JP7793964B2 - Braking control device - Google Patents

Description

The present invention relates to a vehicle braking control device.

As disclosed in Patent Document 1, a vehicle is known that is equipped with a braking system that has separate braking systems: one that adjusts the braking force on the right front wheel and left rear wheel, and another that adjusts the braking force on the left front wheel and right rear wheel. Hereinafter, this type of braking system may also be referred to as a "brake system with cross-connected braking systems." Furthermore, a vehicle equipped with a braking system with cross-connected braking systems may also be referred to as a "vehicle equipped with a cross-connected braking system."

The braking control device disclosed in Patent Document 1 can determine whether one of the braking systems in a braking system having cross-connected braking systems has failed. The failure determination is made by detecting hydraulic fluid leakage in the braking system. Specifically, the braking control device disclosed in Patent Document 1 determines that a failure has occurred if it detects a drop in the fluid level in the reservoir tank when the brake pedal is not depressed.

Japanese Patent Application Laid-Open No. 2020-138657

In a vehicle equipped with a cross-connected braking system, if one of the braking systems fails, a difference in braking force occurs between the left and right wheels during braking, causing a yaw moment in the vehicle. Therefore, in a vehicle equipped with a cross-connected braking system, a one-system failure affects the vehicle's straight-line stability.

When detecting the fluid level in a reservoir tank, such as in the braking control device disclosed in Patent Document 1, it is thought that the accuracy of detecting the fluid level will vary depending on factors such as vehicle sway and road inclination. This variation in the accuracy of detecting the fluid level may reduce the accuracy of determining whether one system has failed.

There is a need for more accurate detection of single-system failures, which can cause unstable behavior in vehicles equipped with cross-connected braking systems.

The braking control device for solving the above problem is applied to the braking system of a vehicle having a first braking system that adjusts the braking force applied to the right front wheel and left rear wheel, and a second braking system that adjusts the braking force applied to the left front wheel and right rear wheel. The braking control device includes a wheel speed detection unit that detects the speed of each wheel as the wheel speed, a yaw rate detection unit that detects the yaw rate actually acting on the vehicle as a yaw rate detection value, and a failure determination unit that, when braking the vehicle, determines whether either the first braking system or the second braking system is in a failure state based on the wheel speed and the yaw rate detection value, and determines that a single system failure exists if either the first braking system or the second braking system is in a failure state.

When a vehicle equipped with a braking device having the above-mentioned first and second braking systems experiences a one-system failure, it is thought that a magnitude relationship will arise between the left and right wheel speeds during braking, since the first and second braking systems are independent. Therefore, by using the wheel speeds and the yaw rate actually acting on the vehicle, it is possible to capture the behavior of a vehicle experiencing a one-system failure.

With the above configuration, it is possible to determine whether a single-system failure has occurred using the wheel speed and yaw rate detection value. In other words, by using the wheel speed and yaw rate detection value, it is possible to capture changes in the behavior of a vehicle in a single-system failure state and accurately determine whether a single-system failure has occurred.

FIG. 1 is a schematic diagram showing an embodiment of a braking control device, a braking device that is an object to be controlled by the braking control device, and a vehicle equipped with the braking device. FIG. 2 is a flowchart showing the flow of the malfunction determination process executed by the brake control device. FIG. 3 is a schematic diagram of a vehicle in a one-system failure state. FIG. 4 is a diagram showing the wheel speed and the detected yaw rate when braking a vehicle in a one-system failure state. FIG. 5 is a flowchart showing the flow of processing when the brake control device performs rear lift suppression control. FIG. 6 is a flowchart showing the flow of the compensation control process executed by the braking control device. FIG. 7 is a diagram showing the transition of the braking force adjusted by the compensation control. FIG. 8 is a diagram showing the transition of the braking force adjusted by the compensation control.

An embodiment of a brake control device will be described below with reference to FIGS.
FIG. 1 shows a braking control device 10 and a vehicle 90 to which the braking control device 10 is applied.
The vehicle 90 is, for example, a four-wheel vehicle, and includes a right front wheel 91fr, a left rear wheel 91rl, a left front wheel 91fl, and a right rear wheel 91rr.

The vehicle 90 is equipped with a braking device 80. The vehicle 90 is equipped with a brake operating member 92 that can be operated by the driver of the vehicle 90. For example, the brake operating member 92 is a brake pedal. By operating the brake operating member 92, the driver can generate a braking force via the braking device 80 and brake the vehicle 90.

The vehicle 90 may be equipped with an alarm device 93. The alarm device 93 is a device intended to notify the driver or other personnel of the state of the vehicle 90. Examples of the alarm device 93 include a warning light, a display device, a speaker device, etc.

<Brake device>
An example of the braking device 80 is a friction braking device. Figure 1 shows a hydraulic braking device as an example of a friction braking device.

Brake device 80 has a first brake system 71 that adjusts the braking force applied to the right front wheel 91fr and the left rear wheel 91rl, and a second brake system 72 that adjusts the braking force applied to the left front wheel 91fl and the right rear wheel 91rr. The first brake system 71 and the second brake system 72 are independent brake systems. In other words, brake device 80 is a so-called cross-piped brake system. Cross-piping is also sometimes called X-piping or diagonal piping. Brake device 80 is an example of a brake system with cross-connected brake systems. Vehicle 90 is an example of a vehicle equipped with a cross-connected brake system.

The braking device 80 includes a hydraulic pressure generating device 81. The braking device 80 includes a brake actuator 70. The braking device 80 includes a braking mechanism corresponding to each wheel.
The first brake mechanism 84a, second brake mechanism 84b, third brake mechanism 84c, and fourth brake mechanism 84d shown in FIG. 1 correspond to the right front wheel 91fr, left rear wheel 91rl, left front wheel 91fl, and right rear wheel 91rr, respectively. Each brake mechanism can apply a braking force to the corresponding wheel. Each brake mechanism is composed of a wheel cylinder, a rotating body that rotates integrally with the wheel, and a friction material that can be pressed against the rotating body. An example of a brake mechanism is a disc brake. The brake mechanism may also be a drum brake. Each brake mechanism can generate a friction braking force on the corresponding wheel in response to the hydraulic pressure in the wheel cylinder. Each brake mechanism is configured so that the force that presses the friction material against the rotating body increases as the hydraulic pressure in the wheel cylinder increases. In other words, each brake mechanism can apply a greater braking force to the wheel as the hydraulic pressure in the wheel cylinder increases.

The hydraulic pressure generating device 81 includes a booster 82 and a master cylinder 83. The booster 82 assists the operation of the brake operating member 92 and transmits the assisted operating force to the master cylinder 83. The booster 82 can be a vacuum booster, hydraulic booster, electric booster, or the like. The master cylinder 83 generates hydraulic pressure in response to the operation of the brake operating member 92. The hydraulic pressure generated by the master cylinder 83 is referred to as the MC pressure Pmc. The master cylinder 83 is a tandem master cylinder. The master cylinder 83 pumps hydraulic fluid from one port to the first brake system 71 in an amount corresponding to the MC pressure Pmc. The master cylinder 83 pumps hydraulic fluid from the other port to the second brake system 72 in an amount corresponding to the MC pressure Pmc.

The brake actuator 70 is disposed between the master cylinder 83 and the wheel cylinders. The brake actuator 70 includes a first hydraulic circuit 61 and a second hydraulic circuit 62 as hydraulic fluid flow paths. The brake actuator 70 also includes multiple solenoid valves, a drive motor 77, a first pump 73 and a second pump 74 operated by the drive motor 77, a first reservoir 75 and a second reservoir 76.

The first hydraulic circuit 61 will now be described. The first hydraulic circuit 61 constitutes the first braking system 71. The first hydraulic circuit 61 has a first path 63a connected to the first braking mechanism 84a. The first hydraulic circuit 61 has a second path 63b connected to the second braking mechanism 84b.

A normally open first pressure increase valve 64a and a normally closed first pressure reduction valve 65a are arranged in the first path 63a. The wheel cylinder of the first braking mechanism 84a is connected between the first pressure increase valve 64a and the first pressure reduction valve 65a.

A normally open second pressure increase valve 64b and a normally closed second pressure reduction valve 65b are arranged in the second path 63b. The wheel cylinder of the second braking mechanism 84b is connected between the second pressure increase valve 64b and the second pressure reduction valve 65b.

