Abstract:
There is provided a brake control device for a brake device including a basic fluid pressure generating unit which generates a basic fluid pressure according to an operation to a brake operating member, a hydraulic pump which generates a fluid pressure to generate a differential pressure, a pressure regulating valve which regulates the differential pressure, and a friction brake mechanism which applies a fluid pressure obtained by adding the differential pressure to the basic fluid pressure, to a wheel cylinder. The control device includes a regulating unit which regulates the differential pressure according to an operating amount correlated value and which decreases the differential pressure according to an increase of the basic fluid pressure due to an increase of the operating amount correlated value when the vehicle is stopped and the differential pressure is regulated to a value larger than 0 by an operation to the brake operating member.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based on and claims priority under 35 U.S.C.§119 to Japanese Patent Application 2012-016863, filed on Jan. 30, 2012, the entire content of which is incorporated herein by reference. 
       BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a brake control device for a vehicle. 
         [0004]    2. Description of Related Art 
         [0005]    There has been known a brake device including a master cylinder which generates a basic fluid pressure (master cylinder fluid pressure) according to an operation to a brake pedal by a driver, a hydraulic pump which generates a differential pressure generation fluid pressure to generate a differential pressure (a linear valve differential pressure) to be added to the basic fluid pressure, a pressure regulating valve (normally open linear electromagnetic valve) which regulates the differential pressure using the differential pressure generation fluid pressure, and a friction brake mechanism which applies a wheel cylinder fluid pressure, obtained by adding the differential pressure to the basic fluid pressure, to a wheel cylinder, thereby generating a friction braking force according to the wheel cylinder fluid pressure (for example, see JP-A-2007-276500). 
         [0006]    In the brake device disclosed in JP-A-2007-276500, in order to reduce the load of the linear electromagnetic valve, the wheel cylinder fluid pressure is limited to “the minimum fluid pressure necessary for maintaining a vehicle in a stopped state on a sloping road” when the vehicle is in a stopped state. 
         [0007]    Meanwhile, in the above brake device, when the vehicle is in a stopped state, the differential pressure is always maintained at a value larger than 0 while the driver is operating the brake pedal. In other words, the liner electromagnetic valve is always driven. Therefore, in terms of durability of the linear electromagnetic valve, there is a room for improvement in that brake device. 
       SUMMARY 
       [0008]    Accordingly, an object of the present invention is to provide a brake control device for a vehicle which applies a wheel cylinder fluid pressure, obtained by adding a differential pressure regulated by a pressure regulating valve to a basic fluid pressure, to a wheel cylinder, while improving the durability of the pressure regulating valve. 
         [0009]    According to an illustrative embodiment of the present invention, there is provided a brake control device for a vehicle, which is applied to a brake device including a basic fluid pressure generating unit which generates a basic fluid pressure according to an operation to a brake operating member by a driver, a hydraulic pump which generates a differential pressure generation fluid pressure to generate a differential pressure to be added to the basic fluid pressure, a pressure regulating valve which uses the differential pressure generation fluid pressure to regulate the differential pressure, and a friction brake mechanism which applies a wheel cylinder fluid pressure obtained by adding the differential pressure to the basic fluid pressure, to a wheel cylinder, thereby generating a friction braking force according to the wheel cylinder fluid pressure. The brake control device includes a differential pressure regulating unit which regulates the differential pressure by controlling the pressure regulating value according to an operating amount correlated value which is correlated with an operating amount of the brake operating member, and a determining unit which determines whether the vehicle is in a stopped state. The differential pressure regulating unit decreases the differential pressure according to an increase of the basic fluid pressure due to an increase of the operating amount correlated value in a case where it is determined that the vehicle is in the stopped state and the differential pressure is regulated to a value larger than 0 by an operation to the brake operating member. 
         [0010]    According to the above configuration, while the vehicle is in a stopped state and the driver is operating the brake operating member (the differential pressure is greater than 0), if the operating amount of the brake operating member increases (typically, a depressing operation to the brake pedal is made), the differential pressure decreases by the increase amount of the basic fluid pressure. In other words, whenever the operating amount increases (whenever the basic fluid pressure increases), the differential pressure decreases. As a result, at the time when the accumulative increase amount of the basic fluid pressure reaches the magnitude of the differential pressure at the stop start timing of the vehicle, it may be possible to make the differential pressure 0 (and thus to stop the driving of the pressure regulating valve). 
