U.S. patent application number 09/171582 was filed with the patent office on 2002-05-09 for brake force control apparatus.
This patent application is currently assigned to KENYON & KENYON. Invention is credited to HASHIMOTO, YOSHIYUKI, SHIMIZU, SATOSHI.
Application Number | 20020053828 09/171582 |
Document ID | / |
Family ID | 14304690 |
Filed Date | 2002-05-09 |
United States Patent
Application |
20020053828 |
Kind Code |
A1 |
SHIMIZU, SATOSHI ; et
al. |
May 9, 2002 |
BRAKE FORCE CONTROL APPARATUS
Abstract
When a normal braking is performed, a master cylinder (32) and
wheel cylinders (44FR, 44FL, 44RR, 44RL) are set to a conducting
state. When an emergency braking is performed, the master cylinder
and the wheel cylinders are joined together so that a large brake
force can be generated. If the wheel cylinders are connected to an
accumulator soon after the emergency braking, an increase in the
wheel cylinder pressure can be prevented. Even after the emergency
braking, the wheel cylinders and the master cylinder are maintained
in the conducting state while the master cylinder pressure is
rapidly increasing.
Inventors: |
SHIMIZU, SATOSHI;
(SUSONO-SHI, JP) ; HASHIMOTO, YOSHIYUKI;
(SUSONO-SHI, JP) |
Correspondence
Address: |
KENYON & KENYON
1500 K STREET, N.W., SUITE 700
WASHINGTON
DC
20005
US
|
Assignee: |
KENYON & KENYON
|
Family ID: |
14304690 |
Appl. No.: |
09/171582 |
Filed: |
January 15, 1999 |
PCT Filed: |
April 22, 1997 |
PCT NO: |
PCT/JP97/01380 |
Current U.S.
Class: |
303/113.4 ;
303/155 |
Current CPC
Class: |
B60T 8/3275 20130101;
B60T 7/042 20130101; B60T 13/686 20130101 |
Class at
Publication: |
303/113.4 ;
303/155 |
International
Class: |
B60T 008/34 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 23, 1996 |
JP |
8-101593 |
Claims
1. A brake force control apparatus having an operation fluid
pressure generating mechanism (32; 206) generating an operation
fluid pressure depending on a degree of operation of a brake pedal
by a driver, a high-pressure source (12, 20; 226) generating a
control fluid pressure higher than that of the fluid pressure
generated by said operation fluid pressure generating mechanism, a
switch mechanism (46, 48, 54; 214) for selectively connecting one
of the operation fluid pressure generating mechanism to the
high-pressure source and a wheel cylinder (44FR, 44FL, 44RR, 44RL;
218), and emergency braking detection means (102, 106) for
detecting execution of an emergency braking, wherein when an
emergency braking is performed by the driver, a brake assist
control for boosting a wheel cylinder pressure is executed in such
a way that the high-pressure source serves as a fluid pressure
source, there are provided start timing detection means (104, 108,
110, 112, 114) for detecting, as a start timing, a time when a
controlled pressure increasing slope obtained by boosting the wheel
cylinder pressure with the high-pressure source used as the fluid
pressure source exceeds a normal pressure increasing slope obtained
by boosting the wheel cylinder pressure with the master cylinder
used as the fluid pressure source, and brake assist control
execution means (116; 120, 128) for starting the brake assist
control after the emergency braking is detected and the start
timing is then detected.
2. The brake force control apparatus as claimed in claim 1, wherein
said start timing detection means comprises delay time counting
means (104, 108, 110, 112, 114) for detecting the start timing when
a predetermined delay time (D, DL, DS) elapses after the emergency
braking is detected.
3. The brake force control apparatus as clamed in claim 1, wherein
said start timing detection means comprises first delay time
setting means (104, 108, 110) for setting the predetermined delay
time on the basis of a differential pressure (.DELTA.PEM) generated
between the operation fluid pressure and the wheel cylinder
pressure when the emergency braking is detected.
4. The brake force control apparatus as claimed in claim 1, wherein
said start timing detection means comprises a second delay time
setting means (104, 110) for setting the predetermined delay time
on the basis of a brake operation speed (.DELTA.PM/C) generated
before the emergency braking is detected.
5. The brake force control apparatus as claimed in claim 1, wherein
said start timing detection means comprises a third delay time
setting means (104, 110) for setting the predetermined delay time
on the basis of a maximum value of a brake operation speed
generated before the emergency braking is detected.
6. The brake force control apparatus as claimed in claim 1, wherein
said start timing detection means comprises a fourth delay time
setting means (108, 110) for setting the predetermined delay time
on the basis of an amount of brake operation before the emergency
braking is detected.
7. The brake force control apparatus as claimed in claim 1, wherein
said start timing detection means comprises a fifth delay time
setting means (108, 110) for setting the predetermined delay time
on the basis of an amount of brake operation when the emergency
braking is detected.
8. The brake force control apparatus as claimed in claim 1, wherein
said start timing detection means comprises a sixth delay time
setting means for setting the predetermined delay time on the basis
of a time (T.alpha..beta.) necessary for a brake operation speed to
exceed a first predetermined value (.alpha.) and then become equal
to or less than a second predetermined value (.beta.).
9. The brake force control apparatus as claimed in claim 1, wherein
the switch mechanism comprises a front wheel switch mechanism (46,
48) that selectively connects one of the operation fluid pressure
generating means and the high-pressure source to wheel cylinders
(4FR, 44FL) of front wheels, and a rear wheel switch mechanism (54)
that selectively connects one of the operation fluid pressure
generating means and the high-pressure source to wheel cylinders
(4RR, 44RL) of rear wheels, and wherein said brake assist control
execution means comprises front wheel control execution means (120)
that starts the brake assist control of front wheels when said
start timing is detected after the emergency braking is detected,
and rear wheel control execution means (122, 124, 126, 128) that
starts the brake assist control of rear wheels when a predetermined
rear wheel delay time (DR) elapses after the brake assist control
of the front wheels is started.
10. The brake force control apparatus as claimed in claim 9,
wherein said rear wheel control execution means comprises first
rear wheel delay time setting means (122, 124) for setting the
predetermined rear wheel delay time on the basis of a condition of
a road on which the vehicle is traveling.
11. The brake force control apparatus as claimed in claim 9,
wherein said rear wheel control execution means comprises second
rear wheel delay time setting means (122, 124) for setting the
predetermined rear wheel delay time on the basis of a deceleration
(DVSO) generated on the vehicle after the brake assist control of
the front wheels.
12. A brake force control apparatus having a high-pressure source
(12, 20; 226) which generates a predetermined control fluid
pressure, a front wheel fluid pressure control mechanism (46, 48)
that controls a wheel cylinder pressure of front wheels with the
high-pressure source used as a fluid pressure source, a rear wheel
fluid pressure control mechanism (54) that controls a rear wheel
cylinder pressure of rear wheels with the high-pressure source used
as the fluid pressure source, and emergency braking detection means
(102, 106) for detecting execution of an emergency braking, wherein
when an emergency braking is performed by the driver, a brake
assist control for generating a wheel cylinder pressure higher than
that generated at a normal time, there are provided front wheel
control execution means (120) for starting the brake assist control
of the front wheels after the emergency braking is detected, and
rear wheel control execution means (122, 124, 126, 128) for
starting the brake assist control of the rear wheels when a
predetermined rear wheel delay time (DR) elapses after the brake
assist control of the front wheel is started.
13. The brake assist control apparatus as claimed in claim 12,
wherein the rear wheel control execution means comprises first rear
wheel delay time setting means (122, 124) for setting the
predetermined rear wheel delay time on the basis of a condition of
a road on which the vehicle is traveling.
14. The brake force control apparatus as claimed in claim 12,
wherein said rear wheel control execution means comprises second
rear wheel delay time setting means (122, 124) for setting the
predetermined rear wheel delay time on the basis of a deceleration
(DVSO) generated on the vehicle after the brake assist control of
the front wheels.
Description
TECHNICAL FIELD
[0001] The present invention relates to a brake force control
apparatus and, more particularly, to a brake force control
apparatus which executes a brake assist control that generates a
brake force greater than that generated at an ordinary time, when
an emergency braking is performed by a driver of a vehicle.
[0002] Conventionally, for example, as disclosed in Japanese
Laid-Open Patent Application 4-121260, a brake force control
apparatus is known which generates, when an emergency braking is
required, a brake force greater than that generated in a normal
time. The above-mentioned conventional apparatus is equipped with a
brake booster, which generates a pushing force having a given power
ratio with respect to a brake pressing force Fp. The pushing force
generated by the brake booster is transferred to a master cylinder.
The master cylinder generates a master cylinder pressure PM/C based
on the pushing force of the brake booster, that is, the brake
pressing force Fp.
[0003] The above-conventional apparatus is equipped with a fluid
pressure generating mechanism, which generates an assist hydraulic
pressure in which a pump is used as a fluid pressure source. The
fluid pressure generating mechanism generates the assist hydraulic
pressure based on a driving signal supplied from a control circuit.
When the brake pedal is operated at a speed higher than a
predetermined speed, the control circuit determines that an
emergency braking is carried out by the driver, and requests a
fluid pressure generating mechanism to the maximum assist hydraulic
pressure. The maximum assist hydraulic pressure generated by the
fluid pressure generating mechanism is supplied to a change valve
together with the master cylinder pressure PM/C. The change valve
supplies the higher one of the assist hydraulic pressure generated
by the fluid pressure generating mechanism and the master cylinder
pressure PM/C toward wheel cylinders.
[0004] According to the conventional apparatus, if the brake pedal
is operated at a speed equal to or less than the given operating
speed, the master cylinder pressure PM/C adjusted to a level
depending on the brake pressing force Fp is supplied to the wheel
cylinders. Hereinafter, the control of realizing the
above-mentioned state will be referred to as a normal control. If
the brake pedal is operated at a speed higher than the given
operating speed, a high assist hydraulic pressure is supplied to
the wheel cylinders in which the pump serves as a fluid pressure
source. Hereinafter, the control of realizing the above-mentioned
state will be referred to as a brake assist control. Hence,
according to the conventional apparatus, the brake force is
controlled to a level based on the brake pressing force FP at
ordinary time, and to rapidly increase the brake force after
emergency braking is executed.
[0005] The above-mentioned conventional apparatus is equipped with
a change valve. The change valve is a mechanism, which selects a
state in which the master cylinder serves as a fluid pressure
source or a state in which the fluid pressure generating mechanism
serves as a fluid pressure source. More particularly, the change
valve selectively realizes a state in which the wheel cylinders are
coupled to the master cylinder (hereinafter referred to as a first
state) and a state in which the wheel cylinders are coupled to the
fluid pressure generating mechanism (hereinafter referred to as
second state).
