BRAKE CONTROL SYSTEM

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0001] The invention relates to a brake control system that controls braking force applied to a wheel of a vehicle. 2. Description of the Related Art

[0002] Japanese Patent Application Publication No. 2005-35393 (JP-A-2005-35393), for example, describes a brake control system that applies braking force to a vehicle by generating hydraulic pressure corresponding to the operating force of a brake pedal in a hydraulic circuit, and supplying that hydraulic pressure to a wheel cylinder of each wheel through a brake line.

[0003] A reservoir tank that stores brake fluid (i.e., hydraulic fluid), a pump that draws up brake fluid from the reservoir tank, and a hydraulic pressure source that includes an accumulator that stores brake fluid that has been pressurized by the pump are all provided in this hydraulic circuit. Electromagnetic valves that are pressure increase valves and pressure decrease valves and the like are provided between the wheel cylinders of the wheels and the hydraulic pressure source. The brake control system controls the hydraulic pressure to apply the appropriate braking force to each wheel by adjusting the amount of brake fluid supplied to and discharged from the wheel cylinders by controlling these electromagnetic valves open and closed. Brake control response is also ensured by accumulating pressure in the accumulator and constantly maintaining high pressure in the hydraulic pressure source in this way. [0004] This kind of brake control is also performed in a variety of types of running control that maintain the running stability of the vehicle and a variety of types of slip control according to the road surface conditions and the like, in addition to control according to operation of the brake pedal. For example, in various types of control such as ABS (Anti-lock Brake System), TRC (Traction Control), and VSC (Vehicle Stability

Control) which are designed to keep the wheels from slipping on the road surface, braking force beyond that obtained with normal brake control may be required.

[0005] These kinds of slip control and running control are not executed very often in view of the total running time of the vehicle, but the hydraulic pressure in the accumulator must be kept high just in case these controls are executed. Therefore, the hydraulic pressure source that includes the accumulator is large by necessity. Also, a piston-type pump that can withstand high pressure is typically used as the pump for accumulating pressure, but the design of this kind of pump increases the hydraulic pressure pulsations which tends to produce noise.

SUMMARY OF THE INVENTION

[0006] This invention thus provides a brake control system that enables a hydraulic pressure source including an accumulator to be made small and display more stable behavior.

[0007] A first aspect of the invention relates to a brake control system which has a power hydraulic pressure source that accumulates pressure by supplying hydraulic fluid to an accumulator by driving a pump, and which generates braking force by supplying the accumulated hydraulic pressure to a wheel cylinder of each wheel. This brake control system includes a braking state determining portion and a control portion. The braking state determining portion determines whether normal braking which requires braking force able to be generated by hydraulic pressure that is within a set range of hydraulic pressure accumulated by the accumulator, or emergency braking which requires more braking force than is required for normal braking, is being performed. The control portion executes normal control that supplies the hydraulic pressure in the accumulator to the wheel cylinders when it is determined that normal braking is being performed, and executes specific control that drives the pump to supply additional hydraulic pressure to the wheel cylinders when it is determined that emergency braking is being performed.

[0008] Normal braking in this case may be braking that is performed relatively

often and includes brake control that is performed when a brake operating member is operated normally, for example. Normal braking may be executed when the target deceleration is approximately low to medium. On the other hand, emergency braking may be braking that is performed relatively infrequently, such as when braking suddenly or when ABS is activated, for example. Emergency braking may be executed when the target deceleration and the target deceleration slope (i.e., the hydraulic pressure slope) are high.

[0009] In this aspect, the accumulator serves as a hydraulic pressure source during normal braking, and the necessary wheel cylinder pressure is ensured using the accumulated hydraulic pressure. However, even during normal braking, the pump is driven to accumulate pressure if the hydraulic pressure in the accumulator falls below the set range. On the other hand, during emergency braking, a large amount of braking force is temporarily required so the hydraulic pressure of the set range that can be accumulated by the accumulator is not enough (i.e., more braking force than may be obtained by the hydraulic pressure of the set range that may be accumulated by the accumulator is temporarily required). Therefore, in addition to the accumulated pressure in the accumulator, the pump is driven to supply additional hydraulic pressure to the wheel cylinders to compensate for the insufficiency in the hydraulic pressure supplied by the accumulator. As a result, the necessary wheel cylinder pressure can be ensured to obtain the desired braking force even during emergency braking.

[0010] According to this aspect, the hydraulic pressure in the hydraulic pressure source can be kept low during normal braking because additional hydraulic pressure is able to be supplied by driving the pump temporarily during emergency braking. That is, the set range of accumulated pressure in the accumulator is able to be kept low based on the fact that normal braking is performed frequently, while the supply of hydraulic pressure during emergency braking, which requires a large amount of braking force, is still able to be ensured. Even if hydraulic pressure exceeding that set range was applied to the accumulator during emergency braking, it would be infrequent and only temporary and thus would not greatly affect the durability of the accumulator.

Therefore, the accumulator can be made smaller and less expensive.

[0011] Also, in the brake control system according to the first aspect described above, the set range of hydraulic pressure accumulated by the accumulator may be approximately 8 to 12 MPa. [0012] Further, in the brake control system according to the foregoing structure, the additional hydraulic pressure provided by driving the pump when emergency braking is being performed may be supplied to the wheel cylinders without being introduced into the accumulator.

[0013] Moreover, in the brake control system according to the foregoing structure, the pump may be a gear pump. Compared with a piston-type pump, for example, a gear pump is able to better suppress hydraulic pressure pulsations, thereby reducing operating noise. Also, vibration is reduced during operation which enables more stable behavior of the power hydraulic pressure source to be ensured. This kind of gear pump uses the shapes of the teeth to carry the hydraulic fluid so the seal is typically not as good as it in a piston-type pump or the like. Therefore, when the hydraulic pressure is high, some hydraulic fluid may leak back in the reverse direction which is why there has been limited acceptance of gear pumps as pumps to be used to accumulate pressure in accumulators. However, this brake control system is designed so that the set range of the accumulated pressure in the accumulator can be kept low, as described above, so even if a gear pump is used, leaking of hydraulic fluid, wearing of the sealing surface of the tooth portions, and seizure can all be suppressed. As a result, a gear pump is able to be practically applied to a brake system with an accumulator.

[0014] Also, in the brake control system according to the structure described above, when it is determined that normal braking is being performed, the control portion may operate a motor which drives the pump, at a slower speed than when emergency braking is being performed. The motor is operated during normal braking more to accumulate pressure in the accumulator on the premise that hydraulic pressure will be supplied to the wheel cylinders than to actually supply hydraulic pressure to the wheel cylinders. Therefore, the motor speed is reduced when the pump is driven to

accumulate pressure during normal braking because hydraulic fluid does not need to be supplied as fast as it does during emergency braking. Accordingly, the load on the motor at this time is reduced so the durability of the pump can be improved. In addition, the consumption current and incoming current of the motor can be reduced. [0015] Also, in the brake control system according to the structure described above, the accumulator may include accumulation suppressing means for suppressing accumulation in the accumulator when the hydraulic pressure in the accumulator becomes equal to or greater than a preset reference pressure. The reference pressure in this case may be an upper limit value of the set range of the accumulated pressure in the accumulator. Also, the accumulation suppressing means may be means for interrupting the flow of hydraulic fluid into the accumulator when the hydraulic pressure in the accumulator is equal to or greater than that reference pressure. The accumulation suppressing means may be mechanically structured to autonomously activate, or may be such that a solenoid type electromagnetic valve is provided to which electronic control is applied. Suppressing accumulation in this way makes it possible to prevent or inhibit the accumulator from being damaged by high hydraulic pressure.

