Patent Publication Number: US-2011049972-A1

Title: Stroke simulator and brake control apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a stroke simulator capable of generating a reaction force in response to the operation of a brake pedal and a brake control apparatus using the stroke simulator. 
     2. Description of the Related Art 
     Stroke simulators have hitherto been used in brake control apparatuses in order to generate a reaction force in response to the operation of a brake pedal (See Patent Documents 1 and 2, for instance). 
     PRIOR ART DOCUMENTS 
     [Patent Documents] 
     [Patent Document 1] Japanese Patent Application Publication No. 2007-203859. 
     [Patent Document 2] Japanese Patent Application Publication No. 2006-248473. 
     With conventional stroke simulators, however, it is necessary to raise the spring constant of a spring provided in the stroke simulator in order to create a desired pedal feeling counter to a high hydraulic pressure generated by a master cylinder. To make the spring constant larger, the wire diameter or the size of the spring must be made larger, which will result in a large size of the stroke simulator. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the foregoing circumstances, and a purpose thereof is to provide a small stroke simulator and a brake control apparatus using such a stroke simulator. 
     In order to resolve the foregoing problems, a stroke simulator according to one embodiment of the present invention is a stroke simulator operative to generate a reaction force in response to an operation of a brake pedal, and the stroke simulator comprises: a housing; a piston disposed slidably in the housing, the piston dividing the interior of the housing into a first volumetric chamber and a second volumetric chamber; an elastic member disposed in at least one of the first volumetric chamber and the second volumetric chamber, the elastic member generating a reaction force in response to the operation of the brake pedal by elastic deformation caused by the sliding of the piston; and supply ports one each provided for the first volumetric chamber and the second volumetric chamber, the supply ports capable of supplying an operating oil pressure into the respective volumetric chambers when the brake pedal is operated. 
     By employing this embodiment, the operating oil pressure is supplied to both the first volumetric chamber and the second volumetric chamber when the brake pedal is stepped on. Thus, it suffices if the elastic member can be deformed elastically against a difference between the force received by a pressure receiving face that faces the first volumetric chamber of the piston and the force received by a pressure receiving face that faces the second volumetric chamber thereof. This allows the use of a stroke simulator spring of small wire diameter and small size, so that the stroke simulator can be made smaller. 
     The piston may comprise the area of a pressure receiving face on a side of the first volumetric chamber and the area of a pressure receiving face on a side of the second chamber which differs from the area of a pressure receiving face on a side of the first volumetric chamber. In such a case, even when the pressure is the same in both the first volumetric chamber and the second volumetric chamber, the piston can be slid because the area of the first volumetric chamber side pressure receiving face differs from the area of the second volumetric chamber side pressure receiving face. As a result, the reaction force in response to the operation of the brake pedal can be generated. 
     Another embodiment of the present invention relates to a brake control apparatus. This apparatus comprises: a wheel cylinder configured to apply a braking force to a wheel by supplying an operating oil pressure thereto; a brake pedal operated by a driver; a master cylinder configured to send out an operating oil pressurized in response to a press of the brake pedal; a master cut valve configured to shut off a flow between the master cylinder and the wheel cylinder; and a stroke simulator disposed between the master cylinder and the master cut valve, the stroke simulator generating a reaction force in response to an operation of the brake pedal. The stroke simulator includes: a housing; a piston disposed slidably in the housing, the piston dividing the interior of the housing into a first volumetric chamber and a second volumetric chamber; an elastic member disposed in at least one of the first volumetric chamber and the second volumetric chamber, the elastic member generating a reaction force in response to the operation of the brake pedal by elastic deformation caused by the sliding of the piston; and supply ports one each provided for the first volumetric chamber and the second volumetric chamber, the supply ports capable of supplying the operating oil pressure into the respective volumetric chambers when the brake pedal is operated. 
