Vehicle braking control device

Abstract

To provide a braking control device of a vehicle that can execute turning support control by which a turning radius of the vehicle is reduced, which can achieve both durability and reduction in size and weight.SOLUTION: A braking control device executes turning support control by which a turning radius of a vehicle is reduced by applying braking torque to a turning inner wheel of the vehicle. The braking control device comprises: a first unit that increases liquid pressure of a wheel cylinder and applies braking torque; a second unit that applies braking torque independently from the first unit; and a controller that controls the first unit and the second unit. The controller makes only the first unit apply braking torque when starting the turning support control, and performs specific determination of whether an operation situation of the first unit is severe or not after starting the turning support control, and when affirming the specific determination, makes the second unit apply braking torque.SELECTED DRAWING: Figure 7

Description

Technical Field

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

Background Art

[0002] In Patent Document 1, in order to turn a vehicle with a smaller turning radius regardless of the vehicle and road surface conditions, "when a turning assist control request occurs while the vehicle is traveling at an extremely low speed, the wheel speed of the rear wheel on the inside of the turn is zero. When it is assumed that the first rotational moment M1 generated in the vehicle with the rear wheel on the inside of the turn as a fulcrum and the second rotational moment M2 generated in the vehicle with the front wheel on the inside of the turn as a fulcrum when the wheel speed of the front wheel on the inside of the turn is assumed to be zero are calculated respectively. Further, when it is determined that the magnitude |M1| of the first rotational moment is greater than or equal to the magnitude |M2| of the second rotational moment, the target wheel speed of the rear wheel on the inside of the turn is set to zero, and when it is determined that the magnitude |M1| of the first rotational moment is less than the magnitude |M2| of the second rotational moment, the target wheel speed of the front wheel on the inside of the turn is set to zero, and turning assist control is executed" is described.

[0003] In Patent Document 1, for reducing the turning radius of the vehicle, the target wheel speed of the front or rear wheel on the inside of the turn is set to zero, and the wheel is locked. By the way, in a braking control device that uses the pressure (hydraulic pressure) of the brake fluid, a certain level of high hydraulic pressure is required to cope with wheel lock. In the braking control device, a situation where a high hydraulic pressure continues to act for a long time is assumed, and its durability must be determined. In the braking control device, there is a trade-off relationship between the robustness of the device and its size and weight. For this reason, in the braking control device, it is desired to achieve miniaturization and weight reduction while ensuring durability.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] An object of the present invention is to provide a braking control device for a vehicle capable of executing turning assistance control for reducing the turning radius of the vehicle, in which the durability of the device and the reduction in size and weight can be achieved simultaneously.

Means for Solving the Problems

[0006] The braking control device according to the present invention performs turning assistance control for reducing the turning radius of the vehicle by applying a braking torque (Tqu) to the inner turning wheel (WHu) of the vehicle, and increases the hydraulic pressure of the wheel cylinder (CW) of the vehicle to apply the braking torque (Tqu). It includes a first unit (YA), a second unit (YB) that applies the braking torque (Tqu) separately from the first unit (YA), and a controller (ECU) that controls the first unit (YA) and the second unit (YB).

[0007] In the braking control device according to the present invention, when starting the turning assistance control, the controller (ECU) applies the braking torque (Tqu) only by the first unit (YA), and after starting the turning assistance control, determines whether the operating state of the first unit (YA) is severe. If the determination (S230) is affirmative, the second unit (YB) applies the braking torque (Tqu). Specifically, from the time point (u6) when the determination (S230) is affirmative, the controller (ECU) decreases the first component (Ta) of the braking torque (Tqu) by the first unit (YA) and increases the second component (Tb) of the braking torque (Tqu) by the second unit (YB).

[0008] The braking control device SC is configured to include two different units, namely the first and second units YA and YB. When the load state of the first unit YA becomes severe (i.e., when the specific determination is affirmed), the load is shared by the second unit YB. Therefore, in the braking control device SC capable of executing turning assistance control for reducing the turning radius of the vehicle, durability and miniaturization and weight reduction can be preferably achieved simultaneously.

Brief Description of Drawings

[0009]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Embodiments for Carrying Out the Invention

[0010] Hereinafter, embodiments of the braking control device SC for a vehicle according to the present invention will be described with reference to the drawings.

[0011] <Symbols of Components, etc.> In the following description, components such as members, signals, values, etc. with the same symbol, such as "CW", have the same function. The subscripts "f" and "r" attached to the ends of various symbols related to the wheels are collective symbols indicating whether the element is related to the front wheel or the rear wheel. Specifically, "f" indicates an element related to the front wheel, and "r" indicates an element related to the rear wheel. For example, in the wheel cylinder CW, it is denoted as the front-wheel wheel cylinder CWf and the rear-wheel wheel cylinder CWr. Further, the subscripts "f" and "r" may be omitted. When these are omitted, each symbol represents its general term.

[0012] <Vehicle JV equipped with braking control device SC> Referring to the configuration diagram of FIG. 1, the entire vehicle JV equipped with the braking control device SC according to the present invention will be described. The vehicle JV is provided with an acceleration operation member AP, a braking operation member BP, a steering operation member SH, and various sensors (BA, etc.). The acceleration operation member (for example, an accelerator pedal) AP is a member that the driver operates to accelerate the vehicle JV and control the speed of the vehicle JV (vehicle body speed Vx). The braking operation member (for example, a brake pedal) BP is a member that the driver operates to decelerate the vehicle JV. The steering operation member (for example, a steering wheel) SH is a member that the driver operates to turn the vehicle JV.

[0013] The vehicle JV is provided with various sensors listed below. The detection signals (Ba, etc.) of these sensors are input to a controller ECU for braking (also simply referred to as the "braking controller") described later. - An acceleration operation amount sensor AA that detects the operation amount (acceleration operation amount) Aa of the acceleration operation member AP, a braking operation amount sensor BA that detects the operation amount (braking operation amount) Ba of the braking operation member BP, and a steering operation amount sensor SA that detects the operation amount (steering operation amount, for example, steering angle) Sa of the steering operation member SH. - A wheel speed sensor VW that detects the rotational speed (wheel speed) Vw of the wheel WH. - In the vehicle JV (particularly, the vehicle body), a yaw rate sensor YR for detecting the yaw rate Yr, a longitudinal acceleration sensor GX for detecting the longitudinal acceleration Gx, and a lateral acceleration sensor GY for detecting the lateral acceleration Gy.

[0014] In addition, various switches such as a switch XC for crawl control and a switch XA for turning assist control are provided. These switches XC and XA are operated by the driver. Then, an operation signal Xc (crawl control signal) from the crawl control switch XC and an operation signal Xa (turning assist control signal) from the turning assist control switch XA are input to the brake controller ECU.

[0015] The vehicle JV is equipped with a braking device SX and a brake control device SC. In the brake control device SC, a so-called front-rear type (also referred to as "Type II") is adopted as the two braking systems.

[0016] The braking device SX is supplied with a brake fluid pressure Pw generated by the brake control device SC. Then, by the braking device SX, a braking torque Tq is applied to the wheel WH according to the brake fluid pressure Pw, and a braking force Fx is generated. The braking device SX includes a rotating member (e.g., a brake disk) 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. A wheel cylinder CW is provided in the brake caliper CP. The wheel cylinder CW is supplied with a brake fluid BF adjusted to the brake fluid pressure Pw from the brake control device SC. By the brake fluid pressure Pw, a friction member (e.g., a brake pad) MS is pressed against the rotating member KT. Since the rotating member KT and the wheel WH are fixed to rotate integrally, a braking torque Tq (and as a result, a braking force Fx) is generated on the wheel WH by the frictional force generated at this time.

[0017] The braking control device SC adjusts the actual braking hydraulic pressure Pw according to the operation amount Ba of the braking operation member BP, and supplies the braking hydraulic pressure Pw to the braking device SX (especially the wheel cylinder CW) via the front and rear wheel connecting paths HSf and HSr. The braking control device SC is composed of a master cylinder CM, a fluid unit HU, and a braking controller ECU. The fluid unit HU is composed of two units (the first and second units) YA and YB. The components of the braking control device SC (such as the solenoid valves and electric motors 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 electric motors. Each of the braking controller ECU, the prime mover controller ECP (described later), and the power transmission controller ECT (described later) is connected to the communication bus BS. Therefore, information (detection values, calculation values) is shared among these controllers via the communication bus BS. For example, in the braking controller ECU, the vehicle body speed Vx is calculated based on the wheel speed Vw. The vehicle body speed Vx is transmitted to other controllers through the communication bus BS. The braking controller ECU receives inputs such as the acceleration operation amount Aa, the braking operation amount Ba, the steering operation amount Sa, the yaw rate Yr, the longitudinal acceleration Gx, the lateral acceleration Gy, the wheel speed Vw, the operation signal Xc (for crawl control), and the operation signal Xa (for turning assist control). Based on these signals, the fluid unit HU is controlled by the braking controller ECU. Details of the braking control device SC will be described later.

