Brake control system for a vehicle

The present invention is directed to a brake control system for a vehicle, wherein a first valve device for opening or closing a main passage, which communicates a master cylinder with wheel brake cylinders, a hydraulic pressure pump for supplying the hydraulic pressure to the main passage, and a second valve device for opening or closing an auxiliary passage, which communicates the inlet of the pressure pump with the master cylinder, are disposed in each hydraulic pressure circuit of a dual hydraulic pressure circuit system. In the case where the slip rate of at least one of the front wheels is smaller than the slip rate of at least one of the wheels belonging to a different hydraulic pressure circuit from the hydraulic pressure circuit to which the one of the front wheels belongs, when the brake pedal is being operated, the hydraulic braking pressure discharged from the pressure pump is applied to one of the wheel brake cylinders operatively mounted on at least one of the front wheels, by controlling the second valve device for example.

This application claims priority under 35 U.S.C. Sec. 119 to No.9-342069
 filed in Japan on Nov. 26, 1997, the entire content of which is herein
 incorporated by reference.
 BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a brake control system for a vehicle with
 a dual hydraulic pressure circuit system, wherein the braking force is
 applied appropriately to wheels in each hydraulic pressure circuit, when a
 brake pedal is depressed.
 2. Description of the Related Arts
 Recently, instead of a proportioning valve, it is proposed to employ
 electromagnetic valves for a braking force distribution control. For
 example, a Japanese Patent Laid-open Publication No.6-144179 discloses
 that a rear standard wheel speed is adjusted to be equal to or greater
 than a front standard wheel speed by actuating pressure control valves
 (inlet valve and outlet valve) connected to rear wheel brake cylinders to
 control the braking force applied to the rear wheels, so that a front and
 rear braking force distribution is approximated to an ideal front and rear
 braking force distribution. According to the publication, the braking
 force applied to the rear wheels is controlled such that the rear standard
 wheel speed is adjusted to be equal to or greater than the front standard
 wheel speed. In other words, the rear standard wheel speed is controlled
 not to be smaller than the front standard wheel speed. Therefore, if the
 braking force applied to the front wheels were reduced due to fade or the
 like in the front braking system, a decreasing rate of the front wheel
 speed would be reduced, so that the braking force to the rear wheels might
 be reduced excessively. As a result, the vehicle speed may not be
 decreased appropriately.
 SUMMARY OF THE INVENTION
 Accordingly, it is an object of the present invention to provide a brake
 control system for a vehicle with a dual hydraulic pressure circuit
 system, wherein the wheel speed is reduced appropriately even in the case
 where a slip rate of at least one of the front wheels of the vehicle is
 smaller than the slip rate of a wheel belonging to a different hydraulic
 pressure circuit from the hydraulic pressure circuit to which the one of
 the front wheels belongs.
 In accomplish the above and other objects, a brake control system for a
 vehicle includes wheel brake cylinders operatively mounted on front and
 rear wheels of the vehicle, respectively, a master cylinder for
 pressurizing brake fluid to supply braking pressure to the wheel brake
 cylinders in response to depression of a brake pedal, a pair of main
 passages for communicating the master cylinder with the wheel brake
 cylinders to provide a dual hydraulic pressure circuit system. An
 auxiliary pressure source is provided for pressurizing the brake fluid to
 supply the hydraulic braking pressure to the main passages. Wheel speed
 sensors are provided for detecting wheel speeds of the wheels, and a slip
 rate calculation device is provided for calculating slip rates of the
 wheels on the basis of the wheel speeds detected by the wheel speed
 sensors. And, a controller is adapted to apply the hydraulic braking
 pressure discharged from the auxiliary pressure source to one of the wheel
 brake cylinders operatively mounted on at least one of the front wheels,
 in the case where the slip rate of the at least one of the front wheels is
 smaller than the slip rate of at least one of the wheels belonging to a
 different hydraulic pressure circuit from the hydraulic pressure circuit
 to which the at least one of the front wheels belongs, at least when the
 brake pedal is being operated.
