Braking system

Abstract

A braking system 20 comprises: a pressure regulation unit 50 that generates a reference liquid pressure and also generates a rear wheel liquid pressure in accordance with the reference liquid pressure; a front wheel liquid pressure adjustment part 74 configured so as to be able to set the front wheel liquid pressure lower than the rear wheel liquid pressure when the reference liquid pressure is generated; and a control device 80 that controls a regeneration device 90, the pressure regulation unit 50, and the front wheel liquid pressure adjustment part 74. The control device 80 executes, when a vehicle braking force request value is reduced under a situation where the vehicle braking force request value is equal to or more than a first braking force, a first reduction process for reducing a regenerative braking force applied to front wheels FL, FR and holding the front wheel liquid pressure while reducing the rear wheel liquid pressure by reducing the reference liquid pressure.

Description

Braking system

[0001] The present invention relates to a braking system that is applied to a vehicle equipped with a regenerative device that applies regenerative braking force to the front wheels.

[0002] Patent Document 1 discloses a braking system for use in a vehicle equipped with a regenerative device capable of applying regenerative braking force to the front wheels. The braking system includes a pressure regulating unit that generates a reference hydraulic pressure and a front wheel hydraulic pressure adjusting unit that adjusts the front wheel hydraulic pressure, which is the hydraulic pressure in the front wheel cylinder, when the reference hydraulic pressure is generated. When the hydraulic pressure in the rear wheel wheel cylinder is set to the rear wheel hydraulic pressure, the braking system can generate rear wheel hydraulic pressure corresponding to the reference hydraulic pressure. Furthermore, the braking system can lower the front wheel hydraulic pressure below the rear wheel hydraulic pressure by operating the front wheel hydraulic pressure adjusting unit when the reference hydraulic pressure is generated.

[0003] When the regenerative braking force cannot be made equal to or greater than the vehicle braking force demand value during vehicle braking, the control device of the braking system operates the pressure regulating unit to implement regenerative cooperative control that increases at least the rear wheel hydraulic pressure out of the front wheel hydraulic pressure and the rear wheel hydraulic pressure. The vehicle braking force demand value is a demand value for braking force to be applied to the vehicle.

[0004] Japanese Patent Application Laid-Open No. 2019-182278

[0005] In the above-described braking system, when regenerative braking force is applied to the front wheels and frictional braking force is applied to both the front and rear wheels, the rear wheel hydraulic pressure is higher than the front wheel hydraulic pressure. If the vehicle braking force requirement value is reduced in this state, the control device reduces the reference hydraulic pressure. Depending on the configuration of the pressure regulating unit, if the reference hydraulic pressure is reduced when the rear wheel hydraulic pressure is higher than the front wheel hydraulic pressure, the front wheel hydraulic pressure may not be reduced. In this case, the braking force applied to the rear wheels is reduced while the sum of the braking forces applied to the front wheels is maintained. If only the braking force applied to the rear wheels is reduced in this way, the vehicle may exhibit a tendency to significantly understeer.

[0006] A braking system for solving the above problem is applied to a vehicle including front wheel cylinders and rear wheel cylinders, front wheels to which a frictional braking force corresponding to the front wheel hydraulic pressure which is the hydraulic pressure in the front wheel cylinders is applied, rear wheels to which a frictional braking force corresponding to the rear wheel hydraulic pressure which is the hydraulic pressure in the rear wheel cylinders is applied, and a regenerative device that applies regenerative braking force to the front wheels, and controls a vehicle braking force that is the braking force applied to the vehicle. The braking system includes a pressure regulating unit that generates a reference hydraulic pressure and generates the rear wheel hydraulic pressure corresponding to the reference hydraulic pressure, a front wheel hydraulic pressure adjusting unit that is configured to make the front wheel hydraulic pressure less than the rear wheel hydraulic pressure when the reference hydraulic pressure is generated, and a control device that controls the regenerative device, the pressure regulating unit, and the front wheel hydraulic pressure adjusting unit based on a vehicle braking force required value that is a required value for the vehicle braking force. The control device controls the regenerative device, the pressure regulating unit and the front wheel hydraulic pressure adjusting unit so that, when the vehicle braking force required value is equal to or greater than a first braking force, a regenerative braking force is applied to the front wheels and a frictional braking force is applied to both the front wheels and the rear wheels, and when the vehicle braking force required value is reduced while the vehicle braking force required value is equal to or greater than the first braking force, the control device reduces the regenerative braking force applied to the front wheels and executes a first reduction process to maintain the front wheel hydraulic pressure while reducing the rear wheel hydraulic pressure by reducing the reference hydraulic pressure.

[0007] The braking system described above has the effect of being able to prevent the vehicle from exhibiting a tendency to understeer significantly when the vehicle braking force is reduced while regenerative braking force is being applied to the front wheels.

[0008] FIG. 1 is a schematic diagram showing a vehicle equipped with a braking system of a first embodiment. FIG. 2 is a schematic diagram showing a brake actuator provided in the braking system of the first embodiment. FIG. 3 is a flowchart showing a first half of regenerative cooperative control executed by a control device provided in the braking system of the first embodiment. FIG. 4 is a flowchart showing a second half of regenerative cooperative control executed by a control device provided in the braking system of the first embodiment. FIG. 5 is a timing chart during vehicle braking in the braking system of the first embodiment. FIG. 6 is a graph showing transitions of state points indicated by front wheel braking force and rear wheel braking force in the braking system of the first embodiment. FIG. 7 is a flowchart showing a portion of regenerative cooperative control executed by a control device provided in a braking system of a second embodiment. FIG. 8 is a timing chart during vehicle braking in the braking system of the second embodiment. FIG. 9 is a graph showing transitions of state points indicated by front wheel braking force and rear wheel braking force in the braking system of the second embodiment.

[0009] First Embodiment A first embodiment of a braking system mounted on a vehicle will be described below with reference to FIGS. 1 to 6. FIG.

[0010] 1 illustrates a plurality of wheels, a plurality of friction brakes 10, a regenerative device 90, and a braking system 20. The plurality of wheels includes two front wheels FL, FR and two rear wheels RL, RR.

[0011] <Friction Brake> One friction brake 10 is provided for each wheel. Each of the multiple friction brakes 10 has a wheel cylinder 11 to which brake fluid is supplied, a rotating body 12 that rotates integrally with the wheel, and a friction material 13 that is pressed against the rotating body 12. The friction brake 10 is configured so that the greater the hydraulic pressure in the wheel cylinder 11, the more strongly the friction material 13 can be pressed against the rotating body 12. The friction brake 10 applies a friction braking force to the wheel that corresponds to the hydraulic pressure in the wheel cylinder 11.

[0012] The wheel cylinder 11 provided in the friction brake 10 for the front wheels FL, FR corresponds to the "front wheel cylinder." The wheel cylinder 11 provided in the friction brake 10 for the rear wheels RL, RR corresponds to the "rear wheel cylinder." The hydraulic pressure in the wheel cylinder 11 corresponding to the front wheel cylinder is called the "front wheel hydraulic pressure PwcF." The hydraulic pressure in the wheel cylinder 11 corresponding to the rear wheel cylinder is called the "rear wheel hydraulic pressure PwcR." A frictional braking force corresponding to the front wheel hydraulic pressure PwcF is applied to the front wheels FL, FR. A frictional braking force corresponding to the rear wheel hydraulic pressure PwcR is applied to the rear wheels RL, RR. The frictional braking force applied to the front wheels FL, FR is called the "front wheel frictional braking force FxMF." The frictional braking force applied to the rear wheels RL, RR is called the "rear wheel frictional braking force FxMR."

[0013] <Regenerative Device> The regenerative device 90 includes a motor generator 91 for the front wheels FL, FR, and a regenerative control unit 92 that controls the motor generator 91. When the motor generator 91 functions as an electric motor, driving force is transmitted from the motor generator 91 to the front wheels FL, FR. On the other hand, when the motor generator 91 functions as a generator, a regenerative braking force FxR corresponding to the amount of power generated by the motor generator 91 is applied to the front wheels FL, FR.

[0014] An example of the regeneration control unit 92 is an electronic control device. In this case, the regeneration control unit 92 has a CPU and a memory. The memory stores a control program executed by the CPU. The CPU executes the control program, allowing the regeneration control unit 92 to control the motor generator 91.

[0015] The regenerative control unit 92 is configured to be able to send and receive various information to and from a control device 80 of the braking system 20, which will be described later in detail. As will be described later in detail, when braking the vehicle, the regenerative control unit 92 adjusts the regenerative braking force FxR by sending and receiving information to and from the control device 80.

[0016] When the braking force applied to the vehicle is defined as "vehicle braking force Fx," the sum of the regenerative braking force FxR, the front wheel friction braking force FxMF, and the rear wheel friction braking force FxMR corresponds to the vehicle braking force Fx. In other words, the total sum of the braking forces applied to the multiple wheels FL, FR, RL, and RR is the vehicle braking force Fx.

[0017] <Brake System> The brake system 20 applies frictional braking force to the multiple wheels FL, FR, RL, and RR by adjusting the hydraulic pressure in the multiple wheel cylinders 11. The brake system 20 includes a brake operating member 21, a hydraulic pressure generator 22, a brake actuator 23, and an atmospheric pressure reservoir 24. The brake operating member 21 is a member that can be operated by the driver of the vehicle. An example of the brake operating member 21 is a brake pedal. The atmospheric pressure reservoir 24 stores brake fluid, and the interior of the atmospheric pressure reservoir 24 is open to the atmosphere.

[0018] The hydraulic pressure generating device 22 is configured to be able to generate hydraulic pressure according to the amount of operation of the brake operating member 21. The hydraulic pressure generating device 22 includes a master device 30 and a pressure adjusting unit 50. The master device 30 can supply brake fluid to the brake actuator 23. The pressure adjusting unit 50 can supply brake fluid to both the master device 30 and the brake actuator 23.

[0019] <Master Device> The master device 30 includes a master cylinder 31, a stroke simulator 32, a plurality of flow paths 331, 332, and 333 connected to the master cylinder 31, and a plurality of control valves 341 and 342 that control the flow of brake fluid. The stroke simulator 32 can generate a reaction force according to the amount of operation of the brake operating member 21.

[0020] The master cylinder 31 includes a main cylinder 41 and a cover cylinder 42. The master cylinder 31 includes a master piston 43 and an input piston 44. The master cylinder 31 includes a master spring 45 that biases the master piston 43, and an input spring 46 that biases the input piston 44. The master piston 43 and the input piston 44 can move relative to the main cylinder 41 and the cover cylinder 42.

[0021] The main cylinder 41 of the master cylinder 31 has a plate-shaped bottom wall 411 and a first peripheral wall 412 extending from the bottom wall 411 along the axis of the bottom wall 411. The main cylinder 41 further has a second peripheral wall 413 extending from the rear end of the first peripheral wall 412 along the axis of the first peripheral wall 412, and a first annular wall 414 extending from the rear end of the second peripheral wall 413 toward the axis of the second peripheral wall 413. Each of the first peripheral wall 412 and the second peripheral wall 413 is cylindrical. A hole is formed in the first annular wall 414 into which the rear end of the master piston 43 (described later) is inserted. The inner diameter of the first peripheral wall 412 is smaller than the inner diameter of the second peripheral wall 413.

[0022] Within the main cylinder 41, a master chamber Rm is defined by a bottom wall 411, a first peripheral wall 412, and a master piston 43. Hereinafter, in the master cylinder 31, the leftward direction in FIG. 1, i.e., the movement direction of the master piston 43 that reduces the volume of the master chamber Rm, will be referred to as the "forward direction." On the other hand, the opposite direction to the forward direction will be referred to as the "rearward direction." The rearward direction is also the movement direction of the master piston 43 that increases the volume of the master chamber Rm.

[0023] Within the main cylinder 41, a first fluid chamber R1 is defined by the second peripheral wall 413 and the master piston 43, and a servo chamber Rs is defined by the second peripheral wall 413, the first annular wall 414, and the master piston 43. The master chamber Rm is formed at a position near the front end of the master cylinder 31. The first fluid chamber R1 is formed rearward of the master chamber Rm. The servo chamber Rs is formed rearward of the first fluid chamber R1. Within the main cylinder 41, the master chamber Rm, the first fluid chamber R1, and the servo chamber Rs are not connected to one another.

[0024] The cover cylinder 42 of the master cylinder 31 has a cylindrical third peripheral wall 421 and a second annular wall 422 extending from the rear end of the third peripheral wall 421 toward the axis of the third peripheral wall 421. The third peripheral wall 421 is attached to the first annular wall 414 so that its axis coincides with that of the second peripheral wall 413 of the main cylinder 41. The second annular wall 422 has a hole into which the rear end of the input piston 44 (described later) is inserted.

[0025] Within the cover cylinder 42, a second fluid chamber R2 is defined by the first annular wall 414, the third peripheral wall 421, and the second annular wall 422 of the main cylinder 41. In the master cylinder 31, the second fluid chamber R2 is formed rearward of the servo chamber Rs.