[0018] The vehicle JV is equipped with a prime mover control device GC and a power transmission device TS. The vehicle JV is a four-wheel drive vehicle in which all four wheels WH are drive wheels (wheels to which the drive torque Td is transmitted to generate the driving force Fd).

[0019] The prime mover control device GC is composed of a prime mover PG and a controller ECP for the prime mover that controls it (simply referred to as the "prime mover controller" as well). The prime mover PG is a general term for devices that convert various energies existing in nature into mechanical work (mechanical energy). Taking the case where an internal combustion engine (gasoline engine) is adopted as the prime mover PG as an example for explanation. The prime mover PG generates power (driving torque Td) for driving four wheels WH. The prime mover PG is controlled by a prime mover controller (engine controller) ECP, and its output is adjusted. Specifically, the prime mover PG includes a throttle device TH, a fuel injection device FI, and an engine speed sensor NE. The throttle opening Th is controlled by the throttle device TH, and the fuel injection amount Fi is controlled by the fuel injection device FI respectively. And based on the engine speed Ne detected by the speed sensor NE, at least one of the throttle opening Th and the fuel injection amount Fi is controlled by the prime mover controller ECP. As a result, the output of the prime mover PG is adjusted.

[0020] The output (rotational power) of the prime mover control device GC (especially the prime mover PG) is input to the power transmission device TS. And the output of the prime mover PG is transmitted to the four wheels WH via the power transmission device TS, and a driving force Fd is generated at each of the wheels WH. The power transmission device TS includes a power transmission mechanism TD and a controller ECT for power transmission that controls it (simply referred to as the "power transmission controller" as well). The power transmission mechanism TD is composed of a main transmission MH, a sub-transmission FH, a front-wheel differential mechanism DF, a center differential mechanism DC, and a rear-wheel differential mechanism DR. The main transmission MH is an automatic transmission that shifts gears according to the driving state of the vehicle. The output of the prime mover PG is input to the sub-transmission FH via the main transmission MH. The sub-transmission FH enables switching between a high-speed gear and a low-speed gear for four-wheel drive.

[0021] The output from the auxiliary transmission FH is input to the respective differential mechanisms DF (front-wheel differential gear), DC (center differential gear), and DR (rear-wheel differential gear). The front-wheel drive torque Tdf is transmitted to the left and right front wheels WHf via the front-wheel differential mechanism DF and the front-wheel drive shaft. Also, the rear-wheel drive torque Tdr is transmitted to the left and right rear wheels WHr via the center differential mechanism DC, the rear-wheel differential mechanism DR, and the rear-wheel drive shaft. Since the power generated by the prime mover PG is transmitted to the front wheels WHf and the rear wheels WHr via the differential mechanisms DF, DC, and DR, a rotational speed difference (i.e., differential) between the respective wheels WH is allowed. Each component (such as MH) of the power transmission mechanism TD is controlled by the power transmission controller ECT. Specifically, the main transmission MH, the auxiliary transmission FH, and the differential mechanisms DF, DC, and DR are each controlled by the power transmission controller ECT.

[0022] <First Configuration Example of the First Unit YA> Referring to the schematic diagram of FIG. 2, a first configuration example of the first unit YA included in the fluid unit HU will be described. The first unit YA is a pressurizing source for increasing the hydraulic pressure (brake hydraulic pressure) Pw of the four wheel cylinders CW. In the example, the first unit YA is integrated with the master cylinder CM. And a front-rear type brake piping system is adopted. The first unit YA is composed of an apply unit AU including the master cylinder CM and a pressurizing unit KU. The apply unit AU and the pressurizing unit KU are controlled by the brake controller ECU. Specifically, the brake 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 are input to the controller ECU, and based on these signals, the drive signal Vn of the input valve VN, the drive signal Vr of the release valve VR, the drive signal Uz of the pressure increasing valve UZ, the drive signal Ug of the pressure reducing valve UG, and the drive signal Ma of the electric motor MA for pressure accumulation are calculated. Then, the solenoid valves "VN, VR, UZ, UG" constituting the first unit YA and the electric motor MA for pressure accumulation are controlled (driven) according to the drive signals "Vn, Vr, Uz, Ug, Ma".

[0023] As will be described later, the fluid unit HU, wheel cylinder CW, etc. are connected by the communication path HS, input path HN, pressure reducing path HG, reflux path HK, and servo path HV. These are fluid paths through which the brake fluid BF is moved. Examples of the fluid path (HS, etc.) include fluid piping, flow paths within the fluid unit HU, hoses, and the like.

[0024] ≪Apply Unit AU≫ The apply unit AU is composed of a master reservoir RV, master cylinder CM, first and second master pistons NP, NS, first and second master springs DP, DS, input cylinder CN, input piston NN, input spring DN, input valve VN, release valve VR, stroke simulator SS, and simulator hydraulic pressure sensor PS.

[0025] The master reservoir RV (also referred to as the "atmospheric pressure reservoir") is a tank for the working liquid, and the brake fluid BF is stored therein. The master reservoir RV is connected to the master cylinder CM (particularly, the front and rear wheel master chambers Rmf, Rmr).

[0026] The master cylinder CM is a cylinder member having a bottom. Inside the master cylinder CM, the first and second master pistons NP, NS are inserted, and the inside is sealed by a seal member SL and divided into the front and rear wheel master chambers Rmf, Rmr. The master cylinder CM is of the so-called tandem type. The front and rear wheel master chambers Rmf, Rmr (= Rm) are finally connected to the front and rear wheel cylinder CWf, CWr (= CW) via the front and rear wheel communication paths HSf, HSr (= HS) and the second unit YB, respectively. 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), the brake fluid BF with a hydraulic pressure Pm (referred to as the "supply hydraulic pressure" and being the "front and rear wheel supply 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 supply hydraulic pressure Pmf and the rear wheel supply hydraulic pressure Pmr are equal.

[0027] The first master piston NP is provided with a flange portion (flange) Tp. By this flange portion Tp, the inside of the master cylinder CM is further partitioned into a servo chamber Ru and a rear chamber Ro. The servo chamber Ru is arranged to face the front wheel master chamber Rmf with the first master piston NP interposed therebetween. Also, the rear chamber Ro is sandwiched between the front wheel master chamber Rmf and the servo chamber Ru and is disposed therebetween. The servo chamber Ru and the rear chamber Ro are also sealed by the seal member SL in the same manner as described above.

[0028] The input cylinder CN is fixed to the master cylinder CM. An input piston NN is inserted into the input cylinder CN and sealed by the seal member SL, thereby forming an input chamber Rn. The input piston NN is mechanically connected to the braking operation member BP via a clevis (U-shaped link).

[0029] The apply unit AU is provided with the input chamber Rn, the servo chamber Ru, the rear chamber Ro, and the hydraulic pressure chambers of the front and rear wheel master chambers Rmf, Rmr. Here, the "hydraulic pressure chamber" is a chamber filled with the braking fluid BF and sealed by the seal member SL. The volume of each hydraulic pressure chamber is changed by the movement of the input piston NN, the first and second master pistons NP, NS.

[0030] The input chamber Rn and the rear chamber Ro are connected via an input passage HN. And an input valve VN is provided in the input passage HN. The input passage HN is connected to the master reservoir RV via an open valve VR between the rear chamber Ro and the input valve VN. The input valve VN and the open valve VR are two-position solenoid valves (also referred to as "on-off valves") having an open position (communicating state) and a closed position (blocking state). A normally-closed solenoid valve is adopted as the input valve VN. A normally-open solenoid valve is adopted as the open valve VR. The input valve VN and the open valve VR are driven (controlled) by drive signals Vn, Vr from the braking controller ECU.

