Adaptive mode anti-lock brake controller

A wheel lock control system is described which utilizes both independent and select low modes of wheel lock controlled braking. When braking on a surface in which the coefficients of friction between the two sides of the vehicle are substantially different or when the vehicle is undergoing severe steering maneuvers while braking, the wheel lock control system utilizes a select low mode of braking for the vehicle rear wheels so as to maintain vehicle stability. However, when the four wheels are being braked on a substantially uniform coefficient of friction surface, the wheel lock control system utilizes an independent mode of braking for each of the wheels of the vehicle so as to minimize the vehicle braking distance.

BACKGROUND OF THE INVENTION 
This invention relates to an anti-lock control system for vehicle wheel 
brakes. 
When the brakes of a vehicle are applied, a braking force between the wheel 
and the road surface is generated that is dependent upon various 
parameters including the road surface condition and the amount of slip 
between the wheel and the road surface. For a given road surface, the 
force between the wheel and the road surface increases with increasing 
slip values to a peak force occurring at a critical wheel slip value. As 
the value of wheel slip increases beyond the critical slip value, the 
force between the wheel and the road surface decreases. Stable braking 
results when the slip value is equal to or less than the critical slip 
value. However, when the slip value becomes greater than the critical slip 
value, braking becomes unstable resulting in sudden wheel lockup, reduced 
vehicle stopping distance and a deterioration in the lateral stability of 
the vehicle. 
Numerous wheel lock control systems have been proposed to prevent the 
wheels from locking up while being braked. These systems generally prevent 
a wheel from locking by controlling the applied brake pressure when an 
incipient wheel lockup condition is sensed so as to maintain substantially 
the maximum possible braking force between the tire and road surface while 
at the same time preventing the wheel from operating in the unstable 
braking region. 
Some of the known wheel lock control systems utilize an "independent" mode 
of braking wherein each of the front and rear vehicle wheels are 
controlled independently so as to establish the maximum possible braking 
force at each wheel during wheel lock controlled braking. By so maximizing 
the braking forces at each wheel, the stopping distance of the vehicle is 
minimized. However, under certain conditions, this mode of operation can 
lead to reduced vehicle stability. The conditions for this occurrence are 
either grossly different coefficients of friction between right and left 
sides of the vehicle, or severe steering maneuvers while braking. 
Other forms of known wheel lock control systems utilize what is referred to 
as the "select low" mode of wheel lock controlled braking which provides 
for improved vehicle stability and steerability when the vehicle is being 
braked on a split coefficient of friction surface. In this form of wheel 
lock control system, the front brakes are typically controlled 
independently as above described but the rear wheel brakes are controlled 
such that the brakes of the rear wheel being braked on a high coefficient 
of friction surface is regulated in response to the conditions of the rear 
wheel being braked on a lower coefficient of friction surface. While this 
mode of braking increases the vehicle stopping distance, vehicle stability 
and steerability is improved. For example the lateral friction force of 
the rear wheel being braked on the high coefficient of friction surface is 
increased since it is being controlled to a low slip value. 
SUMMARY OF THE INVENTION 
The present invention provides for an improved wheel lock control system 
which utilizes both independent and select low modes of wheel lock 
controlled braking. When braking on a surface in which the coefficients of 
friction between the two sides of the vehicle are substantially different 
or when the vehicle is undergoing severe steering maneuvers while braking, 
the wheel lock control system of this invention utilizes a select low mode 
of braking for the vehicle rear wheels so as to maintain vehicle 
stability. However, when the four wheels are being braked on a 
substantially uniform coefficient of friction surface, the wheel lock 
control system of this invention utilizes an independent mode of braking 
for each of the wheels of the vehicle so as to minimize the vehicle 
braking distance. 
In the embodiment hereinafter described, a difference in the coefficients 
of friction between the right and left sides of the vehicle or a severe 
steering maneuver while braking is sensed based on the difference in the 
determined braking force of the front wheels while braking.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
A wheel under the influence of braking has two major torques acting on it: 
brake torque and tire torque. Brake torque arises from the application of 
brake pressure through the brake mechanism and tire torque is generated by 
the friction of the tire-road interface as wheel slip occurs. 
Brake torque T.sub.b is assumed to be proportional to brake pressure 
P.sub.b with a known brake gain K.sub.b and is defined by the expression 
EQU T.sub.b =P.sub.b K.sub.b. (1) 
Tire torque T.sub.t is related to the brake force coefficient .mu. between 
the tire and the road surface, the normal load N on the tire and the wheel 
rolling radius R and is defined by the expression 
EQU T.sub.t =.mu.NR. (2) 
For the free body consisting of the brake, wheel, and tire, the equation of 
motion is 
EQU I.sub.w .omega.+T.sub.b -T.sub.t =0 (3) 
where I.sub.w is the wheel moment of inertia and .omega. is the wheel 
angular acceleration. When the difference between the tire torque and the 
brake torque is positive, the wheel accelerates; and when negative, the 
wheel decelerates. 
Combining expressions 1 and 3, tire torque T.sub.t is defined as 
EQU T.sub.t =I.sub.w .omega.+P.sub.b K.sub.b. (4) 
As can be seen, the tire torque can be calculated from values that are 
either known or can be measured. The wheel moment of inertia I.sub.w and 
the brake gain K.sub.b are known values, the value of brake pressure 
P.sub.b can be measured and .omega. can be determined by differentiating 
the value of wheel speed which can be measured. 
