Note: Descriptions are shown in the official language in which they were submitted.
CA 02297741 2000-01-25
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METHOD AND ApPARATUS
FOR DETERMINING VEHICLE BRAKE EFFECTIVENESS
FIELD OF THE INVENTION
This invention relates to a method and
apparatus for determining vehicle brake
effectiveness and changes in vehicle mass.
BACKGROUND OF THE INVENTION
In order for a driver to operate a vehicle
safely he must know and have confidence in the
stopping distance, and thereby the deceleration
rate, of the vehicle in all environments. Drivers
learn to judge this distance based on vehicle speed,
road slope, road surface and load conditions.
Two conditions that can alter the driver's
perception of a safe stopping distance are a non-
functioning or partially functioning brake system or
an unknown overload condition. As brakes do not
wear linearly and can be affected by temperature,
brake lining condition, moisture, adjustment, and
mechanical problems, stopping distance can change
over a short period. In addition, a vehicle may
take on an unexpectedly heavy load, thus affecting
stopping distance or vehicle deceleration rate.
Various systems for checking the effectiveness
of vehicle braking systems are known. Common
methods include visual inspection, mechanical
devices (roller testers, plate testers), brake
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pressure gauges, and weigh scales to assure proper
vehicle mass.
U.S. Patent 5,299,452 (Caron et al.) determines
vehicle braking effectiveness by first measuring
engine torque to automatically calculate vehicle
mass and then employing a complex averaging method
to attempt to mitigate the effects of road slope,
head winds, tail winds and the like. The accuracy
of vehicle mass calculated by this method, using
engine torque, is dependant on constant engine
horsepower, transfer of horsepower to the wheels,
fuel consumption, altitude, road surface and a level
acceleration area. Changing vehicle conditions and
variations in normal driving conditions such as
uneven terrain, can significantly distort the
calculation of vehicle mass using this method. In
addition, estimating brake effectiveness based on
level road conditions does not account for
travelling up or down a slope. A brake
effectiveness test that does not take into
consideration road slope does not offer the real
time feedback necessary to provide a true safety
application.
It would therefore be desirable to provide a
method and apparatus for providing feedback on
vehicle brake effectiveness for vehicles that do not
decelerate in the expected rate over a large
spectrum of environments.
It would further be desirable to provide a
method and apparatus of assessing vehicle brake
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effectiveness which takes into consideration
travelling up or down a slope, differences in drag
forces such as air friction and engine friction and
engine brake engagement.
It would also be desirable to provide a method
and apparatus of assessing vehicle brake
effectiveness which includes a means of manually or
automatically inputting the actual mass of the
vehicle for use in calculating vehicle brake
effectiveness.
It would further be desirable to provide a
method and apparatus of assessing vehicle brake
effectiveness based on a comparison with known
vehicle deceleration rates for a known vehicle mass
at 100% braking effectiveness.
It would also be desirable to provide a method
and apparatus of assessing vehicle brake
effectiveness based on a comparison with the minimum
acceptable deceleration rate for the vehicle.
It would also be desirable to provide a method
and apparatus of assessing vehicle brake
effectiveness based on driver expected deceleration
rates or on vehicle braking effectiveness history.
Data collected on historical braking effectiveness
can provide trends which may be critical in
assessing real time vehicle safety and monitoring
vehicle maintenance requirements.
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A further desirable object is to provide an
apparatus and method for alerting an operator to a
deterioration in braking effectiveness or to a mass
overload situation so that corrective action could
be taken before the problem becomes critical.
It is also desirable that a method and
apparatus be provided which will permit easy access
to stored vehicle brake effectiveness data by
certain authorized government regulatory agencies
for the purpose of accessing vehicle safety.
While other methods of assuring effective
braking for the main purpose of driver feedback and
safety have a certain degree of efficiency in
certain environments or braking areas, they do not
provide the advantages of the improved methods and
apparatus of the present invention as hereafter more
fully described.
SUMARY OF T8E INVSNTIOIi
Determination of vehicle brake effectiveness is
accomplished by comparing predicted vehicle
deceleration rates and driver expected deceleration
rates, with actual measured vehicle deceleration.
Predicted and expected deceleration rates are
adjusted for variations in driving conditions such
as slope and drag.
If vehicle brake effectiveness is considered to
be 100%, relative gross vehicle weight (GVW) can
also be estimated by a comparison of predicted
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vehicle deceleration rates with the actual measured
vehicle deceleration rate.
An input device, which may be an interactive
display/input device, allows the operator to input
load information manually, or automatically through
a radio frequency (RF) input interface or other
standard communication means connected to an
automated weigh scale. In this way, mass stored in
memory provides an accurate figure, which can always
be assured. As mass has a great impact on
deceleration it is important that this figure be
accurate and consistent. A minimum acceptable
deceleration rate is also input and fixed in memory
for comparison to actual vehicle deceleration rates.
