U.S. patent application number 09/802859 was filed with the patent office on 2001-10-18 for automotive brake control system with anti-skid braking device.
This patent application is currently assigned to UNISIA JECS CORPORATION. Invention is credited to Hano, Sunao, Inoue, Gen, Ohtsu, Nobuyuki, Yamaura, Tamotsu.
Application Number | 20010032045 09/802859 |
Document ID | / |
Family ID | 18591930 |
Filed Date | 2001-10-18 |
United States Patent
Application |
20010032045 |
Kind Code |
A1 |
Hano, Sunao ; et
al. |
October 18, 2001 |
Automotive brake control system with anti-skid braking device
Abstract
An automotive brake control system with an anti-brake skid (ABS)
unit which controls a wheel-brake cylinder pressure to each
individual wheel cylinder of the road wheels to prevent a wheel
lock-up condition during braking, and an ABS control unit being
configured to be connected electrically to at least wheel speed
sensors and the ABS unit to execute skid control having at least a
reduce-pressure operating mode and a pressure build-up operating
mode, when the wheel speed sensor signals indicate that at least
one of the road wheels is locking up. The ABS control unit includes
a road-surface-condition change determination section which
determines, based on both a time length of brake-fluid-pressure
control continuously executed during the skid control and a
recovery acceleration of the wheel speed of the road wheel
subjected to the skid control, whether there is a road-surface .mu.
change from low-.mu. road to high-.mu. road.
Inventors: |
Hano, Sunao; (Kanagawa,
JP) ; Yamaura, Tamotsu; (Kanagawa, JP) ;
Inoue, Gen; (Kanagawa, JP) ; Ohtsu, Nobuyuki;
(Kanagawa, JP) |
Correspondence
Address: |
Richard L. Schwaab
FOLEY & LARDNER
Washington Harbour
3000 K Street, N.W., Suite 500
Washington
DC
20007-5109
US
|
Assignee: |
UNISIA JECS CORPORATION
|
Family ID: |
18591930 |
Appl. No.: |
09/802859 |
Filed: |
March 12, 2001 |
Current U.S.
Class: |
701/80 ; 701/71;
701/78 |
Current CPC
Class: |
B60T 8/172 20130101;
B60T 2210/124 20130101 |
Class at
Publication: |
701/80 ; 701/71;
701/78 |
International
Class: |
B60T 008/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 16, 2000 |
JP |
2000-073733 |
Claims
What is claimed is:
1. An automotive brake control system comprising: vehicle sensors
which detect at least each wheel speed of road wheels to generate
wheel speed sensor signals; an anti-brake skid unit which controls
a wheel-brake cylinder pressure to each individual wheel cylinder
of the road wheels to prevent a wheel lock-up condition during
braking; a control unit being configured to be connected
electrically to the vehicle sensors and the anti-brake skid unit to
execute skid control having at least a reduce-pressure operating
mode and a pressure build-up operating mode, when the wheel speed
sensor signals indicate that at least one of the road wheels is
locking up: said control unit comprising a road-surface-condition
change determination section which determines, based on both a time
length of brake-fluid-pressure control continuously executed during
the skid control and a recovery acceleration of the wheel speed of
the road wheel subjected to the skid control, whether there is a
change of a road surface condition from a road surface having a low
friction coefficient to a road surface having a high friction
coefficient.
2. The automotive brake control system as claimed in claim 1,
wherein the recovery acceleration is a wheel acceleration that the
wheel speed of the road wheel, subjected to the skid control,
increases toward and recovers to a vehicle speed during the
reduce-pressure operating mode of the skid control.
3. The automotive brake control system as claimed in claim 2,
wherein the road-surface-condition change determination section
determines that there is the change of the road surface condition
from the road surface having the low friction coefficient to the
road surface having the high friction coefficient when a value of
the recovery acceleration of the wheel speed exceeds a specified
acceleration threshold value.
4. The automotive brake control system as claimed in claim 3,
wherein the specified acceleration threshold value is set to an
acceleration value ranging from 3 G to 4 G, where G is a
gravitational acceleration unit.
5. The automotive brake control system as claimed in claim 3,
wherein the road-surface-condition change determination section
determines that there is the change of the road surface condition
from the road surface having the low friction coefficient to the
road surface having the high friction coefficient, when a mean
value of the recovery acceleration of the wheel speed exceeds the
specified acceleration threshold value.
6. The automotive brake control system as claimed in claim 3,
wherein the time length of brake-fluid-pressure control is a time
interval of pressure build-up executed during the pressure build-up
operating mode of the skid control.
7. The automotive brake control system as claimed in claim 6,
wherein the anti-brake skid unit comprises pressure build-up and
reduce-pressure solenoid valves to control the wheel-brake cylinder
pressure to each of the wheel cylinders of the road wheels, and the
time interval of pressure build-up corresponds to a number of
pulses of a pulse signal applied to the pressure build-up solenoid
valve, and the road-surface-condition change determination section
determines that there is the change of the road surface condition
from the road surface having the low friction coefficient to the
road surface having the high friction coefficient, when the number
of pulses of the pulse signal exceeds a predetermined pulse
number.
8. The automotive brake control system as claimed in claim 3,
wherein the specified acceleration threshold value is set as a
variable value based on a slip time interval from a time when the
wheel speed is reduced to below a reduce-pressure threshold value
below which the reduce-pressure operating mode of the skid control
is started to a time when the wheel speed recovers near to the
vehicle speed by way of the reduce-pressure operating mode.
9. The automotive brake control system as claimed in claim 8,
wherein the time length of brake-fluid-pressure control is a time
interval of pressure reduction executed during the reduce-pressure
operating mode of the skid control, and the road-surface-condition
change determination section determines that there is the change of
the road surface condition from the road surface having the low
friction coefficient to the road surface having the high friction
coefficient, when the time interval of pressure reduction exceeds a
predetermined time interval.
10. The automotive brake control system as claimed in claim 3,
wherein the road-surface-condition change determination section
inhibits the change of the road surface condition from the road
surface having the low friction coefficient to the road surface
having the high friction coefficient from being determined based on
both the time length of brake-fluid-pressure control and the
recovery acceleration of the wheel speed, when the road wheel is
locking up.
11. The automotive brake control system as claimed in claim 10,
wherein the control unit comprises: (1) a vehicle speed calculation
section which calculates the vehicle speed based on the wheel speed
sensor signals; and (2) a reduce-pressure threshold value
calculation section which calculates, based on the vehicle speed, a
reduce-pressure threshold value below which the reduce-pressure
operating mode of the skid control is started; and wherein the
road-surface-condition change determination section is programmed
to perform the following, deriving, at each execution cycle of the
brake-fluid-pressure control, a spin-up speed that corresponds to a
value of the vehicle speed calculated at the time when the wheel
speed recovers near to the vehicle speed by way of the
reduce-pressure operating mode after the wheel speed has been
reduced to below the reduce-pressure threshold value; calculating a
time rate of change of the spin-up speed; and determining, based on
the time rate of change of the spin-up speed, whether there is the
change of the road surface condition from the road surface having
the low friction coefficient to the road surface having the high
friction coefficient.
12. The automotive brake control system as claimed in claim 11,
wherein the road-surface-condition change determination section
determines that there is the change of the road surface condition
from the road surface having the low friction coefficient to the
road surface having the high friction coefficient, when the time
rate of change of the spin-up speed exceeds a predetermined
acceleration value ranging from 0.3 G to 0.6 G, where G is a
gravitational acceleration unit.
13. An automotive brake control system comprising: vehicle sensors
for detecting at least each wheel speed of road wheels to generate
wheel speed sensor signals; an anti-brake skid unit for controlling
a wheel-brake cylinder pressure to each individual wheel cylinder
of the road wheels to prevent a wheel lock-up condition during
braking; a control unit being configured to be connected
electrically to the vehicle sensors and the anti-brake skid unit to
execute skid control having at least a reduce-pressure operating
mode and a pressure build-up operating mode, when the wheel speed
sensor signals indicate that at least one of the road wheels is
locking up: said control unit comprising a road-surface-condition
change determination means for determining, based on both a time
length of brake-fluid-pressure control continuously executed during
the skid control and a recovery acceleration of the wheel speed of
the road wheel subjected to the skid control, whether there is a
change of a road surface condition from a road surface having a low
friction coefficient to a road surface having a high friction
coefficient.
14. An automotive brake control system comprising: vehicle sensors
which detect at least each wheel speed of road wheels to generate
wheel speed sensor signals; an anti-brake skid unit which controls
a wheel-brake cylinder pressure to each individual wheel cylinder
of the road wheels to prevent a wheel lock-up condition during
braking; a control unit being configured to be connected
electrically to the vehicle sensors and the anti-brake skid unit to
execute skid control having at least a reduce-pressure operating
mode and a pressure build-up operating mode, when the wheel speed
sensor signals indicate that at least one of the road wheels is
locking up: said control unit comprising a road-surface-condition
change determination section which determines, based on both a time
length of brake-fluid-pressure control continuously executed during
the skid control and a recovery acceleration of the wheel speed of
the road wheel subjected to the skid control, whether there is a
change of a road surface condition from a road surface having a low
friction coefficient to a road surface having a high friction
coefficient, the road-surface-condition change determination
section being provided individually for at least left and right
wheels of the road wheels.
15. The automotive brake control system as claimed in claim 14,
wherein the road-surface-condition change determination section
determines that there is the change of the road surface condition
from the road surface having the low friction coefficient to the
road surface having the high friction coefficient, when either one
of the following two criteria for determining is satisfied: (i) a
first criterion that a time length of brake-fluid-pressure control
continuously executed during the skid control is greater than or
equal to a first predetermined threshold value; and (ii) a second
criterion that the time length of brake-fluid-pressure control is
greater than or equal to a second predetermined threshold value,
based on and set to be less than the first predetermined threshold
value, and that the recovery acceleration of the wheel speed
exceeds a predetermined acceleration threshold value.
16. The automotive brake control system as claimed in claim 15,
wherein the road-surface-condition change determination section
finally determines that there is the change of the road surface
condition from the road surface having the low friction coefficient
to the road surface having the high friction coefficient, when the
road-surface-condition change determination section associated with
a first one of the left and right wheels determines that there is
the change of the road surface condition and the
road-surface-condition change determination section associated with
the second wheel determines that there is the change of the road
surface condition.
17. The automotive brake control system as claimed in claim 16,
wherein the road-surface-condition change determination section
comprises a temporary road-surface-condition change determination
section which determines temporarily that there is the change of a
road surface condition from the road surface having the low
friction coefficient to the road surface having the high friction
coefficient when the time length of brake-fluid-pressure control is
greater than or equal to a second predetermined threshold value,
and the road-surface-condition change determination section finally
determines that there is the change of the road surface condition
from the road surface having the low friction coefficient to the
road surface having the high friction coefficient, when the
road-surface-condition change determination section associated with
the first wheel determines that there is the change of the road
surface condition and the temporary road-surface-condition change
determination section associated with the second wheel determines
temporarily that there is the change of the road surface
condition.
18. The automotive brake control system as claimed in claim 17,
wherein the time length of brake-fluid-pressure control is a time
interval of pressure build-up executed during the pressure build-up
operating mode of the skid control.
19. The automotive brake control system as claimed in claim 17,
wherein the time length of brake-fluid-pressure control is a time
interval of pressure reduction executed during the reduce-pressure
operating mode of the skid control.