The brake friction coefficient term .mu. of the tire torque T.sub.t is a 
nonlinear function of the magnitude of slip between the wheel and the road 
surface during braking and is dependent upon the road surface condition. 
FIG. 1 illustrates the brake friction coefficient .mu. as a function of 
percentage-wheel slip for two road surface conditions. For a given road 
surface, it can be seen that as wheel slip is increased in response to 
increased brake torque T.sub.b, the brake friction coefficient .mu. and 
therefore the tire torque T.sub.t increases until a critical slip value at 
which the brake friction coefficient and the tire torque are at a maximum. 
A further increase in wheel slip results in a decrease in the tire torque 
due to a decrease in the brake friction coefficient and high wheel 
deceleration values. The maximum tire torque resulting in a maximum 
braking effort for a given road surface is achieved when the brake torque 
T.sub.b produces the critical wheel slip value. When the braking effort 
produces a wheel slip exceeding the critical slip value, the braking 
operation becomes unstable and typically results in sudden wheel lockup 
which in turn results in increased stopping distance and a deterioration 
in the steering and lateral stability of the vehicle. 
In general, the brake control system identifies the value of the braking 
pressure P.sub.b that produces the maximum tire torque T.sub.t. This is 
accomplished by continuously calculating the tire torque value T.sub.t of 
equation (4) during braking. Any time the calculated value is larger than 
any previously calculated value, the value of the tire torque and the 
braking pressure P.sub.b is stored so that the maximum tire torque and 
brake pressure producing it are known. When an incipient wheel lockup is 
detected, the brake pressure is dumped to allow the wheel speed to recover 
and the brake pressure is thereafter reapplied to the stored value to 
establish a braking condition in which the wheel slip is substantially at 
the critical slip value for the existing road surface condition. This 
results in substantially the maximum possible tire torque T.sub.t and 
minimum stopping distance for the road surface condition. 
A general overview of the wheel lock control system is illustrated in FIG. 
2. The control system for the brake of a single wheel is illustrated, it 
being understood that the brakes of the remaining wheels of the vehicle 
are similarly controlled. A standard wheel brake 10 for a wheel 11 is 
actuated by controlled hydraulic pressure from one of two sources. The 
primary source is a motor driven actuator 12 and the secondary source is a 
standard master cylinder 14 controlled directly by the vehicle brake pedal 
16. A normally open electromagnetic valve 18 is energized when the 
actuator 12 is operative to control the hydraulic pressure to the brake 10 
so as to decouple the master cylinder 14 and brake pedal 16 from the 
hydraulic pressure output of the actuator 12. When the electromagnetic 
valve 18 is deenergized, the hydraulic pressure to brake 10 may be 
modulated directly by the brake pedal 16 and master cylinder 14. 
The valve 18 is deenergized only during limited vehicle operating 
conditions such as low vehicle speed or during failed conditions of the 
primary hydraulic pressure source to permit brake pressure modulation by 
the master cylinder 14. 
An electronic controller 20 is responsive to the outputs of a brake pedal 
force sensor 22 providing a signal that is a measure of the operator 
applied brake pedal force F, a wheel speed sensor 24 that provides a 
signal that is a measure of wheel speed .omega., and a pressure sensor 26 
that provides a signal that is a measure of the hydraulic brake pressure 
P.sub.b applied to the brake 10 from the master cylinder 14 or the 
actuator 12. The electronic controller 20 is responsive to those signals 
to (a) energize the valve 18 when the wheel speed .omega. exceeds a value 
corresponding to a low vehicle speed such as 3 mph, (b) control the 
actuator 12 so as to apply a hydraulic pressure P.sub.b to the brake 10 
that is proportional to the brake force F times a gain constant G for 
providing power assist during normal braking conditions, and (c) limit the 
pressure P.sub.b applied to the brake 10 for wheel lock control. 
Referring to FIG. 3, the actuator 12 in the preferred embodiment includes a 
DC torque motor 28 whose output shaft drives an input gear 30 which in 
turn rotatably drives an output gear 32. The drive member 34 of a ball 
screw actuator is secured for rotation with the output gear 32. The drive 
member 34 engages and axially positions the driven member 36 of the ball 
screw actuator. The driven member 36 drives a piston 38 to control the 
hydraulic pressure output of the actuator 12. The torque output of the 
motor 28 is translated to a directly related hydraulic pressure P.sub.b 
output of the actuator 12 that is applied to the brake 10. 
As illustrated in FIG. 4, the electronic controller 20 takes the form of a 
digital computer 40 and a motor control circuit 41. The digital computer 
is standard in form and includes a central processing unit (CPU) which 
executes an operating program permanently stored in a read-only memory 
(ROM) which also stores tables and constants utilized in controlling the 
hydraulic pressure input to the brake 10. Contained within the CPU are 
conventional counters, registers, accumulators, flag flip flops, etc. 
along with a clock which provides a high frequency clock signal. 
The computer 40 also includes a random access memory (RAM) into which data 
may be temporarily stored and from which data may be read at various 
address locations determined in accord with the program stored in the ROM. 
A power control unit (PCU) receives battery voltage and provides regulated 
power to the various operating circuits in the electronic controller 20. 
The computer 40 further includes an input/output circuit (I/O) that in turn 
includes a discrete output section controlled by the CPU to provide a 
control signal to the valve 18. In controlling the brake 10, the computer 
outputs a digital signal to the motor control circuit 41 via the I/O 
representing a desired value of the hydraulic brake pressure. The motor 
control circuit 41 converts the digital signal representing the desired 
pressure to an analog signal which is compared with the actual measured 
value of the brake pressure P.sub.b. By standard closed loop adjustment 
that may include both proportional and integral terms, the motor 28 
current is controlled so that the actual measured brake pressure P.sub.b 
is made equal to the desired pressure. 