     By employing this embodiment, the operating oil pressure is supplied from the master cylinder to both the first volumetric chamber and the second volumetric chamber of the stroke simulator when the brake pedal is pressed. Thus, it suffices if the elastic member of the stroke simulator can be deformed elastically against a difference between the force received by a pressure receiving face that faces the first volumetric chamber of the piston and the force received by a pressure receiving face that faces the second volumetric chamber thereof. This allows the use of an elastic member of small wire diameter and small size, so that a brake control apparatus using a small stroke simulator can be achieved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments will now be described by way of examples only, with reference to the accompanying drawings which are meant to be exemplary, not limiting and wherein like elements are numbered alike in several Figures in which: 
         FIG. 1  is an illustration showing a brake control apparatus according to an embodiment of the present invention; 
         FIG. 2  is an illustration for explaining the structures of a master cylinder and a stroke simulator in greater detail; 
         FIG. 3  is an illustration for explaining operations of a brake control apparatus according to an embodiment of the present invention; 
         FIG. 4  is an illustration for explaining relational expressions pertaining to a brake control apparatus according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinbelow, a detailed description will be given of best modes for carrying out the invention with reference to the drawings. 
       FIG. 1  is a diagram showing a brake control apparatus  10  according to an embodiment of the present invention. The brake control apparatus  10  shown in  FIG. 1  constitutes an electronically controlled brake system for a vehicle and controls optimally the brakes of the four wheels of the vehicle based on the amount of operation of a brake pedal  12  stepped on by a driver. 
     The brake pedal  12  is connected to a master cylinder  14  that sends out an operating oil pressurized in response to pedal operation by the driver. The brake pedal  12  is provided with a stroke sensor  46  for detecting a pedal stroke caused by the pedal operation. 
     The master cylinder  14  has a first master hydraulic pressure chamber  78  and a second master hydraulic pressure chamber  80 . A reservoir tank  26  for storing the operating oil is connected above the master cylinder  14 . The first master hydraulic chamber  78  and the second master hydraulic chamber  80  communicate with the reservoir tank  26  when the pressing of the brake pedal  12  is released. 
     A brake hydraulic control pipe  18  for a right front wheel is connected to the first master hydraulic pressure chamber  78  of the master cylinder  14  via a first output port  14   a.  The brake hydraulic control pipe  18  is connected to a wheel cylinder  20 FR, for the right front wheel, which applies a braking force to the right front wheel. Also, a brake hydraulic control pipe  16  for a left front wheel is connected to the second master hydraulic pressure chamber  80  of the master cylinder  14  via a second output port  14   b.  The brake hydraulic control pipe  16  is connected to a wheel cylinder  20 FL, for 
     the left front wheel, which applies a braking force to the left front wheel. 
     A right master cut valve  22 FR is provided at a midway point of the brake hydraulic control pipe  18  for the right front wheel, whereas a left master cut valve  22 FL is provided at a midway point of the brake hydraulic control pipe  16  for the left front wheel. Both the right master cut valve  22 FR and the left master cut valve  22 FL are normally-opened type electromagnetic on-off valves which are opened when power is not being applied and closed when power is on. 
     A right master pressure sensor  48 FR for detecting a master cylinder pressure on a right front wheel side is provided at a midway point of the brake hydraulic control pipe  18  for the right front wheel, whereas a left master pressure sensor  48 FL for measuring a master cylinder pressure on a left front wheel side is provided at a midway point of the brake hydraulic control pipe  16  for the left front wheel. 
     In the brake control apparatus  10 , when the brake pedal  12  is stepped on by the driver, the stroke sensor  46  detects the amount of pedal operation. However, it is also possible to obtain the pedal operating force (pedaling force) applied to the brake pedal  12  from the master cylinder pressure detected by the right master pressure sensor  48 FR and the left master pressure sensor  48 FL. Therefore, it is preferable from a failsafe point of view that the master cylinder pressure is monitored by the two pressure sensors  48 FR and  48 FL by assuming the failure of the stroke sensor  46 . Hereinbelow, the right master pressure sensor  48 FR and the left master pressure sensor  48 FL will be generically referred to as “master pressure sensor  48 ” or “master pressure sensors  48 ” as appropriate. 
