Braking device and control method for braking device

Abstract

A braking device (100) comprises: an electric cylinder (51); a servo pressure sensor (58); a braking actuator (70); a memory in which a first characteristic and a second characteristic are stored; and a processing circuit. The processing circuit is configured to: select the second characteristic if a servo pressure detection value is larger than a target servo pressure in a situation where it is determined that the servo pressure has changed due to operation of the braking actuator (70); derive motor torque corresponding to the target servo pressure on the basis of the selected second characteristic; and drive an electric motor of the electric cylinder (51) on the basis of the target value corresponding to the derived motor torque.

Description

Brake device and brake device control method

[0001] The present disclosure relates to a braking device that adjusts 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 braking device includes a supply flow path connecting an output port of the electric cylinder to a wheel cylinder, a holding valve installed in the supply flow path, a return flow path connecting a portion of the supply flow path between the wheel cylinder and the holding valve to a reservoir tank, and a pressure reducing valve installed in the return flow path. The holding valve is a normally open solenoid valve. The pressure reducing valve is a normally closed solenoid valve.

[0003] Some braking systems equipped with electric cylinders also include an actuator that increases the wheel pressure, which is the hydraulic pressure in the wheel cylinder. Such braking systems include a brake fluid line connecting the electric cylinder and the wheel cylinder, and the actuator is connected to the brake fluid line. The wheel pressure is adjusted by operating at least one of the electric cylinder and the actuator.

[0004] DE 102012200494

[0005] A known example of such an actuator includes a brake fluid holding valve, a brake fluid passage between the brake fluid holding valve and the electric cylinder, and a pump that discharges brake fluid into the brake fluid passage. In a braking device equipped with such an actuator, when the pump operates with the brake fluid holding valve closed, the servo pressure, which is the hydraulic pressure in the brake fluid passage between the brake fluid holding valve and the electric cylinder, changes due to the operation of the actuator. If the servo pressure changes due to the operation of the actuator, there is a risk that the controllability of the servo pressure using the electric cylinder will be reduced. If the controllability of the servo pressure is reduced, there is a risk that the controllability of the wheel pressure will also be reduced.

[0006] According to one aspect of the present disclosure, there is provided 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, and the braking device includes: an electric cylinder including an electric motor, a cylinder having an output port, and a piston, the electric cylinder configured to discharge brake fluid from the output port as the piston moves forward within the cylinder in response to driving of the electric motor, and to draw brake fluid from the output port as the piston moves backward within the cylinder in response to driving of the electric motor, a servo flow path connected to the output port of the electric cylinder, a servo pressure sensor configured to detect servo pressure, which is the hydraulic pressure of the brake fluid flowing through the servo flow path, 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, a brake actuator connected to the supply flow path and configured to adjust the wheel pressure, a memory that stores a first characteristic representing a relationship between the servo pressure and motor torque, which is the output torque of the electric motor, when the servo pressure is increased, and a second characteristic representing a relationship between the motor torque and servo pressure when the servo pressure is decreased, and a processing circuit configured to operate the electric cylinder.The processing circuit is configured to execute a determination process for determining whether the servo pressure has changed due to operation of the brake actuator; a first selection process for selecting the second characteristic if a target servo pressure, which is a target value of the servo pressure, has decreased in a situation where it is determined that the servo pressure has not changed due to operation of the brake actuator, and selecting the first characteristic if the target servo pressure has not decreased; a second selection process for selecting the second characteristic if a servo pressure detection value, which is the servo pressure detected by the servo pressure sensor, is greater than the target servo pressure in a situation where it is determined that the servo pressure has changed due to operation of the brake actuator, and selecting the first characteristic if the servo pressure detection value is equal to or less than the target servo pressure; and a motor drive process for deriving a motor torque corresponding to the target servo pressure based on the characteristic selected in the first selection process or the second selection process, and driving the electric motor based on a target value corresponding to the derived motor torque.

[0007] 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, the electric cylinder configured to discharge brake fluid from the output port as the piston moves forward within the cylinder in response to driving of the electric motor, and to draw brake fluid from the output port as the piston moves backward within the cylinder in response to driving of the electric motor, a servo flow path connected to the output port of the electric cylinder, a servo pressure sensor configured to detect a servo pressure, which is the hydraulic pressure of the brake fluid flowing through the servo flow path, 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 a brake actuator connected to the supply flow path and configured to adjust the wheel pressure. A first characteristic represents a relationship between the servo pressure and a motor torque, which is an output torque of the electric motor, when the servo pressure is increased, and a second characteristic represents a relationship between the motor torque and the servo pressure when the servo pressure is decreased. The control method includes determining whether the servo pressure has changed due to operation of the brake actuator; selecting the second characteristic when a target servo pressure, which is a target value of the servo pressure, has decreased in a situation where it is determined that the servo pressure has not changed due to operation of the brake actuator, while selecting the first characteristic when the target servo pressure has not decreased; selecting the second characteristic when a servo pressure detection value, which is the servo pressure detected by the servo pressure sensor, is greater than the target servo pressure in a situation where it is determined that the servo pressure has changed due to operation of the brake actuator, while selecting the first characteristic when the servo pressure detection value is equal to or less than the target servo pressure; deriving a motor torque corresponding to the target servo pressure based on the selected characteristic; and driving the electric motor based on a target value corresponding to the derived motor torque.

