Braking control device for vehicle

Abstract

This braking control device comprises: a control cylinder that generates servo pressure by movement of a control piston driven by an electric motor; a solenoid valve of a normally open type provided in a liquid-pressure transmission path from the control cylinder to a wheel cylinder; and a controller that controls the electric motor and the solenoid valve. The wheel pressure of the wheel cylinder is adjusted by the servo pressure. When a temperature related value related to the temperature of a constituting member for the electric motor exceeds a threshold value, the controller closes the solenoid valve to reduce the servo pressure to a lower limit pressure set on the basis of a maximum rating capable of continuously energizing the constituting member. Further, when the differential pressure between the wheel pressure and the lower limit pressure is small, the controller reduces a valve current supplied for closing the solenoid valve as compared to when the differential pressure is large.

Description

Vehicle braking control device

[0001] The present disclosure relates to a braking control device for a vehicle.

[0002] The applicant has developed a braking control device as described in Patent Document 1. Specifically, the braking control device includes an electric cylinder that discharges brake fluid from an output port at a hydraulic pressure corresponding to the drive of an electric motor, a master cylinder configured so that brake fluid flows out of the master chamber as a result of movement of a master piston in response to an increase in hydraulic pressure in a servo chamber, and brake fluid flows into the master chamber as a result of movement of the master piston in response to a decrease in hydraulic pressure in the servo chamber, a first flow path connecting the master chamber with a wheel cylinder for a front wheel, a sixth flow path connecting the output port with a wheel cylinder for a rear wheel, a fifth flow path connecting the sixth flow path with the servo chamber, and a differential pressure adjustment valve provided in the fifth flow path that adjusts the differential pressure between a first hydraulic pressure which is the hydraulic pressure in the sixth flow path and a second hydraulic pressure which is the hydraulic pressure in the servo chamber.

[0003] However, such braking control devices can have heat generation problems in the electric motor, its drive circuit, etc. For example, Patent Document 2 discloses an electric booster mechanism that includes a holding valve (also called a "solenoid valve") that controls communication between a wheel cylinder that generates braking force on the wheels and a master cylinder, and that closes the holding valve and reduces the amount of current supplied to the electric motor when the brakes are applied continuously and the pressure in the wheel cylinder continues to increase. In such a device, the power consumption of the solenoid valve must be taken into consideration.

[0004] JP 2024-82901 A JP 2009-040122 A

[0005] In view of the above problems, an object of the present invention is to provide a vehicle braking control device that suppresses heat generation in components related to an electric motor by maintaining wheel pressure with an electromagnetic valve, and that can suppress power consumption of the electromagnetic valve.

[0006] The vehicle braking control device (SC) of the present invention comprises an electric cylinder (DN) that generates servo pressure (Pa) by the movement of a control piston (NC) driven by an electric motor (MA), a normally open solenoid valve (UZ) provided in a hydraulic pressure transmission path (HS) from the electric cylinder (DN) to a wheel cylinder (CW), and a controller (EE, etc.) that controls the electric motor (MA) and the solenoid valve (UZ), and adjusts the wheel pressure (Pw) of the wheel cylinder (CW) using the servo pressure (Pa).

[0007] In the vehicle brake control device (SC) according to the present invention, when a temperature-related value (Xm) relating to the temperature of a component (MA, DR, etc.) associated with the electric motor (MA) exceeds a threshold value (xm), the controller (EE, etc.) closes the solenoid valve (UZ) to reduce the servo pressure (Pa) to a lower limit pressure (px) set based on the maximum rating to which current can be continuously applied to the component (MA, DR, etc.). Furthermore, when a differential pressure (sPx) between the wheel pressure (Pw) and the lower limit pressure (px) is small, the controller (EE, etc.) reduces the valve current (Iz) supplied to close the solenoid valve (UZ) compared to when the differential pressure (sPx) is large.

[0008] The valve current Iz required to close the pressure regulating valve UZ is determined by the pressure difference between the wheel pressure Pw and the servo pressure Pa. With the above configuration, the decrease in the servo pressure Pa is limited to the lower limit pressure px, so the pressure difference is kept low to a certain extent. This reduces the valve current Iz, thereby suppressing power consumption of the pressure regulating valve UZ.

[0009] In the vehicle brake control device (SC) according to the present invention, the solenoid valves (UZ) include a front solenoid valve (UZf) provided for the front wheel cylinder (CWf) of the wheel cylinders (CW), and a rear solenoid valve (UZr) provided for the rear wheel cylinder (CWr) of the wheel cylinders (CW), and when the controller (EE, etc.) reduces the servo pressure (Pa), it closes the front solenoid valve (UZf) and keeps the rear solenoid valve (UZr) open. With this configuration, it is no longer necessary to supply power to the rear wheel pressure regulating valve UZr, thereby reducing power consumption accordingly.

[0010] 1 is a schematic diagram for explaining a first embodiment of a brake control device SC (particularly, an upper unit SA) of a vehicle. FIG. 2 is a schematic diagram for explaining an example configuration of a lower unit SZ of the brake control device SC. FIG. 3 is a flow diagram for explaining a pressure regulation control process including a specific process. FIG. 4 is a time series diagram for explaining the operation of the specific process. FIG. 5 is a schematic diagram for explaining a second embodiment of the upper unit SA. FIG. 6 is a flow diagram for explaining a process for selecting a pressure regulation valve to be closed when a specific process is executed in a third embodiment of the brake control device SC. (a) to (c) are time series diagrams for explaining the operation of the specific process in the third embodiment of the brake control device SC.

[0011] <Symbols of components, etc., and subscripts at the end of symbols> In the following description, components, arithmetic processes, signals, characteristics, and values ​​with the same symbol, such as "CW," have the same function. The subscripts "f" and "r" at the end of the symbol for each wheel are generic symbols that indicate whether the symbol relates to the front or rear wheel system. For example, a wheel cylinder CW provided on each wheel is written as "front wheel cylinder CWf, rear wheel cylinder CWr." Furthermore, the subscripts "f" and "r" at the end of a symbol can be omitted. When the subscripts "f" and "r" are omitted, each symbol represents a generic term. For example, "CW" is a generic term for wheel cylinders provided on the front and rear wheels of a vehicle. Furthermore, the generic term "CW" can also be written as "CW (= CWf, CWr)."

[0012] In the brake control device SC, the upper unit SA, the lower unit SZ, and the wheel cylinders CW are connected by a fluid path (communication path HS). Furthermore, in the upper unit SA and the lower unit SZ, various components (CC, etc.) are connected by fluid paths. Here, the "fluid path" is a path for moving the brake fluid BF, and corresponds to piping, flow paths in the actuator, hoses, etc. In the following explanation, the communication path HS, reservoir path HR, input path HN, servo path HU, supply path HH, etc. are fluid paths.

[0013] In the hydraulic pressure transmission path (fluid paths HU, HS, etc.) related to the servo pressure Pa, the side closer to the hydraulic pressure generating unit PU (i.e., the side farther from the wheel cylinder CW) is referred to as the "upper" side. In contrast, the side farther from the hydraulic pressure generating unit PU (i.e., the side closer to the wheel cylinder CW) is referred to as the "lower" side.

[0014] <First Embodiment of Brake Control Device SC> A first embodiment of a brake control device SC (particularly, an upper unit SA) for a vehicle will be described with reference to the schematic diagram of Figure 1. The brake control device SC is composed of an upper unit SA and a lower unit SZ. For example, the brake control device SC is applied to a hybrid vehicle or an electric vehicle equipped with an electric motor for driving.

[0015] The front and rear wheels WHf, WHr (=WH) of a vehicle are equipped with braking devices SX (=SXf, SXr). The braking device SX is composed of a brake caliper, a friction member (e.g., brake pad), and a rotating member KT (e.g., brake disc). The brake caliper (not shown) is provided with a wheel cylinder CW. Hydraulic pressure Pw (referred to as "wheel pressure") in the wheel cylinder CW presses a friction member (not shown) against the rotating member KT fixed to each wheel WH, applying a braking torque Tb to the wheel. As a result, a frictional braking force Fe (also referred to as "hydraulic braking force") is generated in the wheel WH. Therefore, the braking device SX can be described as "a device that generates a frictional braking force Fe using the wheel pressure Pw" or "a device that converts the wheel pressure Pw into a frictional braking force Fe."

[0016] A vehicle is equipped with a brake operating member BP and various sensors (such as SP). The brake operating member BP (e.g., a brake pedal) is an operating member used by the driver to decelerate the vehicle. The vehicle is provided with an operation displacement sensor SP that detects an operation displacement Sp of the brake operating member BP. The operation displacement Sp is one of the state quantities (state variables) that indicate the operation amount of the brake operating member BP, and in a brake-by-wire type brake control device SC, it is a signal that indicates the driver's intention to brake (i.e., a braking command). In addition to the operation displacement sensor SP, a hydraulic pressure Pn (referred to as "input pressure") in an input chamber Rn (described later) is used as another state quantity that indicates the braking operation amount. The input pressure Pn is detected by an input pressure sensor PN. The operation displacement Sp, input pressure Pn, etc. are collectively referred to as the "braking operation amount Ba." The operation displacement sensor SP and the input pressure sensor PN that detect the operation displacement Sp and the input pressure Pn (i.e., the braking operation amount Ba) are referred to as a "braking operation amount sensor BA."

[0017] A vehicle is equipped with various sensors for braking control (i.e., individual control of wheel pressure Pw) such as antilock brake control and anti-skid control. Specifically, each wheel WH is equipped with a wheel speed sensor VW for detecting its rotational speed Vw (referred to as "wheel speed"). Also equipped are a steering amount sensor for detecting the steering amount Sw (e.g., operation angle) of a steering operation member (e.g., a steering wheel), a yaw rate sensor for detecting the yaw rate Yr of the vehicle, a longitudinal acceleration sensor for detecting the longitudinal acceleration Gx (also referred to as "deceleration") of the vehicle, and a lateral acceleration sensor for detecting the lateral acceleration Gy of the vehicle (all not shown).

[0018] The vehicle is equipped with a brake control device SC. The brake control device SC employs a so-called front and rear type (also called "type II") dual brake system. The brake control device SC adjusts the wheel pressure Pw of each wheel cylinder CW.

