Rear wheel braking force control method and an apparatus therefor

A control method and an apparatus therefor, in which the braking force distribution to rear wheels is increased in a normal state, while the braking force distribution to the rear wheels is reduced to prevent the rear wheels from being locked in an early stage when the road surface is slippery. There are provided a pressure sensor (74) for detecting a master cylinder pressure, proportioning valves (57.sub.1, 57.sub.2) arranged in passages which connect a master cylinder and rear wheel cylinders and adapted to operate so that the ratios of the wheel cylinder pressures to the master cylinder pressure are low in a region where the master cylinder pressure is not lower than a predetermined pressure, normally-open switching valves (62, 63) by-passing the two valves, and a controller (71). The controller closes the switching valves to actuates the proportioning valves when the master cylinder pressure detected by the pressure sensor is not lower than a set pressure which lowers depending on the slipperiness of the road surface.

TECHNICAL FIELD 
The present invention relates to a rear wheel braking force control method 
for controlling the distribution of front and rear wheel braking forces 
and an apparatus therefor, and more particularly, to a rear wheel braking 
force control method and an apparatus therefor, capable of reducing the 
share of the braking effect on the front wheel side while preventing the 
rear wheels from being locked. 
BACKGROUND ART 
In a typical braking system of a vehicle, a brake fluid pressure 
(hereinafter referred to as master cylinder pressure), which is produced 
in a master cylinder in response to a driver's depression of a brake 
pedal, is transmitted to wheel cylinders for the four wheels, whereby a 
braking force is applied to each wheel. If a large braking force acts on 
each wheel in response to a deep depression of the brake pedal, while the 
vehicle furnished with the braking system of this type is running, the 
deceleration of the vehicle becomes higher to reduce the rear wheel load, 
so that the ground contact performance of the rear wheels is lowered. If 
the master cylinder fluid pressure is distributed substantially equally to 
the front and rear wheel cylinders with the ground contact performance of 
the rear wheels thus lowered, the rear wheels are locked first, so that 
the braking stability of the vehicle is deteriorated. 
In order to avoid the deterioration of the braking stability due to the 
rear-wheel-first locking, as is conventionally known, proportioning 
control valves (PCVs) are incorporated in the braking system. For example, 
each of two PCVs is disposed in the middle of a duct which connects each 
corresponding one of fluid pressure generator sections of the master 
cylinder and its corresponding rear wheel. The PCVs transmit the master 
cylinder pressure directly to the wheel cylinders for the rear wheels if 
the braking force is small. If the master cylinder pressure attains a 
level not lower than a set pressure, on the other hand, the PCVs lower the 
rate of increase of the fluid pressure transmitted to the rear wheel 
cylinders. 
Thus, in the braking system furnished with the PCVs, the rear wheel braking 
force increases at a high rate as the front wheel braking force increases, 
in a small braking force region where the input fluid pressure to the PCVs 
is not higher than a set pressure. In a large braking force region where 
the input fluid pressure to the PCVs is higher than the set pressure, on 
the other hand, the rear wheel braking force increases at a low rate as 
the front wheel braking force increases. In other words, if a curve 
indicative of the braking force distribution characteristics of the 
braking system furnished with the PCVs is drawn on a graph whose axes of 
ordinate and abscissa represent the rear wheel braking force and front 
wheel braking force, respectively, then this braking force distribution 
curve is composed of a first straight line with a large inclination, which 
corresponds to the small braking force region, and a second straight line 
with a small inclination, which corresponds to the large braking force 
region. 
The braking force distribution characteristics of the conventional braking 
system are set so that the braking force distribution ratio for the rear 
wheels is lower than in the case of a braking force distribution (ideal 
braking force distribution) such that the four wheels are simultaneously 
locked when the vehicle is braked. In this manner, the braking force 
stability is prevented from being lowered by the rear-wheel-first locking. 
Thus, the conventional braking force distribution curve is situated closer 
to the axis of abscissa than an ideal braking force distribution curve, 
that is, the rear wheel braking force always takes a value smaller than 
that of the ideal braking force. Meanwhile, the rear wheels cannot always 
be locked if they are subjected to a braking force of a value greater than 
a value which is determined by the conventional or ideal braking force 
distribution curve. In other words, even if there is enough room for the 
increase of the rear wheel braking force, the conventional braking system 
produces an overall braking force by increasing the share of the front 
wheel braking force correspondingly. 
If the share of the front wheel braking force is excessively increased in 
this manner, wear of braking pads of a front wheel brake unit is 
increased, and besides, heat release from the brake increases. 
Accordingly, the braked vehicle is liable to nose diving, as well as a 
fade, such that the friction coefficient of the brake pads is suddenly 
reduced, and a vapor lock which is attributable to an increase of the 
brake fluid temperature. Thus, the braking stability is lowered. 
If the share of the rear wheel braking force is increased, however, the 
rear wheels becomes liable to be locked, so that their locking should be 
prevented. 
In consideration of these circumstances, technical ideas which are designed 
so that the rear wheel braking force distribution can be increased without 
causing the rear wheels to be locked are disclosed in Published Unexamined 
Japanese Patent Application Nos. 1-257652 (DE3742173, FR2624462, or 
GB2213543), 3-125657 (GB2236156 or DE3931858), and 3-208760 (DE4029332, 
GB2238092, or FR2654401). These prior art examples are provided with an 
anti-lock device and solenoid-operated valves for normally neutralizing 
the action of the proportioning control valves, so that the rear wheels 
can be prevented from locking while enjoying an increase of the braking 
force distribution thereto. 
In these prior art examples, however, the proportioning control valves can 
be actuated only in case of trouble of the anti-lock device, so that the 
functions of the proportioning control valves cannot be effectively 
utilized. Thus, the braking force cannot be properly distributed to the 
rear wheels. 
DISCLOSURE OF THE INVENTION 
The object of the present invention is to provide a rear wheel braking 
force control method and an apparatus therefor, in which the braking force 
distribution to rear wheels can be increased in a normal state, while the 
braking force distribution to the rear wheels is reduced to prevent the 
rear wheels from being locked in an early stage when the road surface is 
slippery. 
In order to achieve the above object, according to the present invention, 
there is provided a rear wheel braking force control method for a vehicle, 
for controlling the operation of solenoid-operated valves arranged in 
passages connecting a master cylinder and wheel cylinders for rear left 
and right wheels, the solenoid-operated valves serving to selectively make 
effective or ineffective the action of proportioning valves in the 
passages to control wheel cylinder pressures so that the rate of increase 
of the wheel cylinder pressures is lower than the rate of increase of a 
master cylinder pressure. 
The method of the present invention comprises a road surface condition 
detecting process for detecting information associated with the 
slipperiness of a road surface, a process for detecting the degree of 
braking to which the vehicle is braked, a set braking degree determining 
process for setting a set braking degree on the basis of the information 
detected in the road surface condition detecting process and adjusting the 
set braking degree to a low level when the road surface is slippery, and a 
distribution control process for actuating the solenoid-operated valves so 
that the action of the proportioning valves is made ineffective when the 
braking degree detected in the braking degree detecting process is lower 
than the set braking degree, and actuating the solenoid-operated valves so 
that the action of the proportioning valves is made effective when the 
braking degree is equal to or higher than the set braking degree. 
According to the present invention, moreover, there is provided a rear 
wheel braking force control apparatus for a vehicle, which has 
proportioning valves, arranged in passages connecting a master cylinder 
and wheel cylinders for rear left and right wheels, for controlling wheel 
cylinder pressures so that the rate of increase of the wheel cylinder 
pressures is lower than the rate of increase of a master cylinder 
pressure, solenoid-operated valves disposed in the passages for 
selectively making the pressure control action of the proportioning valves 
effective or ineffective, and control means for controlling the operation 
of the solenoid-operated valves. 
The apparatus of the present invention comprises road surface condition 
detecting means for detecting information associated with the slipperiness 
of a road surface, and means for detecting the degree of braking to which 
the vehicle is braked. The control means sets a set braking degree on the 
basis of the information detected by the road surface condition detecting 
means, adjusts the set braking degree to a low level when the road surface 
is slippery, actuates the solenoid-operated valves so that the action of 
the proportioning valves is made ineffective when the braking degree 
detected by the braking degree detecting means is lower than the set 
braking degree, and actuates the solenoid-operated valves so that the 
action of the proportioning valves is made effective when the braking 
degree is equal to or higher than the set braking degree. 
According to the present invention, the share of the front wheel braking 
force can be reduced by actuating the solenoid-operated valves so that the 
action of the proportioning valves is made ineffective when the detected 
braking degree is lower than the set braking degree so that there is 
enough room for the increase of the rear wheel braking force, and the 
solenoid-operated valves are actuated so that the action of the 
proportioning valves is made effective when the braking degree is not 
lower than the set braking degree so that there is no room for the 
increase of the rear wheel braking force. Accordingly, the rear wheels can 
be prevented from locking in an early stage by the function of the 
proportioning valves. 
When the road surface is slippery, in particular, the aforesaid set braking 
degree is adjusted to a low level on the basis of the information detected 
by the road surface condition detecting means. When the road surface is 
slippery, therefore, the proportioning valves are actuated in an early 
stage to reduce the braking force distribution to the rear wheels, so that 
the rear wheels can be securely prevented from locking in an early stage. 
Thus, according to the present invention, the share of the front wheel 
braking force can be reduced while effectively utilizing the function of 
the proportioning valves to prevent early locking of the rear wheels, and 
besides, the braking degree for the start of operation of the 
proportioning valves can be properly controlled depending on the road 
surface conditions, so that the control effect is marked. More 
specifically, the braking degree for the start of operation of the 
proportioning valves lowers depending on the slipperiness of the road 
surface. When the road surface is slippery, therefore, early locking of 
the rear wheels can be prevented by actuating the proportioning valves in 
an early stage, and when the road surface is not slippery, the braking 
force distribution to the rear wheels can be made relatively high, whereby 
the share of the front wheel braking force can be reduced. Accordingly, 
wear of a front wheel brake unit can be reduced so that the interval of 
replacement of brake pads can be extended. Also, heat release from the 
front wheel brake unit is reduced so that the anti-fade properties, and 
therefore reliability, are improved, and the possibility of nose diving 
can be lowered to improve the braking stability. 