[0016] The brake control system according to the structure described above may also include a detecting portion that detects the hydraulic pressure in the accumulator, and the control portion may stop driving the pump when the hydraulic pressure in the accumulator becomes equal to or greater than the reference pressure when normal braking or no braking is being performed. The detecting portion may be arranged in the accumulator itself and detect the hydraulic pressure, or it may be arranged in a hydraulic fluid passage that is connected to the accumulator and detect when the hydraulic pressure in the passage has reached the reference pressure. In this structure, the pump stops being driven when the hydraulic pressure in the accumulator becomes equal to or greater than the reference value when normal braking or no braking is being performed so the pump and actuators (e.g., control valves and the like) that operate when the pump is driven are operated less frequently, which inhibits deterioration of those parts.

[0017] Also, in the brake control system according to the structure described above, the control portion may estimate that the hydraulic pressure in the accumulator has become equal to or greater than the reference pressure based on a change over time in a control current value of a motor that drives the pump. That is, when the hydraulic pressure inside the accumulator increases, the load on the pump also increases as a reaction so the control current of the motor that drives the pump increases. In particular, if there is an electromagnetic valve right before the accumulator, the control current of the motor may spike when this valve closes or when the accumulator fills up with hydraulic fluid (or when the piston is at full stroke in the case of a piston-type accumulator) and the hydraulic pressure reaches the upper limit value. Therefore, it is estimated that the hydraulic pressure has become equal to or greater than the reference pressure when the change over time in the motor control current value changes drastically. According to this structure, even if the detecting portion fails, it is possible to detect when the hydraulic pressure has become equal to or greater than the reference pressure so the pump can be stopped.

[0018] Also, in the brake control system according to the structure described above, the reference pressure may be 12 MPa.

[0019] Also, in the brake control system according to the structure described above, the control portion may operate the gear pump in reverse when reducing the hydraulic pressure in the wheel cylinders. That is, the gear pump is mechanically able to operate in reverse. Wheel cylinder pressure is typically decreased through a pressure decrease valve, but operating the gear pump in reverse in this way promotes that pressure decrease by discharging hydraulic fluid from the wheel cylinder, which enables the wheel cylinder pressure to be reduced even faster. [0020] Also, the brake control system according to the structure described above may also include i) a manual hydraulic pressure source which introduces hydraulic fluid stored in a reservoir tank into a master cylinder, pressurizes the hydraulic fluid according to an operating amount of a brake operating member, and sends the pressurized hydraulic fluid to a specific wheel cylinder which is one or more of the wheel cylinders,

ii) a hydraulic circuit which forms both a supply passage for supplying hydraulic pressure generated by at least one of the power hydraulic pressure source and the manual hydraulic pressure source to the wheel cylinders, and a discharge passage for returning the hydraulic fluid in the wheel cylinders to the reservoir tank when reducing the braking force, and iii) a pressure increase valve which is arranged between the power hydraulic pressure source and each of the wheel cylinders and supplies hydraulic fluid from the power hydraulic pressure source to the corresponding wheel cylinder by a valve opening operation. The hydraulic pressure from the power hydraulic pressure source may be supplied to the specific wheel cylinder through a specific pressure increase valve which is one or more the pressure increase valves.

[0021] The manual hydraulic pressure source in this case may .be a hydraulic pressure source that acts as a failsafe at times such as when there is an abnormality in the system, e.g., when a power hydraulic pressure source such as a vehicle power supply or a motor for driving a pump or the like is not functioning properly. That is, with normal brake control, the necessary wheel cylinder pressure is ensured by the power hydraulic pressure source, but when there is an abnormality in the system for example such that the power hydraulic pressure source is unable to supply sufficient hydraulic pressure, the hydraulic pressure of at least a specific wheel cylinder may be ensured by the manual hydraulic pressure source. With this structure, hydraulic pressure from the power hydraulic pressure source can be supplied through all of the pressure increase valves, while hydraulic fluid is also supplied to the specific wheel cylinder through the specific pressure increase valve. That is, hydraulic pressure can be supplied to the specific wheel cylinder from both the power hydraulic pressure source and the manual hydraulic pressure source. [0022] Also, in the brake control system according to the structure described above, the valve opening pressure of the pressure increase valve which is not the specific pressure increase valve may be set lower than the valve opening pressure of the specific pressure increase valve. Setting the valve opening pressure lower in this case may include setting the valve opening pressures of the pressure increase valves so that they

differ in terms of mechanical structure by, for example, making the loads of urging members that urge the valve portions in the valve closing direction different. Alternatively, when both pressure increase valves are electromagnetic valves, the valve opening pressure may be relatively reduced by applying a small amount of current that is less than the valve opening current to the pressure increase valve that is not the specific pressure increase valve beforehand. For example, if the pressure upstream of the pressure increase valve continues to increase as a result of the pump being continuously - driven, hydraulic fluid will start to flow out from both pressure increase valves, at which time foreign matter in the hydraulic fluid may become stuck in a valve portion. If this happens in the specific pressure increase valve and hydraulic pressure is then supplied from the manual hydraulic pressure source, that hydraulic fluid may flow back to the accumulator side through the specific pressure increase valve. Therefore, with this structure, the pressure increase valve that is not the specific pressure increase valve is made to open easier than the specific pressure increase valve so if there is foreign matter in the hydraulic fluid, it will be led to that pressure increase valve side instead of the specific pressure increase valve side, thereby reducing the chances of foreign matter becoming stuck in the specific pressure increase valve. As a result, braking force from at least the manual hydraulic pressure source can be ensured.

[0023] Also, in the brake control system according to the structure described above, the accumulator may include i) a housing which is connected to a high pressure passage that connects the pump and the pressure increase valves together, and ii) a pressure-sensitive member which divides space inside the housing into a reference pressure chamber formed of an enclosed space and an accumulation chamber formed of an open space that is open to the high pressure passage, and keeps the hydraulic pressure within the set range by changing the volume of the accumulation chamber by being displaced according to differential pressure between the reference pressure chamber and the accumulation chamber. A charged pressure in the reference pressure chamber may be set to be equal to or greater than an assumed hydraulic pressure at the time of a servo short-circuit. This assumed hydraulic pressure at the time of a servo short-circuit may

be set appropriately from the standpoint of a failsafe and the like. For example, if the brake control system fails, it may be assumed that a pressure increase valve is leaking due to foreign matter being stuck in the valve. In this case, it is assumed that hydraulic pressure will be supplied to the wheel cylinders from the manual hydraulic pressure source, but the hydraulic pressure supplied may be hydraulic pressure which is equal to the hydraulic pressure that may be leaking from the pressure increase valve at that time.