[0019] The brake control device SC is composed of two brake units SA and SZ. The upper unit SA is composed of an upper actuator YA and an upper controller EA. The upper actuator YA is controlled by the upper controller EA. The lower unit SZ is composed of a lower actuator YZ and a lower controller EZ. The lower actuator YZ is controlled by the lower controller EZ. Here, the upper and lower actuators YA and YZ are also referred to as "upper and lower fluid units." Furthermore, the upper and lower controllers EA and EZ are also referred to as "upper and lower control units."

[0020] The upper unit SA (particularly, the upper controller EZ) and the lower unit SZ (particularly, the lower controller EZ) are connected to a communication bus BS. Signals are transmitted between the multiple controllers (EA, EZ, etc.) via the communication bus BS. That is, the multiple controllers can transmit signals (detected values, calculated values, control flags, etc.) to the communication bus BS and can also receive signals from the communication bus BS.

[0021] <Configuration of Upper Unit SA> The configuration of the upper unit SA according to the first embodiment will be described. The upper unit SA generates a servo pressure Pa in response to operation of the brake operating member BP (brake pedal). The servo pressure Pa causes a supply pressure Ps (=Psf, Psr) to be output from the upper unit SA to the lower unit SZ. Specifically, in the system related to the front wheel cylinder CWf, the master pressure Pm is output as the front wheel supply pressure Psf. Furthermore, in the system related to the rear wheel cylinder CWr, the servo pressure Pa is output as the rear wheel supply pressure Psr. The lower unit SZ adjusts the front and rear wheel supply pressures Psf and Psr (=Ps), ultimately supplying front and rear wheel pressures Pwf and Pwr (=Pw) ​​to the front and rear wheel cylinders CWf and CWr (=CW). The upper unit SA is comprised of an upper actuator YA and an upper controller EA.

[0022] <Upper Actuator YA> The upper actuator YA (upper fluid unit) is composed of a hydraulic pressure generating unit PU, an apply unit AP, and an input unit NR.

[0023] [Hydraulic Pressure Generating Unit PU] The hydraulic pressure generating unit PU uses an electric motor MA as a power source to generate a servo pressure Pa. The hydraulic pressure generating unit PU is also referred to as an "electric cylinder DN." The electric cylinder DN includes an electric motor MA, a rotation angle sensor KA, a motor temperature sensor TM, a reducer GS, a conversion mechanism GH, a control cylinder CC, and a control piston NC.

[0024] The electric motor MA is a power source (pressure source) for generating the servo pressure Pa. "Power" refers to the energy required to move the movable members (GS, GH, NC, etc.) in the electric cylinder DN. For example, power is defined as a physical quantity, namely, energy per unit time (also called "power"). Rotational power (also called "first rotational power") is output from the electric motor MA. The rotational power of the electric motor MA is the shaft torque of the electric motor MA multiplied by the rotational speed of the electric motor MA (particularly, the motor shaft). Note that the linear power of a linearly moving member (described below) is the thrust of the linearly moving member (a force acting in the direction of the central axis) multiplied by the linear speed of the linearly moving member (speed in the direction along the central axis).

[0025] A three-phase brushless motor is used as the electric motor MA. The electric motor MA is provided with a rotation angle sensor KA and a motor temperature sensor TM. The rotation angle sensor KA detects the position Ka (rotation angle) of the motor shaft. The motor temperature sensor TM detects the temperature Tm of the electric motor MA (e.g., the motor coil).

[0026] The motor coils are supplied with power from the upper controller EA (particularly, the drive circuit DR). A permanent magnet is fixed to the outer periphery of the motor shaft. In the three-phase brushless motor MA, the magnetic pole position of the permanent magnet (i.e., the motor rotation angle Ka) is detected by a rotation angle sensor KA. The upper controller EA then drives the switching elements of the drive circuit DR (also called the "inverter circuit") based on the motor rotation angle Ka, switching the three-phase motor current Im (a collective term for the currents flowing through the U, V, and W phases). The rotational power of the electric motor MA is output from the electric motor MA to the reducer GS.

[0027] The reducer GS reduces the speed of the first rotational power output from the electric motor MA. Specifically, the input shaft of the reducer GS is fixed to the motor shaft. Also, the output shaft of the reducer GS is fixed to the rotating member of the conversion mechanism GH. The reducer GS reduces the speed input from the electric motor MA and increases the torque input from the electric motor MA. The reduced rotational power (also referred to as "second rotational power") is then output from the reducer GS to the conversion mechanism GH.

[0028] The conversion mechanism GH is composed of a rotating member that performs rotational motion and a linearly moving member that performs linear motion. In the conversion mechanism GH, the rotational power output from the reducer GS is input to the rotating member. The second rotational power input to the rotating member is then converted into linear power for the linearly moving member. The conversion mechanism GH is also called a "rotation-linear conversion mechanism." A rotation stopper member is engaged with the linearly moving member. This prevents the rotational motion of the linearly moving member, so that the linearly moving member moves along the rotation axis of the rotating member.

[0029] A linear motion member of the conversion mechanism GH transmits linear power to the control piston NC. The control piston NC is inserted into the control cylinder CC. A control chamber Rc (hydraulic pressure chamber) is formed inside the control cylinder CC by the control piston NC. More specifically, the outer peripheral surface of the control piston NC and the inner peripheral surface of the control cylinder CC are sealed by two seal members SL. This makes the control chamber Rc liquid-tight. The hydraulic pressure in the control cylinder CC (i.e., the control chamber Rc) is the servo pressure Pa. In other words, the electric cylinder DN uses the electric motor MA as a power source to output the servo pressure Pa.

[0030] The control cylinder CC is connected to a servo chamber Ru (described later) of the apply unit AP via a servo path HU (fluid path). The control cylinder CC is also connected to a rear wheel cylinder CWr via a rear wheel communication path HSr (fluid path) and the lower unit SZ. The hydraulic pressure generating unit PU is provided with a servo pressure sensor PA to detect a servo pressure Pa (hydraulic pressure generated by the electric cylinder DN).

[0031] FIG. 1 shows a state in which the electric cylinder DN does not generate servo pressure Pa. The control cylinder CC has a through-hole between two seal members SL. The control piston NC also has a through-hole. A supply path HH (fluid path) connected to a master reservoir RV is connected to the through-hole of the control cylinder CC. In the state shown in the figure, the control chamber Rc is connected to the master reservoir RV via the through-hole and the supply path HH, and the servo pressure Pa is "0 (atmospheric pressure)." The position of the control piston NC in this state is referred to as the "initial position." In the initial position, the control piston NC is displaced to its maximum in the backward direction Hb, and the volume of the control chamber Rc is maximized.

[0032] When an increase in servo pressure Pa is required, the rotational power of the electric motor MA is increased. This rotational power is transmitted to the conversion mechanism GH via the reducer GS and output as linear power of the linearly moving member. Then, the control piston NC is pressed by the linearly moving member, causing the control piston NC to move forward in the direction Ha (a direction in which the volume of the control chamber Rc decreases). This movement first blocks communication between the control chamber Rc and the master reservoir RV. When the control piston NC is further moved forward in the direction Ha, the servo pressure Pa (internal pressure of the control chamber Rc) increases from "0 (atmospheric pressure)." Brake fluid BF pressurized to the servo pressure Pa is output (pumped) from the control chamber Rc of the control cylinder CC.

[0033] When it is necessary to maintain the servo pressure Pa, the rotation of the electric motor MA is stopped. The movement of the control piston NC is stopped, and the servo pressure Pa is maintained constant. When it is necessary to decrease the servo pressure Pa, the rotational power of the electric motor MA is reduced. The servo pressure Pa causes the electric motor MA to rotate in the reverse direction, so the control piston NC is moved in the backward direction Hb (the direction in which the volume of the control chamber Rc increases). The brake fluid BF is returned toward the control chamber Rc, so the servo pressure Pa is decreased.

[0034] [Apply Unit AP] The apply unit AP is composed of a single-type master cylinder CM and a master piston NM. The master piston NM is inserted into the single-type master cylinder CM. The interior of the master cylinder CM is divided into three hydraulic chambers Rm, Ru, and Rs by the master piston NM. The master chamber Rm is formed by the master cylinder CM and the master piston NM. Furthermore, the interior of the master cylinder CM is divided into a servo chamber Ru and a reaction chamber Rs by a flange portion Tu of the master piston NM. Here, the pressure-receiving area rm of the master chamber Rm and the pressure-receiving area ru of the servo chamber Ru are set equal to each other.

[0035] A servo pressure Pa is supplied to the servo chamber Ru from the hydraulic pressure generating unit PU (electric cylinder DN). The servo pressure Pa causes the apply unit AP to output a master pressure Pm as a front wheel supply pressure Psf. Here, "master pressure Pm" is the internal pressure of the master chamber Rm. When "Pa = 0" (e.g., when braking is not in progress), the master piston NM is in its most retracted position (i.e., the position where the volume of the master chamber Rm is at its maximum). In this state, the master chamber Rm of the master cylinder CM is in communication with the master reservoir RV. Therefore, the master pressure Pm is "0 (atmospheric pressure)."

[0036] Brake fluid BF is stored inside the master reservoir RV (also called the "atmospheric pressure reservoir"). When the servo pressure Pa increases from "0," the master piston NM is pressed and moves in the forward direction Da (the direction in which the volume of the master chamber Rm decreases). This movement blocks communication between the master chamber Rm and the master reservoir RV. When the master piston NM then moves further in the forward direction Da, the master pressure Pm increases from "0 (atmospheric pressure)." As a result, brake fluid BF pressurized to the master pressure Pm is output (pressurized and pumped) from the master chamber Rm of the master cylinder CM toward the lower unit SZ. Note that since "rm = ru," if the sliding resistance of the seal member SL is ignored, "Pa = Pm."

[0037] [Input Unit NR] The input unit NR realizes regenerative cooperative control. "Regenerative cooperative control" cooperates the friction braking force Fe (braking force due to wheel pressure Pw) and the regenerative braking force Fg (braking force due to the regenerative device) so that the kinetic energy of the vehicle can be efficiently recovered as electrical energy during braking. In regenerative cooperative control, the brake operating member BP is operated, but a state is created in which the wheel pressure Pw is not generated. The input unit NR is composed of an input cylinder CN, an input piston NN, a first control valve VA, a second control valve VB, a stroke simulator SS, and an input pressure sensor PN.