     The stroke simulator  24  creates a reaction force corresponding to a pressing operation of the brake pedal  12  by the driver. The stroke simulator  24  has two volumetric chambers which are a first volumetric chamber  178  and a second volumetric chamber  180   
     The first volumetric chamber  178  of the stroke simulator  24  is connected, upstream of the left master cut valve  22 FL, to the brake hydraulic control pipe  16  for the left front wheel. That is, the first volumetric chamber  178  of the stroke simulator  24  is connected to the second master hydraulic pressure chamber  80  of the master cylinder  14  via the second output port  14   b.  Also, the second volumetric chamber  180  of the stroke simulator  24  is connected, upstream of the right master cut valve  22 FR, to the brake hydraulic control pipe  18  for the right front wheel. That is, the second volumetric chamber  180  of the stroke simulator  24  is connected to the first master hydraulic pressure chamber  78  of the master cylinder  14  via the first output port  14   a.    
     A simulator cut valve  23  is provided midway in a flow passage connecting the second master hydraulic pressure chamber  80  of the master cylinder and the first volumetric chamber  178  of the stroke simulator  24 . The simulator cut valve  23  is a normally-closed type electromagnetic on-off valve which is opened when power is on and closed when power is not being applied (e.g., when abnormality occurs). 
     Connected to the reservoir tank  26  is one end of a hydraulic supply-exhaust pipe  28 , and the other end of the hydraulic supply-exhaust pipe  28  is connected to a suction opening of an oil pump  34  which is driven by a motor  32 . A discharge opening of the oil pump  34  is connected to a high-pressure pipe  30 , and connected to this high-pressure pipe  30  are an accumulator  50  and a relief valve  53 . In the present embodiment, the oil pump  34  to be used is a reciprocating pump equipped with two or more pistons (not shown) which are each reciprocated by the motor  32 . The accumulator  50  to be used is one that stores the pressure energy of operating oil converted into a pressure energy of a filler gas such as nitrogen. 
     The accumulator  50  stores the operating oil whose pressure is raised to about 14 to 22 MPa, for instance, by the oil pump  34 . A valve outlet of the relief valve  53  is connected to the hydraulic supply-exhaust pipe  28 . If the pressure of the operating oil in the accumulator  50  rises abnormally to about 25 MPa, for instance, the relief valve  53  will open to return the high-pressure operating oil to the hydraulic supply-exhaust pipe  28 . Further, an accumulator pressure sensor  51 , which detects the exit pressure of the accumulator  50 , namely, the pressure of operating oil in the accumulator  50 , is provided on the high-pressure pipe  30 . These components, such as the motor  32 , the oil pump  34  and the accumulator  50 , function as hydraulic power sources capable of delivering the operating oil pressurized by the supply of power independently from the operation of the brake pedal  12 . 
     And the high-pressure pipe  30  is connected to the wheel cylinder  20 FR for the right front wheel, the wheel cylinder  20 FL for the left front wheel, the wheel cylinder  2 ORR for the right rear wheel, and the wheel cylinder  2 ORL for the left rear wheel via pressure increasing valves  40 FR,  40 FL,  40 RR and  40 RL, respectively. Hereinbelow, the wheel cylinders  20 FR to  20 RL will be generically referred to as “wheel cylinder  20 ” or “wheel cylinders  20 ” as appropriate. Also, the pressure increasing valves  40 FR to  40 RL will hereinbelow be generically referred to as “pressure increasing valve  40 ” or “pressure increasing valves  40 ” as appropriate. The pressure increasing valves  40  are each a normally-closed type electromagnetic flow control valve (linear valve) which is closed when power is not being applied and which is used to increase the pressure of the wheel cylinder as needed. Note that a disk brake unit is provided for each wheel of a vehicle (not shown) and a braking force is generated by pressing a brake pad against a disk by the operation of the wheel cylinder  20 . 
     The wheel cylinder  20 FR for the right front wheel and the wheel cylinder  20 FL for the left front wheel are connected to the hydraulic supply-exhaust pipe  28  via pressure reducing valves  42 FR and  42 FL, respectively. The pressure reducing valves  42 FR and  42 FL are normally-closed type electromagnetic flow control valves (linear valves) which are used to reduce the pressure of the wheel cylinders  20 FR and  20 FL as needed. On the other hand, the wheel cylinder  20 RR for the right rear wheel and the wheel cylinder  20 RL for the left rear wheel are connected to the hydraulic supply-exhaust pipe  28  via the hydraulic supply-exhaust pipe  28  via pressure reducing valves  42 RR and  42 RL, respectively. Hereinbelow, the pressure reducing valves  42 FR to  43 RL will be generically referred to as “pressure reducing valve  42 ” or “pressure reducing valves  42 ” as appropriate. 