[0024] With this structure, the hydraulic pressure in the accumulation chamber of the accumulator is adjusted by the displacement of the pressure-sensitive member. Meanwhile, the charged pressure in the reference pressure chamber is set to be equal to or greater than the assumed charged pressure at the time of a servo short-circuit. Therefore, if there is a servo short-circuit, the pressure-sensitive member will not be displaced and cause the accumulator to draw in hydraulic fluid so hydraulic fluid can be prevented from flowing back through the pressure increase valves. When hydraulic pressure is supplied from to the wheel cylinders from the manual hydraulic pressure source, the efficiency with which it is supplied will also not decrease.

[0025] Also, in the brake control system according to the structure described above, the accumulator may also include i) a housing which is connected to a high pressure passage that connects the pump and the pressure increase valves together, and ii) a pressure-sensitive member which divides space inside the housing into a reference pressure chamber formed of an enclosed space and an accumulation chamber formed of an open space that is open to the high pressure passage, and keeps the hydraulic pressure within the set range by changing the volume of the accumulation chamber by being displaced according to differential pressure between the reference pressure chamber and the accumulation chamber. Also, a check valve which prevents hydraulic fluid from flowing back to the accumulator side from the pressure increase valves may be arranged in the high pressure passage. This structure makes it possible to prevent hydraulic fluid from flowing back to the accumulator side irrespective of the operating state of the pressure increase valves.

[0026] Also, in the brake control system according to the structure described

above, the control portion may drive the pump and open only the pressure increase valve corresponding to the wheel cylinder where the pressure is to be increased, only when a pressure increase condition is satisfied during emergency braking. That is, even during emergency braking, it is not necessary to increase the wheel cylinder pressure when maintaining or decreasing the hydraulic pressure. This structure makes it possible to suppress needless power consumption by not driving the pump unnecessarily when a pressure increase is not required, for example.

[0027] According to the brake control system of the invention, the hydraulic pressure source including the accumulator is able to be made small.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The foregoing and/or further objects, features and advantages of the invention will become apparent from the following description of preferred embodiments with reference to the accompanying drawings, in which like numerals are used to represent like elements and wherein:

FIG. 1 is a system diagram of a brake control system according to a first example embodiment of the invention, which mainly shows a hydraulic circuit of the brake control system; FIG. 2 is a diagram schematically showing the structure of an accumulator and the surrounding area;

FIG. 3 is a graph illustrating a brake control method using a power hydraulic pressure source;

FIG. 4 is a view of a control map used to determine failure of an accumulator pressure sensor;

FIG. 5 is a flowchart schematically showing the main steps in a hydraulic control routine;

FIG. 6 is a diagram schematically showing the structure of an accumulator and the surrounding area according to a second example embodiment of the invention;

FIGS. 7Ato 7C are diagrams illustrating operation of the accumulator;

FIG. 8 is a diagram showing the structure of an accumulator according to a modified example of the first example embodiment;

FIG. 9 is a diagram showing the structure of an accumulator according to a modified example of the second example embodiment;

FIG. 10 is a diagram schematically showing the structure of an accumulator and the surrounding area according to another modified example of the first example embodiment; and

FIGS HA and HB are diagrams showing the structure of accumulators according to two other modified examples of the first example embodiment.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0029] FIG. 1 is a system diagram of a brake control system according to a first example embodiment of the invention, which mainly shows the hydraulic circuit of the brake control system.

[0030] The brake control system 10 is a vehicular electronically controlled brake (ECB) system that independently and optimally applies brake pressure to the brakes of four wheels of a vehicle according to the operation (i.e., depression) of a brake pedal 12, which serves as a brake operating member, by a driver. This brake pedal 12 is connected to a master cylinder 14 that sends out brake fluid, i.e., hydraulic fluid, according to the depression operation by the driver. Also, a stroke sensor 46 which detects the depression stroke of the brake pedal 12 is provided with the brake pedal 12. Furthermore, a reservoir tank 26 is connected to the master cylinder 14. One output port of the master cylinder 14 is connected via a shut-off valve 23 to a stroke simulator 24 that creates reaction force corresponding to the operating force applied to the brake pedal 12 by the driver. Incidentally, the shut-off valve 23 is a normally closed electromagnetic valve which is closed when de-energized and opens when operation of the brake pedal 12 by the driver is detected.

[0031] The one output port of the master cylinder 14 is also connected to a right front- wheel brake pressure control line 16 which is connected to a right front-wheel wheel cylinder 20FR that applies braking force to a right front wheel, not shown. Also, another output port of the master cylinder 14 is connected to a left front-wheel brake pressure control line 18 which is connected to a left front- wheel wheel cylinder 20FL that applies braking force to a left front wheel, not shown. A right electromagnetic shut-off valve 22FR is arranged midway in the right front-wheel brake pressure control line 16, and a left electromagnetic shut-off valve 22FL is arranged midway in the left front-wheel brake pressure control line 18. The right electromagnetic shut-off valve 22FR and the left electromagnetic shut-off valve 22RL are both normally open electromagnetic valves that are open when de-energized and close when operation of the brake pedal 12 by the driver is detected. Hereinafter, the right electromagnetic shut-off valve 22FR and the left electromagnetic shut-off valve 22RL will be collectively referred to as "shut-off valves 22" where appropriate. [0032] Also, a right master pressure sensor 48FR that detects the master cylinder pressure on the right front wheel side is arranged midway in the right front-wheel brake pressure control line 16. Similarly, a left master pressure sensor 48FL that detects the master cylinder pressure on the left front wheel side is arranged midway in the left front-wheel brake pressure control line 18. In this brake control system 10, when the driver depresses the brake pedal 12, that depression amount is detected by the stroke sensor 46. However, the force with which the brake pedal 12 is depressed (i.e., the depression force) can also be obtained from the master cylinder pressure detected by the right master pressure sensor 48FR and the left master pressure sensor 48FL. Monitoring the master cylinder pressure using both of the pressure sensors 48FR and 48FL in this way is preferable from a safety standpoint in case the stroke sensor 46 was to fail. Hereinafter, the right master pressure sensor 48FR and the left master pressure sensor 48FL will be collectively referred to as "master pressure sensors 48" where appropriate.

