Actuation controller for air bag device

In an air bag system of the type that inflates one air bag using a plurality of inflators, an air bag inflation control apparatus is provided that can make a proper determination of inflator activation mode and a correct decision to activate or not activate the inflators according to the severity of a collision. More specifically, the invention provides an activation control apparatus for an air bag system having a plurality of inflators for one air bag, which, upon detection of a vehicle collision, controls activation of the inflators in accordance with the severity of the collision, comprising: a first acceleration sensor 1, mounted in a position within a passenger compartment, for constantly detecting acceleration G at the mounting position; and a second acceleration sensor 2, mounted in a position within a crush zone in a forward part of a vehicle, for constantly detecting acceleration G' at the mounting position, and wherein: a decision to activate or not activate the plurality of inflators and determination of the activation mode of the inflators are determined by utilizing differences in characteristics among various values in various types of collision and by combining the various values as appropriate, the various values consisting of a first time-integrated value V1 obtained by performing time integration based on an acceleration signal from the first acceleration sensor 1 and a second time-integrated value V1' obtained by performing time integration based on an acceleration signal from the second acceleration sensor 2, and of an integrated value difference between the second time-integrated value and the first time-integrated value, Vd=V1'-V1, or the amount of change of the integrated value difference, (Gd=d(Vd)/dt).

TECHNICAL FIELD
 The present invention relates to an activation control apparatus for an air
 bag system for activating the air bag system by detecting a vehicle
 collision, and more particularly, in an air bag system of the type that
 inflates one air bag using a plurality of inflators, the invention relates
 to a novel air bag inflation control apparatus that can make a proper
 determination of inflator activation mode and a correct decision to
 activate or not activate the inflators according to the severity of a
 collision.
 BACKGROUND ART
 Air bag systems generally used in the past are of the type that inflates
 one air bag using a single inflator. In this type of air bag system, the
 change of acceleration of a vehicle is constantly monitored using an
 acceleration sensor mounted in a passenger compartment, and the resulting
 acceleration signal is processed by performing appropriate mathematical
 operations such as first integration or second integration; then, the
 result is compared with a predetermined threshold value and, if it exceeds
 the threshold value, an activation signal is issued to an inflator
 ignition circuit to activate the inflator and thus inflate the air bag.
 Since this type of air bag system is designed, based on safety standards,
 to produce maximum performance in a frontal collision at 50 km/h, the air
 bag is inflated with designated characteristics if only the threshold
 value is exceeded, regardless of the severity of the collision or the
 position or posture of the vehicle occupant. Therefore, in a low-speed or
 medium-speed collision, the air bag is inflated with a inflating energy
 excessive for occupant protection, giving rise to the possibility that, if
 the occupant is seated close to the air bag, or in the case of a small
 sized occupant, the occupant may be injured by the inflation of the air
 bag.
 As for the acceleration sensor used to determine whether to activate or not
 activate the inflator, there are two types according to the sensor
 mounting position: one is the integral type in which the sensor is
 assembled into the air bag module and mounted in the steering wheel, and
 the other is the separate type in which the sensor is on the driver's seat
 side in the passenger compartment. In the case of the integral type, the
 acceleration sensor detects the impact of the collision transmitted
 through the steering shaft, and in the case of the separate type, the
 acceleration sensor is mounted on a bracket attached to the vehicle body
 and detects the impact of the collision transmitted to the inside of the
 passenger compartment through the vehicle body; in either case, the
 decision whether to inflate or not inflate the air bag is made based on
 the change of acceleration detected by the acceleration sensor mounted
 within the passenger compartment that has a structure of high rigidity and
 is less subjected to deformation in the event of a collision.
 Some vehicle types in which impacts at the front of the vehicle are
 difficult to transmit to the inside of the passenger compartment employ a
 system that uses an electronic acceleration sensor mounted inside the
 passenger compartment in combination with a mechanical sensor mounted in a
 crush zone, such as an engine compartment, in the forward part of the
 vehicle, but since the mechanical sensor, because of its characteristics,
 is only capable of making an ON/OFF decision and is used in conjunction
 with a collision discrimination system that uses the acceleration sensor
 mounted inside the passenger compartment, if a localized impact such as
 hammering is input to the mechanical sensor, an erroneous activation may
 result.
 In recent years, there has been proposed a system generally known as the
 "smart air bag system" which employs a plurality of inflators and controls
 the mode of air bag inflation in an optimum manner by controlling the
 output level of the inflators in accordance with the type of collision and
 the condition of the occupants. To implement this system, an ignition
 decision with a timing earlier than the conventional ignition decision
 timing becomes necessary to perform computations for inflator output
 control, but such an early decision system has yet to be proposed.
 The present invention has been devised to address the above problems, and
 an object of the invention is to provide a novel air bag activation
 control apparatus that can make the ignition decision earlier and more
 timely than previous systems, and that drastically reduces the possibility
 of erroneous activation by correctly discriminating impacts even in
 situations of improper use (hereinafter called the "abuse") such as
 hammering or rough road driving that could result in an erroneous
 activation if the discrimination were made replying only on the passenger
 compartment acceleration sensor.
 DISCLOSURE OF THE INVENTION
 The present invention has been devised in view of the above situation, and
 its feature is that a second electronic acceleration sensor is mounted in
 the crush zone in the forward part of a vehicle to supplement the first
 electronic acceleration sensor mounted, as in a conventional system,
 inside the vehicle's passenger compartment, with provisions made to make a
 decision as to whether to inflate or not inflate the air bag (to activate
 or not activate inflators) and determine the inflation mode of the air bag
 (inflator activation mode) by utilizing the differences in characteristics
 between acceleration signals generated by the respective acceleration
 sensors in various types of collision. In a specific method of
 computation, the decision as to whether to activate or not activate the
 inflators and determination of the inflator activation mode are made by
 utilizing the differences in characteristics among various values in
 various types of collision and by combining the various values as
 appropriate, the various values including a first time-integrated value
 obtained by performing time integration based on the acceleration signal
 from the first acceleration sensor, a second time-integrated value
 obtained by performing time integration based on the acceleration signal
 from the second acceleration sensor, an integrated value difference
 between the first time-integrated value and the second time-integrated
 value, and the amount of change of the integrated value difference.
 The method of the present invention can be roughly divided into two
 methods: the first method that utilizes the characteristics of the
 integrated value difference between the first time-integrated value and
 the second time-integrated value, and the second method that only utilizes
 the differences in characteristics between the first time-integrated value
 and the second time-integrated value in various types of collision.
 The first method provides two methods for determining the inflator
 activation mode: one is to compare the integrated value difference between
 the first time-integrated value and the second time-integrated value with
 a predetermined threshold value given as a function of time, and to
 determine the inflator activation mode based on the result of the
 comparison, and the other is to compare the second time-integrated value
 with a predetermined threshold value given as a function of time, and to
 determine the inflator activation mode based on the result of the
 comparison. There are two modes of inflator activation, a rapid inflation
 mode for rapidly inflating the air bag and a moderate inflation mode for
 inflating the air bag at a moderate speed.
 Whether to activate or not activate the inflators is determined in one of
 the following eight ways.
 (a) The decision whether to activate or not activate the inflators is made
 by comparing the integrated value difference between the first
 time-integrated value and the second time-integrated value with a
 predetermined threshold value given as a function of time.
 (b) The decision whether to activate or not activate the inflators is made
 by comparing the amount of change of the integrated value difference (time
 differential of the difference between the integrated values) with a
 predetermined threshold value given as a function of time.
 (c) The decision whether to activate or not activate the inflators is made
 by using the above two methods (a) and (b) in parallel and by judging
 whether at least one or the other of the methods satisfies the condition
 for inflator activation.
 (d) In addition to using the above two methods (a) and (b), the first
 time-integrated value is compared with a predetermined threshold value
 given as a function of time, and the decision whether to activate or not
 activate the inflators is made by judging whether at least one of methods
 (a) and (b) satisfies the condition for inflator activation and, at the
 same time, whether the first time-integrated value is not less than a
 predetermined threshold value given as a function of time.
 (e) The decision whether to activate or not activate the inflators is made
 by comparing the time-integrated value difference with a predetermined
 threshold value set as a function of the first time-integrated value.
 (f) The decision whether to activate or not activate the inflators is made
 by using the above two methods (b) and (e) in parallel and by judging
 whether at least one or the other of the methods satisfies the condition
 for inflator activation.
 (g) The decision whether to activate or not activate the inflators is made
 by comparing the second time-integrated value with a predetermined
 threshold value set as a function of the first time-integrated value.
 (h) The decision whether to activate or not activate the inflators is made
 by using the above two methods (b) and (g) in parallel and by judging
 whether at least one or the other of the methods satisfies the condition
 for inflator activation.
 The second method of the invention further includes two type of methods
 (method A and method B). In the method A, whether to activate or not
 activate the inflators (that is, whether to inflate or not inflate the air
 bag) is determined by performing a prescribed computation based on the
 acceleration signal from the first acceleration sensor mounted inside the
 passenger compartment, while the inflator activation mode (that is, the
 air bag inflation mode) is determined by performing a prescribed
 computation based on the acceleration signal from the second acceleration
 sensor mounted in the crush zone.
 On the other hand, in the method B, the time-integrated value based on the
 acceleration signal from the second acceleration sensor is compared with a
 predetermined velocity threshold value given as a function of time, and
 the inflator activation mode is determined based on the result of the
 comparison, while the same time-integrated value is compared with a
 predetermined threshold value given as a function of the time-integrated
 value based on the acceleration signal from the first acceleration sensor
 mounted inside the passenger compartment, and the decision to activate or
 not activate the inflators is made based on the result of the comparison.
 The method A includes a method in which the computation for determining the
 inflator activation mode based on the second acceleration sensor is
 suspended for a predetermined period of time from the starting of the
 computation based on the second acceleration sensor, thereby preventing an
 erroneous activation in the early stages of collision, or the value
 computed based on the first acceleration sensor mounted inside the
 passenger compartment is used in combination, to enhance the accuracy of
 the inflator activation mode determination.
 In each of the methods A and B, there are two modes of inflator activation,
 a rapid inflation mode for activating the inflators in such a manner as to
 cause the air bag to inflate rapidly, and a moderate inflation mode for
 activating the inflators in such a manner as to cause the air bag to
 inflate at a moderate speed. The rapid inflation is accomplished either by
 activating all the inflators simultaneously or by activating all the
 inflators by slightly displacing ignition timing between the inflators,
 while the moderate inflation is accomplished either by activating only a
 specified number of inflators or by igniting the inflators in sequence
 with a longer ignition timing difference between each inflator. By
 combining these inflator activation modes, the moderate and rapid
 inflation of the air bag are combined as needed and selected appropriately
 according to the type of vehicle and the type of vehicle body structure.
 In either method, the present invention can prevent an erroneous activation
 due to rough road driving or abuse or in a deer collision (a collision
 with a deer or like animal--the same applies hereinafter) where inflator
 activation is not needed, and can make a correct determination of the
 ignition timing in a high-speed frontal collision (a frontal collision in
 high-speed driving--the same applies hereinafter) and in a high-speed
 oblique collision (a collision at an oblique angle from the front in
 high-speed driving--the same applies hereinafter).
 In each of the above methods, it will be preferable from the viewpoint of
 optimizing the inflator activation if provisions are made so that when an
 inflator activation instruction is issued with the inflator activation
 mode determined as the moderate inflation mode, first a specified number
 of the inflators are activated in accordance with the moderate inflation
 mode, while allowing the computation to continue for the inflator
 activation mode determination, and if, as the result of the computation,
 the mode switches to the rapid inflation mode, the remaining inflators are
 activated.
