Apparatus for increasing brake cylinder pressure by controlling pump motor and reducing the pressure by controlling electric energy applied to control valve

Abstract: Hydraulic pressure control apparatus for controlling a pressure of a brake cylinder pressure, including a pump device (30) connected to the brake cylinder (18, 24, 50, 52) and including at least one set of a pump (32, 34) for delivering a pressurized fluid to the brake cylinder and a pump operating electric motor (38, 40), a low-pressure source (31) for storing the fluid discharged from the brake cylinder, a solenoid-operated pressure control valve (58, 62) disposed between the low-pressure source and the brake cylinder and operated with an electric energy applied thereto, for controlling the brake cylinder pressure depending upon the electric energy, and a controller (160, 71, 72, 74) including a pressure increase control device (S35, S42, S49, S68, S91) for increasing the brake cylinder pressure by controlling the electric motor, and a pressure reduction control device (S36, S44, S45, S66, S92) for reducing the brake cylinder pressure by controlling the electric energy applied to the control valve.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a hydraulic pressure control apparatus for controlling a fluid pressure of a working fluid in a hydraulically operated brake cylinder. Discussion of the Related Art DE 4 029 793 A discloses such a hydraulic pressure control apparatus using an electrically operated hydraulic pressure source as a high-pressure source. This hydraulic pressure control apparatus includes (1) the above-indicated high-pressure source which includes a pump, an electric motor for operating the pump, and an accumulator for storing a working fluid pressurized by the pump, for supplying the pressurized fluid to a brake cylinder, (2) a low-pressure reservoir for storing the fluid discharged from the brake cylinder, (3) a solenoid-operated pressure increase control valve disposed between the high-pressure source and the brake cylinder, and electrically controlled for controlling the pressure of the fluid in the brake cylinder, co a level depending upon an electric energy applied thereto, (4) a solenoid-operated pressure reduction control valve disposed between the low-pressure-reservoir and the brake cylinder, and electrically controlled for controlling the pressure of the fluid in the brake cylinder, to a level depending upon an electric energy applied thereto, and (5) a control device for controlling the pressure increase control valve and the pressure reduction control valve to regulate the pressure of the fluid in the brake cylinder, while controlling the electric motor to hold the pressure of the fluid in the accumulator at a predetermined level. When the fluid pressure in the brake cylinder is increased under the control of this hydraulic pressure control apparatus, the electric energy to be applied to the solenoid-operated pressure increase control valve is controlled by the hydraulic pressure control apparatus, with the solenoid-operated pressure reduction control valve held in its closed state. The pressure of the fluid to be supplied from the accumulator to the brake cylinder through the pressure increase control valve is controlled according to the controlled electric energy applied to the pressure increase control valve. When the fluid pressure in the brake cylinder is reduced under the control of the hydraulic pressure control apparatus, the electric energy to be applied to the pressure reduction control valve is controlled by the apparatus with the pressure increase control valve held in its closed state. The pressure of the fluid in the brake cylinder is reduced to a level corresponding to the controlled electric energy. Thus, the pressure of the fluid in the brake cylinder is controlled by controlling the electric energies to be applied to the pressure increase control valve and the pressure reduction control valve. The assignee of the present invention proposed a hydraulic pressure control apparatus as disclosed in Japanese Patent Application JP-A-2000-00 1162 (which has not been laid open at the time the present invention was made). The hydraulic pressure control apparatus disclosed in this Application includes a normally open, solenoid-operated shut-off valve which is disposed between the brake cylinder and a reservoir and which is placed in its open state when an electric energy is not applied thereto and in its closed state when the electric energy is applied thereto. In a conventional hydraulic pressure, a solenoid-operated shut-off valve disposed between the brake cylinder and the reservoir is a normally closed valve which is placed in its closed state when no electric energy is applied thereto, in order to prevent delayed initiation of a braking effect provided by the brake cylinder. After each braking operation of the brake cylinder by releasing of a brake operating member, for instance, this normally closed shut-off valve is held in the open state for a predetermined time which is estimated to be required for substantially zeroing the pressure of the fluid in the brake cylinder. Then, the electric energy is completely removed from the shut-off valve, so that the shut-off valve is returned to the original closed state. However, the brake cylinder may have a residual fluid pressure even after the expiration of the above-indicated predetermined time, that is, even after the pressurized fluid has been discharged for the predetermined time. This residual fluid pressure may cause a so-called "brake drag", namely, some degree of braking even after the braking operation in question. This brake drag can be avoided where the shut-off valve is a normally open valve which is returned to the open state by removal of an electric energy after each braking operation. On the other hand, however, the normally open shut-off valve must be quickly brought to the closed state when it is required to increase the fluid pressure in the brake cylinder. To this end, the normally open shut-off valve disclosed in the above-identified Patent Application is energized with a maximum electric energy by the pressure control apparatus, when the fluid pressure in the brake cylinder is increased. Accordingly, this normally open shut-off valve requires a comparatively large amount of consumption of the electric energy, and suffers from a relatively large operating noise, a relatively short life, and other problems. To avoid friction-induced delay in brake activation WO 93/00236A, showing in the embodiment of figure 4 all the features of the preamble of claim 1, discloses to anticipate potential brake activation, subsequently rotate a pump already at low speed and change the correspondence of a brake cylinder from a master cylinder to the pump at a predetermined pump pressure. When hydraulic pressure control is carried out according to the driver's wish no solenoid-operated pressure control valve is used for pressure reduction. SUMMARY OF THE INVENTION It is an object of the present invention to provide a novel hydraulic pressure control apparatus which is different from the hydraulic pressure control apparatus described above, namely, provide a hydraulic pressure control apparatus capable of controlling the fluid pressure in the brake cylinder, without controlling the electric energies to be applied to the pressure increase control valve and the pressure reduction control valve. The above object may be achieved according to the following modes of the present invention, each of which is numbered like the appended claims, so as to indicate possible combinations of elements or features in preferred forms of the invention. (1) A hydraulic pressure control apparatus for a vehicle for controlling a fluid pressure of a working fluid in a hydraulically operated brake cylinder, said apparatus comprising: (a) a pump device connected to said brake cylinder and including at least one set of a pump for pressurizing the fluid and delivering the pressurized fluid to said brake cylinder and an electric motor for operating said pump; (b) a low-pressure reservoir for storing the fluid discharged from said brake cylinder; (c) a solenoid-operated pressure control valve disposed between said low-pressure reservoir and said brake cylinder and operated with an electric energy applied thereto, for controlling a pressure of the fluid in said brake cylinder depending upon an amount of said electric energy; and (d) a controller including pressure increase control means for increasing the pressure of the fluid in said brake cylinder by controlling said electric motor, and pressure reduction control means for reducing the pressure of the fluid in the brake cylinder by controlling the amount of said electric energy applied to said solenoid-operated pressure control valve, wherein a plurality of brake cylinders are connected through respective branch passages to said pump device such that said plurality of brake cylinders are parallel with each other, and said solenoid operated pressure control valve is provided in each of a plurality of pressure reducing passages respectively connecting said branch passages to said low pressure reservoir; and wherein said apparatus further comprises a flow restrictor device provided in a portion of each of said branch passages which is located between said pump device and a pair of connection thereof to a corresponding one of said pressure reducing passages, said apparatus being characterized in that said controller includes a motor control device which controls said electric motor on the basis of a highest one of target values of the fluid pressures in said plurality of brake cylinders to which the actual fluid pressure in said brake cylinders are controlled independently of each other, and in that said pressure reduction control means controls the amount of said electric energy applied to said solenoid operated pressure control valves, according to a wish of a driver of the vehicle, to reduce the pressure of fluid in said brake cylinders. In a most typical form of the hydraulic pressure control apparatus according to the above mode (1) of the present invention, a solenoid-operated pressure control valve is not provided between the pump device and the brake cylinder, and the pump for increasing the fluid pressure in the brake cylinder is connected directly to the brake cylinder. The brake cylinder is connected to the low-pressure source through the solenoid-operated pressure control valve. In the present hydraulic pressure control apparatus, the fluid pressure in the brake cylinder is increased by the pressurized fluid delivered thereto from the pump, and the increase in the fluid pressure in the brake cylinder is controlled by controlling the electric motor which is operated to drive the pump. Further, the fluid pressure in the brake cylinder is reduced with the fluid discharged from the brake cylinder to the low-pressure source, and the reduction in the brake cylinder pressure is controlled by controlling the electric energy applied to the solenoid-operated pressure control valve. Thus, the fluid pressure in the brake cylinder is controlled by the present hydraulic pressure control apparatus, by controlling the electric motor for operating the pump and by controlling the electric energy applied to the solenoid-operated pressure control valve. In this respect, the present hydraulic pressure control apparatus is different from the hydraulic pressure control apparatus as disclosed in JP-A-10-322803, wherein the electric energies applied to both of the solenoid-operated pressure increase control valve and pressure reduction control valve are controlled to control the fluid pressure in the brake cylinder. While the solenoid-operated pressure control valve is capable of controlling the fluid pressure in the brake cylinder according to the amount of electric energy applied thereto, the pressure control valve may be adapted to reduce the brake cylinder pressure with either an increase or a decrease in the electric energy applied thereto. The pump device is connected to a plurality of brake cylinders so that the control device controls the fluid pressures in all of brake cylinders in the same manner. As the pump device is connected to a plurality of brake cylinders, a flow restrictor or a pump cut-off valve is provided between the pump device and each of the brake cylinders, in order to permit the fluid pressures in the individual brake cylinders to be controlled independently of each other, as described below. The pump cut-off valve has an open state for connecting the brake cylinder to the pump device, and a closed state for disconnecting the brake cylinder from the pump device. (2) A hydraulic pressure control apparatus according to the above mode (1), wherein the pressure reduction control means includes means for turning off the electric motor when the pressure of the fluid in the brake cylinder is reduced. By turning off the electric motor, the pressurized fluid is not delivered from the pump to the brake cylinder. This arrangement assures more rapid reduction of the fluid pressure in the brake cylinder, than an arrangement wherein the electric motor is held on during the pressure reduction. The controller may further include pressure hold control means for holding the pressure of the fluid in the brake cylinder by turning off the electric motor and placing the solenoid-operated pressure control valve in a closed state for inhibiting a discharge flow of the fluid from the brake cylinder toward the low-pressure source. By placing the solenoid-operated pressure control valve in the closed state with the electric motor held off, the fluid flows into and from the brake cylinder are both inhibited, so that the fluid pressure in the brake cylinder can be held constant with improved stability. (3) A hydraulic pressure control apparatus according to the above mode (1) or (2), wherein the controller activates one of the pressure increase control means and the pressure reduction control means, depending upon at least one of a pressure difference between an actual value and a target value of the fluid pressure in the brake cylinder and a rate of change of the target value . Where the pressure increase control means or the pressure reduction control means is activated depending upon the pressure difference, the controller may be adapted to activate the pressure control means when the pressure difference ΔP (= Pref - P*) between the target value Pref and the actual value P* of the brake cylinder pressure is larger than a predetermined threshold EPS, namely, when ΔP > EPS, and activate the pressure reduction control means when the pressure difference ΔP is not larger than the threshold EPS, namely, when ΔP ≦ EPS. Where the controller includes the pressure hold means indicated above, the controller activates the pressure increase control means when the pressure difference ΔP is larger than a predetermined positive threshold value EPS1, namely, when ΔP > EPS1, activates the pressure reduction control means when the pressure difference ΔP is smaller than a predetermined negative threshold value -EPS2, namely, when ΔP < -EPS2, and activates the pressure hold means when the pressure difference ΔP is in a range between the negative and positive threshold values -EPS2 and EPS1, namely, when -EPS2 ≦ ΔP • EPS1. Where one of the pressure increase control means, pressure hold control means and pressure reduction control means is activated depending upon the rate of change ΔPref of the target brake cylinder pressure as well as the pressure difference ΔP, the controller may be adapted to activate the pressure hold control means rather than the pressure increase control means when the pressure difference ΔP is larger than the positive threshold value EPS1 while the rate of change ΔPref is not higher than a predetermined positive threshold value RPS1, namely, when ΔP > EPS1, and ΔPref ≦ RPS1, and also activate the pressure hold means rather than the pressure reduction means when the pressure difference ΔP is smaller than the negative threshold value - EPS2 while the rate of change ΔPref is not lower than a predetermined negative threshold value -RPS2, namely, when ΔP < -EPS2, and ΔPref ≧ -RPS2. The