For economic considerations it is desirable
that the device obtain data from various existing
sensors which gather and use the data for other
purposes. This eliminates the need for the
installation of additional sensors and control
devices. Most modern tractor semi-trailers are
equipped with an electronic control unit (ECU) which
generates a signal representative of vehicle wheel
speed, distance, RPM, gear ratio and brake system
pressure. In some older vehicles which do not have
the necessary sensors or ECU, it may be necessary to
install sensors and ECUs to gather and process the
required data.
Pressure transducers located throughout the
vehicle measure pressure applied to the brake air
chambers when the vehicle is being decelerated
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during an application of its brakes. This data is
sent to the vehicle ECU via the automatic braking
system (ABS) and a signal representative of the
brake system pressure is generated. At the same
time, or in the alternative, a brake treadle
pressure sensor may be used to determine brake
system pressure.
Road slope is calculated by comparing
measurements from a low-G accelerometer with wheel
based deceleration rates taken from the ABS. Slope
angle is used to adjust deceleration rates for a
more accurate comparison with actual and expected
values.
In addition, the main ECU provides data on
vehicle speed, engine torque, engine RPM and engine
brake engagement (exhaust brake) which is used to
adjust deceleration for drag forces. Engine brake
engagement is monitored to assess the validity of
any results. If the engine brake is engaged,
results are invalidated and discarded.
Actual deceleration is compared to predicted
deceleration which has been adjusted for slope and
drag. Actual deceleration is also compared to
driver expected deceleration rates and a minimum
acceptable deceleration rate. Historic deceleration
data stored by the computer can be used to assess
vehicle brake effectiveness over time and determine
maintenance schedules or provide data for government
safety inspections. Comparisons are highly accurate
due to the various adjustments for slope and drag
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and the accurate value for vehicle mass which has
been manually or automatically input. The computer
collects and stores data on vehicle brake
effectiveness over time for the purposes of
historical comparison, maintenance assessment and
brake inspection.
The invention provides feedback on vehicle
deceleration rate and signals a trend of degrading
brake effectiveness during vehicle deceleration.
Deviations from expected values could be the result
of brake problems, normal wear or an unknown vehicle
mass overload condition.
Results are automatically stored in memory for
future reference and historical brake effectiveness
comparisons, and may be transferred via various
communications interferes to external devices for
other purposes such as maintenance control,
government monitoring or corporate data gathering.
It is therefore an object of the present
invention to provide an apparatus and method for
determining vehicle brake effectiveness.
It is a further object of the present invention
to provide a method and apparatus to give feedback
on vehicle brake effectiveness for vehicles that do
not decelerate in the expected rate over a large
spectrum of environments.
It is a still further object of the present
invention to provide a method and apparatus of
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assessing vehicle brake effectiveness which takes
into consideration travelling up or down a slope,
differences in drag forces such as air friction and
engine friction and engine brake engagement.
It is also an object of the present invention
to provide a method and apparatus of assessing
vehicle brake effectiveness which includes a means
of manually or automatically inputting the actual
mass of the vehicle for use in calculating vehicle
brake effectiveness.
It is a further object of the present invention
to provide a method and apparatus of assessing
vehicle brake effectiveness based on a comparison
with known vehicle deceleration rates for a known
vehicle mass at 100% braking effectiveness.
It is an object of a preferred embodiment of
the present invention to provide a method and
apparatus of assessing vehicle brake effectiveness
based on a comparison with the minimum acceptable
deceleration rate for the vehicle.
It is another object of a preferred embodiment
of the present invention to provide a method and
apparatus of assessing vehicle brake effectiveness
based on a comparison with driver expected
deceleration rates or with vehicle braking
effectiveness history.
It is yet another object of a preferred
embodiment of the present invention to provide a
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method and apparatus of determining a potential vehicle mass
overload condition that affects the vehicle deceleration
rate.
A further object of a preferred embodiment of the
present invention is to provide an apparatus and method for
alerting an operator to a deterioration in braking
effectiveness or to a mass overload problem so that
corrective action can be taken before the problem becomes
critical.
It is also an object of a preferred embodiment of the
present invention to provide a method and apparatus which
will permit easy access to the stored data on vehicle brake
effectiveness for the purposes of vehicle maintenance or for
providing the data to certain authorized government
regulatory agencies for the purpose of assessing vehicle
safety or vehicle load conditions.
According to one aspect then, there is provided an
apparatus for calculating effectiveness of a vehicle braking
system during deceleration of the vehicle, comprising: a
computer; a means for inputting a baseline deceleration rate
of the vehicle to the computer; a means for inputting the
mass of the vehicle to the computer; a low-g accelerometer
for measuring a deceleration rate of the vehicle and means
for inputting the deceleration rate of the vehicle to the
computer; and a means for measuring brake system pressure of
the vehicle during deceleration of the vehicle and for
inputting the brake system pressure of the vehicle to the
computer, the computer having means for calculating the
effectiveness of the vehicle braking system from data
representing the baseline deceleration rate of the vehicle,
the deceleration rate of the vehicle, the brake system
pressure of the vehicle during deceleration of the vehicle,
and the mass of the vehicle, and generating a signal
representative of the effectiveness of the vehicle braking
system, wherein the baseline deceleration rate of the
vehicle is a deceleration rate calculated for a constant
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predetermined brake system pressure and vehicle test mass at
a time when the vehicle braking system is functioning at
100% braking effectiveness, wherein the baseline
deceleration rate of the vehicle is calculated by conducting
and averaging data obtained during a plurality of test
decelerations of the vehicle.
