three-position valve consisting of a pressure reducing position connecting 0 and 32, and is configured to switch the position of the valve according to a control signal from an electronic control circuit 50 described later. The brake fluid stored in the reservoirs 30 and 32 is stored in the pump 3
It is configured to be returned to the master cylinder 24 side by 4, 36.

When the hydraulic pressure of the master cylinder 24 reaches a predetermined value between the control valves 28 and 29 corresponding to the left and right rear wheels 3 and 4 and the hydraulic brakes 20 and 21, the hydraulic brakes 20, The rate of pressure increase of 21 is reduced,
Well-known proportioning valves 38 and 40 for preventing the left and right rear wheels 3 and 4 from locking are provided respectively.

Further, a brake switch 42 for detecting a depression operation of the brake pedal 22 by the driver is arranged near the brake pedal 22, and the brake switch 42 and the wheel speed sensors 14 to 17 described above are provided. The control valves 26 to 29 are connected to the electronic control circuit 50.

The electronic control circuit 50 is connected to the CPU 5
2, a ROM 54, a RAM 56, an input circuit 58, an output circuit 60 and the like, which are connected to each other via a common bus 62, and which are well-known microcomputers.
Receives the detection signals from the wheel speed sensors 14 to 17 and the brake switch 42 via the input circuit 58 and outputs the control signals to the control valves 26 to 29 via the output circuit 60, whereby the hydraulic brakes 18 to The brake hydraulic pressure of 21 is adjusted and the rotation speed of the wheels 1-4 is controlled.

In the thus constructed anti-skid control device of this embodiment, when the driver depresses the brake pedal 22, hydraulic pressure is generated in the master cylinder 24, and the valve positions of the control valves 26 to 29 are increased. This hydraulic pressure is transmitted to the hydraulic brakes 18 to 21 when
4 is braked. Further, when the valve positions of the control valves 26 to 29 are in the holding position, the hydraulic pressures of the hydraulic brakes 18 to 21 are held regardless of the operation of the brake pedal 22, and when the valve positions are in the depressurizing position, similarly. Regardless of the operation of the brake pedal 22, the hydraulic pressure of the hydraulic brakes 18-21 is reduced.

Next, the anti-skid control processing executed by the electronic control circuit 50 will be described with reference to the flow chart shown in FIG. It should be noted that this process is started when an ignition switch (not shown) is turned on.
As shown in FIG. 2, when this process is started, first, at step 110, the valve positions of the control valves 26 to 29 are set to the hydraulic brakes 18 to 21 according to the depression operation of the brake pedal 22.
Initial setting such as setting to a pressure increasing position where the oil pressure to be increased is made, and a determination flag described later is cleared. At the subsequent step 120, it is determined whether control is started. Here, the brake pedal 22 is depressed and the output of the brake switch 42 is in the ON state, and the slip ratio of one of the wheels is larger than the predetermined value due to the process separately executed by the electronic control circuit 50. When the wheel acceleration is detected to be larger than the preset negative acceleration, the process proceeds to step 130 and the control is started.

In step 130, the wheel speed sensor 14
To 17 are input, and the wheel speeds VFR, VFL, VRR, and VRL of the wheels 1 to 4 are calculated based on the detection signals. In the subsequent step 140, based on the calculation result in step 130, Wheel acceleration AFR, A of each wheel 1 to 4
FL, ARR, ARL are calculated, and in step 150, the estimated vehicle speed VW is calculated based on the calculation result in step 130.
Is calculated. Here, the largest wheel speed is set as the estimated vehicle body speed VW.