[0026] The master piston 43 is housed in the master cylinder 31. When the master piston 43 moves in the axial direction, the master piston 43 moves forward and rearward along the inner circumferential surfaces of the first circumferential wall 412, the second circumferential wall 413, and the first annular wall 414 of the main cylinder 41. The rear end of the master piston 43 protrudes rearward beyond the first annular wall 414 and is located in the second fluid chamber R2.

[0027] The input piston 44 is disposed rearward of the master piston 43 within the master cylinder 31. The input piston 44 moves forward and rearward along the inner circumferential surfaces of the third circumferential wall 421 and the second annular wall 422 of the cover cylinder 42. The brake operating member 21 is connected to the rear end of the input piston 44. Therefore, the input piston 44 moves forward, that is, in a direction approaching the master piston 43, in accordance with the amount of operation of the brake operating member 21. Furthermore, a gap is formed between the input piston 44 and the master piston 43 in the second fluid chamber R2.

[0028] The master spring 45 is disposed in the master chamber Rm of the main cylinder 41. The master spring 45 biases the master piston 43 rearward. Therefore, when the master piston 43 moves forward, the master spring 45 is elastically compressed.

[0029] The input spring 46 is disposed in the second fluid chamber R2 of the cover cylinder 42. The input spring 46 biases the input piston 44 rearward. Therefore, when the input piston 44 moves forward, the input spring 46 is elastically compressed.

[0030] In the master cylinder 31, the master chamber Rm is connected to the atmospheric pressure reservoir 24. More specifically, a portion of the master chamber Rm near the rear end is connected to the atmospheric pressure reservoir 24 via a port formed in the first peripheral wall 412 of the main cylinder 41. Therefore, when the master piston 43 moves forward from the initial position shown in FIG. 1 , the connection between the master chamber Rm and the atmospheric pressure reservoir 24 is interrupted. As a result, the hydraulic pressure in the master chamber Rm increases as the master piston 43 moves forward. For example, when the hydraulic pressure in the servo chamber Rs increases, the hydraulic pressure in the servo chamber Rs moves the master piston 43 forward. This causes brake fluid to flow out of the master chamber Rm, increasing the hydraulic pressure in the master chamber Rm. On the other hand, when the hydraulic pressure in the servo chamber Rs decreases, the hydraulic pressure in the master chamber Rm moves the master piston 43 rearward. This causes brake fluid to flow into the master chamber Rm, decreasing the hydraulic pressure in the master chamber Rm.

[0031] The first flow path 331 connects the master chamber Rm and the brake actuator 23. More specifically, the first flow path 331 is connected to the master chamber Rm and a second hydraulic circuit 612 (described later) of the brake actuator 23. The second flow path 332 connects the first fluid chamber R1 and the second fluid chamber R2. The third flow path 333 connects the atmospheric pressure reservoir 24 and the second flow path 332.

[0032] The first control valve 341 is a normally closed solenoid valve. The second control valve 342 is a normally open solenoid valve. The first control valve 341 is disposed in the second flow path 332, between the connection point with the third flow path 333 and the second fluid chamber R2. The second control valve 342 is provided in the third flow path 333. When the control device 80 of the braking system 20 is operating, the first control valve 341 is opened, while the second control valve 342 is closed.

[0033] The stroke simulator 32 is disposed in the second flow path 332 between the first fluid chamber R1 and the first control valve 341. <Pressure Adjusting Unit> The pressure adjusting unit 50 includes an electric cylinder 51. The pressure adjusting unit 50 can adjust the front wheel hydraulic pressure PwcF and the rear wheel hydraulic pressure PwcR by operating the electric cylinder 51.

[0034] The pressure regulating unit 50 has a fourth flow path 54, a fifth flow path 55, and a sixth flow path 58 as flow paths for brake fluid. The fourth flow path 54 is connected to an input port 515 of the electric cylinder 51 and the atmospheric pressure reservoir 24. The fifth flow path 55 is connected to a servo chamber Rs of the master cylinder 31 and an output port 516 of the electric cylinder 51. The sixth flow path 58 is connected to a first hydraulic pressure circuit 611 of the brake actuator 23 (described later) and to the fifth flow path 55. Therefore, the electric cylinder 51 can supply brake fluid discharged from the output port 516 to both the servo chamber Rs and the first hydraulic pressure circuit 611 of the brake actuator 23.

[0035] When the hydraulic pressure in the fifth flow path 55 is referred to as the "servo pressure," this servo pressure corresponds to the "reference hydraulic pressure" generated by the pressure adjustment unit 50. The flow path connecting the pressure adjustment unit 50 and the servo chamber Rs is referred to as the "first supply fluid path," and the flow path branching off from the first supply fluid path is referred to as the "second supply fluid path." In this case, the fifth flow path 55 corresponds to the first supply fluid path, and the sixth flow path 58 corresponds to the second supply fluid path.

[0036] The electric cylinder 51 includes a cylinder 511, a piston 512, a first electric motor 513, and a conversion mechanism 514. The piston 512 is slidably provided within the cylinder 511. The first electric motor 513 is a power source for the electric cylinder 51. The conversion mechanism 514 converts the rotational motion of the output shaft of the first electric motor 513 into linear motion of the piston 512.

[0037] A hydraulic pressure chamber Re, into which brake fluid is introduced, is defined inside the cylinder 511 by the peripheral wall of the cylinder 511 and the piston 512. The position of the piston 512 inside the cylinder 511 can be changed by driving the first electric motor 513. Hereinafter, the movement direction of the piston 512 that reduces the volume of the hydraulic pressure chamber Re will be referred to as the "forward direction Za," while the direction opposite to the forward direction Za will be referred to as the "reverse direction Zb." The reverse direction Zb is also the movement direction of the piston 512 that increases the volume of the hydraulic pressure chamber Re.

[0038] An input port 515 and an output port 516 are formed in the peripheral wall of the cylinder 511 as ports connecting the hydraulic chamber Re with the outside. A through hole 517 is formed in the piston 512. The position of the piston 512 furthest in the backward direction Zb is the most retracted position. The through hole 517 is formed in a position that allows communication between the input port 515 and the hydraulic chamber Re when the piston 512 is in the most retracted position. As a result, when the piston 512 is in the most retracted position, the hydraulic chamber Re of the cylinder 511 is in communication with the atmospheric pressure reservoir 24 via the through hole 517, the input port 515, and the fourth flow path 54. The input port 515 is open when the piston 512 is in the most retracted position, and is closed by the piston 512 when the piston 512 moves in the forward direction Za from the most retracted position. Even after the input port 515 is closed by the piston 512, if the piston 512 moves forward in the Za direction, the hydraulic pressure in the hydraulic chamber Re increases.

[0039] The output port 516 is connected to the master cylinder 31 and the sixth flow path 58 via the fifth flow path 55. The output port 516 is always open regardless of the position of the piston 512. Therefore, when the input port 515 is blocked by the piston 512, the brake fluid in the hydraulic chamber Re is discharged from the output port 516 to the fifth flow path 55 when the piston 512 moves forward in the Za direction.

[0040] 2, the brake actuator 23 is configured to be able to individually adjust the hydraulic pressures PwcF, PwcR of the plurality of wheel cylinders 11. The brake actuator 23 can increase the hydraulic pressures PwcF, PwcR of the wheel cylinders 11 without increasing the hydraulic pressure regulated by the pressure regulating unit 50. In other words, the braking system 20 has a redundant configuration in which the pressure regulating unit 50 is located upstream and the brake actuator 23 is located downstream.

[0041] The brake actuator 23 has a first hydraulic pressure circuit 611 and a second hydraulic pressure circuit 612. Two wheel cylinders 11 for the rear wheels RL and RR are connected to the first hydraulic pressure circuit 611. Two wheel cylinders 11 for the front wheels FL and FR are connected to the second hydraulic pressure circuit 612.

[0042] The first hydraulic circuit 611 has a first connecting fluid line 621 and a first differential pressure control valve 631 provided in the first connecting fluid line 621. The first connecting fluid line 621 is a brake fluid line connecting the two wheel cylinders 11 for the rear wheels RL and RR to the sixth flow path 58. The first differential pressure control valve 631 is a normally open linear solenoid valve. The first differential pressure control valve 631 can adjust the differential pressure between the portion of the first connecting fluid line 621 on the sixth flow path 58 side and the portion on the wheel cylinder 11 side. For example, the first differential pressure control valve 631 can generate a larger differential pressure as the current flowing through its solenoid increases.

[0043] The portion of the first connecting fluid passage 621 closer to the wheel cylinder 11 than the first differential pressure control valve 631 branches into two passages 62a and 62b. The passage 62a is connected to the wheel cylinder 11 for the rear-left wheel RL, while the passage 62b is connected to the wheel cylinder 11 for the rear-right wheel RR. A pressure retention valve 641 is installed in each of the two passages 62a and 62b. That is, the pressure retention valve 641 is located in the portion of the first connecting fluid passage 621 closer to the wheel cylinder 11 than the first differential pressure control valve 631. The pressure retention valve 641 is a normally-open linear solenoid valve that adjusts the pressure difference between the portion of the passage 62a or 62b closer to the first differential pressure control valve 631 and the portion of the passage 62a or 62b closer to the wheel cylinder 11. The pressure retention valve 641 can generate a larger pressure difference as the current flowing through its solenoid increases.

[0044] A bypass flow path 721, which is a flow path that bypasses the pressure retention valve 641, is connected to each of the multiple paths 62a, 62b. A first end of the bypass flow path 721 is connected to a portion of each of the paths 62a, 62b that is closer to the first differential pressure control valve 631 than the pressure retention valve 641. A second end of the bypass flow path 721 is connected to a portion of each of the paths 62a, 62b that is closer to the wheel cylinder 11 than the pressure retention valve 641. A check valve 731 is provided in each of the multiple bypass flow paths 721. That is, the check valve 731 is arranged in parallel with the pressure retention valve 641.

[0045] An example of the check valve 731 is a one-way valve. The check valve 731 closes when the hydraulic pressure on the first differential pressure control valve 631 side of the holding valve 641 is equal to or greater than the rear wheel hydraulic pressure PwcR. On the other hand, the check valve 731 opens when the hydraulic pressure on the first differential pressure control valve 631 side of the holding valve 641 is less than the rear wheel hydraulic pressure PwcR. When the check valve 731 opens in this way, the check valve 731 allows the flow of brake fluid from the wheel cylinder 11 to the first differential pressure control valve 631 side.

[0046] The first hydraulic circuit 611 has a first pressure-reducing reservoir 651 that stores brake fluid, and a first pressure-reducing fluid passage 661 that is connected to the first pressure-reducing reservoir 651. The first pressure-reducing fluid passage 661 is a brake fluid passage that connects the first pressure-reducing reservoir 651 to the portions of the passages 62a and 62b that are closer to the wheel cylinder 11 than the holding valve 641. A first pressure-reducing valve 671, which is a normally-closed solenoid valve, is installed in each of the portions of the first pressure-reducing fluid passage 661 that are connected to the passage 62a and the passage 62b.

[0047] The first hydraulic circuit 611 has a first pump 681 powered by the second electric motor 69, a first return fluid path 701, and a first flow control unit 711. The first pump 681 pumps up the brake fluid in the first reduced-pressure reservoir 651 and discharges the brake fluid to a portion of the first connecting fluid path 621 between the first differential pressure control valve 631 and the holding valve 641. The first return fluid path 701 is a brake fluid path that is connected to the first reduced-pressure reservoir 651 and a portion of the first connecting fluid path 621 that is closer to the sixth flow path 58 than the first differential pressure control valve 631.

[0048] The first flow control unit 711 controls the flow of brake fluid through the first return fluid path 701. An example of the first flow control unit 711 is a one-way valve. The second hydraulic circuit 612 has a second connecting fluid path 622 and a second differential pressure control valve 632 provided in the second connecting fluid path 622. The second connecting fluid path 622 is a brake fluid path connecting the two wheel cylinders 11 for the front wheels FL and FR to the first flow path 331. The second differential pressure control valve 632 is a normally open linear solenoid valve. The second differential pressure control valve 632 can adjust the differential pressure between the portion of the second connecting fluid path 622 on the first flow path 331 side and the portion on the wheel cylinder 11 side. For example, the second differential pressure control valve 632 can generate a larger differential pressure as the current flowing through its solenoid increases.

[0049] The portion of the second connecting fluid path 622 closer to the wheel cylinder 11 than the second differential pressure control valve 632 branches into two paths 62c and 62d. Path 62c is connected to the wheel cylinder 11 for the front left wheel FL, while path 62d is connected to the wheel cylinder 11 for the front right wheel FR. A pressure holding valve 642 is installed in each of the two paths 62c and 62d. The pressure holding valve 642 is a normally open linear solenoid valve that adjusts the pressure difference between the portion of the path 62c or 62d on the second differential pressure control valve 632 side and the portion on the wheel cylinder 11 side. The pressure holding valve 642 can generate a larger pressure difference as the current flowing through its solenoid increases.