[0006] The first and second states can also be realized by a
two-position switch valve. The change valve has a complex structure
and is expensive than the two-position switch valve. Hence, when
the two-position switch valve is used instead of the change valve,
the conventional apparatus can be produced at a reduced cost.
[0007] Anyway, the above-mentioned conventional apparatus starts to
increase the wheel cylinder pressure at the same time as emergency
braking is performed. However, if the switching between the fluid
liquid sources is implemented by the two-position switch valve, the
switching time is required to be determined taking into
consideration the output characteristic of the fluid pressure
generating mechanism and the motion performance of the vehicle.
[0008] More particularly, at the time when emergency braking is
recognized, the brake pedal is being operated at a high speed, and
thus a rapid increase in the master cylinder pressure PM/C takes
place. The speed at which the assist hydraulic pressure generated
by the fluid pressure source is limited to an appropriate value due
to he capability of the pump or the like.
[0009] Hence, if the fluid pressure source is switched to the fluid
pressure generating mechanism from the master cylinder immediately
after the emergency braking is recognized, a decrease in the
increasing speed of the wheel cylinder pressure, as compared to a
case where the master cylinder is maintained as the fluid pressure
source. In order to prevent occurrence such a problem at the time
of emergency braking, it is desired that the fluid pressure source
be switched to the fluid pressure generating mechanism from the
master cylinder when an appropriate delay time elapses after the
emergency braking is recognized.
[0010] In the above-mentioned apparatus, when the brake assist
control is started, the ground contact ability of tires is
partially consumed in order to produce brake force. Hence, if the
brake assist control is executed while the vehicle is turned, the
maximum value of cornering forces which can be produced by the
tires becomes less than that a value obtained when the brake assist
control is not executed.
[0011] The maximum value of the cornering forces generated in the
tires increases as the load exerted on the tires increases. Hence,
when the brake assist control is active in which the weight of the
vehicle is shifted toward the front wheels, the maximum value of
the cornering forces which can be produced by the rear tires is
greatly decreased.
[0012] The above-mentioned conventional apparatus executes the
brake assist control without any consideration of the turning state
of the vehicle. Hence, in the vehicle equipped with the
above-mentioned conventional apparatus, the maximum value of the
cornering forces generated by the rear wheels may greatly be less
than the maximum value of the cornering forces generated by the
front wheels.
[0013] In order to suppress the decrease in the cornering forces of
the rear wheels, it is desirable that the brake assist control to
the rear wheels be started with an appropriate delay time after the
brake assist control to the front wheels is started.
[0014] The conventional apparatus in which the brake assist control
to the front wheels and that to the rear wheels are simultaneously
started is not an ideal one capable of maintaining the stable
behavior of the vehicle which is turning.
DISCLOSURE OF INVENTION
[0015] The present invention is made in view of the above-mentioned
point, and it is the first object of the present invention to
provide a brake force control apparatus in which the brake assist
control is started at a timing different from the timing at which
an emergency braking is recognized and preferable braking
performance can thus be provided.
[0016] The brake force control apparatus directed to achieving the
above object includes an operation fluid pressure generating
mechanism that generates an operation fluid pressure depending on a
degree of operation of a brake pedal by a driver, a high-pressure
source generating a control fluid pressure higher than that of the
fluid pressure generated by said operation fluid pressure
generating mechanism, a switch mechanism for selectively connecting
one of the operation fluid pressure generating mechanism to the
high-pressure source and a wheel cylinder, and emergency braking
detection means for detecting execution of an emergency braking.
When an emergency braking is performed by the driver, a brake
assist control for boosting a wheel cylinder pressure in such a way
that the high-pressure source serves as a fluid pressure
source.
[0017] The brake force control apparatus which achieves the above
object is configured so that, as a start timing, a time is detected
at which a controlled pressure increasing slope obtained by
boosting the wheel cylinder pressure with the high-pressure source
used as the fluid pressure source exceeds a normal pressure
increasing slope obtained by boosting the wheel cylinder pressure
with the master cylinder used as the fluid pressure source. Also,
the brake assist control is started after the emergency braking is
detected and the start timing is then detected.
[0018] In the brake force control apparatus according to the
present invention, a fluid pressure based on a brake operation
force is supplied, at the normal time, to the wheel cylinders from
the operation fluid pressure generating mechanism. After the brake
assist control is started, the high-pressure source is used as the
fluid source, and a fluid pressure higher than that at the normal
time is supplied to the wheel cylinders. In the present invention,
the brake assist control employs, as the fluid pressure source, the
operation fluid pressure generating mechanism, and starts after a
situation is created in which the wheel cylinder pressure can
rather be boosted rapidly with the high-pressure source used as the
fluid pressure source. Hence, it is possible to prevent an event in
which an increase in the wheel cylinder pressure is prevented due
to execution of the brake assist control.
[0019] A second object of the present invention is to provide a
brake force control apparatus in which the brake assist control of
the front wheels and the brake assist control of the rear wheels
are started at respective, different times and preferable braking
performance can be realized.
[0020] The brake force control apparatus which achieves the above
object is equipped with a high-pressure source which generates a
predetermined control fluid pressure, a front wheel fluid pressure
control mechanism that controls a wheel cylinder pressure of front
wheels with the high-pressure source used as a fluid pressure
source, a rear wheel fluid pressure control mechanism that controls
a rear wheel cylinder pressure of rear wheels with the
high-pressure source used as the fluid pressure source, and
emergency braking detection means for detecting execution of an
emergency braking. When an emergency braking is performed by the
driver, a brake assist control for generating a wheel cylinder
pressure higher than that generated at a normal time is
generated.
[0021] The brake assist control of the front wheels is started
after the emergency braking is detected. Then the brake assist
control of the rear wheels is started when a predetermined rear
wheel delay time elapses after the brake assist control of the
front wheel is started. The brake assist control of the rear wheels
in the present invention is started when the predetermined rear
wheel delay time elapses after the brake assist control of the
front wheels is started. When the brake assist control of the front
wheels is started, the vertical load of the rear wheels is
decreased due to load shifting, and thus the cornering forces that
can be generated by the rear wheels is reduced. Hence, if the brake
assist controls of the front and rear wheels are simultaneously
started, the cornering forces generated by the rear wheels may
abruptly be decreased. According to the present invention, it is
possible to suppress decrease in the cornering forces of the rear
wheels resulting from the start of the brake assist control. Hence,
even if an emergency braking is performed when the vehicle is
turning, a large braking force can be ensured without change of the
turning behavior.
BRIEF DESCRIPTION OF DRAWINGS
[0022] FIG. 1 is a system structure diagram of a brake force
control apparatus according to an embodiment of the present
invention,
[0023] FIG. 2 is an illustration for showing a change in a brake
pressing force achieved under various circumstances,
[0024] FIG. 3(A) is a diagram showing variations in a master
cylinder pressure PM/C and a wheel cylinder pressure PW/C when an
emergency braking is performed by a beginner-grade driver,
[0025] FIG. 3(B) is a diagram showing variations in a variation
ratio .DELTA.PM/C of the master cylinder pressure PM/C when an
emergency braking is performed by a beginner-grade driver,
[0026] FIG. 4 is a flowchart of an example of a control routine
executed by the brake force control apparatus shown in FIG. 1,
[0027] FIG. 5 is a diagram showing a relationship between a maximum
value .DELTA.PM/C of the variation ratio .DELTA.PM/C of the master
cylinder pressure PM/C and an emergency braking time differential
pressure .DELTA.PEM,
[0028] FIG. 6 is a diagram showing a relationship between an
emergency braking time master pressure PM/C and the emergency
braking time differential pressure .DELTA.PEM,
[0029] FIG. 7 shows an example of a map used to calculate a delay
time D by the brake force control apparatus shown in FIG. 1,
[0030] FIG. 8 shows another example of the map used to calculate
the delay time D by the brake force control apparatus shown in FIG.
1,
[0031] FIG. 9 is a flowchart of an example of another control
routine executed by the brake force control apparatus shown in FIG.
1,
[0032] FIG. 10 is a diagram showing a variation in a vehicle
deceleration performed, when a brake assist control is executed, in
a vehicle equipped with a brake force control apparatus according
to a second embodiment of the present invention,
[0033] FIG. 11 shows an example of a map referred to when a rear
wheel delay time is calculated in the control routine shown in FIG.
9, and
[0034] FIG. 12 is a system structure diagram of a brake force
control apparatus according to a third embodiment of the present
invention.
BEST MODE FOR CARRYING OUT THE INVENTION
[0035] FIG. 1 is a system structure diagram of a brake force
control apparatus according to an embodiment of the present
invention. The brake force control apparatus shown in FIG. 1 is
controlled by an electronic control unit 10 (hereinafter, referred
to as ECU 10). The brake force control apparatus comprises a pump
12. The pump 12 has a motor 14 as a power source thereof. An inlet
port 12a of the pump 12a communicates with a reservoir tank 16. An
accumulator 20 communicates with an outlet port 12b of the pump via
a check valve 18. The pump 12 delivers brake fluid in the reservoir
tank 16 from the outlet port 12b so that a predetermined pressure
is always accumulated in the accumulator 20.
[0036] The accumulator 20 communicates with a high-pressure port
24a of a regulator 24 via a high-pressure passage 22, and
communicates with a regulator switching solenoid 26 (hereinafter,
referred to as STR 26). The regulator 24 has a low-pressure port
24b and a control fluid pressure port 24c. The low-pressure port
24b communicates with the reservoir tank 16 via a low-pressure
passage 28. The control fluid pressure port 24c communicates with
the STR 26 via a control fluid pressure passage 29. The STR 26 is a
two-position solenoid valve which selectively set one of the
control fluid pressure passage 29 and the high-pressure passage 22
in a conductive state, and sets the control fluid pressure passage
29. In a conductive state and sets the high-pressure passage 22 in
a closed state in a normal state.
[0037] A brake pedal 30 is connected to the regulator 24, and a
master cylinder is mounted to the regulator 24. The regulator 24
has a fluid pressure chamber therein The fluid pressure chamber
always communicates with the control fluid pressure port 24c, and
selectively communicates with the high-pressure port 24a or the
low-pressure port 24b in accordance with an operational state of
the brake pedal 30. The regulator 24 is configured so that a
pressure inside the fluid pressure chamber is adjusted to a fluid
pressure corresponding to a brake pressing force FP exerted on the
brake pedal 30. Accordingly, the fluid pressure corresponding to
the brake pressing force FP always appears at the control fluid
pressure port 24c of the regulator 24. Hereinafter, this fluid
pressure is referred to as a regulator pressure PRE.
[0038] The brake pressing force FP exerted on the brake pedal 30 is
mechanically transmitted to the master cylinder 32 via the
regulator 24. Additionally, a force corresponding to the fluid
pressure inside the fluid pressure chamber of the regulator 24,
that is, a force corresponding to the regulator pressure PRE, is
transmitted to the master cylinder 32.
