Brake system

Abstract

To decrease vehicle brake force while maintaining stability of vehicle behavior in a state in which regenerative brake force is being applied to rear wheels.SOLUTION: A brake system 20 comprises: a pressure adjustment unit 50 which generates reference fluid pressure, and generates front wheel fluid pressure according to the reference fluid pressure; a rear wheel fluid pressure adjustment unit 74 which is constituted so as to adjust rear wheel fluid pressure to fluid pressure under the front wheel fluid pressure, when the reference fluid pressure occurs; and a controller 80 which controls a regeneration device 90, the pressure adjustment unit 50 and the rear wheel fluid pressure adjustment unit 74. When a vehicle brake force request value decreases in the state in which the vehicle brake force request value is equal to or more than first brake force, the controller 80 executes first reduction processing which decreases regenerative brake force assigned to rear wheels RL, RR, and holds the rear wheel fluid pressure while decreasing the front wheel fluid pressure by decreasing the reference fluid pressure.SELECTED DRAWING: Figure 1

Description

[Technical field]

[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 rear wheels. [Background technology]

[0002] Patent Document 1 discloses a braking system applied to a vehicle equipped with a regenerative device capable of applying regenerative braking force to the front wheels. The braking system includes a pressure adjusting unit that adjusts the front wheel hydraulic pressure, which is the hydraulic pressure in the front wheel cylinders, and the rear wheel hydraulic pressure, which is the hydraulic pressure in the rear wheel cylinders. The required value of the braking force to be applied to the vehicle is referred to as the vehicle braking force required value.

[0003] When the control device of the braking system cannot make the regenerative braking force greater than or equal to the vehicle braking force requirement value during vehicle braking, the control device of the braking system performs regenerative cooperative control by activating the pressure regulating unit to increase at least the rear wheel hydraulic pressure of the front wheel hydraulic pressure and the rear wheel hydraulic pressure. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2019-182278 A Summary of the Invention [Problem to be solved by the invention]

[0005] In recent years, vehicles equipped with a regenerative device that can apply regenerative braking force to the rear wheels have been developed. There is a demand for the realization of a regenerative cooperative control that can ensure the stability of the vehicle behavior even when braking such vehicles. [Means for solving the problem]

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

[0007] The above braking system has the effect of reducing the vehicle braking force while maintaining the stability of the vehicle behavior in a situation where regenerative braking force is applied to the rear wheels. [Brief description of the drawings]

[0008] [Figure 1] FIG. 1 is a schematic diagram showing the configuration of a vehicle equipped with a braking system according to a first embodiment. [Diagram 2] FIG. 2 is a schematic diagram showing the configuration of the brake actuator provided in the brake system of the first embodiment. [Diagram 3]FIG. 3 is a flowchart showing the first half of the regenerative cooperative control executed by the control device provided in the braking system of the first embodiment. [Figure 4] FIG. 4 is a flowchart showing the latter half of the regenerative cooperative control executed by the control device provided in the braking system of the first embodiment. [Diagram 5] FIG. 5 is a timing chart when braking the vehicle in the braking system of the first embodiment. [Figure 6] 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. [Figure 7] FIG. 7 is a flowchart showing a part of the regenerative cooperative control executed by the control device provided in the braking system of the second embodiment. [Figure 8] FIG. 8 is a timing chart when braking the vehicle in the braking system of the second embodiment. [Figure 9] FIG. 9 is a graph showing transitions of state points indicated by the front wheel braking force and the rear wheel braking force in the braking system of the second embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] (First embodiment) Hereinafter, a first embodiment of a braking system mounted on a vehicle will be described with reference to Figs.

[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 brakes 10 are configured such that the greater the hydraulic pressure in the wheel cylinder 11, the stronger the friction material 13 can be pressed against the rotating body 12. The friction brakes 10 apply 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 of the wheel cylinder 11 corresponding to the front wheel cylinder is called the "front wheel hydraulic pressure PwcF." The hydraulic pressure of the wheel cylinder 11 corresponding to the rear wheel cylinder is called the "rear wheel hydraulic pressure PwcR." A frictional braking force according to the front wheel hydraulic pressure PwcF is applied to the front wheels FL, FR. A frictional braking force according 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 rear wheels RL and RR, and a regenerative control unit 92 that controls the motor generator 91. When the motor generator 91 functions as an electric motor, a driving force is transmitted from the motor generator 91 to the rear wheels RL and RR. On the other hand, when the motor generator 91 functions as a generator, a regenerative braking force FxR according to the amount of power generated by the motor generator 91 is applied to the rear wheels RL and RR.

[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 regeneration control unit 92 can control the motor generator 91 by the CPU executing the control program.

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

[0016] When the braking force applied to the vehicle is defined as the "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 sum of the braking forces applied to the multiple wheels FL, FR, RL, and RR is the vehicle braking force Fx.

[0017] <Braking system> The braking 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 braking 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 inside of the atmospheric pressure reservoir 24 is open to the atmosphere.

[0018] The hydraulic pressure generating device 22 is configured 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 regulating unit 50. The master device 30 can supply brake fluid to the brake actuator 23. The pressure regulating 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 for controlling 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. The first annular wall 414 has a hole 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 "front." On the other hand, the opposite direction to the front will be referred to as the "rearward." 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 toward 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 is provided with a hole into which the rear end of an input piston 44, which will be 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 backward along the inner circumferential surface of the first peripheral wall 412, the inner circumferential surface of the second peripheral wall 413, and the inner circumferential surface of 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 in the master cylinder 31. The input piston 44 moves forward and rearward along the inner circumferential surface of the third circumferential wall 421 and the inner circumferential surface of 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 in a direction approaching the master piston 43 according to 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 urges 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 cut off. 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 master piston 43 moves forward due to the hydraulic pressure in the servo chamber Rs. As a result, brake fluid flows out of the master chamber Rm, and the hydraulic pressure in the master chamber Rm increases. On the other hand, when the hydraulic pressure in the servo chamber Rs decreases, the master piston 43 moves backward due to the hydraulic pressure in the master chamber Rm. As a result, brake fluid flows into the master chamber Rm, and the hydraulic pressure in the master chamber Rm decreases.

[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 a second hydraulic circuit 612 (described later) of the brake actuator 23 and the master chamber Rm. 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 between the second fluid chamber R2 and a connection point of the second flow path 332 with the third flow path 333. 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 R 1 and the first control valve 341 . <Pressure adjustment unit> The pressure regulating unit 50 includes an electric cylinder 51. The pressure regulating unit 50 can regulate 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 includes a fourth flow path 54, a fifth flow path 55, and a sixth flow path 58 as flow paths for the 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 circuit 611 of the brake actuator 23 (described later) and the fifth flow path 55. Therefore, the electric cylinder 51 can supply the brake fluid discharged from the output port 516 to both the servo chamber Rs and the first hydraulic circuit 611 of the brake actuator 23.

[0035] When the hydraulic pressure of the fifth flow path 55 is defined as a "servo pressure," this servo pressure corresponds to a "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 defined as a "first supply liquid path," and the flow path branching off from the first supply liquid path is defined as a "second supply liquid path." In this case, the fifth flow path 55 corresponds to the first supply liquid path, and the sixth flow path 58 corresponds to the second supply liquid 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 provided in a state in which it can slide 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 the 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 is referred to as the "forward direction Za," while the opposite direction to the forward direction Za is 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 on the most backward direction Zb side where the piston 512 can move is defined as the most backward position. At this time, the through hole 517 is formed at a position where the input port 515 and the hydraulic chamber Re can be communicated when the piston 512 is at the most backward position. As a result, when the piston 512 is at the most backward position, the hydraulic chamber Re of the cylinder 511 is communicated with the atmospheric pressure reservoir 24 via the through hole 517, the input port 515, and the fourth flow passage 54. The input port 515 is opened when the piston 512 is at the most backward position, and is configured to be closed by the piston 512 when the piston 512 moves in the forward direction Za from the most backward position. When the piston 512 moves in the forward direction Za even after the input port 515 is closed by the piston 512, the hydraulic pressure of 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 in the forward direction Za.