[0008] Fig. 1 is a schematic diagram showing an embodiment of a braking device mounted on a vehicle. Fig. 2 is a schematic diagram showing a part of a brake actuator provided in the braking device of Fig. 1. Fig. 3 is a diagram showing the relationship between servo pressure and motor torque in the braking device of Fig. 1. Fig. 4 is a block diagram showing a plurality of processes for driving an electric motor, which is a power source for an electric cylinder, in the braking device of Fig. 1. Fig. 5 is a timing chart when the brake actuator starts to operate while a wheel cylinder is being adjusted by operation of an electric cylinder provided in the braking device of Fig. 1. Fig. 6 is a diagram showing how the operating point showing the relationship between servo pressure and motor torque changes in the graph showing the relationship between servo pressure and motor torque of Fig. 3.

[0009] 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 6. 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.

[0010] 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.

[0011] <Friction Brake> The plurality of friction brakes 15 each generate friction braking forces FxMF, FxMR at the corresponding wheels 11f, 11r. The friction brakes 15 have wheel cylinders 16, rotating bodies 17, and friction units 18. Because the rotating bodies 17 rotate together with the wheels 11f, 11r, the friction units 18 press against the rotating bodies 17, generating friction braking forces FxMF, FxMR at the wheels 11f, 11r. The force pressing the friction units 18 against the rotating bodies 17 increases as the wheel pressure Pw, which is the hydraulic pressure in the wheel cylinders 16, increases. Therefore, the friction brakes 15 can generate larger friction braking forces FxMF, FxMR at the wheels 11f, 11r as the wheel pressure Pw increases.

[0012] <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.

[0013] <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.

[0014] 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.

[0015] <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.

[0016] 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.

[0017] 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.

[0018] 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.

[0019] 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.

[0020] 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.

[0021] 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.

[0022] The master piston 43 is accommodated in the master cylinder 31 with an annular seal interposed between the inner circumferential surface of the circumferential 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 axially, the master piston 43 slides via the seal while maintaining a predetermined gap between the inner circumferential surface of the circumferential wall 412 and the inner circumferential surface of the first annular wall 413. More specifically, the inner circumferential surface of the annular seal provided on the inner circumferential surface of the circumferential wall 412 of the main cylinder 41 and the outer circumferential surface of the master piston 43 slide in contact with each other, and the outer circumferential surface of the seal provided on the outer circumferential surface of the master piston 43 facing the inner circumferential surface of the first annular wall 413 slides in contact with the inner circumferential surface of the first annular wall 413. The rear end of the master piston 43 protrudes rearward from the first annular wall 413 and is located in the second fluid chamber R2.

[0023] The input piston 44 is housed in the master cylinder 31 with an annular seal interposed between it and the inner circumferential surface of the second annular wall 422 of the cover cylinder 42. Therefore, when the input piston 44 moves axially, the input piston 44 slides via the seal while maintaining a predetermined gap between it and 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.

[0024] 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.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] 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.

[0029] 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."

[0030] <Pressurizing Unit> The pressurizing unit 50 includes an electric cylinder 51. The pressurizing unit 50 can adjust the wheel pressure Pw of the plurality of wheel cylinders 16 by operating the electric cylinder 51.

[0031] 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. In other words, the fifth flow path 55 corresponds to the "servo flow path." 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.

[0032] The hydraulic pressure of the brake fluid discharged from the output port 516 by the electric cylinder 51 is referred to as the "servo pressure." The pressurizing unit 50 is equipped with hydraulic pressure sensors that detect the servo pressure, including a servo pressure sensor 58. For example, the servo pressure sensor 58 is connected to the fifth flow path 55. Hereinafter, the servo pressure based on the detection signal of the servo pressure sensor 58 is referred to as the "servo pressure detection value PscE."

[0033] 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 via an annular seal. 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, which is the rotation angle of the output shaft of the first electric motor 513, increases. On the other hand, when the first electric motor 513 rotates in the reverse direction, the motor rotation angle θ decreases.

[0034] 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.

[0035] An annular seal is provided on the inner peripheral surface of the cylinder 511. The outer peripheral surface of the piston 512 contacts the inner peripheral surface of the annular seal. When the piston 512 moves in the forward direction Za and the backward direction Zb, the piston 512 slides while the outer peripheral surface of the piston 512 contacts the inner peripheral surface of the annular seal, thereby maintaining a fluid-tight seal between the cylinder 511 and the piston 512 when servo pressure is generated in the hydraulic pressure chamber Re. An example of such a seal is a cup seal.

[0036] The peripheral wall of the cylinder 511 is formed with ports, including an input port 515 and an output port 516, that connect the hydraulic chamber Re to the outside. A through-hole 517 is formed in the piston 512. The most retracted position of the piston 512 in the direction Zb is referred to as its most retracted position. The through-hole 517 is positioned to allow communication between the input port 515 and the hydraulic chamber Re when the piston 512 is at the most retracted position. As a result, when the piston 512 is at the most retracted position, the hydraulic chamber Re of the cylinder 511 is in communication 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 at the most retracted position and is closed by the piston 512 when the piston 512 moves forward in the 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 direction Za.

