Abstract:
A braking force control apparatus adapted to generate a braking force, which is larger than that generated in a regular case, when a predetermined braking operation is carried out, this apparatus aiming at preventing the occurrence of an unnecessarily large sensible deceleration during a low-speed travel of a vehicle. ECU ( 10 ) is adapted to judge whether or not an emergency braking operation has been executed on the basis of a master cylinder pressure (Pmc) and its rate of change (dPmc). When a judgement that an emergency braking operation has been carried out is given, a wheel cylinder pressure (Pwc) is quickly increased by supplying an accumulator pressure to the wheel cylinder. When a vehicle speed exceeds a predetermined level during the execution of the emergency braking operation, the wheel cylinder pressure (Pwc) is speedily increased ( 116, 118 ) by a braking assist regular control operation. When the vehicle speed is not higher than a predetermined level during the execution of the emergency braking operation, the wheel cylinder pressure (Pwc) is slowly increased ( 116, 120 ) by a braking assist starting special control operation.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to a braking force control apparatus, and more particularly to a braking force control apparatus which generates a braking force larger than that generated during a normal control, when a braking operation that satisfies a predetermined execution condition is performed. 
     DESCRIPTION OF THE RELATED ART 
     As disclosed in Japanese Laid-Open Patent Application No. 4-121260, there is known a braking force control apparatus which generates an increased braking force larger than a braking force during a normal control, when it is detected that an emergency braking operation is performed on an automotive vehicle. The conventional apparatus of the above publication is provided with a brake booster which generates a boosted pressure in response to a braking operation force Fp on a brake pedal of the vehicle, or the boosted pressure being equal to the braking operation force Fp multiplied by a given magnification factor. The boosted pressure is delivered from the brake booster to a master cylinder. The master cylinder generates a master cylinder pressure Pmc in response to the boosted pressure delivered from the brake booster, and the master cylinder pressure Pmc is proportional to the braking operation force Fp. 
     Further, the conventional apparatus of the above publication is provided with a high-pressure source having a pump which generates a brake-assist pressure. The high-pressure source generates a brake-assist pressure in accordance with a drive signal supplied by a control circuit. When a speed of the braking operation of the brake pedal exceeds a given speed, it is determined that an emergency braking operation is performed by a vehicle operator, and the control circuit supplies a drive signal to the high-pressure source, the drive signal requesting a maximum brake-assist pressure to be generated by the high-pressure source. Both the brake-assist pressure generated by the high-pressure source and the master cylinder pressure Pmc generated by the master cylinder are supplied to a switching valve, and the switching valve delivers a larger one of the brake-assist pressure and the master cylinder Pmc to wheel cylinders of the vehicle. 
     In the conventional apparatus of the above publication, when the speed of the braking operation is below the given speed, the master cylinder pressure Pmc, which is proportional to the braking operation force Fp, is supplied to the wheel cylinders. Hereinafter, the control that is performed to generate the braking force by the braking operation under such a condition will be called a normal control. On the other hand, when the speed of the braking operation is above the given speed, the brake-assist pressure, which is generated by the high-pressure source, is supplied to the wheel cylinders. Hereinafter, the control that is performed to generate an increased braking force larger than the braking force generated during the normal control, under such a condition, will be called a brake-assist control. 
     In the conventional apparatus of the above publication, when the braking operation of the brake pedal is performed at a normal speed, the braking force is controlled to the magnitude that is proportional to the braking operation force Fp, and, when the emergency braking operation of the brake pedal is performed, the braking force is quickly increased to be larger than the braking force during the normal control. 
     In the conventional apparatus of the above publication, the braking force acting on the vehicle after the brake-assist control is started is speedily increased to a maximum braking force at the time of a maximum braking operation force acting on the brake pedal. Generally, a deceleration that the vehicle occupant senses in response to the maximum braking operation force acting on the brake pedal when the vehicle is running at a high speed is smaller than that when the vehicle is running at a low speed. If the brake-assist control is performed for both during the high-speed running and during the low-speed running in the same manner, the ride comfort as a result of the execution of the brake-assist control during the low-speed running is likely to become degraded. 
     In the conventional apparatus of the above publication, when a braking operation that satisfies a predetermined execution condition is performed, the brake-assist control is always performed in the same manner regardless of whether the vehicle speed is high or low. Therefore, when the brake-assist control is performed during the low-speed running, the conventional apparatus can speedily increase the braking force that acts on the vehicle after the brake-assist control is started. However, the conventional apparatus produces an unnecessarily large deceleration after the brake-assist control is started in such a case, and the ride comfort will be degraded. 
     SUMMARY OF THE INVENTION 
     A general object of the present invention is to provide an improved braking force control apparatus in which the above-described problems are eliminated. 
     Another, more specific object of the present invention is to provide a braking force control apparatus which changes a rate of increase of a braking force accompanied by a start of a brake-assist control, in accordance with a vehicle speed, preventing an unnecessarily large deceleration from being produced by the brake-assist control during a low-speed running of the vehicle. 
     In order to achieve the above-mentioned objects, one aspect of the present invention is to provide a braking force control apparatus which includes a means for performing a normal control to generate a braking force on a vehicle in accordance with a braking operation force, and a means for performing a brake-assist control to generate an increased braking force larger than the braking force generated during the normal control, characterized in that the apparatus comprises a braking force increasing characteristic change means for changing a rate of increase of the braking force accompanied by a start of the brake-assist control, in accordance with a vehicle speed. 
     According to the above-described braking force control apparatus of the present invention, when a predetermined braking operation is performed, the execution of the normal control is stopped and the execution of brake-assist control is started. After the start of the execution of the brake-assist control, the braking force on the vehicle is increased. The vehicle occupant senses a large deceleration if the braking force on the vehicle is too quickly increased. The sensed deceleration of the vehicle occupant depends upon the vehicle speed. The lower the vehicle speed, the larger the sensed deceleration. As the braking force control apparatus of the present invention is provided with the braking force increasing characteristic change means, it is possible to change the rate of increase of the braking force produced after the start of the brake-assist control, in accordance with the vehicle speed. Hence, the braking force control apparatus of the present invention is effective in achieving the functions of the brake-assist control in an appropriate manner for all the ranges of the vehicle speed without degrading the ride comfort of the vehicle occupant. 
     In a preferred embodiment of the present invention, the braking force control apparatus may be constructed such that the braking force increasing characteristic change means decreases the rate of increase of the braking force in accordance with a decrease in the vehicle speed. 
     According to the above-described braking force control apparatus of the present invention, the rate of increase of the braking force produced after the start of the brake-assist control will decrease as the vehicle speed becomes lower. Hence, it is possible to prevent an unnecessarily large deceleration from being produced after the start of the brake-assist control during a low-speed running of the vehicle. 
     In another preferred embodiment of the present invention, the braking force control apparatus may be constructed such that the braking force increasing characteristic change means decreases a rate of increase of a braking force on rear wheels of the vehicle in accordance with a decrease in the vehicle speed. 
     According to the above-described braking force control apparatus of the present invention, the rate of increase of the braking force on the rear wheels produced after the start of the brake-assist control will decrease as the vehicle speed becomes lower. The rate of increase of the entire braking force on the vehicle produced after the start of the brake-assist control will also decrease as the vehicle speed becomes lower. Hence, it is possible to prevent an unnecessarily large deceleration from being produced after the start of the brake-assist control during a low-speed running of the vehicle. 
     Further, in a preferred embodiment of the present invention, the braking force control apparatus may be constructed such that the braking force increasing characteristic change means delays a time to start increasing a braking force on rear wheels of the vehicle after the start of the brake-assist control from a time to start increasing a braking force on front wheels of the vehicle after the start of the brake-assist control by a delay time, wherein the delay time is increased in accordance with a decrease in the vehicle speed. 
     According to the above-described braking force control apparatus of the present invention, the time to start increasing the braking force on the rear wheels is delayed from the time to start increasing the braking force on the front wheels by the delay time, and the delay time is increased in accordance with a decrease in the vehicle speed. The braking force control apparatus of the present invention initially generates a relatively large braking force on the front wheels and a relatively small braking force on the rear wheels. Hence, the braking force control apparatus of the present invention is effective in providing a good vehicle running stability when an emergency braking operation is performed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, features and advantages of the present invention will be more apparent from the following detailed description when read in conjunction with the accompanying drawings in which: 
     FIG. 1 is a system block diagram of a braking force control apparatus to which one of a first embodiment and a second embodiment of the present invention is applied; 
     FIG. 2 is a diagram for explaining changes of a braking operation force on a brake pedal with respect to the elapsed time in various situations; 
     FIG. 3 is a flowchart for explaining a brake-assist execution condition judgment procedure performed by the braking force control apparatus of FIG. 1; 
     FIG. 4 is a flowchart for explaining a brake-assist starting specific control procedure performed by the braking force control apparatus of FIG. 1; 
     FIG. 5 is a time chart for explaining changes of a wheel cylinder pressure with respect to the elapsed time in the braking force control apparatus of FIG. 1; 
     FIG. 6 is a flowchart for explaining another brake-assist execution condition judgment procedure performed by the braking force control apparatus of FIG. 1; 
     FIG. 7 is a flowchart for explaining a brake-assist starting independent control procedure performed by the braking force control apparatus of FIG. 1; 
     FIG. 8 is a diagram showing a map read by the braking force control apparatus during the brake-assist starting independent control procedure of FIG. 7; 
     FIG. 9 is a system block diagram of a braking force control apparatus to which one of a third embodiment and a fourth embodiment of the present invention is applied; 
     FIG. 10 is a flowchart for explaining a brake-assist starting specific control procedure performed by the braking force control apparatus of FIG. 9; and 
     FIG. 11 is a flowchart for explaining a brake-assist starting independent control procedure performed by the braking force control apparatus of FIG.  9 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A description will now be given of the preferred embodiments of the present invention with reference to the accompanying drawings. 
     FIG. 1 shows one embodiment of the braking force control apparatus of the present invention. The braking force control apparatus of FIG. 1 is incorporated in an automotive vehicle, and controlled by an electronic control unit  10  (hereinafter, called ECU  10 ). 
     In FIG. 1, input signal paths through which signals supplied by certain elements of the braking force control apparatus are sent to the ECU  10 , and output signal paths through which signals supplied by the ECU  10  are sent to certain elements of the braking force control apparatus are indicated by the dotted-line arrows in FIG.  1 . Further, brake fluid paths through which brake fluid is supplied between the elements of the braking force control apparatus are indicated by the solid lines or the one-dot chain lines in FIG.  1 . 
     The braking force control apparatus includes a pump  12 . The pump  12  is provided with an actuating motor  14 . The actuating motor  14  actuates the pump  12  so that the pump  12  supplies a high-pressure brake fluid. The pump  12  has an inlet port  12   a  which is connected to a reservoir tank  16 . The pump  12  has an outlet port  12   b  which is connected to an accumulator  20  via a check valve  18 . The pump  12  produces a high-pressure brake fluid from the brake fluid received from the reservoir tank  16 , and supplies the high-pressure brake fluid from the outlet port  12   b  to the accumulator  20  so that the accumulator  20  stores the high-pressure brake fluid supplied by the pump  12 . The check valve  18  allows only a flow of the high-pressure brake fluid from the pump  12  to the accumulator  20 , and inhibits a counter flow of the brake fluid from the accumulator  20  to the pump  12 . 
     The accumulator  20  is connected through a high-pressure line  22  to a high-pressure port  24   a  of a regulator  24 . The accumulator  20  is further connected through the high-pressure line  22  to a regulator switching solenoid  26  (hereinafter, called STR  26 ). The regulator  24  has a low-pressure port  24   b  which is connected through a low-pressure line  28  to the reservoir tank  16 . The regulator  24  has a controlled-pressure port  24   c  which is connected through a controlled-pressure line  29  to the STR  26 . The STR  26  is a two-position solenoid valve which selectively opens one of the high-pressure line  22  and the controlled-pressure line  29  and closes the other. The STR  26  is normally set in a first position so that the STR  26  opens the controlled-pressure line  29  and closes the high-pressure line  22 . When a drive signal is supplied to the STR  26  by the ECU  10 , the STR  26  is set in a second position so that the STR  26  closes the controlled-pressure line  29  and opens the high-pressure line  22 . A brake pedal  30  is connected to the regulator  24 , and a master cylinder  32  is fixed to the regulator  24 . The regulator  24  contains a pressure chamber therein, and the controlled-pressure port  24   c  is open to the pressure chamber of the regulator  24 . In the regulator  24 , one of the high-pressure port  24   a  and the low-pressure port  24   b  is selectively connected to the pressure chamber in response to a condition (a speed or a quantity) of the braking operation of the brake pedal  30 . 
     The regulator  24  is arranged such that the internal pressure of the pressure chamber is adjusted to a brake fluid pressure proportional to the braking operation force Fp on the brake pedal  30 . Hence, the brake fluid pressure proportional to the braking operation force Fp is present at the controlled-pressure port  24   c  of the regulator  24 . Hereinafter, this brake fluid pressure will be called the regulator pressure Pre. 
     The braking operation force Fp exerted on the brake pedal  30  is mechanically transmitted to the master cylinder  32  via the regulator  24 . In addition, a force proportional to the regulator pressure Pre at the controlled-pressure port  24   c  of the regulator  24  is transmitted to the master cylinder  32 . Hereinafter, this force will be called the brake-assist force Fa. Hence, when the brake pedal  30  is depressed, a resultant force of the braking operation force Fp and the brake-assist force Fa is transmitted to the master cylinder  32 . 