Preferably, in the apparatus of the present invention, rainfall detecting 
means is used as the road surface condition detecting means. By adjusting 
the set braking degree to a low level in rainy weather, early locking of 
the rear wheels, which is liable to be caused in rainy weather, can be 
prevented effectively. 
Preferably, moreover, outside air temperature detecting means is used as 
the road surface condition detecting means. By adjusting the set braking 
degree to a low level when the outside air temperature is low, the braking 
force distribution to the rear wheels can be prevented from unduly 
increasing on a snow-covered road, frozen road, etc. 
Preferably, road surface friction coefficient detecting means is used as 
the road surface condition detecting means. More accurate control can be 
achieved by adjusting the set braking degree to a low level when the road 
surface friction coefficient is low. 
Further preferably, outside air temperature detecting means and windshield 
wiper operation period detecting means are used as the road surface 
condition detecting means. More accurate control can be achieved by 
determining the set braking degree by fuzzy inference based on the outside 
air temperature and windshield wiper operation period. 
Further preferably, rear wheel load detecting means is provided so that 
control based on the load and road surface conditions can be effected to 
ensure more appropriate control by determining a reference level for the 
braking degree in accordance with a detected rear wheel load and then 
determining the set braking degree by compensating the reference degree on 
the basis of the information detected by the road surface condition 
detecting means.

BEST MODE OF CARRYING OUT THE INVENTION 
First, the aforementioned conventional braking system will be described in 
detail. 
Referring to FIG. 27, the conventional X-piping braking system for general 
use in an FF (front-engine front-drive) car is provided with a brake pedal 
11. A treading force on the brake pedal 11 is amplified by means of an 
intensifier 12, and then transmitted to a master cylinder 13 of a tandem. 
The master cylinder 13 includes two fluid pressure generator sections (not 
shown) for generating a brake fluid pressure corresponding to the depth of 
depression of the brake pedal 11. One of the fluid pressure generator 
sections is connected to a wheel cylinder 15.sub.1 for a front left wheel 
by means of a duct 14, and is also connected to a wheel cylinder 15.sub.4 
for a rear right wheel by means of a duct 16 which diverges from the 
middle of the duct 14. A PCV 17.sub.2 is disposed in the middle of the 
duct 16. The other fluid pressure generator section is connected to a 
wheel cylinder 15.sub.2 for a front right wheel by means of a duct 18, 
and is also connected to a wheel cylinder 15.sub.3 for a rear left wheel 
by means of a duct 19 which diverges from the middle of the duct 18. A PCV 
17.sub.1 is disposed in the middle of the duct 19. 
The PCVs 17.sub.1 and 17.sub.2 are proportioning valves which directly 
transmit a fluid pressure produced in the master cylinder 13 unless the 
fluid pressure is higher than a set pressure. In a pressure range 
exceeding the set pressure, however, the PCVs 17.sub.1 and 17.sub.2 lower 
the rate of increase of the fluid pressure for the rear wheels, which 
accompanies an increase of the master cylinder pressure, thereby 
establishing a bent-line relationship between the front and rear wheel 
braking forces. These valves themselves are conventional ones. 
As shown in FIGS. 29 and 30, each of the PCVs 17.sub.1 and 17.sub.2 
includes a valve housing 31. Defined in the housing 31 is a stepped 
cylindrical valve chamber 32 which is formed of large- and small-diameter 
chambers 33 and 34. An outlet port 38, through which the fluid pressure to 
be supplied to the wheel cylinder concerned is taken out, is formed in the 
housing 31, opening to one end face of the cylinder chamber 33 in the 
housing 31. Also, an inlet port 39, through which the fluid pressure from 
the master cylinder 13 is taken in, is formed opening to one side of the 
peripheral surface of the cylinder chamber 34. A cylindrical valve plug 
35, having a diameter a little greater than that of the cylinder chamber 
34, is disposed in the cylinder chamber 33 for axial movement. A hole h is 
bored through the valve plug 35. Two opposite ends of the hole h open 
individually to the outer peripheral surface and the outlet-side end face 
of the plug 35 so that hydraulic oil can flow through the hole h. Further, 
a plunger 36, which is arranged integrally with the valve plug 35, extends 
along the valve axis in the cylinder chamber 34. One end portion of the 
plunger 36 is slidably fitted in a guide hole 37, which is bored through 
the housing 31. 
Two opposite ends of a spring 40 set in the cylinder chamber 34 
individually engage one end face of the valve plug 35 and that portion of 
the housing 31 which defines the end face of the chamber 34, thereby 
continually urging the plug 35 toward the outlet port 38. Normally, 
therefore, a gap A is defined between the peripheral edge portion of the 
valve plug 35 and the end portion of the cylinder chamber 34, so that the 
valve is open. Thus, an input fluid pressure Pi is transmitted as an 
output fluid pressure Po through the gap A and the hole h. 
If the pressure receiving areas of the valve plug 35 on the sides of the 
outlet port 38 and the cylinder chamber 34 are So and Si, respectively, 
and if the urging force of the spring 40, input fluid pressure, and output 
fluid pressure are F, Pi, and Po, respectively, the valve plug 35 moves 
horizontally, depending on the relation between "Pi.multidot.Si+F" and 
"Po.multidot.So." In an initial state 
("Po.multidot.So"&lt;"Pi.multidot.Si+F"), as mentioned before, the gap A is 
opened by the urging force of the spring 40, so that the input fluid 
pressure Pi is delivered directly as the output fluid pressure Po. Thus, 
the output fluid pressure Po increases in accordance with the depth of 
depression of the brake pedal 11. 
If the output fluid pressure Po is raised so that "Po.multidot.So" 
increases, "Po.multidot.So"&gt;"Pi.multidot.Si+F" is obtained when a set 
pressure P1 is attained by the input fluid pressure Pi. Accordingly, the 
valve plug 35 moves against the urging force of the spring 40 toward the 
cylinder chamber 34, so that the gap A is closed by the peripheral edge 
portion of the plug 35, as shown in FIG. 30, whereby the output fluid 
pressure Po is maintained. When the brake pedal 11 is further depressed to 
increase the input fluid pressure Pi so that 
"Po.multidot.So"&lt;"Pi.multidot.Si+F" is obtained again, the gap A is opened 
again, as shown in FIG. 29, and the output fluid pressure Po increases 
corresponding to the increase of the pressure Pi. When the gap A is closed 
again as the output fluid pressure Po increases, the pressure P0 is 
maintained. In the region where the input fluid pressure Pi thus exceeds 
the set pressure P1, the gap A is repeatedly opened and closed, so that 
the output fluid pressure Po gently increases. Thus, in this region, the 
output fluid pressure Po changes in a manner such that its inclination 
with respect to the input fluid pressure Pi becomes smaller, as shown in 
FIG. 31. The magnitude of the set pressure P1 and the inclination of the 
output fluid pressure Po with respect to the input fluid pressure Pi, in 
the region where the input pressure Pi is higher than the set pressure P1, 
are unconditionally determined according to the mechanical constants of 
the PCVs, such as the urging force F of the spring 40, pressure receiving 
areas Si and So, etc. 
Referring now to FIG. 28, the relationship between a set braking force 
distribution, set for a vehicle in accordance with the mechanical 
requirements of the PCVs 17.sub.1 and 17.sub.2, and an ideal braking force 
distribution will be described. In FIG. 28, line A is a set braking force 
distribution line which, having a bend point, represents the set braking 
force distribution, and line B is an ideal braking force distribution 
curve representing the ideal braking force distribution which depends on 
the specifications of the vehicle. Here the ideal braking force 
distribution means a braking force distribution to the front and rear 
wheels such that the four wheels are simultaneously locked by braking. 
An intersecting point P11 between the ideal braking force distribution 
curve B and a dashed line indicative of the deceleration of 0.8 G 
represents the braking force distribution which causes the front and rear 
wheels to be simultaneously locked by hard braking with the deceleration 
of 0.8 G. Likewise, an intersecting point P12 between the ideal, braking 
force distribution curve B and a dashed line indicative of the 
deceleration of 0.4 G represents the braking force distribution which 
causes the front and rear wheels to be simultaneously locked by braking 
with the deceleration of 0.4 G. At every point on the dashed straight line 
for the deceleration of 0.8 G or 0.4 G, the same combined braking force 
(sum of braking forces for front and rear wheels) is required for the 
braking with the deceleration of 0.8 G or 0.4 G. A deceleration produced 
by ordinary braking ranges from 0.2 G to 0.3 G. The two-dot chain line 
represents the braking force to lock the front or rear wheels when the 
friction coefficient .mu. of a road surface is 0.8 or 0.4. Here the 
friction coefficient .mu. of a dry surface of an asphalt road is about 0.8 
in fine weather. Specifically, the point P11 indicates the ideal braking 
force distribution for the front and rear wheels which are simultaneously 
locked on a road surface with the friction coefficient .mu. of 0.8 by hard 
braking with the deceleration of 0.8 G. Likewise, the point P12 indicates 
the ideal braking force distribution for the front and rear wheels which 
are simultaneously locked on a road surface with the friction coefficient 
.mu. of 0.4 by braking with the deceleration of 0.4 G. 
As mentioned before, there is the ideal braking force distribution curve B 
which indicates that the front and rear wheels are simultaneously locked. 
Actually, however, the braking force for the rear wheels is adjusted to a 
smaller value than that of the ideal braking force. This is because the 
braking stability is lowered if the rear wheels are locked earlier than 
the front wheels. Thus, the set braking force is adjusted so that the 
straight line A for the rear wheel braking force never clears the ideal 
braking force distribution curve B. 
If braking with the deceleration of 0.38 G is effected on a road surface 
with the friction coefficient .mu. of 0.4, a braking force distribution is 
made such that the combined braking force is represented by an 
intersecting point P13 between a straight line for 0.38 G and the set 
braking force distribution line A. Even though the rear wheel braking 
force is increased to the level of the braking force distribution at an 
intersecting point P15, however, the rear wheels are never locked. If 
braking with the deceleration of 0.38 G is effected on a road surface with 
the friction coefficient .mu. of 0.8, moreover, the rear wheels are never 
locked even though the rear wheel braking force is increased to the level 
of the braking force distribution at an intersecting point P14 between the 
straight line for 0.38 G and a straight line for the friction coefficient 
.mu. of 0.8. Thus, even when the braking is effected with the same 
deceleration, the front wheel braking force can be reduced by Bf, and the 
rear wheel braking force can be increased by Br beyond the level of the 
ideal braking force distribution, depending on the state of the road 
surface. In other words, although there is enough room for the increase of 
the rear wheel braking force, depending on the vehicle running conditions 
and road surface conditions, as long as the set braking force distribution 
line A is used, the combined braking force is produced by correspondingly 
increasing the front wheel braking force. 