     Wheel cylinder pressure sensors  44 FR,  44 FL,  44 RR and  44 RL, which detect the wheel cylinder pressure, or the pressure of operating oil working on their corresponding wheel cylinders  20 , are disposed in the vicinity of the wheel cylinders  20 FR to  20 RL for the right front wheel, the left front wheel, the right rear wheel and the left rear wheel, respectively. Hereinbelow, the wheel cylinder pressure sensors  44 FR to  44 RL will be generically referred to as “wheel cylinder pressure sensor  44 ” or “wheel cylinder pressure sensors  44 ” as appropriate. 
     The above-described right master cut valve  22 FR, left master cut valve  22 FL, pressure increasing valves  40 FR to  44 RL, pressure reducing valves  42 FR to  42 RL, oil pump  34 , accumulator  50  and the like constitute a hydraulic actuator  100 . The hydraulic actuator  100  is controlled by an electronic control unit (hereinafter referred to as “ECU”)  200 . 
     The ECU  200  functions as a means for controlling the pressures of the wheel cylinders in the wheel cylinders  20 FR to  20 RL. The ECU  200  includes a CPU performing various arithmetic processings, a ROM for storing various control programs, a RAM used as a work area for data storage and program execution, nonvolatile memories such as a backup RAM capable of holding memory contents in the event of a stoppage of the engine, an I/O interface, an A/D converter for retrieving the signals after analog signals inputted from various sensors and the like have been converted into digital signals, a counting timer, and so forth. 
     Electrically connected to the ECU  200  are various actuator-type components containing the hydraulic actuators  100  such as the above-described right master cut valve  22 FR, left master cut valve  22 FL, simulator cut valve  23 , pressure increasing valves  40 FR to  44 RL and pressure reducing valves  42 FR to  42 RL. 
     Also, electrically connected to the ECU  200  are various sensor- and switch-type components that output signals used for the control. That is, the signals indicating the pressures of the wheel cylinders in the wheel cylinders  20 FR to  20 RL are inputted to the ECU  200  from the wheel cylinder pressure sensors  44 FR to  44 RL. 
     Also, the signal indicating a pedal stroke of the brake pedal  12  is inputted to the ECU  200  from the stroke sensor  46 . The signals indicating the pressures of the master cylinders are inputted o the ECU  200  from the right master pressure sensor  48 FR and the left master pressure sensor  48 FL. The signal indicating the pressure of the accumulator is inputted to the ECU  200  from the accumulator pressure sensor  51 . 
     Further, though not shown, the signal indicating the wheel speed of each wheel is inputted to the ECU  200  from a wheel speed sensor provided for each wheel. Also, the signal indicating a yaw rate is inputted to the ECU  200  from a yaw rate sensor, and the signal indicating the steering angle of a steering wheel is inputted to the ECU  200  from a steering angle sensor. 
     In the brake control apparatus  10  configured as above, when the brake pedal  12  is stepped on, the ECU  200  calculates a target deceleration of a vehicle from the pedal stroke and the master cylinder pressure indicating an actuating quantity (e.g., pressing level) of the brake pedal  12 . Then the ECU  200  evaluates a target hydraulic pressure, which is a target value of the wheel cylinder pressure of each wheel, in accordance with the thus calculated deceleration. Then the ECU  200  controls the opening degree of the pressure increasing valves  40  and the pressure reducing valves  42  in such a manner that the wheel cylinder pressure of each wheel is equal to the target hydraulic pressure. 
     On the other hand, the right master cut valve  27 FR and the left master cut valve  27 FL at this time are set in a closed state, whereas the simulator cut value  23  is set in an open state. As a result, the operating oil sent out from the master cylinder  14  as the brake pedal  12  is pressed by the driver will flow into the stroke simulator  24 . This will create a pedal reaction force in response to the pedaling force  12  of the brake pedal  12 . 
     If the accumulator pressure is less than a lower limit of control range, the ECU  200  will raise the accumulator pressure by driving the oil pump  34 . If the accumulator pressure is within the control range, the driving of the oil pump  34  will be stopped. 