     The master cylinder  32  includes a first pressure chamber  32   a  (“No. 1”) and a second pressure chamber  32   b  (“No. 2”) provided therein. In the master cylinder  32 , a master cylinder pressure Pmc, which is proportional to the resultant force of the braking operation force Fp and the brake-assist force Fa, is produced in both the first pressure chamber  32   a  and the second pressure chamber  32   b . A proportioning valve  34  (hereinafter, called the P valve  34 ) is connected to both the first pressure chamber  32   a  and the second pressure chamber  32   b  of the master cylinder  32 . Hence, both the master cylinder pressure Pmc produced in the first pressure chamber  32   a  and the master cylinder pressure Pmc produced in the second pressure chamber  32   b  are supplied to the P valve  34 . 
     A first pressure line  36  and a second pressure line  38  are connected to the P valve  34 . When the master cylinder pressure Pmc is below a reference pressure, the P valve  34  supplies the master cylinder pressure Pmc to both the first pressure line  36  and the second pressure line  38 . When the master cylinder pressure Pmc is above the reference pressure, the P valve  34  supplies the master cylinder pressure Pmc to the first pressure line  36  and supplies a reduced pressure to the second pressure line  38 . The reduced pressure, supplied to the second pressure line  38  in this case, is equal to the master cylinder pressure Pmc multiplied by a given reduction ratio. 
     A hydraulic pressure sensor  40  is connected to the brake fluid path between the P valve  34  and the second pressure chamber  32   b  of the master cylinder  32 . The hydraulic pressure sensor  40  outputs a signal, indicative of the master cylinder pressure Pmc, to the ECU  10 . The ECU  10  detects the master cylinder pressure Pmc, produced in the master cylinder  32 , based on the signal supplied by the hydraulic pressure sensor  40 . 
     A third pressure line  42  is connected to the STR  26 . As described above, the STR  26  selectively opens one of the high-pressure line  22  and the controlled-pressure line  29  and closes the other. The brake fluid pressure from one of the high-pressure line  22  and the controlled-pressure line  29  is supplied to the third pressure line  42  according to the position of the STR  26 . In the present embodiment, the brake fluid pressure from one of the first pressure line  36  connected to the P valve  34  and the third pressure line  42  connected to the STR  26 , is supplied to both a wheel cylinder  44 FR and a wheel cylinder  44 FL, which are respectively provided on a front-right wheel (“FR”) and a front-left wheel (“FL”) of the vehicle. Further, in the present embodiment, the brake fluid pressure from one of the second pressure line  38  connected to the P valve  34  and the third pressure line  42  connected to the STR  26 , is supplied to both a wheel cylinder  44 RR and a wheel cylinder  44 RL, which are respectively provided on a rear-right wheel (“RR”) and a rear-left wheel (“RL”) of the vehicle. 
     A first pressure-assisting solenoid- 46  (hereinafter, called SA- 1   46 ) and a second pressure-assisting solenoid  48  (hereinafter, called SA- 2   48 ) are connected to the first pressure line  36 . A front-right pressure-holding solenoid  50  (hereinafter, called SFRH  50 ), a front-left pressure-holding solenoid  52  (hereinafter, called SFLH  52 ), and a third pressure-assisting solenoid  54  (hereinafter, called SA- 3   54 ) are connected to the third pressure line  42 . 
     The SFRH  50  is a two-position solenoid valve which is normally set in a valve-open position. The SFRH  50  is connected through a pressure adjustment line  56  to both the SA- 1   46  and a front-right pressure-reducing solenoid  58  (hereinafter, called SFRR  58 ). When a drive signal is supplied to the SFRH  50  by the ECU  10 , the SFRH  50  is set in a valve-closed position so that the SFRH  50  is isolated from or closes the pressure adjustment line  56 . A check valve  60  is provided in a bypass line between the third pressure line  42  and the pressure adjustment line  56 . The check valve  60  allows only a flow of the brake fluid from the pressure adjustment line  56  to the third pressure line  42 , and inhibits a counter flow of the brake fluid from the third pressure line  42  to the pressure adjustment line  56 . 
     The SA- 1   46  is a two-position solenoid valve which selectively connects one of the first pressure line  36  and the pressure adjustment line  56  to the wheel cylinder  44 FR. The SA- 1   46  is normally set in a first position so that the SA- 1   46  connects the first pressure line  36  to the wheel cylinder  44 FR. When a drive signal is supplied to the SA- 1   46  by the ECU  10 , the SA- 1   46  is a set in a second position so that the SA- 1   46  connects the pressure adjustment line  56  to the wheel cylinder  44 FR. The SFRR  58  is a two-position solenoid valve which disconnects the pressure adjustment line  56  from or connects the pressure adjustment line  56  to the reservoir tank  16 . The SFRR  58  is normally set in a valve-closed position so that the SFRR  58  disconnects the pressure adjustment line  56  from the reservoir tank  16 . When a drive signal is supplied to the SFRR  58  by the ECU  10 , the SFRR  58  is set in a valve-open position so that the SFRR  58  connects the pressure adjustment line  56  to the reservoir tank  16 . 
     The SFLH  52  is a two-position solenoid valve which is normally set in a valve-open position. The SFLH  52  is connected through a pressure adjustment line  62  to both the SA- 2   48  and a front-left pressure-reducing solenoid  64  (hereinafter, called SFLR  64 ). When a drive signal is supplied to the SFLH  52  by the ECU  10 , the SFLH  52  is set in a valve-closed position so that the SFLH  52  is isolated from or closes the pressure adjustment line  62 . A check valve  66  is provided in a bypass line between the third pressure line  42  and the pressure adjustment line  62 . The check valve  66  allows only a flow of the brake fluid from the pressure adjustment line  62  to the third pressure line  42 , and inhibits a counter flow of the brake fluid from the third pressure line  42  to the pressure adjustment line  62 . 
     The SA- 2   48  is a two-position solenoid valve which selectively connects one of the first pressure line  36  and the pressure adjustment line  62  to the wheel cylinder  44 FL. The SA- 2   48  is normally set in a first position so that the SA- 2   48  connects the first pressure line  36  to the wheel cylinder  44 FL. When a drive signal is supplied to the SA- 2   48  by the ECU  10 , the SA- 2   48  is set in a second position so that the SA- 2   48  connects the pressure adjustment line  62  to the wheel cylinder  44 FL. The SFLR  64  is a two-position solenoid valve which disconnects the pressure adjustment line  62  from or connects the pressure adjustment line  62  to the reservoir tank  16 . The SFLR  64  is normally set in a valve-closed position so that the SFLR  64  disconnects the pressure adjustment line  62  from the reservoir tank  16 . When a drive signal is supplied to the SFLR  64  by the ECU  10 , the SFLR  64  is set in a valve-open position so that the SFLR  64  connects the pressure adjustment line  62  to the reservoir tank  16 . 
     The second pressure line  38  at the output of the P valve  34  is connected to the SA- 3   54 . A rear-right pressure-holding solenoid  68  (hereinafter, called SRRH  68 ) and a rear-left pressure-holding solenoid  70  (hereinafter, called SRLH  70 ) are connected to the downstream side of the SA- 3   54 . The SRRH  68  and the SRLH  70  are respectively provided for the wheel cylinder  44 RR and the wheel cylinder  44 RL. 
     The SA- 3   54  is a two-position solenoid valve which selectively connects one of the second pressure line  38  and the third pressure line  42  to the SRRH  68  and the SRLH  70 . The SA- 3   54  is normally set in a first position so that the SA- 3   54  connects the second pressure line  38  to the SRRH  68  and the SRLH  70 . When a drive signal is supplied to the SA- 3   54  by the ECU  10 , the SA- 3   54  is set in a second position so that the SA- 3   54  connects the third pressure line  42  to the SRRH  68  and the SRLH  70 . 
     The SRRH  68  is a two-position solenoid valve which is normally set in a valve-open position. The downstream side of the SRRH  68  is connected through a pressure adjustment line  72  to both the wheel cylinder  44 RR and a rear-right pressure-reducing solenoid  74  (hereinafter, called SRRR  74 ). When a drive signal is supplied to the SRRH  68  by the ECU  10 , the SRRH  68  is set in a valve-closed position so that the SRRH  68  is isolated from or closes the pressure adjustment line  72 . The SRRR  74  is a two-position solenoid valve which disconnects the pressure adjustment line  72  from or connects the pressure adjustment line  72  to the reservoir tank  16 . The SRRR  74  is normally set in a valve-closed position so that the SRRR  74  disconnects the pressure adjustment line  72  from the reservoir tank  16 . When a drive signal is supplied to the SRRR  74  by the ECU  10 , the SRRR  74  is set in a valve-open position so that the SRRR  74  connects the pressure adjustment line  72  to the reservoir tank  16 . A check valve  76  is provided in a bypass line between the SA- 3   54  and the pressure adjustment line  72 . The check valve  76  allows only a flow of the brake fluid from the pressure adjustment line  72  to the SA- 3   54 , and inhibits a counter flow of the brake fluid from the SA- 3   54  to the pressure adjustment line  72 . 
     The SRLH  70  is a two-position solenoid valve which is normally set in a valve-open position. The downstream side of the SRLH  70  is connected through a pressure adjustment line  78  to both the wheel cylinder  44 RL and a rear-left pressure-reducing solenoid  80  (hereinafter, called SRLR  80 ). When a drive signal is supplied to the SRLH  70  by the ECU  10 , the SRLH  70  is set in a valve-closed position so that the SRLH  70  is isolated from or closes the pressure adjustment line  78 . The SRLR  80  is a two-position solenoid valve which disconnects the pressure adjustment line  78  from or connects the pressure adjustment line  78  to the reservoir tank  16 . The SRLR  80  is normally set in a valve-closed position so that the SRLR  80  disconnects the pressure adjustment line  78  from the reservoir tank  16 . When a drive signal is supplied to the SRLR  80  by the ECU  10 , the SRLR  80  is set in a valve-open position so that the SRLR  80  connects the pressure adjustment line  78  to the reservoir tank  16 . A check valve  82  is provided in a bypass line between the SA- 3   54  and the pressure adjustment line  78 . The check valve  82  allows only a flow of the brake fluid from the pressure adjustment line  78  to the SA- 3   54 , and inhibits a counter flow of the brake fluid from the SA- 3   54  to the pressure adjustment line  78 . 
     In the braking force control apparatus of FIG. 1, a brake switch  84  is provided in the vicinity of the brake pedal  30 . When the brake pedal  30  is depressed by the vehicle operator, the brake switch  84  outputs an ON signal to the ECU  10 . The ECU  10  determines whether the braking operation is performed by the vehicle operator, based on the signal supplied by the brake switch  84 . 
     In the braking force control apparatus of FIG. 1, a wheel speed sensor  86 FR, a wheel speed sensor  86 FL, a wheel speed sensor  86 RR and a wheel speed sensor  86 RL are provided in the vicinity of the front-right wheel FR, the front-left wheel FL, the rear-right wheel RR and the rear-left wheel RL of the vehicle, respectively. Hereinafter, these wheel speed sensors will be collectively referred to as the wheel speed sensors  86 . Each of the wheel speed sensors  86  outputs a signal, indicative of the wheel speed of the related one of the wheels FR, FL, RR and RL, to the ECU  10 . The ECU  10  detects the respective wheel speeds of the wheels FR, FL, RR and RL, based on the signals supplied by the wheel speed sensors  86 . 
     In the braking force control apparatus of FIG. 1, the ECU  10  supplies the respective drive signals to the STR  26 , the SA- 1   46 , the SA- 2   48 , the SA- 3   54 , the SFRH  50 , the SFLH  52 , the SFRR  58 , the SFLR  64 , the SRRH  68 , the SRLH  70 , the SRRR  74  and the SRLR  80  in a controlled manner based on the signals supplied by the hydraulic pressure sensor  40 , the brake switch  84  and the wheel speed sensors  86 . 
     Next, a description will be given of the operation of the braking force control apparatus of the present embodiment. When the operating condition of the vehicle is found stable, the normal control is performed by the braking force control apparatus of the present embodiment to generate a braking force in accordance with the braking operation force Fp on the brake pedal  30 . 
     In order to perform the normal control by the braking force control apparatus, the ECU  10  supplies no drive signals to the STR  26 , the SA- 1   46 , the SA- 2   48 , the SA- 3   54 , the SFRH  50 , the SFLH  52 , the SFRR  58 , the SFLR  64 , the SRRH  68 , the SRLH  70 , the SRRR  74  and the SRLR  80  so that the above solenoids are set in the positions as shown in FIG.  1 . 
     More specifically, when the above solenoids of the braking force control apparatus are in the positions shown in FIG. 1, the wheel cylinders  44 FR and  44 FL are connected to the first pressure line  36 , and the wheel cylinders  44 RR and  44 RL are connected to the second pressure line  38 . In this condition, the master cylinder pressure Pmc from the master cylinder  32  is supplied to and received by the wheel cylinders  44 FR,  44 FL,  44 RL and  44 RR (hereinafter, these wheel cylinders will be collectively called the wheel cylinders  44 ). Hence, in each of the respective wheels FR, FL, RR and RL of the vehicle, the braking force in accordance with the braking operation force Fp is generated. 
