Vehicular brake system

Abstract: When a vacuum booster (1) fails, an pressure amplifying mechanism (36 to 39, 43, 44) increases a wheel brake pressure. After that, even when it is detected that the vehicle is brought into a stopped state, the state in which an increased brake fluid pressure is applied to wheel cylinders (21 to 24) is held. If a condition for terminating control to maintain a stationary state of the vehicle is satisfied, that is, if a predetermined time period has elapsed since when the vehicle stops, the operation of the pressure amplifying mechanism (36 to 39, 43, 44) is stopped to reduce the wheel cylinder pressure.

Description: The present invention relates to a vehicular brake system according to the preamble of claim 1 in which a boosting effect of a brake booster for assisting a brake pedal depressing operation to enhance a braking performance is compensated when the brake booster has failed. Conventionally, a brake booster for boosting a brake pedal depressing force (for implementing a so-called brake assist operation) is mounted in a vehicle in order to reduce a driver's load to depress the brake pedal. As a brake booster of this type, there has been known a vacuum booster for boosting the brake pedal depressing force. The vacuum booster applies a boosted large force to a master cylinder by using the difference between a negative pressure of an intake side of an engine for example and an atmospheric pressure. Because the vacuum booster cannot exhibit a desired boosting effect when it fails, there has been discussed lately control for determining whether or not the vacuum booster fails and for executing another brake assist operation for increasing brake fluid pressure by driving a pump and others to enhance a braking force when the vacuum booster is determined to fail. However, in the above-mentioned technology in which the vacuum booster and another brake assist device are used together, there has been a possibility that the vehicle starts to move by its gravity when the vacuum booster has failed and the vehicle is stopped on a steep slope for example. The reason is that, when the brake assist operation for increasing the brake fluid pressure by driving the pump and the like is finished in response to the vehicle stopping, the braking force due to another brake assist device becomes small rapidly, and because the vacuum booster fails, a driver cannot depress the brake pedal sufficiently to generate braking force for keeping the vehicle resting on the slope. A vehicular bracke system according to the preamble of claim 1 is known from JP-A-8-295231. Accordingly, it is an object of the present invention to solve the above-mentioned problem by providing a vehicular brake system which can perform favorable brake control at the time of the vehicle stopping by use of another brake assist device when the boosting effect of the brake booster cannot be fully exhibited. There has been also studied a so-called pressure amplifying assist brake (PAB) for increasing the wheel cylinder pressure by use of a plurality of cut-off valves and a pump even when the vacuum booster has failed. In detail, the wheel cylinder is increased by cutting off a flow path from a master cylinder to the wheel cylinder by a master cut-off valve (SMC valve), by opening a flow path from the master cylinder to an intake side of a pump by a cut-off valve (SRC valve) and by driving the pump in this state. The magnitude of the pressure increased by the PAB is controlled by the opened/closed state of the SMC valve and the SRC valve or by the driving state of the pump. When the pressure increase is controlled by the opened/closed state of the SMC valve, or more concretely, when the SMC valve is turned OFF (when the flow path is opened) for example, there is a case where pulsations of brake fluid pressure occur. When it is detected by the fluid pressure sensor that the fluid pressure has been increased (due to the increase of pressure caused by the instantaneous pulsation, not due to the normal increase of pressure) for example, the SMC valve, the SRC valve, the pump and others are driven wastefully to reduce the fluid pressure to an adequate level. That is, there has been a problem that when the pulsations of the fluid pressure occur, inadequate control for decreasing and increasing the fluid pressure is carried out. To achieve the object, in the present invention, a brake fluid pressure generating device generates a first brake fluid pressure due to the boosting effect of a brake booster for boosting a driver's brake pedal depressing force when the driver steps on a brake pedal. A wheel braking force generating device generates a wheel braking force by receiving the first brake fluid pressure via a conduit. Then, when a first detecting device detects a failure or a deterioration of function of the brake booster, a pressure increasing device increases the brake fluid pressure applied to the wheel braking force generating device to a second brake fluid pressure which is higher than the first brake fluid pressure. Further, when a second detecting device detects that the vehicle is stopped (after operation of the pressure increasing device), a holding device holds the brake fluid pressure applied to the wheel braking force generating device to be higher than the first brake fluid pressure until when