[0033] Meanwhile, one end of a hydraulic pressure supply and discharge line

28 is connected to the reservoir tank 26, while the other end of the hydraulic pressure supply and discharge line 28 is connected to an inlet port of a pump 34 that is driven by a motor 32. The outlet port of the pump 34 is connected to a high pressure line 30 that forms a hydraulic pressure passage. This high pressure line 30 is connected to an accumulator 50 and a relief valve 53. The accumulator 50, the pump 34, and the motor 32 together make up a power hydraulic pressure source capable of accumulating brake fluid hydraulic pressure. Communication between the inlet port of the pump 34 and the hydraulic pressure supply and discharge line 28 is effectively blocked off when the pump 34 is not being driven. In this example embodiment, the pump 34 is a gear pump that is rotatably driven by the motor 32, and the motor 32 is a brushless motor that is controlled by so-called PWM control, but this is well known and so will not be described here. Further, the accumulator 50 in this example embodiment is an accumulator that converts the pressure energy of the brake fluid into pressure energy of a filler gas such as nitrogen and stores it. [0034] The accumulator normally stores brake fluid that has been pressurized by the pump 34 so that the internal pressure (hereinafter, referred to as the "accumulator pressure") is within a predetermined set range (such as between approximately 8 to 12 MPa). A valve outlet of the relief valve 53 is connected to the hydraulic pressure supply and discharge line 28 such that if the hydraulic pressure within the high pressure line 30 becomes abnormally high, e.g., approximately 25 MPa, the relief valve 53 will open and the high pressure brake fluid will return to the reservoir tank 26 through the hydraulic pressure supply and discharge line 28. Also, an accumulator pressure sensor 51 (which may be regarded as a "detecting portion" of the invention) which detects the pressure of the hydraulic fluid inside the hydraulic pressure line 30 (this pressure is equivalent to the accumulator pressure in this example embodiment) is provided in the high pressure line 30.

[0035] The high pressure line 30 is connected to the right front-wheel wheel cylinder 20FR via a pressure increase valve 40FR, the left front-wheel wheel cylinder 20FL via a pressure increase valve 40FL, a right rear- wheel wheel cylinder 20RR via a

pressure increase valve 40RR, and a left rear-wheel wheel cylinder 20RL via a pressure increase valve 40RL. Hereinafter, the wheel cylinders 20FR to 20RL will collectively be referred to as the "wheel cylinders 20" where appropriate, and the pressure increase valves 40FR to 40RL will collectively be referred to as the "pressure increase valves 40" where appropriate. The pressure increase valves 40 are all normally closed electromagnetic flowrate control valves (linear valves) that are closed when de-energized and are used to increase the pressure in the wheel cylinders 20 as necessary. Incidentally, a disc brake unit, which generates braking force by pushing a brake pad against a disc through operation of the wheel cylinder 20, is provided for each wheel of the vehicle, not shown.

[0036] Also, the right front-wheel wheel cylinder 20FR is connected to the hydraulic pressure supply and discharge line 28 via a pressure decrease valve 42FR, and the left front-wheel wheel cylinder 20FL is connected to the hydraulic pressure supply and discharge line 28 via a pressure decrease valve 42FL. These pressure decrease valves 42FR and 42FL are normally closed electromagnetic flowrate control valves (linear valves) that are used to decrease the pressure in the wheel cylinders 20FR and 20FL as necessary. Meanwhile, the right rear-wheel wheel cylinder 20RR is connected to the hydraulic pressure supply and discharge line 28 via a pressure decrease valve 42RR which is a normally open electromagnetic flowrate control valve, and the left rear-wheel wheel cylinder 20RL is connected to the hydraulic pressure supply and discharge line 28 via a pressure decrease valve 42RL which is also a normally open electromagnetic flowrate control valve. Hereinafter, these pressure decrease valves 42FR to 42RL will collectively be referred to as "pressure decrease valves 42" where appropriate.

[0037] A right front-wheel wheel cylinder pressure sensor 44FR is provided near the right front- wheel wheel cylinder 20FR, a left front- wheel wheel cylinder pressure sensor 44FL is provided near the left front-wheel wheel cylinder 20FL, a right rear-wheel wheel cylinder pressure sensor 44RR is provided near the right rear-wheel wheel cylinder 20RR, and a left rear-wheel wheel cylinder pressure sensor 44RL is provided near the left rear-wheel wheel cylinder 20RL. Each of these wheel cylinder

pressure sensors 44FR to 44RL detects the wheel cylinder pressure of, i.e., the brake fluid pressure applied to, the wheel cylinder 20 near which it is provided.

[0038] The right and left electromagnetic shut-off valves 22FR and 22FL, the pressure increase valves 40FR to 40RL, the pressure decrease valves 42FR to 42RL, the pump 34, and the accumulator 50 and the like together form a hydraulic actuator 80 of the brake control system 10. This hydraulic actuator SO is controlled by an electronic control unit (hereinafter referred to as "ECU") 200. This ECU 200 includes a CPU that executes various calculations, ROM in which various control programs are stored, RAM which is used as a work area for executing programs and storing data, an input/output interface, and memory and the like. The ECU 200 in this example embodiment may be regarded as a "control portion" and a "braking state determining portion" of the invention.

[0039] Next, the structure and operation of the power hydraulic pressure source will be described. FIG. 2 is a diagram schematically showing the structure of the accumulator and the surrounding area. The pump 34 is a gear pump that pumps brake fluid using two gears. In this example embodiment, the pump 34 is an external gear pump in which a pair of same-shape gears 62 with external teeth are arranged juxtaposed in a housing 61. When these gears are rotated by the motor 32, brake fluid is drawn in from the reservoir tank 26 and discharged to the high pressure line 30 where the accumulator 50 is arranged. That is, brake fluid introduced through the inlet of the housing 61 is split in two and carried by the rotating gears 62 toward the outsides of where the gears mesh together. The brake fluid then moves through gaps formed between the inner peripheral surface of the housing 61 and the gears 62 to an outlet where it is discharged. Incidentally, in this example embodiment, an external gear pump is used, but an internal gear pump that is formed of a gear with external teeth and a gear with internal teeth may also be used. These types of gear pumps are well known so detailed descriptions thereof will be omitted.

[0040] The accumulator 50 is a bellows-type accumulator which includes a housing 71 and a metal bellows 72 which hermetically divides the space within the

housing 71 into an enclosed space and an open space. One end of the bellows 72 is fixed to the housing 71, while the other end is attached to an end plate 73 that seals off the inside. The enclosed space inside of the bellows 72 forms a gas chamber 74 (which may be regarded as a "reference pressure chamber" of the invention) which is filled with high pressure gas such as nitrogen. The open space outside the bellows 72 forms an accumulation chamber 75 which is connected to the high pressure line 30 via a connecting line 76. High pressure brake fluid that has been discharged from the pump 34 is stored under pressure in this accumulation chamber 75. Also, a columnar stopper 77 that extends in the axial direction of the bellows 72 is arranged inside the bellows 72. One end of the stopper 77 is fixed to the housing 71. A pad 78 of rubber or the like is fitted to the other end of the stopper 77. An O-ring 79 for sealing is arranged on the end portion of the housing 71 on the side of the connecting line 76.

[0041] The bellows 72 expends or contracts according to the differential pressure between the accumulation chamber 75 and the gas chamber 74, and as it does so, its volume changes. However, the displacement of the bellows 72 in the direction of contraction is restricted by the end plate 73 being retained by the stopper 77. That is, the volume of the accumulation chamber 75 is limited by the stopper 77. The bellows 72 is expanded and contracted within the allowable range such that the pressure in the gas chamber 74 becomes equal to the pressure in the accumulation chamber 75. That is, as the volume of the accumulation chamber 75 increases, the volume of the gas chamber 74 decreases such that the pressure of the gas chamber 74 is maintained at the same pressure as the pressure of the accumulation chamber 75. As increasing the wheel cylinder pressure, brake fluid in the accumulation chamber 75 is forced into the high pressure line 80 via the connecting line 76 and supplied to the wheel cylinders 20. When the accumulator pressure drops, the bellows 72 expands so the volume of the accumulation chamber 75 decreases, but once the end plate 73 is displaced to the point where it is retained by the O-ring 79, that minimum volume is maintained.