     In the braking force control apparatus of the present embodiment, when it is found that any of the wheels of the vehicle will be locked, it is determined that anti-lock braking system (ABS) control execution conditions are satisfied. After this determination is made, the execution of the ABS control of the braking force control apparatus is started. 
     The ECU  10  calculates respective wheel speeds Vwfr, Vwfl, Vwrr and Vwrl (hereinafter, these wheel speeds will be collectively called the wheel speeds Vw) of the vehicle wheels based on the signals supplied by the wheel speed sensors  86 . By using a known vehicle speed estimation method, the ECU  10  determines an estimated vehicle speed Vso from the calculated wheel speeds Vw. If the braking force is exerted on the vehicle by the braking operation, the ECU  10  calculates a slip ratio S of each of the vehicle wheels from the related wheel speed Vw and the estimated vehicle speed Vso in accordance with the following formula: 
     
       
           S =( Vso−Vw )·100 /Vso   (1) 
       
     
     Then, the ECU  10  determines whether the ABS control execution conditions are satisfied based on the slip ratio S of each of the vehicle wheels. When the slip ratio S is found to be above a reference value, it is determined that the ABS control execution conditions are satisfied. When this determination is made, the ECU  10  supplies the drive signals to the SA- 1   46 , the SA- 2   48  and the SA- 3   54 . When the drive signal is supplied to the SA- 1   46 , the SA- 1   46  is set in the second position so that the SA- 1   46  connects the pressure adjustment line  56  to the wheel cylinder  44 FR. The SA- 1   46  closes off or disconnects the first pressure line  36  from the wheel cylinder  44 FR. When the drive signal is supplied to the SA- 2   48 , the SA- 2   48  is set in the second position so that the SA- 2   48  connects the pressure adjustment line  62  to the wheel cylinder  44 FL. The SA- 2   48  closes off or disconnects the first pressure line  36  from the wheel cylinder  44 FL. When the drive signal is supplied to the SA- 3   54 , the SA- 3   54  is set in the second position so that the SA- 3   54  connects the third pressure line  42  to the SRRH  68  and the SRLH  70 . The SA- 3   54  closes off or disconnects the second pressure line  38  from the SRRH  68  and the SRLH  70 . 
     When the solenoids  46 ,  48  and  54  are set in the second positions as described above, the SFRH  50 , the SFLH  52 , the SRRH  68  and the SRLH  70  (these solenoids will be called the pressure-holding solenoids SH), as well as the SFRR  58 , the SFLR  64 , the SRRR  74  and the SRLR  80  (these solenoids will be called the pressure-reducing solenoids SR) are connected to the respective wheels cylinders  44 , and the regulator pressure Pre from the regulator  24  is supplied to the upstream sides of the pressure-holding solenoids SH through the third pressure line  42  and the STR  26 . 
     During the ABS control of the braking force control apparatus of the present embodiment wherein the solenoids  46 ,  48  and  54  are set in the second positions as described above, the pressure-holding solenoids SH and the pressure-reducing solenoids SR may be controlled by the ECU  10  such that the pressure-holding solenoids SH are set in the valve-open positions and the pressure-reducing solenoids SR are set in the valve-closed positions. When the ECU  10  performs this control procedure in the braking force control apparatus, a wheel cylinder pressure Pwc of the related one of the wheel cylinders  44  is increased up to the regulator pressure Pre. This control procedure will be called (1) a pressure-increasing control mode. 
     Alternatively, during the ABS control of the braking force control apparatus of the present embodiment wherein the solenoids  46 ,  48  and  54  are set in the second positions as described above, the pressure-holding solenoids SH and the pressure-reducing solenoids SR may be controlled by the ECU  10  such that the pressure-holding solenoids SH are set in the valve-closed positions and the pressure-reducing solenoids SR are set in the valve-closed positions. When the ECU  10  performs this control procedure in the braking force control apparatus, the wheel cylinder pressure Pwc of the related one of the wheel cylinders  44  is held at the same level without increase or decrease. Hereinafter, this control procedure will be called (2) a pressure-holding control mode. 
     Alternatively, during the ABS control of the braking force control apparatus of the present embodiment wherein the solenoids  46 ,  48  and  54  are set in the second positions as described above, the pressure-holding solenoids SH and the pressure-reducing solenoids SR may be controlled by the ECU  10  such that the pressure-holding solenoids SH are set in the valve-closed positions and the pressure-reducing solenoids SR are set in the valve-open positions. When the ECU  10  performs this control procedure in the braking force control apparatus, the wheel cylinder pressure Pwc of the related one of the wheel cylinders  44  is decreased. This control procedure will be called (3) a pressure-decreasing control mode. 
     In the braking force control apparatus of the present embodiment, the ECU  10  suitably performs one of (1) the pressure-increasing control mode, (2) the pressure-holding control mode and (3) the pressure-decreasing control mode so as to maintain the slip ratio S of each of the vehicle wheels FR, FL, RR and RL below the reference value, preventing all the vehicle wheels from being locked during the braking operation. 
     It is necessary to quickly decrease the wheel cylinder pressure Pwc of the related one of the wheel cylinders  44  after the vehicle operator releases the braking operation force on the brake pedal  30  during the ABS control. In the braking force control apparatus of the present embodiment, the check valves  60 ,  66 ,  76  and  82  are provided in the brake fluid paths connected to the wheel cylinders  44 , so as to allow only the flow of the brake fluid from the pressure adjustment lines  56 ,  62 ,  72  and  78  (connected to the wheel cylinders  44 ) to the third pressure line  42 . As the check valves  60 ,  66 ,  76  and  82  function in this manner, it is possible for the braking force control apparatus of the present embodiment to quickly decrease the wheel cylinder pressure Pwc after the vehicle operator releases the braking operation force on the brake pedal  30  during the ABS control. 
     During the ABS control of the braking force control apparatus of the present embodiment, the wheel cylinder pressure Pwc of the related one of the wheel cylinders  44  is suitably adjusted by supplying the regulator pressure Pre from the regulator  24  to the wheel cylinders  44 . More specifically, when the brake fluid from the pump  12  is delivered to the wheel cylinders  44 , the wheel cylinder pressure Pwc is increased, and, when the brake fluid within the wheel cylinders  44  is returned to the reservoir tank  16 , the wheel cylinder pressure Pwc is decreased. If the increase of the wheel cylinder pressure Pwc is performed by using the master cylinder  32  as the only brake fluid pressure source, the brake fluid contained in the master cylinder  32  is gradually decreased through a repeated execution of the pressure-increasing control mode and the pressure-decreasing control mode. In such a condition, the master cylinder  32  may be malfunctioning due to a too small amount of the brake fluid contained in the master cylinder  32 . 
     In order to avoid the malfunction of the master cylinder  32  mentioned above, in the braking force control apparatus of the present embodiment, the increase of the wheel cylinder pressure Pwc is performed by selectively using one of the master cylinder  32  and the pump  12  as the brake fluid pressure source. If the increase of the wheel cylinder pressure Pwc is performed by using the pump  12  as the brake fluid pressure source, the present embodiment can avoid the malfunction of the master cylinder  32 . It is possible for the braking force control apparatus of the present embodiment to maintain a stable operating condition even when the ABS control is continuously performed over an extended period of time. 
     As described above, the execution of the ABS control of the braking force control apparatus of the present embodiment is started when it is found that any of the wheels of the vehicle will be locked. In other words, the prerequisite condition to start the execution of the ABS control of the braking force control apparatus of the present embodiment is that the vehicle operator exerts an adequate braking operation force Fp on the brake pedal  30  so as to produce a large slip ratio S of any of the vehicle wheels which can be detected by the braking force control apparatus. 
     FIG. 2 shows changes of the braking operation force Fp on the brake pedal  30  with respect to the elapsed time in various situations. A change of the braking operation force Fp exerted on the brake pedal  30  by an experienced vehicle operator who is intended to perform an emergency braking operation, and a change of the braking operation force Fp exerted on the brake pedal  30  by a beginner who is intended to perform the emergency braking operation, are indicated by the curve “A” and the curve “B” in FIG. 2, respectively. Generally, it is necessary that the braking operation force Fp during the emergency braking operation is large enough to start the execution of the ABS control of the braking force control apparatus. 
     As indicated by the curve “A” of FIG. 2, in the case of the experienced vehicle operator, when a condition requiring the emergency braking has occurred, the braking operation force Fp on the brake pedal  30  is quickly raised to an adequately large level, and the braking operation force Fp is maintained at the adequately large level over a certain period of time. In response to the braking operation of the brake pedal  30 , an adequately large master cylinder pressure Pmc from the master cylinder  32  is supplied to the wheel cylinders  44 , and the ABS control of the braking force control apparatus can be started. 
     However, as indicated by the curve “B” of FIG. 2, in the case of the beginner, when the condition requiring the emergency braking has occurred, the braking operation force Fp may not be maintained at the adequately large level over a certain period of time although the braking operation force Fp is initially raised to the adequately large level. Hence, in response to the braking operation of the brake pedal  30  by the beginner, an adequately large master cylinder pressure Pmc from the master cylinder  32  may not be supplied to the wheel cylinders  44 , and the ABS control of the braking force control apparatus cannot be started. 
     Generally, beginners who are less experienced in vehicle operation tend to unintentionally release the brake pedal  30  during the emergency braking operation. In the braking force control apparatus of the present invention, a braking force control procedure is performed by the ECU  10  when a brake releasing operation of the brake pedal  30  is determined as being an intentional operation, and this braking force control procedure allows the adequately large master cylinder pressure Pmc of the master cylinder  32  to be supplied to the wheel cylinders  44  even if the braking operation force Fp is not raised to the adequately large level as indicated by the curve “B” in FIG.  2 . Hereinafter, this braking force control procedure will be called a brake-assist (BA) control. 
     Before starting the brake-assist control in the braking force control apparatus of the present invention, it is necessary to determine, with accuracy, whether a braking operation of the brake pedal  30  is intended to perform the emergency braking operation or not. 
     In FIG. 2, changes of the braking operation force Fp on the brake pedal  30  (which is intended to perform a normal braking operation) with respect to the elapsed time in various situations are indicated by the curves “C” and “D”. As indicated by the curves “A” through “D”, a rate of change of the braking operation force Fp during the normal braking operation is smaller than a rate of change of the braking operation force Fp during the emergency braking operation. In addition, a convergence value of the braking operation force Fp during the normal braking operation is smaller than that of the braking operation force Fp during the emergency braking operation. 
     The braking force control apparatus of the present invention takes account of the differences between the braking operation force Fp during the normal braking operation and the braking operation force Fp during the emergency braking operation as shown in FIG.  2 . When a rate of change of the braking operation force Fp during an initial period of the braking operation is above a certain reference value and the braking operation force Fp is raised to an adequately large level (which falls within a region (I) above the borderline indicated by a dotted line in FIG.  2 ), the ECU  10  of the braking force control apparatus of the present invention determines that the braking operation of the brake pedal  30  is intended to perform the emergency braking operation. 
     On the other hand, when the rate of change of the braking operation force Fp during the initial period of the braking operation is not above the reference value, or when the braking operation force Fp is not raised to the adequately large level (which falls within a region (II) below the borderline indicated by the dotted line in FIG.  2 ), the ECU  10  of the braking force control apparatus of the present invention determines that the braking operation of the brake pedal  30  is intended to perform the normal braking operation. 
     In the braking force control apparatus of the present invention, the ECU  10  makes a determination as to whether a speed of the braking operation of the brake pedal  30  is above a given speed, and makes a determination as to whether a quantity of the braking operation of the brake pedal  30  is above a reference quantity. In accordance with the results of the determinations, the ECU  10  can determine whether the braking operation of the brake pedal  30  is intended to perform the emergency braking operation or the normal braking operation. 
     In the braking force control apparatus of FIG. 1, the speed and the quantity of the braking operation of the brake pedal  30  are detected by using the master cylinder pressure Pmc as the parameter to define the braking operation speed or the braking operation quantity. The master cylinder pressure Pmc is detected by the ECU  10  based on the signal supplied by the hydraulic pressure sensor  40 . The master cylinder pressure Pmc varies in accordance with the braking operation speed or quantity, and a rate of change (dPmc) of the master cylinder pressure Pmc is in correspondence with the braking operation speed. Accordingly, before starting the brake-assist (BA) control, the braking force control apparatus of the present embodiment can determine, with accuracy, whether the braking operation of the brake pedal  30  is intended to perform the emergency braking operation or not. Hereinafter, this function of the braking force control apparatus of the present embodiment will be called a brake-assist control start judgment means. The ECU  10  acts as the brake-assist control start judgment means. 
     Alternatively, in the braking force control apparatus of the present invention, the brake-assist control start judgment means may be constituted by using another quantity of the braking operation of the brake pedal  30  other than the master cylinder pressure Pmc or the rate of change dPmc thereof described above with the present embodiment. 
     Next, a description will be given of the operation of the braking force control apparatus of the present embodiment after it is determined that the brake-assist (BA) control should be started. As described above, in the present embodiment, when the speed of the braking operation of the brake pedal  30  (or the rate of change dPmc of the master cylinder pressure) is above the given speed and the quantity of the braking operation of the brake pedal  30  (or the master cylinder pressure Pmc) is above the reference quantity, the ECU  10  determines that the braking operation of the brake pedal  30  is intended to perform the emergency braking operation. 