absolute value of the positive threshold EPS1 and the absolute value of the negative threshold -EPS2 may be equal to each other, and the absolute value of the positive threshold RPS1 and the absolute value of the negative threshold -RPS2 may be equal to each other. (4) A hydraulic pressure control apparatus according to any one of the above modes (1)-(3), wherein the controller includes a motor control device for controlling the electric motor to produce a larger output torque when a rate of increase of a target value of the fluid pressure in the brake cylinder is higher than a predetermined threshold, than when the rate of increase is not higher than the predetermined threshold. By increasing the output torque of the electric motor of the pump device, the flow rate or pressure of the pressurized fluid delivered from the pump can be increased. Accordingly, the rate of increase of the fluid pressure in the brake cylinder can be increased, with a result of reducing or eliminating a delay in the operation to increase the brake cylinder pressure. The rate of increase of the target value of the brake cylinder pressure exceeds the predetermined threshold when a normal operation to activate the brake cylinder is initiated (with an operation of a brake operating member) or when an emergency operation to activate the brake cylinder is initiated. In such an event, the motor control device is activated to control the electric motor to produce the larger output torque. The threshold of the increase rate of the target brake cylinder pressure may be an increase rate of the fluid pressure in the brake cylinder when the electric motor produces the largest output torque during a normal operation thereof , or an increase rate of the fluid pressure in the brake cylinder when the nominal voltage of a battery used for the hydraulic pressure control apparatus is applied to the electric motor. When the increase rate of the target value of the brake cylinder pressure is higher than the threshold, the voltage applied to the electric motor is made higher than a voltage normally applied to the electric motor, or higher than the nominal voltage of the battery. Alternatively, an electric current applied to the electric motor may be made larger than an electric current normally applied to the motor. In such a case, the output torque of the electric motor may not be controlled. However, the control of the output torque is not essential, provided the fluid pressure in the brake cylinder can be increased at a sufficiently high rate. As is apparent from the above explanation, the motor control device permits a rapid increase of the brake cylinder pressure, an increase in the output torque of the electric motor, and an increase in the delivery rate or pressure of the pump. The motor control device adapted to increase the output torque of the electric motor when the increase rate of the target value of the brake cylinder pressure is higher than the threshold is capable of increasing the delivery rate or pressure of the pump device, permitting an increased rate of increase of the fluid pressure in the brake cylinder, without increasing the capacity of the electric motor, and therefore without increasing the cost of manufacture of the pump device. To increase the brake cylinder pressure at a rate higher than the threshold, by increasing the delivery rate or pressure of the pump device with a normal control of the electric motor, it is necessary to use an electric motor having a larger capacity, which is inevitably expensive. The present arrangement to increase the output torque of the electric motor when the increase rate of the target brake cylinder pressure is higher than the threshold does not require an increase in the capacity and cost of manufacture of the electric motor. The feature according to the above mode (4) is available in a hydraulic pressure control apparatus which is adapted to increase the brake cylinder pressure by controlling the electric energy applied to a solenoid-operated pressure increase control valve and at the same time controlling the electric motor of the pump device or a hydraulic pressure control apparatus which is adapted to increase and reduce the brake cylinder pressure by controlling electric motors of a pressure increasing pump and a pressure reducing pump of the pump device, respectively. (5) A hydraulic pressure control apparatus according to the above mode (4), further comprising a battery for supplying power to said electric motor, and wherein the motor control device comprises a voltage increasing device for applying to the electric motor a voltage higher t han a nominal voltage of the battery when the rate of increase of the target value of the fluid pressure in the brake cylinder is higher than the prede termined threshold. The voltage increasing device may include a driver circuit incorporating a coil, a capacitor and a switching element such as a transistor, or alternatively a DC-DC converter or a booster converter. (6) A hydraulic pressure control apparatus according to any one of the above modes (1), (2) and (3), wherein the controller includes a motor start control device for applying a higher voltage to the electric motor when the electric motor is turned on, and holding the higher voltage applied to the electric motor while a predetermined condition is satisfied, the higher voltage being higher than a voltage to be applied to the electric motor after the predetermined condition is satisfied. Since the higher voltage is applied to the electric motor when the electric motor is turned on, the electric motor can be brought to a desired operating state (with desired operating speed and output torque) in a short time. Accordingly, the delivery rate and pressure of the pump device can be rapidly increased to the desired values. An electric motor cannot be brought to a desired operating state due to an inertia if the stationary electric motor is activated by application of a normal voltage. In the present arrangement according to the above mode (6), the electric motor is activated by application of the relatively high voltage, in order to rapidly increase the delivery rate and pressure of the pump device, making it possible to reduce a delay in the operation to increase the brake cylinder pressure. The higher voltage is held applied to the electric motor until the predetermined condition is satisfied, for instance, until a predetermined time has passed after starting of the electric motor, or until the brake cylinder pressure has been increased to a predetermined level. (7) A hydraulic pressure control apparatus according to any one of the above modes (1)-(6), wherein the controller includes pressure hold control means for turning off the electric motor and placing the solenoid-operated pressure control valve in a state for inhibiting a discharge flow of the fluid from the brake cylinder, when a rate of change of a target value of the fluid pressure in the brake cylinder is held within a predetermined range. In the hydraulic pressure control apparatus according to the above mode (7), the fluid pressure in the brake cylinder is not changed with a change in the rate of change of the target value of the brake cylinder pressure, but is held constant, while the rate of change is held within the predetermined range. For instance, the predetermined range is a range in which the target value of the brake cylinder pressure changes due to a small variation of the operating amount (operating force or stroke) of the brake operating member which is not recognized by the operator of the brake operating member. In this range within which the operator does not intend to change the braking force produced by the brake cylinder, it is desirable to inhibit an operation of the pump device, which would merely results in a waste of an electric energy and generation of a operating noise of the pump device. Where the brake cylinder is used to brake a wheel of an automotive vehicle, the rate of change of the target value of the brake cylinder pressure may change during running of the vehicle without a change in the operating amount of the brake operating member. In this case, a change in the fluid pressure in the brake cylinder results in a change in the deceleration value of the vehicle, which would be unexpected to the vehicle operator. The present arrangement prevents this problem. Further, the present arrangement eliminates a need for the vehicle operator to make efforts in maintaining the operating amount of the brake operating member constant, leading to easier manipulation of the brake operating member. (8) A hydraulic pressure control apparatus according to the above mode (7), wherein the controller includes pressure holding condition relaxing means for changing the predetermined range such that the predetermined range is wider when the pressure hold control means is in operation for a relatively long time than when the pressure hold control means is in operation for a relatively short time. When the pressure hold control means is in operation for a relatively long time, the operator generally desires to maintain the braking force generated by the brake cylinder. In this case, therefore, it is desirable to restrict a change in the fluid pressure in the brake cylinder. Since this arrangement results in an increase in the time during which the electric motor is held off, the electric energy consumption by the pump device is accordingly reduced. The predetermined range indicated above may be increased either in steps or continuously with an increase in t he time of operation of the pressure hold control means. (9) A hydraulic pressure control apparatus according to any one of the above modes (1)-(8), wherein the controller includes pressure hold control means for turning off the electric motor and placing the solenoid-operated pressure control valve in a state for inhibiting a discharge flow of the fluid from the brake cylinder, when a predetermined condition is satisfied, and pressure holding condition relaxing means for changing the predetermined condition such that the predetermined condition is more relaxed when the pressure hold control means is in operation for a relatively long time than when the pressure hold control means is in operation for a relatively short time. Where the predetermined condition is satisfied when the pressure difference ΔP between the target value and the actual value of the fluid pressure in the brake cylinder is held within a predetermined range (EPS1 ≧ ΔP ≧ -EPS2), the predetermined condition can be relaxed by increasing the upper limit EPS1 of the predetermined range and/or reducing the lower limit -EPS2 of the predetermined range. The predetermined condition may be satisfied when the pressure difference ΔP is held within a first predetermined range while the rate of change of the target value of the fluid pressure in the brake cylinder is held within a second predetermined range. The predetermined condition may be satisfied when the rate of change of the target value of the fluid pressure in the brake cylinder is held within a predetermined range. In these cases, too, the predetermined condition can be changed by changing at least one of the upper and lower limits of the predetermined range (each of the first and second predetermined ranges), as indicated above. (10) A hydraulic pressure control apparatus according to any one of the above modes (1)-(9), wherein the brake cylinder is activated to brake a wheel of an automotive vehicle, and the controller includes pressure hold control means for turning off the electric motor and placing the solenoid-operated pressure control valve in a state for inhibiting a discharge flow of the fluid from the brake cylinder, when a predetermined condition is satisfied, and pressure holding condition relaxing means for changing the predetermined condition such that the predetermined condition is more relaxed when the automotive vehicle is stationary than when the automotive vehicle is running. While the vehicle is stationary, the need of increasing or reducing the fluid pressure in the brake cylinder with a change in the operating amount of the brake operating member is not so high. It is sufficient to hold the brake cylinder pressure constant while the vehicle is stationary. In the apparatus according to the above mode (10), the predetermined condition for holding the brake cylinder pressure is relaxed while the vehicle is stationary, so that the operator does not have to make efforts in maintaining the operating amount of the brake operating member constant. Further, the relaxation of the predetermined condition results in an increase in the opportunity of operation of the pressure hold control means, leading to reduction of the electric energy consumption. The relaxation of the predetermination condition as described above with respect to the above mode (9) applies to this mode (10). (11) A hydraulic pressure control apparatus according to any one of the above modes (1)-(10), wherein the brake cylinder is activated to brake a wheel of an automotive vehicle, and the controller includes: pressure increasing means for increasing the fluid pressure in the brake cylinder by controlling the electric motor of the pump device while a predetermined pressure increasing condition is satisfied; compulsory pressure holding means for holding the fluid pressure in the brake cylinder by holding the electric motor off and placing the solenoid-operated pressure control valve in a state for inhibiting a discharge flow of the fluid from the brake cylinder, after a rotating speed of the wheel has been once substantially zeroed, even while the predetermined pressure increasing condition is satisfied; and disabling means for disabling the compulsory pressure holding means to operate, when the rotating speed of the wheel has exceeded a predetermined threshold and when an amount of operation of a brake operating member has been increased while the compulsory pressure holding means is operating. While the automotive vehicle is stationary, the need of increasing the fluid pressure in the brake cylinder with a change in the operating amount of the brake operating member is not so high. It is sufficient to hold the brake cylinder pressure constant while the vehicle is stationary. In the apparatus according to the above mode (11), the compulsory pressure holding means is provided for holding the brake cylinder pressure after the rotating speed of the wheel has been substantially zeroed, even while the predetermined pressure increasing condition is satisfied. However, the braking force generated by the brake cylinder must be increased with an increase in the operating amount of the brake operating member, if the vehicle begins to be moved for some reason or other, for instance, due to a collision of another vehicle with the back of the vehicle in question during stopping at a traffic signal, or due to stopping of the vehicle on an uphill or downhill road surface, without releasing of the brake operating member. To this end, the disabling means is provided for disabling the compulsory pressure holding means, when the rotating speed of the wheel has exceeded the predetermined threshold and when the amount of operation of the brake operating member has been increased while the compulsory pressure holding means is operating. (12) A hydraulic pressure control apparatus according to any one of the above modes (1)-(11), wherein the controller includes pressure increase commanding means for commanding an increase in the fluid pressure in the brake cylinder when a predetermined pressure increase condition is satisfied, and motor pre-starting means for turning on the electric motor when the predetermined pressure increase condition is expected to be satisfied in the near future. A delay in the operation to increase the fluid pressure in the brake cylinder can be reduced if the electric motor of the pump device is started or turned on before generation of a command to increase the brake cylinder pressure, when the predetermined pressure increase condition is expected to be satisfied in the near future. The motor pre-starting means may include detecting means for detecting a sign indicating a high possibility that the pressure