 In the above method, it will also be preferable to hold off the
 determination of the inflator activation mode for moderate inflation and
 allow the computation to continue until a predetermined time elapses after
 the computation based on the second acceleration sensor is started, since
 the accuracy of the inflator activation mode determination can then be
 enhanced.

BEST MODE FOR CARRYING OUT THE INVENTION
 Embodiments of the present invention will be described below with reference
 to the accompanying drawings. FIG. 1 is a block diagram showing a first
 embodiment of the air bag activation control apparatus of the present
 invention; this embodiment concerns the basic mode for implementing the
 first method previously described. In the figure, two acceleration sensors
 are installed which consist of the first acceleration sensor 1 mounted
 inside the passenger compartment, as in a conventional system, and the
 second acceleration sensor 2 mounted in the crush zone. The crush zone
 here refers to a space within the vehicle body, located forward of the
 passenger compartment, and has the effect of mitigating damage to the
 passenger compartment in the event of a collision by crushing prior to the
 deformation of the passenger compartment; generally, the forward part of
 the engine compartment serves as the crush zone.
 Acceleration signals G and G' generated by the acceleration sensors 1 and 2
 are coupled via a computation circuit 6 to a reset circuit 20 and two
 inflator trigger circuit 21 and 22. Each trigger circuit is configured to
 ignite its associated inflator (not shown) to inflate the air bag 23.
 To describe the computation circuit 6, the acceleration signal G generated
 by the first acceleration sensor 1 in the passenger compartment is fed to
 a block 3 which is a computation start point detection circuit; when time
 t0 at which the acceleration value G exceeds a predetermined acceleration
 G1 is detected, a prescribed computation based on the acceleration G is
 started at that point in time. The block 4 that follows is a subtracting
 means which subtracts a predetermined acceleration G2 from the
 acceleration value G after the computation start time t0 and thereby
 offsets the acceleration value G to eliminate noise and minute variations
 in acceleration. Next, the decreased acceleration G3 output from the
 subtracting means 4 is fed into an integrating means 5 which integrates
 the acceleration G3 over time to compute a first time-integrated value V1.
 On the other hand, the acceleration signal G' generated by the second
 acceleration sensor 2 mounted in the crush zone is fed to a block 3' which
 is a computation start point detection circuit; when time t0' at which the
 acceleration value G' detected by the second acceleration sensor 2 exceeds
 a predetermined acceleration G1' is detected, a prescribed computation
 based on the acceleration value G' is started at that point in time, and
 in the following block 4' which is a subtracting means, a predetermined
 acceleration G2' is subtracted from the acceleration value G' after the
 computation start time t0', to compute a decreased acceleration G3', and
 the acceleration G3' is integrated over time by an integrating means 5' to
 compute a second time-integrated value V1'.
 Here, an explanation will be given of the difference between the first
 time-integrated value V1, obtained by performing time integration based on
 the acceleration signal from the first acceleration sensor 1 mounted
 inside the passenger compartment, and the second time-integrated value
 V1', obtained by performing time integration based on the acceleration
 signal from the second acceleration sensor 2 mounted in the crush zone.
 FIGS. 12(A) and 12(B) are diagrams showing the change over time of the
 above values V1 and V1': part (A) is a V1-t diagram and (B) is a V1'-t
 diagram, and the time axis t is shown on the same scale for both diagrams.
 It can be seen clearly from the two diagrams that, for all types of
 collision, the second time-integrated value V1' based on the second
 acceleration sensor 2 in the crush zone reaches a greater value at an
 earlier point in time than does the first time-integrated value V1 based
 on the first acceleration sensor 1.
 Especially, in a high-speed frontal collision, which is one of the severe
 collision types, V1' quickly rises to a large value and, likewise, in a
 high-speed oblique collision, which is also one of the severe collision
 types, V1, quickly rises in the early stage, as in the high-speed frontal
 collision; on the other hand, V1 rises slowly during the early stage and,
 at an intermediate point, begins to rise quickly. Further, in the case of
 a medium-speed centerpole collision (a frontal collision at a medium speed
 against a pole-like object such as an iron pole), since a small area
 centering around the part that hit the pole is heavily crushed, deforming
 only the bumper or the front end part of the vehicle in the early stages
 of collision, for some time after the collision V1 shows a value lower
 than that in a low-speed frontal collision that basically does not require
 air bag inflation, and it is not until after an appreciable length of time
 that the value V1 becomes higher than that in the low-speed frontal
 collision. Accordingly, if the decision is made based on V1, the air bag
 may not inflate at all or, if it does inflate, it may be too late to
 provide the intended protection; in contrast, V1' shows a value higher
 than that in the low-speed frontal collision from the beginning. In the
 case of a collision with deer, only a slight change appears in the value
 of V1, but V1' shows a value as large as a maximum value in the low-speed
 collision. This is because when a vehicle collides with a deer or the
 like, since the vehicle hits the deer away from it at the instant of the
 collision, a relatively large variation in acceleration occurs in the
 crush zone with the crush zone suffering some degree of deformation, but
 the impact is absorbed by the crush zone so that little change in
 acceleration occurs in the passenger compartment. In the case of rough
 road driving, V1 and V1' show similar waveforms between them. This is
 because no deformation is caused to the vehicle body during rough road
 driving, causing no appreciable differences due to the mounting position
 of the acceleration sensors.
 As described above, in medium-to-high speed collisions that cause localized
 crushing or deformation of the crush zone in the forward part of the
 vehicle in the initial stage of collision, V1' rises earlier than V1,
 whereas in rough road driving, etc. that do not cause deformation to the
 crush zone, V1 and V1' show substantially the same waveforms. In a
 low-speed collision that causes a lesser degree of deformation to the
 crush zone, V1' has a tendency to rise earlier than V1, but since the
 degree of deformation is smaller, the difference is correspondingly
 smaller.
 From these phenomena, it can be seen that unique differences exist
 according to the type of collision between the second time-integrated
 value V1' based on the second acceleration sensor 2 mounted in the crush
 zone according to the present invention and the first time-integrated
 value V1 based on the first acceleration sensor 1 mounted, as in a
 conventional system, inside the passenger compartment. Therefore, by
 obtaining the difference between the two integrated values, the degree of
 collision severity can be determined more clearly according to the degree
 of deformation of the crush zone. The most notable feature of the present
 invention is that proper discrimination of the collision type and proper
 determination of the air bag inflation mode are made by making use of the
 above characteristics.
 Next, a description will be given of how the decision to activate or not
 activate the inflators and determination of the activation mode are made
 in the first method of the present invention. As shown in FIG. 1, first a
 subtracting means 7 subtracts the first time-integrated value V1, obtained
 by performing time integration based on the first acceleration sensor 1,
 from the second time-integrated value V1', obtained by performing time
 integration based on the second acceleration sensor 2, and thus obtains
 the difference Vd between the integrated values. The integrated value
 difference Vd is then compared in a comparator 13 with a second velocity
 threshold value Vs2 preset in a block 12 as a threshold value that varies
 as a function of time; when Vd&lt;Vs2, it is determined that the degree of
 collision severity is not high, and an inflator activation mode signal
 (K=1) for causing the air bag to inflate at a moderate speed is sent from
 a block 18 to a block 15. On the other hand, when Vd.gtoreq.Vs2, it is
 determined that the degree of collision severity is high, and an inflator
 activation mode signal (K=2) for causing the air bag to inflate rapidly is
 sent from a block 19 to the block 15.
 Here, the relationship between the integrated value difference Vd and the
 second velocity threshold value Vs2 will be described below with reference
 to FIG. 13. FIG. 13 is a diagram showing the relationship between the
 integrated value difference Vd and time t in various types of collision.
 As can be seen from the figure, for the high-speed frontal and high-speed
 oblique collisions which are severe collisions, Vd shows large values in
 the early stages of collision, compared with the temporal change of the
 first time-integrated value V1 shown in FIG. 12(A); furthermore, these
 values rise quickly with a distinctly recognizable time difference
 compared with the value for the medium-speed centerpole collision, another
 collision event that requires air bag inflation. If the second velocity
 threshold value Vs2 is made up of Th4 which shows a constant value
 regardless of time during the early stages, Th5 which gradually increases
 after the passage of a predetermined time, and Th6 which shows a high
 value during the remaining stage, as illustrated, then in a severe
 collision event, such as a high-speed frontal or oblique collision, the
 inflator activation mode can be determined in the early stages of
 collision.
 Next, the inflator activation mode will be described. There are two modes
 for air bag inflation: moderate inflation in which the air bag is inflated
 at a moderate speed, and rapid inflation in which the air bag is rapidly
 inflated. Whether the air bag is inflated moderately or rapidly is
 determined by controlling the number of inflators activated or the
 activation timing of the inflators, or by controlling both. This will be
 described below.
 (a) In the first method, the air bag is inflated moderately or rapidly by
 controlling the number of inflators activated; in the moderate inflation
 mode, only the first inflator is ignited, while in the rapid inflation
 mode, both the first and second inflators are ignited. In the rapid
 inflation mode, finer control of the inflation is possible by providing a
 difference (including zero difference) in ignition timing between the
 first inflator and the second inflator.
 (b) In the second method, the air bag is inflated moderately or rapidly by
 controlling the ignition timing of the first and second inflators; in the
 moderate inflation mode, the air bag is inflated at a moderate speed by
 increasing the ignition timing difference between the first inflator and
 the second inflator, while in the rapid inflation mode, the air bag is
 fired rapidly by reducing (or zeroing) the ignition timing difference
 between the two inflators.
 Next, the method of deciding activation or non-activation of the inflators
 will be described. The integrated value difference Vd is fed to a
 comparator 10 where it is compared with a first velocity threshold value
 Vs1 preset in a block 11 as a threshold value that varies as a function of
 time; when Vd.gtoreq.Vs1, it is determined that inflator activation is
 needed, and an inflator activation instruction signal is sent to the block
 15. Based on the inflator activation mode signal (K=1 or 2) supplied from
 the block 18 or 19, the block 15 directs the inflator trigger circuits 21
 and 22 to output trigger signals by which the inflators (not shown) are
 activated to inflate the air bag 23. In FIG. 1, the line for supplying the
 moderate inflation signal (K=1) from the block 15 directly to the first
 inflator trigger circuit 21 is for the case where only the first inflator
 is activated in the moderate inflation mode, while the line for supplying
 the moderate inflation signal (K=1) and rapid inflation signal (K-2) from
 the block 15 to the inflator trigger circuits 21 and 22 is for the case
 where the two inflators are activated by displacing the ignition timing
 between them.
 When the integrated value difference is smaller than the first velocity
 threshold value (Vd&lt;Vs1), a comparing means 14 compares Vd with a value
 preset at or near zero (0); if Vd is not larger than the preset value (for
 example, not larger than zero), the system is reset by the system reset
 circuit 20, and if Vd is larger than the preset value (for example, larger
 than zero), the computation in the computation circuit 6 is continued.