[0050] A bypass flow path 722, which is a flow path that bypasses the hold valve 642, is connected to each of the multiple paths 62c, 62d. A first end of the bypass flow path 722 is connected to a portion of each of the paths 62c, 62d that is closer to the second differential pressure control valve 632 than the hold valve 642. A second end of the bypass flow path 722 is connected to a portion of each of the paths 62c, 62d that is closer to the wheel cylinder 11 than the hold valve 642. A check valve 732 is installed in each of the multiple bypass flow paths 722. That is, the check valve 732 is arranged in parallel with the hold valve 642.

[0051] An example of the check valve 732 is a one-way valve. The check valve 732 closes when the hydraulic pressure on the second differential pressure control valve 632 side of the holding valve 642 is equal to or greater than the front wheel hydraulic pressure PwcF. On the other hand, the check valve 732 opens when the hydraulic pressure on the second differential pressure control valve 632 side of the holding valve 642 is less than the front wheel hydraulic pressure PwcF. When the check valve 732 opens, the check valve 732 allows the flow of brake fluid from the wheel cylinder 11 to the second differential pressure control valve 632 side.

[0052] As described above, the plurality of holding valves 642 are normally-open linear solenoid valves. The second hydraulic circuit 612 is connected to the master chamber Rm of the master unit 30 via the first flow path 331. Therefore, by adjusting the magnitude of the current flowing through the solenoids of the plurality of holding valves 642, the front wheel hydraulic pressure PwcF becomes lower than the servo pressure in the fifth flow path 55, more specifically, the hydraulic pressure in the master chamber Rm. As will be described in detail later, the servo pressure corresponds to the rear wheel hydraulic pressure PwcR. Therefore, by operating the holding valves 642 when servo pressure is generated, the front wheel hydraulic pressure PwcF becomes lower than the rear wheel hydraulic pressure PwcR. Therefore, the plurality of holding valves 642 and the plurality of check valves 732 constitute an example of a "front wheel hydraulic pressure adjustment unit 74" configured to make the front wheel hydraulic pressure PwcF lower than the rear wheel hydraulic pressure PwcR when servo pressure is generated.

[0053] The second hydraulic circuit 612 has a second pressure-reducing reservoir 652 that stores brake fluid, and a second pressure-reducing fluid path 662 that is connected to the second pressure-reducing reservoir 652. The second pressure-reducing fluid path 662 is a brake fluid path that connects the second pressure-reducing reservoir 652 to the portions of paths 62c and 62d that are closer to the wheel cylinder 11 than the holding valve 642. A second pressure-reducing valve 672, which is a normally-closed solenoid valve, is installed in each of the portions of the second pressure-reducing fluid path 662 that are connected to path 62c and the portion of the second pressure-reducing fluid path 662 that are connected to path 62d.

[0054] The second hydraulic circuit 612 has a second pump 682 powered by the second electric motor 69, a second return fluid path 702, and a second flow control unit 712. The second pump 682 pumps up the brake fluid from the second reduced-pressure reservoir 652 and discharges the brake fluid to a portion of the second connecting fluid path 622 between the second differential pressure control valve 632 and the holding valve 642. The second return fluid path 702 is a brake fluid path that is connected to the second reduced-pressure reservoir 652 and a portion of the second connecting fluid path 622 that is closer to the first flow path 331 than the second differential pressure control valve 632.

[0055] The second flow control unit 712 controls the flow of brake fluid through the second return fluid line 702. An example of the second flow control unit 712 is a one-way valve. <Detection System of Brake System> As shown in FIG. 1 , the detection system of the brake system has a plurality of sensors that output signals according to the detection results to the control device 80. The plurality of sensors include a plurality of hydraulic pressure sensors and a stroke sensor SE1. The plurality of hydraulic pressure sensors include a master pressure sensor 351, an input pressure sensor 352, and a servo pressure sensor 353.

[0056] The master pressure sensor 351 detects the hydraulic pressure in the master chamber Rm. For example, the master pressure sensor 351 is provided in the first flow path 331. The hydraulic pressure in the master chamber Rm based on the detection signal of the master pressure sensor 351 is referred to as the "master pressure."

[0057] The input pressure sensor 352 detects the fluid pressure in the second fluid chamber R2. For example, the input pressure sensor 352 is connected to a position in the second flow path 332 between the first control valve 341 and the second fluid chamber R2. The fluid pressure in the second fluid chamber R2 based on the detection signal of the input pressure sensor 352 is referred to as the "input fluid pressure."

[0058] The servo pressure sensor 353 detects the servo pressure, which is the hydraulic pressure of the brake fluid discharged from the electric cylinder 51 to the sixth flow path 58. For example, the servo pressure sensor 353 is provided in a portion of the fifth flow path 55 between the connection portion with the sixth flow path 58 and the output port 516. The discharge hydraulic pressure of the electric cylinder 51 based on the detection signal of the servo pressure sensor 353 is referred to as the "servo pressure Psc."

[0059] The stroke sensor SE1 detects the amount of operation of the brake operating member 21. <Control Device> As shown in FIG. 1 , the control device 80 is configured to be able to communicate with the regeneration control unit 92 via an in-vehicle network. The control device 80 includes a processing circuit 81. An example of the processing circuit 81 is an electronic control device. In this case, the processing circuit 81 includes a CPU 82 and a memory 83. The memory 83 stores a control program executed by the CPU 82. The CPU 82 executes the control program, causing the processing circuit 81 to operate the braking system 20.

[0060] <Regenerative Cooperative Control> The regenerative cooperative control executed by the processing circuit 81 of the control device 80 when braking the vehicle will be described with reference to Figures 3 and 4. The regenerative cooperative control is braking control that adjusts the regenerative braking force FxR, the front wheel friction braking force FxMF, and the rear wheel friction braking force FxMR. When a braking request is made, the processing circuit 81 repeatedly executes the regenerative cooperative control at predetermined control intervals.

[0061] In step S11, the processing circuit 81 derives a vehicle braking force request value FxTr, which is a request value for the vehicle braking force Fx. For example, when the driver is operating the brake operating member 21, the processing circuit 81 derives the vehicle braking force request value FxTr so that the greater the amount of operation of the brake operating member 21, the greater the value of the vehicle braking force request value FxTr. Furthermore, when a deceleration request is received from another on-board control device, the processing circuit 81 sets a value corresponding to the deceleration request as the vehicle braking force request value FxTr.

[0062] In the next step S13, the processing circuit 81 determines whether the vehicle braking force request value FxTr is increasing. For example, if the latest value of the vehicle braking force request value FxTr is greater than the previous value of the vehicle braking force request value FxTr, it can be assumed that the vehicle braking force request value FxTr is increasing. Also, for example, if the latest value of the vehicle braking force request value FxTr is equal to or less than the previous value of the vehicle braking force request value FxTr, it can be assumed that the vehicle braking force request value FxTr is not increasing. The latest value of the vehicle braking force request value FxTr is the vehicle braking force request value FxTr derived in the current control cycle. The previous value of the vehicle braking force request value FxTr is the vehicle braking force request value FxTr derived in the previous control cycle. If the processing circuit 81 determines that the vehicle braking force request value FxTr is increasing (S13: YES), it proceeds to step S15. On the other hand, when the processing circuit 81 determines that the vehicle braking force request value FxTr has not increased (S13: NO), the processing circuit 81 proceeds to step S31.

[0063] In step S15, the processing circuit 81 determines whether the vehicle braking force request value FxTr is less than the regenerative braking force upper limit value FxRL. The regenerative braking force upper limit value FxRL is the upper limit of the regenerative braking force FxR that the regenerative device 90 can apply to the vehicle. If the vehicle braking force request value FxTr is less than the regenerative braking force upper limit value FxRL (S15: YES), the processing circuit 81 proceeds to step S17. On the other hand, if the vehicle braking force request value FxTr is equal to or greater than the regenerative braking force upper limit value FxRL (S15: NO), the processing circuit 81 proceeds to step S19.

[0064] In step S17, the processing circuit 81 executes a first increase process. In the first increase process, the processing circuit 81 increases the regenerative braking force FxR in response to an increase in the vehicle braking force request value FxTr, but does not increase the front wheel hydraulic pressure PwcF or the rear wheel hydraulic pressure PwcR.

[0065] Specifically, in step S171, the processing circuit 81 sets the vehicle braking force request value FxTr to the regenerative braking force target value FxRTr. The regenerative braking force target value FxRTr is a target value of the regenerative braking force FxR to be applied to the front wheels FL, FR. In the following step S173, the processing circuit 81 transmits the regenerative braking force target value FxRTr to the regenerative control unit 92 of the regenerative device 90.

[0066] When the regenerative control unit 92 receives the regenerative braking force target value FxRTr, it controls the motor generator 91 to generate power based on the regenerative braking force target value FxRTr. This allows the regenerative control unit 92 to increase the regenerative braking force FxR applied to the front wheels FL, FR in accordance with an increase in the regenerative braking force target value FxRTr.

[0067] In the next step S175, the processing circuit 81 sets the front wheel hydraulic pressure target value PwcFTr and the rear wheel hydraulic pressure target value PwcRTr to 0 (zero). The front wheel hydraulic pressure target value PwcFTr is the target value for the front wheel hydraulic pressure PwcF. The rear wheel hydraulic pressure target value PwcRTr is the target value for the rear wheel hydraulic pressure PwcR.

[0068] In step S177, the processing circuit 81 operates the braking system 20 based on the front wheel hydraulic pressure target value PwcFTr and the rear wheel hydraulic pressure target value PwcRTr. Specifically, the processing circuit 81 operates the electric cylinder 51 of the pressure regulating unit 50 based on the front wheel hydraulic pressure target value PwcFTr and the rear wheel hydraulic pressure target value PwcRTr. In this case, the processing circuit 81 operates the electric cylinder 51 so that the servo pressure Psc becomes 0 (zero). Then, the processing circuit 81 ends the first increase process. Thereafter, the processing circuit 81 temporarily ends the regenerative cooperative control.

[0069] In step S19, the processing circuit 81 determines whether the vehicle braking force request value FxTr is less than the first braking force Fx1. A braking force greater than the regenerative braking force target value FxRTr is set as the first braking force Fx1.

[0070] The first braking force Fx1 will now be described with reference to FIG. 6 . The horizontal axis of FIG. 6 represents the front wheel braking force FxAF, which is the sum of the braking forces applied to the front wheels FL and FR. The vertical axis of FIG. 6 represents the rear wheel braking force FxAR, which is the sum of the braking forces applied to the rear wheels RL and RR. In this example, the front wheel braking force FxAF is equal to the sum of the front wheel friction braking force FxMF and the regenerative braking force FxR. The rear wheel braking force FxAR is equal to the rear wheel friction braking force FxMR. The dashed line in FIG. 6 represents the ideal braking force distribution line L1. The ideal braking force distribution line L1 represents the relationship between the front wheel braking force FxAF and the rear wheel braking force FxAR when the front wheels FL and FR and the rear wheels RL and RR simultaneously lock during vehicle braking. The dashed line in FIG. 6 represents the equal-pressure braking force distribution line L2. The equal pressure braking force distribution line L2 is a line showing the relationship between the front wheel braking force FxAF and the rear wheel braking force FxAR when the front wheel hydraulic pressure PwcF and the rear wheel hydraulic pressure PwcR are increased while maintaining the same pressure.

[0071] For example, the first braking force Fx1 is set to satisfy the following conditions (A1) and (A2): (A1) The front wheel braking force FxAF is equal to the regenerative braking force target value FxRTr at the time when the first increase process is switched to the second increase process.

[0072] (A2) In the graph shown in Fig. 6, the state point P, which is the point indicating the front wheel braking force FxAF and the rear wheel braking force FxAR, is a point on the equal-pressure braking force distribution line L2. Returning to Fig. 3, if the processing circuit 81 determines in step S19 that the vehicle braking force request value FxTr is less than the first braking force Fx1 (S19: YES), the processing circuit 81 proceeds to step S21. On the other hand, if the processing circuit 81 determines that the vehicle braking force request value FxTr is equal to or greater than the first braking force Fx1 (S19: NO), the processing circuit 81 proceeds to step S23.

[0073] In step S21, the processing circuit 81 executes a second increase process. In the second increase process, the processing circuit 81 increases the servo pressure Psc in response to an increase in the vehicle braking force request value FxTr, thereby increasing the rear wheel hydraulic pressure PwcR, while maintaining the front wheel hydraulic pressure PwcF and maintaining the regenerative braking force FxR.

[0074] More specifically, in step S211, the processing circuit 81 transmits an instruction to maintain the regenerative braking force FxR to the regenerative control unit 92 of the regenerative device 90. Upon receiving the instruction, the regenerative control unit 92 operates the motor generator 91 so as to maintain the regenerative braking force FxR.