The I/O also includes an input counter section which receives a pulse 
output from the wheel speed sensor 24 having a frequency representing 
wheel speed .omega.. Wheel speed .omega. is then determined by counting 
clock pulses between wheel speed pulses. 
An analog-to-digital unit (ADU) is included which provides for the 
measurement of analog signals. The analog signals representing conditions 
upon which the hydraulic brake pressure to the brake 10 is based are 
supplied to the ADU. In the present embodiment, those signals include the 
brake pressure value P.sub.b from the pressure sensor 26 and the output of 
the brake pedal force sensor 22 providing a measure of the pedal force F. 
The analog signals are sampled and converted under the control of the CPU 
and stored in ROM designated RAM memory locations. 
Referring to FIG. 5, when power is first applied to the system such as when 
the vehicle ignition switch is rotated to its "on" position, the computer 
program is initiated at point 42 and then proceeds to a step 44 where the 
computer 40 provides for system initialization. For example, at this step 
initial values stored in the ROM are entered into ROM designated RAM 
memory locations and counters, flags and timers are initialized. 
After the initialization step 44, the program proceeds to a step 46 where 
the program conditions the controller 20 to allow interrupts to occur and 
then to a background loop 48 which is continuously repeated. This loop may 
include, for example, diagnostic routines. In the preferred embodiment of 
this invention, an interrupt is provided by the CPU at 5 millisecond 
intervals. Following each interrupt, the execution of the background loop 
48 is interrupted and the routines for establishing the hydraulic brake 
pressure to each of the front and rear wheel brakes are executed. 
Control of the front wheel brakes of the vehicle will first be described. 
The front wheel brakes are controlled by the controller 20 independently 
in identical manner by identical program routines. FIGS. 6-8 combined 
illustrate the routines executed by the electronic controller in 
controlling the hydraulic brake pressure P.sub.b to the brake of one of 
the front wheels. 
Referring to FIG. 6, the five millisecond interrupt routine is illustrated. 
This routine is entered at step 50 and proceeds to a step 52 where the 
last measured speed .omega. of the wheel is saved and the new values of 
the speed .omega. of the wheel, brake pedal force F and brake line 
pressure P.sub.b to the wheel brake are read and stored in ROM designated 
RAM memory locations. Next, the program proceeds to a step 54 where it is 
determined whether or not the operator is commanding brake application. 
The brakes are considered applied if the value of the brake pedal force F 
is greater than zero. If the brakes are not applied, the program proceeds 
to a step 56 where a brake pressure command value P.sub.c for the wheel 
brake is set equal to zero. Also at this step, the speed of the vehicle as 
represented by the speed .omega..sub.v of a hypothetical unbraked wheel is 
set equal to the wheel speed measured at step 52. Since the brakes are not 
applied, the wheel slip is substantially at zero so that the actual and 
hypothetical wheel speeds can be equated. 
From step 56, the program proceeds to a step 58 where a D-flag (represented 
by the state of a flip-flop or a RAM memory location) is reset to 
condition the program to execute an identification routine (illustrated in 
FIG. 7) which identifies the brake pressure producing the critical wheel 
slip value and therefore the maximum possible braking effort and which 
establishes the identified brake pressure following the sensing of an 
incipient wheel lockup condition. As will be described, the D-flag is set 
following the sensing of an incipient wheel lockup condition in accord 
with the principles of this invention to condition the program to execute 
a dump routine (illustrated in FIG. 8) to release the brake pressure and 
allow the wheel speed to recover. Also at step 58, the maximum allowable 
brake line pressure P.sub.m is set equal to a calibration constant K.sub.p 
such as 1500 psi and a RAM memory location storing the value of the 
maximum calculated tire torque value T.sub.tm is set equal to zero. 
Thereafter, the program exits interrupt routine for the respective wheel. 
The foregoing steps 52 thru 58 are continuously repeated at 5 millisecond 
intervals as long as the vehicle operator does not command brake 
application. However, when a force F is applied to the brake pedal, the 
program proceeds from step 54 to a series of steps that provide an 
estimation of the value of vehicle speed .omega..sub.v as represented by 
the speed of a hypothetical unbraked wheel. It is noted that the initial 
value of .omega..sub.v was set equal to the actual wheel speed .omega. at 
step 56 prior to operation of the brake pedal 16. This series of steps 
begins at step 59 where the rate of change in wheel speed .omega. is 
determined from the old value of wheel speed saved at step 52 and the new 
value stored at step 52. The determined rate of change of wheel speed is 
then compared with a constant deceleration of 1 g at step 60. The 1 g 
deceleration value represents the maximum possible vehicle deceleration. 
When wheel deceleration is less than 1 g, it is assumed that the vehicle 
is decelerating at the same rate as the wheel 11. If, however, the wheel 
deceleration exceeds 1 g, it is assumed that the vehicle deceleration 
remains at the maximum value of 1 g. 
If the wheel deceleration is less than or equal to 1 g, the program 
proceeds from step 60 to a step 62 where .omega. is compared to zero. If 
the comparison indicates wheel deceleration, the program proceeds to step 
64 where the rate of change of vehicle speed .omega..sub.v is set equal to 
the actual measured rate of change of wheel speed. If, however, the 
comparison at step 62 indicates no change in wheel speed or wheel 
acceleration, the program proceeds to a step 66 where the rate of change 
of vehicle speed .omega..sub.v is set equal to zero. 
