Vehicle brake control device

Abstract

To provide a braking control device having two pressurizing sources which are separately used, which can improve a performance of safety control.SOLUTION: A braking control device executes safety control by increasing individually a braking fluid pressure of a plurality of wheel cylinders. The braking control device is provided with: a first unit, constituted of a piston that is moved using a first electric motor as a power source and a cylinder into which the piston is inserted, which increases the braking fluid pressure by discharging brake fluid from the cylinder by moving the piston; and a second unit, constituted of pressure regulation valves provided between the first unit and the wheel cylinders and a fluid pump that is driven by a second electric motor, which increases the braking fluid pressure by making the pressure regulation valves squeeze a flow of brake fluid discharged by the fluid pump. A reference fluid pressure for the safety control is supplied by the second unit, when a turning state amount representing a degree of movement in a lateral direction of a vehicle is less than a predetermined amount, and the reference fluid pressure is supplied by the first unit when the turning state amount is above or equal to the predetermined amount.SELECTED DRAWING: Figure 5

Description

[Technical field]

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

[0002] The applicant has developed a vehicle brake control device that is composed of a plurality of control devices as described in Patent Document 1, which has a simple configuration and ensures reliability. Specifically, the brake control device includes an "operation displacement sensor that detects the operation displacement of a brake operating member, a "first hydraulic unit that reads the operation displacement as a first displacement processing value via a first displacement signal line and adjusts the brake hydraulic pressure based on this," a "second hydraulic unit that reads the operation displacement as a second displacement processing value via a second displacement signal line and adjusts the brake hydraulic pressure based on this," and a communication bus that transmits signals between the first hydraulic unit and the second hydraulic unit. Here, the first hydraulic unit obtains the second displacement processing value from the second hydraulic unit via the communication bus and executes a judgment of whether the first displacement processing value is appropriate. Then, the first hydraulic unit adjusts the brake hydraulic pressure based on the first displacement processing value when the first displacement processing value is appropriate, and adjusts the brake hydraulic pressure based on the second displacement processing value when the first displacement processing value is inappropriate.

[0003] Patent Document 1 describes that the first hydraulic unit generates hydraulic pressure in wheel cylinders provided on the four wheels of a vehicle in place of a master cylinder, and the second hydraulic unit is for executing vehicle stability control (also simply called "stability control"). The brake control device is provided with two hydraulic pressure generation sources, and in executing stability control, it is desirable to effectively combine the two hydraulic pressure generation sources (also called "pressurization sources") to improve the performance. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent Publication No. 2018-016162 Summary of the Invention [Problem to be solved by the invention]

[0005] An object of the present invention is to provide a brake control device for a vehicle having two pressure sources, which can be used selectively to improve the performance of vehicle stability control. [Means for solving the problem]

[0006] The brake control device according to the present invention performs stability control to improve the stability of a vehicle (JV) by individually increasing the brake fluid pressure (Pw) in wheel cylinders (CW) provided on a plurality of wheels (WH) of the vehicle (JV), and is a "first unit (YA) that is composed of pistons (NP, NS) that are moved using a first electric motor (MA) as a power source, and cylinders (CM) into which the pistons (NP, NS) are inserted, and that increases the brake fluid pressure (Pw) by discharging brake fluid (BF) from the cylinders (CM) due to the movement of the pistons (NP, NS)." and "a second unit (YB) which is composed of a pressure regulating valve (UB) provided in a communication path (HS) connecting the first unit (YA) and the wheel cylinder (CW) and a fluid pump (QB) driven by a second electric motor (MB) separate from the first electric motor (MA), and which increases the brake fluid pressure (Pw) by throttling the flow of brake fluid (BF) discharged by the fluid pump (QB) using the pressure regulating valve (UB)," and a controller (ECU) which controls the first and second units (YA, YB) to perform the stability control.

[0007] In the braking control device of the present invention, when a turning state quantity (Gs) indicating the degree of lateral movement of the vehicle (JV) is less than a predetermined amount (gs), the controller (ECU) supplies a reference hydraulic pressure (Pd, Pe) corresponding to the maximum hydraulic pressure (Ptx) required for the stability control via the second unit (YB), and when the turning state quantity (Gs) is equal to or greater than the predetermined amount (gs), the controller (ECU) supplies the reference hydraulic pressure (Pd, Pe) via the first unit (YA).

[0008] In the braking control device of the present invention, the controller (ECU) supplies a reference hydraulic pressure (Pd, Pe) corresponding to the maximum hydraulic pressure (Ptx) required for the stability control through the second unit (YB) until a turning state quantity (Gs) indicating the degree of lateral movement of the vehicle (JV) reaches a predetermined amount (gs), and after the turning state quantity (Gs) reaches the predetermined amount (gs), supplies the reference hydraulic pressure (Pd, Pe) through the first unit (YA).

[0009] The brake control device SC is provided with two pressure sources, a first unit YA and a second unit YB. The first unit YA has excellent pressure boost responsiveness, and the second unit YB has excellent pressure regulation performance. According to the above configuration, in normal stability control, the reference hydraulic pressure is supplied by the second unit YB. Then, when it is determined that there is a high probability that a sudden vehicle behavior may occur based on the turning state quantity Gs (i.e., when the turning state quantity Gs is equal to or greater than a predetermined quantity gs), the reference hydraulic pressure is switched to the first unit YA. This switching improves the pressure boost responsiveness, so that vehicle stability can be reliably ensured even when a sudden change in behavior occurs. By making use of the characteristics of the first and second units YA and YB, the two pressure sources YA and YB are used appropriately, thereby improving the performance of the stability control. [Brief description of the drawings]

[0010] [Figure 1] 1 is a configuration diagram for explaining an entire vehicle JV equipped with a braking control device SC. [Diagram 2] 11 is a schematic diagram for explaining a configuration example of a first unit YA. FIG. [Diagram 3] FIG. 13 is a schematic diagram (hydraulic circuit diagram) for explaining a configuration example of a second unit YB. [Figure 4] FIG. 4 is a flow chart for explaining a method of adjusting the brake fluid pressure Pw in the stability control. [Diagram 5] FIG. 11 is a flow chart for explaining a process for adjusting the reference fluid pressure. [Figure 6] FIG. 6 is a time series diagram for explaining the adjustment operation of the reference fluid pressure.

[0011] Hereinafter, an embodiment of a vehicle braking control device SC according to the present invention will be described with reference to the drawings.

[0012] <Symbols of components> In the following explanation, components, signals, values, etc. with the same symbol, such as "CW", have the same function. The subscripts "f" and "r" at the end of various symbols related to wheels are generic symbols that indicate whether the element is related to the front or rear wheels. Specifically, "f" indicates "element related to the front wheels" and "r" indicates "element related to the rear wheels". For example, a wheel cylinder CW is written as "front wheel cylinder CWf, rear wheel cylinder CWr". Furthermore, the subscripts "f" and "r" may be omitted. When these are omitted, each symbol represents its generic name.

[0013] Furthermore, in the communication path HS, the side farther from the wheel cylinder CW is referred to as the "upper" and the side closer to the wheel cylinder CW is referred to as the "lower". For example, the movement of the brake fluid BF by the fluid pump QB is described as "the brake fluid BF is sucked in from the upper part of the pressure regulating valve UB and discharged to the lower part of the pressure regulating valve UB". The first and second units YA and YB are also referred to as the "upper unit YA and lower unit YB". Furthermore, the adjustment (increase, etc.) of the brake fluid pressure Pw by the first unit YA is referred to as the "upper pressure adjustment", and the adjustment (increase, etc.) of the brake fluid pressure Pw by the second unit YB is referred to as the "lower pressure adjustment".

[0014] <Vehicle equipped with brake control device SC JV> The entire vehicle JV equipped with the brake control device SC according to the present invention will be described with reference to the configuration diagram in Fig. 1. The vehicle JV is equipped with a brake operating member BP, a steering operating member SH, and various sensors (BA, etc.). The brake operating member (e.g., brake pedal) BP is a member that the driver operates to decelerate the vehicle JV. The steering operating member (e.g., steering wheel) SH is a member that the driver operates to turn the vehicle JV.

[0015] The vehicle JV is provided with various sensors as listed below. Detection signals (Ba, etc.) of these sensors are input to a controller ECU, which will be described later. A braking operation amount sensor BA that detects an operation amount (braking operation amount) Ba of the braking operation member BP, and a steering operation amount sensor SA that detects an operation amount (steering operation amount, for example, a steering angle) Sa of the steering operation member SH. Wheel speed sensor VW detects the rotational speed (wheel speed) Vw of the wheels WH. In the vehicle JV (particularly the vehicle body), there are a yaw rate sensor YR that detects a yaw rate Yr, a longitudinal acceleration sensor GX that detects longitudinal acceleration Gx, and a lateral acceleration sensor GY that detects lateral acceleration Gy.

[0016] The vehicle JV is equipped with a braking device SX and a braking control device SC. The braking control device SC employs a so-called front and rear type (also called "type II") as two braking systems.

[0017] A brake fluid pressure Pw generated by the brake control device SC is supplied to the brake device SX. Then, the brake device SX generates a braking force Fb on the wheel WH according to the brake fluid pressure Pw. The brake device SX includes a rotating member (e.g., a brake disc) KT and a brake caliper CP. The rotating member KT is fixed to the wheel WH of the vehicle, and the brake caliper CP is provided so as to sandwich the rotating member KT. The brake caliper CP is provided with a wheel cylinder CW. The brake control device SC supplies brake fluid BF adjusted to the brake fluid pressure Pw to the wheel cylinder CW. The brake fluid pressure Pw presses a friction member (e.g., a brake pad) MS against the rotating member KT. Since the rotating member KT and the wheel WH are fixed so as to rotate integrally, a frictional force generated at this time generates a braking torque Tb (as a result, a braking force Fb) on the wheel WH.

[0018] The brake control device SC adjusts the actual brake fluid pressure Pw according to the operation amount Ba of the brake operating member BP, and supplies the brake fluid pressure Pw to the brake device SX (particularly, the wheel cylinder CW) via the front and rear wheel connection paths HSf and HSR. That is, the brake control device SC generates the brake fluid pressure Pw according to the brake operation amount Ba, and decelerates the vehicle JV. Furthermore, the brake control device SC performs vehicle stability control (also simply referred to as "stability control") that improves the stability of the vehicle JV by independently and individually adjusting the brake fluid pressure Pw of each wheel cylinder CW.

[0019] The brake control device SC is composed of a master cylinder CM, a fluid unit HU, and a controller ECU. The fluid unit HU is composed of two units (first and second units) YA and YB. The components of the brake control device SC (such as the solenoid valves and electric motor included in the first and second units YA and YB) are controlled by the controller ECU. The controller ECU is composed of a microprocessor MP that performs signal processing, and a drive circuit DD that drives the solenoid valves and the electric motor.