[0040] <Braking actuator> 2, the brake actuator 23 is configured to be able to individually adjust the hydraulic pressures PwcF, PwcR of the multiple 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 brake system 20 has a redundant configuration in which the pressure regulating unit 50 is on the upstream side and the brake actuator 23 is on the downstream side.

[0041] The brake actuator 23 has a first hydraulic pressure circuit 611 and a second hydraulic pressure circuit 612. The two wheel cylinders 11 for the rear wheels RL and RR are connected to the first hydraulic pressure circuit 611. The 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 passage 621 and a first differential pressure control valve 631 provided in the first connecting fluid passage 621. The first connecting fluid passage 621 is a brake fluid passage connecting two wheel cylinders 11 for the rear wheels RL and RR and 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 a portion of the first connecting fluid passage 621 on the sixth flow path 58 side and a portion of 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 first connecting fluid passage 621, at a portion 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 left rear wheel RL, while the passage 62b is connected to the wheel cylinder 11 for the right rear wheel RR. A holding valve 641 is provided in each of the two passages 62a and 62b. That is, the holding valve 641 is provided in a portion of the first connecting fluid passage 621 closer to the wheel cylinder 11 than the first differential pressure control valve 631. The holding valve 641 is a normally open linear solenoid valve that adjusts the differential pressure between the portion of the passage 62a and 62b on the first differential pressure control valve 631 side and the portion of the passage 62a and 62b on the wheel cylinder 11 side. The holding valve 641 can generate a larger differential pressure as the current flowing through its solenoid increases.

[0044] A bypass flow passage 721, which is a flow passage that bypasses the holding valve 641, is connected to each of the multiple paths 62a, 62b. A first end of the bypass flow passage 721 is connected to a portion of the paths 62a, 62b that is closer to the first differential pressure control valve 631 than the holding valve 641. A second end of the bypass flow passage 721 is connected to a portion of the paths 62a, 62b that is closer to the wheel cylinder 11 than the holding valve 641. A check valve 731 is provided in each of the multiple bypass flow passages 721. That is, the check valve 731 is arranged in parallel with the holding 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 higher 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 lower than the rear wheel hydraulic pressure PwcR. When the check valve 731 opens in this manner, 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] As described above, the plurality of holding valves 641 are normally open linear solenoid valves. The first hydraulic circuit 611 is connected to the fifth flow path 55 via the sixth flow path 58. Therefore, by adjusting the magnitude of the current flowing through the solenoids of the plurality of holding valves 641, the rear wheel hydraulic pressure PwcR becomes lower than the servo pressure of the fifth flow path 55. As will be described in detail later, the servo pressure corresponds to the front wheel hydraulic pressure PwcF. Therefore, by operating the holding valve 641 when servo pressure is generated, the rear wheel hydraulic pressure PwcR becomes lower than the front wheel hydraulic pressure PwcF. Therefore, the plurality of holding valves 641 and the plurality of check valves 731 constitute an example of a "rear wheel hydraulic pressure adjustment unit 74" configured to make the rear wheel hydraulic pressure PwcR lower than the front wheel hydraulic pressure PwcF when servo pressure is generated.

[0047] 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 a portion of the passages 62a, 62b that is closer to the wheel cylinder 11 than the retention valve 641. A first pressure-reducing valve 671, which is a normally-closed solenoid valve, is provided in the portion of the first pressure-reducing fluid passage 661 that is connected to the passage 62a, and in the portion of the first pressure-reducing fluid passage 661 that is connected to the passage 62b.

[0048] 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 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, and the first reduced pressure reservoir 651.

[0049] 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 passage 622 and a second differential pressure control valve 632 provided in the second connecting fluid passage 622. The second connecting fluid passage 622 is a brake fluid passage connecting the two wheel cylinders 11 for the front wheels FL, FR and 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 passage 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.

[0050] The portion of the second connecting fluid passage 622 that is closer to the wheel cylinder 11 than the second differential pressure control valve 632 branches into two passages 62c and 62d. The passage 62c is connected to the wheel cylinder 11 for the left front wheel FL, while the passage 62d is connected to the wheel cylinder 11 for the right front wheel FR. A holding valve 642 is provided in each of the two passages 62c and 62d. The holding valve 642 is a normally open linear solenoid valve that adjusts the pressure difference between the portion of the passage 62c or 62d that is closer to the second differential pressure control valve 632 and the portion that is closer to the wheel cylinder 11. The holding valve 642 can generate a larger pressure difference as the current flowing through its solenoid increases.

[0051] A bypass flow path 722 that is a flow path that bypasses the retention valve 642 is connected to each of the multiple paths 62c and 62d. A check valve 732 is provided in each of the multiple bypass flow paths 722. That is, the check valve 732 is disposed in parallel with the retention valve 642.

[0052] The second hydraulic circuit 612 has a second pressure-reducing reservoir 652 that stores brake fluid, and a second pressure-reducing fluid passage 662 connected to the second pressure-reducing reservoir 652. The second pressure-reducing fluid passage 662 is a brake fluid passage that connects the second pressure-reducing reservoir 652 to a portion of the passages 62c, 62d that is closer to the wheel cylinder 11 than the retention valve 642. A second pressure-reducing valve 672, which is a normally-closed solenoid valve, is provided in the portion of the second pressure-reducing fluid passage 662 that is connected to the passage 62c, and in the portion of the second pressure-reducing fluid passage 662 that is connected to the passage 62d.

[0053] 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 in 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 connected to a portion of the second connecting fluid path 622 closer to the first flow path 331 than the second differential pressure control valve 632, and the second reduced pressure reservoir 652.

[0054] The second flow control unit 712 controls the flow of brake fluid through the second return fluid path 702. An example of the second flow control unit 712 is a one-way valve. <Detection system for braking system> 1, the detection system of the braking system has a plurality of sensors that output signals according to the detection results to the control device 80. The plurality of sensors includes a plurality of hydraulic pressure sensors and a stroke sensor SE1. The plurality of hydraulic pressure sensors includes a master pressure sensor 351, an input pressure sensor 352, and a servo pressure sensor 353.

[0055] 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."

[0056] 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 between the first control valve 341 and the second fluid chamber R2 in the second flow path 332. 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."

[0057] The servo pressure sensor 353 detects a 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 a 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 a "servo pressure Psc."

[0058] 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 a 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.

[0059] <Regenerative Cooperative Control> 3 and 4, the regenerative cooperative control executed by the processing circuit 81 of the control device 80 during vehicle braking will be described. The regenerative cooperative control is a 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 there is a braking request, the processing circuit 81 repeatedly executes the regenerative cooperative control at predetermined control intervals.

[0060] In step S11, the processing circuit 81 derives a vehicle braking force required value FxTr, which is a required value of 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 required 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 required value FxTr. In addition, when a deceleration request is received from another on-board control device, the processing circuit 81 sets a value according to the deceleration request as the vehicle braking force required value FxTr.

[0061] In the next step S13, the processing circuit 81 judges whether the vehicle braking force requirement value FxTr has increased. For example, if the latest value of the vehicle braking force requirement value FxTr is greater than the previous value of the vehicle braking force requirement value FxTr, the vehicle braking force requirement value FxTr can be regarded as having increased. Also, for example, if the latest value of the vehicle braking force requirement value FxTr is equal to or less than the previous value of the vehicle braking force requirement value FxTr, the vehicle braking force requirement value FxTr can be regarded as having not increased. The latest value of the vehicle braking force requirement value FxTr is the vehicle braking force requirement value FxTr derived in the current control cycle. The previous value of the vehicle braking force requirement value FxTr is the vehicle braking force requirement value FxTr derived in the previous control cycle. If the processing circuit 81 judges that the vehicle braking force requirement value FxTr has increased (S13: YES), the processing circuit 81 shifts the processing to step S15. On the other hand, if the processing circuit 81 judges that the vehicle braking force requirement value FxTr has not increased (S13: NO), the processing circuit 81 shifts the processing to step S31.