         [0011]    As s result, it is possible to reduce the driving time of the pressure regulating valve as compared to a case where the linear electromagnetic valves are continuously driven while the vehicle is in a stopped state and the driver is operating a brake pedal. As a result, the durability of the pressure regulating valve can be improved. Further, when the operating amount increases, the differential pressure decreases by the increase amount of the basic fluid pressure. Therefore, the wheel cylinder fluid pressure (which is the sum of the basic fluid pressure and the differential pressure) does not decrease (or it is difficult for the wheel cylinder fluid pressure to decrease). Therefore, in a case where the vehicle is in a stopped state on a sloping road, it may be possible to suppress the vehicle from slipping down due to an increase in the operating amount. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The foregoing and additional features and characteristics of this disclosure will become more apparent from the following detailed description considered with the reference to the accompanying drawings, wherein: 
           [0013]      FIG. 1  is a view illustrating a schematic configuration of a vehicle including a brake control device according to a first illustrative embodiment of the present invention; 
           [0014]      FIG. 2  is a view illustrating a schematic configuration of a brake fluid pressure control unit shown in  FIG. 1 ; 
           [0015]      FIG. 3  is a graph illustrating the relation between a command current and a command differential pressure for a normally open linear electromagnetic valve shown in  FIG. 2 ; 
           [0016]      FIG. 4  is a graph illustrating the relation between a stroke of a brake pedal, and a master cylinder fluid pressure, a wheel cylinder fluid pressure and a linear valve differential pressure in a case of using the brake control device shown in  FIG. 1 ; 
           [0017]      FIG. 5  is a view for explaining how the relation between a stroke and the linear valve differential pressure changes according to depressing of the brake pedal in a case of using the brake control device shown in  FIG. 1 ; 
           [0018]      FIG. 6  is a graph illustrating exemplary changes of the master cylinder fluid pressure, the wheel cylinder fluid pressure, and the linear valve differential pressure when a running vehicle is stopped by depressing of the brake pedal in a case of using the brake control device shown in  FIG. 1 ; 
           [0019]      FIG. 7  is a graph illustrating exemplary changes of the master cylinder fluid pressure, the wheel cylinder fluid pressure, and the linear valve differential pressure while the brake pedal is further depressed from the state shown in  FIG. 6  when the vehicle is in a stopped state; 
           [0020]      FIG. 8  is a graph illustrating exemplary changes of the master cylinder fluid pressure, the wheel cylinder fluid pressure, and the linear valve differential pressure if the brake pedal is returned from the state shown in  FIG. 7  when the vehicle is in a stopped state; 
           [0021]      FIG. 9  is a graph of a comparative example corresponding to  FIG. 8 ; 
           [0022]      FIG. 10  is a time chart illustrating exemplary changes of various values in a case of using the brake control device shown in  FIG. 1 ; 
           [0023]      FIG. 11  is a view for explaining a change of the relation between the stroke and the linear valve differential pressure, corresponding to the example shown in  FIG. 10 ; 
           [0024]      FIG. 12  is a graph corresponding to  FIG. 4  in a case of using a brake control device according to a modified illustrative embodiment of the present invention; and 
           [0025]      FIG. 13  is a graph corresponding to  FIG. 5  in a case of using a brake control device according to another illustrative embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    Hereinafter, illustrative embodiments of a brake control device for a vehicle will be described with reference to the accompanying drawings. 
       First Illustrative Embodiment 
       [0027]      FIG. 1  shows a schematic configuration of a vehicle including a brake device  10  including a brake control device according to an illustrative embodiment of the present invention. Hereinafter, a symbol ‘**’ added to the ends of various variables and reference numerals is a comprehensive symbol of “fr”, “fl”, “rr” and “rl” which are added to the ends of various variables and reference numerals for representing which of wheels FR, FL, RR, and RL the various variables and reference numerals relate to. 