 Therefore, in the case where a difference is caused between the slip rates
 of the front wheels in a diagonal hydraulic pressure circuit system for
 example, the hydraulic braking pressure discharged from a master cylinder
 in response to depression of a brake pedal is supplied to the wheel brake
 cylinder operatively mounted on one of the front wheels having the larger
 slip rate, whereas the hydraulic braking pressure discharged from the
 auxiliary pressure source is added to the master cylinder pressure, with
 respect to the front wheel having the smaller slip rate.
 Preferably, the controller is adapted to equalize the hydraulic braking
 pressure in the wheel brake cylinders operatively mounted on the rear
 wheels, on the basis of one of the rear wheels having a larger slip rate
 than the other one of the rear wheels.
 In order to equalize the hydraulic braking pressure in the wheel brake
 cylinders operatively mounted on the rear wheels, on the basis of one of
 the rear wheels having a larger slip rate than the other wheel, the brake
 control system may further include a modulator which is disposed between
 the master cylinder and the wheel brake cylinders in each of the hydraulic
 pressure circuits for modulating the hydraulic braking pressure in each of
 the wheel brake cylinders in accordance with a pressure mode selected from
 at least a pressure increase mode and a pressure decrease mode.

DESCRIPTION OF THE PREFERRED EMBODIMENT
 Referring to FIG. 1, there is schematically illustrated a brake control
 system according to the present invention, wherein wheel brake cylinders
 Wfl, Wfr, Wrl, Wrr are operatively mounted on the front wheels FL, FR and
 rear wheels RL, RR of the vehicle, respectively. The wheel FL designates
 the wheel at the front left side as viewed from the position of a driver's
 seat, the wheel FR designates the wheel at the front right side, the wheel
 RL designates the wheel at the rear left side, and the wheel RR designates
 the wheel at the rear right side. According to the present embodiment, the
 hydraulic pressure circuit is divided into two circuits to provide a
 diagonal circuit (X-circuit) system, or front-rear dual circuit system.
 Wheel speed sensors WS1 to WS4 are provided for the wheels FR, RL, FL and
 RR, respectively, and connected to the electronic controller ECU, and by
 which a signal having pulses proportional to a rotational speed of each
 wheel, i.e., a wheel speed signal is fed to the electronic controller ECU.
 There are also provided a brake switch BS which turns on when the brake
 pedal BP is depressed, and turns off when the brake pedal BP is released.
 Also, the pressure sensor PS is connected to the electronic controller
 ECU, so that the signal indicative of the master cylinder pressure is
 input to the electronic controller ECU. The electronic controller ECU is
 provided with a microcomputer (not shown) which includes a central
 processing unit (CPU), memories (ROM, RAM), an input port, an output port,
 and etc. The signals detected by each of the wheel speed sensors WS1 to
 WS4, brake switch BS and etc. are fed to the input port via respective
 amplification circuits (not shown) and then to the central processing
 unit. Then, control signals are fed from the output port to the solenoid
 valves as described later via the respective driving circuits (not shown).
 In the electronic controller ECU, the memory (ROM) memorizes a program
 corresponding to flowcharts shown in FIGS. 2-4, the central processing
 unit (CPU) executes the program while the ignition switch (not shown) is
 closed, and the memory (RAM) temporarily memorizes variable data needed to
 execute the program.
 According to the present embodiment, a master cylinder MC is activated
 through a vacuum booster VB in response to depression of a brake pedal BP
 to pressurize the brake fluid in a low-pressure reservoir LRS and
 discharge the master cylinder pressure to the hydraulic pressure circuits
 for the wheels FR, RL and the wheels FL, RR, respectively. The master
 cylinder MC is of a tandem type having two pressure chambers communicated
 with two hydraulic pressure circuits, respectively. That is, a first
 pressure chamber MCa is communicated with the hydraulic pressure circuit
 for the wheels FR, RL, and a second pressure chamber MCb is communicated
 with the hydraulic pressure circuit for the wheels FL, RR.
