Method and system for detecting vehicle roll-over

A method and system for detecting a vehicle roll-over utilizes a z-axis accelerometer (12) and a processor (20) for analyzing the accelerometer output (18) to provide early detection/prediction of a vehicle roll-over. The output signal (18) is substantially a positive constant value equal to the force of gravity when the vehicle (16) is in a normal upright condition, and as the vehicle (16) experiences a roll-over, the output signal transitions to a substantially negative constant value when the vehicle has rolled 180.degree. upside down. To provide early roll-over detection and/or prediction, the z-axis data is averaged (102) and compared to a first threshold value (104) set to a g-force value within a range of 0 to -1 g. Estimating a slope value (step 110), i.e. the change in z-axis data over time, can provide an additional discrimination measure to improve overall reliability, and to provide a qualitative measure of the roll over.

BACKGROUND OF THE INVENTION 
The present invention relates generally to vehicle safety device actuation 
systems, and more particularly to a method and system for providing early 
and reliable vehicle roll-over detection. 
Generally, typical vehicle roll-over detection systems utilize 
electro-mechanical sensors, such as mercury filled inclination sensors 
and/or gyroscopes, which are arranged to close a switch contact in 
response to the forces generated by a vehicle roll-over. Known systems 
have not proven wholly satisfactory because these sensors tend to rely on 
a mechanical action responsive to the vehicle rolling over which makes a 
high sensitivity and accuracy in roll-over detection difficult to obtain. 
Specifically, known systems tend to use sensors having thresholds which 
are tripped very late in the roll-over to provide a high degree of 
accuracy in the roll-over detection. However, waiting until very late in 
the roll-over to confirm a detection significantly degrades roll-over 
sensitivity. Reduced sensitivity correspondingly reduces the period of 
time in which appropriate vehicle safety devices can be actuated in 
response to a roll-over detection. The mechanical action is also subject 
to performance degradation over time. 
Furthermore, some roll-over arrangements have attempted to increase 
reliability by utilizing a plurality of sensors positioned in different 
locations on the vehicle. However, each individual sensor tends to suffer 
from the sensitivity/reliability problem described above, while the 
overall system significantly increases the cost and physical space 
requirements for vehicle roll-over detection. 
SUMMARY OF THE INVENTION 
Therefore, an object of the present invention is to provide a vehicle 
roll-over detection method and system which significantly improves 
sensitivity and reliability. 
A further object of the present invention is to provide a vehicle roll-over 
detection method and system which can provide early and reliable vehicle 
roll-over detection using a single sensor. 
Another object of the present invention is to provide a vehicle roll-over 
detection method and system which utilizes signal processing on a sensor 
output signal to provide early and reliable roll-over detection. 
Another object of the present invention is to provide an improved vehicle 
roll-over detection method and system which utilizes a plurality of 
different signal processing techniques on an accelerometer output to 
provide early and reliable roll-over detection. 
In accordance with the present invention, a method for detecting roll-over 
of a vehicle comprises the steps of receiving vehicle data from a signal 
output by an accelerometer whose sensing axis is oriented substantially 
parallel to a gravitational force, averaging the vehicle data to filter 
out noise, and determining whether the averaged vehicle data exceeds a 
first predetermined threshold value. The first threshold value is 
representative of a g-force indicative of the vehicle beginning to 
roll-over. At least one vehicle safety device is actuated if the first 
predetermined threshold is exceeded. 
The method can further comprise the steps of estimating a slope value 
representative of a change over time in the averaged data, determining 
whether the slope value exceeds a second predetermined threshold value, 
and actuating the at least safety device if both the first and second 
threshold values are exceeded. In addition, a step of further analyzing 
the received data using at least one additional discrimination process can 
be performed, wherein the at least one additional discrimination process 
has a corresponding threshold value, and the at least one vehicle safety 
device is actuated only when all threshold values have been exceeded. 
In operation, the accelerator output signal is substantially a positive 
constant value equal to the force of gravity when the vehicle is in a 
normal upright condition. When the vehicle experiences a roll-over, the 
output signal transitions to about zero as the vehicle has rolled on a 
side, and to a substantially negative constant value when the vehicle has 
rolled upside down. To provide early roll-over detection and/or 
prediction, the first threshold value is set to a g-force value within a 
range of 0 to -1 g. The estimate of the slope value can provide an 
additional discrimination measure to improve overall reliability, and can 
provide a qualitative measure of the roll-over, i.e., the greater the 
negative slope value, the more violent the roll-over. 