[0075] In the next step S213, the processing circuit 81 increases the rear wheel hydraulic pressure target value PwcRTr and maintains the front wheel hydraulic pressure target value PwcFTr at 0. The processing circuit 81 sets the rear wheel hydraulic pressure PwcR when the rear wheel friction braking force FxMR is equal to the value obtained by subtracting the regenerative braking force FxR from the vehicle braking force request value FxTr as the rear wheel hydraulic pressure target value PwcRTr.

[0076] In step S215, the processing circuit 81 activates the brake system 20 based on the front wheel hydraulic pressure target value PwcFTr and the rear wheel hydraulic pressure target value PwcRTr. Specifically, the processing circuit 81 sets a hydraulic pressure corresponding to the rear wheel hydraulic pressure target value PwcRTr as the servo pressure target value PscTr, which is the target value of the servo pressure Psc. For example, the processing circuit 81 sets the servo pressure Psc for making the rear wheel hydraulic pressure PwcR equal to the rear wheel hydraulic pressure target value PwcRTr as the servo pressure target value PscTr. The processing circuit 81 activates the electric cylinder 51 based on this servo pressure target value PscTr. As a result, the processing circuit 81 increases the servo pressure Psc in response to an increase in the vehicle braking force request value FxTr, thereby increasing the rear wheel hydraulic pressure PwcR and the rear wheel frictional braking force FxMR.

[0077] Furthermore, the processing circuit 81 operates the front wheel hydraulic pressure adjustment unit 74 so that the front wheel hydraulic pressure PwcF does not increase even when the servo pressure Psc increases. For example, the processing circuit 81 maintains the front wheel hydraulic pressure PwcF by closing the plurality of maintenance valves 642 of the front wheel hydraulic pressure adjustment unit 74. In this case, the processing circuit 81 may increase the current flowing through the solenoid of the maintenance valve 642 as the pressure difference between the servo pressure Psc and the front wheel hydraulic pressure target value PwcFTr increases. After completing the second increase process, the processing circuit 81 temporarily terminates the regenerative cooperative control.

[0078] In step S23, the processing circuit 81 executes a third increase process. In the third increase process, the processing circuit 81 increases the servo pressure Psc in response to an increase in the vehicle braking force request value FxTr, thereby increasing the front wheel hydraulic pressure PwcF and the rear wheel hydraulic pressure PwcR while maintaining the regenerative braking force FxR.

[0079] More specifically, in step S231, the processing circuit 81 transmits an instruction to maintain the regenerative braking force FxR to the regenerative control unit 92 of the regenerative device 90, similar to step S211. In the following step S233, the processing circuit 81 increases the front wheel hydraulic pressure target value PwcFTr and the rear wheel hydraulic pressure target value PwcRTr. At this time, the processing circuit 81 increases the front wheel hydraulic pressure target value PwcFTr and the rear wheel hydraulic pressure target value PwcRTr so as to satisfy the following conditions (B1) and (B2).

[0080] (B1) The difference between the front wheel hydraulic pressure PwcF and the rear wheel hydraulic pressure PwcR at the start of the third increase process is maintained. (B2) The sum of the front wheel friction braking force FxMF, the rear wheel friction braking force FxMR, and the regenerative braking force FxR is equal to the vehicle braking force request value FxTr.

[0081] The difference between the rear wheel hydraulic pressure PwcR and the front wheel hydraulic pressure PwcF at the start of the third increase process is also referred to as the "reference hydraulic pressure difference ΔPwcB." In step S235, the processing circuit 81 activates the brake system 20 based on the front wheel hydraulic pressure target value PwcFTr and the rear wheel hydraulic pressure target value PwcRTr. For example, the processing circuit 81 sets the servo pressure Psc for equalizing the rear wheel hydraulic pressure PwcR with the rear wheel hydraulic pressure target value PwcRTr as the servo pressure target value PscTr. The processing circuit 81 activates the electric cylinder 51 based on this servo pressure target value PscTr. As a result, the processing circuit 81 increases the servo pressure Psc in response to an increase in the vehicle braking force request value FxTr, thereby increasing the rear wheel hydraulic pressure PwcR and the rear wheel frictional braking force FxMR.

[0082] The processing circuit 81 also operates the front wheel hydraulic pressure adjuster 74 so that the front wheel hydraulic pressure PwcF also increases in response to an increase in the servo pressure Psc. For example, the processing circuit 81 sets the magnitude of the current flowing through the solenoids of the plurality of holding valves 642 to a value corresponding to the reference hydraulic pressure difference ΔPwcB. This allows the processing circuit 81 to increase the front wheel hydraulic pressure PwcF and the front wheel friction braking force FxMF in response to an increase in the servo pressure Psc. The processing circuit 81 then ends the third increase process. Thereafter, the processing circuit 81 temporarily ends the regenerative cooperative control.

[0083] In step S31, the processing circuit 81 determines whether the vehicle braking force request value FxTr has decreased. For example, if the latest value of the vehicle braking force request value FxTr is smaller than the previous value of the vehicle braking force request value FxTr, the vehicle braking force request value FxTr can be considered to have decreased. Furthermore, if the latest value of the vehicle braking force request value FxTr is equal to or greater than the previous value of the vehicle braking force request value FxTr, the vehicle braking force request value FxTr can be considered not to have decreased. If the processing circuit 81 determines that the vehicle braking force request value FxTr has decreased (YES in S31), the processing proceeds to step S41 in FIG. 4. On the other hand, if the processing circuit 81 determines that the vehicle braking force request value FxTr has not decreased (NO in S31), the processing circuit 81 can determine that the vehicle braking force request value FxTr is being maintained, and therefore proceeds to step S33.

[0084] In step S33, similar to step S211, the processing circuit 81 transmits an instruction to the regenerative control unit 92 of the regenerative device 90 to maintain the regenerative braking force FxR. In the following step S35, the processing circuit 81 maintains the front wheel hydraulic pressure target value PwcFTr and the rear wheel hydraulic pressure target value PwcRTr. In the next step S37, the processing circuit 81 operates the braking system 20 based on the front wheel hydraulic pressure target value PwcFTr and the rear wheel hydraulic pressure target value PwcRTr. Specifically, the processing circuit 81 operates the electric cylinder 51 so as to maintain the servo pressure Psc. The processing circuit 81 operates the front wheel hydraulic pressure adjustment unit 74 so that the difference between the front wheel hydraulic pressure PwcF and the rear wheel hydraulic pressure PwcR is maintained at the reference hydraulic pressure difference ΔPwcB. The processing circuit 81 then temporarily terminates the regenerative cooperative control.

[0085] In step S41, the processing circuit 81 determines whether the vehicle braking force request value FxTr is equal to or greater than the first braking force Fx1. If the processing circuit 81 determines that the vehicle braking force request value FxTr is equal to or greater than the first braking force Fx1 (YES in step S41), the processing proceeds to step S43. On the other hand, if the processing circuit 81 determines that the vehicle braking force request value FxTr is less than the first braking force Fx1 (NO in step S41), the processing proceeds to step S45.

[0086] In step S43, the processing circuit 81 executes a first decrease process. In the first decrease process, the processing circuit 81 decreases the regenerative braking force FxR and decreases the servo pressure Psc to decrease the rear wheel hydraulic pressure PwcR while maintaining the front wheel hydraulic pressure PwcF.

[0087] More specifically, in step S431, the processing circuit 81 decreases the regenerative braking force target value FxRTr so as to satisfy the following conditions (C1) and (C2).

[0088] (C1) The sum of the regenerative braking force FxR, the front wheel friction braking force FxMF, and the rear wheel friction braking force FxMR is equal to the vehicle braking force request value FxTr. (C2) The distribution ratio of the front wheel braking force FxAF and the rear wheel braking force FxAR is maintained at the reference front / rear distribution ratio.

[0089] The reference front / rear distribution ratio is the distribution ratio between the front wheel braking force FxAF and the rear wheel braking force FxAR when the state point P is located on the equal-pressure braking force distribution line L2 in Fig. 6. In other words, when the distribution ratio between the front wheel braking force FxAF and the rear wheel braking force FxAR is the reference front / rear distribution ratio, the state point P is located on the equal-pressure braking force distribution line L2.

[0090] In the following step S433, the processing circuit 81 transmits the regenerative braking force target value FxRTr to the regenerative control unit 92. Upon receiving the regenerative braking force target value FxRTr, the regenerative control unit 92 causes the motor generator 91 to generate power based on the regenerative braking force target value FxRTr. As a result, the regenerative control unit 92 reduces the regenerative braking force FxR in accordance with the reduction in the regenerative braking force target value FxRTr.

[0091] In step S435, the processing circuit 81 decreases the rear wheel hydraulic pressure target value PwcRTr while maintaining the front wheel hydraulic pressure target value PwcFTr. The processing circuit 81 decreases the front wheel hydraulic pressure target value PwcFTr so as to satisfy the following condition (D1).

[0092] (D1) The rear wheel friction braking force target value FxMRTr is equal to the vehicle braking force request value FxTr minus the front wheel braking force FxAF. The rear wheel friction braking force target value FxMRTr is the target value of the rear wheel friction braking force FxMR.

[0093] In step S437, the processing circuit 81 operates the brake system 20 based on the front wheel hydraulic pressure target value PwcFTr and the rear wheel hydraulic pressure target value PwcRTr. For example, the processing circuit 81 sets the servo pressure Psc for making the rear wheel hydraulic pressure PwcR equal to the rear wheel hydraulic pressure target value PwcRTr as the servo pressure target value PscTr. The processing circuit 81 operates the electric cylinder 51 based on this servo pressure target value PscTr. As a result, the processing circuit 81 reduces the servo pressure Psc in response to a decrease in the vehicle braking force request value FxTr, thereby reducing the rear wheel hydraulic pressure PwcR and the rear wheel friction braking force FxMR.

[0094] Furthermore, the processing circuit 81 operates the front wheel hydraulic pressure adjustment unit 74 so that the front wheel hydraulic pressure PwcF does not decrease in response to a decrease in the servo pressure Psc as described above. For example, the processing circuit 81 maintains the magnitude of the current flowing through the solenoids of the plurality of holding valves 642. This keeps the plurality of holding valves 642 closed, allowing the processing circuit 81 to maintain the front wheel hydraulic pressure PwcF and the front wheel friction braking force FxMF even when the servo pressure Psc is decreased. The processing circuit 81 then ends the first decrease process. Thereafter, the processing circuit 81 temporarily ends the regenerative cooperative control.

[0095] In step S45, the processing circuit 81 determines whether the vehicle braking force request value FxTr is equal to or greater than the second braking force Fx2. A braking force smaller than the first braking force Fx1 is set as the second braking force Fx2. For example, the second braking force Fx2 is set to the vehicle braking force Fx when the rear wheel hydraulic pressure PwcR becomes equal to the front wheel hydraulic pressure PwcF when only the rear wheel friction braking force FxMR is reduced among the front wheel friction braking force FxMF, the rear wheel friction braking force FxMR, and the regenerative braking force FxR. In other words, the second braking force Fx2 is the vehicle braking force Fx when the rear wheel hydraulic pressure PwcR becomes equal to the front wheel hydraulic pressure PwcF when the rear wheel hydraulic pressure PwcR is reduced by executing a second reduction process described below. If the vehicle braking force request value FxTr is equal to or greater than the second braking force Fx2 (S45: YES), the processing circuit 81 proceeds to step S47. On the other hand, if the vehicle braking force request value FxTr is less than the second braking force Fx2 (S45: NO), the processing circuit 81 proceeds to step S49.

[0096] In step S47, the processing circuit 81 executes a second decrease process. In the second decrease process, the processing circuit 81 maintains the regenerative braking force FxR and maintains the front wheel hydraulic pressure PwcF while reducing the servo pressure Psc to reduce the rear wheel hydraulic pressure PwcR.

[0097] More specifically, in step S471, the processing circuit 81 transmits an instruction to the regenerative control unit 92 of the regenerative device 90 to maintain the regenerative braking force FxR, similar to step S211. In the following step S473, the processing circuit 81 decreases the rear wheel hydraulic pressure target value PwcRTr while maintaining the front wheel hydraulic pressure target value PwcFTr. At this time, the processing circuit 81 decreases the rear wheel hydraulic pressure target value PwcRTr so as to satisfy the following condition (E1):

[0098] (E1) The rear wheel friction braking force target value FxMRTr is equal to the vehicle braking force request value FxTr minus the front wheel braking force FxAF. In step S475, the processing circuit 81 activates the brake system 20 based on the front wheel hydraulic pressure target value PwcFTr and the rear wheel hydraulic pressure target value PwcRTr. For example, the processing circuit 81 sets the servo pressure Psc for making the rear wheel hydraulic pressure PwcR equal to the rear wheel hydraulic pressure target value PwcRTr as the servo pressure target value PscTr. The processing circuit 81 activates the electric cylinder 51 based on this servo pressure target value PscTr. As a result, the processing circuit 81 reduces the servo pressure Psc in response to a decrease in the vehicle braking force request value FxTr, thereby reducing the rear wheel hydraulic pressure PwcR and the rear wheel friction braking force FxMR.

