Abstract:
A braking control apparatus for vehicles is provided with a braking control for performing such control that when deceleration d determined by a deceleration determining device is not less than a predetermined value f(v) according to vehicle speed v determined by a vehicle speed determining device, a braking force exerted on a rear wheel is limited as compared with a braking force exerted on a front wheel. This predetermined value f(v) is set so as to become smaller with increase in the vehicle speed v.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a braking control apparatus and method for vehicles and, more particularly, to the apparatus and method for controlling braking forces exerted respectively on the front wheels and on the rear wheels of an automobile. 
     2. Related Background Art 
     There are known braking controllers for controlling the braking forces distributed to the front wheels and to the rear wheels during braking. The technology disclosed in Japanese Patent Application Laid-Open No. 5-213169 is such a technique that when the deceleration reaches a predetermined value during braking, the hydraulic pressure to transmit the braking force to the rear wheels is maintained at the level at that time. The above application describes that this technique can prevent the rear wheels from locking prior to the front wheels and thus effectively prevent a sideslip of the vehicle during braking. 
     SUMMARY OF THE INVENTION 
     However, the braking force exerted on the rear wheels differs depending upon conditions of the vehicle even at fixed deceleration, and it is not always certain that the rear wheels lock. If the braking force is limited in spite of some margin of braking force left before locking of the rear wheels, the braking distance will become longer because of insufficient braking force. 
     In view of the above problem, an object of the present invention is to provide a braking control apparatus and method for vehicles satisfying both braking performance and stability of vehicle. 
     A braking control apparatus for vehicles according to the present invention comprises (1) vehicle speed determining means for determining a speed v of a vehicle, (2) deceleration determining means for determining a deceleration d of the vehicle, and (3) a braking control means for performing such control that when the deceleration d is not less than a predetermined value f(v) set according to the vehicle speed v so as to become smaller with increase in the vehicle speed, a braking force exerted on a rear wheel is limited as compared with a braking force exerted on a front wheel. 
     Further, a braking control method for vehicles according to the present invention comprises steps of (1) determining a speed v and a deceleration d of a vehicle, (2) determining if the deceleration d determined satisfies such a control condition that the deceleration d is not less than a predetermined value f(v) set according to the vehicle speed so as to become smaller with increase in the vehicle speed, and (3) performing such control that when said control condition is satisfied, a braking force exerted on a rear wheel is limited as compared with a braking force exerted on a front wheel. 
     In the apparatus and method according to the present invention, the braking force is controlled based on such control that the braking force exerted on the rear wheel is limited as compared with the braking force exerted on the front wheel when the deceleration is not less than the predetermined value according to the vehicle speed. The vehicle weight is shifted more toward the front wheel with increase in the deceleration. In the present invention, the braking force distributed to the front wheel is increased in the large deceleration region, thereby stabilizing the vehicle behavior. The predetermined value is set so as to become smaller with increase in the vehicle speed, whereby the braking-force control is carried out even at small decelerations in the high speed region where the vehicle tends to become instable, in order to keep the vehicle stable, and whereby the braking-force control is not carried out before the deceleration becomes relatively large, in the low speed region where the vehicle is relatively stable, in order to ensure the braking performance. 
     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram to show the structure of the braking control apparatus for vehicles of a first embodiment according to the present invention; 
     FIG. 2 is a diagram to show the structure of a braking system of a vehicle equipped with the control apparatus of FIG. 1; 
     FIG. 3 is a flowchart to show proportioning control carried out by the apparatus of FIG. 1; 
     FIG. 4 is a diagram to illustrate a high-speed proportioning control region; and 
     FIG. 5A to FIG. 5C are graphs to show temporal changes in the vehicle speed, the deceleration, and braking pressure, respectively, during the braking control carried out by the vehicle braking control apparatus according to the present invention. 
     FIG. 6 is a diagram to show the structure of the braking control apparatus for vehicles of a second embodiment according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A preferred embodiment of the present invention will be described hereinafter with reference to the accompanying drawings. FIG. 1 is a block diagram showing a main part of a vehicle employing a first embodiment of a braking control apparatus in accordance with the present invention. FIG. 2 is a schematic view showing the configuration of a brake actuator in the vehicle of FIG.  1 . 