[0037] 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.

[0038] In the hydraulic pressure generating device 20, when brake fluid is being discharged from the output port 516 of the electric cylinder 51, the brake fluid flows through the sixth flow path 56 toward the wheel cylinder 16 for the rear wheel 11r. Also, the brake fluid flows through the first flow path 331 toward the wheel cylinder 16 for the front wheel 11f. In this respect, the sixth flow path 56 and the first flow path 331 correspond to "supply flow paths." On the other hand, when the electric cylinder 51 is drawing brake fluid through the output port 516, the brake fluid flows through the sixth flow path 56 toward the fifth flow path 55, and the brake fluid flows through the first flow path 331 toward the master chamber Rm.

[0039] <Brake Actuator> The brake actuator 70 is configured to be able to individually adjust the wheel pressures Pw of the multiple 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.

[0040] As shown in FIG. 2 , the first hydraulic circuit 711 has a connection flow path 721 connected to the sixth flow path 56, which is an example of a supply flow path, and a differential pressure control valve 73 installed in the connection flow path 721. The connection flow path 721 is a brake fluid path connecting the two wheel cylinders 16 for the rear wheels 11r to the sixth flow path 56. The differential pressure control valve 73 is a normally open linear solenoid valve. The differential pressure control valve 73 can adjust the differential pressure between the portion of the connection flow path 721 connected to the sixth flow path 56 and the portion connected to the wheel cylinders 16. For example, the differential pressure control valve 73 can generate a larger differential pressure as the current flowing through its solenoid increases.

[0041] The connecting flow path 721, located closer to the wheel cylinders 16 than the differential pressure control valve 73, branches into two paths 72a and 72b. Path 72a is connected to one of the two wheel cylinders 16 for the rear wheels 11r, while path 72b is connected to the other of the two wheel cylinders 16 for the rear wheels 11r. A holding valve 74 is installed in each of the two paths 72a and 72b. That is, the holding valve 74 is installed in a portion of the connecting flow path 721 closer to the wheel cylinders 16 than the differential pressure control valve 73. The holding valve 74 is a normally open solenoid valve. When the holding valve 74 is closed, it restricts the supply of brake fluid to the wheel cylinders 16. As a result, an increase in the wheel pressure Pw in the wheel cylinders 16 for the rear wheels 11r is restricted. That is, the holding valve 74 is an solenoid valve installed in the connecting flow path 721 that closes to restrict an increase in the wheel pressure Pw.

[0042] The first hydraulic circuit 711 has a reduced pressure reservoir 751 that stores brake fluid, and a reduced pressure fluid path 761 that is connected to the reduced pressure reservoir 751. The reduced pressure fluid path 761 is a brake fluid path that connects the reduced pressure reservoir 751 to the portions of the paths 72a and 72b that are closer to the wheel cylinder 16 than the holding valve 74. A pressure reducing valve 77, which is a normally closed solenoid valve, is installed in each of the portions of the reduced pressure fluid path 761 that are connected to the path 72a and the path 72b.

[0043] The first hydraulic circuit 711 has a pump 791 and a return fluid path 801. The pump 791 is an electric pump powered by the second electric motor 78. The pump 791 pumps up brake fluid from the reduced-pressure reservoir 751 and discharges the brake fluid to a portion of the connecting flow path 721 between the differential pressure control valve 73 and the holding valve 74. The return fluid path 801 is a brake fluid path that is connected to the reduced-pressure reservoir 751 and a portion of the connecting flow path 721 that is closer to the sixth flow path 56 than the differential pressure control valve 73.

[0044] The configuration of the second hydraulic pressure circuit 712 is substantially the same as the configuration of the first hydraulic pressure circuit 711. Therefore, a detailed description of the configuration of the second hydraulic pressure circuit 712 will be omitted here. The second hydraulic pressure circuit 712 includes a differential pressure control valve 73, a plurality of holding valves 74, a pressure-reducing valve 77, a pressure-reducing reservoir 751, and a pump 791 powered by a second electric motor 78.

[0045] <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.

[0046] The control device 200 includes a plurality of electronic control devices. Hereinafter, the electronic control devices will be 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.

[0047] The first ECU 210 has a CPU 211, a first memory 212, and a second memory 213. The second ECU 220 has a CPU 221, a first memory 222, and a second memory 223. The CPUs 211 and 221 are "processing circuits." Specifically, the CPU 211 of the first ECU 210 corresponds to the "first processing circuit," and the CPU 221 of the second ECU 220 corresponds to the "second processing circuit." The first memories 212 and 222 store control programs executed by the CPUs 211 and 221. The second memories 213 and 223 store the results of calculations by the CPUs 211 and 221.

[0048] <Control of Electric Cylinder> First, the characteristics of the electric cylinder will be described. The output torque of the first electric motor 513 is referred to as the "motor torque Tmt." The rotation direction of the first electric motor 513 for increasing the servo pressure Psc is referred to as the "forward direction," while the rotation direction of the first electric motor 513 for decreasing the servo pressure Psc is referred to as the "reverse direction." The reverse direction is the opposite direction to the forward direction.