20. The automotive brake control system as claimed in claim 15,
wherein the recovery acceleration of the wheel speed of the road
wheel subjected to the skid control is a mean value of a plurality
of wheel acceleration data continuously calculated for a
predetermined time interval.
21. The automotive brake control system as claimed in claim 15,
wherein the recovery acceleration of the wheel speed of the road
wheel subjected to the skid control is a maximum value of a
plurality of wheel acceleration data continuously calculated for a
predetermined time interval.
Description
TECHNICAL FIELD
[0001] The present invention relates to an automotive brake control
system with an anti-skid braking device (ABS unit) which acts to
prevent a wheel lock-up condition during braking and to provide
maximum effective braking, and particularly to such an anti-skid
braking system (ABS system) capable of variably controlling a rate
of pressure build-up depending on the changes in road conditions,
that is, a friction coefficient of the road surface.
BACKGROUND ART
[0002] ABS systems usually employ rotational wheel speed sensors
that monitor each wheel's rotational speed and send a signal back
to an on-board computer to indicate a deceleration rate of each
wheel. If the wheel speed sensor signals indicate that a wheel or
wheels are locking up, the ABS device comes into operation to
momentarily reduce a wheel-brake cylinder pressure of the wheel
subjected to skid control and thereafter to again build up the
wheel-cylinder pressure when the wheel speed recovers near to the
vehicle speed and thus there is no risk of the wheels to lock up.
In this manner, the ABS system operates to prevent skidding and to
shorten a braking distance as much as possible, and thus to provide
maximum effective braking during anti-skid operation. In recent
years, there have been proposed and developed various ABS systems
in which a rate of pressure build-up following the
pressure-reduction operating mode (or reduce-pressure mode) can be
varied depending upon road surface conditions, during skid control.
In such ABS systems, the rate of pressure build-up is determined or
set depending on the road-surface friction coefficient (hereinafter
is referred to as a "road-surface .mu."), and thus it is possible
to provide the shortest possible braking distance without incurring
wheel lock-up and vehicle skidding, during braking on both a
high-.mu. road surface condition (e.g., dry pavement) and a
low-.mu. road surface condition (e.g., snow or icy roads). For
instance, during braking on the high-.mu. road, it is required to
set the rate of pressure build-up following the reduce-pressure
mode at a comparatively great value, so as to properly reduce a
recovery time that the wheel-brake cylinder pressure recovers near
to a suitable brake fluid pressure, and thus to shorten the
stopping distance. Conversely, during braking on the low-.mu. road,
it is required to set the rate of pressure build-up at a
comparatively small value, so as to prevent the wheel from again
starting to lock due to an improperly high pressure build-up rate
during one cycle of the pressure build-up mode. Skid control based
on the relatively great pressure build-up rate programmed to be
suitable for the high-.mu. road surface condition is often called
as a high-.mu. control mode (simply, high-.mu. control), whereas
skid control based on the relatively small pressure build-up rate
programmed to be suitable for the low-.mu. road surface condition
is often called as a low-.mu. control mode (simply, low-.mu.
control). On a moment's thought, the road surface conditions are
not constant. That is, during vehicle driving, low-.mu. roads are
usually sprinkled or dispersed. Therefore, there is an increased
tendency for the road surface condition to change from the low-.mu.
road surface condition to the high-.mu. road surface condition.
SUMMARY OF THE INVENTION
[0003] It is an object of the invention to provide an automotive
brake control system with an anti-skid braking device, which is
capable of accurately detecting a difference between low-.mu. road
and high-.mu. a road.
[0004] In order to accomplish the object of the present invention,
an automotive brake control system comprises vehicle sensors which
detect at least each wheel speed of road wheels to generate wheel
speed sensor signals, an anti-brake skid unit which controls a
wheel-brake cylinder pressure to each individual wheel cylinder of
the road wheels to prevent a wheel lock-up condition during
braking, a control unit being configured to be connected
electrically to the vehicle sensors and the anti-brake skid unit to
execute skid control having at least a reduce-pressure operating
mode and a pressure build-up operating mode, when the wheel speed
sensor signals indicate that at least one of the road wheels is
locking up, the control unit comprising a road-surface-condition
change determination section which determines, based on both a time
length of brake-fluid-pressure control continuously executed during
the skid control and a recovery acceleration of the wheel speed of
the road wheel subjected to the skid control, whether there is a
change of a road surface condition from a road surface having a low
friction coefficient to a road surface having a high friction
coefficient.
[0005] According to another aspect of the invention, an automotive
brake control system comprises vehicle sensors for detecting at
least each wheel speed of road wheels to generate wheel speed
sensor signals, an anti-brake skid unit for controlling a
wheel-brake cylinder pressure to each individual wheel cylinder of
the road wheels to prevent a wheel lock-up condition during
braking, a control unit being configured to be connected
electrically to the vehicle sensors and the anti-brake skid unit to
execute skid control having at least a reduce-pressure operating
mode and a pressure build-up operating mode, when the wheel speed
sensor signals indicate that at least one of the road wheels is
locking up, the control unit comprising a road-surface-condition
change determination means for determining, based on both a time
length of brake-fluid-pressure control continuously executed during
the skid control and a recovery acceleration of the wheel speed of
the road wheel subjected to the skid control, whether there is a
change of a road surface condition from a road surface having a low
friction coefficient to a road surface having a high friction
coefficient.
[0006] According to a still further aspect of the invention, an
automotive brake control system comprises vehicle sensors which
detect at least each wheel speed of road wheels to generate wheel
speed sensor signals, an anti-brake skid unit which controls a
wheel-brake cylinder pressure to each individual wheel cylinder of
the road wheels to prevent a wheel lock-up condition during
braking, a control unit being configured to be connected
electrically to the vehicle sensors and the anti-brake skid unit to
execute skid control having at least a reduce-pressure operating
mode and a pressure build-up operating mode, when the wheel speed
sensor signals indicate that at least one of the road wheels is
locking up, the control unit comprising a road-surface-condition
change determination section which determines, based on both a time
length of brake-fluid-pressure control continuously executed during
the skid control and a recovery acceleration of the wheel speed of
the road wheel subjected to the skid control, whether there is a
change of a road surface condition from a road surface having a low
friction coefficient to a road surface having a high friction
coefficient, the road-surface-condition change determination
section being provided individually for at least left and right
wheels of the road wheels.
[0007] The other objects and features of this invention will become
understood from the following description with reference to the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 is a system block diagram illustrating one embodiment
of an ABS unit of the automotive brake control system of the
invention.
[0009] FIG. 2 is a block diagram illustrating electronic brake
control unit (ECU) inputs and outputs employed in the automotive
brake control system of the embodiment.
[0010] FIG. 3 is a flow chart illustrating a main ABS control flow
executed by the ABS system of the embodiment.
[0011] FIG. 4 is a flow chart illustrating a control mode setting
subroutine executed by the ABS system of the embodiment and related
to step S108 of FIG. 3.
[0012] FIG. 5 is a time chart showing the operation and effect of
the automotive brake control system of the embodiment.
[0013] FIG. 6 is a flow chart illustrating a modified skid control
routine executed by the ABS system of the embodiment.
[0014] FIG. 7 is a flow chart illustrating arithmetic processing
for pseudo vehicle speed Vi.
[0015] FIG. 8 is a flow chart illustrating a first arithmetic
calculation for vehicle deceleration rate VIK.
[0016] FIG. 9 is a flow chart illustrating arithmetic processing
for reduce-pressure threshold value .lambda.1.
[0017] FIG. 10 is a flow chart illustrating a subroutine for
solenoid reduce-pressure control.
[0018] FIG. 11 is a flow chart illustrating a subroutine for
solenoid pressure build-up control.
[0019] FIG. 12 is a flow chart illustrating a second arithmetic
calculation for vehicle deceleration rate VIK.
[0020] FIG. 13 is a preprogrammed recovery acceleration threshold
value .alpha..sub.max versus slip time Lo.mu.T characteristic
map.
[0021] FIGS. 14A-14D are time charts illustrating simulation test
results obtained by the ABS system of the embodiment capable of
performing the routine shown in FIG. 12, during driving on low-.mu.
road.
[0022] FIGS. 15A-15D are time charts illustrating simulation test
results obtained by the ABS system of the embodiment capable of
performing the routine shown in FIG. 12, during driving on
middle-.mu. road.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0023] Referring now to the drawings, particularly to FIG. 1, the
automotive brake control system of the invention is exemplified in
an automotive vehicle equipped with a three-channel ABS front-rear
split brake-circuit layout. In FIG. 1, WC denotes a wheel cylinder.
Reference signs FL, FR, and R in parentheses are inserted at the
underside of reference sign WC to indicate front-left, front-right,
and rear wheel-brake cylinders, respectively. Actually, the rear
section of the dual braking system includes rear-left and
rear-right wheel cylinders and two hydraulic brake circuits for
them. For the purpose of illustrative simplicity, regarding the
rear section of the dual braking system, FIG. 1 merely shows only
one rear wheel-brake cylinder WC.sub.(R) and its rear-brake
hydraulic circuit. This is because in the three-channel ABS system
for parallel brake-circuit division the rear brakes are controlled
together. As will be hereinafter described in detail, the
fundamental concept of the invention can be applied to a
four-channel ABS system in which the front brakes are controlled
individually and also the rear brakes are controlled individually.
The front wheel-brake cylinders WC.sub.(FL) and WC.sub.(FR) are
fluidly connected through a hydraulic brake circuit 1 to a master
cylinder MC (constructed by a typical tandem master cylinder with
two pistons in tandem), while the rear wheel-brake cylinders
WC.sub.(RL) and WC.sub.(RR) are fluidly connected through a
hydraulic brake circuit 2 to the master cylinder MC. The hydraulic
brake circuit 1, which is included in the primary section (front
section) and extends towards the front-left and front-right
wheel-brake cylinders WC.sub.(FL) and WC.sub.(FR), is divided at
the branch point 1d into two branch circuits 1L and 1R associated
with the respective front wheel cylinders WC.sub.(FL) and
WC.sub.(FR). An inflow valve 5 is fluidly disposed in each of the
branch circuits 1L and 1R. In the same manner, an inflow valve 5 is
fluidly disposed in the rear brake circuit 2. The inflow valve 5 is
often called as a "pressure build-up valve". Each of the three
inflow valves 5 is comprised of a normally-open, two-port
two-position, electromagnetic directional control valve. Therefore,
when de-energized, the inflow valve 5 is held at its open position
by way of the bias of a return spring. Conversely, when energized,
the inflow valve 5 is shifted to its closed position against the
spring bias by way of electromagnetic force created by the
electromagnetic solenoid. Reference sign 1h denotes a bypass line
bypassing the associated inflow valve 5. A one-way check valve 1g
is fluidly disposed in each of the three bypass lines 1h to permit
brake-fluid flow from the downstream side (the wheel-cylinder side)
to the upstream side (the master-cylinder side) in one direction
only. A drain circuit 10 for the primary section is connected to
the downstream side of each of the two inflow valves 5 associated
with the front wheel cylinders WC.sub.(FL) and WC.sub.(FR) for
communicating the front brake circuit 1 with a reservoir 7
therethrough. Likewise, a drain circuit 10 for the secondary
section is connected to the downstream side of the one inflow valve
5 associated with the rear wheel cylinder WC.sub.(R) for
communicating the rear brake circuit 2 with a reservoir 7
therethrough. Two outflow valves 6 are fluidly disposed in the
drain circuit 10 of the primary section in such a manner as to be
connected to the outlets of the front-left and front-right inflow
valves 5. In the same manner, an outflow valve 6 is fluidly
disposed in the drain circuit 10 of the secondary section in such a
manner as to be connected to the outlet of the rear inflow valve 5.