[0038] The input cylinder CN is fixed to the master cylinder CM. An input piston NN is inserted into the input cylinder CN. The input piston NN is mechanically connected to a brake operating member BP (brake pedal) so as to move in conjunction with the movement of the brake operating member BP. A gap Ln (also referred to as the "separation distance") is defined between the end face of the input piston NN and the end face of the master piston NM. The separation distance Ln is adjusted by the servo pressure Pa, thereby realizing regenerative cooperative control.

[0039] The input chamber Rn of the input unit NR is connected to the reaction force chamber Rs of the apply unit AP via an input path HN (fluid path). A normally closed first control valve VA is provided in the input path HN. The input path HN is connected to a master reservoir RV via a reservoir path HR (fluid path) between the first control valve VA and the reaction force chamber Rs. A normally open second control valve VB is provided in the reservoir path HR. On-off solenoid valves are used for the first and second control valves VA and VB. A stroke simulator SS is connected to the input path HN between the first control valve VA and the reaction force chamber Rs.

[0040] When power is not supplied to the first and second control valves VA and VB, the first control valve VA is closed and the second control valve VB is open. Closing the first control valve VA seals the input chamber Rn, creating a fluid lock. This causes the master piston NM to displace integrally with the brake operating member BP. Opening the second control valve VB also connects the stroke simulator SS and the reaction chamber Rs to the master reservoir RV.

[0041] When power is supplied to the first and second control valves VA and VB, the first control valve VA is opened and the second control valve VB is closed. This allows the master piston NM to be displaced separately from the brake operating member BP. Since the input chamber Rn is connected to the stroke simulator SS, the operating force for the brake operating member BP is generated by the stroke simulator SS. An input pressure sensor PN is provided in the input line HN between the input chamber Rn and the first control valve VA to detect the input pressure Pn. The input pressure Pn is also the hydraulic pressure within the stroke simulator SS.

[0042] <<Upper Controller EA>> The upper actuator YA is controlled by the upper controller EA (upper control unit). The upper controller EA is composed of a microprocessor MP and a drive circuit DR. The upper controller EA is connected to a communication bus BS so that signals (detected values, calculated values, control flags, etc.) can be shared with other controllers (EZ, etc.).

[0043] The upper controller EA directly receives various signals such as the operation displacement Sp (detection value of the operation displacement sensor SP), the input pressure Pn (detection value of the input pressure sensor PN), the servo pressure Pa (detection value of the servo pressure sensor PA), and the motor rotation angle Ka (detection value of the rotation angle sensor KA). Furthermore, the upper controller EA also receives various signals such as the master pressure Pm (supply pressure) and the vehicle speed Vx from the communication bus BS. The upper controller EA also outputs an execution flag FL (described later) of a specific process to the communication bus BS. The lower controller EZ controls the pressure regulating valve UZ based on the execution flag FL acquired from the communication bus BS.

[0044] The upper controller EA (particularly the microprocessor MP) is programmed with a pressure regulation control algorithm. Pressure regulation control is a control for adjusting the wheel pressure Pw (=Pwf, Pwr). Pressure regulation control is performed based on the various signals (Sp, Pa, etc.) described above. Based on the pressure regulation control algorithm, the drive circuit DR drives the electric motor MA and various solenoid valves (VA, etc.). The drive circuit DR includes an inverter circuit configured with switching elements (e.g., MOS-FETs) to drive the electric motor MA. The drive circuit DR also includes switching elements to drive the various solenoid valves. Additionally, the drive circuit DR includes a motor current sensor IM that detects the current Im (motor current) supplied to the electric motor MA, and a temperature sensor TD that detects the temperature Td (circuit temperature) of the drive circuit DR. For example, the circuit temperature sensor TD detects the temperature of the switching elements (e.g., MOS-FETs) of the inverter circuit as the circuit temperature Td.

[0045] The upper controller EA calculates drive signals Va and Vb for the first and second control valves VA and VB, and a drive signal Ma for the electric motor MA. The switching elements are then driven in response to the various drive signals (Ma, etc.). Specifically, in controlling the solenoid valves, power is supplied to the first and second control valves VA and VB based on the drive signals Va and Vb. This opens the first control valve VA and closes the second control valve VB. Additionally, the drive signal Ma is determined based on a pressure regulation control algorithm, and the electric motor MA is controlled based on the drive signal Ma.

[0046] <Lower Unit SZ> The configuration of the lower unit SZ will be described with reference to the schematic diagram of FIG. 2. The lower unit SZ is a general-purpose unit for performing antilock brake control, traction control, anti-skid control, etc. The master pressure Pm and servo pressure Pa are input from the upper unit SA to the lower unit SZ as front and rear wheel supply pressures Psf and Psr (i.e., Psf = Pm, Psr = Pa). The front and rear wheel supply pressures Psf and Psr (= Ps) are then adjusted (increased or decreased) by the lower unit SZ and output as hydraulic pressures Pwf and Pwr (front and rear wheel pressures) for the front and rear wheel cylinders CWf and CWr. The lower unit SZ is composed of a lower actuator YZ and a lower controller EZ.

[0047] The lower actuator YZ (lower fluid unit) is provided in the communication passage HS between the upper actuator YA and the wheel cylinder CW. The lower actuator YZ is composed of a pressure regulating valve UZ, a supply pressure sensor PS, a fluid pump QZ, an electric motor MZ, a pressure regulating reservoir RZ, an inlet valve VI, and an outlet valve VO.

[0048] Pressure regulating valves UZ (=UZf, UZr) are provided in the communication passage HS (=HSf, HSr). The pressure regulating valves UZ are normally open linear solenoid valves. The front and rear wheel pressure regulating valves UZf, UZr (corresponding to "solenoid valves") enable the front and rear wheel pressures Pwf, Pwr to be increased from the front and rear wheel supply pressures Psf, Psr. In the lower unit SZ, the front and rear wheel pressure regulating valves UZf, UZr allow the front and rear wheel pressures Pwf, Pwr to be individually adjusted.

[0049] The communication passage HS (= HSf, HSr) is provided with a bypass path (fluid path) connecting the upper and lower parts of the pressure regulating valves UZ (= UZf, UZr). The bypass path is provided with an upper check valve GU (= GUf, GUr). The check valve GU allows fluid to flow toward the wheel cylinder CW but blocks fluid to flow from the wheel cylinder CW. In other words, the upper check valve GU (corresponding to a "check valve") is disposed to bypass the pressure regulating valve UZ and allows fluid pressure to be transmitted from the electric cylinder DN (particularly the control cylinder CC) to the wheel cylinder CW but blocks fluid pressure from the wheel cylinder CW to the electric cylinder DN.

[0050] Front and rear wheel supply pressure sensors PSf and PSr (=PS) are provided above the pressure regulating valve UZ (at the portion of the communication passage HS closer to the upper actuator YA) to detect front and rear wheel supply pressures Psf and Psr (=Ps). The supply pressures Ps (=Psf, Psr) detected by the supply pressure sensors PS (=PSf, PSr) are input to the lower controller EZ. Here, the rear wheel supply pressure sensor PSr can be omitted.

[0051] A return path HZ (=HZf, HZr) connects the upper and lower parts of the pressure regulating valve UZ (=UZf, UZr). The return path HZ (fluid path) is provided with a fluid pump QZ (=QZf, QZr) and a pressure regulating reservoir RZ (=RZf, RZr). The fluid pump QZ is driven by an electric motor MZ. The electric motor MZ is a power source for increasing the supply pressure Ps (=Psf, Psr). Because the electric motor MZ and the fluid pump QZ are included in the lower unit SZ, they are also referred to as the "lower electric motor MZ" and the "lower fluid pump QZ."

[0052] When the electric motor MZ is driven, the fluid pump QZ draws brake fluid BF from the upper portion of the pressure regulating valve UZ and discharges it from the lower portion of the pressure regulating valve UZ. This generates a circulating flow KZ (indicated by dashed arrows) of brake fluid BF through the pressure regulating reservoir RZ in the communication line HS and the return line HZ. When the pressure regulating valve UZ narrows the flow path of the communication line HS and throttles the circulating flow KZ of brake fluid BF, the resulting orifice effect increases the hydraulic pressure Pp (also referred to as "regulation pressure") below the pressure regulating valve UZ from the hydraulic pressure Ps (supply pressure) above the pressure regulating valve UZ. The magnitude relationship between the supply pressure Ps and the regulation pressure Pp is such that the regulation pressure Pp is equal to or greater than the supply pressure Ps (i.e., "Pp≧Ps"). In other words, the regulation pressure Pp can be increased from the supply pressure Ps.

[0053] Within the lower actuator YZ, the front and rear wheel communication passages HSf and HSr are each branched into two passages connected to the front and rear wheel cylinders CWf and CWr. A normally-open inlet valve VI and a normally-closed outlet valve VO are provided for each wheel cylinder CW to individually adjust the wheel pressure Pw. Specifically, the inlet valve VI is provided in the branched communication passage HS (i.e., on the side of the communication passage HS closer to the wheel cylinder CW). The communication passage HS is connected to the pressure regulating reservoir RZ below the inlet valve VI (the portion of the communication passage HS closer to the wheel cylinder CW) via a pressure reduction passage HG (a fluid passage). The pressure reduction passage HG is provided with an outlet valve VO. The inlet valve VI and the outlet valve VO are on-off solenoid valves.

[0054] The communication passage HS (= HSf, HSr) has a fluid path that bypasses the inlet valve VI (= VIf, VIr), and a lower check valve GV (= GVf, GVr) is disposed in this fluid path. The flow direction of the lower check valve GV is opposite to that of the upper check valve GU. In other words, the lower check valve GV allows flow from the wheel cylinder CW but prevents flow toward the wheel cylinder CW.