 In the hydraulic pressure circuit for the wheels FR, RL, the first pressure
 chamber MCa is communicated with wheel brake cylinders Wfr, Wrl
 respectively, through a main passage MF and its branch passages MFr, MFl.
 A normally open first solenoid valve SC1 (hereinafter, simply referred to
 as a solenoid valve SC1) is disposed in the main passage MF, to act as a
 so-called cut-off valve. Also, the first pressure chamber MCa is
 communicated through an auxiliary passage MFc with check valves CV5, CV6,
 which will be described later. A normally closed second solenoid valve SI1
 (hereinafter, simply referred to a solenoid valve SIl) is disposed in the
 auxiliary passage MFc. Each of the solenoid valves SC1, SI1 is formed by a
 two-port two-position solenoid operated valve. In the main passage MF, a
 pressure sensor is connected to detect the master cylinder pressure, which
 is output to an electronic controller ECU, as a signal varied in response
 to depression of the brake pedal BP. As for the sensor for detecting the
 operating condition of the brake pedal BP, a stroke sensor for detecting
 the stroke of the brake pedal BP may be employed.
 And, normally open two-port two-position solenoid valves PCd, PC2
 (hereinafter, simply referred to as solenoid valves PC1, PC2) are disposed
 in the branch passages MFr, MFl, respectively, and in parallel therewith
 check valves CV1, CV2, respectively. The check valve CV1, CV2 are provided
 for allowing the flow of the brake fluid toward the master cylinder MC and
 preventing the flow toward the wheel brake cylinders Wfr, Wrl. The brake
 fluid in the wheel brake cylinders Wfr, Wrl is returned to the master
 cylinder MC, and then to the low-pressure reservoir LRS through the check
 valves CV1, Cv2 and the solenoid valve SC1 placed in its open position as
 shown in FIG. 1. Accordingly, if the brake pedal BP is released, the
 braking pressure in the wheel brake cylinders Wfr, Wrl is rapidly reduced
 to the pressure lower than the pressure at the master cylinder MC. And,
 normally closed two-port two-position solenoid valves PC5, PC6
 (hereinafter, asimply referred to as solenoid valves PC5, PC6) are
 disposed in the branch passages RFr, RF1, respectively, which merge into
 the drain passage RF connected to the reservoir RS1.
 In the hydraulic pressure circuit for the wheels FR, RL, the solenoid
 valves PC1, PC2, and solenoid valves PC5, PC6 form the modulators of the
 present invention, respectively. A hydraulic pressure pump HP1 is disposed
 in a passage MFp connected to the branch passages MFr, MFl at the upstream
 of the solenoid valves PC1, PC2, and an outlet of the pressure pump HP1 is
 connected to the solenoid valves PC1, PC2 through a check valve CV7. The
 pressure pump HP1 and a pressure pump HP2 in the other hydraulic pressure
 circuit are driven by a single electric motor M to introduce the brake
 fluid from the inlets, pressurize the brake fluid to a predetermined
 pressure, and discharge it from the outlets. The reservoir RS1 is disposed
 independently from the low-pressure reservoir LRS of the master cylinder
 MC, and provided with a piston and spring to function as an accumulator
 for storing a necessary volume of the brake fluid for various controls.
 The master cylinder MC is connected to a position between the check valves
 CV5 and CV6 disposed at the inlet side of the pressure pump HP1 through
 the passage MFc. The check valve CV5 is provided for preventing the flow
 of the brake fluid toward the reservoir RS1 and allowing the reverse flow.
 The check valves Cv6, CV7 are provided for restricting the flow of the
 brake fluid discharged from the pressure pump HP1 to a predetermined
 direction, and normally formed within the pressure pump HP1 in a body.
 Accordingly, the solenoid valve SI1 is normally placed in the closed
 position as shown in FIG. 1, where the communication between the master
 cylinder MC and the inlet of the pressure pump HP1 is blocked, and
 switched to the open position where the master cylinder MC is communicated
 with the inlet of the pressure pump HP1.