[0062] 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 value 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 shifts the processing 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 shifts the processing to step S19.

[0063] 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 requirement value FxTr, but does not increase the front wheel hydraulic pressure PwcF and the rear wheel hydraulic pressure PwcR.

[0064] More specifically, in step S171, the processing circuit 81 sets the vehicle braking force requirement value FxTr as 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 rear wheels RL and RR. 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.

[0065] 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. This enables the regenerative control unit 92 to increase the regenerative braking force FxR applied to the rear wheels RL, RR in response to an increase in the regenerative braking force target value FxRTr.

[0066] 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 of the front wheel hydraulic pressure PwcF. The rear wheel hydraulic pressure target value PwcRTr is the target value of the rear wheel hydraulic pressure PwcR.

[0067] 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 processing. After that, the processing circuit 81 temporarily ends the regenerative cooperative control.

[0068] 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.

[0069] Here, the first braking force Fx1 will be described with reference to FIG. 6. The horizontal axis of FIG. 6 is the front wheel braking force FxAF, which is the sum of the braking forces applied to the front wheels FL, FR. The vertical axis of FIG. 6 is the rear wheel braking force FxAR, which is the sum of the braking forces applied to the rear wheels RL, RR. In this example, the front wheel braking force FxAF is equal to the front wheel friction braking force FxMF. The rear wheel braking force FxAR is equal to the sum of the rear wheel friction braking force FxMR and the regenerative braking force FxR. The dashed line in FIG. 6 is the ideal braking force distribution line L1. The ideal braking force distribution line L1 is a line showing the relationship between the front wheel braking force FxAF and the rear wheel braking force FxAR when the front wheels FL, FR and the rear wheels RL, RR are locked simultaneously during vehicle braking. The dashed line in FIG. 6 is the same 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.

[0070] For example, the first braking force Fx1 is set so as to satisfy the following conditions (A1) and (A2). (A1) The rear wheel braking force FxAR 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.

[0071] (A2) In the graph shown in FIG. 6, state point P, which is a point indicating the front wheel braking force FxAF and the rear wheel braking force FxAR, is a point on equal pressure braking force distribution line L2. 3, in step S19, if the processing circuit 81 determines that the vehicle braking force requirement value FxTr is less than the first braking force Fx1 (S19: YES), the processing proceeds to step S21. 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 first braking force Fx1 (S19: NO), the processing proceeds to step S23.

[0072] 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 requirement value FxTr to increase the front wheel hydraulic pressure PwcF, while maintaining the rear wheel hydraulic pressure PwcR and maintaining the regenerative braking force FxR.

[0073] More specifically, in step S211, the processing circuit 81 transmits to the regenerative control unit 92 of the regenerative device 90 an instruction to hold the regenerative braking force FxR. When the regenerative control unit 92 receives the instruction, it operates the motor generator 91 so as to maintain the regenerative braking force FxR.

[0074] In the next step S213, the processing circuit 81 increases the front wheel hydraulic pressure target value PwcFTr and maintains the rear wheel hydraulic pressure target value PwcRTr at 0. The processing circuit 81 sets the front wheel hydraulic pressure PwcF when the front wheel friction braking force FxMF is equal to the vehicle braking force request value FxTr minus the regenerative braking force FxR as the front wheel hydraulic pressure target value PwcFTr.

[0075] In step S215, 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 sets a hydraulic pressure corresponding to the front wheel hydraulic pressure target value PwcFTr as the servo pressure target value PscTr, which is a target value of the servo pressure Psc. For example, the processing circuit 81 sets the servo pressure Psc for making the front wheel hydraulic pressure PwcF equal to the front wheel hydraulic pressure target value PwcFTr as the servo pressure target value PscTr. The processing circuit 81 operates the electric cylinder 51 based on the 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 requirement value FxTr, thereby increasing the front wheel hydraulic pressure PwcF and the front wheel friction braking force FxMF.

[0076] Furthermore, the processing circuit 81 operates the rear wheel hydraulic pressure adjustment unit 74 so that the rear wheel hydraulic pressure PwcR does not increase even if the servo pressure Psc increases. For example, the processing circuit 81 maintains the rear wheel hydraulic pressure PwcR by closing the multiple retention valves 641 of the rear wheel hydraulic pressure adjustment unit 74. At this time, it is preferable that the processing circuit 81 increases the current flowing through the solenoid of the retention valve 641 the greater the differential pressure between the servo pressure Psc and the rear wheel hydraulic pressure target value PwcRTr. Then, when the processing circuit 81 ends the second increase process, it temporarily ends the regenerative cooperative control.

[0077] 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 requirement value FxTr, thereby increasing the front wheel hydraulic pressure PwcF and the rear wheel hydraulic pressure PwcR, while maintaining the regenerative braking force FxR.

[0078] More specifically, in step S231, the processing circuit 81 transmits an instruction to the regenerative control unit 92 of the regenerative device 90 to hold the regenerative braking force FxR, similarly to step S211 described above. In the next 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).

[0079] (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.

[0080] 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 also referred to as the "reference hydraulic pressure difference ΔPwcB." In step S235, 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. For example, the processing circuit 81 sets the servo pressure Psc for making the front wheel hydraulic pressure PwcF equal to the front wheel hydraulic pressure target value PwcFTr as the servo pressure target value PscTr. The processing circuit 81 operates the electric cylinder 51 based on the 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 front wheel hydraulic pressure PwcF and the front wheel friction braking force FxMF.

[0081] The processing circuit 81 also operates the rear wheel hydraulic pressure adjusting unit 74 so that the rear wheel hydraulic pressure PwcR 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 multiple retention valves 641 to a value corresponding to the reference hydraulic pressure difference ΔPwcB. This allows the processing circuit 81 to increase the rear wheel hydraulic pressure PwcR and the rear wheel friction braking force FxMR in response to an increase in the servo pressure Psc. Then, the processing circuit 81 ends the third increase processing. After that, the processing circuit 81 temporarily ends the regenerative cooperative control.

[0082] In step S31, the processing circuit 81 judges whether the vehicle braking force requirement value FxTr has decreased. For example, if the latest value of the vehicle braking force requirement value FxTr is smaller than the previous value of the vehicle braking force requirement value FxTr, the vehicle braking force requirement value FxTr can be regarded as having decreased. Also, for example, if the latest value of the vehicle braking force requirement value FxTr is equal to or greater than the previous value of the vehicle braking force requirement value FxTr, the vehicle braking force requirement value FxTr can be regarded as not having decreased. If the processing circuit 81 judges that the vehicle braking force requirement value FxTr has decreased (S31: YES), the processing circuit 81 shifts the processing to step S41 in FIG. 4. On the other hand, if the processing circuit 81 judges that the vehicle braking force requirement value FxTr has not decreased (S31: NO), the processing circuit 81 can judge that the vehicle braking force requirement value FxTr is held, and shifts the processing to step S33.

[0083] In step S33, the processing circuit 81 transmits an instruction to the regenerative control unit 92 of the regenerative device 90 to hold the regenerative braking force FxR, similarly to step S211. In the following step S35, the processing circuit 81 holds 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 that the servo pressure Psc is held. The processing circuit 81 operates the rear 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 held at the reference hydraulic pressure difference ΔPwcB. Then, the processing circuit 81 temporarily ends the regenerative cooperative control.