[0055] The inlet valve VI and the outlet valve VO can individually adjust the wheel pressure Pw for each wheel cylinder CW. When the inlet valve VI and the outlet valve VO are not powered and are deactivated, the inlet valve VI is open and the outlet valve VO is closed. In this state, the wheel pressure Pw is equal to the regulated pressure Pp. To decrease the wheel pressure Pw, the inlet valve VI is closed and the outlet valve VO is opened. This prevents brake fluid BF from flowing into the wheel cylinder CW and causes the brake fluid BF in the wheel cylinder CW to flow into the pressure regulating reservoir RZ, thereby decreasing the wheel pressure Pw. To increase the wheel pressure Pw, the inlet valve VI is opened and the outlet valve VO is closed. This prevents brake fluid BF from flowing into the pressure regulating reservoir RZ and causes the regulated pressure Pp from the pressure regulating valve UZ to be supplied to the wheel cylinder CW, thereby increasing the wheel pressure Pw. However, the upper limit of the increase is the regulated pressure Pp. To maintain the wheel pressure Pw, both the inlet valve VI and the outlet valve VO are closed. Since the wheel cylinder CW is fluidly sealed, the wheel pressure Pw is maintained constant.

[0056] <Lower Controller EZ> The lower controller EZ (lower control unit) controls the lower actuator YZ. Like the upper controller EA, the lower controller EZ is composed of a microprocessor MP and a drive circuit DR. The lower controller EZ is connected to a communication bus BS. Therefore, the upper controller EA and the lower controller EZ can share signals via the communication bus BS.

[0057] The lower controller EZ (particularly, the microprocessor MP) receives inputs of sensor signals such as the supply pressure Ps, wheel speed Vw, steering amount Sw, yaw rate Yr, longitudinal acceleration Gx, and lateral acceleration Gy. Furthermore, the lower controller EZ calculates the vehicle's traveling speed Vx (also referred to as "vehicle speed") based on the wheel speed Vw. The lower controller EZ executes antilock brake control (also known as ABS control) to prevent the wheels WH from locking, traction control to prevent the drive wheels from spinning, and anti-skid control (also known as ESC) to prevent understeer and oversteer and improve the directional stability of the vehicle.

[0058] The lower controller EZ drives the electric motor MZ and various solenoid valves (UZ, etc.) that constitute the lower actuator YZ via a drive circuit DR in accordance with a control algorithm programmed in the microprocessor MP. The drive circuit DR of the lower controller EZ includes an H-bridge circuit configured with switching elements (e.g., MOS-FETs) to drive the electric motor MZ. The drive circuit DR also includes switching elements to drive the various solenoid valves (UZ, etc.). In addition, the drive circuit DR includes a motor current sensor (not shown) that detects the current Im supplied to the electric motor MZ and a valve current sensor IZ (not shown) that detects the current Iz (also referred to as the "valve current") supplied to the pressure regulating valve UZ. Based on the control algorithm, a drive signal Mz for the electric motor MZ, a drive signal Uz for the pressure regulating valve UZ, a drive signal Vi for the inlet valve VI, and a drive signal Vo for the outlet valve VO are calculated. Based on the drive signal (Uz, etc.), the electric motor MZ and the solenoid valves UZ, VI, and VO are controlled by the drive circuit DR.

[0059] <Pressure Regulation Control Processing> An example of the pressure regulation control processing will be described with reference to the flowchart of FIG. 3. In pressure regulation control, the processing of each step is executed for each calculation cycle. Note that pressure regulation control includes a specific processing for suppressing temperature rise of components related to the electric motor MA. Specifically, the processing from step S130 to step S230 corresponds to the specific processing. The specific processing is permitted when the vehicle is stopped. That is, when the vehicle traveling speed Vx (vehicle speed) is "0", the specific processing can be executed. On the other hand, when the vehicle speed Vx is greater than "0", the execution of the specific processing is prohibited.

[0060] The specific process is executed by the upper unit SA and the lower unit SZ. That is, the control process for the electric motor MA is executed by the upper controller EA. The upper controller EA drives the electric motor MA in the upper actuator YA. On the other hand, the control process for the pressure regulating valve UZ is executed by the lower controller EZ. The lower controller EZ drives only the pressure regulating valve UZ in the lower actuator YZ. That is, during the specific process, the drive of the lower electric motor MZ, inlet valve VI, and outlet valve VO is stopped. An instruction to open or close the pressure regulating valve UZ is transmitted from the upper controller EA to the lower controller EZ by the execution flag FL.

[0061] In pressure regulation control, first, power is supplied to the first and second control valves VA, VB. The normally closed first control valve VA is opened, and the normally open second control valve VB is closed. This allows the master piston NM and the brake operating member BP to be displaced separately, so that the front and rear wheel pressures Pwf, Pwr can be adjusted independently of the operation of the brake operating member BP. At this time, the operating force of the brake operating member BP is generated by the stroke simulator SS.

[0062] In step S110, various signals are read in the upper controller EA. Signals of the braking operation amount Ba (collectively referring to Sp and Pn), servo pressure Pa, and vehicle speed Vx are input to the upper controller EA. The braking operation amount Ba is acquired by the operation amount sensor BA. The servo pressure Pa is acquired by the servo pressure sensor PA. The vehicle speed Vx is calculated by the lower controller EZ based on the wheel speed Vw, and is acquired by the upper controller EA via the communication bus BS.

[0063] In step S120, a command pressure Pd is calculated based on the braking operation amount Ba and a calculation map Zpd. The "command pressure Pd" is a target value of the wheel pressure Pw requested by the driver. In pressure regulation control, the wheel pressure Pw is adjusted via the servo pressure Pa, but the servo pressure Pa (actual value) is ultimately adjusted by a target pressure Pt (described later). Therefore, the command pressure Pd corresponds to an intermediate target value in pressure regulation control. Specifically, the command pressure Pd is calculated to be "0" according to the calculation map Zpd when the braking operation amount Ba is less than a predetermined amount bo. When the braking operation amount Ba is equal to or greater than the predetermined amount bo, the command pressure Pd is calculated to increase from "0" as the braking operation amount Ba increases. Here, the predetermined amount bo corresponds to the play of the brake operating member BP, etc., and is preset as a predetermined value (constant) (see the command pressure calculation block PD for the above).

[0064] In step S130, it is determined whether a specific process is being executed. The specific process is a control process for preventing components related to the electric motor MA from overheating. Here, the "components related to the electric motor MA" include the electric motor MA itself, the drive circuit DR (particularly the switching elements) that drives the electric motor MA, the electrical wiring, etc. If the specific process is not being executed, step S130 is negative, and the process proceeds to step S140. If the specific process is already being executed, step S130 is positive, and the process proceeds to step S150.

[0065] In step S140, it is determined whether or not the specific process should be started. This determination process is called a "start determination." The specific process is determined to be started when the following conditions are satisfied. Note that the vehicle being stopped is a prerequisite for the execution of the specific process (i.e., a condition for permitting the specific process).

[0066] [Start condition] The temperature-related value Xm, which is related to the temperature of the components (MA, DR, etc.) of the electric motor MA, exceeds the start threshold value xm. Here, the "temperature-related value Xm" corresponds to at least one of the temperature Tm of the electric motor MA (e.g., the motor coil) and the temperature Td of the drive circuit DR (e.g., the switching element). The integrated value of the motor current Im over time may also be used as the temperature-related value Xm. The start threshold value xm is a predetermined value (constant) that is set in advance.

[0067] If the temperature-related value Xm has not reached the start threshold value xm, the result of step S140 is negative, and the process proceeds to step S180. On the other hand, if the temperature-related value Xm is greater than the start threshold value xm, the result of step S140 is positive, and the process proceeds to step S160. At this time, the execution flag FL is switched from "0" to "1." The "execution flag FL" is a control flag that indicates the execution status of a specific process. Here, "FL = 0" indicates that the specific process is not being executed, and "FL = 1" indicates that the specific process is being executed. The execution flag FL also serves as a control flag that instructs the lower controller EZ to close the pressure regulating valve UZ.

[0068] In step S150, it is determined whether the specific process should be terminated. This determination process is called an "end determination." The specific process is determined to be terminated when the following conditions are satisfied:

[0069] [Ending Condition] The command pressure Pd calculated from the braking operation amount Ba becomes smaller than the maintained wheel pressure Pw. Here, an estimated value based on the servo pressure Pa is used for the wheel pressure Pw. Note that, when estimating the wheel pressure Pw, a decrease in the wheel pressure Pw due to leakage from the pressure regulating valve UZ may or may not be taken into account. If leakage is ignored, the wheel pressure Pw is estimated to be the servo pressure Pa when the pressure regulating valve UZ is closed (at the start of the specific processing) or when the servo pressure Pa is re-increased and reaches the command pressure Pd. If leakage is taken into account, the wheel pressure Pw is estimated based on the elapsed time from the time when the pressure regulating valve UZ is closed or when the re-increase of the servo pressure Pa is completed. In either case, the wheel pressure Pw is determined based on the servo pressure Pa.

[0070] If the termination condition is not satisfied, step S150 is negative and the process proceeds to step S160. At this time, execution of the specific process continues, and the execution flag FL remains at "1." If the termination condition is satisfied, step S150 is positive and the process proceeds to step S230. At this time, the execution flag FL is switched from "1 (executed)" to "0 (not executed)."

[0071] In step S160, a target current It is calculated so that the pressure regulating valve UZ is closed. The "target current It" is a target value corresponding to the valve current Iz (actual value) to be supplied to the pressure regulating valve UZ. For example, in step S160, a hydraulic pressure difference sPx (also referred to as a "specific differential pressure") between the command pressure Pd and a lower limit pressure px (described later) is calculated (i.e., "sPx = Pd - px"). At this time, since the wheel pressure Pw has been increased to the command pressure Pd by the servo pressure Pa, the specific differential pressure sPx is also the hydraulic pressure difference between the wheel pressure Pw (= Pa) and the lower limit pressure px (i.e., "sPx = Pw - px").