[0039] The master cylinder 32 is provided with a first fluid
pressure chamber 32a and a second fluid pressure chamber 32b
therein. A master cylinder pressure PM/C corresponding to a
resultant force of the brake pressing force FP and a brake assist
force FA is generated in the first fluid pressure chamber 32a and
the second fluid pressure chamber 32b. Both the master cylinder
pressure PM/C generated in the first fluid pressure chamber 32a and
the master cylinder pressure PM/C generated in the second fluid
pressure chamber 32b are supplied to a proportioning valve 34
(hereinafter, referred to as P valve 34).
[0040] The P valve 34 communicates with a first fluid pressure
passage 36 and a second fluid pressure passage 38. The P valve 34
supplies the master cylinder pressure PM/C to the first fluid
pressure passage 36 and the second fluid pressure passage 38
without change in a range where the master cylinder pressure PM/C
is less than a predetermined value. Additionally, the P valve 34
supplies the master cylinder pressure PM/C to the first fluid
pressure passage 36 without change and supplies a fluid pressure
obtained by decreasing the master cylinder pressure PM/C by a
predetermined ratio to the second fluid pressure passage 38 in a
range where the master cylinder pressure PM/C is less than a
predetermined value.
[0041] A hydraulic pressure sensor 40, which outputs an electric
signal corresponding to the master cylinder pressure PM/C, is
provided between the second fluid pressure chamber 32b of the
master cylinder 32 and the P valve 34. An output signal of the
hydraulic pressure sensor 40 is supplied to the ECU 10. The ECU 10
detects the master cylinder pressure PM/C generated in the master
cylinder 32 based on the output signal of the hydraulic pressure
sensor 40.
[0042] The above-mentioned STR 26 communicates with a third fluid
pressure passage 42. The third fluid pressure passage 42
communicates with one of the control fluid pressure passage 29 and
the high-pressure passage 22 in accordance with a state of the STR
26. In the present embodiment, wheel cylinders 44FL and 44FR
provided to left and right front wheels FL and FR are provided with
a brake fluid pressure from the first fluid pressure passage 36
communicating with the P valve 34 or the third fluid pressure
passage 42 communicating with the STR 26. Additionally, wheel
cylinders 44RL and 44RR provided to left and right rear wheels RL
and RR are provided with a brake fluid pressure from the second
fluid pressure passage 38 communicating with the P valve 34 or the
third fluid pressure passage 42 communicating with the STR 26.
[0043] The first fluid pressure passage 36 communicates with a
first assist solenoid valve 46 (hereinafter referred to as
SA.sub.-1 46) and a second assist solenoid valve 48 (hereinafter,
referred to as SA.sub.-2 48). On the other hand, the third fluid
pressure passage 42 communicates with a right front holding
solenoid valve 50 (hereinafter, referred to as SFRH 50), a left
front holding solenoid valve 52 (hereinafter, referred to as SFLH
52) and a third assist solenoid valve 54 (hereinafter, referred to
as SA.sub.-3 54).
[0044] The SFRH 50 is a two-position solenoid valve which maintains
an open state in a normal state. The SFRH 50 communicates with the
SA.sub.-1 46 and a right front wheel pressure decreasing solenoid
valve 58 (hereinafter, referred to as SFRR 58) via a pressure
adjusting fluid pressure passage 56. A check valve 60 permitting a
fluid flow only in a direction from the pressure adjusting fluid
pressure passage 56 to the third fluid pressure passage 42 is
provided, in parallel, between the third fluid pressure passage 42
and the pressure adjusting fluid pressure passage 56.
[0045] The SA.sub.-1 46 is a two-position solenoid valve which
selectively renders one of the first fluid pressure passage 36 and
the pressure adjusting fluid pressure passage 56 to be communicated
with the wheel cylinder 44FR, and renders the first fluid pressure
passage 36 and the wheel cylinder 44FR to be in a communicating
state in a normal state (OFF state). On the other hand, the SFRR 58
is a two-position solenoid valve which renders the pressure
adjusting fluid pressure passage 56 and the reservoir tank 16 to be
in a connected state or a disconnected state. The SFRR 58 renders
the pressure adjusting fluid pressure passage 56 and the reservoir
tank 16 to be in a disconnected state in a normal state (OFF
state).
[0046] The SFLH 52 is a two-position solenoid valve which maintains
an open state in a normal state. The SFLH 52 communicates with the
SA.sub.-2 48 and a left front wheel pressure decreasing solenoid
valve 64 (hereinafter, referred to as SFLR 64) via a pressure
adjusting fluid pressure passage 62. A check valve 66 permitting a
fluid flow only in a direction from the pressure adjusting fluid
pressure passage 62 to the third fluid pressure passage 42 is
provided, in parallel, between the third fluid pressure passage 42
and the pressure adjusting fluid pressure passage 62.
[0047] The SA.sub.-2 48 is a two-position solenoid valve which
selectively renders one of the first fluid pressure passage 36 and
the pressure adjusting fluid pressure passage 62 to be communicated
with the wheel cylinder 44FL, and renders the first fluid pressure
passage 36 and the wheel cylinder 44FL to be in a communicating
state in a normal state (OFF state). On the other hand, the SFLR 64
is a two-position solenoid valve which renders the pressure
adjusting fluid pressure passage 62 and the reservoir tank 16 to be
in a connected state or a disconnected state. The SFLR 64 renders
the pressure adjusting fluid pressure passage 62 and the reservoir
tank 16 to be in a disconnected state from each other in a normal
state (OFF state).
[0048] The second fluid pressure passage 38 communicates with the
above-mentioned SA.sub.-3 54. The downstream side of the SA.sub.-3
54 communicates with a right rear wheel holding solenoid valve 68
(hereinafter, referred to as SRRH 68) provided in correspondence
with a wheel cylinder 44RR of the right rear wheel RR and a left
rear wheel holding solenoid valve 70 (hereinafter, referred to as
SRLR 70) provided in correspondence with a wheel cylinder 44RL of
the left rear wheel RL. The SA.sub.-3 54 is a two-position solenoid
valve which selectively selectively renders one of the second fluid
pressure passage 38 and the third fluid pressure passage 42 to be
communicated with the SRRH 68 and the SRLR 70, and renders the
second fluid pressure passage 38, the SRRH 68 and the SRLR 70 in a
communicating state in a normal state (OFF state).
[0049] The downstream side of the SRRH 66 communicates with the
wheel cylinder 44RR and a right rear wheel pressure decreasing
solenoid valve 74 (hereinafter, referred to as SRRR 74) via a
pressure adjusting fluid pressure passage 72. The SRRR 74 is a
two-position solenoid valve which renders the pressure adjusting
fluid pressure passage 72 and the reservoir tank 16 in a
communicating state or a disconnected state, and renders the
pressure adjusting fluid pressure passage 72 and the reservoir tank
16 in the disconnected state in a normal state (OFF state).
Additionally, a check valve 76 permitting a fluid flow only in a
direction from the pressure adjusting fluid pressure passage 72 to
the SA.sub.-3 54 is provided, in parallel, between the SA.sub.-3 54
and the pressure adjusting fluid pressure passage 72.
[0050] Similarly, the downstream side of the SRLH 70 communicates
with the wheel cylinder 44RL and a left rear wheel pressure
decreasing solenoid valve 80 (hereinafter, referred to as SRLR 80)
via a pressure adjusting fluid pressure passage 78. The SRLR 80 is
a two-position solenoid valve which renders the pressure adjusting
fluid pressure passage 78 and the reservoir tank 16 in a
communicating state or a disconnected state, and renders the
pressure adjusting fluid pressure passage 78 and the reservoir tank
16 in the disconnected state in a normal state (OFF state).
Additionally, a check valve 82 permitting a fluid flow only in a
direction from the pressure adjusting fluid pressure passage 78 to
the SA.sub.-3 54 is provided, in parallel, between the SA.sub.-3 54
and the pressure adjusting fluid pressure passage 78.
[0051] In the system according to the present embodiment, a brake
switch 84 is provided near the brake pedal 30. The brake switch 84
is a switch that generates an ON output when the brake pedal 30 is
pressed. The output signal of the brake switch 84 is supplied to
the ECU 10. The ECU 10 determines whether or not a braking
operation is performed by the driver based on the output signal of
the brake switch 84.
[0052] Additionally, in the system according to the present
embodiment, wheel velocity sensors 86FL, 86FR, 86RL and 86RR
(hereinafter, these are referred to as 86** as a whole) are
provided near the left and right front wheels FL and FR and the
left and right rear wheels RL and RR, each of the sensors
generating a pulse signal when the respective wheel rotates a
predetermined angle. The output signals of the wheel velocity
sensors 86** are supplied to the ECU 10. The ECU 10 detects a wheel
velocity of each of the wheels FL, FR, RL and RR based on the
output signals of the wheel velocity sensors 86**.
[0053] The ECU 10 supplies, if necessary, drive signals to the
above-mentioned STR 26, SA.sub.-1 46, SA.sub.-2 48, SA.sub.-3 54,
SFRH 50, SFLH 52, SFRR 58, SFLR 64, SRRH 68, SRLH 70, SRRR 74 and
SRLR 80 based on the output signal of the brake switch 84.
[0054] A description will now be given of an operation of the brake
force control apparatus according to the present embodiment. The
brake force control apparatus according to the present embodiment
performs the normal control for generating a brake force
corresponding to the brake pressing force FP exerted on the brake
pedal 30 when the vehicle is in a stable state. The normal control
can be achieved, as shown in FIG. 1, by turning off all of the STR
26, SA.sub.-1 46, SA.sub.-2 48, SA.sub.-3 54, SFRH 50, SFLH 52,
SFRR 58, SFLR 64, SRRH 68, SRLH 70, SRRR 74 and SRLR 80 based on
the output signal of the brake switch 84.
[0055] That is, in the state shown in FIG. 1, the wheel cylinders
44FR and 44FL communicate with the first fluid pressure passage 36,
and the wheel cylinders 44RR and 44RL communicate with the second
fluid pressure passage 38. In this case, the brake fluid flows
between the master cylinder 32 and the wheel cylinders 44FR, 44FL,
44RL and 44RR (hereinafter, these may be referred to as ** as a
whole), and a brake force corresponding to the brake pressing force
FP is generated in each of the wheels FL, FR, RL and RR.
[0056] In the present embodiment, when a possibility for shifting
to a locked state is detected in one of the wheels, it is
determined that a condition for performing an antilock brake
control (hereinafter, referred to as ABS control) is established.
The ECU 10 calculates wheel velocities VWFL, VWFR, VWRL and VWRR
(hereinafter, these are referred to as VW** as a whole) of the
wheels based on output signals of the wheel velocity sensors 86**,
and calculates an assumed value VSO (hereinafter, referred to as an
assumed vehicle speed VSO) of a speed of the vehicle according to a
publicly known method. Then, when the vehicle is in a braking
state, a slip rate S of each wheel is calculated according to the
following equation so as to determine that the wheel may shift to a
locked state when the slip rate S exceeds a predetermined
value.