In the next step 160, step 130
It is determined whether the center differential 8 is in the restrained state or not based on the calculation result in. That is, as shown in the equation (3), the sum of the wheel speeds VFR and VFL of the left and right front wheels 1 and 2 and the left and right rear wheels 3,
Calculate the absolute value of the difference between the wheel speed VRR of 4 and the sum of VRL,
If the calculated value is smaller than the predetermined value Kd, the center differential 8
Indicates that the differential operation is restricted (differential lock), and the process proceeds to step 170. If the calculated value is equal to or more than the predetermined value Kd, it is determined that the differential lock is not performed, and step 2
After clearing (turning off) the judgment flag at 00, step 21
Move to 0.

| (VFR + VFL) − (VRR + VRL) | <Kd (3) In step 170, it is determined based on the calculation result in step 130 whether the vehicle is traveling on the μ-split road. That is, as shown in equation (4), the left and right rear wheels 3, 4
The absolute value of the difference between the wheel velocities VRR and VRL is calculated. If the calculated value is equal to or less than the predetermined value Ks, it is determined that the vehicle is not traveling on the μ-split road, and the process proceeds to step 180. If it is larger, it is determined that the vehicle is traveling on the μ-split road, the determination flag is turned off in step 200, and the process proceeds to step 210.

| VRR−VRL |> Ks (4) In step 180, it is determined whether the vehicle is in the four-wheel drop state based on the calculation result in step 130. That is, as shown in the equation (5), the absolute value of the difference between the wheel speeds VFR and VFL of the left and right front wheels 1 and 2 is calculated, and if the calculated value is larger than the predetermined value Kf, it means that the four wheels are in the depressed state. If the calculated value is equal to or less than the predetermined value Kf, it is determined that the four-wheel drop state is not set, and the determination flag is turned off in step 200, and then the process proceeds to step 210.

| VFR−VFL |> Kf (5) In step 190, a determination flag indicating that the brake hydraulic pressure of the rear wheels 3, 4 should be limited is set (turned on), and the routine proceeds to step 210. In step 210, the wheel speed V of the right front wheel 1 calculated in steps 130 to 150 is calculated.
Based on the FR, the wheel acceleration AFR, and the estimated vehicle speed VW, the control mode of the control valve 26 that controls the hydraulic pressure to the hydraulic brake 18 for braking the right front wheel 1 (that is, any one of the pressure increasing mode, the holding mode, and the pressure reducing mode). Or) is set. Since the control mode setting process is a well-known process in the ABS control, the detailed description thereof will be omitted.

In the following step 220, the above step 1
The wheel speed VFL of the left front wheel 2 calculated from 30 to 150,
Based on the wheel acceleration AFL and the estimated vehicle speed V, the control mode of the control valve 27 that controls the hydraulic pressure to the hydraulic brake 19 for braking the left front wheel 2 is set.

In the following step 230, it is determined whether or not the determination flag is turned on, and if it is not turned on, the process proceeds to step 240. In step 240, it is determined which of the wheel speeds VRR, VRL of the left and right rear wheels 3, 4 is the lower speed based on the calculation result in step 130, and in the following step 250, it is determined in step 240. The wheel speed VRR (or V) of the low speed side wheel 3 (or 4)
RL), the wheel acceleration ARR (or ARL), and the estimated vehicle body speed VW, the control modes of the control valves 28 and 29 for controlling the hydraulic pressure to the hydraulic brakes 20 and 21 for braking the left and right rear wheels 3 and 4 are set. Then, the process proceeds to step 290.

On the other hand, when it is determined in the previous step 230 that the determination flag is on, step 26
After shifting to 0, it is determined which of the wheel speeds VFR, VFL of the left and right front wheels 1 and 2 is the lower speed based on the calculation result in the above step 130, and in the following step 270,
It is determined whether or not the control mode for controlling the control valve 26 (or 27) corresponding to the low speed side wheel 1 (or 2) determined in step 230 is the decrease mode for reducing the hydraulic pressure. Then, if the control mode is not the decrease mode, the routine proceeds to step 240, and the control mode for controlling the rear wheels 3, 4 is set to normal as described above, while the control mode is decreased. If it is the mode, the process proceeds to step 280.

At step 280, the control mode for controlling the control valves 28, 29 corresponding to the left and right rear wheels 3, 4 is set to the reduction mode, and the routine proceeds to step 290. In step 290, the control valves 26 to 29 are driven in accordance with the control mode set in steps 210 and 220 and step 250 or 280 to drive the hydraulic brake 18.
Control oil pressure to ~ 21.