increase condition will be satisfied in the near future. Where the predetermined pressure increase condition is satisfied when the brake operating member is operated, the sign may be a releasing operation of an accelerator pedal of the vehicle. Where the pressure increase condition is satisfied when a predetermined traction control condition to initiate a traction control of the vehicle drive wheels is satisfied, the sign may be a pre-traction control condition of the vehicle which is to be satisfied slightly before the traction control condition is satisfied. (13) A hydraulic pressure control apparatus according to the above mode (12), wherein the controller includes motor stopping means for turning off the electric motor when a predetermined time has passed, without satisfaction of the predetermined pressure increase condition, after the electric motor is turned on by the motor pre-starting means. If the predetermined pressure increase condition is not satisfied within the predetermined time after a sign indicating a high possibility that the pressure increase condition will be satisfied in the near future is detected, namely, after the electric motor is turned on by the motor pre-starting means, it means that the preliminary starting of the electric motor before the pressure increase condition is satisfied is likely to be unnecessary. In such a case, therefore, it is desirable to turn off the electric motor. However, the motor stopping means may be adapted to turn off the electric motor when the sign indicating a high possibility that the pressure increase condition will be satisfied in the near future has become absent. For instance, where the electric motor is turned on when the pre-traction control condition is satisfied, the electric motor may be turned off when the pre-traction control condition has become absent. (14) A hydraulic pressure control apparatus according to the above mode (12) or (13), wherein the brake cylinder is activated to brake a wheel of an automotive vehicle, the apparatus further comprising a downhill running detector for detecting that the automotive vehicle is running downhill, if an actual acceleration value of the vehicle is higher than an estimated acceleration value of the vehicle which is obtained on the basis of a vehicle drive torque produced by a drive power source for driving the vehicle, and wherein the motor pre-starting means including means for turning on the electric motor when at least one sign indicating a high possibility that the predetermined pressure increase condition will be satisfied in the near future is detected, the at least one sign including detection by the downhill running detector that the automotive vehicle is running downhill. The brake operating member is usually likely to be operated during downhill running of the automotive vehicle. Detection of a downhill running of the vehicle may be a sign indicating a high possibility that the predetermined pressure increase condition will be satisfied in the near future. A delay in the operation to increase the brake cylinder pressure is reduced by turning on the electric motor when the downhill running is detected. The downhill running detector may include an actual acceleration obtaining device and an estimated acceleration obtaining device. The actual acceleration obtaining device may include (1) an acceleration sensor for detecting the actual acceleration value of the vehicle, or (2) wheel speed sensors for detecting the rotating speeds of the vehicle wheels, and acceleration calculating means for calculating the actual acceleration value of the vehicle on the basis of the rotating speeds of the wheels detected by the wheel speed sensors. In the latter case, the acceleration calculating means obtains an estimated running speed of the vehicle on the basis of the detected speeds of the wheels, and obtains the acceleration value of the vehicle as a rate of change of the estimated running speed of the vehicle. The estimated acceleration obtaining device may be adapted to estimate the acceleration value of the vehicle on the basis of the drive torque of the vehicle drive power source. As described below with respect to the first preferred embodiment of the invention, the estimated acceleration value of the vehicle may be obtained as (F - F')/M, wherein "F" represents a drive torque produced by the drive power source, "F'" represents a road load, and "M" represents a weight M of the vehicle. The drive power source includes at least one of an internal combustion engine and an electric motor, and the drive torque F is the total torque of the engine and/or the electric motor. When the vehicle is running on a flat road surface, the actual acceleration value and the estimated acceleration value of the vehicle are substantially equal to each other. When the vehicle is running uphill, the actual acceleration value is smaller than the estimated acceleration value. When the vehicle is running downhill, the actual acceleration value is larger than the estimated acceleration value. Based on this fact, a downhill running of the vehicle can be detected. (15) A hydraulic pressure control apparatus according to the above mode (14), wherein the above-indicated at least one sign further including at least one of (a) detection of releasing of an accelerator member provided for accelerating the automotive vehicle, and (b) detection that a gradient of a downhill road surface on which the vehicle is running is higher than a predetermined threshold. Where the above-indicated at least one sign further includes the detection of releasing of the accelerator member, the motor pre-starting means turns on the electric motor of the pump device when the accelerator member is released during a downhill running of the vehicle. The releasing of the accelerator member reflects a desire of the vehicle operator to decelerate the vehicle, and indicates a high possibility of a subsequent operation of the brake operating member. Where the above-indicated at least one sign further includes the detection that the gradient of the downhill road surface is higher than the predetermined threshold, the motor pre-starting means turns on the electric motor of the pump device when the gradient of the downhill road surface on which the vehicle is running is detected to be higher than the threshold. When the downhill road surface has a relatively high gradient, the operator is likely to operate the brake operating member. The downhill road surface gradient higher than the threshold may be detected on the basis of a difference between the actual and estimated acceleration values of the vehicle described above. (16) A hydraulic pressure control apparatus according to any one of the above modes (1)-(15), wherein a plurality of brake cylinders are connected through respective branch passages to the pump device such that the plurality of brake cy linders are parallel with each other, and the solenoid-operated pressure control valve is provided in each of a plurality of pressure reducing passages respectively connecting the branch passages to the low-pressure reservoir, the apparatus further comprising a pump cut-off valve provided in a portion of each of the branch passages which is located between the pump device and a point of connection thereof to a corresponding one of the pressure reducing passages, the pump cut-off valve having a flow permitting state for permitting a flow of the pressurized fluid delivered from the pump into a corresponding one of the brake cylinders, and a flow inhibiting state for inhibiting the flow of the pressurized f luid into the corresponding one of the brake cylinders, the controller including pump cut-off valve control means for placing the pump cut-off valve in the flow inhibiting state when a rate of reduction of a target value of the fluid pressure in each of at least one of the plurality of brake cylinders is higher than a predetermined pressure reduction threshold while a rate of change of the target value of the fluid pressure in each of the other of the brake cylinders is on the side of increasing the fluid pressure as compared with the predetermined pressure reduction threshold. In the hydraulic pressure control apparatus according to the above mode (16), the pump cut-off valve placed in the flow permitting state functions as the flow restrictor device described above with respect to the above mode (1). When the pump cut-off valve is placed in the flow permitting state, the fluid pressure in the corresponding brake cylinder can be reduced by controlling the electric energy applied to the corresponding solenoid-operated pressure control valve. Thus, the fluid pressures in the individual brake cylinders can be controlled independently of each other. When the rate of reduction of the target value of the fluid pressure in one of the brake cylinders is higher than the predetermined pressure reduction threshold, the corresponding pump cut-off valve is brought to the flow inhibiting state, and the electric energy applied to the corresponding solenoid-operated pressure control valve is controlled to reduce the fluid pressure in the brake cylinder in question. Although the fluid pressure in the brake cylinder can be reduced even when the corresponding pump cut-off valve is placed in the flow permitting state, it is difficult to reduce the fluid pressure in the brake cylinder at a sufficiently high rate, that is, without a delay. In view of this drawback, the pump cut-of f valve is placed in the flow inhibiting state to inhibit the flow of the fluid from the pup device into the brake cylinder, for facilitating the reduction of the fluid pressure in the brake cylinder, thereby reducing a delay in the reduction of the brake cylinder pressure. Since the pump cut-off valve is not required to control the brake cylinder pressure, it may be a solenoid-operated shut-off valve. Where the fluid pressure in at least one of the brake cylinders is to be held constant, it is desirable to disconnect the at least one brake cylinder from both the pump device and the low-pressure reservoir. To hold the brake cylinder pressure constant, it is desirable to inhibit the flows of the fluid into and from the brake cylinder. To this end, the pump cut-off valve and the solenoid-operated pressure control valves are both placed in the flow inhibiting state. (17) A hydraulic pressure control apparatus according to any one of the above modes (1)-(16), wherein the pump device includes a low-pressure pump and a high-pressure pump having a higher delivery pressure and a lower delivery flow rate than the low-pressure pump. The use of the low-pressure pump and the high-pressure pump permits the overall delivery flow rate and pressure of the pump device to be controlled over relatively wide ranges, with comparatively small weights and sizes of those two pumps. BRIEF DESCRIPTION OF THE DRAWINGS The above and optional objects, features, advantages and technical and industrial significance of this invention will be further clarified by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings, in which: Fig. 1 is a schematic view illustrating a braking system including a hydraulic pressure control apparatus according to one embodiment of this invention; Fig. 2 is an elevational view in cross section schematically showing a solenoid-operated pressure reduction control valve included in the braking system of Fig. 1; Fig. 3 is an elevational view in cross section schematically showing another solenoid-operated pressure reduction control valve included in the braking system of Fig. 1; Fig. 4 is a flow chart illustrating a brake control routine executed according to a program stored in ROM of the hydraulic pressure control apparatus; Fig. 5 is a graph indicating a relationship between a vehicle speed and a road load, which relationship is represented by a data table stored in the ROM of the hydraulic pressure control apparatus; Fig. 6 is a view schematically indicating different control zones for a pump device controlled by the hydraulic pressure control apparatus; Fig. 7 is a flow chart illustrating a downhill running determining routine executed in step S2 of the flow chart of Fig. 4; Fig. 8 is a view showing an example of preliminary control of a pump motor of the motor device by the hydraulic pressure control apparatus; Fig. 9A and 9B are graphs showing examples of control of the pump motor by the hydraulic pressure control apparatus; Fig. 10 is a flow chart illustrating a common pressure control routine executed in step S11 of the flow chart of Fig. 4; Fig. 11 is a graph showing an example of common pressure control of the pressure reduction control valve by the hydraulic pressure control apparatus; Fig. 12 is a view showing the common pressure control of Fig. 11; Fig. 13 is a flow chart illustrating an independent pressure control routine executed in step S12 of the flow chart of Fig. 4; Fig. 14 is a view showing the independent pressure control of Fig. 13; Fig. 15 is a flow chart illustrating a stationary vehicle braking control routine executed in step S13 of the flow c har t of Fig. 4 ; Fig. 16 is a flow chart illustrating a common pressure control routine executed in place of the routine of Fig. 10, according to a program stored in ROM of a hydraulic pressure control apparatus according to another embodiment of the present invention; Fig. 17 is a view showing a common pressure control in place of that of Fig. 12, which is executed according to a program stored in ROM of a hydraulic pressure control apparatus according to a further embodiment of this invention; Fig. 18 is a flow chart illustrating a motor control routine executed according to a program stored in ROM of a hydraulic pressure control apparatus according to a yet further embodiment of this invention; Fig. 19 is an independent pressure control routine executed in step S12 of the flow chart of Fig. 4, according to a program stored in ROM of a hydraulic pressure control apparatus according to a still further embodiment of this invention; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first to Fig. 1, there is shown a hydraulically operated braking system for an automotive vehicle, which includes a brake operating member in the form of a brake pedal 10, and a master cylinder 12. The master cylinder 12 is of a tandem type having two pressurizing chambers, one of which is connected through a fluid passage 14 to a brake cylinder 18 for a front left wheel 16 of the vehicle, and the other of which is connected through a fluid passage 20 to a brake cylinder 24 for a front right wheel 22 of the vehicle. Thus, the two front wheel brake cylinders 18, 24 are connected to the respective two pressurizing chambers of the master cylinder 12. Solenoid-operated master cylinder cut-off valves 26, 28 are connected to the respective two fluid passages 14, 20. Each of these master cylinder cut-off valves 26, 28 is switchable between a closed state and an open state by energization and de-energization of its solenoid coil. When the pressure of a working fluid in the corresponding wheel brake cylinder 18, 24 is controlled, the master cylinder cut-off valve 26, 28 is held in its closed state with an electric current being applied to the solenoid coil. When the fluid pressure is not controlled, the master cylinder cut-off valve 26, 28 is held in its open state without an electric current being applied to the solenoid coil. Thus, each cut-off valve 26, 28 is a normally open valve, which is brought to the open state in the event of occurrence of any electrical abnormality. The present braking system further includes a pump device 30, which includes a low-pressure source in the form of a master reservoir 31, two pumps 32, 34, and two check valves 35, 36. As shown in Fig . 