[0031] A stroke simulator (also simply referred to as a "simulator") SS is connected to the rear chamber Ro. The simulator SS generates an operating force Fp of the braking operation member BP. Inside the simulator SS, a piston and an elastic body (for example, a compression spring) are provided. When the braking fluid BF flows into the simulator SS, the piston is pushed by the braking fluid BF. Since a force is applied to the piston in a direction to prevent the inflow of the braking fluid BF by the elastic body, the operating force Fp of the braking operation member BP is generated. That is, the operating characteristics (the relationship between the operating displacement Sp and the operating force Fp) of the braking operation member BP are formed by the simulator SS.

[0032] A simulator hydraulic pressure sensor PS is provided to detect the hydraulic pressure of the simulator SS (which is the simulator hydraulic pressure and also the hydraulic pressure of the input chamber Rn and the rear chamber Ro) Ps. The simulator hydraulic pressure sensor PS is one of the above-mentioned braking operation amount sensors BA. The simulator hydraulic pressure Ps is input as the braking operation amount Ba to the controller ECU for braking.

[0033] In the first unit YA, in addition to the simulator hydraulic pressure sensor PS, as a braking operation amount sensor BA, an operation displacement sensor SP for detecting the operation displacement Sp of the braking operation member BP and / or an operation force sensor FP for detecting the operation force Fp of the braking operation member BP are provided. 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.

[0034] ≪Pressurizing Unit KU≫ The pressurizing unit KU generates and adjusts the supply hydraulic pressure Pm. The pressurizing unit KU is composed of a hydraulic accumulator pump QA, a hydraulic accumulator electric motor MA, an accumulator AC, an accumulator hydraulic pressure sensor PC, a pressurizing cylinder CK, a pressurizing piston NK, a pressure increasing valve UZ, a pressure reducing valve UG, and a servo hydraulic pressure sensor PU.

[0035] The pressurizing unit KU is provided with a fluid pump QA for pressure accumulation so as to accumulate the accumulator AC. The fluid pump QA for pressure accumulation is driven by an electric motor MA for pressure accumulation and pumps up 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 to detect the accumulator hydraulic pressure Pc. The electric motor MA for pressure accumulation is controlled by the brake controller ECU so that the accumulator hydraulic pressure Pc is maintained within a predetermined range.

[0036] The pressurizing unit KU is provided with a pressure cylinder CK 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 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 opposite side of 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.

[0037] An annular recess (constriction) is provided on the outer peripheral portion of the pressure piston NK. The annular chamber Rv is formed by this annular recess and the inner peripheral portion of the pressure cylinder CK. Further, a valve body Vv (for example, a spool valve) is formed on the outer peripheral portion of the pressure piston NK. And the brake fluid BF pressurized from the accumulator AC to the accumulator hydraulic pressure Pc is supplied to this valve body Vv. By the valve body Vv, the accumulator hydraulic pressure Pc is adjusted and introduced into the annular chamber Rv. The annular chamber Rv communicates with the pressurizing chamber Rk through a through hole provided in the pressure 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 referred to as "servo hydraulic pressure Pu".

[0038] Specifically, when the pressure 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, the braking fluid BF is supplied from the accumulator AC through the valve body Vv of the pressure 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 pressure chamber Rk) match. That is, the high-pressure 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 finally controls the servo hydraulic pressure Pu to match the target value. Since the pressure chamber Rk and the servo chamber Ru are connected by a fluid passage, the braking fluid BF adjusted to the servo hydraulic pressure Pu is supplied from the pressurizing unit KU to the servo chamber Ru.

[0039] <Second Unit YB> With reference to the schematic diagram of FIG. 3, a configuration example of the second unit YB included in the fluid unit HU will be described. The second unit YB has a different power source (drive source) from the first unit YA. For example, the second unit YB is provided between the first unit YA and the wheel cylinder CW in the communication path HS (fluid path for moving the braking fluid BF). The brake control device SC can adjust (increase, hold, decrease) the supply hydraulic pressure Pm by the second unit YB. That is, the hydraulic pressure Pw of the wheel cylinder CW is finally adjusted by the second unit YB. For example, the second unit YB is used for anti-lock brake control (control to suppress locking of the wheel WH), traction control (control to suppress wheel spin of the wheel WH), and vehicle stability control (control to suppress excessive understeer and oversteer). The second unit YB is composed of a supply hydraulic pressure sensor PM, a pressure regulating valve UB, a fluid pump QB for reflux, an electric motor MB for reflux, a pressure regulating reservoir RC, an inlet valve UI, and an outlet valve VO.

[0040] Similar to the first unit YA, the second unit YB is also controlled by the brake controller ECU. Specifically, in the controller ECU, based on the various signals (such as Ba) described above, drive signals Ub for the pressure regulating valve UB, Ui for the inlet valve UI, Vo for the outlet valve VO, and Mb for the electric motor MB for reflux are calculated. Then, according to these drive signals (such as Ub), the solenoid valves "UB, UI, VO" and the electric motor MB for reflux that make up the second unit YB are controlled (driven).

[0041] The front-wheel and rear-wheel pressure regulating valves UBf, UBr (= UB) are provided in the front-wheel and rear-wheel connecting paths 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 connecting path HS close to the first unit YA) and the lower part of the pressure regulating valve UB (the part of the connecting path HS close to the wheel cylinder CW) are connected by the front-wheel and rear-wheel reflux paths HKf, HKr (= HK). The reflux path HK is provided with front-wheel and rear-wheel reflux fluid pumps QBf, QBr (= QB) and front-wheel and rear-wheel pressure regulating reservoirs RCf, RCr (= RC). The reflux fluid pump QB is driven by the electric motor MB for reflux. A supply hydraulic pressure sensor PM is provided at the upper part of the pressure regulating valve UB to detect the actual hydraulic pressure (supply hydraulic pressure) Pm supplied by the first unit YA.

[0042] When the electric motor MB is rotationally driven, the fluid pump QB sucks the braking fluid BF from above the pressure regulating valve UB and discharges the braking fluid BF below the pressure regulating valve UB. As a result, a reflux KN of the braking fluid BF (i.e., the front wheel and rear wheel refluxes KNf and KNr, which are the flowing of the circulating braking fluid BF) including the pressure regulating reservoir RC occurs in the communication path HS and the reflux path HK. When the reflux KN of the braking fluid BF is restricted by the pressure regulating valve UB, due to the orifice effect, the hydraulic pressure Pq (referred to as the "regulated hydraulic pressure") below the pressure regulating valve UB is increased from the hydraulic pressure Pm (supply hydraulic pressure) above the pressure regulating valve UB. In other words, the hydraulic pressure difference mQ (also referred to as the "differential pressure") between the supply hydraulic pressure Pm and the regulated hydraulic pressure Pq is adjusted by the second unit YB. In the second unit YB, the reflux electric motor MB, the reflux fluid pump QB, and the pressure regulating valve UB are referred to as the "pressure source KB". The second unit YB (particularly, the pressure source KB) has a power source different from that of the first unit YA (particularly, the pressurizing unit KU).

[0043] Inside the second unit YB, the front wheel and rear wheel communication paths HSf and HSr are each branched into two and connected to the front wheel and rear wheel wheel cylinders CWf and CWr, respectively. And for each wheel cylinder CW, an inlet valve UI and an outlet valve VO are provided. The inlet valve UI (electromagnetic 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 open in different directions. Specifically, since the pressure regulating valve UB opens corresponding to the flow of the braking fluid BF from the wheel cylinder CW to the master cylinder CM, 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, since the inlet valve UI opens corresponding to the flow from the master cylinder CM to the wheel cylinder CW, in the pressure regulation by the inlet valve UI, the braking fluid pressure Pw is equal to or lower than the regulated hydraulic pressure Pq (i.e., "Pq≧Pw").

[0044] The inlet valve UI is provided in the branched communication path HS (i.e., the side closer to the wheel cylinder CW with respect to the branch portion of the communication path HS). The communication path HS is connected to the pressure regulating reservoir RC via a pressure reducing path HG at the lower part of the inlet valve UI (the portion of the communication path HS closer to the wheel cylinder CW). And an outlet valve VO, which is a normally closed on-off valve, is arranged in the pressure reducing path HG.