According to further aspect, there is provided a method
for calculating effectiveness of a vehicle braking system in
a vehicle having a brake treadle during deceleration of the
vehicle, comprising: providing a baseline deceleration rate
for the vehicle calculated for a constant predetermined
brake system pressure and vehicle test mass at a time when
the vehicle braking system is functioning at 100% braking
effectiveness, the baseline deceleration rate calculated by
conducting and averaging data obtained during a plurality of
test decelerations of the vehicle; providing the mass of the
vehicle; accelerating the vehicle; applying pressure to the
vehicle brake treadle to decelerate the vehicle; determining
brake system pressure of the vehicle during deceleration of
the vehicle; determining a deceleration rate of the vehicle
using a low-g accelerometer; calculating the effectiveness
of the vehicle braking system from data representing the
baseline deceleration rate of the vehicle, the deceleration
rate of the vehicle, the brake system pressure of the
vehicle during deceleration of the vehicle, and the provided
mass of the vehicle; and generating a signal representative
of the effectiveness of the vehicle braking system.
According to a another aspect, there is provided an
apparatus for calculating effectiveness of a vehicle braking
system during deceleration of the vehicle, comprising: a
computer; a means for inputting the mass of the vehicle to
the computer; a means for measuring brake system pressure of
the vehicle during deceleration of the vehicle and for
inputting the brake system pressure to the computer; a means
for measuring road slope during deceleration of the vehicle
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and for inputting the road slope to the computer; a means to
measuring air friction of the vehicle and engine friction of
the vehicle during deceleration of the vehicle and for
inputting the air friction and the engine friction to the
computer; a means for determining predicted deceleration of
the vehicle and for inputting the predicted deceleration of
the vehicle to the computer; a means for measuring
deceleration of the vehicle and for inputting the
deceleration of the vehicle to the computer; and the
computer having means for calculating, the effectiveness of
the vehicle braking system from data representing the
deceleration of the vehicle, the road slope, the air
friction of the vehicle, the engine friction of the vehicle,
the brake system pressure during deceleration of the
vehicle, the mass of the vehicle, and the predicted
deceleration of the vehicle under comparable conditions, and
generating a signal representative of the effectiveness of
the vehicle braking system.
According to another aspect, there is provided an
apparatus for calculating effectiveness of a vehicle braking
system during deceleration of the vehicle, comprising: a
computer; a means for inputting a baseline deceleration rate
of the vehicle to the computer; a means for inputting the
mass of the vehicle to the computer; a low-g accelerometer
for measuring a deceleration rate of the vehicle and means
for inputting the deceleration rate of the vehicle to the
computer; a slope compensating means for generating a slope
adjusted deceleration rate of the vehicle, and means for
inputting the slope adjusted deceleration rate of the
vehicle to the computer to automatically compensate for the
effect of road slope during deceleration, and a means for
measuring brake system pressure of the vehicle during
deceleration of the vehicle and for inputting the brake
system pressure of the vehicle to the computer, the computer
having means for calculating the effectiveness of the
vehicle braking system from data representing the baseline
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deceleration rate of the vehicle, the slope adjusted
deceleration rate of the vehicle, the brake system pressure
of the vehicle during deceleration of the vehicle, and the
mass of the vehicle, and generating a signal representative
of the effectiveness of the vehicle braking system.
According to a further aspect, there is provided a
method for calculating effectiveness of a vehicle braking
system in a vehicle having a brake pedal during deceleration
of the vehicle, comprising: providing a baseline
deceleration rate for the vehicle; providing the mass of the
vehicle; accelerating the vehicle; applying pressure to the
vehicle brake pedal to decelerate the vehicle; determining
brake system pressure of the vehicle during deceleration of
the vehicle; determining a deceleration rate of the vehicle
using a low-g accelerometer; generating a slope adjusted
deceleration rate of the vehicle to automatically compensate
for the effect of road slope during deceleration of the
vehi-,-le; calculating the effectiveness of the vehicle
braking system from data representing the baseline
deceleration rate of the vehicle, the slope adjusted
deceleration rate of the vehicle, the brake system pressure
of the vehicle during deceleration of the vehicle, and the
mass of the vehicle; and generating a signal representative
of the effectiveness of the vehicle braking system.
