Method for controlling actuation of a vehicle safety device using filtered vehicle deceleration data

The present invention is directed to a filtering technique for improving the capability of crash sensing systems in distinguishing between severe and minor crash events. The onset of a crash event is detected when a sensed vehicle deceleration exceeds a deceleration threshold value. Following the onset of the crash event, a crash severity parameter is calculated at predetermined intervals as a function of vehicle deceleration. At the conclusion of a predetermined time period, a value of the crash severity parameter less than or equal to a parameter threshold value corresponds to a minor crash incident. A value greater than the parameter threshold value corresponds to a potential severe crash event requiring airbag deployment, thereby necessitating further crash severity discrimination. Before continuing, crash severity parameters having specific signal trace characteristics discard deceleration data generated during the initial stages of the crash event, preventing potentially misleading deceleration data from influencing airbag deployment. A severe crash event is characterized by the values of multiple crash severity parameters simultaneously exceeding their respective deployment threshold levels. A value of at least one of the crash severity parameters less than a predetermined reset threshold value corresponds to a minor crash incident.

This invention relates to a method of filtering motor vehicle deceleration 
data used for controlling actuation of a motor vehicle safety device, and 
more particularly, to improve the immunity of a crash sensing system to 
minor incidents, such as rough road events and low speed undercarriage 
hits, which do not require actuating vehicle safety devices, such as 
airbags and seat belt pre-tensioners. 
BACKGROUND OF THE INVENTION 
A vehicle crash sensing system detects and discriminates severe crash 
events which require deployment of an airbag, such as those listed in 
Table 1, from minor crash incidents which do not, such as those listed in 
Table 2. 
TABLE 1 
______________________________________ 
Type of Collision Speed Range (mph) 
______________________________________ 
Full frontal to barrier (F/B) 
12-30 
30.degree. right angle to barrier (R/B) 
20-30 
30.degree. left angle to barrier (L/B) 
20-30 
On-center pole (C/P) 
15-30 
Full frontal to rear of parked car 
60 
______________________________________ 
TABLE 2 
______________________________________ 
Event or Incident Upper Speed (mph) 
______________________________________ 
Undercarriage Hit (U/H) 
20 
Car to Deer (D/H) 50 
Square block road (S/B) 
40 
Chatter bumps 60 
Hood slams N/A 
Door Slams N/A 
Hammer blows (5-8 lbs.) 
N/A 
______________________________________ 
Discrimination is accomplished by means of a vehicle-mounted accelerometer 
and an associated signal processing algorithm contained within a 
microprocessor. Since the total available time for deploying an airbag to 
effectively restrain occupants in a severe crash event is very short, the 
ability to quickly and reliably determine the severity of a collision is 
paramount. Equally important is the system's immunity to inadvertent 
deployment during minor crash incidents. 
Many prior art airbag deployment algorithms have been developed which 
utilize one or more quantities for measuring the severity of a collision. 
These "quantities" or "parameters" have included vehicle velocity change, 
energy, power, power-rate, jerk, predicted occupant/interior contact 
velocity, predicted occupant movement, energy of a vehicle deceleration 
signal, and oscillation measure of the vehicle deceleration signal. The 
value of these quantities are generally calculated as a function of 
successively sampled accelerometer data. Based upon test data obtained 
from the accelerometer during a representative set of minor crash 
incidents, one or more boundary thresholds are set. Airbag deployment is 
initiated whenever the values of some or all of these quantities exceed 
their respective boundary thresholds. 
For example, in U.S. Pat. No. 5,339,242, issued Aug. 16, 1994, to Reid et 
al., a crash sensing system is disclosed in which time-dependent jerk and 
velocity change data represent two crash severity conditions which are 
continually consulted following the onset of a crash event in order to 
determine whether vehicle safety devices should be actuated. 
As illustrated in FIG. 1, many minor crash incidents including, 20 mph 
undercarriage hits (U/H) 10, 50 mph simulated deer hits (D/H) 12 and 
square block rough road incidents (S/B) 14 are characterized by a rapid 
decrease in vehicle velocity over a relatively short duration, which 
thereafter quickly levels off. On the other hand, FIG. 2 illustrates that 
vehicle velocity changes of severe crash events including, 30 mph frontal 
barrier (F/B) 16, 30 mph left angle to barrier (L/B) 18, 30 mph right 
angle to barrier (R/B) 20, 30 mph on center high pole (C/P) 22 and 13 mph 
increase slowly following the onset of the crash event but shortly 
thereafter increases monotonically. 