The outflow valve 6 is often called as a "reduce-pressure valve".
Each of the outflow valves 6 is comprised of a normally-closed,
two-port two-position, electromagnetic directional control valve.
Therefore, when de-energized, the outflow valve 6 is held at its
closed position by way of the bias of a return spring so as to shut
off the associated drain circuit 10. Conversely, when energized,
the outflow valve 6 is shifted to its open position against the
spring bias by way of electromagnetic force created by the
electromagnetic solenoid so as to establish fluid-communication
between the associated drain circuit 10 and inflow valve 5. The
drain circuit 10 of the primary section is connected through a
reflux circuit 11 to the front brake circuit 1 upstream of the
front-left and front-right inflow valves (5, 5). On the other hand,
the drain circuit 10 of the secondary section is connected through
a reflux circuit 11 to the rear brake circuit 2 upstream of the
rear inflow valve 5. A pair of return pumps (4, 4), often called
"ABS pumps", are provided in the two reflux circuits (11, 11), so
as to return the brake fluid stored in the reservoirs (7, 7) into
the respective brake circuits 1 and 2. Each of the return pumps (4,
4) is comprised of a reciprocating plunger type pump in which a
volumetric capacity of a working chamber is variable by
reciprocating motion of a plunger 4p, to suck and discharge the
brake fluid. The plunger type return pumps (4, 4) are driven by
means of a motor M with an eccentric rotary cam. A one-way inflow
check valve 4a is provided in the inlet side of each of the return
pumps (4, 4). A one-way outflow check valve 4b is provided in the
discharge side of each of the return pumps (4, 4). A damper 4d is
further provided in the discharge side of each of the return pumps
(4, 4) to effectively absorb or attenuate pulsation of brake fluid
discharged. In FIG. 1, a portion, denoted by reference sign BU and
contained within one-dotted line, corresponds to the ABS unit
(anti-brake skid unit). The ABS unit is usually housed in a
hydraulic control unit (HCU) housing (not shown).
[0024] With the previously-noted arrangement, as the brake pedal is
depressed, the piston in the master cylinder MC applies pressure to
brake fluid. The pressure forces the brake fluid through the
inlet/outlet port of the master cylinder MC into the hydraulic
brake circuits (1, 2). During normal braking, in the ABS unit BU,
the inflow valves (5, 5, 5) are kept at their open positions and at
the same time the outflow valves (6, 6, 6) are kept at their closed
positions, and thus hydraulic pressure (master-cylinder pressure)
is applied via the hydraulic brake circuit (1, 2) through the
inflow and outflow valves (5, 6) to the wheel-brake cylinders
WC.sub.(FL), WC.sub.(FR), and WC.sub.(R). During skid control, in
order to properly regulate or control the brake-fluid pressure to
each of the wheel-brake cylinders, the inflow and outflow valves
(5, 6) are designed to switch among at least three operating modes,
namely a reduce-pressure mode where the inflow valve 5 of the wheel
subjected to skid control (hereinafter is referred to as
"skid-controlled wheel") is held at the closed position and the
outflow valve 6 of the skid-controlled wheel is held at the open
position to relieve the brake fluid in the wheel-brake cylinder WC
via the drain circuit 10 within toward the reservoir 7 and
consequently to reduce the wheel-cylinder pressure, a hold-pressure
mode where the inflow valve 5 of the skid-controlled wheel remains
closed and the outflow valve 6 of the skid-controlled wheel is also
shifted to the closed position to shut off the brake circuit (1, 2)
and consequently to hold the brake-fluid pressure in the
wheel-brake cylinder WC constant, and a pressure build-up mode
where the inflow valve 5 of the skid-controlled wheel is shifted to
the open position and the outflow valve 6 of the skid-controlled
wheel remains closed and consequently the upstream side (the
master-cylinder side) of the brake circuit (1, 2) is communicated
with the downstream side (the wheel-brake cylinder side) of the
brake circuit (1, 2) to build up the wheel-cylinder pressure. The
brake-fluid reservoir 7 is designed to temporarily store energy by
holding the surplus brake fluid, which will occur anytime that
there is a drop in system pressure (especially during the
pressure-reduction mode). The return pump (4, 4) is designed to
carry or return the brake fluid flowing from the wheel-brake
cylinders via the individual reservoirs into the correct circuit of
the master cylinder (i.e., the upstream side of the directional
control valve 5) during skid control. With the previously
arrangement of the ABS system shown in FIG. 1, the brake-fluid
pressure to each wheel-brake cylinder can be properly regulated or
controlled. The operation (switching among valve positions) of the
directional control valves (5, 6) associated with each wheel-brake
cylinder (WC.sub.(FL), WC.sub.(FR), WC.sub.(R)) and the operation
(switching between inoperative and operative states) of the motor M
of the return pumps (4, 4) are electrically controlled by means of
the ABS system electronic control unit (ECU or CU). Electronic
control unit CU usually comprises a microcomputer. Although it is
not clearly shown in FIG. 2, the electronic control unit CU
includes a central processing unit (CPU) that performs necessary
arithmetic-calculations, processes informational data, compares
signals from a group of engine/vehicle sensors and switches SG to
preprogrammed threshold values, and makes necessary decisions of
acceptance, memories (RAM, ROM), and an input/output interface.
Actually, the control unit CU performs various data processing
actions as shown in FIGS. 3 and 4 which will be fully described
later. The input interface of control unit CU receives input
informational data from various engine/vehicle sensors/switches SG,
that is, three wheel speed sensors (SS, SS, SS), a brake switch BS,
and an acceleration sensor GS. The three wheel speed sensors SS
placed at the respective road wheels (FL, FR, R) are provided to
continuously monitor or detect each individual wheel's rotational
speed and relay this signal to the input interface of electronic
control unit CU. Brake switch BS is designed to generate a
switched-ON signal (or a high-voltage signal) when the brake pedal
is depressed and thus the brakes are applied. Conversely, when the
brake pedal is undepressed and thus the brakes are released, the
input interface of CU receives a switched-OFF signal (or no
electrical signal) from the brake switch BS. Acceleration sensor GS
is provided to monitor or detect the longitudinal acceleration
exerted on the vehicle (corresponding to a component of the vector
acceleration of a point in the vehicle in the X-direction and the
lateral acceleration exerted on the vehicle (corresponding to a
component of the vector acceleration of a point in the vehicle
perpendicular to the vehicle x-axis and parallel to the road
plane). The memories store preprogrammed or predetermined data such
as various threshold values and temporarily stores the results of
arithmetic-calculations and the necessary decisions made by the
CPU. The output interface of CU is configured to be electronically
connected to each of the directional control valves (5, 6), to
produce a control command signal to the directional control valve
associated with each individual wheel-brake cylinder as well as the
return pumps (4, 4), on the basis of the results of
arithmetic-calculations and decisions performed by the CPU.
[0025] Hereunder described in detail in reference to the flow chart
shown in FIG. 3 is the main skid control routine executed by the
brake control system of the embodiment and preprogrammed in the CPU
of electronic control unit CU. The control routine shown in FIG. 3
is executed as time-triggered interrupt routines to be triggered
every predetermined time intervals.
[0026] At step S101, data stored in the memories, in particular RAM
are all initialized.
[0027] At step S102, the more recent wheel speed indicative data
signals (that is, the latest up-to-date information being received
from the three wheel speed sensors SS), are read, and then a wheel
speed Vw at each road wheel (a front-right wheel speed VwFR, a
front-left wheel speed VwFL, and a rear wheel speed VwR) is
arithmetically calculated.
[0028] Then, at step S103, an acceleration/deceleration rate
(simply an acceleration/deceleration .DELTA.Vw) at each road wheel,
that is, a time rate of change in each of the wheel speeds VwFL,
VwFR and VwR (a front-right wheel acceleration/deceleration
.DELTA.VwFR, a front-left wheel acceleration/deceleration
.DELTA.VwFL, and a rear wheel acceleration/deceleration .DELTA.VwR)
is arithmetically calculated.
[0029] At step S104, a pseudo vehicle speed Vi is computed or
determined by a predetermined or pre-programmed arithmetic
processing. For example, the pseudo vehicle speed Vi can be
determined as the highest one of all of the wheel speeds (VwFL,
VwFR, VwR). Alternatively, the pseudo vehicle speed Vi may be
determined as the higher one of the driven wheel speeds.
[0030] At step S105, a test is made to determine whether the ABS
system is in operation. When the answer to step S105 is in the
negative (NO), that is, the ABS system in conditioned in the
inoperative state, step S106 occurs. At step S106, a check is made
to determine whether or not skid control should be started.
Actually, the check of step S106 is based on the ABS operating mode
determined at step 108 (described later). When the answer to step
S106 is negative, the routine returns to step S102. Conversely,
when the answer to step S106 is affirmative, the routine proceeds
from step S106 to step S108. On the other hand, when the answer to
step S105 is in the affirmative (YES), that is, the ABS system is
in operation, step S107 occurs. At step S107, a check is made to
determine whether the skid control should be executed continuously.
When the answer to step S107 is negative (NO), the routine flows
from step S107 to step S102 to terminate the skid control.
Conversely, when the answer to step S107 is affirmative (YES), the
routine proceeds from step S107 to step S108. At step S108, first,
a road condition (that is, a road-surface .mu.) of the road surface
on which the vehicle is now running is decided or determined, as
described later. Secondly, one of the three operating modes of skid
control, namely the reduce-pressure mode, hold-pressure mode, and
pressure build-up mode, is selected depending on wheel speeds Vw,
acceleration/deceleration rates .DELTA.Vw, pseudo vehicle speed Vi.
Thirdly, the presence or absence of the so-called .mu. jump, that
is, the absence or presence of the road-surface .mu. change from
low-.mu. to high-.mu. is determined, as will be fully described
later in reference to the flow chart shown in FIG. 4.
[0031] After step S108, step S109 occurs. At step S109, a check is
made to determine whether the reduce-pressure operating mode is
selected. When the answer to step S109 is affirmative (YES), step
S110 occurs. Conversely, when the answer to step S109 is negative
(NO), step S114 occurs. Although it is not shown in the flow chart
of FIG. 3, a hold-pressure mode decision box is provided between
steps S109 and S114, so as to check whether the hold-pressure mode
is selected. When the hold-pressure mode is selected, the CU
outputs command signals corresponding to the hold-pressure
operating mode to the inflow and outflow solenoid valves.
Conversely, when the answer to step S109 is negative and
additionally the answer to the hold-pressure mode decision box (not
shown) is negative, step S114 occurs. For the purpose of
illustrative simplicity, the hold-pressure mode decision box is
omitted.