[0045] The inlet valve UI and the outlet valve VO are individually controlled so that the braking hydraulic pressure Pw is adjusted separately for each wheel cylinder CW. To decrease the braking hydraulic pressure Pw, the inlet valve UI is closed and the outlet valve VO is opened. The inflow of the braking fluid BF into the wheel cylinder CW is blocked and the braking fluid BF in the wheel cylinder CW flows out to the pressure regulating reservoir RC, so the braking hydraulic pressure Pw is decreased. To increase the braking hydraulic pressure Pw, the inlet valve UI is opened and the outlet valve VO is closed. The outflow of the braking 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 the braking hydraulic pressure Pw is increased. To hold the braking hydraulic pressure Pw, both the inlet valve UI and the outlet valve VO are closed. Since the wheel cylinder CW is fluidly sealed, the braking hydraulic pressure Pw is maintained constant.

[0046] <Turn Assist Control> The turn assist control will be described. The "turn assist control" reduces the turning radius of the vehicle JV by applying a braking torque Tqu to the inner turning wheel WHu. For example, the turn assist control is executed on the premise that the crawl control is being executed. Here, the "crawl control" maintains the vehicle body speed Vx at a low speed and constant on an unpaved road (also referred to as "off-road").

[0047] ≪Crawl Control≫ First, the crawl control will be described. The crawl control controls the braking force Fx and the driving force Fd by the braking control device SC and the prime mover control device GC even when the acceleration operation member (accelerator pedal) AP and the braking operation member (brake pedal) BP are not operated, and maintains the vehicle body speed Vx at a predetermined set vehicle speed vc. The crawl control is instructed by an operation signal Xc (switch signal for crawl control) from a crawl control switch XC operated by the driver. When the operation signal (switch signal) Xc indicates an on state, the crawl control is executed, but when the operation signal Xc is in an off state, it is not executed. In addition to instructing the necessity of executing the crawl control by the switch XC, the set speed vc by the crawl control is also instructed. That is, the operation signal Xc includes information on the target value (set speed) vc of the vehicle body speed Vx of the vehicle JV. In the crawl control, the skidding of the vehicle JV is suppressed, and the vehicle body speed Vx coincides with and is maintained at a predetermined constant low speed (set speed) vc. The output of the prime mover PG is adjusted and the braking hydraulic pressure Pw of each wheel WH is individually adjusted so that this function is realized. On sandy ground, dirt roads, rocky roads, muddy roads, etc., delicate operations of the acceleration operation member AP and the braking operation member BP are required. However, with the crawl control, the driver can concentrate on the operation of the steering operation member SH, and the running performance on rough terrain and the like is improved.

[0048] The adjustment process of the braking hydraulic pressure Pw in the creep control will be described. In the creep control, the target hydraulic pressure Pt of each wheel cylinder CW is calculated based on the deviation hV between the actual vehicle body speed Vx and the set speed vc (the target value of the vehicle body speed Vx in the creep control). That is, in the creep control, the braking hydraulic pressure Pw is adjusted individually for each wheel cylinder CW. Specifically, the deviation hV between the vehicle body speed Vx and the set speed vc is calculated (i.e., "hV = Vx - vc"). Based on the speed deviation hV, the target total braking force Fvt, which is the target value of the braking force acting on the entire vehicle JV (the sum of the braking forces of the four wheels), is calculated. Further, the distribution ratio Hw of each wheel WH is determined, and the target total braking force Fvt is multiplied by the ratio Hw to determine the target value (target braking force) Fxt of the braking force Fx of each wheel WH (i.e., "Fxt = Fvt·Hw"). Finally, the target braking force Fxt is converted into the dimension of the hydraulic pressure in each wheel cylinder CW based on the specifications of the braking device SX, the braking control device SC, etc., and the target hydraulic pressure Pt corresponding to the braking hydraulic pressure Pw is calculated. For example, the distribution ratio Hw can be set to "0.25" so as to be uniform among the four wheels WH. Also, the distribution ratio Hwf of the front wheels WHf may be set to be larger than the distribution ratio Hwr of the rear wheels WHr.

[0049] The maximum value among the four target hydraulic pressures Pt is determined as the maximum target hydraulic pressure Ptx. Based on the maximum target hydraulic pressure Ptx, the first unit YA is controlled. Specifically, the first unit YA (particularly, the pressurizing unit KU) is controlled so that the actual hydraulic pressure Pwx (referred to as the "selected braking hydraulic pressure") of the wheel cylinder (referred to as the "selected wheel cylinder CWx") corresponding to the maximum target hydraulic pressure Ptx approaches and matches the maximum target hydraulic pressure Ptx. At this time, the inlet valve UIx (referred to as the "selected inlet valve") and the outlet valve VOx (referred to as the "selected outlet valve") corresponding to the selected wheel cylinder CWx are de-energized. Therefore, the selected braking hydraulic pressure Pwx matches the supply hydraulic pressure Pm.

[0050] Of the four wheel cylinders CW, for the remaining three wheel cylinders that do not correspond to the selected wheel cylinder CWx (referred to as "non - selected wheel cylinders CWz"), the actual hydraulic pressure (referred to as "non - selected braking hydraulic pressure Pwz") is controlled by the non - selected inlet valve UIz and the non - selected outlet valve VOz. Here, the "non - selected inlet valve UIz" is the remaining three inlet valves among the four inlet valves UI that do not correspond to the selected inlet valve UIx. Also, the "non - selected outlet valve VOz" is the remaining three outlet valves among the four outlet valves VO that do not correspond to the selected outlet valve VOx.

[0051] Specifically, for the adjustment of the non - selected braking hydraulic pressure Pwz, one of the three control modes, namely the "decrease mode", the "increase mode", and the "hold mode", is selected. In the decrease mode, when it is necessary to decrease the non - selected braking hydraulic pressure Pwz, the non - selected inlet valve UIz is closed and the non - selected outlet valve VOz is opened. At the upper part of the non - selected inlet valve UIz (the side close to the first unit YA), the supply hydraulic pressure Pm corresponding to the maximum target hydraulic pressure Ptx is supplied, but since the non - selected inlet valve UIz is closed, this supply is blocked. Then, since the non - selected outlet valve VOz is opened, the braking fluid BF in the non - selected wheel cylinder CWz flows out to the pressure - regulating reservoir RC, and the non - selected braking hydraulic pressure Pwz is decreased.

[0052] In the increase mode, when it is necessary to increase the non - selected braking hydraulic pressure Pwz, the non - selected outlet valve VOz is closed and the non - selected inlet valve UIz is opened. The closing of the non - selected outlet valve VOz blocks the outflow of the braking fluid BF to the pressure - regulating reservoir RC. Then, through the non - selected inlet valve UIz, the supply hydraulic pressure Pm is supplied to the non - selected wheel cylinder CWz, so the braking hydraulic pressure Pwz is increased. Note that in the adjustment of the non - selected braking hydraulic pressure Pwz, since the supply hydraulic pressure Pm (= Pwx) generated by the first unit YA is the basis, the upper limit of the non - selected braking hydraulic pressure Pwz is the selected braking hydraulic pressure Pwx (that is, "Pwz ≦ Pwx").

[0053] In the hold mode, when it is necessary to hold the non-selective braking hydraulic pressure Pwz, both the non-selective inlet valve UIz and the non-selective outlet valve Voz are closed. Since the non-selective wheel cylinder CWz is fluidly sealed, the non-selective braking hydraulic pressure Pwz is maintained constant. Note that in the adjustment of the non-selective braking hydraulic pressure Pwz, the hold mode may be omitted. In this case, the braking hydraulic pressure Pwz of the non-selective wheel cylinder CWz is adjusted by repeating the decrease mode and the increase mode.

[0054] <<Processing of Turning Assistance Control>> Next, with reference to the flowchart of FIG. 4, the processing of the turning assistance control will be described. The turning assistance control is instructed by the signal Xa of the control switch XA. The algorithm of the turning assistance control is programmed in the microprocessor MP of the braking controller ECU. Note that the steering operation amount Sa is a state quantity (variable) having positive and negative signs in order to represent the turning direction of the vehicle. However, in order to avoid complexity in the description, the absolute value thereof is used hereinafter.