1, the two pumps 32, 34 are disposed in parallel connection with each other. The pump 32 is a high-pressure small-capacity gear pump (hereinafter referred to as "high-pressure pump" where appropriate), while the pump 34 is a low-pressure large-capacity gear pump (hereinafter referred to as "low-pressure pump" where appropriate). The high-pressure pump 32 has a relatively higher maximum delivery pressure and a relatively small maximum delivery amount per unit time, while the low-pressure pump 34 has a relatively lower maximum delivery pressure and a relatively larger maximum delivery amount per unit time. The high-pressure pump 32 is operated by a high-pressure pump motor 38, while the low-pressure pump 34 is operated by a low-pressure pump motor 40. The check valve 35 is provided to prevent the delivery pressure of the low-pressure pump 34 from acting on the high-pressure pump 32, while the check valve 36 is provided to prevent the delivery pressure of the high-pressure pump 34 from acting on the low-pressure pump 34. Further, a pressure relief valve (not shown) is connected in parallel with each of the high-pressure and low-pressure pumps 32, 34, for limiting the delivery pressure of the pump 32, 34 to the predetermined maximum level. The pressure and flow rate of the working fluid delivered from each pump 32, 34 (delivery pressure and rate of the pump 32, 34) can be adjusted by controlling the corresponding pump motor 38, 40. Both of the high-pressure and low-pressure pumps 32, 34 need not be gear pumps. That is, at least one of these pumps 32, 34 may be a plunger pump. If a solenoid-operated shut-off valve is provided in place of the check valve 36, the fluid pressure in the wheel brake cylinders 18, 24 may be controlled by operating the low-pressure pump 34 in the reverse direction. A pump passage 42 is connected to the delivery side of the pump device 30. The pump passage 42 is split into four branch passages 44 which are respectively connected at their ends to the above-indicated brake cylinders 18, 24 for the front wheels 16, 22, and brake cylinders 50, 52 for rear left and right wheels 46, 48. Thus, the four wheel brake cylinders 18, 24, 50, 52 are connected to the pump device 30 such that the wheel brake cylinders 18, 24, 50, 52 are connected in parallel with each other. To each of the branch passages 44 of the pump passage 42, there is connected a solenoid-operated pump cut-off valve 54 having an open state for connection of the corresponding wheel brake cylinder to the pump device 30 and a closed state for disconnection of the wheel brake cylinder from the pump device 30. Each pump cut-off valve 54 is a normally open valve which is held in the open state without an electric current being applied to its solenoid coil. A pressure reducing passage 56 is connected at one end thereof to each of the branch passages 44 for the front wheels 16, 22, at a point 55 between the pump cut-off valve 54 and the wheel brake cylinder 18, 24, and at the other end to the master reservoir 31. To this pressure reducing passage 56, there is connected a pressure reduction control valve 58 for each front wheel brake cylinder 18, 22. Similarly, a pressure reducing passage 60 is connected at one end thereof to each of the branch passages 44 for the rear wheels 46, 48, at a point 59 between the pump cut-off valve 54 and the wheel brake cylinder 50, 52, and at the other end to the master reservoir 31. To this pressure reducing passage 60, there is connected a pressure reduction control valve 62 for each rear wheel brake cylinder 50, 52. The pressure reduction control valves 58, 62 will be described. In the present braking system, the four branch passages 44 of the pump passage 42 are provided with the respective pump cut-off valves 54, as described above, so that the fluid pressures in the wheel brake cylinders 18, 24, 50, 52 can be controlled independently of each other. When the pump cut-off valves 54 are placed in the open state, they also function as flow restrictors, so that a pressure difference may be present across each pump cut-off valve 54. Since the solenoid-operated pressure reduction control valve 58, 62 is located on one of the opposite sides of the corresponding pump cut-off valve 54 which is nearer to the wheel brake cylinder 18, 24, 50, 52, the fluid pressures in the wheel brake cylinders 18, 24, 50, 52 can be controlled independently of each other. That is, by controlling the pressure reduction control valve 58, 62, the fluid pressure in the portion of the branch passage 44 on one of the opposite sides of the pump cut-off valve 54 which is nearer to the wheel brake cylinder 18, 24, 50, 52 can be controlled to be higher or lower than that in the portion of the branch passage 44 which is nearer to the pump device 30. The fluid pressures in the portions of the branch passages 44 which are nearer to the wheel brake cylinder 18, 24, 50, 52 can be controlled independently of each other by controlling the solenoid-operated pressure reduction control valves 58, 62. The front wheel brake cylinders 18, 24 are connected to both of the master cylinder 12 and the pump device 30, while the rear wheel brake cylinders 50, 52 are connected to the pump device 30 but are not connected to the master cylinder 12. In view of this arrangement, the solenoid-operated pressure reduction control valves 58 for the front wheel brake cylinders 18, 24 that can be operated by the master cylinder 12 are adapted to be normally closed, while the solenoid-operated pressure reduction control valves 62 for the rear wheel brake cylinders 50, 52 which are always operated by the pump device 30 are adapted to be normally open. Accordingly, the rear wheel brake cylinders 50, 52 do not suffer from so-called "brake drag" or a residual braking pressure, while the front wheel brake cylinders 18, 24 can be actuated by the master cylinder 12 even in the event of an electrical trouble, namely, even in the event of a failure of the pump device 30 and an electrical abnormality of the pressure reduction control valves 58. To the fluid passage 30 indicated above, there is connected a stroke simulator 70 which functions to prevent an extremely short operating stroke of the brake pedal 10 when the wheel brake cylinders 18, 24, 50, 52 are disconnected from the master cylinder 12 by closing of the master cut-off valves 26, 28 and are connected to the pump device 30. The stroke simulator 70 may be connected to the fluid passage 14, or may connected directly to at least one of the pressurizing chambers of the master cylinder 12. Two or three stroke simulators 70 may be connected to the appropriate ones of the fluid passages 14, 20 and the pressurizing chambers. The braking system includes a stroke sensor 71 for detecting the operating stroke of the brake pedal 10, a master cylinder pressure sensor 72 for detecting the fluid pressure in the master cylinder 12, a pump pressure sensor 74 for detecting the delivery pressure of the pump device 30, and wheel brake cylinder pressure sensors 75-78 for detecting the fluid pressures in the wheel brake cylinders 18, 24, 50, 52. Both of the stroke sensor 71 and the master cylinder pressure sensor 72 are not essential. Only one of these two sensors 71, 72 may be provided. As shown in Fig. 2, each of the solenoid-operated pressure reduction control valves 58 for the front wheel brake cylinders 18, 24 includes a seating valve 82 and an electromagnetic force generating device 84. The seating valve 82 includes a valve member 90, a valve seat 92, an electromagnetically movable member 94 movable with the valve member 90, and an elastic member in the form of a spring 96 which serves as a biasing device for biasing the electromagnetically movable member 94 in a direction for moving the valve member 90 to be seated on the valve seat 92. The electromagnetic force generating device 84 includes a coil 100, a holder member 102 made of a resin for holding the coil 100, a first magnetic path forming body 104 and a second magnetic path forming body 106. By energizing the coil 100, there is produced a magnetic field. A substantive portion of the magnetic flux in the magnetic field passes through the first magnetic path forming body 104, the electromagnetically movable member 94, and an air gap between the movable member 94 and the second magnetic path forming body 106. A magnetic force which acts between the electromagnetically movable member 94 and the second magnetic path forming body 106 can be changed by controlling the electric current to be applied to the coil 100. This magnetic force increases with an increase in the amount of electric current to be applied to the coil 100. Since the electric current amount and the magnetic force generated has a known relationship, the force for biasing the electromagnetically movable member4 94 can be changed by continuously changing the electric current to be applied to the coil 100, according to the known relationship. This force (which will be referred to as "electromagnetic drive force Fs") acts on the electromagnetically movable member 94 in the direction away from the valve member 90, that is, in the direction for moving the valve member 90 away from the valve seat 92. This direction in which the electromagnetic drive force Fs acts on the movable member 94 is opposite to a direction in which a biasing force Fk of the spring 96 acts on the movable member 94. As indicated above, the electromagnetic drive force Fs and the biasing force Fk of the spring 96 act on the valve member 90 through the electromagnetically movable member 94 in the pressure reduction control valve 58. In addition, a force Fp based on a difference between fluid pressures in an input port 108 and an output port 108 of the seating valve 82 acts on the valve member 90 in the direction for moving the valve member 90 away from the valve seat 92, that is, in the direction in which the electromagnetic drive force Fs acts on the valve member 90. When a sum of the electromagnetic drive force Fs and the force Fp (hereinafter referred to as "pressure difference force") is larger than the biasing force Fk of the spring 96 (where Fs + Fp > Fk), the pressure reduction control valve 58 (seating valve 82) is held in the open state with the valve member 90 being spaced apart from the valve seat 92. When the sum of the electromagnetic drive force F s and the pressure difference force Fp is smaller than the biasing force Fk (where Fs + Fp < Fk), the pressure reduction control valve 58 is held in the closed state with the valve member 90 seated on the valve seat 92. Since the biasing force Fk of the spring 96 is determined to be larger than the pressure difference force Fp corresponding to the maximum delivery pressure of the pump device 30, the pressure reduction control valve 58 is held in the closed state when the electromagnetic drive force Fs is zero. That is, the pressure reduction control valve 58 is a normally closed valve, as indicated above. In the present braking system, the electric current to be applied to the coil 100 of the pressure reduction control valve 58 is continuously controlled to regulate the fluid pressure in the corresponding front wheel brake cylinder 18, 24. The difference between the fluid pressures in the input and output ports 108, 109 of the pressure reduction control valve 58 corresponds to a difference between the fluid pressure in the wheel brake cylinder 18, 24 and the fluid pressure in the master reservoir 31. Since the fluid pressure in the master reservoir 31 can be considered to be equal to the atmospheric pressure, the pressure difference force Fp corresponds to the fluid pressure in the wheel brake cylinder 18, 24. The sum of the pressure difference force Fp and the electromagnetic drive force Fs can be controlled by controlling the electromagnetic drive force Fs. Therefore, the fluid pressure in the wheel brake cylinder 18, 24 can be controlled by controlling the electromagnetic drive force Fs. The fluid pressure in the wheel brake cylinder 18, 24 can be reduced by increasing the electromagnetic drive force Fs. Described more precisely, the electric current to be applied to the coil 100 is controlled so that the actual fluid pressure in the wheel brake cylinder 18, 24 changes toward a determined target value. As shown in Fig. 3, each of the solenoid-operated pressure reduction control valves 62 for the rear wheel brake cylinders 50, 52 includes a seating valve 130 and an electromagnetic force generating device 132, like the pressure reduction control valves 58. The seating valve 130 includes a valve seat 134, a valve member 136 movable to be seated on and unseated from the valve seat 134, an elastic member in the form of a spring 138 serving as a biasing device for biasing the valve member 136 in a direction away from the valve seat 134, a drive member 140 for moving the valve member 136, and an electrically movable member 142. The drive member 140, valve member 136 and electromagnetically movable member 142 are movable as a unit. The electromagnetic force generating device 132 includes a coil 144, a holder member 145 for holding the coil 144, a first magnetic path forming body 146, and a second magnetic path forming body 148 fixed to the housing of the control valve 62. When the coil 144 is energized, a magnetic field is produced with a magnetic flux passing through the first magnetic path forming body 146, the electromagnetically movable member 142, and an air gap between the movable member 142 and the second magnetic path forming body 148. As a result, an electromagnetic drive force Fs acts on the electromagnetically movable member 142 in a direction for moving the movable member 142 toward the second magnetic path forming body 148, so that the movable member 142 is moved to move the valve member 136 toward the valve seat 134. When the electromagnetic drive force Fs is zeroed, the valve member 136 is moved away from the valve seat 134 by a biasing force Fk of the spring 138. The pressure reduction control valve 62 is held in its open state when the electromagnetic drive f orce Fs is smaller than a sum of the pressure difference force Fp and the biasing force Fk (where Fs < Fp + Fk). Since the biasing force Fk is very small, it can be ignored. When the electromagnetic drive force Fs is zero, the control valve 62 is held in the open state. That is, the control valve 62 is a normally open valve. When the electromagnetic drive force Fs is larger than the sum of the pressure difference force Fp and the biasing force Fk (where Fs > Fp + Fk), the control valve 62 is held in the closed state. The fluid pressure in the wheel brake cylinder 50, 52 can be reduced by reducing the electromagnetic drive force Fs. In the present embodiment, the electric current to be applied to the coil 144 is controlled so that the actual fluid pressure in the wheel brake cylinder 50, 52 changes toward a determined target value. The braking system includes a control device 160 constituted principally by a computer incorporating a processing unit (PU) 152, a random-access memory (RAM) 153, a read-only memory (ROM) 154, an input portion 155 and an output portion 156. The input portion 155 is adapted to receive output signals of the above-indicated stroke sensor 71, master cylinder pressure sensor 72, pump pressure sensor 74 and wheel brake cylinder pressure sensors 75-78, and other sensors and switches such as wheel speed sensors 162-165 for detecting the rotating speeds of the wheels 16, 22, 46, 48, a brake switch 166 for detecting an operation of the brake pedal 10, a throttle opening sensor 168 for detecting an amount of operation of an accelerator pedal (not shown), a steering angle sensor 170 for detecting an operating angle of a steering wheel, and a yaw rate sensor 172 for detecting a yaw rate of the vehicle. The throttle opening sensor 168 may serve as a device for detecting an operation of the accelerator pedal. The input portion 155 is also connected to a power unit control device 174 which is provided for controlling a power unit of the vehicle including a drive power source and a transmission. The output portion 156 is connected to driver circuits (not shown) for the solenoid coils of the solenoid operated valves 26, 28, 54, 58, 62, and driver circuits 176, 178 for the high-pressure pump motor 38 and low-pressure pump motor 40. The ROM 154 stores various control programs including a program for executing a brake control routine illustrated in the flow chart of Fig. 4, a program for executing a normal braking pressure control routine, a program for executing an emergency braking control routine, a program for executing an anti-lock braking pressure control routine, a program for executing a vehicle stability control routine, and a program for executing a