[0072] In step S160, a valve-closing current Ih is calculated based on the specific differential pressure sPx. The "valve-closing current Ih" is a target value corresponding to the minimum supply current Iz required to close the pressure regulating valve UZ against the specific differential pressure sPx. Specifically, the valve-closing current Ih is determined to be larger as the specific differential pressure sPx increases according to a preset calculation map Zih (see the valve-closing current calculation block IH). Furthermore, in step S160, a margin current ic is added to the valve-closing current Ih to more reliably close the pressure regulating valve UZ, thereby determining a target current It (i.e., It = Ih + ic). Here, the "target current It" is a target value of the supply current Iz (valve current) to the pressure regulating valve UZ to close the pressure regulating valve UZ in the specific process. The "margin current ic" is a preset constant that increases the valve-closing current Ih to allow for a margin in the target current It. The target current It (target value) is determined in accordance with the valve-closing current Ih so that it increases as the specific differential pressure sPx increases.

[0073] In step S170, it is determined whether or not the servo pressure Pa needs to be increased. In the specific process (particularly, the reduction process), the pressure regulating valve UZ is closed to reduce the servo pressure Pa while the wheel cylinder CW is fluid-locked, in order to suppress temperature increases in the components related to the electric motor MA. However, even if the pressure regulating valve UZ is closed, leakage of brake fluid BF can occur. Initially, when the pressure regulating valve UZ is closed, the wheel pressure Pw is maintained at the command pressure Pd, but gradually decreases over time T. In the "pressure increase process," the target pressure Pt (and thus the servo pressure Pa) is again increased to compensate for the decrease in wheel pressure Pw caused by the leakage of brake fluid BF.

[0074] In step S170, if at least one of the following conditions is met: "the pressure reduction time Tk (described below) has not reached the first predetermined time tx" and "the holding time Tj (described below) has continued for the second predetermined time tz," step S170 is negative, and processing proceeds to step S190. On the other hand, if "the pressure reduction time Tk is equal to or greater than the first predetermined time tx" and "the holding time Tj is less than the second predetermined time tz," step S170 is positive, and processing proceeds to step S210. Here, the first predetermined time tx is a threshold value corresponding to the pressure reduction time Tk and is a predetermined value (constant). Similarly, the second predetermined time tz is a threshold value corresponding to the holding time Tj and is also a predetermined value (constant).

[0075] In steps S180, S190, S210, and S230, a target pressure Pt corresponding to each process is determined based on the command pressure Pd. The command pressure Pd is a target value for the servo pressure Pa (and consequently, the wheel pressure Pw) commanded by the driver. However, in certain processes, the servo pressure Pa may be reduced from the command pressure Pd to suppress heat generation. The "target pressure Pt" is a final target value corresponding to the servo pressure Pa for commanding a reduction in hydraulic pressure. In the above processes, the servo pressure Pa is controlled based on the target pressure Pt. Specifically, the electric cylinder DN (particularly the electric motor MA) adjusts the servo pressure Pa so that it approaches and matches the target pressure Pt.

[0076] In step S180, normal pressure regulation control (i.e., pressure regulation control when no specific process is being executed) is executed. This control process is referred to as "normal process." In normal process, the command pressure Pd is determined as the target pressure Pt (i.e., "Pt = Pd"). Note that in normal process, the operation of the lower actuator YZ is stopped. Therefore, no power is supplied to the pressure regulator valve UZ, and the normally open pressure regulator valve UZ is in a fully open state.

[0077] Steps S190 to S230 are specific processing steps. The specific processing steps include a pressure reduction process, a pressure increase process, and a termination process. In the specific processing steps, the relationship between the command pressure Pd and the target pressure Pt is such that the target pressure Pt is equal to or less than the command pressure Pd (i.e., Pt≦Pd).

[0078] In step S190, a pressure reduction process is executed as part of the specific process. This process reduces the load on the electric motor MA to suppress heat generation in components related to the electric motor MA. Specifically, in step S190, a target pressure Pt (a target value for controlling the servo pressure Pa) is determined based on the command pressure Pd and the calculation map Zpg so that the target pressure Pt decreases from the command pressure Pd to the lower limit pressure px at a pressure reduction gradient dg. For example, in the calculation map Zpg, the point at which the first affirmative determination in step S140 is made (the corresponding calculation cycle, when the execution flag FL is switched from "0" to "1") is set as the starting point (i.e., "T = 0"), and the target pressure Pt is calculated as the time T elapses from that point. Here, the pressure reduction gradient dg (the amount of hydraulic pressure reduction per unit time) is a predetermined constant. The lower limit pressure px is also a predetermined constant (see the target pressure calculation block PT).

[0079] The lower limit pressure px is set based on the rating (also referred to as the "continuous rating") at which continuous current can be applied to the components of the electric motor MA. For example, the continuous rating is the maximum value of the motor current Im that can be continuously supplied to the electric motor MA. In this case, the continuous rating is also referred to as the "current capacity." The lower limit pressure px is determined as the hydraulic pressure equivalent to the continuous rating (current capacity) of the components of the electric motor MA. In particular, the lower limit pressure px is determined according to the continuous rating of the component that is most thermally severe among the components of the electric motor MA. As an example, the lower limit pressure px is set in accordance with the continuous rating of a switching element (such as a MOS-FET) that switches the three-phase motor current Im.

[0080] During the pressure reduction process, the current Im supplied to the electric motor MA is reduced, thereby suppressing heat generation in the components related to the electric motor MA. During the pressure reduction process, the pressure regulating valve UZ remains closed, and the wheel pressure Pw in the wheel cylinder CW is sealed (a so-called fluid lock state). Therefore, even if the servo pressure Pa decreases due to a decrease in the motor current Im, the wheel pressure Pw is maintained approximately constant.

[0081] In step S200, a pressure reduction time Tk is calculated. The "pressure reduction time Tk" is the elapsed time related to the pressure reduction process. For example, the pressure reduction time Tk is determined as the elapsed time from the point (corresponding calculation cycle) when the target pressure Pt (resulting in the servo pressure Pa) starts to decrease. The pressure reduction time Tk is used in the determination of step S170.

[0082] In step S210, a pressure increase process is performed in the specific process. In this process, the servo pressure Pa, which has been reduced to the lower limit pressure px, is increased back to the command pressure Pd so that the wheel pressure Pw coincides with the command pressure Pd. Specifically, in step S210, the target pressure Pt is determined based on the command pressure Pd and the calculation map Zpz, so that the target pressure Pt increases from the lower limit pressure px to the command pressure Pd at a pressure increase gradient dz. Here, the pressure increase gradient dz (the amount of hydraulic pressure increase per unit time) is a predetermined constant. Even during this process, the pressure regulating valve UZ remains closed. Therefore, the brake fluid BF pressurized to the servo pressure Pa flows into the wheel cylinder CW through the check valve GU. In other words, the wheel pressure Pw is increased via the check valve GU. This ensures that the wheel pressure Pw is increased without being reduced.

[0083] In step S220, a holding time Tj is calculated. The "holding time Tj" is the elapsed time related to the pressure increase process. For example, the holding time Tj is determined as the elapsed time from the point (corresponding calculation cycle) when the target pressure Pt (resulting in the servo pressure Pa) matches the command pressure Pd. The holding time Tj is used in the determination of step S170.

[0084] In step S230, the specific process is terminated. The termination is performed when the brake operating member BP is returned and the command pressure Pd becomes less than the wheel pressure Pw. In the termination, the pressure regulating valve UZ is gradually opened to reduce the wheel pressure Pw. After the wheel pressure Pw matches the servo pressure Pa, the electric cylinder DN reduces the servo pressure Pa (=Pw).

[0085] <Operation of the Specific Processing> The operation of the specific processing will be described with reference to the time series diagram of Figure 4 (a diagram showing the transition of each state quantity over time T). The diagram assumes a situation in which, after the vehicle stops, the operation amount Ba of the brake operating member BP is increased, and then the temperature-related value Xm exceeds the start threshold value xm. Note that the valve current Iz is controlled to match the target current It, so the diagrams overlap. Similarly, the servo pressure Pa is controlled to match the target pressure Pt, so the diagrams overlap. Furthermore, in the specific processing, the pressure regulating valve UZ is driven, but the drive of the other components (MZ, VI, VO, etc.) of the lower actuator YZ is stopped.

[0086] Before time t0, normal processing based on the braking operation amount Ba of value ba generates a servo pressure Pa (=Pw) ​​of value pa, and the vehicle is decelerated. At time t0, the vehicle stops. At time t0, the condition "Vx = 0" is satisfied, and the specific processing is permitted. Specifically, at time t0, the permission flag FK is switched from "0" to "1." Here, the "permission flag FK" is a control flag that indicates whether the specific processing is permitted or prohibited. The permission flag FK is set to "0" to indicate prohibition, and "1" to indicate permission.

[0087] At time t1, the braking operation amount Ba is increased from value ba. As a result, the command pressure Pd is increased from value pa. As the command pressure Pd increases, the temperature-related value Xm gradually increases. At time t2, the braking operation amount Ba is maintained at value bb, and the command pressure Pd is maintained at value pb.

[0088] At time t3, the temperature-related value Xm exceeds the start threshold value xm (a predetermined constant). The temperature-related value Xm is determined based on at least one of the temperature Tm (motor temperature) of the electric motor MA, the temperature Td (circuit temperature) of the drive circuit DR, and the motor current Im (particularly, its time-integrated value). At time t3, the determination in step S140 is satisfied, and the pressure reduction process associated with the specific process is initiated. At time t3, a target current It is calculated to reliably close the pressure regulating valve UZ. Specifically, the target current It is set to a value ix by adding a margin current ic to the valve-closing current Ih. A valve current Iz corresponding to the target current It is supplied to the pressure regulating valve UZ to close the pressure regulating valve UZ. The valve-closing current Ih is determined based on a specific differential pressure sPx (the difference between the wheel pressure Pw and the lower limit pressure px), and is set to a larger value as the specific differential pressure sPx increases. Therefore, when the specific differential pressure sPx between the wheel pressure Pw and the lower limit pressure px is small, the valve current Iz for closing the pressure regulating valve UZ is set smaller than when the specific differential pressure sPx is large.

