Braking device and control method for braking device

Abstract

A braking device (100) comprises: an electric cylinder (51); a supply flow path through which a brake fluid flows toward a wheel cylinder (16) when the brake fluid is discharged from an output port (516) of the electric cylinder (51); an orifice (60) provided in the supply flow path; and a processing circuit. The processing circuit is configured to set the lower limit value for motor rotation speed on the basis of a negative pressure suppression characteristic and an orifice differential pressure, and to drive an electric motor of the electric cylinder (51) so that the motor rotation speed does not fall below the lower limit value when a piston (512) is moved in the backward direction (Zb) by driving the electric motor.

Description

Brake device and brake device control method

[0001] The present disclosure relates to a braking device that adjusts wheel pressure, which is hydraulic pressure in a wheel cylinder, by operating an electric cylinder, and a method for controlling the braking device.

[0002] Patent Document 1 discloses a braking device equipped with an electric cylinder. The electric cylinder includes a cylinder and a piston that moves forward or backward within the cylinder in response to the drive of an electric motor. A hydraulic chamber is defined within the cylinder by the cylinder and the piston. When the piston moves forward, brake fluid from the hydraulic chamber is discharged toward a wheel cylinder via an output port of the electric cylinder. A rear chamber is also formed within the cylinder of the electric cylinder on the opposite side of the hydraulic chamber from the piston. The braking device further includes a rear hydraulic passage connecting the rear chamber to a reservoir and an orifice that generates flow resistance for the brake fluid flowing through the rear hydraulic passage.

[0003] While the electric cylinder is operating to generate wheel pressure in the wheel cylinder, the power supply to the electric motor may be interrupted. In this case, brake fluid flows forcefully from the wheel cylinder into the electric cylinder through the output port. In the braking device described above, an orifice is provided in the rear fluid passage. Therefore, when the piston retracts due to the force of the brake fluid, the hydraulic pressure in the rear chamber is less likely to decrease. This prevents the piston from retracting forcefully.

[0004] Japanese Patent Application Laid-Open No. 2022-138690

[0005] Unlike the braking device of Patent Document 1, it is also possible to provide an orifice in the supply passage connecting the electric cylinder and the wheel cylinder. In this case, when the power supply from the power source to the electric motor is cut off while the electric cylinder is generating wheel pressure, the orifice in the supply passage acts to reduce the flow rate of brake fluid flowing into the cylinder of the electric cylinder through the supply passage. As a result, the piston in the electric cylinder is prevented from moving back too forcefully.

[0006] The control unit of the braking system may reduce the target wheel pressure, which is a target value of the wheel pressure. In this case, the control unit drives the electric motor so that the piston retracts in accordance with the reduction in the target wheel pressure. Therefore, the greater the reduction rate of the target wheel pressure, the greater the retraction rate of the piston.

[0007] When the piston retracts due to the drive of the electric motor, brake fluid flows from the supply passage through the output port into the hydraulic chamber in the cylinder, reducing the amount of brake fluid in the wheel cylinder. This reduces wheel pressure. The faster the piston retracts, the faster the wheel pressure decreases.

[0008] However, if an orifice is provided in the supply passage, the flow rate of brake fluid toward the electric cylinder does not increase in the portion of the supply passage between the orifice and the electric cylinder. Therefore, when the piston retracts at a high speed, the flow rate of brake fluid flowing from the supply passage through the output port into the hydraulic chamber in the cylinder may be lower than the flow rate corresponding to the piston's retraction speed. In this case, the amount of brake fluid flowing from the output port into the hydraulic chamber is too small compared to the expansion of the hydraulic chamber volume as the piston retracts. As a result, the pressure in the hydraulic chamber decreases, which may cause negative pressure to develop in the cylinder.

[0009] According to one aspect of the present disclosure, there is provided a braking device that generates a braking force at a wheel according to a wheel pressure, which is a hydraulic pressure in a wheel cylinder. The braking device is an electric cylinder including an electric motor, a cylinder having an output port, and a piston, wherein the electric cylinder is configured such that the piston moves forward within the cylinder based on the drive of the electric motor to eject brake fluid from the output port, and the piston moves backward within the cylinder based on the drive of the electric motor to draw brake fluid from the output port into the cylinder; a supply flow path through which brake fluid flows toward the wheel cylinder when brake fluid is ejected from the output port of the electric cylinder; an orifice provided in the supply flow path that generates flow resistance to the brake fluid flowing through the supply flow path when the wheel pressure is reduced; a memory that stores negative pressure suppression characteristics that indicate the relationship between the motor rotation speed, which is the rotational speed of the electric motor, and an orifice differential pressure, which is the differential pressure between both sides of the orifice in the supply flow path, such that negative pressure is not generated in the cylinder when the piston is moved backward; and a processing circuit configured to operate the electric cylinder. The processing circuit is configured to execute a lower limit setting process that sets a lower limit value for the motor rotation speed based on the negative pressure suppression characteristic and the orifice differential pressure, and a motor driving process that drives the electric motor so that the motor rotation speed does not fall below the lower limit value when the piston is moved in the backward direction by driving the electric motor.

[0010] According to another aspect of the present disclosure, there is provided a method for controlling a braking device that generates a braking force at a wheel corresponding to a wheel pressure, which is a hydraulic pressure in a wheel cylinder. The braking device includes an electric cylinder including an electric motor, a cylinder having an output port, and a piston, wherein the electric cylinder is configured such that the piston moves forward within the cylinder to discharge brake fluid from the output port, and the piston moves backward within the cylinder to draw brake fluid into the cylinder from the output port, a supply flow path through which brake fluid flows toward the wheel cylinder when brake fluid is discharged from the output port of the electric cylinder, and an orifice provided in the supply flow path that generates flow resistance against the brake fluid flowing through the supply flow path when the wheel pressure is reduced. The negative pressure suppression characteristic is a relationship between a motor rotation speed, which is the rotation speed of the electric motor, and an orifice differential pressure, which is the differential pressure across the orifice in the supply flow path, such that negative pressure is not generated in the cylinder when the piston moves backward. The control method includes setting a lower limit value for the motor rotation speed based on the negative pressure suppression characteristic and the orifice differential pressure, and driving the electric motor so that the motor rotation speed does not fall below the lower limit value when driving the electric motor to move the piston in the backward direction.

[0011] FIG. 1 is a schematic diagram showing an embodiment of an on-vehicle braking system. FIG. 2 is a diagram schematically showing an electric cylinder provided in the braking system of FIG. 1 and its surroundings. FIG. 3 is a block diagram showing a plurality of processes executed when driving a first electric motor provided in the electric cylinder of FIG. 2. FIG. 4 is a block diagram showing details of the target rotation angle setting process shown in FIG. 3. FIG. 5 is a graph showing the N / T characteristic of the first electric motor provided in the electric cylinder of FIG. 2. FIG. 6 is a diagram showing a negative pressure suppression characteristic map, a system limit rotation speed map, and a pressure reduction required rotation speed map in the braking system of FIG. 1. FIG. 7 is a timing chart when the electric cylinder of FIG. 2 is operated during vehicle braking. FIG. 8 is a diagram showing how a second rotation angle lower limit value is set in the second limit value setting process in a modified example.

[0012] An embodiment of a braking device and a method for controlling the braking device applied to a vehicle will be described below with reference to Figures 1 to 7. Figure 1 shows a vehicle equipped with a braking device 100. The vehicle has wheels including two front wheels 11f and two rear wheels 11r. The vehicle also has friction brakes 15, the number of which is the same as the number of wheels.

[0013] The friction braking force generated at the rear wheels 11r is referred to as the "rear wheel friction braking force FxMR," and the friction braking force generated at the front wheels 11f is referred to as the "front wheel friction braking force FxMF." The sum of the friction braking forces generated in the vehicle is referred to as the "vehicle friction braking force FxM." The sum of the rear wheel friction braking force FxMR and the front wheel friction braking force FxMF is the vehicle friction braking force FxM.