[0042] The motor 32 is controlled so that the accumulator pressure detected by the accumulator pressure sensor 51 is kept within a predetermined set range. That is, if

the accumulator pressure falls below a lower limit value Pamin (8 MPa in this example embodiment) of that set range, the ECU 200 starts current flowing to the motor 32 to drive the pump 34 and increase the accumulator pressure. If the accumulator pressure exceeds an upper limit value Pamax (12 MPa in this example embodiment) of that set range, the ECU 200 stops the flow of current to the motor 32 to stop the pump 34. In this example embodiment, the set range of the accumulator pressure is large enough to cover the normal region but- is kept smaller than the typical set range (such as approximately 14 to 22 MPa). This prevents high pressure from being constantly applied to the accumulator 50, thereby improving its durability. When a large amount of braking force is required, the ECU 200 immediately drives the pump 34 to compensate for the insufficiency of the accumulator pressure (i.e., to make up the difference between the required hydraulic pressure and the accumulator pressure). That is, the necessary wheel cylinder pressure is ensured by supplying brake fluid discharged from the pump 34 as well as brake fluid from the accumulator 50 to the wheel cylinders 20. [0043] In this example embodiment, the charged pressure in the gas chamber

74 of the accumulator 50 is set to a value equal to or greater than the hydraulic pressure at the time of a servo short-circuit (e.g., 5 to 8 MPa). For example, if the motor 32 fails and the hydraulic pressure of the power hydraulic pressure source is unable to be maintained, the ECU 200 detects that abnormality and executes a failsafe routine to ensure braking force by opening the shut-off valve 22. If at this time foreign matter is stuck in the valve portions of the front wheel side pressure increase valves 40FR and 40FL, for example, such that those valves are unable to be kept closed, brake fluid may flow back through those pressure increase valves 40FR and 40FL. That is, if the charged pressure in the gas chamber 74 is low, some of the brake fluid supplied from the master cylinder 14 may flow into the accumulator 50 through those pressure increase valves 40FR and 40FL, preventing the wheel cylinder pressure from being increased or maintained. Therefore, the charged pressure of the filler gas in the gas chamber 74 is set to be equal to or greater than the assumed hydraulic pressure at the time of a servo short-circuit. Accordingly, even if there is a servo short circuit, the end plate 73 will not

be displaced in the direction of contraction so backflow of brake fluid through the pressure increase valves 40FR and 40FL can be prevented or minimized. As a result, the necessary wheel cylinder pressure can be maintained and braking force ensured.

[0044] FIG. 3 is a graph illustrating a brake control method using the power hydraulic pressure source. The horizontal axis of the graph represents time and the vertical axis represents the wheel cylinder pressure (W/C pressure) to be controlled. The solid line in the graph indicates the region where hydraulic pressure is controlled by the accumulator 50, and the alternate long and short dash line indicates the region where additional hydraulic pressure is controlled by driving the pump 34. [0045] In this example embodiment, during normal braking such as when the brake pedal is depressed normally when the vehicle is running, brake control is performed using the hydraulic pressure accumulated in the accumulator 50. That is, in the normal region where the target deceleration is approximately low to medium, brake control is performed by supplying the brake fluid stored in the accumulator 50 to the wheel cylinders 20. As shown in the drawing, the wheel cylinder pressure is ensured using the hydraulic pressure accumulated in the accumulator 50 up to the pressure Pamax (such as 12 MPa) that is able to be provided by the accumulator 50.

[0046] On the other hand, during emergency braking such as when ABS or the like activates, a large amount of braking force is required, which may not be able to be generated with only the pressure Pamax that can be provided by the hydraulic pressure accumulated in the accumulator 50 at any one give point in time. Therefore, when necessary the pump 34 is immediately driven to discharge additional brake fluid which is supplied to the wheel cylinders 20. That is, in a specific region where the target deceleration is high, the required wheel cylinder pressure Pwc is ensured using the hydraulic pressure obtained from driving the pump 34 in addition to the hydraulic pressure accumulated in the accumulator 50. In this case as well, the accumulator pressure is used as the initial response to braking. From another perspective, the hydraulic pressure is boosted by driving the pump 34 during emergency braking, in which case the required braking force is large, which enables the accumulator pressure to

be kept within the normal range, thus making it possible to reduce the size of the accumulator 50 and the like.

[0047] In this example embodiment, the volume of the accumulation chamber 75 is limited by restricting the displacement of the bellows 72 in the direction of contraction by the stopper 77, as shown in FIG. 2. That is, once the accumulator pressure reaches the upper limit value Pamax of the set range, the end plate 73 is retained by the stopper 77 so the volume of the accumulation chamber 75 will not get any larger. This inhibits additional brake fluid supplied by driving the pump 34 during emergency braking from entering the accumulator 50, thus reducing the amount of hydraulic fluid consumed which enables brake fluid to be supplied to the wheel cylinders 20 more efficiently and improves pump performance.

[0048] However, if the motor 32 continues to operate unnecessarily after the volume of the accumulation chamber 75 is restricted in this way, it may adversely affect the control valve that is connected to the hydraulic pressure line 30, i.e., it may cause wearing on the seal surface and produce a noise (flow noise) when hydraulic pressure is relieved, for example. That is, excess hydraulic fluid is relieved through the pressure increase valves 40 or the relief valve 53. However, because the pressure increase valves 40 in particular are used frequently in brake control as well, needless activation of these valves may cause their seal portions and the like to deteriorate, reducing the life of the valves. Therefore, even during emergency braking, the motor 32 may be quickly turned off once the required hydraulic pressure is obtained.

[0049] Also, in this example embodiment, the motor 32 is not driven unnecessarily when the brake pedal 12 is not being depressed even if the accumulator pressure sensor 51 fails. FIG. 4 is a view of a control map used to determine failure of the accumulator pressure sensor 51. The horizontal axis in the drawing represents time and the vertical axis represents the control current of the motor 32 and the accumulator pressure. The solid line in the drawing indicates the change in the control current of the motor 32 and the alternate long and short dash line indicates the change in (i.e., the estimated value of) the accumulator pressure.