[0020] <First Unit YA> A configuration example of a first unit (upper unit) YA included in the hydraulic unit HU will be described with reference to the schematic diagram of FIG. 2. The first unit YA is a pressurizing source for increasing the hydraulic pressure (braking hydraulic pressure) Pw of the four wheel cylinders CW. In this example, the first unit YA is integrated with a master cylinder CM. A front-rear type braking system is adopted. The first unit YA is composed of an apply unit AU including a master cylinder CM, and a pressurizing unit KU. The apply unit AU and the pressurizing unit KU are controlled by a controller ECU. In detail, the controller ECU receives the braking operation amount Ba (at least one of the simulator hydraulic pressure Ps, the operation displacement Sp, and the operation force Fp), the wheel speed Vw, the accumulator hydraulic pressure Pc, the servo hydraulic pressure Pu, and the supply hydraulic pressure Pm, and calculates the drive signal Vn of the input valve VN, the drive signal Vr of the release valve VR, the drive signal Uz of the booster valve UZ, the drive signal Ug of the pressure reducing valve UG, and the drive signal Ma of the pressure accumulating electric motor MA based on these signals. Then, in response to the drive signals "Vn, Vr, Uz, Ug, Ma", the solenoid valves "VN, VR, UZ, UG" constituting the first unit YA and the electric motor MA for accumulating pressure are controlled (driven).

[0021] As described later, the fluid unit HU, wheel cylinders CW, etc. are connected by a communication path HS, an input path HN, a pressure reduction path HG, and a return path HK. These are fluid paths through which the brake fluid BF moves. Fluid paths (HS, etc.) include fluid piping, flow paths in the fluid unit HU, hoses, etc.

[0022] <<Apply Unit AU>> The apply unit AU is composed of a master reservoir RV, a master cylinder CM, first and second master pistons NP, NS, first and second master springs DP, DS, an input cylinder CN, an input piston NN, an input spring DN, an input valve VN, a release valve VR, a stroke simulator SS, and a simulator hydraulic pressure sensor PS.

[0023] The master reservoir (also called the "atmospheric pressure reservoir") RV is a tank for hydraulic fluid, and brake fluid BF is stored inside it. The master reservoir RV is connected to the master cylinder CM (particularly the front and rear master chambers Rmf and Rmr).

[0024] The master cylinder CM is a cylinder member having a bottom. The first and second master pistons NP and NS are inserted into the master cylinder CM, and the inside is sealed by a seal member SL to separate the master chambers Rmf and Rmr. The master cylinder CM is a so-called tandem type. The first and second master springs DP and DS are provided in the master chambers Rmf and Rmr. The first and second master springs DP and DS press the first and second master pistons NP and NS in the backward direction Hb (the direction in which the volume of the master chamber Rm increases, the opposite direction to the forward direction Ha). The front and rear master chambers Rmf and Rmr (=Rm) are ultimately connected to the front and rear wheel cylinders CWf and CWr (=CW), respectively, via the front and rear wheel communication passages HSf and HSR (=HS) and the second unit YB. When the first and second master pistons NP, NS are moved in the forward direction Ha (the direction in which the volume of the master chamber Rm decreases), brake fluid BF at hydraulic pressure Pm (called "supplied hydraulic pressure" and "front and rear wheel supplied hydraulic pressures Pmf, Pmr") is supplied from the first unit YA (particularly the master cylinder CM) to the second unit YB. Here, the front wheel supplied hydraulic pressure Pmf and the rear wheel supplied hydraulic pressure Pmr are equal.

[0025] The first master piston NP is provided with a flange. The flange further divides the inside of the master cylinder CM into a servo chamber Ru and a rear chamber Ro. The servo chamber Ru is disposed opposite the front wheel master chamber Rmf across the first master piston NP. The rear chamber Ro is sandwiched between the front wheel master chamber Rmf and the servo chamber Ru and disposed therebetween. The servo chamber Ru and the rear chamber Ro are also sealed by the seal member SL in the same manner as above.

[0026] The input cylinder CN is fixed to the master cylinder CM. An input piston NN is inserted inside the input cylinder CN and sealed by a seal member SL to form an input chamber Rn. The input piston NN is mechanically connected to the brake operating member BP via a clevis (U-shaped link). A flange is provided on the input piston NN. An input spring DN is provided between the flange and the mounting surface of the input cylinder CN relative to the master cylinder CM. The input spring DN presses the input piston NN in the backward direction Hb.

[0027] The apply unit AU is provided with hydraulic chambers, namely, an input chamber Rn, a servo chamber Ru, a rear chamber Ro, and front and rear master chambers Rmf and Rmr. Here, the "hydraulic chambers" are chambers filled with brake fluid BF and sealed by a seal member SL. The volume of each hydraulic chamber is changed by the movement of the input piston NN and the first and second master pistons NP and NS. The hydraulic chambers are arranged along the central axis Jm of the master cylinder CM in the order of the input chamber Rn, servo chamber Ru, rear chamber Ro, front master chamber Rmf, and rear master chamber Rmr, from the side closest to the brake operating member BP.

[0028] The input chamber Rn and the rear chamber Ro are connected via an input path HN. An input valve VN is provided in the input path HN. The input path HN is connected to the master reservoir RV between the rear chamber Ro and the input valve VN via a release valve VR. The input valve VN and the release valve VR are two-position solenoid valves (also called "on-off valves") having an open position (communication state) and a closed position (shut-off state). A normally closed solenoid valve is used as the input valve VN. A normally open solenoid valve is used as the release valve VR. The input valve VN and the release valve VR are driven (controlled) by drive signals Vn and Vr from the brake controller ECU.

[0029] A stroke simulator (also simply referred to as "simulator") SS is connected to the rear chamber Ro. An operating force Fp of the brake operating member BP is generated by the simulator SS. A piston and an elastic body (e.g., a compression spring) are provided inside the simulator SS. When brake fluid BF flows into the simulator SS, the piston is pushed by the brake fluid BF. A force is applied to the piston by the elastic body in a direction that prevents the inflow of the brake fluid BF, and thus an operating force Fp of the brake operating member BP is generated. In other words, the operating characteristic of the brake operating member BP (the relationship between the operating displacement Sp and the operating force Fp) is formed by the simulator SS.

[0030] A simulator hydraulic pressure sensor PS is provided to detect the hydraulic pressure Ps of the simulator SS (which is the simulator hydraulic pressure and is also the hydraulic pressure in the input chamber Rn and the rear chamber Ro). The simulator hydraulic pressure sensor PS is one of the above-mentioned braking operation amount sensors BA. The simulator hydraulic pressure Ps is input to the braking controller ECU as the braking operation amount Ba.

[0031] In addition to the simulator hydraulic pressure sensor PS, the first unit YA is provided with a braking operation amount sensor BA, which includes an operation displacement sensor SP that detects an operation displacement Sp of the brake operating member BP and / or an operation force sensor FP that detects an operation force Fp of the brake operating member BP. That is, as the braking operation amount sensor BA, at least one of the simulator hydraulic pressure sensor PS, the operation displacement sensor SP (stroke sensor), and the operation force sensor FP is adopted. Therefore, the braking operation amount Ba is at least one of the simulator hydraulic pressure Ps, the operation displacement Sp, and the operation force Fp.

[0032] <Pressure unit KU> The pressurizing unit KU generates and adjusts the supply hydraulic pressure Pm. The pressurizing unit KU is composed of an accumulator fluid pump QA, an accumulator electric motor MA (corresponding to the "first electric motor"), an accumulator AC, an accumulator hydraulic pressure sensor PC, a pressurizing cylinder CK, a pressurizing piston NK, a pressure booster valve UZ, a pressure reducing valve UG, and a servo hydraulic pressure sensor PU.

[0033] In the pressurizing unit KU, a fluid pump QA for pressure accumulation is provided so as to accumulate the accumulator AC. The fluid pump QA for pressure accumulation is driven by an electric motor MA (first electric motor) for pressure accumulation, and sucks the brake fluid BF from the master reservoir RV. Then, the brake fluid BF discharged from the fluid pump QA is stored in the accumulator AC. The accumulator AC stores the brake fluid BF pressurized up to the accumulator hydraulic pressure Pc. An accumulator hydraulic pressure sensor PC is provided so as to detect the accumulator hydraulic pressure Pc.

[0034] The electric motor MA (first electric motor) for pressure accumulation is controlled by the brake controller ECU so that the accumulator hydraulic pressure Pc is maintained within a predetermined range. Specifically, when the accumulator hydraulic pressure Pc is less than the lower limit value pl, the electric motor MA is driven at a predetermined rotational speed. Also, when the accumulator hydraulic pressure Pc is greater than or equal to the upper limit value pu, the electric motor MA is stopped. Here, the lower limit value pl and the upper limit value pu are predetermined values (constants) set in advance, and are in the relationship of "pl < pu". By controlling the electric motor MA, the accumulator hydraulic pressure Pc is maintained within the range from the lower limit value pl to the upper limit value pu.

[0035] In the pressurizing unit KU, a pressure cylinder CK is provided so as to adjust the accumulator hydraulic pressure Pc from the accumulator AC and supply it to the servo chamber Ru. A pressure piston NK is inserted into the pressure cylinder CK. By the pressure piston NK, the inside of the pressure cylinder CK is partitioned into three hydraulic pressure chambers Rp (pilot chamber), Rv (annular chamber), and Rk (pressurizing chamber) sealed by a seal member SL. The pilot chamber Rp and the pressurizing chamber Rk are arranged so as to sandwich the pressure piston NK. That is, the pilot chamber Rp is located on the side opposite to the pressurizing chamber Rk with respect to the pressure piston NK in the pressure cylinder CK. The pilot chamber Rp is supplied with a pilot hydraulic pressure Pp adjusted by a pressure increasing valve UZ and a pressure reducing valve UG described later.

[0036] An annular recess (constricted portion) is provided on the outer periphery of the pressurizing piston NK. The annular recess and the inner periphery of the pressurizing cylinder CK form an annular chamber Rv. Furthermore, a valve body Vv (e.g., a spool valve) is formed on the outer periphery of the pressurizing piston NK. Brake fluid BF pressurized to accumulator hydraulic pressure Pc is supplied from the accumulator AC to this valve body Vv. The accumulator hydraulic pressure Pc is adjusted by the valve body Vv and introduced into the annular chamber Rv. The annular chamber Rv is communicated with the pressurizing chamber Rk via a through hole provided in the pressurizing piston NK. Therefore, the hydraulic pressure in the annular chamber Rv and the hydraulic pressure in the pressurizing chamber Rk are the same. This hydraulic pressure is called "servo hydraulic pressure Pu."

[0037] Specifically, when the pressurizing piston NK is moved by the hydraulic pressure (pilot hydraulic pressure) Pp in the pilot chamber Rp, the opening amount of the valve body Vv changes. Then, brake fluid BF is supplied from the accumulator AC through the valve body Vv of the pressurizing piston NK so that the pilot hydraulic pressure Pp (hydraulic pressure in the pilot chamber Rp) and the servo hydraulic pressure Pu (hydraulic pressure in the annular chamber Rv and the pressurizing chamber Rk) match. In other words, the high accumulator hydraulic pressure Pc is throttled by the valve body Vv and adjusted to the servo hydraulic pressure Pu. A servo hydraulic pressure sensor PU is provided to detect the actual servo hydraulic pressure Pu. The detected servo hydraulic pressure Pu is input to the brake controller ECU. The controller ECU adjusts the pilot hydraulic pressure Pp based on the servo hydraulic pressure Pu, and ultimately controls the servo hydraulic pressure Pu to match a target value. The pressurizing chamber Rk and the servo chamber Ru are connected by a fluid path, so that the brake fluid BF adjusted to the servo hydraulic pressure Pu is supplied from the pressurizing unit KU to the servo chamber Ru.