[0014] <Friction Brake> The multiple friction brakes 15 generate friction braking forces FxMF and FxMR at the corresponding wheels 11f and 11r. The friction brakes 15 each include a wheel cylinder 16, a rotating body 17, and a friction portion 18. Because the rotating body 17 rotates together with the wheels 11f and 11r, the friction portion 18 presses against the rotating body 17, generating friction braking forces FxMF and FxMR at the wheels 11f and 11r. The force pressing the friction portion 18 against the rotating body 17 increases as the wheel pressure Pw, which is the hydraulic pressure in the wheel cylinder 16, increases. Therefore, the friction brake 15 can generate larger friction braking forces FxMF and FxMR at the wheels 11f and 11r as the wheel pressure Pw increases. The wheel pressure Pw of the wheel cylinder 16 for the rear wheel 11r is sometimes referred to as the "wheel pressure Pwr," and the wheel pressure Pw of the wheel cylinder 16 for the front wheel 11f is sometimes referred to as the "wheel pressure Pwf."

[0015] <Brake Device> The brake device 100 adjusts the vehicle friction braking force FxM by controlling the wheel pressure Pw of the plurality of wheel cylinders 16. The brake device 100 includes a hydraulic pressure generating device 20, a brake actuator 70, and a control device 200. The hydraulic pressure generating device 20 and the brake actuator 70 are each configured to be able to control the wheel pressure Pw of the plurality of wheel cylinders 16. The control device 200 controls the operation of the hydraulic pressure generating device 20 and the brake actuator 70.

[0016] <Hydraulic Pressure Generator> The hydraulic pressure generator 20 includes a reservoir tank 21, a brake operating member 22, a master device 30, and an electric pressure unit 50. The reservoir tank 21 stores brake fluid and is open to the atmosphere.

[0017] The brake operating member 22 is a member that is operated by the driver of the vehicle to decelerate the vehicle. An example of the brake operating member 22 is a brake pedal. The driver's operation of the brake operating member 22 is referred to as a "braking operation." When a braking operation is being performed, the hydraulic pressure generating device 20 can generate wheel pressures Pw in the multiple wheel cylinders 16 according to the amount of operation of the brake operating member 22.

[0018] <Master Device> The master device 30 includes a master cylinder 31, a stroke simulator 32, a plurality of flow paths 331, 332, and 333 connected to the master cylinder 31, and a plurality of control valves 341 and 342 that control the flow of brake fluid. The master device 30 includes sensors that detect hydraulic pressure, including a plurality of hydraulic pressure sensors 351 and 352. Detection signals from the plurality of hydraulic pressure sensors 351 and 352 are input to the control device 200.

[0019] The stroke simulator 32 is capable of generating a reaction force corresponding to the amount of operation of the brake operating member 22. The master cylinder 31 includes a main cylinder 41, a cover cylinder 42, a master piston 43, and an input piston 44. The master piston 43 and the input piston 44 can each move relative to the main cylinder 41 and the cover cylinder 42. 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.

[0020] The main cylinder 41 has a plate-shaped bottom wall 411, a cylindrical peripheral wall 412 extending from the bottom wall 411 along the axis of the bottom wall 411, and a first annular wall 413 extending from the rear end of the peripheral wall 412 toward the axis of the peripheral wall 412. A hole is formed in the first annular wall 413, into which the rear end of a master piston 43 (described later) is inserted.

[0021] Within the main cylinder 41, a master chamber Rm is defined by a bottom wall 411, a peripheral wall 412, and a master piston 43. Hereinafter, in the master cylinder 31, the leftward direction in FIG. 1, i.e., the direction in which the master piston 43 moves to reduce the volume of the master chamber Rm, will be referred to as the "forward" direction. On the other hand, the opposite direction to the forward direction will be referred to as the "rearward" direction. The rearward direction is also the direction in which the volume of the master chamber Rm increases.

[0022] A first fluid chamber R1 is defined behind the master chamber Rm within the main cylinder 41 by the peripheral wall 412 and the master piston 43. A servo chamber Rs is defined behind the first fluid chamber R1 within the main cylinder 41 by the peripheral wall 412, the first annular wall 413, and the master piston 43. 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.

[0023] The cover cylinder 42 has a cylindrical peripheral wall 421 and a second annular wall 422 extending from the rear end of the peripheral wall 421 toward the axis of the peripheral wall 421. The peripheral wall 421 is attached to the first annular wall 413 so that its axis coincides with that of the peripheral wall 412 of the main cylinder 41. The second annular wall 422 has a hole into which the rear end of the input piston 44 (described later) is inserted.

[0024] Within the cover cylinder 42, a second fluid chamber R2 is defined by the peripheral wall 421, the second annular wall 422, and the first annular wall 413 of the main cylinder 41. In the master cylinder 31, the second fluid chamber R2 is located rearward of the servo chamber Rs.

[0025] The master piston 43 is housed in the master cylinder 31 in surface contact with the inner circumferential surface of the peripheral wall 412 of the main cylinder 41 and the inner circumferential surface of the first annular wall 413. Therefore, when the master piston 43 moves in the axial direction, the master piston 43 slides on the inner circumferential surface of the peripheral wall 412 and the inner circumferential surface of the first annular wall 413. The rear end of the master piston 43 protrudes rearward beyond the first annular wall 413 and is located in the second fluid chamber R2.

[0026] The input piston 44 is housed in the master cylinder 31 in surface contact with the inner circumferential surface of the second annular wall 422 of the cover cylinder 42. Therefore, when the input piston 44 moves in the axial direction, the input piston 44 slides on the inner circumferential surface of the second annular wall 422. The rear end of the input piston 44 protrudes rearward beyond the second annular wall 422. The brake operating member 22 is connected to the rear end of the input piston 44. In the second fluid chamber R2, a gap is formed between the input piston 44 and the master piston 43. When the brake operating member 22 is operated, the input piston 44 moves in a direction approaching the master piston 43.

[0027] The master spring 45 is disposed between the bottom wall 411 of the main cylinder 41 and the master piston 43. The master spring 45 biases the master piston 43 rearward, so when the master piston 43 moves forward, the master spring 45 is elastically compressed.

[0028] The input spring 46 is disposed between the first annular wall 413 of the main cylinder 41 and the input piston 44. The input spring 46 biases the input piston 44 rearward, so that when the input piston 44 moves forward, the input spring 46 is elastically compressed.

[0029] In the master cylinder 31, the master chamber Rm is connected to the reservoir tank 21. More specifically, a portion of the master chamber Rm near the rear end is connected to the reservoir tank 21 via a port formed in the 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 reservoir tank 21 is released. From this point on, the hydraulic pressure in the master chamber Rm increases as the master piston 43 moves forward. For example, when the hydraulic pressure in the servo chamber Rs increases, the hydraulic pressure in the servo chamber Rs moves the master piston 43 forward. This increases the hydraulic pressure in the master chamber Rm.

[0030] The first flow path 331 connects a second hydraulic circuit 712 of the brake actuator 70 (described later) to the master chamber Rm. The second flow path 332 connects the first fluid chamber R1 to the third flow path 333. The stroke simulator 32 is also connected to the second flow path 332. The third flow path 333 connects the second fluid chamber R2 to the reservoir tank 21.

[0031] 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 provided in a portion of the third flow path 333 closer to the second liquid chamber R2 than the connection point with the second flow path 332. The second control valve 342 is provided in a portion of the third flow path 333 on the opposite side of the connection point with the second flow path 332 from the first control valve 341. When the control device 200 is operating, the first control valve 341 is opened and the second control valve 342 is closed.

[0032] The master hydraulic pressure sensor 351 detects the hydraulic pressure in the master chamber Rm. For example, the master hydraulic pressure sensor 351 is provided in the first flow path 331. The input hydraulic pressure sensor 352 detects the hydraulic pressure in the second hydraulic chamber R2. For example, the input hydraulic pressure sensor 352 is provided in a portion of the third flow path 333 between the connection point with the second hydraulic chamber R2 and the first control valve 341. In the following description, the hydraulic pressure based on the detection signal of the master hydraulic pressure sensor 351 will be referred to as the "master pressure Pmc." The hydraulic pressure based on the detection signal of the input hydraulic pressure sensor 352 will be referred to as the "input hydraulic pressure Pgs."

