Patent Publication Number: US-2011066345-A1

Title: Brake System

Description:
TECHNICAL FIELD 
     The present invention relates to a brake system that controls a deceleration of a vehicle by controlling an actuator that boosts a master cylinder. 
     BACKGROUND ART 
     A known example of a brake system that performs cooperative control of a hydraulic brake and a regenerative brake includes, as described in Patent Document 1, a BBW (Brake-By-Wire) in which a brake pedal is electrically connected to an actuator. 
     Such a brake system includes, for example, a control device for controlling a frictional brake actuator that generates a braking force by pressurizing hydraulic oil and a regenerative brake actuator that generates a braking force by regeneration. Based on a stroke amount of a brake pedal, a vehicle speed, or the like, the control device determines a distribution of braking forces to be generated by the frictional brake actuator and the regenerative brake actuator, and outputs a control signal to each actuator. 
     In addition, Patent Document 2 describes an electrically-driven brake booster used in a brake mechanism of an automobile that utilizes an electrically-driven actuator as a booster. 
     Patent Document 1 JP Patent Application Publication No. 2005-329740 A (2005) 
     Patent Document 2 JP Patent Application Publication No. 2007-191133 A (2007) 
     DISCLOSURE OF THE INVENTION 
     Problems to be Solved by the Invention 
     The electrical connection between the brake pedal and the actuator in the brake system described in Patent Document  1  prevents an unnecessary reaction force or the like from being outputted to the brake pedal. However, the brake system according to Patent Document 1 has a higher production cost than a conventional brake system using a negative-pressure booster, and is low in reliability since the brake pedal and a mechanism for generating hydraulic pressure are electrically connected to each other. 
     The brake system described in Patent Document 2 features a brake pedal and a frictional brake actuator mechanically connected to each other, and adheres to a structure of a conventional brake system using a negative-pressure booster. Therefore, the brake system has a lower production cost and higher reliability than the brake system according to Patent Document 1. However, since the brake pedal and the frictional brake actuator are mechanically connected to each other in the brake system according to Patent Document 2, the brake system is susceptible to changes in hydraulic pressure of the frictional brake actuator during regenerative cooperative control and a reaction force of the brake pedal is liable to variation. Given that many drivers operate a brake pedal using a pedal depressing force, a variation in a pedal reaction force is accompanied by a fluctuation in a pedal stroke amount. In Patent Document 2, since an output of the frictional brake actuator is determined based on a pedal depressing force and an input rod displacement amount, a fluctuation in deceleration occurs. Since such fluctuations in the pedal reaction force and a deceleration are totally unrelated to the intentions of a driver, the respective fluctuations must be either reduced or suppressed. 
     An object of the present invention is to provide a brake control technique that enables suppression of fluctuations in deceleration not intended by a driver. 
     Means for Solving the Problems 
     In order to achieve the object described above, a brake system according to the present invention includes a pedal and an actuator that generates hydraulic pressure, wherein the brake system controls a braking force based on a pedal reaction force. 
     In addition to the feature described above, the brake system according to the present invention controls braking force based on a displacement amount of a piston that pressurizes a master cylinder. 
     Furthermore, the brake system according to the present invention controls a braking force based on a pedal reaction force and on a hydraulic pressure generated by the actuator. 
     Moreover, the brake system according to the present invention includes a control device that stores braking force characteristics based on a pedal reaction force and on a displacement amount of the piston that pressurizes the master cylinder. 
     In addition, the brake system according to the present invention includes a control device that stores braking force characteristics based on a pedal reaction force and on a hydraulic pressure generated by the actuator. 
     Furthermore, the brake system according to the present invention includes: a hydraulic braking device having a pedal, a master pressure generating device, and a wheel pressure generating device; and a regenerative braking device, wherein the brake system adjusts a total braking force based on a pedal reaction force and a displacement amount of a piston that pressurizes a master cylinder in order to maintain the total braking force at approximately a constant level when a transition is made from regenerative braking to frictional braking in response to a decrease in vehicle speed. 
     Moreover, in addition to the features described above, the brake system according to the present invention includes: means for calculating a maximum regenerative braking force based on a vehicle speed and/or a gear position; and means for calculating a regenerative braking force limit based on a vehicle speed, wherein the regenerative braking force limit is to be set as a regenerative braking force when the maximum regenerative braking force is greater than the regenerative braking force limit, the maximum regenerative braking force is to be set as a regenerative braking force when the maximum regenerative braking force is smaller than the regenerative braking force limit, the regenerative braking device is to output the regenerative braking force and the hydraulic braking device is to output a difference between the total braking force and the regenerative braking force when the total braking force is greater than the regenerative braking force, while the total braking force is to be outputted solely by the regenerative braking device when the total braking force is smaller than the regenerative braking force. 
     Furthermore, an automobile according to the present invention is mounted with any of the brake systems described above. 
     Advantage of the Invention 
     According to the present invention, since a braking force fluctuation and a deceleration fluctuation during a transition period from a regenerative brake to a hydraulic brake can be suppressed, brake operations of vehicles such as a hybrid vehicle mounted with a hydraulic brake and a regenerative brake, an electric car, and the like can be operated in a stable and simple manner. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an explanatory diagram illustrating a configuration of a vehicle to which the present invention has been applied. 
         FIG. 2  is an explanatory diagram illustrating a functional configuration of a brake system according to the present invention. 
         FIG. 3  is an explanatory diagram illustrating a configuration of a master pressure generating device and a wheel pressure generating device according to the present invention. 
         FIG. 4  is a flowchart illustrating basic operations of the brake system according to the present invention. 
         FIG. 5  is a graph illustrating a maximum regenerative braking force outputted by a regenerative braking device based on a vehicle speed and a gear position in the brake system according to the present invention. 
         FIG. 6  is a graph illustrating a limit of a regenerative braking force outputted by the regenerative braking device based on a vehicle speed in the brake system according to the present invention. 