     When it is determined that the braking operation of the brake pedal  30  is intended to perform the emergency braking operation, the ECU  10  supplies the drive signals to the STR  26 , the SA- 1   46 , the SA- 2   48  and the SA- 3   54 . 
     When the drive signal is supplied to the STR  26  by the ECU  10 , the STR  26  is set in the second position so that the STR  26  closes the controlled-pressure line  29  connected to the regulator  24 , and connects the high-pressure line  22  to the third pressure line  42 . The accumulator pressure Pace from the accumulator  20  is supplied to the third pressure line  42  through the STR  26 . When the drive signal is supplied to the SA- 1   46 , the SA- 1   46  is set in the second position so that the SA- 1   46  connects the pressure adjustment line  56  to the wheel cylinder  44 FR. The SA- 1   46  closes off or disconnects the first pressure line  36  from the wheel cylinder  44 FR. When the drive signal is supplied to the SA- 2   48 , the SA- 2   48  is set in the second position so that the SA- 2   48  connects the pressure adjustment line  62  to the wheel cylinder  44 FL. The SA- 2   48  closes off or disconnects the first pressure line  36  from the wheel cylinder  44 FL. When the drive signal is supplied to the SA- 3   54 , the SA- 3   54  is set in the second position so that the SA- 3   54  connects the third pressure line  42  to the SRRH  68  and the SRLH  70 . The SA- 3   54  closes off or disconnects the second pressure line  38  from the SRRH  68  and the SRLH  70 . 
     Hence, when the drive signals are supplied to the STR  26 , the SA- 1   46 , the SA- 2   48  and the SA- 3   54 , all the wheel cylinders  44  are connected to both the pressure-holding solenoids SH and the pressure-reducing solenoids SR, and the accumulator pressure Pacc is supplied to the upstream sides of the pressure-holding solenoids SH through the STR  26 . 
     Immediately when it is determined that the braking operation of the brake pedal  30  is intended to perform the emergency braking operation, the ECU  10  does not yet supply the drive signals to the pressure-holding solenoids SH or the pressure-reducing solenoids SR. The accumulator pressure Pacc is supplied to the wheel cylinders  44  through the pressure-holding solenoids SH. Consequently, the wheel cylinder pressure Pwc of each of the wheel cylinders  44  is quickly increased toward the accumulator pressure Pacc. 
     Accordingly, it is possible for the braking force control apparatus of the present embodiment to quickly increase the wheel cylinder pressure Pwc of each of the wheel cylinders  44  when the emergency braking operation is performed, regardless of the magnitude of the braking operation force Fp. Therefore, in the braking force control apparatus of the present embodiment, after the condition requiring the emergency braking has occurred, it is possible to quickly generate an increased braking force larger than that generated during the normal control, even if the vehicle operator is a beginner. 
     After the accumulator pressure Pacc is continuously supplied to the wheel cylinders  44 , the increased braking force is generated on the vehicle, and a relatively large slip ratio S of the vehicle wheels FR, FL, RR and RL is produced. It is then determined that the ABS control execution conditions are satisfied. After this determination is made, the execution of the ABS control of the braking force control apparatus of the present embodiment is started. As described above, the ECU  10  suitably performs one of (1) the pressure-increasing control mode, (2) the pressure-holding control mode and (3) the pressure-decreasing control mode so as to maintain the slip ratio S of each of the vehicle wheels FR, FL, RR and RL below the reference value, preventing all the vehicle wheels from being locked during the braking operation. 
     When the ABS control is performed following the emergency braking operation, the wheel cylinder pressure Pwc of each of the wheel cylinders  44  is increased by the supply of the accumulator pressure Pacc from the pump  12  or the accumulator  20  to the wheel cylinders  44 , while the wheel cylinder pressure Pwc is reduced by the returning flow of the brake fluid within the wheel cylinders  44  to the reservoir tank  16 . It is possible to prevent the malfunctioning of the master cylinder  32  even when the repeated execution of the pressure-increasing mode control and the pressure-reducing mode control is performed during the ABS control. 
     When the vehicle operator starts releasing the brake pedal  30  after the brake-assist (BA) control was started by the emergency braking operation, it is necessary to terminate the brake-assist (BA) control. In the braking force control apparatus of the present embodiment, during the execution of the brake-assist (BA) control, the ECU  10  supplies the drive signals to the STR  26 , the SA- 1   46 , the SA- 2   48  and the SA- 3   54 . When the drive signals are supplied to the solenoids  26 ,  46 ,  48 , and  54  by the ECU  10 , the solenoids  26 ,  46 ,  48  and  54  are set in the second positions as described above. In this condition, the internal pressure chamber of the regulator  24  is isolated from the wheel cylinders  44  and the pump  12 , and both the first pressure chamber  32   a  and the second pressure chamber  32   b  of the master cylinder  32  are isolated from the wheel cylinders  44  and the pump  12 . 
     Hence, in the braking force control apparatus of the present embodiment, during the execution of the brake-assist (BA) control, the master cylinder pressure Pmc varies in proportion with the braking operation force Fp on the brake pedal  30 . By monitoring the master cylinder pressure Pmc which is detected based on the signal supplied by the hydraulic pressure sensor  40 , the ECU  10  can easily determine whether a brake releasing operation of the brake pedal  30  is performed by the vehicle operator. When it is determined that the brake releasing operation is performed, the ECU  10  stops supplying the drive signals to the STR  26 , the SA- 1   46 , the SA- 2   48  and the SA- 3   54 . Hence, the brake-assist (BA) control is terminated and the normal control is restarted. 
     A deceleration that the vehicle occupant senses when quickly increasing the braking force acting on the vehicle after the start of the brake-assist (BA) control will increase as the vehicle speed becomes low. Herein-after, this deceleration will be called the sensed deceleration. If a rate of increase of the braking force as a result of the execution of the brake-assist (BA) control when the vehicle is running at a low speed is the same as that when the vehicle is running at a high speed, the ride comfort as a result of the execution of the brake-assist (BA) control during the low-speed running is likely to be significantly degraded. On the other hand, if the rate of increase of the braking force during the low-speed running is made lower than that during the high-speed running, after an emergency braking operation is performed, it is possible to appropriately exert an adequate braking force on the vehicle against the condition requiring the emergency braking during the low-speed running. 
     In the braking force control apparatus of the present embodiment, the rate of increase of the braking force accompanied by the start of the brake-assist (BA) control is changed in accordance with the vehicle speed. Specifically, the braking force control apparatus is adapted to lower the rate of increase of the braking force accompanied by the start of the brake-assist (BA) control as the vehicle speed at that time is decreased. According to the braking force control apparatus of the present embodiment, it is possible to prevent an unnecessarily large deceleration from being produced by the brake-assist (BA) control during the low-speed running of the vehicle. 
     Next, a description will be given of a control procedure performed by the ECU  10  of the braking force control apparatus in order to achieve the above-mentioned function of changing a rate of increase of the braking force in response to a vehicle speed, with reference to FIG.  3  through FIG.  5 . 
     FIG. 3 is a flowchart for explaining a brake-assist execution condition judgment procedure performed by the ECU  10  of the braking force control apparatus of FIG.  1 . The procedure of FIG. 3 is started every time a braking operation of the brake pedal  30  is performed. At the start of the brake-assist execution condition judgment procedure, the ECU  10  performs step  100  of the procedure of FIG.  3 . When this procedure is performed for the first time after the detection of the braking operation of the brake pedal  30 , all settings of flags used in this procedure and values of parameters temporarily stored in this procedure are initialized. 
     Step  100  determines whether the brake-assist (BA) control is currently performed. As the procedure of FIG. 3 is performed to make a determination as to whether the brake-assist (BA) control execution conditions are satisfied by the braking operation, the execution of the procedure of FIG. 3 is useless if the BA control is already being performed. When the result at the step  100  is affirmative (the BA control is currently performed), the procedure of FIG. 3 at the present cycle ends, and the subsequent steps of the procedure of FIG. 3 are not performed. On the other hand, when the result at the step  100  is negative (the BA control is not currently performed), the ECU  10  performs a next step  102  of the procedure of FIG.  3 . 
     Step  102  determines whether an emergency braking operation of the brake pedal  30  is detected. In the step  102 , after the ON signal output by the brake switch  84  is detected, the ECU  10  determines whether both a master cylinder pressure Pmc that is above the reference pressure and a master cylinder pressure change rate dPmc that is above a reference change rate are detected based on the signal supplied by the hydraulic pressure sensor  40 . When the result at the step  102  is negative (the emergency braking operation is not detected), the procedure of FIG. 3 at the present cycle ends, and the subsequent steps of the procedure of FIG. 3 are not performed. On the other hand, when the result at the step  102  is affirmative (the emergency braking operation is detected), the ECU  10  performs a next step  104  of the procedure of FIG.  3 . 
     Step  104  stores a maximum rate of change (MAXdPmc) of the master cylinder pressure Pmc in a memory of the ECU  10 . In the step  104 , when a presently-detected master cylinder pressure change rate dPmc is larger than a maximum rate of change (MAXdPmc) of the master cylinder pressure Pmc previously stored in the memory of the ECU  10 , the previous maximum change rate MAXdPmc is renewed by the presently-detected master cylinder pressure change rate dPmc, and the new maximum change rate is stored in the memory of the ECU  10  at the present cycle. When the presently-detected master cylinder pressure change rate dPmc is not larger than the previously-stored maximum change rate MAXdPmc, the renewal of the maximum change rate MAXdPmc is not performed, and the previously-stored maximum rate of change MAXdPmc is stored in the memory of the ECU  10  without change. After the step  104  is performed, the ECU  10  performs a next step  106  of the procedure of FIG.  3 . 
     Step  106  determines whether the master cylinder pressure change rate dPmc is smaller than a predetermined reference value β. The reference value β is a threshold value that is used to make a determination as to whether a rate of increase of the master cylinder pressure Pmc during the emergency braking operation has changed to a low rate. When the result at the step  106  is negative (dPmc≧β), it is determined that the rate of increase of the master cylinder pressure Pmc is being rapidly increased. In this case, the control is transferred back to the step  104 , and the ECU  10  performs the step  104  again. On the other hand, when the result at the step  106  is affirmative (dPmc&lt;β), it is determined that the rate of increase of the master cylinder pressure Pmc has changed to the low rate. In this case, the ECU  10  performs a next step  108  of the procedure of FIG.  3 . After the steps  104  and  106  are performed, the maximum change rate MAXdPmc of the master cylinder pressure Pmc, produced during the emergency braking operation between the time the depression of the brake pedal  30  was started and the time the rate of increase of the master cylinder pressure Pmc has changed to the low rate, is stored in the memory of the ECU  10 . 
     The above-mentioned condition of the step  106  is satisfied when the rate of increase of the master cylinder pressure Pmc has changed to the low rate after the master cylinder pressure Pmc was quickly increased by the emergency braking operation. Hence, when the result at the step  106  is affirmative, an adequately high master cylinder pressure Pmc is already produced in the master cylinder  32 . 
     Generally, in the braking force control apparatus of the present embodiment, there is a delay time between the time the master cylinder pressure Pmc was increased to the adequately high level and the time the wheel cylinder pressure Pwc is increased to an adequately high level. For this reason, when the condition of the step  106  has just been satisfied, there is a difference between the master cylinder pressure Pmc and the wheel cylinder pressure Pwc. Hereinafter, this pressure difference will be called the emergency braking pressure difference dPem. When the emergency braking pressure difference dPem is at a large level, the master cylinder pressure Pmc from the master cylinder  32  is continuously supplied to the wheel cylinders  44  until the difference |Pmc−Pmc| is reduced to a certain level. The wheel cylinder pressure Pwc can be more smoothly increased to the adequately high level by this method, rather than using the accumulator pressure Pacc from the accumulator  20  or the pump  12  being supplied to the wheel cylinders  44 . Therefore, in the present embodiment, after an estimated delay time elapses since the time the condition of the step  106  is satisfied, the execution of the brake-assist (BA) control is started. 
     The time required to increase the wheel cylinder pressure Pwc to the adequately high level by supplying the master cylinder pressure Pmc to the wheel cylinders  44  will increase as the emergency braking pressure difference dPem becomes high. The emergency braking pressure difference dPem will increase as the master cylinder pressure Pmc at the time the condition of the step  106  has just been satisfied (hereinafter, this master cylinder pressure will be called the emergency braking master cylinder pressure Pmcem) becomes high. Further, the emergency braking pressure difference dPem will increase as the maximum change rate MAXdPmc of the master cylinder pressure Pmc produced during the emergency braking operation from the time the depression of the brake pedal  30  was started to the time the rate of increase of the master cylinder pressure Pmc has changed to the low rate (which maximum change rate is stored in the step  104 ) becomes high. Hence, in the present embodiment, the delay time is determined in the following steps  108  and  110  based on the emergency braking master cylinder pressure Pmcem and the maximum change rate MAXdPmc. 
     Step  108  stores the master cylinder pressure Pmc, which is detected based on the signal supplied by the hydraulic pressure sensor  40  at the time the condition of the step  106  is satisfied, in the memory of the ECU  10  as the emergency braking master cylinder pressure Pmcem (Pmcem←Pmc). After the step  108  is performed, the ECU  10  performs a next step  110  of the procedure of FIG.  3 . 