[0033] 1 and 2, the pressurizing unit 50 includes an electric cylinder 51. The pressurizing unit 50 can adjust the wheel pressures Pw of the plurality of wheel cylinders 16 by operating the electric cylinder 51.

[0034] The pressurizing unit 50 has brake fluid flow paths including a fourth flow path 54, a fifth flow path 55, and a sixth flow path 56. The fourth flow path 54 is connected to an input port 515 of the electric cylinder 51 and the reservoir tank 21. 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 56 is connected to a first hydraulic pressure circuit 711 of the brake actuator 70 (described later) and the fifth flow path 55. Therefore, the electric cylinder 51 can supply brake fluid discharged from the output port 516 to both the servo chamber Rs and the first hydraulic pressure circuit 711.

[0035] The pressurizing unit 50 is equipped with a servo pressure sensor 58 and a rotation angle sensor 59. The discharge pressure of the brake fluid from the electric cylinder 51 via the output port 516 is referred to as the "servo pressure." The servo pressure sensor 58 detects the servo pressure. For example, the servo pressure sensor 58 is connected to the fifth flow path 55. The rotation angle sensor 59 detects the motor rotation angle θ, which is the rotation angle of the output shaft of the first electric motor 513. Hereinafter, the servo pressure based on the detection signal of the servo pressure sensor 58 will be referred to as the "servo pressure detection value PscE." The motor rotation angle θ based on the detection signal of the rotation angle sensor 59 will be referred to as the "rotation angle detection value θE."

[0036] The electric cylinder 51 includes a cylinder 511, a piston 512, a first electric motor 513, and a conversion mechanism 514. The piston 512 is slidably disposed within the cylinder 511. The first electric motor 513 is the power source of the electric cylinder 51. The conversion mechanism 514 converts the rotation of the output shaft of the first electric motor 513 into linear movement of the piston 512. Of the forward and reverse rotation directions of the first electric motor 513, the forward direction is the rotation direction when the electric cylinder 51 increases the hydraulic pressure of the brake fluid. The reverse direction is the rotation direction when the electric cylinder 51 decreases the hydraulic pressure of the brake fluid. When the first electric motor 513 rotates in the forward direction, the motor rotation angle θ increases. Furthermore, when the first electric motor 513 rotates in the forward direction, the motor rotation speed Nmt, which is the rotational speed of the first electric motor 513, becomes a positive value. On the other hand, when the first electric motor 513 rotates in the reverse direction, the motor rotation angle θ decreases. Furthermore, when the first electric motor 513 rotates in the reverse direction, the motor rotation speed Nmt, which is the rotation speed of the first electric motor 513, becomes a negative value.

[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 direction of linear movement of the piston 512 when reducing the volume of the hydraulic pressure chamber Re will be referred to as the "forward direction Za," and the direction opposite to the forward direction Za will be referred to as the "rearward direction Zb." The rearward direction Zb is also the direction of linear movement of the piston 512 when increasing the volume of the hydraulic pressure chamber Re.

[0038] 2 , a restricting member 518 that restricts movement of the piston 512 in the backward direction Zb is disposed in the electric cylinder 51 at a position further in the backward direction Zb than the piston 512. For example, the restricting member 518 is an elastic body such as a disc spring. When the piston 512 moves in the backward direction Zb and comes into contact with the restricting member 518, further movement of the piston 512 in the backward direction Zb is restricted by the restricting member 518. The position of the piston 512 when further movement in the backward direction Zb is restricted by the restricting member 518 is referred to as a "rearmost retreat position," which is the furthest position in the backward direction Zb within the movable range of the piston 512.

[0039] Ports, including an input port 515 and an output port 516, connecting the hydraulic chamber Re to the outside are formed in the peripheral wall of the cylinder 511. A through-hole 517 is formed in the piston 512. The through-hole 517 is positioned so that the input port 515 and the hydraulic chamber Re can communicate with each other when the piston 512 is in the most retracted position. As a result, when the piston 512 is in the most retracted position, the hydraulic chamber Re of the cylinder 511 communicates with the reservoir tank 21 via the through-hole 517, the input port 515, and the fourth flow path 54. The input port 515 is open when the piston 512 is in the most retracted position and is closed by the piston 512 when the piston 512 moves forward in the forward direction Za from the most retracted position. Even after the input port 515 is closed by the piston 512, the hydraulic pressure in the hydraulic chamber Re increases when the piston 512 moves forward in the forward direction Za.

[0040] The output port 516 is connected to the master cylinder 31 and the sixth flow path 56 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 closed by the piston 512, the piston 512 moves in the forward direction Za within the cylinder 511 in response to the drive of the first electric motor 513, causing the brake fluid in the hydraulic chamber Re to be discharged from the output port 516 to the fifth flow path 55. On the other hand, the piston 512 moves in the backward direction Zb within the cylinder 511 in response to the drive of the first electric motor 513, causing the brake fluid in the fifth flow path 55 to be drawn into the hydraulic chamber Re through the output port 516.

[0041] In the hydraulic pressure generating device 20, when brake fluid is discharged from the output port 516 of the electric cylinder 51, the brake fluid flows through the fifth flow path 55. A portion of the brake fluid flowing through the fifth flow path 55 flows toward the wheel cylinder 16 for the rear wheel 11r via the sixth flow path 56. The remaining brake fluid flows into the servo chamber Rs of the master device 30. As a result, the hydraulic pressure in the servo chamber Rs increases, causing the master piston 43 to move forward, thereby increasing the hydraulic pressure in the master chamber Rm. This causes the brake fluid in the master chamber Rm to flow through the first flow path 331 toward the wheel cylinder 16 for the front wheel 11f. Therefore, the fifth flow path 55, the sixth flow path 56, and the first flow path 331 correspond to the "supply flow path" through which brake fluid flows toward the wheel cylinder 16 when brake fluid is discharged from the output port 516 of the electric cylinder 51.

[0042] On the other hand, when the electric cylinder 51 is drawing brake fluid through the output port 516, brake fluid flows out from each of the wheel cylinder 16 for the rear wheel 11r and the wheel cylinder 16 for the front wheel 11f. As a result, brake fluid flows through the sixth flow path 56 toward the fifth flow path 55. Also, brake fluid flows through the first flow path 331 toward the master chamber Rm. As a result, the hydraulic pressure in the master chamber Rm increases, causing the master piston 43 to move in the backward direction Zb. As a result, brake fluid in the servo chamber Rs flows out into the fifth flow path 55. As a result, brake fluid flows through the fifth flow path 55 toward the electric cylinder 51.

[0043] Hereinafter, the direction in which the brake fluid flows in the supply flow path when the wheel pressure Pw is increased will be referred to as the "pressure-up direction Ya," and the direction opposite to the pressure-up direction Ya will be referred to as the "depressurization direction Yb." In this case, the depressurization direction Yb is the direction in which the brake fluid flows in the supply flow path when the wheel pressure Pw is decreased.

[0044] <Orifice> The hydraulic pressure generating device 20 includes an orifice 60 provided in a supply flow path. In the present embodiment, as shown in Fig. 2 , the orifice 60 is installed in the fifth flow path 55, which is an example of a supply flow path. For example, the orifice 60 is disposed in a portion of the fifth flow path 55 between a connection point 55a with the sixth flow path 56 and the output port 516 of the electric cylinder 51.

[0045] The orifice 60 is configured to generate flow resistance to the brake fluid flowing in the pressure reduction direction Yb through the fifth flow path 55 when the wheel pressure Pw is reduced. For example, the orifice 60 may be configured so that the flow resistance to the brake fluid flowing in the pressure reduction direction Yb through the sixth flow path 56 is greater than the flow resistance to the brake fluid flowing in the pressure increase direction Ya through the sixth flow path 56.