[0049] The motor torque Tmt is converted via the conversion mechanism 514 into a force that causes the piston 512 to pressurize the brake fluid in the hydraulic chamber Re. Mechanical loss occurs in the power transmission path from the first electric motor 513 to the piston 512. The magnitude of the mechanical loss differs between when the first electric motor 513 rotates in the forward direction and when the first electric motor 513 rotates in the reverse direction. Therefore, the first characteristic LR1 and the second characteristic LR2 are different from each other. The first characteristic LR1 represents the relationship between the motor torque Tmt and the servo pressure Psc when the first electric motor 513 rotates in the forward direction to increase the servo pressure Psc. The second characteristic LR2 represents the relationship between the motor torque Tmt and the servo pressure Psc when the first electric motor 513 rotates in the reverse direction to decrease the servo pressure Psc.

[0050] Fig. 3 shows an example of the first characteristic LR1 and the second characteristic LR2. In the example shown in Fig. 3, the first characteristic LR1 and the second characteristic LR2 are maps showing the relationship between the motor torque Tmt and the servo pressure Psc. In both the first characteristic LR1 and the second characteristic LR2, the servo pressure Psc increases as the motor torque Tmt increases.

[0051] When the servo pressure Psc is increased, the direction in which the resistance force corresponding to the mechanical loss acts is the backward direction Zb, which is the opposite direction to the forward direction Za, which is the direction of movement of the piston 512. Therefore, the motor torque Tmt required to set the servo pressure Psc to its target value increases. As a result, as shown by the first characteristic LR1, the gradient of change in the servo pressure Psc relative to a change in the motor torque Tmt becomes relatively gentle.

[0052] When the servo pressure Psc is reduced, the direction in which the resistance force corresponding to the mechanical loss acts is the forward direction Za, which is the opposite direction to the backward direction Zb, which is the direction of movement of the piston 512. Therefore, the motor torque Tmt required to set the servo pressure Psc to its target value is reduced. As a result, as shown by the second characteristic LR2, the gradient of change in the servo pressure Psc relative to a change in the motor torque Tmt is steeper than when the servo pressure Psc is increased.

[0053] An example of a resistance force corresponding to mechanical loss is a friction force that occurs in a direction opposite to the direction of movement of the piston 512 at the contact portion between the outer circumferential surface of the piston and the annular seal when the piston 512 moves in the forward direction Za and the backward direction Zb. Such a resistance force is also referred to as "sliding resistance." For example, when the piston 512 is moved in the forward direction Za against the servo pressure, the piston 512 will not move in the forward direction Za unless a motor torque Tmt is generated such that the force pushing the piston 512 is greater than at least the value obtained by adding the pressing force on the piston 512 by the servo pressure and the resistance force corresponding to mechanical loss. On the other hand, when the piston 512 is moved in the backward direction Zb against the servo pressure, the resistance force corresponding to mechanical loss acts in a direction opposite to the pressing force on the piston 512 by the servo pressure. Therefore, the piston 512 will move in the backward direction Zb if the motor torque Tmt is set such that the force pushing the piston 512 is smaller than the value obtained by subtracting the resistance force corresponding to mechanical loss from the pressing force on the piston 512 by the servo pressure.

[0054] The first characteristic LR1 and the second characteristic LR2 are stored in the first memory 212 of the first ECU 210. In this respect, the first memory 212 corresponds to a "memory." With reference to FIG. 4, a plurality of processes executed by the CPU 211 when operating the electric cylinder 51 will be described.

[0055] The CPU 211 executes the control program stored in the first memory 212 to execute a target setting process M11, a determination process M13, a selection process M15, and a reference motor torque derivation process M17. The CPU 211 also executes a deviation derivation process M19, a compensation torque derivation process M21, a target torque setting process M23, and an output process M25.

[0056] <Target Setting Process> In the target setting process M11, the CPU 211 sets a target servo pressure PscTr, which is a target for the servo pressure Psc. The CPU 211 executes the target setting process M11 at each predetermined control cycle. For example, the CPU 211 sets the target servo pressure PscTr so that it increases as the vehicle friction braking force target value FxMTr, which is the target value for the vehicle friction braking force FxM, increases.

[0057] <Determination Process> In the determination process M13, the CPU 211 determines whether or not the servo pressure Psc has changed due to the operation of the brake actuator 70. The CPU 211 executes the determination process M13 every time the target servo pressure PscTr is set by executing the target setting process M11.

[0058] Here, the first hydraulic pressure circuit 711 of the brake actuator 70 is connected to the sixth flow path 56, and the sixth flow path 56 is connected to the fifth flow path 55. Therefore, when the hydraulic pressure in the connecting flow path 721 of the first hydraulic pressure circuit 711, which is connected to the sixth flow path 56, changes, the hydraulic pressure in the sixth flow path 56 and the fifth flow path 55, i.e., the servo pressure Psc, may change.