[0055] In step S110, various signals including the turning assistance control switch signal Xa, the crawl control execution flag FC, the turning assistance control execution flag FS, the steering operation amount Sa (for example, the steering angle), the wheel speed Vw, the supply hydraulic pressure Pm, and the servo hydraulic pressure Pu are read. The switch signal Xa indicates the necessity of the turning assistance control transmitted from the turning assistance control switch XA. When it is in the on state, the turning assistance control is required, and when it is in the off state, the turning assistance control is not required. The execution flag FC is a control flag representing the execution state of the crawl control, where "FC = 1" represents being in execution and "FC = 0" represents non-execution. Similarly, the execution flag FS is a control flag representing the execution state of the turning assistance control, where "FS = 1" represents being in execution and "FS = 0" represents non-execution. The wheel speed Vw is calculated based on the detection value of the wheel speed sensor VW, the supply hydraulic pressure Pm is calculated based on the detection value of the supply hydraulic pressure sensor PM, and the servo hydraulic pressure Pu is calculated based on the detection value of the servo hydraulic pressure PU, respectively.

[0056] In step S120, based on the execution flag FS for turning assistance control, it is determined whether "turning assistance control is being executed or not". If the execution flag FS is "0 (non-execution state)", step S120 is negated and the process proceeds to step S130. On the other hand, if the execution flag FS is "1 (execution state)", step S120 is affirmed and the process proceeds to step S140.

[0057] In step S130, based on the turning assistance control switch signal Xa and the steering operation amount Sa (absolute value), it is determined whether "turning assistance control is to be started or not (start determination)". If at least one of "the switch signal Xa is in the off state" and "the steering operation amount Sa is less than the start predetermined amount sx" is satisfied, step S130 is negated and the process returns to step S110. On the other hand, if "the switch signal Xa is in the on state" and "the steering operation amount Sa is greater than or equal to the start predetermined amount sx", step S130 is affirmed and the process proceeds to step S150. Here, the start predetermined amount sx is a control start threshold value corresponding to the steering operation amount Sa and is a predetermined value (positive constant) set in advance.

[0058] In step S140, based on the switch signal Xa and the steering operation amount Sa (absolute value), it is determined whether "turning assistance control is to be terminated or not (end determination)". If at least one of "the switch signal Xa is in the off state" and "the steering operation amount Sa is less than the end predetermined amount sz" is satisfied, step S140 is affirmed and the process proceeds to step S160. On the other hand, if "the switch signal Xa is in the on state" and "the steering operation amount Sa is greater than or equal to the end predetermined amount sz", step S140 is negated and the process proceeds to step S150. Here, the end predetermined amount sz is a control end threshold value corresponding to the steering operation amount Sa and is a predetermined value (positive constant) set in advance. Note that in terms of the magnitude relationship between the start predetermined amount sx and the end predetermined amount sz, the end predetermined amount sz is a value smaller than the start predetermined amount sx.

[0059] In step S150, a braking torque Tqu (referred to as "inner turning braking torque") is applied to the inner turning wheel WHu so as to reduce the turning radius of the vehicle JV. Specifically, the target hydraulic pressure Ptu (referred to as "inner turning target hydraulic pressure") of the wheel cylinder CWu (referred to as "inner turning wheel cylinder") provided in the inner turning wheel WHu is increased at a preset pressure increasing gradient ka (a predetermined constant). Accordingly, the actual hydraulic pressure Pwu (referred to as "inner turning braking hydraulic pressure") of the inner turning wheel cylinder CWu is increased. When the inner turning wheel WHu locks, the inner turning target hydraulic pressure Ptu is maintained constant and the inner turning braking hydraulic pressure Pwu is held. In other words, the hydraulic pressures Ptu (target value) and Pwu (actual value) of the inner turning wheel cylinder CWu are increased until the inner turning wheel WHu locks, and are maintained at a constant value after locking. When the inner rear wheel WHur on the inner turning side is selected as the inner turning wheel WHu, since the effect of reducing the turning radius is high, in the turning assistance control, at least the hydraulic pressures Ptur (target value) and Pwur (actual value) of the inner rear wheel WHur on the inner turning side are increased.

[0060] In step S160, the turning assistance control is terminated. The inner turning braking torque Tqu that has been applied to reduce the turning radius of the vehicle JV is decreased. In the decrease of the inner turning braking torque Tqu (that is, the inner turning braking hydraulic pressure Pwu), a limit is provided to the decreasing gradient so that the vehicle behavior does not change suddenly, and it is gradually decreased.

[0061] <Operation of Turning Assistance Control> With reference to the time-series diagram of FIG. 5 (a diagram showing the transition of various state quantities as time T elapses), an operation example of the turning assist control will be described. In this example, it is assumed that the steering operation member SH (steering wheel) is operated in the left-turning direction when the crawl control is being executed and the switch signal Xa for the turning assist control is in the ON state. Also, in the turning assist control, an example is given of applying a braking torque Tqur by increasing the braking hydraulic pressure Pwur to the turning inner rear wheel WHur (i.e., the rear wheel on the left side with respect to the traveling direction of the vehicle JV) where the control effect is high. Here, since the actual braking hydraulic pressure Pwur is controlled to match the target hydraulic pressure Ptur, in the diagram, the target value Ptur and the actual value Pwur overlap.

[0062] Until time t0, the operation member SH is held in the straight-ahead direction and the steering operation amount Sa is "0 (neutral position)". At time t0, the operation member SH starts to be operated in the left direction and the steering operation amount Sa increases from "0". Since the vehicle body speed Vx is maintained at the set speed vc (the target speed of the crawl control set by the switch XC) by the crawl control, until time t0, the wheel speed Vwur of the left rear wheel is constant at the set speed vc. Here, the left rear wheel corresponds to the turning inner rear wheel WHur because the vehicle JV turns left and the turning assist control is started. The turning inner rear wheel WHur is also simply referred to as the "inner rear wheel". Also, the wheel speed Vwur of the inner rear wheel WHur is the wheel speed corresponding to the turning inner rear wheel after the start of the turning assist control and is also simply referred to as the "inner rear wheel speed".

[0063] At time t1, the steering operation amount Sa reaches the starting predetermined amount sx, the condition of step S130 is satisfied, and the turning assist control is started. The target hydraulic pressure Ptur of the left rear wheel is increased at the pressure increasing gradient ka. Accordingly, the braking hydraulic pressure Pwur of the left rear wheel is increased at the pressure increasing gradient ka. Here, the braking hydraulic pressure Pwur of the inner rear wheel WHur is the braking hydraulic pressure corresponding to the inner rear wheel during turning after the start of the turning assist control, and is also simply referred to as the "inner rear wheel hydraulic pressure". The pressure increasing gradient ka is a predetermined value (constant) set in advance. At time t1, the execution flag FS for the turning assist control is switched from "0 (non-operating)" to "1 (operating)".

[0064] After time t1, as the inner rear wheel hydraulic pressure Pwur increases, the inner rear wheel speed Vwur decreases. At time t2, the inner rear wheel WHur becomes in a locked state and the inner rear wheel speed Vwur becomes "0". Since the inner rear wheel hydraulic pressure Pwur does not need to be increased any further, it is maintained at the value pa from time t2. At time t2, the lock duration Tk (referred to as the "lock continuation time") of the inner rear wheel WHur is calculated.

[0065] From time t3, the steering operation member SH is held and the steering operation amount Sa is maintained at the value sa. At time t4, the lock duration Tk reaches the predetermined time tk. The predetermined time tk is a predetermined value (constant) set in advance. At time t4, the hydraulic pressures Ptur (target value) and Pwur (actual value) of the inner rear wheel WHur during turning are decreased so that the locked state of the inner rear wheel WHur is once released. The decompression of the inner rear wheel hydraulic pressure Pwur is to prevent a flat spot (a flat deformed part appears on a part of the tread surface of the tire) from occurring on the wheel (tire). Specifically, when the condition of "Tk≧tk" is satisfied, the inner rear wheel hydraulic pressure Pwur is decreased by the value pd from the value pa at the decompression gradient kb (a predetermined constant set in advance). As a result, the inner rear wheel speed Vwur is increased from "0" to the value vb (refer to time t5).

[0066] When at least one of "the hydraulic pressures Ptur and Pwur of the inner rear wheels during turning are decreased by a predetermined hydraulic pressure pd" and "the speed Vwur of the inner rear wheels is increased by a predetermined speed vb" is satisfied at time point t5, the hydraulic pressures Ptur and Pwur of the inner rear wheels are increased again at a pressure increasing gradient kc. Here, the predetermined hydraulic pressure pd, the predetermined speed vb, and the pressure increasing gradient kc are predetermined values (constants) set in advance. Then, when the inner rear wheel WHur during turning is locked again at time point t6, the hydraulic pressures Ptur and Pwur of the inner rear wheels are held. At the same time, the count of the lock duration Tk is started. After time point t6, the operations (lock operation and its release operation) from time point t2 to t6 are repeatedly alternated by holding, decreasing, and increasing the hydraulic pressures Ptur and Pwur of the inner rear wheels during turning. That is, the first lock operation is executed between time point t2 and time point t4, then the lock is released, the second lock operation is executed between time point t6 and time point t7, then the lock is released, and the third lock operation is started from time point t9.