The first reservoir 75 temporarily stores the hydraulic fluid that has flowed out through the first pressure reducing valve 65a and the hydraulic fluid that has flowed out through the second pressure reducing valve 65b.
The first pump 73 draws hydraulic fluid temporarily stored in a first reservoir 75 and discharges it between the first pressure increase valve 64 a and the second pressure increase valve 64 b in the first hydraulic circuit 61 and the master cylinder 83 .

An MC pressure sensor SE7 that detects the hydraulic pressure of the hydraulic fluid pumped from the master cylinder 83 is connected to the first hydraulic circuit 61 in the first brake system 71. The detection signal from the MC pressure sensor SE7 is input to the brake control device 10. Based on the detection signal from the MC pressure sensor SE7, the brake control device 10 obtains the MC pressure Pmc. The MC pressure sensor SE7 is an example of a hydraulic pressure detection device that can detect the hydraulic pressure generated by the master cylinder 83, i.e., the MC pressure Pmc. The first brake system 71, which is equipped with the MC pressure sensor SE7, is a brake system that can detect the MC pressure Pmc.

The second hydraulic circuit 62 will now be described. The second hydraulic circuit 62 constitutes the second brake system 72. Unlike the first hydraulic circuit 61, the second hydraulic circuit 62 does not include an MC pressure sensor SE7. The second hydraulic circuit 62 includes a third path 63c connected to the third brake mechanism 84c. The second hydraulic circuit 62 includes a fourth path 63d connected to the fourth brake mechanism 84d.

A normally open third pressure increase valve 64c and a normally closed third pressure reduction valve 65c are arranged in the third path 63c. The wheel cylinder of the third braking mechanism 84c is connected between the third pressure increase valve 64c and the third pressure reduction valve 65c.

A normally open fourth pressure increase valve 64d and a normally closed fourth pressure reduction valve 65d are arranged in the fourth path 63d. The wheel cylinder of the fourth brake mechanism 84d is connected between the fourth pressure increase valve 64d and the fourth pressure reduction valve 65d.

The second reservoir 76 temporarily stores the hydraulic fluid that has flowed out through the third pressure reducing valve 65c and the hydraulic fluid that has flowed out through the fourth pressure reducing valve 65d.
The second pump 74 draws hydraulic fluid temporarily stored in the second reservoir 76 and discharges it between the third and fourth pressure increase valves 64 c and 64 d in the second hydraulic circuit 62 and the master cylinder 83 .

Hydraulic fluid is supplied from the master cylinder 83 to the wheel cylinders via the brake actuator 70. The braking device 80 is configured so that the braking force applied to the wheels in accordance with the MC pressure Pmc generated by the master cylinder 83 is greater than the braking force applied to the front wheels than the braking force applied to the rear wheels.

By controlling the first pressure increase valve 64a and the first pressure reduction valve 65a, the fluid pressure in the wheel cylinder of the first brake mechanism 84a can be adjusted separately from the fluid pressure in the other wheel cylinders. Similarly, by controlling the respective pressure increase valves and pressure reduction valves, the fluid pressure in the wheel cylinders of the second brake mechanism 84b, the third brake mechanism 84c, and the fourth brake mechanism 84d can be adjusted separately from the fluid pressure in the other wheel cylinders.

Sensor
The vehicle 90 is equipped with various sensors. Fig. 1 shows, as examples of the various sensors, a first wheel speed sensor SE1, a second wheel speed sensor SE2, a third wheel speed sensor SE3, a fourth wheel speed sensor SE4, a yaw rate sensor SE5, and a G sensor SE6. Detection signals from the various sensors are input to the braking control device 10.

The first to fourth wheel speed sensors SE1 to SE4 are sensors that detect the speed of each wheel. The first wheel speed sensor SE1, second wheel speed sensor SE2, third wheel speed sensor SE3, and fourth wheel speed sensor SE4 correspond to the right front wheel 91fr, left rear wheel 91rl, left front wheel 91fl, and right rear wheel 91rr, respectively. The braking control device 10 can obtain the wheel speeds based on the detection signals from the first to fourth wheel speed sensors SE1 to SE4.

The yaw rate sensor SE5 is a sensor that detects the yaw rate of the vehicle 90. Based on the detection signal from the yaw rate sensor SE5, the braking control device 10 can obtain a yaw rate detection value Yrs. The yaw rate detection value Yrs is set to take a positive value when the vehicle 90 is turning left, i.e., when the vehicle 90 is turning counterclockwise when viewed from above. On the other hand, when the vehicle 90 is turning right, i.e., when the vehicle 90 is turning clockwise when viewed from above, the yaw rate detection value Yrs is set to take a negative value.

The G sensor SE6 is a sensor that detects the acceleration of the vehicle 90 in the longitudinal direction. Based on the detection signal from the G sensor SE6, the braking control device 10 can calculate the vehicle body deceleration G. The vehicle body deceleration G takes on a larger value as the speed of the vehicle 90 changes in the deceleration direction.

Braking control device
The braking control device 10 is a processing circuit configured with multiple functional units that execute various controls. Fig. 1 shows, as examples of the functional units, an acquisition unit 11, a calculation unit 12, an inter-system wheel speed difference determination unit 13, a malfunction diagnosis unit 14, and a braking control unit 15. The functional units included in the braking control device 10 are capable of transmitting and receiving information to and from each other.

The acquisition unit 11 will now be described. The acquisition unit 11 can acquire the yaw rate detection value Yrs. The acquisition unit 11 is an example of a yaw rate detection unit. The acquisition unit 11 can acquire the wheel speed of each wheel. The acquisition unit 11 is an example of a wheel speed detection unit.

The calculation unit 12 will now be described. The calculation unit 12 is capable of calculating the vehicle deceleration G. The calculation unit 12 is capable of calculating the vehicle speed VS based on the wheel speed of each wheel.
The calculation unit 12 can calculate an estimated yaw rate based on the wheel speeds based on the difference between the left and right wheel speeds. For example, the calculation unit 12 can calculate a yaw rate calculation value Yrw based on the difference between the wheel speed VWrl of the left rear wheel 91rl and the wheel speed VWrr of the right rear wheel 91rr. The calculation unit 12 is an example of a yaw rate calculation unit.

The inter-system wheel speed difference determination unit 13 can select one of the first brake system 71 and the second brake system 72 of the vehicle 90 and determine whether the selected brake system is in a state where there is an inter-system wheel speed difference. Hereinafter, the selected one brake system may be referred to as the target system. The other brake system that is not the target system may be referred to as the non-target system.

The following describes the state where there is a wheel speed difference between the systems. The state where there is a wheel speed difference between the systems is a state where there is a predetermined wheel speed difference between the wheel speed of the front wheels belonging to the target system and the wheel speed of the front wheels belonging to the non-target system, and a state where there is a predetermined wheel speed difference between the wheel speed of the rear wheels belonging to the target system and the wheel speed of the rear wheels belonging to the non-target system.

A state in which a predetermined wheel speed difference exists means that there is a predetermined deviation between the wheel speed in the target system and the wheel speed in the non-target system, and the magnitude relationship between the wheel speeds in the target system and the wheel speeds in the non-target system is also in a predetermined state. In this embodiment, a state in which a predetermined wheel speed difference exists means, for example, with respect to the front wheels, a state in which the wheel speed of the front wheels to which braking force is applied by the target system is lower than the wheel speed of the front wheels to which braking force is applied by the non-target system. For example, with respect to the rear wheels, a state in which a predetermined wheel speed difference exists means a state in which the wheel speed of the rear wheels to which braking force is applied by the target system is lower than the wheel speed of the rear wheels to which braking force is applied by the non-target system.

An example of the determination made by the inter-system wheel speed difference determination unit 13 will be described using a specific example. For example, when the first braking system 71 is selected as the target system, the inter-system wheel speed difference determination unit 13 determines that the first braking system 71 is in a state where an inter-system wheel speed difference exists if the wheel speed of the left front wheel 91fl, to which braking force is applied by the second braking system 72, is greater than the wheel speed of the right front wheel 91fr, to which braking force is applied by the first braking system 71, plus a predetermined front wheel speed difference threshold, and the wheel speed of the right rear wheel 91rr, to which braking force is applied by the second braking system 72, is greater than the wheel speed of the left rear wheel 91rl, to which braking force is applied by the first braking system 71, plus a predetermined rear wheel speed difference threshold.