     Step  110  determines a delay time D based on the emergency braking master cylinder pressure Pmcem (stored in the step  108 ) and the maximum change rate MAXdPmc (stored in the step  104 ). The delay time D is determined by the ECU  10  by reading out a map, which is stored, in advance, in the memory of the ECU  10 , in accordance with the emergency braking master cylinder pressure Pmcem and the maximum change rate MAXdPmc. Specifically, the delay time D will increase to a relatively long time (D 1 ) as both the emergency braking master cylinder pressure Pmcem and the maximum change rate dPem are increased (or the emergency braking pressure difference dPem is increased). The delay time D will decrease to a relatively short time (Ds) as both the emergency braking master cylinder pressure Pmcem and the maximum change rate dPem are decreased (or the emergency braking pressure difference dPem is decreased). 
     After the step  110  is performed, the ECU  10  at step  112  decrements the delay time D (D←(D−1)). After the step  112  is performed, the ECU  10  at step  114  determines whether the time to start the execution of the brake-assist (BA) control is reached by detecting whether the decremented delay time D is equal to zero “0”. When the result at the step  114  is negative, it is determined that the time to start the execution of the BA control is not yet reached. The control is transferred back to the step  112 , and the ECU  10  performs the step  112  again. When the result at the step  114  is affirmative after the steps  112  and  114  are repeated, it is determined that the time to start the execution of the BA control is reached. In this case, the ECU  10  performs a next step  116  of the procedure of FIG.  3 . 
     Step  116  determines whether a vehicle speed V (detected based on the signals supplied by the wheel speed sensors  86 ) is larger than a reference speed V 1 . The reference speed V 1  is a threshold value that is predetermined according to the experiments of the inventors. When V&gt;V 1 , it is determined that the vehicle is running at a high speed, and the sensed deceleration when quickly increasing the braking force acting on the vehicle after the start of the brake-assist (BA) control will not be so large as to degrade the ride comfort of the vehicle occupant. Therefore, when the result at the step  116  is affirmative (V&gt;V 1 ), the ECU  10  performs a next step  118  of the procedure of FIG.  3 . 
     Step  118  starts the execution of the brake-assist (BA) control in the braking force control apparatus of the present embodiment in a regular manner. Hereinafter, the BA control which is started in the step  118  will be called the BA regular control. In the step  118 , the ECU  10  supplies the respective drive signals to the STR  26 , the SA- 1   46 , the SA- 2   48  and the SA- 3   54 . The STR  26  is set in the second position so that the controlled-pressure line  29  from the regulator  24  is closed and the high-pressure line  22  from the accumulator  20  is opened by the STR  26 . The SA- 1   46  is set in the second position so that the SA- 1   46  connects the pressure adjustment line  56  to the wheel cylinder  44 FR. The SA- 2   48  is set in the second position so that the SA- 2   48  connects the pressure adjustment line  62  to the wheel cylinder  44 FL. The SA- 3   54  is set in the second position so that the SA- 3   54  connects the third pressure line  42  to the SRRH  68  and the SRLH  70 . Hence, after the execution of the BA regular control is started in the step  118 , the wheel cylinder pressure Pwc of each of the wheel cylinders  44  will be quickly increased toward the accumulator pressure Pacc. After the step  118  is performed, the procedure of FIG. 3 at the present cycle ends. 
     On the other hand, when the result at the step  116  is negative (V≦V 1 ), it is determined that the vehicle is running at a low speed, and the sensed deceleration when quickly increasing the braking force acting on the vehicle after the start of the brake-assist (BA) control will be large enough to degrade the ride comfort of the vehicle occupant. Therefore, when the result at the step  116  is negative (V≦V 1 ), the ECU  10  performs a next step  120  of the procedure of FIG.  3 . 
     Step  120  sets a BA starting specific control start flag (XBASTS) to “1” (XBASTS←“1”). The BA starting specific start flag XBASTS is set to “1” when starting the execution of a specifically-controlled brake-assist (BA) control in the braking force control apparatus of the present embodiment. Hereinafter, the specifically-controlled BA control, performed after the step  120  is performed, will be called the BA starting specific control. The BA starting specific start flag XBASTS is reset to “0” when the ABS control execution conditions are satisfied during the BA control or when the execution of the BA control is terminated. When the step  120  is performed, the procedure of FIG. 3 at the present cycle ends. 
     FIG. 4 is a flowchart for explaining a brake-assist (BA) starting specific control procedure performed by the ECU  10  of the braking force control apparatus of FIG.  1 . The first embodiment of the present invention is constituted by the ECU  10  of the braking force control apparatus of FIG. 1 when performing the control procedures of FIG.  3  and FIG.  4 . 
     The BA starting specific control is performed in the braking force control apparatus of the present embodiment in order to lower in an appropriate manner the rate of increase of the braking force accompanied by the start of the BA control. The BA starting specific control is achieved by the ECU  10  when performing a brake-assist (BA) starting specific control procedure shown in FIG.  4 . The control procedure shown in FIG. 4 is an interrupt-initiated routine which is periodically initiated at intervals of a predetermined time. As shown in FIG. 4, at the start of the BA starting specific control procedure, the ECU  10  of the braking force control apparatus of the present embodiment performs step  130  of the procedure of FIG.  4 . 
     Step  130  determines whether the BA starting specific start flag XBASTS is equal to 1. When the result at the step  130  is negative (the flag XBASTS is not equal to 1), the procedure of FIG. 4 at the present cycle ends, and the subsequent steps of the procedure of FIG. 4 are not performed. When the result at the step  130  is affirmative (XBASTS=1), the ECU  10  performs a next step  132  of the procedure of FIG.  4 . 
     Step  132  supplies the respective drive signals to the STR  26 , the SA- 1   46 , the SA- 2   48  and the SA- 3   54 . After the step  132  is performed, the brake fluid paths through which the accumulator pressure Pacc is supplied to each of the wheel cylinders  44  are opened by the STR  26 , the SA- 1   46 , the SA- 2   48  and the SA- 3   54 . After the step  132  is performed, the ECU  10  performs a next step  134  of the procedure of FIG.  4 . 
     Step  134  determines whether a timer T STS  is above a predetermined reference time T STS0 . During the operation of the ECU  10 , the timer T STS  is automatically incremented to a given upper limit that is larger than the reference time T STS0 . During the execution of the procedure of FIG. 4, the timer T STS  is reset to zero at a given timing. Only when the step  134  is performed for the first time after the start of the procedure of FIG. 4, the timer T STS  is set to the upper limit, and the result at the step  134  is affirmative (T STS ≧T STS0 ). When the result at the step  134  is affirmative, the ECU  10  performs a next step  136  of the procedure of FIG.  4 . 
     Step  136  sets all the pressure-holding solenoids SH in the valve-open positions (or the OFF states with no drive signal supplied to the pressure-holding solenoids SH). The pressure-holding solenoids SH open the pressure adjustment lines  56 ,  62 ,  72  and  78 , and the accumulator pressure Pacc is supplied to each of the wheel cylinders  44  so that the wheel cylinder pressure Pwc of each of the wheel cylinders  44  will be quickly increased. After the step  136  is performed, the ECU  10  performs a next step  138  of the procedure of FIG.  4 . 
     Step  138  resets the timer T STS  to zero. After the step  138  is performed, the procedure of FIG. 4 at the present cycle ends. After the timer T STS  is reset to zero, the timer T STS  is automatically incremented from zero to the upper limit. In the subsequent cycles, the result at the step  134  will be negative (T STS &lt;T STS0 ) until the reference time T STS0  elapses. When the result at the step  134  is negative, the ECU  10  performs a next step  140  of the procedure of FIG.  4 . 
     Step  140  determines whether the timer T STS  is above a given hold time T HOLD . The hold time T HOLD  is predetermined to be smaller than the reference time T STS0  (T HOLD &lt;T STS0 ). When the result at the step  140  is negative (T STS &lt;T HOLD ), it is determined that the hold time T HOLD  has not yet elapsed after the timer T STS  is reset to zero. In this case, the procedure of FIG. 4 at the present cycle ends, and the subsequent step is not performed. On the other hand, when the result at the step  140  is affirmative (T STS ≧T HOLD ), it is determined that the hold time T STS  elapses after the timer T STS  is reset to zero. In this case, the ECU  10  performs a next step  142  of the procedure of FIG.  4 . 
     Step  142  sets all the pressure-holding solenoids SH in the valve-closed positions (or the ON states with the respective drive signal being supplied to the pressure-holding solenoids SH). The pressure-holding solenoids SH close the pressure adjustment lines  56 ,  62 ,  72  and  78 , and all the wheel cylinders  44  are isolated from the STR  26 . The supply of the accumulator pressure Pacc to each of the wheel cylinders  44  is stopped, and the wheel cylinder pressure Pwc of each of the wheel cylinders  44  is held at the same level without being increased. 
     After the step  142  is performed, the procedure of FIG. 4 at the present cycle ends. In the subsequent cycles the result at the step  140  will be affirmative (T STS ≧T HOLD ) until the reference time T STS0  elapses. The step  142  is continuously performed for such a duration so that the wheel cylinder pressure Pwc is held at the same level. Further, when the timer T STS  is incremented to exceed the reference time T STS0 , the result at the step  134  will be affirmative. At that time, the step  136  is performed again so that all the pressure-holding solenoids SH are set in the valve-open positions, and the wheel cylinder pressure Pwc of each of the wheel cylinders  44  will be quickly increased toward the accumulator pressure Pacc. 
     In the above-described procedure of FIG. 4, after the flag XBASTS is set to  1 , the wheel cylinder pressure Pwc is continuously increased toward the accumulator pressure Pacc until the hold time T HOLD  elapses after the timer T STS  is reset to zero. When the hold time T HOLD  passed but the reference time T STS0  has not yet elapsed, the supply of the accumulator pressure Pacc to each of the wheel cylinders  44  is stopped and the wheel cylinder pressure Pwc is held at the same level by the pressure-holding solenoids SH. In this manner, the pressure increasing operation of the wheel cylinders  44  and the pressure holding operation of the wheel cylinders  44  are repeated every time the reference time T STS  elapses after the timer T STS  is reset to zero. Therefore, by performing the BA starting specific control procedure of FIG. 4, it is possible for the braking force control apparatus of the present embodiment to lower in an appropriate manner the rate of increase of the braking force accompanied by the start of the BA starting specific control, in comparison with the rate of increase of the braking force accompanied by the start of the BA regular control. 
     FIG. 5 is a time chart for explaining changes of a wheel cylinder pressure Pwc with respect to the elapsed time in the braking force control apparatus of FIG.  1 . In FIG. 5, an emergency braking operation of the brake pedal  30  is started at a time “to”. The curve “A”, shown in FIG. 5, indicates an increasing characteristic of the wheel cylinder pressure Pwc produced when the execution of the brake-assist (BA) regular control is started at a time “t 1 ” following the time “to” the emergency braking operation is started. The braking force control apparatus of the present embodiment achieves the increasing characteristic of the wheel cylinder pressure Pwc indicated by the curve A, when the vehicle is running at a high speed above the reference speed V 1  and the emergency braking operation is performed. It is possible for the braking force control apparatus of the present embodiment to quickly generate a large braking force after the start of the brake-assist control. 
     The curve “B”, shown in FIG. 5, indicates an increasing characteristic of the wheel cylinder pressure Pwc produced when the execution of the brake-assist (BA) starting specific control is started at the time “t 1 ” following the time “to” the emergency braking operation is started. The braking force control apparatus of the present embodiment achieves the increasing characteristic of the wheel cylinder pressure Pwc indicated by the curve B, when the vehicle is running at a low speed below the reference speed V 1  and the emergency braking operation is performed. It is possible for the braking force control apparatus of the present embodiment to lower in an appropriate manner the rate of increase of the braking force accompanied by the start of the BA starting specific control, in comparison with the rate of increase of the braking force accompanied by the start of the BA regular control. It is possible to prevent an unnecessarily large deceleration from being produced by the BA control during the low-speed running of the vehicle. 
     As described above, the braking force control apparatus of the present embodiment can quickly increase the wheel cylinder pressure Pwc toward the accumulator pressure Pacc when an emergency braking operation is performed during a high-speed running of the vehicle. Further, when an emergency braking operation is performed during a low-speed running of the vehicle, the braking force control apparatus of the present embodiment can increase the wheel cylinder pressure Pwc at a lowered rate of increase while preventing an unnecessarily large deceleration from being produced by the BA control. Therefore, the braking force control apparatus of the present embodiment is effective in achieving the functions of the BA control in an appropriate manner for all the ranges of the vehicle speed V without degrading the ride comfort of the vehicle occupant. 
     In the above-described embodiment, the determination as to whether the emergency braking operation of the brake pedal by the vehicle operator is an intentional operation is made based on the master cylinder pressure Pmc and the master cylinder pressure change rate dPmc. However, the basic parameter for making the determination according to the present invention is not limited to the master cylinder pressure Pmc and the master cylinder pressure change rate dPmc. 
     When the braking operation of the brake pedal  30  is performed, not only the master cylinder pressure Pmc, but also the braking operation force Fp on the brake pedal  30  or a stroke L of the brake pedal  30  varies in accordance with a quantity of the braking operation. Further, when the braking force is exerted on the vehicle as a result of the braking operation of the brake pedal  30 , a deceleration G of the vehicle is produced. By taking account of these factors, the determination as to whether the braking operation is an emergency braking operation or a normal braking operation, and the determination as to whether the braking operation is an intentional operation may be made based on any of the basic parameters including: (1) the master cylinder pressure Pmc; (2) the braking operation force Fp; (3) the brake pedal stroke L; (4) the vehicle deceleration G; (5) the estimated vehicle speed Vso; and (6) the wheel speed Vw. 