 Next, the relationship between the value Vd and the first velocity
 threshold value Vs1 will be described with reference to FIG. 13. As shown,
 in the early stages of collision, the threshold value Vs1 is set at Th1
 higher than the level of the deer collision; with this threshold value
 level, a severe collision event such as a high-speed frontal or high-speed
 oblique collision is discriminated in the early stages to issue an
 inflator activation command. In the following middle stage of collision,
 Vs1 is set as a gradually decreasing threshold value Th2 downward to the
 right (decreasing with time) and leading to a lower threshold value Th3 in
 the remaining stage; using this gradually decreasing threshold value Th2,
 a moderate-severity collision such as a medium-speed centerpole collision
 is discriminated to issue an inflator activation command. The lower
 threshold value Th3 in the remaining stage is used to decide whether to
 activate or not activate the inflators in a low-speed frontal collision
 event, and is set at such a value that does not trigger air bag inflation
 in a frontal collision at a speed lower than a predetermined speed.
 In this figure also, the time axis is shown on the same scale as the time
 axis in the V1-t diagram of FIG. 12(A). As is apparent from a comparison
 between the two figure, in a severe collision such as a high-speed frontal
 or high-speed oblique collision, since activation commands can be issued
 to the inflators in the very early stages of collision, not only can the
 air bag be inflated without delay but, because of increased time margin
 for the determination of the ignition mode of each inflator,
 correspondingly longer computation time can be spared for the inflation
 mode control, increasing the accuracy of the computation for the air bag
 inflation control.
 In the case of rough road driving, since the waveforms of V1 and V1' are
 substantially the same, as shown in FIGS. 12(A) and 12(B), the difference
 Vd is very small. As a result, if the decision is made based on Vd, an
 erroneous activation of the air bag during rough road driving can be
 completely prevented. Likewise, in a minor-severity collision that only
 causes a minor deformation to the vehicle body, the difference between the
 time-integrated values based on the respective acceleration sensors is
 small; in this case also, an erroneous activation can be prevented in a
 reliable manner. In view of this, if the second acceleration sensor 2 is
 mounted in a position within the crush zone where no deformation is caused
 in a minor severity collision such as a low-speed frontal collision, the
 waveforms of V1 and V1' become substantially the same in the case of
 low-speed frontal collisions, and the difference Vd can thus be held to a
 small value. This serves to prevent an erroneous activation in a low-speed
 collision in a more reliable manner.
 FIG. 2 shows a modified example of the first embodiment shown in FIG. 1.
 The difference from FIG. 1 is that when Vd&lt;Vs2 as the result of the
 comparison between the integrated value difference Vd and the second
 velocity threshold value Vs2 in the comparator 13, elapsed time to from
 the starting time t0' of the computation based on the second acceleration
 sensor 2 is compared in a time comparator 8 with a predetermined first
 time threshold value ts1 before sending the result to the moderate
 inflation signal output device 18; then, when t'.gtoreq.ts1, that is, only
 when the predetermined time has elapsed, the moderate inflation signal is
 sent from the block 18 to the block 15, but when the elapsed time is less
 than the predetermined time (t1&lt;ts1), the determination of the K value
 is held off and the computation is continued. In other respects, the
 configuration is the same; therefore, the same elements are designated by
 the same reference numerals, and detailed descriptions thereof will not be
 repeated here.
 More specifically, in the example of FIG. 2, when the integrated value
 difference Vd and the second velocity threshold value Vs2 are compared in
 the comparator 13, if Vd&lt;Vs2, then the elapsed time t' from the time
 that the computation based on the second acceleration sensor 2 was started
 is compared in the block 8 with the predetermined the first time threshold
 value ts1; then, if t'&lt;ts1, the determination of the K value is held
 off and a K value undetermined signal (K=0) is sent from a block 24 to the
 block 15 which, when the signal K=0 is received, allows the computation to
 continue further. On the other hand, when the predetermined time has
 elapsed, i.e., t'.gtoreq.ts1, the determination of the K value so far held
 off is now made, and the resultant signal is sent to the block 18 which
 then sends the moderate inflation signal (K=1) to the block 15 as is done
 in the foregoing example. The reason for this is that even in a severe
 collision event such as a high-speed frontal or high-speed oblique
 collision, the value of Vd to be compared for the determination remains
 small in too early a stage of collision and, if the determination is made
 too early at this point in time, an erroneous determination may be made to
 inflate the air bag in the moderate inflation mode when the air bag should
 be inflated in the rapid inflation mode; accordingly, when Vd&lt;Vs2, the
 case which normally commands a moderate inflation, the determination is
 held off and the computation is continued until the predetermined time
 elapses, in order to enhance reliability in determining the inflator
 activation mode.
 On the other hand, when the result of the comparison between the integrated
 value difference Vd and the second velocity threshold value Vs2 in the
 comparator 13 shows Vd.gtoreq.Vs2, that is, when it is determined that the
 air bag should be inflated in the rapid inflation mode (K=2), the signal
 is immediately sent to the block 15 without judging the elapsed time in
 the time comparator 8.
 FIG. 3 shows another modified example of the first embodiment, illustrating
 an alternative method of determining the inflator activation mode. The
 difference from FIG. 2 is that the determination of the inflator
 activation mode is made not by comparing the integrated value difference
 Vd with the second velocity threshold value Vs2, but by comparing the
 second time-integrated value V1 based on the second acceleration sensor 2
 with a third velocity threshold value Vs3 preset as a threshold value that
 varies as a function of time. In other respects, the configuration is the
 same; therefore, the same elements are designated by the same reference
 numerals, and detailed descriptions thereof will not be repeated here.
 More specifically, in FIG. 3, the second time-integrated value V1' is fed
 to a comparator 17 where it is compared with the third velocity threshold
 value Vs3, preset as a function of time, supplied from a block 16, and the
 inflator activation mode is selected based on the result of the
 comparison. When the second time-integrated value is equal to or larger
 than the third velocity threshold value (V1'.gtoreq.Vs3), an instruction
 is given to the block 19 to output the rapid inflation inflator activation
 mode signal (K=2); on the other hand, when the second time-integrated
 value is smaller than the third velocity threshold value (V1'&lt;Vs3), the
 output is sent to the time comparator 8 and the determination of the
 moderate inflation mode is held off until the predetermined time ts
 elapses, as in the case of FIG. 2.
 Either activation mode signal is transmitted to the block 15 which, upon
 receiving the inflator activation signal from the comparator 10, supplies
 the activation mode signal to the first inflator trigger circuit 21 and/or
 the second inflator trigger circuit 22, and in accordance with the
 specified activation mode, the inflators are activated to inflate the air
 bag 23, as in the case of FIG. 2.
 The method of determining the inflator activation mode based on the second
 time-integrated value V1' will be described with reference to FIGS. 12(A)
 and 12(B). As is apparent from the figures, in all types of collision, V1'
 shows higher values than V1 in the early stages of collision. Accordingly,
 if the third velocity threshold value Vs3 is set at a relatively low value
 Th7 in the early stage, a gradually increasing value Th8 in the middle
 stage, and a high value Th9 in the remaining stage, as shown in FIG.
 12(B), then in a severe collision such as a high-speed frontal or
 high-speed oblique collision, the inflator activation mode (K=2) for
 rapidly inflating the air bag is selected at time t1 or time t2, i.e., at
 a very early point in time in the collision; here, if the time threshold
 value ts in the time comparator 8 is set as a value greater than t1 and
 t2, as shown, the selection of the moderate inflation mode is held off
 until the time ts. As is apparent from the comparison with FIG. 12(A)
 shown on the same time scale, if the determination were made based only on
 the acceleration sensor 1 mounted inside the passenger compartment, it
 would be difficult to make a correct determination at time ts in the case
 of the high-speed frontal collision or the high-speed oblique collision,
 since, in this case, the time ts is the time at which the integrated value
 is just beginning to rise; on the other hand, if V1' is used, it is clear
 that a correct discrimination can be made at time ts. It will therefore be
 understood that if both the inflator activation/non-activation decision
 and the inflation mode determination were to be made based only on V1, it
 would take considerable time, making it difficult to make a proper
 decision or determination at proper time.
 FIG. 4 shows another embodiment of the present invention. The difference
 from FIG. 1 is that the integrated value difference Vd is differentiated
 in time (d(Vd)/dt) by a differentiator 30 to compute the amount of change,
 Gd, of Vd, based on which the inflator activation/non-activation decision
 is made; that is, the amount of change, Gd, of the difference is supplied
 to a comparator 34 where it is compared with a difference change threshold
 value Gs preset in a block 32 as a threshold value that varies as a
 function of time, and a decision whether to activate or not activate the
 air bag is made based on the result of the comparison. When the amount of
 change of the difference is equal to or larger than the difference change
 threshold value (Gd.gtoreq.Gs), it is determined that inflator activation
 is needed, and an inflator activation instruction signal is sent to the
 block 15; then, in accordance with the inflator activation mode signal
 (K=1 or 2) selected based on the result of the comparison between the
 integrated value difference Vd and the second velocity threshold value Vs2
 in the comparator 13, the block 15 directs the first inflator trigger
 circuit 21 and/or the second inflator trigger circuit 22 to output the
 trigger signal, as in the foregoing example. On the other hand, when the
 difference change amount is smaller than the difference change threshold
 value (Gd&lt;Gs), the comparator 14 compares Vd with the value preset at
 or near zero (0); if Vd is not larger than the preset value (for example,
 not larger than zero), the system is reset by the system reset circuit 20,
 and if Vd is larger than the preset value (for example, larger than zero),
 the computation in the computation circuit 6 is continued. This operation
 is also the same as in the foregoing example.
 The relationship between the value Gd and the difference change threshold
 value Gs as a time function will be described with reference to FIG. 14.
 FIG. 14 is a diagram showing the relationship between the change of
 amount, Gd, of the difference and time t in various types of collision. As
 shown, in the early stages of collision, the threshold value Gs is set at
 Th10, higher than the level of the deer collision, to prevent an erroneous
 activation of the air bag in the event of a collision with a deer or the
 like; the high threshold value Th10 is followed by a rightwardly falling
 steep threshold value Th11 leading to a low threshold value Th12 and,
 using the threshold value Th11, a severe collision event such as a
 high-speed frontal or high-speed oblique collision is discriminated in the
 early stages to issue an inflator activation command. A medium-speed
 centerpole collision can be detected in the early part of the low
 threshold value Th12 so that the medium-speed centerpole collision can
 also be detected in the early stages. The low threshold value Th12 in the
 remaining stage is used to decide whether to activate or not activate the
 air bag in a low-speed frontal collision, and is set at such a value that
 does not trigger air bag inflation in a collision at a speed lower than a
 predetermined speed.
 The time axis of FIG. 14 is also shown on the same scale as the time axis
 of the Vd-t diagram of FIG. 13. As can be seen from a comparison between
 the two figures, when the decision is made based on Gd, the air bag
 activation command can be issued at an earlier point in time, making it
 possible to quickly decide to activate the inflators in the event of a
 severe collision, and allowing a sufficient time margin from the time the
 activation/non-activation decision is made to the time the inflators are
 ignited. The advantage of this is that a complex computation can be
 performed for the determination of the air bag inflation mode. In the case
 of rough road driving, since the value of Gd remains very small, as in the
 case of the value of Vd, if the decision is made based on Gd, an erroneous
 activation of the air bag during rough road driving can be completely
 prevented, as in the case of using Vd.