Furthermore, for example, when the inter-system wheel speed difference determination unit 13 selects the second braking system 72 as the target system, it determines that the second braking system 72 is in a state where an inter-system wheel speed difference exists if the wheel speed of the right front wheel 91fr, to which braking force is applied by the first braking system 71, is greater than the value obtained by adding a predetermined front wheel speed difference threshold to the wheel speed of the left front wheel 91fl, to which braking force is applied by the second braking system 72, and if the wheel speed of the left rear wheel 91rl, to which braking force is applied by the first braking system 71, is greater than the value obtained by adding a predetermined rear wheel speed difference threshold to the wheel speed of the right rear wheel 91rr, to which braking force is applied by the second braking system 72.

The front wheel speed difference threshold and rear wheel speed difference threshold can be set as appropriate. The front wheel speed difference threshold and rear wheel speed difference threshold can be set to values greater than or equal to "0." The front wheel speed difference threshold and rear wheel speed difference threshold may be set to different values or to the same value.

The malfunction diagnosis unit 14 will now be described. The malfunction diagnosis unit 14 has the function of diagnosing whether the first braking system 71 and the second braking system 72 of the braking device 80 are in a malfunction state. A malfunction state is a state in which the function of applying braking force to the wheels cannot be performed due to an abnormality occurring in at least one component from the hydraulic line connected to the port of the master cylinder 83 to the braking mechanism. The malfunction diagnosis unit 14 executes a malfunction determination process to determine whether the braking system is in a malfunction state. The malfunction diagnosis unit 14 is an example of a malfunction determination unit that determines whether either one of the first braking system 71 and the second braking system 72 is in a malfunction state. The malfunction diagnosis unit 14 is an example of a malfunction determination unit that determines whether either one of the first braking system 71 and the second braking system 72 is in a malfunction state. When either one of the first braking system 71 and the second braking system 72 is in a malfunction state, the malfunction diagnosis unit 14 determines that a single-system malfunction has occurred.

The failure diagnosis unit 14 can also identify the braking system that is in a failed state between the first braking system 71 and the second braking system 72 as the failed system. The failure diagnosis unit 14 can also identify the braking system that is not in a failed state between the first braking system 71 and the second braking system 72 as the remaining system. The failure diagnosis unit 14 is also an identification unit that identifies the failed system and the remaining system.

The braking control unit 15 has the function of controlling the braking device 80. For example, by controlling the solenoid valves of the brake actuator 70, it is possible to adjust the braking force of each wheel separately. Examples of controls performed by the braking control unit 15 include anti-lock brake control, rear lift suppression control, and compensation control.

Anti-lock brake control is a control that prevents wheels from locking while braking the vehicle 90. Hereinafter, anti-lock brake control will be referred to as ABS control. The braking control unit 15 starts ABS control when an ABS start condition is met. The ABS start condition is determined to be met, for example, when the wheel speed drops relative to the vehicle speed and the amount of wheel slip is equal to or greater than a threshold value.

Rear lift suppression control is control that suppresses the occurrence of rear lift. Rear lift is a phenomenon in which the rear wheels of the vehicle 90 lift off the road surface when the load on the rear wheels decreases due to a pitching moment while the vehicle 90 is braking. In rear lift suppression control, a target deceleration GT is set as a target value for the vehicle body deceleration G. The braking control unit 15 suppresses the braking force when the vehicle body deceleration G is greater than the target deceleration GT.

Compensation control is performed to compensate for a one-system failure state. In the event of a one-system failure state, the braking control unit 15 stabilizes the vehicle 90 by performing compensation control corresponding to the remaining system, depending on which of the first braking system 71 and the second braking system 72 is the remaining system. Compensation control is, for example, control that adjusts the braking force of the front wheels in the remaining system. As will be described in detail later, the braking control unit 15 performs first compensation control when the first braking system 71 is the remaining system, and performs second compensation control, which is different from the first compensation control, when the second braking system 72 is the remaining system. The braking control unit 15 is an example of a compensation control unit.

<Failure determination process>
An example of the malfunction determination process executed by the malfunction diagnosis unit 14 will be described with reference to Fig. 2. This process routine is repeatedly executed at predetermined intervals.

When this processing routine is started, first, in step S101, the malfunction diagnosis unit 14 determines whether or not the determination start condition is satisfied. For example, the malfunction diagnosis unit 14 determines that the determination start condition is satisfied when the following (Condition 1), (Condition 2), and (Condition 3) are all satisfied:
(Condition 1) The vehicle 90 is being braked by the driver operating the brake operating member 92.
(Condition 2) The vehicle speed VS is greater than a specified determination vehicle speed.
(Condition 3) The vehicle deceleration G is greater than a specified determination deceleration.

If the judgment start condition is not met (S101: NO), the malfunction diagnosis unit 14 temporarily terminates this processing routine. If the judgment start condition is met (S101: YES), the malfunction diagnosis unit 14 proceeds to step S102.

In step S102, the malfunction diagnosis unit 14 determines whether the turning direction of the vehicle 90 indicated by the detected yaw rate value Yrs is opposite to the turning direction of the vehicle 90 indicated by the calculated yaw rate value Yrw. Specifically, it determines whether the detected yaw rate value Yrs is greater than "0", i.e., a positive value, and the calculated yaw rate value Yrw is less than "0", i.e., a negative value.

In the processing of step S102, if the detected yaw rate value Yrs is greater than "0" and the calculated yaw rate value Yrw is less than "0" (S102: YES), the malfunction diagnosis unit 14 proceeds to step S103. On the other hand, if the situation is different from when the detected yaw rate value Yrs is greater than "0" and the calculated yaw rate value Yrw is less than "0" (S102: NO), the malfunction diagnosis unit 14 proceeds to step S107.

In step S103, the malfunction diagnosis unit 14 determines whether the second brake system 72 is in a state where there is an inter-system wheel speed difference. That is, in step S103, the malfunction diagnosis unit 14 uses the inter-system wheel speed difference determination unit 13 to select the second brake system 72 as the target system. The inter-system wheel speed difference determination unit 13 determines whether a predetermined wheel speed difference exists between the front wheels and the rear wheels between the wheel speeds of the wheels belonging to the second brake system 72 selected as the target system and the wheel speeds of the wheels belonging to the first brake system 71, which is the non-target system.

As an example, the inter-system wheel speed difference determination unit 13 determines that the second brake system 72 is in a state where an inter-system wheel speed difference exists when the wheel speed of the right front wheel 91fr belonging to the first brake system 71 is greater than the value obtained by adding the wheel speed of the left front wheel 91fl belonging to the second brake system 72 to the front wheel speed difference threshold, and the wheel speed of the left rear wheel 91rl belonging to the first brake system 71 is greater than the value obtained by adding the wheel speed of the right rear wheel 91rr belonging to the second brake system 72 to the rear wheel speed difference threshold.

The inter-system wheel speed difference determination unit 13 does not determine that an inter-system wheel speed difference exists if there is no predetermined wheel speed difference between the wheel speed of the front wheel belonging to the target system and the wheel speed of the front wheel belonging to the non-target system. Therefore, if the wheel speed of the right front wheel 91fr belonging to the first braking system 71 is equal to or less than the value obtained by adding the wheel speed of the left front wheel 91fl belonging to the second braking system 72 to the front wheel speed difference threshold, it is not determined that an inter-system wheel speed difference exists. The inter-system wheel speed difference determination unit 13 does not determine that an inter-system wheel speed difference exists if there is no predetermined wheel speed difference between the wheel speed of the rear wheel belonging to the target system and the rear wheel belonging to the non-target system. Therefore, if the wheel speed of the left rear wheel 91rl belonging to the first braking system 71 is equal to or less than the value obtained by adding the wheel speed of the right rear wheel 91rr belonging to the second braking system 72 to the rear wheel speed difference threshold, it is not determined that an inter-system wheel speed difference exists.

If the second braking system 72 is in a state where there is a wheel speed difference between the systems (S103: YES), the malfunction diagnosis unit 14 proceeds to step S104. On the other hand, if the second braking system 72 is not in a state where there is a wheel speed difference between the systems (S103: NO), the malfunction diagnosis unit 14 proceeds to step S107.