a predetermined releasing condition is satisfied. That is, there is a possibility that the braking force is insufficient especially when an external force which might move the vehicle of a stationary state is applied to the vehicle (one example of the external force is gravity acting to the vehicle on a slope) if the operation of the increasing device is finished in response to the vehicle stopping after the increasing device increased the brake fluid pressure so as to compensate the failure or the function deterioration of the brake booster. However, in the present invention, the operation of the increasing device is not stopped until the predetermined releasing condition (i.e. the condition for stopping the operation of the increasing device) in which the vehicle can keep its stationary state is satisfied. In other words, the operation of the increasing device is not finished provided that the vehicle stops. Therefore, because the enough braking force is assured even when the vehicle stops on a slope for example when the brake booster is failed or its function is deteriorated. As a result, the vehicle will not start to move by gravity acting thereto and safety can be enhanced. It should be noted that the brake fluid pressure held by the holding device may be a pressure of a degree which, at least, can stop the vehicle on a slope and, for example, the second brake fluid pressure may be reduced more or less as the brake fluid pressure held by the holding device. Also, the brake fluid pressure held by the holding device may be equal to the second brake fluid pressure created by the pressure increasing device. That is, while the brake fluid pressure held by the holding device may be a pressure which can at least keep the vehicle in a stationary state when it stops on a slope for example, it is preferable for the holding device to hold high brake fluid pressure such as the second brake fluid pressure which is established during brake control because the ability for stopping the vehicle is enhanced. Preferably, the holding device holds the high brake fluid pressure applied to the wheel braking force generating device for a predetermined time period since when the second detecting device detects that the vehicle is in a stationary state. That is, because it is considered that any operation for keeping the vehicle in the stationary state is normally made (e.g. by manipulating a parking brake lever) when the predetermined time period elapses since when the vehicle has stopped, the high brake fluid pressure applied to the wheel braking force generating device is held for the predetermined time period. Thereby, the vehicle will not start to move due to the external force (gravity) acting thereto even if it stops on a slope. The holding device may hold the high brake fluid pressure applied to the wheel braking force generating device until when an accelerator pedal is depressed to cause the vehicle to start since when the second detecting device detects that the vehicle has stopped. That is, because a force for starting the vehicle is generated when the accelerator pedal is depressed, there is no problem even if the brake fluid pressure is reduced at that time. Further, the brake fluid pressure needs to be reduced in order to start the vehicle smoothly. Therefore, the high wheel cylinder pressure is held since when the vehicle has stopped state until when the accelerator pedal is depressed, which is a period during which the stationary state of the vehicle needs to be maintained. Thereby, the vehicle will not start to move due to the external force (gravity) acting thereto even if it stops on a slope. The holding device may hold the high brake fluid pressure applied to the wheel braking force generating device until when the driver returns the brake pedal since when the second detecting device detects that the vehicle has stopped. That is, because a period from when the vehicle has stopped till when the brake pedal is returned is considered to be a period during which the driver wants to maintain the stationary state of the vehicle, the high brake fluid pressure applied to the wheel braking force generating device is held during such a period. Thereby, the vehicle will not start to move due to the external force (gravity) acting thereto even if it stops on a slope. It should be noted that the brake pedal is returned at a vehicle starting time or a vehicle parking time when the parking brake is manipulated, for example. The holding device may hold the high brake fluid pressure applied to the wheel braking force generating device until when the driver operates the parking brake since when the second detecting device detects that the vehicle has stopped. That is, because a period from when the vehicle has stopped till when the parking brake is operated is considered to be a period during which the driver wants to maintain the stationary state of the vehicle, the high wheel cylinder pressure applied to the wheel braking force generating device is held during such a period. Thereby, the vehicle will not start to move due to the external force (gravity) acting thereto even if it stops on a slope. When the vehicle is provided with an automatic transmission, the holding device may holds the high brake fluid pressure applied to the wheel braking force generating device until when the driver sets