[0089] From time t3, the target pressure Pt is reduced at a pressure reduction gradient dg as time T passes in accordance with the calculation map Zpg. In addition, calculation of the pressure reduction time Tk begins at time t3. After time t3, the wheel cylinder CW is sealed by the pressure regulating valve UZ, so even if the servo pressure Pa decreases, the wheel pressure Pw does not decrease substantially. However, due to leakage from the pressure regulating valve UZ, the wheel pressure Pw decreases, albeit slightly. At time t4, the target pressure Pt is maintained at the lower limit pressure px in accordance with the calculation map Zpg. From time t4, the state of "Pt = px" continues.

[0090] Due to leakage from the pressure regulating valve UZ, the wheel pressure Pw decreases from the command pressure Pd. At time t5, the pressure reduction time Tk reaches the first predetermined time tx. At time t5, the determination in step S170 is satisfied, and the pressure increase process related to the specific process is initiated. At this time, the pressure reduction time Tk is reset to "0." From time t5, the target pressure Pt is increased at a pressure increase gradient dz as time T passes, according to the calculation map Zpz. The pressure regulating valve UZ remains closed even after time t5. However, when the target pressure Pt (and thus the servo pressure Pa) becomes greater than the wheel pressure Pw, the servo pressure Pa is supplied to the wheel cylinder CW through the check valve GU.

[0091] At time t6, the target pressure Pt reaches the command pressure Pd. At time t6, calculation of the holding time Tj begins. From time t6, the state "Pt = Pd" continues. At time t7, the holding time Tj reaches the second predetermined time tz. As a result, the determination in step S170 is negative, and the pressure reduction process related to the specific process is restarted. At this time, the holding time Tj is reset to "0". As described above, from time t7, the target pressure Pt is reduced based on the calculation map Zpg. Similarly, at time t7, calculation of the pressure reduction time Tk, which had been reset to "0", begins.

[0092] At time t8, the brake operating member BP is returned, and the braking operation amount Ba, which has been maintained at value bb, begins to decrease. As the braking operation amount Ba decreases, the command pressure Pd decreases. At time t9, the command pressure Pd becomes smaller than the wheel pressure Pw (estimated value), and the termination condition is met. At time t9, the pressure regulating valve UZ, which had been closed, begins to open. Specifically, at time t9, the target current It is reduced by the margin current ic, and the target current It is set equal to the valve-closing current Ih. After time t9, the target current It is determined based on the hydraulic pressure difference sPt between the command pressure Pd and the target pressure Pt and the calculation map Zih. As a result, the valve current Iz is gradually reduced, and the pressure regulating valve UZ is opened to achieve the hydraulic pressure difference sPt. As a result, the wheel pressure Pw decreases in line with the command pressure Pd.

[0093] At time t10, the command pressure Pd (and consequently the wheel pressure Pw) reaches the target pressure Pt (and consequently the servo pressure Pa), which has been maintained constant. At time t10, the supply of power to the pressure regulating valve UZ is stopped, and the pressure regulating valve UZ is fully opened. From time t10 onwards, the target pressure Pt is set equal to the command pressure Pd. As a result, after time t10, the wheel pressure Pw (= Pa) decreases as the command pressure Pd decreases.

[0094] <<Functions and Effects of the Specific Processing>> The valve current Iz required to close the pressure regulating valve UZ depends on the differential pressure between the high-pressure side hydraulic pressure (i.e., wheel pressure Pw) and the low-pressure side hydraulic pressure (i.e., servo pressure Pa) for the pressure regulating valve UZ. Specifically, the greater the differential pressure, the greater the valve current Iz required to close the pressure regulating valve UZ. For example, if the servo pressure Pa is reduced to "0 (atmospheric pressure)" during the specific processing, the differential pressure between the wheel pressure Pw and the servo pressure Pa increases, requiring a large valve current Iz. However, since the brake control device SC only reduces the servo pressure Pa to the lower limit pressure px, the difference sPx between the wheel pressure Pw and the lower limit pressure px (specific differential pressure) is kept low to a certain extent. This minimizes the valve current Iz, thereby reducing power consumption of the pressure regulating valve UZ during the specific processing.

[0095] The lower limit pressure px is preset as a predetermined value based on the continuous rating at which current can be continuously applied to the components (MA, DR, etc.) of the electric motor MA. Even if the electric motor MA is driven at the continuous rating, the temperature at which heat generation and heat dissipation / cooling balance is below an allowable temperature, so even if the servo pressure Pa is maintained at the lower limit pressure px, no thermal problems will occur.

[0096] The pressure regulating valves UZ include a front wheel pressure regulating valve UZf provided for the front wheel cylinder CWf of the wheel cylinders CW, and a rear wheel pressure regulating valve UZr provided for the rear wheel cylinder CWr of the wheel cylinders CW. When the brake control device SC executes the specific process, a valve current Iz is supplied to the front wheel pressure regulating valve UZf and the rear wheel pressure regulating valve UZr to close them. This maintains the front wheel pressure Pwf and the rear wheel pressure Pwr. This reduces power consumption related to the front wheel pressure regulating valve UZf and the rear wheel pressure regulating valve UZr. Since braking force is maintained even when the servo pressure Pa decreases, the vehicle can be reliably maintained in a stopped state even when the specific process is executed.

[0097] The pressure regulating valve UZ (solenoid valve) is composed of a valve element driven by a solenoid and a valve seat against which the valve element can contact. The pressure regulating valve UZ regulates the hydraulic pressure Pp by the gap between the valve seat and the valve element. During specific processing, the valve seat and the valve element are brought into tight contact, and the pressure regulating valve UZ is closed. However, even in this state, a small gap exists between the valve seat and the valve element, which may result in leakage of brake fluid BF. The degree of hydraulic leakage when the pressure regulating valve UZ is closed depends on the differential pressure between the high-pressure side hydraulic pressure (i.e., wheel pressure Pw) and the low-pressure side hydraulic pressure (i.e., servo pressure Pa) relative to the pressure regulating valve UZ. For example, during specific processing, if the servo pressure Pa (= Ps) is reduced to "0 (atmospheric pressure)," the differential pressure between the wheel pressure Pw and the servo pressure Pa increases, and leakage from the pressure regulating valve UZ becomes significant. However, in the brake control device SC, the servo pressure Pa is only reduced to the lower limit pressure px, so that the reduction in the wheel pressure Pw due to fluid leakage is suppressed.

[0098] Furthermore, the specific processing in the brake control device SC includes a pressure increase processing in addition to a pressure decrease processing. The pressure increase processing increases the wheel pressure Pw that has decreased to the command pressure Pd. That is, the pressure increase processing can compensate for the decrease in wheel pressure Pw caused by fluid leakage from the pressure regulating valve UZ.

[0099] Additionally, in the brake control device SC, the wheel pressure Pw is increased through the check valve GU during the pressure increase process. That is, the pressure regulating valve UZ remains closed even during the pressure increase process. Because the wheel pressure Pw does not decrease due to the opening of the pressure regulating valve UZ, the wheel pressure Pw can be reliably increased. The check valve GU is disposed to bypass the pressure regulating valve UZ, and allows pressure to be transmitted from the electric cylinder DN (particularly the control cylinder CC) to the wheel cylinder CW, but prevents pressure from being transmitted from the wheel cylinder CW to the electric cylinder DN.

[0100] <Modification of the First Embodiment> A modification of the first embodiment will be described. In the above-described embodiment, a single-type master cylinder CM is used, and master pressure Pm is transmitted to the front wheel cylinder CWf, and servo pressure Pa is transmitted to the rear wheel cylinder CWr. Alternatively, a tandem-type master cylinder CM may be used. In this configuration, two hydraulic chambers, front and rear master chambers Rmf and Rmr, are formed in the master cylinder CM. Then, servo pressure Pa is supplied to the servo chamber Ru to generate front and rear master pressures Pmf and Pmr, which are transmitted to the front and rear wheel cylinders CWf and CWr.

[0101] In the above-described embodiment, the pressure-receiving area rm (master area) of the master chamber Rm and the pressure-receiving area ru (servo area) of the servo chamber Ru in the apply unit AP are set equal. However, the master area rm and the servo area ru do not have to be equal. In a configuration in which the master area rm and the servo area ru are different, the master pressure Pm (= Psf) can be converted to the servo pressure Pa based on the area ratio between the servo area ru and the master area rm (i.e., conversion based on "Pm rm = Pa ru").

[0102] In the specific process described above, the front wheel pressure Pwf and the rear wheel pressure Pwr are controlled in the same manner. Specifically, when the specific process is initiated, both the front wheel pressure regulator valve UZf and the rear wheel pressure regulator valve UZr are closed, and the servo pressure Pa is adjusted by the electric cylinder DN. Alternatively, the front wheel pressure regulator valve UZf may be closed but the rear wheel pressure regulator valve UZr may be open. The pressure regulator valves UZ include a front wheel pressure regulator valve UZf (corresponding to a "front wheel solenoid valve") provided for the front wheel cylinder CWf and a rear wheel pressure regulator valve UZr (corresponding to a "rear wheel solenoid valve") provided for the rear wheel cylinder CWr. However, when the specific process is executed, the front wheel pressure regulator valve UZf is closed but the rear wheel pressure regulator valve UZr remains open. In this configuration, the front wheel cylinder CWf is sealed by closing the front wheel pressure regulating valve UZf, and the front wheel pressure Pwf is maintained substantially constant (see the transition of "Pw" in FIG. 4), while the rear wheel pressure Pwr increases or decreases in conjunction with the servo pressure Pa (see the transition of "Pa" in FIG. 4). During the specific processing, the rear wheel pressure regulating valve UZr remains de-energized, thereby reducing power consumption accordingly. Note that, since the contribution of the front wheel pressure Pwf to the generation of braking force is significantly greater than the contribution of the rear wheel pressure Pwr, maintaining the front wheel pressure Pwf allows the vehicle to be sufficiently maintained in a stopped state.

[0103] Alternatively, in the specific processing, the rear wheel pressure regulator valve UZr may be closed, but the front wheel pressure regulator valve UZf may be left open. In this configuration, the rear wheel cylinder CWr is sealed by the rear wheel pressure regulator valve UZr being closed, so the rear wheel pressure Pwr is maintained substantially constant (see the transition of "Pw" in FIG. 4). Meanwhile, the front wheel pressure Pwf increases or decreases in conjunction with the servo pressure Pa (see the transition of "Pa" in FIG. 4). In the specific processing, the front wheel pressure regulator valve UZf remains de-energized, thereby reducing power consumption accordingly.