[0099] The processing circuit 81 also operates the front wheel hydraulic pressure adjuster 74 so that the front wheel hydraulic pressure PwcF does not decrease in response to a decrease in the servo pressure Psc. For example, the processing circuit 81 maintains the magnitude of the current flowing through the solenoids of the plurality of holding valves 642. This keeps the plurality of holding valves 642 closed, allowing the processing circuit 81 to maintain the front wheel hydraulic pressure PwcF and the front wheel friction braking force FxMF even when the servo pressure Psc is decreased. The processing circuit 81 then ends the second decrease process. After that, the processing circuit 81 temporarily ends the regenerative cooperative control.

[0100] In step S49, the processing circuit 81 determines whether the vehicle braking force request value FxTr is equal to or greater than the third braking force Fx3. A vehicle braking force Fx smaller than the second braking force Fx2 is set as the third braking force Fx3. For example, the vehicle braking force Fx when the vehicle braking force request value FxTr is equal to the regenerative braking force upper limit value FxRL is set as the third braking force Fx3. In other words, the third braking force Fx3 is the vehicle braking force Fx when both the front wheel hydraulic pressure PwcF and the rear wheel hydraulic pressure PwcR become 0 (zero) by executing a third reduction process, which will be described later. If the vehicle braking force request value FxTr is equal to or greater than the third braking force Fx3 (S49: YES), the processing circuit 81 proceeds to step S51. On the other hand, if the vehicle braking force request value FxTr is less than the third braking force Fx3 (S49: NO), the processing circuit 81 proceeds to step S53.

[0101] In step S51, the processing circuit 81 executes a third decrease process. In the third decrease process, the processing circuit 81 maintains the regenerative braking force FxR and decreases both the front wheel hydraulic pressure PwcF and the rear wheel hydraulic pressure PwcR by decreasing the servo pressure Psc.

[0102] More specifically, in step S511, the processing circuit 81 transmits an instruction to maintain the regenerative braking force FxR to the regenerative control unit 92 of the regenerative device 90, similar to step S211. In the following step S513, the processing circuit 81 decreases the front wheel hydraulic pressure target value PwcFTr and the rear wheel hydraulic pressure target value PwcRTr. At this time, the processing circuit 81 decreases the front wheel hydraulic pressure target value PwcFTr and the rear wheel hydraulic pressure target value PwcRTr so as to satisfy the following conditions (F1) and (F2).

[0103] (F1) The rear wheel hydraulic pressure target value PwcRTr is equal to the front wheel hydraulic pressure target value PwcFTr. (F2) The sum of the front wheel friction braking force FxMF and the rear wheel friction braking force FxMR is equal to the vehicle braking force request value FxTr minus the regenerative braking force FxR.

[0104] In step S515, the processing circuit 81 operates the brake system 20 based on the front wheel hydraulic pressure target value PwcFTr and the rear wheel hydraulic pressure target value PwcRTr. For example, the processing circuit 81 sets the servo pressure Psc for making the rear wheel hydraulic pressure PwcR equal to the rear wheel hydraulic pressure target value PwcRTr as the servo pressure target value PscTr. The processing circuit 81 operates the electric cylinder 51 based on this servo pressure target value PscTr. As a result, the processing circuit 81 reduces the servo pressure Psc in response to a decrease in the vehicle braking force request value FxTr, thereby reducing the rear wheel hydraulic pressure PwcR and the rear wheel frictional braking force FxMR.

[0105] The front wheel hydraulic pressure adjusting unit 74 has a check valve 732 arranged in parallel with the holding valve 642. Therefore, when a decrease in servo pressure Psc causes the hydraulic pressure in the hydraulic passage between the holding valve 642 and the second differential pressure control valve 632 to fall below the front wheel hydraulic pressure PwcF, the check valve 732 opens. This allows the brake fluid in the wheel cylinder 11 corresponding to the front wheel cylinder to flow out via the check valve 732. As a result, the processing circuit 81 can reduce the front wheel hydraulic pressure PwcF and the front wheel frictional braking force FxMF by reducing the servo pressure Psc without changing the magnitude of the current flowing through the solenoid of the holding valve 642.

[0106] When the processing circuit 81 reduces the servo pressure Psc by the electric cylinder 51, the processing circuit 81 ends the third reduction process. Then, the processing circuit 81 temporarily terminates the regenerative cooperative control. In step S53, the processing circuit 81 executes a fourth reduction process to reduce the regenerative braking force FxR. More specifically, in step S531, the processing circuit 81 reduces the regenerative braking force target value FxRTr in accordance with the decrease in the vehicle braking force request value FxTr. In the following step S533, the processing circuit 81 transmits the regenerative braking force target value FxRTr to the regenerative control unit 92, similar to step S173 above. The processing circuit 81 ends the fourth reduction process. Then, the processing circuit 81 temporarily terminates the regenerative cooperative control.

[0107] When the regenerative control unit 92 receives the regenerative braking force target value FxRTr, it causes the motor generator 91 to generate power based on the regenerative braking force target value FxRTr. As a result, the regenerative control unit 92 reduces the regenerative braking force FxR in accordance with the reduction in the regenerative braking force target value FxRTr.

[0108] <Functions and Effects of the Present Embodiment> The functions and effects of the braking system 20 during vehicle braking will be described with reference to Figures 5 and 6. Figure 5A shows the trends in the vehicle braking force request value FxTr and the vehicle braking force Fx. Figure 5B shows the trends in the regenerative braking force target value FxRTr and the regenerative braking force FxR. Figure 5C shows the trends in the front wheel hydraulic pressure target value PwcFTr, the rear wheel hydraulic pressure target value PwcRTr, the front wheel hydraulic pressure PwcF, and the rear wheel hydraulic pressure PwcR.

[0109] Since a vehicle braking request is generated from timing T0, the processing circuit 81 of the control device 80 increases the vehicle braking force request value FxTr. During the period from timing T0 to timing T1, the vehicle braking force request value FxTr is equal to or less than the regenerative braking force upper limit value FxRL. When the vehicle braking force request value FxTr is increased while the vehicle braking force request value FxTr is smaller than the regenerative braking force upper limit value FxRL, the processing circuit 81 executes a first increase process. The first increase process increases the regenerative braking force FxR in response to an increase in the vehicle braking force request value FxTr, while not increasing the front wheel hydraulic pressure PwcF or the rear wheel hydraulic pressure PwcR. Therefore, during the first increase process, the processing circuit 81 increases the regenerative braking force target value FxRTr so that the regenerative braking force target value FxRTr remains equal to the vehicle braking force request value FxTr. When the control device 80 transmits the regenerative braking force target value FxRTr to the regenerative control unit 92, the regenerative device 90 applies a regenerative braking force FxR corresponding to the regenerative braking force target value FxRTr to the front wheels FL, FR.

[0110] Note that during the period from timing T0 to timing T1, the front wheel braking force FxAF is increased, but the rear wheel braking force FxAR is not increased. The state point P when no vehicle braking request is generated is referred to as the "initial state point P0." During the period from timing T0 to timing T1, as shown in FIG. 6, the state point P moves from the initial state point P0 in the direction of the first arrow Y1. The first arrow Y1 indicates the direction along the horizontal axis in FIG. 6 in which the front wheel braking force FxAF is increased.

[0111] During the period from timing T0 to timing T1, braking force is applied only to the front wheels FL, FR out of the front wheels FL, FR and the rear wheels RL, RR, but the vehicle braking force Fx is not so large. Therefore, even if only the front wheel braking force FxAF increases out of the front wheel braking force FxAF and the rear wheel braking force FxAR, the vehicle does not tend to understeer significantly.

[0112] Returning to FIG. 5 , even after timing T1 has passed, the processing circuit 81 continues to increase the vehicle braking force request value FxTr. At timing T1, the regenerative braking force target value FxRTr becomes equal to the regenerative braking force upper limit value FxRL. Therefore, at timing T1, the processing circuit 81 switches the processing from the first increase processing to the second increase processing. That is, when the vehicle braking force request value FxTr is increased under the condition that the regenerative braking force FxR has reached the regenerative braking force upper limit value FxRL, the processing circuit 81 executes the second increase processing. The second increase processing is a processing for maintaining the front wheel hydraulic pressure PwcF and the regenerative braking force FxR while increasing the rear wheel hydraulic pressure PwcR by increasing the servo pressure Psc in accordance with the increase in the vehicle braking force request value FxTr.

[0113] When the rear wheel hydraulic pressure PwcR increases, the rear wheel friction braking force FxMR increases. Meanwhile, the front wheel hydraulic pressure PwcF is maintained at 0 (zero) and the regenerative braking force FxR is maintained. Therefore, the rear wheel braking force FxAR increases while the front wheel braking force FxAF is maintained.

[0114] In FIG. 6 , the state point P at the end of the first increase process is referred to as the "first state point PA." In this case, when the second increase process is started, the state point P moves from the first state point PA in the direction of the second arrow Y2. The second arrow Y2 indicates the direction along the vertical axis in FIG. 6 in which the rear wheel braking force FxAR is increased. As shown in FIG. 6 , when the state point P moves in the direction of the second arrow Y2 as the vehicle braking force request value FxTr increases, the state point P approaches the equal-pressure braking force distribution line L2. This allows the brake system 20 to suppress the vehicle from exhibiting a tendency to significantly understeer due to an increase in the vehicle braking force Fx.

[0115] Returning to FIG. 5 , even after timing T2 has passed, the processing circuit 81 continues to increase the vehicle braking force request value FxTr. At timing T2, the vehicle braking force request value FxTr becomes equal to the first braking force Fx1. Therefore, at timing T2, the processing circuit 81 switches the processing from the second increase processing to the third increase processing. That is, when the vehicle braking force request value FxTr is increased under the circumstances where the regenerative braking force FxR has reached the regenerative braking force upper limit value FxRL and the vehicle braking force request value FxTr is equal to or greater than the first braking force Fx1, the processing circuit 81 executes the third increase processing. The third increase processing is a processing for maintaining the regenerative braking force FxR while increasing the front wheel hydraulic pressure PwcF and the rear wheel hydraulic pressure PwcR by increasing the servo pressure Psc in accordance with the increase in the vehicle braking force request value FxTr.

[0116] When the rear wheel hydraulic pressure PwcR increases, the rear wheel friction braking force FxMR increases. When the front wheel hydraulic pressure PwcF increases, the front wheel friction braking force FxMF increases. Therefore, even if the regenerative braking force FxR is maintained, both the front wheel braking force FxAF and the rear wheel braking force FxAR increase.

[0117] In Figure 6, the state point P at the end of the second increase process is referred to as the "second state point PB." The second state point PB is an operating point on the equal-pressure brake-force distribution line L2. When the third increase process is started, the state point P moves from the second state point PB in the direction of the third arrow Y3. The third arrow Y3 is a direction along the equal-pressure brake-force distribution line L2 that increases the rear wheel brake force FxAR. Therefore, when the state point P moves in the direction of the third arrow Y3 as the vehicle braking force request value FxTr increases, the state point P moves on the equal-pressure brake-force distribution line L2.

[0118] 5, at time T3, the processing circuit 81 starts to hold the vehicle braking force request value FxTr. Therefore, the processing circuit 81 holds the front wheel friction braking force FxMF, the rear wheel friction braking force FxMR, and the regenerative braking force FxR.

[0119] After that, at timing T4 onwards, the processing circuit 81 reduces the vehicle braking force request value FxTr. During the period from timing T4 to timing T5, the vehicle braking force request value FxTr is equal to or greater than the first braking force Fx1. Therefore, the processing circuit 81 executes the first reduction process. That is, when the vehicle braking force request value FxTr is reduced under conditions in which the vehicle braking force request value FxTr is equal to or greater than the first braking force Fx1, the processing circuit 81 executes the first reduction process. The first reduction process is a process in which the regenerative braking force FxR is reduced and the servo pressure Psc is reduced to reduce the rear wheel hydraulic pressure PwcR, while maintaining the front wheel hydraulic pressure PwcF.

[0120] When the rear wheel hydraulic pressure PwcR is reduced, the rear wheel friction braking force FxMR is reduced. When the front wheel hydraulic pressure PwcF is maintained, the front wheel friction braking force FxMF is also maintained. However, because the regenerative braking force FxR is reduced, both the front wheel braking force FxAF and the rear wheel braking force FxAR are reduced.

[0121] Consider the case where, from timing T4, a reduction process is executed to reduce the rear wheel braking force FxAR while maintaining the regenerative braking force FxR. In this case, the regenerative braking force FxR is not reduced, so the regenerative energy recovery efficiency is higher than when the first increase process is executed. However, because the proportion of the front wheel braking force FxAF in the vehicle braking force Fx increases, there is a risk that the vehicle will exhibit a significant tendency toward understeer.

