Brake system control

A brake system control apparatus comprising: a fluid brake system including a master cylinder containing a hydraulic fluid having a hydraulic pressure, a hydraulic pump having an output hydraulically coupled to the master cylinder, a motor coupled to and providing motive force to the hydraulic pump, a current sensing device coupled to the motor and providing an output signal indicative of electric current through the motor; and an electronic controller coupled to the current sensing device comprising a first sampler taking a first sample of the output signal a first time period after a start up of the motor, a second sampler taking a second sample of the output current a second time period after start up of the motor and a divider for determining a ratio of the first and second samples wherein the ratio is indicative of a hydraulic pressure in the master cylinder at the start up of the motor.

This invention relates to a brake system control. 
BACKGROUND OF THE INVENTION 
Many vehicles produced by automotive manufacturers include anti-lock 
braking control systems (ABS) typically designed to detect and prevent the 
occurrence of a wheel lock-up condition during commanded braking of the 
vehicle. The wheel lock-up condition is prevented by cycling or modulation 
of the hydraulic pressure to the wheel brake(s) in a known manner by, for 
example, providing a series of releases and applies of the hydraulic brake 
pressure. 
Many ABS systems use solenoid valves that are controllably opened and 
closed to increase (apply) and decrease (release) the hydraulic brake 
pressure to the vehicle brakes. In one known method, an apply solenoid is 
used to control application of brake pressure from a high pressure 
hydraulic source, such as a fluid pump, to the wheel brakes and a release 
solenoid is used to control the release of brake pressure from the fluid 
brakes to a low pressure brake fluid reservoir or accumulator. 
In some systems, during the ABS event, the substantially steady state 
running current or speed of the hydraulic pump motor is monitored to 
determine master cylinder pressure, which is used as a control input for 
the ABS system in a known manner. One challenge to this approach is that 
different pumps and pump motors, even of the same design, have variations 
from part to part that affect the relationship between pump running 
current or speed and pressure output from the pump. For example, tolerance 
variations in wire used to wind the motor coils can affect the impedance 
of a motor and therefore also affect the relationship between motor 
current and output load. Similarly, variations in mechanical parts can 
affect the internal friction of the motor and of the pump and can also 
affect the relationship between motor current and output load (i.e., 
hydraulic pressure at the pump outlet). 
SUMMARY OF THE PRESENT INVENTION 
It is an object of this invention to provide a brake system control 
according to claim 1. 
Advantageously, this invention provides a brake system control that 
determines master cylinder pressure in the vehicle brake system 
immediately upon activation of the vehicle ABS system. Advantageously, 
this invention determines master cylinder pressure without requiring a 
master cylinder pressure sensor and without requiring the ABS pump motor 
to reach its substantially steady state running condition. 
Advantageously, this invention recognizes that the start up profile of the 
electric current through the ABS pump motor, which is activated upon first 
detection of an incipient wheel lock up condition, varies based on the 
master cylinder pressure to which the pump output is exposed. 
Advantageously, this invention recognizes that by measuring electric 
current through the pump motor at two different points in time during the 
start-up of the pump motor and before the pump motor has reached a 
substantially steady state operation, a ratio of the two measurements may 
be used as an estimation of master cylinder pressure to which the pump is 
exposed. This approach has been found to both provide an extremely fast 
means for estimating master cylinder pressure and to eliminate errors in 
pump output pressure measurement caused by part-to-part variability, 
including variations in electrical impedance and variations in internal 
mechanical losses of the pump and motor. 