         FIG. 7  is a graph illustrating a frictional braking force outputted by the master pressure generating device based on an input rod displacement amount in the brake system according to the present invention. 
         FIG. 8  is a graph illustrating an ideal output during execution of the flowchart illustrated in  FIG. 4  when a frictional braking force and a regenerative braking device are approximately equal to each other in the brake system according to the present invention. 
         FIG. 9  is a graph illustrating an actual output when a master pressure generating device  200  and a regenerative braking device  18  are controlled according to the flowchart illustrated in  FIG. 4  in a case where a frictional braking force and a regenerative braking device are approximately equal to each other in the brake system according to the present invention. 
         FIG. 10  is a graph illustrating an actual output when a wheel pressure generating device  300  and the regenerative braking device  18  are controlled according to the flowchart illustrated in  FIG. 4  in a case where a frictional braking force and a regenerative braking device are approximately equal to each other in the brake system according to the present invention. 
         FIG. 11  is a graph illustrating characteristics of a total braking force outputted by the brake system based on a pedal reaction force and a piston displacement amount used in the brake system according to the present invention. 
         FIG. 12  is a flowchart illustrating operations of the brake system according to the present invention. 
         FIG. 13  is a graph illustrating an actual output when the master pressure generating device  200  and the regenerative braking device  18  ace controlled according to the total braking force characteristics illustrated in  FIG. 11  and the flowchart illustrated in  FIG. 12  in a case where a frictional braking force and a regenerative braking force are approximately equal to each other in the brake system according to the present invention. 
         FIG. 14  is a graph illustrating characteristics of a total braking force outputted by the brake system based on a pedal reaction force and on a hydraulic pressure increased/reduced by the wheel pressure generating device  300  used in the brake system according to the present invention. 
         FIG. 15  is a graph illustrating an actual output when the wheel pressure generating device  300  and the regenerative braking device  18  are controlled according to the total braking force characteristics illustrated in  FIG. 14  and the flowchart illustrated in  FIG. 12  in a case where the frictional braking force and a regenerative braking device are approximately equal to each other in the brake system according to the present invention. 
     
    
    
     DESCRIPTION OF SYMBOLS 
     
         
           10  vehicle 
           15   a,    15   b,    15   c,    15   d  wheel 
           16  brake pedal 
           17  electrical storage device 
           18  regenerative braking device 
           20   a,    20   b,    20   c,    20   d  disk rotor 
           21   a,    21   b,    21   c,    21   d  brake caliper 
           31  brake sensor 
           100  brake control device 
           110  CPU 
           111  braking force calculating unit 
           112  communication control unit 
           200  master pressure generating device 
           201  master pressure controller 
           210  master pressure generating mechanism 
           300  wheel pressure generating device 
           301  wheel pressure controller 
           310  wheel pressure generating mechanism 
       
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Hereinafter, an embodiment according to the present invention will be described with reference to  FIGS. 1 to 15 . 
     While the present embodiment is an example where the present invention is applied to an FF (front-engine, front-wheel drive) vehicle, the example is not restrictive and the present invention is also applicable to vehicles such as a 4WD (four-wheel drive) vehicle and an FR (front-engine, rear-wheel drive) vehicle. 
     As illustrated in  FIG. 1 , a vehicle  10  according to a first embodiment includes an engine  11 , a torque converter  12 , a transmission  13 , drive shafts  14  and  19 , wheels  15   a  to  15   d , a brake pedal  16 , disk rotors  20   a  to  20   d,  brake calipers  21   a  to  21   d,  a brake control device  100 , a master pressure generating device  200  that generates hydraulic pressure for operating the brake calipers  21   a  to  21   d,  a wheel pressure generating device  300  that similarly generates hydraulic pressure for operating the brake calipers  21   a  to  21   d , an electrical storage device  17 , and a regenerative braking device  18  that applies braking force to rear wheels  15   c  and  15   d.    
     The engine  11  is an internal-combustion engine that causes an explosion of an air-fuel mixture inside a combustion chamber to generate power. A movement of a piston caused by the explosion is converted into a rotational movement of a crankshaft via a con rod. The crankshaft transfers power to front wheels  15   a  and  15   b  via the torque converter  12 , the transmission  13 , and the drive shaft  14 . 
     The torque converter  12  is provided between the engine  11  and the transmission  13 . Through the use of a working fluid such as oil, the torque converter  12  functions as a clutch that intermittently transfers rotational torque outputted from the engine  11  to the transmission  13 , and also amplifies the rotational torque before transferring the same to the transmission  13 . 
     The transmission  13  is provided between the torque converter  12  and the drive shaft  14  and has a plurality of gears that correspond to respective shift stages of, for example, five forward stages (first to fifth speeds) and one reverse stage. 
     The drive shaft  14  is a rotary shaft that couples the transmission  13  to the front wheels  15   a  and  15   b,  and transfers the rotational driving force of the engine  11  to the front wheels  15   a  and  15   b.    
     The brake pedal  16  is to be operated by a driver when decelerating the vehicle  10 . A depressing force of the driver is transferred to the master pressure generating device  200  via the brake pedal  16 . Hydraulic pressure generated at the master pressure generating device  200  is transferred to the brake calipers  21   a  to  21   d  via the wheel pressure generating device  300  and operates the brake calipers  21   a  to  21   d . The wheel pressure generating device  300  either transfers the hydraulic pressure generated at the master pressure generating device  200  to the brake calipers  21   a  to  21   d  without modification, or transfers the hydraulic pressure to the brake calipers  21   a  to  21   d  after further pressurization. 