 In parallel with the solenoid valve SC1, is disposed a relief valve RV1
 which prevents the brake fluid in the master cylinder MC from flowing
 toward the solenoid valves PC1, PC2, and allows the brake fluid to flow
 toward the master cylinder MC when the braking pressure at the solenoid
 valves PC1, PC2 is more than the braking pressure at the master cylinder
 MC by a predetermined pressure difference, and a check valve AV1 which
 allows the flow of the brake fluid toward the wheel brake cylinders Wfr,
 Wrl, and prevents its reverse flow. The relief valve RV1 is provided for
 returning the brake fluid to the low-pressure reservoir LRS through the
 master cylinder MC when the pressurized braking pressure discharged from
 the pressure pump HP1 is more than the braking pressure discharged from
 the master cylinder MC by the predetermined pressure difference, thereby
 to modulate the braking pressure in the main passage MF not to exceed a
 predetermined pressure. Because of the check valve AV1, even if the
 solenoid valve SC1 is in its closed position, when the brake pedal BP is
 depressed, the hydraulic braking pressure in the wheel brake cylinders
 Wfr, Wrl is increased. A damper DP1 is disposed at the outlet side of the
 pressure pump HP1, and a proportioning valve PVl is disposed in a passage
 connected to the rear wheel brake cylinder Wrl.
 In the hydraulic pressure circuit for the wheels FL, RR, are disposed a
 reservoir RS2, damper DP2, proportioning valve PV2, normally open two-port
 two-position solenoid valve SC2 (first solenoid valve), normally closed
 two-port two-position solenoid valves SI2 (second solenoid valve), PC7,
 PC8, normally open two-port two-position solenoid valves PC3, PC4, check
 valves CV3, CV4, CV8-CV10, relief valve RV2, and check valve AV2. The
 pressure pump HP2 is driven by the electric motor M together with the
 pressure pump HP1, both of the pumps HP1 and HP2 will be driven
 continuously after the motor M starts to operate them. In the following
 flowcharts, the valves or the like for use in the two hydraulic pressure
 circuits are represented by adding "*" to each reference. The solenoid
 valves SC1, SC2, SI1, SI2 and PC1-PC8 are controlled by the electronic
 controller ECU to perform the control modes such as the anti-skid control
 mode.
 In operation, every valves are placed in their normal positions and the
 motor M is stopped as shown in FIG. 1, during the normal braking
 operation. When the brake pedal BP is depressed in the conditions as shown
 in FIG. 1, the master cylinder MC is actuated to discharge the master
 cylinder pressure from the first and second pressure chambers MCa, MCb to
 the hydraulic pressure circuit for the wheels FR, RL, and the hydraulic
 pressure circuit for the wheels FL, RR, respectively, and supply the
 pressure into the wheel brake cylinders Wfr, Wrl, Wfl, Wrr, through the
 solenoid valves SC1, SC2 and the solenoid valves PC1-PC8. Since the
 hydraulic pressure circuits for the wheels FR, RL and wheels FL, RR are
 substantially the same, only the hydraulic pressure circuit for the wheels
 FR, RL will be explained hereinafter.
 During the braking operation, when the wheel FR tends to be locked for
 example, and the anti-skid control is initiated, the solenoid valve PCd is
 changed to its closed position, and the solenoid valve PC5 is placed in
 its open position, while the solenoid valve SC1 is placed in its open
 position. As a result, the brake fluid in the wheel brake cylinder Wfr is
 drained into the reservoir RS1 through the solenoid valve PC5 to reduce
 the pressure in the wheel brake cylinder Wfr. When a pulse pressure
 increase mode is selected for the wheel brake cylinder Wfr, the solenoid
 valve PC5 is placed in its closed position and the solenoid valve PC1 is
 placed in its open position, so that the master cylinder pressure is
 supplied from the master cylinder MC to the wheel brake cylinder Wfr
 through the solenoid valve Pcd in its open position. Then, the solenoid
 valve PC1 is opened and closed alternately, so that the pressure in the
 wheel brake cylinder Wfr is increased and held repeatedly like pulses
 thereby to be increased gradually. When a rapid pressure increase mode is
 selected for the wheel brake cylinder Wfr, the solenoid valve PC5 is
 placed in the closed position, and then the solenoid valve PC1 is placed
 in its open position, so that the master cylinder pressure is supplied
 from the master cylinder MC to the wheel brake cylinder Wfr. When the
 brake pedal BP is released and the master cylinder pressure comes to be
 lower than the pressure in the wheel brake cylinder Wfr, the brake fluid
 in the wheel brake cylinder Wfr is returned to the master cylinder MC
 through the check valve CV1 and the solenoid valve SC1 in its open
 position, and consequently to the low pressure reservoir LRS. Thus, an
 independent braking force control is performed with respect to each wheel.