a shift lever at the position for parking since when the second detecting device detects that the vehicle has stopped. That is, because a period from when the vehicle has stopped till when the shift lever is set at the position for parking (normally indicated by "P") is considered to be a period during which the driver wants to maintain the stationary state of the vehicle, the high wheel cylinder pressure applied to the wheel braking force generating device is held during such a period. Thereby, the vehicle will not start to move due to the external force (gravity) acting thereto even if it stops on a slope. These and other objects, features and characteristics of the present invention will be appreciated from a study of the following detailed description, the appended claims, and drawings, all of which form a part of this application. In the drawings: FIG. 1 is a schematic structural diagram showing a vehicular brake system of a first embodiment; FIG. 2 is a block diagram showing an electrical structure of the first embodiment; FIG. 3 is a flow chart showing control processing steps of the first embodiment; FIG. 4A is a graph showing changes of wheel speed and vehicle-body speed and FIG. 4B is a graph showing changes of brake fluid pressure by the control processing of the first embodiment; Preferred embodiments according to the present invention will be explained below in detail with reference to the drawings. [First Embodiment] The schematic structure of the vehicular brake system will be explained based on FIG. 1 at first. It is noted that the vehicular brake system is equipped with a brake booster and a pressure amplifying mechanism comprising a pump and others in order to carry out brake assist control. As shown in FIG. 1, the vehicular brake system of the present embodiment has a vacuum booster 1 as a brake booster. The vacuum booster 1 is connected with a brake pedal 2 at its input side and with a tandem type master cylinder 3 at its output side. The master cylinder 3 is connected with a master reservoir 4 and a hydraulic pressure control circuit 6 composed of first and second longitudinal conduit systems A and B. The brake booster 1 is provided with a vacuum chamber (constant-pressure chamber) 11 and a variable-pressure chamber 12 which are partitioned by a diaphragm 8. The vacuum booster 1 exhibits a boosting effect as the brake pedal 2 is depressed. That is, it increases a force applied to a piston 7 of the master cylinder 3 by utilizing a difference of pressures in the both chambers 11 and 12. A negative pressure (intake negative pressure) generated in an intake manifold of an engine not shown is introduced into the vacuum chamber 11 and the atmospheric pressure is introduced into the variable-pressure chamber 12. In the hydraulic pressure control circuit 6, a wheel cylinder 21 of the front right (FR) wheel communicates with a wheel cylinder 22 of the front left (FL) wheel via the first conduit system A. Further, a wheel cylinder 23 of the rear right (RR) wheel communicates with a wheel cylinder 24 of the rear left (RL) wheel via the second conduit system B. The first conduit system A is provided with a known pressure-increasing valve 25 and a pressure-reducing valve 31 for controlling the hydraulic pressure of the wheel cylinder 21 of the wheel FR and with a pressure-increasing valve 26 and a pressure-reducing valve 32 for controlling the hydraulic pressure of the wheel cylinder 22 of the wheel FL. The second conduit system B is provided with a pressure-increasing valve 27 and a pressure-reducing valve 33 for controlling the hydraulic pressure of the wheel cylinder 23 of the wheel RR and with a pressure-increasing valve 28 and a pressure-reducing valve 34 for controlling the hydraulic pressure of the wheel cylinder 24 of the wheel RL. The structure of the first conduit system A will be further explained. The first conduit system A is provided with a master cylinder cut-off valve (SMC valve) 36 for communicating/cutting off a fluid path 51 thereof on the side of the master cylinder 3 from each of the pressure-increasing valves 25 and 26. It is also provided with a reservoir 41 for temporarily storing the brake fluid discharged from each of the pressure-reducing valves 31 and 32 and a hydraulic pump 43 for pressurizing and feeding the brake fluid to the fluid path 51. It is also provided with a fluid path 53 for supplying the brake fluid directly from the master cylinder 3 to the hydraulic pump 43 in increasing the brake fluid pressure of the wheel cylinder. A cut-off valve (SRC valve) 38 for communicating/cutting off the fluid path 53 is disposed in the fluid path 53. The hydraulic pump 43 is driven by a pump motor 46 and serves as a pressure amplifying mechanism together with the SMC valve 36 and the SRC valve 38. The pressure amplifying mechanism can increase the wheel cylinder pressure more than the master cylinder pressure by driving the pump 43 while opening the SRC valve 38 and cutting off the SMC valve 36. It is noted that an accumulator 47 for suppressing pulsations of the internal hydraulic pressure is provided in the fluid path for discharging the brake fluid from the pump 43. Similarly to the first conduit system A described above, the second conduit system B is provided with pressure-increasing valves 27 and 28, pressure-reducing valves 33 and 34, a SMC