[0059] The second hydraulic circuit 712 of the brake actuator 70 is connected to the first flow path 331, which is in turn connected to the master chamber Rm. Therefore, when the hydraulic pressure in the connecting flow path of the second hydraulic circuit 712 that is connected to the first flow path 331 changes, the hydraulic pressure in the master chamber Rm changes. When the hydraulic pressure in the master chamber Rm changes, the hydraulic pressure in the servo chamber Rs and the servo pressure Psc, which is the hydraulic pressure in the fifth flow path 55 that is connected to the servo chamber Rs, may change.

[0060] When antilock brake control or traction control is performed, at least one of the pump 791 and the holding valve 74 operates. When at least one of the pump 791 and the holding valve 74 operates in this manner, the servo pressure Psc may change due to the operation of the brake actuator 70. When the pump 791 operates, the brake fluid discharged from the pump 791 is supplied to the connecting flow path 721. As a result, the servo pressure Psc may change. When the holding valve 74 operates, the smaller the opening of the holding valve 74, the greater the restriction on the supply of brake fluid to the wheel cylinder 16. As a result, the servo pressure Psc may change.

[0061] Therefore, when the CPU 211 receives from the second ECU 220 information that braking control involving operation of the pump 791 is being performed, the CPU 211 determines that the servo pressure Psc has changed due to the operation of the brake actuator 70. Furthermore, when the CPU 211 receives from the second ECU 220 information that braking control involving operation of the hold valve 74 is being performed, the CPU 211 determines that the servo pressure Psc has changed due to the operation of the brake actuator 70. On the other hand, when the CPU 211 does not receive from the second ECU 220 either information that braking control involving operation of the pump 791 is being performed or information that braking control involving operation of the hold valve 74 is being performed, the CPU 211 determines that the servo pressure Psc has not changed due to the operation of the brake actuator 70.

[0062] <Selection Process> In the selection process M15, the CPU 211 selects either the first characteristic LR1 or the second characteristic LR2. The CPU 211 executes the selection process M15 at every predetermined control cycle.

[0063] The selection process M15 includes a first selection process M151 and a second selection process M152. The CPU 211 executes the first selection process M151 when it determines that the servo pressure Psc has not changed due to the operation of the brake actuator 70. On the other hand, the CPU 211 executes the second selection process M152 when it determines that the servo pressure Psc has changed due to the operation of the brake actuator 70.

[0064] In the first selection process M151, the CPU 211 selects either the first characteristic LR1 or the second characteristic LR2 based on the trend of the target servo pressure PscTr. Specifically, if the latest value of the target servo pressure PscTr is less than the previous value of the target servo pressure PscTr, the target servo pressure PscTr is considered to be decreasing. Therefore, the CPU 211 selects the second characteristic LR2. On the other hand, if the latest value of the target servo pressure PscTr is equal to or greater than the previous value of the target servo pressure PscTr, the target servo pressure PscTr is considered not to be decreasing. Therefore, the CPU 211 selects the first characteristic LR1.

[0065] In the second selection process M152, the CPU 211 compares the target servo pressure PscTr with the detected servo pressure value PscE to select either the first characteristic LR1 or the second characteristic LR2. Specifically, when the detected servo pressure value PscE is greater than the target servo pressure PscTr, it is preferable to operate the electric cylinder 51 so as to decrease the detected servo pressure value PscE. Therefore, the CPU 211 selects the second characteristic LR2. On the other hand, when the detected servo pressure value PscE is equal to or less than the target servo pressure PscTr, it is preferable to operate the electric cylinder 51 so as to increase the detected servo pressure value PscE. Therefore, the CPU 211 selects the first characteristic LR1.

[0066] In the reference motor torque derivation process M17, the CPU 211 derives a reference motor torque TmtB, which is a reference value of the motor torque Tmt, based on the characteristics selected in the selection process M15. The CPU 211 executes the reference motor torque derivation process M17 at every predetermined control period.

[0067] For example, if the characteristic selected in selection process M15 is the first characteristic LR1, the CPU 211 derives the motor torque Tmt corresponding to the target servo pressure PscTr based on the first characteristic LR1 shown in Figure 3. For example, if the characteristic selected in selection process M15 is the second characteristic LR2, the CPU 211 derives the motor torque Tmt corresponding to the target servo pressure PscTr based on the second characteristic LR2 shown in Figure 3. The CPU 211 then sets the motor torque Tmt derived based on the first characteristic LR1 or the second characteristic LR2 as the reference motor torque TmtB.

[0068] <Deviation Derivation Process> In deviation derivation process M19, the CPU 211 derives the deviation ΔPsc between the target servo pressure PscTr and the detected servo pressure value PscE. The CPU 211 executes the deviation derivation process M19 for each predetermined control period. For example, the CPU 211 derives the deviation ΔPsc by subtracting the detected servo pressure value PscE from the target servo pressure PscTr.

[0069] <Compensation Torque Derivation Process> In the compensation torque derivation process M21, the CPU 211 derives a compensation torque ΔTmt, which is a compensation value for the motor torque Tmt, based on the deviation ΔPsc. The CPU 211 executes the compensation torque derivation process M21 at every predetermined control cycle. The CPU 211 derives, as the compensation torque ΔTmt, an increase in the motor torque Tmt required to bring the deviation ΔPsc closer to 0 (zero). For example, the CPU 211 can derive the compensation torque ΔTmt by feedback control using the deviation ΔPsc as an input.