As also described before under the caption "BACKGROUND OF THE INVENTION," 
the aforementioned conventional braking systems have some problems, such 
as increased load on the front wheel braking unit. According to the 
alternative conventional examples which are arranged so that the rear 
wheel braking force distribution can be increased without causing the rear 
wheels to be locked, moreover, the functions of the proportioning valves 
cannot be effectively utilized, as mentioned before under the caption 
"BACKGROUND OF THE INVENTION." 
The following is a description of a braking system furnished with a rear 
wheel braking force control apparatus according to a first embodiment of 
the present invention. 
As shown in FIG. 1, the braking system comprises a brake pedal 51, an 
intensifier 52, and a master cylinder 53 (which correspond to the elements 
11, 12 and 13, respectively, shown in FIG. 27), wheel cylinders 55.sub.1, 
55.sub.2, 55.sub.3 and 55.sub.4 (which correspond to the elements 
15.sub.1, 15.sub.2, 15.sub.3 and 15.sub.4, respectively), ducts 54, 56, 58 
and 59 (which correspond to the elements 14, 16, 18 and 19, respectively), 
and PCVs 57.sub.1 and 57.sub.2 (which correspond to the elements 17.sub.1 
and 17.sub.2, respectively). The ducts constitute a first passage which 
connects the master cylinder 53 and the wheel cylinders 55.sub.3 and 
55.sub.4 for rear left and right wheels. 
Thus, in this braking system, which comprises the brake pedal 51, a 
treading force on the brake 51 is amplified by means of the intensifier 
52, and then transmitted to the master cylinder 53 of a tandem. The master 
cylinder 53 includes two fluid pressure generator sections (not shown) for 
generating a brake fluid pressure corresponding to the depth of depression 
of the brake pedal 51. One of the fluid pressure generator sections is 
connected to the wheel cylinder 55.sub.1 for a front left wheel by means 
of the duct 54, and is also connected to the wheel cylinder 55.sub.4 for 
the rear right wheel by means of the duct 56, which diverges from the 
middle of the duct 54, and the PCV 57.sub.2. The other fluid pressure 
generator section is connected to the wheel cylinder 55.sub.2 for a front 
right wheel by means of the duct 58, and is also connected to the wheel 
cylinder 55.sub.3 for the rear left wheel by means of the duct 59, which 
diverges from the middle of the duct 18, and the PCV 57.sub.1. 
The PCVs 57.sub.1 and 57.sub.2 are proportioning valves which serve to 
directly transmit the master cylinder pressure when the braking force is 
relatively small, and to lower the rate of increase of the rear wheel 
cylinder pressure compared with that of the master cylinder pressure when 
the master cylinder pressure exceeds a set pressure. Since these valves 
are constructed in the same manner as the one described with reference to 
FIGS. 27 to 31, a detailed description of the valves is omitted. 
A by-pass pipe 60 is provided between the upper-and lower-course sides of 
the duct 59 with respect to the PCV 57.sub.1. Likewise, a by-pass pipe 61 
is provided between the upper- and lower-course sides of the duct 56 with 
respect;to the PCV 57.sub.2. The by-pass pipes 60 and 61 are provided with 
PCV by-pass valves 62 and 63, respectively, formed of normally-open 
solenoid-operated valves. 
The on-off operation of the PCV by-pass valves 62 and 63 is controlled by 
means of a controller 71 as control means, which is formed of a 
microcomputer and its peripheral circuit. The controller 71 is connected 
with a brake switch 72 for outputting an on-signal in response to the 
depression of the brake pedal 51 by means of a driver, a vehicle velocity 
sensor 73 for detecting a vehicle velocity Vs, a pressure sensor 74 for 
detecting a braking pressure P or fluid pressure delivered from the master 
cylinder 53, a raindrop sensor 75 for detecting rainfalls. The raindrop 
sensor 75 is adapted to deliver an off-signal in fine weather, and an 
on-signal in rainy weather. The controller 71 is further connected with an 
outside air temperature sensor 76 for detecting an air temperature T, a 
helm sensor 77 for detecting a helm H.theta. of a steering wheel, and a 
user seating sensor 78 attached to each seat and used to detect a user's 
seating. The raindrop sensor 75 and the air temperature sensor 76 
constitute road surface condition detecting means. Although the pressure 
sensor 74 is attached to one line of a two-way duct according to the 
present embodiment, it may alternatively be attached to each line. 
Referring now to FIG. 2, the raindrop sensor 75 will be described in 
detail. 
In FIG. 2, numerals 81' and 82' denote electrodes which face each other. 
Comb-shaped conductors 83' extend from the one electrode 81' toward the 
other electrode 82', while comb-shaped conductors 84' extend from the 
electrode 82' toward the electrode 81'. Each conductor 83' is located 
between its corresponding pair of conductors 84'. The raindrop sensor 75, 
which is used with voltage applied between the electrodes 81' and 82', 
detects a rainfall when it is subjected to a current flow which is 
attributable to a short circuit caused between terminals a and b by 
raindrops. 
The controller 71, which is provided with memory means 71a for storing maps 
shown in FIGS. 5 to 7, executes the control shown in the flow chart of 
FIG. 8. More specifically, when a set pressure (Pb or Pc mentioned later) 
is attained by the pressure detected by the pressure sensor 74, the 
controller 71 closes the PCV bypass valves 62 and 63, thereby causing the 
PCVs 57.sub.1 and 57.sub.2 to fulfill their valve functions. 
Thus, in the present embodiment, the PCV by-pass valves 62 and 63, formed 
of normally-open solenoid-operated valves are disposed in the by-pass 
pipes 60 and 61 which bypass the PCVs 57.sub.1 and 57.sub.2 attached to 
the braking system, so that the master cylinder pressure is transmitted 
directly to the wheel cylinders for the rear wheels before the set 
pressure is attained by the braking pressure detected by the pressure 
sensor 74, and that the PCV by-pass valves 62 and 63 are closed, thereby 
causing the PCVs 57.sub.1 and 57.sub.2 to fulfill their functions, when 
the set pressure is exceeded by the braking pressure detected by the 
pressure sensor 74. 
The hatched region of FIG. 3 represents a basic range of the rear wheel 
braking force which is controllable by means of the apparatus of the 
present embodiment. Thus, according to the present embodiment, the rear 
wheels can be subjected to a higher braking force than the braking force 
represented by the ideal braking force distribution curve B. Further, a 
bent line C formed of straight lines a and b represents a braking force 
distribution obtained when the PCV by-pass valves 62 and 63 are kept 
closed. The leading edge portion of the line C, represented by the 
straight lines a, is steeper than that of the bent line A (FIG. 28) which 
represents the braking force distribution characteristic of the 
conventional apparatus. This is so because the ratio of the pressure 
receiving areas of the wheel cylinders 55.sub.3 and 55.sub.4 for the rear 
wheels to the pressure receiving areas of the wheel cylinders 55.sub.1 and 
55.sub.2 for the front wheels is as high as about 50 to 50, which is 
higher than in the conventional case. The braking force distribution 
characteristics observed at the bend point and after the bend point is 
passed are attributable to the structural arrangement of the PCVs 57.sub.1 
and 57.sub.2. 
As shown in FIG. 4, the bent line C of FIG. 3 also represents the 
input-output characteristics of the PCVs 57.sub.1 and 57.sub.2. When the 
PCV by-pass valves 62 and 63 are closed, the output fluid pressure of each 
of these valves is determined by the straight line a before the input 
fluid pressure of each of the PCVs 57.sub.1 and 57.sub.1 attains P1, and 
by the straight line b when the input fluid pressure exceeds P1. If the 
input fluid pressure is increased with the valves 62 and 63 open, on the 
other hand, the output fluid pressure increases in the manner indicated by 
an extension (given by broken line) of the straight line a even when P1 is 
exceeded by the input fluid pressure. Thus, if the PCV by-pass valves 62 
and 63 are closed when P2, which is higher than P1, is attained by the 
input fluid pressure, for example, the output fluid pressure increases 
along the extension of the straight line a until the input fluid pressure, 
after exceeding P1, attains P2. When the input fluid pressure exceeds P2, 
the output fluid pressure is kept at a point f corresponding to a broken 
line ef which passes through a point e on the extension corresponding to 
P2 and crosses the straight line b. When the input fluid pressure exceeds 
a value corresponding to the intersecting point f between the broken line 
ef and the straight line b, moreover, the output fluid pressure increases 
along the straight line b. The output fluid pressure is thus kept in the 
input fluid pressure region corresponding to the broken line error the 
following reason. If the output fluid pressure Po is higher than a value 
for a normal control state Indicated by the straight line b, 
"Po.multidot.So"&gt;"Pi.multidot.Si+F" is obtained, so that the gap A is 
closed, as shown in FIG. 30. 
Referring now to FIG. 8, the operation of the control apparatus according 
to the first embodiment of the present invention, constructed in the 
aforesaid manner, will be described. 
First, the controller 71 opens the PCV by-pass valves 62 and 63, thereby 
preventing the PCVs 57.sub.1 and 57.sub.2 from fulfilling their functions. 
In order to determine whether the vehicle is practically running or not, 
moreover, the controller 71 determines whether or not the vehicle velocity 
Vs detected by means of the vehicle sensor 73 is not lower than 3 km/h. If 
it is concluded that the vehicle velocity Vs is not lower than 3 km/h, the 
controller 71 further determines whether the brake switch 72 is on or not 
(Steps S11 to S13). 
If the result of decision in Step S13 is positive (YES), the controller 71 
successively reads the braking pressure detected by the pressure sensor 
74, the output of the raindrop sensor 75, and the outside air temperature 
T detected by the outside air temperature sensor 76 (Steps S14 to S16). 