Returning to step 60, if it is determined that the wheel deceleration is 1 
g or greater, the program proceeds to a step 68 where .omega..sub.v is set 
equal to the maximum possible vehicle deceleration of 1 g. 
From the respective steps 64, 66 or 68, the program proceeds to a step 70 
where vehicle speed .omega..sub.v is estimated. This estimation is based 
on an initial value of vehicle speed .omega..sub.v-1 determined during the 
previous execution of the interrupt routine and the rate of change of 
vehicle speed determined at step 64, 66 or 68 over the five millisecond 
interval .DELTA.t between interrupt periods. 
From step 70, the program proceeds to step 72 where the actual wheel speed 
.omega. measured at step 52 is compared to the vehicle speed .omega..sub.v 
determined at step 70. If the wheel speed is equal to or greater than the 
vehicle speed (which cannot occur during braking of the wheel), the value 
of vehicle speed is corrected at step 74 by setting the vehicle speed 
.omega..sub.v equal to wheel speed .omega. and the initial vehicle speed 
.omega..sub.v-1 to be used at step 70 in the next execution of the 
interrupt routine is set equal to wheel speed .omega.. If at step 72 the 
wheel speed .omega. is determined to be less than the vehicle speed 
.omega..sub.v, the program proceeds to a step 76 where the initial vehicle 
speed .omega..sub.v-1 to be used at step 70 during the next execution of 
the interrupt routine set equal to the value of vehicle speed determined 
at step 70. 
Following step 74 or step 76, the program proceeds to a step 78 where the 
vehicle speed is compared to a calibration constant such as 3 mph. If the 
vehicle speed is less than 3 mph, the program proceeds to a step 80 where 
the commanded brake line pressure P.sub.c is set equal to the value of the 
brake pedal force F times a gain constant G for providing power assisted 
braking. Thereafter, the program proceeds to a step 82 where the valve 18 
of FIG. 2 is deenergized and then to the step 58 previously described. 
If the vehicle speed is greater than 3 mph, the program proceeds from step 
78 to step 84 where the valve 18 is energized to decouple the master 
cylinder 14 from the actuator 12. Brake application is thereafter provided 
solely via the actuator 12 as controlled by the electronic controller 20. 
From step 84, the program proceeds to a step 86 where the state of the 
D-flag is sampled. If the D-flag is reset to condition the program to 
execute the identify routine, the program proceeds to a step 88 where the 
identify routine is executed. 
If step 86 determines that the D-flag is set, the program is conditioned to 
execute a dump routine, and the program proceeds to a step 90 where the 
dump routine is executed. During this routine, the pressure to the brake 
10 is released to allow the speed of the wheel 11 to recover from an 
incipient lockup condition. Following the steps 88 or 90, the program 
exits the interrupt routine for the respective wheel. 
The program executes an interrupt routine as described for each of the 
remaining vehicle wheels following each 5 millisecond interrupt after 
which the program returns to the background loop 48 of FIG. 5. 
Referring to FIG. 7, the identify routine 88 of FIG. 6 is illustrated. This 
routine (A) provides for power assisted braking, (B) identifies the brake 
line pressure producing the critical wheel slip corresponding to the 
maximum possible braking force between the tire and the road surface, (C) 
senses an incipient wheel lockup condition and conditions the program to 
execute the dump routine to allow wheel recovery from the lockup condition 
and (D) reestablishes the brake line pressure to substantially the 
identified pressure producing the critical slip value. 
The identify routine is entered at point 92 and proceeds to a step 94 where 
the value of the tire torque T.sub.t is calculated in accord with the 
equation (4) from the wheel deceleration .omega. determined at step 59, 
the brake line pressure P.sub.b measured at step 52 and the known values 
of wheel inertia I.sub.w and brake gain K.sub.b. From step 94, the program 
proceeds to steps 96 and 98 that function to identify the brake pressure 
producing the maximum value of tire torque and to determine the decrease 
in tire torque from the peak value that represents an incipient wheel 
lockup condition. At step 96, the tire torque T.sub.t calculated at step 
94 is compared with the largest previously calculated value T.sub.tm 
stored in memory. If the value calculated at step 94 is greater than the 
stored value T.sub.tm, the program proceeds to a step 98 where the stored 
value T.sub.tm is set equal to the larger value calculated at step 94, a 
stored value of brake line pressure P.sub.bm representing the brake line 
pressure corresponding in time to the stored maximum calculated value of 
tire torque is set equal to the brake line pressure P.sub.b measured at 
step 52 and a stored value of the decrease in tire torque T.sub.DEL from 
the stored peak value T.sub.tm that represents an incipient wheel lockup 
condition is updated. In this embodiment, T.sub.DEL is a predetermined 
percentage of the peak calculated tire torque value T.sub.tm. Accordingly, 
the value of T.sub.DEL stored at step 98 is set equal to T.sub.tm 
/K.sub.DEL where K.sub.DEL is a calibration constant establishing the 
percentage drop in tire torque as the wheel slip exceeds the critical slip 
value that represents an impending wheel lockup condition. For 
illustration purposes only, K.sub.DEL may be 4.0 establishing a 25% 
decrease in tire torque T.sub.t. 