[0046] 1, the brake actuator 70 is configured to be able to individually adjust the wheel pressures Pw of the plurality of wheel cylinders 16, independent of the pressurizing unit 50. The brake actuator 70 has a first hydraulic pressure circuit 711 and a second hydraulic pressure circuit 712. The first hydraulic pressure circuit 711 is connected to the sixth flow path 56 and is connected to the two wheel cylinders 16 for the rear wheels 11r. The second hydraulic pressure circuit 712 is connected to the first flow path 331 and is connected to the two wheel cylinders 16 for the front wheels 11f.

[0047] An example of the configuration of the first hydraulic pressure circuit 711 and the second hydraulic pressure circuit 712 is disclosed in Japanese Patent Application Laid-Open No. 2023-20360. Of course, the first hydraulic pressure circuit 711 and the second hydraulic pressure circuit 712 may have a configuration different from that disclosed in the publication.

[0048] <Control Device> The control device 200 will be described with reference to Figure 1. Detection signals are input to the control device 200 from a plurality of sensors. The plurality of sensors includes a plurality of hydraulic pressure sensors 351, 352, 58 as well as an operation amount sensor 22a. The operation amount sensor 22a detects the amount of operation of the brake operating member 22 by the driver. The control device 200 then operates the hydraulic pressure generating device 20 and the brake actuator 70 based on the detection signals from the plurality of sensors 351, 352, 58, 22a.

[0049] In this embodiment, the control device 200 includes a plurality of electronic control devices. Hereinafter, the electronic control devices are referred to as "ECUs." Each of the plurality of ECUs is configured to be able to send and receive various information and commands via an in-vehicle network. Of the plurality of ECUs, a first ECU 210 controls the hydraulic pressure generating device 20, and a second ECU 220 controls the brake actuator 70.

[0050] Each of the multiple ECUs has a CPU, a first memory, and a second memory. The first memory stores a control program executed by the CPU and maps referenced by the CPU. The second memory stores calculation results of the CPU. Hereinafter, the CPU constituting the first ECU 210 will be referred to as "CPU 211," the first memory as "first memory 212," and the second memory as "second memory 213." In this case, the CPU 211 corresponds to the "processing circuit" that operates the electric cylinder 51.

[0051] <Operation Process of Electric Cylinder> A plurality of processes executed by the first ECU 210 when operating the electric cylinder 51 will be described with reference to FIGS. 3 to 6 .

[0052] The CPU 211 of the first ECU 210 operates the electric cylinder 51 by executing the control program stored in the first memory 212. For example, the CPU 211 executes a standard fluid volume deriving process M11, a standard total fluid volume deriving process M13, a standard position deriving process M15, an estimated fluid volume deriving process M17, an estimated total fluid volume deriving process M19, a position conversion process M21, and a position deviation deriving process M23. Furthermore, the CPU 211 executes a target position setting process M25, a filter process M27, a target rotation angle setting process M29, and a motor drive process M31.

[0053] <Standard Fluid Volume Derivation Process> In the standard fluid volume derivation process M11, the CPU 211 derives a standard fluid volume Qw, which is a base value for the amount of brake fluid supplied to the wheel cylinders 16. The CPU 211 executes the standard fluid volume derivation process M11 at predetermined calculation intervals.

[0054] The standard fluid volume Qw ​​is the amount of brake fluid required to adjust the wheel pressure Pw to the target wheel pressure. Therefore, the CPU 211 derives the standard fluid volume Qw ​​so that it increases as the target wheel pressure increases. In this embodiment, the CPU 211 derives the standard fluid volume Qw ​​including a first standard fluid volume Qwr and a second standard fluid volume Qwf. The first standard fluid volume Qwr is the standard fluid volume for the wheel cylinder 16 of the rear wheel 11r. The second standard fluid volume Qwf is the standard fluid volume for the wheel cylinder 16 of the front wheel 11f.

[0055] For example, the CPU 211 derives the first standard fluid volume Qwr and the second standard fluid volume Qwf based on a first map MP1. The first map MP1 is a map showing the relationship between brake fluid pressure and brake fluid volume. The wheel cylinder 16 for the rear wheel 11r and the wheel cylinder 16 for the front wheel 11f have different volumes. Therefore, the first map MP1 includes a first map MP1r for the rear wheel 11r and a first map MP1f for the front wheel 11f. The first map MP1r shows the relationship between the amount of brake fluid supplied to the wheel cylinder 16 for the rear wheel 11r and the wheel pressure Pwr of the wheel cylinder 16. The CPU 211 derives the brake fluid volume corresponding to the target rear wheel pressure PwrTr as the first standard fluid volume Qwr based on the first map MP1r for the rear wheel 11r. The target rear wheel pressure PwrTr is a target value for the wheel pressure Pwr of the wheel cylinder 16 for the rear wheel 11r.

[0056] The first map MP1f shows the relationship between the amount of brake fluid supplied to the wheel cylinder 16 for the front wheel 11f and the wheel pressure Pwf of the wheel cylinder 16. Based on the first map MP1f for the front wheel 11f, the CPU 211 derives a brake fluid amount corresponding to the target front wheel pressure PwfTr as a second standard fluid amount Qwf. The target front wheel pressure PwfTr is a target value for the wheel pressure Pwf of the wheel cylinder 16 for the front wheel 11f.

[0057] <Normal Total Fluid Volume Derivation Process> In the normal total fluid volume derivation process M13, the CPU 211 derives a normal total fluid volume QwA, which is a base value for the sum of the brake fluid volumes supplied to the multiple wheel cylinders 16. The CPU 211 executes the normal total fluid volume derivation process M13 each time the first normal fluid volume Qwr and the second normal fluid volume Qwf are derived in the normal fluid volume derivation process M11. For example, the CPU 211 derives the sum of the first normal fluid volume Qwr and the second normal fluid volume Qwf as the normal total fluid volume QwA.

[0058] <Standard Position Derivation Process> In the standard position derivation process M15, the CPU 211 derives the standard position PSA of the piston 512 by converting the standard total fluid volume QwA into the position of the piston 512 in the cylinder 511. The CPU 211 executes the standard position derivation process M15 each time the standard total fluid volume QwA is derived in the standard total fluid volume derivation process M13.

[0059] There is a correlation between the amount of movement of the piston 512 in the forward direction Za and the amount of brake fluid discharged from the electric cylinder 51. Therefore, the CPU 211 derives, as the standard position PSA, the position of the piston 512 corresponding to the standard total fluid volume QwA, based on the relationship between the amount of movement of the piston 512 in the forward direction Za and the amount of brake fluid discharged from the electric cylinder 51. In this case, the CPU 211 can derive, as the standard position PSA, the position of the piston 512 that has moved further in the forward direction Za as the standard total fluid volume QwA increases.

[0060] In the estimated fluid volume derivation process M17, the CPU 211 derives the estimated fluid volume Qwe, which is an estimate of the amount of brake fluid stored in the wheel cylinder 16. The CPU 211 executes the estimated fluid volume derivation process M17 at predetermined calculation intervals.

[0061] The CPU 211 derives an estimated fluid volume Qwe that includes a first estimated fluid volume Qwer, which is the estimated fluid volume in the wheel cylinder 16 for the rear wheel 11r, and a second estimated fluid volume Qwef, which is the estimated fluid volume in the wheel cylinder 16 for the front wheel 11f.

[0062] For example, the CPU 211 derives the first estimated fluid amount Qwer and the second estimated fluid amount Qwef based on the first map MP1. In this case, the CPU 211 derives the brake fluid amount corresponding to the wheel pressure Pwr as the first estimated fluid amount Qwer based on the first map MP1r for the rear wheels 11r. The CPU 211 derives the brake fluid amount corresponding to the wheel pressure Pwf as the second estimated fluid amount Qwef based on the first map MP1f for the front wheels 11f.