     The brake is made up of disk rotors  20   a  to  20   d  and the brake calipers  21   a  to  21   d . The respective disk rotors  20   a  to  20   d  are fixed to the respective wheels  15   a  to  15   d  and rotate integrally with the wheels  15   a  to  15   d.  Although not shown, each of the brake calipers  21   a  to  21   d  is made up of a cylinder, a piston, a pad, and the like. The pistons in the cylinders are moved by hydraulic oil from the master pressure generating device  200  and the wheel pressure generating device  300 , and press pads coupled to the pistons against the disk rotors  20   a  to  20   d . When the pads press against the disk rotors  20   a  to  20   d,  a frictional force is generated between the pads and the disk rotors  20   a  to  20   d.  The frictional force acts as a braking force on the respective wheels  15   a  to  15   d,  and further generates a braking force between the respective wheels  15   a  to  15   d  and the road surface. 
     The regenerative braking device  18  is connected to drive shafts  19  respectively extending from left and right rear wheels  15   c  and  15   d,  and during a braking process, generates electricity according to a rotation of the drive shafts  19  and supplies the generated electricity to the electrical storage device  17 . At the same time, rotational resistance during the generation of electricity provides a braking force to the left and right rear wheels  15   c  and  15   d.    
     As illustrated in  FIG. 2 , the electrical storage device  17  is provided with a voltmeter  36  for detecting a voltage of the electrical storage device. The voltmeter  36  is connected to an interface  101  of the brake control device  100  in the same manner as other sensors. 
     In the present embodiment, among the components of the vehicle described above, the brake system is constituted by the brake pedal  16 , the disk rotors  20   a  to  20   d,  the brake calipers  21   a  to  21   d , the master pressure generating device  200 , the wheel pressure generating device  300 , the brake control device  100 , a brake sensor to be described later, and the regenerative braking device  18 . 
     As illustrated in  FIG. 2 , the brake control device  100  is a computer including a CPU that performs various arithmetic processing, the interface  101  that receives/transmits signals from/to the outside, a ROM  102  that stores, in advance, various programs to be executed by the CPU, various data, and the like, and a RAM  103  to be used as a workspace by the CPU. 
     The CPU functionally includes braking force calculating means  111  that calculates a target deceleration based on information from the various sensors, communication control means  112  that determines a braking force distribution between frictional braking and regenerative braking based on the target deceleration calculated by the braking force calculating means  111  and on information from the various sensors, and a communication control unit that controls communication with the outside. The respective functional units  111  and  112  are both activated when the CPU  110  executes programs stored in the ROM  102 . 
     The various sensors include the brake sensor  31 , a vehicle speed sensor  32  that detects a speed of the vehicle  10 , a longitudinal acceleration sensor  33  that detects an acceleration being generated in a longitudinal direction of the vehicle  10 , a wheel speed sensor  34  that detects speeds of the respective wheels  15   a  to  15   d,  and a gear position sensor  35  that detects a gear position of the transmission  13 . The various sensors are all connected to the interface  101  of the brake control device  100 . 
     The brake sensor  31  that detects a required braking force of the driver is, as illustrated in  FIG. 3 , a stroke sensor that detects a displacement amount of an input rod  214  coupled to the brake pedal  16 . A plurality of stroke sensors may be combined to make up the brake sensor  31 . Accordingly, a fail-safe can be secured because even when a signal from one sensor ceases, a driver&#39;s brake request can be detected and recognized by the remaining sensors. In addition, the brake sensor  31  may also be a depressing force sensor that detects a depressing force applied to the brake pedal  16 , or a combination of the depressing force sensor and a stroke sensor. 
     The master pressure generating device  200  includes a master pressure controller  201  that receives a drive control signal from the brake control device  100  and a master pressure generating mechanism  210  controlled by the master pressure controller  201 . 
     In addition, the wheel pressure generating device  300  includes a wheel pressure controller  301  that receives a drive control signal from the brake control device  100  and a wheel pressure generating mechanism  310  controlled by the wheel pressure controller  301 . 
     As illustrated in  FIG. 3 , the master pressure generating mechanism  210  includes a return spring storage cylinder  211 , a master cylinder  212  internally filled with hydraulic oil, a reservoir tank  213  that stores hydraulic oil to be supplied to the inside of the master cylinder  212 , the input rod  214  as first pressurizing means having one end coupled to the brake pedal  16  and another end facing the inside of the master cylinder  212 , and a motor pressurizing mechanism  220  as second pressurizing means. 
     The inside of the reservoir tank  213  is divided by a partition wall, not shown, to provide the reservoir tank  213  with two fluid chambers. The respective fluid chambers are connected to respective fluid chambers  215  and  216 , to be described later, in the master cylinder  212 . 
     The motor pressurizing mechanism  220  includes a pressurizing motor  221  that is driven by a drive signal from the master pressure controller  201 , a deceleration mechanism  230  that amplifies a rotational torque of the pressurizing motor  221 , a rotation-to-translation conversion mechanism  240  that converts a rotational force into a translational force, a movable member  250  that moves linearly while in contact with the rotation-to-translation conversion mechanism  240 , a primary piston  251  that is pressed by the movable member  250  and forms a primary fluid chamber  215  in the master cylinder  212 , a secondary piston  252  that forms a secondary fluid chamber  216  in the master cylinder  212 , and a return spring  255  which is arranged inside the return spring storage cylinder  211  and which attempts to restore the movable member  250  pressed by the rotation-to-translation conversion mechanism  240  to its original position. 
     The deceleration mechanism  230  amplifies a rotational torque of the pressurizing motor  221  precisely by a deceleration ratio thereof Suitable deceleration methods include gear deceleration and pulley deceleration. The present embodiment adopts a pulley deceleration system that includes a driving side pulley  231  attached to a rotational shaft of the pressurizing motor  221 , a driven side pulley  232 , and a belt  233  that bridges the driving side pulley  231  and the driven side pulley  232 . When the pressurizing motor  221  has a sufficiently large rotational torque and does not require torque amplification by deceleration, the deceleration mechanism  230  may be omitted and the pressurizing motor  221  may be directly coupled to the rotation-to-translation conversion mechanism  240 . Accordingly, various problems related to reliability, quietness, mountability, and the like that arise due to the interposition of the deceleration mechanism  230  can be avoided. 