[0122] That is, in FIG. 6 , the state point P immediately before the start of the decrease in the vehicle braking force request value FxTr is designated the "third state point PC." In this state, if the rear wheel friction braking force FxMR is decreased while the regenerative braking force FxR and the front wheel friction braking force FxMF are maintained, the state point P moves from the third state point PC in the direction of the dashed arrow YA. The dashed arrow YA indicates the direction along the vertical axis in FIG. 6 that decreases the rear wheel braking force FxAR. Then, as shown in FIG. 6 , when the vehicle braking force Fx is relatively large, the state point P deviates significantly from the equal-pressure braking force distribution line L2 in the direction that increases the braking force distribution between the front wheels FL and FR. Therefore, as the vehicle braking force Fx decreases, the vehicle may exhibit a tendency toward significant understeer. If the vehicle exhibits a tendency toward significant understeer during cornering, the driver, for example, increases the steering amount in order to maintain the vehicle's course.

[0123] In this regard, in the braking system 20, the processing circuit 81 starts the first decrease process from timing T4. When the first decrease process is executed, both the front wheel braking force FxAF and the rear wheel braking force FxAR are decreased as described above.

[0124] That is, in FIG. 6 , state point P moves from the third state point PC in the direction of the fourth arrow Y4. The fourth arrow Y4 is a direction along the equal-pressure brake-force distribution line L2 that reduces the rear wheel brake force FxAR. Therefore, when state point P moves in the direction of the fourth arrow Y4 as the vehicle brake force request value FxTr decreases, state point P moves on the equal-pressure brake-force distribution line L2. In other words, state point P is prevented from deviating from the equal-pressure brake-force distribution line L2 when the vehicle brake force Fx is relatively large. Therefore, the brake system 20 can prevent the vehicle from exhibiting a significant tendency to understeer when reducing the vehicle brake force Fx under conditions in which regenerative braking force FxR is applied to the front wheels FL, FR.

[0125] Returning to FIG. 5 , even after timing T5 has passed, the processing circuit 81 continues to reduce the vehicle braking force request value FxTr. At timing T5, the vehicle braking force request value FxTr becomes equal to the first braking force Fx1. Therefore, at timing T5, the processing circuit 81 switches the processing from the first reduction processing to the second reduction processing. That is, when the vehicle braking force request value FxTr is reduced under the condition that the vehicle braking force request value FxTr is less than the first braking force Fx1 but is equal to or greater than the second braking force Fx2, the processing circuit 81 executes the second reduction processing. The second reduction processing is a processing that maintains the regenerative braking force FxR and maintains the front wheel hydraulic pressure PwcF while reducing the rear wheel hydraulic pressure PwcR by reducing the servo pressure Psc.

[0126] When the rear wheel hydraulic pressure PwcR is reduced, the rear wheel friction braking force FxMR is reduced. When the front wheel hydraulic pressure PwcF is maintained, the front wheel friction braking force FxMF is maintained. In this case, since the regenerative braking force FxR is also maintained, the rear wheel braking force FxAR is reduced while the front wheel braking force FxAF is maintained.

[0127] The vehicle braking force request value FxTr at the end of the first reduction process is equal to the first braking force Fx1. Therefore, in Figure 6, the state point P at the end of the first reduction process is the second state point PB. When the second reduction process is started, the state point P moves from the second state point PB in the direction of the fifth arrow Y5. The fifth arrow Y5 indicates the direction along the vertical axis in Figure 6 in which the rear wheel braking force FxAR is reduced.

[0128] Here, when state point P moves in the direction of the fifth arrow Y5 as the vehicle braking force request value FxTr decreases, state point P deviates from equal-pressure braking force distribution line L2. However, at this point, vehicle braking force Fx is relatively small. Therefore, even if state point P deviates from equal-pressure braking force distribution line L2, the vehicle's tendency to understeer does not increase significantly. In other words, the braking system 20 can reduce vehicle braking force Fx while suppressing an increase in the vehicle's tendency to understeer.

[0129] Returning to FIG. 5 , even after timing T6 has passed, the processing circuit 81 continues to reduce the vehicle braking force request value FxTr. At timing T6, the vehicle braking force request value FxTr becomes equal to the second braking force Fx2. Also, at timing T6, the front wheel hydraulic pressure PwcF and the rear wheel hydraulic pressure PwcR become equal to each other, as shown in FIG. 5C . Therefore, at timing T6, the processing circuit 81 switches the processing from the second reduction processing to the third reduction processing. That is, when the vehicle braking force request value FxTr is reduced under the condition that it is less than the second braking force Fx2 but greater than or equal to the third braking force Fx3, the processing circuit 81 executes the third reduction processing. The third reduction processing is a processing in which the regenerative braking force FxR is maintained while reducing both the front wheel hydraulic pressure PwcF and the rear wheel hydraulic pressure PwcR by reducing the servo pressure Psc.

[0130] When the rear wheel hydraulic pressure PwcR is reduced, the rear wheel friction braking force FxMR is reduced. When the front wheel hydraulic pressure PwcF is reduced, the front wheel friction braking force FxMF is reduced. Therefore, even if the regenerative braking force FxR is maintained, both the front wheel braking force FxAF and the rear wheel braking force FxAR are reduced.

[0131] The front wheel hydraulic pressure adjusting section 74 of the braking system 20 includes a holding valve 642 and a check valve 732 arranged in parallel with the holding valve 642. The check valve 732 opens when the hydraulic pressure on the master unit 30 side of the holding valve 642 is less than the front wheel hydraulic pressure PwcF. When the check valve 732 opens, brake fluid flows from the wheel cylinders 11 for the front wheels FL and FR toward the master unit 30 via the check valve 732. Therefore, in the braking system 20, by reducing the servo pressure Psc through the operation of the pressure adjusting unit 50, both the front wheel hydraulic pressure PwcF and the rear wheel hydraulic pressure PwcR are reduced. This simplifies control compared to reducing the front wheel hydraulic pressure PwcF by reducing the magnitude of the current flowing through the solenoid of the holding valve 642.

[0132] 6, the state point P at the end of the second decrease process is the fourth state point PE. Therefore, when the third decrease process is started, the state point P moves from the fourth state point PE in the direction of the sixth arrow Y6. The sixth arrow Y6 indicates the direction in which both the front wheel braking force FxAF and the rear wheel braking force FxAR are reduced.

[0133] Returning to FIG. 5 , even after timing T7, the processing circuit 81 continues to reduce the vehicle braking force request value FxTr. At timing T7, the vehicle braking force request value FxTr becomes equal to the third braking force Fx3. In this case, as shown in FIG. 5C , both the front wheel hydraulic pressure PwcF and the rear wheel hydraulic pressure PwcR become 0 (zero). That is, because both the front wheel friction braking force FxMF and the rear wheel friction braking force FxMR are 0 (zero), the regenerative braking force FxR becomes equal to the vehicle braking force request value FxTr. Therefore, at timing T7, the processing circuit 81 shifts the processing from the third reduction processing to the fourth reduction processing. That is, when the vehicle braking force request value FxTr is reduced under conditions in which the vehicle braking force request value FxTr is less than the third braking force Fx3, the processing circuit 81 executes the fourth reduction processing. The fourth reduction processing is a process for reducing the regenerative braking force FxR. In the example shown in FIG. 5, the vehicle braking force request value FxTr becomes 0 (zero) at timing T8.

[0134] In Figure 6, the state point P at the end of the third decrease process is the fifth state point PF. The fifth state point PF is a point on the horizontal axis. Therefore, as the regenerative braking force target value FxRTr is decreased, the state point P moves from the fifth state point PF in the direction of the seventh arrow Y7. The seventh arrow Y7 indicates the direction along the horizontal axis in which the front wheel braking force FxAF is reduced. Then, the state point P returns to the initial state point P0.

[0135] Second Embodiment A second embodiment of the braking system will be described with reference to Figures 7 to 9. In the second embodiment, the upper limit of the regenerative power that the regenerative device can apply to the front wheels is large, so the regenerative cooperative control is partially different from that in the first embodiment. In the following description, differences from the first embodiment will be mainly described, and the same components as those in the first embodiment will be designated by the same reference numerals, and redundant description will be omitted.

[0136] <Regenerative Device> The regenerative device 90 of the vehicle is configured to apply a regenerative braking force FxR greater than the first braking force Fx1 to the front wheels FL, FR. In other words, the regenerative braking force upper limit value FxRL is greater than the first braking force Fx1.

[0137] <Regenerative Cooperative Control> The regenerative cooperative control executed in this embodiment will be described with reference to FIG. 7 . In step S11, the processing circuit 81 derives the vehicle braking force request value FxTr. In the next step S13, the processing circuit 81 determines whether the vehicle braking force request value FxTr is increasing. If the processing circuit 81 determines that the vehicle braking force request value FxTr is increasing (S13: YES), the processing circuit 81 proceeds to step S16. On the other hand, if the processing circuit 81 determines that the vehicle braking force request value FxTr is not increasing (S13: NO), the processing circuit 81 proceeds to step S31. The processing flow from step S31 onwards is the same as in the first embodiment, and therefore will not be described again.

[0138] In step S16, the processing circuit 81 determines whether the vehicle braking force request value FxTr is less than the switching regenerative braking force FxRA. A braking force smaller than the regenerative braking force upper limit value FxRL and the first braking force Fx1 is set as the switching regenerative braking force FxRA.

[0139] In a vehicle equipped with the brake system 20, the regenerative braking force FxR is applied only to the front wheels FL, FR out of the front wheels FL, FR and the rear wheels RL, RR. Therefore, if the regenerative braking force FxR applied to the front wheels FL, FR is increased to the regenerative braking force upper limit value FxRL by the first increase process described below, there is a risk that the vehicle's tendency to understeer will increase. Therefore, a braking force that keeps the vehicle's tendency to understeer within an acceptable range even when the regenerative braking force FxR is applied to the front wheels FL, FR by executing the first increase process is set as the switchover regenerative braking force FxRA.

[0140] If the processing circuit 81 determines that the vehicle braking force requirement value FxTr is less than the switching regenerative braking force FxRA (S16: YES), the processing proceeds to step S17. On the other hand, if the processing circuit 81 determines that the vehicle braking force requirement value FxTr is equal to or greater than the switching regenerative braking force FxRA (S16: NO), the processing proceeds to step S19.

[0141] In step S17, the processing circuit 81 executes a first increment process. The content of the first increment process is the same as that executed in the first embodiment, so a description of the first increment process will be omitted.

[0142] In step S19, the processing circuit 81 determines whether the vehicle braking force request value FxTr is less than the first braking force Fx1. If the processing circuit 81 determines that the vehicle braking force request value FxTr is less than the first braking force Fx1 (S19: YES), the processing circuit 81 proceeds to step S21. On the other hand, if the processing circuit 81 determines that the vehicle braking force request value FxTr is equal to or greater than the first braking force Fx1 (S19: NO), the processing circuit 81 proceeds to step S25.

[0143] In step S21, the processing circuit 81 executes a second increment process. The details of the second increment process are the same as those executed in the first embodiment, and therefore a description of the second increment process will be omitted.

[0144] In step S25, the processing circuit 81 determines whether the vehicle braking force request value FxTr is less than the regenerative braking force upper limit value FxRL. If the vehicle braking force request value is less than the regenerative braking force upper limit value FxRL (S25: YES), the processing circuit 81 proceeds to step S27. On the other hand, if the vehicle braking force request value is equal to or greater than the regenerative braking force upper limit value FxRL (S25: NO), the processing circuit 81 proceeds to step S29.

[0145] In step S27, the processing circuit 81 executes a third increase process. The content of this third increase process differs from the content of the third increase process of the first embodiment. In the third increase process, the processing circuit 81 increases the regenerative braking force FxR in response to an increase in the vehicle braking force request value FxTr, and increases the servo pressure Psc in response to an increase in the vehicle braking force request value FxTr, thereby increasing the rear wheel hydraulic pressure PwcR while maintaining the front wheel hydraulic pressure PwcF.

[0146] More specifically, in step S271, the processing circuit 81 increases the regenerative braking force target value FxRTr. For example, the processing circuit 81 sets a value that allows the distribution ratio between the front wheel braking force FxAF and the rear wheel braking force FxAR to maintain the reference front / rear distribution ratio as the regenerative braking force target value FxRTr. In the following step S273, the processing circuit 81 transmits the regenerative braking force target value FxRTr to the regenerative control unit 92.

[0147] When the regenerative control unit 92 receives the regenerative braking force target value FxRTr, it controls the motor generator 91 to generate power based on the regenerative braking force target value FxRTr. This allows the regenerative control unit 92 to increase the regenerative braking force FxR in accordance with an increase in the regenerative braking force target value FxRTr.