[0063] <Estimated Total Fluid Volume Deriving Process> In the estimated total fluid volume derivation process M19, the CPU 211 derives an estimated total fluid volume QweA, which is an estimate of the sum of the brake fluid volumes supplied to the multiple wheel cylinders 16. The CPU 211 executes the estimated total fluid volume derivation process M19 each time the first estimated fluid volume Qwer and the second estimated fluid volume Qwef are derived in the estimated fluid volume derivation process M17. For example, the CPU 211 derives the sum of the first estimated fluid volume Qwer and the second estimated fluid volume Qwef as the estimated total fluid volume QweA.

[0064] <Position Conversion Process> In the position conversion process M21, the CPU 211 derives the estimated position PSeA of the piston 512 by converting the estimated total liquid volume QweA into the position of the piston 512 in the cylinder 511. The CPU 211 executes the position conversion process M21 each time the estimated total liquid volume QweA is derived in the estimated total liquid volume derivation process M19.

[0065] As described above, there is a correlation between the amount of movement of the piston 512 in the forward direction Za and the amount of brake fluid discharged from the electric cylinder 51. Therefore, the CPU 211 derives, as the estimated position PSeA, the position of the piston 512 corresponding to the estimated total fluid volume QweA, based on the relationship between the amount of movement of the piston 512 in the forward direction Za and the amount of brake fluid discharged from the electric cylinder 51. In this case, the CPU 211 can derive, as the estimated position PSeA, the position of the piston 512 that has moved further in the forward direction Za as the estimated total fluid volume QweA increases.

[0066] <Position Deviation Deriving Process> In the position deviation derivation process M23, the CPU 211 derives a position deviation ΔPS, which is the deviation between the position detection value PS, which is the actual position of the piston 512 in the cylinder 511 of the electric cylinder 51, and the estimated position PSeA. The CPU 211 executes the position deviation derivation process M23 every time the estimated position PSeA is derived in the position conversion process M21. For example, the CPU 211 derives the value obtained by subtracting the estimated position PSeA from the position detection value PS as the position deviation ΔPS.

[0067] In the electric cylinder 51, there is a correlation between the motor rotation angle θ and the position of the piston 512. Therefore, the CPU 211 acquires the position of the piston 512 corresponding to the motor rotation angle θ as the position detection value PS based on the relationship between the motor rotation angle θ and the position of the piston 512.

[0068] In the target position setting process M25, the CPU 211 sets a target position PSTr of the piston 512 in the cylinder 511 of the electric cylinder 51. The CPU 211 executes the target position setting process M25 at predetermined calculation intervals. For example, the CPU 211 sets the sum of the reference position PSA and the position deviation ΔPS as the target position PSTr.

[0069] <Filtering Process> In the filtering process M27, the CPU 211 removes noise contained in the target position PSTr. The CPU 211 executes the filtering process M27 every time the target position PSTr is set in the target position setting process M25.

[0070] The target position signal indicates the change in the target position PSTr. The target position signal includes noise. Therefore, the CPU 211 can derive the target position PSTr from which the noise has been removed by, for example, passing the target position signal through a low-pass filter.

[0071] <Target Rotation Angle Setting Process> In the target rotation angle setting process M29, the CPU 211 sets a target rotation angle θTr that is a target for the motor rotation angle θ. The CPU 211 executes the target rotation angle setting process M29 at predetermined calculation intervals.

[0072] The target rotation angle setting process M29 will be described in detail with reference to Fig. 4. The target rotation angle setting process M29 includes a first limit value setting process M51, a second limit value setting process M53, a selection process M55, a target conversion process M57, a gradient limiting process M59, and a characteristic correction process M65.

[0073] <First Limit Value Setting Process> In the first limit value setting process M51, the CPU 211 sets upper and lower limit values ​​for the motor rotation speed Nmt based on the N / T characteristic determined from the specifications of the first electric motor 513. The N / T characteristic indicates the relationship between the motor rotation speed, which is the rotation speed of the electric motor, and the motor torque. The upper limit value set here is referred to as the "rotation speed upper limit value NmtL1." The lower limit value set here is referred to as the "first rotation speed lower limit value NmtL2a." The CPU 211 sets the rotation speed upper limit value NmtL1 and the first rotation speed lower limit value NmtL2a based on the N / T characteristic, the estimated torque TmtE, which is an estimated value of the motor torque Tmt, and the target rotation speed NmtTr, which is the target value of the motor rotation speed Nmt.

[0074] An example of a method for setting the rotation speed upper limit value NmtL1 and the first rotation speed lower limit value NmtL2a will be described with reference to Fig. 5. The solid line in Fig. 5 indicates the available output torque Tav determined from the target rotation speed NmtTr. The available output torque Tav is the upper limit of the torque that can be output from the first electric motor 513. The larger the target rotation speed NmtTr, the smaller the available output torque Tav.

[0075] The CPU 211 derives the available output torque Tav corresponding to the previous value NmtTr(n-1) of the target rotation speed NmtTr as the available output torque Tav1. The previous value NmtTr(n-1) of the target rotation speed is the target rotation speed NmtTr derived in the previous calculation cycle. The CPU 211 derives the value obtained by subtracting the estimated torque TmtE from the available output torque Tav1 as the surplus acceleration torque Ta.

[0076] The surplus acceleration torque Ta is a motor torque that can be used to increase or decrease the motor rotation speed Nmt. Therefore, it can be said that the greater the surplus acceleration torque Ta, the greater the change in the motor rotation speed Nmt that the CPU 211 can make.

[0077] The CPU 211 sets the motor rotation speed Nmt higher than the previous target rotation speed value NmtTr(n-1) as the rotation speed upper limit value NmtL1. The CPU 211 sets the motor rotation speed Nmt lower than the previous value NmtTr(n-1) as the first rotation speed lower limit value NmtL2a. At this time, the CPU 211 sets the rotation speed upper limit value NmtL1 so that the deviation between the previous value NmtTr(n-1) and the rotation speed upper limit value NmtL1 increases as the acceleration surplus torque Ta increases. Furthermore, the CPU 211 sets the first rotation speed lower limit value NmtL2a so that the deviation between the previous value NmtTr(n-1) and the first rotation speed lower limit value NmtL2a increases as the acceleration surplus torque Ta increases.

[0078] 4, in a second limit value setting process M53, the CPU 211 sets the second rotation speed lower limit NmtL2b based on the orifice differential pressure DP. The orifice differential pressure DP is the differential pressure between both sides of the orifice 60 in the fifth flow path 55, which is an example of a supply flow path.

[0079] The CPU 211 derives the orifice differential pressure DP based on the detected servo pressure value PscE. The detected servo pressure value PscE is the brake fluid pressure between the orifice 60 and the electric cylinder 51 in the fifth flow path 55. For example, if the fluid pressure in the servo chamber Rs can be detected or estimated, the CPU 211 can derive the differential pressure between the fluid pressure in the servo chamber Rs and the detected servo pressure value PscE as the orifice differential pressure DP. For example, the CPU 211 may estimate the fluid pressure in the servo chamber Rs based on the master pressure Pmc.

[0080] Referring to FIG. 6 , an example of a method for setting the second rotation speed lower limit NmtL2b based on the orifice differential pressure DP will be described. FIG. 6 shows the relationship between the motor rotation speed Nmt and the orifice differential pressure DP when the first electric motor 513 is driven to move the piston 512 in the reverse direction Zb. FIG. 6 also shows a negative pressure suppression characteristic map MP11 and a system limit rotation speed map MP12. The negative pressure suppression characteristic map MP11 corresponds to a "negative pressure suppression characteristic" that shows the relationship between the motor rotation speed Nmt and the orifice differential pressure DP at which negative pressure is not generated in the cylinder 511 when the piston 512 is moved in the reverse direction Zb. The negative pressure suppression characteristic map MP11 shows a characteristic in which the motor rotation speed Nmt increases as the orifice differential pressure DP increases. This is because the flow rate of brake fluid flowing from the fifth flow path 55 into the electric cylinder 51 increases as the orifice differential pressure DP increases. The higher the flow rate of the brake fluid flowing into the electric cylinder 51, the less likely negative pressure is generated in the hydraulic chamber Re of the electric cylinder 51 even if the speed at which the piston 512 moves in the backward direction Zb is high.