[0032] At step S110, a check is made to determine whether a
temporary low-to-high-.mu. mode-switching flag f_j0 is reset to
"0". The setting or resetting of the temporary low-to-high-.mu.
mode-switching flag f_j0 is dependent on the result of comparison
between the number PN of pressure build-up and 70% of a
pulse-number threshold value N1 (described later). As will be fully
discussed later, the temporary low-to-high-.mu. mode-switching flag
f_j0 is necessary to reset the pressure build-up start timing of
the wheel-brake cylinder corresponding to a slower one of the left
and right wheels, after the reduce-pressure operating mode of skid
control. When the temporary low-to-high-.mu. mode-switching flag
f_j0 is set to "1", step S111 occurs. Conversely, when the
temporary low-to-high-.mu. mode-switching flag f_j0 is reset to
"0", the routine proceeds from step S110 to step S113. At step
S113, the CU generates command signals corresponding to the
reduce-pressure operating mode to the inflow and outflow solenoid
valves.
[0033] At step S111, a test is made to determine whether a maximum
wheel acceleration .DELTA.VwMAX exceeds a predetermined
acceleration threshold value (or a specified acceleration threshold
value) .alpha..sub.max. The maximum wheel acceleration .DELTA.VwMAX
means a maximum value of a wheel recovery acceleration that the
wheel speed Vw of the skid-controlled road wheel increases and
recovers to the pseudo vehicle speed Vi during the reduce-pressure
mode of skid control. Thus, the predetermined acceleration
threshold value .alpha..sub.max is often called a "recovery
acceleration threshold .alpha..sub.max". In case of
.DELTA.VwMAX>.alpha..sub.max, the routine proceeds from step
S111 to step S112. At step S112, a low-to-high-.mu. mode-switching
flag f_j is set to "1". As discussed above, in case of f_j0=1, or
in case of f_j0=0 and .DELTA.VwMAX.ltoreq..alpha..sub.max, or after
the low-to-high-.mu. mode-switching flag f_j has been set at step
S112, the routine proceeds to step S113, so as to output command
signals corresponding to the reduce-pressure operating mode to the
inflow and outflow solenoid valves. Note that the low-to-high-.mu.
mode-switching flag f_j is different from the temporary
low-to-high-.mu. mode-switching flag f_j0. As will be fully
discussed later, the low-to-high-.mu. mode-switching flag f_j is
also needed to reset the pressure build-up start timing of the
wheel-brake cylinder, after the reduce-pressure operating mode of
skid control. The aforementioned predetermined acceleration
threshold value amp corresponds to a minimum acceleration value
necessary for the wheel speed Vw to recover to the pseudo vehicle
speed (the vehicle speed) Vi during execution cycle of the ABS
reduce-pressure mode on a high-.mu. road. The specified
acceleration threshold value .alpha..sub.max is actually set as a
variable value based on the slip time interval (Lo.mu.T) from the
time when the wheel speed Vw is reduced to below the
reduce-pressure threshold value .lambda.1 below which the
reduce-pressure operating mode of the skid control is started to
the time when the wheel speed recovers near to the vehicle speed
(pseudo vehicle speed Vi) in accordance with the reduce-pressure
operating mode (see FIG. 13). In the system of the shown
embodiment, the predetermined acceleration threshold value
.alpha..sub.max is set to an acceleration value ranging from 3G to
4G (where G means a gravitational acceleration unit).
[0034] On the other hand, when the answer to step S109 is negative
(NO) and additionally the pressure build-up mode is selected, the
routine proceeds from step S109 to step S114. At step S114, the
pulse-number threshold value is set to a predetermined value N1.
The pulse-number threshold value N1 is set to a predetermined value
corresponding to a pressure-build-up time interval given when the
pressure build-up is made according to the low-.mu. skid-control
mode on a high-.mu. road. In the shown embodiment, the pulse-number
threshold value N1 is set to "9". Threshold values N1 vary,
depending on the specification, type, total weight and size of
vehicle. After step S114, step S115 occurs. At step S115, the
pressure build-up number PN is counted up. The number PN of
pressure build-up corresponds to a time interval of pressure
build-up from the start of pressure build-up to the end of pressure
build-up. Then, at step S116, the pressure build-up number PN is
compared to the pulse-number threshold value N1. In case of
PN.gtoreq.N1, the routine proceeds to step S117. At step S117, the
low-to-high-.mu. mode-switching flag f_j is set to "1". Conversely,
in case of PN<N1, the routine proceeds from step S116 to step
S118. At step S118, the pressure build-up number PN is compared to
a predetermined value N1*0.7 slightly smaller than the pulse-number
threshold value N1. In the system of the embodiment, the
predetermined value N1*0.7 is set to 70% of the pulse-number
threshold value N1. In case of PN.gtoreq.N1*0.7, the routine
proceeds from step S118 to step S119. At step S119, the temporary
low-to-high-.mu. mode-switching flag f_j0 is set to "1".
Conversely, in case of PN<N1*0.7, the routine proceeds from step
S118 to step S120. At step S120, the CU outputs command signals
corresponding to the pressure build-up operating mode to the inflow
and outflow solenoid valves.
[0035] Details of the control mode setting subroutine executed at
step S108 of FIG. 3 are hereunder described in detail in reference
to the flow chart shown in FIG. 4.
[0036] Step S206 is a very important step. Thus, step S206 is
discussed first of all. At step S206, either of the reduce-pressure
mode and the pressure build-up mode is selected or set depending on
the road surface condition, that is, the road-surface .mu. of the
road surface on which the vehicle is now running. In the
road-surface .mu. determination procedure of step S206, the CU
measures a time interval T from a time when the pressure reduction
starts to execute to a time when the start of pressure build-up is
decided or judged, at the initial pressure build-up executed after
the reduce-pressure mode. The time interval T is hereinafter
referred to as a "recovery time of wheel speed" during which the
wheel is skidding. The recovery time T essentially corresponds to a
"slip time Lo.mu.T" as shown in FIGS. 12 and 13. Thereafter, the
processor of CU arithmetically calculates a road-surface-condition
decision value DDM by dividing the maximum wheel acceleration
(recovery acceleration) .DELTA.VwMAX by the recovery time T
(Lo.mu.T) measured. That is, the road-surface-condition decision
value DDM is represented by the expression DDM=.DELTA.VwMAX/T. When
the road-surface-condition decision value DDM is below a
predetermined criterion, the CU determines that the road-surface
.mu. of the road surface on which the vehicle is now running, is a
low-.mu. road. When the road-surface-condition decision value DDM
is above the predetermined criterion, the CU determines that the
road-surface .mu. of the road surface on which the vehicle is now
running, is a high-.mu. road. Instead of the use of a sole
criterion distinguishing high-.mu. road from low-.mu. road, a first
criterion distinguishing high-.mu. road from middle-.mu. road and a
second criterion distinguishing middle-.mu. road from low-.mu. road
may be used (see FIG. 13). In the system of the embodiment shown in
FIGS. 3 and 4, in calculating the road-surface-condition decision
value DDM, the recovery time T as well as the recovery acceleration
.DELTA.VwMAX is used, and the recovery time T is put on the
denominator, as can be seen from the expression DDM=.DELTA.VwMAX/T.
Thus, even when the wheel acceleration/deceleration rate .DELTA.Vw
is remarkably varied and affected by unevenness of the road surface
or by superimposition of noise and thus the maximum wheel
acceleration (recovery acceleration) .DELTA.VwMAX becomes great, a
response (sensitivity) of determination for the road surface
condition is optimized by way of the use of the recovery time T put
on the denominator of the above expression. The high-accuracy
road-surface-condition determination can be assured. Furthermore,
in step S206 of FIG. 4 (or in step S108 of FIG. 3), a
reduce-pressure threshold value .lambda.1 and a hold-pressure
threshold value .lambda.2 are computed on the basis of the pseudo
vehicle speed Vi computed at step S104 of FIG. 3. In selecting or
determining the ABS operating mode, based on the parameters Vw,
.DELTA.Vw, and Vi, comparison between the calculated
reduce-pressure threshold value .lambda.1 and each wheel speed Vw
and comparison between the calculated hold-pressure threshold value
.lambda.2 and each wheel acceleration/deceleration .DELTA.Vw are
made. In the shown embodiment, when a predetermined condition, that
is, an inequality Vw<.lambda.1 is satisfied, the electronic
control unit CU selects the reduce-pressure mode. When an
inequality Vw.gtoreq..lambda.1 and an inequality
.DELTA.Vw.gtoreq..lambda.2 are both satisfied, the electronic
control unit CU selects the pressure build-up mode. When the
inequality Vw.gtoreq..lambda.1 and inequality
.DELTA.Vw<.lambda.2 are both satisfied, the electronic control
unit CU selects the hold-pressure mode. That is, the wheel speed Vw
becomes higher than the reduce-pressure threshold value .lambda.1
and thus the wheel speed Vw approaches to the vehicle speed (i.e.,
the pseudo vehicle speed Vi), the ABS operating mode is shifted via
the hold-pressure mode to the pressure build-up mode. As a result
of the pressure build-up, when the wheel speed Vw becomes below the
reduce-pressure threshold value .lambda.1, the ABS operating mode
is shifted again to the reduce-pressure mode. In this manner, the
reduce-pressure mode, the hold-pressure mode, and the pressure
build-up mode will repeat to prevent vehicle skidding. Note that,
in the system of the embodiment shown in FIGS. 3 and 4, in the
pressure build-up operating mode, the CU selects, depending on the
road surface condition determined, one of a low-.mu. a control mode
(low-.mu. control) that a pressure build-up rate of the initial
pressure build-up is set to a relatively small pressure build-up
rate programmed to be suitable for the low-.mu. road surface
condition and a high-.mu. control mode (high-.mu. control) that a
pressure build-up rate of the initial pressure build-up is set to a
relatively great pressure build-up rate programmed to be suitable
for the high-.mu. road surface condition. Additionally, note that
in the system of the embodiment shown in FIGS. 3 and 4, in the
pressure build-up operating mode, a timing of switching between
low-.mu. control and high-.mu. control is dependent upon the
temporary low-to-high-.mu. mode-switching flag f_j0 as well as the
low-to-high-.mu. mode-switching flag f_j.
[0037] Steps S201 through S205 correspond to the road-surface .mu.
change (the road-surface .mu. jump) determination procedure. This
road-surface .mu. change determination procedure of steps S201-S205
is executed as soon as the CU determines the low-.mu. road and the
low-.mu. control is started.
[0038] At step S201, a check is made to determine whether the ABS
system is in the low-.mu. control. When the answer to step S201 is
affirmative, that is, during the low-.mu. control, step S202
occurs. Conversely, when the ABS system is out of the low-.mu.
control, that is, during high-.mu. control, step S206 occurs.
[0039] At step S202, a check is made to determine whether the
low-to-high-.mu. mode-switching flag f_j for at least one of the
front-left and front-right road wheels is set to "1". When the
low-to-high-.mu. mode-switching flag f_j for at least one of the
front-left and front-right road wheels is reset to "0", that is, in
case of f_j.sub.(1)=0 and f_j.sub.(.sub.2)=0, the subroutine flows
to step S206. Conversely, when the low-to-high-.mu. mode-switching
flag f_j for at least one of the front-left and front-right road
wheels is set to "1", that is, in case of f_j.sub.(1)=1 and/or
f_j.sub.(.sub.2)=1, the subroutine flows to step S203. The flag
f_j.sub.(1) means the low-to-high-.mu. mode-switching flag f_j for
a first one of the front-left and front-right road wheels, whereas
the flag f_j.sub.(2) means the low-to-high-.mu. mode-switching flag
f_j for the second front road wheel.