       FIG. 2  is an illustration for explaining the structures of a master cylinder  14  and a stroke simulator  24  in greater detail. The master cylinder  14  includes a master housing  60 , a first master piston  62 , and a second master piston  64 . 
     The master cylinder  14  has the first master piston  62  slidably housed in the master housing  60 . Further, inside the master housing  60 , the second master piston  64  is housed slidably in a position forward of the first master piston  62 . With the two pistons inserted in the master housing  60  as described above, a first master hydraulic pressure chamber  78  is formed between the first master piston  62  and the second master piston  64 , and a second master hydraulic pressure chamber  80  is formed between the second master piston  64  and the bottom of the master housing  60 . It should be noted that in this patent specification, the term “forward” refers to the direction in which the first master piston  62  moves when the brake pedal  12  is stepped on, and the term “backward” refers to the direction in which the first master piston  62  moves when the brake pedal  12  returns to a predetermined initial position after the stepping-on is released. 
     Disposed at the backward end of the first master piston  62  is a piston rod  70  which connects the first master piston  62  with the brake pedal  12 . Also, a first master spring  66  is disposed at a predetermined mounting load between the first master piston  62  and the second master piston  64 , and a second master spring  68  is disposed at a predetermined mounting load between the second master piston  64  and the bottom of the master housing  60 . 
     A first output port  14   a  of the master cylinder  14  communicates with the first master hydraulic pressure chamber  78 , and a brake hydraulic control pipe  18  for the right front wheel is connected to the first output port  14   a.  A second output port  14   b  of the master cylinder  14  communicates with the second master hydraulic pressure chamber  80 , and a brake hydraulic control pipe  16  for the left front wheel is connected to the second output port  14   b.    
     The stroke simulator  24  includes a stroke simulator housing  160 , a stroke simulator piston  162 , and a stroke simulator spring  166 . 
     The stroke simulator piston  162  is slidably housed in the stroke simulator housing  160 . The stroke simulator piston  162  divides the interior of the stroke simulator housing  160  into a first volumetric chamber  178  and a second volumetric chamber  180 . Inside the second volumetric chamber  180 , the stroke simulator spring  166  is provided in such a manner as to bias the stroke simulator piston  162  toward the first volumetric chamber  178 . In other words, the stroke simulator spring  166  is provided to bias the stroke simulator piston  162  in such a direction as to reduce the volume of the first volumetric chamber  178 . The stroke simulator spring  166  generates a reaction force in response to the operation of the brake pedal  12  by undergoing elastic deformation caused by the sliding of the stroke simulator piston  162 . 
     The stroke simulator piston  162  is such that there is a difference between the area of a first volumetric chamber side pressure receiving face  162   a  facing the first volumetric chamber  178  and the area of a second volumetric chamber side pressure receiving face  162   b  facing the second volumetric chamber  180 . In the present embodiment, as shown in  FIG. 2 , the stroke simulator piston  162  is formed such that the area of the first volumetric chamber side pressure receiving face  162   a  is larger than the area of the second volumetric chamber side pressure receiving face  162   b.    
     The first volumetric chamber  178  and second volumetric chamber  180  of the stroke simulator  24  are provided with a first supply port  164  and a second supply port  165 , respectively, for supplying the operating oil pressures into the respective volumetric chambers. 
     The first supply port  164  of the first volumetric chamber  178  is connected to the brake hydraulic control pipe  16  in a position upstream of the left master cut valve. That is, the first volumetric chamber  178  of the stroke simulator  24  is connected to the second master hydraulic pressure chamber  80  of the master cylinder  14  via the first supply port  164 . Note that the simulator cut valve, which is to be provided between the brake hydraulic control pipe  18  and the stroke simulator  24 , is not shown in  FIG. 2 . 
     The second supply port  165  of the second volumetric chamber  180  is connected to the brake hydraulic control pipe  18  in a position upstream of the right master cut valve. That is, the second volumetric chamber  180  of the stroke simulator  24  is connected to the first master hydraulic pressure chamber  78  of the master cylinder  14  via the second supply port  165 . 
       FIG. 3  is an illustration for explaining operations of a brake control apparatus according to the present embodiment. When the brake pedal  12  is stepped on by the driver, the right master cut valve and the left master cut valve, as described already, are closed and the simulator cut valve is opened. Accordingly, the pressing of the brake pedal  12  by the driver causes the operating oil pressure sent out from the second master hydraulic pressure chamber  80  of the master cylinder  14  to be supplied to the first volumetric chamber  178  of the stroke simulator  24  via the first supply port  164 . 