S=(VSO-VW**).multidot.100/VSO (1)
[0057] When the condition for performing the ABS control is
established, the ECU 10 outputs the drive signals to the SA.sub.-1
46, SA.sub.-2 48 and SA.sub.-3 54. As a result, when the SA.sub.-1
46 is turned on, the wheel cylinder 44FR is disconnected from the
first fluid pressure passage 36 and connected to the pressure
adjusting fluid pressure passage 56. Additionally, when the
SA.sub.-2 48 is turned on, the wheel cylinder 44FL is disconnected
from the first fluid pressure passage 36 and connected to the
pressure adjusting fluid pressure passage 62. Further, when the
SA.sub.-3 54 is turned on, the upstream side of the SRRH 68 and the
SRLH 70 is disconnected from the second fluid pressure passage 38
and connected to the third fluid pressure passage 42.
[0058] In this case, all wheel cylinders 44** communicate with
respective holding solenoid valves SFRH 50, SFLH 52, SRRH 68 and
SRLH 70 (hereinafter, these are referred to as holding solenoid
S**H) and respective pressure decreasing solenoid valves SFRR 58,
SFLR 64, SRRR 74 and SRLR 80 (hereinafter, these are referred to as
pressure decreasing solenoid S**R), and a regulator pressure PRE is
introduced to the upstream side of each of the holding solenoids
S**H via the third fluid pressure passage 42 and the STR 26.
[0059] In the above-mentioned condition, a wheel cylinder pressure
PW/C of the respective wheel cylinders 44** is increased with the
regulator pressure PRE as an upper limit by the holding solenoids
S**H being in an open state and the pressure decreasing solenoids
S**R being in a closed state. Hereinafter, this state is referred
to as a pressure increasing mode {circle over (1)}. Additionally,
the wheel cylinder pressure PW/C of the respective wheel cylinders
44** is maintained without being increased or decreased by the
holding solenoids S*H being in a closed state and the pressure
decreasing solenoids S**R being in the closed state. Hereinafter,
this state is referred to as a holding mode {circle over (2)}.
Further, the wheel cylinder pressure PW/C of the respective wheel
cylinders 44** is decreased by the holding solenoids S**H being in
the closed state and the pressure decreasing solenoids S**R being
in the open state. Hereinafter, this state is referred to as a
pressure decreasing mode {circle over (3)}. The ECU 10 achieves, if
necessary, the above-mentioned pressure increasing mode {circle
over (1)}, holding mode if and pressure decreasing mode {circle
over (3)} so that a slip rate S of each wheel during a braking time
becomes an appropriate value, that is, so that each wheel does not
shift to the locked state.
[0060] When a depression of the brake pedal 30 is released by the
driver during execution of the ABS control, the wheel cylinder
pressure PW/C must be immediately decreased. In the system
according to the present embodiment, the check valves 60, 66, 76
and 82 are provided in hydraulic pressure paths corresponding to
each of the wheel cylinders 44**, each of the check valves 60, 66,
76 and 82 permitting a fluid flow only in the directions from the
wheel cylinders 44** to the third fluid pressure passage 42. Thus,
according to the system of the present embodiment, the wheel
cylinder pressures PW/C of all of the wheel cylinders 44** can be
immediately decreased after the depression of the brake pedal 30 is
released.
[0061] In the system according to the present embodiment, when the
ABS control is performed, the wheel cylinder pressure PW/C is
increased by the brake fluid being supplied from the regulator 24
to the wheel cylinders 44**, that is, by the brake fluid being
supplied from the pump 12 to the wheel cylinders 44**, and is
decreased by the brake fluid in the wheel cylinders 44** flowing to
the reservoir tank 16. When the increase in the wheel cylinder
pressure PW/C is performed by using the master cylinder 32 as a
fluid pressure source and if the pressure increasing mode and the
pressure decreasing mode are repeatedly performed, the brake fluid
in the master cylinder 32 gradually decreases and a so-called
bottoming of the master cylinder may occur.
[0062] On the other hand, if the pump 12 is used as a fluid
pressure source so as to increase the wheel cylinder pressure PW/C,
as in the system according to the present embodiment, such a
bottoming can be prevented. Thus, in the system according to the
present embodiment, a stable operational state can be maintained if
the ABS control is continued for a long time.
[0063] In the system according to the present embodiment, the ABS
control is started when a possibility for shifting to the locked
state is detected in one of the wheels. Accordingly, in order to
start the ABS control, as a precondition, a braking operation
having a level at which a large slip rate S is generated in one of
the wheels must be performed.
[0064] FIG. 2 shows changes in the brake pressing force FP applied
to the brake pedal 30 with respect to time under various
conditions. Curves indicated by {circle over (1)} and {circle over
(2)} in FIG. 2 represent changes in the pressing force F when an
emergency braking is performed by a highly skilled driver
(hereinafter, referred to as a high-grade driver) and an unskilled
driver or a driver lacking (hereinafter, referred to as a
beginner-grade driver), respectively. The emergency braking
operation is an operation performed when is it desired to rapidly
decelerate a vehicle. Accordingly, the brake pressing force
associated with the emergency braking operation is preferably a
force sufficiently large as the ABS control is performed.
[0065] As shown by the curve {circle over (1)}, when the driver of
the vehicle is a high-grade driver, the brake pressing force FP is
immediately and rapidly increased in response to establishment of a
condition in which an emergency braking is required, and a large
brake pressing force FP can be maintained for a long time. If such
a brake pressing force FP is exerted on the brake pedal 30, a
sufficiently high brake fluid pressure can be provided from the
master cylinder 32 to each of the wheel cylinders 44** so as to
start the ABS control.
[0066] However, as shown by the curve {circle over (2)} when the
driver of the vehicle is a beginner-grade driver, the brake
pressing force FP may not be increased to a sufficiently high value
in response to the condition in which an emergency braking is
required. If the brake pressing force FP exerted on the brake pedal
30 is not sufficiently increased as shown by the curve {circle over
(2)} after an emergency braking is required, the wheel cylinder
pressure PW/C in each of the wheels 44** is not sufficiently
increased, which results in a possibility that the ABS control is
not started.
[0067] As mentioned above, when the driver of the vehicle is a
beginner-grade driver, the braking ability of the vehicle may not
be sufficiently performed even when an emergency braking operation
is performed despite that the vehicle has a good braking ability.
Accordingly, the system according to the present embodiment is
provided with a brake assist function for sufficiently increasing
the wheel cylinder pressure PW/C even if the brake pressing force
FP is not sufficiently increased when the brake pedal is operated
with an intention to perform an emergency braking. Hereinafter, a
control performed by the ECU 10 to achieve such a function is
referred to as a brake assist control.
[0068] In the system according to the present embodiment, when
performing the brake assist control, an accurate determination must
be made as to whether, when the brake pedal 30 is operated, the
operation is intended to perform an emergency braking operation or
to perform a regular braking operation.
[0069] Curves indicated by shown {circle over (3)} and {circle over
(4)} in FIG. 2 show changes in the brake pressing force FP when the
driver operates the brake pedal with an intention to perform a
normal braking operation under various conditions. As shown by the
curves {circle over (1)} to {circle over (4)}, a change in the
brake pressing force FP associated with the normal braking
operation is gentle as compared to a change in the brake pressing
force FP associated with an emergency braking operation.
Additionally, a convergent value of the brake pressing force FP
associated with the normal braking operation is not so large as a
convergent value of the brake pressing force FP associated with an
emergency braking operation.
[0070] Giving attention to those differences, when the brake
pressing force FP is increased to a sufficiently large value at a
rate of change exceeding a predetermined value after a braking
operation is started, that is, when the brake pedal 30 is operated
so that the brake pressing force FP reaches an area indicated by
(I) in FIG. 2, it can be determined that an emergency braking is
performed.
[0071] Additionally, when the rate of change of the brake pressing
force FP is smaller than the predetermined value or when the
convergent value of the brake pressing force FP is smaller than the
predetermined value, that is, when the brake pedal 30 is operated
so that the brake pressing force FP always changes within an area
indicated by (II) in FIG. 2, it can be determined that a normal
braking operation is performed.
[0072] Accordingly, in the system according to the present
embodiment, an operational speed and an amount of operation of the
brake pedal are detected or assumed, and, then, it is determined
whether or not the operational speed exceeds a predetermined value
and whether or not the amount of operation exceeds a predetermined
value, and, thereby, it can be determined whether or not the
operation on the brake pedal 30 is intended to perform an emergency
braking.
[0073] In the present embodiment, the speed and magnitude of the
operation of the brake pedal 30 are detected as a parameter that is
the master cylinder pressure PM/C detected by the hydraulic
pressure sensor 40 (hereinafter the parameter used for this
application is referred to as a basic parameter), The master
cylinder pressure PM/C indicates a value based on the magnitude of
the operation of the brake pedal 30, and varies with a variation
ratio PM/C based on the operation speed of the brake pedal 30.
Hence, according to the apparatus of the present embodiment, when
the braking operation is performed by the driver, it is possible to
precisely determine whether the operation is an emergency operation
or normal braking operation.
[0074] A description will be given of an operation of the system
according to the embodiment in a case where it is determined an
emergency braking is performed by the ECU 10. The ECU 10 determines
that an emergency braking is performed when the master cylinder
pressure PM/C that has a value greater than a predetermined value
is detected and the variation ratio .DELTA.PM/C that has a value
greater than a predetermined value is detected after the brake
pedal 30 .is pressed. When it is determined that the emergency
braking is performed, the ECU 10 sends a drive signal to the STR
26, SA.sub.-1 46, SA.sub.-2 48 and SA.sub.-3 54.
[0075] When the STR 26 is turned ON in response to the above drive
signal, the third fluid pressure passage 42 and the high-pressure
22 are joined together. In this case, an accumulator pressure PACC
is introduced into the third fluid pressure passage 42. When the
SA.sub.-1 46 and SA.sub.-2 48 are turned on in response to the
drive signal, the wheel cylinders 44FR and 44FL are jointed to the
pressure-adjusting fluid pressure passages 56 and 62, respectively.
Further, when the SA.sub.-3 54 is turned ON, the upstream portions
of the SRRH 68 and SRLH 70 are jointed to the third fluid pressure
passage 42. In this case, a state is formed in which all the wheel
cylinders 44** are jointed to the respective holding solenoids S**H
and the pressure decreasing solenoids S**R, and the accumulator
pressure PACC is introduced into the upstream portions of all the
holding solenoids S**H.