According to yet another aspect, there is provided an
apparatus for calculating effectiveness of a vehicle braking
system during deceleration of the vehicle, comprising: a
computer; a means for inputting a baseline deceleration rate
of the vehicle to the computer; a means for inputting the
mass of the vehicle to the computer; a means for measuring a
deceleration rate of the vehicle and means for inputting the
decell-eration rate of the vehicle to the computer; a means
for measuring road slope during deceleration of the vehicle
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and for inputting the road slope to the computer; and a
means for measuring brake system pressure of the vehicle
during deceleration of the vehicle and for inputting the
brake system pressure of the vehicle to the computer, the
computer having means for calculating the effectiveness of
the vehicle braking system from data representing the
baseline deceleration rate of the vehicle, the mass of the
vehicle, the deceleration rate of the vehicle, the road
slope, and the brake system pressure of the vehicle during
deceleration of the vehicle, and generating a signal
representative of the effectiveness of the vehicle braking
system.
According to a further aspect, there is provided a
method for calculating effectiveness of a vehicle braking
system in a vehicle having a brake treadle during
deceleration of the vehicle, comprising: providing a
baseline deceleration rate for the vehicle; providing the
mass of the vehicle; accelerating the vehicle; applying
pressure to the vehicle brake treadle to decelerate the
vehicle; determining a brake system pressure of the vehicle
during deceleration of the vehicle; determining a
deceleration rate of the vehicle; determining road slope
during deceleration of the vehicle; calculating the
effectiveness of the vehicle braking system from data
representing the baseline deceleration rate of the vehicle,
the mass of the vehicle, the deceleration rate of the
vehicle, the road slope, and the brake system pressure of
the vehicle during deceleration of the vehicle; and
generating a signal representative of the effectiveness of
the vehicle braking system.
BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments of the present invention will now
be described and will be better understood
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when read in conjunction with the accompanying
drawings, in which:
Fig. 1 is a flow diagram illustrating the
method of the present invention;
Fig. 2 is block diagram illustrating the major
elements of the present invention;
Fig. 3 is a perspective view of a typical
tractor semi-trailer vehicle;
Fig. 4 is a schematic view of the brake control
system of the tractor as shown in Fig. 3.
Fig. 5 is a schematic view of the brake control
system of semitrailer as shown in Fig. 3.
Fig. 6 is a schematic view of a free body
acceleration diagram showing the measurement of
slope angle.
Fig. 7 is a block diagram illustrating the
major elements of a preferred embodiment of the
present invention.
Fig. 8 is a flow diagram illustrating the
method of the preferred embodiment of the present
invention shown in Fig. 7.
Similar reference numerals are used in the
Figures to denote similar components.
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DSTAILED DESCRIPTION OF THE INVENTION
The apparatus for determining vehicle brake
effectiveness 100 is best understood by reference to
Figs. 2, 3, 4 and 5. Fig. 3 shows a typical tractor
semi-trailer vehicle 40 comprising a tractor 15 and
a trailer 20.
As shown in Figs. 4 and 5 tractor 15 and
trailer 20 include a drive axle pressure transducer
50, a steering axle pressure transducer 60, and a
trailer pressure transducer 70. These transducers
monitor air pressure delivered to the corresponding
brake air chambers and provide a signal to the
automatic brake system (ABS) 120 (Fig. 2). The
signal provided to ABS 120 by pressure transducers
50, 60 and 70 is representative of overall brake
system pressure and is used to determine the
predicted vehicle deceleration rate of the vehicle
as will be described below. At the same time, or in
the alternative, brake system pressure can be
determined by a brake treadle pressure sensor 130
(Fig. 2) on the brake treadle itself. Tractor 15
includes an electronic control unit (ECU) 30
connected to ABS 120. ECU 30, ABS 120, pressure
transducers 50, 60, and 70 and brake treadle
pressure sensor 130 are standard equipment on most
tractor semi-trailer vehicles, however, vehicles not
having such equipment can be easily retrofitted.
With reference now to Fig. 2, the apparatus for
determining vehicle brake effectiveness 100 also
includes a computer 110 comprising memory, a central
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processing unit (CPU) and mass storage. Included in
the circuits of computer 110 are drag calculation
circuits 150 for calculating vehicle drag force and
a slope sensor 140 which includes a low-G
accelerometer. Computer 110 is connected to
existing vehicle automatic brake control system
(ABS) 120 via existing vehicle electronic control
unit (ECU) 30. A means 160 for inputting actual
vehicle mass (M) or maximum gross vehicle weight is
provided. Mass may be input manually by the
operator or entered automatically directly from
automated weigh scales. A further input means 190
is provided to input driver expected deceleration
rate and minimum acceptable deceleration rate to
computer 110.
The apparatus for determining vehicle brake
effectiveness 100 is provided with an output
interface 170 including a data transfer circuit,
communication means and means for saving data to a
storage device. It will be clear to those skilled
in the art that output interface 170 may include
various communication means including a physical
connector or an automatic remote radio frequency
(RF) output which can be used to send and receive
data without the need for the vehicle to stop.
Included as well is an indicating device 180 which
may include a video display terminal (VDT), a series
of lights or analog dials, or an audio signalling
means to provide output on vehicle brake
effectiveness and a warning of vehicle brake
deterioration to the vehicle operator.