In further accordance with the present invention, a system for detecting 
roll-over of a vehicle comprises an accelerometer mounted to the vehicle 
in such a manner that the accelerometer's sensing axis is oriented 
substantially parallel to a gravitational force, and a processor means for 
analyzing a signal output by the accelerometer. The processor means 
comprises a means for receiving the accelerometer output signal as vehicle 
data, a means for filtering noise from the vehicle data, a means for 
comparing the filtered vehicle data to a first predetermined threshold, 
and a means for producing a roll-over detection signal if the first 
threshold is exceeded. A means for actuating at least one vehicle safety 
device actuates the device(s) in response to the roll-over detection 
signal. 
The processor means can further comprise a means for estimating a slope 
value representative of a change over time in the averaged data, and a 
means for comparing the slope value to a second predetermined threshold 
value. The means for producing the roll-over detection signal will only 
produce the signal if both the first and second threshold values are 
exceeded. 
The present invention will be more fully understood upon reading the 
following detailed description of the preferred embodiment in conjunction 
with the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
FIG. 1 shows a block diagram circuit of a vehicle roll-over detection 
system 10 in accordance with the present invention. Roll-over detection 
system 10 is primarily formed from a z-axis accelerometer 12 coupled to a 
control unit 14. Accelerometer 12 is positioned within a vehicle 16 so 
that the accelerometer's sensing axis is oriented parallel to a 
gravitational force, or perpendicular to the normal plane of travel (i.e., 
the "z-axis"). 
The z-axis accelerometer 12 produces an analog output 18 responsive to the 
orientation of the vehicle 16 relative to the ground. More specifically, 
as seen in FIG. 2(a), the z-axis accelerometer output 18 normally produces 
a relatively steady state +1 g output responsive to the earth's 
gravitational field when all of the vehicle's wheels are properly 
contacting the ground. However, as seen in FIG. 2(b), the output 18 
translates from a +1 g output to a -1 g output during a vehicle roll-over. 
The actual roll-over and corresponding sensor output translation lasts for 
a time period of T.sub.2 -T.sub.1 seconds, which is typically on the order 
of .apprxeq.3 seconds. 
The accelerometer output 18 is supplied to a processor unit 20 located 
within control unit 14 for monitoring and evaluation. The overall 
operation of processor 20 is more fully described hereinbelow in context 
with FIGS. 3 and 4. The processor unit 20 utilizes a memory arrangement 22 
for tracking and/or storing accelerometer output data samples received 
over periods of time. The memory arrangement 22 can be implemented in a 
manner well known to one having ordinary skill in the art, such as by 
using one or more RAM or EEPROM memory chips. 
The processor generates a roll-over detection output for controlling the 
triggering of an actuation circuit 24. In response to a roll-over 
detection signal, the actuation circuit 24 is arranged to operate one or 
more vehicle safety devices such as an air bag inhibitor 26, roll-over bar 
28, and a seat belt pretensioner 30. One of ordinary skill will appreciate 
that other devices can be operated, such as unlocking vehicle door locks 
and disabling the vehicle fuel supply system. 
As shown in FIG. 3, the processor 20 of the present invention receives 
z-axis data at step 100, averages the individual z-axis data samples over 
a predetermined sampling time period at step 102, and if the averaged 
z-axis data indicates a force approximately equal to a -1 g measure at 
step 104, a roll-over has been detected. If the average data indicates a 
force greater than a -1 g measure, the processor 20 then generates a new 
average using the next z-axis data sample. 
The average of the z-axis data samples at step 102 improves discrimination 
reliability by filtering out external noise from the accelerometer output 
18. More specifically, external noise sources such as rough road 
conditions, or electromagnetic interference (EMI) noise generated by other 
electrical components, typically produce noise having a z-axis component 
greater than 1 Hz. Averaging the z-axis data samples over a sampling time 
period of at least 1 second effectively filters out all noise having a 
frequency greater than 1 Hz. The filtering can be achieved with a simple 
average, median filter, or digital filter arrangement. 