[0076] In the ECU 10, immediately after the execution of the
emergency braking is detected, all the holding solenoids S**H and
all the pressure decreasing solenoids S**R are maintained in the
OFF state. Hence, as described above, when the accumulator pressure
PACC is introduced into the upstream portions of the holding
solenoids S*H, the fluid pressure is supplied to the wheel
cylinders 44** as it is. Hence, the wheel cylinder pressure PW/C of
all the wheel cylinders 44** is increased toward the accumulator
pressure PACC.
[0077] As described above, according to the system of the present
embodiment, when the emergency braking is performed, the wheel
cylinder pressure PW/C of all the wheel cylinders 44** can rapidly
be increased irrespective of the magnitude of the brake pressing
force FP. Hence, according to the system of the present invention,
even if the driver is a beginner-grade driver, a large braking
force can rapidly be produced after a situation necessary for an
emergency braking occurs.
[0078] When the accumulator pressure PACC is started to be applied
to the wheel cylinders 44**, the slip ratios S of the wheels FL,
FR, RL and RR are abruptly increased, and then the condition for
execution of the ABS control stands. When the condition for
execution of the ABS control is satisfied, the ECU 10 realizes
{circle over (1)} the pressure increasing mode. {circle over (2)}
holding mode and {circle over (3)} pressure decreasing mode so that
the slip ratios of all the wheels fall within an appropriate range,
that is, all the wheels are not prevented from being locked.
[0079] If the ABS control is executed after the above-mentioned
brake assist control, the wheel cylinder pressure PW/C is increased
so that the brake fluid is supplied to the wheel cylinders 44**
from the pup 12 and the accumulator 20. Thus, even if the
pressure-increasing mode and the pressure-decreasing mode are
alternately performed, so that the so-called bottoming of the
master cylinder 32 may not occur.
[0080] When the emergency braking is performed and thus the brake
assist control is executed, it is required to terminate the brake
assist control when the brake pedal 30 is released from the pressed
state. In the system of the present embodiment, the STR 26,
SA.sub.-1 46, SA.sub.-2 48 and SA.sub.-3 54 are maintained in the
ON states as have been described. In the case where the STR 26,
SA.sub.-1 46, SA.sub.-2 48 and SA.sub.-3 54 are maintained in the
ON state, the fluid pressure chamber within the regulator 24 and
the first and second fluid pressure chambers 32a and 32b of the
master cylinder 32 are substantially closed spaces.
[0081] In this case, the accumulator pressure PACC is applied to
the wheel cylinders of the wheels, while the master cylinder
pressure PM/C depending on the brake pressing force FP is applied
to the hydraulic pressure sensor 40. Hence, the ECU 10 can
accurately determine, based on the detection value of the hydraulic
sensor 40, whether the brake pedal 30 is released from the pressed
state. When it is detected that the brake pedal 30 is released from
the pressed state, the ECU 10 stops supplying the drive signal to
the STR 26, SA.sub.-1 46, SA.sub.-2 48 and SA.sub.-3 54 and returns
the brake force control apparatus to the state in which the normal
control is performed (hereinafter, the above state will be referred
to as a normal brake state).
[0082] The brake force control apparatus of the present embodiment
is characterized in that execution of the brake assist control is
started when a predetermined delay time elapses after an emergency
braking required to start execution of the brake assist control is
detected. A description will now be given of the above feature.
[0083] FIG. 3(A) shows variations in the master cylinder pressure
PM/C and the wheel cylinder pressure PW/C which occur when a
beginner-grade driver performs an emergency braking. FIG. 3(B)
shows the variation ratio .DELTA.PM/C of the master cylinder
pressure PM/C observed when a beginner-grade driver performs an
emergency braking.
[0084] When an emergency braking is performed at time t0 by the
beginner-grade driver, the master cylinder pressure PM/C is varied
as indicated by curve {circle over (1)} shown in FIG. 3(A). In this
case, the variation ratio .DELTA.PM/C is changed as indicated by
curve {circle over (2)} shown in FIG. 3(B). As shown in curve
{circle over (2)}, the variation ratio .DELTA.PM/C increases
rapidly and then decreases rapidly.
[0085] A first predetermined value shown in FIG. 3(B) is a
threshold value related to the variation ratio .DELTA.PM/C and used
to discriminate the normal braking and emergency braking over each
other. When the braking operation executed by the driver is an
emergency braking, it takes a very short time for the variation
ratio .DELTA.PM/C to exceed first predetermined value .alpha. (time
t1) after the emergency braking is started (time t0).
[0086] Hence, it is possible to determine, within a very short time
after the braking is started, whether the braking is the normal
braking or emergency braking. Thus, when the emergency braking is
performed by the driver, it is possible to start the brake assist
control immediately after the braking is started.
[0087] However, as indicated by curve {circle over (1)}, at the
time (time t1) when the braking by the driver is recognized as
being an emergency braking, the master cylinder pressure PM/C is
increasing with a sharp slope. Hence, at this stage, it is
advantageous to maintain the master cylinder 32 as the fluid
pressure source rather than the pump 12 and the accumulator 20 in
terms of a rapid increase in the wheel cylinder pressure PW/C.
[0088] Hence, in order to rapidly raise up the wheel cylinder
pressure PW/C after the emergency braking is performed, it is
preferable not to start the brake assist control until a certain
delay time elapses after the emergency braking is started, more
particularly, the increasing slope of the master cylinder pressure
PM/C becomes gentle to some extent.
[0089] A second predetermined value .beta. shown in FIG. 3(B) is a
threshold value used to discern whether the increasing slope of the
master cylinder pressure PM/C becomes gentle. That is, in the
present embodiment, at the time when the variation ratio
.DELTA.PM/C indicated by curve {circle over (2)} becomes equal to
or less than .beta., it can be determined that the increasing slope
of the master cylinder pressure PM/C indicated by curve {circle
over (1)} becomes gentle.
[0090] Hence, in the brake force control apparatus of the present
embodiment, when an emergency braking is performed, it can be
determined that the brake assist control should not be executed
until .DELTA.PM/C<.beta. is satisfied. FIG. 3(B) shows that the
variation ratio .DELTA.PM/C reduces to the second predetermined
value .beta. at time t2.
[0091] The brake fluid flowing out of the master cylinder 32 flows
in the wheel cylinders 44** through brake hoses. The wheel cylinder
pressure PW/C increases after the brake fluid flows therein to some
extent. The brake fluid flowing out of the master cylinder 32 is
slightly consumed due to an expansion of the brake hoses in the
progress of reaching the wheel cylinders 44**. Hence, there is a
time lag until an increase in the wheel cylinder pressure PW/C is
started after an increase in the master cylinder pressure PM/C is
started.
[0092] Curve {circle over (3)} of the one-dot chained line shown in
FIG. 3(A) shows a variation in the wheel cylinder pressure PW/C
observed when the master cylinder pressure PM/C is changed as
indicated by curve {circle over (1)} due to execution of the normal
control. Curve {circle over (3)} exemplarily indicates a case where
an increase in the master cylinder pressure PM/C is started at time
t0 and then an increase in the wheel cylinder pressure PW/C is
started bust before time t2.
[0093] The variation in the master cylinder pressure PM/c indicated
by curve {circle over (3)} is realized in the case where, at time
t2, the brake fluid necessary for initial press has already flowed
in the wheel cylinders 44** and the brake fluid consumed due to
expansion of the brake hoses have already been supplied from the
master cylinder 32 Hence, if the brake assist control is started at
time t2, the wheel cylinder pressure PW/C can rapidly be increased
in the case where the pump 12 and the accumulator 20 serve as the
fluid pressure source.
[0094] Curve {circle over (4)} indicated by the two-dot chained
line shown in FIG. 3(A) shows a variation in the wheel cylinder
pressure PW/C observed when the brake assist control is started at
time t2, that is, when it is determined that an increase in the
master cylinder pressure PM/C becomes gentle. As shown in curve
{circle over (4)}, if the brake assist control is started at that
timing, the wheel cylinder pressure PW/C can sufficiently be
increased even if the master cylinder pressure PM/C is not
sufficiently increased to a level close to the upper limit value
(see curve {circle over (1)}) after the emergency braking is
performed.
[0095] However, at the stage of time t2, the master cylinder
pressure PM/C is considerably higher than the wheel cylinder
pressure PW/C, and thus a relatively large quantity of brake fluid
flows in the wheel cylinders 44** from the master cylinder 32 even
if the master cylinder pressure PM/C is not increased but the
normal control is maintained. The quantity of brake fluid, which
can be, supplied from the pump 12 and the accumulator 20 to the
wheel cylinders 44** is limited to a level based on the capability
of the pump 12 and the capacity of the accumulator 20. Hence, if
there is a great difference between the master cylinder pressure
PM/C and the wheel cylinder pressure PW/C, as indicated by curves
{circle over (3)} and {circle over (4)}, the wheel cylinder
pressure PW/C can rapidly be increased when the normal control is
retained, as compared to the case where the brake assist control is
started.
[0096] Hence, in the brake control apparatus of the present
embodiment, in order to rapidly increase the wheel cylinder
pressure PW/C after an emergency braking is performed, it is
preferable that the brake assist control be not executed as long as
the variation ratio .DELTA.PM/C obtained when the variation ratio
.DELTA.PM/C obtained after the brake assist control is started
(hereinafter, the above variation will be referred to as pressure
increasing slope .DELTA.PM/C) is greater than the variation ratio
.DELTA.PM/C obtained when the normal control is maintained
(hereinafter, the above variation ratio will be referred to as
normal pressure increasing slope .DELTA.PM/C).
[0097] The brake force control apparatus of the present embodiment
realizes the above-mentioned functions by starting execution of the
brake assist control when the predetermined delay time D elapses
after the time then an emergency braking is started and the
variation ratio .DELTA.PM/C of the master cylinder pressure PM/C
becomes less than the first predetermined value .beta.
(hereinafter, the above time will be referred to as emergency
braking state detection time). Curve {circle over (5)} shown in
FIG. 3(A) indicates a variation in the wheel cylinder pressure PW/C
generated when the brake assist control is started at time t3 when
the delay time D elapses after time t2.
[0098] As indicated by curve {circle over (5)}, when the brake
assist control is started when the delay time D elapses after the
emergency braking state detection time, the resultant wheel
cylinder pressure PM/C is always greater than the wheel cylinder
pressure PW/C generated when the normal control is maintained.
Thus, according to the brake force control apparatus of the present
embodiment, when an emergency braking is performed, both the
capability of the master cylinder 32 and the capability of the pump
12 and the accumulator 20 are effectively utilized to rapidly
increase the wheel cylinder pressure PW/C.
[0099] FIG. 4 is a flowchart of an example of a control routine
executed by the ECU 10 so as to implement the above-mentioned
functions. When the present routine is activated, a process of step
100 is executed.
[0100] At step 100, it is determined whether the brake assist
control is being executed. The present routine is a routine
directed to determining the timing at which the brake assist
control is started. Hence, if it is determined that the brake
assist control has been executed, the routine is ended without any
process. If the brake assist control has not been activated, a
process of step 102 is next executed.