[0038] <<Operation of the first unit YA>> When braking is not being applied (i.e., when the brake operating member BP is not being operated), the pistons "NN, NP, NS" are pressed by the springs "DN, DP, DS" and returned to their initial positions (the positions where they are moved furthest in the backward direction Hb). In this state, the front and rear wheel master chambers Rmf, Rmr are in communication with the master reservoir RV, and the front and rear wheel supply hydraulic pressures Pmf, Pmr are "0 (atmospheric pressure)". In addition, at the initial positions of the pistons, there is a gap between the input piston NN and the first master piston NP. Similarly, when braking is not being applied, the boost valve UZ is closed and the pressure reducing valve UG is open, so that the pilot chamber Rp is in communication with the master reservoir RV, and the pilot hydraulic pressure Pp is "0 (atmospheric pressure)". The pressurizing piston NK is pressed against the bottom of the pressurizing cylinder CK by the compression spring DK, and the valve body Vv (spool valve) is closed. Since the pressurized chamber Rk and the master reservoir RV are in communication with each other, the servo hydraulic pressure Pu is also "0." Furthermore, when braking is not being applied, the input valve VN and the release valve VR are open, and the rear chamber Ro and the input chamber Rn are in communication with the master reservoir RV, so the internal pressures Po and Pn are also "0." In other words, when braking is not being applied, the state is "Pmf=Pmr=Pp=Pu=Po=Pn=0."

[0039] During braking (i.e., when the brake operating member BP is operated), the input valve VN is opened and the release valve VR is closed. That is, the input chamber Rn and the rear chamber Ro are in communication with each other, and the communication between the rear chamber Ro and the master reservoir RV is interrupted and the two chambers are in a non-communicating state. As the operation amount Ba of the brake operating member BP increases, the input piston NN is moved in the forward direction Ha, and brake fluid BF is discharged from the input chamber Rn. This brake fluid BF is absorbed by the stroke simulator SS, so that the hydraulic pressure Pn (input hydraulic pressure) in the input chamber Rn and the hydraulic pressure Po (rear hydraulic pressure) in the rear chamber Ro are increased, and an operating force Fp is generated in the brake operating member BP. At this time, the pressure increasing valve UZ and the pressure reducing valve UG are controlled according to the braking operation amount Ba (at least one of the simulator hydraulic pressure Ps, the operation displacement Sp, and the operating force Fp), and the hydraulic pressure Pp (pilot hydraulic pressure) in the pilot chamber Rp is increased. In response to an increase in the pilot hydraulic pressure Pp, the valve body Vv opens, and the hydraulic pressure Pu (servo hydraulic pressure) in the annular chamber Rv and the pressurizing chamber Rk increases. This servo hydraulic pressure Pu is supplied to the servo chamber Ru, so that the first master piston NP is pressed in the forward direction Ha and moves in the forward direction Ha. As the first master piston NP moves in the forward direction Ha, the hydraulic pressures Pmf and Pmr (=Pm) supplied to the front and rear wheels increase. Then, the brake fluid BF adjusted to the supply hydraulic pressure Pm by the first unit YA is supplied to the second unit YB, and finally the brake hydraulic pressure Pw in the wheel cylinder CW is increased.

[0040] Since the braking control device SC is a so-called brake-by-wire type, when the vehicle is an electric vehicle (for example, an electric vehicle or a hybrid vehicle), regenerative cooperative control is executed. Since there is a gap between the input piston NN and the first master piston NP, the relative positional relationship between the input piston NN and the first and second master pistons NP, NS can be arbitrarily adjusted within the range of this gap by controlling the servo hydraulic pressure Pu. For example, when only braking force by regenerative braking is required, "Pu=0" is set, and the supply hydraulic pressure Pm from the front and rear wheel master chambers Rmf, Rmr is kept at "0". No braking force is generated by friction between the rotating member KT and the friction member, and the braking force Fb is generated only by the regenerative braking force of the driving electric motor functioning as a generator.

[0041] <Second Unit YB> With reference to the schematic diagram of FIG. 3, a configuration example of the second unit (lower unit) YB included in the fluid unit HU will be described. The second unit YB is provided between the first unit YA and the wheel cylinder CW in the communication path HS (a fluid path for moving the brake fluid BF). The brake control device SC can adjust (increase, maintain, decrease) the supply fluid pressure Pm by the second unit YB. The second unit YB is composed of a supply fluid pressure sensor PM, a pressure regulating valve UB, a return fluid pump QB, a return electric motor MB (corresponding to a "second electric motor"), a pressure regulating reservoir RC, an inlet valve UI, and an outlet valve VO.

[0042] Like the first unit YA, the second unit YB is also controlled by the brake controller ECU. In detail, the controller ECU calculates a drive signal Ub for the pressure regulating valve UB, a drive signal Ui for the inlet valve UI, a drive signal Vo for the outlet valve VO, and a drive signal Mb for the recirculation electric motor MB based on the various signals (Ba, etc.) described above. Then, the solenoid valves "UB, UI, VO" constituting the second unit YB and the recirculation electric motor MB are controlled (driven) in response to these drive signals (Ub, etc.).

[0043] Front and rear wheel pressure regulating valves UBf, UBr (=UB) are provided in the front and rear wheel communication passages HSf, HSR (=HS). The pressure regulating valve UB (solenoid valve) is a normally open linear valve (also called a "differential pressure valve" or "proportional valve"). The upper part of the pressure regulating valve UB (the part of the communication passage HS closer to the first unit YA) and the lower part of the pressure regulating valve UB (the part of the communication passage HS closer to the wheel cylinder CW) are connected by the front and rear wheel return passages HKf, HKr (=HK). The return passage HK is provided with front and rear wheel return fluid pumps QBf, QBr (=QB) and front and rear wheel pressure regulating reservoirs RCf, RCr (=RC). The return fluid pump QB is driven by a return electric motor MB (second electric motor). A supply fluid pressure sensor PM is provided above the pressure regulating valve UB to detect the actual fluid pressure (supply fluid pressure) Pm supplied by the first unit YA.

[0044] When the electric motor MB (second electric motor) is driven to rotate, the fluid pump QB sucks in the brake fluid BF from the upper part of the pressure regulating valve UB and discharges the brake fluid BF to the lower part of the pressure regulating valve UB. As a result, a return flow KN of the brake fluid BF (i.e., front and rear wheel return flows KNf and KNr, which are a circulating flow of brake fluid BF) is generated in the communication path HS and the return path HK, including the pressure regulating reservoir RC. When the return flow KN of the brake fluid BF is throttled by the pressure regulating valve UB, the hydraulic pressure Pq (referred to as the "regulated hydraulic pressure") at the lower part of the pressure regulating valve UB is increased from the hydraulic pressure Pm (supply hydraulic pressure) at the upper part of the pressure regulating valve UB due to the orifice effect. In other words, the second unit YB can increase the front and rear wheel brake hydraulic pressures Pwf and Pwr (=Pw) ​​from the supply hydraulic pressure Pm. In the second unit YB, the reflux electric motor MB, the reflux fluid pump QB, and the pressure adjusting valve UB are referred to as the "pressurizing source KB."

[0045] Inside the second unit YB, the front and rear wheel communication passages HSf and HSR are each branched into two and connected to the front and rear wheel cylinders CWf and CWr. An inlet valve UI and an outlet valve VO are provided for each wheel cylinder CW. The inlet valve UI (solenoid valve) is a normally open linear valve, similar to the pressure regulating valve UB. However, the pressure regulating valve UB and the inlet valve UI have different opening directions. In detail, the pressure regulating valve UB opens in response to the flow of brake fluid BF from the wheel cylinder CW to the master cylinder CM, so that in the pressure regulation by the pressure regulating valve UB, the regulated hydraulic pressure Pq is equal to or higher than the supply hydraulic pressure Pm (i.e., "Pq ≧ Pm"). On the other hand, the inlet valve UI opens in response to the flow from the master cylinder CM to the wheel cylinder CW, so that in the pressure regulation by the inlet valve UI, the brake hydraulic pressure Pw is equal to or lower than the regulated hydraulic pressure Pq (i.e., "Pq ≧ Pw").

[0046] The inlet valve UI is provided in the branched communication passage HS (i.e., on the side closer to the wheel cylinder CW than the branching portion of the communication passage HS). The communication passage HS is connected to a pressure regulating reservoir RC via a pressure reduction passage HG at a lower portion of the inlet valve UI (the portion of the communication passage HS closer to the wheel cylinder CW). An outlet valve VO, which is a normally closed on-off valve, is disposed in the pressure reduction passage HG.

[0047] The inlet valve UI and the outlet valve VO are individually controlled so that the brake fluid pressure Pw is adjusted separately for each wheel cylinder CW. To reduce the brake fluid pressure Pw, the inlet valve UI is closed and the outlet valve VO is opened. The inflow of brake fluid BF into the wheel cylinder CW is blocked, and the brake fluid BF in the wheel cylinder CW flows out to the pressure regulating reservoir RC, so that the brake fluid pressure Pw is reduced. To increase the brake fluid pressure Pw, the inlet valve UI is opened and the outlet valve VO is closed. The outflow of brake fluid BF to the pressure regulating reservoir RC is blocked, and the regulated hydraulic pressure Pq from the pressure regulating valve UB is supplied to the wheel cylinder CW, so that the brake fluid pressure Pw is increased. To maintain the brake fluid pressure Pw, both the inlet valve UI and the outlet valve VO are closed. The wheel cylinder CW is fluidically sealed, so that the brake fluid pressure Pw is maintained constant.

[0048] <Adjustment of brake fluid pressure Pw in vehicle stability control> The process of adjusting the brake fluid pressure Pw in the vehicle stability control will be described with reference to the flow diagram of Fig. 4. In the stability control, the fluid pressures Pw in the four wheel cylinders CW provided on the vehicle JV are independently and individually controlled to impart a yawing moment to the vehicle JV and decelerate the vehicle JV. By imparting this moment, excessive oversteer and excessive understeer are suppressed in the vehicle JV, and the vehicle JV is stabilized. Vehicle stability control is well known, so it will be described briefly.

[0049] Before describing the adjustment process of the brake fluid pressure Pw, the following will be defined. In stability control, an individual target fluid pressure Pt is calculated for the wheel cylinder CW provided on each wheel WH, and each internal pressure (brake fluid pressure) Pw is individually controlled. In the wheel cylinder CW, the one corresponding to the maximum value Ptx (referred to as the "maximum target fluid pressure") among the multiple target fluid pressures Pt is referred to as the "selected wheel cylinder CWx." Meanwhile, among the multiple wheel cylinders CW, the ones other than the selected wheel cylinder CWx are referred to as the "non-selected wheel cylinder CWz." In other words, in the vehicle JV, one of the four wheel cylinders CW is the selected wheel cylinder CWx, and the remaining three are non-selected wheel cylinders CWz.

[0050] Furthermore, among the components related to the wheel cylinder CW, the subscript "x" indicates that the component corresponds to the selected wheel cylinder CWx, and the subscript "z" indicates that the component corresponds to the non-selected wheel cylinder CWz. Therefore, the actual hydraulic pressure of the selected wheel cylinder CWx is referred to as the "selected brake hydraulic pressure Pwx," the actual hydraulic pressure of the non-selected wheel cylinder CWz is referred to as the "non-selected brake hydraulic pressure Pwz," and the target hydraulic pressure of the non-selected wheel cylinder CWz is referred to as the "non-selected target hydraulic pressure Ptz." Note that since the maximum target hydraulic pressure Ptx is the maximum value of the target hydraulic pressures Pt, the magnitude relationship between the non-selected target hydraulic pressure Ptz (resulting in the actual non-selected brake hydraulic pressure Pwz) and the maximum target hydraulic pressure Ptx (resulting in the actual selected brake hydraulic pressure Pwx) is "Ptz≦Ptx, Pwz≦Pwx."