     Next, a description will be given of the second embodiment of the present invention, with reference to FIG.  6  and FIG.  7 . The second embodiment of the present invention is constituted by the ECU  10  of the braking force control apparatus of FIG. 1 when performing the control procedures of FIG.  6  and FIG.  7 . 
     FIG. 6 is a flowchart for explaining another brake-assist execution condition judgment procedure performed by the ECU  10  of the braking force control apparatus of the present embodiment. Similar to the procedure of FIG. 3, the procedure of FIG. 6 is started every time a braking operation of the brake pedal  30  is performed. When this procedure is performed for the first time after the detection of the braking operation of the brake pedal  30 , all settings of flags used in this procedure and values of parameters temporarily stored in this procedure are initialized. In FIG. 6, the steps which are the same as corresponding steps in FIG. 3 are designated by the same reference numerals, and a description thereof will be omitted. 
     In the procedure of FIG. 6, the steps  100  through  114  are performed in order to determine whether the brake-assist (BA) control execution conditions are satisfied, and to determine whether the time to start the execution of the brake-assist (BA) control is reached, similar to the corresponding steps in the procedure of FIG.  3 . When the result at the step  114  is affirmative (the time to start the execution of the BA control is reached), the ECU  10  performs a next step  150  of the procedure. 
     Step  150  determines whether a vehicle speed V (detected based on the signals supplied by the wheel speed sensors  86 ) is larger than a predetermined reference speed V 2 . The reference speed V 2  is a threshold value that is predetermined according to the experiments of the inventors. The reference speed V 2  in the present embodiment is used to make a determination as to whether the wheel cylinder pressure Pwc after the start of the BA control should be increased at a normal rate of increase or at a lower rate of increase. 
     When V&gt;V 2 , it is determined that the vehicle is running at a high speed, and the sensed deceleration when quickly increasing the braking force acting on the vehicle after the start of the BA control will not be so large as to degrade the ride comfort of the vehicle occupant. Therefore, when the result at the step  150  is affirmative (V&gt;V 2 ), the ECU  10  performs the step  118  which is the same as the step  118  of the procedure of FIG.  3 . After the execution of the BA regular control is started in the step  118 , the wheel cylinder pressure Pwc of each of the wheel cylinders  44  will be quickly increased toward the accumulator pressure Pacc. After the step  118  is performed, the procedure of FIG. 6 at the present cycle ends. 
     On the other hand, when the result at the step  150  is negative (V≦V 2 ), it is determined that the vehicle is running at a low speed and the sensed deceleration when quickly increasing the braking force acting on the vehicle after the start of the brake-assist (BA) control will be large enough to degrade the ride comfort of the vehicle occupant. Therefore, when the result at the step  150  is negative (V≦V 2 ), the ECU  10  performs a next step  152  of the procedure of FIG.  6 . 
     Step  152  sets a BA starting independent control start flag (XBASTI) to “1” (XBASTI←“1”). The BA starting independent control start flag XBASTI is set to “1” when starting the execution of an independently-controlled brake-assist (BA) control in the braking force control apparatus of the present embodiment. Hereinafter, the independently-controlled BA control, subsequently performed after the step  152  is performed, will be called the BA starting independent control. The BA starting independent control start flag XBASTI is reset to “0” when the ABS control execution conditions are satisfied during the BA control or when the execution of the BA control is terminated. After the step  152  is performed, the procedure of FIG. 6 at the present cycle ends. 
     FIG. 7 is a flowchart for explaining a brake-assist (BA) starting independent control procedure performed by the ECU  10  of the braking force control apparatus of FIG.  1 . 
     The BA starting independent control is performed by the ECU  10  wherein the time the braking force on the rear wheels RR and RL is increased by the start of the BA control is delayed from the time the braking force on the front wheels FR and FL is increased by the start of the BA control, in order to lower in an appropriate manner the rate of increase of the braking force accompanied by the start of the BA control. The BA starting independent control is achieved by the ECU  10  when performing a brake-assist (BA) starting independent control procedure shown in FIG.  7 . The control procedure shown in FIG. 7 is an interrupt-initiated routine which is periodically initiated at intervals of a predetermined time. As shown in FIG. 7, at the start of the BA starting independent control procedure, the ECU  10  performs step  160  of the procedure of FIG.  7 . 
     Step  160  determines whether the BA starting independent control start flag XBASTI is equal to 1. When the result at the step  160  is negative (the flag XBASTI is not equal to 1), the procedure of FIG. 7 at the present cycle ends, and the subsequent steps of the procedure of FIG. 7 are not performed. When the result at the step  160  is affirmative (XBASTI=1), the ECU  10  performs a next step  162  of the procedure of FIG.  7 . 
     Step  162  supplies the respective drive signals to the STR  26 , the SA- 1   46 , the SA- 2   48  and the SA- 3   54 . After the step  162  is performed, the brake fluid paths through which the accumulator pressure Pacc is supplied to each of the wheel cylinders  44  are opened by the STR  26 , the SA- 1   46 , the SA- 2   48  and the SA- 3   54 . After the step  162  is performed, the ECU  10  performs a next step  164  of the procedure of FIG.  7 . 
     Step  164  determines whether a timer T STI  is above a predetermined reference time T STIO . During the operation of the ECU  10 , the timer T STI  is automatically incremented to a given upper limit that is larger than the reference time T STIO . During the execution of the procedure of FIG. 7, the timer T STI  is reset to zero at a controlled timing. Only when the step  164  is performed for the first time after the start of the procedure of FIG. 7, the timer T STI  is set to the upper limit, and the result at the step  164  is affirmative (T STI ≧T STIO ). When the result at the step  164  is affirmative, the ECU  10  performs a next step  166  of the procedure of FIG.  7 . 
     Step  166  sets the front-right and front-left pressure-holding solenoids SFRH  50  and SFLH  52  in the valve-open positions (or the OFF states with no drive signal supplied to the solenoids  50  and  52 ), and sets the rear-right and rear-left pressure-holding solenoids SRRH  68  and SRLH  70  in the valve-closed positions by supplying the drive signals to the solenoids  68  and  70  (or the ON states). Only the front-wheel-related pressure-holding solenoids  50  and  52  open the pressure adjustment lines  56  and  62 , and the accumulator pressure Pacc is supplied to each of the front wheel cylinders  44 FR and  44 FL so that the wheel cylinder pressure Pwc of each of the front wheel cylinders  44 FR and  44 FL will be quickly increased. However, the wheel cylinder pressure Pwc of each of the rear wheel cylinders  44 RR and  44 RL is maintained at the same level by the rear-wheel-related pressure-holding solenoids  68  and  70 . After the step  166  is performed, the ECU  10  performs a next step  168  of the procedure of FIG.  7 . 
     Step  168  resets the timer T STI  to zero. After the step  168  is performed, the procedure of FIG. 7 at the present cycle ends. After the timer T STI  is reset to zero, the timer T STI  is automatically incremented from zero to the upper limit. In the subsequent cycles, the result at the step  164  will be negative (T STI &lt;T STIO ) until the reference time T STIO  elapses. When the result at the step  164  is negative, the ECU  10  performs a next step  170  of the procedure of FIG.  7 . 
     Step  170  determines whether the timer T STI  is above a delay time T ALL . The delay time T ALL  is predetermined to be smaller than the reference time T STIO  (T ALL &lt;T STIO ). In the present embodiment, the delay time T ALL  defines a period during which the increasing of the wheel cylinder pressure Pwc of each of the rear wheel cylinders  44 RR and  44 RL after the start of the BA control, is inhibited. The ECU  10  determines a delay time T ALL  based on the vehicle speed V by reading a map from the memory of the ECU  10 . In this map, as shown in FIG. 8, the delay time T ALL  will decrease as the vehicle speed V becomes high. The lower the vehicle speed V, the larger the delay time T ALL . In FIG. 8, “V 2 ” indicates a reference speed of the vehicle speed V, which corresponds to the threshold value used in the procedure of FIG.  6 . 
     When the result at the step  170  is negative (T STI &lt;T ALL ), it is determined that the delay time T ALL  has not yet elapsed after the timer T STI  is reset to zero. In this case, the procedure of FIG. 7 at the present cycle ends, and the subsequent steps are not performed. Hence, before the delay time T ALL  elapses after the timer T STI  is reset to zero, the solenoids  50  and  52  are set in the valve-open positions so as to allow the increasing of the wheel cylinder pressure Pwc of each of the front wheel cylinders  44 FR and  44 FL, and the solenoids  68  and  70  are set in the valve-closed positions so as to maintain the wheel cylinder pressure Pwc of each of the rear wheel cylinders  44 RR and  44 RL at the same level. On the other hand, when the result at the step  170  is affirmative (T STI ≧T ALL ), it is determined that the delay time T STI  elapses after the timer T STI  is reset to zero. In this case, the ECU  10  performs a next step  172  of the procedure of FIG.  7 . 
     Step  172  sets all the pressure-holding solenoids SH in the valve-open positions (or the OFF states with no drive signal being supplied to the pressure-holding solenoids SH). The pressure-holding solenoids SH open the pressure adjustment lines  56 ,  62 ,  72  and  78 , and the supply of the accumulator pressure Pace to each of the wheel cylinders  44  is allowed, and the wheel cylinder pressure Pwc of each of the wheel cylinders  44  is quickly increased toward the accumulator pressure Pacc. After the step  172  is performed, the ECU  10  performs a next step  174  of the procedure of FIG.  7 . 
     Step  174  sets the timer T STI  to the delay time T ALL  (T STI ←T ALL ). After the step  174  is performed, the procedure of FIG. 7 at the present cycle ends. The timer T ALL  is automatically incremented after the step  174  is performed. In the subsequent cycles the result at the step  170  will be affirmative (T STI ≧T ALL ) until the flag XBASTI is reset to zero. The steps  160 - 164  and the steps  170 - 174  are continuously repeated for such a duration so that the wheel cylinder pressure Pwc is quickly increased toward the accumulator pressure Pacc. 
     In the above-described procedure of FIG. 7, after the flag XBASTI is set to 1, only the wheel cylinder pressure Pwc of each of the front wheel cylinders  44 FR and  44 FL is increased toward the accumulator pressure Pacc until the delay time T ALL  elapses after the timer T STI  is reset to zero. When the delay time T ALL  passed but the flag XBASTI is not reset to zero, the supply of the accumulator pressure Pacc to all the wheel cylinders  44  is allowed by the pressure-holding solenoids SH and the wheel cylinder pressure Pwc of each of the wheel cylinders  44  is quickly increased toward the accumulator pressure Pacc by the pressure-holding solenoids SH. In this manner, the time to start increasing the braking force on the rear wheels RR and RL after the start of the BA control is delayed from the time to start increasing the braking force on the front wheels FR and FL after the start of the BA control. Therefore, by performing the BA starting independent control procedure of FIG. 7, it is possible for the braking force control apparatus of the present embodiment to lower in an appropriate manner the rate of increase of the entire braking force accompanied by the start of the BA starting independent control, in comparison with the rate of increase of the braking force accompanied by the start of the BA regular control. 
     As described above, the braking force control apparatus of the present embodiment can increase the wheel cylinder pressure Pwc toward the accumulator pressure Pacc at a lowered rate of increase when an emergency braking operation is performed during a low-speed running of the vehicle. As shown in FIG. 8, the delay time T ALL  between the time to start increasing the braking force on the rear wheels RR and RL after the start of the BA control and the time to start increasing the braking force on the front wheels FR and FL after the start of the BA control will increase as the vehicle speed V becomes low. Therefore, the braking force control apparatus of the present embodiment is effective in achieving the functions of the BA control for all the ranges of the vehicle speed V without degrading the ride comfort of the vehicle occupant. 
     In the above-described embodiment, by performing the BA starting independent control procedure of FIG. 7 when an emergency braking operation is detected, the braking force control apparatus generates a relatively large braking force on the front wheels FL and FR and a relatively small braking force on the rear wheels RL and RR. The braking force control apparatus of the present embodiment is effective in providing a vehicle running stability when the emergency braking operation is performed. 
     In the above-described procedure of FIG. 7, in order to lower the rate of increase of the braking force on the rear wheels RL and RR, the time to start increasing the braking force on the rear wheels RR and RL is delayed from the time to start increasing the braking force on the front wheels FR and FL. However, the present invention is not limited to this embodiment. Alternatively, another method of lowering the rate of increase of the wheel cylinder pressure Pwc of each rear wheel cylinder may be used. Further, in the above-described procedure of FIG. 7, the delay time T ALL  which defines a period during which the increasing of the wheel cylinder pressure Pwc of each rear wheel cylinder, after the start of the BA control, is inhibited is determined based on the vehicle speed V. However, the present invention is not limited to this embodiment. Alternatively, the delay time T ALL  may be preset to a constant value. 
     Next, a description will be given of another embodiment of the braking force control apparatus of the present invention. 
     FIG. 9 is a system block diagram of a braking force control apparatus to which one of a third embodiment and a fourth embodiment of the present invention is applied. For the sake of simplicity of description, a configuration of the braking force control apparatus having only one wheel cylinder provided for only one wheel of an automotive vehicle is illustrated in FIG.  9 . 