[0084] In step S41, the processing circuit 81 determines whether the vehicle braking force requirement value FxTr is equal to or greater than the first braking force Fx1. If the processing circuit 81 determines that the vehicle braking force requirement value FxTr is equal to or greater than the first braking force Fx1 (S41: YES), the processing circuit 81 proceeds to step S43. On the other hand, if the processing circuit 81 determines that the vehicle braking force requirement value FxTr is less than the first braking force Fx1 (S41: NO), the processing circuit 81 proceeds to step S45.

[0085] 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 front wheel hydraulic pressure PwcF while maintaining the rear wheel hydraulic pressure PwcR.

[0086] More specifically, in step S431, the processing circuit 81 reduces the regenerative braking force target value FxRTr. The processing circuit 81 reduces the regenerative braking force target value FxRTr so as to satisfy the following conditions (C1) and (C2).

[0087] (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 a reference front / rear distribution ratio.

[0088] 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 brake 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 brake force distribution line L2.

[0089] In the following step S433, the processing circuit 81 transmits the regenerative braking force target value FxRTr to the regenerative control unit 92. When the regenerative control unit 92 receives 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 response to the reduction in the regenerative braking force target value FxRTr.

[0090] In step S435, the processing circuit 81 decreases the front wheel hydraulic pressure target value PwcFTr while maintaining the rear wheel hydraulic pressure target value PwcRTr. The processing circuit 81 decreases the front wheel hydraulic pressure target value PwcFTr so as to satisfy the following condition (D1).

[0091] (D1) The front wheel friction braking force target value FxMFTr is equal to the vehicle braking force request value FxTr minus the rear wheel braking force FxAR. The front wheel friction braking force target value FxMFTr is a target value of the front wheel friction braking force FxMF.

[0092] In step S437, 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. For example, the processing circuit 81 sets the servo pressure Psc for making the front wheel hydraulic pressure PwcF equal to the front wheel hydraulic pressure target value PwcFTr 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 front wheel hydraulic pressure PwcF and the front wheel friction braking force FxMF.

[0093] Furthermore, the processing circuit 81 operates the rear wheel hydraulic pressure adjusting unit 74 so that the rear wheel hydraulic pressure PwcR does not decrease in response to the 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 multiple retention valves 641. This maintains the multiple retention valves 641 in a closed state, so that the processing circuit 81 can maintain the rear wheel hydraulic pressure PwcR and the rear wheel friction braking force FxMR even if the servo pressure Psc is decreased. Then, the processing circuit 81 ends the first decrease processing. After that, the processing circuit 81 temporarily ends the regenerative cooperative control.

[0094] In step S45, the processing circuit 81 judges whether the vehicle braking force requirement 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, when only the front wheel friction braking force FxMF is reduced among the front wheel friction braking force FxMF, the rear wheel friction braking force FxMR, and the regenerative braking force FxR, the vehicle braking force Fx when the front wheel hydraulic pressure PwcF becomes equal to the rear wheel hydraulic pressure PwcR is set as the second braking force Fx2. In other words, the second braking force Fx2 is the vehicle braking force Fx when the front wheel hydraulic pressure PwcF becomes equal to the rear wheel hydraulic pressure PwcR when the front wheel hydraulic pressure PwcF is reduced by executing a second reduction process described later. When the vehicle braking force requirement value FxTr is equal to or greater than the second braking force Fx2 (S45: YES), the processing circuit 81 shifts the processing to step S47. On the other hand, when the vehicle braking force request value FxTr is less than the second braking force Fx2 (S45: NO), the processing circuit 81 shifts the process to step S49.

[0095] 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 rear wheel hydraulic pressure PwcR while reducing the servo pressure Psc to reduce the front wheel hydraulic pressure PwcF.

[0096] Specifically, in step S471, the processing circuit 81 transmits an instruction to the regenerative control unit 92 of the regenerative device 90 to hold the regenerative braking force FxR, similarly to step S211 described above. In the next step S473, the processing circuit 81 decreases the front wheel hydraulic pressure target value PwcFTr while maintaining the rear wheel hydraulic pressure target value PwcRTr. At this time, the processing circuit 81 decreases the front wheel hydraulic pressure target value PwcFTr so as to satisfy the following condition (E1).

[0097] (E1) The front wheel friction braking force target value FxMFTr is equal to the vehicle braking force request value FxTr minus the rear wheel braking force FxAR. In step S475, 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. For example, the processing circuit 81 sets the servo pressure Psc for making the front wheel hydraulic pressure PwcF equal to the front wheel hydraulic pressure target value PwcFTr 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 front wheel hydraulic pressure PwcF and the front wheel friction braking force FxMF.

[0098] Furthermore, the processing circuit 81 operates the rear wheel hydraulic pressure adjusting unit 74 so that the rear wheel hydraulic pressure PwcR 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 multiple retention valves 641. This maintains the multiple retention valves 641 in a closed state, so that the processing circuit 81 can maintain the rear wheel hydraulic pressure PwcR and the rear wheel friction braking force FxMR even if the servo pressure Psc is reduced. Then, the processing circuit 81 ends the second reduction process. After that, the processing circuit 81 temporarily ends the regenerative cooperative control.

[0099] In step S49, the processing circuit 81 judges whether the vehicle braking force requirement 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 requirement value FxTr is equal to the regenerative braking force upper limit value FxRL is set as the third braking force Fx3. That is, 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 described later. If the vehicle braking force requirement value FxTr is equal to or greater than the third braking force Fx3 (S49: YES), the processing circuit 81 shifts the processing to step S51. On the other hand, if the vehicle braking force requirement value FxTr is less than the third braking force Fx3 (S49: NO), the processing circuit 81 shifts the processing to step S53.

[0100] 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.

[0101] More specifically, in step S511, the processing circuit 81 transmits an instruction to the regenerative control unit 92 of the regenerative device 90 to hold the regenerative braking force FxR, similarly to step S211 described above. In the next 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).

[0102] (F1) The front wheel hydraulic pressure target value PwcFTr is equal to the rear wheel hydraulic pressure target value PwcRTr. (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.

[0103] In step S515, 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. For example, the processing circuit 81 sets the servo pressure Psc for making the front wheel hydraulic pressure PwcF equal to the front wheel hydraulic pressure target value PwcFTr 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 front wheel hydraulic pressure PwcF and the front wheel friction braking force FxMF.

[0104] The rear wheel hydraulic pressure adjusting unit 74 has a check valve 731 arranged in parallel with the holding valve 641. Therefore, when the hydraulic pressure in the hydraulic passage between the holding valve 641 and the first differential pressure control valve 631 becomes less than the rear wheel hydraulic pressure PwcR due to a decrease in the servo pressure Psc, the check valve 731 opens. This causes the brake fluid in the wheel cylinder 11 corresponding to the rear wheel cylinder to flow out via the check valve 731. As a result, the processing circuit 81 can reduce the rear wheel hydraulic pressure PwcR and the rear wheel friction braking force FxMR by reducing the servo pressure Psc without changing the magnitude of the current flowing through the solenoid of the holding valve 641.

[0105] The processing circuit 81 ends the third decrease process when the servo pressure Psc is decreased by the electric cylinder 51. After that, the processing circuit 81 temporarily ends 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 response to the reduction in the vehicle braking force requirement 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. The processing circuit 81 ends the fourth reduction process. After that, the processing circuit 81 temporarily ends the regenerative cooperative control.

[0106] When the regenerative control unit 92 receives 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 response to the reduction in the regenerative braking force target value FxRTr.

[0107] <Actions and Effects of the Present Embodiment> The operation and effect of the braking system 20 during vehicle braking will be described with reference to Figures 5 and 6. Figure 5(A) shows the transitions of the vehicle braking force request value FxTr and the vehicle braking force Fx. Figure 5(B) shows the transitions of the regenerative braking force target value FxRTr and the regenerative braking force FxR. Figure 5(C) shows the transitions of 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.