Referring to the map of FIG. 5, the controller 71 then obtains a by-pass 
valve closing pressure Pa above which the PCV by-pass valves 62 and 63 are 
closed depending on the outside air temperature T and rainfall conditions. 
In FIG. 5, a map represented by full line is referred to when the output 
of the raindrop sensor 75 is off (fine), and a map represented by broken 
line is referred to when the output of the raindrop sensor is on (rainy). 
In fine weather, the road surface is less slippery, so that the closing 
pressure Pa is set on a high level. If the outside air temperature T is 
low, moreover, the road surface is more slippery, so that the closing 
pressure Pa is set on a low level. Thus, in fine weather, the closing 
pressure Pa is changed in three stages with points of transition 
corresponding to the outside air temperatures of 0.degree. C. and 
5.degree. C. In rainy weather, the closing pressure is changed in two 
stages with a point of transition corresponding to the outside air 
temperature of 5.degree. C. Accordingly, the more slippery the road 
surface (or the lower the locking limit), the lower the closing pressure 
Pa is. The maximum value (40 kg/cm.sup.2) of the closing pressure Pa is 
set so as to correspond to the braking pressure at a point P10 of FIG. 3. 
Subsequently, an increment in weight caused by users and detected by means 
of the user seating sensor 78 is estimated, and a weight factor K.sub.B is 
obtained with reference to the map of FIG. 6 (Step S18). The weight 
increment is obtained in the following manner. A piezoelectric device is 
previously embedded in each seat, and a user's seating is discriminated in 
accordance with a detection signal from this device. Then, the product of 
the weight of users in the front seats and a first predetermined ratio (%) 
and the product of the weight of users in the rear seats and a second 
predetermined ratio (%) are obtained, and the sum of the two products are 
calculated as the weight increment. As seen from the map of FIG. 6, the 
greater the weight increment, the higher the weight factor K.sub.B 
becomes. This is because the greater a load on the rear wheels, the less 
easily the rear wheels can be locked. 
Subsequently, the controller 71 multiplies the closing pressure Pa obtained 
in Step S17 by the factor K.sub.B obtained with reference to the map of 
FIG. 6, thereby obtaining a load-compensated closing pressure Pb (Step 
S19). Since the load is compensated in this manner, the greater the rear 
wheel load, that is, the less easily the rear wheels can be locked (or the 
higher the locking limit), the higher the closing pressure Pb becomes. 
Then, the controller 71 reads the helm H.theta. detected by the helm sensor 
77, and determines whether or not the helm H.theta. is not higher than 60 
deg. (Steps S20 and S21). If the helm H.theta. is not higher than 60 deg., 
it is further determined whether or not the braking pressure P detected by 
means of the pressure sensor 74 is not lower than the load-compensated 
closing pressure Pb (Step S22). If the braking pressure P is lower than 
the value Pb, the process of Step S11 and the subsequent processes are 
repeated. If the braking pressure P is not lower than the value Pb, the 
PCV by-pass valves 62 and 63 are closed (Step S23). If it is concluded in 
Step S21 that the helm H.theta. is higher than 60 deg., a transverse 
acceleration G.sub.YB acting on the body of the vehicle is calculated on 
the basis of the vehicle velocity V detected by means of the vehicle 
velocity sensor 73 and the helm H.theta. detected by means of the helm 
sensor 77 (Step S24). 
Subsequently, a compensation factor K.sub.G corresponding to the transverse 
acceleration G.sub.YB thus calculated is obtained with reference to a map 
shown in FIG. 7. For example, the compensation factor K.sub.G is set so 
that its values on the outer- and inner-wheel sides increase and decrease, 
respectively, when the transverse acceleration G.sub.YB ranges from 0.2 G 
to 0.6 G, and are kept at the value of 0.6 G when the transverse 
acceleration G.sub.YB is higher than 0.6 G. This is done in consideration 
of the fact that if the vehicle turns more sharply, then the load 
correspondingly moves to the outer-wheel side so that the outer wheels can 
be locked less easily than the inner wheels. Then, the load-compensated 
closing pressure Pb calculated in Step S19 is multiplied by the 
compensation factor K.sub.G obtained in this manner, whereby a 
transverse-G-compensated closing pressure Pc is calculated (Step S25). 
More specifically, the transverse G is compensated in a manner such that 
the more sharply the vehicle turns, the higher the closing pressure on the 
outer-wheel side corresponding to the higher locking limit is, and the 
lower the closing pressure on the inner-wheel side corresponding to the 
lower locking limit is. 
The inner-wheel-side valve, out of the PCV by-pass valves 62 and 63, is 
closed when the braking pressure P detected by means of the pressure 
sensor 74 attains the closing pressure for the inner wheels, and the 
outer-wheel,side valve, out of the PCV by-pass valves 62 and 63, is closed 
when the braking pressure P attains the closing pressure for the outer 
wheels, thereafter (Steps S26 and S27). Thus, the braking force on the 
inner wheels, which are more easily locked when the vehicle turns, is 
restrained from increasing by closing the inner-wheel-side PCV by-pass 
valves earlier than the outer-wheel-side PCV by-pass valves while the 
vehicle is turning. 
Referring now to FIGS. 9 to 11, a rear wheel braking force control 
apparatus according to a second embodiment of the present invention will 
be described. 
The basic configuration of the apparatus of the present embodiment is 
identical with that of the first embodiment. In FIG. 9, like numerals are 
used to designate common elements corresponding to the elements shown in 
FIG. 1, and a detailed description of those common elements is omitted. In 
the first embodiment, the pressure value set on the basis of the raindrop 
sensor output, indicative of a rainfall, and the outside air temperature 
is compensated in accordance with the rear wheel load and the transverse 
G, whereby the by-pass valve closing pressure is set. The present 
embodiment differs from the first embodiment mainly in that the pressure 
value obtained on the basis of the rear wheel load is compensated in 
accordance with the vehicle velocity, slip factor, outside air 
temperature, degree of road roughness, and occurrence of a rainfall, 
whereby the by-pass valve closing pressure is set. 
In connection with this difference, the apparatus of the present embodiment 
is furnished with neither the helm sensor 77 nor the user seating sensor 
78 shown in FIG. 1, but comprises a rear wheel stroke sensor 81 for 
detecting the stroke of the rear wheels and a wheel speed sensor 83 for 
detecting a wheel speed V.sub.WS of the driven wheels (rear wheels). The 
rear wheel stroke sensor 81 delivers a stroke signal V.sub.ST, which 
increases as the rear wheel load increases, to the controller 71, while 
the wheel speed sensor 83 supplies the detected wheel speed V.sub.WS to 
the controller 71. The wheel speed sensor 73 detects the vehicle velocity 
V.sub.S by detecting the rotational rate of the vehicle drive system, the 
vehicle velocity V.sub.S substantially corresponding to the rotational 
rate of the driving wheels. 
Referring now to FIGS. 10 and 11, the contents of control by means of the 
controller 71 will be described. 
in FIGS. 10 and 11, the stroke signal V.sub.ST detected by the rear wheel 
stroke sensor 81 is supplied to a low-pass filter 91. The high-frequency 
fluctuation component of the signal V.sub.ST is removed in this filter, 
and is supplied to a rear wheel load estimator section 92, as a stroke 
signal V.sub.ST ' which increases with the increase of the rear wheel 
load. The rear wheel load L.sub.R, obtained in response to the signal 
V.sub.ST ' by means of the rear wheel load estimator section 92, is 
delivered to a closing pressure setter section 93 for setting a closing 
pressure P.sub.OL which is used to close the PCV by-pass valves 62 and 63. 
The closing pressure P.sub.OL is set so that the greater the rear wheel 
load, the higher it is. This is because the rear wheels are less liable to 
lock if the rear wheel load becomes greater, as mentioned before. 
The closing pressure P.sub.OL set by means of the closing pressure setter 
section 93, along with the vehicle velocity V.sub.S from the vehicle 
velocity sensor 73, is delivered to a vehicle velocity compensator section 
94. In the compensator section 94, the closing pressure P.sub.OL from the 
closing pressure setter section 93 is multiplied by a coefficient Kv, 
which varies depending on the vehicle velocity Vs, and the resulting value 
or compensated closing pressure P.sub.OV is delivered to a slip factor 
compensator section 95. The coefficient Kv is set so that the higher the 
vehicle velocity Vs, the lower it is. This is intended to leave a margin 
for stability, in consideration of the fact that the higher the vehicle 
velocity, the greater the influence of lowered stability, attributable to 
an excessively large rear wheel braking force, is. 
In the slip factor compensator section 95, the vehicle-velocity-compensated 
closing pressure P.sub.OV is multiplied by a coefficient K.sub.S, which 
varies depending on a slip factor S computed by means of a slip factor 
computing section (mentioned later), whereby the closing pressure P.sub.OV 
is compensated. The illustrated map is stored in the memory means 71a of 
the controller 71. The higher the slip factor S, the more easily the rear 
wheels are locked. In the map, therefore, the coefficient K.sub.S is set 
so that it is lowered as the slip factor becomes higher, and is kept at a 
fixed value when a certain value is exceeded by the slip factor S. 
A slip-factor-compensated closing pressure P.sub.OS, outputted from the 
slip factor compensator section 95, is delivered to an outside air 
temperature compensator section 96 shown in FIG. 11, and the compensated 
closing pressure P.sub.OS is multiplied by a coefficient Kt which varies 
depending on the outside air temperature T detected by the outside air 
temperature sensor 76. As seen from the map of the block 96, the 
coefficient Kt is adjusted to a small value in the region where the 
outside air temperature T is low, and to a large value in the region where 
the outside air temperature T is high. This is done because the lower the 
outside air temperature T, the more slippery the road surface is, and the 
more easily the rear wheels is locked. 
An air-temperature-compensated closing pressure P.sub.OT is delivered to a 
rough road compensator section 98, whereupon it is multiplied by a 
coefficient Kr. The compensator section 98 is supplied with a level 
frequency H from a rough road detector section, which is indicative of the 
roughness of the road, and will be mentioned later. The higher the level 
frequency H, the higher the degree of roughness of the road is, and the 
more easily the rear wheels are locked. As seen from the map of the block 
98, therefore, the coefficient Kr is set so that the higher the frequency 
H, the lower it is. 