     The rotation-to-translation conversion mechanism  240  converts a rotational power of the pressurizing motor  221  into a translational power and presses the primary piston  251  via the movable member  250 . Suitable conversion mechanisms include a rack-and-pinion and a ball screw. The present embodiment adopts a ball screw system including a ball screw nut  241  that is rotated by the driven side pulley  232  and a ball screw shaft  242  whose translational movement is caused by a rotational movement of the ball screw nut  241 . 
     One end of the input rod  214  is coupled to the brake pedal  16  and the other end faces the inside of the primary fluid chamber  215  in the master cylinder  212 . When the brake pedal  16  is depressed and the input rod  214  makes a rectilinear movement, the hydraulic pressure in the primary fluid chamber  215  rises and the secondary piston  252  is pressed, causing the hydraulic pressure in the secondary fluid chamber  216  to also rise. As a result, hydraulic oil is supplied to a first master pipe  261  connecting the primary fluid chamber  215  and the wheel pressure generating mechanism  310  and to a second master pipe  262  connecting the secondary fluid chamber  216  and the wheel pressure generating mechanism  310 , and the hydraulic oil is then delivered to the respective brake calipers  21   a  to  21   d  via the wheel pressure generating device  300 . Therefore, a predetermined braking force can be secured even when the motor pressurizing mechanism  220  is unable to operate normally due to a failure or the like. 
     In addition, as described above, when the brake pedal  16  is depressed, the hydraulic pressure in the primary fluid chamber  215  rises and the hydraulic pressure acts as a brake pedal reaction force. Therefore, by adopting the structure of the present embodiment, a mechanism such as a screw for generating a brake pedal reaction force becomes unnecessary. Accordingly, a contribution can be made to reducing the size and weight of the brake system. 
     The pressurizing motor  221  is operated by a drive signal from the master pressure controller  201  and generates a desired rotational torque. While a DC motor, a DC brushless motor, an AC motor or the like is suitable as the pressurizing motor  221 , a DC brushless motor is most preferable in terms of controllability, quietness, and durability. The pressurizing motor  221  includes a position sensor and is configured so that a position signal from the position sensor is inputted to the master pressure controller  201 . Accordingly, the master pressure controller  201  is capable of calculating a rotational angle of the pressurizing motor  221  based on the position signal from the position sensor, and further calculating a translation amount of the rotation-to-translation conversion mechanism  240  or, in other words, a displacement amount of the primary piston  251 . 
     The rotational torque of the pressurizing motor  221  is amplified by the deceleration mechanism  230  and rotates the ball screw nut  241  of the rotation-to-translation conversion mechanism  240 . The rotation of the ball screw nut  241  causes a translational movement of the ball screw shaft  242 , which in turn presses against the primary piston  251  via the movable member  250 . 
     In addition, an end of the return spring  255  is in contact with the movable member  250  on a side opposite to the ball screw shaft  242 , and the other end of the return spring  255  is in contact with an inner wall of the return spring storage cylinder  211 . Therefore, a force in the opposite direction of the thrust force of the ball screw shaft  242  acts on the ball screw shaft  242  via the movable member  250 . Accordingly, in a state where the pressurizing motor  221  is driven, the primary piston  251  is pressed, and a master pressure (a pressure within the master cylinder  212 ) is being pressurized, even if the pressurizing motor  221  stops due to a failure or the like and a return control applied to the ball screw shaft  242  is disabled, the ball screw shaft  242  is returned to its initial position by an elastic force of the return spring  255  and the master cylinder pressure can be lowered to around zero. As a result, a drag on the braking force due to a failure of the pressurizing motor  221  can be avoided. 
     When the primary piston  251  is pressed, the hydraulic pressure in the primary fluid chamber  215  rises, in turn pressing the secondary piston  252  and causing the hydraulic pressure in the secondary fluid chamber  216  to also rise. As a result, hydraulic oil is supplied to the first master pipe  261  connecting the primary fluid chamber  215  and the wheel pressure generating mechanism  310  and to the second master pipe  262  connecting the secondary fluid chamber  216  and the wheel pressure generating mechanism  310 , and the hydraulic oil is then delivered to the respective brake calipers  21   a  to  21   d  via the wheel pressure generating device  300 . In other words, hydraulic oil is delivered to the respective brake calipers  21   a  to  21   d  via the master pipes  261  and  262  and the wheel pressure generating device  300  even when the input rod  214  is pressed by the depressing force of the driver or when the primary piston  251  is pressed by the drive of the pressurizing motor  221 . 
     The present embodiment adopts a tandem system provided with the primary piston  251  and the secondary piston  252 . The reason for this is to secure a certain level of master pressure even if oil leaks from the master cylinder  212 . For example, when an oil leak occurs in the primary fluid chamber  215 , due to the configuration illustrated in  FIG. 3 , the primary piston  251  directly presses the secondary piston  252  so as to ensure that the hydraulic pressure in the secondary fluid chamber  216  rises. 
     In the present embodiment, by displacing the primary piston  251  according to a displacement amount of the input rod  214  resulting from a braking operation of the driver, pressurization of the hydraulic pressure in the primary fluid chamber  215  due to the input rod  214  can be further amplified. The amplification ratio (hereunder, referred to as a “boosting ratio”) is determined by a ratio of a displacement amount of the input rod  214  to that of the primary piston  251 , a ratio of a cross-sectional area of the input rod  214  (hereunder, referred to as “AIR”) to that of the primary piston  251  (hereunder, referred to as “APP”), or the like. In particular, when displacing the primary piston  251  by the same amount as the displacement amount of the input rod  214 , the boosting ratio is uniquely determined as (AIR+APP)/AIR. More specifically, by setting AIR and APP based on a necessary boosting ratio and controlling the primary piston  60  so that the displacement amount thereof becomes equal to the displacement amount of the input rod  214 , a constant boosting ratio can always be obtained. A displacement amount of the input rod  214  is detected by the brake sensor  31  and a displacement amount of the primary piston  251  is calculated by the master pressure controller  201  based on a signal from a position sensor of the pressurizing motor  221 . 