 In the example of FIG. 4, the determination of the inflator activation mode
 is made by comparing Vd with the second velocity threshold value Vs2, but
 it will be appreciated that this can also be accomplished by comparing the
 second time-integrated value V1' with the third velocity threshold value
 Vs3 as a time function, as in the example of FIG. 3. Furthermore, in the
 example of FIG. 4, even when the inflator activation mode is determined to
 be the moderate inflation mode (K=1), the signal is immediately sent to
 the block 15, but it will be appreciated that the time comparator 8 and
 the time threshold value setting device 9 may be included, as shown in
 FIGS. 2 and 3, so that the determination of the moderate inflation is held
 off until the predetermined time ts elapses from the time t0' at which the
 computation based on the second acceleration sensor 2 was started.
 FIG. 5 is a block diagram showing another embodiment of the present
 invention, in which the inflator activation/non-activation decision
 process using Vd is added to the method of FIG. 4. That is, the integrated
 value difference Vd and its amount of change Gd are input to a comparator
 35 where they are compared with the first velocity threshold value Vs1 as
 a time function and the difference change threshold value Gs,
 respectively, and when either Vd.gtoreq.Vs1 or Gd.gtoreq.Gs or both are
 satisfied, it is determined that inflator activation is needed, and the
 inflator activation instruction signal is sent to the block 15; then, when
 the inflator activation mode signal (K=1 or 2), determined based on the
 result of the comparison between Vd and the second velocity threshold
 value Vs2 in the comparator 13 as the inflator activation mode determining
 circuit, is input to the block 15, trigger instruction signals are sent to
 the first and second inflator trigger circuits 21 and 22 in accordance
 with the specified activation mode. On the other hand, when the result of
 the comparison in the comparator 35 shows Vd&lt;Vs1, then Vd is compared
 with the preset value in the comparator 14 and, depending on the result of
 the comparison, it is determined whether to reset the system or to
 continue the computation, as in the case of the foregoing embodiment.
 The method that determines that inflator activation is needed when either
 Vd.gtoreq.Vs1 or Gd a Gs is satisfied is the same as the method of FIG. 3
 or 4, but this has the advantage of making various sensitivity settings
 possible. Further, the method that activates the inflators only when both
 conditions are satisfied has the effect of enhancing reliability because
 the decision is made doubly.
 FIG. 6 is a block diagram showing another embodiment of the present
 invention in which the inflator activation/non-activation decision is made
 using the first time-integrated value V1 based on the first acceleration
 sensor 1 in the passenger compartment, in addition to the integrated value
 difference Vd and the amount of change, Gd, of the integrated value shown
 in FIG. 5. That is, a comparator 36 not only compares Vd and Gd with their
 respective threshold values Vs1 and Gs, but also compares the first
 time-integrated value V1 with a fourth velocity threshold value Vs4 preset
 in a block 35 as a threshold value that varies as a function of time, and
 only when at least one of the conditions of Vd.gtoreq.Vs1 and Gd.gtoreq.Gs
 are satisfied, and the condition V1.gtoreq.Vs4 are satisfied, is an
 inflator activation permit signal sent to the block 15. When the
 activation mode signal (K=1 or 2) and the activation permit signal are
 input, the block 15 sends the trigger signal to the inflator circuits, as
 in the case of the foregoing embodiment. When Vd&lt;Vs1, the operation is
 the same as previously described, and therefore, the description will not
 be repeated here.
 The reason that the first time-integrated value V1 is also used when making
 the decision is that by setting the fourth velocity threshold value Vs4
 for the first time-integrated value V1 at a relatively low value, as shown
 in FIG. 12(A), the decision can, in effect, be made based on Vd and Gd
 and, at the same time, an erroneous activation due to Vd and Gd can be
 prevented.
 FIG. 7 is a block diagram showing another embodiment of the present
 invention, illustrating an alternative method of inflator
 activation/non-activation decision. In the examples of FIGS. 1 to 6, Vd
 was compared with the threshold value given as a time function, but the
 feature of the present embodiment is that Vd is compared with a velocity
 function threshold value defined as a function of the first
 time-integrated value V1. That is, in FIG. 7, the difference Vd between
 V1' and V1, obtained in the subtracting means 7, is fed to a comparator
 40, while the first time-integrated value V1 is fed to a block 41 which
 computes a fifth velocity threshold value Vs5 (=f(V1)) preset as a
 function of the first time-integrated value V1 and supplies the fifth
 velocity threshold value Vs5 to the comparator 40. The comparator 40
 compares the two values and, when the integrated value difference is equal
 to or larger than the fifth velocity threshold value (Vd.gtoreq.Vs5),
 sends an inflator trigger permit signal to the block 15. On the other
 hand, when the integrated value difference is smaller than the fifth
 velocity threshold value (Vd&lt;Vs5), Vd is supplied to the comparator 14
 where it is compared with a value preset at or near zero (0); when Vd&lt;0
 (or the preset value near zero), a signal is sent to the system reset
 circuit 20 to reset the system, but when Vd.gtoreq.0 (or the preset value
 near zero), the computation is continued, as in the case of the foregoing
 embodiment.
 The comparison between the value Vd and the velocity function threshold
 value Vs5 in the present embodiment will be described below. FIG. 15 is a
 diagram showing the relationship between Vd and V1 in various types of
 collision. As shown, the fifth velocity threshold value Vs5 as a velocity
 function is hyperbolic in shape; the curve segment a rising along the Vd
 axis is so set as to be able to discriminate a deer collision, while the
 minimum value of V1 is set equal to the level of the threshold value Vs4
 which is shown as a constant value in FIG. 12(A). On the other hand, the
 progressively decreasing curve b along the V1 axis is so set as to be able
 to discriminate a low-speed frontal collision. According to this
 discrimination method, the threshold value is given, not as a function of
 time, but as a function of the first time-integrated value V1 so that a
 stable discrimination result can be obtained independently of time.
 FIG. 8 is a block diagram showing another embodiment of the present
 invention, in which the inflator activation/non-activation decision is
 made based on the comparison between the amount of change, Gd, of the
 integrated value difference and the difference change threshold value Gs,
 in addition to the comparison between the integrated value difference Vd
 and the fifth velocity threshold value Vs5 as a velocity function shown in
 FIG. 7. That is, in FIG. 8, the integrated value difference Vd, the fifth
 velocity threshold value Vs5, the amount of change, Gd, of the difference,
 and the difference change threshold value Gs are input to a comparator 43
 where the respective comparisons are made, and when either one of the
 conditions, Vd.gtoreq.Vs5 or Gd a Gs, or both conditions are satisfied,
 the inflator trigger permit signal is sent to the block 15. When
 Vd&lt;Vs5, the system reset circuit 20 is activated to reset the system or
 the computation is continued, depending on the value of Vd at that time,
 as in the case of the foregoing embodiment.
 In this way, when the comparison between Gd and the difference change
 threshold value Gs as a time function is used in addition to the fifth
 velocity threshold value Vs5 given as a velocity function that has
 stability not dependent on time, the scope of the decision can be enlarged
 and reliability enhanced. On the other hand, if provisions are made to
 permit inflator activation only when both conditions are satisfied, since
 a severe collision event such as a high-speed frontal or high-speed
 oblique collision can be discriminated in the early stages of collision by
 comparing Gd with its difference change threshold value Gs, as shown in
 FIG. 14, a collision discrimination with further enhanced reliability
 becomes possible since the capability to discriminate the severity of the
 collision in early stages is combined with the reliability achieved by the
 use of the velocity function threshold value.
 FIG. 9 is a block diagram showing another embodiment of the present
 invention, illustrating still another example of inflator
 activation/non-activation decision, in which the second time-integrated
 value V1' is compared with a sixth velocity threshold value Vs6=(V1)) set
 as a function of the first time-integrated value V1, rather than comparing
 the integrated value difference Vd with the fifth velocity threshold value
 Vs5 as a time function shown in FIG. 8. That is, the first time-integrated
 value V1 obtained by time integration in the time-integrating means 5 is
 sent to a block 44 which then computes the sixth velocity threshold value
 Vs6 preset as a function of the first time-integrated value V1, and
 supplies it to a comparator 45. The comparator 45 is also supplied with
 the second time-integrated value V1' from the integrator 5', the amount of
 change, Gd, of the difference from the block 30, and the difference change
 threshold value Gs from the block 32, and compares the second
 time-integrated value V1' with the sixth threshold value Vs6 set as a
 velocity function and the amount of change, Gd, of the integrated value
 difference with the threshold value Gs as a time function of Gd, and when
 either of the two conditions, Gd.gtoreq.Gs or V1.gtoreq.Vs6, or both
 conditions are satisfied, the inflator activation permit signal is sent to
 the block 15. This also means that the decision whether to activate or not
 activate the inflators can be made based only on the comparison between
 V1' and Vs6, but the present embodiment shows the case where the decision
 is made based on two comparisons, that is, one between Gd and Gs and the
 other between V1' and Vs6.
 As the result of the comparison in the comparator 45, if V1'&lt;Vs6, the
 value of V1' is sent to a comparator 46 where it is compared with the
 value preset at or near zero (0); if V1'&lt;0 (or preset value near zero),
 a signal is sent to the system reset circuit 20 to reset the system, while
 if V1'.gtoreq.0 (or preset value near zero), the computation is continued.
 That is, in FIGS. 1 to 8, the decision whether to reset the system or to
 continue the computation was made based on the value of Vd at that time,
 but in the present embodiment, the decision is made based on the value of
 V1'. Whether to use Vd or V1' is at the designer's discretion, and
 whichever easier in system design should be chosen. This means that the
 decision whether to reset the system or to continue the computation can
 also be made based on the value of Gd at that time.
 Next, the relationship between the value V1' and the threshold value Vs6 as
 a velocity function of V1' in the present embodiment will be described
 with reference to FIG. 16. FIG. 16 is a diagram showing the relationship
 between V1' and V1 in various types of collision. In the figure, the
 dashed line drawn at 45 degrees to the axes represents V1'=V1, and in each
 type of collision, the condition V1'=V1 is eventually reached. As can also
 be seen from FIGS. 12(A) and 12(B), in every collision type, V1' shows a
 higher value than V1 from the instant of collision, and has the
 characteristic of approaching V1 as the time elapses; accordingly, all the
 lines are located above the 45-degree line. Also, the sixth velocity
 threshold value Vs6 set as a function of V1 is shown in the form of a
 hyperbolic function of V1 located between the 45-degree line and the V1'
 axis, and the smallest V1 value on the curve c along the V1' axis is set
 at a value approximately equal to the level of the threshold value Vs4
 shown as a constant value in FIG. 12(A) so that a deer collision can
 likewise be discriminated. On the other hand, the curve segment d along
 the 45-degree line is so set as to be able to discriminate a low-speed
 collision. In this case also, since the threshold value is given, not as a
 function of time, but as a velocity function of the first time-integrated
 value V, a stable discrimination independent of time can be achieved, as
 in the case of the foregoing embodiment that uses the fifth velocity
 threshold value Vs5.
 In the case of FIG. 9, as in the case of FIG. 8, since the discrimination
 based on the comparison between Gd and the difference change threshold
 value Gs as a time function is used in combination with the discrimination
 based on the threshold value Vs6 as a velocity function having stability
 not dependent on the time, the effect is that the scope of the
 discrimination is enlarged, enhancing the discrimination capability. On
 the other hand, if provisions are made to issue activation permit signals
 to the inflators only when both conditions are satisfied, not only can a
 severe collision event such as a high-speed frontal or high-speed oblique
 collision be discriminated in the early stages of collision, but the
 effect of further enhancing the reliability of collision discrimination
 can be expected since the capability to discriminate the severity of the
 collision in early stages is combined with the reliability achieved by the
 use of the velocity function threshold value.