[0050] As shown in the drawing, in the accumulation process of the accumulator 50, the increase in hydraulic pressure in the hydraulic pressure line 30 is absorbed by the expansion of the accumulation chamber 75, so the rotational load on the motor 32 is small and the slope of the supplied current (i.e., the current slope) is also gradual. However, after that accumulation has ended such that the accumulation chamber 75 is filled with brake fluid and restricted from expanding any further by the stopper 77, the hydraulic pressure in the hydraulic pressure line 30 rapidly increases thus increasing the rotational load on the motor 32. As a result, the current slope of the motor 32 rapidly becomes greater i.e., steeper, as shown in the drawing. That is, the change in the current slope rapidly changes after the accumulator 50 is filled with brake fluid compared to before the accumulator pressure 50 is filled with brake fluid. In the example shown in the drawing, at time tl the accumulation chamber 75 becomes full with brake fluid, marking a point of inflexion where the current slope changes greatly. Therefore, in this example embodiment, this current slope is monitored and when it becomes greater than a predetermined value, it is estimated that the accumulator pressure has reached the upper limit value Pamax and the motor 32 is stopped. As a result, the motor 32 will not be run unnecessarily if the accumulator pressure sensor 51 fails. Incidentally, the current slope (i.e., the predetermined value) which is the reference for determining whether the accumulator pressure sensor 51 has failed may be set appropriately through testing or the like.

[0051] Next, a hydraulic pressure control routine of this example embodiment will be described. FIG. 5 is a flowchart schematically illustrating the main steps in the hydraulic pressure control routine. This routine is repeatedly executed in predetermined cycles after an ignition switch has been turned on. [0052] First, the ECU 200 calculates, according to a calculation process based on a predetermined program, a target wheel cylinder pressure Pwc which is a target value to which the wheel cylinder pressure is to be controlled (step SlO). Then the ECU 200 determines whether that target wheel cylinder pressure is less than a preset hydraulic pressure Pl. Here, a hydraulic pressure (such as 12 MPa) that is able to be provided by

the accumulator 50 is set as the hydraulic pressure Pl so that normal brake control is executed by the hydraulic pressure accumulated in the accumulator 50.

[0053] If the target wheel cylinder pressure Pwc is less than the hydraulic pressure Pl (i.e., Yes in step S12), the ECU 200 determines whether the differential pressure between the accumulator pressure Pace detected by the accumulator pressure sensor 51 and the target wheel cylinder pressure Pwc is greater than a preset differential pressure δP. If the accumulator pressure Pace is greater than the target wheel cylinder pressure Pwc, hydraulic pressure can be supplied so the differential pressure δP is set to zero. Alternatively, however, the differential pressure δP may also be set to a predetermined value (a positive value) taking into account pressure loss in the hydraulic circuit and the like.

[0054] If the differential pressure between those two pressures is greater than the set differential pressure δP (i.e., Yes in step S14), then normal brake control is executed (step S 16). That is, a first motor relay control which keeps the accumulator pressure within that set range is executed, and open/close control of the pressure increase valves 40 and the pressure decrease valves 42 is executed to realize the target wheel cylinder pressure. In the first motor relay control, if the accumulator pressure detected by the accumulator pressure sensor 51 falls below the lower limit value Pamin, current starts to be supplied to the motor 32 which then drives the pump 34 to increase the accumulator pressure. If the accumulator pressure exceeds the upper limit value Pamax, current stops being supplied to the motor 32 to stop the pump 34.

[0055] If, on the other hand, the target wheel cylinder pressure Pwc is greater than the hydraulic pressure Pl in step S12 (i.e., No in step S12), emergency brake control is executed (step S 18). That is, a second motor relay control which compensates for the insufficiency in the accumulator pressure, which has been set low as described above, using additional hydraulic pressure provided by driving the pump 34 is executed, and open/close control of the pressure increase valves 40 and the pressure decrease valves 42 is executed to realize the target wheel cylinder pressure. In the second motor relay control, if there is a demand to increase the pressure when the target wheel cylinder

pressure Pwc exceeds the upper limit value Pamax of the accumulator pressure, the pump 34 is driven to compensate for the lack of hydraulic pressure. When the wheel cylinder pressure reaches the target wheel cylinder pressure Pwc as a result, the pump 34 is then stopped. Even if the target wheel cylinder pressure Pwc exceeds the upper limit value Pamax of the accumulator pressure, the pump 34 is not driven if the wheel cylinder pressure has already reached that target wheel cylinder pressure Pwc and there is a demand to maintain the hydraulic pressure, or there is a demand to reduce the pressure. In this case as well, if the accumulator pressure falls below the lower limit value Pamin, current starts to be supplied to the motor 32 which then drives the pump 34 to increase the accumulator pressure. Also, when the current slope δI of the motor 32 is greater than a predetermined value α, the accumulator pressure sensor 51 may be failing so current stops being supplied in order to stop the pump 34.

[0056] Also, even if the differential pressure between the accumulator pressure Pace and the target wheel cylinder pressure Pwc is less than the set differential pressure δP in step S 14 (i.e., No in step S 14), then the emergency brake control is executed to compensate for that lack of hydraulic pressure (step S18).

[0057] As described above, in this example embodiment, during emergency braking, additional hydraulic pressure can be supplied by driving the pump 34 so the hydraulic pressure in the power hydraulic pressure source during normal braking can be kept lower. That is, the set range of the accumulated pressure in the accumulator 50 is kept lower based on the fact that normal braking is performed frequently, while the necessary wheel cylinder pressure can be ensured if emergency braking is required. Even if hydraulic pressure that exceeds that set range was applied to the accumulator 50 during emergency braking, it is infrequent and only temporary and thus does not greatly affect the durability of the accumulator 50. Therefore, the accumulator 50 can be made smaller. Also, the normal accumulator pressure is kept low so the starting current and the rated current of the motor 32 that drives the pump 34 can be kept low, which also enables the motor 32 and the pump 34 to be smaller. Accordingly, the drain on the battery power can be reduced so the entire power hydraulic pressure source can be made

smaller.

[0058] Also, reducing the set range of the accumulation pressure of the accumulator 50 in this way enables a gear pump to be used for the pump 34. As a result, fluid pulsations are able to be suppressed so less operating noise is produced compared to when a typical piston-type pump is used. Also, reduced vibration during operation ensures more stable behavior of the power hydraulic pressure source.

[0059] Also, in a typical brake control system, the set range of the accumulator pressure is high. As a result, there is a large differential pressure between that accumulator pressure and the wheel cylinder pressure so the flowrate of the brake fluid that flows through the valve portion of the pressure increase valves is fast (i.e., there is greater flow mass) so cavitation tends to occur. However, in this example embodiment, this tendency can be suppressed by keeping the set range of the accumulator pressure low. Also, in a typical brake control system, the pressure increase valves control a relatively large differential pressure so the gain of the control current must be set large with respect to the flow mass. However, in this example embodiment, the gain can be set small so controllability is able to be improved.

[0060] Next, a second example embodiment of the invention will be described. This second example embodiment is generally similar to the first example embodiment except for that the structure of the accumulator is different. Therefore, constituent portions that are substantially the same as those in the first example embodiment will be denoted by the same reference characters when necessary and descriptions of those portions will be omitted where appropriate. FIG. 6 is a diagram schematically showing the structure of an accumulator and the surrounding area according to this second example embodiment. [0061] The accumulator 250 of this example embodiment is not provided with the stopper 77 inside the bellows 72 as in the first example embodiment. Instead, a plate-shaped stopper 272 (which may be regarded as "accumulation suppressing means") is connected via a shaft 271 to the end plate 73 on the side of the end plate 73 that is opposite the bellows 72. That is, in this example embodiment, the stopper is provided in

the hydraulic circuit instead of in the gas chamber 74. The shaft 271 for is shaped like a column that is longer and has a smaller cross-section than the connecting line 76. One end of the shaft 271 is fixed to the center of the end plate 73, while the other end is fixed to the center of the stopper 272. An O-ring 273 is fitted on the surface, near the outer peripheral edge, of the stopper 272 on the same side as the shaft 271.