[0067] <Shared control> Referring to the flowchart of FIG. 6, the shared control will be described. In the "shared control", the pressure source for applying the braking torque Tqu to the inner wheel WHu during turning is shared not only by the first unit YA but also by the second unit YB.

[0068] In the turning assistance control, the turning inner brake hydraulic pressure Pwu is increased until at least the rear wheels among the turning inner wheels are in a locked state so as to reduce the turning radius of the vehicle JV. At this time, the wheel cylinder CW provided for the wheel to be put in the locked state corresponds to the selected wheel cylinder CWx, and its hydraulic pressure Pwx is adjusted by the first unit YA. In other words, the first unit YA is required to output a high hydraulic pressure for locking the turning inner wheel WHu. Since the turning assistance control may be operated on an off-road with a winding curve where curves are continuous, its operation time may be long. Therefore, a control (i.e., shared control) is adopted to share the turning assistance control with the second unit YB so as to relieve the load of the first unit YA.

[0069] In step S210, various signals including the servo hydraulic pressure Pu, the supply hydraulic pressure Pm, the adjustment hydraulic pressure Pq, the braking hydraulic pressure Pw, the control operation flag FS of the turning assist control, etc. are read. The servo hydraulic pressure Pu is calculated based on the detected value of the servo hydraulic pressure sensor PU, and the supply hydraulic pressure Pm is calculated based on the detected value of the supply hydraulic pressure sensor PM. The adjustment hydraulic pressure Pq is calculated based on the supply hydraulic pressure Pm and the driving state of the pressure regulating valve UB. An adjustment hydraulic pressure sensor PQ may be provided to detect the adjustment hydraulic pressure Pq. Also, the braking hydraulic pressure Pw is calculated based on the driving states of the pressure regulating valve UB, the inlet valve UI, and the outlet valve VO. A braking hydraulic pressure sensor PW may be provided to detect the braking hydraulic pressure Pw.

[0070] In step S220, based on the execution flag FS for the turning assist control, it is determined whether "the turning assist control is being executed or not". When the execution flag FS is "0 (not executed)", step S220 is negated and the process returns to step S210. On the other hand, when the execution flag FS is "1 (executing)", step S220 is affirmed and the process proceeds to step S230.

[0071] In step S230, it is determined whether "the operating state of the first unit YA is severe or not". This determination is referred to as the "specific determination", and its determination condition is referred to as the "specific condition". Therefore, in step S230, it is determined whether "the specific condition is satisfied or not". Hereinafter, examples of the specific conditions will be listed.

[0072] Specific condition (1): It is determined according to whether "the locked state of the turning inner wheel WHu (for example, the inner rear wheel WHur) has passed the first predetermined time tj from the time when it first occurred (referred to as the "first lock time point") or not". Therefore, step S230 is satisfied at the time when the first predetermined time tj has passed from the first lock time point. Here, the first predetermined time tj is a preset predetermined value (constant).

[0073] Specific condition (2): It is determined according to "whether or not the second predetermined time ti has elapsed since the start of the turning support control (referred to as the "control start time point"). Therefore, step S230 is satisfied at the time when the second predetermined time ti has elapsed since the control start time point. Here, the second predetermined time ti is a preset predetermined value (constant).

[0074] Specific condition (3): It is determined according to "whether or not the third predetermined time tx has elapsed since the time when the braking torque Tqu of the inner turning wheel WHu (for example, the inner rear wheel WHur) first becomes equal to or greater than the predetermined torque tq (referred to as the "high load generation time point"). Therefore, step S230 is satisfied at the time when the third predetermined time tx has elapsed since the high load generation time point. Here, each of the predetermined torque tq and the third predetermined time tx is a preset predetermined value (constant). Note that since the inner turning braking torque Tqu corresponds to the inner turning brake hydraulic pressure Pwu, specific condition (3) may be determined according to "whether or not the third predetermined time tx has elapsed since the time when the hydraulic pressures Ptu (target value) and Pwu (actual value) of the inner turning wheel WHu (for example, the inner rear wheel WHur) first become equal to or greater than the predetermined hydraulic pressure px (that is, the high load generation time point). Here, the predetermined hydraulic pressure px is a preset predetermined value (constant) and is a value corresponding to the predetermined torque tq.

[0075] When step S230 is negated, the process proceeds to step S240. On the other hand, when step S230 is affirmed, the process proceeds to step S250.

[0076] In step S240, the braking torque Tqu is applied to the inner turning wheel WHu only by the first unit YA. When step S230 is negated, it means that the load on the first unit YA is not so large. Therefore, similar to the crawl control, the inner turning braking torque Tqu (for example, the inner braking hydraulic pressure Pwu) is increased only by the first unit YA. Note that at the start of the turning support control, the braking torque Tqu is applied to the inner turning wheel only by the first unit YA.

[0077] In step S250, the component Ta (referred to as the "first component") of the turning inner braking torque Tqu by the first unit YA is decreased. When step S230 is affirmed, it means that the load on the first unit YA is large. Therefore, the first component Ta by the first unit YA is decreased. For example, the decrease of the first component Ta is executed by decreasing the supply hydraulic pressure Pm generated by the first unit YA.

[0078] In step S260, the component Tb (referred to as the "second component") of the turning inner braking torque Tqu by the second unit YB is increased so as to compensate for the decrease of the first component Ta. For example, the increase of the second component Tb is executed by the second unit YB increasing the supply hydraulic pressure Pm to the adjustment hydraulic pressure Pq (that is, increasing the hydraulic pressure difference mQ). Here, since the increase of the second component Tb complements the decrease of the first component Ta, the absolute value of the first component Ta and the absolute value of the second component Tb are equal. Therefore, as much as the supply hydraulic pressure Pm is decreased, the hydraulic pressure difference mQ is increased, so the relationship of "Ta + Tb = 0" is maintained.

[0079] In turning assist control, a large turning inner braking torque Tqu is required such that the turning inner wheel WHu is locked. If the durability of the braking control device is determined so that this braking torque Tqu can be generated only by the first unit YA over a long time, the size and weight of the braking control device (especially the first unit YA) will be increased. In the braking control device SC, when the load situation of the first unit YA becomes severe, the load of the first unit YA is shared by the second unit YB, and the load of the first unit YA is reduced. Here, the second unit YB has a power source different from that of the first unit YA and can apply the turning inner braking torque Tqu. Although durability (robustness) and miniaturization and weight reduction are in a trade-off relationship, since the two units are preferably used, this trade-off relationship can be achieved in the braking control device SC in which turning assist control can be executed.

[0080] <Operation of sharing control> Referring to the time-series diagram of FIG. 7 (transition diagram of various state quantities with respect to time T), an operation example of the shared control will be described. The shared control is to reduce the load on the first unit YA by sharing the load with the second unit YB when applying the braking torque Tqu to the inner turning wheel WHu. Similar to the time-series diagram of FIG. 5, in the diagram, the situation in the left turning direction is assumed, and for the inner turning rear wheel WHur (i.e., the left rear wheel), the inner turning rear wheel braking torque Tqur is applied by increasing the inner turning rear wheel braking hydraulic pressure Pwur (inner rear wheel hydraulic pressure). In the operation example, the specific determination is made based on "whether or not the first predetermined time tj has elapsed since the first occurrence of the locked state of the inner turning rear wheel WHur (the first lock time point)" (refer to the above specific condition (1)). Also, since the actual braking hydraulic pressure Pwur matches the target hydraulic pressure Ptur, the target value Ptur and the actual value Pwur overlap in the diagram.

[0081] At time point u1, the turning assistance control is started. At the beginning of the control start, only the first unit YA increases the inner rear wheel hydraulic pressure Pwur (i.e., the inner rear wheel braking torque Tqur). Specifically, from time point u1, the inner rear wheel target hydraulic pressure Ptur is increased at the pressure increasing gradient ka, and the supply hydraulic pressure Pm is increased at the pressure increasing gradient ka by the first unit YA. At this time, since the second unit YB is not operated, the pressure regulating valve UB is in the non-energized state (i.e., the fully open state), and the hydraulic pressure difference mQ remains "0".