 FIG. 10 is a block diagram showing another embodiment of the present
 invention, illustrating yet another example of inflator
 activation/non-activation and inflator activation mode determination. In
 the examples shown in FIGS. 1 to 9, once the trigger command is issued
 from the block 15 to the inflator trigger circuits 21 and 22 in accordance
 with the inflator activation mode (K=1 or 2), the activation mode thus
 determined can no longer be changed. On the other hand, if too much
 emphasis is placed on the optimization of the activation mode, there
 arises the possibility of missing the inflator activation timing. In view
 of this, in the present embodiment, during the period when the moderate
 inflation signal K=1 is output as the result of the inflator activation
 mode determination, if it is determined that inflator activation is needed
 as the result of the inflator activation/non-activation decision, the
 first inflator is immediately activated while allowing the computation to
 continue for the inflator activation mode determination; then, if the
 activation mode determination changes to the rapid inflation mode as the
 result of the computation, the second inflator is immediately activated.
 More specifically, in FIG. 10, when Vd&lt;Vs2 as the result of the
 comparison between the integrated value difference Vd and the second
 velocity threshold value Vs2 in the block 13, the determination of the
 moderate inflation mode is held off until the predetermined time ts1
 elapses, and at the end of the predetermined time ts1, the block 18
 outputs the moderate inflation signal K=1 which is sent to the block 15.
 When t&lt;ts1, the block 24 sends the K value undetermined signal (K=0) to
 the block 15 which, when K=0 is input, allows the computation to continue,
 as in the case of FIGS. 2 and 3. On the other hand, in the moderate
 inflation mode, that is, when the moderate inflation signal K=1 is being
 applied to the block 15, the inflator activation/non-activation decision
 system operates as follows: first, a block 51 checks the activation status
 of the first inflator, and if the first inflator is in a non-activated
 state, the block 10 compares the integrated value difference Vd with the
 first velocity threshold value Vs1 to decide whether to activate or not
 activate the inflator; if it is determined that there is a need to
 activate the inflator, the resultant signal is sent to the block 15. Since
 the inflator activation mode is the moderate inflation mode, only the
 first inflator is activated in accordance with the moderate inflation
 signal, while allowing the computation to continue for the inflator
 activation mode determination. As the result of the computation, if
 Vd.gtoreq.Vs2 in the block 13, the block 19 outputs the rapid inflation
 signal (K=2), and a block 50 checks the activation status of the first
 inflator; here, since the first inflator is already activated, the second
 inflator is immediately activated.
 On the other hand, if Vd.gtoreq.Vs2 before activating the first inflator,
 and the rapid inflation signal K=2 is output from the block 19, then since
 the activation status of the first inflator checked in the block 50 is
 "not activated", the rapid inflation signal K=2 is sent to the block 15 in
 the same manner as described above; also, in the inflator
 activation/non-activation decision making circuit, the activation status
 of the first inflator checked in the block 51 is "not activated", and
 therefore, when Vd.gtoreq.Vs1 as the result of the comparison between the
 integrated value difference Vd and the first velocity threshold value Vs1
 in the block 10, the inflator activation signal is sent to the block 15 so
 that both the first and second inflators are activated in accordance with
 the already issued rapid inflation signal in the same manner as earlier
 described.
 As is apparent from the above description, in the methods of FIGS. 1 to 9,
 the various threshold values must be chosen so that the determination of
 the inflator activation mode is completed before the decision is made as
 to whether to activate or not activate the inflators, but it is difficult
 to satisfy all such conditions for all types of vehicle body structure and
 all types of collision. Considering that unpredictable situations can
 happen, it can be said that the method of the present embodiment that
 waits the decision to switch or not switch to the rapid inflation mode
 until the last moment after activating the inflator for moderate
 inflation, not fixing the inflator activation mode once determined, is a
 versatile method that can be applied to extensive types of body structure
 and a variety of collision types.
 FIG. 11 is a block diagram showing a modified example of the embodiment of
 FIG. 10. Differences from FIG. 10 are that the inflator
 activation/non-activation decision is made by comparing the amount of
 change, Gd, of the integrated value difference with its threshold value
 Gs, and that the blocks 8 and 9 are omitted that were used in FIG. 10 to
 hold off the determination of the moderate inflation mode until the
 predetermined time elapses after the start of the computation based on the
 second acceleration sensor. In other respects, the configuration is the
 same as that of FIG. 10; that is, as in the case of FIG. 10, when it is
 determined that there is a need to activate the inflator in the moderate
 inflation mode, the first inflator is immediately activated while allowing
 the computation to continue for the inflator activation mode
 determination, and when the inflator activation mode switches to the rapid
 inflation mode as the result of the computation, the second inflator is
 immediately activated.
 As described so far, in the above-described embodiments, the second
 acceleration sensor is mounted in the crush zone to supplement the first
 acceleration sensor mounted, as in a conventional system, in the passenger
 compartment, the most notable feature being that the decision to activate
 or not activate the inflators and the determination of the inflator
 activation mode are made based on the differences in characteristics among
 the integrated values, the difference between the integrated values, the
 amount of change of the integrated value difference, etc. Arising from the
 differences between the acceleration values detected by the respective
 sensors in various types of collision. Specific implementations of the
 method have been described with reference to FIGS. 1 to 11, but it will be
 appreciated that the present invention is not limited to the illustrated
 examples, but that various other modifications are possible without
 departing from the spirit and scope of the present invention.
 For example, the first to fourth velocity threshold values Vs1 to Vs4, each
 provided as a threshold value varying as a function of time, may include a
 constant-value threshold value as a special case of the time function, and
 the difference change threshold value Gs, also given as a function of
 time, may likewise include a constant-value threshold value. Furthermore,
 the time function threshold values Vs3, Vs1, and Gs, given in FIGS. 12(B),
 13, and 14 and each shown as consisting of three straight line segments,
 can each be represented by a curved line varying as a function of time;
 conversely, the velocity threshold values Vs5 and Vs6 given in FIGS. 15
 and 16, each shown as a curve varying as a function of the first
 time-integrated value V1, can each be replaced by a combination of
 straight lines representing the function of V1.
 In the examples of FIGS. 4 to 11, the inflator activation mode is
 determined by comparing Vd with the threshold value Vs2 as a time function
 of Vd, but this can also be accomplished by comparing the second
 time-integrated value V1' with the third velocity threshold value Vs3, a
 time function of V1', as practiced in the example of FIG. 3. Further, in
 the examples of FIGS. 4 to 9 and 11, when the result of the inflator
 activation mode determination shows moderate inflation (K=1), the
 corresponding signal is immediately sent to the block 15, but instead, as
 shown in FIGS. 2, 3, and 10, provisions may be made so that, when the
 result of the inflator activation mode determination satisfies the
 moderate inflation condition, the decision is held off and the computation
 is continued until the predetermined time ts1 elapses from the starting
 time t0' of the computation based on the second acceleration sensor 2, and
 upon the expiration of the predetermined time the moderate inflation
 signal (K=1) is output, thereby enhancing the reliability of the inflator
 activation mode determination. Furthermore, rather than arranging this
 time-judging circuit between the inflator activation mode determining
 means 13 or 17 and the moderate inflation setting device 18, as shown in
 FIGS. 2, 3, and 10, the time-judging circuit may be arranged, for example,
 between the K value discriminating circuit 15 and the first inflator
 trigger circuit 21 so that the output of the first inflator trigger signal
 for moderate inflation is held off until the predetermined time elapses.
 The only requirement here is that the inflator activation by the moderate
 inflation signal be held off until the predetermined time ts elapses, and
 the same effect can be achieved as long as this requirement is satisfied.
 Next, the second method of the present invention will be described. In this
 method, the determination of the air bag inflation mode and the decision
 whether to activate or not activate the inflators are made based on the
 differences in characteristics between the first time-integrated value V1
 and second time-integrated value V1' obtained by performing time
 integration based on the acceleration signals supplied from the first
 acceleration sensor 1 and second acceleration sensor 2. FIG. 17 shows a
 typical block diagram, in which the same constituent elements as those in
 FIGS. 1 to 11 are designated by the same reference numerals, and detailed
 descriptions of such elements will not be repeated here. To describe the
 computation circuit 6 shown here, the block 3 detects the time t0 at which
 the acceleration value G detected by the first acceleration sensor 1
 mounted inside the passenger compartment exceeds the predetermined
 acceleration G1, and the computation for collision discrimination is
 started at this time t0. A block 4a is a peak-cut means which computes
 acceleration G3 greater than predetermined acceleration G2 by cutting any
 acceleration values G after the computation starting time t0 that are not
 greater than G2 (acceleration not greater than G2 is assumed to be G2).
 Next, a time integrating means 5a integrates the acceleration G3 over time
 to compute the time-integrated value V, and the velocity subtracting means
 5b that follows subtracts a predetermined velocity change value .DELTA.V
 per unit time from the time-integrated value V, as needed, to compute the
 first time-integrated value V1 as a decreased integrated value. The
 velocity change value .DELTA.V may be a constant value or a value of a
 time function.
 Here, the peak-cut means in the block 4a and the velocity subtracting means
 in the block 5b provide means for distinctly discriminating between a
 high-speed oblique collision and a low-speed frontal collision, as
 described in detail in Japanese Patent No. 2543839 and Unexamined Patent
 Publication No. 4-321455, and correspond to the offsetting means in the
 block 4 shown in FIGS. 1 to 11. Accordingly, the blocks 3 to 5b in FIG. 17
 provide the same effect as that of the blocks 3 to 5 in FIGS. 1 to 11, and
 it will therefore be recognized that either configuration can be used.
 On the other hand, the acceleration signal G' detected by the second
 acceleration sensor 2 mounted in the crush zone is sent to the block 3'
 which detects the time t0' at which the acceleration signal G' exceeds the
 predetermined acceleration G1', upon which the computation for the air bag
 inflation control is started. A block 4a' is a peak-cut means which
 computes acceleration G3' greater than predetermined acceleration G2' by
 cutting any acceleration value values G' after the time t0' that are not
 greater than G2' (acceleration not greater than G2' is assumed to be G2').
 Next, a time integrating means 5a' integrates the acceleration G3' over
 time to compute the time-integrated value V', and the velocity subtracting
 means 5b' that follows subtracts a predetermined velocity change value
 .DELTA.V' per unit time from the time-integrated value V', as needed, to
 compute the second time-integrated value V1' as a decreased integrated
 value. The velocity change value .DELTA.V' may be a constant value or a
 value of a time function.
 Here, the peak-cut means in the block 4a' and the velocity subtracting
 means in the block 5b' correspond to the offsetting means in the block 4'
 shown in FIGS. 1 to 11, as noted above. Accordingly, the blocks 3' to Sb'
 in FIG. 17 provide the same effect as that of the blocks 3' to 5' in FIGS.
 1 to 11, and either configuration may be used, as described above.
 The differences in characteristics between the first time-integrated value
 V1 based on the acceleration signal G from the first acceleration sensor 1
 and the second time-integrated value V1' based on the acceleration signal
 G' from the second acceleration sensor 2 are as previously described with
 reference to FIG. 12; the same description will not be repeated here, but
 the differences can be summarized as follows:
 (a) In a collision event such as a high-speed frontal or high-speed oblique
 collision or a medium-speed centerpole collision, that requires air bag
 inflation, the waveform of the second time-integrated value V1' rises
 quickly in the early stages of collision, compared with the waveform of
 the first time-integrated value V1.