[0101] At step 102, it is determined whether an emergency braking
has been performed. More particularly, it is determined whether,
after an ON output of the brake switch 84 is issued, the variation
ratio .DELTA.PM/C exceeding the first predetermined value .alpha.
occurs in master cylinder pressure PM/C. If it is determined that
the emergency braking has not been performed, the routine is ended
without any process. In contrast, if it is determined that the
emergency braking has been performed, a process of step 104 is next
executed.
[0102] At step 104, the process is executed in which a maximum
value MAX.DELTA.PM/C of the variation ratio .DELTA.PM/C occurring
in master cylinder pressure PM/C is stored. More particularly, if
the variation ratio .DELTA.PM/C detected by he present-time process
is greater than that detected in the last-time process, the
detection value detected by the present-time process is stored as
the updated maximum value MAX.DELTA.PM/C. When the process of step
104 ends, a process of step 106 is next executed.
[0103] At step 106, it is determined whether the variation ratio
.DELTA.PM/C of the master cylinder pressure PM/C is less than the
second predetermined value .beta., that is, whether a variation in
increase of the master cylinder pressure PM/C becomes gentle. If it
is determined that a condition .DELTA.PM/C<.beta. is not yet
satisfied, the processes of steps 104 and 106 are repeatedly
carried out until the above condition is satisfied. When it is
determined that the condition .DELTA.PM/C<.beta. is satisfied, a
process of step 108 is then executed.
[0104] According to the above-mentioned process, at the time when
the condition .DELTA.PM/C<.beta. is satisfied, the maximum
variation ratio .DELTA.PM/C generated in the master cylinder
pressure PM/C obtained prior to the emergency braking state
detection time is stored as the maximum value MAX.DELTA.PM/C.
[0105] FIG. 5 shows a relationship between the above-mentioned
maximum value MAX.DELTA.PM/C and an emergency braking time
differential pressure .DELTA.PEM. The emergency braking time
differential pressure .DELTA.PEM is the difference (corresponding
to the differential pressure .vertline.PM/C-PW/C.vertline. at time
t2 shown in FIG. 3) between the master cylinder pressure PM/C and
the wheel cylinder pressure PW/C obtained when the condition of
step 106 is satisfied (the emergency braking state detection time).
As shown in FIG. 5, the emergency braking time differential
pressure .DELTA.PEM increases as the above-mentioned maximum value
MAX.DELTA.PM/C increases.
[0106] At step 108, the detected value PM/C of the hydraulic
pressure sensor 40 is stored as master cylinder pressure PM/C
master cylinder pressure PM/C obtained at the emergency braking
state detection time.
[0107] FIG. 6 shows a relationship between the master cylinder
pressure PM/C obtained at the time of emergency braking and the
emergency braking time differential pressure .DELTA.PEM. As shown
in FIG. 6, the emergency braking time differential pressure
.DELTA.PEM increases as the master cylinder pressure PM/C obtained
at the time of emergency braking increases. When the process of
step 108 ends, a process of step 110 is then executed.
[0108] The delay time D to be ensured between the emergency braking
state detection time and the time of starting the brake assist
control should be the time necessary for the controlled pressure
increasing slope PM/C to exceed the normal pressure increasing
slope .DELTA.PW/C after the emergency braking state detection time
is recognized. The above time becomes long as the emergency braking
time differential pressure .DELTA.PEM increases, and becomes short
as the emergency braking time differential pressure .DELTA.PEM
decreases.
[0109] As shown in FIG. 5, the emergency braking time differential
pressure .DELTA.EM increases as the maximum value MAX.DELTA.PM/C
increases. Hence, it is appropriate that the delay time D is
elongated as the maximum value MAX.DELTA.PM/C increases. Further,
as shown in FIG. 6, the emergency braking time differential
pressure .DELTA.EM increases as the emergency braking time master
pressure PM/CEM increases. Hence, it is appropriate that the delay
time D becomes long as the emergency braking time master pressure
PM/CEM increases.
[0110] At step 110, the delay time D is calculated based on the
above-mentioned maximum value MAX.DELTA.PM/C and the emergency
braking time master pressure PM/CEM. When the process of step 110
ends, a process of step 112 is then executed.
[0111] FIG. 7 shows an example of a map, which is referred to when
the delay time D is calculated. As shown in FIG. 7, the delay time
is set to a comparatively long delay time DL or a comparatively
short delay time DS on the basis of the maximum value
MAX.DELTA.PM/C and the emergency braking time master pressure
PM/CEM.
[0112] More particularly, the delay time D is set to the
comparatively long time DL if the maximum value MAX.DELTA.PM/C and
the emergency braking time master pressure PM/CEM are great, that
is, when the emergency braking time differential pressure
.DELTA.PEM is large. In contrast, the delay time D is set to the
comparatively short time DS if the maximum value MAX.DELTA.PM/C and
the emergency braking time master pressure PM/CEM are small, that
is, when the emergency braking time differential pressure
.DELTA.PEM is small.
[0113] At step 112, the down counting of the delay time D is
carried out. When the process of step 112 ends, a process of step
114 is then executed.
[0114] At step 114, it is determined whether the start timing of
the brake assist control comes, that is, whether the down counting
of the delay time D is completed. If it is determined that the down
counting of the delay time D has not yet been completed, the
process of step 112 is executed again. In contrast, it is
determined that the down counting is completed, a process of step
116 is then executed.
[0115] At step 116, a process for starting the brake assist control
is executed. When the process of step 116 ends, the routine in
progress is ended.
[0116] According to the above process, if it takes a comparatively
long time for the controlled pressure increasing slope .DELTA.PW/C
to exceed the normal pressure increasing slope .DELTA.PW/C after
the emergency braking state detection time is recognized after the
emergency braking state detection time is detected, the delay time
D can be set to the comparatively long time DL. Hence, according to
the brake force control apparatus of the present embodiment, after
the emergency braking is performed by the driver, it is possible to
rapidly increase the wheel cylinder pressure PW/C by utilizing both
the capability of the master cylinder 32 and the capabilities of
the pump 12 and the accumulator 20.
[0117] By the way, in the present embodiment, by referring to the
map of FIG. 7, the delay time D can be set in the two-stage
formation. However, the method of setting the delay time D is not
limited.
[0118] FIG. 8 shows another example of a delay time map applicable
to the brake force control apparatus of the present embodiment. The
map of FIG. 8 is a map directed to continuously changing the delay
time D in correspondence with the magnitude of the maximum value
MAX.DELTA.PM/C and the magnitude of the emergency braking time
master pressure PM/CEM. By using the map of FIG. 8, it is possible
to more precisely set the starting time of the brake assist control
and more accurately control the pressure increasing characteristic
of the wheel cylinder pressure PW/C.
[0119] In the above-mentioned embodiment, the delay time D is
estimated based on the um value MAX.DELTA.PM/C and the emergency
braking time master pressure PM/CEM. However, the method of
calculating the delay time D is not limited to the above.
[0120] That is, in the brake force control apparatus of the present
embodiment, the emergency braking time differential pressure
.DELTA.PEM can be estimated to be great if the time T .beta. it
takes for the variation ratio .DELTA.PM/C to exceed the first
predetermined value .alpha. and then becomes equal to or less than
the second predetermined value .beta. (the time between t1 and t2
shown in FIG. 3) is long. Further, it is possible to estimate the
emergency braking time differential pressure .DELTA.PEM to be small
if the above time T .beta. is short (see FIG. 5).
[0121] Hence, the delay time D may be calculated on the basis of
the time T .beta. it takes for the variation ratio .DELTA.PM/C to
exceed the first predetermined value .alpha. and then becomes equal
to or less than the second predetermined value .beta.. Although the
above-mentioned embodiment employs the combination of the maximum
value MAX.DELTA.PM/C and the emergency braking time master pressure
PM/CEM, the present invention is not limited to the above but may
be configured so that the delay time D can be calculated based on
any of the maximum value MAX.DELTA.PM/C, the emergency braking time
master pressure PM/CEM, and the time T .beta..
[0122] Further, in the above-mentioned embodiment, the delay time
is changed in accordance with the emergency braking time
differential pressure .DELTA.PEM. However, the present invention is
not limited to the above, and may be configured so that the brake
assist control is necessarily started when the constant delay time
D elapses after the emergency braking state detection time.
[0123] Moreover, in the above-mentioned embodiment, the emergency
braking state detection time is defined as the time when the
variation ratio .DELTA.PM/C reduces to .beta. after becoming equal
to or greater than .alpha., and the down counting is started after
the emergency braking state detection time. However, the emergency
braking state detection time is not limited to the above. That is,
the time when an emergency braking is detected (when PM/C>) is
defined as the emergency braking state detection time, and then the
down counting of the delay time D may be started.
[0124] Furthermore, in the above-mentioned embodiment, the
emergency braking and the normal braking are discriminated over
each other on the basis of the master cylinder pressure PM/C.
However, the basis parameter used for the above discrimination is
not limited to the master cylinder pressure PM/C.
[0125] More particularly, wen the brake pedal 30 is operated, the
brake pressing force FP exerted on the brake pedal 30 and the
magnitude of stroke of the brake pedal 30 are varied in addition to
variation in the master cylinder pressure PM/C. Also, when the
brake pedal 30 is operated and brake force is thus exerted on the
vehicle, a deceleration G is generated on the vehicle. Hence, the
discrimination over the emergency braking and the normal braking
can be estimated based on, in addition to the aforementioned master
cylinder pressure PM/C {circle over (1)}, the brake pressing force
FP {circle over (2)}, pedal stroke L {circle over (3)}, vehicle
deceleration G {circle over (4)}, estimated vehicle velocity VSO
{circle over (5)}, and the wheel velocity VW** {circle over
(6)}.
[0126] The brake pressing force FP {circle over (2)} and pedal
stroke L {circle over (3)} among the parameters {circle over (1)},
{circle over (2)}, {circle over (3)}, {circle over (4)}, {circle
over (5)} and {circle over (6)} are sensitive to the operation of
the brake pedal 30 as in the case of the master cylinder pressure
PM/C {circle over (1)}. Hence, if these parameters are used as the
basic parameters, the parameters are monitored so that it is easily
possible to determine whether the pressing of the brake pedal 30 is
released.
[0127] In contrast, the vehicle deceleration G {circle over (4)},
estimated vehicle velocity VSO {circle over (5)} and the wheel
velocity VW** {circle over (6)} among the parameters {circle over
(1)}, {circle over (2)}, {circle over (3)}, {circle over (4)},
{circle over (5)} and {circle over (6)} are not changed until the
brake forces of the wheels start to change. Hence, even if the
brake pedal 30 is released from the pressed state during execution
of the brake assist control, a large variation in the above
parameters do not occur until the brake assist control is ended.