[0051] Among the components provided corresponding to each of the four wheel cylinders CW, the one corresponding to the selected wheel cylinder CWx is given the suffix "x," and the one corresponding to the non-selected wheel cylinder CWz is given the suffix "z." For example, among the four inlet valves UI and outlet valves VO, the elements corresponding to the selected wheel cylinder CWx are called the "selected inlet valve UIx" and the "selected outlet valve VOx." On the other hand, among the four inlet valves UI and outlet valves VO, the elements corresponding to the non-selected wheel cylinder CWz are called the "non-selected inlet valve UIz" and the "non-selected outlet valve VOz."

[0052] Furthermore, the brake control device SC employs two brake fluid paths, with the suffix "x" attached to components of the path including the selected wheel cylinder CWx and the suffix "z" attached to components of the path not including the selected wheel cylinder CWx. For example, among the two communication paths HS, the pressure regulating valve UB, the pressure regulating reservoir RC, and the fluid pump QB, the elements including the selected wheel cylinder CWx are referred to as the "selected communication path HSx," the "selected pressure regulating valve UBx," the "selected pressure regulating reservoir RCx," and the "selected fluid pump QBx." On the other hand, among the two communication paths HS, the pressure regulating valve UB, the pressure regulating reservoir RC, and the fluid pump QB, the elements not including the selected wheel cylinder CWx are referred to as the "non-selected communication path HSz," the "non-selected pressure regulating valve UBz," the "non-selected pressure regulating reservoir RCz," and the "non-selected fluid pump QBz."

[0053] The adjustment process of the brake fluid pressure Pw in the vehicle stability control will be described. In step S110, various signals including the wheel speed Vw, the vehicle body speed Vx, the steering operation amount Sa, the yaw rate Yr, the lateral acceleration Gy, the servo fluid pressure Pu, the supply fluid pressure Pm, etc. are read. Here, the vehicle body speed Vx is calculated by the controller ECU based on the wheel speed Vw and a known method.

[0054] In step S120, a stability index Js is calculated based on the steering operation amount Sa, the yaw rate Yr, and the lateral acceleration Gy. The "stability index Js" is an index that indicates the degree of stability of the vehicle JV (i.e., the degree of oversteer or understeer). In detail, the closer the stability index Js is to "0", the higher the stability of the vehicle JV is, and the larger the magnitude (absolute value) of the stability index Js is, the more the stability of the vehicle JV is impaired.

[0055] The stability index Js is calculated based on the deviation hJ (=Ja-Jt) between a target vehicle yaw amount (target yaw amount) Jt and an actual yaw amount Ja. Here, the "yaw amount" is a state quantity that represents the yawing motion of the vehicle JV. For example, the physical quantity as the yaw amount is calculated as at least one of the yaw rate, the vehicle body sideslip angle (also simply referred to as "sideslip angle"), and the vehicle body sideslip angular velocity (also simply referred to as "sideslip angular velocity"). Note that when the yaw deviation hJ has a negative sign (i.e., "hJ<0"), it indicates understeer, and when the yaw deviation hJ has a positive sign (i.e., "hJ>0"), it indicates oversteer.

[0056] For example, the yaw rate is adopted as the physical quantity of the stability index Js. Specifically, the target yaw rate Yt is calculated as the target yawing amount Jt based on the steering operation amount (steering angle) Sa and the vehicle speed Vx. Then, the deviation hY (yaw rate deviation) between the actual yaw rate Yr (detection value of the yaw rate sensor YR) and the target yaw rate Yt is adopted as the stability index Js (i.e., "Js=Yr-Yt").

[0057] Also, the stability index Js may be calculated as the interrelationship between multiple physical quantities, rather than a single physical quantity. For example, the deviation hβ between the actual sideslip angle βa and the target sideslip angle βt (sideslip angle deviation) is calculated based on the steering operation amount Sa, the yaw rate Yr, and the lateral acceleration Gy (i.e., "hβ=βa-βt"). Then, the stability index Js is calculated based on the interrelationship between the yaw rate deviation hY and the sideslip angle deviation hβ (see formula (1)). Js = K1·hY + K2·hβ (where K1 and K2 are coefficients) … Equation (1)

[0058] In step S130, based on the execution flag FE, it is determined whether "stability control is in execution or not". The execution flag FE is a control flag that indicates the execution state of the stability control. Specifically, when the stability control is being executed, the execution flag FE is set to "1 (in execution)", and when the stability control is not being executed, the execution flag FE is set to "0 (not executed)". If "FE = 0" and the stability control is not executed, the process proceeds to step S140. On the other hand, if "FE = 1" and the stability control is already in execution, the process proceeds to step S150.

[0059] In step S140, based on the stability index Js, it is determined whether "the start condition of the stability control is satisfied or not (start determination)". Specifically, when the magnitude of the stability index Js is equal to or greater than the start threshold jx, the execution of the stability control is started. Here, the start threshold jx is a predetermined value (constant) set in advance. If "Js ≥ jx", the start of the stability control is affirmed, and the process proceeds to step S160. If "Js < jx", the start of the stability control is negated, and the process returns to step S110. Note that in step S140, in addition to the start determination, based on the stability index Js, it is also determined whether the behavior of the vehicle JV is understeer or oversteer.

[0060] In step S150, based on the stability index Js, it is determined whether "the end condition of stability control is satisfied or not (end determination)". Specifically, when the magnitude of the stability index Js is less than the end threshold jz, the execution of stability control is terminated. Here, the end threshold jz is a preset value (constant). In the magnitude relationship between the start threshold jx and the end threshold jz, the end threshold jz is a value smaller than the start threshold jx. When "Js < jz", the end of stability control is affirmed, and the process returns to step S110. When "Js ≥ jz", the end of stability control is negated, and the process proceeds to step S160.

[0061] In step S160, based on the stability index Js, the target hydraulic pressure Pt corresponding to the hydraulic pressure Pw of each wheel cylinder CW is calculated. That is, in stability control, the target hydraulic pressure Pt is calculated for each wheel cylinder CW, and the actual braking hydraulic pressure Pw is adjusted individually. Specifically, a calculation map when the behavior of the vehicle JV is oversteer and a calculation map when it is understeer are set separately. Then, based on the stability index Js, the steering characteristics of the vehicle JV (understeer behavior or oversteer behavior), and the preset calculation map, the target hydraulic pressure Pt is calculated such that the larger the stability index Js, the larger the target hydraulic pressure Pt. Note that for a wheel WH (i.e., wheel cylinder CW) that is unnecessary for generating the yawing moment of the vehicle JV, an increase in the braking hydraulic pressure Pw is unnecessary. Therefore, for example, when not braking (i.e., "Ba = 0"), the target hydraulic pressure Pt of the corresponding wheel cylinder CW is set to "0".

[0062] When the vehicle behavior is oversteer, at least the target hydraulic pressure Pt corresponding to the outer front wheel in the turning direction of the vehicle JV is calculated based on the stability index Js. On the other hand, when the vehicle behavior is understeer, at least the target hydraulic pressure Pt corresponding to the inner rear wheel in the turning direction of the vehicle JV is calculated based on the stability index Js.

[0063] In step S170, a target value Pd of the reference hydraulic pressure is calculated based on the target hydraulic pressure Pt. Then, based on this target value (target reference hydraulic pressure) Pd, the actual reference hydraulic pressure Pe is adjusted. Specifically, the maximum value of the target hydraulic pressures Pt of each wheel cylinder CW is determined as the maximum target hydraulic pressure Ptx (i.e., "Ptx=MAX(Pt)"). Then, the target reference hydraulic pressure Pd is calculated to be equal to the maximum target hydraulic pressure Ptx (i.e., "Pd=Ptx"). In other words, the target value Pd of the reference hydraulic pressure is the maximum target hydraulic pressure required for stability control. The reference hydraulic pressure Pe is the actual value (i.e., the control result) corresponding to the target reference hydraulic pressure Pd (maximum target hydraulic pressure). In other words, the maximum hydraulic pressure generated by the brake control device SC during the execution of stability control is the reference hydraulic pressure Pd (target value), Pe (actual value), and all brake hydraulic pressures Pw are adjusted to hydraulic pressures equal to or lower than the reference hydraulic pressure Pe as the base pressure.

[0064] In step S180, the brake fluid pressure Pw is adjusted using the reference fluid pressure Pe as a base pressure (base fluid pressure). The fluid pressure Pwx (=Pe) of the selected wheel cylinder CWx is adjusted by at least one of the first unit YA and the second unit YB, as described below. At this time, no power is supplied to the selected inlet valve UIx and the selected outlet valve VOx, and they are in an inoperative state. On the other hand, the brake fluid pressure Pwz of the non-selected wheel cylinder CWz is adjusted in the range from "0" to the reference fluid pressure Pe by the non-selected inlet valve UIz and the non-selected outlet valve VOz. The reference fluid pressure Pe is supplied to the upper part of the non-selected inlet valve UIz, and the non-selected brake fluid pressure Pwz is adjusted by opening and closing the non-selected inlet valve UIz and the non-selected outlet valve VOz.

[0065] In detail, one of three control modes, namely, a "decrease mode", an "increase mode", and a "hold mode", is selected for adjusting the non-selected brake fluid pressure Pwz. In the decrease mode, when it is necessary to decrease the non-selected brake fluid pressure Pwz, the non-selected inlet valve UIz is closed and the non-selected outlet valve VOz is opened. A reference hydraulic pressure Pe corresponding to the target reference hydraulic pressure Pd is supplied to an upper portion of the non-selected inlet valve UIz (between the pressure regulating valve UB and the inlet valve UI in the communication passage HS), but this supply is prevented because the non-selected inlet valve UIz is closed. Then, because the non-selected outlet valve VOz is opened, the brake fluid BF in the non-selected wheel cylinder CWz flows out to the pressure regulating reservoir RC, and the non-selected brake fluid pressure Pwz is decreased.

[0066] In the increase mode, when it is necessary to increase the non-selected brake fluid pressure Pwz, the non-selected outlet valve VOz is closed and the non-selected inlet valve UIz is opened. Closing the non-selected outlet valve VOz prevents the brake fluid BF from flowing out to the pressure regulating reservoir RC. Then, the brake fluid BF adjusted to the reference hydraulic pressure Pe is introduced into the non-selected wheel cylinder CWz through the non-selected inlet valve UIz, so that the non-selected brake fluid pressure Pwz is increased. However, the non-selected brake fluid pressure Pwz is only increased to the reference hydraulic pressure Pe.

[0067] In the holding mode, when it is necessary to hold the non-selected brake fluid pressure Pwz, both the non-selected inlet valve UIz and the non-selected outlet valve VOz are closed. Since the non-selected wheel cylinder CWz is fluidly sealed, the non-selected brake fluid pressure Pwz is maintained constant. The holding mode may be omitted in adjusting the non-selected brake fluid pressure Pwz. In this case, the decrease mode and the increase mode are repeated to adjust the brake fluid pressure Pwz of the non-selected wheel cylinder CWz. In the wheel cylinder CWz where the non-selected target pressure Ptz is calculated to be "0", the inlet valve UIz is closed to keep the non-selected brake fluid pressure Pwz at "0".