More particularly then, according to a preferred example, this invention 
provides a brake system control apparatus comprising: a fluid brake system 
including a master cylinder and containing a hydraulic fluid having a 
hydraulic pressure, a hydraulic pump having an output hydraulically 
coupled to the master cylinder, a motor coupled to and providing motive 
force to the hydraulic pump and a current sensing device coupled to the 
motor and providing an output signal indicative of an electric current 
through the motor; and an electronic controller coupled to the current 
sensing device, including a first sampler receiving a first sample of the 
output signal a first time period after a start up of the motor, a second 
sampler receiving a second sample of the output signal a second time 
period after the start up of the motor and a divider for determining a 
ratio of the first and second samples wherein the ratio is indicative of 
the hydraulic pressure in the master cylinder within 0.15 seconds of the 
start up of the motor.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to FIG. 1, the graph illustrates four motor current profiles 
corresponding to the current (filtered) through an ABS pump motor 
immediately following activation of the motor. Traces 300, 302, 304 and 
305 illustrate current profiles when the master cylinder pressure to which 
the pump output is coupled ranges of 3,000 psi (trace 300) to 250 psi 
(trace 305). As the current profiles illustrate, within about 10 
milliseconds of pump activation, the currents represented by traces 300, 
302 and 304 peak and after about 90 milliseconds the currents begin 
separating into three separate bands. Determining the pressure at the 
output of the pump and therefore the master cylinder pressure may be done 
on a single pump by monitoring the pump current during the steady state 
running of the pump, i.e., at least 300 milliseconds after the pump has 
been activated. However, accuracy will be compromised in different systems 
because of part-to-part variability in pump electrical and physical 
tolerances, which will affect the pump current. 
Referring now also to FIG. 2, a correlation between pump current ratios and 
pump output load (and therefore master cylinder pressure) is shown. The 
traces 306, 308 and 310 illustrate the ratios of the peak pump currents to 
currents detected after the peak currents for a pump with output pressures 
of 3,000 psi, 1,500 psi and 250 psi, respectively. As FIG. 2 illustrates, 
the by time t.sub.X, within 90 milliseconds of pump start up, three 
distinct pressure bands are observable. By the time t.sub.Y, 250 
milliseconds after pump start-up, four distinct pressure bands are 
observable. Because the graph in FIG. 2 plots ratios, part to part 
variability factors that affect pump current are canceled out and the 
bands delineated by FIG. 2 hold true from part to part. 
Referring now to FIG. 3, the vehicle brake system shown takes advantage of 
the relationships described above with reference to FIGS. 1 and 2. The 
vehicle hydraulic brake system shown schematically includes brake pedal 
10, master cylinder 12, solenoid actuator valves represented by block 22, 
the vehicle brakes represented by block 24, an accumulator represented by 
block 26 and hydraulic pump 29, which receives its motive force from 
electric motor 30. 
During normal braking operation, when the vehicle operator depresses brake 
pedal 10, the master cylinder 12 responds by increasing the hydraulic 
pressure in hydraulic lines 14, which are coupled to hydraulic lines 20 
connected to the actuator valves 22. The actuator valves 22 are normally 
open and during normal non-ABS braking, hydraulic pressure from the master 
cylinder is coupled directly through the valves to the vehicle brakes 24. 
The electronic controller 56 monitors a variety of vehicle data through 
input BUS 42, including wheel speed information from a plurality of wheel 
speed sensors of a known type (not shown) that provide output signals 
indicative of the rotational velocity of the individual wheels. When the 
controller 56, in response to the information on BUS 42, determines an 
incipient wheel lock up condition of one or more of the vehicle wheels, 
the controller provides an output command through line 58 to transistor 
34, turning on transistor 34, which then provides current to electric 
motor 30. The electric motor 30 then immediately begins operating 
hydraulic pump 29. 
In response to the control commands on lines 60, the actuator valves 22 
selectively release hydraulic pressure from the brakes 24 corresponding to 
the wheels for which an incipient lock-up condition is detected into the 
accumulator 26, which stores the hydraulic fluid until it is pumped 
through pump 29 via the pump's input hydraulic line 28. In a known manner, 
the actuator valves 22 selectively apply hydraulic pressure, pumped from 
accumulator 26, through pump 29, past check valve 18, into the hydraulic 
circuit leading to hydraulic line 20 at the input to the actuator valves 
22, to the brakes 24. By responding to the data on BUS 42 to selectively 
provide the control signals on lines 60, controller 56 controls the 
actuator valves 22 to achieve the controlled release and apply of 
hydraulic fluid to the vehicle brakes 24 as desired during anti-lock brake 
operation. Except for the modifications disclosed herein, anti-lock brake 
control of the type used with the apparatus shown in FIG. 1 is well known 
to those skilled in the art and the details of the well known portions of 
the control need not be repeated herein. 