pump motor control routine. The ROM 154 further stores various data tables or maps as shown in Figs. 5 and 6, which are used for controlling the braking system. The control device 160 obtains a force acting on the brake pedal 10, on the basis of the output signals of the stroke sensor 71 and the master cylinder pressure sensor 72. In an initial portion of an operation of the brake pedal 10, an increase in the f luid pressure in the master cylinder 12 is more or less delayed due to a phenomenon of so-called "fast fill". In view of this tendency, the force acting on the brake pedal 10 is obtained on the basis of the output signal of the stroke sensor 71 while the fluid pressure in the master cylinder 12 is relatively low, that is, before the master cylinder pressure has been raised to a predetermined threshold. After the master cylinder pressure has exceeded the threshold, the force acting on the brake pedal 10 is obtained on the basis of the output signal of the master cylinder pressure sensor 71. If one of the stroke sensor 71 and the master cylinder pressure sensor 72 is defective, the force acting on the brake pedal 10 is obtained on the output signal of the other sensor 71, 72. The control device 160 further obtains a slipping state of each wheel 16, 22, 46, 48 being braked, on the basis of the rotating speed of the wheel detected by the corresponding wheel speed sensor 162, 163, 164, 165, and an estimated vehicle running speed obtained based on the detected wheel speeds. The control device 160 effects the anti-lock braking pressure control on the basis of the obtained slipping state of each wheel during brake application. The control device 160 further obtains an actual acceleration value of the vehicle, based on an amount of increase of the estimated vehicle running speed per unit time. In the present braking system, the wheel speed sensors 162-165 also serve as a sensor for obtaining the vehicle acceleration. The control device 160 effects the vehicle stability control by applying a positive brake to an appropriate one of the wheels 16, 22, 46, 48 so as to improve the running stability of the vehicle, on the basis of the output signals of the steering angle sensor 170 and the yaw rate sensor 172. The control device 160 receives from the power unit control device 174 information indicative of a drive torque being applied to the vehicle. The drive power source of the vehicle includes an internal combustion engine and/or an electric motor. The vehicle is driven by a total torque generated by at least one of the engine and the electric motor. Each of the driver circuits 176, 178 includes a normal voltage control circuit for controlling a voltage to be applied to the corresponding -high-pressure or low-pressure pump motor 38, 40, and a maximum voltage application circuit for applying the maximum voltage to the pump motor 38, 40. Normally, the voltage controlled by the normal voltage control circuit is applied to the pump motor 38, 40. When a required rate of increase of the fluid pressure in the wheel brake cylinders 18, 24, 50, 52 is higher than a predetermined upper limit, the maximum voltage which is higher than the nominal voltage of a battery used in the braking system is applied to the pump motor 38, 40 through the maximum voltage application circuit. The maximum voltage application circuit of the driver circuits 176, 178 may be a circuit including a coil, a capacitor and a switching element such as a transistor. Alternatively, the maximum voltage application circuit may include a DC-DC converter, or a boosting converter. An operation of the present braking system constructed as described above will be described. When the brake pedal 10 is depressed, a normal braking pressure control routine is executed. This normal braking pressure control routine is formulated to first determine a target value of the fluid pressure in the wheel brake cylinders 18, 24, 50, 52 (hereinafter referred to as "target wheel brake cylinder pressure"), depending upon a depression force acting on the brake pedal 10. The routine is further formulated to control the pump device 30 and the pressure reduction control valves 58, 62, so as to reduce a difference between the target wheel brake cylinder pressure and an actual value of the fluid pressure in the wheel brake cylinders 18, 24, 50, 52 (hereinafter referred to as "actual wheel brake cylinder pressure"). In the present embodiment, the voltages applied to the high-pressure and low-pressure pump motors 38, 40 of the pump device 30 and the electric currents applied to the coils 100, 144 of the pressure reduction control valves 58, 60 are controlled while the master cylinder cut-off valves 26, 28 are held in the closed state and while the pump cut-off valves 54 are held in the open state, so that the fluid pressures in all of the wheel brake cylinders 18, 24, 50, 52 are controlled in the same manner. This control will be referred to as "common pressure control" of the wheel brake cylinders. When the amount of slipping of a certain wheel 16, 22, 46, 48 being braked has become excessively large with respect to the friction coefficient of the road surface, the control device 160 initiates the anti-lock braking pressure control routine, wherein a target value of the fluid pressure in the corresponding wheel brake cylinder 18, 24, 50, 52 is determined so as to hold the amount of slipping of the excessively slipping wheel within an optimum range. The anti-lock braking pressure control routine is formulated to control the pump device 30 and the corresponding pressure reduction control valve 58, 60 and pump cut-off valve 54, while the master cylinder cut-off valves 26, 28 are held in the open state. When the wheel brake cylinder pressure in question is rapidly reduced or held at the present level, the corresponding pump cut-off valve 54 is held in the closed state. When the wheel brake cylinder pressure is slowly reduced, or increased, the corresponding pump cut-off valve 54 is held in the open state. When the amount of spinning or drift-out motion of the vehicle during turning has exceeded a predetermined threshold SVs or SVd, the control device 160 initiates the vehicle stability control routine. The amount of spinning or drift-out motion of the vehicle is obtained on the basis of the output signals of the steering angle sensor 170 and the yaw rate sensor 172. The vehicle stability control routine is formulated to first obtain the target fluid pressure in each wheel brake cylinder, so that the appropriate wheel or wheels is/are braked so as to give the turning vehicle a yaw moment which is effective to reduce the amount of spinning or drift-out motion. The pump device 30, the pressure reduction control valves 58, 62 and the pump cut-off valves 54 are controlled so as to establish the target wheel brake cylinder pressures. When the operating speed of the brake pedal 10 is higher than a predetermined threshold, the emergency braking control routine is initiated. The target wheel brake cylinder pressure is obtained depending upon the operating speed of the brake pedal 10, and the fluid pressures in all of the wheel brake cylinders 18, 24, 50, 52 are controlled to the same target value. When an operation to reduce the wheel brake cylinder pressure is terminated, the master cylinder cut-off valves 26, 28 are returned to the open state, and the coils 100, 144 of the pressure reduction control valves 58, 62 are de-energized. The fluid is returned from the wheel brake cylinders 18, 24 to the master reservoir 31 through the master cylinder cut-off valves 26, 28 and master cylinder 12, while the fluid is returned from the wheel brake cylinders 50, 52 to the master reservoir 31 through the pressure reduction control valves 62. In the event of occurrence of an electrical abnormality, the solenoid coils of the master cylinder cut-off valves 26, 28 and the coils 100, 144 of the pressure reduction control valves 58, 62 are de-energized, so that the master cylinder cut-off valves 26, 28 are returned to the open state, and the pressure reduction control valves 58 are returned to the closed state while the pressure reduction control valves 62 are returned to the open state. With the pressure reduction control valves 58 placed in the closed state, the wheel brake cylinders 18, 24 can be actuated by the pressurized fluid delivered from the master cylinder 12. Although the pump cut-off valves 54 are placed in the open state, the check valves 35, 36 prevent flows of the fluid from the master cylinder 12 (wheel brake cylinders 18, 24) into the master reservoir 31 through the pump device 30. Thus, the vehicle can be braked by the front wheel brake cylinders 18, 22 actuated by the master cylinder 12, even in the event of occurrence of an electrical abnormality in the braking system. The pump device 30 is controlled according to the pump motor control routine. In the present embodiment, the delivery pressure and rate of the low-pressure pump 34 are controlled in zone 1 indicated in Fig. 6, by controlling the low-pressure pump motor 40. With the high-pressure pump motor 38 as well as the low-pressure pump motor 40 being controlled, the delivery pressures and rates of the high-pressure and low-pressure pumps 32, 34 are controlled in zone 2. The control of the pump device 30 in. zone 3 may be achieved by applying the maximum voltage to at least one of the high-pressure and low-pressure pump motors 38, 40. In this case, the delivery pressure and rate of the pump device 30 are maximized, but are difficult to be controlled. When the common pressure control is effected according to the normal braking pressure control routine or emergency braking control routine, for example, at least one of the high-pressure and low-pressure pump motors 38, 40 is controlled according to the control data map indicated in Fig. 6, so that the delivery pressure of the pump device 30 as detected by the pump pressure sensor 74 changes toward a determined target value. When the force acting on the brake pedal 10 is relatively small, the low-pressure pump motor 40 is operated and controlled. When the force acting on the brake pedal 10 has increased to a predetermined threshold, the high-pressure pump motor 38 is also operated and controlled. When an "independent pressure control" is effected according to the anti-lock braking pressure control routine or vehicle stability control routine, the delivery pressure of the pump device 30 is controlled so as to be equal to a highest one of the target fluid pressures of t he wheel brake cylinders 18, 24, 50, 52. This control of the delivery pressure of the pump device 30 permits the fluid pressure in any wheel brake cylinder 18, 24, 50, 52 to be controlled to the target value. When the required rate of increase of the wheel brake cylinder pressure is higher than a predetermined threshold, the maximum voltage higher than the nominal voltage of the battery used in the braking system is applied to the high-pressure and low-pressure pump motors 38, 40, to maximize the delivery of each pump 32, 34, for maximizing the actual rate of increase of the wheel brake cylinder pressure. The required rate of increase of the wheel brake cylinder pressure is represented by a difference obtained by subtracting the target wheel brake cylinder pressure determined in the last control cycle from the target wheel brake cylinder pressure determined in the present control cycle. The pump device 30, the pump cut-off valves 54 and the pressure reduction control valves 58, 62 are controlled according to the brake control routine illustrated in the flow chart of Fig. 4. The brake control routine is initiated with step S1 to determine whether the fluid pressure in any one of the wheel brake cylinders 18, 24, 50, 52 is in the process of being controlled. This determination is effected by determining whether the target fluid pressure of any wheel brake cylinder is higher than the atmospheric pressure. Since the target fluid pressure of each wheel brake cylinder is determined in the above-described various control routines such as the normal braking pressure control routine, it is possible to determine that the fluid pressure in any wheel brake cylinder is being controlled, if the target fluid pressure of any wheel brake cylinder is higher than the atmospheric pressure. If a negative decision (NO) is obtained in step S1, that is, if the fluid pressure in any wheel brake cylinder is not being controlled, the control flow goes to steps S2 and S3 to determine whether an increase of the fluid pressure in the wheel brake cylinders will be required in the near future. Since the fluid pressure in the wheel brake cylinders is required to be increased if the brake pedal 10 is depressed, steps S2 and S3 are provided to determine whether the vehicle is in a condition in which the brake pedal 10 is expected to be depressed. That is, the brake pedal 10 is expected to be depressed if the accelerator pedal is placed in its non-operated or fully released position while the vehicle is running downhill. In this case, the low-pressure pump motor 40 is started to be ready for activating the wheel brake cylinders 18, 24, 50, 52 as soon as the brake pedal 10 is actually depressed. This arrangement is effective to avoid a delayed braking effect. Namely, step S2 is provided to determine whether the vehicle is running downhill, and step S3 is provided to determine whether the accelerator pedal is in the fully released position. If an affirmative decision (YES) is obtained in both of these steps S2 and S3, the control flow goes to step S4 in which the low-pressure pump motor 40 is turned on. If a negative decision (NO) is obtained in step S2 or S3, the control flow goes to step S5 to reset the braking system to the initial state, that is, reset the master cylinder cut-off valves 26, 28 to the open state, reset the pump cut-off valves 54 to the open state, de-energize the coils 100, 144 of the pressure reduction control valves 58, 62, turn' off the pump device 30, and reset the various control parameters to the initial values. This resetting operation in step S5 is similar to that implemented when the braking system is initialized. The low-pressure pump motor 40 is held in the operated state as long as the vehicle is running downhill with the accelerator pedal being released. When the road surface is determined not to be a downhill road surface, or when the accelerator pedal is depressed again, the control flow goes to step S5 in which the low-pressure pump motor 40 is turned off. The determination step S2 is implemented according to a routine illustrated in the flow chart of Fig. 7. This routine is initiated with step S21 to obtain an estimated running speed of the vehicle on the basis of the output signals of the wheel speed sensors 162-165, and calculate a rate of change of the estimated vehicle running speed. This rate of change represents an actual acceleration value α of the vehicle. Step S21 is followed by step S22 to obtain an estimated acceleration value α' of the vehicle on the basis of information representative of a vehicle drive torque F received from the power unit control device 174. The estimated acceleration value α' is calculated according to the following equation: α' = (F - F')/M In the above equation, "M" represents a weight of the vehicle, and "F'" represents a road load which increases with a vehicle running speed V, as indicated by a curve shown in Fig. 5. A predetermined relationship between the road load F' and the vehicle running speed V is represented by a data map stored in the ROM 154. The road load F' is determined on the basis of the estimated vehicle running speed V and according to the predetermined relationship. The value (F - F') represents an effective drive torque which is considered to decrease with an increase in the road load which increases with the vehicle running speed V. If the actual vehicle acceleration α is equal to the estimated value α', the vehicle is considered to be running on a flat road surface. If the actual acceleration value α is smaller than the estimated value α', the vehicle is considered to be running on an uphill road surface. If the actual acceleration value α is larger th an the estimated value α', the vehicle is considered to be