     The supply of the operating oil pressure increases the volume of the first volumetric chamber  178 , and the stroke simulator piston  162  moves in such a manner as to reduce the volume of the second volumetric chamber  180 . As a result, the stroke simulator spring  166  is deformed elastically, and a reaction force in response to it is applied to the brake pedal  12 . 
     Further, according to the present embodiment, the second volumetric chamber  180  of the stroke simulator  24  is connected to the first master hydraulic pressure chamber  78  of the master cylinder  14 , so that when the brake pedal  12  is pressed, the operating oil pressure is also supplied to the second volumetric chamber  180 . This operating oil pressure supplied to the second volumetric chamber  180  generates such a force as to push the stroke simulator piston  162  toward the first volumetric chamber  178 . 
     Note here that in the present embodiment as described above, the stroke simulator  24  is formed such that the area of the first volumetric chamber side pressure receiving face  162   a  is larger than the area of the second volumetric chamber side pressure receiving face  162   b . Accordingly, even when the same operating pressure has occurred in the first master hydraulic pressure chamber  78  and the second master hydraulic pressure chamber  80  of the master cylinder  14 , a difference can be created between the force the first volumetric chamber side pressure receiving face  162   a  receives from the hydraulic oil and the force the second volumetric chamber side pressure receiving face  162   b  receives from it. Thus, a reaction force due to the elastic deformation of the stroke simulator spring  166  can be obtained. 
     With a conventional stroke simulator, the first volumetric chamber  178  is connected to the second master hydraulic pressure chamber  80  of the master cylinder  14  whereas the second volumetric chamber  180  is connected to a reservoir tank or the like. In such a case, it is necessary to set the spring constant of the stroke simulator spring  166  high in order to achieve a desired pedal feeling counter to the high master cylinder pressure applied to the first volumetric chamber  178  when the brake pedal  12  is pressed. To make the spring constant larger, the wire diameter or the size of the spring must be made larger, which will result in a large size of the stroke simulator. 
     In contrast to that, in a brake control apparatus  10  according to the present embodiment, the second volumetric chamber  180  of the stroke simulator  24  is connected to the first master hydraulic pressure chamber  78  of the master cylinder  14 , so that when the brake pedal  12  is pressed, the operating oil pressure is also supplied to the second volumetric chamber  180 . And this generates such a force as to push the stroke simulator piston  162  toward the first volumetric chamber  178 . It can be considered that this force assists the biasing force of the stroke simulator spring  166 . Hence, the stroke simulator spring  166  is acceptable if it can be deformed elastically against a difference between the force the first volumetric chamber side pressure receiving face  162   a  receives from the operating oil and the force the second volumetric chamber side pressure receiving face  162   b  receives from the operating oil. This allows the use of a stroke simulator spring  166  of small wire diameter and small size, so that the stroke simulator  24  can be made smaller. 
       FIG. 4  is an illustration for explaining relational expressions pertaining to the brake control apparatus  10  according to the present embodiment. Here, the stroke of the piston rod  70  is denoted as strk_rod, and the force inputted to the piston rod  70  as F rod. Also, as regards the master cylinder  14 , the spring constant of the first master spring  66  is denoted as k_mc 1 , the spring constant of the second master spring  68  as k_mc 2 , the sectional area of the first master hydraulic pressure chamber  78  as sa_mc 1 , the sectional area of the second master hydraulic pressure chamber  80  as sa_mc 2 , the hydraulic pressure of the first master hydraulic pressure chamber  78  as p_mc 1 , and the hydraulic pressure of the second master hydraulic pressure chamber  80  as p_mc 2 . Also, as regards the stroke simulator  24 , the spring constant of the stroke simulator spring  166  is denoted as k_ss, the area of the first volumetric chamber side pressure receiving face  162   a  as sa_ss 1 , the area of the second volumetric chamber side pressure receiving face  162   b  as sa_ss 2 , and the stroke of the stroke simulator piston  162  as strk_ss. 