[0108] Since a request for vehicle braking is generated from timing T0, the processing circuit 81 of the control device 80 increases the vehicle braking force request value FxTr. In 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 under a condition in which 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 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. Therefore, in 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 is maintained 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 according to the regenerative braking force target value FxRTr to the rear wheels RL, RR.

[0109] During the period from timing T0 to timing T1, the rear wheel braking force FxAR is increased but the front wheel braking force FxAF is not increased. The state point P when no request for vehicle braking is generated is called 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 vertical axis of Fig. 6 in which the rear wheel braking force FxAR is increased.

[0110] 6, the region on the side where the rear wheel braking force FxAR is greater than the ideal braking force distribution line L1 is an unstable region USR where the vehicle behavior is likely to become unstable. In the period from timing T0 to timing T1, state point P is included in the unstable region USR.

[0111] However, in the period from timing T0 to timing T1, the vehicle braking force Fx is not so large. Therefore, even if only the rear wheel braking force FxAR of the front wheel braking force FxAF and the rear wheel braking force FxAR increases, the stability of the vehicle behavior is actually maintained.

[0112] Returning to FIG. 5, even after timing T1 has passed, the processing circuit 81 increases 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. In other words, 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 increasing the front wheel hydraulic pressure PwcF by increasing the servo pressure Psc in response to an increase in the vehicle braking force request value FxTr, while maintaining the rear wheel hydraulic pressure PwcR and maintaining the regenerative braking force FxR.

[0113] When the front wheel hydraulic pressure PwcF is increased, the front wheel friction braking force FxMF is increased. Meanwhile, the rear wheel hydraulic pressure PwcR is maintained at 0 (zero) and the regenerative braking force FxR is maintained. Therefore, the front wheel braking force FxAF increases, while the rear wheel braking force FxAR 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 horizontal axis of FIG. 6 in which the front wheel braking force FxAF is increased. As shown in FIG. 6, when the state point P moves in the direction of the second arrow Y2 in accordance with an increase in the vehicle braking force requirement value FxTr, the state point P passes through the ideal braking force allocation line L1. In other words, the state point P leaves the unstable region USR. This allows the braking system 20 to suppress a decrease in the stability of the vehicle behavior caused by an increase in the vehicle braking force Fx.

[0115] Returning to FIG. 5, even after the timing T2 has passed, the processing circuit 81 increases the vehicle braking force request value FxTr. At the timing T2, the vehicle braking force request value FxTr becomes equal to the first braking force Fx1. Therefore, at the 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 a situation in which 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 response to an increase in the vehicle braking force request value FxTr.

[0116] When the front wheel hydraulic pressure PwcF is increased, the front wheel friction braking force FxMF is increased. When the rear wheel hydraulic pressure PwcR is increased, the rear wheel friction braking force FxMR is increased. 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 Fig. 6, the state point P at the end of the second increase process is referred to as the "second state point PB." In this case, 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 braking-force distribution line L2 that increases the front wheel braking force FxAF. 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 braking-force distribution line L2.

[0118] 5, at time T3, the processing circuit 81 starts to hold the vehicle braking force requirement 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, from timing T4 onwards, the processing circuit 81 reduces the vehicle braking force requirement value FxTr. In the period from timing T4 to timing T5, the vehicle braking force requirement 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 requirement value FxTr is reduced under a condition in which the vehicle braking force requirement 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 front wheel hydraulic pressure PwcF, while maintaining the rear wheel hydraulic pressure PwcR.

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

[0121] Here, let us consider a case where a reduction process is executed from timing T4 to reduce the front wheel braking force FxAF while maintaining the regenerative braking force FxR. In this case, the regenerative braking force FxR is not reduced, so the recovery efficiency of the regenerative energy is higher than when the first increase process is executed. However, there is a risk that the stability of the vehicle behavior may decrease.

[0122] That is, in FIG. 6, state point P immediately before the start of the decrease in vehicle braking force request value FxTr is set as "third state point PC." In this state, if the front wheel friction braking force FxMF is decreased while the regenerative braking force FxR and the rear wheel friction braking force FxMR are maintained, state point P moves from the third state point PC in the direction of dashed arrow YA. The dashed arrow YA indicates the direction along the horizontal axis of FIG. 6 in which the front wheel braking force FxAF is decreased. Then, as shown in FIG. 6, state point P enters the unstable region USR when the vehicle braking force Fx is relatively large. Therefore, there is a risk that the stability of the vehicle behavior will decrease as the vehicle braking force Fx decreases.

[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 braking force distribution line L2 in which the front wheel braking force FxAF is reduced. Therefore, when state point P moves in the direction of the fourth arrow Y4 as the vehicle braking force request value FxTr decreases, state point P moves on the equal pressure braking force distribution line L2. In other words, when the vehicle braking force Fx is relatively large, state point P is prevented from entering the unstable region USR. Therefore, the brake system 20 can reduce the vehicle braking force Fx while maintaining the stability of the vehicle behavior in a situation where regenerative braking force FxR is applied to the rear wheels RL, RR.

[0125] Returning to FIG. 5, even after the timing T5 has passed, the processing circuit 81 continues to reduce the vehicle braking force requirement value FxTr. At the timing T5, the vehicle braking force requirement value FxTr becomes equal to the first braking force Fx1. Therefore, at the 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 requirement value FxTr is reduced under a condition in which the vehicle braking force requirement value FxTr is less than the first braking force Fx1 and 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 in which the regenerative braking force FxR is maintained, and the servo pressure Psc is reduced to reduce the front wheel hydraulic pressure PwcF while maintaining the rear wheel hydraulic pressure PwcR.

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

[0127] The vehicle braking force requirement value FxTr at the end of the first decrease process is equal to the first braking force Fx1. Therefore, in Fig. 6, the state point P at the end of the first decrease process is the second state point PB. Then, when the second decrease 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 horizontal axis in Fig. 6 in which the front wheel braking force FxAF is reduced.

[0128] Here, when state point P moves in the direction of the fifth arrow Y5 in accordance with a decrease in the vehicle braking force requirement value FxTr, state point P may pass through the ideal braking force distribution line L1 and enter the unstable region USR. However, at the point when state point P enters the unstable region USR, the vehicle braking force Fx is relatively small. Therefore, even if state point P is in the unstable region USR, the stability of the vehicle behavior does not actually decrease, and stability is ensured. In other words, the braking system 20 can reduce the vehicle braking force Fx while suppressing a decrease in the stability of the vehicle behavior.

[0129] Returning to FIG. 5, even after the timing T6 has passed, the processing circuit 81 continues to reduce the vehicle braking force requirement value FxTr. At the timing T6, the vehicle braking force requirement value FxTr becomes equal to the second braking force Fx2. Also, at the timing T6, the front wheel hydraulic pressure PwcF and the rear wheel hydraulic pressure PwcR become equal to each other as shown in FIG. 5(C). Therefore, at the 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 requirement value FxTr is reduced under a condition in which the vehicle braking force requirement value FxTr is less than the second braking force Fx2 and is equal to or greater than 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 and both the front wheel hydraulic pressure PwcF and the rear wheel hydraulic pressure PwcR are reduced by reducing the servo pressure Psc.