running on a downhill road surface. Step S22 is followed by step S23 to determine whether the actual acceleration value α is larger than the estimated value α'. If an affirmative decision (YES) is obtained in step S23, the control flow goes to step S24 to determine that the vehicle is running downhill. When the vehicle is driven by both the engine and the electric motor, the drive torque F is obtained based on the operating states of the engine and electric motor. When the vehicle is driven by the engine or electric motor, the drive torque F is obtained based on the operating state of the engine or electric motor. The arrangement to start the low-pressure pump motor 40 while the accelerator pedal is held released during downhill running of the vehicle permits the wheel brake cylinder pressures to be rapidly increased immediately after the brake pedal 10 is operated, as indicated in the graph of Fig. 8. This arrangement is effective to reduce a delayed increase in the wheel brake cylinder pressures. When the fluid pressure in any wheel brake cylinder is being controlled, that is, if an affirmative decision (YES) is obtained in step S1, the control flow goes to step S6 to determine whether the vehicle is stationary or in a standstill state. This determination is effected by determining whether the estimated vehicle running speed is lower than a predetermined threshold. In most cases, a negative decision (NO) is obtained immediately after initiation of control of the wheel brake cylinder pressure, and the control flow goes to step S7 to read the target wheel brake cylinder pressure Pref which has been determined according to a suitable pressure control program, and to obtain a change rate ΔPref of the target wheel brake cylinder pressure Pref. Then, the control flow goes to step S8 to determine whether the obtained change rate ΔPref is higher than a predetermined increase rate threshold. In most cases, the change rate ΔPref is higher than the predetermined increase rate threshold immediately after the brake pedal 10 has been depressed. If the change rate ΔPref is higher than the threshold, that is, if an affirmative decision (YES) is obtained in step S8, the control flow goes to step S9 in which the maximum voltage (higher than the nominal voltage of the battery) is applied to the high-pressure and low-pressure pump motors 38, 40, and the pressure reduction control valves 58, 62 are closed. With the maximum voltage being applied to the pump motors 38, 40, the output torques of the motors 38, 40 are increased to increase the delivery rate of the pump device 30, making it possible to rapidly increase the wheel brake cylinder pressure, as indicated in the graph of Figs. 9A, 9B. When an emergency brake is applied, or immediate after the brake pedal 10 has been operated, the change rate ΔPref of the target wheel brake cylinder pressure Pref is higher than the predetermined increase rate threshold, so that the pressure increasing delay can be reduced. In the present embodiment, the increase rate threshold is determined to be higher than the highest value that can be achieved in the zone 2 of Fig. 6. The maximum voltage is kept applied to the pump motors 38, 40 as long as the change rate ΔPref is higher than the threshold. Since the delivery rate of the pump device 30 can be increased by applying the maximum voltage to the high-pressure and low-pressure pump motors 38, 40, the wheel brake cylinder pressure can be increased at a sufficiently high rate, without having to increase the capacities of the motors 38, 40. Although the delivery rate of the pump device 30 can be increased by increasing the capacities of the pump motors 38, 40, this results in an increased cost of manufacture of the pump device 30. In view of this, the present embodiment is adapted such that the delivery pressure and rate of the pump device 30 are maximized only when a relatively high rate of increase of the wheel brake cylinder pressure is required. While the delivery pressure and rate of the pump device 30 can be controlled when they are maximized, the rate of increase of the wheel brake cylinder pressure need not be controlled, as long as the wheel brake cylinder pressure can be increased at a sufficiently high rate. The present arrangement makes it possible to increase the wheel brake cylinder pressure at a rate higher than the predetermined increase rate threshold, without increasing the cost of manufacture of the pump device 30. While steps S7-S9 are arranged to apply the maximum voltage to the pump motors 38, 40 while the rate of change ΔPref of the target wheel brake cylinder pressure Pref is higher than the threshold, the maximum voltage may be applied to the pump motors 38, 40 until the wheel brake cylinder pressure has been increased to a predetermined level. When the change rate ΔPref of the target wheel brake cylinder pressure is lower than the predetermined increase rate threshold, a negative decision (NO) is obtained in step S8, and the control flow goes to step S10 to determine whether the fluid pressure in any wheel brake cylinder is controlled according to the anti-lock braking pressure control routine or vehicle stability control routine, in other words, whether the "independent pressure control" of the wheel brake cylinder pressures is effected. In the independent pressure control, the fluid pressures in the individual wheel brake cylinders 18, 24, 50, 52 are controlled independently of each other. If a negative decision (NO) is obtained in step S10, the "common pressure control" is effected. In the common pressure control, the fluid pressures in all of the wheel brake cylinders 18, 24, 50, 52 are controlled in the same manner. Where the common pressure control is effected, that is, if a negative decision (NO) is obtained in step S10, the control flow goes to step S11 in which the common pressure control is effected for all wheel brake cylinders, according to a common pressure control routine illustrated in the flow chart of Fig. 10. The common pressure control routine of Fig. 10 is initiated with step S31 to obtain a pressure difference ΔP between the target and actual wheel brake cylinder pressure values. Then, the control flow goes to steps S32 and S33 to select one of pressure increase, pressure hold and pressure reduction modes, depending upon the pressure difference ΔP as compared with a predetermined threshold EPS, as indicated in the graph of Fig. 11. When the pressure difference ΔP is in a range corresponding to the pressure hold mode, that is, when the absolute value of the pressure difference ΔP is equal to or smaller than the threshold EPS, namely, is in an insensitive zone indicated in Fig. 11, the control flow goes to step S34 to select the pressure hold mode in which the wheel brake cylinder pressure is held at the present level. When the pressure difference ΔP is in a range corresponding to the pressure increase mode, that is, is larger than the threshold EPS, the control flow goes to step S35 to select the pressure increase mode in which the wheel brake cylinder pressure is increased. When the pressure difference ΔP is in a range corresponding to the pressure reduction mode, that is, is not larger than the threshold EPS, the control flow goes to step S26 to select the pressure reduction mode in which the wheel brake cylinder pressure is reduced. In the common pressure control, the pump cut-off valves 54 are held in the open state. In the pressure increase mode, the delivery pressure of the pump device 30 is controlled to be the target wheel brake cylinder pressure Pref by controlling at least one of the high-pressure and low-pressure pump motors 38, 40, while the pressure reduction control valves 58, 62 are held in the closed state, as indicated in Fig. 12. In the pressure hold mode, the pump motors 38, 40 are held off, and the pressure reduction control valves 58, 62 are held in the closed state, as indicated in Fig. 12. In this pressure hold mode, the pressurized fluid is not supplied to the wheel brake cylinders 19, 24, 50, 52, so that the pump cut-off valves 54 need not be closed to prevent an increase in the wheel brake cylinder pressure. In the pressure reduction mode, the pump motors 38, 40 are held off, and the pressure reduction control valves 58, 62 are controlled so as to zero the pressure difference ΔP, as indicated in Fig. 12. In the common pressure control, the wheel brake cylinder pressure is increased in the pressure increase mode by controlling the delivery pressure and rate of the pump device 30, is held in the pressure hold mode with the pump device 30 held off and the pressure reduction control valves 58, 62 held in the closed state, and is reduced in the pressure reduction mode by controlling the pressure reduction control valves 58, 62 with the pump device 30 held off. Thus, the common pressure control is effected to control the wheel brake cylinder pressure by controlling the pump device 30 and the pressure reduction control valves 58, 62, without controlling pressure increase control valves as well as the pressure reduction control valves as required in the conventional hydraulic pressure control apparatus. As indicated above, the pump cut-off valves 54 are held in the open state in the common pressure control. In other words, the common pressure control does not require the pump cut-off valves 54. If the anti-lock braking pressure control or vehicle stability control is effected, an affirmative decision (YES) is obtained in step S10, and the control flow goes to step S12 in which the independent pressure control is effected according to a routine illustrated in the flow chart of Fig. 13. This routine is initiated with step S41 to obtain a highest value Prefmax of the target fluid pressures Pref of the wheel brake cylinders 18, 24, 50, 52. Then, step S42 is implemented to control at least one of the pump motors 38, 40 on the basis of the highest target fluid pressure Prefmax of the wheel brake cylinders 18, 24, 50, 52. In the independent pressure control, the pump device 30 is held in the operated state. Step S42 is followed by step S43 to determine whether the change rate ΔPref of the target pressure of any wheel brake cylinder is lower than a negative threshold value, that is, whether the fluid pressure in any wheel brake cylinder is required to be reduced at a rate higher than a predetermined threshold. When the fluid pressure in any wheel brake cylinder is required to be rapidly reduced, an affirmative decision (YES) is obtained in step S43, and the control flow goes to steps S44 and S45 to close the corresponding pump cut-off valve 54 and control the corresponding pressure reduction control valve 58, 62, so that the actual fluid pressure in the wheel brake cylinder in question is reduced to the target value. In the independent pressure control in which at least one of the pump motors 38, 40 is controlled depending upon the highest target fluid pressure Prefmax, the wheel brake cylinder whose fluid pressure is reduced is desirably disconnected from the pump device 30. Accordingly, the corresponding pump cut-off valve 54 is closed to permit rapid reduction of the fluid pressure in the wheel brake cylinder in question. When a negative decision (NO) is obtained in step S43, that is, when the fluid pressure in any wheel brake cylinder is not required to be rapidly reduced, the control flow goes to step S46 to obtain the pressure difference ΔP, and to steps S47 and S48 to select one of the pressure increase, hold and reduction modes, depending upon the obtained pressure difference ΔP. When the pressure difference ΔP is in a range corresponding to the pressure increase mode, the control flow goes step S49 to select the pressure increase mode in which the corresponding pump cut-off valve 54 is held in the open state, and the corresponding pressure reduction control valve 58, 62 is controlled as needed, so that the actual wheel brake cylinder is increased to the target value Pref, as indicated in Fig. 14. Since the pump device 30 delivers the pressurized fluid having the highest target pressure value Prefmax, the corresponding pressure control valve 58, 62 may be required to be controlled. However, the control valve 58, 62 may be held in the closed state. When the pressure difference ΔP is in a range corresponding to the pressure hold mode, the control flow goes step S50 to select the pressure hold mode in which the corresponding pump cut-off valve 54 and pressure reduction control valve 58, 62 are both held in the closed state, as indicated in Fig. 14, to prevent flows of the fluid into the wheel brake cylinder in question for thereby holding the wheel brake cylinder pressure with high stability. When the pressure difference ΔP is in a range corresponding to the slow pressure reduction mode, the control flow goes step S51 to select the slow pressure reduction mode in which the corresponding pump cut-off valve 54 is opened, and the corresponding pressure reduction control valve 58, 62 is controlled, as indicated in Fig. 14, so that the actual fluid pressure in the wheel brake cylinder in question is slowly reduced to the target value. As described above, the independent pressure control is effected while the delivery pressure of the pump device 30 is held at the highest target wheel brake cylinder pressure Prefmax. Accordingly, the wheel brake cylinder whose pressure is rapidly reduced is disconnected from the pump device 30, so that the wheel brake cylinder pressure in question can be reduced at a high rate, without delayed pressure reduction. Further, the wheel brake cylinder whose pressure is held is disconnected from both the pump device 30 and the master reservoir 31, so that the wheel brake cylinder pressure in question can be held constant. It is also noted that the pump cut-off valves 54 which also function as flow restrictors permit a fluid pressure difference between the different wheel brake cylinders. When the vehicle has been brought to a stop due to activation of the wheel brake cylinders 18, 24, 50, 52, an affirmative decision (YES) is obtained in step S6, and the control flow goes to step S13 in which a stationary vehicle braking control is effected according to a stationary vehicle braking control routine illustrated in the flow chart of Fig. 15. This routine is initiated with step S61 in which the threshold EPS for selecting one of the pressure increase, hold and reduction modes is changed to a larger value EPS', in order to increase the insensitive zone. Then, the control flow goes to step S62 to obtain the pressure difference ΔP, and to steps S63 and S64 to select one of the pressure increase, hold and reduction modes, depending upon the pressure difference ΔP. When the pressure difference ΔP is in a range corresponding to the pressure hold mode, the control flow goes to step S65 to select the pressure increase mode in which the wheel brake cylinder pressure is held at the present level. When the pressure difference ΔP is in a range corresponding to the pressure reduction mode, the control flow goes to step S66 in which the wheel brake cylinder pressure is reduced. When the pressure difference ΔP is in a range corresponding to the pressure increase mode, the control flow goes to step S67 to determine whether the wheel speed is lower than a threshold (which is almost zero). If the wheel speed is not lower than the threshold, the control flow goes to step S68 in which the wheel brake cylinder pressure is increased. If the wheel speed is lower than the threshold, the control flow goes to step S65 in which the wheel brake cylinder pressure is held at the present level. Where a negative decision (NO) is obtained in step S67, it means that the wheel is rotated with the brake pedal 10 being depressed, due to some reason such as collision of ano ther automotive vehicle with the back of the stationary automotive vehicle in question during stopping at a traffic signal. In this case, the braking force (wheel brake cylinder pressure) is increased in step S68. When the vehicle is stationary with the wheel speed lower than the threshold, the wheel brake cylinder pressure is held constant, in order to reduce the required electric energy consumed by the pump device 30 while the vehicle is stationary with the brake pedal 10 