Lines 32 and 36 connect the high and low voltage sides of transistor 34 to 
the inputs of differential amplifier 37, which provides an output signal 
indicative of the voltage across transistor 34. The voltage across 
transistor 34 varies with the current used by motor 30 and transistor 34 
is thus used as a current sensing device in the circuit of the electric 
motor 30. Analog-to-digital converter 38 converts the signal output from 
amplifier 37 into a digital format and provides that signal to the input 
circuit 40 for use within the electronic controller 56. 
In addition to the input circuitry 40, the electronic controller 56 
includes a microprocessor 46, ROM 48, RAM 50, output interface circuitry 
44 and command and data buses 52 and 54. All of the components of the 
controller 56 are of a type well known to those skilled in the art. In 
general, microprocessor 46 performs a series of commands stored in 
permanent memory 48 to receive the input data through input interface 
circuitry 40 and perform the control steps described herein along with 
known anti-lock brake control steps and provide the control commands for 
actuator valves 22 to the output interface circuitry 44. 
Immediately after detection of an incipient wheel lock-up condition of one 
or more of the vehicle wheels, the controller 56 outputs a signal through 
line 58 to the transistor 34, turning on the transistor 34, in turn 
turning on the motor 30 and starting pump 29. After start-up of the motor, 
the microprocessor 46 continually samples and low pass filters the signal 
output from analog to digital converter 38. After expiration of a first 
time period t.sub.1 (i.e., t.sub.1 =0.010 seconds) after start-up of the 
motor 30 , the microprocessor 46 stores a first sample of the low pass 
filtered signal as a representation of current through motor 30 at time 
t.sub.1. In general, the period t.sub.1 is set to capture an approximate 
peak of the current through motor 30. After expiration of a second time 
period t.sub.2 (i.e., t.sub.2 =0.090 seconds) after start-up of the motor 
30, the microprocessor 46 stores a second sample of the low pass filtered 
signal as a representation of current through motor 30 at time t.sub.2. 
Microprocessor 46 processes those two sampled signals, determines an 
indication of the hydraulic pressure within the master cylinder 12 based 
on those two sampled signals and uses that information in determining 
commands for actuator valves 22 to perform the anti-lock brake control of 
the wheel brakes 24. According to the example described herein, the 
initial pressure of master cylinder 12 at the activation of the anti-lock 
braking control can be determined in one of three pressure bands within 
about 100 milliseconds of the activation of the pump motor 30. If desired, 
the voltage across transistor 34 can be continuously monitored and 
thereafter used to obtain increased resolution of master cylinder pressure 
as the pump motor continues to run. 
Referring now to FIG. 4, the schematic shown illustrates the initial master 
cylinder pressure estimation according to this invention. The data input, 
for example, as provided by the analog-to-digital converter 38 (FIG. 3), 
is low pass filtered by filter 80 and then provided to a saturation 
function block 82, which limits the filter output to predetermined maximum 
and minimum values bounding the expected operating range of the data 
received by the controller 56 and filtered by filter 80. The predetermined 
maximum and minimum values can be easily determined by one skilled in the 
art by monitoring the current output of the motor in a test vehicle during 
example braking conditions and setting the maximum and minimum values at 
the maximum and minimum current values observed in a normally operating 
system. 
Block 84 represents a generic function block that may be implemented to 
scale the output of block 82 or provide a nonlinear function thereto, if 
desired, to transform the information from block 82 to a scale or other 
form of a type more desirable to the system designer. The output of block 
84 is provided to sample and hold block 98 and to block 104 which are 
enabled by timers 92 and 94, respectively. Timers 92 and 94 in turn are 
initiated by the signal on line 96 provided by the ABS control algorithm 
86 when an incipient wheel lock-up condition triggering an ABS control 
event is first detected. 
With the initiation signal on line 96, the timers 92 and 94 begin running. 
Timer 92 times out first (for example after 0.010 seconds) and provides a 
signal to sample and hold block 98, commanding the sample and hold 
function to sample the output of block 84 and hold that sampled signal as 
an input to block 104. In this manner, block 98 traps the approximate peak 
motor current. A short time later, timer 94 times out (for example, after 
0.090 seconds), enabling block 104 to continuously sample the output of 
block 84 and provide at its output a ratio of the data captured by sample 
and hold block 98 and output by block 84. 