As can be realized from the above, if vehicle velocity change (calculated 
as a function of acceleration) is used as a crash severity parameter, 
accelerometer data generated during the initial stages of a crash event 
(e.g., up to the first 25 msec.) can be misleading. A nondeployment type 
minor crash incidem may initially produce a higher deceleration signal 
value (and corresponding higher velocity change value) than a deployment 
type severe crash event. Lowering threshold levels to increase deployment 
sensitivity (i.e., reduce deploymere time) may result in the initially 
higher velocity change values of the minor crash incidents exceeding the 
boundary threshold levels, inadvertently deploying the airbag. 
Use of crash severity parameters other than vehicle velocity change 
characterized by similar severe-versus-minor crash event signal traces as 
those of the vehicle velocity change parameter will similarly be 
susceptible to the potentially misleading acceleration data generated 
during the initial stages of the crash event. 
SUMMARY OF THE INVENTION 
The present invention is directed to a filtering technique for improving 
the capability of crash sensing systems in distinguishing between severe 
and minor crash events. In so doing, the respective boundary threshold 
levels can be lowered to reduce deployment time without negatively 
impacting the system's immunity to minor crash incidents. This filtering 
technique can be an add-on feature to existing crash sensing methods to 
achieve improved performance. 
According to one aspect of the present invention, the onset of a crash 
event is sensed by a vehicle-mounted accelerometer when the detected 
vehicle deceleration exceeds a predetermined deceleration threshold value. 
Following the onset of the crash event, a change in vehicle velocity is 
calculated at predetermined intervals as a function of the deceleration 
data obtained from the accelerometer, each incremental change being added 
to the previous change to arrive at a resultant summation of changes in 
vehicle velocity occurring since the onset of the crash event. 
At the conclusion of a predetermined time period, the resultant sum change 
in vehicle velocity is compared to a predetermined threshold value. A 
value less than or equal to the threshold value corresponds to a 
nondeployment type minor crash incident. A value greater than the 
threshold value corresponds to a potential deployment type severe crash 
event requiring further crash severity discrimination for determining the 
necessity of airbag deployment. First, however, the value of the vehicle 
velocity change parameter is reset to zero. This eliminates the effects of 
deceleration data generated during the initial stages of the crash event 
upon subsequent calculations of the vehicle velocity change value, thereby 
preventing potentially misleading deceleration data from influencing 
airbag deployment. 
According to another aspect of the present invention, a severe crash event 
is characterized by the values of multiple crash severity parameters 
simultaneously exceeding their respective deployment threshold levels, one 
of the crash severity parameters being the vehicle velocity change. A 
value of at least one of the crash severity parameters less than a 
predetermined reset threshold value corresponds to a nondeployment type 
minor crash incident (e.g., rough road incident or low speed undercarriage 
hit). 
These and other aspects and advantages of the invention may be best 
understood by reference to the following detailed description of the 
preferred embodiment when considered in conjunction with the accompanying 
drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The filter of the present invention is not sufficient by itself to provide 
complete discrimination between severe and minor crash events for 
deployment of an airbag. Rather, it is intended to enhance a main crash 
sensing system. According to the crash sensing system of the present 
invention, discrimination between deployment and nondeployment type crash 
events is based upon a two-pan criteria: (i) oscillation measure of the 
vehicle acceleration trace and (ii) vehicle velocity change of the vehicle 
after impact. 
The formula for calculating oscillation measure (OM) is: 
EQU OM =.intg. da(t)/dt dt (1) 
The derivative of acceleration gives the rate of change of acceleration 
(referred to as the jerk). The total area under the jerk curve is an 
indication of how the acceleration is oscillating over time. Total area is 
the integral of the jerk curve (using absolute value to counter 
subtracting area when jerk goes negative). If an acceleration contains 
high oscillation (both frequency and magnitude) then this oscillation 
measurement becomes very large. 
The second criteria involves detecting vehicle velocity changes occurring 
after impact, calculated in accordance with the following formula: 
EQU Velocity Change =.intg.a(t)dt (2) 
In accordance with the crash sensing system, airbag deployment is triggered 
if a crash event exceeds both an Oscillation Boundary Curve (OBC) and a 
Velocity Boundary Curve (VBC), indicative of a severe crash event. 