     The wheel pressure generating mechanism  310  includes outlet gate valves  310   a  and  310   b  that control the supply of hydraulic oil from the master pressure generating mechanism  210  to the respective brake calipers  21   a  to  21   d , inlet gate valves  311   a  and  311   b  that control the supply of hydraulic oil from the master pressure generating mechanism  210  to pumps, to be described later, inlet valves  312   a  to  312   d  that control the supply of hydraulic oil having passed through the outlet gate valves  310   a  and  310   b  and hydraulic oil from the pumps to the respective brake calipers  21   a  to  21   d,  outlet valves  313   a  to  313   d  that control pressure reduction of the hydraulic pressure on the brake calipers  21   a  to  21   d,  pumps  314   a  and  314   b  that boost hydraulic oil supplied from the master pressure generating mechanism  210  via the inlet gate valves  311   a  and  311   b , a pump motor  315  that drives the pumps  314   a  and  314   b,  a master pressure sensor  316  that detects a master pressure, and reservoir tanks  317   a  and  317   b.    
     A hydraulic pressure control unit for anti-lock brake control, a hydraulic pressure control unit for vehicle behavior stabilization control, a hydraulic pressure control unit for brake-by-wire, or the like can be adopted as the wheel pressure generating mechanism  310  described above. 
     The wheel pressure generating mechanism  310  is constituted by two systems, namely, a first brake system that controls the supply of hydraulic pressure to the FL (front left) wheel brake caliper  21   a  and the RR (rear right) wheel brake caliper  21   d , and a second brake system that controls the supply of hydraulic pressure to the FR (front right) wheel brake caliper  21   b  and the RL (rear left) wheel brake caliper  21   c.    
     The first brake system is made up of the outlet gate valve  310   a,  the inlet gate valve  311   a,  the inlet valves  312   a  and  312   d,  the outlet valves  313   a  and  313   d,  and the reservoir tank  317   a.  In addition, the second brake system is made up of the outlet gate valve  310   b,  the inlet gate valve  311   b,  the inlet valves  312   b  and  312   c,  the outlet valves  313   b  and  313   c,  and the reservoir tank  317   b.  The first master pipe  261  connected to the primary fluid chamber  215  of the master pressure generator  210  is connected to the outlet gate valve  310   a  and the inlet gate valve  311   a  of the first brake system, and the second master pipe  262  connected to the secondary fluid chamber  216  of the master pressure generator  210  is connected to the outlet gate valve  310   b  and the inlet gate valve  311   b  of the second brake system. 
     By providing two brake systems in this manner, even if one of the brake systems fails, a braking force of two wheels at diagonally opposing corners can be secured by the other normally-operating brake system and the behavior of the vehicle can be kept stable. 
     The outlet gate valves  310   a  and  310   b,  the inlet gate valves  311   a  and  311   b,  the inlet valves  312   a  to  312   d,  and the outlet valves  313   a  to  313   d  are all electromagnetic valves which include a solenoid and which are opened and closed by passing a current to the solenoid. The opening/closing of each valve is controlled by the wheel pressure controller  301 . The outlet gate valves  310   a  and  310   b  and the inlet valves  312   a  to  312   d  are valves that enter an open state when currents to the valves are interrupted and enter a closed state when the currents flow through the valves, while the inlet gate valves  311   a  and  311   b  and the outlet valves  313   a  to  313   d  are valves that enter a closed state when currents to the valves are interrupted and enter an open state when the currents flow through the valves. 
     While a plunger pump, a trochoid pump, a gear pump or the like is suitable as the pumps  314   a  and  314   b,  a gear pump is most desirable in terms of quietness. The pump motor  315  is operated by a drive signal from the wheel pressure controller  301  and drives the pumps  314   a  and  314   b  that are coupled to the pump motor  315 . While a DC motor, a DC brushless motor, an AC motor or the like is suitable as the pump motor  315 , a DC brushless motor is most desirable in terms of controllability, quietness, and durability. 
     The master pressure sensor  316  is connected to the second master pipe  262  connected to the secondary fluid chamber  216  of the master pressure generating mechanism  210 . A master pressure detected by the master pressure sensor  316  is sent to the wheel pressure controller  301 . Moreover, the number of master pressure sensors  316  and installation positions thereof are to be appropriately determined from the perspectives of controllability, fail-safe, and the like. 
     Next, operations of the wheel pressure generating mechanism  310  will be described. Hereinafter, only operations of the first brake system will be described. Since operations of the second brake system are the same as the operations of the first brake system, a description thereof will be omitted. 
     First, a case will be described where a hydraulic pressure boosted by the master pressure generating mechanism  210  is supplied as-is to the FL wheel brake caliper  21   a  and the RR wheel brake caliper  21   d  without further boosting. In this case, the inlet gate valve  311   a  and the outlet valves  313   a  and  313   d  are in a closed state, and the outlet gate valve  310   a  and the inlet valves  312   a  and  312   d  are in an open state. 
     Hydraulic oil from the master pressure generating mechanism  210  via the first master pipe  261  is sent to the brake calipers  21   a  and  21   d  via the outlet gate valve  310   a  and the inlet valves  312   a  and  312   d.  In other words, hydraulic oil from the master pressure generating mechanism  210  is supplied to the brake calipers  21   a  and  21   d  without being boosted by the pump  314   a.    
     As described above, the outlet gate valves  310   a  and  310   b  and the inlet valves  312   a  to  312   d  enter an open state when currents to the valves are interrupted, while the inlet gate valves  311   a  and  311   b  and the outlet valves  313   a  to  313   d  enter a closed state when currents to the valves are interrupted in the present embodiment. The states of the respective valves during the current interruption are the same as the states of the respective valves when hydraulic oil from the master pressure generating mechanism  210  is supplied as-is to the brake calipers  21   a  and  21   d  without being boosted by the pump  314   a.  Therefore, hydraulic oil can be supplied from the master pressure generating mechanism  210  to the brake calipers  21   a  and  21   d  even when the power supply system fails and currents cannot be supplied to the respective valves. In other words, even in the event of failure of the wheel pressure generating mechanism  310 , pressure of the hydraulic oil sent to the brake calipers  21   a  and  21   d  can be controlled by the master pressure generating mechanism  210 . 