A rough-road-compensated closing pressure P.sub.OH is delivered to a wet 
road compensator section 100, whereupon it is multiplied by a coefficient 
K.sub.W, thereby providing a closing pressure P.sub.OK. The wet road 
compensator section 100 is supplied with the output signal from the 
raindrop sensor 75. As seen from the map of the block 100, the coefficient 
K.sub.W is set so as to be changed to a smaller value when the output of 
the raindrop sensor 75 is on (indicative of a rainfall). This is because 
the rear wheels are liable to be locked on a rain-wet road such that the 
raindrop sensor 75 is turned on. 
An electrical signal V.sub.P, which corresponds to the brake fluid pressure 
detected by means of the pressure sensor 74, is delivered to a converter 
section 102, whereupon it is converted into a brake fluid pressure Pr. In 
a subtracter section 103, the closing pressure P.sub.OK delivered from the 
wet road compensator section 100 is subtracted from the brake fluid 
pressure Pr. A value indicative of the result of this subtraction is 
delivered to a discriminator section 104, whereupon it is determined 
whether or not there is a relation Pr.gtoreq.P.sub.OK. If the decision 
result is positive, a processor section 105 is activated to close the PCV 
by-pass valves 62 and 63. 
As shown in FIG. 10, the signal V.sub.ST from the rear wheel stroke sensor 
81 is delivered to a differentiator section 111 to be differentiated 
thereby. The output of the differentiator section 111 is delivered to a 
low-pass filter 112, whereupon its high-frequency component is cut. 
Further, the output of the low-pass filter 112 is delivered to a rough 
road detector section 113, whereupon the number of times a predetermined 
level is exceeded within a predetermined period of time is calculated as 
the frequency H which corresponds to the level of roughness of the road. 
The frequency H is delivered to the aforesaid rough road compensator 
section 98. Also, the vehicle velocity Vs corresponding to the speed of 
the driving wheels, detected by means of the vehicle velocity sensor 73, 
and the wheel speed V.sub.WS of the driven wheels, detected by means of 
the wheel speed sensor 83, are supplied to a slip factor calculator 
section 121, whereupon the slip factor S (=Vs-V.sub.WS)/Vs) is calculated. 
The calculated slip factor S is delivered to the aforesaid slip factor 
compensator section 95. A voltage V.sub.T, which is proportional to the 
outside air temperature T from the outside air temperature sensor 76, is 
delivered to a converter section 131, whereupon it is converted into the 
outside air temperature T, which is delivered to the outside air 
temperature compensator section 96. 
The following is a description of the operation of the rear wheel braking 
force control apparatus according to the present embodiment constructed in 
this manner. 
The rear wheel stroke signal V.sub.ST delivered from the rear wheel stroke 
sensor 81 is applied to the low-pass filter 91, whereupon its 
high-frequency pressure fluctuation is cut. Then, the signal V.sub.ST is 
applied to the rear wheel load estimator section 92. In this estimator 
section 92, the load L.sub.R acting on the rear wheels is estimated. The 
rear wheel load L.sub.R is delivered to the closing pressure estimator 
section 93, whereupon the closing pressure P.sub.OL for closing the PCV 
by-pass valves 62 and 63, which corresponds to the rear wheel load 
L.sub.R, is obtained. Thereafter, the closing pressure P.sub.OL is 
multiplied successively by the coefficients Kv, Ks, Kt, Kr and K.sub.W in 
the vehicle velocity compensator section 94, slip factor compensator 
section 95, outside air temperature compensator section 96, rough road 
compensator section 98, and wet road compensator section 100, 
respectively. By doing this, the final closing pressure P.sub.OK is 
obtained. 
If the discriminator section 104 concludes that the brake fluid pressure Pr 
detected by means of the pressure sensor 74 is equal to or higher than the 
closing pressure P.sub.OK, the PCV by-pass valves 62 and 63 are closed, 
and processing for activating the PCVs 57.sub.1 and 57.sub.2 is executed. 
As a result, if the PCV by-pass valves 62 and 63 are closed at the point e 
(FIG. 4) which corresponds to the input fluid pressure P3 (=P.sub.OK), for 
example, the output fluid pressure is kept at the pressure at the point e 
even though the input fluid pressure increases thereafter. After the input 
fluid pressure becomes equal to or higher than the pressure at the point 
f, thereafter, the output fluid pressure increases along the straight line 
b. The output fluid pressure is kept in this manner because a relation 
Po.multidot.So&gt;(Pi.multidot.Si+F) is maintained, as shown in FIG. 30, 
since the output fluid pressure Po is higher than the value for the normal 
control state, as indicated by the straight line b. 
According to the present embodiment, as described above, the closing 
pressure P.sub.OK is compensated in accordance with the slipperiness of 
the road surface by multiplying an input pressure signal by its 
corresponding one of the coefficients Ks, Kt and Kw in each of the 
sections including the slip factor compensator section 95, outside air 
temperature compensator section 96, and wet compensator section 100. On a 
road surface which is liable to cause the rear wheels to lock, therefore, 
the closing pressure P.sub.OK is reduced to prevent the rear wheels from 
locking. 
Referring now to FIGS. 12 to 14, a rear wheel braking force control 
apparatus according to a third embodiment of the present invention will be 
described. 
The basic configuration of the apparatus of the present embodiment is 
identical with that of the second embodiment. In FIG. 13, like numerals 
are used to designate common elements corresponding to the elements shown 
in FIG. 9, and a detailed description of those common elements is omitted. 
The present embodiment differs from the second embodiment mainly in that 
the apparatus is mounted in a vehicle furnished with an active suspension, 
and that a rainfall is detected on the basis of the operating state of a 
windshield wiper. 
In FIG. 13, reference numeral 81a denotes an active suspension actuator 
pressure sensor for detecting a pressure Va of a rear wheel actuator of an 
active suspension. The active suspension is a suspension in which a fluid 
is fed into and discharged from fluid spring chambers, which are provided 
individually for suspension units of the vehicle, so that the load bearing 
capacity of each suspension unit can be varied to effect absorption of 
vibration, attitude control such as roll control for turning, vehicle 
height adjustment, etc. The pressure sensor 81a detects the pressure in 
each fluid spring chamber, and supplies the detected pressure Va to the 
controller 71. Reference numeral 82 denotes a vehicle height switch for 
assigning the vehicle height level of the vehicle which carries the active 
suspension. An L vehicle height lower than a standard vehicle height and 
an H vehicle height higher than the standard vehicle height, as well as 
the standard vehicle height, can be selected by manually operating the 
switch 82. 
Referring now to FIGS. 13 and 14, the contents of control by means of the 
controller 71 will be described. 
In FIGS. 13 and 14, the pressure Va detected by means of the pressure 
sensor 81 is applied to the low-pass filter 91, and a pressure Va' from 
the filter 91, whose high-frequency pressure fluctuation is cut, along 
with a control signal from the vehicle height switch 82, is applied to the 
rear wheel load estimator section 92. The estimator section 92 estimates 
the rear wheel load L.sub.R which corresponds to the pressure Va', 
depending on the vehicle height, H, standard, or L, assigned by the 
control signal from the vehicle height switch 82. A map illustrated in the 
block 92 of FIG. 13 and various other maps mentioned later are stored in 
the memory means 71a of the controller 71. The pressure-load (Va'-L.sub.R) 
map is arranged so that the load L.sub.R for the L vehicle height is 
greater than the load L.sub.R for the standard vehicle height or H vehicle 
height at the same pressure Va'. This is attributable to the fact that if 
the H vehicle height is selected, for example, the pressure of the 
pressure sensor 81a increases to supply the fluid to the fluid spring 
chambers, in order to increase the vehicle height. Thus, even though the 
load L.sub.R is fixed, the output of the pressure sensor 81a is higher if 
the H vehicle height is selected. 
The rear wheel load L.sub.R obtained by means of the rear wheel load 
estimator section 92 is delivered to the closing pressure setter section 
93 for obtaining a closing pressure P.sub.OL which is used to close the 
PCV by-pass valves 62 and 63. As in the case of the second embodiment, the 
closing pressure P.sub.OL is set so that the greater the load, the higher 
it is. As in the case of the second embodiment, the closing pressure 
P.sub.OL thus obtained is compensated with use of the coefficient Kv, 
which varies depending on the vehicle velocity Vs, in the vehicle velocity 
compensator section 94, and is then compensated with use of the 
coefficient K.sub.S, which varies depending on the slip factor S, in the 
slip factor compensator section 95. As in the case of the second 
embodiment, the vehicle-velocity-compensated and slip-factor-compensated 
closing pressure P.sub.OS is further compensated with use of the 
coefficient Kt, which varies depending on the outside air temperature T, 
in the outside air temperature compensator section 96, and is then 
compensated with use of the coefficient Kr, which varies depending on the 
level frequency H indicative of the road roughness, in the rough road 
compensator section 98. 
The compensated closing pressure P.sub.OH from the rough road compensator 
section 98 is delivered to a first wet road compensator section 99, and is 
multiplied by a coefficient K.sub.W1. The compensator section 99 is 
supplied with a control signal (on/off) from a windshield wiper switch 84. 
As seen from the map of the block 99, the coefficient K.sub.W1 is set so 
as to be changed to a smaller value when the windshield wiper switch 84 is 
turned on. Also, the coefficient K.sub.W1 is set so as to take a smaller 
value when the windshield wiper is operated in a "Hi mode," as indicated 
by broken line, than when the wiper is operated intermittently. This is 
because the rear wheels are liable to be locked on a rain-wet road such 
that the windshield wiper switch 84 is turned on, and that the "Hi mode," 
compared with the intermittent mode, is used when the road surface is more 
slippery due to a heavier rain. 
A compensated closing pressure P.sub.W1 from the first wet road compensator 
section 99 is delivered to a second wet road compensator section 100, 
which corresponds to the wet road compensator section 100 of the second 
embodiment, and is multiplied by a coefficient K.sub.W2. The second wet 
road compensator section 100 is supplied with the output signal from the 
raindrop sensor 75. As seen from the map of the block 100, the coefficient 
K.sub.W2, like the coefficient K.sub.W of the second embodiment, is set so 
as to be changed to a smaller value when the raindrop sensor output is 
turned on. 