being depressed. It will be understood from the foregoing description of the present embodiment that the control device 160, the master cylinder pressure sensor 72, stroke sensor 71 and pump pressure sensor 74 cooperate with each other to constitute a major portion of a controller for controlling the pump device 30 and the pressure reduction control valves 58, 62 controlling the fluid pressures in the wheel brake cylinders 18, 24, 50, 52. Further, a,portion of the control device 160 assigned to implement steps S8 and S9 constitutes a motor controller for controlling the pump motors 38, 40 of the pump device 30. It will also be understood that the brake switch 166, stroke sensor 71 and a portion of the control device 160 assigned to execute the normal braking pressure control routine and the emergency braking control routine cooperate to constitute a pressure increase commanding device. Further, a portion of the control device 160 assigned to implement steps S2-S4 constitutes motor pre-starting means for starting the pump motors 38, 40 before a condition for increasing the wheel brake cylinder pressure is satisfied. The motor pre-starting means is adapted to start the pump motors 38, 40 while the vehicle is running downhill. It is also noted that a portion of the control device 160 assigned to implement steps S44 and S50 constitutes pump cut-off valve control means for controlling the pump cut-off valves 54 so as to inhibit a flow of the fluid from the pump device 30 into the wheel brake cylinder 18, 24, 50, 52. The stationary vehicle braking control routine of Fig. 15 is formulated to increase the threshold EPS of the pressure difference ΔP used for selecting one of the pressure increase, hold and reduction modes, and to select the pressure hold mode in step S65 when the wheel speed is lower than the predetermined threshold even when the pressure difference ΔP is larger than the increased threshold EPS'. However, steps S65 and S67 may be eliminated so that the pressure increase mode is selected as long as the pressure difference ΔP is larger than the increased threshold EPS'. An increase in the threshold EPS results in increasing the chance of selecting the pressure hold mode. Further, the common pressure control in step S11 of Fig. 4 may be adapted to select the pressure hold mode when the change rate ΔPref of the target wheel brake cylinder pressure Pref is lower than a predetermined lower limit. In this case, a common pressure control routine illustrated in the flow chart of Fig. 16 is executed in place of the common pressure control routine of Fig. 10, according to a second embodiment of this invention. The common pressure control routine of Fig. 16 according to the second embodiment of this invention is initiated with step S81 to determine whether the change rate ΔPref of the target wheel brake cylinder pressure Pref is within a predetermined range between lower and upper limits RPFL(t) and RPFU(t). If the change rate ΔPref is within the predetermined range, the control flow goes to step S82 to select the pressure hold mode. If the change rate ΔPref is within the prede termined range between the lower and upper limits RPFL(t) and RPFU(t), it means that a change in the target wheel brake cylinder pressure Pref is caused by only a small amount of variation in the depression force of the brake pedal 10 which is not recognized by the vehicle operator. In this case, the wheel brake cylinder pressure is not changed according to a change in the target wheel brake cylinder pressure. In the pressure hold mode in step S82, the pump device 30 is held off, and the pump cut-off valves 54 are held in the open state while the pressure reduction control valves 58, 62 are held in the open state. Then, the control flow goes to step S83 to determine whether a predetermined time Ts has passed after the reduction of the wheel brake cylinder pressure is initiated in step S82. If a negative decision (NO) is obtained in step S83, the control flow goes to step S84 in which the lower and upper limits RPFL(t) and RPFU(t) remain RPFL(1) and RPFU(1). If an affirmative decision (YES) is obtained in step S83, the control flow goes to step S85 in which the lower and upper limits RPFL(t) and RPFU(t) are changed f rom RPFL(1) and RPFU(1) to RPFL(2) and RPFU(2). The lower limit RPFL(2) is smaller than the lower limit RPFL(1), and the upper limit RPFU(2) is larger than the upper limit RPFU(1). Accordingly, the range between the lower and upper limits RPFL(t) and RPFU(t) is enlarged, and the pressure hold mode is more likely to be selected. When the change rate ΔPref is not within the predetermined range between the lower and upper limits RPFL(t) and RPFU(t), a negative decision (NO) is obtained in step S81, and the control flow goes to step S86 to reset a timer, to set "t" to "1'', and then goes to steps S88-S92 which are the same as teps S31-S36 of Fig. 10. It will be understood that a portion of the control device 160 assigned to implement steps S81-S85 constitutes pressure hold control means, while a portion of the control device 160 assigned to implement steps S83-S85 constitutes pressure holding condition relaxing means. Although the lower and upper limits RPFL(t) and RPFU(t) are reduced and increased, respectively, in step S85, only one of these lower and upper limits may be changed in step S85. In this case, too, the range within which the pressure hold mode is selected is enlarged. While the above two embodiments are adapted to select one of the pressure increase mode, pressure hold mode and pressure reduction mode depending upon the pressure difference ΔP between the target wheel brake cylinder pressure Pref and the actual wheel brake cylinder pressure, the rate of change ΔPref of the target wheel brake cylinder as well as the pressure difference ΔP is taken into account to select one of the pressure increase, hold and reduction modes. An example of this arrangement according to a third embodiment of the present invention is shown in Fig. 17. In the third embodiment of Fig. 17, the pressure hold mode rather than the pressure increase mode is selected when the change rate ΔPref is not higher than a predetermined threshold EPS even while the pressure difference ΔP is larger than the predetermined threshold EPS. Further, the pressure hold mode rather than the pressure reduction mode is selected when the change rate ΔPref is not lower than the threshold EPS even while the pressure difference ΔP is lower than a predetermined threshold -EPS- Thus, the present embodiment is comparatively likely to select the pressure hold mode, permitting reduction of the electric energy consumption. The change rate ΔPref may not be taken into account to select the pressure control mode, when the vehicle is stationary. It is possible to use different absolute values of the threshold of the change rate ΔPref for selecting the pressure increase mode and the pressure reduction mode. While the brake control routine of Fig. 4 is adapted to start the low-pressure pump motor 40 when the accelerator pedal is released during downhill running of the vehicle, the pump motor 40 may be started when the gradient of the downhill road surface has become higher than a predetermined upper limit. The gradient of the road surface may be obtained based on a difference between the actual vehicle acceleration value and the estimated vehicle acceleration value. The pump motor 40 may be started when the accelerator pedal is released irrespective of the gradient of the road surface . Alternatively, the pump motor 40 may be started when a condition which is similar to and which is satisfied prior to the condition for initiating the vehicle stability control is satisfied. For instance, the pump motor 40 may be started when the amount of spinning or drift-out motion of the vehicle has exceeded a threshold SVs' or SVd' which is smaller than the threshold SVs or SVd which is used for initiating the vehicle stability control. The pump motor 40 may be started when the selection of the pressure increase mode in the near future is expected during an operation in the pressure hold mode or pressure reduction mode according to the normal braking pressure control routine. In the pressure hold or reduction mode in the normal braking pressure control, the high-pressure and low-pressure pump motors 38, 40 are held off. If the low-pressure pump motor 40 is started upon detection of a sign indicating a near future change from the pressure hold or reduction mode to the pressure increase mode, a delay in the pressure increasing operation may be reduced. For instance, the selection of the pressure increase mode in the near future is expected when the pressure difference ΔP has exceeded a threshold EPS* which is smaller than the threshold EPS indicated above. In this case, a determination as to whether the pressure difference ΔP has exceeded the threshold EPS* is effected in step S34 of the flow chart of Fig. 10. In the brake control routine of Fig. 4, the maximum voltage higher than the nominal voltage of the battery used in the braking system is applied to the high-pressure and low-pressure pump motors 38, 40 when the increase rate ΔPref of the target wheel brake cylinder pressure has become higher than the predetermined threshold. However, the maximum voltage may be applied to only the low-pressure pump motor 40 when this motor 40 is commanded to be turned on. The maximum voltage may be held applied to the motor 40 for a predetermined time, or until the increase rate ΔPref has been lowered to a predetermined value or until the wheel brake cylinder pressure has been increased to a predetermined level. For instance, the control device 160 may be adapted to execute a motor control routine illustrated in the flow chart of Fig. 18 according to a fourth embodiment of this invention. This motor control routine is executed as an interruption routine at a predetermined time interval. The routine is initiated with step S101 to determine whether a MOTOR START flag is set at "1". If a negative decision (NO) is obtained in step S101, the control flow goes to step S102 to determine whether the low-pressure pump motor 40 has been commanded to be turned on (started). If an affirmative decision (YES) is obtained in step S102, the control flow goes to step S103 to set the MOTOR START flag to "1" and start a timer, and then to step S104 to apply the maximum voltage to the pump motor 40. Step S104 is followed by step S105 to determine whether a predetermined time has elapsed after starting of the timer, namely, after the maximum voltage has been applied to the pump motor 40. If a negative decision (NO) is obtained in step S105, the control flow returns to step S101. In this case, an affirmative decision (YES) is obtained in step s101, and the control flow goes to steps S104 and S105. Steps S101, S104 and S105 are repeatedly implemented until an affirmative decision (YES) is obtained in step S105, that is, until the maximum voltage is held applied to the pump motor 40 for the predetermined time. When the affirmative decision (YES) is obtained in step S105, the control flow goes to step S106 to reset the MOTOR START flag to "0". Then, the pump device 30 is controlled as described above by reference to Figs. 12 and 14. The pump motor 40 is commanded to be started in the common pressure control with the brake pedal 10 being depressed, when the pressure hold or reduction mode is changed to the pressure increase mode. At this time, the affirmative decision (YES) is obtained in step S102, and the maximum voltage is applied to the pump motor 40 in step S104, so that a delay in the operation of the low-pressure pump 34 is reduced. Further, an arrangement described below is also possible. That is, the maximum voltage is applied to only the low-pressure pump motor 40 when only the pump motor 40 is turned on while the high-pressure pump motor 38 is held off while the increase rate ΔPref of the target wheel brake cylinder pressure Pref is higher than the predetermined threshold. When the high-pressure pump motor 38 is also turned on, the maximum voltage is also applied to this pump motor 38. It is noted that the maximum voltage to be applied to the pump motor 38 and/or pump motor 40 need not be higher than the nominal voltage of the battery, but may be higher than the voltage normally applied to the motors 38, 40. In this case, the maximum voltage application circuit of each driver circuit 176, 178 is adapted such that a voltage higher than the voltage generated by the normal voltage control circuit and lower than the nominal battery voltage is applied to the pump motor 38 and/or the pump motor 40. While the brake control routine of Fig. 4 includes steps S2-S4 for starting the pump motor 40 before the selection of the pressure increase mode and step S9 for applying the maximum voltage to the pump motors 38, 40, steps S2-S4 or step S9 may be eliminated. That is, either one of the motor pre-starting control (S2-S4) and the motor voltage increasing control (S9) may be provided, for the purpose of reducing a delay in the pressure increasing operation. Although the brake control routine of Fig. 4 includes the motor pre-starting control (S2-S4), the motor voltage increasing control (S9), the stationary vehicle braking control (S13) and the common pressure control (S11), at least one of these controls may be provided. In the independent pressure control of Fig. 13, the pump device 30 is controlled depending upon the highest value Prefmax of the target fluid pressures Pref of the four wheel brake cylinders 18, 24, 50, 52, the pump device 30 may be held off while the pressure hold or reduction mode is selected for all the wheel brake cylinders 18, 24, 50, 52. For instance, the independent pressure control routine may be formulated as illustrated in the flow chart of Fig. 19 according to a fifth embodiment of this invention. The routine is initiated with step S121 to obtain the pressure difference ΔP and select one of the pressure increase, hold and reduction modes for each of the wheel brake cylinders depending upon the pressure difference ΔP as compared with the threshold. Then, the control flow goes to step S122 to determine whether the pressure hold or reduction mode is selected for all the wheel brake cylinders. If the pressure control mode is selected for any of the wheel brake cylinders, a negative decision (NO) is obtained in step S122, and the control flow goes to step S123 in which the pump device 30 is controlled based on the highest value Prefmax of the target fluid pressures Pref of the four wheel brake cylinders, and the pressure reduction valves 58, 62 are controlled. If the pressure hold or reduction mode is selected for all of the four wheel brake cylinders, an affirmative decision (YES) is obtained in step S122, and the control flow goes to step S124 in which the pump motor 30 is turned off, and the pump cut-off valves 54 and the pressure reduction control valves 58, 62 are controlled to control the wheel brake cylinder pressures. As described above, the pump cut-off valves 54 and the pressure reduction control valves 58, 62 are held in the closed state, when the pressure hold mode is selected, and the pressure reduction control valves 58, 62 are controlled with the pump cut-off valves 54 held in the open state, when the pressure reduction mode is selected. In the fifth embodiment of Fig. 19 wherein the pump device 30 is held off when the pressure hold or reduction mode is selected for all the wheel brake cylinders, the electric energy consumption can be reduced. Other changes or modifications regarding the control of the braking system are possible. For instance, different threshold values may be used for selecting the pressure increase and reduction modes. In the above embodiments, the pump device 30 includes both of the high-pressure pump 32 and the low-pressure pump 34. However, the pump device may use a single pump. Further, the control device 160 may be arranged to be capable of effecting a traction control for positively applying brakes to drive wheels of the vehicle in order to prevent slipping of the vehicle during starting and acceleration of the vehicle. The master cylinder 12 need not be of tandem type, provided it has two pressurizing chambers.