The foregoing sequence of steps 96 and 98 are repeated with each execution 
of the identify routine as long as the tire torque is increasing. If step 
96 should determine that the calculated value of tire torque T.sub.t is 
less than the stored maximum calculated value T.sub.tm, step 98 is 
bypassed. This will occur when the brake pressure P.sub.b results in a 
wheel slip that exceeds the critical value which in turn results in a 
decrease in the tire torque. The stored value of brake pressure P.sub.bm 
then represents the brake line pressure establishing the critical wheel 
slip value and therefore the maximum braking effort and the stored value 
of T.sub.DEL is the decrease in tire torque representing an incipient 
wheel lockup condition. 
The present invention utilizes the values of P.sub.bm for each of the front 
wheels to detect braking on a split coefficient of friction surface 
wherein the left and right wheels of the vehicle are being braked on 
surfaces having different coefficient of friction or to detect a severe 
steering maneuver while braking. This is based on the fact that the value 
of P.sub.bm for each of the front wheels will be substantially equal to 
one another when both wheels are at their respective critical slip values 
when being braked on surfaces having substantially equal coefficients of 
friction and in the absence of a severe steering maneuver. A difference in 
the stored values of P.sub.bm for each of the front wheels is indicative 
of the braking of the front wheels on a split coefficient of friction 
surface with the magnitude of the difference representing the difference 
between the coefficients of friction of the surfaces on the left and right 
sides of the vehicle. 
The program next determines whether or not an incipient wheel lock 
condition exists. At step 99 the ratio .omega./.omega..sub.v is compared 
with a reference value S.sub.L above which stable braking takes place. In 
one embodiment, S.sub.L may equal 0.92 representing 8% wheel slip. A ratio 
less than S.sub.L indicates a potential for unstable braking. 
Particularly, if the wheel slip exceeds the value represented by S.sub.L 
and the wheel is decelerating, a decrease in the tire torque T.sub.t to a 
value below the stored maximum tire torque value T.sub.tm by an amount 
equal to T.sub.DEL is a result of wheel slip exceeding the critical slip 
value as the wheel decelerates toward a lockup condition. 
If step 99 determines that a potential exists for unstable braking, the 
program proceeds to determine if an incipient wheel lockup condition 
exists based on the decrease in the tire torque from the peak value (if 
the wheel is decelerating) or based on the magnitude of wheel slip. Step 
100 determines if wheel acceleration is negative. If negative the program 
proceeds to step 101 to determine if the tire torque T.sub.t calculated at 
step 94 is less than the peak tire torque T.sub.tm stored at step 98 by 
the value T.sub.DEL or greater. If the tire torque T.sub.t has not 
decreased from the peak value by the value T.sub.DEL, representing stable 
braking based on this parameter or if wheel acceleration is not less than 
0 as determined at step 100, the program proceeds to step 102 where the 
ratio .omega./.omega..sub.v is compared with a reference value S.sub.m 
(such as 0.7) which represents a wheel slip value that exceeds the largest 
possible critical wheel slip value for any road surface condition. A ratio 
less than S.sub.m indicates that braking has become unstable and an 
incipient wheel lockup condition exists. 
If either of the steps 99 and 102 indicates a stable braking condition, the 
program proceeds to a step 104 where the value of the operator requested 
brake pressure that is equal to the applied pedal force F times the power 
assist gain factor G is compared with a maximum allowable brake line 
pressure P.sub.m. If the product is less than the maximum value, the 
program proceeds to a step 106 where the commanded brake pressure value 
P.sub.c is adjusted toward the operator requested pressure in accord with 
a first order lag filter equation to provide power assisted braking. 
Thereafter, the program exits the identify routine and returns to the 
background loop 48. 
If at step 104 it is determined that the operator requested brake pressure 
is greater than the maximum allowable pressure P.sub.m, the program 
proceeds to a pressure ramp routine where, through repeated executions of 
the identify routine, the maximum allowable brake pressure P.sub.m and the 
commanded brake line pressure P.sub.c are ramped up at rates dependent 
upon the tire-road interface condition until step 104 detects that the 
maximum allowable brake pressure P.sub.m has become greater than the 
operator requested pressure or, if the operator requested brake pressure 
results in an unstable braking condition, until the commanded brake 
pressure results in an incipient wheel lockup condition at which time the 
brake pressure establishing the critical slip value has been identified by 
the steps 96 and 98 as well as the value of T.sub.DEL to be used at step 
101 in determining whether or not an incipient wheel lockup condition 
exists. The brake pressure identified is used to reestablish the commanded 
brake pressure after the wheel recovers from the incipient lockup 
condition. The result of the ramping of the brake pressure is a periodic 
low frequency reidentification of the brake pressure producing the 
critical wheel slip value. 
The routine for ramping the brake pressure begins at a step 108 where the 
value of a time t.sub.1 in a RAM timing register is compared to zero. The 
initial value of time t.sub.1 establishes a delay in the ramping of the 
commanded brake pressure P.sub.c. Thereafter, the time t.sub.1 functions 
in establishing the ramp rate. If the time t.sub.1 is greater than zero, 
the program proceeds to a step 110 where the time t.sub.1 is decremented. 