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Road slope sensor 140 produces a signal
representing road inclination angle 6(Fig. 6).
Road slope sensor 140 includes a low-G accelerometer
to measure vehicle acceleration. Circuits within
computer 110 compare vehicle acceleration measured
by the low-G accelerometer to wheel based
acceleration measured by ABS 120 to calculate road
slope angle 6. With reference to the free body
diagram shown in Fig. 6, inclination angle e is
calculated using the following formula:
e = SIN"1 [ [Af-AX ] /g];
where: 8= inclination angle or slope;
Af = forward vehicle acceleration obtained
from a measurement of the change in
wheel based speed over time,
calculated by ABS 120 using the
formula:
A~ = dVF/dt
where VF = wheel speed in the forward
direction and t = time;
AX = forward vehicle acceleration as
measured by the low-G accelerometer;
g = gravity (Constant)
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The measurement of vehicle acceleration A. as
calculated by ABS 120 from the derivative of
steering wheel speed is an accurate measurement of
vehicle acceleration when the wheels are not
slipping. It will be appreciated that under most
conditions, steering wheels of tractor semi-trailers
are not capable of locking. Wheel based
acceleration measured in this way is unaffected by
road slope.
By contrast, vehicle acceleration AX measured by
the low-G accelerometer on the slope sensor 140 is
affected by slope angle. If the vehicle is
travelling on an upward slope, as shown in Fig. 6,
the force of gravity (g) acting on the low-G
accelerometer will have two components; g,, acting
perpendicular to the direction of travel of the
vehicle will have no effect on the acceleration rate
measured by the low-G accelerometer; and g., acting
180 degrees opposite to the direction of travel of
the vehicle will have a negative effect on the
acceleration rate measured by the low-G
accelerometer. Since wheel based acceleration A_, as
measured by ABS 120, is unaffected by gravity, the
difference between wheel based acceleration A. and
the acceleration measured by the low-G accelerometer
A., is, by the above-noted formula, an accurate
measure of the angle of inclination A or road slope.
If the vehicle is travelling on an downward
slope the force of gravity (g) acting on the low-G
accelerometer will again have two components; gv
acting perpendicular to the direction of travel of
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the vehicle will have no effect on the acceleration
measured by the low-G accelerometer; and g.,, this
time acting in the direction of travel of the
vehicle, will have a positive effect on the
acceleration measured by the low-G accelerometer.
Again, since wheel based acceleration, as measured
by ABS 120, is unaffected by gravity, the difference
between wheel based acceleration AF and acceleration
measured by the low-G accelerometer A. is, according
to the above-noted formula, the angle of inclination
6 or road slope.
In another embodiment of the present invention
(not shown), a second low-G accelerometer is mounted
on the slope sensor circuit 140 and positioned to
provide readings for lateral acceleration during
cornering. Lateral acceleration readings are used
to warn the vehicle operator of a potential roll-
over condition during excessively sharp cornering.
Drag friction caused by air resistance and
engine friction is determined by drag calculation
circuits 150 in computer 110. Drag calculation
circuits 150 use vehicle speed, engine RPM, engine
torque and engine brake output as provided by ECU 30
and ABS 120 to calculate a figure for vehicle drag.
Monitoring of engine brake engagement is important
since any engagement of the engine brake will
corrupt the test results and prevent any meaningful
comparison with known standards. If the engine
brake is engaged, results are invalidated and
discarded.
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Actual vehicle mass is input to computer 110
manually by the operator or automatically from
automated road scales. Also included is a value for
the maximum legal gross vehicle weight (GVW) which
is the maximum load legally allowed for a particular
vehicle. The system will always calculate brake
effectiveness based on the maximum legal gross
vehicle weight figure, but will also calculate brake
effectiveness based on other mass values provided.
Operation of the apparatus for determining
vehicle brake effectiveness 100 is best understood
by reference to Figure 1. A driver expected
deceleration rate (DED) 200 is input by the operator
via keyboard, read/write identification tag or card
or semi-conductor chip. This rate is one which the
operator, may have obtained from the last vehicle
driven and represents a safe estimated deceleration
rate against which the operator wishes results
compared. Vehicle mass (M) 202 is input manually by
the operator or automatically by automated weigh
scales. A figure for maximum legal GVW may also be
input at this time or can be stored permanently by
computer 110. Brake pressure 204 is applied, and
brake system pressure (SP) 206 is determined, from a
measurement of brake treadle pressure and/or brake
air chamber pressures, during a deceleration
procedure. All measurements of air chamber
pressures from transducers 40, 50 and 60 are sampled
every 1/100 second and averaged for the test period
which may range from a minimum .3 seconds to a
maximum 3 seconds. At the same time, or in the
alternative, brake system pressure 206 is determined
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by brake treadle pressure sensor 130. Road slope
angle 6 is calculated 208 and drag force (R) is
determined 210. It should be noted that road slope
angle 9 and drag force (R) are monitored
continuously by computer 110 so that figures are
immediately available when required. A figure for
vehicle wheel based deceleration (AF) 212 is obtained
from ABS 120 and a figure for actual vehicle
deceleration (AD) is obtained from the low-G
accelerometer on the slope sensor 140.