     Next, a case will be described where hydraulic pressure boosted by the master pressure generating mechanism  210  is supplied to the FL wheel brake caliper  21   a  and the RR wheel brake caliper  21   d  after subjected to further boosting by the pump  314   a.  In this case, the inlet gate valve  311   a  and the inlet valves  312   a  and  312   d  are in an open state, and the outlet gate valve  310   a  and the outlet valves  313   a  and  313   d  are in a closed state. 
     Hydraulic oil supplied from the master pressure generating mechanism  210  via the first master pipe  261  is sent to the pump  314   a  via the inlet gate valve  311   a  to be boosted. The hydraulic oil boosted by the pump  314   a  is sent to the brake calipers  21   a  and  21   d  via the inlet valves  312   a  and  312   d.  Moreover, hydraulic oil can be sent from the pump  314   a  to the brake calipers  21   a  and  21   d  even when the master pressure generating mechanism  210  fails and hydraulic oil cannot be supplied from the master pressure generating mechanism  210 . In this case, the inlet gate valve  311   a  and the outlet gate valve  310   a  enter a closed state. 
     As described above, the present embodiment adopts a configuration wherein even if one of the master pressure generating device  200  and the wheel pressure generating device  300  becomes defective, output from the other is not prevented. 
     Next, a case will be described where a hydraulic pressure applied to the brake calipers  21   a  and  21   d  is reduced. In this case, while the outlet valves  313   a  and  313   d  are in an open state and the other valves are either in an open or closed state as situations demand, the inlet valves  312   a  and  312   d  are basically in a closed state. 
     Hydraulic oil retained in the brake calipers  21   a  and  21   d  flows into the reservoir tank  317   a  respectively via the outlet valves  313   a  and  313   d.  The hydraulic oil in the reservoir tank  317   a  is to be used when boosting the hydraulic oil from the master pressure generating mechanism  210  at the pump  314   a.    
     Operations of the brake control device  100  will now be described according to the flowchart illustrated in  FIG. 4 . 
     In step S 1 , the communication control unit  112  of the brake control device  100  acquires, at predetermined time intervals, various vehicle environmental information from the respective sensors and the like, and stores the information in the RAM  103 . In this case, the predetermined time interval is set to a millisecond. The respective sensors and the like include, in addition to the aforementioned brake sensor  31 , the vehicle speed sensor  32 , the longitudinal acceleration sensor  33 , the wheel speed sensor  34 , the gear position sensor  35 , and the voltmeter  36 , the master pressure controller  201  and the wheel pressure controller  301 . Basically, the respective sensors  31  to  36  constantly output detected values when the ignition is turned on, and the interface  101  receives output from the respective sensors  31  to  36  at predetermined time intervals. In addition, basically, the master pressure controller  201  constantly detects a hydraulic pressure inside the master cylinder and a displacement amount of the primary piston  251  when the ignition is turned on, and the interface  101  receives the values of fluid pressure and the displacement amount. Moreover, various vehicle environmental information from the respective sensors  31  to  36  acquired over a predetermined number of times is stored in the RAM  103  in order to recognize changes in vehicle environmental information. 
     Next, in step S 2 , the braking force calculating unit  111  calculates a maximum regenerative braking force Fr_max based on a vehicle speed and a gear position acquired in step S 1 . The maximum regenerative braking force is the greatest regenerative braking force that can be generated by the regenerative braking device  18  and is determined based on a vehicle speed and a gear position. Methods of determining the maximum regenerative braking force include storing table data illustrated in  FIG. 5  in the ROM  102  in advance and referencing the table data. 
     Next, in step S 3 , a regenerative braking force limit Fr_limit is calculated based on the vehicle speed acquired in step S 1 . A power generating efficiency of the regenerative braking device  18  declines significantly as the wheels  15   c  and  15   d  slow down. Therefore, a regenerative braking force is limited at or below a vehicle speed where the power generating efficiency declines. 
     Methods of determining the regenerative braking force limit Fr_limit include storing table data illustrated in  FIG. 6  in the ROM  102  in advance and referencing the table data.  FIG. 6  illustrates that the regenerative braking force limit is gradually lowered from a vehicle speed Vs to a vehicle speed Ve and is set to 0 at the vehicle speed Ve. The period from the vehicle speed Vs to the vehicle speed Ve is a period where a switchover occurs from a regenerative braking force to a frictional braking force, to be described later. Moreover, the vehicle speed Vs and the vehicle speed Ve are determined based on the performance of the regenerative braking device  18 . 
     In addition, the regenerative braking force Fr_limit is set to 0 regardless of a vehicle speed V when a voltage value indicated on the voltmeter  36  reaches a predetermined voltage value or, in other words, when the amount of electricity stored in the electrical storage device  17  reaches a predetermined amount because power generated by the regenerative braking device  18  can no longer be stored. However, depending on the type of the electrical storage device  17 , the method described above cause may shorten the life span of the electrical storage device  17 . Therefore, a method may alternatively be adopted in which the regenerative braking force Fr_limit is gradually reduced to 0 from a predetermined stored electricity amount. 
     Next, in step S 4 , the sizes of the maximum regenerative braking force Fr_max and the regenerative braking force limit Fr_limit are compared. When the maximum regenerative braking force Fr_max is equal to or greater than the regenerative braking force limit Fr_limit, in step S 5 , Fr_limit is substituted into the regenerative braking force Fr so that a braking force equal to or under the regenerative braking force limit is outputted. When the maximum regenerative braking force Fr_max is lower than the regenerative braking force limit Fr_limit, in step S 6 , Fr_max is substituted into the regenerative braking force Fr because the maximum regenerative braking force is equal to or lower than the regenerative braking force limit. 