[0068] For example, assume that excessive oversteer occurs during non-braking, and an increase in the brake hydraulic pressure Pw of the front and rear wheels on the outside of the turn is required, while the brake hydraulic pressure Pw of the front and rear wheels on the inside of the turn is required to remain at "0". In this case, the target hydraulic pressure Pt of the outside front wheel, which has the greatest effect of suppressing oversteer, is calculated as the maximum target hydraulic pressure Ptx. Then, "Pd=Ptx" is determined, and at least one of the first and second units YA and YB applies pressure so that the target reference hydraulic pressure Pd is achieved. Therefore, the selected brake hydraulic pressure Pwx of the outside front wheel (i.e., the reference hydraulic pressure Pe) is controlled to coincide with the target reference hydraulic pressure Pd. Moreover, the non-selected brake hydraulic pressure Pwz of the outside rear wheel of the turn is adjusted to be equal to or lower than the reference hydraulic pressure Pe by the non-selected inlet valve UIz and the non-selected outlet valve VOz. In addition, the non-selected brake fluid pressure Pwz of the front and rear wheels on the inside of the turn, which has been determined to be "Ptz = 0", is maintained at "0" by closing the non-selected inlet valve UIz.

[0069] <Adjustment process of reference fluid pressure Pe> The adjustment process of the reference hydraulic pressure Pe will be described with reference to the flow chart of FIG. 5. As described above, the target reference hydraulic pressure Pd is calculated so as to coincide with the maximum target hydraulic pressure Ptx required for vehicle stability control. The reference hydraulic pressure Pe is adjusted (increased, maintained, decreased) by at least one of the first unit YA and the second unit YB so as to approach and coincide with the target reference hydraulic pressure Pd. Here, the adjustment of the reference hydraulic pressure Pe by the first unit (upper unit) YA is referred to as "upper pressure adjustment", and the adjustment of the reference hydraulic pressure Pe by the second unit (lower unit) YB is referred to as "lower pressure adjustment". In other words, in "upper pressure adjustment", the first unit YA (particularly, the pressurizing unit KU) is used as the pressurizing source, and in "lower pressure adjustment", the second unit YB (particularly, the pressurizing source KB) is used as the pressurizing source.

[0070] In step S210, the lower pressure adjustment is performed. In the vehicle stability control, the pressure adjustment is basically performed by the second unit YB. Therefore, when the stability control is started, the reference hydraulic pressure Pe (finally, the brake hydraulic pressure Pw) is always increased by the lower pressure adjustment.

[0071] Specifically, in step S210, the electric motor MB is energized to drive the fluid pump QB. Then, a target value Ibt of the amount of current Ib to the pressure regulating valve UB is calculated based on the target reference hydraulic pressure Pd. The drive circuit DD is provided with an amount of current sensor IB (e.g., a current sensor) that detects the actual amount of current Ib. A switching element (such as a MOS-FET) of the drive circuit DD is controlled so that the actual amount of current Ib coincides with the target amount of current Ibt. This is based on the fact that the relationship (so-called IP characteristic) between the amount of current Ib and the differential pressure mQ (hydraulic pressure difference between the upper and lower parts of the pressure regulating valve UB) is known in the pressure regulating valve UB. In step S210, the upper pressure adjustment is not performed, so that the supply hydraulic pressure Pm adjusted by the first unit YA remains at "0" when the brake operating member BP is not being operated and braking is not being applied.

[0072] As the amount of current Ib (current value) increases, the opening amount of the pressure regulating valve UB decreases. As a result, the return flow KN of the brake fluid BF discharged by the fluid pump QB is throttled by the pressure regulating valve UB, and the fluid pressure (adjusted fluid pressure) Pq at the lower part of the pressure regulating valve UB is increased by a pressure mQ compared to the fluid pressure (supply fluid pressure) Pm at the upper part of the pressure regulating valve UB. In other words, the pressure difference (differential pressure) mQ between the supply fluid pressure Pm and the adjusted fluid pressure Pq is adjusted by the pressure regulating valve UB. In other words, in the lower pressure adjustment, the amount of current Ib to the pressure regulating valve UB is adjusted so that the actual reference fluid pressure Pe (=Pm+mQ) approaches and coincides with the target reference fluid pressure Pd. Note that when braking is not in progress, "Pm=0", so "Pe=mQ".

[0073] An adjusted hydraulic pressure sensor PQ (not shown) may be provided below the pressure regulating valve UB (between the pressure regulating valve UB and the inlet valve UI), and the amount of electricity Ib supplied to the pressure regulating valve UB may be controlled so that the detected value (adjusted hydraulic pressure) Pq of the sensor coincides with the target reference hydraulic pressure Pd. In any case, in step S210, the hydraulic pressure difference mQ is adjusted by the pressure source KB of the second unit YB (i.e., the electric motor MB, the fluid pump QB, and the pressure regulating valve UB) to adjust the reference hydraulic pressure Pe.

[0074] In step S220, various signals including the steering operation amount Sa, the yaw rate Yr, the lateral acceleration Gy, the vehicle speed Vx, the servo hydraulic pressure Pu, the supply hydraulic pressure Pm, etc. are read.

[0075] In step S230, a turning state quantity Gs is calculated based on at least one of the steering operation quantity Sa, the yaw rate Yr, and the lateral acceleration Gy. The "turning state quantity Gs" is a state quantity (state variable) that indicates the degree (magnitude, level) of the lateral movement of the vehicle JV, and indicates the probability (occurrence probability) of a sudden change in vehicle behavior. For example, "lateral acceleration" is adopted as the physical quantity of the turning state quantity Gs. Then, the detection value (lateral acceleration) Gy by the lateral acceleration sensor GY is adopted as the turning state quantity Gs[Gy]. Here, "Gs[ ]" indicates that the turning state quantity Gs is determined (calculated) based on the state quantity in [ ]. In other words, the turning state quantity Gs is calculated based on the lateral acceleration Gy.

[0076] There is a predetermined relationship between the lateral acceleration Gy and the yaw rate Yr. Therefore, the turning state quantity Gs[Yr] may be calculated based on the yaw rate Yr. Specifically, the turning state quantity Gs[Yr] according to the yaw rate Yr is calculated by the following formula (2). Gs[Yr]=Yr×Vx …Equation (2)

[0077] Furthermore, the turning state amount Gs can be calculated based on the steering operation amount Sa. Specifically, the turning state amount Gs[Sa] corresponding to the steering operation amount Sa is calculated based on the following formula (3). Gs[Sa]=(Vx 2 ·Sa) / {L·Ns·(1+Kh·Vx 2 )} …Equation (3) Here, L is the wheel base, Ns is the steering gear ratio, and Kh is the stability factor, each of which is a predetermined value that has been set in advance.

[0078] The turning state quantity Gs may be calculated by combining the above-mentioned various turning state quantities Gs[Gy], Gs[Yr], and Gs[Sa]. Therefore, the turning state quantity Gs is calculated based on at least one of the lateral acceleration Gy, the yaw rate Yr, and the steering operation quantity Sa.

[0079] In step S240, it is determined whether "the state is lower pressure regulating or not (pressure regulation determination)" based on the state flag FK. The state flag FK is a control flag that indicates the pressure regulating state. The state flag FK indicates the lower pressure regulating state when it is "0" and the upper pressure regulating state when it is "1". If "FK=0", that is, the state is lower pressure regulating, the process proceeds to step S250. On the other hand, if "FK=1", that is, the state is upper pressure regulating, the process proceeds to step S260.

[0080] In step S250, a determination is made based on the turning state quantity Gs as to "whether or not the adjustment of the reference hydraulic pressure Pe should be switched from the lower pressure adjustment to the upper pressure adjustment (switching determination)." Specifically, in the switching determination in step S250, a determination is made as to "whether or not the turning state quantity Gs is equal to or greater than a first predetermined quantity gs (i.e., whether or not the turning state quantity Gs has increased to or greater than the first predetermined quantity gs)." Here, the first predetermined quantity gs is a threshold value for determination, and is set as a predetermined value (constant) that has been set in advance.

[0081] In the above switching judgment, a permission condition (condition for judgment possibility) for the turning direction of the vehicle JV is added. For example, a sudden change in vehicle behavior is likely to occur when the steering operation member SH is operated in one direction (e.g., left turning direction) and then in the opposite direction (e.g., right turning direction) as in slalom driving. Such an operation is called "transient steering". In transient steering, the turning state amount Gs increases or decreases according to the operation of the steering operation member SH. For this reason, in the switching judgment of step S250, it is set as a condition that the direction of the turning state amount Gs and the turning direction of the vehicle JV match. Specifically, the turning direction of the vehicle JV is expressed by positive and negative signs in each state amount (Sa, Yr, Gy, etc.). For example, a positive sign (+) is associated with a left turning direction, and a negative sign (-) is associated with a right turning direction. Then, when at least one of the following conditions is satisfied: "the sign of the steering operation amount Sa and the sign of the yaw rate Yr are the same" and "the sign of the steering operation amount Sa and the sign of the lateral acceleration Gy are the same", the execution of the above switching judgment is permitted.

[0082] The first predetermined amount gs may be calculated based on the operation speed dS. In this case, in step S250, the operation speed dS is calculated by time-differentiating the steering operation amount Sa based on the steering operation amount Sa. That is, the operation speed dS is a time change amount of the steering operation amount Sa, for example, an operation angular velocity. Then, as shown in the balloon ZA, the first predetermined amount gs is determined based on the operation speed dS (steering angular velocity) and the calculation map Zgs. In detail, the first predetermined amount gs is determined according to the calculation map Zgs so that the larger the operation speed dS, the smaller the first predetermined amount gs becomes. This is based on the fact that the larger the operation speed dS, the more likely it is that a sudden vehicle behavior (yawing behavior and rolling behavior) will occur. When the operation speed dS is large, the first predetermined amount gs is reduced, and therefore step S250 is satisfied with a smaller turning state amount Gs.

[0083] Furthermore, the first predetermined amount gs may be calculated based on the peak value dSp of the operation speed dS so that the phase difference (time difference in dynamic change) between the operation speed dS and the turning state amount Gs is compensated. The operation speed peak value dSp is calculated based on a comparison at each calculation cycle, in which the operation speed dS is stored in chronological order. That is, in the calculation cycle, a magnitude comparison is performed between the previous value of the operation speed dS and the current value of the operation speed dS, and when the current value is greater than the previous value, the current value is sequentially stored as the peak value dSp of the operation speed dS, thereby calculating the operation speed peak value dSp. As shown in [ ] in the balloon section ZA, the first predetermined amount gs is determined based on the operation speed peak value dSp and the calculation map Zgs. The first predetermined amount gs is calculated so that the larger the operation speed peak value dSp is, the smaller the first predetermined amount gs is calculated according to the calculation map Zgs. Although the turning state quantity Gs occurs with a time delay relative to the operation speed dS, the influence of the phase difference between the operation speed dS and the turning state quantity Gs is compensated for by adopting the operation speed peak value dSp.