A closing pressure P.sub.W2 outputted from the second wet road compensator 
section 100 is delivered to a hard braking compensator section 101 shown 
in FIG. 14, and is multiplied by a coefficient K.sub.P to calculate the 
closing pressure P.sub.OK. The hard braking compensator section 101 is 
supplied with a time-dependent changing rate Pr' of the braking pressure, 
which is indicative of hard braking. As seen from the map of the block 
101, the coefficient K.sub.P is set so as to become smaller as the 
time-dependent changing rate Pr' of the braking pressure increases, and to 
be fixed to a low value when the time-dependent changing rate Pr' exceeds 
a certain level, whereby the closing pressure is compensated for reduction 
when the vehicle is braked hard. 
The electrical signal Vp, which corresponds to the brake fluid pressure 
detected by means of the pressure sensor 74, is delivered to the converter 
section 102, whereupon it is converted into the brake fluid pressure Pr. 
In the subtracter section 103, the closing pressure P.sub.OK is subtracted 
from the brake fluid pressure Pr. The value indicative of the result of 
this subtraction is delivered to the discriminator section 104, whereupon 
it is determined whether or not there is the relation Pr.gtoreq.P.sub.OK. 
If the decision result is positive, the processor section 105 is activated 
to close the PCV by-pass valves 62 and 63. 
The brake fluid pressure Pr from the converter section 102 is delivered to 
a differentiator section 106, whereupon the time-dependent changing rate 
Pr' is obtained and delivered to the aforesaid hard braking compensator 
section 101. 
Like the stroke sensor signal V.sub.ST of the second embodiment, the 
pressure Va of the actuator of the active suspension, detected by means of 
the pressure sensor 81a, is delivered to the differentiator section 111 to 
be differentiated thereby. The output of the differentiator section 111 is 
delivered to the low-pass filter 112, whereupon its high-frequency 
component is cut. Further, the output of the low-pass filter 112 is 
delivered to the rough road detector section 113, whereupon the number of 
times the predetermined level is exceeded within the predetermined period 
of time is calculated as the frequency H which corresponds to the level of 
roughness of the road. The frequency H is delivered to the aforesaid rough 
road compensator section 98. 
As in the case of the second embodiment, the slip factor S 
(=(Vs-V.sub.WS)/Vs) is calculated according to the vehicle velocity Vs and 
the wheel speed V.sub.WS in the slip factor calculator section 121, to be 
delivered to the slip factor compensator section 95. Also, the output 
voltage V.sub.T from the outside air temperature sensor 76 is converted 
into the outside air temperature T in the converter section 131, whereupon 
it is delivered to the outside air temperature compensator section 96. 
The following is a description of the operation of the rear wheel braking 
force control apparatus according to the present embodiment constructed in 
this manner. 
The pressure Va of the actuator of the active suspension, detected by means 
of the pressure sensor 81a, is applied to the low-pass filter 91, 
whereupon its high-frequency pressure fluctuation is cut. Then, the 
pressure Va is applied to the rear wheel load estimator section 92, 
whereupon the load L.sub.R acting on the rear wheels is estimated in 
accordance with the vehicle height selected by means of the vehicle height 
switch 82. The rear wheel load L.sub.R is delivered to the closing 
pressure setter section 93, whereupon the closing pressure P.sub.OL for 
closing the PCV by-pass valves 62 and 63, which corresponds to the rear 
wheel load L.sub.R, is obtained. Thereafter, the closing pressure P.sub.OL 
is multiplied successively by the coefficients Kv, Ks, Kt, Kr, K.sub.W1, 
K.sub.W2 and Kp in the vehicle velocity compensator section 94, slip 
factor compensator section 95, outside air temperature compensator section 
96, rough road compensator section 98, first and second wet road 
compensator sections 99 and 100, and hard braking compensator section 111, 
respectively. By doing this, the final closing pressure P.sub.OK is 
obtained. 
If the discriminator section 104 concludes that the brake fluid pressure Pr 
detected by means of the pressure sensor 74 is equal to or higher than the 
closing pressure P.sub.OK, the PCV by-pass valves 62 and 63 are closed, 
and processing for activating the PCVs 57.sub.1 and 57.sub.2 is executed. 
As in the cases of the first and second embodiments, if the PCV by-pass 
valves 62 and 63 are closed, for example, at the point e which corresponds 
to the input fluid pressure P3 (=P.sub.OK), as shown in FIG. 4, the output 
fluid pressure is kept at the point e before the input fluid pressure 
becomes equal to or higher than the pressure at the point f. 
Thus, according to the present embodiment, the closing pressure can be also 
compensated in accordance with the operating state of the windshield wiper 
switch 84, as well as the slip factor and the raindrop sensor output, 
whereby the rear wheels can be prevented from locking on a slippery road 
surface. 
Referring now to FIGS. 15 and 16, a rear wheel braking force control 
apparatus according to a fourth embodiment of the present invention will 
be described. The basic configuration of the apparatus of the present 
embodiment is identical with that of the third embodiment. In FIG. 15, 
like numerals are used to designate common elements corresponding to the 
elements shown in FIG. 12, and a detailed description of those common 
elements is omitted. The present embodiment differs from the third 
embodiment mainly in that the closing pressure is compensated in 
accordance with road surface conditions discriminated on the basis of the 
power steering output. 
In connection with this difference, the apparatus of the present embodiment 
is furnished with a power steering pressure sensor 85 for detecting a 
power steering pressure P.sub.PS, as well as the active suspension 
actuator pressure sensor 81a, vehicle height switch 82, wheel speed sensor 
83, windshield wiper switch 84, etc. which are provided individually for 
the apparatus of the third embodiment. The power steering pressure 
P.sub.PS detected by means of the sensor 85, along with the wheel speed 
V.sub.WS detected by means of the sensor 83 and the on/off 
(operating/nonoperating) signal from the switch 84, is supplied to the 
controller 71. 
Referring now to FIGS. 16 to 18, the contents of control by means of the 
controller 71 will be described. 
As in the case of the third embodiment, the rear wheel load L.sub.R is 
estimated in the rear wheel load estimator section 92 from the low-pass 
filter output Va', which corresponds to the pressure sensor output Va, and 
the control signal from the vehicle height switch 82, as shown in FIGS. 16 
to 18. Then, the closing pressure P.sub.OL for closing the PCV by-pass 
valves 62 and 63 is set in accordance with the estimated rear wheel load 
L.sub.R in the closing pressure setter section 93. As in the case of the 
third embodiment, the closing pressure P.sub.OL thus obtained is 
compensated with use of the coefficient Kv, which varies depending on the 
vehicle velocity Vs, in the vehicle velocity compensator section 94, then 
compensated with use of the coefficient K.sub.S, which varies depending on 
the slip factor S, in the slip factor compensator section 95, and further 
compensated with use of the coefficient Kt, which varies depending on the 
outside air temperature T, in the outside air temperature compensator 
section 96. 
The compensated closing pressure P.sub.OT outputted from the outside air 
temperature compensator section 96 is delivered to a low-.mu. road 
compensator section 97, whereupon it is multiplied by a coefficient 
K.sub.PS. The low-.mu. road compensator section 97 is supplied with a 
deviation P.sub.PSL (mentioned later) between the actual power steering 
pressure P.sub.PS and an estimated power steering pressure P.sub.PS '. As 
seen from the map of the block 97, the coefficient K.sub.PS is set so that 
the greater the deviation P.sub.PSL, the smaller it is. This is because 
the actual power steering pressure P.sub.PS is lower than the estimated 
power steering pressure P.sub.PS ' on a low-.mu. road. 
Further, a compensated closing pressure P.sub.OP from the low-.mu. road 
compensator section 97 is delivered to the rough road compensator section 
98, and is multiplied by the coefficient Kr which varies depending on the 
level frequency H from the rough road detector section, which is 
indicative of the roughness of the road, and will be mentioned later. 
As in the case of the third embodiment, the compensated closing pressure 
P.sub.OH from the rough road compensator section 98 is compensated with 
use of the coefficient K.sub.W1, which varies depending on the control 
signal from the windshield wiper switch 84, in the first wet road 
compensator section 99, compensated with use of the coefficient K.sub.W2, 
which varies depending on the output signal from the raindrop sensor 75, 
in the second wet road compensator section 100, and further compensated 
with use of the coefficient K.sub.P, which varies depending on the 
time-dependent changing rate Pr' of the braking pressure, which is 
indicative of hard braking, in the hard braking compensator section 101. 
Then, the output signal Vp from the pressure sensor 74 is converted into 
the brake fluid pressure Pr in the converter section 102, and the closing 
pressure P.sub.OK is subtracted from the brake fluid pressure Pr in the 
subtracter section 103. If it is concluded in the discriminator section 
104 that there is the relation Pr.gtoreq.P.sub.OK, the PCV by-pass valves 
62 and 63 are closed by means of the processor section 105. 
As in the case of the third embodiment, moreover, the time-dependent 
changing rate Pr', obtained in the differentiator section 106 on the basis 
of the brake fluid pressure Pr from the converter section 102, is 
delivered to the hard braking compensator section 101. Then, the frequency 
H, calculated in accordance with the pressure sensor output Va supplied to 
the rough road detector section 113 through the differentiator section 111 
and the low-pass filter 112, is delivered to the rough road compensator 
section 98. Further, the slip factor S (=(Vs-V.sub.WS)/Vs), calculated 
according to the vehicle velocity Vs and the wheel speed V.sub.WS in the 
slip factor calculator section 121, is delivered to the slip factor 
compensator section 95. The outside air temperature sensor output V.sub.T 
is converted into the outside air temperature T in the converter section 
131, whereupon it is delivered to the outside air temperature compensator 
section 96. 
As shown in FIG. 17, the helm H.theta. of the steering wheel, detected by 
means of the helm sensor 77, is supplied to a power steering pressure 
estimator section 132. A map illustrated in the estimator section 132 
indicates a power steering pressure P.sub.P ' which is required to rotate 
the steering wheel for the helm H.theta.. The power steering pressure 
P.sub.P ' is delivered to a vehicle velocity compensator section 133, 
whereupon it is multiplied by a coefficient K.sub.PV which varies 
depending on the vehicle velocity Vs detected by means of the vehicle 
velocity sensor 73. A velocity-responsive power steering system is 
controlled so that the manual steering effort for the steering 
wheel:becomes heavier as the vehicle velocity increases. Accordingly, the 
power steering pressure lowers as the vehicle velocity increases. Thus, 
the coefficient K.sub.PV is set so as to become smaller with the increase 
of the vehicle velocity. 