Thereafter, at step 112, the program proceeds to adjust the commanded 
brake pressure P.sub.c toward a predetermined fraction FRAC of the maximum 
allowable brake pressure P.sub.m in accord with the filter equation 
EQU P.sub.c =(Z.sub.p .multidot.P.sub.co)+(Z.sub.z .multidot.P.sub.m 
.multidot.FRAC) (5) 
where Z.sub.p and Z.sub.z are values established as will be described based 
on the value of the stored peak brake pressure P.sub.bm so that P.sub.c is 
ramped at a rate dependent upon the road-tire friction coefficient and 
P.sub.co is the prior value of P.sub.c. The time constant of this 
expression is generally small so that the brake pressure P.sub.b is 
quickly ramped toward the maximum allowable pressure P.sub.m. By setting 
the maximum allowable brake pressure P.sub.m to the stored pressure 
P.sub.bm after an incipient wheel lockup condition is sensed (as will be 
described), the commanded pressure established upon repeated executions of 
step 112 will be the predetermined fraction FRAC of the pressure producing 
the critical wheel slip. In one embodiment, FRAC is 0.9 so that the 
resultant brake pressure produces substantially the critical wheel slip 
value. 
As long as an incipient wheel lock condition is not detected and the 
operator requested brake pressure is greater than the maximum allowable 
brake line pressure P.sub.m, the steps 108 thru 112 are repeated at the 
five millisecond interrupt interval until t.sub.1 has been decremented to 
zero. After t.sub.1 has been decremented to zero, the program proceeds 
from step 108 to step 114 where the time t.sub.2 in a RAM timing register 
is compared to zero. If the time t.sub.2 is greater than zero, the program 
proceeds to a step 116 where the time t.sub.2 is decremented. 
Following step 116 or step 114, the program proceeds to a step 118 where 
the maximum allowable brake pressure is incremented and the time t.sub.1 
is set equal to K.sub.n (t.sub.2 +1). Thereafter, the steps 114 thru 118 
will be bypassed upon repeated executions of the identify routine until 
t.sub.1 is again decremented to zero. From this it can be seen that the 
maximum allowable brake pressure P.sub.m is periodically incremented at 
intervals determined by K.sub.n and t.sub.2. When t.sub.2 is decremented 
to zero, the maximum allowable brake line pressure P.sub.m is incremented 
with each K.sub.n executions of the identify routine. The initial value of 
t.sub.2 is based on the stored peak brake pressure P.sub.bm as will be 
described so that P.sub.m and therefore P.sub.c is ramped at a rate 
dependent upon the tire-road friction coefficient. 
Following step 118, the program proceeds to step 112 where the commanded 
brake line pressure P.sub.c is again set as previously described. Repeated 
executions of the foregoing steps function to increase the commanded brake 
pressure P.sub.c exponentially. This increase will be continued until (A) 
an incipient wheel lock condition is forced so as to force a 
reidentification of the brake pressure producing the critical slip value 
via the steps 96 and 98 or (B) the operator requested brake pressure 
becomes less than the maximum allowable pressure P.sub.m. 
If the commanded brake pressure P.sub.c is increased to a point resulting 
in the wheel slip value becoming greater than the critical slip value, the 
wheels then quickly approach a lockup condition. This incipient wheel lock 
condition is detected at step 101 or step 102. When the incipient wheel 
lockup condition is detected, the brake line pressure P.sub.bm in memory 
at that time is the brake line pressure producing the critical wheel slip 
value and therefore the maximum possible tire torque. 
After a wheel lockup condition has been sensed, the program proceeds to a 
step 120 where the time t.sub.2 is compared with a time t.sub.k1. As will 
be seen, these two values will be equal only if a wheel lockup condition 
is sensed within a predetermined constant time period t.sub.k2 (such as 
500 ms) after the brake pressure is reestablished after recovery from an 
incipient wheel lockup condition. A wheel lockup occurring within this 
period after reapplication of the brake pressure implies the application 
of an unstable brake pressure producing an incipient wheel lockup 
condition. If this condition exists, the program proceeds to a step 122 
where the brake pressure P.sub.bm, stored at step 98 and identified as the 
pressure establishing the critical wheel slip value, is compared with the 
commanded brake pressure P.sub.c which resulted in the incipient wheel 
lockup condition. If greater, the program proceeds to a step 124 where the 
stored value of P.sub.bm is corrected to the commanded pressure P.sub.c. 
This condition represents an error in the calculation of the tire torque 
either through changes in the brake line coefficients or errors in various 
constants used in the determination of the calculation of the tire torque 
T.sub.t. Since the brake line pressure producing the critical slip value 
can never be greater than the commanded brake line pressure P.sub.c that 
resulted in an incipient wheel lock condition, the value of P.sub.bm is 
reduced to the value of P.sub.c causing the incipient wheel lock 
condition. 
From step 120 if the time t.sub.2 is not equal to t.sub.k1, from step 122 
if P.sub.bm is less than P.sub.c, or from step 124, the program proceeds 
to a step 125 where the value of t.sub.k1 is set equal to k.sub.t1 
(1-P.sub.bm /K.sub.p) where k.sub.t1 is a calibration constant and k.sub.p 
is the limit of the brake pressure as described with respect to step 58. 
From the above expression, it can be seen that t.sub.k1 varies inversely 
with the brake pressure P.sub.bm producing the maximum braking effort. As 
will be seen, this results in a rate of increase in the brake pressure via 
the steps 114, 116 and 118 that varies directly with the peak calculated 
tire torque T.sub.tm stored at step 98. 