In step S104, the malfunction diagnosis unit 14 determines whether the MC pressure Pmc is smaller than a first determination value KP1.
The MC pressure Pmc and the first determination value KP1 will now be described. If the first brake system 71 fails, the MC pressure Pmc acquired based on the detection signal from the MC pressure sensor SE7 included in the first brake system 71 will differ from the hydraulic pressure actually generated by the master cylinder 83. For example, even if the master cylinder 83 is generating hydraulic pressure during braking, the MC pressure Pmc will be acquired as "0." That is, if the first brake system 71 fails, the MC pressure Pmc will be smaller than when the first brake system 71 is functioning. The first determination value KP1 is set as a threshold value for determining whether the hydraulic pressure generated by the master cylinder 83 can be detected as the MC pressure Pmc when the master cylinder 83 is generating hydraulic pressure. The first determination value KP1 is a value calculated in advance through experiments or the like. In other words, the first determination value KP1 is set as a value indicating the possibility of a failure in the first brake system 71 when the MC pressure Pmc is smaller than the first determination value KP1. As an example, the first determination value KP1 is a value slightly larger than "0".

In step S104, if the MC pressure Pmc is less than the first determination value KP1 (S104: YES), the malfunction diagnosis unit 14 proceeds to step S105. On the other hand, if the MC pressure Pmc is greater than or equal to the first determination value KP1 (S104: NO), the malfunction diagnosis unit 14 proceeds to step S107.

In step S105, the malfunction diagnosis unit 14 determines whether the detected yaw rate value Yrs is greater than the first Yr threshold value KY1. The first Yr threshold value KY1 is a positive value. The first Yr threshold value KY1 is set as a threshold value for determining whether the detected yaw rate value Yrs indicates that the vehicle 90 is turning left. In other words, if the detected yaw rate value Yrs is greater than the first Yr threshold value KY1, it indicates that the vehicle 90 is turning left. If the detected yaw rate value Yrs is greater than the first Yr threshold value KY1 (S105: YES), the malfunction diagnosis unit 14 proceeds to step S106. On the other hand, if the detected yaw rate value Yrs is equal to or less than the first Yr threshold value KY1 (S105: NO), the malfunction diagnosis unit 14 proceeds to step S107.

The behavior of a vehicle 90 in a one-system failure state will now be described using Figures 3 and 4. Figure 3 illustrates a vehicle 90 in a state in which the first braking system 71 is the failed system and the second braking system 72 is the remaining system. Figure 4(a) shows the progression of the wheel speed of the vehicle 90 in the state illustrated in Figure 3. Figure 4(b) shows the progression of the detected yaw rate value Yrs of the vehicle 90 in the state illustrated in Figure 3. Figure 4 shows an example in which braking begins at time t1.

Because the first braking system 71 is in a failed state, even if braking of the vehicle 90 is initiated, no braking force is applied to the right front wheel 91fr and the left rear wheel 91rl, as shown in FIG. 3. Meanwhile, braking force is applied to the left front wheel 91fl and the right rear wheel 91rr by the second braking system 72. The left front wheel braking force BFfl shown in FIG. 3 indicates the magnitude of the braking force applied to the left front wheel 91fl. The right rear wheel braking force BFrr shown in FIG. 3 indicates the magnitude of the braking force applied to the right rear wheel 91rr. Note that because the first braking system 71 is in a failed state, the MC pressure sensor SE7 has difficulty accurately detecting the hydraulic pressure actually generated by the master cylinder 83.

The detected yaw rate value Yrs of the vehicle 90 shown in Figure 3 will now be described. Because the left front wheel braking force BFfl is greater than the right rear wheel braking force BFrr, the yaw rate actually acting on the vehicle 90 indicates a leftward turn, i.e., a counterclockwise turn, as indicated by the solid arrow in Figure 3, as the detected yaw rate value Yrs. In other words, the vehicle 90 will be deflected to the left.

The calculated yaw rate Yrw for the vehicle 90 shown in FIG. 3 will be described. As described above, braking force is applied to the right rear wheel 91rr, while no braking force is applied to the left rear wheel 91rl. Therefore, as shown by the solid line in FIG. 4(a), the wheel speed VWrr of the right rear wheel 91rr is smaller than the wheel speed VWrl of the left rear wheel 91rl, as shown by the dashed line. In other words, a magnitude relationship exists between the wheel speeds VWrr and VWrl of the right rear wheel 91rr and the left rear wheel 91rl. The calculated yaw rate Yrw, calculated based on the difference between the wheel speeds VWrr and VWrl of the right rear wheel 91rr and the left rear wheel 91rl, is a negative value indicating a turn to the right, as indicated by the dashed arrow in FIG. 3. On the other hand, the detected yaw rate Yrs is a positive value indicating a turn to the left, as shown in FIG. 4(b).

At this time, the wheel speed VWrr of the right rear wheel 91rr is smaller than the wheel speed VWrl of the left rear wheel 91rl. Furthermore, the wheel speed of the left front wheel 91fl is smaller than the wheel speed of the right front wheel 91fr. Based on this wheel speed difference, the inter-system wheel speed difference determination unit 13 determines that a predetermined wheel speed difference exists between the wheel speeds of the front and rear wheels belonging to the second braking system 72 and the wheel speeds of the front and rear wheels belonging to the first braking system 71. In other words, the inter-system wheel speed difference determination unit 13 determines that the second braking system 72, to which the right rear wheel 91rr and the left front wheel 91fl belong, is in an inter-system wheel speed difference state. The situation in which the second braking system 72 is determined to be in an inter-system wheel speed difference state occurs when braking force is applied only to both wheels of the vehicle 90 belonging to the second braking system 72, as illustrated in FIG. 3 .

Returning to FIG. 2, in step S106, the failure diagnosis unit 14 sets the first system failure flag F1 ON. The initial value of the first system failure flag F1 is OFF. When the first system failure flag F1 is ON, the first system failure flag F1 is a flag indicating that the first brake system 71, which is equipped with the MC pressure sensor SE7, is in a failed state. In other words, the failure diagnosis unit 14 identifies the first brake system 71 as the failed system and sets the first system failure flag F1 ON. This is because, when the vehicle 90 exhibits behavior that would occur if the first brake system 71 were in a failed state, as illustrated in FIG. 3, a positive judgment is made in the processing of steps S102 to S105, and the processing proceeds to step S106. Note that when the first system failure flag F1 is set ON, the failure diagnosis unit 14 can be said to have identified the second brake system 72 as the remaining system. When the failure diagnosis unit 14 turns on the first system failure flag F1, it ends this processing routine.

In step S107, the malfunction diagnosis unit 14 determines whether the turning direction of the vehicle 90 indicated by the detected yaw rate value Yrs is opposite to the turning direction of the vehicle 90 indicated by the calculated yaw rate value Yrw. Specifically, it determines whether the detected yaw rate value Yrs is less than "0", i.e., a negative value, and the calculated yaw rate value Yrw is greater than "0", i.e., a positive value.

In the processing of step S107, if the detected yaw rate value Yrs is less than "0" and the calculated yaw rate value Yrw is greater than "0" (S107: YES), the malfunction diagnosis unit 14 proceeds to step S108. On the other hand, if the situation is different from when the detected yaw rate value Yrs is less than "0" and the calculated yaw rate value Yrw is greater than "0" (S107: NO), the malfunction diagnosis unit 14 proceeds to step S112.

When processing moves from step S102 to step S107, processing moves to step S112 in the following cases. That is, after a negative determination is made in step S102, a negative determination is also made in step S107. One example is when either the detected yaw rate value Yrs or the calculated yaw rate value Yrw is "0." Another example is when the turning direction of the vehicle 90 indicated by the detected yaw rate value Yrs and the turning direction of the vehicle 90 indicated by the calculated yaw rate value Yrw are the same.

In step S108, the malfunction diagnosis unit 14 determines whether the first brake system 71 is in a state where there is an inter-system wheel speed difference. That is, in step S108, the malfunction diagnosis unit 14 uses the inter-system wheel speed difference determination unit 13 to select the first brake system 71 as the target system. The inter-system wheel speed difference determination unit 13 determines whether a predetermined wheel speed difference exists between the front wheels and the rear wheels between the wheel speeds of the wheels belonging to the first brake system 71 selected as the target system and the wheel speeds of the wheels belonging to the second brake system 72, which is the non-target system.

As an example, the inter-system wheel speed difference determination unit 13 determines that the first brake system 71 is in a state where an inter-system wheel speed difference exists when the wheel speed of the left front wheel 91fl belonging to the second brake system 72 is greater than the value obtained by adding the wheel speed of the right front wheel 91fr belonging to the first brake system 71 to the front wheel speed difference threshold, and the wheel speed of the right rear wheel 91rr belonging to the second brake system 72 is greater than the value obtained by adding the wheel speed of the left rear wheel 91rl belonging to the first brake system 71 to the rear wheel speed difference threshold.