[0040] At step S203, a check is made to determine whether the
low-to-high-.mu. mode-switching flag f_j.sub.(2) for the second
front road wheel is set to "1". When the second low-to-high-.mu.
mode-switching flag f_j.sub.(2) is set, the subroutine proceeds
from step S203 to step S205. At step S205, the processor of CU
switches from the low-.mu. control to high-.mu. control. When the
second low-to-high-.mu. mode-switching flag f_j.sub.(2) is reset,
the subroutine proceeds from step S203 to step S204. At step S204,
a check is made to determine whether the temporary low-to-high-.mu.
mode-switching flag f_j0 for the second front road wheel is set to
"1". When the second temporary low-to-high-.mu. mode-switching flag
f_j0.sub.(2) is reset to "0", the subroutine proceeds from step
S204 to step S206. When the second temporary low-to-high-.mu.
mode-switching flag f_j0.sub.(2) is set to "1", the subroutine
proceeds from step S204 to step S205, so as to switch to the
high-.mu. control.
[0041] As can be seen from the flow chart of FIG. 4, during the
low-.mu. control (at the ABS system pressure build-up control
mode), switching operation from the low-.mu. control to high-.mu.
control occurs (i) when the low-to-high-.mu. mode-switching flag
f_j.sub.(1) for the first front road wheel is set to "1" and the
low-to-high-.mu. mode-switching flag f_j.sub.(2) for the second
front road wheel is set to "1", that is, in case of f_j.sub.(1)=1
and f_j.sub.(2)=1, or (ii) when the low-to-high-.mu. mode-switching
flag f_j.sub.(1) for the first front road wheel is set to "1" and
the temporary low-to-high-.mu. mode-switching flag f_j0.sub.(2) for
the second front road wheel is set to "1", that is, in case of
f_j.sub.(1)=1 and f_j0.sub.(2)=1. On the other hand, setting of the
low-to-high-.mu. mode-switching flag f_j to "1" occurs (a) when the
number PN of pressure build-up is greater than or equal to the
pulse-number threshold value N1, that is, PN.gtoreq.N1, and (b)
when the maximum wheel acceleration (recovery acceleration)
.DELTA.VwMAX exceeds the predetermined acceleration threshold value
.alpha..sub.max when the wheel speed Vw of the skid-controlled road
wheel is recovering to the vehicle speed (pseudo vehicle speed Vi)
in the reduce-pressure operating mode continuously executed after
the number PN of pressure build-up is greater than or equal to
N1.times.70% (70% of pulse-number threshold value N1), that is,
PN.gtoreq.N1*0.7, and thus the temporary low-to-high-.mu.
mode-switching flag f_j0 for the skid-controlled road wheel is set.
The first low-to-high-.mu. mode-switching condition (a) corresponds
to the flow from step S109 through steps S114, S115 and S116 to
step S117 (i.e.,
S109.fwdarw.S114.fwdarw.S115.fwdarw.S116.fwdarw.S117)- . The second
low-to-high-.mu. mode-switching condition (b) corresponds to the
flow from step S116 via step S118 to step S119, and to the
subsequent flow from step S116 via step S118 to step S119 (i.e.,
S116.fwdarw.S118.fwdarw.S119.fwdarw.S120.fwdarw.S102.fwdarw.S103.fwdarw.S-
104.fwdarw.S105.fwdarw.S107.fwdarw.S108.fwdarw.S109.fwdarw.S110.fwdarw.S11-
1.fwdarw.S112).
[0042] The operation of the ABS system of the embodiment is
hereinbelow described in detail in reference to the time chart
shown in FIG. 5. The time chart of FIG. 5 shows a particular case
that skid control is started during braking on low-.mu. road and
thereafter the road surface condition is changed from low-.mu. road
to high-.mu. road.
[0043] In FIG. 5, the low-.mu. control is executed for the time
interval from T1 to T2. Z1, Z2 and Z3 respectively denote first,
second, and third brake-fluid pressure build-up controls. When the
third pressure build-up control Z3 is executing, the road surface
condition changes from low-.mu. road to high-.mu. road, and thus
the number PN of pressure build-up increases. However, during
driving, low-.mu. and high-.mu. road surfaces are generally
sprinkled or dispersed, and thus the points of .mu. jump from
low-.mu. to high-.mu. vary intermittently. In such a case, there is
a decreased tendency for the number PN of pressure build-up to
exceed the pulse-number threshold value N1 readily. The example of
skid control of FIG. 5 shows such a case that the pressure build-up
number PN does not exceed the pulse-number threshold value N1
readily. In this case, if the pressure build-up is attained so that
the pressure build-up number PN exceeds the second threshold N1*0.7
(70% of pulse-number threshold value N1) when the pressure build-up
number PN does not yet exceed the pulse-number threshold value N1,
the temporary low-to-high-.mu. mode-switching flag f_j0 is set to
"1" (see the flow from step S116 via step S118 to step S119 in FIG.
3). Thereafter, when the wheel speed Vw of the skid-controlled
wheel becomes less than the reduce-pressure threshold value
.lambda.1 and thus the reduce-pressure mode is executed under a
condition where the temporary low-to-high-.mu. mode-switching flag
f_j0 is set, the wheel speed Vw of the skid-controlled wheel
recovers near to the vehicle speed (Vi) rapidly. At this time, as
soon as the maximum wheel acceleration (recovery acceleration)
.DELTA.VwMAX exceeds the predetermined acceleration threshold value
.alpha..sub.max, the processor of CU determines that there is a
.mu. change from low-.mu. road to high-.mu. road and therefore the
low-to-high-.mu. mode-switching flag f_j is set at once, so as to
switch from low-.mu. control to high-.mu. control (see the flow
from step S109 via step S110 through step S111 to step S112 in FIG.
3). Thus, after the time T3 (flag setting point of f_j=1), the
pressure build-up rate tends to rise according to the high-.mu.
control. As a consequence, it is possible to prevent the driver
from experiencing undesired free-running feel, and also it is
possible to reduce a braking distance as much as possible.
[0044] As will be appreciated from the above, before the pressure
build-up numbers PN for left and right road wheels both exceed the
pulse-number threshold value N1, the system of the embodiment shown
in FIGS. 3-5 can accurately rapidly detect a road-surface .mu.
change from low-.mu. to high-.mu. (.mu. jump) depending on the
temporary low-to-high-.mu. mode-switching flag f_j0 as well as the
low-to-high-.mu. mode-switching flag f_j, and thus optimally time
the switching from low-.mu. control to high-.mu. control. Also,
according to the system of the embodiment shown in FIGS. 3-5, even
when the road surface conditions of left and right road wheels are
different from each other and pressure build-up timings for the
left and right wheels differ from each other and thus the pressure
build-up numbers PN for the left and right wheels do not
simultaneously exceed the threshold N1, the low-to-high-.mu.
mode-switching flag f_j can be set to "1" under a predetermined
condition defined by f_j0=1 and .DELTA.VwMAX>.alpha..sub.max.
Therefore, the system of the embodiment can accurately detect the
road-surface .mu. change even during vehicle driving under the road
surface condition where low-.mu. and high-.mu. road surfaces are
sprinkled. Additionally, according to the system of the embodiment,
detection or determination of the road-surface .mu. change can be
made to each of the left and right road wheels (see the flow shown
in FIG. 4). In other words, the road-surface-condition change
determination section is provided individually for the left and
right road wheels. This avoids the road-surface .mu. change from
being misjudged, thus ensuring more precise skid control.
[0045] In the system of the embodiment, immediately when a
low-to-high-.mu. mode-switching flag f_j.sub.(1) for a first one of
front-left and front-right road wheels is set and additionally the
pressure build-up corresponding to 70% of a predetermined
pulse-number threshold value N1 is attained and thus a temporary
low-to-high-.mu. mode-switching flag f_j0.sub.(2) for the second
front road wheel is set, switching from low-.mu. control to
high-.mu. control. The second threshold is not limited to 70% of a
predetermined pulse-number threshold value N1. The second threshold
may be set to a predetermined value less than the first threshold
value (the predetermined pulse-number threshold value N1).
Moreover, in the system of the embodiment, as a recovery
acceleration value of wheel speed Vw, the maximum wheel
acceleration .DELTA.VwMAX is used. The recovery acceleration value
of wheel speed Vw is not limited to the maximum wheel acceleration
.DELTA.VwMAX. Instead of the use of the maximum wheel acceleration
.DELTA.VwMAX of the wheel acceleration data .DELTA.Vw calculated
from the wheel speed sensor signal, a simple mean
(.DELTA.Vw.sub.(1)+.DELTA.Vw.sub.(2)+ . . .
+.DELTA.Vw.sub.(n-1)+.DELTA.Vw.sub.(n))/n of a plurality of wheel
acceleration data continuously calculated (.DELTA.Vw.sub.(1),
.DELTA.Vw.sub.(2), . . . , .DELTA.Vw.sub.(n-1) and
.DELTA.Vw.sub.(n)) may be used as the recovery acceleration value.
Denoted by "n" is the number of wheel acceleration data
continuously calculated and stored in predetermined memory
addresses of the computer memory for a predetermined time interval
such as n.times.10 msec. Alternatively, a weighted mean
(w.sub.1.multidot..DELTA.Vw.sub.(1)+w.sub.2.multidot..DELTA.Vw.sub.(2)+
. . .
+W.sub.(n-1).multidot..DELTA.v.sub.(n-1)+W.sub.(n).multidot..DELTA.Vw-
.sub.(n))/(w.sub.1+w.sub.2+ . . . +w.sub.(n-1)+w.sub.(n)) of a
plurality of wheel acceleration data continuously calculated
(.DELTA.Vw.sub.(1), .DELTA.Vw.sub.(2), . . . , .DELTA.Vw.sub.(n-1)
and .DELTA.Vw.sub.(n)) may be used as the recovery acceleration
value, if each wheel acceleration data .DELTA.Vwi is to have weight
w.sub.i, that is, taking into account the type of vehicle.
[0046] Japanese Patent Provisional Publication No. 3-79460 teaches
the use of a so-called pressure build-up time interval from the
start of pressure build-up to the end of pressure build-up. The ABS
system ECU (electronic control unit) or ECM (electronic control
module) as described in the Japanese Patent Provisional Publication
No. 3-79460 determines that there is a lack of the rate of pressure
build-up when the pressure build-up time intervals of both of
front-left and front-right road wheels exceed a predetermined
threshold during the low-.mu. control, and that the road surface
condition has changed from the low-.mu. road to high-.mu. road.
Thus, the system operates to regulate the hydraulic brake pressure
of the wheel-brake cylinder subjected to skid control in accordance
with the high-.mu. control instead of the low-.mu. control, so as
to increasingly compensate for the pressure build-up rate. Assuming
that the ECU or ECM erroneously determines that the road surface
condition has changed from the low-.mu. road to high-.mu. road even
when the vehicle is still running on the low-.mu. road, the wheel
has an increased tendency to lock up owing to the increasingly
compensated pressure build-up rate, and thus there is a possibility
that the driver loses directional stability of the vehicle. To
ensure more precise skid control and to avoid misjudgment, a
determination of the road-surface .mu. change is generally based on
the pressure build-up time intervals of two road wheels (left and
right road wheels) but not on the pressure build-up time interval
of a single road wheel. However, in case that the ECU uses the
pressure build-up time intervals of the left and right road wheels
in order to precisely determine the presence or absence of the
road-surface .mu. change from low-.mu. to high-.mu. (often called
".mu. jump"), there is the following drawback.