At step 126, the values of Z.sub.p and Z.sub.z to be used in the filter 
equation (5) in step 112 are established. Z.sub.p is set equal to the 
expression (K.sub.z -P.sub.bm /K.sub.p)/K.sub.z where K.sub.z is a 
calibration constant. In one embodiment, K.sub.z was selected to be 5.0 
resulting in Z.sub.p being equal to approximately 0.8 when braking on a 
surface having a high coefficient. Z.sub.z is set equal to 1-Z.sub.p. As 
can be seen, the values of Z.sub.p and Z.sub.z are dependent upon the 
identified brake pressure P.sub.bm producing the peak tire torque T.sub.tm 
such that the filter equation (5) has a time constant that decreases with 
increasing values of P.sub.bm. This results in a more rapid application of 
brake pressure for road surfaces having a higher coefficient of friction. 
At step 127, the D-flag is set to condition the program to execute the dump 
routine and certain initial conditions for reapplication of brake pressure 
are established. The initial conditions include setting the maximum 
allowable brake pressure P.sub.m equal to the stored value of brake 
pressure P.sub.bm (the brake pressure identified as producing the critical 
wheel slip value), setting the time t.sub.1 equal to the constant t.sub.k2 
and setting the time t.sub.2 equal to the value t.sub.k1 previously 
described which makes the initial value of t.sub.2 dependent upon P.sub.bm 
to control the rate of increase of P.sub.m as a function of the road 
surface condition as previously described. 
The program next proceeds to a step 128 where the dump routine is executed. 
Thereafter, during executions of the 5 ms interrupt routine of FIG. 6, the 
identify routine is bypassed via the step 86 and the dump routine 90 is 
executed until the D-flag is again reset. 
The dump routine executed at step 128 of the identify routine of FIG. 7 and 
at step 90 of the interrupt routine of FIG. 6 is illustrated in FIG. 8. 
This routine is entered at point 130 and proceeds to step 131 where wheel 
slip represented by the ratio of wheel speed .omega. to the speed 
.omega..sub.v of the hypothetical unbraked wheel is compared to a constant 
S.sub.k representing wheel speed approaching vehicle speed. S.sub.k may 
be, for example, 0.92 representing a wheel slip of 8 percent. If the ratio 
is less than S.sub.k, the program proceeds to a step 132 where wheel 
acceleration .omega. is compared with a low value .omega..sub.L such as a 
value representing 1 g. If the wheel speed has not yet begun to accelerate 
at this level in its recovery from the incipient lockup condition, the 
program proceeds to a step 134 where the commanded brake pressure P.sub.c 
is set to zero to allow the wheel speed to recover from the incipient 
wheel lockup and toward vehicle speed. From step 134, the program compares 
at step 136 the time t.sub.R that the brake pressure has been dumped with 
a maximum allowable time K.sub.R beyond which the brake pressure is to be 
reapplied even if recovery from the lockup condition has not been 
detected. If the time period K.sub.R has not been exceeded, the time 
t.sub.R is incremented at step 138 and the program returns to the 
background loop 48 of FIG. 5. 
Returning to step 132, if the wheel acceleration .omega. has exceeded 
.omega..sub.L, the commanded brake pressure P.sub.c is set at step 140 
equal to the then existing brake pressure P.sub.b to effect a hold of the 
brake pressure until wheel speed recovery is detected. 
At step 142, the present wheel acceleration .omega..sub.t is compared to 
the previous wheel acceleration .omega..sub.t-1. If wheel acceleration is 
increasing indicating that the wheel slip is still decreasing toward the 
critical slip value, the program proceeds to the step 136 previously 
described. 
If step 131 detects wheel speed recovery based on wheel slip decreasing to 
a value below that represented by S.sub.k or if step 142 detects that the 
wheel slip is less than the critical slip value represented by a decrease 
in wheel acceleration or if step 136 detects a brake pressure dump 
duration exceeding K.sub.R, the program proceeds to a step 144 where the 
D-flag is reset to condition the program to execute the identify routine 
of FIG. 7. Also at this step, the maximum value of calculated tire torque 
T.sub.tm is set to zero so that the identify routine is conditioned to 
reidentify the brake pressure establishing the critical wheel slip value, 
the hypothetical unbraked wheel speed .omega..sub.v is set equal to the 
last measured wheel speed .omega. and the time t.sub.R is reset. The 
program then exits the dump routine of FIG. 8 and returns to the 
background loop 48. 
During the following executions of the interrupt routine of FIG. 6 at the 5 
millisecond interrupt intervals, the program executes the identify routine 
at step 88 until the D-flag is again set at step 127 after an incipient 
wheel lockup condition is sensed. 
Control of the rear wheel brakes of the vehicle will now be described. The 
rear wheels are each controlled by identical program routines. The routine 
for controlling the hydraulic brake pressure to the brake of a rear wheel 
is the same as the above described routine for a front wheel brake except 
for the modification to the identify routine of FIG. 7 as illustrated in 
FIG. 9. The added program steps of FIG. 9 to the identify routine provide 
for the control of the operating mode of the rear wheel brakes in accord 
with the principles of this invention. Particularly, the routine senses 
when the vehicle wheels are being braked on a substantially uniform 
coefficient of friction surface and controls the braking of the rear wheel 
in an independent mode so as to minimize the vehicle stopping distance and 
senses when the coefficient of friction between the two sides of the 
vehicle are substantially different or when the vehicle is undergoing 
severe steering maneuvers while braking and controls the braking of the 
rear wheels in a select low mode where both wheels are jointly controlled 
based on the rear wheel being braked on the lowest coefficient of friction 
surface. 