[0070] <Target Torque Setting Process> In the target torque setting process M23, the CPU 211 sets a target torque TmtTr, which is a target value of the motor torque Tmt, based on the reference motor torque TmtB and the compensation torque ΔTmt. The CPU 211 executes the target torque setting process M23 at every predetermined control period. For example, the CPU 211 sets the sum of the reference motor torque TmtB and the compensation torque ΔTmt as the target torque TmtTr.

[0071] <Output Process> In output process M25, the CPU 211 sets a command value for the first electric motor 513 based on the target torque TmtTr. The CPU 211 executes output process M25 at each predetermined control cycle. For example, the CPU 211 sets at least one of a current command value and a voltage command value so that the motor torque Tmt becomes the target torque TmtTr. Then, the CPU 211 operates the inverter of the first electric motor 513 based on at least one of the current command value and the voltage command value.

[0072] In this embodiment, the reference motor torque derivation process M17, the target torque setting process M23, and the output process M25 constitute a "motor drive process M30" for driving the first electric motor 513.

[0073] <Functions and Effects of the Present Embodiment> The functions and effects of the present embodiment will be described with reference to Figures 5 and 6. The dashed line in Figure 5A indicates the transition of the target servo pressure PscTr. The solid lines in Figure 5A, 5B, and 5C indicate the transition of the detected servo pressure value PscE, motor torque Tmt, and motor rotation angle θ in this embodiment. The two-dot chain lines in Figure 5A, 5B, and 5C indicate the transition of the detected servo pressure value PscE, motor torque Tmt, and motor rotation angle θ in a comparative example. In the comparative example, the first selection process M151 selects either the first characteristic LR1 or the second characteristic LR2 regardless of whether the detected servo pressure value PscE has changed due to operation of the brake actuator 70.

[0074] At time t0, the servo pressure Psc has not changed due to the operation of the brake actuator 70. Therefore, the CPU 211 selects the first characteristic LR1 when increasing the servo pressure Psc in response to an increase in the target servo pressure PscTr. Then, the CPU 211 derives the motor torque Tmt corresponding to the target servo pressure PscTr as the reference motor torque TmtB based on the first characteristic LR1.

[0075] In this case, the reference motor torque TmtB increases as the target servo pressure PscTr increases. Therefore, the target torque TmtTr, which is set by the CPU 211 based on the reference motor torque TmtB, also increases. The CPU 211 then drives the first electric motor 513 based on the target torque TmtTr. As a result, the motor torque Tmt and the motor rotation angle θ increase as the target servo pressure PscTr increases. Then, in the electric cylinder 51, the piston 512 moves in the forward direction Za within the cylinder 511. This causes brake fluid to be discharged from the output port 516 to the fifth flow path 55, increasing the detected servo pressure value PscE. When the detected servo pressure value PscE increases, brake fluid is also supplied to the multiple wheel cylinders 16. As a result, the wheel pressure Pw of the multiple wheel cylinders 16 increases.

[0076] In the example shown in Figure 5, braking control such as anti-lock brake control is initiated at timing t1 during the period in which the target servo pressure PscTr is maintained. Then, the detected servo pressure value PscE changes due to the operation of the brake actuator 70. For example, the period during which the anti-lock brake control is performed includes a period in which the brake fluid is discharged from the pump 791 with the holding valve 74 closed. Therefore, even though the motor torque Tmt and motor rotation angle θ of the first electric motor 513 are maintained, the detected servo pressure value PscE increases as shown in Figure 5A.

[0077] 6 shows a graph illustrating the relationship between motor torque Tmt and servo pressure Psc. In this graph, the point determined by motor torque Tmt and servo pressure Psc is referred to as the "operating point." When the electric cylinder 51 begins to operate at time t0, the operating point moves generally along the line representing the first characteristic LR1. Then, when the brake actuator 70 begins to operate at time t1, the detected servo pressure value PscE increases while the motor torque Tmt is maintained, and the operating point moves as indicated by arrow A1.

[0078] In the comparative example, even if the detected servo pressure value PscE increases due to the operation of the brake actuator 70, the CPU 211 executes the first selection process M151. In other words, because the target servo pressure PscTr remains unchanged, the CPU 211 selects the first characteristic LR1. Therefore, the reference motor torque TmtB does not change. When a deviation occurs between the target servo pressure PscTr and the detected servo pressure value PscE, the compensation torque ΔTmt derived by the CPU 211 begins to change. Therefore, the operating point moves from the position at time t2 in FIG. 5 as indicated by arrow A2. In other words, although the motor torque Tmt decreases in response to a decrease in the target torque TmtTr, the detected servo pressure value PscE is maintained.