Thus, when the vehicle deceleration G {circle over (4)}, estimated
vehicle velocity VSO {circle over (5)} and the wheel velocity VW**
{circle over (6)} are used as the basic parameters, a pressing
switch is provided which detects whether the pressing force FP is
exerted on the brake pedal 30. Then, a decision is made as to
whether the brake assist control is ended based on the output state
of the pressing switch.
[0128] A description will now be given, with reference to FIGS. 9
through 11, of a second embodiment of the present invention.
[0129] In order to maintain the vehicle in the stable turning
behavior, it is necessary for the front and rear wheels FL, FR, RL
and RR to generate the respective cornering forces based on the
vehicle velocity and the turning radius. More particularly, if the
front wheels FL and Fr generate insufficient cornering forces, the
vehicle will tend to drift out, in contrast, if the rear wheels RL
and RR generate insufficient cornering forces, the vehicle will
tend to spin.
[0130] The maximum value of the cornering forces that can be
generated by the wheels depends on the performance of tires, the
coefficient of friction against the road surface, and the loads
exerted on the wheels. In addition, the maximum value of the
cornering forces becomes great as a small degree of the ground
contact ability of the wheels is consumed in order to generate the
drive force and brake force. Hence, if the braking operation is
performed while the vehicle is turning and thus the ground contact
ability of the tires is partially consumed to generate the braking
force, the maximum value of the cornering forces which can be
generated by the tires will be reduced.
[0131] If the brake force is exerted on the vehicle, the load of
the vehicle shifts to the front wheels FL and FR. When the load of
the vehicle shifts to the front wheels FL and FR, the vertical load
exerted on the rear wheels RL and RR is decreased, and thus the
ground contact ability of the rear wheels is decreased. Hence, if
the braking is performed while the vehicle is turning, the maximum
value of the cornering forces that can be generated by the rear
wheels RL and RR will greatly be decreased as compared to the
situation before the brake force is generated.
[0132] By the way, in the brake force control device according to
the aforementioned first embodiment, the brake assist control of
the front wheels FL and FR and the brake assist control of the rear
wheels RL and RR are simultaneously started when the predetermined
delay time D elapses after the emergency braking is detected. In
this regard, the brake force control apparatus of the first
embodiment has a characteristic which is likely to degrade the
turning behavior of the vehicle due to the start of the brake
assist control in response to the emergency braking which is
performed when the vehicle is turning.
[0133] The brake force control apparatus of the present embodiment
is directed to overcoming the above problem of the brake force
control apparatus of the first embodiment, and is characterized in
that the brake assist control of the front wheels FL and FR is
started, and the brake assist control of the rear wheels RL and RR
is then started with an appropriate delay time (hereinafter, the
above delay time will be referred to as rear wheel delay time
DR).
[0134] The brake force control apparatus of the present embodiment
can be realized by causing the ECU 10 used in the system structure
shown in FIG. 1 to execute a routine shown in FIG. 9 rather than
the routine shown in FIG. 4.
[0135] FIG. 9 is a flowchart of a characterizing part of the
routine executed in the present embodiment. After the routine of
FIG. 9 is activated, the ECU 10 executes the processes of the steps
100 to 114 as in the case of the first embodiment. If it is
determined that the brake assist control should be started now, a
process of step 120 is executed.
[0136] At step 120, a process is started which starts the brake
assist control of the two front wheels. More particularly, the
process for turning ON STR 26 and turning ON SA.sub.-1 46 and
SA.sub.-2 48 is started. When the above process is executed, the
wheel cylinder pressure PW/C of the front wheels FL and FR is
increased toward the accumulator pressure PACC. When the process of
the present step ends, a process of step 122 is then executed.
[0137] At step 122, it is determined whether or not the variation
ratio .DELTA.DVSO of a deceleration velocity DVSO generated in the
vehicle is equal to or greater than a predetermined value .gamma..
The deceleration DVSO is an estimated vehicle velocity VSO.
[0138] Curve {circle over (1)} shown in FIG. 10 indicates a
variation in the deceleration DVSO observed when the vehicle is
running on a road having a high friction coefficient. When the
vehicle is traveling on the road with a high friction coefficient,
the deceleration DVSO is abruptly increased after time t0, in
response to the start of the brake assist control of the two front
wheels. In other words, in the system of the present embodiment, it
can be determined that the vehicle is traveling on the road with a
high coefficient of friction if the deceleration DVSO is abruptly
increased after time t0.
[0139] Curves {circle over (2)} and {circle over (3)} shown in FIG.
10 respectively indicate variations in the deceleration DVSO
respectively observed when the vehicle is traveling on a road
having a middle coefficient of friction and a road having a low
coefficient of friction. When the vehicle is traveling on a road
having a middle or low coefficient of friction, the deceleration
DVSO gradually increases after the time t0, as compared to that
observed when the vehicle is traveling on the road having a high
coefficient of friction. In other words, when the deceleration DVSO
gradually increases after the time t0, it can be determined that
the vehicle is traveling on the road having a low coefficient of
friction. In this case, the coefficient of friction against the
vehicle traveling road can be estimated based on the deceleration
DVSO that occurs after the time t0.
[0140] The predetermined value .gamma. used at step 122 is a
threshold value provided to determine whether the vehicle is
traveling on a road of a high coefficient of friction. Hence, if it
is determined at step 122 that .DELTA.DVSO .gamma. is not
satisfied, it can be concluded that the vehicle was traveling on a
road with a coefficient of friction lower than that the high
coefficient of friction when the brake assist control was started.
In this case, a process of step 124 is executed.
[0141] When the vehicle is traveling on the road with a low
coefficient of friction, the turning behavior of the vehicle is
likely to be changed due to the start of the brake assist control.
Thus, if it is estimated that the vehicle is traveling on the road
having a low coefficient of friction, it is preferable that the
brake assist control of the rear wheels RL and RR is not started
for a while after the brake assist control of the front wheels FL
and FR is started.
[0142] At step 124, the rear wheel delay time DR is computed which
should be ensured between the time when the brake assist control of
the two front wheels is started and the time when the brake assist
control of the two rear wheels is started. When the process of step
124 ends, a process of step 126 is then executed.
[0143] FIG. 11 shows an example of a map that is to be referred to
when the rear-wheel delay time DR is calculated at step 124. The
rear-wheel delay time DR is set to a long time as the variation
ratio .DELTA.DVSO of the deceleration is small, that is, as the
coefficient of friction against the road is small. Hence, according
to the system of the present embodiment, the timing at which the
brake assist control of the rear wheels is started can be delayed
as the coefficient of friction against the road on which the
vehicle is traveling is small.
[0144] At step 126, it is determined that the rear-wheel delay time
DR elapses. The process of step 126 is repeatedly carried out until
it is determined that the rear-wheel delay time DR elapses. When it
is determined that the rear-wheel delay time DR elapses, a process
of step 128 is executed.
[0145] At step 128, the process for starting the brake assist
control of the two rear wheels is executed. More particularly, the
process for turning ON SA.sub.-3 53 is performed. After the above
process is executed, the wheel cylinder pressure PW/C of the rear
wheels RL and RR is increased toward the accumulator pressure PACC.
When the process of step 128 ends, the routine in progress is
ended.
[0146] In the present routine, if it is determined at step 122 that
the variation ratio .DELTA.DVSO of the deceleration is greater than
the predetermined value .gamma., it is possible to determine that
the vehicle was traveling on a high-friction-coefficient road when
the brake assist control of the front wheels was started. When the
vehicle is traveling on a high-friction-coefficient road, a large
change of the turning behavior of the vehicle does not occur even
if the brake assist control of the rear wheels is started soon
after the brake assist control of the front wheels is started.
Hence, if it is determined at step 122 that .DELTA.DVSO .gamma. is
satisfied, steps 124 and 126 are bypassed, and a process of step
128 is executed.
[0147] Curve {circle over (1)} shown in FIG. 10 indicates a
variation in the variation ratio .DELTA.DVSO of deceleration that
is realized when it is determined that .DELTA.DVSO .gamma. is
satisfied (the condition prescribed at step 122) after the brake
assist control of the front wheels is started at time t0. As
indicated by curve {circle over (1)}, the deceleration DVSO of the
vehicle is rapidly increased to a greater value.
[0148] Curve {circle over (2)} shown in FIG. 10 indicates a
variation in the variation ratio .DELTA.DVSO of deceleration that
is realized when it is determined that .DELTA.DVSO .gamma. is not
satisfied at time t1 and the rear-wheel delay time DR is set to a
comparatively short time DR1. Curve {circle over (3)} indicates a
variation in the variation ratio .DELTA.DVSO of deceleration that
is realized when it is determined that .DELTA.DVSO .gamma. is not
satisfied at time ti and the rear-wheel delay time DR is set to a
comparatively long time DR2.
[0149] As indicated by curves {circle over (1)} {circle over (2)}
and {circle over (3)}, according to the brake force control
apparatus of the present embodiment, as the coefficient of friction
against the road on which the vehicle is traveling becomes smaller,
the brake assist control of the two front wheels is active for a
longer time. While the brake assist control is being performed to
only the two front wheels, it is possible to generate sufficiently
large cornering forces on the rear wheels RL and RR. Hence,
according to the brake force control apparatus of the present
embodiment, it is possible to stably maintain the turning behavior
of the vehicle without being affected by the coefficient of
friction against the road and to thus generate a large brake force
when an emergency braking is required.
[0150] By the way, in the aforementioned embodiment, the brake
assist control of the rear wheels RL and RR is necessarily executed
after the rear-wheel delay time DR irrespective of whether the
vehicle is turning, if the coefficient of friction against the road
is low. However, the present invention is not limited to the above,
but may be configured so that the start timing of the brake assist
control of the rear wheels is delayed only if it is determined that
the vehicle is turning by referring to a steering angle sensor or a
yaw rate sensor.
[0151] A description will now be given, with reference to FIG. 12,
of a third embodiment of the present invention. FIG. 12 is a
diagram of a system configuration of the brake force control
apparatus according to the present embodiment. In FIG. 12, only a
part of the configuration related to a single wheel is depicted for
the convenience of description.
[0152] The brake force control apparatus shown in FIG. 12 is
controlled by the ECU 200. The brake force control apparatus of the
present embodiment is equipped with a brake pedal 202. A brake
switch 203 is disposed in the vicinity of the brake pedal 202. The
brake switch 203 generates an ON output when the brake pedal 202 is
pressed. The output signal of the brake switch 203 is supplied to
the ECU 200. The ECU 200 determines, based on the output signal of
the brake switch 203, whether the braking is performed.
[0153] The brake pedal 202 is joined to a vacuum booster 204. The
vacuum booster 204 operates with a drive source, which is
depression at a manifold of an internal combustion engine. When the
brake pressing force FP is applied to the brake pedal 30, the
vacuum booster 204 generates an assist force FA having a given
power ratio with respect to the brake pressing force FP. A master
cylinder 206 is fixed to the vacuum booster 204. The resultant of
the brake pressing force FP and the assist force FA is input to the
master cylinder 206.