[0148] In the next step S275, the processing circuit 81 increases the rear wheel hydraulic pressure target value PwcRTr and maintains the front wheel hydraulic pressure target value PwcFTr at 0 (zero). The processing circuit 81 derives a value that allows the distribution ratio between the front wheel braking force FxAF and the rear wheel braking force FxAR to be maintained at the reference front / rear distribution ratio as the rear wheel friction braking force target value FxMRTr. The processing circuit 81 then converts the rear wheel friction braking force target value FxMRTr into a rear wheel hydraulic pressure, and sets the converted value as the rear wheel hydraulic pressure target value PwcRTr.

[0149] In step S277, the processing circuit 81 operates the brake system 20 based on the front wheel hydraulic pressure target value PwcFTr and the rear wheel hydraulic pressure target value PwcRTr. For example, the processing circuit 81 sets the servo pressure Psc for making the rear wheel hydraulic pressure PwcR equal to the rear wheel hydraulic pressure target value PwcRTr as the servo pressure target value PscTr. The processing circuit 81 operates the electric cylinder 51 based on this servo pressure target value PscTr. As a result, the processing circuit 81 increases the servo pressure Psc in response to an increase in the vehicle braking force request value FxTr, thereby increasing the rear wheel hydraulic pressure PwcR and the rear wheel friction braking force FxMR.

[0150] The processing circuit 81 also operates the front wheel hydraulic pressure adjuster 74 so as not to increase the front wheel hydraulic pressure PwcF even when the servo pressure Psc is increased as described above. For example, the processing circuit 81 maintains the front wheel hydraulic pressure PwcF by closing the plurality of holding valves 642 of the front wheel hydraulic pressure adjuster 74. In this case, the processing circuit 81 may increase the magnitude of the current flowing through the solenoid of the holding valve 642 as the pressure difference between the servo pressure Psc and the front wheel hydraulic pressure target value PwcFTr increases. The processing circuit 81 then terminates the third increase process. The processing circuit 81 then temporarily terminates the regenerative cooperative control.

[0151] In step S29, the processing circuit 81 executes a fourth increase process. In the fourth increase process, the processing circuit 81 increases both the front wheel hydraulic pressure PwcF and the rear wheel hydraulic pressure PwcR by increasing the servo pressure Psc in accordance with an increase in the vehicle braking force requirement value FxTr while maintaining the regenerative braking force FxR.

[0152] More specifically, in step S291, the processing circuit 81 transmits an instruction to the regenerative control unit 92 of the regenerative device 90 to maintain the regenerative braking force FxR, similar to step S211. In the following step S293, the processing circuit 81 increases the front wheel hydraulic pressure target value PwcFTr and the rear wheel hydraulic pressure target value PwcRTr. At this time, the processing circuit 81 increases the front wheel hydraulic pressure target value PwcFTr and the rear wheel hydraulic pressure target value PwcRTr so as to satisfy the following conditions (G1), (G2), and (G3).

[0153] (G1) The difference between the front wheel hydraulic pressure PwcF and the rear wheel hydraulic pressure PwcR at the start of the fourth increase process is maintained. (G2) The sum of the front wheel friction braking force FxMF, the rear wheel friction braking force FxMR, and the regenerative braking force FxR is equal to the vehicle braking force request value FxTr.

[0154] (G3) The distribution ratio between the front wheel braking force FxAF and the rear wheel braking force FxAR is the reference front / rear distribution ratio. Note that the difference between the rear wheel hydraulic pressure PwcR and the front wheel hydraulic pressure PwcF at the start of the fourth increase process is also referred to as the "reference hydraulic pressure difference ΔPwcB1."

[0155] In step S295, the processing circuit 81 operates the brake system 20 based on the front wheel hydraulic pressure target value PwcFTr and the rear wheel hydraulic pressure target value PwcRTr. For example, the processing circuit 81 sets the servo pressure Psc for making the rear wheel hydraulic pressure PwcR equal to the rear wheel hydraulic pressure target value PwcRTr as the servo pressure target value PscTr. The processing circuit 81 operates the electric cylinder 51 based on this servo pressure target value PscTr. As a result, the processing circuit 81 increases the servo pressure Psc in response to an increase in the vehicle braking force request value FxTr, thereby increasing the rear wheel hydraulic pressure PwcR and the rear wheel friction braking force FxMR.

[0156] The processing circuit 81 also operates the front wheel hydraulic pressure adjuster 74 so that the front wheel hydraulic pressure PwcF increases in response to an increase in the servo pressure Psc as described above. For example, the processing circuit 81 sets the magnitude of the current flowing through the solenoids of the plurality of holding valves 642 to a value corresponding to the reference hydraulic pressure difference ΔPwcB1. This allows the processing circuit 81 to increase the servo pressure Psc in response to an increase in the vehicle braking force request value FxTr, thereby increasing the front wheel hydraulic pressure PwcF and the front wheel friction braking force FxMF. The processing circuit 81 then terminates the fourth increase process. Thereafter, the processing circuit 81 temporarily terminates the regenerative cooperative control.

[0157] <Actions and Effects of the Present Embodiment> With reference to Figures 8 and 9, the actions and effects of the braking system 20 during vehicle braking will be described, focusing on those that differ from the first embodiment. Figure 8(A) shows the trends in the vehicle braking force request value FxTr and the vehicle braking force Fx. Figure 8(B) shows the trends in the regenerative braking force target value FxRTr and the regenerative braking force FxR. Figure 8(C) shows the trends in the front wheel hydraulic pressure target value PwcFTr, the rear wheel hydraulic pressure target value PwcRTr, the front wheel hydraulic pressure PwcF, and the rear wheel hydraulic pressure PwcR.

[0158] Since a request for vehicle braking is issued from timing T10, the processing circuit 81 of the control device 80 increases the vehicle braking force request value FxTr. During the period from timing T10 to timing T11, the vehicle braking force request value FxTr is equal to or less than the switchover regenerative braking force FxRA. If the vehicle braking force request value FxTr is increased under conditions in which the vehicle braking force request value FxTr is smaller than the switchover regenerative braking force FxRA, the processing circuit 81 executes a first increase process. The first increase process is a process in which the regenerative braking force FxR is increased in response to an increase in the vehicle braking force request value FxTr, while the front wheel hydraulic pressure PwcF and the rear wheel hydraulic pressure PwcR are not increased.

[0159] Note that, during the period from timing T10 to timing T11, the front wheel braking force FxAF is increased but the rear wheel braking force FxAR is not increased. Therefore, during the period from timing T10 to timing T11, the state point P moves from the initial state point P0 in the direction of the first arrow Y11, as shown in Fig. 9. The first arrow Y11 indicates the direction along the horizontal axis in Fig. 9 in which the front wheel braking force FxAF is increased.

[0160] In the period from timing T10 to timing T11, state point P deviates from equal-pressure brake-force distribution line L2 in Figure 9. However, the vehicle braking force Fx is not particularly large during this period. Therefore, even if only the front wheel braking force FxAF increases out of the front wheel braking force FxAF and the rear wheel braking force FxAR, as indicated by the first arrow Y11, the vehicle is unlikely to understeer.

[0161] Returning to FIG. 8 , even after timing T11 has passed, the processing circuit 81 continues to increase the vehicle braking force request value FxTr. At timing T11, the regenerative braking force target value FxRTr becomes equal to the switching regenerative braking force FxRA. Therefore, at timing T11, the processing circuit 81 switches the processing from the first increase processing to the second increase processing. That is, when the vehicle braking force request value FxTr is increased under the circumstances where the regenerative braking force FxR has reached the switching regenerative braking force FxRA and the vehicle braking force request value FxTr is less than the first braking force Fx1, the processing circuit 81 executes the second increase processing. The second increase processing is a processing for increasing the servo pressure Psc in response to an increase in the vehicle braking force request value FxTr, thereby increasing the rear wheel hydraulic pressure PwcR, while maintaining the front wheel hydraulic pressure PwcF and maintaining the regenerative braking force FxR.

[0162] When the rear wheel hydraulic pressure PwcR increases, the rear wheel friction braking force FxMR increases. Meanwhile, the front wheel hydraulic pressure PwcF is maintained at 0 (zero) and the regenerative braking force FxR is maintained. Therefore, the rear wheel braking force FxAR increases while the front wheel braking force FxAF is maintained.

[0163] In FIG. 9 , the state point P at the end of the first increase process is referred to as the "first state point PA1." In this case, when the second increase process is started, the state point P moves from the first state point PA1 in the direction of the second arrow Y12. The second arrow Y12 indicates the direction along the vertical axis in FIG. 9 in which the rear wheel braking force FxAR is increased. As shown in FIG. 9 , when the state point P moves in the direction of the second arrow Y12 as the vehicle braking force request value FxTr increases, the state point P approaches the equal-pressure braking force distribution line L2. This allows the brake system 20 to suppress an increase in the vehicle's tendency to understeer due to an increase in the vehicle braking force Fx.

[0164] Returning to FIG. 8 , even after timing T12 has passed, the processing circuit 81 continues to increase the vehicle braking force request value FxTr. At timing T12, the vehicle braking force request value FxTr is greater than the first braking force Fx1. Furthermore, the regenerative braking force FxR is equal to the switching regenerative braking force FxRA. Therefore, at timing T12, the processing circuit 81 switches the processing from the second increase processing to the third increase processing. That is, when the vehicle braking force request value FxTr is increased under the circumstances where the vehicle braking force request value FxTr is equal to or greater than the first braking force Fx1, while the regenerative braking force FxR is equal to or greater than the switching regenerative braking force FxRA and less than the regenerative braking force upper limit value FxRL, the processing circuit 81 executes the third increase processing. The third increase process is a process in which the regenerative braking force FxR is increased in accordance with an increase in the vehicle braking force requirement value FxTr, and the servo pressure Psc is increased in accordance with an increase in the vehicle braking force requirement value FxTr, thereby increasing the rear wheel hydraulic pressure PwcR while maintaining the front wheel hydraulic pressure PwcF.

[0165] When the rear wheel hydraulic pressure PwcR is increased, the rear wheel friction braking force FxMR is increased and the regenerative braking force FxR is also increased. Therefore, even if the front wheel hydraulic pressure PwcF is not increased, both the front wheel braking force FxAF and the rear wheel braking force FxAR are increased.

[0166] 9, the state point P at the end of the second increase process is referred to as the "second state point PB1." In this case, when the third increase process is started, the state point P moves from the second state point PB1 in the direction of the third arrow Y13. The third arrow Y13 indicates the direction along the equal-pressure brake-force distribution line L2 in which the rear wheel brake force FxAR is increased. Therefore, when the state point P moves in the direction of the third arrow Y13 as the vehicle braking force request value FxTr increases, the state point P moves on the equal-pressure brake-force distribution line L2.

[0167] Returning to FIG. 8 , even after timing T13 has passed, the processing circuit 81 continues to increase the vehicle braking force request value FxTr. At timing T13, the regenerative braking force FxR becomes equal to the regenerative braking force upper limit value FxRL. Therefore, at timing T13, the processing circuit 81 switches the processing from the third increase processing to the fourth increase processing. That is, when the vehicle braking force request value FxTr is increased under the circumstances where the vehicle braking force request value FxTr is equal to or greater than the first braking force Fx1 while the regenerative braking force FxR has reached the regenerative braking force upper limit value FxRL, the processing circuit 81 executes the fourth increase processing. The fourth increase processing is a processing for increasing both the front wheel hydraulic pressure PwcF and the rear wheel hydraulic pressure PwcR by increasing the servo pressure Psc in accordance with the increase in the vehicle braking force request value FxTr while maintaining the regenerative braking force FxR.

[0168] When the front wheel hydraulic pressure PwcF increases, the front wheel friction braking force FxMF increases. When the rear wheel hydraulic pressure PwcR increases, the rear wheel friction braking force FxMR increases. Therefore, even if the regenerative braking force FxR is maintained, both the front wheel braking force FxAF and the rear wheel braking force FxAR increase.

[0169] 9, the state point P at the end of the third increase process is referred to as the "third state point PC1." In this case, when the fourth increase process is started, the state point P moves from the third state point PC1 in the direction of the third arrow Y13. In other words, the state point P moves on the equal-pressure brake-force distribution line L2.

[0170] 8, at time T14, the processing circuit 81 holds the vehicle braking force request value FxTr. To this end, the processing circuit 81 holds the front wheel friction braking force FxMF, the rear wheel friction braking force FxMR, and the regenerative braking force FxR.

[0171] After that, from timing T15 onwards, the processing circuit 81 reduces the vehicle braking force request value FxTr. In the period from timing T15 to timing T16, the processing circuit 81 executes a first reduction process. In the period from timing T16 to timing T17, the processing circuit 81 executes a second reduction process. In the period from timing T17 to timing T18, the processing circuit 81 executes a third reduction process. Then, in the period from timing T18 to timing T19, the processing circuit 81 reduces the regenerative braking force target value FxRTr towards 0 (zero).