[0079] In contrast, in this embodiment, when the detected servo pressure value PscE begins to increase due to operation of the brake actuator 70, the CPU 211 executes the second selection process M152. Therefore, after timing t1, when the detected servo pressure value PscE becomes greater than the target servo pressure PscTr, the CPU 211 selects the second characteristic LR2. Then, after timing t2, the reference motor torque TmtB decreases, and the target torque TmtTr also decreases. Therefore, the operating point shifts as indicated by arrow A3 in FIG. 6 . That is, because the motor torque Tmt decreases significantly, the first electric motor 513 of the electric cylinder 51 rotates in the reverse direction. Then, because the motor rotation angle θ decreases, the piston 512 moves in the backward direction Zb. As a result, the detected servo pressure value PscE decreases. That is, at timing t3, the detected servo pressure value PscE roughly matches the target servo pressure PscTr. Therefore, the CPU 211 can control the servo pressure Psc with high precision even when the brake actuator 70 is operating.

[0080] Therefore, the braking device 100 can improve the controllability of the servo pressure Psc by operating the electric cylinder 51, thereby increasing the accuracy of adjusting the wheel pressures Pw of the multiple wheel cylinders 16.

[0081] The present embodiment can further provide the following effects. (1) In the braking system 100, the second ECU 220, which is separate from the first ECU 210 that controls the electric cylinder 51, controls the brake actuator 70. Therefore, the first ECU 210 cannot grasp the detailed operating state of the brake actuator 70. However, information that enables the first ECU 210 to grasp whether the brake actuator 70 is operating or whether braking control that operates the brake actuator 70 is being executed is transmitted from the second ECU 220 to the first ECU 210.

[0082] Therefore, the first ECU 210 determines whether the brake actuator 70 is operating based on the information transmitted by the second ECU 220. Then, based on the result of this determination, the first selection process M151 or the second selection process M152 is selected. This allows the first ECU 210 to accurately adjust the servo pressure Psc even if it does not know the detailed operating state of the brake actuator 70.

[0083] (2) Consider a case where the CPU 211 executes the second selection process M152 even when the target servo pressure PscTr is reduced under conditions in which the brake actuator 70 does not increase the detected servo pressure value PscE. In this case, when the target servo pressure PscTr decreases and becomes less than the detected servo pressure value PscE, the CPU 211 selects the second characteristic LR2. The CPU 211 then sets the target torque TmtTr using the reference motor torque TmtB based on the second characteristic LR2. The CPU 211 then drives the first electric motor 513 based on the target torque TmtTr. This reduces the detected servo pressure value PscE. When the detected servo pressure value PscE becomes equal to or less than the target servo pressure PscTr, the CPU 211 selects the first characteristic LR1. The CPU 211 then sets the target torque TmtTr using the reference motor torque TmtB based on the first characteristic LR1. The CPU 211 then drives the first electric motor 513 based on this target torque TmtTr. This increases the detected servo pressure value PscE. That is, periods in which the CPU 211 selects the first characteristic LR1 and periods in which the CPU 211 selects the second characteristic LR2 alternate. As a result, the detected servo pressure value PscE may decrease in a stepped manner.

[0084] In this regard, in the braking device 100, when the target servo pressure PscTr is decreased under conditions in which the brake actuator 70 does not increase the detected servo pressure value PscE, the CPU 211 executes the first selection process M151. In this case, because the target servo pressure PscTr is decreasing, the CPU 211 selects the second characteristic LR2. More specifically, while the target servo pressure PscTr is decreasing, the second characteristic LR2 continues to be selected, and the first characteristic LR1 is not selected. Therefore, the detected servo pressure PscE is decreased at a rate corresponding to the rate at which the target servo pressure PscTr is decreasing. Therefore, even when the target servo pressure PscTr is decreased under conditions in which the brake actuator 70 does not increase the detected servo pressure value PscE, the braking device 100 can accurately adjust the servo pressure Psc.

[0085] <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.

[0086] The hydraulic pressure generating device may have a configuration different from that of the hydraulic pressure generating device 20 shown in Fig. 1 as long as it includes the pressurizing unit 50. For example, the fifth flow path 55 may be connected to the second hydraulic pressure circuit 712 instead of the servo chamber Rs of the master device 30.

[0087] The brake actuator may have a configuration different from that shown in Fig. 2 as long as the servo pressure Psc can be changed due to the operation of the hold valve 74. The brake actuator may have a configuration different from that shown in Fig. 2 as long as the servo pressure Psc can be changed due to the operation of the pump 791.

[0088] The first characteristic LR1 does not have to be the map shown in Fig. 2 as long as it shows the relationship between the motor torque Tmt and the servo pressure Psc when the servo pressure Psc is increased. For example, the first characteristic LR1 may be a relational expression that shows the relationship between the motor torque Tmt and the servo pressure Psc when the servo pressure Psc is increased.

[0089] The second characteristic LR2 does not have to be the map shown in Fig. 2 as long as it indicates the relationship between the motor torque Tmt and the servo pressure Psc when the servo pressure Psc is reduced. For example, the second characteristic LR2 may be a relational expression that indicates the relationship between the motor torque Tmt and the servo pressure Psc when the servo pressure Psc is reduced.

[0090] 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):

[0091] (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.