     The braking control method in accordance with the present invention is carried out by a brake control unit  1  shown in FIG.  1 . Respective output signals from a stop switch  41  for detecting driver&#39;s ON/OFF operations of a brake pedal  4 , a master cylinder pressure sensor  21  for detecting the hydraulic pressure of a master cylinder  30  which will be mentioned later, a wheel speed sensor  42  for detecting the wheel speed of each wheel, an engine speed sensor  43  for detecting the engine speed, a parking brake switch  44  for detecting the ON/OFF state of parking brake, and a shift position sensor  45  for detecting the shift state of the transmission are fed into the brake control unit  1 . The brake control unit  1  is configured so as to carry out so-called Electronic Braking Distribution or EBD control, by controlling the brake actuator  2 , for regulating the hydraulic pressures applied to wheel cylinders  50 FR,  50 FL,  50 RR,  50 RL of brakes attached to their corresponding wheels, thereby being able to independently control the respective braking forces exerted on the wheels. 
     As shown in FIG. 2, the brake actuator  2  is connected to the master cylinder  30 , into which the stepping force of the driver exerted on the brake pedal  4  is fed after being amplified by a brake booster  32 . 
     The brake actuator  2  has two systems having substantially the same configuration, i.e., a system connected to the front left wheel cylinder  50 FL and rear right wheel cylinder  50 RR, and a system connected to the front right wheel cylinder  50 FR and rear left wheel cylinder  50 RL; whereas the master cylinder pressure sensor  21  is connected to only one of the systems. 
     Each system (identified with a or b) has one piece each of master cylinder (M/C) cut solenoid valve  21  and suction solenoid valve  22 , two pieces each of holding solenoid valves  23  and pressure reducing solenoid valves  24  provided so as to correspond to the respective wheel cylinders  50 , and one piece each of pump  25  and reservoir  26 . (The constituents provided one by one in each system are identified with a or b as with the respective system, whereas those provided so as to correspond to the wheel cylinders  50  are identified with letters FL, FR, RR, and RL which indicate the wheel positions as with the wheel cylinders  50 .) 
     FIG. 2 shows a state where the brake actuator  2  is not operated (where the respective signals fed into the valves  21  to  24  are turned OFF). Here, the M/C solenoid valves  21  and holding solenoid valves  23  are in their open state, whereas the suction solenoid valves  22  and pressure reducing solenoid valves  24  are in their closed state, whereby a pressure identical to that at the master cylinder  30  which is amplified the stepping force applied to the brake pedal  4  by the brake booster  32  is exerted on each wheel cylinder  50 . 
     When the brake actuator  2  is in operation, any of three kinds of control, i.e., pressure enhancing mode (including the time when no EBD control is carried out), holding mode, and pressure reducing mode, can be applied to each wheel cylinder  50 . In the pressure enhancing mode, the M/C cut solenoid valve  21 , holding solenoid valve  23 , and pressure reducing solenoid valve  24  are kept in their OFF state, whereas the suction solenoid valve  22  is turned ON. As a consequence, the M/C cut solenoid valve  21 , holding solenoid valve  23 , and suction solenoid valve  22  attain their open state, whereas the pressure reducing solenoid valve  24  is in its closed state, whereby the hydraulic pressure supplied to the respective wheel cylinder  50  can be enhanced in response to the driver&#39;s braking operation. Here, the hydraulic pressure amplified by the pump  25  may be supplied to each wheel cylinder  50 , so that a hydraulic pressure at the master cylinder pressure or higher can be exerted on the respective wheel cylinder  50 . The pressure accumulated at the reservoir  26  can be utilized as well. 
     In the holding mode, the M/C cut solenoid valve  21 , suction solenoid valve  22 , and pressure reducing solenoid valve  24  are turned OFF, whereas the holding solenoid valve  23  is turned ON. As a consequence, the M/C cut solenoid valve  21  attains its open state, whereas the suction solenoid valve  22 , holding solenoid valve  23 , and pressure reducing solenoid valve  24  are in their closed state, so that the supply of hydraulic fluid to the respective wheel cylinder  50  is stopped, whereby the hydraulic pressure exerted on the respective wheel cylinder  50  is held. 