Predicted deceleration (PD) 216 is calculated
by a comparison of measured brake system pressure
(SP) with a table of known deceleration data stored
by computer 110 obtained from tests at 100% brake
effectiveness. Predicted deceleration (PD) is
adjusted for slope 218 and for drag 220. Actual
vehicle deceleration rate (AD) is compared to the
adjusted value of predicted vehicle deceleration
(NPD) 222 to provide a figure for vehicle brake
effectiveness (BE) compared to known values. Actual
vehicle deceleration (AD) is also compared to the
vehicle minimum acceptable deceleration rate (MD)
224, adjusted driver expected deceleration (NDED)
226, and historical stored values (HD) 228. Output
230 is provided to an indicating device 180 (Fig. 2)
to provide a means of warning the operator of
potential brake problems or a mass overload
condition, or for a comparison with historical
braking effectiveness data. Output 230 is also sent
to a storage device connected to computer 110 for
use in future historical comparisons of brake
effectiveness data. Output 230 can be in the form
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of a visual display or an audible signal or both.
Output 230 is also provided to an output interface
170 (Fig. 2) which is used to communicate data from
the invention to remote devices for monitoring of
brake maintenance or to provide data on brake
effectiveness to government regulatory agencies.
To determine a potential mass overload
situation, the operator compares the reading for
vehicle brake effectiveness taken immediately before
loading to a reading for vehicle brake effectiveness
obtained immediately after loading. Any significant
difference between the two readings would be due to
either a mass overload situation or faulty brakes.
To confirm the cause, the operator could weigh the
load or disconnect the load and perform another
brake effectiveness test. If brake effectiveness
returns to the reading prior to loading, a mass
overload would be confirmed. If not, a brake
problem would be indicated. Alternatively, if the
operator does not have any immediate prior readings
for vehicle brake effectiveness, a comparison can be
made with stored test data for unloaded and maximum
load conditions which were obtained at 100% vehicle
brake effectiveness. Any significant differences
would indicate a mass overload situation or a brake
problem.
As noted, the apparatus for determining vehicle
brake effectiveness 100 can assess vehicle brake
effectiveness by any one of four different methods:
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1) compare actual vehicle deceleration (AD)
with adjusted predicted vehicle deceleration rate
(NPD) for the same-mass at 100% brake effectiveness
222;
2) compare actual vehicle deceleration (AD)
with a minimum acceptable vehicle deceleration rate
(MD) 224;
3) compare actual vehicle deceleration (AD)
with a driver expected vehicle deceleration rate
(DED) entered by the operator via keyboard,
read/write identification tag or card or semi-
conductor chip 226; and
4) compare actual vehicle deceleration (AD)
with historical data of deceleration rates (HD) for
that particular vehicle 228.
In accordance with method number one, in order
to compare actual vehicle deceleration (AD) with
known vehicle deceleration rates at different
vehicle masses and brake system pressures, computer
110 is provided with storage for a table of test
data representing deceleration rates for the current
vehicle in which brake effectiveness is known to be
100%. At 100% brake effectiveness, braking force
has a constant relationship with brake system
pressure.
Test data is obtained by decelerating test
vehicles on a flat surface using various brake
system pressures (SP). Deceleration rates are
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measured with an accelerometer. This produces a
brake system pressure versus deceleration rate curve
for a given vehicle mass. Net braking force (F) is
derived by the formula:
F = SP x k = M x D
where M= actual vehicle mass, D deceleration rate
and k represents the constant relationship between
brake system pressure and force. The results are
stored as a table by computer 110. It should be
obvious to one skilled in the art that a
computerized algorithm or graph stored in computer
110 could perform the same function as the above-
described table.
Once actual brake system pressure (SP) has been
measured during an actual vehicle braking procedure,
computer 110 locates the same brake system pressure
in its table of stored test data and determines the
predicted vehicle deceleration (PD) by the formula:
PD = F / M
The result is a predicted deceleration rate (PD)
which is then adjusted for road slope using the
following formula and the slope angle calculation
described above, to give a slope adjusted predicted
deceleration rate (SPD):
SPD = PD -[g x sin 6]
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Variation in drag force (R), such as that
caused by air resistance and engine friction have a
very limited effect on deceleration rates in most
vehicles. Nevertheless, these forces can be
estimated by monitoring vehicle speed, engine RPM
and engine torque. Net predicted deceleration is
calculated by the following formula:
NPD = SPD - [R / M]
Vehicle brake effectiveness (BE) is calculated
as follows:
BE = AD / NPD
where AD is actual vehicle deceleration as measured
by the low-G accelerometer. The result can be shown
as a percentage or represented by a graph and an
audible or visual warning can be sent to the vehicle
operator to warn of a brake effectiveness reading
outside a particular pre-determined range.