         [0028]    The brake device  10  includes a brake fluid pressure control unit  30  which generates a friction braking force (friction braking torque) according to a wheel cylinder fluid pressure at wheels**. As shown in  FIG. 2 , the brake fluid pressure control unit  30  includes a brake fluid pressure generating unit  32  which generates a fluid pressure according to a stroke (or a depressing force) of a brake pedal BP, brake fluid pressure regulating units  33  to  36  which regulate a wheel cylinder fluid pressure to be supplied to wheel cylinders W** disposed at the respective wheels**, and a reflux brake fluid supplying unit  37 . Friction braking torques according to the wheel cylinders W** are given to the wheels**. 
         [0029]    The brake fluid pressure generating unit  32  includes a vacuum booster VB which operates according to the brake pedal BP, and a master cylinder MC which is connected to the vacuum booster VB. The vacuum booster VB is configured to use an air pressure (negative pressure) of the inside of a suction pipe of an engine (not shown) to assist an operation force of the brake pedal BP at a predetermined ratio, and transmit the assisted operation force to the master cylinder MC. 
         [0030]    The master cylinder MC has two output ports, receives supply of brake fluid from a reservoir RS, and generates a fluid pressure (master cylinder fluid pressure Pm) according to the assisted operation force from the two ports. The configurations and operations of the master cylinder MC and the vacuum booster VB are known, and thus will not be described in detail here. The master cylinder fluid pressure Pm corresponds to a “basic fluid pressure”. 
         [0031]    A normally open linear electromagnetic valve PC  1  is interposed between one port of the master cylinder MC and the upstream portion of the brake fluid pressure regulating units  33  and  34 , and a normally open linear electromagnetic valve PC 2  is interposed between the other port of the master cylinder MC and the upstream portion of the brake fluid pressure regulating units  35  and  36 . The linear electromagnetic valves PC 1  and PC 2  will be described in detail later. 
         [0032]    The brake fluid pressure regulating units  33  to  36  are configured by pressure booster valves PU** which are normally open electromagnetic on-off valves of a two-port, two-position switching type, and pressure reducing valves PD** which are normally closed electromagnetic on-off valves of a two-port, two-position switching type. Each pressure booster valve PU** is capable of connecting or disconnecting the upstream portion of a corresponding regulating unit of the brake fluid pressure regulating units  33  to  36  to or from a corresponding wheel cylinder W**. Each pressure reducing valve PD** is capable of connecting or disconnecting a corresponding wheel cylinder W** to or from a corresponding reservoir of the reservoirs RS 1  and RS 2 . As a result, it is possible to increase, maintain, and reduce the fluid pressures (wheel cylinder fluid pressures Pw**) of the wheel cylinders W** by controlling the pressure booster valves PU** and the pressure reducing valves PD**. 
         [0033]    The reflux brake fluid supplying unit  37  includes a DC motor MT, and two hydraulic pumps (gear pumps) HP 1  and HP 2  which are driven at the same time by the DC motor MT. The hydraulic pumps HP 1  and HP 2  are configured to draw the brake fluid in the reservoirs RS 1  and RS 2  returned from the pressure reducing valves PD** and supply the drawn brake fluid to the upstream portions of the brake fluid pressure regulating units  33  to  36 . 
         [0034]    Next, the normally open linear electromagnetic valves PC 1  and PC 2  will be described. On the valve body of each normally open linear electromagnetic valve PC 1  or PC 2 , an opening direction force based on a biasing force from a coil spring (not shown) always acts, and an opening direction force based on a differential pressure (linear valve differential pressure ΔP) obtained by subtracting the master cylinder fluid pressure Pm from the pressure of the upstream portion of corresponding regulating units of the brake fluid pressure regulating units  33  to  36  , and a closing direction force based on pulling power which proportionally increases according to a current (a command current Id) supplied to the normally open linear electromagnetic valve PC 1  or PC 2  act. 