The inter-system wheel speed difference determination unit 13 does not determine that an inter-system wheel speed difference exists if there is no predetermined wheel speed difference between the wheel speed of the front wheel belonging to the target system and the wheel speed of the front wheel belonging to the non-target system. Therefore, if the wheel speed of the left front wheel 91fl belonging to the second braking system 72 is equal to or less than the value obtained by adding the wheel speed of the right front wheel 91fr belonging to the first braking system 71 to the front wheel speed difference threshold, it does not determine that an inter-system wheel speed difference exists. The inter-system wheel speed difference determination unit 13 does not determine that an inter-system wheel speed difference exists if there is no predetermined wheel speed difference between the wheel speed of the rear wheel belonging to the target system and the rear wheel belonging to the non-target system. Therefore, if the wheel speed of the right rear wheel 91rr belonging to the second braking system 72 is equal to or less than the value obtained by adding the wheel speed of the left rear wheel 91rl belonging to the first braking system 71 to the rear wheel speed difference threshold, it does not determine that an inter-system wheel speed difference exists.

If the first brake system 71 is in a state where there is a wheel speed difference between the systems (S108: YES), the malfunction diagnosis unit 14 proceeds to step S109. On the other hand, if the first brake system 71 is not in a state where there is a wheel speed difference between the systems (S108: NO), the malfunction diagnosis unit 14 proceeds to step S112.

In step S109, the malfunction diagnosis unit 14 determines whether the characteristic value PG is smaller than the second determination value KP2.
The characteristic value PG and the second determination value KP2 will be described. The characteristic value PG is a value that indicates the relationship between the vehicle deceleration G and the total braking force of the vehicle 90. The characteristic value PG can be calculated by dividing the vehicle deceleration G by the MC pressure Pmc. If the second braking system 72 has failed, the vehicle deceleration G will be smaller than the vehicle deceleration G when the second braking system 72 is not failed, assuming that the MC pressure Pmc is the same value. In other words, if the second braking system 72 has failed, the characteristic value PG will be smaller than when the second braking system 72 is not failed. The second determination value KP2 is set as a value that indicates the possibility of a failure of the second braking system 72 when the characteristic value PG is smaller than the second determination value KP2. The second determination value KP2 is a value calculated in advance through experiments, etc.

In step S109, if the characteristic value PG is smaller than the second determination value KP2 (S109: YES), the malfunction diagnosis unit 14 proceeds to step S110. On the other hand, if the characteristic value PG is equal to or greater than the second determination value KP2 (S109: NO), the malfunction diagnosis unit 14 proceeds to step S112.

In step S110, the malfunction diagnosis unit 14 determines whether the detected yaw rate value Yrs is smaller than the second Yr threshold value KY2. The second Yr threshold value KY2 is a negative value. The second Yr threshold value KY2 is set as a threshold value for determining whether the detected yaw rate value Yrs indicates that the vehicle 90 is turning right. In other words, if the detected yaw rate value Yrs is smaller than the second Yr threshold value KY2, it indicates that the vehicle 90 is turning right. If the detected yaw rate value Yrs is smaller than the second Yr threshold value KY2 (S110: YES), the malfunction diagnosis unit 14 proceeds to step S111. On the other hand, if the detected yaw rate value Yrs is equal to or greater than the second Yr threshold value KY2 (S110: NO), the malfunction diagnosis unit 14 proceeds to step S112.

Here, unlike the example shown in Figure 3, we will describe a vehicle 90 in a state where the first braking system 71 is the remaining system and the second braking system 72 is the failed system. In this state, the relationship between the detected yaw rate value Yrs and the calculated yaw rate value Yrw is as follows: The detected yaw rate value Yrs is a negative value indicating a turn to the right, i.e., a clockwise turn, while the calculated yaw rate value Yrw is a positive value indicating a turn to the left. Furthermore, the wheel speeds of the left rear wheel 91rl and the right front wheel 91fr decrease. This determines that the first braking system 71, to which the left rear wheel 91rl and the right front wheel 91fr belong, is in a state where there is a wheel speed difference between the systems. This state is caused by braking force being applied only to the two wheels of the vehicle 90 that belong to the first braking system 71. Consequently, the characteristic value PG is smaller than when there is no single-system failure.

Returning to FIG. 2, in step S111, the failure diagnosis unit 14 sets the second system failure flag F2 ON. The second system failure flag F2 has an initial value of OFF. The second system failure flag F2 is a flag indicating that the second brake system 72 is in a failed state when the second system failure flag F2 is ON. In other words, the failure diagnosis unit 14 identifies the second brake system 72 as the failed system and sets the second system failure flag F2 ON. This is because, when the vehicle 90 exhibits behavior that would occur if the second brake system 72 were in a failed state, as described above, a positive judgment is made in the processing of steps S107 to S110, and the processing proceeds to step S111. Note that when the second system failure flag F2 is set ON, the failure diagnosis unit 14 can be said to have identified the first brake system 71 as the remaining system. When the failure diagnosis unit 14 turns on the second system failure flag F2, it ends this processing routine.

In step S112, the malfunction diagnosis unit 14 leaves the first system malfunction flag F1 and the second system malfunction flag F2 at their initial values of OFF. The malfunction diagnosis unit 14 then terminates this processing routine.

The relationship between the first Yr threshold value KY1 and the second Yr threshold value KY2 will now be described. As an example, the first Yr threshold value KY1 and the second Yr threshold value KY2 can be set so that the absolute value of the first Yr threshold value KY1 is smaller than the absolute value of the second Yr threshold value KY2. In this case, comparing the processing of step S105 with the processing of step S110, step S105 is more likely to make a positive determination even when the yaw rate detection value Yrs is close to "0." In other words, the processing of step S105 has a higher sensitivity for determining whether the vehicle 90 is turning than the processing of step S110. As a result, the sensitivity for determining that the first braking system 71 is in a malfunction state is higher. As another example, the absolute value of the first Yr threshold value KY1 and the absolute value of the second Yr threshold value KY2 may be equal.

In the above example, the malfunction diagnosis unit 14 causes the inter-system wheel speed difference determination unit 13 to select, as the target system, the braking system to which the inner rear wheels belong when it is assumed that the vehicle 90 is turning in the turning direction indicated by the calculated yaw rate value Yrw. In other words, the malfunction diagnosis unit 14 causes the inter-system wheel speed difference determination unit 13 to select, as the target system, the braking system to which the inner front wheels belong when it is assumed that the vehicle 90 is turning in the turning direction indicated by the detected yaw rate value Yrs. Therefore, in the processing of step S103, the second braking system 72 is selected as the target system as described above. In the processing of step S108, the first braking system 71 is selected as the target system as described above.

The failure diagnosis unit 14 may control the alarm device 93 when the first system failure flag F1 or the second system failure flag F2 is ON. In other words, the failure diagnosis unit 14 may control the alarm device 93 when one system is in a failure state, to notify the driver or the like that a braking system has failed.

Compensation Control
The compensation control executed by the braking control unit 15 in the case of a one-system failure will be described with reference to FIGS. 5 and 6. FIG.

Figure 5 shows an example of the processing performed by the brake control unit 15 when performing rear lift suppression control. The processing flow shown in Figure 5 is an example of a specific implementation of first compensation control when the first braking system 71 is the remaining system. This processing routine is repeatedly executed at predetermined intervals until the vehicle 90 comes to a stop.

When this processing routine starts, first in step S201, the brake control unit 15 determines whether the start conditions for rear lift suppression control are met. For example, the brake control unit 15 determines that the start conditions for rear lift suppression control are met when the MC pressure Pmc is greater than the third determination value KP3. The third determination value KP3 can be a value calculated in advance through experiments or the like as a value at which rear lift is likely to occur when the MC pressure Pmc is greater than the third determination value KP3.

If the start conditions for rear lift suppression control are not met (S201: NO), the braking control unit 15 temporarily terminates this processing routine. On the other hand, if the start conditions for rear lift suppression control are met (S201: YES), the braking control unit 15 proceeds to step S202.

In step S202, the braking control unit 15 determines whether the second system failure flag F2 is ON. If the second system failure flag F2 is ON, i.e., if the first braking system 71 is the remaining system and the second braking system 72 is the failed system (S202: YES), the braking control unit 15 proceeds to step S203. In step S203, the braking control unit 15 sets the second increase amount KF2b as the increase amount KF2. Then, the braking control unit 15 proceeds to step S204.