The filtering technique of the present invention is contained within the 
airbag deployment methodology represented by the microprocessor program 
instructions illustrated in FIG. 3. Referring to FIG. 3, reference numeral 
30 generally refers to the filter portion of the crash sensing methodology 
comprising steps 36-46. Referring to FIG. 3, the crash sensing system 
begins at step 31 with initialization, which includes the resetting of all 
applicable timers and memory locations containing the values of crash 
sensing parameters. After initialization 31, an acceleration signal is 
read in step 32 from a vehicle-mounted accelerometer and compared in step 
34 with a threshold deceleration value. If the detected deceleration 
exceeds the threshold deceleration value, the vehicle is assumed to be 
undergoing an impact and crash discrimination calculations are initiated, 
beginning with the filter 30 of the present invention. 
The filtering technique 30 begins at step 36 in which a crash event timer 
is activated (i.e., begins to time-out). The timer value will vary between 
vehicle models depending upon the particular structural design of the 
vehicle, but will generally be between 1-25 msec. At step 38, a crash 
severity parameter (in this case, the change in vehicle velocity) is 
initially calculated as a function of the acceleration signal read at step 
32. The integral of the acceleration signal (which is equal to the change 
in vehicle velocity) is a simplified means of indicating the kinetic 
energy of the vehicle impact. 
The vehicle velocity change is repetitively calculated in step 38, each 
time being updated with new acceleration data read in step 42, until the 
event timer completely times-out as determined by step 40. Once the event 
timer has timed-out 40, the then-existing value of the energy measure 
calculated in step 38 is compared in step 44 with a threshold value 
obtained from a look up table in step 43 which contains time-dependant 
velocity change values. Alternatively, the value obtained from the look up 
table can be a single constant value or calculated in accordance with a 
formula. If the threshold value for the look up table time entry point 
following the onset of the crash event is exceeded, as determined by the 
COME function of step 44, the current vehicle impact is determined to 
be a potentially severe crash event requiring further crash 
discrimination. Failure of the crash severity parameter value to exceed 
the threshold value in step 44 is indicative of a minor nondeployment type 
crash event. If such is the case, the algorithm is reinitialized at step 
31. 
As an option, to ensure that the vehicle is still undergoing a crash event, 
step 44 could also compare the current vehicle deceleration with the 
deceleration threshold value. If the current vehicle deceleration does not 
exceed the deceleration threshold value, the algorithm is reinitialized at 
step 31. 
Once a potentially severe crash event is identified by step 44, further 
discrimination calculations must be performed to determine the necessity 
and timing of airbag deployment. This is generally achieved by comparing 
values of multiple crash severity parameters with corresponding threshold 
boundary curves. As noted above, the acceleration data obtained prior to 
the expiration of the event timer in step 40 may be misleading, depending 
upon the crash severity parameters being utilized. Those parameters, such 
as vehicle velocity change, having signal traces characterized by (i) for 
minor crash incidents - an initially rapid response in a relatively short 
duration which then levels off, and (ii) for severe crash events - an 
initially slow response which thereafter increases rapidly, are 
particularly susceptible to acceleration data generated during the initial 
stages of the crash event. For this reason, the value of these crash 
severity parameters are reset to zero in step 46 so as not to influence 
subsequently calculated values of the crash severity parameter, which 
would thereby affect its value with respect to the boundary curves and 
ultimately influence deployment. 
Further discrimination and deployment calculations are conducted in 
accordance with a two-part criteria, including: (i) an oscillation measure 
value (OM) with respect to an oscillation boundary curve (OBC), and (ii) 
the change in vehicle velocity with respect to a velocity boundary curve 
(VBC). 
The oscillation measure portion of the algorithm begins at step 47 by 
rereading accelerometer data. Note that the accelerometer data used for 
calculating oscillation measure is not discarded by filter 30. Rather, 
calculation of oscillation measure begins immediately after the event 
timer is actuated in step 36. This is because the signal trace of the 
oscillation measure of the acceleration data does not demonstrate the 
"acceleration-sensitive" characteristics that parameters such as the 
vehicle velocity change. 