         [0035]    As a result, as shown in  FIG. 3 , a command differential pressure ΔPd which is a command value of the linear valve differential pressure ΔP is determined to proportionally increase according to the command current Id. Here, a reference symbol “I 0 ” represents a current value corresponding to the biasing force of the coil spring. The normally open linear electromagnetic valves PC 1  and PC 2  are closed when the command differential pressure ΔPd is larger than the linear valve differential pressure ΔP, and are opened when the command differential pressure ΔPd is smaller than the linear valve differential pressure ΔP. As a result, when the hydraulic pump HP 1  or HP 2  is being driven, brake fluid of the upstream portion of corresponding regulating units of the brake fluid pressure regulating units  33  to  36  flows toward a corresponding port of the master cylinder MC through a corresponding electromagnetic valve of the normally open linear electromagnetic valves PC 1  and PC 2 , so that the linear valve differential pressure ΔP can be regulated to coincide with the command differential pressure ΔPd. The brake fluid flowing toward the corresponding port of the master cylinder MC returns to a corresponding reservoir of the reservoirs RS 1  and RS 2 . 
         [0036]    In other words, if the motor MT (accordingly, the hydraulic pumps HP 1  and HP 2 ) is driven, the linear valve differential pressure ΔP can be controlled according to the command current Id of the normally open linear electromagnetic valves PC  1  and PC 2 . The pressures of the upstream portions of the brake fluid pressure regulating units  33  to  36  become a value (Pm+ΔP) obtained by adding the linear valve differential pressure ΔP to the master cylinder fluid pressure Pm. After the driving of the hydraulic pumps HP 1  and HP 2  is stopped in a state where the linear valve differential pressure ΔP is regulated to a value larger than 0, it is possible to regulate the linear valve differential pressure ΔP only in the reduction direction by controlling the command current Id in a reduction direction. 
         [0037]    If the normally open linear electromagnetic valves PC 1  and PC 2  are made a non-excited state (that is, if the command current Id is set to “0”), the normally open linear electromagnetic valves PC 1  and PC 2  maintain an open state by the biasing forces of the coil springs. In this case, the linear valve differential pressure ΔP becomes “0”, and the pressures of the upstream portions of the brake fluid pressure regulating units  33  to  36  become equal to the master cylinder fluid pressure Pm. 
         [0038]    According to the above-described configuration, the brake fluid pressure control unit  30  is configured by a fluid pressure circuit of two systems, that is, a system relative to front right and left wheels FR and FL and a system relative to rear right and left wheels RR and RL. In the brake fluid pressure control unit  30 , when all electromagnetic valves are in the non-excited state, the wheel cylinder fluid pressures Pw** are regulated to the same value as the master cylinder fluid pressure Pm. 
         [0039]    In contrast, in this state, if the motor MT (accordingly, the hydraulic pumps HP 1  and HP 2 ) is driven and the normally open linear electromagnetic valves PC 1  and PC 2  are controlled, the wheel cylinder fluid pressures Pw** can be regulated to a fluid pressure (Pm+ΔP). Further, if the pressure booster valves PU** and the pressure reducing valves PD** are controlled, the wheel cylinder fluid pressures Pw** can be regulated independently for each wheel. In other words, regardless of an operation to the brake pedal BP by the driver, a braking force to be applied to the wheels** can be regulated independently for each wheel. 
         [0040]    Referring back to  FIG. 1 , the brake device  10  includes wheel speed sensors  41 ** which detect the rotation speeds of the wheels, a brake switch  42  which selectively outputs a signal according to existence or non-existence of an operation to the brake pedal BP, a front/rear acceleration sensor  43  which detects acceleration in the front/rear direction of the vehicle body (front/rear acceleration), and a master cylinder fluid pressure sensor  44  (see  FIG. 2 ) which detects the master cylinder fluid pressure Pm. 
         [0041]    The wheel speed sensors  41 ** function as sensors for detecting whether the vehicle is in a stopped state. The front/rear acceleration sensor  43  has a function of outputting a value according to the tiling angle of the vehicle body in a vehicle body pitching direction. Therefore, the front/rear acceleration sensor  43  also functions as a gradient sensor for detecting the gradient of a road surface in the vehicle body pitching direction when the vehicle is in a stopped state. 