 According to the present embodiment as constituted above, a program routine
 for various controls including the auxiliary brake control, anti-skid
 control and so on is executed by the electronic controller ECU, as will be
 described hereinafter with reference to FIGS. 2-5. The program routine
 starts when an ignition switch (not shown) is turned on. At the outset,
 the program provides for initialization of the system at Step 101 to clear
 various data. At Step 102, the signals detected by the wheel speed sensors
 WS1 to WS4 are read by the electronic controller ECU. Then, the program
 proceeds to Step 103 where the wheel speed Vw** (** represents one of the
 wheels FL, FR, RL, RR) of each wheel is calculated, and differentiated to
 provide the wheel acceleration DVw**. At Step 104, the maximum of the
 wheel speeds Vw** for four wheels is calculated to provide an estimated
 vehicle speed Vso on a gravity center of the vehicle (Vso=MAX[Vw**]), and
 an estimated vehicle speed Vso** is calculated for each wheel,
 respectively, on the basis of the wheel speed Vw**. The estimated vehicle
 speed Vso** may be normalized to reduce the error caused by a difference
 between the wheels located on the inside and outside of the curve while
 cornering. Furthermore, the estimated vehicle speed Vso is differentiated
 to provide an estimated vehicle deceleration DVso on the gravity center of
 the vehicle. In this respect, the estimated vehicle deceleration is used
 for the convenience of explanation. When its sign is opposite, it
 indicates an estimated vehicle acceleration. At Step 105, also calculated
 is an actual slip rate Sa** for each wheel, on the basis of the wheel
 speed Vw** and the estimated vehicle speed Vso** (or, the estimated and
 normalized vehicle speed) which are calculated at Steps 103 and 104,
 respectively, in accordance with the following equation:
EQU Sa** (Vso**-Vw**)/Vso**
 Then, at Step 106, a coefficient of friction .mu. against a road surface is
 calculated on the basis of the vehicle deceleration DVso. In order to
 detect the coefficient of friction, various devices may be employed, such
 as a sensor for directly detecting the coefficient of friction against the
 road surface, for example. And, the program proceeds to Step 107, where it
 is determined whether the starting conditions for the anti-skid control
 mode have been fulfilled or not. If it has been fulfilled, the program
 proceeds to Step 108 where the anti-skid control mode is set, and a
 desired slip rate therefor is set. Otherwise, the program returns to Step
 109 where the auxiliary brake control is performed, as will be described
 later. And, after the abnormality is checked at Step 110, the braking
 force to each wheel is controlled according to the hydraulic pressure
 servo control, the program returns to Step 102.
 FIG. 3 shows the auxiliary brake control executed at Step 109 in FIG. 2. At
 Step 201, a slip rate of the smaller value between the slip rates SaFR,
 SaFL (i.e., MIN(SaFR, SaFL)) is calculated. Next, at Step 202, a slip rate
 of the larger value between the slip rates SaFR, SaFL (i.e., MAX(SaFR,
 SaFL)) is calculated. Then, a difference between the slip rate (MAX(SaFR,
 SaFL)) and slip rate (MIN(SaFR, SaFL)) is compared with a predetermined
 value Ks. If it is determined that the difference is equal to or smaller
 than the predetermined value Ks, the program returns to the main routine.