 (b) Of the collision types that do not require air bag inflation, the
 low-speed frontal collision is low in severity, causing deformation only
 to the bumper at the front end of the vehicle; as a result, there occurs
 no appreciable difference between the two waveforms.
 (c) In the case of rough road driving, since no deformation is caused to
 the crush zone, the two waveforms are substantially the same.
 (d) In the case of a deer collision, the crush zone suffers minor
 deformation, but since the vehicle hits the animal away from it at the
 instant of the collision, the deformation lasts only briefly; as a result,
 while the second time-integrated value V1' exhibits a large value
 momentarily, the first time-integrated value V1 shows only a slight
 variation.
 In view of the above-described characteristics of the first and second
 time-integrated values V1 and V1', if provisions are made to select the
 inflation mode of the air bag system by discriminating the severity of the
 collision in the very early stages of collision based on the second
 time-integrated value V1' that is sensitive to the severity of the
 collision, and to decide whether to inflate or not inflate the air bag
 based on the first time-integrated value V1 that shows the condition
 within the passenger compartment, then the degree of the collision
 severity can be determined in the early stages of collision, allowing a
 time margin for the subsequent computation of the air bag inflation mode;
 since the computation of the inflation mode is completed by the time it is
 determined that air bag inflation is needed, the air bag can be
 immediately inflated without any time delay due to the computation for the
 inflation mode control.
 Next, a detailed description of the computation will be given with
 reference to FIG. 17. First, the second time-integrated value V1' (either
 the time-integrated value V' before the subtraction or the decreased
 integrated value V1' obtained by subtracting .DELTA.V' from it may be
 used, but in the following description, the value is represented by V1'
 unless otherwise stated) is fed to a comparator 60 where the value is
 compared with an eighth velocity threshold value Vs8 preset in a block 61
 as a time function representing collision severity, and when the second
 time-integrated value V1' is smaller than the eighth velocity threshold
 value Vs8 (V1'&lt;Vs8), it is determined that the collision is not so
 severe, and the moderate inflation signal (K=1) for inflating the air bag
 at a moderate speed is output from the block 18; on the other hand, when
 the second time-integrated value V1' is equal to or larger than the eighth
 velocity threshold value Vs8 (V1'.gtoreq.Vs8), it is determined that the
 collision is severe, and the rapid inflation signal (K=2) for rapidly
 inflating the air bag is output from the block 19. The eighth velocity
 threshold value Vs8 may be set as a constant value, or may be given as a
 time function Vs8(t), like Vs3 shown in FIG. 12(b), in which case the same
 value as Vs3 may be used.
 Next, the first time-integrated value V1 (either the time-integrated value
 V before the subtraction or the decreased integrated value V1 obtained by
 subtracting .DELTA.V from it may be used, but in the following
 description, the value is represented by V1 unless otherwise stated) is
 compared in a block 62 with a seventh threshold value Vs7 given as a time
 function, and when V1&lt;Vs7, the resultant signal is sent to the
 comparator 46; if V1 is equal to or smaller than the value preset at or
 near zero (V1.gtoreq.0), a signal is sent to the system reset circuit 20
 to stop the computation, otherwise (V1&gt;0) the computation is continued.
 On the other hand, when V1.gtoreq.Vs7, it is determined that air bag
 inflation is needed, and the signal is sent to the block 15 which, based
 on the air bag inflation mode determining signal (K=1 or K=2) input to it
 from the block 18 or 19, issues a trigger signal to the inflator(s). More
 specifically, when the air bag inflation mode is the moderate inflation
 mode (K=1), the trigger signal is sent from the first inflator trigger
 circuit 21 to the first inflator to ignite only the first inflator,
 thereby inflating the air bag 23 at a moderate speed. On the other hand,
 in the case of the rapid inflation mode (K=2), the trigger signal is
 issued to both the first inflator trigger circuit 21 and the second
 inflator trigger circuit 22 to ignite the two inflators together, thereby
 rapidly inflating the air bag 23. Here, the block 15 is configured to
 issue the trigger signal to the inflator trigger circuit(s) 21, 22 only
 when both the inflator activation signal from the comparator 62 and the
 inflator activation mode signal from the block 18 or 19 are input, and not
 to issue the trigger signal when only one or the other of the signals is
 input. This arrangement is the same as that in the previously described
 configuration.
 In the moderate inflation mode (K=1) described above, it is also possible
 to activate both the first and second inflators by displacing the ignition
 timing between them; in this case, in the rapid inflation mode (K=2), the
 two inflators are activated either simultaneously or by displacing the
 ignition timing only slightly. One way to ignite the inflators by
 displacing the ignition timing is to preset ignition timings appropriate
 to the slow and rapid inflation modes, respectively, and to ignite the
 inflators with the preset timing difference. Alternatively, the ignition
 timing difference may be determined based on the time, after the starting
 of the computation, at which the first time-integrated value V1 exceeds
 the seventh velocity threshold value Vs7, the time function, when V1 is
 compared with Vs7 in the block 62; that is, when V1 exceeded Vs7 in the
 very early stages of collision, this means a severe collision and,
 therefore, the ignition timing difference between the inflators is made
 small, and when V1 exceeded Vs7 in relatively early stages after that, the
 inflators are ignited with a slightly larger time difference, for example,
 with a time difference of a few milliseconds.
 FIG. 18 shows another embodiment of the present invention, wherein the same
 constituent elements as those in FIG. 17 are designated by the reference
 numerals. In the configuration shown here, the second time-integrated
 value V1' obtained in the subtracting means 5b' is input to an inflator
 trigger determination computing circuit 70 which, based on the second
 time-integrated value V1', makes a first decision as to whether to
 activate or not activate the inflators; then, based on the result of this
 decision, the value of the seventh velocity threshold value Vs7 in the
 block 71 is varied, and this varied seventh velocity threshold value Vs7
 is input to the comparator 62 for comparison with the first
 time-integrated value V1.
 More specifically, since the crush zone is the first part that is damaged
 in the event of a collision, the acceleration detected by the second
 acceleration sensor 2 settles down earlier than the acceleration detected
 by the first acceleration sensor 1 mounted, as in a conventional system,
 inside the passenger compartment, as can be seen from FIG. 12. Therefore,
 if the decision as to whether to activate or not activate the inflators is
 made based on the acceleration signal from the second acceleration sensor
 2, the activation/non-activation decision can be made earlier than would
 be possible if the decision were made based on the signal from the first
 acceleration sensor mounted, as in a conventional system, inside the
 passenger compartment. In view of this, in the present embodiment, the
 first decision as to whether to activate or not activate the air bag
 system is made in the block 70 by using the second time-integrated value
 V1' computed based on the change of the acceleration detected by the
 second acceleration sensor 2. Various systems (algorithms) proposed in the
 art and implemented in practice for making such decisions using an
 acceleration signal from a passenger compartment acceleration sensor can
 be used for the decision making circuit in the block 70; though not
 specifically limited, it is preferable to use the algorithm previously
 proposed by the applicant of the present invention and implemented in
 practice (the algorithm described, for example, in Japanese Patent No.
 2543839 and Unexamined Patent Publication No. 3-253441).
 Next, when it is determined that inflator activation is needed as the
 result of the first decision in the block 70, the value of the seventh
 velocity threshold value Vs7 to be input to the comparator 62 is set at a
 relatively low value in the block 71, but when it is determined that
 inflator activation is not needed, on the other hand, the value of the
 seventh velocity threshold value Vs7 is set at a relatively high value;
 further, if, in the block 70, it is determined in early stages that
 inflator activation is needed, the value of Vs7 is set at an extremely low
 value. In this way, the seventh velocity threshold value Vs7 is set as a
 value that varies as a function of the second time-integrated value V1',
 that is, Vs7=f(V1').
 The comparator 62 compares the first time-integrated value V1 with the
 seventh velocity threshold value Vs7, and when V1.gtoreq.Vs7, the first
 inflator and/or the second inflator are ignited in accordance with the K
 value supplied as a slow/rapid inflation indicator to the block 15, as in
 the case of FIG. 17.
 FIG. 19 shows a modified example of FIG. 17, wherein the same constituent
 elements as those in FIG. 17 are designated by the same reference numerals
 and will not be described in detail here. In the case of FIG. 17, since
 the second time-integrated value V1' is constantly compared with the eight
 velocity threshold value Vs8 used to judge the severity of the collision,
 in the case of a high-speed oblique collision the degree of the collision
 severity is determined in the very early stages of the collision, as can
 be seen from FIG. 12(b). In the case of a very high-speed collision, this
 means that the determination is made at a much earlier point in time, and
 depending on the situation, the result of the determination may lack
 stability. Furthermore, in situations where the second acceleration sensor
 2 is hit directly by a thin pole or projection in a low-speed collision,
 or where the vehicle body on which the acceleration sensor 2 is mounted
 hits the ground during rough road driving, exerting a large acceleration
 to the sensor 2, there occurs the possibility that the situation may be
 judged as being a severe collision when it is actually a collision of
 minor severity that does not require air bag inflation.
 FIG. 19 shows the method that solves the above problems. In the computation
 circuit for performing the computation based on the acceleration signal
 from the second acceleration sensor 2, before the inflator activation mode
 is determined in the block 60 based on the second time-integrated value
 V1', the elapsed time t' from the starting of the computation is compared
 in a time comparator 65 with a second time threshold value ts2preset in a
 block 64, and when t'&lt;ts2, the inflator activation mode determination
 by the comparator 60 is held off. That is, even when the computation based
 on the acceleration sensor 2 is started, the signal for the inflator
 activation mode determination is not sent to the comparator 60 but the
 computation is continued until the predetermined second time threshold
 value ts2 is reached. Since the inflator activation mode determination is
 not made in the very early stages of collision, this arrangement serves to
 prevent a collision that causes an input of a large abrupt acceleration,
 as described above, or an impact applied during rough road driving, from
 being erroneously judged as being a severe collision, and thus the
 stability of the collision severity discrimination is increased.
 On the other hand, when it is determined in the time comparator 65 that
 t'.gtoreq.ts2 (predetermined time has elapsed), the second time-integrated
 value V1' is sent to the comparator 60 for comparison with the eighth
 velocity threshold value Vs8; here, the slow or rapid inflation K value
 may be set in the block 18 or 19 based on the result of the comparison, as
 in the case of FIG. 17, but in the present embodiment, an additional step
 is included in the activation mode determination process. That is, when
 V1'&lt;Vs8, the result is sent to the block 18 to set the air bag
 inflation mode signal to moderate inflation (K=1), as in the foregoing
 example, but when V1'.gtoreq.Vs8, the result is sent to a comparator 66
 where the first time-integrated value V1 is further compared with a 10th
 velocity threshold value Vs10 preset in a block 79 as a time function,
 based on which result the activation mode is determined. When V1&lt;Vs10
 in the comparator 66, the event is determined to be a relatively minor
 collision, and the result is sent to the block 18 to set the air bag
 inflation mode signal to moderate inflation (K=1), but when
 V1.gtoreq.Vs10, the result is sent to the block 19 to set the air bag
 inflation mode signal to rapid inflation (K=2). This arrangement prevents
 the rapid air bag inflation mode from being selected due to a localized
 impact to the crush zone, thus minimizing the risk of occupant injury due
 to a rapid inflation of the air bag. The second time threshold value ts2
 may be set at the same value as the first time threshold value ts1
 previously explained in the examples of FIGS. 2, 3, and 10.