         [0042]    The brake device  10  further includes an electronic control device  50 . The electronic control device  50  is a micro-computer including a CPU  51 , a ROM  52 , a RAM  53 , a backup RAM  54 , an interface  55 , and so on. 
         [0043]    The interface  55  is connected to the sensors  41  to  44 , supplies signals from the sensors  41  to  44  to the CPU  51 , and transmits a drive signal to the electromagnetic valves (the normally open linear electromagnetic valves PC 1  and PC 2 , the pressure booster valves PU**, and the pressure reducing valves PD**) of the brake fluid pressure control unit  30 , and the motor MT based on an instruction of the CPU  51 . Hereinafter, a case where all of the pressure booster valves PU** and the pressure reducing valves PD** are in the non-excited state, that is, a case where “PW**=Pm+ΔP” is satisfied will be described. 
         [0044]    (Characteristics of Master Cylinder Fluid Pressure, Linear Valve Differential Pressure, and Wheel Cylinder Fluid Pressure) 
         [0045]    In the brake device  10  having the above-described, during running of the vehicle, with respect to a stroke of the brake pedal BP (hereinafter, also referred to simply as “a stroke”), the master cylinder fluid pressure Pm, the linear valve differential pressure ΔP (the command differential pressure ΔPd), and the wheel cylinder fluid pressure Pw (=Pm+ΔP) are regulated as shown in  FIG. 4 . 
         [0046]    Specifically, the master cylinder fluid pressure Pm (see a broken line) is maintained at 0 when the stroke is less than Sb, and increases from 0 according to an increase of the stroke when the stroke is equal to or greater than Sb. The linear valve differential pressure ΔP (see a solid line) is maintained at  0  when the stroke is less than Sa (&lt;Sb), increases from 0 to Pb according to an increase of the stroke when the stroke is between Sa and Sb, and has a constant value Pb regardless of the stroke when the stroke is equal to or greater than Sb. As a result, the wheel cylinder fluid pressure Pw (see a dotted line) is maintained at 0 when the stroke is less than Sa, and increases from 0 according to an increase of the stroke when the stroke is equal to or greater than Sa. Further, the gradient of the increase of the master cylinder fluid pressure Pm and the gradient of the increase of the linear valve differential pressure ΔP coincide with each other. Therefore, the gradient of the increase of the wheel cylinder fluid pressure Pw is constant within a range where the stroke is equal to or greater than Sa. 
         [0047]    In the brake device  10 , the above-described characteristics shown in  FIG. 4  are also applied in principal even after the vehicle is stopped by an operation to the brake pedal BP. Further, in the brake device  10 , while the vehicle is stopped, in a state where the stroke is greater than Sb (in a state where the master cylinder fluid pressure Pm increases according to the increase of the stroke and the linear valve differential pressure ΔP is greater than 0), when the brake pedal BP is depressed (that is, increasing of the stroke), the linear valve differential pressure ΔP (command differential pressure ΔPd) decreases by the increase amount of the master cylinder fluid pressure Pm, as shown in  FIG. 5 . 
         [0048]      FIG. 6  shows exemplary changes in the master cylinder fluid pressure Pm, the linear valve differential pressure ΔP, and the wheel cylinder fluid pressure Pw in a case where the vehicle is stopped on a sloping road (or a flat road) by depressing the brake pedal BP during running of the vehicle. In this example, in a state where the stroke is maintained at S 1  (&gt;Sb) (that is, the master cylinder fluid pressure Pm is P 2 , the linear valve differential pressure ΔP is Pb, and the wheel cylinder fluid pressure Pw is P 1  (=P 2 +ΔPb)), the vehicle transitions from the running state to the stopped state. 
         [0049]    In other words, when the wheel cylinder fluid pressure Pw is P 1 , the vehicle does not slide down on the sloping road. 