The compensated estimated power steering pressure P.sub.PS ' from the 
vehicle velocity compensator section 133 is applied to a deviation 
calculator section 134, whereupon the actual power steering pressure 
P.sub.PS, detected by means of the power steering pressure sensor 85, is 
subtracted from the estimated power steering pressure P.sub.PS ', whereby 
the deviation P.sub.PSL is calculated. The deviation P.sub.PSL, which 
increases as the friction coefficient .mu. of the road surface lowers, is 
applied to the aforesaid low-.mu. road compensator section 97. 
The following is a description of the operation of the rear wheel braking 
force control apparatus according to the present embodiment constructed in 
this manner. 
The pressure Va of the actuator of the active suspension, detected by means 
of the pressure sensor 81a, is applied to the low-pass filter 91, 
whereupon its high-frequency pressure fluctuation is cut. Then, the 
pressure Va is applied to the rear wheel load estimator section 92, 
whereupon the load L.sub.R acting on the rear wheels is estimated 
depending on the vehicle height selected by means of the vehicle height 
switch 82. The rear wheel load L.sub.R is delivered to the closing 
pressure setter section 93, whereupon the closing pressure P.sub.OL for 
closing the PCV by-pass valves 62 and 63, which corresponds to the rear 
wheel load L.sub.R, is obtained. Thereafter, the closing pressure P.sub.OL 
is multiplied successively by the coefficients Kv, Ks, Kt, K.sub.PS, Kr, 
K.sub.W1, K.sub.W2 and Kp in the vehicle velocity compensator section 94, 
slip factor compensator section 95, outside air temperature compensator 
section 96, low-.mu. road compensator section 97, rough road compensator 
section 98, first and second wet road compensator sections 99 and 100, and 
hard braking compensator section 101, respectively, whereupon the final 
closing pressure P.sub.OK is obtained. If it is detected in the 
discriminator section 104 that the brake fluid pressure Pr detected by 
means of the pressure sensor 74 is equal to or higher than the closing 
pressure P.sub.OK, the PCV by-pass valves 62 and 63 are closed, and 
processing for activating the PCVs 57.sub.1 and 57.sub.2 is executed. 
Thus, according to the present embodiment, the closing pressure P.sub.OK 
for closing the PCV by-pass valves 62 and 63 is subjected to reductive 
compensation when a low-.mu. road is discriminated by a low actual power 
steering output, as well as the compensation according to the third 
embodiment, based on the slip factor, outside air temperature, raindrop 
sensor output, and windshield wiper switch output. Accordingly, the rear 
wheels can be more securely prevented from locking on a slippery road 
surface, if any, and the rear wheel braking force can be increased to a 
level not lower than the ideal braking force distribution, within a range 
such that the rear wheels cannot be locked in an early stage. 
Referring now to FIGS. 19 to 23, a rear wheel braking force control 
apparatus according to a fifth embodiment of the present invention will be 
described. 
The rear wheel braking force control apparatus according to the present 
embodiment differs from the apparatuses of the foregoing embodiments 
mainly in that the apparatus is used with an anti-skid braking system 
(ABS), and that the closing pressure, which decides the valve closing 
timing for the by-pass valves, is set according to fuzzy inference based 
on the outside air temperature and windshield wiper operation period. 
As shown in FIG. 19, the vehicle braking system comprises a master cylinder 
201 of a tandem. The master cylinder 201 operates in response to the 
depression of a brake pedal 202 through the medium of a vacuum-type brake 
booster 203. A pair of main brake lines 204 and 205 extend from each of 
pressure chambers of the master cylinder 201. The one main brake line 204 
diverges into front and rear wheel brake lines 206 and 207. The front 
wheel brake line 206 is connected to a wheel cylinder 208FL for the front 
left wheel, while the rear wheel brake line 207 is connected to a wheel 
cylinder 208RR for the rear right wheel. Likewise, the other main brake 
line 205 diverges into front and rear wheel brake lines 209 and 210, which 
are connected to wheel cylinders 208FR and 208RL for the front right and 
rear left wheels, respectively. 
An anti-skid valve (ABS valve) 211 is disposed in the middle of each wheel 
brake line. These ABS valves 211 control the braking pressures of the 
wheel cylinders 208, which correspond to the ABS valves, in response to 
command signals from an ABS controller, which will be mentioned later. A 
pump system for supplying the brake fluid to the wheel cylinders 208 
through the ABS valves 211, return lines for releasing the brake fluid 
from the wheel cylinders through the ABS valves 211. etc. are not shown in 
FIG. 19. 
On the lower-course side of the ABS valves 211, proportioning valves (PCVs) 
212L and 212R for use as control valves are arranged in the middle of the 
rear wheel brake lines 207 and 210, respectively. These PCVs 212 serve to 
lower the rate of increase of the pressure (rear wheel braking pressure) 
transmitted to the rear wheel cylinders 208R, compared with that on the 
front wheel side, when a set pressure P.sub.o is attained by the master 
cylinder pressure supplied from the master cylinder 201, as shown in FIG. 
20. The PCVs 212, whose construction is not illustrated, have their set 
pressure P.sub.o unconditionally determined by the set load of valve 
springs, as in the cases of the foregoing embodiments. 
Further, the rear wheel brake lines 207 and 210 are provided with by-pass 
lines 213 for by-passing the PCVs 212, individually. By-pass valves 214L 
and 214R, formed of normally-close solenoid-operated switching valves, are 
arranged in the middle of the by-pass lines 213, individually. 
Each ABS valve 211 is connected electrically to an ABS controller 215 for 
anti-skid brake control, while each by-pass valve 214 is connected 
electrically to a BV controller 216. Both these controllers 215 and 216, 
which are formed of a microcomputer and its peripheral circuits each, are 
connected, as required, to each other by means of a communication circuit 
so that data, control signals, etc. can be transferred between them. 
The ABS controller 215 is connected with switches and brakes, such as a 
brake switch 217 for detecting the depression of the brake pedal 2, a G 
sensor 218 for detecting the vehicle body deceleration, wheel speed 
sensors 219 for detecting the respective speeds of the individual wheels, 
and a vehicle velocity sensor 220 for detecting the vehicle velocity. The 
wheel speed sensors 219, which are shown as one block in FIG. 19, are 
provided individually for the wheels. 
The ABS controller 215, like the controller 71 shown in FIG. 15, for 
example, computes the difference between a reference vehicle body speed 
and each wheel speed, that is, the slip factor for each wheel, in 
accordance with sensor signals from each wheel speed sensor 219 and the 
vehicle velocity sensor 220, and delivers ABS control signals F.sub.LA, 
F.sub.RA, R.sub.LA and R.sub.RA corresponding to the thus computed slip 
factors to their corresponding ABS valves 211. Depending on their signal 
levels, the ABS control signals function as pressure boosting signals for 
loading the brake fluid into the wheel cylinders 208, holding signals for 
holding the brake fluid in the wheel cylinders, or pressure reducing 
signals for unloading the brake fluid from the wheel cylinders. In 
accordance with the ABS control signals, the ABS valves 212 are switched 
in a conventional manner. The ABS controller 215 of the present embodiment 
delivers a holding signal to one of the ABS valve 211 when a satisfactory 
value for the ABS starting conditions is exceeded by the slip factor of 
each corresponding wheel. If the slip factor further increases so that the 
tendency of the wheels to be locked grows, thereafter, the controller 215 
delivers a pressure reducing signal. 
The BV controller 216, like the controller 71 shown in FIG. 15, for 
example, fetches an on/off signal from the brake switch 217 with every 
predetermined sampling period, and compares the master cylinder pressure 
with a target switching pressure (closing pressure), which Will be 
mentioned later, thereby controlling the operation of the by-pass valves 
214L and 214R. Thus, the by-pass valves 214L and 214R are opened 
immediately when the brake switch 217 is turned on. When the master 
cylinder pressure attains the target switching pressure (mentioned later) 
or when the brake switch 217 is turned off, the by-pass valves 214 are 
closed. For this by-pass valve control, as shown in FIG. 19, the BV 
controller 216 is connected electrically with a pressure sensor 221 for 
detecting the pressure inside the main brake line 204, which is indicative 
of the master cylinder pressure. 
As will be described in detail later, moreover, the BV controller 216 
variably controls the target switching pressure depending on the road 
surface conditions, according to the fuzzy inference based on the outside 
air temperature and windshield wiper operation period. For this fuzzy 
inference, the BV controller 216 is connected electrically to an outside 
air temperature sensor 222 and a rotary encoder 223 for detecting the 
rotating speed of a windshield wiper motor (not shown). The controller 216 
receives the outside air temperature T from the outside air temperature 
sensor 222, and detects the windshield wiper operation period W in 
accordance with the wiper motor rotating speed detected by means of the 
rotary encoder 223. 
Functionally, as shown in FIG. 21, the BV controller 216 is provided with a 
controlled variable computing block 224 and a controlled variable command 
value calculating block 225. In the computing block 224, the outside air 
temperature T and the windshield wiper operation period W as inputs are 
fuzzified or made to be fuzzy variables, and inference based on the fuzzy 
rules mentioned later is carried out. In the calculating block 225, the 
result of the fuzzy inference is defuzzified or made to be non-fuzzy, 
whereupon the target switching pressure is finally set. A control routine 
corresponding to FIG. 21 is always executed in predetermined cycles by 
means of the BV controller 216 without regard to braking operation of the 
vehicle, while the vehicle is running. 
For the fuzzy inference, the BV controller 216 is loaded with, for example, 
nine fuzzy rules (FIG. 22) which are described in IF-THEN form. Each fuzzy 
rule includes the outside air temperature T and windshield wiper operation 
period W as two items (fuzzy variables) of its antecedent, and a 
compensation value (controlled variable command value) .DELTA.P as one 
item of its consequent. This compensation value .DELTA.P is used to 
compensate the target switching pressure for reduction, as mentioned 
later. In FIG. 22, each of symbols S, M, N, BD, MD and ZO represents a 
label which is indicative of a fuzzy subset (hereinafter referred to 
simply as fuzzy set) in the whole space or universe of discourse (carrier 
set) for its corresponding one of the outside air temperature T, 
windshield wiper operation period W, and compensation value .DELTA.P. Each 
fuzzy set is represented by a membership function mentioned later. 