     The following relational expressions (1) to (6) hold for the master cylinder  14  and the stroke simulator  24  shown in  FIG. 4 . 
     (1) Expression of equilibrium of forces at the stroke simulator piston  162 : 
       sa_ss1×p_mc2=sa_ss2×p_mc1+k_ss×strk_ss
 
     (2) Expression of equilibrium of forces at the first master piston  62 : 
       k_mc1×strk_mc1+p_mc1×sa_mc1=F_rod
 
     (3) Expression of equilibrium of forces at the second master piston  64 : 
       k_mc2×strk_mc2+p_mc2×sa_mc2=p_mc1×sa_mc2+k_mc1×strk_mc1
 
     (4) Expression of equilibrium of the amount of operating oil in the first master hydraulic pressure chamber  78 : 
       strk_mc1×sa_mc1=−strk_ss×sa_ss2
 
       strk_mc1=−strk_ss×sa_ss2/sa_mc1
 
     (5) Expression of equilibrium of the amount of operating oil in the second master hydraulic pressure chamber  80 : 
       strk_mc2×sa_mc2=strk_ss×sa_ss1
 
       strk_mc2=strk_ss×sa_ss1/sa_mc2
 
     (6) Relational expression of stroke: 
       strk_rod=strk_mc1+strk_mc2 
     The first term sa_ss 2 ×p_mc 1  of the right-hand side of expression (1) is the term which does not exist with a conventional stroke simulator. That is, the expression of equilibrium of forces at the stroke simulator piston of the conventional stroke simulator will be as expressed in expression (7) below. 
       sa_ss1×p_mc2=k_ss×strk_ss   (7)
 
     The stroke strk ss of the stroke simulator piston  162  has the limits, so that when the hydraulic pressure p_mc 2  in the second master hydraulic pressure chamber  80  is high, the spring constant k_ss of the stroke simulator spring  166  must be made large to satisfy expression (7). 
     Expression (8) below is one with the first term of the right-hand side of expression (1) transpose to the left-hand side. 
       sa_ss1×p_mc2−sa_ss2×p_mc1=k_ss×strk_ss   (8)
 
     In the present embodiment, it is so arranged that the operating oil pressure is supplied from the first master hydraulic pressure chamber  78  to the second volumetric chamber  180 . As a result, a force (sa_ss 2 ×p_mc 1 ) is generated that reduces the force (sa_ss 1 ×p_mc 2 ) which pushes the stroke simulator piston  162  toward the second volumetric chamber  180 . The occurrence of this force allows the spring constant k_ss of the stroke simulator spring  166  to become smaller. In other words, a small stroke simulator spring  166  can be used. 
     The expressions (1) to (6) may be rearranged into expression (9) below. 
       F_rod==strk_rod/(sa_ss1/sa_mc2−sa_ss2/sa_mc1)×{k_mc1×sa_ss2/sa_mc1+(k mc1×sa _ss2/sa_mc1−k_mc2×sa_ss1/sa_mc2+k_ss/sa_ss1×sa_mc2)/(sa_ss2/sa_ss1×sa_mc2−sa_mc2)×sa_mc1}  (9)
 
     As shown by the expression (9), the relationship between the stroke strk rod of the piston rod  70  and the force F rod inputted to the piston rod  70  can be expressed by another design parameters (kmc 1 , kmc 2 , etc.), which indicate the feasibility of the brake control apparatus  10  according to the present embodiment. 
     The present invention has been described by referring to the embodiments and such description is for illustrative purposes only. It is understood by those skilled in the art that any arbitrary combinations of the embodiments and any arbitrary combinations of the constituting elements and processes could be developed as modifications and that such modifications are also within the scope of the present invention. 
     In the above-described embodiment, a single stroke simulator spring  166  is used. However, one having multistage spring characteristics or nonlinear spring characteristics may be used if an improved feeling of brake operation by the driver is to be achieved. 
     Also, in above-described embodiment, the second master hydraulic pressure chamber  80  is connected to the first volumetric chamber  178 , and the first master hydraulic pressure chamber  78  is connected to the second volumetric chamber  180 . However, the arrangement may be such that the first master hydraulic pressure chamber  78  is connected to the first volumetric chamber  178 , and the second master hydraulic pressure chamber  80  is connected to the second volumetric chamber  180 .