In the following step 300, it is determined whether or not the control is finished. Here, it is determined whether or not the depression operation on the brake pedal 22 is released, the output of the brake switch 42 is turned off, and whether or not the stop of the vehicle is confirmed. If the control is continued, step 130 is performed.
Returning to step 300, step 130 to step 300 are repeatedly executed until it is determined that the control is terminated in step 300. When it is determined that the control is terminated, the procedure returns to step 110 and the initialization is performed again. In step 120, the process waits until the next control starts.

That is, in steps 160 to 190, the determination flag is turned on when the four wheels are in the depressed state, which requires the restriction of the brake hydraulic pressure of the rear wheels 3 and 4, and in steps 230 to 230.
When the determination flag is turned on by 280, the control mode of the rear wheels 3 and 4 is set to the pressure reduction mode only when the control mode of the low speed side wheels of the front wheels 1 and 2 is the pressure reduction mode. The brake hydraulic pressure of the wheels 3, 4 is limited, and in other cases, the control mode of the rear wheels 3, 4 is set according to the rotation state of the low speed side wheels, that is, the select low control. Further, in steps 210 and 220, the front wheel 1,
The control mode of No. 2 is for each wheel 1, regardless of the determination flag.
2 is set according to the rotation state.

Note that step 130 is wheel speed detecting means, step 150 is vehicle speed estimating means, and step 16 is
0 corresponds to restraint state detecting means, steps 170 and 180 correspond to four-wheel drop determining means, steps 210 to 290 correspond to braking force control means, and steps 260 to 280 correspond to limit control means.

Next, FIGS. 3, 4 and 5 are explanatory views showing a specific operation of the anti-skid control device of the present embodiment.
Note that FIG. 3 shows a state in which the center differential 8 is in a restrained state and a braking operation is performed while going straight on an extremely low μ road (uniform road) where the friction coefficient μ of the road surface is extremely low, and FIG. 3 shows the state when the brake operation is performed when traveling straight on an extremely low μ road as in FIG. 3, but shows the case where the center differential 8 is in the free state in which the differential operation is not restricted, and FIG. The center differential 8 is in a restrained state, the friction coefficient μ of the road surface on which the right front and rear wheels 1, 3 travel is high, and the friction coefficient μ of the road surface on which the left front and rear wheels 2, 4 travel is low. It shows the state when performing.

First, the case where the center differential 8 is in a restrained state during traveling on a uniform road will be described with reference to FIG. As shown in FIG. 3, the brake operation is performed, and the antiskid control is started at time T1 due to the behavior of the front wheels 1 and 2. Here, since the center differential 8 is in a restrained state, the sum of wheel speeds of the front wheels 1 and 2 (VFR + VFL) and the sum of wheel speeds of the rear wheels 3 and 4 (VRR + VRL) become equal, and therefore the difference | ( VFR + VFL)-(VRR + VRL) | does not exceed the judgment value Kd (step 160-Yes),
Further, since the road is a uniform road, the wheel speeds VRR and VRL of the left and right rear wheels 3 and 4 are subjected to the same control by the select low control.
Are approximately equal, and the wheel speed difference of the rear wheels | VRR-V
RL | never exceeds the judgment value Ks (step 17).
0-No). Therefore, the determination flag is turned on / off depending on the behavior of the front wheels 1 and 2, that is, the wheel speed difference | VFR-VFL |
Is determined by

Then, the wheel speed difference between the front wheels | VFR-VFL |
Is displaced within the judgment value Kf (periods T1 and T2)
Since the determination flag is turned off assuming that the four wheels are not in the depressed state, the control mode of the rear wheels 3 and 4 is set independently of the front wheels 1 and 2 by the normal select low control.

Thereafter, when the four wheels fall, the speed difference between the wheel speeds VFR and VFL of the front wheels increases, and at time T2 the wheel speed difference | VFR-VFL | of the front wheels exceeds the judgment value Kf. Is turned on. Then, during the ON period of the determination flag, while the control mode of the low-speed side wheel of the front wheels (here, the left front wheel 2) is set to the pressure reduction mode (periods T2 to T3).
The control mode of the rear wheels 3 and 4 is also set to the pressure reducing mode. Further, when the control mode of the low-speed side wheels of the front wheels is set to the holding mode at time T3, the control mode of the rear wheels 3 and 4 is the normal front select wheel control even if the determination flag is ON. It is set independently of 1 and 2.