 Next, in the comparator 62, the first time-integrated value V1 is compared
 with the seventh velocity threshold value Vs7 to decide whether to
 activate or not activate the inflators, as in the foregoing example, and
 when it is determined that inflator activation is needed (V1.gtoreq.Vs7),
 the inflator activation signal is sent to the block 15 which, when both
 the inflator activation signal and the inflator activation mode signal
 from the block 18 or 19 are input, issues the trigger signal to the
 inflator trigger circuit(s) 21, 22, as in the foregoing example.
 In this way, the inflator activation mode is determined using the second
 time-integrated value V1' in conjunction with the first time-integrated
 value V1; this prevents an erroneous determination from being made in such
 situations as bottom hitting over a rough road that causes a large
 acceleration change only in the second time-integrated value or a
 low-speed collision against a thin pole that causes a localized
 deformation to the acceleration sensor mounting part. The effect of this
 is improved stability in the inflator activation mode determination.
 FIG. 20 shows a modification of the example of FIG. 19, wherein the same
 constituent elements as those in FIG. 19 are designated by the same
 reference numerals and will not be described in detail here. In the
 illustrated example, provisions are made to prevent a delay in inflator
 ignition timing in an extremely severe collision; as shown, the second
 time comparator 65 is preceded by a comparator 67 which compares the
 second time-integrated value V1 with an 11th velocity threshold value Vs11
 preset in a block 68 as a function of time, and when the second
 time-integrated value V1' is equal to or larger than the 11th velocity
 threshold value Vs11 (V1'.gtoreq.Vs11), the result is sent to the block 19
 which thereupon issues a rapid air bag inflation signal (K=2). That is,
 when the second integrated-time value V1' shows an extremely large value,
 a rapid inflation command is issued, overriding the holding period during
 which the determination of the degree of collision severity by the time
 comparator 65 is on hold. The 11th velocity threshold value Vs11 in the
 block 68 is, therefore, set at a large value sufficient to prevent an
 erroneous activation.
 FIG. 21 shows a new embodiment wherein the concept of the time threshold
 value comparison shown in FIGS. 19 and 20 is applied to the method of FIG.
 17. This embodiment is the same as the embodiment of FIG. 17 in that the
 second time-integrated value V1' is compared in the block 60 with the
 eighth velocity threshold value Vs8 and, when it is equal to or greater
 than the threshold value, i.e., V1'.gtoreq.Vs8, it is determined that the
 situation requires a rapid inflation of the air bag, and the rapid
 inflation signal (K=2) is sent from the block 19 to the block 15, but
 differs in that when the second time-integrated value V1' is smaller than
 the threshold value (V1'&lt;Vs8), the elapsed time t' from the starting of
 the computation based on the second acceleration sensor 2 is compared in
 the block 65 with the second time threshold value ts2; furthermore, when
 the elapsed time t' is equal to or larger than the second time threshold
 value ts2 (t' 2 ts2), the mode signal is set to moderate inflation, and
 the signal (K=1) is sent to the block 15, but when t'&lt;ts2, the mode
 signal is set to activation mode undetermined, and the mode undetermined
 signal K=0 is sent from a block 69 to the block 15.
 In the block 15, when the activation mode undetermined signal K=0 is input,
 the computation is allowed to continue, regardless of the presence or
 absence of the inflator activation/non-activation decision signal from the
 block 62. On the other hand, when the result of the inflator
 activation/non-activation decision in the block 62 shows the first
 time-integrated value V1.gtoreq.the seventh velocity threshold value Vs7,
 and when the signal K=1 or 2 is input to the block 15, the inflator(s) are
 activated in accordance with the designated activation mode, as in the
 case of the foregoing example.
 In this way, in determining the inflator activation mode, determination of
 only the rapid inflation can be effected before the prescribed time ts2
 elapses from the starting time of the computation, but determination of
 the moderate inflation is held off and the computation is continued during
 that period. This prevents the moderate inflation signal from being
 determined by making a decision too early in the early stages of
 collision, and further improves the occupant protection performance of the
 air bag.
 FIG. 22 shows still another embodiment of the present invention. In the
 methods of FIGS. 17 to 21, the inflator activation/non-activation decision
 is made based on the first time-integrated value V1, but the method of
 FIG. 22 significantly differs in that the second time-integrated value V1'
 is also used for the inflator activation/non-activation decision.
 More specifically, the illustrated method is the same in that the inflator
 inflation mode is determined by comparing the second time-integrated value
 V1' with the eighth velocity threshold value Vs8, but fundamentally
 differs in that the second time-integrated value V1' is also sent to a
 comparator 72 where it is compared with a ninth velocity threshold value
 Vs9 supplied from a block 73, to determine whether to activate or not
 activate the inflator(s). Particularly, this ninth velocity threshold
 value Vs9 is set as a function of the first time-integrated value V1
 (Vs9=f(V1)), and when the second time-integrated value is equal to or
 larger than the threshold value (V1'&gt;Vs9), it is determined that
 inflator activation is needed, and the resultant signal is sent to the
 block 15. If the block 15 is supplied at this time with the activation
 mode signal (K=1 or K=2) from the inflator activation mode setting device
 18 or 19, the block 15 sends a trigger signal to the first trigger circuit
 21 and/or the second trigger circuit 22 according to the specified
 activation mode. On the other hand, when the second time-integrated value
 is smaller than the ninth velocity threshold value (V1'&lt;Vs9), the
 resultant signal is sent to a comparator 74; if the second time-integrated
 value V1' is smaller than the value preset at or near zero (0), a signal
 is sent to the system reset circuit to reset the system, but if V1 is
 equal to or larger than the preset value, the computation is continued.
 The relationship between the ninth velocity threshold value Vs9 (V1) used
 in the present embodiment and the second time-integrated value V1' will be
 described with reference to the previously given FIG. 16. Like the
 previously described sixth velocity threshold value Vs6(V1) shown in the
 same figure, the ninth velocity threshold value Vs9(V1) as a function of
 V1 is set as a hyperbolic function of V1 located between the 45-degree
 line and the V1, axis, and its value is also set at approximately the same
 level as the sixth velocity threshold value Vs6; that is, the smallest V1
 value on the curve c along the V1' axis is set as a value slightly higher
 than the level of the deer collision shown in FIG. 12(A), while the curve
 d along the 45-degree line is so set as to be able to discriminate the
 low-speed frontal collision. In this way, by setting the threshold value
 as a function of the first time-integrated value V1, not as a function to
 time, stable discrimination not dependent on time can be expected.
 FIG. 23 shows a modification of the example of FIG. 19, wherein the same
 constituent elements as those in FIG. 19 are designated by the same
 reference numerals and will not be described in detail here. In the
 embodiments shown in FIGS. 17, 18, and 22, once the trigger command is
 issued from the block 15 to the inflator trigger circuits 21 and 22 in
 accordance with the inflator activation mode (K=1 or 2), the activation
 mode thus determined can no longer be changed. On the other hand, if too
 much emphasis is placed on the optimization of the activation mode, there
 arises the possibility of missing the inflator activation timing. In view
 of this, in FIGS. 19 and 20, determination of the inflator activation mode
 is held off during the predetermined period of time, while in FIG. 21,
 determination only of the moderate inflation is held off during the
 predetermined period of time; by contrast, in the present embodiment, when
 the result of the inflator activation/non-activation decision in the block
 62 shows "activation needed", first only the first inflator is activated
 based on the moderate inflation command and, thereafter, if the inflator
 activation mode determination in the block 60 changes to the rapid
 inflation mode (K=2), the second inflator is activated at that instant in
 time.
 More specifically, in FIG. 23, in the early stages of computation
 immediately after the collision, since the value of the second
 time-integrated value V1' has not yet accumulated to the point that
 exceeds the threshold value, the result of the inflator activation mode
 determination in the block 60 is V1'&lt;Vs8, and the block 15 is therefore
 supplied with the moderate inflation signal K=1 from the block 18. On the
 other hand, the value of the first time-integrated value V1 is sent to a
 block 75 which then checks whether the first inflator is activated or not;
 if the first inflator is not activated, the block 62 compares V1 with the
 seventh velocity threshold value Vs7 to make a decision as to whether to
 activate or not activate the inflator. In the early stages of the
 computation, since the first time-integrated value V1 is also in the
 process of accumulating by time integration, the value is still below the
 threshold value Vs7, therefore, it is determined that V1&lt;Vs7, and the
 signal is sent to the block 46; here, if V1.gtoreq.0, the system is reset
 to stop the computation, as previously described, but when V1&gt;0, the
 computation is continued. As the computation continues, the first
 time-integrated value V1 increases with time, and when V1.gtoreq.Vs7, the
 inflator activation signal is sent to the block 15. By this time, the
 moderate inflation signal (K=1) has been input to the block 15 from the
 block 18; therefore, the block 15 instructs the first inflator trigger
 circuit 21 to ignite the first inflator which is thus ignited, and the air
 bag 23 starts to inflate at a moderate speed with a relatively small
 amount of gas released from the first inflator.
 While the trigger signal is sent from the block 15 to the first inflator
 trigger circuit 21, the computation is further continued, and as a result,
 the second time-integrated value V1' increases as the time integration
 progresses; when the value reaches or exceeds the eighth velocity
 threshold value (V1'.gtoreq.Vs8), the inflator activation mode signal
 switches to the rapid inflation signal K=2 which is output from the block
 19 and sent to a block 76 which then checks whether the first inflator is
 activated or not. At this time, since the first inflator is already
 activated, as described above, the block 76 immediately sends a trigger
 command signal to the second inflator trigger circuit 22 to ignite the
 second inflator, and the air bag is thus rapidly inflated with a large
 amount of gas, combining the gas released from the first inflator with the
 gas released from the second inflator.
 On the other hand, if the block 60 determines that the inflator activation
 mode should be set as the rapid inflation mode (K=2) before the block 62
 determines that inflator activation is needed, since the first inflator
 activation status checked by the block 76 is "not activated", the signal
 (K=2) is sent to the block 15, and thereafter, when the block 62
 determines that inflator activation is needed, the block 15 sends the
 trigger signal to both of the inflator trigger circuits 21 and 22 in
 accordance with the rapid inflation mode, and the air bag 23 is thus
 inflated in the same manner as in FIGS. 17 to 22.
 As is apparent from the above description, in the methods of FIGS. 17 to
 22, the value of the eighth velocity threshold value used for the inflator
 activation mode determination and the value of the seventh velocity
 threshold value used for the inflator activation/non-activation decision
 are selected so that the inflator activation mode determination in the
 block 60 is completed earlier than the inflator activation/non-activation
 decision in the block 62, but it is not possible to satisfy such
 conditions for all types of vehicle body structure and all types of
 collision. Considering that unpredictable situations can happen, it can be
 said that the method of the present embodiment that considers the
 possibility of rapid inflation becoming necessary, rather than fixing the
 inflator activation mode once determined, is a versatile method that is
 substantially unaffected by the vehicle body structure and that can be
 applied to extraordinary collision types as well.
 FIG. 24 is a block diagram showing a modified example of the method of FIG.