valve 37, a reservoir 42, a hydraulic pump 44, an accumulator 48, a SRC valve 39, fluid paths 52 and 53 and the like at the same locations. As shown in FIG. 2, an ECU (electric control unit) 61 for controlling the vehicular brake system described above is mainly composed of a microcomputer equipped with a known CPU 61a, a ROM 61b, a RAM 61c, an input/output section 61d, a bus line 61e and the like. Signals of wheel speed sensors 62 disposed at each wheel which detect wheel speeds thereof, of a brake switch 63 for detecting that the brake pedal 2 has been depressed, of an accelerator switch 64 for detecting that an accelerator pedal not shown is depressed, of a parking switch 66 for detecting that a parking brake lever not shown is pulled, of a gear position sensor 67 for detecting that a shift lever not shown is set at the position for parking in case of a vehicle provided with an automatic transmission, of a constant-pressure chamber pressure sensor 68 for detecting a pressure within the constant-pressure chamber 11 of the vacuum booster 1, of a variable-pressure chamber pressure sensor 69 for detecting a pressure within the variable-pressure chamber 12, of a M/C pressure sensor 71 for detecting the master cylinder pressure, of a W/C pressure sensor 72 for detecting the wheel cylinder pressure and the like are fed to the ECU 61. The ECU 61 outputs control signals for driving the pressure-increasing valves 25 through 28, the pressure-reducing valves 31 through 34, the SMC valves 36 and 37 and the SRC valves 38 and 39 which are electromagnetic valves, and control actuators such as the pump motor 46. Next, control processing steps of the present embodiment will be explained based on a flow chart in FIG. 3 and on graphs in FIGS. 4A and 4B. At first, it is determined whether or not the brake pedal 2 is being depressed by determining whether or not the brake switch 63 is ON in Step 100 in FIG. 3. When it is determined to be Yes here, the processing advances to Step 110 and when it is determined to be No, the same determination is repeated again. In Step 110, the pressure (negative pressure) P of the constant-pressure chamber 11 is detected based on the signal from the constant-pressure chamber pressure sensor 68. In the following Step 120, it is determined whether or not the brake booster (V/B) 1 fails by determining whether or not the constant-pressure chamber pressure P which is negative pressure exceeds a determination value kP. That is, when no adequate negative pressure is supplied to the constant-pressure chamber 11, it is determined that the vacuum booster 1 fails. When it is determined to be Yes here, the processing advances to Step 130 and when it is determined to be No, the processing returns to Step 100 described above. It is noted that a deterioration of the function of a degree which will not invite the failure of the brake booster 1 may be also detected by setting the level of the determination value low, i.e. by setting the value for discriminating the deterioration of the function to be lower than the value of kP. The brake fluid pressure may be amplified in the same manner with the case of the failure of the brake booster 1 in response to this result (however, the degree of amplification of the pressure is small). Because the brake booster 1 is determined to be failed in Step 130, a pressure amplifying mechanism (an increasing device) is started to complement the boosting effect of the vacuum booster 1. That is, the amplification of the brake fluid pressure is carried out by the pressure amplifying mechanism. Specifically, the SMC valves 36 and 37 are closed to cut off the fluid paths 51 and 52 extending from the master cylinder 3 to the wheel cylinders 21 through 24 and the SRC valves 38 and 39 are opened to communicate the fluid paths 53 and 54 from the master cylinder 3 to the intake side of the hydraulic pumps 43 and 44. In this state, the hydraulic pumps 43 and 44 are driven by turning on the pump motor 46. Thereby, the wheel cylinder pressure may be increased while complementing the boosting effect of the failed vacuum booster 1. In the following Step 140, wheel speeds Vw of the four wheels is detected based on the signals from wheel speed sensors 62 disposed at each of four wheels. In the following Step 150, a speed value Vw which corresponds to a vehicle-body speed Vb is selected among the wheel speeds Vw of the four wheels and is set as a selected wheel speed Vws. For example, an average value or a maximum value of the wheel speeds Vw of the four wheels is set as the selected wheel speed Vws. In the following Step 160, it is determined whether or not the vehicle has stopped by determining whether or not the selected wheel speed Vws is less than a determination value kVw (e.g. 4 km/s). When it is determined to be Yes here, the processing advances to Step 170 and when it is determined to be No on the other hand, the processing returns to Step 130 described above. In Step 170, it is determined whether or not a condition for terminating control for a stopped (stationary) state of the vehicle, i.e. control for preventing the braking force from being insufficient at the time of the vehicle stopping is satisfied. Specifically, it is determined whether or not a predetermined time T1 has elapsed since when the vehicle has stopped. When it is determined to be Yes here, the