By the control in the periods T2 to T3, the brake hydraulic pressure of the rear wheels 3, 4 is reduced, and the sneaking of the rear wheel 3, 4 brake torque to the front wheels 1, 2 is suppressed, whereby the four wheels are reduced. The drop condition is alleviated, and the front wheel speed difference | V
When FR-VFL | becomes smaller than the comparison value Kf, the determination flag is turned off at time T4.

As shown in FIG. 4, when the center differential 8 is in the free state, the sum of wheel speeds of the front wheels 1 and 2 (VFR + VFL) and the sum of wheel speeds of the rear wheels 3 and 4 (VRR + VR) are normally used.
L) does not match and the difference between them | (VFR + VFL)-(VR
R + VRL) | exceeds the determination value Kd (step 160-No), the wheel speed difference of the front wheels | VFR-VFL |
Even if the value exceeds the judgment value, the four wheels are not judged to be in the depressed state (the judgment flag is turned on).

Further, as shown in FIG. 5, when the μ-split road surface is run, the select low control of the rear wheels 3 and 4 causes
Since the brake oil pressure is controlled so that the left rear wheel 4 on the low μ road side has a predetermined slip ratio, the right rear wheel 3 on the high μ road side is controlled.
Is decompressed more than necessary and hardly slips,
The wheel speed VRR substantially matches the actual vehicle speed. As a result, a speed difference occurs between the left and right rear wheels 3 and 4, and the wheel speed difference | VRR-VRL | of the rear wheels exceeds the judgment value Ks (step 170-Yes). Therefore, the wheel speed difference | VFR of the front wheels.
Even if -VFL | exceeds the judgment value, it is not judged that the four wheels are in the depressed state (the judgment flag is on).

As described above, in the anti-skid control device of the present embodiment, when it is determined that the vehicle is in the four-wheel drop state, when the control mode of the low-speed side wheels of the front wheels 1 and 2 is the pressure reducing mode. The control mode of the rear wheels 3 and 4 is forcibly set to the pressure reducing mode to reduce the brake hydraulic pressure of the rear wheels 3 and 4.

Therefore, according to the anti-skid control device of this embodiment, it is possible to suppress the brake torque of the rear wheels 3 and 4 from sneaking into the front wheels 1 and 2 when the four wheels are dropped, and the control is performed in the pressure reducing mode. Wheel speeds VFR and VFL of the front wheels 1 and 2
Can be reliably and promptly returned to a speed at which the slip ratio at which a sufficient braking force is obtained can be obtained. As a result, it is possible to perform good anti-skid control without causing the four-wheel drop state to deteriorate and lock all four wheels.

Further, according to this embodiment, if the center differential 8 is in the restrained state as in the conventional apparatus, the rear wheels 3, 4 are unconditionally.
Not limiting the brake hydraulic pressure of the vehicle, but a peculiar phenomenon that occurs when the four wheels are depressed, that is, the wheel speed VF of the front wheels 1 and 2.
The brake hydraulic pressure of the rear wheels 3 and 4 is limited only when the difference between R and VFL is detected, and except when the μ-split road where the same phenomenon occurs is excluded, that is, only when really necessary. As a result, the brake oil pressure of the rear wheels 3 and 4 is not unnecessarily reduced and the braking distance is not extended when the four wheels are not in the depressed state, and effective anti-skid control is performed. You can

Further, according to the anti-skid control device of the present embodiment, the center differential 8 is restrained, the vehicle is running, and 4
Since the wheel drop condition is determined based on the wheel speeds VFR, VFL, VRR, VRL used in normal anti-skid control, the number of signals used for control is increased as compared with the conventional device. Without, rather, center differential 8
The switch and the signal line for connecting the output of the switch to the electronic control circuit 50 can be reduced as compared with the case where the restraint state is detected by a switch that mechanically detects, and the device configuration is simple. Can be one.