[0172] In FIG. 9 , the state point P at the end of the fourth increase process is the fourth state point PD1 on the equal-pressure brake force distribution line L2. When the first decrease process is executed, the state point P moves from the fourth state point PD1 in the direction of the fourth arrow Y14. The fourth arrow Y14 indicates the direction along the equal-pressure brake force distribution line L2 in which the rear wheel braking force FxAR is reduced. When the state point P reaches the second state point PB1, the second decrease process is executed. Therefore, the state point P moves from the second state point PB1 in the direction of the fifth arrow Y15. The fifth arrow Y15 indicates the direction along the vertical axis in FIG. 9 in which the rear wheel braking force FxAR is reduced. Then, when the third decrease process is executed, the state point P moves in the direction of the sixth arrow Y16. The sixth arrow Y16 indicates the direction in which both the front wheel braking force FxAF and the rear wheel braking force FxAR are reduced. Finally, when the fourth decrease process is executed, the state point P moves in the direction of the seventh arrow Y17. The seventh arrow Y17 indicates the direction along the horizontal axis in Figure 9 in which the front wheel braking force FxAF is decreased. Then, the state point P returns to the initial state point P0.

[0173] (Modifications) The above-described embodiments can be modified as follows: The above-described embodiments and the following modifications can be combined with each other within the scope of technical compatibility.

[0174] In the second embodiment, when the vehicle braking force requirement value FxTr is greater than or equal to the first braking force Fx1 while the regenerative braking force FxR is greater than or equal to the switching regenerative braking force FxRA and less than the regenerative braking force upper limit value FxRL, and the vehicle braking force requirement value FxTr is increased, the processing circuit 81 may be configured to execute the fourth increase process.

[0175] Although the braking system 20 implements the second embodiment when the vehicle's regenerative device 90 is capable of applying a regenerative braking force FxR greater than the first braking force Fx1 to the front wheels FL, FR, this is not limitative. Even when the braking system 20 is capable of applying a regenerative braking force FxR greater than the first braking force Fx1 to the front wheels FL, FR, the braking system 20 may execute the same regenerative cooperative control as the first embodiment when a predetermined condition is met.

[0176] - In the third reduction process, the processing circuit 81 may gradually reduce the magnitude of the current flowing to the solenoid of the holding valve 642 of the front wheel hydraulic pressure adjustment unit 74, thereby reducing the front wheels FL, FR in accordance with the decrease in servo pressure Psc.

[0177] If the rear wheel hydraulic pressure PwcR becomes equal to the front wheel hydraulic pressure PwcF by reducing the rear wheel hydraulic pressure PwcR through the second reduction process, the processing circuit 81 does not need to perform the third reduction process. For example, the processing circuit 81 may perform a process to reduce all of the front wheel hydraulic pressure PwcF, the rear wheel hydraulic pressure PwcR, and the regenerative braking force FxR in response to a decrease in the vehicle braking force request value FxTr.

[0178] The pressure adjustment unit may be a unit other than the pressure adjustment unit 50 that includes the electric cylinder 51 as a pressure source. For example, the pressure adjustment unit may include an electric pump as a pressure source. An example of such a pressure adjustment unit is disclosed in Japanese Patent Application Laid-Open No. 2019-182278.

[0179] The hydraulic pressure generating device may have a different configuration from the hydraulic pressure generating device 22 shown in Fig. 1. In the hydraulic pressure generating device 22, brake fluid output from the pressure adjusting unit 50 is supplied to the servo chamber Rs of the master cylinder 31. Then, as the hydraulic pressure in the servo chamber Rs increases, brake fluid is supplied from the master cylinder 31 to the wheel cylinders 11 for the front wheels FL and FR. However, the hydraulic pressure generating device may be configured to supply brake fluid output from the pressure adjusting unit to the wheel cylinders 11 for the front wheels FL and FR without passing through the master cylinder 31.

[0180] The processing circuitry 81 is not limited to a circuit having a CPU and ROM and executing software processing. In other words, the processing circuitry 81 may have any one of the following configurations (a), (b), and (c):

[0181] (a) The processing circuitry 81 includes one or more processors that execute various processes according to a computer program. The processor includes a CPU and memory such as RAM and ROM. The memory stores program code or instructions configured to cause the CPU to execute processes. Memory, i.e., computer-readable media, includes any available media that can be accessed by a general-purpose or special-purpose computer.

[0182] (b) The processing circuit 81 includes one or more dedicated hardware circuits that perform various processes. Examples of dedicated hardware circuits include application specific integrated circuits (ASICs) or FPGAs. ASIC stands for "Application Specific Integrated Circuit," and FPGA stands for "Field Programmable Gate Array."

[0183] (c) The processing circuitry 81 includes a processor that executes some of the various processes in accordance with a computer program, and dedicated hardware circuits that execute the remaining processes of the various processes.

[0184] <Other Technical Ideas> The following describes technical ideas that can be understood from the above-described multiple embodiments and modified examples. [Appendix 1] The braking system preferably includes: a master cylinder having a master piston that separates a servo chamber and a master chamber, wherein brake fluid flows out of the master chamber as the master piston moves in response to an increase in hydraulic pressure in the servo chamber, and brake fluid flows into the master chamber as the master piston moves in response to a decrease in hydraulic pressure in the servo chamber; a first supply fluid path connecting the pressure regulating unit and the servo chamber; and a second supply fluid path branching from the first supply fluid path, wherein the brake fluid flowing into the second supply fluid path from the first supply fluid path is supplied to the rear wheel cylinder, and the brake fluid flowing out of the master chamber is supplied to the front wheel cylinder.

[0185] It should be noted that the expression "at least one" used in this specification means "one or more" of the desired options. As an example, the expression "at least one" used in this specification means "only one option" or "both of two options" if the number of options is two. As another example, the expression "at least one" used in this specification means "only one option" or "any combination of two or more options" if the number of options is three or more.

Claims

1. A braking system that is applied to a vehicle equipped with front wheel cylinders and rear wheel cylinders, front wheels to which a frictional braking force corresponding to the front wheel hydraulic pressure, which is the hydraulic pressure in the front wheel cylinders, is applied, rear wheels to which a frictional braking force corresponding to the rear wheel hydraulic pressure, which is the hydraulic pressure in the rear wheel cylinders, is applied, and that controls a vehicle braking force, which is the braking force applied to the vehicle, comprising: a pressure regulating unit that generates a reference hydraulic pressure and generates the rear wheel hydraulic pressure corresponding to the reference hydraulic pressure; a front wheel hydraulic pressure adjusting section that is configured to make the front wheel hydraulic pressure less than the rear wheel hydraulic pressure when the reference hydraulic pressure is generated; and a control device that controls the regenerative device, the pressure regulating unit, and the front wheel hydraulic pressure adjusting section based on a vehicle braking force requirement value, which is a required value for the vehicle braking force, wherein the control device: a first reduction process that reduces the regenerative braking force applied to the front wheels and reduces the reference hydraulic pressure to reduce the rear wheel hydraulic pressure while maintaining the front wheel hydraulic pressure, when the vehicle braking force requirement value is reduced in a situation where the vehicle braking force requirement value is reduced and the vehicle braking force requirement value is reduced, and when the vehicle braking force requirement value is reduced in a situation where the vehicle braking force requirement value is reduced and the vehicle braking force requirement value is reduced, the regenerative braking force applied to the front wheels and the reference hydraulic pressure to reduce the rear wheel hydraulic pressure while maintaining the front wheel hydraulic pressure.

2. The braking system described in claim 1, wherein when the vehicle braking force requirement value is reduced under a condition where the vehicle braking force requirement value is less than the first braking force and equal to or greater than a second braking force that is smaller than the first braking force, the control device maintains the regenerative braking force applied to the front wheels and executes a second reduction process that maintains the front wheel hydraulic pressure while reducing the reference hydraulic pressure to reduce the rear wheel hydraulic pressure.

3. The braking system described in claim 2, wherein when the vehicle braking force requirement value is reduced under a condition where the vehicle braking force requirement value is less than the second braking force and equal to or greater than a third braking force that is smaller than the second braking force, the control device maintains the regenerative braking force applied to the front wheels and executes a third reduction process that reduces both the front wheel hydraulic pressure and the rear wheel hydraulic pressure by reducing the reference hydraulic pressure.

4. A braking system as described in claim 3, wherein the second braking force is the vehicle braking force when the rear wheel hydraulic pressure becomes equal to the front wheel hydraulic pressure after being reduced by executing the second reduction process.

5. The braking system described in claim 4, wherein the third braking force is the vehicle braking force when both the front wheel hydraulic pressure and the rear wheel hydraulic pressure become 0 (zero) by executing the third reduction process, and the control device executes a fourth reduction process to reduce the regenerative braking force applied to the front wheels when the vehicle braking force request value is reduced under circumstances where the vehicle braking force request value is less than the third braking force.

6. A braking system as claimed in claim 3 or claim 4, comprising a master device that generates the front wheel hydraulic pressure according to the reference hydraulic pressure, wherein the front wheel hydraulic pressure adjustment section has a retention valve which is a linear electromagnetic valve provided in a hydraulic path connecting the master device and the front wheel cylinder and adjusts the pressure difference between a portion on the master device side and a portion on the front wheel cylinder side, and a check valve arranged in parallel with the retention valve, wherein the check valve is configured to close when the hydraulic pressure on the master device side of the retention valve is equal to or higher than the front wheel hydraulic pressure, and to open when the hydraulic pressure on the master device side of the retention valve is lower than the front wheel hydraulic pressure, thereby allowing brake fluid to flow from the front wheel cylinder to the master device, and wherein the control device, when reducing both the front wheel hydraulic pressure and the rear wheel hydraulic pressure in the third reduction process, operates the pressure adjustment unit so that the master pressure, which is the hydraulic pressure generated by the master device, is lower than the front wheel hydraulic pressure.

7. The control device performs a first increase process in which, when the vehicle braking force requirement value is increased under a condition where the vehicle braking force requirement value is smaller than a regenerative braking force upper limit value, which is the upper limit value of the regenerative braking force that can be applied to the front wheels, the regenerative braking force applied to the front wheels is increased in accordance with the increase in the vehicle braking force requirement value, while not increasing either the front wheel hydraulic pressure or the rear wheel hydraulic pressure; and a second increase process in which, when the vehicle braking force requirement value is increased under a condition where the regenerative braking force applied to the front wheels has reached the regenerative braking force upper limit value, the reference hydraulic pressure is increased in accordance with the increase in the vehicle braking force requirement value, thereby increasing the rear wheel hydraulic pressure, while maintaining the front wheel hydraulic pressure and maintaining the regenerative braking force applied to the front wheels.

2. The braking system according to claim 1, wherein, when the vehicle braking force requirement value is increased under a condition in which the regenerative braking force applied to the front wheels has reached the regenerative braking force upper limit value and the vehicle braking force requirement value is equal to or greater than the first braking force, a third increase process is executed to increase both the front wheel hydraulic pressure and the rear wheel hydraulic pressure in accordance with an increase in the vehicle braking force requirement value while maintaining the regenerative braking force applied to the front wheels.

8. A switching regenerative braking force is set as a braking force smaller than a regenerative braking force upper limit value, which is an upper limit value of the regenerative braking force that the regenerative device can apply to the front wheels, and the control device performs: a first increase process in which, when the vehicle braking force request value is increased under a condition where the vehicle braking force request value is smaller than the switching regenerative braking force, the regenerative braking force applied to the front wheels is increased in accordance with the increase in the vehicle braking force request value, while not increasing either the front wheel hydraulic pressure or the rear wheel hydraulic pressure; and a second increase process in which, when the vehicle braking force request value is increased under a condition where the regenerative braking force applied to the front wheels has reached the switching regenerative braking force and the vehicle braking force request value is less than the first braking force, the reference hydraulic pressure is increased in accordance with the increase in the vehicle braking force request value, thereby increasing the rear wheel hydraulic pressure, while maintaining the front wheel hydraulic pressure and maintaining the regenerative braking force applied to the front wheels.

2. The brake system according to claim 1, wherein, when the vehicle braking force request value is increased in a situation where the vehicle braking force request value is equal to or greater than the first braking force while the regenerative braking force applied to the front wheels is equal to or greater than the switching regenerative braking force and less than the regenerative braking force upper limit value, the brake system executes a third increase process to increase the regenerative braking force applied to the front wheels in accordance with the increase in the vehicle braking force request value and to increase the reference hydraulic pressure in accordance with the increase in the vehicle braking force request value, thereby increasing the rear wheel hydraulic pressure while maintaining the front wheel hydraulic pressure; and a fourth increase process to increase both the front wheel hydraulic pressure and the rear wheel hydraulic pressure by increasing the reference hydraulic pressure in accordance with the increase in the vehicle braking force request value, when the vehicle braking force request value is increased in a situation where the vehicle braking force request value is equal to or greater than the first braking force while the regenerative braking force applied to the front wheels has reached the regenerative braking force upper limit value, while maintaining the regenerative braking force applied to the front wheels.

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