processing advances to Step 180 and when it is determined to be No on the other hand, the processing returns to Step 130 described above. Because the condition for terminating control for the stopped state of the vehicle is met, the operation of the increasing device is stopped (released) in Step 180 and the processing is ended. In detail, the SMC valves 36 and 37 are opened to communicate the fluid path 51 and 52 from the master cylinder 3 to the wheel cylinders 21 through 24, the SRC valves 38 and 39 are closed to cut off the fluid path 53 and 54 from the master cylinder 3 to the intake side of the hydraulic pumps 43 and 44, and the hydraulic pumps 43 and 44 are stopped by turning off the pump motor 46. Thereby, control for increasing the wheel cylinder pressure (pressure amplifying control) carried out for complementing the boosting effect of the failed vacuum booster 1 may be stopped. As a result, the amplification of the wheel cylinder pressure may be stopped and the braking force of the wheel may be reduced after the predetermined time T1 since when the vehicle has stopped by the above-mentioned processing. Next, an effect obtained by the present embodiment will be explained based on FIGS. 4A and 4B. As shown in FIGS. 4A and 4B, when the brake pedal is depressed at time t0, the wheel speed Vw and the vehicle-body speed Vb gradually drop in response to the depression of the brake pedal 2. If the vacuum booster 1 fails at time t1, the amplification of the wheel cylinder pressure is started by the pressure amplifying mechanism in order to compensate the boosting effect thereof. Accordingly, although the master cylinder pressure (M/C pressure) barely increases (depending on the state of the failure), the wheel cylinder pressure (W/C pressure) increases thereafter. When the wheel speed Vw (or more specifically, the selected wheel speed Vws) decreases below the determination value kVw at time t2, it is determined that the vehicle is put into the stopped state. The amplification of the wheel cylinder pressure by the pressure amplifying mechanism is stopped and the wheel cylinder pressure is reduced at time t3 at which the predetermined time T1 has elapsed from the determination of the stopped state (made at time t2). That is, the pressure amplifying mechanism (the increasing device) for compensating the boosting effect of the vacuum booster 1 is driven when it is determined that the vacuum booster 1 has failed. Further, when the vehicle is determined to be in the stopped state after that, the operation of the pressure amplifying mechanism (the increasing device) is stopped not simultaneously with such a determination but after an elapse of the predetermined time T1 from the determination. This is because the braking force might become insufficient because of the failure of the vacuum booster 1 when the increasing device is stopped immediately after the vehicle is determined to be stopped. For example, when the vehicle is stopped on a slope, the vehicle may be unable to keep its stationary state by gravity applied thereto. As a countermeasure for that, the increasing device is stopped after the elapse of the predetermined time T1 since the vehicle stops. During such a period, it can be considered that the driver will take any action for maintaining the stopped state of the vehicle like pulling a parking brake lever for example. As a result, the present embodiment can obtain a remarkable effect that the vehicle will not start to move unintentionally even if it stops on a slope and it is very safe even when the vacuum booster 1 fails. [Second Embodiment] Next, a second embodiment will be explained below. However, because only the conditions for terminating control for the stopped state of the vehicle is different from that of the first embodiment, that point will be explained in detail hereinafter. In the second embodiment, the amplification of the wheel cylinder pressure by the pressure amplifying mechanism described above is continued until when the accelerator pedal (not shown) is depressed to start the vehicle from its stationary state even if the vehicle is detected to be in the stopped state. When it is detected that the accelerator pedal is depressed based on the signal from the accelerator switch 64, the amplification of the wheel cylinder pressure by the pressure amplifying mechanism is stopped to reduce the wheel cylinder pressure. That is, because a force for starting the vehicle is applied when the accelerator pedal is depressed, the vehicle does not move unintentionally even if the brake fluid pressure is reduced at that point of time. In other words, the vehicle will not move by gravity applied thereto even when the vehicle stops on a slope. Further, the brake fluid pressure needs to be reduced in order to start the vehicle smoothly. Therefore, the high wheel cylinder pressure is held until when the accelerator pedal is depressed in the present embodiment. Thereby, the same effect with the first embodiment described above can by obtained by the second embodiment. Further, because the braking force of the wheel is reduced after the operation for starting the vehicle is started, not reducing it when the predetermined time T1 has elapsed, the safety is enhanced further. [Third Embodiment] Next, a third embodiment will be explained below. However, because only the