On the other hand, if the second system failure flag F2 is OFF in the processing of step S202 (S202: NO), the braking control unit 15 proceeds to step S206. In step S203, the braking control unit 15 sets the first increase amount KF2a as the increase amount KF2. That is, when the second braking system 72 is not in a failure state, the increase amount KF2 is set to the first increase amount KF2a. After setting the increase amount KF2, the braking control unit 15 proceeds to step S207.

The increase amount KF2 is a value that sets the magnitude of the increase in braking force per unit time when increasing braking force during rear lift suppression control. For example, the second increase amount KF2b is a value smaller than the first increase amount KF2a. In this case, if the second braking system 72 is in a failed state, the increase amount KF2 will be smaller than when the second braking system 72 is not in a failed state. As a result, the increase in braking force is suppressed when one system is in a failed state.

In step S204, the braking control unit 15 determines whether the processing of step S204 is being performed for the first time after the malfunction determination. If the processing of step S204 is being performed for the first time since the second system malfunction flag F2 was set ON, the braking control unit 15 determines that this is the first processing after the malfunction determination.

If this is the initial processing after the malfunction determination (S204: YES), the brake control unit 15 proceeds to step S205. In step S205, the brake control unit 15 reduces the front wheel target braking force BFfT by the initial reduction amount KF1. As a result, the braking force applied to the front wheels by the brake device 80 is reduced. Specifically, the braking force of the right front wheel 91fr, which is the wheel targeted by the first brake system 71, the remaining system, is reduced at this time.

The initial decrease amount KF1 may be set, for example, so that reducing the front wheel target braking force BFfT by the initial decrease amount KF1 reduces the vehicle body deceleration G of the vehicle 90 by a specified percentage compared to before the front wheel target braking force BFfT was reduced. The specified percentage can be approximately 60%. The upper limit of the specified percentage is, for example, 70%. The lower limit of the specified percentage is, for example, 50%. The braking control unit 15 can calculate the initial decrease amount KF1. As another example, the initial decrease amount KF1 may be a fixed value calculated in advance through experiments, etc.

The braking control unit 15 reduces the front wheel target braking force BFfT by the initial reduction amount KF1, and then ends this processing routine.
If the process in step S204 is not the first process after the malfunction determination, that is, if it is the second or subsequent process after the malfunction determination (S204: NO), the braking control unit 15 proceeds to step S207.

In step S207, the braking control unit 15 determines whether the vehicle body deceleration G is smaller than the value obtained by subtracting the sensitivity KG from the target deceleration GT. The target deceleration GT is a target value for the vehicle body deceleration G calculated to suppress the occurrence of rear lift. The sensitivity KG is a value that sets the hysteresis width. The sensitivity KG is set so that, when the vehicle body deceleration G is controlled to follow the target deceleration GT, the braking force does not increase and decrease more than necessary when the vehicle body deceleration G is near the target deceleration GT. The sensitivity KG is, for example, a value calculated in advance. The sensitivity KG can also be calculated to be a larger value as the target deceleration GT increases.

If the vehicle deceleration G is less than the value obtained by subtracting the sensitivity KG from the target deceleration GT (S207: YES), the brake control unit 15 proceeds to step S208. In step S208, the brake control unit 15 uses the increase amount KF2 set in step S203 or step S206 to increase the front wheel target braking force BFfT by the increase amount KF2. As a result, the braking force applied to the front wheels by the brake device 80 is increased. Specifically, the braking force of the right front wheel 91fr, which is the wheel targeted by the first brake system 71, the remaining system, is increased at this time. After increasing the front wheel target braking force BFfT by the increase amount KF2, the brake control unit 15 terminates this processing routine.

On the other hand, in the processing of step S207, if the vehicle body deceleration G is equal to or greater than the value obtained by subtracting the sensitivity KG from the target deceleration GT (S207: NO), the braking control unit 15 proceeds to step S209. In step S209, the braking control unit 15 determines whether the vehicle body deceleration G is greater than the target deceleration GT.

If the vehicle body deceleration G is greater than the target deceleration GT (S209: YES), the brake control unit 15 proceeds to step S210. In step S210, the brake control unit 15 reduces the front wheel target braking force BFfT by the reduction amount KF3. As a result, the braking force applied to the front wheels by the brake device 80 is reduced. Specifically, the braking force of the right front wheel 91fr, which is the wheel targeted by the first brake system 71, the remaining system, is reduced at this time. After reducing the front wheel target braking force BFfT by the reduction amount KF3, the brake control unit 15 terminates this processing routine.

If vehicle body deceleration G is equal to or less than target deceleration GT (S209: NO), the brake control unit 15 proceeds to step S211. In step S211, the brake control unit 15 maintains the front wheel target braking force BFfT without increasing or decreasing it. That is, if vehicle body deceleration G is equal to or greater than the value obtained by subtracting sensitivity KG from target deceleration GT and vehicle body deceleration G is equal to or less than target deceleration GT, the brake control unit 15 maintains the front wheel target braking force BFfT. Thereafter, the brake control unit 15 terminates this processing routine.

The processing flow shown in Figure 6 is an example of the second compensation control when the second braking system 72 is the remaining system. This processing routine is repeatedly executed at predetermined intervals until the vehicle 90 comes to a stop.

When this processing routine starts, first in step S301, the braking control unit 15 determines whether the first system failure flag F1 is ON. If the first system failure flag F1 is OFF (S301: NO), the braking control unit 15 temporarily terminates this processing routine.

On the other hand, if the first system failure flag F1 is ON, i.e., if the first brake system 71 is the failed system and the second brake system 72 is the remaining system (S301: YES), the brake control unit 15 proceeds to step S302.

In step S302, the braking control unit 15 determines whether ABS control is being performed. If ABS control is not being performed (S302: NO), the braking control unit 15 proceeds to step S304. In step S304, the braking control unit 15 performs constant pressure increase control. Thereafter, the braking control unit 15 terminates this processing routine.

Constant pressure increase control will now be described. Constant pressure increase control limits the rate of increase in braking force on the left front wheel 91fl, the wheel controlled by the second brake system 72 (the remaining system), to a specified gradient. In constant pressure increase control, the brake control unit 15 controls the brake device 80 so that the braking force on the left front wheel 91fl continues to increase at a constant gradient. The braking force on the left front wheel 91fl can be adjusted by controlling the third pressure increase valve 64c and the third pressure reduction valve 65c. For example, in constant pressure increase control, the brake control unit 15 repeatedly holds the third pressure increase valve 64c open for a predetermined valve opening time, and then holds the third pressure increase valve 64c closed for a predetermined holding time. The gradient of increase is set to prevent excessive braking force. It is preferable that the gradient of increase be large enough to minimize the yaw moment of the vehicle 90 in a one-system failure state when the driver steers the vehicle 90, whose braking force has been increased by the execution of constant pressure increase control.

If ABS control is being performed in the processing of step S302 (S302: YES), the braking control unit 15 proceeds to step S303. In step S303, the braking control unit 15 ends constant pressure increase control. Thereafter, the braking control unit 15 ends this processing routine. In other words, if ABS control is started while constant pressure increase control is being performed, the braking control unit 15 ends constant pressure increase control.

In the process of step S302, the braking control unit 15 may determine whether the conditions for starting ABS control are met. In this case, if the conditions for starting ABS control are met, the process may proceed to step S303, and if the conditions for starting ABS control are not met, the process may proceed to step S304.

<Action and Effects>
The operation and effects of this embodiment will be described.
The braking control device 10 can determine whether the turning direction indicated by the detected yaw rate value Yrs is opposite to the turning direction indicated by the calculated yaw rate value Yrw, thereby determining whether the vehicle 90 is in a one-system failure state. That is, the braking control device 10 can accurately determine whether the vehicle 90 is in a one-system failure state by detecting changes in the behavior of the vehicle 90 when the vehicle 90 is in a one-system failure state.

The brake control device 10 selects one of the first and second brake systems 71 and 72 as the target system. The conditions for determining a single-system failure include a state in which there is an inter-system wheel speed difference, where a predetermined wheel speed difference exists between the front and rear wheels in the target system and the front and rear wheels in the non-target system. This improves the accuracy of determining whether a single-system failure has occurred. In other words, it is possible to prevent a false determination that a single-system failure has occurred when a single-system failure is not occurring.

The first brake system 71 is a brake system capable of detecting the MC pressure Pmc. Therefore, by determining whether the MC pressure Pmc is detected, as in the processing of step S104, it is possible to ensure accuracy in determining whether a single-system failure has occurred. Therefore, in the brake control device 10, the first Yr threshold value KY1 and the second Yr threshold value KY2 are set so that the absolute value of the first Yr threshold value KY1 is smaller than the absolute value of the second Yr threshold value KY2. This increases the sensitivity with which a failure in the first brake system 71 is determined. In other words, if the first brake system 71 fails, it becomes possible to identify the failed system more quickly.