[0082] At time point u2, the inner turning rear wheel WHur locks. At time point u2, the inner rear wheel target hydraulic pressure Ptur (and as a result, the inner rear wheel braking hydraulic pressure Pwur) is maintained constant at the value pa. Also, at time point u2, the calculation of the duration Tj (the first duration) of the locked state of the inner turning rear wheel WHur is started. From time point u2, as described with reference to FIG. 5, the processes of holding, decreasing, and increasing the inner rear wheel hydraulic pressures Ptur (target value) and Pwur (actual value) (similar to the processes after time point t2 in FIG. 5) are repeatedly executed.

[0083] At time u6, the first continuous time Tj reaches the first predetermined time tj (a preset constant), and at step S230, a specific condition (the load state of the first unit YA has become severe) is satisfied. The point in time when step S230 is satisfied for the first time is referred to as the "specific point in time". At the specific point in time u6, the supply fluid pressure Pm (i.e., the first component Ta) by the first unit YA begins to decrease. Then, in order to compensate for the decrease in the supply fluid pressure Pm, the hydraulic pressure difference mQ (i.e., the second component Tb) by the second unit YB begins to increase. Specifically, after time u6, the supply fluid pressure Pm is decreased at a gradient kd, and the hydraulic pressure difference mQ is increased at a gradient ke. Here, the decrease gradient kd and the increase gradient ke are preset predetermined values (constants), and their absolute values are equal (i.e., "|kd| = |ke|").

[0084] At time u7, the supply fluid pressure Pm is maintained constant at the value pc, and the hydraulic pressure difference mQ is maintained constant at the value pq. At this time, the relationship is "pa = pc + pq". At time u8, in order to avoid a flat spot due to the lock of the inner rear wheel WHur during turning, the hydraulic pressure difference mQ is decreased and then increased again by the second unit YB.

[0085] For turning assist control such as off-road driving where curves are continuous, it may be operated for a long time. If the first unit YA is configured to be able to handle such a situation, there is a risk of increasing the overall size of the device. In the braking control device SC, two units YA and YB are appropriately used to achieve a trade-off between durability and miniaturization and weight reduction. Specifically, when the load on the first unit YA is not so severe, the turning assist control is executed only by the first unit YA. When the load state of the first unit YA becomes severe, the load on the first unit YA is reduced, and that load is shared by the second unit YB. Thereby, in the braking control device SC capable of executing the turning assist control, durability can be ensured while achieving its miniaturization and weight reduction.

[0086] In the operation example, as the specific determination, specific condition (1) is adopted, but specific condition (2) may also be adopted. In this case, the duration Ti (second duration) from the time point u1 (control start time point) when the turning assistance control is started is calculated. Then, at the time point u6 (specific time point) when the second duration Ti reaches the second predetermined time ti (predetermined constant), the supply hydraulic pressure Pm (first component Ta) is decreased and the hydraulic pressure difference mQ (second component Tb) is increased. Also, specific condition (3) may be adopted for the specific determination. In this case, the duration Tx (third duration) from the time point x2 (high load generation time point) when the inner rear wheel hydraulic pressures Ptur, Pwur (corresponding to the turning inner braking torque Tqu) first become equal to or higher than the predetermined hydraulic pressure px is calculated. Then, at the time point u6 (specific time point) when the third duration Tx reaches the third predetermined time tx (predetermined constant), the supply hydraulic pressure Pm (first component Ta) is decreased and the hydraulic pressure difference mQ (second component Tb) is increased. Even when specific conditions (2) and (3) are adopted, the same effects as in the case of specific condition (1) are achieved.

[0087] <Other Embodiments> Hereinafter, other embodiments of the braking control device SC will be described. Also in other embodiments, since the load on the first unit YA is reduced by the above-described sharing control, the above-described effects (that is, the durability of the braking control device SC and the compatibility of miniaturization and weight reduction) are achieved.

[0088] ≪Adoption of the reflux type pressurizing unit KU and the single type master cylinder CM as the first unit YA≫ With reference to the schematic diagram of FIG. 8, a second configuration example of the first unit YA will be described. In the first configuration example of the first unit YA, an accumulator type is adopted as the pressurizing unit KU (pressurizing source) of the first unit YA, and a tandem type is adopted as the master cylinder CM. Instead of this, in the second configuration example, a reflux type pressurizing unit KU and a single type master cylinder CM are adopted. As described above, components such as members, signals, and values to which the same reference signs are attached have the same functions.

[0089] In the reflux type pressure unit KU, in the fluid pump QC driven by the electric motor MC, the suction part (the part that sucks the brake fluid BF) and the discharge part (the part that discharges the brake fluid BF) are connected by the reflux path HK. And the reflux path HK is connected to the master reservoir RV, and the fluid pump QC can suck the brake fluid BF from the master reservoir RV. In the reflux path HK, a normally open linear valve UC (a solenoid valve similar to the pressure regulating valve UB of the second unit YB) is provided to regulate the brake fluid BF discharged by the fluid pump QC. Here, the hydraulic pressure regulated by the linear valve UC corresponds to the servo hydraulic pressure Pu in the first configuration example. The reflux path HK is connected to the servo chamber Ru via the servo path HV between the discharge part of the fluid pump QC and the linear valve UC. Also, the servo path HV is connected to the rear wheel wheel cylinder CWr via the rear wheel connection path HSr. Therefore, in the second configuration example, the servo hydraulic pressure Pu is supplied to the servo chamber Ru and the rear wheel wheel cylinder CWr.

[0090] The first unit YA can be configured by a combination of "one of the tandem type and the single type of the master cylinder CM" and "one of the accumulator type and the reflux type of the pressure unit KU". That is, one of "tandem type master cylinder CM + accumulator type pressure unit KU (illustrated in FIG. 2)", "tandem type master cylinder CM + reflux type pressure unit KU", "single type master cylinder CM + accumulator type pressure unit KU", and "single type master cylinder CM + reflux type pressure unit KU (illustrated in FIG. 7)" is adopted as the first unit YA.

[0091] ≪Adoption of the electric cylinder type as the first unit YA≫ In the first and second configuration examples of the first unit YA, as the pressure source KU of the first unit YA, a fluid pump QC driven by an electric motor MC was adopted. Instead of this, the braking hydraulic pressure Pw may be increased by directly driving a piston inserted into a cylinder by an electric motor. That is, a so-called electric cylinder type may be adopted as the first unit YA. Since the configuration of the electric cylinder type is known, for example, in "WO2012 / 046703" etc., the configuration will be briefly described below. The first unit YA of the electric cylinder type is composed of a pressure regulating cylinder, a pressure regulating piston, a linear motion conversion mechanism, and an electric motor.

[0092] Separate from the master cylinder CM, a pressure regulating cylinder (also referred to as a "slave cylinder") is provided. 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 electric motor via a rotation-linear motion conversion mechanism (for example, a screw mechanism). Here, the linear motion conversion mechanism converts the rotational power of the electric motor into the linear power (thrust) of the pressure regulating piston.

[0093] The pressure regulating piston is driven by the electric motor. Specifically, when the electric motor rotates, its power is converted into the linear power of the pressure regulating piston by the linear motion conversion mechanism. The inside of the pressure regulating cylinder is partitioned 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 a communication path HS and the second unit YB. Therefore, when the electric motor is driven, the volume of the pressure regulating chamber decreases, so the braking fluid BF is pumped from the pressure regulating chamber to the wheel cylinder CW at the supply hydraulic pressure Pm. That is, in the first unit YA of the electric cylinder type, the supply hydraulic pressure Pm is directly controlled (regulated) by adjusting the output of the electric motor without using the solenoid valves UZ and UG.

[0094] ≪Adoption of an Electric Parking Brake Device as the Second Unit YB≫ In the configuration example of the second unit YB described above, the one using the brake fluid BF used for vehicle stability control or the like is adopted. Instead of this, an electric parking brake device may be adopted as the second unit YB. Since the configuration of the electric parking brake device is known, for example, in "Japanese Patent Application Laid-Open No. 2018-086879" or the like, it will be briefly described below.