[0047] For instance suppose a split-.mu. road where the left-hand
side road-surface .mu. is different from the right-hand side
road-surface .mu.. In case that the low-.mu. road surfaces are
sprinkled and additionally the sprinkled range of the left-hand
side road surface is different from that of the right-hand side
road surface, there is a tendency for the pressure build-up time
interval for one of left and right road wheels to be kept at a time
interval less than the predetermined threshold, even when the
pressure build-up time interval for the other road wheel reaches
the predetermined threshold. In this case, the ECU determines that
there is no .mu. change from low-.mu. road to high-.mu. road, and
therefore the low-.mu. control is continued. As a result the driver
experiences undesired free-running feel.
[0048] Accordingly, the system of the embodiment provides an
automotive brake control system with an anti-skid braking device,
which avoids the aforementioned disadvantages.
[0049] Referring now to FIG. 6, there is shown a modified skid
control routine executed by the processor of the microcomputer
employed in the electronic control unit CU. The modified skid
control routine shown in FIG. 6 is also executed as time-triggered
interrupt routines to be triggered every predetermined time
intervals such as 10 msec. Step S1 of the modified skid control
routine corresponds to steps S102 and S103 shown in FIG. 3, and
step S2 of the modified routine almost corresponds to step S104 of
FIG. 3.
[0050] At step S1, the more recent wheel speed indicative data
signals are read, and then a wheel speed Vw at each road wheel
(i.e., VwFR, VwFL, and VwR) is arithmetically calculated. Then, an
acceleration/deceleration rate at each road wheel (i.e.,
.DELTA.VwFR, .DELTA.VwFL, and .DELTA.VwR) is arithmetically
calculated.
[0051] At step S2, a pseudo vehicle speed Vi is computed or
determined by a predetermined or pre-programmed arithmetic
processing. Detailed description of the arithmetic processing for
pseudo vehicle speed Vi will be hereinafter described in detail in
reference to the flow chart shown in FIG. 7.
[0052] At step S3, a vehicle deceleration rate VIK is computed or
determined by a predetermined or pre-programmed arithmetic
processing. Detailed description of the arithmetic processing for
vehicle deceleration rate VIK will be hereinafter described in
detail in reference to the flow charts shown in FIGS. 8 and 12.
[0053] At step S4, a reduce-pressure threshold value .lambda.1 and
a hold-pressure threshold value .mu.2 are computed on the basis of
the pseudo vehicle speed Vi computed at step S2 of FIG. 6. For the
sake of simplicity, detailed description of the arithmetic
processing for reduce-pressure threshold value .lambda.1 will be
hereinafter described in detail in reference to the flow chart
shown in FIG. 9.
[0054] At step S5, a check is made to determine whether the wheel
speed Vw is less than the reduce-pressure threshold value
.lambda.1. In case of Vw<.lambda.1, the routine proceeds from
step S5 to step S6. Conversely, in case of Vw.gtoreq..lambda.1, the
routine proceeds from step S5 to step S7.
[0055] At step S6, a backward timer AS is set to a predetermined
value such as 150. Then, at step S8, a check is made to determine
whether the wheel acceleration .DELTA.Vw is greater than a
predetermined value such as 0.8 G (where G means a gravitational
acceleration unit). When the answer to step S8 is in the negative
(NO), that is, in case of .DELTA.Vw<0.8 G, the CU determines
that pressure reduction is required to avoid a wheel lock-up
condition, and thus the routine flows to step S9. At step S9, the
CU outputs command signals corresponding to the reduce-pressure
operating mode to the inflow and outflow .solenoid valves.
Conversely, when the answer to step S8 is in the affirmative (YES),
that is, in case of .DELTA.Vw>0.8 G, the CU determines that
pressure reduction is not required and that the wheel speed
recovers to the vehicle speed, and thus the routine flows to step
S11. At step S11, the CU outputs command signals corresponding to
the hold-pressure operating mode to the inflow and outflow solenoid
valves. The predetermined value for the backward timer AS is set to
a preset value (such as 150) enough to return the brake fluid
stored in the reservoirs (7, 7) into the brake circuits 1 and 2
during the reduce-pressure operating mode of ABS system
operation.
[0056] Returning to step S5, in case of Vw.gtoreq..lambda.1, the
routine proceeds to step S7. At step S7, a check is made to
determine whether the wheel acceleration .DELTA.Vw is less than the
hold-pressure threshold value .lambda.2. In case of
.DELTA.Vw<.lambda.2, the routine flows from step S7 to step S11,
to start the hold-pressure operating mode. In case of
.DELTA.Vw.gtoreq..lambda.2, the routine flows from step S7 to step
S10. At step S10, the CU outputs command signals corresponding to
the pressure build-up operating mode to the inflow and outflow
solenoid valves.
[0057] After steps S9, S10, or S11, the routine proceeds via step
S12 to step S13. At step S12, a test is made to determine whether a
predetermined time interval (10 msec) has expired from the start of
the ABS system operating mode (either of the pressure-reduce mode,
pressure-hold mode, and pressure build-up mode). When the
predetermined time interval (10 msec) has expired, the backward
timer AS is decremented. Therefore, timer AS is not decremented
until the predetermined time interval (10 msec) expires from the
start of the ABS system operating mode.
[0058] Referring now to FIG. 7, there is shown the arithmetic
calculation subroutine for the pseudo vehicle speed Vi.
[0059] At step S21, a highest one (VwMAX1 or simply, VwMAX) of all
of the wheel speeds Vw sensed is set as a selected wheel speed (a
reference wheel speed) VFS. The selected reference wheel speed VFS,
corresponding to the highest wheel speed of the wheel speed data
signals is often called a "select-HIGH wheel speed" VwMAX1.
[0060] At step S22, a check is made to determine whether a
skid-control-state indicative signal AS is "0". AS=0 means that the
ABS system is conditioned in an inoperative state. AS.noteq.0 means
that the ABS system is in operation. In case of AS=0, the
subroutine proceeds from step S22 to step S23. However, in case of
AS.noteq.0, the subroutine proceeds from step S22 to step S24.
[0061] When the ABS system is inoperative, at step S23, the
reference wheel speed VFS is set to a higher one (VwMAX2) of the
wheel speeds Vw of the two driven road wheels. At the same time, at
step S23, a so-called low-.mu. flag Lo.mu.F is reset to "0". The
low-.mu. flag Lo.mu.F is used to determine whether the road
condition is a low-.mu. road surface condition. Lo.mu.F=1 means
that the road condition of the road surface on which the vehicle is
now running is a low-.mu. road (such as snow or icy roads).
Lo.mu.F=0 means that the road condition of the road surface on
which the vehicle is now running is a middle-.mu. road (such as wet
roads) or a high-.mu. road (such as dry pavement). As described
later in reference to the flow chart shown in FIG. 8, the low-.mu.
flag Lo.mu.F is set to "1", for example, when a pressure-reduction
time interval DECT (described later) becomes above a predetermined
time interval such as 100 msec (see steps S36 and S37 of FIG.
8).
[0062] At step S24, the selected reference wheel speed VFS, set to
either the wheel speed VwMAX1 or the wheel speed VwMAX2, is
compared to the pseudo vehicle speed Vi. In case of Vi.gtoreq.VFS,
step S25 occurs. Conversely, in case of Vi<VFS, step S26
occurs.
[0063] At step S25, the current value Vi.sub.(n) of pseudo vehicle
speed Vi is calculated based on the previous value Vi.sub.(n-1) of
pseudo vehicle speed Vi and the vehicle deceleration rate VIK, from
the following expression.
Vi.sub.(n)=Vi.sub.(n-1)-(VIK+0.3 G)k
[0064] where k denotes a predetermined value, and G means a
gravitational acceleration unit.
[0065] At step S26, a wheel-acceleration-period limiter value
(simply, a limit value) x is set to 2 km/h. Thereafter, step S27
occurs. The limit value x is provided to prevent excessive wheel
acceleration.
[0066] At step S27, in the same manner as step S22, a check is made
to determine whether the skid-control-state indicative signal AS is
"0". In case of AS=0, that is, when the ABS system is inoperative,
the subroutine proceeds from step S27 to step S28. At step S28, the
limit value x is decreasingly compensated for and thus set to 0.143
km/h less than 2 km/h. In case of AS.noteq.0, that is, when the ABS
system is operative, the subroutine proceeds from step S27 to step
S29.
[0067] Thereafter, at step S29, the current value Vi.sub.(n) of
pseudo vehicle speed Vi is calculated based on the previous value
Vi.sub.(n-1) of pseudo vehicle speed Vi and the limit value x from
the following expression.
Vi.sub.(n)=Vi.sub.(n-1)-x
[0068] After steps S25 or S29, the program exits the
pseudo-vehicle-speed (Vi) calculation routine, and thus returns to
the main routine.
[0069] Referring now to FIG. 8, there is shown a first arithmetic
calculation subroutine for vehicle deceleration rate VIK.
[0070] At step S31, a check is made to determine the presence or
absence of a transition from the ABS inoperative state (AS=0) to
the ABS operative state (AS.noteq.0). As soon as the ABS system
comes into operation, the subroutine flows from step S31 to step
S32. In the absence of the transition from the ABS inoperative
state to the ABS operative state, the subroutine flows from step
S31 to step S33.
[0071] Step S32, the pseudo vehicle speed Vi is set to a
vehicle-deceleration starting-period vehicle speed V0. The speed
value V0 corresponds to a value of pseudo vehicle speed Vi
calculated at the time when the wheel speed Vw begins to decelerate
after braking action. At the same time, a counted value of a timer
T0 is initialized to "0".
[0072] At step S33, the timer T0 is incremented, so as to measure
an elapsed time from the time when the transition indicated by
AS=0.fwdarw.AS.noteq.0 has occurred.
[0073] Thereafter, at step S34, a check is made to determine the
presence or absence of a transition from the state defined by
Vi<VFS to the state defined by Vi.gtoreq.VFS. As soon as the
transition indicated by Vi<VFS.fwdarw.Vi.gtoreq.VFS occurs, the
subroutine proceeds from step S34 to step S35. However, in the
absence of the transition indicated by
Vi<VFS.fwdarw.Vi.gtoreq.VFS, the subroutine proceeds from step
S34 to step S36.
[0074] At step S35, the vehicle deceleration rate VIK is calculated
based on the vehicle-deceleration starting-period vehicle speed V0,
the pseudo vehicle speed Vi, and the counted value of the timer T0,
from the following expression.
VIK=(V0-Vi)/T0
[0075] Then, at step S36, the pressure-reduction time interval
DECT, during which the ABS system is in the reduce-pressure
operating mode, is compared to a predetermined time interval such
as 100 msec. In case of DECT.gtoreq.100 msec, step S37 occurs. At
step S37, the low-.mu. flag Lo.mu.F is set to "1". After step S37,
or in case of DECT>100 msec at step S36, the program exits the
vehicle-deceleration-rate (VIK) calculation routine, and thus
returns to the main routine.
[0076] Referring now to FIG. 9, there is shown the reduce-pressure
threshold value .lambda.1 arithmetic-calculation subroutine.