conditions for terminating control for the stopped state of the vehicle is different from that of the first embodiment, that point will be explained in detail hereinafter. In the third embodiment, the amplification of the wheel cylinder pressure by the pressure amplifying mechanism described above is continued until when the brake pedal 2 is actually returned by the passenger even if the vehicle is detected to be in the stopped state. That is, when it is detected that the brake pedal 2 is returned based on the signal from the brake switch 63, the amplification of the wheel cylinder pressure by the pressure amplifying mechanism is stopped to reduce the wheel cylinder pressure. In other words, because a period from when the vehicle has stopped till when the brake pedal 2 is returned is considered to be a period during which the passenger wants to maintain the stopped state of the vehicle, the high wheel cylinder pressure is held until when the brake pedal 2 is actually returned. As a result, the same effect with the first embodiment described above can be obtained by the third embodiment. Further, because the braking force of the wheel is reduced after when the intention of completely stopping the vehicle or of starting the vehicle again is exhibited by the driver, not reducing the braking force of the wheel when the predetermined time T1 has elapsed since when the vehicle has stopped, the safety is enhanced further. [Fourth Embodiment] Next, a fourth embodiment will be explained below. However, because only the conditions for terminating control for the stopped state of the vehicle is different from.that of the first embodiment, that point will be explained in detail hereinafter. In the fourth embodiment, the amplification of the wheel cylinder pressure by the pressure amplifying mechanism described above is continued until when a parking brake lever (not shown) is actually operated (pulled) by a driver even when the vehicle is detected to be in the stopped state. That is, when it is detected that the parking brake lever has been pulled based on the signal from the parking switch 66, the amplification of the wheel cylinder pressure by the pressure amplifying mechanism is stopped to reduce the wheel cylinder pressure. Because a period from when the vehicle has stopped till when the parking brake lever is pulled is considered to be a period during which the passenger wants to maintain the stopped state of the vehicle, the high wheel cylinder pressure is held until when the parking brake lever is pulled in the present embodiment. Thereby, the same effect with the first embodiment described above can be obtained by the fourth embodiment. Further, because the braking force of the wheel is reduced after when the operation for completely stopping the vehicle is made, not reducing the braking force of the wheel when the predetermined time T1 has elapsed, the safety is enhanced further. [Fifth Embodiment] Next, a fifth embodiment will be explained below. However, because only the conditions for terminating control for the stopped state of the vehicle is different from that of the first embodiment, that point will be explained in detail hereinafter. In the fifth embodiment, when the vehicle has an automatic transmission, the amplification of the wheel cylinder pressure by the pressure amplifying mechanism described above is continued until when the driver actually sets a shift lever (not shown) at a position for parking even when the vehicle is detected to be in the stopped state. That is, when it is detected that the lever has been set at the position for parking based on the signal from the gear position sensor 67, the amplification of the wheel cylinder pressure by the pressure amplifying mechanism is stopped to reduce the wheel cylinder pressure. Because a period from when the vehicle has stopped till when the shift lever is set at the position for parking is considered to be a period during which the driver wants to maintain the stopped state of the vehicle, the high wheel cylinder pressure is held until when the lever is set at the position for parking in the present embodiment. Thereby, the same effect with the first embodiment described above can be obtained. Further, because the braking force of the wheel is reduced after when the operation for completely stopping the vehicle is made, not reducing the braking force of the wheel when the predetermined time T1 has elapsed, the safety is enhanced further. It is noted that the present invention is not limited to the above-described embodiments and it may be practiced in various modes within the scope of the appended claims. For instance, although a vacuum booster 1 using the negative pressure of the engine and the atmospheric pressure has been described as a brake booster in the first through fifth embodiments, one using pressure from another pressure source such as an accumulator may be adopted as the brake booster. Further, a hydro-booster is also applicable as the brake booster. Although the determination of the failure and the function deterioration of the vacuum booster is made based on the state of the pressure of the constant-pressure chamber in the above-mentioned first through fifth embodiments, various methods other than that such as the following methods (A) through (C) may be adopted: (A) When the difference (P1 - P2) between