For purposes of differentiating the parameters associated with the four 
vehicle wheels, the designations lf, rf, lr and rr identifying the left 
front, right front, left rear and right rear wheels, respectively, will be 
added to the subscripts of the parameters previously referred to with 
respect to FIGS. 6-8. 
Referring to FIG. 9, after the step 112 or the step 128 previously 
described, the program for controlling a rear wheel brake proceeds to a 
step 146 where the value of brake line pressure P.sub.bmlf stored at step 
98 in the routine controlling the left front wheel brake is compared with 
the value of P.sub.bmrf stored at step 98 in the routine controlling the 
right front wheel brake. These values will be substantially equal if the 
two front wheels are being braked on substantially equal coefficient of 
friction surfaces in the absence of a severe steering maneuver but will be 
unequal by an amount related to the difference in the coefficient of 
friction between the left and right sides of the vehicle or by an amount 
related to the severity of a steering maneuver while braking. Other 
parameters, such as T.sub.tmlf and T.sub.tmrf may also be utilized to 
indicate braking on a split coefficient of friction surface or a severe 
steering maneuver while braking. 
The values of P.sub.bmlf and P.sub.bmrf will be substantially equal when 
the vehicle is being braked on a split coefficient of friction surface or 
while undergoing a severe steering maneuver until the slip of the front 
wheel on the lowest coefficient of friction surface or on the inside of a 
turn during a severe steering maneuver exceeds the critical slip value at 
which time its determined value of tire torque T.sub.tm and the 
corresponding stored value of P.sub.bm is at its peak. The stored value of 
P.sub.bm of the front wheel on the highest coefficient of friction surface 
or on the outside of a turn during a severe steering maneuver will 
continue to increase as the determined value of tire torque increases. 
Therefore, a rear wheel having a lower value of P.sub.bm (or T.sub.tm) is 
indicative of the wheel being braked on a road surface having a lower 
coefficient of friction surface or having a lower maximum possible tire 
torque due to a turning maneuver. The magnitude of the difference is 
indicative of the difference in the coefficients of friction or the degree 
or a severe steering maneuver while braking. 
If P.sub.bmlf is equal or greater than P.sub.bmrf, the program proceeds to 
a step 148 where P.sub.bmrf is subtracted from P.sub.bmlf to obtain the 
difference .DELTA.P. If P.sub.bmrf is less that P.sub.bmlf the program 
proceeds from step 146 to a step 150 where Pbmlf is subtracted from 
P.sub.bmrf to obtain the diferrence .DELTA.P. As previously indicated, the 
value of .DELTA.P is an indicator of the difference in the coefficient of 
friction between the left and right sides of the vehicle or an indicator 
of a severe steering maneuver. 
From step 148 or 150, the program proceeds to a step 152 where .DELTA.P is 
compared with a calibration constant K.sub.D. A value of .DELTA.P equal to 
or greater than K.sub.D represents a large difference in the coefficients 
of friction between the left and right sides of the vehicle or represents 
a severe steering maneuver of the vehicle while braking, both of which 
could result in reduced vehicle stability. If this condition exists, the 
program proceeds to a step 154 where the value of brake line pressure 
P.sub.bmlr stored at step 98 in the routine controlling the left rear 
wheel is compared with the value of P.sub.bmrr stored at step 98 in the 
routine controlling the right rear wheel to determine which rear wheel is 
being braked on the highest coefficient of friction surface. 
In general, P.sub.bmlr and P.sub.bmrr will be substantially equal until the 
slip of the rear wheel on the lowest coefficient of friction surface 
exceeds the critical slip value at which time the corresponding stored 
value of P.sub.bm is at its peak. The stored value of P.sub.bm of the rear 
wheel on the highest coefficient of friction surface will continue to 
increase as the determined value of tire torque increases. Therefore, a 
rear wheel having a lower value of P.sub.bm is indicative of the wheel 
being braked on a road surface having a lower coefficient of friction 
surface. 
If P.sub.bmlr is greater than P.sub.bmrr indicating that the left rear 
wheel is being braked on the higher coefficient of friction surface, the 
program proceeds to a step 156 where the commanded brake pressure 
P.sub.clr to the left rear brake is set equal to the value of the 
commanded brake pressure P.sub.crr to the right rear brake determined at 
step 112, 134 or 140 of the routine controlling the right rear brake. 
If at step 154 P.sub.bmrr is determined to be greater than P.sub.bmlr 
indicating that the right rear wheel is being braked on the higher 
coefficient of friction surface, the program proceeds to a step 158 where 
the commanded brake pressure P.sub.crr to the right rear brake is set 
equal to the value of the commanded brake pressure P.sub.clr to the left 
rear brake determined at step 112, 134 or 140 of the routine controlling 
the left rear brake. From steps 156 or 158, the program exits the identify 
routine. 
Through the steps 152, 154, 156 and 158, the pressure applied to the rear 
wheels during wheel lock controlled braking are controlled in the select 
low mode where the braking pressure applied to the rear wheel being braked 
on the highest coefficient of friction surface is controlled to the 
pressure established for the rear wheel being braked on the lowest 
coefficient of friction surface. However, if at step 152 it is determined 
that the coefficients of friction between the sides of the vehicle are not 
greatly different, the program exits the identify routine and the rear 
wheels are braked in an independent mode in the same manner as the front 
wheels so as to minimize the vehicle braking distance. 
The foregoing description of a preferred embodiment for the purpose of 
explaining the principles of this invention is not to be considered as 
limiting or restricting the invention since many modifications may be made 
by the exercise of skill in the art without departing from the scope of 
the invention.