[0077] At step S41, an offset value xx is set to 8 km/h. As can be
appreciated from the following step S42, the offset value xx varies
depending on the vehicle deceleration rate VIK and the low-.mu.
flag Lo.mu.F.
[0078] At step S42, a check is made to determine whether a union
defined by VIK<0.4 G .orgate. Lo.mu.F=1 is satisfied. When the
union defined by logical expression VIK<0.4 G .orgate. Lo.mu.F=1
is satisfied, that is, in case of VIK<0.4 G and/or Lo.mu.1, the
subroutine proceeds from step S42 to step S43. At step S43, the
offset value xx is decreasingly compensated for and thus set to 4
km/h. Conversely, when the union defined by VIK<0.4 G .orgate.
Lo.mu.F=1 is unsatisfied, that is, in case of VIK.gtoreq.0.4 G and
Lo.mu.F=0, the subroutine proceeds from step S42 to step S44.
[0079] At step S44, the reduce-pressure threshold value .lambda.1
is computed based on the pseudo vehicle speed Vi and the offset
value xx, from the following expression.
.lambda.1=0.95.times.Vi-xx
[0080] According to the routine of FIG. 9, the reduce-pressure
threshold value .lambda.1 is properly determined depending upon the
road surface condition, that is, depending upon whether the
low-.mu. flag Lo.mu.F is set (low-.mu. road) or reset (middle-.mu.
road or high-.mu. road), and upon the result of comparison between
the vehicle deceleration rate VIK and the predetermined value such
as 0.4 G (where G means a gravitational acceleration unit).
Thereafter, the program exits the reduce-pressure threshold value
(.lambda.1) calculation routine, and thus returns to the main
routine.
[0081] Referring now to FIG. 10, there is shown the solenoid
reduce-pressure control subroutine for the inflow and outflow
solenoid valves (5, 6).
[0082] At step S51, a pressure build-up time interval INCT is
initialized to "0".
[0083] At step S52, a target pressure-reduction time interval GAW
is calculated based on a rate-of-change VWD30 of wheel speed Vw per
30 msec, and the vehicle deceleration rate VIK, from the following
expression.
GAW=VWD30+.alpha./VIK
[0084] where .alpha. is a predetermined fixed value.
[0085] Thereafter, at step S53, a check is made to determine
whether an intersection defined by VIK.gtoreq.0.4 G .andgate.
Lo.mu.F=1 is satisfied. When the intersection defined by
VIK.gtoreq.0.4 G .andgate. Lo.mu.F=1 is satisfied, that is, in case
of VIK.gtoreq.0.4 G and Lo.mu.F=1, the subroutine proceeds to step
S54. Conversely, when the intersection defined by VIK.gtoreq.0.4 G
.andgate. Lo.mu.F=1 is unsatisfied, that is, in case of VIK<0.4
G or Lo.mu.F=0, the subroutine proceeds to step S55.
[0086] At step S54, the target pressure-reduction time interval GAW
is calculated based on only the rate-of-change VWD30 of wheel speed
Vw per 30 msec, from the following expression.
GAW=VWD30.times..alpha./0.1 G
[0087] The use of the aforementioned rate-of-change VWD30 of wheel
speed Vw per 30 msec is superior to the use of a rate-of-change
VWD10 of wheel speed Vw per 10 msec, a rate-of-change VWD20 of
wheel speed Vw per 20 msec, or a rate-of-change VWD40 of wheel
speed Vw per 40 msec, from the viewpoint of the responsiveness and
noise reduction of skid control. In more detail, the rate-of-change
VWD10 of wheel speed Vw per 10 msec and the rate-of-change VWD20 of
wheel speed Vw per 20 msec are inferior to the rate-of-change VWD30
of wheel speed Vw per 30 msec from the viewpoint of noise
reduction. On the other hand, the rate-of-change VWD40 of wheel
speed Vw per 40 msec is inferior to the rate-of-change VWD30 of
wheel speed Vw per 30 msec from the viewpoint of the
responsiveness. For the reasons discussed above, in the modified
system shown in FIGS. 6 through 12, the rate-of-change VWD30 of
wheel speed Vw per 30 msec is used for the solenoid
pressure-reduction control subroutine of FIG. 10, the solenoid
pressure-build-up control subroutine of FIG. 11 (described later),
and the vehicle-deceleration rate (VIK) calculation subroutine of
FIG. 12 (described later).
[0088] At step S55, the CU outputs command signals corresponding to
the reduce-pressure operating mode to the inflow and outflow
solenoid valves, and simultaneously the pressure-reduction time
interval DECT is incremented.
[0089] At step S56, the pressure-reduction time interval DECT is
compared to the target pressure-reduction time interval GAW. In
case of DECT.gtoreq.GAW, step S57 occurs. At step S57, the CU
outputs command signals corresponding to the hold-pressure
operating mode to the inflow and outflow solenoid valves, and
simultaneously the pressure-reduction time interval DECT is
decremented. The previously-discussed intersection defined by the
logical expression VIK.gtoreq.0.4 G .andgate. Lo.mu.F=1 (see step
S53) means that the CU has determined that the current road surface
condition is a low road (Lo.mu.F=1) before the wheel speed Vw
reaches a spin-up speed VP. The spin-up speed corresponds to a
value VP of pseudo vehicle speed Vi calculated at the time when the
pseudo vehicle speed changes from an increasing state to a
decreasing state at each execution cycle of skid control. At the
initial cycle of skid control, the vehicle deceleration rate VIK is
set to an initial value such as a deceleration rate ranging from
0.6 G to 1.4 G, and therefore the initial value of vehicle
deceleration rate VIK is greater than 0.4 G at the initial stage of
skid control. Thus, under such a condition, the intersection
defined by the logical expression VIK.gtoreq.0.4 G .andgate.
Lo.mu.F=1 is satisfied. In this case, the target pressure-reduction
time interval GAW can be set to a pressure-reduction rate (or a
pressure-reduction time interval) defined by the expression
GAW=VWD30.times..alpha./0.1 G and programmed to be suitable for the
low-.mu. road surface condition (see the flow from step S53 to step
S54). Otherwise, the target pressure-reduction time interval GAW
can be set to a pressure-reduction rate (or a pressure-reduction
time interval) defined by the expression
GAW=VWD30.times..alpha./VIK and programmed to be suitable for road
surface conditions except for the low-.mu. road surface condition.
After step S57, or in case of DECT<GAW at step S56, the program
exits the solenoid pressure-reduction control routine, and thus
returns to the main routine.
[0090] Referring now to FIG. 11, there is shown the solenoid
pressure-build-up control subroutine for the inflow and outflow
solenoid valves (5, 6).
[0091] At step S61, a pressure-reduction time interval DECT is
initialized to "0".
[0092] At step S62, a target pressure-build-up time interval ZAW is
calculated based on the rate-of-change VWD30 of wheel speed Vw per
30 msec, and the vehicle deceleration rate VIK, from the following
expression.
ZAW=VWD30.times..beta..times.VIK
[0093] where .beta. is a predetermined fixed value. The
above-mentioned values .alpha. and .beta. are tuned values
pre-programmed or predetermined to be provide optimal skid
control.
[0094] Thereafter, at step S63, a check is made to determine
whether an intersection defined by VIK.gtoreq.0.4 G .andgate.
Lo.mu.F=1 is satisfied. When the intersection defined by
VIK.gtoreq.0.4 G .andgate. Lo.mu.F=1 is satisfied, that is, in case
of VIK.gtoreq.0.4 G and Lo.mu.F=1, the subroutine proceeds to step
S64. Conversely, when the intersection defined by VIK.gtoreq.0.4 G
.andgate. Lo.mu.F=1 is unsatisfied, that is, in case of VIK<0.4
G or Lo.mu.F=0, the subroutine proceeds to step S65.
[0095] At step S64, the target pressure-build-up time interval ZAW
is calculated based on only the rate-of-change VWD30 of wheel speed
Vw per 30 msec, from the following expression.
ZAW=VWD30.times..beta..times.0.1 G
[0096] At step S65, the CU outputs command signals corresponding to
the pressure build-up operating mode to the inflow and outflow
solenoid valves, and simultaneously the pressure-build-up time
interval INCT is incremented.
[0097] At step S66, the pressure-build-up time interval INCT is
compared to the target pressure-build-up time interval ZAW. In case
of INCT.gtoreq.ZAW, step S67 occurs. At step S67, the CU outputs
command signals corresponding to the hold-pressure operating mode
to the inflow and outflow solenoid valves, and simultaneously the
pressure-build-up time interval INCT is decremented. The
previously-discussed intersection defined by the logical expression
VIK.gtoreq.0.4 G .andgate. Lo.mu.F=1 (see step S63) means that the
CU has determined that the current road surface condition is a low
road (Lo.mu.F=1) before the wheel speed Vw reaches the spin-up
speed VP. At the initial stage of skid control, the intersection
defined by the logical expression VIK.gtoreq.0.4 G .andgate.
Lo.mu.F=1 is satisfied. In this case, the target pressure-build-up
time interval ZAW can be set to a pressure-build-up rate (or a
pressure-build-up time interval) defined by the expression
ZAW=VWD30=.beta.+0.1 G and programmed to be suitable for the
low-.mu. road surface condition (see the flow from step S63 to step
S64). Otherwise, the target pressure-build-up time interval ZAW can
be set to a pressure-build-up rate (or a pressure-build-up time
interval) defined by the expression ZAW=VWD30.times..beta.=VIK and
programmed to be suitable for road surface conditions except for
the low-.mu. road surface condition. After step S67, or in case of
INCT<ZAW at step S66, the program exits the solenoid
pressure-build-up control routine, and thus returns to the main
routine.
[0098] Referring now to FIG. 12, there is shown a second arithmetic
calculation subroutine for vehicle deceleration rate VIK. As
discussed later, the vehicle deceleration rate VIK calculation
subroutine of FIG. 12 is superior to that of FIG. 8, from the
viewpoint of more precise determination of the road surface
condition. Steps S71, S72, S74, S75, S86 and S87 of the second
vehicle deceleration rate VIK calculation subroutine of FIG. 12
correspond to steps S31, S32, S34, S35, S36 and S37 shown in FIG.
8, and step S73 of FIG. 12 is somewhat similar to step S33 of FIG.
8. Thus, the other steps S76-S85, and S88-S92 will be hereinafter
described in detail with reference to the accompanying drawings,
while detailed description of steps S71-S75 and S86-S87 will be
omitted because the above description thereon seems to be
self-explanatory.
[0099] In the presence of the transition from the ABS inoperative
state (AS=0) to the ABS operative state (AS.noteq.0), the
subroutine proceeds from step S71 to step S72. During the ABS
inoperative state, the subroutine flows from step S71 to step
S73.
[0100] At step S72, the pseudo vehicle speed Vi is set to the
vehicle-deceleration starting-period vehicle speed V0, and at the
same time a counted value of timer T0 is initialized to "0".
[0101] At step S73, the timer T0 is incremented, so as to measure
an elapsed time from the time when the transition indicated by
AS=0.fwdarw.AS.noteq.0 has occurred. At the same time, a spin-up
time (or a derivative time) VPT is incremented. The spin-up time
VPT corresponds to a time interval from a first spin-up point (see
the point corresponding to the first spin-up speed VPA in FIGS.
14A-14D) to a second spin-up point (see the point corresponding to
the second spin-up speed VPB in FIGS. 14A-14D).