 23, in which the output of the block 60 that determines the inflator
 activation mode is directly fed to the block 15, while the inflator
 activation/non-activation decision based on the first time-integrated
 value V1 is made using one of two routes according to the activation
 status of the first inflator. That is, the first time-integrated value V1
 is first sent to the block 75 that judges the activation status of the
 first inflator, but in the early stages immediately after the collision,
 since the first inflator is not activated, the value is sent to the block
 62 for comparison with the seventh velocity threshold value Vs7, as in the
 foregoing example, and when the value reaches or exceeds the threshold
 value, it is determined that inflator activation is needed, and the signal
 is sent to the block 15. Since the inflator activation mode signal from
 the block 18 or 19 is already input here, the trigger signal is sent to
 the inflator(s) in accordance with the activation mode signal; in the
 early stages of collision, when the moderate inflation signal K=1 is
 input, only the first inflator is activated to start the moderate
 inflation of the air bag 23, while on the other hand, the computation for
 the inflator activation/non-activation decision is continued.
 When the first inflator is activated, the first time-integrated value V1 is
 now sent to a block 77, the other inflator activation/non-activation
 decision device, and compared with a 12th velocity threshold value Vs12
 preset in a block 78. The 12th velocity threshold value Vs12 is set at a
 higher value than the seventh velocity threshold value Vs7 supplied to the
 block 62. When it is determined in the block 77 that V1.gtoreq.Vs12, the
 trigger signal is immediately sent to the second inflator trigger circuit
 22 to activate the second inflator, and thus the air bag is rapidly
 inflated with a large amount of high-pressure gas combining with the gas
 released from the earlier ignited first inflator. On the other hand, when
 V1&lt;Vs12, the value is sent to the block 46 where a decision is made as
 to whether to continue or not continue the computation, as in the
 foregoing example.
 More specifically, in the illustrated example, if the inflator activation
 mode determining device 60 determines that the activation mode should be
 set as the rapid inflation (K=2) before the inflator
 activation/non-activation decision device 62 determines that inflator
 activation is needed, the same operation as described in FIG. 17 is
 performed; on the other hand, when the activation mode is determined as
 the moderate inflation (K=1), if it is determined that inflator activation
 is needed, only the first inflator is activated while allowing the
 computation to continue, and thereupon, the activation/non-activation
 decision device is changed from the block 62 with the lower threshold
 value to the block 77 with the higher threshold value; thereafter, when
 the new activation/non-activation decision device determines that inflator
 activation is needed, the second inflator is immediately activated, thus
 switching the activation mode from the moderate inflation to the rapid
 inflation. In this way, if the inflator activation instruction is issued
 when the inflator activation mode is set as the moderate inflation, and
 if, thereafter, a situation occurs that demands switching to the rapid
 inflation mode because of the change of acceleration, the activation mode
 can be switched to the rapid inflation mode in the second inflator
 activation/non-activation decision process using the 12th velocity
 threshold value Vs12; this ensures stable switching from the moderate to
 the rapid inflation mode, regardless of the type of vehicle body structure
 or the type of collision, and occupant safety can thus be enhanced.
 As described above, in the second method of the present invention, the
 decision whether to activate or not activate the inflators and the
 determination of the inflator activation mode are made utilizing the
 differences in output characteristics between the first time-integrated
 value V1 based on the acceleration signal from the first acceleration
 sensor mounted inside the passenger compartment and the second
 time-integrated value V1' based on the acceleration signal from the second
 acceleration sensor mounted in the crush zone; it will be appreciated,
 however, that the method of the invention is not limited to the specific
 examples illustrated in FIGS. 17 to 22, and that various modifications may
 be made in accordance with the spirit of the claims appended hereto. For
 example, the concept of the time threshold value shown in FIGS. 19 to 21,
 that is, the concept of holding off the activation mode determination for
 the designated inflator during the predetermined period of time, can be
 applied to the configuration of FIGS. 18 and 22 to 24; furthermore, it
 will be recognized that the combination of the threshold value for the
 activation mode determination and the threshold value for the
 activation/non-activation decision is not limited to the illustrated
 examples, but various other combinations are possible.
 The above description has dealt with a system in which only the second
 acceleration sensor 2 is mounted in the crush zone and all other computing
 circuits are grouped together as the computation circuit 6 and mounted in
 an appropriate position within the passenger compartment, but
 alternatively, circuitry up to the integrating circuit 5a' or its
 subtracting circuit 5b' for the second acceleration sensor 2 may be
 mounted in the crush zone. The latter arrangement serves to reduce the
 cost of the system because the central computer mounted inside the
 passenger compartment is then supplied with the time-integrated value V'
 or V1' that does not require a high communication speed, not the
 acceleration signal G' that demands a high communication speed.
 Each velocity threshold value may be set as a constant value, but it is
 preferable that each velocity threshold value is set as a function of time
 so that various types of collision can be easily responded to.
 Further, the above description has dealt with examples in which the
 decreased integrated value V1 obtained by the subtracting means 5b is used
 as the first time-integrated value used for comparison in various
 comparators, but instead, the time-integrated value V obtained by the
 integrating means 5a may be used as the first time-integrated value. For
 example, suppose the case where V1 is used as the first time-integrated
 value and a comparison is made with the seventh velocity threshold value;
 here, if V is to be used as the first time-integrated value, since
 V=V1+.DELTA.V, Vs7+.DELTA.V should be used as the new seventh velocity
 threshold value, as comparing V1 with Vs7 is the same as comparing V with
 Vs7+.DELTA.V. Therefore, either V or V1 may be used as the first
 time-integrated value, but the threshold value must be varied accordingly.
 Likewise, instead of using the decreased integrated value V1', the
 time-integrated value V' before the subtraction may be used as the second
 time-integrated value described in the above description, but in this case
 also, the threshold value must be varied accordingly.
 Each of the embodiments shown in FIGS. 1 to 11 and 17 to 24 has been
 described dealing with the configuration using two inflators, but it will
 be appreciated that the present invention is equally applicable to
 configurations using three or more inflators. In the latter case, it is
 also possible to configure the system so that only a specified number of
 inflators are activated in the moderate inflation activation mode.
 In the configuration using two inflators, a performance difference may be
 provided between the two inflators; for example, the performance of the
 first inflator is set at 70% of the total gas output and that of the
 second inflator at 30%, and control is performed so that in a collision of
 highest severity, the two inflators are ignited simultaneously, while in a
 collision of moderate severity, the first inflator is ignited first,
 followed with a certain delay by the second inflator, and in a collision
 of low severity, only the first inflator is ignited.
 Further, rather than using a plurality of independent inflators, a single
 inflator whose housing is partitioned into a plurality of independent
 combustion chambers, each with an independent igniter and capable of being
 activated independently, may be used instead of the inflators in the
 present invention; the term "plurality of inflators" used in the present
 invention embraces all such variations, and it will be appreciated that
 any type of inflator having a plurality of independently ignitable gas
 generators can be used in the present invention, regardless of whether
 they are assembled into one unit or not.
 As has been described above, according to the present invention,
 acceleration sensors are mounted in both the passenger compartment and the
 crush zone, and the decision whether to activate or not activate the
 inflators and the determination of the inflator activation mode are made
 utilizing the differences in characteristics that arise between the
 computed values of the acceleration signals detected by the two sensors in
 various types of collision due to the differences in the characteristics
 of these acceleration signals; this makes it possible to easily
 discriminate soft crashes, represented by impacts during rough road
 driving and low-speed collisions, which have been difficult to
 discriminate with traditional crash detection systems that relay only on
 the acceleration sensor mounted inside the passenger compartment. In
 particular, in the case of rough road driving or abuse that does not cause
 deformation to the crush zone, or in a low-speed collision that causes
 only minor deformation to the vehicle body, the waveforms of the two
 acceleration sensors are substantially the same, and the difference
 between the time-integrated values of the two signals is therefore
 extremely small. Using this difference between the time-integrated values
 directly or indirectly to discriminate between impacts requiring air bag
 inflation and impacts not requiring inflation, it becomes possible to
 perfectly prevent an erroneous activation of the air bag in a soft crash
 event, such as rough road driving or a low-speed collision, that involves
 only minor body deformation.
 Furthermore, noting the characteristic that the second time-integrated
 value V1 based on the acceleration signal from the second acceleration
 sensor mounted in the crush zone increases rapidly in the early stages of
 collision compared with the first time-integrated value V1 based on the
 acceleration signal from the first acceleration sensor mounted inside the
 passenger compartment, the second time-integrated value V1' itself or the
 difference Vd between V1' and V1 or the amount of change, Gd, of the
 difference is compared with a threshold value provided as its time
 function, so that not only a severe collision such as a high-speed frontal
 or high-speed oblique collision, but also a medium-speed centerpole
 collision which the passenger compartment acceleration sensor tends to
 detect belatedly, can be detected in the very early stages of collision.
 As a result, not only the decision whether to inflate or not inflate the
 air bag (activation or non-activation of the inflators) but the
 determination of the air bag inflation mode (inflator activation mode) can
 be made with the correct timing without fear of activation delays.
 Further, when using a plurality of inflators and controlling the inflator
 activation mode by providing a timing difference between the activation of
 one inflator and the activation of the next inflator, since a collision
 can be detected in the very early stages of the collision, sufficient time
 is allowed for computation from the time the collision is detected to the
 time the inflators are activated. Accordingly, complex computation can be
 performed for the inflator activation mode control, and thus the air bag
 inflation mode can be controlled to the optimum mode that matches the type
 of collision.
 By setting the threshold value as a function of the first time-integrated
 value V1, stable discrimination performance not dependent on time can be
 achieved, and further by using it in combination with the earlier
 described time function threshold value, an air bag inflation
 determination system can be constructed that combines an early
 discrimination capability with a reliable determination capability.
 When the inflator activation mode is moderate inflation, the computation is
 continued by holding off the determination of the mode until a
 predetermined time elapses; this prevents the moderate inflation mode to
 be determined too early, and enhances the accuracy of the proper
 activation mode determination.
 Likewise, when the inflator activation mode is determined as moderate
 inflation, if an "activation needed" instruction is issued from the
 inflator activation/non-activation decision making circuit, a specified
 number of inflators are immediately activated in accordance with the
 moderate inflation mode, while allowing the computation to continue for
 the activation mode determination; thereafter, if the mode switches to the
 rapid inflation mode, the remaining inflators are immediately activated.
 In this configuration, air bag inflation can be initiated at an early
 stage, and the air bag inflation speed can be changed to that of rapid
 inflation depending on the change of the condition thereafter. This offers
 the effect of enhancing the accuracy of the activation mode control,
 further ensuring occupant safety.
 In the description so far given, no mention has been made of a control
 method based on combinations of the seating position and posture of the
 occupants when the present invention is applied to the air bag system for
 the front passenger seat or rear seats, but it will be appreciated that
 the invention can also perform control based on such combinations. For
 example, to make a final decision as to whether to inflate or not inflate
 the air bag, an air bag activation/non-activation decision making circuit,
 which makes a decision as to whether to activate or not activate the air
 bag based on the seating position and posture of the occupant, may be
 provided immediately before or after the block 15 that issues the trigger
 signal to the first and second inflators. Provision may also be made to
 make a decision as to whether to inflate the air bag in moderate mode or
 rapid mode based on the seating position and posture of the occupant, and
 to couple the result of the decision to the discrimination system of the
 present invention to control the air bag inflation mode by providing
 priority order between the seating position and posture and the severity
 of the collision. That is, various application modes are possible without
 departing from the spirit of the invention as described in the appended
 claims, and the present invention does not exclude such variations.
 POTENTIAL FOR EXPLOITATION IN INDUSTRY
 As described above, the air bag activation control apparatus according to
 the present invention is capable of inflating the air bag in an optimum
 mode according to various types of collision, and is therefore very useful
 as a vehicle passenger protection apparatus.