The brake control device 10, which can identify a faulty system in a vehicle equipped with a cross-connected brake system, is particularly useful for vehicles with a short wheelbase, such as small trucks, which are prone to poor straight-line stability.

The brake control device 10 can identify whether the failed system is a brake system capable of detecting the MC pressure Pmc, thereby executing compensation control according to the remaining system.
7 shows an example in which the rear lift suppression control is executed when the first brake system 71 is the remaining system. In the example shown in FIG. 7, the rear lift suppression control is started at timing t11. At timing t12, it is determined that one system is in a failure state, and furthermore, the second brake system 72 is identified as the failed system.

In Fig. 7(a), the target deceleration GT is indicated by a broken line, and the vehicle body deceleration G is indicated by a solid line.
In (b) of Fig. 7, the braking force requested by the driver of the vehicle 90 is indicated by a dashed line. The dashed line corresponds to the MC pressure Pmc. In (b) of Fig. 7, the front wheel target braking force BFfT is indicated by a solid line.

In the example shown in Figure 7, during the period from timing t11 to timing t12 before it is determined that one system is in a failure state, the front wheel target braking force BFfT is controlled so that the vehicle deceleration G, shown by the solid line, follows the target deceleration GT.

Since the second braking system 72 is the failed system, the processing of steps S203 to S205 shown in FIG. 5 is executed.
When the second braking system 72 is identified as the failed system at timing t12, the front wheel target braking force BFfT is reduced by the initial reduction amount KF1 (S205). Therefore, after timing t12, the front wheel target braking force BFfT is reduced significantly, as shown in FIG. 7B. The reduction in the front wheel braking force reduces the vehicle body deceleration G, as shown in FIG. 7A. This reduces the yaw moment that would otherwise be caused by the application of a large braking force in a one-system failure state. Thereafter, steps S207 to S211 are executed, and the front wheel target braking force BFfT is adjusted in accordance with the target deceleration GT and the vehicle body deceleration G. This ensures braking force while minimizing a reduction in the stability of the vehicle 90.

The second increase KF2b, which is set as the increase KF when one system fails, is a smaller value than the first increase KF2a (S203). As a result, in the rear lift suppression control when one system fails, the increase in front wheel braking force is suppressed. In other words, a sudden increase in front wheel braking force is suppressed.

The brake control device 10 can control the remaining system using the MC pressure Pmc when one system fails. This makes it possible to adjust the braking force while understanding the absolute magnitude of the braking force applied to the wheels. For example, it is possible not only to increase the front wheel braking force, but also to decrease it.

Figure 8 shows an example in which constant pressure increase control is performed when the second brake system 72 is the remaining system. In the example shown in Figure 8, at timing t21, it is determined that one system has failed, and the first brake system 71 is identified as the failed system. In other words, constant pressure increase control is initiated at timing t21. Furthermore, the ABS start condition is met at timing t22, and ABS control is performed from timing t22 onwards.

In Figure 8(a), the braking force requested by the driver of the vehicle 90 is shown by a dashed line. The dashed line corresponds to the MC pressure Pmc. In Figure 8(a), the front wheel target braking force BFfT is shown by a solid line.

In the example shown in FIG. 8, the first braking system 71 is the failed system, and therefore, as shown in FIG. 8(b), the MC pressure Pmc cannot be detected even though braking is in progress.
Constant pressure increase control is performed during the period from timing t21 when the second compensation control is initiated to timing t22 when the ABS control is initiated. That is, the increase gradient of the front wheel target braking force BFfT is limited. Therefore, as shown in FIG. 8A, the increase rate of the front wheel target braking force BFfT is controlled to be constant. This makes it possible to gradually increase the front wheel braking force while preventing the front wheel braking force from becoming excessively large, even in a state where the MC pressure Pmc cannot be detected. That is, it is possible to ensure braking force while preventing a decrease in the stability of the vehicle 90.

As illustrated in Figures 7 and 8, the brake control device 10 suppresses an increase in braking force through compensation control when one system fails. This makes it easier to stabilize the vehicle 90 when the driver of the vehicle 90 in a one-system failure state performs a steering operation.

(Example of change)
This embodiment can be modified as follows: This embodiment and the following modifications can be combined and implemented within the scope of technical compatibility.

- In the event of a one-system failure, the braking control unit 15 may control the booster valve in the failed system to close. This blocks the hydraulic fluid flow path. For example, if a failure occurs in the flow path near the wheel cylinder, hydraulic fluid may leak out. Closing the booster valve blocks the flow path of the failed system, preventing hydraulic fluid from leaking out.

- In the above embodiment, an example was shown in which the turning direction of the vehicle 90 indicated by the yaw rate detection value Yrs was identified in steps S105 and S110, and the failed system was identified based on the turning direction. Specifically, if the turning direction of the vehicle 90 indicated by the yaw rate detection value Yrs is to the left, the first brake system 71 was identified as the failed system, and the second brake system 72 was identified as the remaining system. Furthermore, if the turning direction of the vehicle 90 indicated by the yaw rate detection value Yrs is to the right, the first brake system 71 was identified as the remaining system, and the second brake system 72 was identified as the failed system.

Alternatively, the failed system can be identified based on the turning direction of the vehicle 90 indicated by the calculated yaw rate value Yrw. In this case, the failed system is identified by taking into account that, in a one-system failure state as described above, the turning direction indicated by the detected yaw rate value Yrs is opposite to the turning direction indicated by the calculated yaw rate value Yrw. That is, if the turning direction indicated by the calculated yaw rate value Yrw is leftward, the second brake system 72 is identified as the failed system. In this case, the first brake system 71 is also identified as the remaining system. If the turning direction indicated by the calculated yaw rate value Yrw is rightward, the first brake system 71 is identified as the failed system. In this case, the second brake system 72 is also identified as the remaining system.

- In the above embodiment, an example was shown in which the calculated yaw rate value Yrw was calculated and used in the malfunction determination process, but calculating the calculated yaw rate value Yrw is not essential. In the malfunction determination process, instead of making a determination based on the turning direction of the vehicle 90 indicated by the calculated yaw rate value Yrw, a determination may be made based on the turning direction of the vehicle 90 estimated from the wheel speed. For example, the turning direction of the vehicle 90 may be estimated based on the magnitude relationship between the wheel speed VWrr of the right rear wheel 91rr and the wheel speed VWrl of the left rear wheel 91rl. Specifically, if the wheel speed VWrr of the right rear wheel 91rr is smaller than the wheel speed VWrl of the left rear wheel 91rl, the turning direction of the vehicle 90 estimated from the wheel speed is rightward. If the wheel speed VWrr of the right rear wheel 91rr is greater than the wheel speed VWrl of the left rear wheel 91rl, the turning direction of the vehicle 90 estimated from the wheel speed is leftward.

The failure determination process shown in Figure 2 is an example. For example, one or more of steps S103 to S105 and steps S108 to S110 in Figure 2 may be omitted. For example, for a vehicle 90 in a one-system failure state, the turning direction indicated by the detected yaw rate value Yrs is opposite to the turning direction indicated by the calculated yaw rate value Yrw. If the turning direction indicated by the detected yaw rate value Yrs is opposite to the turning direction indicated by the calculated yaw rate value Yrw, it can be determined that the vehicle is in a one-system failure state.

- In the above embodiment, an example of identifying a failed system is shown. Identifying a failed system is not essential. The failure diagnosis unit 14 may perform processing to determine whether or not a single system has failed. For example, the failure diagnosis unit 14 may perform processing to determine that a single system has failed when the turning direction indicated by the detected yaw rate value Yrs and the turning direction indicated by the calculated yaw rate value Yrw are opposite directions.

- In the above embodiment, the front wheel speed difference threshold may be a different value depending on whether the front wheels are drive wheels or driven wheels. As an example, the front wheel speed difference threshold when the front wheels are drive wheels may be set to a larger value than the front wheel speed difference threshold when the front wheels are driven wheels. The wheel speed of the drive wheels is affected by engine braking, regenerative braking, etc. With the above configuration, it is possible to suppress erroneous determinations caused by the influence of engine braking, regenerative braking, etc. In other words, it is possible to suppress erroneous determinations of whether a predetermined wheel speed difference has occurred. As with the front wheel speed difference threshold, a different value may be used for the rear wheel speed difference threshold depending on whether the rear wheels are drive wheels or driven wheels.