     In the pressure reducing mode, the M/C cut solenoid valve  21  and suction solenoid valve  22  are turned OFF, whereas the holding solenoid valve  23  and pressure reducing solenoid valve  24  are turned ON. As a consequence, the M/C cut solenoid valve  21  and pressure reducing solenoid valve  24  attain their open state, whereas the suction solenoid valve  22  and holding solenoid valve  23  are in their closed state, so that the hydraulic fluid is led from the pressure reducing solenoid valve  24  to the reservoir  26 , whereby the hydraulic pressure exerted on the respective wheel cylinder  50  is reduced. 
     When operations of these individual valves  21  to  24  are controlled independently of each other, the respective hydraulic pressures supplied to the individual wheel cylinders  50  can be regulated independently of each other, so that each wheel cylinder  50  can be controlled so as to attain any of the pressure enhancing, holding, and pressure reducing modes, whereby the braking forces exerted on the respective wheels can be controlled independently of each other. 
     The braking force proportioning control for the front and rear wheels, which is the feature of the braking control in the vehicle braking control apparatus according to the present invention, will be described below with reference to FIG. 1 to FIG.  4  and FIGS. 5A to  5 C. FIG. 3 is a flowchart to show this proportioning control, FIG. 4 is a diagram to illustrate a high-speed proportioning control region, and FIGS. 5A to  5 C are graphs to show the temporal changes in the vehicle speed, the deceleration, and the brake pressures during the braking control carried out by the vehicle braking control apparatus according to the present invention. 
     Let us suppose that the driver initiates a decelerating operation to step on the brake pedal  4  at the point of time to illustrated in FIGS. 5A to  5 C. With the stepping operation on the brake pedal  4 , the brake control unit  1  controls the brake actuator  2  not operated. So the all wheel cylinders  50  are directly connected to master cylinder  30 . Then the hydraulic pressure Pf supplied to the wheel cylinders  50 FR,  50 FL of the respective front wheels FR, FL and the hydraulic pressure Pr supplied to the wheel cylinders  50 RR,  50 RL of the respective rear wheels RR, RL are increased each as illustrated in FIG.  5 C. This increases the deceleration d as illustrated in FIG. 5B while decreasing the vehicle speed v as illustrated in FIG. 5A, thus effecting retardation of the vehicle. 
     The brake control unit  1  is monitoring each of the vehicle speed v and the deceleration d, based on the output signal of the wheel speed sensor  42 , and, as illustrated in FIG. 3, first checks in step S 1  if the vehicle speed is not less than a predetermined value A and then checks in step S 2  if the deceleration d is not less than f(v), which is a function value determined by the vehicle speed. Here f(v) is a function decreasing with increase in the vehicle speed v, as illustrated in FIG.  4 . This function is properly determined depending upon the type of the vehicle, etc., but a plurality of functions can be used by switching between them in accordance with a shift state measured by the shift position sensor  45  or the like. Alternately, the values of f(v) are stored in memory unit in brake control unit  1  as tabulated data (so called map). When the two conditions above are met, braking-force proportioning to the respective wheels for high-speed driving is carried out in step S 3 . When either of the conditions is not met, braking-force proportioning to the respective wheels for normal driving is carried out in step S 4 . Then the brake control unit  1  holds the brake actuator  2  not operating in step S 5 . 
     Specifically, let us suppose a situation in which the braking forces are enhanced with increase in the hydraulic pressures Pf, Pr supplied to the respective wheel cylinders  50 , so as to increase the deceleration and in which, as a result, the deceleration d, reaches f(v 1 ) at the point (time t 1 ) where the vehicle speed is v 1 , which is still over A, whereby the relation between vehicle speed v and deceleration d goes into the high-speed proportioning region illustrated in FIG.  4 . Then the brake control unit  1  operates brake actuator  2  as holding mode for both rear wheels so that the hydraulic pressure Pr supplied to the wheel cylinders  50 RR and  50 RL of the rear wheels is maintained at the hydraulic pressure P 1  at the point of time t 1 . This prevents the rear wheels from locking prior to the front wheels during high-speed driving, whereby the vehicle behavior can be kept stable. 