Although the LSD is used as the central differential mechanism in the above embodiment, a vis coupling may be used.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of an anti-skid control device according to an embodiment.

FIG. 2 is a flowchart of anti-skid control processing in the embodiment.

FIG. 3 is an explanatory diagram showing an operation under uniform road traveling and center differential restraint.

FIG. 4 is an explanatory diagram showing an operation on uniform road running and center differential free.

FIG. 5 is an explanatory diagram showing an operation under μ-split road traveling and center differential restraint.

FIG. 6 is an explanatory diagram showing the principle of four-wheel drop state detection.

[Explanation of symbols]

1 to 4 ... Wheels 6 ... Internal combustion engine 8 ... Center differential gears 10, 12 ... Propeller shaft 11 ... Front differential gears 14-17 ... Wheel speed sensor 13 ... Rear differential gears 18-21 ... Hydraulic brake 22 ... Brake pedal
24 ... Master cylinder 26-29 ... Control valve 30, 32 ... Reservoir 3
4, 36 ... Pump 38, 40 ... Proportioning valve 42 ... Brake switch 44 ... Front drive shaft 46 ... Rear drive shaft 50 ... Electronic control circuit 52 ... CPU 54 ... ROM
56 ... RAM 58 ... Input circuit 60 ... Output circuit 62 ... Common bus

Claims (5)

[Claims] 1. A front wheel differential mechanism that transmits a driving force from a front wheel drive shaft to a pair of front wheels and absorbs a rotation difference between the front wheels, and a driving force from a rear wheel drive shaft to a pair of rear wheels. A rear wheel differential mechanism that absorbs a rotational difference between the rear wheels, a central differential mechanism that absorbs a rotational difference between the front wheel drive shaft and the rear wheel drive shaft, and a differential operation of the central differential mechanism are restrained. Mounted on a four-wheel drive vehicle having a restraint mechanism for detecting the wheel speed of the front and rear wheels, and the vehicle speed from the wheel speed of the fastest wheel based on the detection result of the wheel speed detection means. Based on the vehicle speed estimation means for estimating and the vehicle speed estimated by the vehicle speed estimation means and the wheel speed of the front and rear wheels detected by the wheel speed detection means, the brake pressure to the front and rear wheels is estimated. Adjust to prevent excessive contact between the front and rear wheels and the road surface. In the anti-skid control device including a braking force control unit that controls so that various slips do not occur, a restraint state that detects the restraint state in which the differential operation of the central differential mechanism is restrained by the restraint mechanism. When the restraint state of the central differential mechanism is detected by the detecting means and the restraint state detecting means, all the wheel speeds of the front and rear wheels have fallen below the actual vehicle body speed based on the detection result of the wheel speed detecting means. A four-wheel drop determination means for determining whether or not a four-wheel drop state is provided is provided, and the four-wheel drop determination means is provided to the braking force control means.
An anti-skid control device comprising limit control means for limiting the brake pressure to the rear wheels when it is determined that the wheels are in a wheel drop state. 2. The anti-skid according to claim 1, wherein the four-wheel drop determination means determines that the four-wheel drop state is in a state where a wheel speed difference between the front wheels is equal to or more than a predetermined value. Control device. 3. The braking force control means performs select low control on the rear wheels to similarly control the brake pressure of both wheels on the basis of the low speed side wheels, and the four-wheel drop determination means determines between the front wheels. The anti-skid control according to claim 1, wherein when the wheel speed difference is a predetermined value or more and the wheel speed difference between the rear wheels is a predetermined value or less, it is determined that the four wheels are in the down state. apparatus. 4. The restraint state detecting means calculates the difference between the sum of wheel speeds between the front wheels and the sum of wheel speeds between the rear wheels based on the detection result of the wheel speed detecting means, and the calculated value is The case where the difference is equal to or less than a predetermined value is detected as a restrained state of the central differential mechanism.
The vehicle state detection device according to any one of 1. 5. The limit control means reduces the brake pressure of the rear wheels at the same time as reducing the brake pressure of the low speed side wheels of the front wheels. Anti-skid control device described in.