[0062] FIGS. 7A to 7C are diagrams illustrating operation of the accumulator. FIG. 7A shows the normal control state in which the hydraulic pressure of the power hydraulic pressure source is within the control range of the accumulator pressure. FIG. 7B shows a high pressure control state in which that hydraulic pressure exceeds the control range, and FIG. 7C shows a zero down state in which that hydraulic pressure is below the control range. Here, the normal control state includes the state during normal braking, and the high pressure control state includes the state during emergency braking. The zero down state is a state in which there is negative pressure in the accumulation chamber 75 and the accumulator 250 is effectively not functioning. [0063] In the normal control state shown in FIG. 7A, the bellows 72 expands and contracts so that the pressures in the accumulation chamber 75 and the gas chamber 74 balance each other out. As a result, the accumulator 250 is maintained in a state where accumulation within the set range is possible. The brake fluid discharged from the pump 34 into the high pressure line 30 is introduced into the accumulation chamber 75 through a gap between the stopper 272 and the hydraulic pressure line 30, and a gap between the shaft 271 and the connecting line 76. During braking, brake fluid in the accumulation chamber 75 flows out into the hydraulic pressure line 30 to be supplied to the wheel cylinders 20.

[0064] On the other hand, in the high pressure control state shown in FIG. IB, brake fluid is added by driving the pump 34 so the hydraulic pressure in the high pressure line 30 becomes fairly high. As a result, the differential pressure between the pressure in the accumulation chamber 75 and the hydraulic pressure in the gas chamber 74 increases such that the bellows 72 is displaced in the direction of contraction and the stopper 272 and the O-ring 273 are displaced toward the connecting line 76 side where

they are retained, thereby sealing off the accumulation chamber 15 from inside the high pressure line 30. That is, when the hydraulic pressure in the accumulator 50 exceeds the upper limit value of the set range (which is 8 to 12 MPa in this example embodiment), any more brake fluid that is introduced into the high pressure line 30 is supplied directly to the wheel cylinders 20 instead of being led into the accumulation chamber 75.

[0065] In the zero down state shown in FIG. 7C, the hydraulic pressure in the gas chamber 74 is greater than the hydraulic pressure in the accumulation chamber 75 so the bellows 72 expands to the maximum. As a result, the stopper 272 is displaced toward the center of the high pressure line 30, creating a wide open communication path between the high pressure line 30 and the communicating line 76. As a result, when the pump 34 is driven again and brake fluid is discharged, that brake fluid flows quickly into the accumulation chamber 75, thus raising the accumulator pressure to within the set range. On the other hand, even if the motor 32 fails or there is a servo short-circuit in this zero down state, the charged pressure in the gas chamber 74 is set equal to or greater than the hydraulic pressure at the time of that servo short-circuit, as described above, so backflow of the brake fluid from the pressure increase valve 40 side can be prevented.

[0066] In this example embodiment, when the accumulator pressure exceeds the set range, the hydraulic fluid passage between the high pressure line 30 and the accumulation chamber 75 is closed off, thereby preventing the pressure in the accumulator 50 from becoming equal to or greater than the upper limit value of the set range. Closing off the hydraulic fluid passage in this way enables the wheel cylinder pressure to be efficiently increased during emergency braking, as well as prevents the pressure in the accumulator 50 from rising unnecessarily, thus improving the durability of the accumulator 50. [0067] Although the invention has been described herein with reference to specific example embodiments, it is not limited to these example embodiments. That is, many modifications and variations therein, such as design changes, will readily occur to those skilled in the art, and all such variations and modifications are included within the intended scope of the invention.

[0068] FIG. 8 is a diagram showing the structure of an accumulator according to a modified example of the first example embodiment. Constituent portions in this example embodiment which are the same as those in the first example embodiment will be denoted by the same reference numerals. In the accumulator 350 of this modified example, a rounded pad 378 is provided on the tip end of the stopper 77. With this design, the area of the pad 378 that receives the pressure from the end plate 73 increases gradually as the hydraulic pressure inside the accumulation chamber 75 increases, which smoothes out the initial rise in that hydraulic pressure. As a result, the accumulator 350 receives less of a shock when the hydraulic pressure increases during emergency braking, so the noise produced when the end plate 73 abuts against the stopper 77 at that time is reduced. Incidentally, the tip end of the pad 378 is curved, but it may also be tapered in a cone-shape or the like.

[0069] Also, in the first example embodiment and the modified example thereof, the stopper 77 of a predetermined length is provided, but the stopper 77 may be shorter than that length as long as the pressure in the gas chamber 74 can be ensured. This enables the accumulator to be made smaller. Alternatively, the accumulation chamber 75 and the gas chamber 74 may be divided by a thin membrane-like diaphragm instead of the bellows. Furthermore, those two chambers may also be divided by only a dividing member such as the end plate 73 instead of the contractible and expandable bellows or the like. For example, a seal ring that can slide against the housing 71 may be arranged on an outer peripheral portion of the end plate 73 and the volume of the accumulation chamber 75 may be changed by displacement of that end plate 73.

[0070] FIG. 9 is a diagram showing the structure of an accumulator according to a modified example of the second example embodiment. Constituent portions in this modified example which are the same as those in the second example embodiment will be denoted by the same reference numerals. In the accumulator 450 in this modified example, the tip end portion of a shaft 471 that is connected to the end plate 73 forms a valve body 472. A valve seat 473 is formed by an open end edge of the connecting line 76 that leads to the high pressure line 30. The valve body 472 is able to be seated on the

valve seat 473 as well as lifted off of the valve seat 473 from the high pressure line 30 side. When the valve body 472 is lifted off of the valve seat 473, the connecting line 76 becomes open to the high pressure line 30. Conversely, when the valve body 472 is seated on the valve seat 473, the connecting line 76 becomes closed off from the high pressure line 30. In the high pressure control state, the differential pressure between the hydraulic pressure in the accumulation chamber 75 and the hydraulic pressure in the gas chamber 74 increases so the bellows 72 becomes displaced in the direction of contraction. As a result, the valve body 472 becomes seated on the valve seat 473, thereby sealing off the accumulation chamber 75 from inside the high pressure chamber 30. As a result, the brake fluid that was introduced into the high pressure line 30 can be more efficiently supplied to the wheel cylinders 20. In this way, in this modified example, the communicating portion between the high pressure line 30 and the accumulator 50 is sealed off by a metal seal instead of a seal ring made of rubber or the like as in the second example embodiment. As a result, the seal portion will not deteriorate as easily, which increases its life.