[0102] At step S74, the presence or absence of a transition from
the state defined by Vi<VFS to the state defined by
Vi.gtoreq.VFS is checked. In other words, a check is made to
determine whether the wheel speed Vw reaches the spin-up speed
point (VP). If the transition indicated by
Vi<VFS.fwdarw.Vi.gtoreq.VFS occurs, the subroutine proceeds from
step S74 to step S75. However, in the absence of the transition
indicated by Vi<VFS.fwdarw.Vi.gtoreq.VFS, the subroutine
proceeds from step S74 to step S78.
[0103] At step S75, the vehicle deceleration rate VIK is calculated
from the expression VIK=(V0-Vi)/T0.
[0104] At step S76, the first spin-up speed VPA (i.e., the previous
value VP.sub.(n-1) of spin-up speed VP) is updated by the second
spin-up speed VPB (i.e., the current value VP.sub.(n) of spin-up
speed VP), the second spin-up speed VPB is updated by the pseudo
vehicle speed indicative data Vi, and thereafter a time rate of
change of spin-up speed VP, that is, a derivative VIKB (=dVP/dt) of
spin-up speed VP is calculated from the following expression.
VIKB=(VPA-VPB)/VPT
[0105] At step S77, the spin-up time VPT is set to "0", the slip
time Lo.mu.T is initialized to "0", a middle-.mu./high-.mu. flag
H.mu.F (simply, a high-.mu. flag H.mu.F) is reset to "0", and a
wheel lock-up indicative flag LOCKF (simply, a wheel-lock-up flag
LOCKF) is reset to "0".
[0106] At step S78, a check is made to determine whether a
condition defined by Lo.mu.T.noteq.0 or Vw<.lambda.1 is
satisfied. When the wheel speed Vw becomes less than the
reduce-pressure threshold value .lambda.1 and thus the CU begins to
measure the slip time Lo.mu.T, the answer to step S78 is
affirmative (YES). At this time, the subroutine proceeds to step
S79. Conversely, when Lo.mu.T=0 and Vw.gtoreq..lambda.1, the
routine flows from step S78 to step S80.
[0107] At step S79, the slip time Lo.mu.T is incremented.
[0108] At step S80, in order to determine whether the road wheel is
locked up, the wheel speed Vw is compared to a predetermined speed
value such as 0 km/h. If at least one of the wheels is locked up
and the condition Vw=0 km/h is satisfied, step S81 occurs. When
Vw.noteq.0 km/h, step S82 occurs.
[0109] At step S81, the wheel-lock-up flag LOCKF is set to "1".
[0110] At step S82, a check is made to determine whether the
transition from the wheel locked-up state defined by LOCKF=1 to the
wheel unlocked state defined by LOCKF=0. As soon as the transition
indicated by LOCKF=1.fwdarw.0 occurs, the routine proceeds to step
S83. However, in the absence of the transition indicated by
LOCKF=1.fwdarw.0, the routine advances from step S82 to step
S86.
[0111] At step S83, the time rate of change VIKB of spin-up speed
VP is compared to a predetermined acceleration value. The
predetermined acceleration value is set to a value ranging from 0.3
G to 0.6 G (more preferably 0.5 G) In the system of the embodiment,
the predetermined acceleration value is set to 0.5 G. When
VIKB<0.5 G and thus the CU determines that there is an increased
tendency for the road wheel to be locked up, the routine proceeds
from step S83 to step S84. At step S84, low-.mu. flag Lo.mu.F is
set to "1". Conversely when VIKB.gtoreq.0.5 G, the routine proceeds
from step S83 to step S85. At step S85, low-.mu. flag Lo.mu.F is
cleared to "0".
[0112] At step S86, the pressure-reduction time interval DECT is
compared to a predetermined time interval such as 100 msec. In case
of DECT.gtoreq.100 msec, step S87 occurs. At step S87, low-.mu.
flag Lo.mu.F is set to "1". In case of DECT<100 msec at step
S86, the routine proceeds to step S88. Note that, in the second
vehicle deceleration rate (VIK) calculation routine of FIG. 12,
setting or resetting of low-.mu. flag Lo.mu.F is dependent upon the
time rate of change VIKB of spin-up speed VP as well as the
pressure-reduction time interval DECT.
[0113] Thereafter, at step S88, the acceleration threshold value
(recovery acceleration threshold) .alpha..sub.max is computed or
map-retrieved from a predetermined or preprogrammed .alpha..sub.max
versus Lo.mu.T characteristic map showing how the recovery
acceleration threshold value .alpha..sub.max varies relative to the
slip time Lo.mu.T. As can be seen from the preprogrammed
.alpha..sub.max versus Lo.mu.T characteristic map shown in FIG. 13,
the recovery acceleration threshold) .alpha..sub.max varies in the
order of the low-.mu. road, middle-.mu. road, and high-.mu. road.
The recovery acceleration threshold value .alpha..sub.max (related
to the wheel recovery acceleration such as .DELTA.VwMAX or the
simple-mean wheel recovery acceleration or the weighted-mean wheel
recovery acceleration) of the middle-.mu. road tends to be less
than that of the high-.mu. road and to be greater than that of the
low-.mu. road. For example, as compared to the low-.mu. road,
during driving on the middle-.mu. road the wheel recovery
acceleration tends to be greater and also the slip time Lo.mu.T
tends to be shorter. So, in the system of the embodiment of FIG.
12, the wheel recovery acceleration threshold value .alpha..sub.max
is variably determined depending on the road surface condition
(road-surface .mu.) and slip time Lo.mu.T. In other words, the road
surface condition can be represented as a function of the wheel
recovery acceleration (.DELTA.VwMAX) and slip time (Lo.mu.T). In
the shown embodiment, the road-surface-condition decision value
DDM, which is obtained by dividing the maximum wheel acceleration
(recovery acceleration) .DELTA.VwMAX by the recovery time Lo.mu.T
measured, can be used to determine the road surface condition (see
step S206 of FIG. 4).
[0114] Then, at step S89, in order to judge the road surface
condition from the result of comparison of the wheel recovery
acceleration (the rate-of-change VWD30 of wheel speed Vw per 30
msec) with the acceleration threshold value .alpha..sub.max, a
check is made to determine whether the rate-of-change VWD30 of
wheel speed Vw per 30 msec is greater than the acceleration
threshold value .alpha..sub.max. When VWD30>.alpha..sub.max step
S90 occurs. At step S90, high-.mu. flag H.mu.F is set to "1". On
the other hand, when the answer to step S89 is negative (NO), that
is, VWD30.ltoreq..alpha..sub.max the routine proceeds from step S89
to step S91.
[0115] At step S91, a test is made to determine when the high-.mu.
flag H.mu.F is set. In case of H.mu.F=1, the routine flows from
step S91 to step S92. At step S92, low-.mu. flag Lo.mu.F is reset
to "0". In contrast, in case of H.mu.F=0, the program exits the
second vehicle deceleration rate VIK calculation routine of FIG.
12, and thus returns to the main routine.
[0116] As discussed above, in the first vehicle-deceleration-rate
VIK calculation subroutine of FIG. 8, low-.mu. flag Lo.mu.F,
indicative of the road surface condition, is set by the
pressure-reduction time interval DECT greater than or equal to the
predetermined time interval (100 msec) (see step S36 of FIG. 8). On
the other hand, in the second vehicle-deceleration-rate VIK
calculation subroutine of FIG. 12, low-.mu. flag Lo.mu.F,
indicative of the road surface condition, is temporarily set by the
pressure-reduction time interval DECT greater than or equal to the
predetermined time interval (100 msec) (see step S86 of FIG. 12),
and thereafter when the wheel-lock-up flag LOCKF is set by the
predetermined condition of Vw=0 km/h (see the flow from step S80 to
step S81), low-.mu. flag Lo.mu.F, temporarily set at step S87, can
be cleared to "0" under the predetermined condition of VIKB>0.5
G (see the flow from step S82 via step S83 to step S85) or remain
set to "1" under the predetermined condition of VIKB<0.5 G (see
the flow from step S82 via step S83 to step S84). That is,
according to the second vehicle-deceleration-rate VIK calculation
subroutine of FIG. 12, by the use of the time rate of change VIKB
of spin-up speed VP, the .mu.-change from low-.mu. road to
high-.mu. road, is rapidly detected. Therefore, the accuracy of
determination for the road surface condition (road-surface .mu.) of
the vehicle-deceleration-rate VIK calculation subroutine of FIG. 12
is higher than that of FIG. 8.
[0117] FIGS. 14A through 14D show simulation test results obtained
by the ABS system of the embodiment capable of performing the
routine shown in FIG. 12, during driving on low-.mu. road, whereas
FIGS. 15A through 15D show simulation test results obtained during
driving on middle-.mu. road. As shown in FIGS. 14A-14D, during the
vehicle driving on low-.mu. road, the slip time Lo.mu.T and the
spin-up time VPT tend to be longer and thus the time rate of change
VIKB (=(VPA-VPB)/VPT) of spin-up speed VP is relatively small.
Therefore, in step S83 of the routine shown in FIG. 12, the
condition of VIKB<0.5 G is satisfied (YES) and thus the routine
flows from step S83 to step S84, to retain low-.mu. flag Lo.mu.F
set (Lo.mu.F=1), and the road surface condition is decided
continually as low-.mu. road. On the other hand, as shown in FIGS.
15A-15D, during the vehicle driving on middle-.mu. road, the slip
time Lo.mu.T and the spin-up time VPT tend to be shorter and thus
the time rate of change VIKB (=(VPA-VPB)/VPT) of spin-up speed VP
is relatively great. Therefore, in step S83 of the routine shown in
FIG. 12, the condition of VIKB<0.5 G is unsatisfied (NO) and
thus the routine proceeds from step S83 to step S85, to clear
low-.mu. flag Lo.mu.F, and the ECU determines the road surface
condition is changed from low-.mu. road to middle-.mu. road (or
high-.mu. road).
[0118] As described previously, according to the ABS system related
to FIGS. 3 and 4, in order to prevent erroneous detection of the
road-surface .mu. when at least one of the road wheels is locking
up, a determination for .mu. change (.mu. jump) from low-.mu. road
to high-.mu. road is based on the temporary low-to-high-.mu.
mode-switching flag f_j0 as well as the low-to-high-.mu.
mode-switching flag f_j, thus ensuring high-accuracy .mu. change
(.mu. jump) decision. On the other hand, according to the ABS
system related to FIGS. 6 through 12, in order to accurately
determine the road-surface .mu. when at least one of the road
wheels is locking up (LOCKF=1), the road-surface-condition change
determination section of the ABS system inhibits the .mu. change
(.mu. jump) from being determined based on both the time length (PN
or DECT or DDM) of brake-fluid-pressure control and the recovery
acceleration (.DELTA.VwMAX or VWD30) of the wheel speed, and then
the determination for .mu. change (.mu. jump) is corrected based on
the time rate of change in spin-up speed VP, that is, the
derivative dVP/dt (=(VPA-VPB)/VPT=VIKB).
[0119] The entire contents of Japanese Patent Application No.
P2000-073733 (filed Mar. 16, 2000) is incorporated herein by
reference.
[0120] While the foregoing is a description of the preferred
embodiments carried out the invention, it will be understood that
the invention is not limited to the particular embodiments shown
and described herein, but that various changes and modifications
may be made without departing from the scope or spirit of this
invention as defined by the following claims.
* * * * *