[0104] In the specific process, when only the rear wheel pressure regulator valve UZr is closed, the electric cylinder DN may be operated so that the rear wheel pressure Pwr is increased to a value higher than the hydraulic pressure at the start of the specific process. The "start of the specific process" mentioned here does not necessarily mean the start of the specific process, but may also include the period immediately before and after the start of the specific process. This makes it easier to maintain a stopped state even if the contribution of the rear wheel pressure Pwr to the generation of braking force is lower than the contribution of the front wheel pressure Pwf.

[0105] In the specific process described above (particularly the pressure increase process), calculation of the pressure decrease time Tk is initiated at the time (corresponding calculation cycle) when the target pressure Pt (resulting in the servo pressure Pa) begins to decrease due to the pressure decrease process. That is, the calculation of the pressure decrease time Tk starts at the time when the pressure decrease starts. Alternatively, the calculation of the pressure decrease time Tk may start at the time (corresponding calculation cycle) when the target pressure Pt (resulting in the servo pressure Pa) reaches the lower limit pressure px. In this configuration, calculation of the pressure decrease time Tk is initiated at time t4 (i.e., the time when the pressure decrease ends and the time when the pressure hold starts). Then, the pressure increase process is initiated at time t5, which is a first predetermined time tx after time t4.

[0106] <Second Embodiment of Brake Control Device SC> A second embodiment of the upper unit SA of the brake control device SC will be described with reference to the schematic diagram of FIG. 5 . In the first embodiment, the servo pressure Pa is transmitted as the supply pressure Ps (=Pm) via the master cylinder CM and the master piston NM. In other words, in the hydraulic pressure transmission path for at least the front wheel cylinder CWf, the apply unit AP is arranged in series with the hydraulic pressure generating unit PU. Instead of this configuration, in the second embodiment, the apply unit AP and the hydraulic pressure generating unit PU are arranged in parallel. That is, in the second embodiment, the apply unit AP (particularly the master cylinder CM) and the hydraulic pressure generating unit PU are each directly connected to the lower unit SZ (particularly the lower actuator YZ). In the second embodiment, the same specific processing as in the first embodiment is also performed.

[0107] In the upper unit SA according to the second embodiment, a shutoff valve VM, a simulator valve VS, and a communication valve VC are provided instead of the input unit NR. The shutoff valve VM is a normally open solenoid valve, while the simulator valve VS and the communication valve VC are normally closed solenoid valves. The shutoff valve VM is provided in a front wheel communication passage HSf that connects the master cylinder CM (particularly the master chamber Rm) and the front wheel cylinders CWf. A stroke simulator SS is connected to the front wheel communication passage HSf between the master cylinder CM and the shutoff valve VM via the simulator valve VS.

[0108] The front and rear wheel communication passages HSf and HSr (fluid passages connecting the front and rear wheel cylinders CWf and CWr) are connected to the control cylinder CC (particularly, the control chamber Rc) via a communication passage HV (fluid passage). The communication passage HV is also a fluid passage connecting the front and rear wheel communication passages HSf and HSr. A communication valve VC is provided in the communication passage HV.

[0109] During pressure regulation control, power is supplied to the shutoff valve VM, simulator valve VS, and communication valve VC. This closes the shutoff valve VM and opens the simulator valve VS and communication valve VC. The master chamber Rm is disconnected from the front wheel cylinder CWf, and servo pressure Pa is supplied to the front wheel cylinder CWf. Since the master chamber Rm is connected to the stroke simulator SS, the operating force of the brake operating member BP (brake pedal) is generated by the stroke simulator SS. The servo pressure sensor PA may be provided in the hydraulic pressure generating unit PU or in the lower actuator YZ. In a configuration in which the servo pressure sensor PA is provided in the lower actuator YZ, the servo pressure Pa is acquired by the upper controller EA via the communication bus BS.

[0110] The second embodiment also executes the same control as the first embodiment (including the above-described modified example), and therefore the second embodiment also achieves the same effects as the first embodiment (such as suppressing heat generation from components related to the electric motor MA through specific processing, suppressing power consumption related to the pressure regulating valve UZ, and compensating for a decrease in wheel pressure Pw due to fluid leakage).

[0111] <Third Embodiment of Brake Control Device SC> A third embodiment of the brake control device SC will be described with reference to Figures 6 and 7. The lower actuator YZ has a front wheel system and a rear wheel system. Of the lower actuator YZ, the portion that adjusts the front wheel cylinder CWf corresponds to the "front wheel system," and the portion that adjusts the rear wheel cylinder CWr corresponds to the "rear wheel system." That is, the front wheel system includes a front wheel pressure regulating valve UZf, a fluid pump QZf, a pressure regulating reservoir RZf, an inlet valve VIf, and an outlet valve VOf. The rear wheel system includes a rear wheel pressure regulating valve UZr, a fluid pump QZr, a pressure regulating reservoir RZr, an inlet valve VIr, and an outlet valve VOr.

[0112] One of the upper controller EA and the lower controller EZ determines whether an abnormality has occurred in the front wheel system. For example, this determination is made by the upper controller EA. The upper controller EA receives the front wheel master pressure Pmf from the lower controller EZ at every predetermined control cycle. Then, when the servo pressure Pa is generated by the operation of the electric cylinder DN, the upper controller EA compares the servo pressure Pa with the front wheel master pressure Pmf to determine whether an abnormality has occurred in the front wheel system.

[0113] If the difference between the rate of increase of the servo pressure Pa and the rate of increase of the front wheel master pressure Pmf is relatively large, there is a possibility that a leak of brake fluid BF is occurring in the front wheel system. If the servo pressure Pa is maintained under such circumstances that a leak of brake fluid BF is occurring in the front wheel system, it can be inferred that a deviation will occur between the servo pressure Pa and the front wheel master pressure Pmf.

[0114] Therefore, for example, the upper controller EA determines that an abnormality has occurred in the front wheel system when at least one of the following two conditions is met: In this case, the upper controller EA determines that no abnormality has occurred in the front wheel system, that is, that the front wheel system is normal, when neither of the following two conditions is met:

[0115] When the servo pressure Pa is increasing, the magnitude of the difference between the rate of increase of the servo pressure Pa and the rate of increase of the front wheel master pressure Pmf is equal to or greater than a predetermined value. When the servo pressure Pa is maintained, the deviation between the servo pressure Pa and the front wheel master pressure Pmf is equal to or greater than a predetermined value.

[0116] When the upper controller EA executes the specific process described above under the condition that it has determined that no abnormality has occurred in the front wheel system, it instructs the lower controller EZ to close the front wheel pressure regulating valve UZf and to keep the rear wheel pressure regulating valve UZr open. When the lower controller EZ receives such an instruction, it closes the front wheel pressure regulating valve UZf to suppress a decrease in the front wheel pressure Pwf, while keeping the rear wheel pressure regulating valve UZr open.

[0117] On the other hand, when the upper controller EA executes specific processing under the condition that it determines that an abnormality has occurred in the front wheel system, it instructs the lower controller EZ to close the rear wheel pressure regulating valve UZr and to keep the front wheel pressure regulating valve UZf open. When the lower controller EZ receives such an instruction, it closes the rear wheel pressure regulating valve UZr to suppress a decrease in the rear wheel pressure Pwr, while keeping the front wheel pressure regulating valve UZf open.

[0118] <Determination of Pressure Regulating Valve to Close When Specific Processing is Executed> A series of processes executed by the upper controller EA to determine whether the front wheel pressure regulating valve UZf or the rear wheel pressure regulating valve UZr should be closed when specific processing is executed will be described with reference to Fig. 6. This series of processes is repeatedly executed at predetermined control intervals.

[0119] In step S310, it is determined whether braking force is being applied by normal processing. If braking force is being applied by normal processing, the result of step S310 is affirmative, and the process proceeds to step S320. On the other hand, if braking force is not being applied by normal processing, the result of step S310 is negative, and the series of processes shown in FIG. 6 is temporarily terminated.

[0120] In step S320, it is determined whether or not an abnormality such as that described above has occurred in the front wheel system. If it is determined that an abnormality has occurred in the front wheel system, step S320 is answered in the affirmative, and the process proceeds to step S330. On the other hand, if it is determined that an abnormality has not occurred in the front wheel system, step S320 is answered in the negative, and the process proceeds to step S340.

[0121] In step S330, the selection flag FL1 is set to "1." The selection flag FL1 being "1" means that the rear wheel pressure regulating valve UZr is closed when the specific process is executed. Then, the series of processes shown in FIG. 6 is temporarily terminated.

[0122] In step S340, the selection flag FL1 is set to "0." The selection flag FL1 being "0" means that the front wheel pressure regulating valve UZf is closed when the specific process is executed. Then, the series of processes shown in FIG. 6 is temporarily ended.

[0123] <Operation of Identification Processing> The operation of the identification processing in the third embodiment, focusing on the differences from the operation of the identification processing in the above embodiments, will be described with reference to Fig. 7. Here, the identification processing when an abnormality occurs in the front wheel system will be described.

[0124] At time t20, operation of the brake operating member BP begins. Then, as shown in Figures 7(a) and 7(b), the servo pressure Pa is increased in accordance with the increase in the braking operation amount Ba by normal processing. Then, brake fluid BF is supplied to both the front wheel system and the rear wheel system.

[0125] In this case, if an abnormality occurs in the front wheel system but not in the rear wheel system, brake fluid BF is supplied to the rear wheel system, and brake fluid BF is supplied to the rear wheel cylinder CWr, which causes the rear wheel pressure Pwr to increase in response to the increase in servo pressure Pa, as shown in Figure 7(b).

[0126] 7, the braking operation amount Ba is maintained at time t21. Therefore, the servo pressure Pa and the rear wheel pressure Pwr are also maintained. If this state continues, the temperature-related value Xm will exceed the start threshold value xm at time t22, and the pressure reduction process related to the specific process will be started.