         [0050]      FIG. 7  shows an example where the brake pedal BP is further depressed from the state (where the stroke is S 1 ) shown in  FIG. 6 . In this example, the stroke increases from S 1  to S 2  (&gt;S 1 ), and accordingly, the master cylinder fluid pressure Pm increases from P 2  to Pb according to the characteristic shown by the broken line in  FIG. 4 . In this case, the linear valve differential pressure ΔP (command differential pressure ΔPd) is not maintained at constant Pb, but decreases from Pb to P 2 . For example, the linear valve differential pressure ΔP decreases such that the characteristic of the linear valve differential pressure ΔP shown in  FIG. 4  is offset by the increase amount of the master cylinder fluid pressure Pm in the reduction direction of the command differential pressure (see  FIG. 5 ). As a result, the wheel cylinder fluid pressure Pw is maintained at constant P 1 . As described above, in the brake device  10 , even if the linear valve differential pressure ΔP decreases by depressing of the brake pedal BP, the wheel cylinder fluid pressure Pw does not decrease. Therefore, the vehicle does not slip down on the sloping road. 
         [0051]      FIG. 8  shows an example where the brake pedal BP is returned from the state (where the stroke is S 2 ) shown in  FIG. 7 . In this example, the stroke decreases from S 2  to 0, and accordingly, the master cylinder fluid pressure Pm decreases from Pb to 0 according to the characteristic shown by the broken line in  FIG. 4 . In this case, the linear valve differential pressure ΔP (command differential pressure ΔPd) is maintained at P 2  within a range in which the stroke is equal to or greater than Sb, and decreases from P 2  to 0 according to the decrease of the stroke within a range in which the stroke is less than Sb (specifically, from S 3  to Sb). As a result, the wheel cylinder fluid pressure Pw decreases according to the decrease of the master cylinder fluid pressure Pm within a range in which the stroke is equal to or greater than Sb, and decreases according to the decrease of the linear valve differential pressure ΔP within the range in which the stroke is less than Sb (specifically, from S 3  to Sb). For example, the characteristic of the linear valve differential pressure ΔP which is offset by the increase amount of the master cylinder fluid pressure Pm according to further depressing of the brake pedal BP in the reduction direction of the command differential pressure is maintained, and the linear valve differential pressure ΔP decreases according to that characteristic. Here, the gradient of the decrease of the master cylinder fluid pressure Pm and the gradient of the decrease of the linear valve differential pressure ΔP coincide with each other. Therefore, the gradient of the decrease of the wheel cylinder fluid pressure Pw is constant within a range in which the stroke is between S 3  and S 2 . 
         [0052]    As described above, in the brake device  10 , when a returning operation to the brake pedal BP is performed, it is possible to smoothly and reliably decrease the wheel cylinder fluid pressure Pw in synchronization with the decrease of the stroke, that is, the intention of the driver to decrease the wheel cylinder fluid pressure. 
         [0053]    Hereinafter, “smooth decreasing” of the wheel cylinder fluid pressure Pw will be additionally described.  FIG. 9  shows a case where in place of the brake device, a comparative example is used in the same situation as that of the example shown in  FIG. 8 . In this comparative example, in a case where a returning operation to the brake pedal BP is performed, the linear valve differential pressure ΔP is determined to a smaller value between a present value and a value determined according to the characteristic shown by the solid line in  FIG. 4 . Therefore, in the example shown in  FIG. 9 , the linear valve differential pressure ΔP is maintained at P 2  even when the stroke becomes less than Sb, and decreases from P 2  toward 0 after the stroke reaches S 4 . 
         [0054]    Therefore, in a process where the stroke decreases from Sb to S 4 , the wheel cylinder fluid pressure Pw does not decrease according to the intention of the driver. As a result, the timing for the vehicle to start to slip down is delayed, and thus a strange feeling (such as a feeling as if the wheel cylinder fluid pressure is remaining, and a jerky feeling) might be caused to the driver. In contrast, in the brake device  10 , as shown in  FIG. 8 , even after the stroke becomes less than Sb, the wheel cylinder fluid pressure Pw continuously and smoothly decreases. Therefore, the above-explained strange feeling is not caused to the driver. 
         [0055]      FIG. 10  shows an example where the brake device  10  is used. In this example, at a time t 1 , the vehicle is stopped on a sloping road (or a flat road) by depressing the brake pedal BP (the stroke becomes greater than Sb) while the vehicle is running In this example, after the time t 1 , the driving of the hydraulic pumps HP 1  and HP 2  stop. However, the hydraulic pumps HP 1  and HP 2  may be continuously driven. 