[0095] The electric parking brake device is provided as the second unit YB on the rear wheel WHr. In the electric parking brake device, the rotational power of the electric motor is converted into linear power by a rotation-linear motion conversion mechanism (for example, a screw mechanism). By this linear power, the friction member MS (brake pad) is pressed against the rotating member KT (brake disk). Thereby, a braking torque Tqr is applied to the rear wheel WHr.

[0096] <Summary of the Embodiment of the Braking Control Device SC and Its Operation and Effects> The embodiments of the braking control device SC will be summarized. The braking control device SC executes turning assistance control. In the turning assistance control, a braking torque Tqu is applied to the inner turning wheel WHu of the vehicle JV so that the turning radius of the vehicle JV is reduced. The braking control device SC includes "a first unit YA that can increase the hydraulic pressure of the wheel cylinder CW to apply the inner turning braking torque Tqu", "a unit different from the first unit YA (particularly, having a separate power source) that can apply the inner turning braking torque Tqu", and "a controller ECU that controls the first and second units YA and YB".

[0097] In the braking control device SC, when the turning assistance control is started by the controller ECU (i.e., at the beginning of control start), the turning inner braking torque Tqu is applied only by the first unit YA. Then, after the turning assistance control is started, a specific determination is made as to "whether or not the operating state (load state) of the first unit YA is severe". If the specific determination is negative, the application of the turning inner braking torque Tqu only by the first unit YA is continued. On the other hand, if the specific determination is affirmative (satisfied), the turning inner braking torque Tqu is applied by the second unit YB. Specifically, from the time when the specific determination is affirmed by the controller ECU, in the turning inner braking torque Tqu, the first component Ta by the first unit YA is decreased and the second component Tb by the second unit YB is increased.

[0098] The braking control device SC includes two units YA and YB in order to realize various functions. For example, the first unit YA is used to realize functions such as service brake and crawl control. The second unit YB is used to realize functions such as vehicle stability control and parking brake. Therefore, each of the first and second units YA and YB is provided to realize different functions. And each of the first and second units YA and YB has a different power source (pressure source).

[0099] In the braking control device SC that executes the turning assistance control, it is required to generate a braking torque Tq that can lock the wheel WH for a long time. If an attempt is made to satisfy this requirement with a single unit (i.e., the first unit YA), the device needs to be enlarged. Therefore, in the braking control device SC, when the load of the first unit YA is not so severe (i.e., when the specific determination is not satisfied), the turning assistance control is executed only by the first unit YA. When the load state of the first unit YA becomes severe (i.e., when the specific determination is satisfied), the load of the first unit YA is reduced and that load is shared by the second unit YB. Thereby, while the requirement for durability is satisfied, the size and weight of the braking control device SC are reduced.

[0100] For example, in the braking control device SC, the first continuous time Tj is calculated from the time when the locked state of the inner turning wheel WHu first occurs (the lock start time). And as a specific condition (the condition for performing specific determination), "whether the first continuous time Tj is equal to or greater than the first predetermined time tj" is adopted. Here, the first predetermined time tj is a predetermined value (constant) set in advance. When the first continuous time Tj reaches the first predetermined time tj (that is, the time when the first predetermined time tj has elapsed since the lock start time), the specific condition of "Tj ≥ tj" is satisfied and the specific determination is affirmed, so the sharing control is started.

[0101] Also, in the braking control device SC, the second continuous time Ti can be calculated from the time when the turning assist control is started (the control start time). And as a specific condition, "whether the second continuous time Ti is equal to or greater than the second predetermined time ti" is adopted. Here, the second predetermined time ti is a predetermined value (constant) set in advance. When the second continuous time Ti reaches the second predetermined time ti (that is, the time when the second predetermined time ti has elapsed since the control start time), the specific condition of "Ti ≥ ti" is satisfied and the specific determination is affirmed, so the sharing control is started.

[0102] Furthermore, in the braking control device SC, the third continuous time Tx may be calculated from the time when the inner turning braking torque Tqu first becomes equal to or greater than the predetermined torque tq (the high load start time). And as a specific condition, "whether the third continuous time Tx is equal to or greater than the third predetermined time tx" is adopted. Here, the third predetermined time tx and the predetermined torque tq are each a predetermined value (constant) set in advance. When the third continuous time Tx reaches the third predetermined time tx (that is, the time when the third predetermined time tx has elapsed since the high load start time), the specific condition of "Tx ≥ tx" is satisfied and the specific determination is affirmed, so the sharing control is started.

Explanation of symbols

[0103] JV… vehicle, SC… brake control device, SX… brake device, CP… brake caliper, CW… wheel cylinder, KT… rotating member (brake disk), MS… friction member (brake pad), ECU… brake controller, BS… communication bus, XC… crawl control switch, XA… turning assist control switch, HU… fluid unit, YA… first unit, CM… master cylinder, MA… electric motor (for pressure accumulation), KU… pressurizing unit, YB… second unit, UB… pressure regulating valve, UI… inlet valve, VO… outlet valve, RC… pressure regulating reservoir, MB… electric motor (for reflux), QB… fluid pump (for reflux), PM… supply hydraulic pressure sensor, Pm… supply hydraulic pressure, Pq… regulated hydraulic pressure, Pw… brake hydraulic pressure (actual hydraulic pressure of CW), Pt… target hydraulic pressure (target value of Pw), mQ… hydraulic pressure difference (difference between supply hydraulic pressure Pm and regulated hydraulic pressure Pq), Tq… braking torque, Vw… wheel speed, Ta… first component (component of braking torque Tq by first unit YA), Tb… second component (component of braking torque Tq by second unit YB).

Claims

1. A braking control device for a vehicle that performs turning assistance control to reduce the turning radius of the vehicle by locking the inner turning wheel of the vehicle by applying a braking torque to the inner turning wheel of the vehicle, a first unit that increases the hydraulic pressure of the wheel cylinder of the vehicle to apply the braking torque, a second unit that applies the braking torque separately from the first unit, and a controller that controls the first unit and the second unit, wherein, the controller, when starting the turning assistance control, applies the braking torque only by the first unit, after starting the turning assistance control, makes a specific determination as to whether the operating state of the first unit is severe, and when the specific determination is affirmative, applies the braking torque by the second unit to continue the state in which the inner turning wheel is locked, the controller affirms the specific determination when a predetermined time has elapsed from a predetermined start time related to the operation of the first unit accompanying the execution of the turning assistance control, the predetermined start time is the time when the locked state of the inner turning wheel first occurs, a braking control device for a vehicle.

2. A braking control device for a vehicle that performs turning assistance control to reduce the turning radius of the vehicle by locking the inner turning wheel of the vehicle by applying a braking torque to the inner turning wheel of the vehicle, a first unit that increases the hydraulic pressure of the wheel cylinder of the vehicle to apply the braking torque, a second unit that applies the braking torque separately from the first unit, and a controller that controls the first unit and the second unit, wherein, the controller, when starting the turning assistance control, applies the braking torque only by the first unit, after starting the turning assistance control, makes a specific determination as to whether the operating state of the first unit is severe, and when the specific determination is affirmative, applies the braking torque by the second unit to continue the state in which the inner turning wheel is locked, the controller affirms the specific determination when a predetermined time has elapsed from a predetermined start time related to the operation of the first unit accompanying the execution of the turning assistance control, The vehicle braking control device, wherein the predetermined start time is the time when the braking torque applied to the inner turning wheel for the first time becomes equal to or greater than a predetermined torque by implementing the turning assistance control.

3. A vehicle braking control device that performs turning assistance control to reduce the turning radius of the vehicle by locking the inner turning wheel of the vehicle by applying a braking torque to the inner turning wheel of the vehicle, a first unit that increases the hydraulic pressure of the wheel cylinder of the vehicle to apply the braking torque; a second unit that applies the braking torque separately from the first unit; a controller that controls the first unit and the second unit; and includes: The controller: When starting the turning assistance control, applies the braking torque only by the first unit; After starting the turning assistance control, makes a specific determination as to whether the operating state of the first unit is severe, and when the specific determination is affirmative, applies the braking torque by the second unit to continue the state where the inner turning wheel is locked; The controller affirms the specific determination when a predetermined time has elapsed from a predetermined start time related to the operation of the first unit accompanying the execution of the turning assistance control; The vehicle braking control device, wherein the predetermined start time is the time when the turning assistance control is started.

4. In the vehicle braking control device according to any one of Claims 1 to 3, the controller decreases a first component of the braking torque by the first unit and increases a second component of the braking torque by the second unit from the time when the specific determination is affirmed.

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