[0084] In step S250, if the turning state quantity Gs is equal to or greater than the first predetermined quantity gs (i.e., if the switching determination is positive), the probability of a sudden change in vehicle behavior is high, so the process proceeds to step S270 to switch from lower pressure regulation to upper pressure regulation. On the other hand, if the turning state quantity Gs is less than the first predetermined quantity gs (i.e., if the switching determination is negative), the probability of a sudden change in vehicle behavior is low, so the process returns to step S220 to continue lower pressure regulation.

[0085] In step S260, it is determined whether or not the reference hydraulic pressure adjustment should be returned from the upper pressure adjustment to the lower pressure adjustment (return determination). Specifically, in the return determination in step S260, it is determined whether or not the turning state quantity Gs has decreased to less than a second predetermined quantity gt. Here, the second predetermined quantity gt is a threshold value corresponding to the turning state quantity Gs, and is set to a value equal to or less than the first predetermined quantity gs.

[0086] Similar to the permission conditions for the switching determination, permission conditions related to the turning direction of the vehicle JV are also added in the return determination. That is, when at least one of the conditions "the signs of the steering operation amount Sa and the yaw rate Yr match" and "the signs of the steering operation amount Sa and the lateral acceleration Gy match" is satisfied, the execution of the return determination is permitted.

[0087] In step S260, when the turning state quantity Gs is less than the second predetermined quantity gt (that is, when the return determination is affirmed), the process proceeds to step S280 so that the pressure regulation is switched from the upper pressure regulation to the lower pressure regulation (that is, returned). On the other hand, when the turning state quantity Gs is greater than or equal to the second predetermined quantity gt (that is, when the return determination is negated), the process proceeds to step S270 so that the upper pressure regulation is continued.

[0088] Furthermore, for the return determination in step S260, at least one of "whether the vehicle body speed Vx has decreased below the predetermined speed vx" and "whether the steering operation amount Sa is within the range of the predetermined operation amount sa (that is, whether the magnitude (absolute value) |Sa| of the steering operation amount Sa is less than sa)" is added. Specifically, step S260 is satisfied when "Gs < gt" and "Vx < vx", but step S260 is negated when it corresponds to any one of "Gs ≧ gt" and "Vx ≧ vx". Similarly, step S260 is satisfied when "Gs < gt" and "|Sa| < sa", but step S260 is negated when it corresponds to any one of "Gs ≧ gt" and "|Sa| ≧ sa". The predetermined speed vx and the predetermined operation amount sa are preset predetermined values (constants).

[0089] In step S270, the first unit YA adjusts the supply hydraulic pressure Pm as the reference hydraulic pressure Pe. In detail, a target value Pv (also called "target servo hydraulic pressure") of the servo hydraulic pressure Pu corresponding to the target reference hydraulic pressure Pd is calculated. Then, the pressure increase valve UZ and the pressure reduction valve UG are controlled (so-called hydraulic pressure feedback control) so that the actual servo hydraulic pressure Pu (detection value of the servo hydraulic pressure sensor PU) coincides with the target servo hydraulic pressure Pv (target value).

[0090] In step S280, the same process as in step S210 (pressure regulation by the pressure regulating valve UB of the second unit YB) is executed. The electric motor MB drives the fluid pump QB. Then, the flow KN of the brake fluid BF discharged by the fluid pump QB is throttled by the pressure regulating valve UB, so that the hydraulic pressure difference mQ is adjusted as the reference hydraulic pressure Pe.

[0091] The reasons and effects of appropriately switching between the upper pressure regulation and the lower pressure regulation based on the turning state quantity Gs will be described below.

[0092] <Characteristics of the 1st and 2nd units YA and YB> The brake control device SC is equipped with two units YA and YB as pressure sources. In the first unit YA, the brake fluid pressure Pw is increased (i.e., pressurized) by pumping brake fluid BF from the cylinder CM as pistons NP and NS move. Meanwhile, in the second unit YB, the brake fluid pressure Pw is increased by the orifice effect that occurs when the flow (circulation flow) KN of the brake fluid BF discharged by the fluid pump QB is throttled. Due to the difference in pressurization method, the first and second units YA and YB have the following features (advantages / disadvantages).

[0093] In the first unit YA, the pistons NP, NS and the cylinder CM are sealed by the seal member SL. Therefore, when the pistons NP, NS are moved, the sliding resistance (frictional force) of the seal member SL acts. Therefore, the second unit YB is more advantageous than the first unit YA in fine pressure adjustment when the brake fluid pressure Pw starts to increase. In particular, in a configuration in which an accumulator AC is used in the pressurizing unit KU, it is difficult to adjust the pressure when an extremely low pressure is adjusted based on the high pressure of the accumulator AC.

[0094] The first unit YA simultaneously adjusts the hydraulic pressures Pw of all (i.e., four) wheel cylinders CW provided on the vehicle JV. In other words, the first unit YA does not adjust the brake hydraulic pressures Pw individually. For this reason, the first unit YA is mainly used for service brakes (service braking). In fulfilling the function of the service brake, the first unit YA can also respond to a sudden operation of the brake operating member BP (i.e., sudden braking). Therefore, the output of the first unit YA (particularly, the pressurizing unit KU) is larger than the output of the second unit YB (particularly, the pressurizing source KB). Furthermore, in a configuration in which the accumulator AC is employed in the pressurizing unit KU, the high pressure stored in the accumulator AC can be utilized, which is advantageous in terms of boost response. In light of these characteristics, the first unit YA and the second unit YB are appropriately used in the adjustment process of the reference hydraulic pressure Pe.

[0095] In vehicle stability control, in order to suppress understeer behavior and allow the vehicle JV to turn smoothly, a large reference hydraulic pressure Pe is not required, but high pressure regulation accuracy is required. For example, this applies to a situation in which the vehicle is traveling on a road surface with a low road friction coefficient (icy road, packed snow road, etc.). On the other hand, when oversteer behavior is suppressed, not only is a large reference hydraulic pressure Pe required, but it is also required to increase it quickly. For example, this applies to a situation in which the steering operation member SH is suddenly operated on a road surface with a high road friction coefficient (paved road such as asphalt road or concrete road). To satisfy these requirements with one unit (generally equivalent to the second unit YB), the unit would be enlarged and the cost would be high. Specifically, it would be necessary to increase the flow rate of the pressure regulating valve UB, increase the output of the electric motor MB, and increase the discharge amount of the fluid pump QB, or add a device (for example, an accumulator) to complement the responsiveness.

[0096] In the brake control device SC, the features of the two pressurization sources (first and second units YA and YB) with different pressurization methods are utilized, and the above requirements are satisfied by appropriately using them. Specifically, in the adjustment process of the reference hydraulic pressure Pe required for vehicle stability control, the lower pressure adjustment by the second unit YB is adopted as a default (initial setting). The second unit YB can adjust the supply hydraulic pressure Pm generated by the first unit YA for each wheel individually so as to execute antilock brake control and traction control in addition to stability control. In the lower pressure adjustment by the second unit YB, the reference hydraulic pressure Pe is increased in order to execute stability control, and this increase is realized by throttling the flow KN of the brake fluid BF discharged by the fluid pump QB by the pressure adjustment valve UB provided in the communication fluid path HS. Since normal stability control is executed by the lower pressure adjustment with relatively high pressure adjustment accuracy, good control performance can be ensured.

[0097] In the braking control device SC, based on "Gs≧gs", a situation in which the vehicle stability is rapidly impaired is determined. In this situation, in order to maintain the stability of the vehicle JV, the reference hydraulic pressure Pe must be rapidly increased (that is, high pressure increase responsiveness is required). When it is determined that the turning state quantity Gs is equal to or greater than the first predetermined quantity gs, the pressure regulation is switched from the lower pressure regulation to the upper pressure regulation. Since the reference hydraulic pressure Pe is increased with high responsiveness by the first unit YA, even if a sudden oversteer behavior occurs, the stability of the vehicle JV can be appropriately ensured.

[0098] In the braking control device SC, when "Gs < gs", since such a high-response pressurization is not required so much, highly accurate control is executed by the lower pressure regulation. On the other hand, when "Gs≧gs", since high-response pressurization is required, the upper pressure regulation is adopted. That is, the two pressurizing sources YA and YB are appropriately used separately, and the performance of the stability control is improved. Incidentally, when the second unit YB attempts to preferably maintain the stability in a situation where the behavior suddenly changes, the second unit YB (particularly, the pressurizing source KB) becomes large-sized. However, since the first unit YA can cope with this situation, it can be said that the second unit YB can be miniaturized by the proper use of the pressurizing sources.

[0099] In the braking control device SC, the first predetermined quantity gs can be calculated based on the operation speed dS (the differential value of the steering operation amount Sa). Specifically, the first predetermined quantity gs is calculated such that the larger the operation speed dS, the smaller the first predetermined quantity gs. This is based on the fact that even with the same turning state quantity Gs, the higher the operation speed dS, the more easily the stability of the vehicle JV is impaired. When the operation speed dS is high, the first predetermined quantity gs is set small, so that it is easier to switch from the lower pressure regulation to the upper pressure regulation. Thereby, the pressure increase responsiveness of the reference hydraulic pressure Pe is further improved.

[0100] Further, an operation speed peak value dSp is calculated based on the operation speed dS, and a first predetermined amount gs can be calculated based on the operation speed peak value dSp. As in the above, the first predetermined amount gs is calculated so that the larger the operation speed peak value dSp, the smaller the first predetermined amount gs becomes. Although there is a time lag (i.e., a phase difference) between the operation speed dS and the turning state amount Gs, the effect of this lag is compensated for by adopting the operation speed peak value dSp as the state amount related to the operation speed dS.

[0101] When switching from lower pressure regulation by the second unit YB to upper pressure regulation by the first unit YA, pressurization by the second unit YB (i.e., pressurization source KB) is not immediately terminated so as to compensate for the boost response of the first unit YA. In detail, pressurization by the pressurization source KB (i.e., lower pressure regulation) is continued until the actual reference hydraulic pressure Pe coincides with the target reference hydraulic pressure Pd. Here, the reference hydraulic pressure Pe is calculated by adding the hydraulic pressure difference mQ to the hydraulic pressure Pm supplied from the first unit YA (i.e., "Pe = Pm + mQ"). Then, at the point in time when the actual reference hydraulic pressure Pe coincides with the target reference hydraulic pressure Pd, the lower pressure regulation is terminated. Even if there is a time delay in pressurization by the first unit YA, pressurization is continued by the second unit YB so as to compensate for this delay. Therefore, even if the vehicle stability is suddenly impaired, the situation can be appropriately dealt with and the stability of the vehicle JV can be maintained.

[0102] <Adjustment of reference fluid pressure Pe> The operation of the adjustment process of the reference hydraulic pressure Pe will be described with reference to the time series diagram of FIG. 6 (a transition diagram of various state quantities (Gs, etc.) corresponding to changes in time T). The diagram assumes a situation in which a fast lane change is made on a road surface with a high friction coefficient (a dry paved road). Specifically, the steering wheel SH is abruptly operated in a left turning direction (indicated by a positive sign), then continuously operated in a right turning direction (indicated by a negative sign), and then operated again in a left turning direction. Note that no braking operation is performed during the lane change.