In accordance with method number two, the
minimum acceptable vehicle deceleration rate (MD),
which is independent of current vehicle mass, road
slope or drag force, is determined. MD represents
the minimum rate at which the vehicle must stop,
regardless of the mass. Vehicle brake effectiveness
compared to minimum acceptable vehicle deceleration
is calculated by the formula:
BE=AD /MD
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and is displayed as a percentage or represented on a
graph. Again, an audible or visual warning can be
sent to the vehicle operator to warn of a brake
effectiveness reading less than the accepted
minimum. This will signal a serious problem with
the vehicle brakes or a mass overload situation.
In accordance with method number three, driver
expected deceleration rate (DED), entered manually
by the operator, is adjusted for slope and drag
using the following formula:
NDED = DED - [g x sin 6) - [R / M]
to arrive at net adjusted driver expected
deceleration (NDED). Vehicle brake effectiveness
compared to net adjusted driver expected
effectiveness is:
BE = AD / NDED
This result is displayed as a percentage or
represented on a graph and warnings are given to the
operator should the brake effectiveness be less than
expected by the operator.
In accordance with method number four, computer
110 compiles and stores historic data of actual
vehicle decelerations (HD), recording actual
deceleration rate, brake system pressure, slope,
drag force, vehicle mass and date. This data is
stored in the same type of table used to store known
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deceleration rates used in method number one
described above.
Historic data is compared to the current data
from a current vehicle deceleration to assess
vehicle brake effectiveness. Current data can be
compared to data from any previous deceleration or
to an average of data for all previous
decelerations. This comparison provides the
operator with a means of assessing current brake
performance based on historical brake performance
for that vehicle.
Historic data can also be used to determine a
brake maintenance schedule or can be sent to remote
devices for monitoring brake effectiveness or to
provide data on brake effectiveness to government
regulatory agencies.
A preferred embodiment of the present invention
is shown in Figure 7 wherein an apparatus for
determining vehicle brake effectiveness 300 is shown
in schematic form. A T-type fitting is inserted
into the vehicle brake line and a brake line
pressure transducer 330 is attached. It will be
readily noted by those skilled in this area that
many vehicles are already equipped with such brake
line pressure transducers. Output from brake line
pressure transducer 330 is connected to a computer
310 which includes a CPU, memory and storage.
Connected to computer 310 is an input device 360
which may be a keyboard, and an output device 380
which may include both a visual display and an audio
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output. Also connected to computer 310 is an output
interface 370 similar to the output interface 170
described above and shown in Figure 2. Also
connected to computer 310 is a low-G accelerometer
340 for measuring vehicle deceleration and an
automatic brake control system (ABS) 320 for
providing wheel based speed. On most new vehicles,
the typical connection for the wheel based speed
signal is the J1708 plug.
In order to calculate vehicle brake
effectiveness using the apparatus shown in Figure 7,
it is first necessary to provide computer 310 with a
baseline deceleration rate at a constant brake line
pressure of 60 psi [BDR(60)] obtained at a time when
the vehicle brakes are known to be functioning at
100% effectiveness. This baseline deceleration
rate, BDR(60), can be input from data obtained on
previous tests or it can be calculated by averaging
the results from a series of three typical ramp-
type, stop sign stops, with the vehicle under three-
quarters to full load conditions.
During a BDR test the driver performs three
test stops. The vehicle test mass (TM), which
should be from three-quarters maximum to maximum,
including the weight of any trailer attached to the
vehicle, is entered manually into computer 310 using
input device 360. Once the vehicle reaches a speed
of approximately 45 miles per hour, the brakes are
applied in a manner which smoothly increases
pressure until the vehicle is stopped. Vehicle
speed, as calculated by ABS 320, and brake line
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pressure as measured by brake line pressure
transducer 330 are displayed on output device 380 to
assist the driver in meeting the required test
criteria which are described in detail below.
Vehicle deceleration is read directly from low-G
accelerometer 340. If the required test criteria
are met, computer 310 stores the values for vehicle
deceleration and brake line pressure over time as
the vehicle comes to a controlled stop. Three
successful tests are averaged and the result is
stored as the baseline deceleration rate (BDR).
Each BDR test must meet strict criteria to be
acceptable. First, brake line pressure must reach
at least 65 psi before the vehicle's speed drops
below 5 miles per hour. Second, the average brake
pressure application rate must fall within a range
of 2 to 6 psi per second. If the average rate falls
outside this range, the test is rejected and the
operator is requested to perform another test.
Third, computer 310 calculates the instantaneous
rate of brake line pressure change over time. This
indicates any sudden increases or decreases in brake
pressure. If the instantaneous rate of brake line
pressure change over time falls outside the range of
-10 psi per second to +40 psi per second, the test
is rejected. To eliminate the effects of unusual
noise spikes, the instantaneous rate of brake line
pressure is calculated by using a seven point
floating average for each sample.