Further, the present invention advantageously provides a method which 
allows early and reliable prediction of a vehicle roll-over. More 
specifically, instead of using a discrimination threshold of -1 g at step 
104, a prediction threshold (TH.sub.1) of between 0 and -1 g provides an 
indication that the vehicle is on its side and heading toward a complete 
180.degree. roll-over. Therefore, adjusting the discrimination analysis by 
using a prediction threshold (TH.sub.1) provides a reliable indication 
that a vehicle is beginning to experience roll-over, thereby allowing the 
system to predict a roll-over to permit earlier deployment of the 
appropriate vehicle safety devices. For example, using a TH.sub.1 
threshold of .apprxeq.-0.24 g provides a good indication that the vehicle 
has just begun to roll from its side toward its roof. 
As shown in FIG. 4, the reliability and performance of the roll-over 
prediction/discrimination analysis of the present invention can be 
enhanced by incorporating additional discrimination analysis subroutines 
into the method of FIG. 3. More specifically, roll-over of the vehicle can 
be verified and/or the severity of the roll-over can be measured by 
analyzing the rate of change (i.e. the slope) of the averaged z-axis data 
samples. Thus, averaged z-axis data samples are buffered at step 106 to 
generate a record of past data samples. The past data sample is then 
combined with the most current averaged z-axis data samples at step 108 so 
that the slope of the z-axis data samples plotted over time can be 
estimated at step 110. The estimated slope is then compared to a 
predetermined threshold value TH.sub.2 at step 112, to provide a 
qualitative evaluation of the roll-over. 
As further shown in FIG. 4, the overall roll-over analysis can be 
supplemented with one or more optional discrimination subroutines, which 
utilize the received vehicle data to generate different event measures. 
Each respective processing subroutine would be supplied with the received 
z-axis data to provide appropriate data processing at a step 114(a) to 
(n), and to determine whether a respective subroutines analysis threshold 
TH.sub.a-n has been exceeded at a step 116(a) to (n). Examples of suitable 
discrimination subroutines are signal processing arrangements which track 
the energy of the received z-axis data in a particular frequency band, 
measure the variance of the z-axis data waveform over time, and/or monitor 
occupant position relative to fixed structure within the vehicle. However, 
one of ordinary skill in the art will readily appreciate that the above 
noted examples of suitable discrimination subroutines are not to be 
construed as limiting. Further, outputs from other binary or analog 
inclination sensors, such as from a mercury switch arranged to close a 
contact if the vehicle has reached a predetermined inclination, can be 
added to the roll-over discrimination analysis. A voting circuit can be 
added to weight and/or analyze the various discrimination subroutine 
outputs, thereby significantly reducing the likelihood of a false 
detection, such as might occur due to the vehicle taking a curve at high 
speed. 
As shown symbolically in FIG. 4, a "yes" determination at each of the steps 
104, 112 and 116(a) to (n) causes a respective input line to an AND gate 
118 to go high. If all of the subroutines have provided an 
indication/prediction that the vehicle is in a roll-over, the AND gate 118 
provides the roll-over detection signal to the actuation circuit 24. The 
optional voting circuit noted above can be incorporated into the AND gate 
operation, or alternatively can provide an input to the AND gate by 
operating as a prioritizing circuit which receives two or more of the 
subroutine outputs. 
Therefore, the present invention provides a vehicle roll-over 
discrimination analysis which advantageously utilizes signal processing of 
a z-axis accelerometer output to allow the present invention to reliably 
predict and detect a vehicle roll-over. This in turn permits early 
deployment of vehicle safety devices and/or provides the discrimination 
analysis additional time to analyze further information before triggering 
deployment of the safety devices. In addition to providing early and 
reliable predictions of a vehicle roll-over, the present invention further 
allows the z-axis data to be qualitatively analyzed to determine roll-over 
severity. 
It will be understood that the foregoing description of the preferred 
embodiment of the present invention is for illustrative purposes only, and 
that the various structural and operational features herein disclosed are 
susceptible to a number of modifications, none of which departs from the 
spirit and scope of the present invention as defined in the appended 
claims.