     As shown in FIG. 9, the braking force control apparatus of the present embodiment is controlled by an electronic control unit  200  (hereinafter, called ECU  200 ). The braking force control apparatus of FIG. 9 includes a brake pedal  202 . A brake switch  203  is provided in the vicinity of the brake pedal  202 . When the brake pedal  202  is depressed by the vehicle operator, the brake switch  203  outputs an ON signal to the ECU  200 . The ECU  200  determines whether the braking operation is performed by the vehicle operator, based on the signal supplied by the brake switch  203 . 
     The brake pedal  202  is connected to a vacuum booster  204 . The vacuum booster  204  serves to increase the braking operation force of the brake pedal  202  by using an intake pressure of air into an internal combustion engine of the vehicle. A master cylinder  206  is fixed to the vacuum booster  204 . When the brake pedal  202  is depressed, a resultant force of the braking operation force Fp, exerted on the brake pedal  202 , and a brake-assist force Fa, produced by the vacuum booster  204 , is transmitted from the vacuum booster  204  to the master cylinder  206 . 
     The master cylinder  206  includes a pressure chamber provided therein. A reservoir tank  208  is provided on the top of the master cylinder  206 . When the braking operation force on the brake pedal  202  is released by the vehicle operator, the reservoir tank  208  is connected to or communicates with the pressure chamber of the master cylinder  206 . When the brake pedal  202  is depressed by the vehicle operator, the reservoir tank  208  is disconnected from or isolated from the pressure chamber of the master cylinder  206 . Hence, the pressure chamber of the master cylinder  206  is replenished with brake fluid from the reservoir tank  208  every time the braking operation force on the brake pedal  202  is released by the vehicle operator. 
     A hydraulic pressure line  210  is connected to the pressure chamber of the master cylinder  206 . A hydraulic pressure sensor  212  is provided at an intermediate portion of the pressure line  210 . The hydraulic pressure sensor  212  outputs a signal, indicative of the master cylinder pressure Pmc, to the ECU  10 . The ECU  200  detects the master cylinder pressure Pmc, produced in the master cylinder  206 , based on the signal supplied by the hydraulic pressure sensor  212 . 
     A pressure-holding solenoid  216  (hereinafter called SH  216 ) is provided in the pressure line  210 . The SH  216  is a two-position solenoid valve which is normally set in a valve-open position so as to connect the master cylinder  206  to a wheel cylinder  218 . When a drive signal is supplied to the SH  216  by the ECU  200 , the SH  216  is set in a valve-closed position so as to disconnect the master cylinder  206  from the wheel cylinder  218 . 
     The wheel cylinder  218  is connected on the downstream side of the SH  216  to the pressure line  210 . A pressure-reducing solenoid  220  (hereinafter called SR  220 ) is also connected on the downstream side of the SH  216  to the pressure line  210 . The SR  220  is a two-position solenoid valve which is normally set in a valve-closed position so as to inhibit a flow of the brake fluid from the wheel cylinder  218  to a downstream portion of the pressure line  210  via the SR  220 . When a drive signal is supplied to the SR  220  by the ECU  200 , the SR  220  is set in a valve-open position so as to allow the flow of the brake fluid from the wheel cylinder  218  to the downstream portion of the pressure line  210  via the SR  220 . In addition, a check valve  222  is provided in a bypass line of the pressure line  210  around the SH  216 , and the bypass line is connected to the wheel cylinder  218 . The check valve  222  allows only a flow of the brake fluid from the wheel cylinder  218  to the pressure line  210 , and inhibits a counter flow of the brake fluid from the pressure line  210  to the wheel cylinder  218 . 
     A wheel speed sensor  219  is provided in the vicinity of the wheel of the vehicle for which the wheel cylinder  218  is provided. The wheel speed sensor  219  outputs a signal, indicative of a wheel speed of the vehicle, to the ECU  200 . The ECU  200  detects the wheel speed of the vehicle wheel based on the signal supplied by the wheel speed sensor  219 . 
     A reservoir  224  is connected to the pressure line  210  on the downstream side of the SR  220 . When the SR  220  is set in the valve-open position, the brake fluid from the SR  220  flows into the reservoir  224 , and stored in the reservoir  224 . In the reservoir  224 , a certain amount of brake fluid is initially stored. A pump  226  is provided in the pressure line  210 , and has an inlet port  226   a  which is connected to the reservoir  224 . The pump  226  has an outlet port  226   b  which is connected to a check valve  228  in the pressure line  210 . The check valve  228  is connected to the upstream side of the SH  216  through the pressure line  210 . The check valve  228  allows only a flow of the brake fluid from the outlet port  216   b  of the pump  226  to the upstream side of the SH  216  in the pressure line  210 , and inhibits a counter flow of the brake fluid from the upstream side of the SH  216  to the outlet port  226   b  of the pump  226 . 
     An intake pressure line  230  and a pressure adjustment line  232  are connected to the vacuum booster  204 . An intake pipe of the engine or the like is connected to the intake pressure line  230 , and an intake pressure from the intake pipe is delivered through the intake pressure line  230  to the vacuum booster  204 . The pressure adjustment line  232  is connected to both an intake pressure valve  234  and an atmospheric pressure valve  236 . The intake pressure valve  234  is provided between the intake pressure line  230  and the pressure adjustment line  232 . The intake pressure valve  234  is a two-position solenoid valve which is normally set in a valve-open position so as to connect the intake pressure line  230  and the pressure adjustment line  232 . When a drive signal is supplied to the valve  234  by the ECU  200 , the valve  234  is set in a valve-closed position so as to disconnect the pressure adjustment line  232  from the intake pressure line  230 . The atmospheric pressure valve  236  is provided between the pressure adjustment line  232  and an atmospheric pressure line which is open to the atmosphere. The atmospheric pressure valve  236  is a two-position solenoid valve which is normally set in a valve-closed position so as to disconnect the pressure adjustment line  232  from the atmospheric pressure line. When a drive signal is supplied to the valve  236  by the ECU  200 , the valve  236  is set in a valve-open position so as to connect the pressure adjustment line  232  and the atmospheric pressure line. 
     The vacuum booster  204  includes an intake pressure chamber and a pressure adjusting chamber both provided therein. In the vacuum booster  204 , the intake pressure chamber and the pressure adjusting chamber are separated from each other by a diaphragm. The intake pressure chamber is connected to the intake pressure line  230 . When the vehicle is normally running, the intake pressure chamber of the vacuum booster  204  is held at a vacuum pressure of the intake pressure of the intake pressure line  230 . The pressure adjusting chamber of the vacuum booster  204  is connected to the pressure adjustment line  232  through a valve device. The valve device is provided in the vacuum booster  204  to adjust an internal pressure of the pressure adjusting chamber in accordance with the braking operation of the brake pedal  202 . 
     The operation of the valve device of the vacuum booster  204  will now be described. When the intake pressure from the intake pressure valve  234  is supplied to the pressure adjustment line  232 , the valve device connects the pressure adjusting chamber to the pressure adjustment line  232  until a difference in pressure between the pressure adjusting chamber and the intake pressure chamber is produced in proportion to the braking operation force Fp on the brake pedal  202  by the vehicle operator. An actuating force which is proportional to the difference in pressure between the pressure adjusting chamber and the intake pressure chamber (or in proportion to the braking operation force Fp) is exerted on the diaphragm between the pressure adjusting chamber and the intake pressure chamber. Therefore, when the brake pedal  202  is depressed, the brake-assist force Fa is produced by the vacuum booster  204  in accordance with the actuating force on the diaphragm, so that a resultant force of the braking operation force Fp and the brake-assist force Fa is transmitted from the vacuum booster  204  to the master cylinder  206 . 
     On the other hand, when the atmospheric pressure from the atmospheric pressure valve  236  is supplied to the pressure adjusting line  232 , the valve device of the vacuum booster  204  connects the pressure adjusting chamber to the pressure adjustment line  232  so that the atmospheric pressure is supplied to the pressure adjusting chamber by the valve device, regardless of whether the braking operation force Fp on the brake pedal  202 . An actuating force which is proportional to the difference in pressure between the pressure adjusting chamber and the intake pressure chamber is exerted on the diaphragm between the pressure adjusting chamber and the intake pressure chamber. At this time, a maximum brake-assist force FaMAX is produced in accordance with the actuating force on the diaphragm by the vacuum booster  204 . 
     Next, a description will be given of the operation of the braking force control apparatus of the present embodiment. 
     Similar to the ECU  10  of FIG. 1 in the previous embodiment, the ECU  200  of FIG. 9 in the present embodiment starts the control procedure of FIG. 3 when a braking operation of the brake pedal  202  is performed. The control procedure of FIG. 3 is performed in order to make a determination as to whether the brake-assist (BA) control execution conditions are satisfied by the braking operation. Namely, after the brake pedal  202  is depressed by the vehicle operator, the ECU  200  determines whether the BA control execution conditions are satisfied by the braking operation, based on the master cylinder pressure Pmc and the master cylinder pressure change rate dPmc. When the ECU  200  determines that the BA control execution conditions are not satisfied, the normal control is executed. When the ECU  200  determines that the BA control execution conditions are satisfied, the execution of one of (1) the brake-assist (BA) regular control and (2) the brake-assist (BA) starting specific control is started in accordance with the vehicle speed V. 
     When the normal control is performed by the ECU  200  of the braking force control apparatus of the present embodiment, the intake pressure valve  234  is set in the valve-open position (or the OFF state) and the atmospheric pressure valve  236  are set in the valve-closed position (or the OFF state). In this condition, the vacuum booster  204  produces the brake-assist force Fa in accordance with the braking operation force Fp as described above. A resultant force of the braking operation force Fp and the brake-assist force Fa is transmitted from the vacuum booster  204  to the master cylinder  206 . In addition, when the resultant force of the braking operation force Fp and the brake-assist force Fa is transmitted to the master cylinder  206 , the master cylinder  206  produces a master cylinder pressure Pmc that is equal to the braking operation force Fp multiplied by a given magnification factor. 
     When the operating condition of the vehicle is found stable, the ECU  200  maintains the pump  226  in the stopped condition, sets the SH  216  in the valve-open position, and sets the SR  220  in the valve-closed position. Hereinafter, this condition of the hydraulic circuit related to the wheel cylinder  218  will be called the normal condition. When the hydraulic circuit related to the wheel cylinder  218  is placed in the normal condition, the master cylinder pressure Pmc from the master cylinder  206  is supplied to the wheel cylinder  218  through the SH  216 . Hence, during the normal control, the wheel cylinder  218  generates a braking force on the vehicle wheel in accordance with the braking operation force Fp on the brake pedal  202 . 
     When the slip ratio S of the vehicle wheel is found to be above a reference value after the braking operation is performed in the braking force control apparatus of the present embodiment, it is determined that the ABS control execution conditions are satisfied. After this determination is made, the execution of the ABS control of the braking force control apparatus is started by the ECU  200  in the present embodiment, similar to the ECU  10  in the previous embodiment. When the brake pedal  202  is depressed, or when the master cylinder pressure Pmc is increased to an adequately high pressure, the ABS control is achieved by the ECU  200 . That is, during the ABS control, the ECU  200  starts the operation of the pump  226 , and supplies the drive signals to the SH  216  and the SR  220  in the following manner. 
     During the ABS control of the present embodiment, if the adequately increased master cylinder pressure Pmc is supplied by the master cylinder  206 , the ECU may control the SH  216  and the SR  220  such that the SH  216  is set in the valve-open position and the SR  220  is set in the valve-closed position. When the ECU  200  performs this control procedure, the wheel cylinder pressure Pwc of the wheel cylinder  218  is increased up to the master cylinder pressure Pmc. Hereinafter, this control procedure will be called (1) a pressure-increasing control mode. 
     Alternatively, during the ABS control of the present embodiment, the ECU  200  may control the SH  216  and the SR  220  such that the SH  216  is set in the valve-closed position and the SR  220  is set in the valve-closed position. When the ECU  200  performs this control procedure, the wheel cylinder pressure Pwc of the wheel cylinder  218  is maintained at the same level without increase or decrease. Hereinafter, this control procedure will be called (2) a pressure-holding control mode. 
     Alternatively, during the ABS control of the present embodiment, the ECU  200  may control the SH  216  and the SR  220  such that the SH  216  is set in the valve-closed position and the SR  220  is set in the valve-open position. When the ECU  200  performs this control procedure, the wheel cylinder pressure Pwc of the wheel cylinder  218  is decreased. Hereinafter, this control procedure will be called (3) a pressure-decreasing control mode. 
     In the braking force control apparatus of the present embodiment, the ECU  200  suitably performs one of (1) the pressure-increasing control mode, (2) the pressure-holding control mode and (3) the pressure-decreasing control mode so as to maintain the slip ratio S of the vehicle wheel below the reference value, preventing the vehicle wheel from being locked during the braking operation. 
     It is necessary to quickly decrease the wheel cylinder pressure Pwc of the wheel cylinder  218  after the vehicle operator releases the braking operation force on the brake pedal  202  during the ABS control. In the braking force control apparatus of the present embodiment, the check valve  222  is provided in the bypass line connected to the wheel cylinder  218  so as to allow only the flow of the brake fluid from the wheel cylinder  218  to the pressure line  210 . As the check valve  222  functions in this manner, it is possible for the braking force control apparatus of the present embodiment to quickly decrease the wheel cylinder pressure Pwc after the vehicle operator releases the braking operation force on the brake pedal  202  during the ABS control. 