Using BDR test data which meets the above
criteria for brake pressure and deceleration over
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time, the baseline deceleration rate of the vehicle
can be calculated for a constant brake application
pressure of 60 psi. This is called the BDR(60). In
the present invention BDR(60) is calculated using a
third order curve fit of BDR test data comprising
deceleration versus pressure, however, it will be
understood by those skilled in the field that other
similar mathematical models could be used to perform
the same task. The result is a single number,
BDR(60), that represents the vehicle deceleration
rate at a constant brake pressure of 60 psi, at a
test mass of TM and 100% vehicle brake
effectiveness.
Once a value for BDR(60) at 100% brake
effectiveness has been obtained, this figure is used
as a baseline for comparison with actual vehicle
deceleration rates (ADR), to determine the current
vehicle brake effectiveness. ADR test results must
meet the same criteria as designed for the BDR test,
that is, a ramp-type, stop sign stop which reaches a
minimum brake application pressure of 65 psi before
the vehicle's speed drops below 5 miles per hours.
The average pressure application rate must fall
within the range of 2 psi per second to 6 psi per
second, and the instantaneous rate of brake line
pressure change must fall within the range of -10
psi per second to +40 psi per second. Any ADR tests
which do not meet this criteria will be rejected and
the operator will be asked to perform another stop
using a smoother, more constant brake application
pressure.
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Once a successful ADR brake test has been
achieved, the ADR at 60 psi, ADR(60), is calculated
using the same method described above for
calculating BDR(60). The result is a single number
which represents vehicle deceleration at a constant
brake pressure of 60 psi.
In this embodiment of the present invention, no
adjustment is made for drag forces (R) caused by air
resistence and engine friction. Since both the BDR
and the ADR test criteria are identical, any drag
forces caused by air resistence or engine friction
are similar and thus self compensating. As
described above, vehicle engine brake is monitored
to ensure that it is not engaged during a test. Any
engagement of the vehicle engine brake would
invalidate the test results.
Since all vehicle deceleration rates are
measured by low-G accelerometer 340 and since
BDR(60) is calculated on level terrain, adjustments
for road slope are also unnecessary. This is made
clear by the following example. If the ADR test is
conducted on a down slope, the extra horizontal
force caused by the pull of gravity on the vehicle
will increase the actual vehicle stopping distance,
thereby decreasing the ADR(60) number since it will
take relatively longer for the vehicle to come to a
stop. However, the gravitational effect of the down
slope will cause an increase in the deceleration
rate ADR(60) as read by low-G accelerometer 340.
The increase in the ADR(60) caused by low-G
accelerometer 340 will be exactly equal to the
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decrease in the ADR(60) caused by the increased
stopping distance due to the downward slope.
Similarly, stopping on an uphill grade results in a
shorter vehicle stopping distance. This tends to
increases the ADR(60), however, gravitational forces
decrease the reading from low-G accelerometer 340,
resulting in a comparable decrease in the ADR(60)
number. The result is that, following adjustment
for mass as described below, the ADR(60) can be
compared directly to the BDR(60), regardless of the
slope on which the ADR(60) is calculated.
In order to calculate the current vehicle brake
effectiveness BE, the ADR(60) is multiplied by the
ratio of actual vehicle mass (M) input by the
operator to the vehicle test mass (TM) from the BDR
test results:
ADR(m60) = ADR(60) x M / TM
The result is a mass adjusted deceleration rate
ADR(m60) which can be directly compared to the
baseline deceleration rate BDR(60) to provide the
current vehicle brake effectiveness:
BE = ADR(m60) / BDR(60)
Brake effectiveness BE is displayed in a
similar manner to that previously described, as a
percentage or represented on a graph, and audio and
visual warnings are provided should the brake
effectiveness drop below a certain predetermined
level.
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Operation of the above-described apparatus for
determining vehicle brake effectiveness 300 is best
understood by reference to Figure 8. A baseline
deceleration rate at a constant brake pressure of 60
psi [BDR(60)] is input by the operator at 400 or
calculated by performing several test stops as
described above. The BDR(60) is determined at a
vehicle test mass (TM) which is between three-
quarters and full vehicle load at a time when the
brakes are known to be operating at 100%
effectiveness. Actual vehicle mass (M) is input
manually by the operator at 402 or automatically by
automated weigh scales. An actual deceleration rate
(ADR) test is conducted by applying brake pressure
at 404 in a ramp-type, stop sign stop which meets
all of the above-described test criteria. Actual
vehicle deceleration over time is measured by low-G
accelerometer 340 at 406 and brake line pressure
over time is measured by brake line pressure
transducer 330 at 408. The actual vehicle
deceleration at 60 psi [ADR(60)] is calculated at
410 and adjusted for the current vehicle mass (M) at
412. Vehicle brake effectiveness BE is calculated
at 414 and the results are output to a visual
display or audio warning device at 416. Results can
also be stored for future reference or output via a
communications port to external reading devices.
Preferred embodiments of the invention have
been described, however, numerous modification,
variations and adaptations, obvious to one skilled
in the art, may be made to the particular
embodiments of the invention described above without
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departing from the spirit and scope of the invention
as defined in the claims.
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