[0081] The system limit rotation speed map MP12 shows the relationship between the orifice differential pressure DP and the protection rotation speed Np, which is the motor rotation speed Nmt set for the purpose of protecting the first electric motor 513. In the system limit rotation speed map MP12, the protection rotation speed Np is set to the motor rotation speed Nmt determined based on the specifications of the first electric motor 513, regardless of the magnitude of the orifice differential pressure DP. Note that the protection rotation speed Np is set to a motor rotation speed lower than the limit value of the motor rotation speed Nmt determined based on the configuration of the first electric motor 513.

[0082] FIG. 6 also shows a pressure reduction required rotation speed map MP13 that shows the relationship between the motor rotation speed Nmt and the orifice differential pressure DP when the servo pressure Psc is reduced at a predetermined reduction rate. The predetermined reduction rate is determined based on the specifications required for the electric cylinder 51. The pressure reduction required rotation speed map MP13 shows a characteristic in which the motor rotation speed Nmt decreases as the orifice differential pressure DP increases. This is because the higher the flow rate of the brake fluid flowing into the electric cylinder 51 from the fifth flow path 55, the more efficiently the brake fluid assists the movement of the piston 512 in the backward direction Zb. As described above, the higher the orifice differential pressure DP, the higher the flow rate of the brake fluid flowing into the electric cylinder 51 from the fifth flow path 55.

[0083] In this embodiment, the CPU 211 derives the motor rotation speed Nmt corresponding to the orifice differential pressure DP as the derived rotation speed NmtA based on the negative pressure suppression characteristic map MP11 and the system limit rotation speed map MP12.

[0084] For example, the CPU 211 derives the motor rotation speed Nmt corresponding to the orifice differential pressure DP as the first derived rotation speed NmtA1 based on the negative pressure suppression characteristic map MP11. The first derived rotation speed NmtA1 corresponds to the "lower limit candidate value." The CPU 211 then derives the smaller of the protection rotation speed Np indicated by the system limit rotation speed map MP12 and the first derived rotation speed NmtA1 as the derived rotation speed NmtA. Therefore, in FIG. 6, the characteristic line showing the relationship between the orifice differential pressure DP and the derived rotation speed NmtA is indicated by the solid line LS1. In other words, the CPU 211 can derive the motor rotation speed Nmt corresponding to the orifice differential pressure DP as the derived rotation speed NmtA based on the negative pressure suppression characteristic map MP11 within a range that does not exceed the protection rotation speed Np.

[0085] The CPU 211 then sets the value obtained by reversing the sign of the derived rotation speed NmtA as the second rotation speed lower limit NmtL2b. <Selection Process> As shown in FIG. 4 , in the selection process M55, the CPU 211 sets the larger of the first rotation speed lower limit NmtL2a and the second rotation speed lower limit NmtL2b as the rotation speed lower limit NmtL2. When the servo pressure Psc is reduced, the first electric motor 513 rotates in the reverse direction, so the motor rotation speed Nmt becomes a negative value. In other words, both the first rotation speed lower limit NmtL2a and the second rotation speed lower limit NmtL2b become negative motor rotation speeds Nmt. Therefore, the CPU 211 sets the rotation speed lower limit NmtL2 to the smaller absolute value of the first rotation speed lower limit NmtL2a and the second rotation speed lower limit NmtL2b.

[0086] In this embodiment, the second limit value setting process M53 and the selection process M55 constitute an example of a "lower limit setting process M61" that sets the rotation speed lower limit value NmtL2 based on the negative pressure suppression characteristic and the orifice differential pressure DP.

[0087] <Target Conversion Process> In the target conversion process M57, the CPU 211 converts the target position PSTr, from which noise has been removed in the filter process M27, into a motor rotation angle θ, and derives the resulting value as a target rotation angle candidate value θTrA.

[0088] <Gradient Limitation Process> In gradient limiting process M59, the CPU 211 sets the target rotation angle θTr based on the target rotation angle candidate value θTrA, the rotation speed upper limit value NmtL1, and the rotation speed lower limit value NmtL2. The target rotation angle θTr set in the previous calculation cycle is the previous value θTr(n-1) of the target rotation angle. The CPU 211 derives the sum of the previous value θTr(n-1) and a first product as the upper limit rotation angle θL1. The first product is the product of the rotation speed upper limit value NmtL1 and the calculation time Ts. The calculation time Ts is the length of the calculation cycle for the target rotation angle θTr. The CPU 211 derives the sum of the previous value θTr(n-1) and a second product as the lower limit rotation angle θL2. The second product is the product of the rotation speed lower limit value NmtL2 and the calculation time Ts. Then, the CPU 211 selects the second largest value from among the target rotation angle candidate value θTrA, the upper limit rotation angle θL1, and the lower limit rotation angle θL2 as the provisional target value θTrB.

[0089] The CPU 211 derives the difference ΔθTr between the previous value θTr(n-1) of the target rotation angle and the provisional target value θTrB. If the absolute value of the difference ΔθTr is equal to or less than the limit value ΔθL, the CPU 211 sets the provisional target value θTrB to the target rotation angle θTr. On the other hand, if the absolute value of the difference ΔθTr is greater than the limit value ΔθL, the CPU 211 sets the target rotation angle θTr so that the absolute value of the difference between the target rotation angle θTr and the previous value θTr(n-1) is equal to the limit value ΔθL.

[0090] <Characteristic Correction Process> In the characteristic correction process M65, the CPU 211 corrects the negative pressure suppression characteristic map MP11 according to the temperature of the brake fluid. The lower the temperature of the brake fluid, the higher the viscosity of the brake fluid. If the viscosity of the brake fluid is high, the flow rate of the brake fluid flowing from the fifth flow path 55 into the electric cylinder 51 tends to be slow when the piston 512 moves in the backward direction Zb due to the driving of the first electric motor 513. Therefore, compared to when the viscosity of the brake fluid is low, the upper limit of the motor rotation speed Nmt required to prevent negative pressure from being generated in the hydraulic chamber Re tends to be lower.

[0091] Therefore, in the characteristic correction process M65, the CPU 211 corrects the negative pressure suppression characteristic map MP11 so that the motor rotation speed Nmt corresponding to the orifice differential pressure DP decreases as the temperature of the brake fluid decreases.

[0092] 3, in a motor drive process M31, the CPU 211 drives the first electric motor 513 based on the target rotation angle θTr. The CPU 211 executes the motor drive process every time the target rotation angle θTr is set in the target rotation angle setting process M29.

[0093] For example, the CPU 211 derives a command value Imt for the first electric motor 513 by feedback control using as input the deviation between the target rotation angle θTr and the detected rotation angle value θE. The CPU 211 drives the first electric motor 513 by operating a driver circuit for the first electric motor 513 based on the command value Imt.

[0094] <Actions and Effects of the Present Embodiment> The actions and effects of the present embodiment will be described with reference to Fig. 7. The solid lines in Fig. 7 show changes in the wheel pressure Pw, the motor rotation speed Nmt of the first electric motor 513, the internal pressure PI of the hydraulic chamber Re of the electric cylinder 51, and the kinetic energy ES generated in the electric cylinder 51 in this embodiment. The two-dot chain lines in Fig. 7 show changes in the wheel pressure Pw, the motor rotation speed Nmt of the first electric motor 513, the internal pressure PI of the hydraulic chamber Re of the electric cylinder 51, and the kinetic energy ES generated in the electric cylinder 51 in a comparative example.

[0095] In the comparative example, the second limit value setting process M53 shown in Fig. 4 is not executed when setting the rotation speed lower limit value NmtL2. That is, in the comparative example, the rotation speed lower limit value NmtL2 is set without taking the orifice differential pressure DP into consideration.