[0103] The diagram shows, from the top, the steering operation amount Sa, the turning state amount Gs, the target reference hydraulic pressure Pd, the supply hydraulic pressure Pm, and the hydraulic pressure difference mQ in that order. The lateral acceleration Gy, which is the detection value of the lateral acceleration sensor GY, is adopted and displayed as the turning state amount Gs. As described above, the target reference hydraulic pressure Pd is calculated to be equal to the maximum value Ptx of the target hydraulic pressures Pt related to the four wheel cylinders CW. The supply hydraulic pressure Pm is generated by the first unit YA, and the hydraulic pressure difference mQ is generated by the second unit YB. Therefore, the supply hydraulic pressure Pm corresponds to the reference hydraulic pressure Pe by the upper pressure adjustment, and the hydraulic pressure difference mQ corresponds to the reference hydraulic pressure Pe by the lower pressure adjustment.

[0104] Up to time t0, the steering operation amount (steering angle) Sa is "0 (the steering operation member SH is in the neutral position, corresponding to the vehicle JV traveling straight ahead)." At time t0, the driver begins to operate the steering operation member SH. In response to this operation, from time t0, the lateral acceleration Gy (= Gs) is increased. Up to time t1, the magnitude of the stability index Js is less than the stability control start threshold amount jx, so stability control is not executed and the target reference hydraulic pressure Pd is calculated to be "0."

[0105] At time t1, the magnitude of the stability index Js transitions to be equal to or greater than the start threshold jx. This satisfies step S140, and the execution of stability control is started. Therefore, at time t1, the target reference hydraulic pressure Pd is increased from "0". At time t1, the switching judgment is not satisfied, so the target reference hydraulic pressure Pd is achieved only by the lower pressure regulation by the second unit YB. That is, at time t1, the driving of the electric motor MB is started, and the fluid pump QB is rotated. Then, the return flow KN of the brake fluid BF discharged by the fluid pump QB is throttled by the pressure regulating valve UB. Due to the orifice effect of the pressure regulating valve UB, a hydraulic pressure difference mQ between the supply hydraulic pressure Pm and the regulated hydraulic pressure Pq is generated. That is, only by the second unit YB (particularly, the pressure source KB), the actual reference hydraulic pressure Pe (=mQ) is increased from "0" so as to approach and match the target reference hydraulic pressure Pd. Until time t2, the turning state quantity Gs (for example, the lateral acceleration Gy) is less than the first predetermined quantity gs, so the lower pressure adjustment continues. At this time, the upper pressure adjustment is not performed, so the supply hydraulic pressure Pm remains at "0".

[0106] At time t2, the turning state quantity Gs transitions from a state where it is less than the first predetermined quantity gs to a state where it is equal to or greater than the first predetermined quantity gs. At time t2, the state is reached where "the direction of the steering operation quantity Sa and the direction of the lateral acceleration Gy coincide with each other, and the turning state quantity Gs is equal to or greater than the first predetermined quantity gs", and step S250 is satisfied. Then, at time t2, the lower pressure regulation is switched to the upper pressure regulation. The actual supply hydraulic pressure Pm (=Pe) is increased from "0" by the upper pressure regulation.

[0107] At time t2, the lower pressure adjustment is not immediately terminated. Although the first unit YA can increase the supply hydraulic pressure Pm with high response, there is a time delay in the increase of the supply hydraulic pressure Pm with respect to the target reference hydraulic pressure Pd. In order to compensate for this time delay, the lower pressure adjustment is continued and the hydraulic pressure difference mQ is increased until the actual reference hydraulic pressure Pe coincides with the target reference hydraulic pressure Pd. In other words, until the time delay of the supply hydraulic pressure Pm is eliminated (between time t2 and t3), the reference hydraulic pressure Pe is increased by both the upper pressure adjustment in the first unit YA and the lower pressure adjustment in the second unit YB. The actual reference hydraulic pressure Pe is calculated as the sum of the supply hydraulic pressure Pm and the hydraulic pressure difference mQ, "Pm+mQ". For example, the supply hydraulic pressure Pm is detected by the supply hydraulic pressure sensor PM, and the hydraulic pressure difference mQ is estimated based on the amount of electricity Ib (current value). Furthermore, in a configuration in which an adjusted hydraulic pressure sensor PQ for detecting the adjusted hydraulic pressure Pq is provided, the detected value Pq by the adjusted hydraulic pressure sensor PQ is used as the actual reference hydraulic pressure Pe.

[0108] At time t3, the actual reference hydraulic pressure Pe coincides with the target reference hydraulic pressure Pd. In other words, since the time delay in the upper pressure regulation has been eliminated, the lower pressure regulation is temporarily stopped and the pressure regulation valve UB is opened. This causes the hydraulic pressure difference mQ to decrease toward "0". After this, the reference hydraulic pressure Pe is achieved by the upper pressure regulation alone. Note that when the lower pressure regulation is stopped, it is desirable to continue driving the electric motor MB. This is to ensure the responsiveness of the hydraulic pressure difference mQ when switching from upper pressure regulation to lower pressure regulation again.

[0109] At time t4, the turning state quantity Gs decreases, transitioning from a state equal to or greater than the second predetermined quantity gt to a state less than the second predetermined quantity gt. At time t4, the state is reached where "the turning state quantity Gs is less than the second predetermined quantity gt, and the vehicle speed Vx is less than a predetermined speed vx (or the steering operation quantity Sa is within the range of the predetermined operation quantity sa)", and step S260 is satisfied. Then, at time t4, a switch is made from upper pressure regulation to lower pressure regulation. Accordingly, the supply hydraulic pressure Pm by the first unit YA is decreased toward "0". Meanwhile, the hydraulic pressure difference mQ by the second unit YB is increased from "0". Thereafter, the reference hydraulic pressure Pe is achieved only by the lower pressure regulation.

[0110] At time t5, the stability control termination condition (the judgment process of step S150) is satisfied, and the reference hydraulic pressure Pe is set to 0. Since the reference hydraulic pressure Pe in the stability control is basically generated by the lower pressure adjustment, pressurization is performed only by the lower pressure adjustment at the end of the stability control, just as at the start of the stability control.

[0111] In the stability control, the reference hydraulic pressure Pe is basically generated by adjusting the hydraulic pressure difference mQ by the second unit YB (lower pressure adjustment). This is based on the fact that the second unit YB is easier to adjust the pressure with smoother and higher accuracy than the first unit YA. However, in a situation where a sudden yawing or rolling behavior occurs (i.e., when "Gs≧gs"), the reference hydraulic pressure Pe is generated by the first unit YA, which has superior boost response, instead of the second unit YB. In this way, the first unit YA and the second unit YB are used depending on the vehicle behavior, thereby improving the performance of the vehicle stability control.

[0112] If the second unit YB is configured to be able to respond to sudden changes in behavior, it becomes necessary to increase the capacity of the pressure regulating valve UB, the electric motor MB, and the fluid pump QB (increasing the flow rate of the pressure regulating valve UB, increasing the output of the electric motor MB, and increasing the discharge volume of the fluid pump QB), or to add an additional boost auxiliary device (adding an electric pump, accumulator, etc. used only for boosting pressure). However, in the brake control device SC, this increase in size is avoided by switching from the lower pressure regulation to the upper pressure regulation described above. In other words, the brake control device SC can be made smaller by utilizing the respective characteristics of the first unit YA and the second unit YB.

[0113] In the above example, switching between lower pressure regulation and upper pressure regulation in lane change behavior on a road surface with a high friction coefficient has been described. On a road surface with a low friction coefficient, the requirement for boost responsiveness is not so high. For this reason, it is preferable that this switching not be performed on a low friction road surface. As described above, the turning state quantity Gs is calculated based on at least one of the lateral acceleration Gy, the yaw rate Yr, and the steering operation amount Sa, but it is preferable that at least the lateral acceleration Gy is included as a state quantity for calculating the turning state quantity Gs. Since the lateral acceleration Gy is generated according to the road surface friction coefficient, the turning state quantity Gs remains small on a road surface with a low friction coefficient. As a result, on a road surface with a low friction coefficient, it is difficult to satisfy the condition "Gs≧gs", and stability control is always performed by the lower pressure regulation.

[0114] <Other embodiments> Other embodiments of the braking control device SC will be described below. The other embodiments also provide the same effects as those described above (improved performance of vehicle stability control, i.e., improved boost response in response to sudden changes in vehicle behavior, and maintaining the compact size of the device SC).

[0115] <Use of electric cylinder type pressure source> In the above embodiment, the accumulator hydraulic pressure Pc stored in the accumulator AC by the first electric motor MA is used as the first unit YA (pressurization source). Alternatively, the first electric motor MA may directly drive a piston inserted in a cylinder to increase the braking hydraulic pressure Pw. A so-called electric cylinder type may be adopted as the first unit YA. The configuration of the electric cylinder type is publicly known, for example, from "WO2012 / 046703" and the like, and therefore, the configuration will be described below in brief without being shown in the figures. The electric cylinder type first unit YA is composed of a pressure regulating cylinder, a pressure regulating piston, a linear motion conversion mechanism, and a first electric motor MA.

[0116] A pressure regulating cylinder (also called a "slave cylinder") is provided in addition to the master cylinder CM. The pressure regulating cylinder has the same configuration as the master cylinder CM and is, for example, a tandem type cylinder. Two pressure regulating pistons are inserted into the pressure regulating cylinder via an elastic body (compression spring). One of the two pressure regulating pistons is connected to the first electric motor MA via a linear motion conversion mechanism (for example, a screw mechanism). Here, the linear motion conversion mechanism converts the rotational power of the electric motor MA into linear power (thrust) of the pressure regulating piston.

[0117] The pressure regulating piston is driven by the first electric motor MA. In detail, when the electric motor MA rotates, its power is converted into linear power of the pressure regulating piston by the linear motion conversion mechanism. The inside of the pressure regulating cylinder is divided into two pressure regulating chambers by two pressure regulating pistons and a seal member. The two pressure regulating chambers are connected to the wheel cylinder CW via the communication passage HS and the second unit YB. Therefore, when the electric motor MA is driven, the volume of the pressure regulating chamber is reduced, and the brake fluid BF is pumped from the pressure regulating chamber to the wheel cylinder CW at the supply hydraulic pressure Pm. In other words, in the electric cylinder type first unit YA, the supply hydraulic pressure Pm is directly controlled (adjusted) by adjusting the output of the electric motor MA without using the solenoid valves UZ and UG. In this configuration, the hydraulic pressure Pwx of the selected wheel cylinder CWx is controlled by the electric motor MA. The hydraulic pressure Pwz in the non-selected wheel cylinder CWz is adjusted by the non-selected inlet valve UIz and the non-selected outlet valve VOz.

[0118] <<Another way of setting the target reference hydraulic pressure Pd (i.e., setting it as "Pd>Ptx")>> In the above-described embodiment, the maximum target hydraulic pressure Ptx is determined as the target reference hydraulic pressure Pd (i.e., "Pd = Ptx"). Then, the hydraulic pressure Pwx (= Pe) of the selection wheel cylinder CWx is adjusted (i.e., increased, held, or decreased) by the first unit YA or the pressure source KB (particularly, the pressure regulating valve UB). Also, the hydraulic pressure Pwz of the non-selection wheel cylinder CWz is adjusted by the non-selection inlet valve UIz and the non-selection outlet valve VOz within the range from "0" to the reference hydraulic pressure Pe with the reference hydraulic pressure Pe (= Pwx) as the base pressure. Instead of this, the target reference hydraulic pressure Pd may be determined to be greater than the maximum target hydraulic pressure Ptx. For example, the target reference hydraulic pressure Pd is calculated as a value that is greater than the maximum target hydraulic pressure Ptx by a predetermined pressure pd (a constant set in advance) (i.e., "Pd = Ptx + pd"). In this case, since the reference hydraulic pressure Pe is greater than the maximum target hydraulic pressure Ptx, the hydraulic pressure Pw of all the wheel cylinders CW including the selection wheel cylinder CWx is adjusted by the inlet valve UI and the outlet valve VO within the range from "0" to the reference hydraulic pressure Pe. Even with this configuration, when "Gs < gs", the target reference hydraulic pressure Pd is achieved by the second unit YB (particularly, the pressure source KB), and the actual reference hydraulic pressure Pe (= mQ) is adjusted. On the other hand, when "Gs ≧ gs", the target reference hydraulic pressure Pd is achieved by the first unit YA, and the actual reference hydraulic pressure Pe (= Pm) is adjusted.