     Next, in step S 7 , a frictional braking force Ff is calculated based on the displacement amount of the input rod  214  acquired in step S 1 . The frictional braking force is a braking force that acts on the respective wheels  15   a  to  15   d  when the master pressure generating device  200  and the wheel pressure generating device  300  are in operation. Methods of determining a frictional braking force include storing table data illustrated in  FIG. 7  in the ROM  102  in advance and referencing the table data.  FIG. 7  illustrates characteristics measured on a dry asphalt road (road surface μ=0.9). 
     Next, in step S 8 , the sizes of the frictional braking force Ff and the regenerative braking force Fr are compared. When the frictional braking force Ff is greater than the regenerative braking force Fr, the braking force (frictional braking force) required by the driver surpasses the regenerative braking force. Therefore, in step S 9 , Ff-Fr is substituted into a frictional braking force output command value Ffo to be transmitted to the master pressure controller  201  and the wheel pressure controller  301  while Fr is substituted into a regenerative braking force output value Fro to be transmitted to the regenerative braking device  18 . 
     When the frictional braking force Ff is equal to or smaller than the regenerative braking force Fr, since a braking force equivalent to the frictional braking force Ff can be outputted by the regenerative braking force Fr alone, in step S 10 , 0 is substituted into the frictional braking force output command value Ffo and Ff is substituted into the regenerative braking force output value Fro. Subsequently, in step S 11 , the communication control unit  112  outputs a braking force signal corresponding to a present braking force to the master pressure generating device  200 , the wheel pressure generating device  300 , and the regenerative braking device  18 . 
     The frictional braking force Ffo is outputted to the master pressure generating device  200  or the wheel pressure generating device  300  but basically to the master pressure generating device  200 . The regenerative braking force Fro is outputted to the regenerative braking device  18 . 
     Hereinafter, a case will be described where the frictional braking force Ffo is outputted to the master pressure generating device  200  and the regenerative braking force Fro is outputted to the regenerative braking device  18 . 
     An execution of the flowchart illustrated in  FIG. 4  results in, for example, the output illustrated in  FIG. 8 .  FIG. 8  illustrates an output in a case where the sizes of a frictional braking force and a regenerative braking force are equal to each other and an input rod displacement amount does not fluctuate. From a vehicle speed Vs to a vehicle speed Ve, the regenerative braking force decreases as the regenerative braking force limit drops while the frictional braking force increases so as to compensate for the decline in the regenerative braking force. In the case illustrated in  FIG. 8 , since the input rod displacement amount or, in other words, the command value does not fluctuate, a total braking force combining the frictional braking force and the regenerative braking force is constant in all areas. 
     However, in reality, fluctuations such as those illustrated in  FIGS. 9 and 10  occur when controlling the master pressure generating device  200  and the regenerative braking device  18  or the wheel pressure generating device  300  and the regenerative braking device  18  according to the flowchart illustrated in  FIG. 4 .  FIG. 9  illustrates a result of controlling the master pressure generating device  200  and the regenerative braking device  18  according to the flowchart illustrated in  FIG. 4 , and  FIG. 10  illustrates a result of controlling the wheel pressure generating device  300  and the regenerative braking device  18  according to the flowchart illustrated in  FIG. 4 . Such fluctuations are caused by a fluctuation in a reaction force of the brake pedal that accompanies fluctuations in a hydraulic pressure in the master cylinder that is generated when generating a frictional braking force, a spring reaction force, or a sliding resistance. 
     The examples illustrated in  FIGS. 9 and 10  are both cases where the brake pedal is depressed at a constant depressing force. In the example illustrated in  FIG. 9 , during the switchover from regenerative braking to frictional braking, the pedal reaction force declines, a pedal displacement amount increases, an input rod displacement amount increases, and a frictional braking force command value increases. As a result, fluctuations occur in the total braking force and the deceleration. 
     In addition, in the example illustrated in  FIG. 10 , during the switchover from regenerative braking to frictional braking, a pedal reaction force increases, a pedal displacement amount decreases, an input rod displacement amount decreases, and a frictional braking force command value decreases. As a result, fluctuations occur in the total braking force and the deceleration. 
     A method of addressing the problem described above by controlling the master pressure generating device  200  and the regenerative braking device  18  will now be described. 
     First, for example, one method involves determining a total braking force that is a sum of a frictional braking force and a regenerative braking force from the pedal reaction force illustrated in  FIG. 11  based on a relationship between an input rod displacement amount Xir and a primary piston displacement amount Xpp. The method takes into consideration fluctuations in a pedal reaction force and a primary piston displacement amount during a switchover period from regenerative braking to frictional braking. While a change in characteristics in which the total braking force increases occurs when the primary piston is displaced so as to output a frictional braking force, such a displacement of the primary piston causes a decrease in the pedal reaction force and reduces the total braking force. 
     Consequently, for example, when the regenerative braking force is approximately equal to the total braking force during regenerative braking, the total braking force does not fluctuate despite fluctuations in the primary piston displacement and the pedal reaction force after the switchover period from regenerative braking to frictional braking. As a result, a fluctuation in deceleration can be suppressed. Moreover, while a total braking force is determined using the table illustrated in  FIG. 11  in the present embodiment, methods of determining a total braking force is not limited thereto and may alternatively be determined using a mathematical expression. 
     Next, operations of the brake control device  100  using the total braking force characteristics illustrated in  FIG. 11  will now be described according to a flowchart illustrated in  FIG. 12 . 
     In the flowchart illustrated in  FIG. 12 , operations in steps S 1  to S 6  and S 11  are basically the same as in the flowchart illustrated in  FIG. 4 . 
     In step S 12 , a total braking force Ft that is a braking force of the entire system and that combines a frictional braking force and a regenerative braking force is determined. 