[0092] That is, the control device 200 may include a processing circuit that controls the electric cylinder 51 and the braking actuator. (b) The control device 200 includes one or more dedicated hardware circuits that perform various processes. Examples of the dedicated hardware circuit include an application specific integrated circuit, i.e., ASIC or FPGA. Note that ASIC is an abbreviation for "Application Specific Integrated Circuit." FPGA is an abbreviation for "Field Programmable Gate Array."

[0093] (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.

[0094] It should be noted that the expression "at least one" used in this specification means "one or more" of the desired options. As an example, the expression "at least one" used in this specification means "only one option" or "both of two options" if the number of options is two. As another example, the expression "at least one" used in this specification means "only one option" or "any combination of two or more options" if the number of options is three or more.

Claims

1. A braking device configured to generate a braking force at a wheel corresponding to wheel pressure, which is 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 so that brake fluid is discharged from the output port by the piston moving forward within the cylinder in response to driving of the electric motor, and so that brake fluid is drawn from the output port by the piston moving backward within the cylinder in response to driving of the electric motor; a servo flow path connected to the output port of the electric cylinder; a servo pressure sensor configured to detect servo pressure, which is the hydraulic pressure of the brake fluid flowing through the servo flow path; 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; a braking actuator connected to the supply flow path and configured to adjust the wheel pressure; a memory that stores a first characteristic indicating the relationship between motor torque, which is the output torque of the electric motor, and servo pressure when increasing the servo pressure, and a second characteristic indicating the relationship between motor torque and servo pressure when decreasing the servo pressure; and a processing circuit configured to operate the electric cylinder, wherein the processing circuit a determination process for determining whether the servo pressure has changed due to the operation of the brake actuator; a first selection process for selecting the second characteristic when a target servo pressure, which is a target value of the servo pressure, has decreased under the condition that it is determined that the servo pressure has not changed due to the operation of the brake actuator, and selecting the first characteristic when the target servo pressure has not decreased; a second selection process for selecting the second characteristic when a servo pressure detection value, which is the servo pressure detected by the servo pressure sensor, is greater than the target servo pressure under the condition that it is determined that the servo pressure has changed due to the operation of the brake actuator, and selecting the first characteristic when the servo pressure detection value is equal to or less than the target servo pressure.a motor drive process that derives a motor torque corresponding to the target servo pressure based on the characteristic selected in the first selection process or the second selection process, and drives the electric motor based on a target value corresponding to the derived motor torque.

2. A braking device as described in claim 1, wherein the brake actuator comprises: a connecting flow path connected to the supply flow path; and an electric pump configured to discharge brake fluid into the connecting flow path; the brake actuator is further configured to increase the wheel pressure by supplying the brake fluid discharged from the pump to the wheel cylinder via the connecting flow path; and the processing circuit is further configured to determine, in the determination process, when the pump is operating, that the servo pressure has changed due to the operation of the brake actuator.

3. A braking device as described in claim 1, wherein the brake actuator comprises a connection flow path connected to the supply flow path and a holding valve which is a normally open solenoid valve installed in the connection flow path, the brake actuator is further configured to be able to restrict the supply of brake fluid to the wheel cylinder via the connection flow path by reducing the opening of the holding valve, and the processing circuit is further configured to determine in the determination process that the servo pressure has changed due to the operation of the brake actuator when the supply of brake fluid to the wheel cylinder is restricted by the operation of the holding valve.

4. A braking device as described in claim 2, wherein the processing circuit is a first processing circuit, the braking device further comprises a second processing circuit that controls the braking actuator, the second processing circuit is configured to transmit information to the first processing circuit when the pump is to be operated, and the first processing circuit is configured to execute the determination process based on information received from the second processing circuit.

5. 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 motor; an electric cylinder including a cylinder having an output port and a piston, the electric cylinder configured so that brake fluid is discharged from the output port by the piston moving forward within the cylinder in response to the driving of the electric motor, and so that brake fluid is drawn from the output port by the piston moving backward within the cylinder in response to the driving of the electric motor; a servo flow path connected to the output port of the electric cylinder; a servo pressure sensor configured to detect servo pressure, which is the hydraulic pressure of the brake fluid flowing through the servo flow path; 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 a braking actuator connected to the supply flow path and configured to be able to adjust the wheel pressure, wherein a first characteristic represents a relationship between the servo pressure and motor torque, which is the output torque of the electric motor, when the servo pressure is increased, and a second characteristic represents a relationship between the motor torque and servo pressure when the servo pressure is decreased, and the control method A method for controlling a braking device, comprising: determining whether the servo pressure has changed due to operation of the brake actuator; selecting the second characteristic if a target servo pressure, which is a target value of the servo pressure, has decreased in a situation where it is determined that the servo pressure has not changed due to operation of the brake actuator, and selecting the first characteristic if the target servo pressure has not decreased; selecting the second characteristic if a servo pressure detection value, which is the servo pressure detected by the servo pressure sensor, is greater than the target servo pressure in a situation where it is determined that the servo pressure has changed due to operation of the brake actuator, and selecting the first characteristic if the servo pressure detection value is equal to or less than the target servo pressure; deriving a motor torque corresponding to the target servo pressure based on the selected characteristic; and driving the electric motor based on a target value corresponding to the derived motor torque.

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