 If it is determined that the difference is larger than the predetermined
 value Ks, the program proceeds to Steps 204-206 where the wheel cylinder
 pressure for each wheel will be controlled. At Step 204, it is determined
 which is larger between the slip rate SaFR of front wheel FR, and the slip
 rate SaFL of front wheel FL. If the slip rate SaFR is smaller than the
 slip rate SaFL, the program proceeds to Step 205 where a pressurizing mode
 is set to the wheel brake cylinder operatively mounted on the wheel FR of
 the smaller slip rate, so that the wheel brake cylinder of the wheel FR
 will be pressurized by the pressure pump HP1 until the wheel cylinder
 pressure will exceed the master cylinder pressure, while the wheel brake
 cylinder of the wheel FL will be pressurized to the master cylinder
 pressure as shown in FIG. 1. Whereas, if the wheel brake cylinder
 operatively mounted on the wheel FL is the smaller one, the program
 proceeds to Step 206 where the pressurizing mode is set to the wheel brake
 cylinder operatively mounted on the wheel FL of the smaller slip rate, so
 that the wheel brake cylinder of the wheel FL will be pressurized until
 the wheel cylinder pressure will exceed the master cylinder pressure,
 while the wheel brake cylinder of the wheel FR will be pressurized to the
 master cylinder pressure.
 Then, at Step 207, a deceleration difference .DELTA.G between a desired
 vehicle deceleration G* and an estimated vehicle deceleration DVso which
 represents the actual vehicle deceleration is calculated
 (.DELTA.G=G*-DVso). The desired vehicle deceleration G* is calculated by
 adding a deceleration .DELTA.g, which is provided in accordance with a
 predetermined hydraulic braking pressure for the auxiliary brake control,
 to the actual vehicle deceleration Gm, which is obtained on the basis of
 the pressure detected by the pressure sensor PS, for example, or the
 stroke of the brake pedal. The program further proceeds to Step 208, where
 the amount of the auxiliary brake control is calculated. For example, a
 duty Di and a duty Dc are set for the solenoid valves SI*, SC*, which are
 disposed in the hydraulic pressure circuit (left side or right side in
 FIG. 1) including the wheel with the pressurizing mode set thereto, as
 shown in FIG. 6. That is, the amount of the auxiliary brake control is set
 by controlling at least the solenoid valve SI*, without controlling the
 solenoid valves PC1, PC5, or valves PC3, PC7 (with these solenoid valves
 held in the conditions as shown in FIG. 1).
 The program proceeds to Step 209, where the slip rates SaRR, SaRL of the
 rear wheels RR, RL are compared in gratitude. If the slip rate SaRL is
 larger than the slip rate SaRR, the program proceeds to Step 210, where
 the slip rates SaR* of the wheels RR, RL under control are set to be the
 larger slip rate SaRL. If the slip rate SaRR is larger than the slip rate
 SaRL, the program proceeds to Step 211, where the slip rates SaR* of the
 wheels RR, RL under control are set to be the larger slip rate SaRR. Thus,
 the wheel brake cylinders of the rear wheels RR, RL are controlled on the
 basis of the wheel brake cylinder in which the hydraulic braking pressure
 is to be decreased, so as to comply with the requirements of the vehicle
 stability. Next, at Step 212, a difference .DELTA.SRR between the slip
 rate SaFR of the front right wheel FR and the slip rate SaRR of the rear
 right wheel RR, and a difference .DELTA.SRL between the slip rate SaFL of
 the front left wheel FL and the slip rate SaRL of the rear left wheel RL
 is calculated. In other words, the slip rate difference between the front
 and rear wheels placed at the same side of the vehicle (left or right) is
 calculated. Then, the program proceeds to Steps 213, 214, where the
 differences .DELTA.SRR, .DELTA.SRL are set to be zero, and the wheel
 cylinder pressure in each wheel brake cylinder of the rear wheels RR, RL
 is controlled by actuating the solenoid valves PC2, PC6 and solenoid
 valves PC4, PC8 to be opened or closed. As a result, the braking force
 distribution is made between the front and rear wheels of the vehicle.