     Methods of determining the total braking force Ft include storing table data illustrated in  FIG. 11  in the ROM  102  in advance and referencing the table data. 
       FIG. 11  illustrates a total braking force to be outputted with respect to a pedal reaction force. A plurality of characteristics exists depending on a relationship between the input rod displacement amount Xir and the primary piston displacement amount Xpp. In the same manner as in the first embodiment, since a pedal reaction force varies depending on a hydraulic pressure in the master cylinder, a spring reaction force, a sliding resistance, or the like, a pedal reaction force can be determined from F=P·Air+Fk+Fo, where P denotes a hydraulic pressure inside the master cylinder, Air denotes a cross-sectional area of the input rod, Fk denotes a spring reaction force, and Fo denotes a reaction force such as a sliding resistance. The cross-sectional area of the input rod Air, the spring reaction force Fk, and the reaction force such as a sliding resistance Fo are all determined according to a specification of the brake system. In addition, during frictional braking where regenerative braking is not used, a characteristic of Xir=Xpp in which the input rod displacement amount Xir and the primary piston displacement amount Xpp are approximately equal to each other is used as an initial characteristic so that a boosting ratio of a hydraulic pressure generated by displacements of the input rod and the primary piston is always constant. The relationship illustrated in  FIG. 11  is a characteristic measured on a dry asphalt road (road surface μ=0.9). 
     Next, in step S 13  in the flowchart illustrated in  FIG. 12 , the sizes of the total braking force Ft and the regenerative braking force Fr are compared. When the total braking force Ft is greater than the regenerative braking force Fr, a braking force that cannot be outputted by a regenerative braking force must be outputted by a frictional braking force. Therefore, in step S 14 , Ft-Fr is substituted into a frictional braking force output command value Ffo to be transmitted to the master pressure controller  201 , and Fr is substituted into a regenerative braking force output value Fro to be transmitted to the regenerative braking device  18 . 
     Conversely, when the total braking force Ft is equal to or smaller than the regenerative braking force Fr, since a braking force equivalent to the total braking force Ft can be outputted by the regenerative braking force Fr alone, in step S 15 , 0 is substituted into the frictional braking force output command value Ffo and Ft is substituted into the regenerative braking force output value Fro. 
     In a case where the master pressure generating device  200  and the regenerative braking device  18  are controlled according to the total braking force characteristics illustrated in  FIG. 11  and to the flowchart illustrated in  FIG. 12 , for example, while an initially selected characteristic among  FIG. 11  in step S 12  in which a total braking force is determined when a regenerative braking force and a total braking force are approximately equal to each other during regenerative braking is the aforementioned characteristic expressed as Xir=Xpp, since the frictional braking force must be set to 0 when the regenerative braking force is greater than the total braking force, Xpp inevitably becomes smaller than Xir. As such, when the regenerative braking force and the total braking force are approximately equal to each other during regenerative braking as is the case with the present example, a characteristic expressed as Xpp=0 is to be selected. 
     When entering the switchover period from regenerative braking to frictional braking, since the regenerative braking force becomes smaller than the total braking force and a frictional braking force must be generated, Xpp becomes greater than 0 and a characteristic that is closer to Xir=Xpp than to Xpp=0 is used. At this point, although the total braking force increases in a case where a pedal reaction force does not change, since the pedal reaction force decreases in the present brake system, the total braking force remains unchanged before and after the switchover period from regenerative braking to frictional braking and, as a result, fluctuations in the deceleration can be suppressed as illustrated in  FIG. 13 . 
     Next, as another method of suppressing fluctuations in a total braking force and a deceleration as illustrated in  FIG. 10 , a method of controlling the wheel pressure generating device  300  and the regenerative braking device  18  will be described. 
     When controlling the wheel pressure generating device  300 , for example, one method involves determining a total braking force that is a sum of a frictional braking force and a regenerative braking force from the pedal reaction force illustrated in  FIG. 14  based on a hydraulic pressure Px that is increased or decreased by the wheel pressure generating device  300 . The method takes into consideration fluctuations in the pedal reaction force and the hydraulic pressure Px that is increased or decreased by the wheel pressure generating device  300  during the switchover period from regenerative braking to frictional braking. While a change to characteristics in which the total braking force decreases occurs when the wheel pressure generating device  300  increases pressure in order to output a frictional braking force, such an increase in pressure by the wheel pressure generating device  300  causes an increase in the pedal reaction force, resulting in an increase total braking force. 
     Consequently, for example, when the regenerative braking force is approximately equal to the total braking force during regenerative braking, the total braking force does not fluctuate despite fluctuations in the hydraulic pressure that is increased or decreased by the wheel pressure generating device  300  or in the pedal reaction force after the switchover period from regenerative braking to frictional braking. As a result, a fluctuation in deceleration can be suppressed. Moreover, while a total braking force is determined using the table illustrated in  FIG. 14  in the present embodiment, methods of determining the total braking force is not limited to such tables and may alternatively be determined using a mathematical expression. 
     Moreover, the method of controlling the wheel pressure generating device  300  only differs from the method of controlling the master pressure generating device  200  in the manner in which a total braking force is determined, and otherwise basically follows the flowchart illustrated in  FIG. 12 . 
     By controlling the wheel pressure generating device  300  and the regenerative braking device  18  using the total braking force characteristics illustrated in  FIG. 14  according to the flowchart illustrated in  FIG. 12 , fluctuations in a total braking force and a deceleration can be suppressed even when a pedal reaction force fluctuates as illustrated in  FIG. 15 . 
     While an apparatus for generating a braking force is made up of the master pressure generating device  200 , the wheel pressure generating device  300 , and the regenerative braking device  18  in the present embodiment, the master pressure generating device  200  may be a negative pressure booster that utilizes a negative pressure of the engine  11 , and the wheel pressure generating device  300  may simply be a hydraulic pipe or an ABS (anti-lock brake system) that prevents locking of wheels.