The output of block 104 is provided to block 106, which is a generic, 
nonlinear function block that may be implemented to convert the ratio 
output from block 104 to a more desirable scale or format for use as an 
indication of pressure in the master cylinder at the initiation of the ABS 
routine. The signal output from block 106 is input to table 108, which 
provides an output signal indicative of one of three or more predetermined 
pressure bands in which the master cylinder pressure may fall. The use of 
pressure bands is preferred where there are resolution limitations of the 
pressure signal provided by block 106. The pressure band selected by table 
108 is the initial master cylinder pressure estimation that is then 
provided to the ABS control algorithm 86, which utilizes the initial 
master cylinder pressure information to control the ABS operation through 
the solenoid actuator valves 22 (FIG. 3). 
As the motor continues to run, the pressure estimation may be continuously 
updated and table 108 may vary as a function of time, for example, 
dividing into four pressure bands after 250 milliseconds of pump 
operation, to increase resolution. 
It is noted that the master cylinder pressure determined as described 
herein may be used in any manner desired by a system designer to 
facilitate or enhance the control provided by the ABS control algorithm 
86, many examples of which are known to those skilled in the art, and no 
specific implementation of the ABS control algorithm 86 is required by 
this invention. 
Referring now to FIG. 5, an example master flow control diagram for 
controller 56 (FIG. 3) is shown. The controller continuously monitors the 
various sensor inputs at block 120 and determines in a known manner at 
block 122 whether or not an incipient wheel lock-up condition warranting 
initiation of ABS control exists. If an ABS control is initiated, block 
124 activates the motor for the hydraulic pump and block 126 performs the 
master cylinder pressure estimation according to the examples set forth 
herein. Block 128 then performs the ABS control functions utilizing the 
master cylinder pressure information estimated at block 126. 
Referring now to FIG. 6, an example of the steps performed at block 126 is 
shown. At block 139, the routine determines if the control loop is in its 
first pass since activation of ABS. If so, the routine continues to block 
140, otherwise the routine continues to block 142. At block 140, two 
timers, TIMER1 and TIMER2, are initialized and then the routine moves to 
block 142 where it reads the voltage level across the pump motor drive 
transistor 34 (FIG. 3), as provided at the output of analog-to-digital 
converter 38 (FIG. 3). The voltage input at block 142 is low pass filtered 
at block 144 through a known digital single pole low pass filter with a 
cut-off frequency at about 1 kHz and provides an output signal VOLTAGEF. 
Then block 146 limits VOLTAGEF so that it is no greater than the values 
MAX and MIN, where MAX and MIN are predetermined values that bound the 
expected range of the signal VOLTAGEF. 
The output of block 146 is provided to block 148 where a temporary value, 
TEMP, is determined as: 
EQU TEMP=SCALE*VOLTAGEF, 
where SCALE is a scale value that converts the signal VOLTAGEF to a range 
more desirable for use by the system. Then block 150 compares the value 
TIMER1 to its time-out value (i.e., 0.010 seconds) and if TIMER1 is not 
equal to its time-out value, then the routine continues to block 154. If 
TIMER1 is equal to its time-out value, the routine continues to block 152 
where it sets the value NUM equal to TEMP. The routine then moves to block 
153 where TIMER1 and TIMER2 are both incremented and then returns to block 
142. 
At block 154, TIMER2 is compared to its time-out value (i.e., 0.090 
seconds). If TIMER2 is not greater than or equal to its time-out value, 
the routine continues to block 153. If TIMER2 is greater than or equal to 
its time-out value, the routine continues to block 156 where the value DEN 
is set equal to TEMP. From block 156 the routine continues to block 158 
where it determines: 
EQU RATIO=NUM/DEN. 
From block 158, the routine continues to block 160 where it determines the 
pressure band from a look-up table as a function of RATIO. In one example, 
the entire master cylinder operating pressure range is divided into at 
least three pressure bands and the pressure band determined at block 160 
designates one of the at least three pressure bands within approximately 
0.100 seconds of motor start-up. 
It may be desirable to add a temperature compensation to the system shown 
in FIG. 2 if it is found that the pressure estimation described herein is 
affected by temperature. A variety of means for implementing temperature 
compensation are available to those skilled in the art. For example, a 
thermistor can be placed in series with transistor 34 to oppose the 
changes in the impedance of transistor 34 as the temperature of transistor 
34 changes.