In FIG. 22, Rule 1, "If T=S and W=S, then .DELTA.P=BD," indicates that if 
the outside air temperature T and the windshield wiper operation period W 
are low and short, respectively, each corresponding to a fuzzy set S, then 
the negative compensation value .DELTA.P is made large. Rules 2 to 9 will 
now be described in brief. 
Rule 2: If the outside air temperature T and the windshield wiper operation 
period W are low and moderate, respectively, then the compensation value 
.DELTA.P is made large. 
Rule 3: If the outside air temperature T and the windshield wiper operation 
period W are low and long, respectively, then the compensation value 
.DELTA.P is made moderate. 
Rule 4: If the outside air temperature T and the windshield wiper operation 
period W are moderate and short, respectively, then the compensation value 
.DELTA.P is made moderate. 
Rule 5: If the outside air temperature T and the windshield wiper operation 
period W are both moderate, then the compensation value .DELTA.P is made 
moderate. 
Rule 6: If the outside air temperature T and the windshield wiper operation 
period W are moderate and long, respectively, then the compensation value 
.DELTA.P is made small. 
Rule 7: If the outside air temperature T and the windshield wiper operation 
period W are high and short, respectively, then the compensation value 
.DELTA.P is made moderate. 
Rule 8: If the outside air temperature T and the windshield wiper operation 
period W are high and moderate, respectively, then the compensation value 
.DELTA.P is made small. 
Rule 9: if the outside air temperature T and the windshield wiper operation 
period W are high and long, respectively, then the compensation value 
.DELTA.P is made small. 
Membership functions for individually defining the three fuzzy sets S, M 
and N for the outside air temperature T, membership functions for 
individually defining the three fuzzy sets S, M and N for the windshield 
wiper operation period W, and membership functions for individually 
defining the three fuzzy sets BD, MD and ZO for the compensation value 
.DELTA.P are predetermined as shown in FIG. 23, and are stored in the 
memory means of the BV controller 216. 
Referring to FIG. 23, the membership function associated with the fuzzy set 
S for the outside air temperature T is set so that its adaptation or 
conformity degree is 1 when the outside air temperature T is not higher 
than a first predetermined temperature, which is lower than 0.degree. C., 
and that the adaptation degree is reduced from 1 to 0 as the outside air 
temperature T increases from the first predetermined temperature to 
0.degree. C. Further, the membership function associated with the fuzzy 
set M for the outside air temperature T is set so that its adaptation 
degree varies within the range from 0 to 1 as the outside air temperature 
T changes from the first predetermined temperature to a second 
predetermined temperature which is higher than 0.degree. C. Also, the 
membership function associated with the fuzzy set N for the outside air 
temperature T is set so that its adaptation degree varies within the range 
from 0 to 1 when the outside air temperature T is not lower than 0.degree. 
C. 
The membership functions associated with the fuzzy sets S, M and N for the 
windshield wiper operation period W and the fuzzy sets BD, MD and ZO for 
the compensation value .DELTA.P are set substantially in the same manner 
as the fuzzy sets S, M and N for the outside air temperature T, as shown 
in FIG. 23. For example, the membership function associated with the fuzzy 
set S for the windshield wiper operation period W is set so that its 
adaptation degree is 1 when the period W is not longer than a first 
predetermined period, which is shorter than 5 cycles per minute, and that 
the adaptation degree varies within the range from 0 to 1 as the 
temperature T changes within the range from the first predetermined period 
to 5 cycles per minute. Further, the membership function associated with 
each fuzzy set for the compensation value .DELTA.P is set so that its 
adaptation degree varies within the range from 0 to 1 as the compensation 
value .DELTA.P changes within a negative region. 
In setting the target switching pressure, the BV controller 216 makes a 
fuzzy inference in conventional steps of procedure on the basis of 
detected conditions, represented by the outside air temperature sensor 
output T and the windshield wiper operation period W calculated from the 
rotary encoder output, and the nine fuzzy rules shown in FIG. 22. In this 
fuzzy inference, a membership value for the detected outside air 
temperature associated with its corresponding one of the membership 
functions for the outside air temperature T (one item of antecedent) and a 
similar membership value for the windshield wiper operation period W 
(another item of antecedent) are obtained with respect to each of the 
fuzzy rules. Then, in order to obtain an inference output on the basis of 
the max-min principle, the corresponding membership function for the 
compensation value .DELTA.P (item of consequent) is top-cut with a smaller 
value (adaptation degree), out of the two membership values, and a figure 
corresponding to the cut membership function is obtained. In order to make 
the inference output non-fuzzy, moreover, the center of gravity of a 
figure obtained by combining the figures corresponding individually to the 
nine rules is calculated as the compensation value (controlled variable 
command value) .DELTA.P. 
When the compensation value .DELTA.P is calculated in this manner, the 
controller 216 calculates the target switching pressure PX according to 
the following equation. 
EQU PX=PX-.DELTA.P. 
Here the initial value of the target switching pressure PX is set at a 
value higher than a set pressure P0 for the PCVs 212, as shown in FIG. 21. 
By the fuzzy inference described above, the target switching pressure PX of 
the by-pass valves 214 can be properly set in accordance with the friction 
coefficient .mu. of the road surface. If the road conditions are such that 
the outside air temperature T and the windshield wiper operation period W 
are low and short, respectively, thus conforming to Rule 1, then it is 
supposed that the friction coefficient of the road surface is very low due 
to the highly possible freezing of the road surface or fallen snow 
thereon. In this case, the target switching pressure PX is sharply reduced 
so that the by-pass valves 214 are closed in an early stage of braking 
operation, thereby restraining the distribution of the rear wheel braking 
force. If the road conditions are such that the outside air temperature T 
and the windshield wiper operation period W are nearly 0.degree. C. and 
short or moderate, respectively, thus conforming to Rule 4 or 5, then it 
is supposed that the road surface is slippery, that is, its friction 
coefficient is low, due to a rainfall, and the target switching pressure 
PX is reduced to a moderate level. By doing this, the distribution of the 
rear wheel braking force can be restrained in response to the lowering of 
the friction coefficient of the road surface. If the road conditions are 
such that the outside air temperature T and the windshield wiper operation 
period W are high and moderate or long, respectively, thus conforming to 
Rule 8 or 9, on the other hand, then it is supposed that the road surface 
is dry, that is, its friction coefficient is high, and the target 
switching pressure PX is slightly reduced or is not reduced. In this case, 
the rear wheel braking force distribution is enhanced. 
The above-described fuzzy control of the target switching pressure PX is 
executed without regard to the braking operation of the vehicle, so that 
the target switching pressure PX set in accordance with the running 
conditions before the start of the braking operation. Thus, the rear wheel 
braking force can be securely prevented from becoming excessively large, 
and besides, the operation frequency of the ABS system on the rear wheel 
side can be lowered. 
According to the present embodiment, if it is supposed, in accordance with 
the fuzzy inference based on the outside air temperature and the 
windshield wiper operation period, that the friction coefficient of the 
road surface is low, the target switching pressure (target value of master 
cylinder pressure), which determines the valve closing timing of the 
by-pass valves, is lowered, so that the rear wheel braking force 
distribution cannot become excessive during the braking operation. Since 
the target switching pressure is variably controlled without regard to the 
braking operation of the vehicle, moreover, the target switching pressure 
can be properly set in accordance with the friction coefficient of the 
road surface estimated before the start of the braking operation. Thus, a 
marked effect can be produced such that the operation frequency of the ABS 
on the rear wheel side cannot increase, also in the case where the 
variable control of the target switching pressure and the ABS control are 
combined together. 
According to the fuzzy inference of the present embodiment, the controlled 
variable command value .DELTA.P is calculated by the max-min principle or 
gravity center method. Alternatively, however, any other method of 
calculation may be used, or any other membership functions than the 
membership functions shown in FIG. 23 may be used. 
FIG. 24 shows a rear wheel braking force control apparatus according to a 
sixth embodiment of the present invention, in which an acceleration sensor 
(G sensor) 79 as braking degree detecting means is used in place of the 
pressure sensor 74 used in the first embodiment. This G sensor 79 serves 
to detect the deceleration of the vehicle body. The processes shown in 
FIG. 25 are executed in the controller 71. In these processes, vehicle 
body decelerations .alpha., which are detected by means of the G sensor 
79, and closing decelerations .alpha.a, .alpha.b and .alpha.c, which are 
set in the controller 71, are used in place of the brake fluid pressures P 
and the closing pressures Pa, Pb and Pc, respectively, according to the 
first embodiment. Since the contents of processing are substantially the 
same as those for the case of the first embodiment, a detailed description 
of these contents is omitted. 
According to the present embodiment, when the deceleration .alpha. of the 
vehicle body attains the predetermined value .alpha.b, the PCV by-pass 
valves 62 and 63 are closed so that the proportioning valves 57.sub.1 and 
57.sub.2 fulfill their functions. When the vehicle turns, moreover, the 
PCV by-pass valves 62 and 63 are closed so that the deceleration of the 
vehicle body at which the proportioning valves start to fulfill their 
functions is one on the inner wheel side and another on the outer wheel 
side. Thus, the same effect of the foregoing first embodiment can be 
obtained. 
The utilization of the vehicle body deceleration, described in connection 
with the present embodiment, may be also applied to the foregoing second 
to fifth embodiments in like manner. 
The present invention is not limited to the individual embodiments 
described above, and various modifications may be effected therein. 
For example, instead of using the X-piping configuration described in 
connection with the foregoing embodiments, a valve arrangement such as the 
one shown in FIG. 26 may be applied to front and rear piping systems which 
are generally used in an FR (front-engine rear-drive) car. Further, valves 
of any other types or properties may be used as the proportioning control 
valves. As for the road surface detecting means, furthermore, the means 
used in the individual embodiments may be used independently or in 
combination with one another. It is to be understood that various other 
modifications may be effected without departing from the spirit of the 
present invention.