     As the vehicle speed decreases with further increase in the deceleration, the deceleration d 2  becomes smaller than f(v 2 ) at the point of time t 2  to leave the high-speed proportioning region illustrated in FIG.  4 . The brake control unit  1  performs such control as to increase the hydraulic pressure Pr supplied to the wheel cylinders  50 RR and  50 RL of the rear wheels, from P 1  in order to return the proportioning control between the front and rear wheels to the normal proportioning control. So the brake actuator  2  does not operate. As the deceleration increases thereby, the deceleration d 3  reaches f(v 3 ) at the point of time t 3  to go again into the high-speed proportioning region illustrated in FIG.  4 . Therefore, the hydraulic pressure Pr supplied to the wheel cylinders  50 RR and  50 RL of the rear wheels is kept at the hydraulic pressure P 3  at the point of time t 3 . 
     This operation is carried out repeatedly and only the normal proportioning control is carried out after the time t e  when the vehicle speed becomes smaller than A. The lower the vehicle speed, the smaller the difference between the hydraulic pressures supplied to the wheel cylinders of the rear wheels and the front wheels; thus the sufficient braking force can be maintained. 
     The above embodiment was described with the control in which the hydraulic pressure Pr applied to the rear wheels underwent stepwise change with time for easier understanding of the description, but the brake control unit  1  may be structured to effect smooth change of the hydraulic pressure Pr in order to prevent passengers from feeling uncomfortable. 
     FIG. 6 is a diagram to show the structure of the brake actuator of second embodiment according to the present invention. 
     A brake pedal  4  for controlling this braking system is coupled to a piston shaft of a master cylinder  30 . Each of two hydraulic lines extending from this master cylinder  30  is connected through a solenoid valve  27   a  or  27   b  to the wheel cylinder  50 FR,  50 FL of the front right wheel FR or the front left wheel FL, respectively. A master pressure sensor  21   a  ( 21   b ) is disposed in this line from the master cylinder  30  to the solenoid valve  27   a  ( 27   b ). 
     On the other hand, a hydraulic line extending from a reservoir tank  31  is connected to a pump  25 , which is driven by a motor, and a hydraulic line extending from the pump  25  is connected through each linear valve  28  to the wheel cylinder  50  of each wheel. An accumulator  26  for accumulating the pressure of hydraulic fluid are placed between the pump  25  and a branch point to the linear valves  28 . Each of pressure reducing valves  29  is connected to a hydraulic line returning from the wheel cylinder  50  to the reservoir tank  31 . 
     The basic operation during braking of this braking system will be described below. When the driver steps on the brake pedal  4 , the piston shaft of the master cylinder  30  is pushed to generate the hydraulic pressure (master pressure) according to a stroke amount. The solenoid valves  27  are kept in their off state during normal operation, so that the master pressure is not transmitted directly to the wheel cylinder  50 FR of the front right wheel FR and to the wheel cylinder  50 FL of the front left wheel FL. The pressure of the hydraulic fluid supplied from the reservoir tank  31  is increased by the pump  25  driven by the motor and this hydraulic fluid is supplied in parallel through the each linear valve  28  to the wheel cylinder  50  of the corresponding wheels. The accumulator  26  functions to maintain the hydraulic fluid at the increased pressure. This braking system can adjust the hydraulic pressures (wheel cylinder pressures) of the respective wheel cylinders  50  independently of each other by independently controlling each linear valve  28  by the braking control unit  1 . The braking system can independently control the braking forces applied to the respective wheels in this way. The hydraulic fluid is returned through the pressure reducing valve  29  connected to each wheel cylinder  50 , to the reservoir tank  31 . 
     With this braking system the brake control method is fairly realized as first embodiment. Various brake systems are applicable for the present invention which can control the rear braking force independently of the front braking force. 
     From the invention thus described, it will be obvious that the invention may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended for inclusion within the scope of the following claims.