[0071] FIG. 10 is a diagram schematically showing the structure of an accumulator and the surrounding area according to another modified example of the first example embodiment. Constituent portions in this modified example which are the same as those in the first example embodiment shown in FIG. 2 will be denoted by the same reference numerals. In this modified example, a check valve 52 is provided in the high pressure line 30 between the pressure increase valves 40 and the accumulator 50. This check valve 52 allows brake fluid to flow from the accumulator 50 side to the pressure increase valve 40 side, while preventing it from flowing in the reverse direction. Providing this kind of check valve 52 prevents the backflow of brake fluid irrespective of the charged pressure in the gas chamber 74. Therefore, even if there is a servo short-circuit when the motor 32 fails or the like, brake fluid is still able to be efficiently supplied to the wheel cylinders 20. Also, the charged pressure in the accumulator 50 can be reduced so the accumulator 50 can also be made smaller.

[0072] FIGS. HA and HB are diagrams showing the structures of

accumulators according to two other modified examples of the first example embodiment. Constituent portions in these modified examples that are the same as those in the first example embodiment shown in FIG. 2 will be denoted by the same reference numerals. In these modified examples, the accumulator is a so-called piston-type accumulator instead of a bellows-type accumulator. As shown in FIG. HA, the accumulator 550 includes a piston 510 that hermetically divides the space within the housing 71 into an enclosed space and- an open space. The piston 510 forms an open cylinder with a bottom. O-rings 512 and 514 which serve as seal members are fitted into two grooves formed around the outer peripheral portion of the piston 510. The O-rings 512 and 514 are arranged at predetermined intervals in the axial direction of the piston 510 and hermetically separates the accumulation chamber 75 from the gas chamber 74. In the gas chamber 74, a spring 516 which serves as an urging member that urges the piston 510 in the direction in which the accumulation chamber 75 contracts, is interposed between the bottom portion of the piston 510 and the housing 71. The open end portion, which is on the opposite side of the piston 510 from the bottom portion, forms a retaining surface 518 that abuts against, and is thus retained by, the bottom surface of the housing 71, thereby restricting displacement in the axial direction of the piston 510.

[0073] The piston 510 changes the volume of the accumulation chamber 75 by being displaced in the axial direction such that the urging force from the spring 516 and the force from the differential pressure between the accumulation chamber 75 and the gas chamber 74 balance each other out. However, even if the accumulator pressure increases so that the differential pressure increases and the piston 510 is displaced in the direction of expansion of the accumulation chamber 75 (i.e., to the right in the drawing), the volume of the accumulation chamber 75 is still limited by the retaining surface 518 being retained by the housing 71. Conversely, when the accumulator pressure drops, the piston 510 is displaced in the direction of contraction of the accumulation chamber 75 (i.e., to the left in the drawing) such that the volume of the accumulation chamber 75 decreases. However, that displacement is restricted by the bottom portion of the piston 510 being retained by the housing 71.

[0074] Incidentally, as shown in FIG. HB, a ring shaped pad 520, for example, may also be provided on the open end portion of the piston 510. As shown in the drawing, having the tip end of the pad 520 be rounded enables the same effects as those obtained in the modified example shown in FIG. 8 to be obtained. That is, it reduces the noise produced when the piston 510 makes a full stroke and abuts against the housing 71. Incidentally, in the modified example shown in the drawing, the piston 510 is directly retained by the housing 71, but the stopper 77 may also be provided as shown in FIGS. 2 and 8. For example, the stopper 77 may be arranged at the axial center of the spring 516 and the bottom of the piston 510 may be retained by the stopper 77. If the stopper 77 was provided in FIG. UA, the open end portion of the piston 510 need not function as the retaining surface 518. Also, if the stopper 77 was provided in FIG. HB, there would be no need to provide the pad 520. Furthermore, the bellows-type accumulator shown in FIGS. 7 and 9 may also instead be piston-type accumulators. For example, the end plate 73 shown in FIGS. 7 and 9 may be replaced by the piston 510 shown in FIGS. HA and HB. Also, the set charged pressure of the accumulator may be set using a spring or nitrogen gas or the like.

[0075] Incidentally, in the example embodiments described above, a gear pump is used as the pump 34, but a piston-type pump may also be used. For example, a reciprocating pump having two or more pistons that each which move in a reciprocating manner by the motor 32 may be used. However, using a gear pump as in the example embodiments described above is preferable because it reduces hydraulic pressure pulsations, resulting in less operating noise.

[0076] Also, although not mentioned in the example embodiments, other failsafe structure may also be employed in case of a failure from foreign matter becoming stuck in the valve portions of the front wheel side pressure increase valves 40FR and 40FL, for example. That is, in this case, despite the brake pedal 12 not being depressed, hydraulic fluid from the accumulator 50 may leak from the pressure increase valves 40FR and 40FL and flow back to the master cylinder 14 side through the shut-off valves 22 which are open. As a result, the master pressure sensor 48 may detect that hydraulic

pressure and the ECU 200 may erroneously determine that the brake pedal 12 is being depressed. Therefore, the rear wheel side pressure increase valves 40RR and 40RL, which are not connected to a manual hydraulic pressure source, may be opened first to make it easier to relieve hydraulic fluid in order to inhibit foreign matter from becoming stuck in the front wheel side pressure increase valves 40FR and 40FL in this way.

[0077] More specifically, a small amount of current that is less than the valve opening current may be constantly applied to the rear wheel side pressure increase valves 40RR and 40RL. As a result, the valve opening pressure of the rear wheel side pressure increase valves 40RR and 40RL becomes less than the valve opening pressure of the front wheel side pressure increase valves 40FR and 40FL, thus making it easier for the rear wheel side pressure increase valves 40RR and 40RL to open. As a result, the hydraulic pressure from the accumulator 50 can be relieved to the hydraulic pressure supply and discharge line 28 through the rear wheel side pressure increase valves 40RR and 40RL. Alternatively, the valve opening pressure of the rear wheel side pressure increase valves 40RR and 40RL may be structurally set lower than the valve opening pressure of the front wheel pressure increase valves 40FR and 40FL. For example, the spring loads of springs or the like that urge the valve bodies of the pressure increase valves in the valve closing direction may be set different from one another to achieve the same effect. The former is advantageous in that the same parts can be used for all of the pressure increase valves 40, while the latter is advantageous in that it reduces power consumption. Incidentally, in this modified example, each of the front wheel side pressure increase valves 40FR and 40FL corresponds to a "specific pressure increase valve", and each of the front wheel side wheel cylinders 20FR and 20FL which correspond to those pressure increase valves 40FR and 40FL corresponds to a "specific wheel cylinder". [0078] Moreover, although not mentioned in the example embodiments, when there is a demand to reduce the pressure during brake control, the pump 34 may be driven in reverse. As a result, hydraulic pressure in the high pressure line 30 can be actively returned to the reservoir tank 26, thereby promoting pressure decrease control.

[0079] While the invention has been described with reference to example

embodiments thereof, it is to be understood that the invention is not limited to the example embodiments or constructions. To the contrary, the invention is intended to cover various modifications and equivalent arrangements. In addition, while the various elements of the example embodiments are shown in various combinations and configurations, which are exemplary, other combinations and configurations, including more, less or only a single element, are also within the spirit and scope of the invention.