Claims: A hydraulic pressure control apparatus for a vehicle, for controlling a fluid pressure of a working fluid in a hydraulically operated brake cylinder, said apparatus comprising: a pump device (30) connected to said brake cylinder (18, 24, 50, 52) and including at least one set of a pump (32, 34) for pressurizing the fluid and delivering the pressurized fluid to said brake cylinder and an electric motor (38, 40) for operating said pump; a low-pressure reservoir (31) for storing the fluid discharged from said brake cylinder; a solenoid-operated pressure control valve (58, 62) disposed between said low-pressure reservoir and said brake cylinder and operated with an electric energy applied thereto, for controlling a pressure of the fluid in said brake cylinder depending upon an amount of said electric energy; and a controller (160, 71, 72, 74-78, 166, 170, 172) including pressure increase control means (S35, S42, S49, S68, S91) for increasing the pressure of the fluid in said brake cylinder by controlling said electric motor, and pressure reduction control means (S36, S44, S45, S66, S92) for reducing the pressure of the fluid in said brake cylinder by controlling the amount of said electric energy applied to said solenoid-operated pressure control valve, wherein a plurality of brake cylinders (18, 24, 50, 52) are connected through respective branch passages (44) to said pump device (30) such that said plurality of brake cylinders are parallel with each other, and said solenoid-operated pressure control valve (58, 62) is provided in each of a plurality of pressure reducing passages (56, 60) respectively connecting said branch passages to said low-pressure reservoir (31); and wherein said apparatus further comprises a flow restrictor device (54) provided in a portion of each of said branch passages (44) which is located between said pump device (30) and a point of connection (55, 59) thereof to a corresponding one of said pressure reducing passages (56, 60), characterized in that said controller includes a motor control device (S42) which controls said electric motor (38, 40) on the basis of a highest one (Prefmax) of target values (Pref) of the fluid pressures in said plurality of brake cylinders, to which the actual fluid pressure in said brake cylinders are controlled independently of each other, and in that said pressure reduction control means (S36, S44, S45, S66, S92) controls the amount of said electric energy applied to said solenoid-operated pressure control valves, according to a wish of a driver of the vehicle, to reduce the pressure of the fluid in said brake cylinders. A hydraulic pressure control apparatus according to claim 1, wherein said pressure reduction control means includes means (S36, S92, S124) for turning off said electric motor (38, 40) when the pressure of the fluid in said brake cylinder is reduced. A hydraulic pressure control apparatus according to claim 1 or 2, wherein said controller activates one of said pressure increase control means and said pressure reduction control means, depending upon at least one of a pressure difference (ΔP) between an actual value and a target value (Pref) of the fluid pressure in said brake cylinder and a rate of change (ΔPref) of said target value. A hydraulic pressure control apparatus according to any one of claims 1-3, wherein said controller includes a motor control device (160, S9) for controlling said electric motor (38, 40) to produce a larger output torque when a rate of increase (ΔPref) of a target value (Pref) of the fluid pressure in said brake cylinder is higher than a predetermined threshold, than when said rate of increase is not higher than said predetermined threshold. A hydraulic pressure control apparatus according to claim 4, further comprising a battery for supplying power to said electric motor (38, 40), and wherein said motor control device includes a voltage increasing device for applying to said electric motor a voltage higher than a nominal voltage of said battery when said rate of increase of said target value of the fluid pressure in said brake cylinder is higher than said predetermined threshold. A hydraulic pressure control apparatus according to any one of claims 1-3, wherein said controller includes a motor start control device (160, S101-S106) for applying a higher voltage to said electric motor (38, 40) when said electric motor is turned on, and holding said higher voltage applied to said electric motor while a predetermined condition is satisfied, said higher voltage being higher than a voltage to be applied to said electric motor after said predetermined condition is satisfied. A hydraulic pressure control apparatus according to any one of claims 1-6, wherein said controller includes pressure hold control means (S81, S82) for turning off said electric motor (38, 40) and placing said solenoid-operated pressure control valve (58, 62) in a state for inhibiting a discharge flow of the fluid from said brake cylinder, when a rate of change (ΔPref) of a target value (Pref) of the fluid pressure in said brake cylinder is held within a predetermined range. A hydraulic pressure control apparatus according to claim 7, wherein said controller includes pressure holding condition relaxing means (S83-S85) for changing said predetermined range such that the predetermined range is wider when said pressure hold control means is in operation for a relatively long time than when said pressure hold control means is in operation for a relatively short time. A hydraulic pressure control apparatus.according to any one of claims 1-8, wherein said controller includes pressure hold control means (S81, S82) for turning off said electric motor (38, 40) and placing said solenoid-operated pressure control valve (58, 62) in a state for inhibiting a discharge flow of the fluid from said brake cylinder, when a predetermined condition is satisfied, and pressure holding condition relaxing means (S83-S85) for changing said predetermined condition such that the predetermined condition is more relaxed when said pressure hold control means is in operation for a relatively long time than when said pressure hold control means is in operation for a relatively short time. A hydraulic pressure control apparatus according to any one of claims 1-9, wherein said brake cylinder is activated to brake a wheel (16, 22, 46, 48) of an automotive vehicle, and said controller includes pressure hold control means (S63, S65) for turning off said electric motor (38, 40) and placing said solenoid-operated pressure control valve (58, 62) in a state for inhibiting a discharge flow of the fluid from said brake cylinder, when a predetermined condition is satisfied, and pressure holding condition relaxing means (S61) for changing said predetermined condition such that the predetermined condition is more relaxed when said automotive vehicle is stationary than when said automotive vehicle is running. A hydraulic pressure control apparatus according to any one of claims 1-10, wherein said brake cylinder is activated to brake a wheel (16, 22, 46, 48) of an automotive vehicle, and said controller includes: pressure increasing means (S64, S68) for increasing the f luid pressure in said brake cylinder by controlling said electric motor (38, 40) of said pump device (30) while a predetermined pressure increasing condition is satisfied; compulsory pressure holding means (S67, S65) for holding the f luid pressure in said brake cylinder by holding said electric motor off and placing said solenoid-operated pressure control valve (58, 62) in a state for inhibiting a discharge flow of the fluid from the brake cylinder, after a rotating speed of said wheel has been once substantially zeroed, even while said predetermined pressure increasing condition is satisfied; and disabling means (S64, S67, S68) for disabling said compulsory pressure holding means to operate, when the rotating speed of the wheel has exceeded a predetermined threshold and when an amount of operation of a brake operating member (10) has been increased while said compulsory pressure holding means is operating. A hydraulic pressure control apparatus according to any one of claims 1-11, wherein said controller includes pressure increase commanding means (160, 71, 166) for commanding an increase in the fluid pressure in said brake cylinder when a predetermined pressure increase condition is satisfied, and motor pre-starting means (160, S2-S4) for turning on said electric motor (40) when said predetermined pressure increase condition is expected to be satisfied in the near future. A hydraulic pressure control apparatus according to claim 12, wherein said controller includes motor stopping means for turning off said electric motor when a predetermined time has passed, without satisfaction of said predetermined pressure increase condition, after said electric motor is turned on by said motor pre-starting means. A hydraulic pressure control apparatus according to claim 12 or 13, wherein said brake cylinder is activated to brake a wheel (16, 22, 46, 48) of an automotive vehicle, said apparatus further comprising a downhill running detector (S21-S24) for detecting that the automotive vehicle is running downhill, if an actual acceleration value of the vehicle is higher than an estimated acceleration value of the vehicle which is obtained on the basis of a vehicle drive torque produced by a drive power source for driving the vehicle, and wherein said motor pre-starting means including means (S2-S4) for turning on said electric motor when at least one sign indicating a high possibility that said predetermined pressure increase condition will be satisfied in the near future is detected, said at least one sign including detection by said downhill running detector that the automotive vehicle is running downhill. A hydraulic pressure control apparatus according to claim 14, wherein said at least one sign further including at leas t one of (a) detection of releasing of an accelerator member provided for accelerating the automotive vehicle, and (b) detection that a gradient of a downhill road surface on which the vehicle is running is higher than a predetermined threshold. A hydraulic pressure control apparatus according to any one of claims 1-15, wherein a plurality of brake cylinders (18, 24, 50, 52) are connected through respective branch passages (44) to said pump device (30) such that said plurality of brake cylinders are parallel with each other, and said solenoid-operated pressure control valve (58, 62) is provided in each of a plurality of pressure reducing passages (56, 60) respectively connecting said branch passages to said low-pressure reservoir (31), said apparatus further comprising a pump cut-off valve (54) provided in a portion of each of said branch passages which is located between said pump device and a point of connection thereof to a corresponding one of said pressure reducing passages, said pump cut-off valve having a flow permitting state for permitting a flow of the pressurized fluid delivered from said pump (32, 34) into a corresponding one of said brake cylinders, and a flow inhibiting s tate for inhibiting the flow of the pressurized fluid into said corresponding one of said brake cylinders, said controller including pump cut-off valve control means (S44) for placing said pump cut-off valve in said flow inhibiting state when a rate of reduction (ΔPref) of a target value (Pref) of the fluid pressure in each of at least one of said plurality of brake cylinders is higher than a predetermined pressure reduction threshold while a rate of change (ΔPref) of said target value of the fluid pressure in each of the other of said brake cylinders is on the side of increasing the fluid pressure as compared with said predetermined pressure reduction threshold. A hydraulic pressure control apparatus according to any one of claims 1-16, wherein said pump device (30) includes a low-pressure pump (34) and a high-pressure pump (32) having a higher delivery pressure and a lower delivery flow rate than said low-pressure pump.

Inventors: NAKAMURA KIYOHARU, OHKUBO MASAYASU, SAKAI AKIRA, SHIMURA AMANE, SOEJIMA SHINICHI, NAKAMURA, KIYOHARU, OHKUBO, MASAYASU,, SAKAI, AKIRA,, SHIMURA, AMANE,, SOEJIMA, SHINICHI,, Ohkubo, Masayasu, c/o Toyota Jidosha K. K., Sakai, Akira, c/o Toyota Jidosha K. K., Shimura, Amane, c/o Toyota Jidosha K. K., Soejima, Shinichi, c/o Toyota Jidosha K. K.
Applicants: TOYOTA MOTOR CO LTD, TOYOTA JIDOSHA KABUSHIKI KAISHA
Application year: 1999
Main classification: B60T8
IPC codes: B60T8, B60T7