Using the acceleration data read in step 47, the oscillation measure is 
calculated in step 50 and compared in step 54 with a time-based 
oscillation threshold value (OBC) obtained from a look up table value at 
step 52 for the appropriate time into the crash event. If the oscillation 
threshold value of step 52 is exceeded as determined by a COME function 
at step 54, the first of the two part criteria of step 58 is met. If the 
oscillation threshold value of step 52 is not exceeded, the oscillation 
measure portion of the algorithm repeats until its value exceeds the 
oscillation threshold value in step 54 or the crash sensing algorithm is 
reinitialized in step 3 1. 
The second of the two-part criteria is the vehicle velocity change value. 
This portion of the algorithm begins at step 48 by rereading the 
accelerometer data. Note that this parameter initially has a value of 
zero, its value being reset in step 46. The velocity change value is 
calculated in step 60 and compared in step 64 with a velocity change 
threshold value (VBC) obtained from a look up table value at step 62 for 
the appropriate time into the crash event. If the velocity change 
threshold value of step 62 is exceeded as determined by a COME function 
in step 64, the second of the two part criteria of step 58 is met. As an 
option, to ensure that the vehicle is still involved in a crash event, 
step 64 could also compare a current deceleration value with the 
deceleration threshold value. If at any time the deceleration value drops 
below the deceleration threshold value, the algorithm is reinitialized in 
step 31. 
If the velocity change threshold value of step 62 is not exceeded, the 
velocity change value of step 60 is compared in step 56 with a time-based 
reset threshold value obtained from a look up table value at step 53 for 
the appropriate time into the crash event. If the vehicle change value of 
step 60 drops below the reset threshold value of step 53, as determined by 
a COME function in step 56, the sensed vehicle impact is determined to 
be of a minor nondeployment type. If such is the case, the algorithm 
reinitializes at step 31. If, however, the velocity change value of step 
60 remains above the reset threshold value of step 53, calculation of the 
velocity change value repeats until the velocity change value drops below 
the reset threshold value or a crash event is no longer sensed by step 64. 
FIG. 4 is a graph of time vs. velocity change curves for a vehicle 
experiencing various types of severe crash events, incorporating multiple 
time-based reset threshold levels 70 and 71. 
Referring back to FIG. 3, if both the oscillation measure and velocity 
change values exceed their respective deployment threshold boundary 
values, a severe deployment-type crash event is identified and an airbag 
deployment enable command is generated at step 66. If only one, or 
neither, of the thresholds are exceeded, no deployment command is enabled. 
In accordance with the present invention, the acceleration data accumulated 
during a predetermined time period following the onset of a crash event is 
used to assist in an initial determination as to the severity of the crash 
event, identifying a certain category of nondeployment type minor crash 
incidents. If unable to identify the current crash event as of this 
category, the algorithm continues by making a definite determination in 
accordance with a two-part criteria. To prevent misleading acceleration 
data from being used in determining deployment, future calculations of 
certain crash severity parameters discard acceleration data generated 
during the predetermined time period. 
As noted earlier, the acceleration data during the initial stages of a 
crash event can be misleading, with the value of deceleration during a 
minor crash incident being greater than that of a severe crash event, 
increasing the values of the corresponding crash severity parameters used 
for deployment of the airbag. For this reason, deployment threshold values 
must be maintained at a certain level to prevent inadvertent deployment. 
However, incorporating the filtering technique 30 of the present invention 
into a crash sensing system, deployment threshold levels can be lowered to 
increase sensitivity of the safety devices without sacrificing immunity to 
inadvertent actuation during minor crash incidents. 
FIGS. 5 and 6 illustrate graphs of time vs. velocity change curves for a 
vehicle experiencing various types of minor and severe crash events, 
respectively, the vehicle employing a crash sensing methodology utilizing 
the filtering technique of the present invention. Comparing FIG. 5 with 
FIG. 2, and FIG. 6 with FIG. 3, the influence of the filtering technique 
of the present invention on the corresponding velocity curves can be seen. 
Note that the velocity boundary curves (VBC) can be lowered considerably. 
Although the crash sensing system of the preferred embodiment utilizes 
vehicle velocity change and oscillation measure as crash severity 
parameters, the filtering technique of the present invention can be 
incorporated into any crash sensing system utilizing crash severity 
parameters other than those described. 
While the present invention has been described in reference to the 
illustrated embodiments, it will be recognized that various modifications 
will occur to those skilled in the art. In this regard, it will be 
understood that methods incorporating such modifications may fall within 
the scope of this invention which is defined by the appended claims.