[0119] ≪Diagonal Fluid Passage≫ In the above-described embodiment, a front-rear type configuration is adopted as the two systems of the brake fluid passages. Instead of this, a diagonal type (also referred to as "X type") brake system may be adopted. In this configuration, of the two hydraulic pressure chambers formed in the master cylinder CM (or the pressure regulating cylinder), one side is connected to the right front wheel cylinder and the left rear wheel cylinder, and the other side is connected to the left front wheel cylinder and the right rear wheel cylinder.

[0120] <Summary of Embodiment of Brake Control Device SC and Its Operation and Effects> The embodiments of the braking control device SC are summarized below. The braking control device SC executes stability control to improve the stability of the vehicle by individually increasing the braking fluid pressure Pw of wheel cylinders CW provided in a plurality of wheels WH. The braking control device SC includes: "a first unit YA composed of pistons NP and NS moved by using a first electric motor MA as a power source and a cylinder CM into which the pistons NP and NS are inserted, and increasing the braking fluid pressure Pw by discharging the braking fluid BF from the cylinder CM by the movement of the pistons NP and NS"; "a pressure regulating valve UB provided in a communication path HS connecting the first unit YA and the wheel cylinder CW, and a fluid pump QB driven by a second electric motor MB different from the first electric motor MA, and increasing the braking fluid pressure Pw by restricting the flow of the braking fluid BF discharged by the fluid pump QB by the pressure regulating valve UB"; and "a controller ECU that controls the first and second units YA and YB to execute stability control".

[0121] In the braking control device SC, the controller ECU calculates a turning state quantity Gs (a state variable representing the probability of the occurrence of a sudden vehicle behavior) representing the degree of lateral movement of the vehicle JV. When the turning state quantity Gs is less than a predetermined quantity gs, the reference fluid pressures Pd (target value) and Pe (actual value) corresponding to the maximum fluid pressure Ptx required for stability control are supplied by the second unit YB. On the other hand, when the turning state quantity Gs is greater than or equal to the predetermined quantity gs, the reference fluid pressures Pd and Pe are supplied by the first unit YA. For example, when "Gs < gs", the controller ECU supplies the reference fluid pressures Pd and Pe only by the second unit YB, and when "Gs ≧ gs", the controller ECU supplies the reference fluid pressures Pd and Pe only by the first unit YA.

[0122] Here, the reference hydraulic pressures Pd and Pe are the hydraulic pressures that serve as the basis for adjusting the braking hydraulic pressure Pw in stability control. In other words, the braking hydraulic pressure Pw of each wheel cylinder CW can be controlled within the range of "0 (atmospheric pressure)" or higher and the reference hydraulic pressure Pe or lower. The target reference hydraulic pressure Pd is determined as "Pd = Ptx" or "Pd > Ptx" based on the maximum target hydraulic pressure Ptx. Therefore, the reference hydraulic pressures Pd and Pe are hydraulic pressures higher than the maximum hydraulic pressures Ptx and Pwx. Specifically, in terms of the reference hydraulic pressure, it is equal to or higher than the maximum target hydraulic pressure Ptx at the target value Pd and equal to or higher than the selective braking hydraulic pressure Pwx at the actual value Pe (i.e., "Pd ≥ Ptx, Pe ≥ Pwx").

[0123] In the braking control device SC, after starting stability control, until the turning state quantity Gs reaches a predetermined quantity gs, the controller ECU supplies the reference hydraulic pressures Pd and Pe by the second unit YB, and after the turning state quantity Gs reaches the predetermined quantity gs, the controller ECU supplies the reference hydraulic pressures Pd and Pe by the first unit YA. For example, after starting stability control, the controller ECU supplies the reference hydraulic pressures Pd and Pe only by the second unit YB while "Gs < gs", but after the turning state quantity Gs reaches the predetermined quantity gs, the controller ECU supplies the reference hydraulic pressures Pd and Pe only by the first unit YA.

[0124] The braking control device SC is provided with two pressure sources, namely the first and second units YA and YB. The first unit YA is excellent in boosting responsiveness, and the second unit YB is excellent in pressure regulation performance. Based on the characteristics of the first and second units YA and YB, in stability control, usually, the reference hydraulic pressure supply (lower pressure regulation) by the second unit YB is performed. Then, based on the turning state quantity Gs, the supply of the reference hydraulic pressure (upper pressure regulation) by the first unit YA is switched from the lower pressure regulation. Here, the turning state quantity Gs is a state quantity representing the possibility of the occurrence of sudden vehicle behavior (yawing behavior, rolling behavior). When the turning state quantity Gs is large (i.e., in the case of "Gs≧gs"), the probability of the occurrence of sudden yawing behavior or the like is high. For this reason, the switching from the lower pressure regulation to the upper pressure regulation is performed. Since the boosting responsiveness in the braking control device SC is improved, even when a sudden behavior change occurs, the vehicle stability is surely ensured and its control performance is improved. When the turning state quantity Gs is small (i.e., in the case of "Gs<gs"), since the probability of the occurrence of sudden behavior is low, the switching is not performed and the lower pressure regulation is continued.

[0125] In the controller ECU, the operation speed dS of the steering operation member SH is acquired. For example, the operation speed dS is calculated by differentiating the operation amount (steering angle) Sa with respect to time. The larger the operation speed dS (for example, the steering angular velocity), the smaller the predetermined quantity gs is calculated so that the predetermined quantity gs becomes smaller. The more likely sudden vehicle behavior is to occur as the operation speed dS is larger. Since the predetermined quantity gs is changed based on the operation speed dS, the switching to the upper pressure regulation is performed earlier. As a result, appropriate switching is performed even for sudden behavior changes.

[0126] In the controller ECU, based on the operation speed dS, the operation speed peak value dSp is calculated. And the larger the operation speed peak value dSp is, the smaller the predetermined quantity gs may be calculated so that the predetermined quantity gs becomes smaller. The turning state quantity Gs occurs with a time delay with respect to the operation speed dS. By adopting the operation speed peak value dSp, this time delay can be compensated.

[0127] In the controller ECU, when switching from lower pressure regulation to upper pressure regulation, the reference hydraulic pressure Pe (i.e., hydraulic pressure difference mQ) continues to increase by lower pressure regulation until the actual reference hydraulic pressure Pe coincides with the target reference hydraulic pressure Pd. Then, at the point when the actual reference hydraulic pressure Pe reaches the target reference hydraulic pressure Pd, the lower pressure regulation is stopped. Although the upper pressure regulation has excellent pressure increase responsiveness, there still exists a time delay. At the time of switching, the pressure increase by the lower pressure regulation continues, so that the time delay is compensated for. [Explanation of symbols]

[0128] JV...vehicle, SC...brake control device, SX...braking device, CP...brake caliper, CW...wheel cylinder, KT...rotating member (brake disc), MS...friction member (brake pad), ECU...braking controller, HU...fluid unit, YA...first unit (upper unit), CM...master cylinder, MA...electric motor (first electric motor), YB...second unit (lower unit), UB...pressure regulating valve, UI...inlet valve, VO...outlet valve, MB...electric motor (second electric motor), QB...fluid pump, PM...supply fluid pressure sensor, Pm...supply fluid pressure (detected value of supply fluid pressure sensor PM), Pq...adjusted fluid pressure, Pw...braking fluid pressure, mQ...fluid pressure difference (difference between adjusted fluid pressure Pq and supply fluid pressure Pm), Pt...target fluid pressure (each wheel cylinder), Ptx...maximum target fluid pressure (target the maximum target hydraulic pressure Pt), Pd...reference hydraulic pressure (target value), Pe...reference hydraulic pressure (actual value), Ib...amount of current (actual amount of current supplied to the pressure regulating valve UB), Ibt...target amount of current (target value corresponding to the amount of current Ib), CWx...selected wheel cylinder (of multiple wheel cylinders, the wheel cylinder corresponding to the maximum target hydraulic pressure Ptx), CWz...non-selected wheel cylinder (of multiple wheel cylinders, the wheel cylinder that does not correspond to the selected wheel cylinder CWx), Js...stability index, Gs...turning state quantity, gs...first predetermined quantity (threshold value for switching from the lower pressure regulating to the upper pressure regulating), gt...second predetermined quantity (threshold value for switching from the upper pressure regulating to the lower pressure regulating), Sa...steering operation quantity (steering angle), dS...steering speed (differential value of the steering operation quantity Sa), dSp...steering speed peak value.

Claims

1. A vehicle brake control device that performs stability control to improve stability of a vehicle by individually increasing brake fluid pressures in wheel cylinders provided on a plurality of wheels of the vehicle, comprising: a first unit including a piston that is moved using a first electric motor as a power source and a cylinder into which the piston is inserted, the first unit increasing the brake fluid pressure by discharging brake fluid from the cylinder as the piston moves; a second unit including a pressure regulating valve provided in a communication passage connecting the first unit and the wheel cylinder, and a fluid pump driven by a second electric motor separate from the first electric motor, the second unit increasing the brake fluid pressure by throttling a flow of brake fluid discharged by the fluid pump using the pressure regulating valve; a controller for controlling the first and second units to perform the stability control; Equipped with A vehicle braking control device, wherein when a turning state quantity representing the degree of lateral movement of the vehicle is less than a predetermined amount, the controller supplies a reference hydraulic pressure corresponding to a maximum hydraulic pressure required for the stability control via the second unit, and when the turning state quantity is equal to or greater than the predetermined amount, the controller supplies the reference hydraulic pressure via the first unit.

2. A vehicle brake control device that performs stability control to improve stability of a vehicle by individually increasing brake fluid pressures in wheel cylinders provided on a plurality of wheels of the vehicle, comprising: a first unit including a piston that is moved using a first electric motor as a power source and a cylinder into which the piston is inserted, the first unit increasing the brake fluid pressure by discharging brake fluid from the cylinder as the piston moves; a second unit including a pressure regulating valve provided in a communication passage connecting the first unit and the wheel cylinder, and a fluid pump driven by a second electric motor separate from the first electric motor, the second unit increasing the brake fluid pressure by throttling a flow of brake fluid discharged by the fluid pump using the pressure regulating valve; a controller for controlling the first and second units to perform the stability control; Equipped with A vehicle braking control device, wherein the controller supplies a reference hydraulic pressure corresponding to a maximum hydraulic pressure required for the stability control through the second unit until a turning state quantity representing the degree of lateral movement of the vehicle reaches a predetermined amount, and supplies the reference hydraulic pressure through the first unit after the turning state quantity reaches the predetermined amount.

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