[0127] Here, of the front wheel pressure regulator valve UZf and the rear wheel pressure regulator valve UZr, the rear wheel pressure regulator valve UZr is closed, while the front wheel pressure regulator valve UZf remains open. At time t22, a target current Itr is calculated to ensure that the rear wheel pressure regulator valve UZr is closed. Specifically, as shown in FIG. 7C, the target current Itr is set to a value ix by adding a margin current ic to the valve-closing current Ih. A valve current Izr corresponding to the target current Itr is supplied to the rear wheel pressure regulator valve UZr, thereby closing the rear wheel pressure regulator valve UZf. The valve-closing current Ih is determined based on a specific differential pressure sPx (the difference between the rear wheel pressure Pwr and the lower limit pressure px), and is set to a larger value as the specific differential pressure sPx increases. Therefore, when the differential pressure sPx between the rear wheel pressure Pwr and the lower limit pressure px is small, the valve current Izr for closing the rear wheel pressure regulating valve UZr is set smaller than when the specific differential pressure sPx is large. On the other hand, since the target current Itf for the front wheel pressure regulating valve UZf remains "0", the front wheel pressure regulating valve UZf is maintained in an open state.

[0128] Due to the relationship between the characteristics of the front wheel braking device SXf and the rear wheel braking device SXr, the contribution of the rear wheel pressure Pwr to the generation of braking force is smaller than the contribution of the front wheel pressure Pwf.

[0129] Therefore, in the specific processing executed in the third embodiment, as shown in Figure 7(b), the servo pressure Pa is increased at time t22. Then, the rear wheel pressure Pwr is also increased following the increase in the servo pressure Pa. As a result, even if the servo pressure Pa is subsequently reduced, the vehicle can be kept stopped by the braking force generated at the rear wheels.

[0130] At time t23 thereafter, as in the first embodiment, a pressure reducing process according to the specific process is executed, thereby reducing the servo pressure Pa to the lower limit pressure px. However, the lower limit pressure px when the front wheel pressure regulating valve UZf is not closed is preferably higher than the lower limit pressure px when the front wheel pressure regulating valve UZf is closed. Of course, the lower limit pressure px when the front wheel pressure regulating valve UZf is closed may be the same as the lower limit pressure px when the front wheel pressure regulating valve UZf is closed.

[0131] After time t23, even though the rear wheel pressure regulator valve UZr is closed, the rear wheel pressure Pwr decreases slightly due to leakage from the rear wheel pressure regulator valve UZr. Therefore, as in the first embodiment, the pressure increase process associated with the specific process is initiated at time t24. This increases the servo pressure Pa. In the pressure increase process of the third embodiment, the servo pressure Pa is increased to the servo pressure Pa at time t23. Brake fluid BF is then supplied to the rear wheel cylinder CWr, thereby restoring the rear wheel pressure Pwr. Therefore, at time t25, the pressure decrease process associated with the specific process is executed again. In other words, while the braking operation amount Ba is substantially constant, the pressure decrease process and the pressure increase process are periodically repeated.

[0132] Therefore, in the third embodiment, even if only the rear wheel pressure regulating valve UZr is closed out of the front wheel pressure regulating valve UZf and the rear wheel pressure regulating valve UZr, the same effects as those of the above embodiments can be obtained.

[0133] When the specific process is executed under the condition that no abnormality is determined to exist in the front wheel system, the front wheel pressure regulator valve UZf is closed and the rear wheel pressure regulator valve UZr is opened. The process flow is substantially the same as that in the first embodiment, and therefore a detailed description thereof will be omitted.

[0134] Other embodiments will be described. Other embodiments also achieve the same effects as those described above. In the above-described embodiment, a disc-type braking device is used as the braking device SX. Alternatively, a drum-type braking device may be used as the braking device SX. In a drum-type braking device SX, the rotating member KT fixed to the wheel WH is a brake drum, and the friction member is a brake lining attached to a brake shoe. In the drum-type braking device SX, as in the disc-type braking device SX, the wheel pressure Pw of the wheel cylinder CW presses the brake lining (friction member) against the brake drum (rotating member), generating a frictional braking force Fe.

[0135] The hydraulic pressure transmission in the brake control device SC involves various resistances, such as pipe friction resistance in the fluid path (e.g., HS), resistance due to the orifice of the solenoid valve (e.g., VI), and sliding resistance of the seal member SL. In hydraulic pressure feedback control, the actual value is controlled to match the target value. However, considering these resistances, the actual value and the target value must be compared at the same location. In the above-described embodiment, the target pressure Pt is determined as a target value corresponding to the servo pressure Pa (actual value). That is, the location where the target value and the actual value are compared (also referred to as the "comparison location") is the location where the servo pressure sensor PA is installed. Alternatively, the comparison location may be any location along the hydraulic pressure transmission path from the discharge port of the electric cylinder DN to the wheel cylinder CW. For example, the wheel cylinder CW may be used as the comparison location, and the target pressure Pt is determined to correspond to the wheel pressure Pw (actual value). The wheel pressure Pw is estimated from the servo pressure Pa after compensating for the hydraulic pressure component due to the resistances. In a configuration in which the lower part of the pressure regulating valve UZ is used as the comparison part, when the pressure regulating valve UZ is closed in the specific process, the wheel pressure Pw is determined assuming that the pressure regulating valve UZ is open. In pressure regulation control, regardless of the location of the comparison part between the target value and the actual value, the target pressure Pt can be said to be the target value for controlling the servo pressure Pa.

[0136] In the above-described embodiment, pressure regulation control including specific processing is performed by two controllers EA and EZ. Alternatively, the upper and lower controllers EA and EZ may be integrated into a single integrated controller EC. Here, the controllers EA, EZ, and EC are referred to as "controller EE." In either case, in the braking control device SC, the electric motor MA and the pressure regulating valve UZ are controlled by the controller EE (i.e., a collective term for the controllers EA, EZ, and EC).

[0137] Summary of the embodiment The brake control device SC is provided with a control cylinder CC, a pressure regulating valve UZ, and a controller EE (a collective term for controllers EA, EZ, and EC), and adjusts the wheel pressure Pw of the wheel cylinder CW by a servo pressure Pa. The control cylinder CC generates the servo pressure Pa by the movement of a control piston NC driven by an electric motor MA. The pressure regulating valve UZ (solenoid valve) is of a normally open type and is provided in a hydraulic pressure transmission path (HS, etc.) from the control cylinder CC to the wheel cylinder CW. The controller EE controls the electric motor MA and the pressure regulating valve UZ.

[0138] In the brake control device SC, the controller EE executes a specific process to close the pressure regulating valve UZ to reduce the servo pressure Pa when a temperature-related value Xm, which is related to the temperature of components associated with the electric motor MA, exceeds a threshold value xm. In the specific process, the servo pressure Pa is reduced to a lower limit pressure px. Here, the lower limit pressure px is preset based on the continuous rating of the components of the electric motor MA. In addition, in the specific process, the valve current Iz supplied to close the pressure regulating valve UZ is adjusted based on a differential pressure sPx (specific differential pressure) between the wheel pressure Pw and the lower limit pressure px. Specifically, when the specific differential pressure sPx is small, the valve current Iz is reduced compared to when the specific differential pressure sPx is large. By appropriately setting the lower limit pressure px, the valve current Iz to the pressure regulating valve UZ is kept to a necessary minimum. As a result, power consumption by the brake control device SC is reduced.

[0139] The brake control device SC is provided with pressure regulator valves UZ, namely, a front wheel pressure regulator valve UZf for the front wheel cylinder CWf and a rear wheel pressure regulator valve UZr for the rear wheel cylinder CWr. When a specific process is executed (i.e., when the servo pressure Pa is to be reduced), the front wheel pressure regulator valve UZf is closed and the rear wheel pressure regulator valve UZr is left open. Since the power consumption of the rear wheel pressure regulator valve UZr is zero, the brake control device SC achieves energy savings. Furthermore, because the front wheel pressure Pwf can generate a greater braking force than the rear wheel pressure Pwr, the vehicle can be reliably maintained stationary even when the specific process is executed using only the front wheel cylinder CWf.

Claims

1. A vehicle braking control device comprising: an electric cylinder that generates servo pressure by moving a control piston driven by an electric motor; a normally open solenoid valve provided in a hydraulic pressure transmission path from the electric cylinder to a wheel cylinder; and a controller that controls the electric motor and the solenoid valve, and that adjusts the wheel pressure of the wheel cylinder by the servo pressure, wherein when a temperature-related value related to the temperature of a component related to the electric motor exceeds a threshold value, the controller closes the solenoid valve to reduce the servo pressure to a lower limit pressure set based on the maximum rating that can be continuously applied to the component, and when the pressure difference between the wheel pressure and the lower limit pressure is small, the controller reduces the valve current supplied to close the solenoid valve compared to when the pressure difference is large.

2. A vehicle braking control device as described in claim 1, wherein the solenoid valves include a front wheel solenoid valve provided for a front wheel cylinder among the wheel cylinders, and a rear wheel solenoid valve provided for a rear wheel cylinder among the wheel cylinders, and when the servo pressure is to be reduced, the controller closes the front wheel solenoid valve and keeps the rear wheel solenoid valve open.

3. A vehicle braking control device as described in claim 1, wherein the solenoid valves include a front wheel solenoid valve provided for a front wheel cylinder among the wheel cylinders, and a rear wheel solenoid valve provided for a rear wheel cylinder among the wheel cylinders, and when the controller reduces the servo pressure and holds it at the lower limit pressure, it closes the rear wheel solenoid valve and keeps the front wheel solenoid valve open.

4. A vehicle brake control device as described in claim 2, wherein, of the hydraulic pressure transmission paths, a system that adjusts the wheel pressure of the front wheel cylinder is a front system, and a system that adjusts the wheel pressure of the rear wheel cylinder is a rear system, the front wheel system has the front wheel solenoid valve, and the rear wheel system has the rear wheel solenoid valve, and the controller determines whether or not an abnormality has occurred in the front wheel system, and if the temperature-related value exceeds the threshold value when it is determined that no abnormality has occurred in the front wheel system, closes the front wheel solenoid valve and keeps the rear wheel solenoid valve open, and if the temperature-related value exceeds the threshold value when it is determined that an abnormality has occurred in the front wheel system, closes the rear wheel solenoid valve and keeps the front wheel solenoid valve open.

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