[0130] When the front wheel hydraulic pressure PwcF is reduced, the front wheel friction braking force FxMF is reduced. When the rear wheel hydraulic pressure PwcR is reduced, the rear wheel friction braking force FxMR 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 rear wheel hydraulic pressure adjusting section 74 of the braking system 20 has a holding valve 641 and a check valve 731 arranged in parallel with the holding valve 641. The check valve 731 opens when the hydraulic pressure on the pressure regulating unit 50 side of the holding valve 641 is less than the rear wheel hydraulic pressure PwcR. When the check valve 731 opens, the brake fluid flows from the wheel cylinders 11 for the rear wheels RL and RR to the pressure regulating unit 50 through the check valve 731. Therefore, in the braking system 20, both the front wheel hydraulic pressure PwcF and the rear wheel hydraulic pressure PwcR are reduced by reducing the servo pressure Psc by the operation of the pressure regulating unit 50. Therefore, the control is simplified compared to the case where the rear wheel hydraulic pressure PwcR is reduced by reducing the magnitude of the current flowing through the solenoid of the holding valve 641.

[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 shows a method of decreasing both the front wheel braking force FxAF and the rear wheel braking force FxAR.

[0133] Returning to FIG. 5, even after the timing T7 has passed, the processing circuit 81 reduces the vehicle braking force requirement value FxTr. At the timing T7, the vehicle braking force requirement 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, since 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 requirement value FxTr. Therefore, at the 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 requirement value FxTr is reduced under the condition that the vehicle braking force requirement 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 processing 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 FIG. 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 vertical axis. Therefore, by decreasing the regenerative braking force target value FxRTr, 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 vertical axis in which the rear wheel braking force FxAR 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 rear wheels is large, so some of the regenerative cooperative control is different from that in the first embodiment. In the following explanation, the differences from the first embodiment will be mainly explained, and the same reference numerals will be used to designate the same members as in the first embodiment, and duplicate explanations will be omitted.

[0136] <Regenerative device> The regenerative device 90 of the vehicle is configured to apply a regenerative braking force FxR larger than the first braking force Fx1 to the rear wheels RL, RR. In other words, the regenerative braking force upper limit value FxRL is larger 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. In step S11, the processing circuit 81 derives the vehicle braking force requirement value FxTr. In the next step S13, the processing circuit 81 determines whether or not the vehicle braking force requirement value FxTr is increasing. If the processing circuit 81 determines that the vehicle braking force requirement value FxTr is increasing (S13: YES), the processing circuit 81 shifts the processing to step S16. On the other hand, if the processing circuit 81 determines that the vehicle braking force requirement value FxTr is not increasing (S13: NO), the processing circuit 81 shifts the processing to step S31. The processing flow after step S31 is the same as in the first embodiment, and therefore will not be described.

[0138] In step S16, the processing circuit 81 determines whether the vehicle braking force requirement 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 braking system 20, the regenerative braking force FxR is applied only to the rear wheels RL and RR among the front wheels FL and FR and the rear wheels RL and RR. Therefore, if the regenerative braking force FxR applied to the rear wheels RL and RR is increased to the regenerative braking force upper limit value FxRL by the first increase process described below, the stability of the vehicle behavior may decrease. Therefore, a braking force that can maintain the stability of the vehicle behavior even if the regenerative braking force FxR is applied to the rear wheels RL and RR by executing the first increase process is set as the switching regenerative braking force FxRA.

[0140] When 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, when 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 the content of the first increment process 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 requirement value FxTr is less than the first braking force Fx1. If the processing circuit 81 determines that the vehicle braking force requirement 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 requirement 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 content of the second increment process is the same as the content of the second increment process executed in the first embodiment, so a description of the second increment process will be omitted.

[0144] In step S25, the processing circuit 81 determines whether the vehicle braking force requirement value FxTr is less than the regenerative braking force upper limit value FxRL. If the vehicle braking force requirement value is less than the regenerative braking force upper limit value FxRL (S25: YES), the processing circuit 81 shifts the process to step S27. On the other hand, if the vehicle braking force requirement value is equal to or greater than the regenerative braking force upper limit value FxRL (S25: NO), the processing circuit 81 shifts the process 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 requirement value FxTr, and increases the servo pressure Psc in response to an increase in the vehicle braking force requirement value FxTr, thereby increasing the front wheel hydraulic pressure PwcF while maintaining the rear wheel hydraulic pressure PwcR.

[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 can maintain the state in which the distribution ratio of the front wheel braking force FxAF and the rear wheel braking force FxAR is 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] 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. This allows the regenerative control unit 92 to increase the regenerative braking force FxR in response to an increase in the regenerative braking force target value FxRTr.

[0148] In the next step S275, the processing circuit 81 increases the front wheel hydraulic pressure target value PwcFTr and holds the rear wheel hydraulic pressure target value PwcRTr at 0 (zero). The processing circuit 81 derives a value that can maintain the state in which the distribution ratio of the front wheel braking force FxAF and the rear wheel braking force FxAR is the above-mentioned reference front / rear distribution ratio as the front wheel friction braking force target value FxMFTr. Then, the processing circuit 81 sets a value obtained by converting the front wheel friction braking force target value FxMFTr into a front wheel hydraulic pressure as the front wheel hydraulic pressure target value PwcFTr.

[0149] In step S277, 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. For example, the processing circuit 81 sets the servo pressure Psc for making the front wheel hydraulic pressure PwcF equal to the front wheel hydraulic pressure target value PwcFTr 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 front wheel hydraulic pressure PwcF and the front wheel friction braking force FxMF.

[0150] The processing circuit 81 also operates the rear wheel hydraulic pressure adjustment unit 74 so as not to increase the rear wheel hydraulic pressure PwcR even if the servo pressure Psc is increased as described above. For example, the processing circuit 81 maintains the rear wheel hydraulic pressure PwcR by closing the multiple retention valves 641 of the rear wheel hydraulic pressure adjustment unit 74. At this time, the processing circuit 81 may increase the magnitude of the current flowing through the solenoid of the retention valve 641 as the differential pressure between the servo pressure Psc and the rear wheel hydraulic pressure target value PwcRTr increases. Then, the processing circuit 81 ends the third increase process. After that, the processing circuit 81 temporarily ends 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 response to an increase in the vehicle braking force requirement value FxTr while maintaining the regenerative braking force FxR.

[0152] Specifically, in step S291, the processing circuit 81 transmits an instruction to the regenerative control unit 92 of the regenerative device 90 to hold the regenerative braking force FxR, similarly to step S211 described above. In the next 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 of front wheel braking force FxAF and rear wheel braking force FxAR is the above standard front / rear distribution ratio. 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 also referred to as the "reference hydraulic pressure difference ΔPwcB1."

[0155] In step S295, 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. For example, the processing circuit 81 sets the servo pressure Psc for making the front wheel hydraulic pressure PwcF equal to the front wheel hydraulic pressure target value PwcFTr 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 front wheel hydraulic pressure PwcF and the front wheel friction braking force FxMF.

[0156] Furthermore, the processing circuit 81 operates the rear wheel hydraulic pressure adjusting unit 74 so that the rear wheel hydraulic pressure PwcR 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 retention valves 641 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 requirement value FxTr, thereby increasing the rear wheel hydraulic pressure PwcR and the rear wheel friction braking force FxMR. Then, the processing circuit 81 ends the fourth increase processing. After that, the processing circuit 81 temporarily ends the regenerative cooperative control.

[0157] <Actions and Effects of the Present Embodiment> With reference to Figures 8 and 9, the operation and effect of the braking system 20 during vehicle braking will be described, focusing on the differences from the first embodiment. Figure 8(A) shows the transitions of the vehicle braking force request value FxTr and the vehicle braking force Fx. Figure 8(B) shows the transitions of the regenerative braking force target value FxRTr and the regenerative braking force FxR. Figure 8(C) shows the transitions of 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 generated from timing T10, the processing circuit 81 of the control device 80 increases the vehicle braking force requirement value FxTr. In the period from timing T10 to timing T11, the vehicle braking force requirement value FxTr is equal to or less than the switchover regenerative braking force FxRA. When the vehicle braking force requirement value FxTr is increased under the condition that the vehicle braking force requirement 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 requirement value FxTr, while the front wheel hydraulic pressure PwcF and the rear wheel hydraulic pressure PwcR are not increased.