[0096] As shown in (A), (B), (C), and (D) in FIG. 7, a braking request is issued to the vehicle, such as when a braking operation is initiated at timing t10. The CPU 211 then increases the target front wheel pressure PwfTr and the target rear wheel pressure PwrTr. The CPU 211 then derives a target position PSTr of the piston 512 of the electric cylinder 51 based on the target front wheel pressure PwfTr and the target rear wheel pressure PwrTr. The CPU 211 then executes the target rotation angle setting process M29 shown in FIG. 4 to set a target rotation angle θTr for the first electric motor 513. At this time, the CPU 211 sets the target rotation angle θTr so as not to exceed the rotation speed upper limit NmtL1 set in the first limit value setting process M51. The CPU 211 then drives the first electric motor 513 based on the target rotation angle θTr.

[0097] As a result, in the electric cylinder 51, the first electric motor 513 rotates in the forward direction, increasing the motor rotation speed Nmt. As a result, the piston 512 begins to move in the forward direction Za at timing t11, immediately after the first electric motor 513 starts to drive, increasing the internal pressure PI of the hydraulic chamber Re of the electric cylinder 51. As the piston 512 moves, kinetic energy ES is generated in the piston 512, the first electric motor 513, and other components that move in conjunction with the movement of the piston 512. This kinetic energy ES correlates with the motor rotation speed Nmt. Furthermore, when the internal pressure PI increases, brake fluid is discharged from the output port 516 of the electric cylinder 51 to the fifth flow path 55. As a result, brake fluid is supplied to the multiple wheel cylinders 16, increasing the wheel pressure Pw. In this embodiment, the kinetic energy ES when the piston 512 moves in the forward direction Za is expressed as a positive value, while the kinetic energy ES when the piston 512 moves in the backward direction Zb is expressed as a negative value.

[0098] 7, the target front wheel pressure PwfTr and the target rear wheel pressure PwrTr are maintained at time t12. Then, immediately thereafter, at time t13, the wheel pressure Pw is also maintained.

[0099] From timing t14 thereafter, the CPU 211 rapidly reduces the target front wheel pressure PwfTr and the target rear wheel pressure PwrTr to 0 (zero). Specifically, the rate at which the target front wheel pressure PwfTr and the target rear wheel pressure PwrTr decrease is greater than the rate at which the target front wheel pressure PwfTr and the target rear wheel pressure PwrTr increase from timing t10.

[0100] In this case, the CPU 211 reduces the target rotation angle θTr of the first electric motor 513 in accordance with the reduction in the target front wheel pressure PwfTr and the target rear wheel pressure PwrTr. This causes the piston 512 to move in the backward direction Zb, thereby reducing the wheel pressure Pw. When reducing the target rotation angle θTr, the CPU 211 sets the target rotation angle θTr so that the motor rotation speed Nmt does not fall below the rotation speed lower limit value NmtL2.

[0101] In the comparative example, the rotation speed lower limit NmtL2 is set without taking the orifice differential pressure DP into consideration. Therefore, if the target front wheel pressure PwfTr and the target rear wheel pressure PwrTr decrease at a high rate, the absolute value of the rotation speed lower limit NmtL2 becomes relatively large. As a result, the decrease rate of the target rotation angle θTr becomes relatively large.

[0102] Since the orifice 60 is disposed in the fifth flow path 55, flow resistance is generated in the brake fluid flowing in the pressure reduction direction Yb through the fifth flow path 55. That is, the flow velocity of the brake fluid flowing in the pressure reduction direction Yb through the fifth flow path 55 is not very high. Therefore, if the speed at which the target rotation angle θTr is reduced is increased, thereby excessively increasing the speed at which the piston 512 moves in the backward direction Zb, the following problem may arise.

[0103] That is, the amount of brake fluid flowing into the hydraulic pressure chamber Re from the output port 516 is small relative to the expansion of the hydraulic pressure chamber Re caused by the movement of the piston 512 in the backward direction Zb. This causes the internal pressure PI of the hydraulic pressure chamber Re to decrease. As a result, negative pressure may be generated in the hydraulic pressure chamber Re, as shown by the two-dot chain line in FIG. 7C . When negative pressure is generated in the hydraulic pressure chamber Re, a small gap may be created due to deformation of the seal member that blocks communication between the input port 515 and the hydraulic pressure chamber Re. This may result in brake fluid flowing from the input port 515 into the hydraulic pressure chamber Re through the gap. If brake fluid flows into the hydraulic pressure chamber Re from the input port 515 through the gap, the rate at which the wheel pressure Pw decreases becomes slower.

[0104] Furthermore, when the first electric motor 513 rotates in the reverse direction, negative kinetic energy ES is generated in the electric cylinder 51, as shown in FIG. 7D . When negative pressure is generated in the hydraulic chamber Re as described above, the piston 512 and the first electric motor 513 move at a high speed in a direction that moves the piston 512 in the backward direction Zb, generating a large amount of negative kinetic energy ES. The absolute value of the negative kinetic energy ES may exceed the threshold value ESth, i.e., the kinetic energy ES may fall below the threshold value ESth. The threshold value ESth is a criterion for determining whether an excessive load has been applied to the electric cylinder 51. Therefore, in order to protect the electric cylinder 51, it is undesirable for the absolute value of the negative kinetic energy ES to exceed the threshold value ESth.

[0105] Furthermore, even if the moving speed of the piston 512 in the backward direction Zb in the electric cylinder 51 is increased, the rate at which the wheel pressure Pw decreases does not increase so much, as shown in FIG. 7A.

[0106] In contrast, in this embodiment, the CPU 211 sets the rotation speed lower limit NmtL2 taking the orifice differential pressure DP into consideration. That is, the CPU 211 sets the rotation speed lower limit NmtL2 based on the negative pressure suppression characteristic map MP11 and the orifice differential pressure DP. This allows the CPU 211 to set the rotation speed lower limit NmtL2 so as not to generate negative pressure in the hydraulic pressure chamber Re. Next, the CPU 211 sets the target rotation angle θTr so that the motor rotation speed Nmt does not fall below the rotation speed lower limit NmtL2. The CPU 211 then rotates the first electric motor 513 in the reverse direction based on the target rotation angle θTr.

[0107] Here, the second rotation speed lower limit NmtL2b is set so that the absolute value of the second rotation speed lower limit NmtL2b decreases as the orifice differential pressure DP decreases, and therefore the rotation speed lower limit NmtL2 is set so that the absolute value of the rotation speed lower limit NmtL2 decreases as the orifice differential pressure DP decreases.

[0108] As a result, as shown in Fig. 7B, when the wheel pressure Pw is reduced, the absolute value of the motor rotation speed Nmt does not become larger than in the comparative example. As a result, although the piston 512 moves in the reverse direction Zb, the speed of the movement does not become larger than in the comparative example. As a result, in the electric cylinder 51, when the piston 512 is moved in the reverse direction Zb by driving the first electric motor 513, as shown in Fig. 7C, the generation of negative pressure in the hydraulic pressure chamber Re can be suppressed.

[0109] Furthermore, by not generating negative pressure in the hydraulic pressure chamber Re in this manner, the absolute value of the negative kinetic energy ES does not exceed the threshold value ESth, as shown in Figure 7(D). Furthermore, in this embodiment, the CPU 211 reduces the target rotation angle θTr at a rate corresponding to the rate of reduction of the target front wheel pressure PwfTr and the target rear wheel pressure PwrTr, as long as the motor rotation speed Nmt does not fall below the rotation speed lower limit value NmtL2. Therefore, as shown in Figure 7(A), the braking system 100 can reduce the wheel pressure Pw at a rate equivalent to that of the comparative example.

[0110] This embodiment further provides the following advantages: (1) When setting the second rotation speed lower limit NmtL2b, the CPU 211 sets the second rotation speed lower limit NmtL2b so that the absolute value of the motor rotation speed Nmt does not exceed the protection rotation speed Np. Therefore, the CPU 211 can drive the first electric motor 513 within a range in which the load on the first electric motor 513 is not excessively large.