[0154] The master cylinder 206 is equipped with a fluid pressure
chamber provided therein. A reservoir tank 208 is arranged on the
upper portion of the master cylinder 206. The fluid pressure
chamber and the reservoir tank 208 are joined together when the
brake pedal 202 is in the released state, and are isolated from
each other when the brake pedal 202 is pressed. Hence, the brake
fluid is supplemented each time the brake pedal 202 is released
from the pressed state.
[0155] A fluid pressure passage 210 is joined to the fluid pressure
chamber of the master cylinder 206. A hydraulic sensor 210, which
generates an electric signal based on the internal pressure of the
fluid pressure passage 210, is disposed to the fluid pressure
passage 210. The output signal of the hydraulic pressure sensor 212
is supplied to the ECU 200. The ECU 200 detects the fluid pressure
generated by the master cylinder 206, namely, the master cylinder
pressure PM/C on the basis of the output signal of the hydraulic
pressure sensor 212.
[0156] A fluid pressure cut solenoid 214 (hereinafter simply
referred to as SC 214) is arranged to the fluid pressure passage
210. The SC 214 is a two-position solenoid valve, which correspond
to a conducting state and shutting state. The SC 214 is turned ON
(the closed state) when the ECU 200 supplies a drive signal
thereto.
[0157] The fluid pressure passage 210 is provided with a holding
solenoid 216 (hereinafter, referred to as SH 216) on the downstream
side of the SC 214. The SH 216 is a two-position solenoid valve
which maintains an open state in a normal state (OFF state) The SH
216 is set to be in an ON state (closed state) by a drive signal
being supplied by the ECU 200.
[0158] The downstream side of the SH 216 communicates with a wheel
cylinder 218 and a pressure decreasing solenoid 220 (hereinafter,
referred to as SR 220). The SR 220 is a two-position solenoid
valve, which maintains a closed state in a normal state (OFF
state). SR 220 is set to be in an ON state (open state) by a drive
signal being supplied by the ECU 200. Additionally, a check valve
222 which permits a fluid flow only in a direction from the wheel
cylinder 218 to the fluid pressure passage 210 is provided between
the wheel cylinder 218 and the fluid pressure passage 210.
[0159] A wheel velocity sensor 219 generates a pulse signal each
time the wheel rotates a predetermined angle is provided near the
wheel cylinder 218. An output signal of the wheel velocity sensor
219 is supplied to the ECU 200. The ECU 200 detects a wheel
velocity based on the output signal of the wheel velocity sensor
219.
[0160] A reservoir 224 is provided on the downstream side of the SR
220. The brake fluid flowing out of the SR 220 when the SR 220 is
set to be in the ON state (open state) is stored in the reservoir
224. It should be noted that the reservoir previously stores a
predetermined amount of brake fluid. The reservoir 224 communicates
with an inlet port 226a of a pump 226. Additionally, an outlet port
226b of the pump 226 communicates with the fluid pressure passage
210 via a check valve 228. The check vale 228 is a one-way valve,
which permits a fluid flow only in a direction from the pump 226 to
the fluid pressure passage 210.
[0161] A fluid pressure passage 230 connected to the reservoir tank
208 is joined to the reservoir 224. A switch solenoid 234
(hereinafter simply referred to as SCH 234) is disposed to the
fluid pressure passage 230. The SCH 234 is a two-position solenoid
valve, which maintains the closed state in the normal state (OFF
state). The SCH 234 is switched to the closed state in response to
a supply of the drive signal from the ECU 200.
[0162] A description will now be given of an operation of the brake
force control apparatus of the present embodiment. In the present
embodiment, the ECU 200 determines whether the brake assist control
should be started and determines the start timing thereof by
executing the routine shown in FIG. 4 of FIG. 9 as in the case of
the aforementioned first or second embodiment.
[0163] More particularly, the ECU 200 starts to count down the
delay time D when the condition .DELTA.PM/C<.beta. is satisfied
after an emergency braking is performed. The ECU 200 starts the
brake assist control when the down counting of the delay time D is
completed. When the ECU 200 executes the routine shown in FIG. 9,
the brake assist control of only the two front wheels is started
and then the brake assist control of the rear wheels is started
after the rear-wheel delay time DR.
[0164] In the system of the present embodiment, when the ECU 200
executes the normal control, the SC 214, SCH 234, SH 216 and SR 220
are all maintained in the OFF state, and the pump 226 is maintained
in the inactive state. Under the above situation, only the master
cylinder 206 can function as the fluid pressure source, and all of
the brake fluid flowing out of the master cylinder 206 is supplied
to the wheel cylinder 218. Hence, in this case, the wheel cylinder
pressure PW/C of the wheel cylinder 218 is adjusted to a fluid
pressure having the given power ratio with respect to the brake
pressing force FP.
[0165] After the braking is started, if a slip ratio S of the
wheels exceeds a predetermined value, the ECU 200 starts the ABS
control as in the case of the ECU 10 of the first embodiment. As
will be described below, the ABS control is implemented by driving
the SH 216 and SR 220 while operating the pump 226 when the brake
pedal 202 is pressed, that is, when the master cylinder pressure
PM/C is appropriately boosted.
[0166] When the master cylinder pressure PM/C is output which is
supplied from the master cylinder 204 and is appropriately boosted,
the SH 216 is switched to the closed state, and the SR 220 is
switched to the closed state. Hence, it is possible to boost the
wheel cylinder pressure PW/C while the master cylinder pressure
PM/C is the upper limit. Hereinafter, the above state will be
referred to as {circle over (1)} pressure-increasing mode. Under
the same environment as described above, if the SH 216 is set to
the closed state and the SR 220 is set to the closed state, the
wheel cylinder pressure PW/C can be maintained. If the SH 216 is
set to the closed state and the SR 220 is set to the open state,
the wheel cylinder pressure FW/C can be reduced. These states will
hereinafter be referred to {circle over (2)} holding mode and
{circle over (3)} pressure-decreasing mode, respectively. The ECU
200 realizes {circle over (1)} pressure-increasing mode, {circle
over (2)} holding mode and {circle over (3)} pressure-decreasing
mode so that the slip ratio S of the wheels becomes equal to an
appropriate value.
[0167] If the brake pedal 202 is released from the pressed state by
the driver while the ABS control is active, it is necessary to
rapidly reduce the wheel cylinder pressure PW/C. In the system of
the present embodiment, a check valve 222 which permits a flow of
fluid from the wheel cylinder 218 to the master cylinder 206 is
disposed to a hydraulic pressure circuit corresponding to the wheel
cylinder 218. Hence, according to the system of the present
embodiment, it is possible to rapidly reduce the wheel cylinder
pressure PW/C of the wheel cylinder 222 after the brake pedal 202
is released from the pressed state.
[0168] While the ABS control is being executed in the system of the
present embodiment, the wheel cylinder pressure PW/C is boosted in
such a way that the master cylinder 206 serves as the fluid
pressure source. Also, the wheel cylinder pressure PW/C can be
reduced by flowing the brake fluid in the wheel cylinder 218 to the
reservoir 224. Hence, if the pressure-increasing mode and the
pressure-decreasing mode are alternately performed, the brake fluid
gradually flows to the reservoir 224 from the master cylinder 206.
However, in the system of the present embodiment, the brake fluid
flowing to the reservoir 224 is pressure-sent to the master
cylinder 206 by the pump 226. Hence, even if the ABS control
continues for a long time, the bottoming of the master cylinder
will not occur.
[0169] As described above, the ECU 200 starts the brake assist
control when the predetermined delay time D elapses after the
emergency braking state required to start execution of the brake
assist control is detected. In the system of the present
embodiment, the brake assist control is realized by turning ON both
the SC 214 and SCH 234, that is, setting the SC 214 and SCH 234 to
the close and open states, respectively, and by activating the pump
226.
[0170] Under the above situation, the master cylinder 206 and the
wheel cylinder 218 is isolated from each other. The pump 226
pressure-sends the brake fluid supplied from the reservoir tank 208
via the fluid pressure passage 230 toward the wheel cylinder 218.
Hence, the wheel cylinder pressure PW/C is boosted in such a way
that the pump 226 serves as the fluid pressure source.
[0171] In the system of the present embodiment, boosting of the
wheel cylinder pressure PW/C by the pump 226 is started after a
state is formed in which the controlled pressure increasing slope
.DELTA.PW/C exceeds the normal pressure increasing slope
.DELTA.PW/C. Thus, even by the brake force control device of the
present embodiment, as in the case of the aforementioned first
embodiment, it is possible to effectively utilize the capability of
the master cylinder 206 and the capability of the pump 226 and to
thus boost the wheel cylinder pressure PW/C rapidly after the
emergency braking is performed.
[0172] When the ECU 200 executes the routine shown in FIG. 9, it is
possible to rapidly set up the brake force of the two front wheels
after the emergency braking is performed and to rapidly set up the
brake force of the two rear wheels after the predetermined time DR
elapses. In this case, as in the case of the aforementioned second
embodiment, it is possible to suppress a variation in the turning
behavior of the vehicle which is caused when an emergency braking
is performed while the vehicle is turning.
[0173] If the wheel cylinder pressure PW/C is rapidly increased as
described above, the slip ratio S of the wheels is abruptly
increased, and then the condition for execution of the ABS control
stands. When the condition for execution of the ABS control stands,
the ECU 200 {circle over (1)} pressure-increasing mode, {circle
over (2)} holding mode and {circle over (3)} pressure-decreasing
mode so that the slip ratio S of the wheels becomes equal to an
appropriate value.
[0174] In the system of the present embodiment, while the brake
assist control is being performed, the SC 214 is maintained in the
ON state as described above. When the SC 214 is the ON state, the
fluid pressure chamber of the master cylinder 206 and the upstream
portion of the SC 214 substantially form a closed space.
[0175] Under the above situation, the master cylinder pressure PM/C
becomes a value depending on the brake pressing force FP. Hence,
the ECU 200 monitors the output signal of the master cylinder
pressure PM/C detected by the hydraulic pressure sensor 212 and
thus determines whether the brake pedal 202 is released from the
pressed state. When the ECU 200 detects that the brake pedal 202 is
released from the pressed state, the ECU 200 stops supplying the
drive signal to the SC 214 and SCH 234, and terminates the brake
assist control.
[0176] By the way, in the above-mentioned third embodiment, the
master cylinder pressure PM/C is used as the basic parameter for
discriminating the normal braking over emergency braking. However,
the basic parameter is not limited to the above, but the brake
pressing force Fr, pedal stroke L, vehicle deceleration G,
estimated vehicle velocity VSO, and the wheel velocity VW** may be
used as the basic parameters as in the case of the first
embodiment.
* * * * *