[0159] During the period from timing T10 to timing T11, the rear wheel braking force FxAR is increased but the front wheel braking force FxAF 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 vertical axis in Fig. 9 in which the rear wheel braking force FxAR is increased.

[0160] In the period from timing T10 to timing T11, the state point P is included in the unstable region USR in Fig. 9. However, the vehicle braking force Fx is not so large in this period. Therefore, even if only the rear wheel braking force FxAR of the front wheel braking force FxAF and the rear wheel braking force FxAR increases as shown by the first arrow Y11, the stability of the vehicle behavior is actually maintained.

[0161] Returning to FIG. 8, even after the timing T11 has passed, the processing circuit 81 increases the vehicle braking force requirement value FxTr. At the timing T11, the regenerative braking force target value FxRTr becomes equal to the switching regenerative braking force FxRA. Therefore, at the 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 requirement value FxTr is increased under the circumstances that the regenerative braking force FxR reaches the switching regenerative braking force FxRA and the vehicle braking force requirement 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 to increase the front wheel hydraulic pressure PwcF by increasing the servo pressure Psc in response to the increase in the vehicle braking force requirement value FxTr, while maintaining the rear wheel hydraulic pressure PwcR and maintaining the regenerative braking force FxR.

[0162] When the front wheel hydraulic pressure PwcF is increased, the front wheel friction braking force FxMF is increased. Meanwhile, the rear wheel hydraulic pressure PwcR is maintained at 0 (zero) and the regenerative braking force FxR is maintained. Therefore, the front wheel braking force FxAF increases, while the rear wheel braking force FxAR 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 horizontal axis in FIG. 9 in which the front wheel braking force FxAF is increased. As shown in FIG. 9, when the state point P moves in the direction of the second arrow Y12 in accordance with an increase in the vehicle braking force requirement value FxTr, the state point P passes through the ideal braking force allocation line L1. In other words, the state point P leaves the unstable region USR. This allows the braking system 20 to suppress a decrease in the stability of the vehicle behavior 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 increases 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. Also, 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. In other words, when the vehicle braking force request value FxTr is increased under a situation in which 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 required value FxTr, and the servo pressure Psc is increased in accordance with an increase in the vehicle braking force required value FxTr, thereby increasing the front wheel hydraulic pressure PwcF while maintaining the rear wheel hydraulic pressure PwcR.

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

[0166] In Fig. 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 braking-force distribution line L2 in which the front wheel braking force FxAF 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 braking-force distribution line L2.

[0167] Returning to FIG. 8, even after the timing T13 has passed, the processing circuit 81 increases the vehicle braking force request value FxTr. At the timing T13, the regenerative braking force FxR becomes equal to the regenerative braking force upper limit value FxRL. Therefore, at the timing T13, the processing circuit 81 switches the processing from the third increase processing to the fourth increase processing. That is, in a situation where the vehicle braking force request value FxTr is increased under the condition that 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 is increased, the front wheel friction braking force FxMF is increased. When the rear wheel hydraulic pressure PwcR is increased, the rear wheel friction braking force FxMR is increased. 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 braking force distribution line L2.

[0170] 8, at time T14, the processing circuit 81 holds the vehicle braking force requirement 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.

[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 becomes the fourth state point PD1 on the equal pressure braking force distribution line L2. Then, 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 braking force distribution line L2 in which the front wheel braking force FxAF is reduced. Then, 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 horizontal axis of FIG. 9 in which the front wheel braking force FxAF is reduced. After that, 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 a method of reducing both the front wheel braking force FxAF and the rear wheel braking force FxAR. 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 vertical axis of Fig. 9 in which the rear wheel braking force FxAR is decreased. Then, the state point P returns to the initial state point P0.

[0173] (Example of change) The above-described embodiments may be modified as follows: The above-described embodiments and the following modifications may be combined with each other to the extent that they are not technically inconsistent.

[0174] In the above-described 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, the processing circuit 81 may be configured to execute a fourth increase process when the vehicle braking force requirement value FxTr is increased.

[0175] The braking system 20 implements the second embodiment when the regenerative device 90 of the vehicle is capable of applying a regenerative braking force FxR larger than the first braking force Fx1 to the rear wheels RL and RR, but is not limited to this. Even when the braking system 20 is capable of applying a regenerative braking force FxR larger than the first braking force Fx1 to the rear wheels RL and RR, the braking system 20 may execute the same regenerative cooperative control as the first embodiment. For example, when the upper limit value of the regenerative braking force FxR is set to a value that ensures the stability of the vehicle, for example, the switching regenerative braking force FxRA, and the rear wheel braking force is increased to be greater than the switching regenerative braking force FxRA that ensures the stability of the vehicle, the regenerative braking force FxR may be maintained at the switching regenerative braking force FxRA and the rear wheel frictional braking force FxMR may be increased. In general, when comparing the anti-lift forces generated in the suspension of the rear wheels RL and RR when the same rear wheel braking force is generated, the anti-lift force generated by the frictional braking force is often larger than the anti-lift force generated by the regenerative braking force. Therefore, for example, in cases where emphasis is placed on suppressing changes in vehicle posture during braking, the braking system 20 may be configured to execute regenerative cooperative control similar to that of the first embodiment described above, even if the vehicle's regenerative device 90 is capable of applying a regenerative braking force FxR greater than the first braking force Fx1 to the rear wheels RL, RR.

[0176] In the third reduction process, the processing circuit 81 may gradually reduce the magnitude of the current flowing to the solenoid of the retention valve 641 of the rear wheel hydraulic pressure adjustment unit 74, thereby reducing the rear wheel hydraulic pressure PwcR in accordance with the reduction in the servo pressure Psc.

[0177] When the front wheel hydraulic pressure PwcF becomes equal to the rear wheel hydraulic pressure PwcR by reducing the front wheel hydraulic pressure PwcF through the execution of the second reduction process, the processing circuit 81 may not execute the third reduction process. For example, the processing circuit 81 may execute 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 reduction in the vehicle braking force requirement value FxTr.

[0178] The pressure regulating unit may be a unit other than the pressure regulating unit 50 having the electric cylinder 51 as a pressure source. For example, the pressure regulating unit may be a unit having an electric pump as a pressure source. An example of such a pressure regulating unit is disclosed in JP 2019-182278 A.

[0179] 1 may be used as the hydraulic pressure generating device. In the hydraulic pressure generating device 22, brake fluid output from the pressure regulating 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, FR. However, the hydraulic pressure generating device may be configured to supply brake fluid output from the pressure regulating unit to the wheel cylinders 11 for the front wheels FL, FR without passing through the master cylinder 31.

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

[0181] (a) The processing circuit 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 codes or instructions configured to cause the CPU to execute processes. The 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 execute various processes. Examples of the dedicated hardware circuits include application specific integrated circuits, i.e., ASIC and FPGA. ASIC is an abbreviation for "Application Specific Integrated Circuit" and "FPGA" is an abbreviation 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 a dedicated hardware circuit that executes the remaining processes among the various processes.