[0111] (2) The higher the viscosity of the brake fluid flowing through the fifth flow path 55, the less brake fluid is likely to flow from the output port 516 into the hydraulic chamber Re when the piston 512 is moved in the reverse direction Zb. Therefore, the CPU 211 corrects the negative pressure suppression characteristic map MP11 in accordance with the temperature of the brake fluid, which correlates with viscosity. This allows the CPU 211 to set the rotation speed lower limit NmtL2 taking into account the viscosity of the brake fluid at that time. Therefore, even when the viscosity of the brake fluid is high, the CPU 211 can suppress the generation of negative pressure in the hydraulic chamber Re when the piston 512 is moved in the reverse direction Zb by driving the first electric motor 513.

[0112] <Modifications> The above embodiment can be modified as follows: The above embodiment and the following modifications can be combined with each other within the scope of technical compatibility.

[0113] The first memory 212 of the first ECU 210 may also store a pressure reduction required rotation speed map MP13. The pressure reduction required rotation speed map MP13 corresponds to a "pressure reduction characteristic" that indicates the relationship between the motor rotation speed Nmt and the orifice differential pressure DP when the servo pressure Psc is reduced at a predetermined reduction rate. In this case, the CPU 211 may set the second rotation speed lower limit NmtL2b in the second limit value setting process M53 based on the negative pressure suppression characteristic map MP11, the pressure reduction required rotation speed map MP13, the orifice differential pressure DP, and the protection rotation speed Np.

[0114] FIG. 8 shows a modified example in which the second rotation speed lower limit NmtL2b is set using the pressure reduction required rotation speed map MP13. In the modified example shown in FIG. 8, a motor rotation speed Nmt lower than the protection rotation speed Np used in the above embodiment is set as the protection rotation speed NpA. For example, the CPU 211 derives the motor rotation speed Nmt corresponding to the orifice differential pressure DP as the first derived rotation speed NmtA1 based on the negative pressure suppression characteristic map MP11. The CPU 211 derives the motor rotation speed Nmt corresponding to the orifice differential pressure DP as the second derived rotation speed NmtA2 based on the pressure reduction required rotation speed map MP13. If the second derived rotation speed NmtA2 is higher than the protection rotation speed NpA, the CPU 211 sets the derived rotation speed NmtA to the smaller of the first derived rotation speed NmtA1 and the second derived rotation speed NmtA2. On the other hand, if the second derived rotation speed NmtA2 is lower than the protection rotation speed NpA, the CPU 211 sets the derived rotation speed NmtA to the second lowest rotation speed among the first derived rotation speed NmtA1, the second derived rotation speed NmtA2, and the protection rotation speed NpA.

[0115] - When deriving the derived rotation speed NmtA, the CPU 211 may set the derived rotation speed NmtA to the smaller of the first derived rotation speed NmtA1 and the second derived rotation speed NmtA2, as long as it does not exceed the protection rotation speed Np.

[0116] The orifice 60 may be provided in a supply flow path other than the fifth flow path 55. For example, the orifice 60 may be provided in the sixth flow path 56 or the first flow path 331.

[0117] The hydraulic pressure generating device may have a configuration different from the hydraulic pressure generating device 20 shown in Fig. 1 as long as it includes the electric cylinder 51 and the orifice 60. For example, the hydraulic pressure generating device may have a configuration in which the fifth flow path 55 is not connected to the servo chamber Rs of the master unit 30, but is connected to the second hydraulic pressure circuit 712 of the brake actuator 70.

[0118] The control device 200 is not limited to a device that includes a CPU and a ROM and executes software processing. That is, the control device 200 may have any one of the following configurations (a), (b), and (c):

[0119] (a) The control device 200 includes one or more processors that execute various processes according to a computer program. The processor includes a CPU and memory such as RAM and ROM. The memory stores program code or instructions configured to cause the CPU to execute processes. Memory, i.e., computer-readable media, includes any available media that can be accessed by a general-purpose or special-purpose computer.

[0120] (b) The control device 200 includes one or more dedicated hardware circuits that execute various processes. Examples of dedicated hardware circuits include application-specific integrated circuits (ASICs) or FPGAs. ASIC is an abbreviation for "Application Specific Integrated Circuit." FPGA is an abbreviation for "Field Programmable Gate Array."

[0121] (c) The control device 200 includes one or more processors that execute some of the various processes in accordance with a computer program, and one or more dedicated hardware circuits that execute the remaining processes of the various processes.

Claims

1. A braking device configured to generate a braking force at a wheel corresponding to the wheel pressure, which is the hydraulic pressure in a wheel cylinder, comprising: an electric cylinder including an electric motor, a cylinder having an output port, and a piston, wherein the electric cylinder is configured such that the piston moves forward within the cylinder based on the drive of the electric motor to discharge brake fluid from the output port, and the piston moves backward within the cylinder based on the drive of the electric motor to draw brake fluid from the output port into the cylinder; a supply flow path through which brake fluid flows toward the wheel cylinder when brake fluid is discharged from the output port of the electric cylinder; an orifice provided in the supply flow path that generates flow resistance to the brake fluid flowing through the supply flow path when the wheel pressure is reduced; a memory that stores negative pressure suppression characteristics that indicate the relationship between the motor rotation speed, which is the rotation speed of the electric motor, and an orifice differential pressure, which is the differential pressure on both sides of the orifice in the supply flow path, such that negative pressure is not generated in the cylinder when the piston moves backward; and a processing circuit configured to operate the electric cylinder. The processing circuit is configured to execute a lower limit setting process that sets a lower limit value for the motor rotation speed based on the negative pressure suppression characteristic and the orifice differential pressure, and a motor driving process that drives the electric motor so that the motor rotation speed does not fall below the lower limit value when the piston is moved in the backward direction by driving the electric motor.

2. The braking device according to claim 1, wherein the processing circuit is further configured to execute a characteristic correction process for correcting the negative pressure suppression characteristic in accordance with the temperature of the brake fluid.

3. A braking device according to claim 1 or claim 2, wherein the motor rotation speed for protecting the electric cylinder is a protection rotation speed, and the processing circuit is further configured to set the lower limit value based on the negative pressure suppression characteristic, the orifice differential pressure, and the protection rotation speed in the lower limit setting process.

4. The braking device according to claim 3, wherein the memory stores a pressure reduction characteristic showing the relationship between the motor rotation speed and the orifice differential pressure when the servo pressure, which is the discharge pressure of the brake fluid of the electric cylinder, is reduced at a predetermined reduction rate, and the processing circuit is further configured to set the lower limit value based on the negative pressure suppression characteristic, the pressure reduction characteristic, the orifice differential pressure, and the protective rotation speed in the lower limit setting process.

5. A braking device according to claim 3, wherein the processing circuit is further configured to, in the lower limit setting process, derive the motor rotation speed corresponding to the orifice differential pressure based on the negative pressure suppression characteristic as a lower limit candidate value, and set the lower limit value based on the lower limit candidate value or the protection rotation speed, whichever has the smaller absolute value.

6. A control method for a braking device configured to generate, at a wheel, a braking force corresponding to wheel pressure, which is hydraulic pressure in a wheel cylinder, the braking device comprising: an electric cylinder including an electric motor, a cylinder having an output port, and a piston, the electric cylinder configured such that brake fluid is discharged from the output port as the piston moves forward within the cylinder based on the drive of the electric motor, and brake fluid is drawn into the cylinder from the output port as the piston moves backward within the cylinder based on the drive of the electric motor; a supply flow path through which brake fluid flows toward the wheel cylinder when brake fluid is discharged from the output port of the electric cylinder; and an orifice provided in the supply flow path, which generates flow resistance to the brake fluid flowing through the supply flow path when the wheel pressure is reduced; a negative pressure suppression characteristic is a relationship between a motor rotation speed, which is the rotation speed of the electric motor, and an orifice differential pressure, which is the differential pressure on both sides of the orifice in the supply flow path, and which indicates a relationship between the motor rotation speed, which is the rotation speed of the electric motor, and the orifice differential pressure, which does not generate negative pressure in the cylinder when the piston moves backward; A method for controlling a braking device, comprising: setting a lower limit value for the motor rotation speed based on the negative pressure suppression characteristic and the orifice differential pressure; and driving the electric motor so that the motor rotation speed does not fall below the lower limit value when driving the electric motor to move the piston in the backward direction.

Images