         [0056]    In this example, after the time t 1 , from a time t 2  to a time t 3 , from a time t 4  to a time t 5 , and from a time t 6  to a time t 7 , the brake pedal is further depressed (increase of the stroke). Therefore, the linear valve differential pressure ΔP (command differential pressure ΔPd) decreases by the increase amount of the master cylinder fluid pressure Pm whenever the stroke increases. As a result, the characteristic of the linear valve differential pressure ΔP before the time t 2  is represented by a “characteristic a” of  FIG. 11  (which is the same as the characteristic shown by the solid line in  FIG. 4 ). The characteristic from the time t 3  to the time t 4  is represented by a “characteristic b” of  FIG. 11 , the characteristic from the time t 5  to the time t 6  is represented by a “characteristic c” of  FIG. 11 , and the characteristic after the time t 7  (that is, after the linear valve differential pressure ΔP reaches 0) is represented by a “characteristic d” (ΔP=0) of  FIG. 11 . 
         [0057]    As described above, in the brake device  10 , while the vehicle is in a stopped state, whenever the master cylinder fluid pressure Pm increases according to the increase of the stroke of the brake pedal BP, the linear valve differential pressure ΔP decreases. As a result, at the time when the accumulative increase amount of the master cylinder fluid pressure Pm reaches the linear valve differential pressure ΔP at the stop start timing of the vehicle, it is possible to make the linear valve differential pressure ΔP to 0. That is, it is possible to stop the driving of the normally open linear electromagnetic valves PC 1  and PC 2 . As s result, it is possible to reduce the driving times of the linear electromagnetic valves as compared to a brake device of JP-A-2007-276500 explained in “BACKGROUND” section, in which the linear electromagnetic valves are continuously driven while the vehicle is in a stopped state and the driver is operating a brake pedal. As a result, the durability of the linear electromagnetic valves can be improved. 
         [0058]    The present invention is not limited to the above-mentioned illustrative embodiment, and various modifications can be made within the scope of the present invention. For example, in the above-described illustrative embodiment, as shown in  FIG. 4 , the master cylinder fluid pressure Pm, the linear valve differential pressure ΔP, and the wheel cylinder fluid pressure Pw are regulated according to the stroke of the brake pedal BP. However, the master cylinder fluid pressure Pm, the linear valve differential pressure ΔP, and the wheel cylinder fluid pressure Pw may be regulated according to depressing force of the brake pedal BP. 
         [0059]    Further, in the above-described illustrative embodiment, the brake fluid pressure control unit  30  is configured by a fluid pressure circuit of two systems, that is, the system relative to the front right and left wheels FR and FL and the system relative to the rear right and left wheels RR and RL. However, the brake fluid pressure control unit  30  may be configured by a fluid pressure circuit of two systems, that is, a system relative to the front left wheel FL and the rear right wheel RR and a system relative to the front right wheel FR and the rear left wheel RL. 
         [0060]    Further, in the above-described illustrative embodiment, as shown in  FIGS. 4 and 8 , the gradient of change of the master cylinder fluid pressure Pm and the gradient of change of the linear valve differential pressure ΔP with respect to the stroke coincide with each other. However, they may not coincide with each other. 
         [0061]    Further, in the above-described illustrative embodiment, as shown in  FIG. 4 , the master cylinder fluid pressure Pm is maintained at 0 when the stroke is less than Sb, and increases from 0 according to the increase of the stroke when the stroke is equal to or greater than Sb. However, the master cylinder fluid pressure Pm may increase from 0 according to the increase of the stroke from 0. 
         [0062]    In this case, as shown in  FIG. 12 , if the linear valve differential pressure ΔP also increases according to the increase of the stroke from 0, it is possible to smoothly increase the wheel cylinder fluid pressure Pw from 0 according to the increase of the stroke from 0. Also, in this case, whenever depressing of the brake pedal BP (that is, increasing of the stroke) is made while the vehicle is in a stopped state, as shown in  FIG. 13 , the linear valve differential pressure ΔP (command differential pressure ΔPd) decreases by the increase amount in the master cylinder fluid pressure Pm.