the pressure P1 in the variable-pressure chamber and the pressure P2 in the constant-pressure chamber is less than a predetermined value, it may be determined that the vacuum booster fails or its function deteriorates. (B) When a difference (PT - P1) between the atmospheric pressure PT of the outside and the pressure P1 of the variable-pressure chamber is below a predetermined reference value, it may be determined that the vacuum booster fails or its function deteriorates. (C) When a difference (PT - P2) between the atmospheric pressure PT and the pressure P2 of the constant-pressure chamber is below a predetermined reference value, it may be determined that the vacuum booster fails or its function deteriorates. While the increasing device composed of the SMC valves 36 and 37, the hydraulic pumps 43 and 44, and the SRC valves 38 and 39 has been exemplified as the pressure amplifying mechanism in the above-mentioned first through fifth embodiments, the following structures (D) and (E) may be adopted for example as long as the wheel cylinder pressure can be increased when the vacuum booster fails or when its function deteriorates. (D) An arrangement in which a proportioning control valve is inversely connected in the place of the SMC valve, i.e. an arrangement in which the high-pressure side (input side) of the proportioning control valve is connected to a wheel cylinder side and the low-pressure side (output side) thereof is connected to a master cylinder side, may be adopted in the above-mentioned embodiments. In this case, it is desirable to set a split-point pressure of the proportioning control valve as low as possible. The pressure at the high-pressure side is equal to the pressure at the low-pressure side when the pressure applied to the proportioning control valve is less than the split-point pressure. Further, the proportioning control valve controls the pressure at the low-pressure side so that the pressure at the low-pressure side is reduced from the pressure at the high-pressure side with a predetermined rate when the pressure applied thereto exceeds the split-point pressure. Accordingly, the proportioning control valve can be used as the pressure amplifying mechanism along with the hydraulic pump. (E) It is possible to arrange such that the intake side of the hydraulic pump is connected to the master reservoir 4, not the master cylinder 3, and the SRC valve is disposed on the fluid path connecting the intake side of the hydraulic pump to the master reservoir 4. In this case, because the wheel cylinder pressure is increased by the brake fluid supplied from the master reservoir 4, not enhancing the wheel cylinder pressure by moving the brake fluid between the master cylinder and the wheel cylinder to the wheel cylinder side like the above-mentioned embodiments, it has such advantages that the wheel cylinder pressure may be increased quickly and the braking performance is high. Although the amplification of the wheel cylinder pressure by the pressure amplifying mechanism has been continued until when the predetermined conditions such as the elapse of a predetermined time T is satisfied since when the vehicle has been determined to be in the stopped state in the first through fifth embodiments, the high wheel cylinder pressure may be maintained by driving the pressure-increasing control valves, instead of amplifying the pressure by the pressure amplifying mechanism. In detail, when the vehicle is determined to be in the stopped state, the high wheel cylinder pressure may be maintained by closing the pressure-increasing control valves while the amplification of the wheel cylinder pressure by the pressure amplifying mechanism is stopped. Although the high wheel cylinder pressure formed during braking is held as it is until when the predetermined conditions such as the elapse of the predetermined time T is satisfied since when the vehicle has been determined to be in the stopped state in the first through fifth embodiments, the wheel cylinder pressure may be reduced to some degree while maintaining the braking force required to stop the vehicle because it is enough to prevent the unintentional move of the vehicle caused by the lack of the braking force during the stopped state. For instance, the wheel cylinder pressure may be reduced by reducing the driving force of the pump, by controlling the closed state of the SRC valve or by opening the pressure-reducing control valve with a predetermined duty ratio. In this case, because the wheel cylinder pressure is reduced to a certain degree, there is such an advantage that the shock of pressure (to the master cylinder side) caused when the SMC valve is opened in response to the stoppage of the pressure amplifying mechanism may be reduced. Further, if the function of the vacuum booster has deteriorated due to the fluctuation of the negative pressure of the intake manifold, the amplification of pressure is finished when it is detected that the function of the vacuum booster is recovered, i.e. that the negative pressure or the engine speed (idling speed) is normalized. It is noted that the normalization may be detected from the negative pressure of the vacuum booster or the like. Although the longitudinal conduit system has been exemplified in the first through fifth embodiments, a diagonal conduit system may be used instead of that.