     During the ABS control of the braking force control apparatus of the present embodiment, the wheel cylinder pressure Pwc of the wheel cylinder  218  is suitably adjusted by supplying the master cylinder pressure Pmc from the master cylinder  206  to the wheel cylinder  218 . When the brake fluid from the master cylinder  206  is delivered to the wheel cylinder  218 , the wheel cylinder pressure Pwc is increased, and, when the brake fluid within the wheel cylinder  218  is delivered to the reservoir  224 , the wheel cylinder pressure Pwc is decreased. If the increase of the wheel cylinder pressure Pwc is performed by using the master cylinder  206  as the only brake fluid pressure source, the brake fluid contained in the master cylinder  206  is gradually decreased through a repeated execution of the pressure-increasing control mode and the pressure-decreasing control mode. However, in the present embodiment, the brake fluid contained in the reservoir  224  is returned back to the master cylinder  206  by the pump  226 . Therefore, it is possible to prevent the master cylinder  206  from malfunctioning due to a too small amount of the brake fluid contained therein even when the ABS control is continuously performed over an extended period of time. 
     Next, a description will be given of the operation of the braking force control apparatus of the present embodiment when the brake-assist (BA) control is performed by the ECU  200 . 
     When the vehicle is running at a high speed above the reference speed V 1  and an emergency braking operation is performed, the ECU  200  starts the execution of the BA control by performing the BA regular control. In the present embodiment, the BA regular control is achieved by the ECU  200  by setting the intake pressure valve  234  in the valve-closed position, setting the atmospheric pressure valve  236  in the valve-open position, setting the SH  216  in the valve-open position, setting the SR  220  in the valve-closed position, and stopping the operation of the pump  226 . 
     When the hydraulic circuit related to the wheel cylinder  218  is placed in the above-mentioned condition, the atmospheric pressure from the atmospheric pressure valve  236  is supplied to the pressure adjustment line  232 . When the atmospheric pressure is supplied to the pressure adjustment line  232 , the atmospheric pressure is supplied to the pressure adjusting chamber of the vacuum booster  204  by the valve device thereof, and the vacuum booster  204  produces the maximum brake-assist force FaMAX. A resultant force of the braking operation force Fp and the maximum brake-assist force FaMAX is transmitted to the master cylinder  206  by the vacuum booster  204 . The master cylinder pressure Pmc from the master cylinder  206  is supplied to the wheel cylinder  218  through the SH  216 . Hence, after the start of the BA regular control, the wheel cylinder pressure Pwc of the wheel cylinder  218  can be quickly increased in accordance with a change of the force transmitted to the master cylinder  206  from the resultant force “Fa+Fp” to the resultant force “FaMAX+Fp”. 
     In the braking force control apparatus of the present embodiment, when an emergency braking operation of the brake pedal  202  is performed, it is possible to quickly increase the wheel cylinder pressure Pwc of the wheel cylinder  218  to the adequately high level. Therefore, in the braking force control apparatus of the present embodiment, after the condition requiring the emergency braking has occurred, it is possible to quickly generate an increased braking force larger than that generated during the normal control even if the vehicle operator is a beginner. 
     When the vehicle is running at a low speed below the reference speed V 1  and an emergency braking operation is performed, the ECU  200  starts the execution of the BA control by performing the BA starting specific control. In the present embodiment, the BA starting specific control is achieved by the ECU  200  by setting the intake pressure valve  234  in the valve-closed position, setting the atmospheric pressure valve  236  in the valve-open position, stopping the operation of the pump  226 , and performing a brake-assist starting specific control procedure shown in FIG.  10 . 
     FIG. 10 is a flowchart for explaining a brake-assist (BA) starting specific control procedure performed by the ECU  200  of the braking force control apparatus of FIG.  9 . The third embodiment of the present invention is constituted by the ECU  200  of the braking force control apparatus of FIG. 9 when performing the control procedures of FIG.  3  and FIG. 10, which will now be described. 
     The BA starting specific control procedure of FIG. 10 is performed in order to lower in an appropriate manner the rate of increase of the wheel cylinder pressure Pwc accompanied by the start of the BA control. The control procedure of FIG. 10 is essentially the same as the control procedure of FIG. 4 except the step  132  shown in FIG. 4 is not included in the control procedure of FIG.  10 . In FIG. 10, the steps which are the same as corresponding steps in FIG. 4 are designated by the same reference numerals. Hence, a description of each of the respective steps of the control procedure of FIG. 10 will be omitted. 
     In the BA starting specific control procedure of FIG. 10, after the flag XBASTS is set to 1, the wheel cylinder pressure Pwc is continuously increased toward the accumulator pressure Pacc until the hold time T HOLD  elapses after the timer T STS  is reset to zero. When the hold time T HOLD  passed but the reference time T STS0  has not yet elapsed, the supply of the accumulator pressure Pacc to the wheel cylinder  218  is stopped and the wheel cylinder pressure Pwc is maintained at the same level. In this manner, the pressure increasing of the wheel cylinder  218  and the pressure holding of the wheel cylinder  218  are repeated every time the reference time T STS  elapses after the timer T STS  is reset to zero. Therefore, by performing the BA starting specific control procedure of FIG. 10, it is possible to lower in an appropriate manner the rate of increase of the braking force accompanied by the start of the BA starting specific control, in comparison with the rate of increase of the braking force accompanied by the start of the BA regular control. Hence, when an emergency braking operation of the brake pedal  202  is performed during a low-speed running of the vehicle, the braking force control apparatus of the present embodiment can increase the wheel cylinder pressure Pwc at a lowered rate of increase while preventing an unnecessarily large deceleration from being produced by the start of the BA control. The braking force control apparatus of the present embodiment is effective in achieving the functions of the BA control without degrading the ride comfort of the vehicle occupant. 
     After the wheel cylinder pressure Pwc is increased by the start of the BA control as described above, an increased braking force is exerted on the vehicle, and a relatively large slip ratio S of the vehicle wheel is produced. Then, the ECU  200  determines that the ABS control execution conditions are satisfied by such an operating condition of the vehicle. After this determination is made, the ECU  200  starts the execution of the ABS control of the braking force controlling apparatus of the present embodiment. As described above, the ECU  200  suitably performs one of (1) the pressure-increasing control mode, (2) the pressure-holding control mode and (3) the pressure-decreasing control mode so as to maintain the slip ratio S of the vehicle wheel below the reference value, preventing the vehicle wheel from being locked during the braking operation. 
     In the braking force controlling apparatus of the present embodiment, when a braking operation force Fp is exerted on the brake pedal  202  by the vehicle operator after the start of the brake-assist control, the master cylinder pressure Pmc is maintained at a level in accordance with the resultant force “FAMAX+Fa” supplied by the vacuum booster  204 . When the braking operation force on the brake pedal  202  is released by the vehicle operator after the start of the brake-assist control, the master cylinder pressure Pmc is decreased to a level in accordance with the maximum brake-assist force “FaMAX” supplied by the vacuum booster  204 . 
     The ECU  200  monitors the signal supplied by the hydraulic pressure sensor  212 , and makes a determination as to whether the braking operation force on the brake pedal  202  is released by the vehicle operator, based on the signal supplied by the hydraulic pressure sensor  212 . When it is determined that the braking operation force is released, the ECU  200  stops supplying the drive signals to the valves  234  and  236  and terminates the brake-assist control. 
     Similar to the previous embodiment of FIG. 1, the braking force controlling apparatus of the present embodiment can quickly generate a large braking force on the vehicle when an emergency braking operation is performed during a high-speed running of the vehicle. Further, when an emergency braking operation is performed during a low-speed running of the vehicle, the braking force control apparatus of the present embodiment can increase the wheel cylinder pressure Pwc at a lowered rate of increase while preventing an unnecessarily large deceleration from being produced by the start of the BA control. Therefore, the braking force control apparatus of the present embodiment is effective in achieving the functions of the BA control for all the ranges of the vehicle speed V without degrading the ride comfort of the vehicle occupant. 
     FIG. 11 is a flowchart for explaining a brake-assist (BA) starting independent control procedure performed by the ECU  200  of the braking force control apparatus of FIG.  9 . The fourth embodiment of the present invention is constituted by the ECU  200  of the braking force control apparatus of FIG. 9 when performing the control procedures of FIG.  6  and FIG. 11, which will now be described. 
     In the present embodiment, the ECU  200  performs the normal control when it is determined that an emergency braking operation is not performed. Further, (1) when the vehicle is running at a high speed above the reference speed V 2  and an emergency braking operation is performed, the ECU  200  starts the execution of the BA control by performing the BA regular control procedure. Further, (2) when the vehicle is running is running a low speed below the reference speed V 2  and an emergency braking operation is performed, the ECU  200  starts the execution of the BA control by performing the BA starting independent control procedure. 
     The normal control and the BA regular control are achieved by the ECU  200  in the present embodiment in the same manner as the ECU  200  in the above-described third embodiment. Hence, a description of these control procedures will be omitted. The BA starting independent control is achieved by the ECU  200  by setting the intake pressure valve  234  in the valve-closed position (or the ON state), setting the atmospheric pressure valve  236  in the valve-open position (or the On state), stopping the operation of the pump  226 , and performing the control procedure of FIG.  11 . 
     The BA starting independent control procedure of FIG. 11 is performed by the ECU  200  wherein the time the braking force on the rear wheels RR and RL is increased by the start of the BA control is delayed from the time the braking force on the front wheels FR and FL is increased by the start of the BA control, in order to lower in an appropriate manner the rate of increase of the braking force accompanied by the start of the BA control. The BA starting independent control is achieved by the ECU  200  when performing the BA starting independent control procedure of FIG.  11 . The control procedure of FIG. 11 is essentially the same as the control procedure of FIG. 7 except the step  162  shown in FIG. 7 is not included in the control procedure of FIG.  11 . In FIG. 11, the steps which are the same as corresponding steps in FIG. 7 are designated by the same reference numerals. Hence, a description of each of the respective steps of the control procedure of FIG. 11 will be omitted. 
     In the BA starting independent control procedure of FIG. 11, after the flag XBASTI is set to 1, only the wheel cylinder pressure Pwc of each of the front wheel cylinders  44 FR and  44 FL is increased toward the accumulator pressure Pacc until the delay time T ALL  elapses after the timer T STI  is reset to zero. When the delay time T ALL  passed but the flag XBASTI is not reset to zero, the supply of the accumulator pressure Pacc to all the wheel cylinders  44  is allowed by the pressure-holding solenoids SH and the wheel cylinder pressure Pwc of each of the wheel cylinders  44  is quickly increased toward the accumulator pressure Pacc by the pressure-holding solenoids SH. In this manner, the time to start increasing the braking force on the rear wheels RR and RL after the start of the BA control is delayed from the time to start increasing the braking force on the front wheels FR and FL after the start of the BA control. Therefore, by performing the BA starting independent control procedure of FIG. 11, it is possible for the braking force control apparatus of the present embodiment to lower in an appropriate manner the rate of increase of the entire braking force accompanied by the start of the BA starting independent control, in comparison with the rate of increase of the braking force accompanied by the start of the BA regular control. 
     Similar to the previous embodiments, the braking force control apparatus of the present embodiment is effective in achieving the functions of the BA control for all the ranges of the vehicle speed V without degrading the ride comfort of the vehicle occupant. Further, in the present embodiment, by performing the BA starting independent control procedure of FIG. 11 when an emergency braking operation is detected, the braking force control apparatus initially generates a relatively large braking force on the front wheels FL and FR and a relatively small braking force on the rear wheels RL and RR. The braking force control apparatus of the present embodiment is effective in providing a vehicle running stability when the emergency braking operation is performed. 
     As described above, the braking force control apparatus of the present invention can change a rate of increase of the braking force produced after the start of the brake-assist control, so as to prevent an unnecessarily large deceleration from being produced by the start of the brake-assist control during a low-speed running of the vehicle. Hence, the braking force control apparatus of the present invention is effective in achieving the functions of the brake-assist control in an appropriate manner for all the ranges of the vehicle speed without degrading the ride comfort of the vehicle occupant. 
     In addition, the braking force control apparatus of the present invention can safely lower the sensed deceleration of the vehicle occupant when the brake-assist control is started during a low-speed running of the vehicle. Hence, the braking force control apparatus of the present invention is effective in achieving the functions of the brake-assist control while providing a good ride comfort of the vehicle occupant for all the ranges of the vehicle speed. 
     Further, the braking force control apparatus of the present invention can change a rate of increase of the braking force exerted on the rear wheels after the start of the brake-assist control, and can change a rate of increase of the entire braking force exerted on the vehicle after the start of the brake-assist control in accordance with the vehicle speed. Hence, the braking force control apparatus of the present invention is effective in achieving the functions of the brake-assist control in an appropriate manner for all the ranges of the vehicle speed without degrading the ride comfort of the vehicle occupant. 
     Further, the braking force control apparatus of the present invention can delay the time to start increasing the braking force on the rear wheels after the start of the brake-assist control from the time to start increasing the braking force on the front wheels after the start of the brake-assist control, and can change a rate of increase of the entire braking force exerted on the vehicle after the start of the brake-assist control in accordance with the vehicle speed. Hence, the braking force control apparatus of the present invention is effective in providing a vehicle running stability when the emergency braking operation is performed. 
     Further, the present invention is not limited to the above-described embodiments, and variations and modifications may be made without departing from the scope of the present invention.