[0184] <Other technical ideas> Next, the technical ideas that can be understood from the above-described embodiments and modifications will be described. (Additional Note) The braking system comprises: a master cylinder having a master piston that divides a servo chamber and a master chamber, the master piston moving in response to an increase in hydraulic pressure in the servo chamber causes brake fluid to flow out of the master chamber, and the master piston moving in response to a decrease in hydraulic pressure in the servo chamber causes brake fluid to flow into the master chamber; a first supply fluid path connecting the pressure regulating unit and the servo chamber; A second supply liquid path branched off from the first liquid path, It is preferable that the brake fluid flowing from the first fluid path to the second 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] The term "at least one" used herein means "one or more" of the desired options. As an example, the term "at least one" used herein means "only one option" or "both of two options" if the number of options is two. As another example, the term "at least one" used herein means "only one option" or "any combination of two or more options" if the number of options is three or more. [Explanation of symbols]

[0186] 11...Wheel cylinder (front wheel cylinder, rear wheel cylinder) 20…Braking system 31...Master cylinder 43…Master piston 50…Pressure adjustment unit 51...Electric cylinder 55...Fifth flow path (first supply liquid path) 58...6th flow path (second supply liquid path) 641...Retention valve 731...Check valve 74…Rear wheel hydraulic pressure adjustment section 80...Control device 81...Processing circuit 90…Regenerative device FL, FR…Front wheel Rm…Master room RL,RR…Rear wheel Rs…Servo room

Claims

1. A braking system is applied to a vehicle including a front wheel cylinder and a rear wheel cylinder, front wheels to which a frictional braking force corresponding to a front wheel hydraulic pressure which is a hydraulic pressure in the front wheel cylinder is applied, rear wheels to which a frictional braking force corresponding to a rear wheel hydraulic pressure which is a hydraulic pressure in the rear wheel cylinder is applied, and a regenerative device which applies a regenerative braking force to the rear wheels, the braking system controlling a vehicle braking force which is a braking force applied to the vehicle, comprising: a pressure adjusting unit that generates a reference hydraulic pressure and generates the front wheel hydraulic pressure corresponding to the reference hydraulic pressure; a rear wheel hydraulic pressure adjusting unit configured to make the rear wheel hydraulic pressure lower than the front wheel hydraulic pressure when the reference hydraulic pressure is generated; a control device that controls the regenerative device, the pressure regulating unit, and the rear wheel hydraulic pressure adjusting unit based on a vehicle braking force required value that is a required value of the vehicle braking force, The control device includes: when the vehicle braking force required value is equal to or greater than a first braking force, controlling the regenerative device, the pressure regulating unit and the rear wheel hydraulic pressure adjusting unit so that a regenerative braking force is applied to the rear wheels and a frictional braking force is applied to both the front wheels and the rear wheels; When the vehicle braking force requirement value is reduced under a condition in which the vehicle braking force requirement value is equal to or greater than the first braking force, a first reduction process is executed to reduce the regenerative braking force applied to the rear wheels and to reduce the reference hydraulic pressure to reduce the front wheel hydraulic pressure while maintaining the rear wheel hydraulic pressure. Braking system.

2. When the vehicle braking force requirement value is reduced under a condition in which the vehicle braking force requirement value is less than the first braking force and is equal to or greater than a second braking force smaller than the first braking force, the control device performs a second reduction process to maintain a regenerative braking force applied to the rear wheels and to maintain the rear wheel hydraulic pressure while reducing the reference hydraulic pressure to reduce the front wheel hydraulic pressure.

2. The braking system of claim 1.

3. When the vehicle braking force requirement value is reduced under a condition in which the vehicle braking force requirement value is less than the second braking force and is equal to or greater than a third braking force smaller than the second braking force, the control device maintains a regenerative braking force applied to the rear wheels and executes a third reduction process in which the reference hydraulic pressure is reduced to reduce both the front wheel hydraulic pressure and the rear wheel hydraulic pressure.

3. The braking system of claim 2.

4. The second braking force is the vehicle braking force when the front wheel hydraulic pressure becomes equal to the rear wheel hydraulic pressure after the front wheel hydraulic pressure is reduced by the execution of the second reduction process.

4. The braking system of claim 3.

5. 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) as a result of the third reduction process being executed, The control device executes a fourth reduction process to reduce the regenerative braking force applied to the rear wheels when the vehicle braking force requirement value is reduced under a condition in which the vehicle braking force requirement value is less than the third braking force.

5. The braking system of claim 4.

6. The rear wheel hydraulic pressure adjusting unit is a pressure retention valve, which is a linear solenoid valve provided in a fluid passage connecting the pressure regulating unit and the rear wheel cylinder, for adjusting a pressure difference between a portion of the pressure regulating unit side and a portion of the rear wheel cylinder side; a check valve disposed in parallel with the retention valve; the check valve is configured to close when the hydraulic pressure on the pressure regulating unit side of the retention valve is equal to or higher than the rear wheel hydraulic pressure, and to open when the hydraulic pressure on the pressure regulating unit side of the retention valve is lower than the rear wheel hydraulic pressure to allow the brake fluid to flow from the rear wheel cylinder to the pressure regulating unit, When both the front wheel hydraulic pressure and the rear wheel hydraulic pressure are to be reduced in the third reduction process, the control device operates the pressure regulating unit so that the reference hydraulic pressure becomes less than the rear wheel hydraulic pressure. A braking system according to claim 3 or claim 4.

7. The control device includes: a first increase process for increasing the regenerative braking force applied to the rear wheels in response to an increase in the vehicle braking force required value when the vehicle braking force required value is increased under a condition in which the vehicle braking force required value is smaller than a regenerative braking force upper limit value, which is an upper limit value of the regenerative braking force that can be applied to the rear wheels, while not increasing either the front wheel hydraulic pressure or the rear wheel hydraulic pressure; a second increase process for increasing the front wheel hydraulic pressure by increasing the reference hydraulic pressure in response to an increase in the vehicle braking force required value when the vehicle braking force required value is increased under a condition in which the regenerative braking force applied to the rear wheels has reached the regenerative braking force upper limit value, and for maintaining the rear wheel hydraulic pressure and maintaining the regenerative braking force applied to the rear wheels; a third increase process for increasing both the front wheel hydraulic pressure and the rear wheel hydraulic pressure by increasing the reference hydraulic pressure in response to an increase in the vehicle braking force required value in a situation where the regenerative braking force applied to the rear wheels has reached the regenerative braking force upper limit value and the vehicle braking force required value is equal to or greater than the first braking force, thereby maintaining the regenerative braking force applied to the rear wheels while increasing the reference hydraulic pressure in response to an increase in the vehicle braking force required value.

2. The braking system of claim 1.

8. a switching regenerative braking force is set as a braking force smaller than a regenerative braking force upper limit value that is an upper limit value of the regenerative braking force that the regenerative device can apply to the rear wheels, The control device includes: a first increase process for increasing the regenerative braking force applied to the rear wheels in response to an increase in the vehicle braking force requirement value when the vehicle braking force requirement value is smaller than the switching regenerative braking force, while not increasing either the front wheel hydraulic pressure or the rear wheel hydraulic pressure; a second increase process for increasing the front wheel hydraulic pressure by increasing the reference hydraulic pressure in response to an increase in the vehicle braking force required value while maintaining the rear wheel hydraulic pressure and maintaining the regenerative braking force applied to the rear wheels, when the vehicle braking force required value is increased under a condition in which the regenerative braking force applied to the rear wheels has reached the switching regenerative braking force and the vehicle braking force required value is less than the first braking force; a third increase process for increasing the regenerative braking force applied to the rear wheels in response to an increase in the vehicle braking force required value while increasing the reference hydraulic pressure in response to an increase in the vehicle braking force required value, in a situation where the vehicle braking force required value is equal to or greater than the first braking force, while the regenerative braking force applied to the rear wheels is equal to or greater than the switchover regenerative braking force and less than the regenerative braking force upper limit value, thereby increasing the front wheel hydraulic pressure while maintaining the rear wheel hydraulic pressure; a fourth increase process for increasing both the front wheel hydraulic pressure and the rear wheel hydraulic pressure by increasing the reference hydraulic pressure in response to an increase in the vehicle braking force requirement value, while maintaining the regenerative braking force to be applied to the rear wheels, when the vehicle braking force requirement value is increased in a situation in which the vehicle braking force requirement value is equal to or greater than the first braking force and the regenerative braking force applied to the rear wheels has reached the regenerative braking force upper limit value.

2. The braking system of claim 1.

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