Claims: A vehicular brake system, comprising: a brake booster (1) for boosting a brake pedal depressing force when a driver steps on a brake pedal; a brake fluid pressure generating device (3) for generating a first brake fluid pressure due to the brake pedal depressing force boosted by a boosting effect of said brake booster; a wheel braking force generating device (21 to 24) for generating a wheel braking force by receiving the first brake fluid pressure; a fluid conduit (51, 52) for communicating said brake fluid pressure generating device with said wheel braking force generating device; a first detecting device (61, 68) for detecting at least one of a failure and a deterioration of function of said brake booster; and a pressure increasing device (36 to 39, 43, 44) for increasing a brake fluid pressure applied to said wheel braking force generating device to a second brake fluid pressure which is higher than said first brake fluid pressure when said first detecting device detects at least one of the failure and the deterioration of function of said brake booster; characterized by : a second detecting device (61, 62) for detecting that the vehicle is stopped; and a holding device (61) for holding the brake fluid pressure applied to said wheel braking force generating device to be higher than said first brake fluid pressure until a predetermined releasing condition is satisfied when said second detecting device detects that the vehicle is stopped after said pressure increasing device increases the brake fluid pressure applied to said braking force generating device to said second brake fluid pressure. The vehicular brake system according to Claim 1, characterized in that the brake fluid pressure held by said holding device (61) is equal to said second brake fluid pressure generated by said pressure increasing device. The vehicular brake system according to Claim 1, characterized in that said holding device (61) holds the brake fluid pressure applied to said wheel braking force generating device for a predetermined time period (T) after said second detecting device detects that the vehicle is in a stopped state. The vehicular brake system according to Claim 1, characterized in that said holding device (61) holds the brake fluid pressure applied to said wheel braking force generating device until when an accelerator pedal is depressed to cause the vehicle to start after said second detecting device detects that the vehicle is in a stopped state. The vehicular brake system according to Claim 1, characterized in that said holding device (61) holds the brake fluid pressure applied to said wheel braking force generating device until when the driver returns said brake pedal (2) after said second detecting device detects that the vehicle is in a stopped state. The vehicular brake system according to Claim 1, characterized in that said holding device (61) holds the brake fluid pressure applied to said wheel braking force generating device until when the driver operates a parking brake after said second detecting device detects that the vehicle is in a stopped state. The vehicular brake system according to Claim 1, characterized in that said vehicle has an automatic transmission, and said holding device (61) holds the brake fluid pressure applied to said wheel braking force generating device until when the driver sets a shift lever of said automatic transmission at a position for parking after said detecting device detects that the vehicle is in a stopped state.

Inventors: ABE YOUICHI, YONEMURA SHUICHI, ABE, YOUICHI, YONEMURA, SHUICHI, Abe, Youichi, c/o DENSO CORPORATION, Yonemura, Shuichi, c/o DENSO CORPORATION
Applicants: DENSO CORP, DENSO CORPORATION
Application year: 1997
Main classification: B60T13
IPC codes: B60T11, B60T8, B60T13, B60T7, B60T17