 Consequently, the wheel cylinder pressure is controlled with respect to
 every wheel except for the front wheel with the larger slip rate, thereby
 to distribute the braking force between the front and rear wheels, and
 between the left and right wheels, appropriately.
 While the diagonal hydraulic pressure circuit system is employed according
 to the present embodiment, the front and rear hydraulic pressure circuit
 system may be employed. In the latter system, the pressurizing mode is set
 to the front wheels FR, FL, so that the wheel brake cylinders Wfr, Wfl are
 pressurized automatically by the pressure pump HP1, provided that the
 following conditions are fulfilled;
EQU [MAX(SaFR, SaRR)-MIN(SaFR, SaRR)]&gt;K1, and/or
EQU [MAX(SaFL, SaRL)-MIN(SaFL, SaRL)]&gt;K2
 FIG. 4 shows the hydraulic servo control executed on the basis of the slip
 rate of each wheel at Step 111 in FIG. 2. At the outset, it is determined
 at Step 301 whether the anti-skid control is being controlled, or not. If
 the result is affirmative, the program proceeds to Step 302 where the slip
 rate servo control for the anti-skid control is performed. If the
 anti-skid control is not being performed, the program proceeds to Step 303
 where it is determined whether the auxiliary brake control is being
 performed or not. If the auxiliary brake control is not being performed,
 the program proceeds to Step 304 where all of the solenoid valves are
 turned off, and returns to the main routine in FIG. 2. When it is
 determined at Step 303 that the auxiliary brake control is being
 performed, the program proceeds to Step 305 where the rear wheel R* (RR or
 RL) is to be controlled or not. If the result is affirmative, the program
 proceeds to Step 306 where a pressure mode is selected in accordance with
 a control map as shown in FIG. 7. The control map has a rapid pressure
 decrease zone, a pulse pressure decrease zone, a pressure hold zone, a
 pulse pressure increase zone, and a rapid pressure increase zone, which
 are provided in advance as shown in FIG. 7, so that any one of the zones
 is selected in accordance with the slip rate SaR* and the vehicle
 deceleration difference .DELTA.G, which are used as parameters of the
 control map, and the pressure mode fallen in that zone is set at step 306.
 Then, the program proceeds to Step 307. If it is determined at Step 305
 that the rear wheel R* is not to be controlled, the program proceeds to
 Step 307, without setting any pressure mode (i.e., solenoids are off).
 At Step 307, the abnormality is determined. If no abnormality is found, the
 program proceeds to Step 308, where the duty of the solenoid valve SI* is
 controlled with respect to the front wheel F* to be controlled, and the
 solenoid valve PC* (PC2, PC4, or PC6, PC8) is actuated with respect to the
 rear wheel R*, in accordance with the pressure mode as described above.
 When any abnormality is found, the program proceeds to Step 309, where the
 solenoid valve SI* and etc. for use in controlling the wheel F* are turned
 off immediately, whereas the pulse pressure increase mode is set for a
 predetermined time period Tp, before they are turned off, with respect to
 the rear wheel R*. As a result, the terminating control in case of
 abnormality is performed smoothly.
 FIG. 5 shows the determination of the abnormality executed at Step 110 in
 FIG. 2, wherein the abnormality in braking operation, such as the fade,
 defect of pad and the like, will be determined. First of all, it is
 determined at Step 401 whether the pressurizing mode is being performed
 with respect to one of the front wheels. If the pressurizing mode is being
 performed, the program further proceeds to Steps 402 and 403, where the
 value of [MAX(SaFR, SaFL)-MIN(SaFR, SaFL)] exceeded the predetermined
 value Kt has lasted for a predetermined time period Tu, then the program
 proceeds to Step 404 where it is determined that the brake system of the
 wheel F* is abnormal. When the results of determination at steps 401-403
 are negative, the program returns to the main routine.
 It should be apparent to one skilled in the art that the above-described
 embodiment is merely illustrative of but one of the many possible specific
 embodiments of the present invention. Numerous and various other
 arrangements can be readily devised by those skilled in the art without
 departing from the spirit and scope of the invention as defined in the
 following claims.