- The braking control device 10, which is a processing circuit, may have any of the following configurations [a] to [c]. [a] A circuit comprising one or more processors that execute various processes according to a computer program. The processor comprises a processing device. Examples of the processing device include a CPU, DSP, and GPU. The processor comprises memory. Examples of the memory include RAM, ROM, and flash memory. The memory stores program code or instructions configured to cause the processing device to execute the processes. Memory, i.e., computer-readable medium, includes any available medium that can be accessed by a general-purpose or special-purpose computer. [b] A circuit comprising one or more hardware circuits that execute various processes. Examples of hardware circuits include an ASIC (Application Specific Integrated Circuit), a CPLD (Complex Programmable Logic Device), and an FPGA (Field Programmable Gate Array). [c] A circuit comprising a processor that executes some of the various processes according to a computer program, and hardware circuits that execute the remaining processes.

Some of the functions implemented by the brake control device 10 may be implemented by another processing circuit connected to the brake control device 10 .
In the above embodiment, the MC pressure sensor SE7 is provided in the first brake system 71. However, the MC pressure sensor SE7 may be provided in the second brake system 72. Furthermore, the brake system equipped with the MC pressure sensor SE7 is not limited to only one of the brake systems. There is no limitation to providing the MC pressure sensor SE7 in both the first brake system 71 and the second brake system 72.

- In the above embodiment, a hydraulic braking device was used as an example of the braking device 80. The braking device to which the brake control device 10 is applied is not limited to a hydraulic braking device, and may be any braking device with cross-connected braking systems. For example, the braking device may be a mechanical friction braking device that presses a friction material against a rotating body by mechanically transmitting the drive force of an electric motor. Specifically, it is sufficient that the circuit that supplies power to the braking mechanism that adjusts the braking force applied to the right front wheel 91fr and the left rear wheel 91rl and the circuit that supplies power to the braking mechanism that adjusts the braking force applied to the left front wheel 91fl and the right rear wheel 91rr are independent. A braking device configured in this way can also be said to be a braking device with cross-connected braking systems.

(technical thought)
The technical ideas that can be understood from the above-described embodiment and modified examples will be described.
[A] A braking control device applied to a braking device of a vehicle having a first braking system that adjusts braking forces applied to a right front wheel and a left rear wheel among the wheels, and a second braking system that adjusts braking forces applied to a left front wheel and a right rear wheel among the wheels,
a wheel speed detection unit that detects the speed of each wheel as a wheel speed;
an inter-system wheel speed difference determination unit that selects one of the first and second brake systems as a target system, determines whether the target system is in an inter-system wheel speed difference state when a predetermined wheel speed difference occurs between the wheel speeds of the front and rear wheels belonging to the target system and the wheel speeds of the front and rear wheels belonging to a brake system other than the target system; and
a failure determination unit that determines whether or not one of the first braking system and the second braking system is in a failure state when braking the vehicle, and determines that a single system failure state exists when one of the first braking system and the second braking system is in a failure state;
The failure determination unit determines that one system is in a failure state when the target system is in the state where there is a wheel speed difference between the systems.

[B] a determination unit that, when the one-system failure state occurs, identifies one of the first brake system and the second brake system that is in a failed state as the failed system, and identifies one of the first brake system and the second brake system that is not in a failed state as the remaining system,
The identification unit
A braking control device as described in [A], in which, when the target system is in a state where there is a wheel speed difference between the systems, the target system is the remaining system, and a braking system other than the target system is identified as the failed system.

REFERENCE SIGNS LIST 10...Brake control device 11...Acquisition unit 12...Calculation unit 13...Inter-system wheel speed difference determination unit 14...Fault diagnosis unit 15...Brake control unit 61...First hydraulic pressure circuit 62...Second hydraulic pressure circuit 70...Brake actuator 71...First braking system 72...Second braking system 80...Brake device 81...Hydraulic pressure generator 83...Master cylinder 84a-84d...First to fourth braking mechanisms 90...Vehicle 91fl...Left front wheel 91fr...Right front wheel 91rl...Left rear wheel 91rr...Right rear wheel SE1-SE4...First to fourth wheel speed sensors SE5...Yaw rate sensor SE6...G sensor SE7...MC pressure sensor

Claims (8)

A braking control device applied to a braking device of a vehicle having a first braking system that adjusts braking forces applied to a right front wheel and a left rear wheel among the wheels, and a second braking system that adjusts braking forces applied to a left front wheel and a right rear wheel among the wheels,
a wheel speed detection unit that detects the speed of each wheel as a wheel speed;
a yaw rate detection unit that detects a yaw rate that actually acts on the vehicle as a yaw rate detection value;
a failure determination unit that determines whether or not one of the first braking system and the second braking system is in a failure state based on the wheel speed and the detected yaw rate when braking the vehicle, and determines that a single system failure has occurred when one of the first braking system and the second braking system is in a failure state ;
a yaw rate calculation unit that calculates, as a yaw rate calculation value, a yaw rate based on a speed difference between the wheel speed of the left rear wheel and the wheel speed of the right rear wheel, based on the wheel speed of the left rear wheel and the wheel speed of the right rear wheel,
The failure determination unit determines that the one-system failure has occurred when the turning direction of the vehicle indicated by the yaw rate detection value is different from the turning direction of the vehicle indicated by the yaw rate calculation value.
Braking control device. 2. The brake control device according to claim 1, wherein the conditions under which the failure determination unit determines that the one-system failure state exists include a state in which the wheel speed of a front wheel to which braking force is applied by one of the first and second brake systems is lower than the wheel speed of a front wheel to which braking force is applied by the other brake system, and a state in which the wheel speed of a rear wheel to which braking force is applied by the one brake system is lower than the wheel speed of a rear wheel to which braking force is applied by the other brake system. an identification unit that, when the one-system failure state occurs, identifies one of the first brake system and the second brake system that is in a failed state as a failed system, and identifies one of the first brake system and the second brake system that is not in a failed state as a remaining system;
The identification unit
When the turning direction of the vehicle indicated by the yaw rate detection value is a leftward direction, the first braking system is identified as the failed system and the second braking system is identified as the remaining system,
2. The brake control device according to claim 1 , wherein when the turning direction of the vehicle indicated by the yaw rate detection value is a rightward direction, the first brake system is identified as the remaining system and the second brake system is identified as the failed system. an identification unit that, when the one-system failure state occurs, identifies one of the first brake system and the second brake system that is in a failed state as a failed system, and identifies one of the first brake system and the second brake system that is not in a failed state as a remaining system;
The identification unit
When the turning direction of the vehicle indicated by the calculated yaw rate is a leftward direction, the first braking system is identified as the remaining system and the second braking system is identified as the failed system,
2. The brake control device according to claim 1 , wherein when the turning direction of the vehicle indicated by the calculated yaw rate is to the right, the first brake system is identified as the failed system and the second brake system is identified as the remaining system. an identification unit that, when the one-system failure state occurs, identifies one of the first brake system and the second brake system that is in a failed state as a failed system, and identifies one of the first brake system and the second brake system that is not in a failed state as a remaining system;
2. The brake control device according to claim 1, further comprising: a compensation control unit that, when the one-system failure state occurs, stabilizes the vehicle by performing control corresponding to the remaining system depending on which of the first brake system and the second brake system is the remaining system. the braking device is a hydraulic braking device that has a hydraulic pressure generating device and applies braking force in accordance with the hydraulic pressure generated by the hydraulic pressure generating device, and a hydraulic pressure detecting device capable of detecting the hydraulic pressure is provided in only one of the first braking system and the second braking system,
The brake control device according to claim 3 , wherein the identifying unit identifies whether the brake system capable of detecting the hydraulic pressure is the failed system or the remaining system. 7. The brake control device according to claim 6, further comprising a compensation control unit that, when the brake system capable of detecting the hydraulic pressure is not the remaining system, suppresses an increase in braking force by controlling the remaining system so that the braking force is increased by limiting the rate at which the braking force increases to a specified gradient . 7. The brake control device according to claim 6, further comprising a compensation control unit that, when the brake system capable of detecting the hydraulic pressure is the remaining system, sets a target deceleration as a target value of vehicle body deceleration based on the hydraulic pressure, and controls the remaining system based on the target deceleration and the vehicle body deceleration, thereby suppressing an increase in braking force.