Control system for actuating vehicle safety devices

In a control system for actuating vehicle safety devices, data representing change in the detected vehicle speed is calculated on the basis of the acceleration sensed by a vehicle acceleration sensor. The data is compared with a reference signal depending upon the vehicle running speed to discriminate whether or not the change in the detected vehicle speed has reached a level determined by the reference signal, whereby the occurrence collision can be detected reliably and without erroneous detection over a wide range of vehicle running speeds.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a control system for actuating vehicle 
safety devices such as an air-bag, a safety belt tightening device and the 
like in a motor vehicle. 
2. Description of the Prior Art 
For ensuring the safety of motor vehicle passengers, there have been 
developed various vehicle safety devices such as the air-bag, the safety 
belt tightening device and the like. For these devices to work 
effectively, it is necessary to detect when a collision has occurred as 
soon as possible, and to actuate the safety device reliably when a 
collision has occurred. On the other hard, it should be ensured the safety 
device will not be actuated owing to an erroneous detecting or 
discriminating operation. 
Various control systems for actuating vehicle safety devices have been 
proposed for meeting these requirements. For example, U.S. Pat. No. 
3,911,391 discloses a control device in which the output from an 
acceleration sensor is integrated when the output level of the 
acceleration sensor exceeds a predetermined level to obtain information 
concerning the vehicle speed thereafter and it is determined that the 
vehicle has collided with an obstruction when the decrease in the vehicle 
speed reaches a prescribed level. Thus, in the disclosed control device, 
the operation necessary for ensuring the safety of the operator and 
passengers of the motor vehicle is actuated when a sudden decrease in the 
vehicle speed has occurred. 
However, in the case of a vehicle with a crushable structure in which the 
impact incurred at the time of a collision can be absorbed to soften the 
impact, the impact is absorbed during the initial period of the collision. 
As a result, the output of an acceleration sensor located in the passenger 
compartment is likely to be small just after a collision has occurred, and 
the acceleration sensor produces a large output only later, after sudden 
acceleration or deceleration of the vehicle body occurs Therefore, in such 
a case, if it is determined that the vehicle has collided with an 
obstruction at the time the output level of the acceleration sensor 
reaches a predetermined level, the safety device may be actuated with a 
relatively large delay time. As a result, the movement speed of the body 
of the driver or passengers per unit time has already become large and the 
quick displacement of the head of the car occupants may have already 
started. Accordingly, when the safety device is actuated it is very 
difficult to keep the displacement of the head within a predetermined 
range which is required for ensuring the safety of the car occupants. To 
overcome this problem, if the prescribed level described above is lowered, 
it is liable to cause an erroneous discrimination and the reliability of 
the control device will be degraded 
Furthermore, in the prior art there has been proposed a control system in 
which a switch type sensor is placed at the forward and central portion of 
the vehicle in order to detect vehicle collision as soon as possible. 
However, the reliability of the sensor of this type is not sufficient and 
the sensor or the wiring therefor will be destroyed when the vehicle 
collides with an obstruction. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an improved control 
system for actuating vehicle safety devices. 
It is another object of the present invention to provide a control system 
for actuating vehicle safety devices which is capable of reliably 
detecting the occurrence of a collision within the period of time required 
for ensuring the protection of the vehicle occupants. 
It is a further object of the present invention to provide a control system 
for actuating vehicle safety devices with high reliability. 
According to the present invention, in a control system for actuating 
vehicle safety devices, such as an air-bag, a safety belt tightening 
device and the like in a motor vehicle, the system comprises a first means 
which includes an acceleration sensor for sensing the acceleration of the 
vehicle and produces data representing change in the detected speed of the 
vehicle obtained based on the acceleration sensed by the acceleration 
sensor, a second means for detecting the running speed of the vehicle, and 
a discriminating means responsive to the first and second means for 
discriminating whether or not a collision has occurred on the basis of the 
change in the detected speed of the vehicle taking into account the 
running speed of the vehicle. The discriminating means may include a 
setting means responsive to the second means for setting a reference level 
in relation to the running speed of the vehicle, and a comparing means for 
comparing the level of the detected speed of the vehicle represented by 
the data with the reference level to discriminate whether or not a 
collision has occurred. 
The acceleration of the vehicle at the time the collision is detected by 
the acceleration sensor and the data representing the change in the 
detected vehicle speed is obtained based on the acceleration detected by 
the acceleration sensor. On the other hand, the running speed of the 
vehicle is detected by the second means, and the discrimination is made by 
the discriminating means as to whether or not the change in the magnitude 
of the detected vehicle speed obtained by the first means exceeds the 
reference level determined in accordance with the running speed of the 
vehicle for determining whether or not collision has occurred Thus, the 
discrimination of the occurrence of collision can be appropriately carried 
out with reference to a reference level properly determined in accordance 
with the running speed at each instant. 
The invention will be better understood and other objects and advantages 
thereof will be more apparent from the following detailed description of 
preferred embodiments with reference to the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In FIG. 1, there is shown an air-bag control system for controlling the 
actuation of an air bag for vehicles in accordance with the present 
invention. An air-bag control system 1 is for controlling the actuating 
current for an electrically fired actuator 3 provided in an air-bag 2 
mounted on a vehicle (not shown), and has an acceleration sensor 4 for 
sensing the acceleration of the vehicle, which may be the conventional 
semiconductor type vehicle acceleration sensor of well-known design for 
detecting the magnitude of the positive/negative change in the speed of a 
vehicle per unit time as an acceleration. The acceleration sensor 4 is 
mounted at, for example, an appropriate place in the engine compartment 
and an output signal AS showing the acceleration acting on the vehicle 
body is produced thereby. 
The output signal AS is supplied to a signal processing unit 5 where it is 
amplified. In the signal processing unit 5 discrimination is made as to 
whether or not the acceleration shown by the output signal AS is in a 
predetermined range falling outside the range of the acceleration 
occurring in normal operation of the vehicle and within the rage of 
acceleration occurring during collision. After the acceleration shown by 
the output signal AS has once entered the predetermined range, the output 
signal AS produced thereafter is integrated and the signal processing unit 
5 produces a detected speed signal VS indicating the change in the vehicle 
speed after the acceleration at a level never incurred in normal operation 
is detected. 
FIG. 2 is a graph showing an example of the change in the acceleration of a 
vehicle detected by the acceleration sensor 4 in the case where the 
vehicle collides with a forward obstruction. In this graph, time t is 
plotted along the abscissas and the curve (a) shows the change in the 
acceleration of the vehicle. Furthermore, the curve (a) starts at 
t=t.sub.0 where the the vehicle acceleration shown by the output signal AS 
has entered the predetermined range, and the integration operation for the 
output signal AS starts in the signal processing unit 5 after t.sub.0. In 
FIG. 2, the detected vehicle speed shown by the detected speed signal VS, 
which is obtained by integrating the output signal AS as described above, 
is shown by the curve (b). 
Returning to FIG. 1, the system 1 is provided with a vehicle speed sensor 
6, which is a well-known speed sensor for detecting the rotational speed 
of the propeller shaft and produces a pulse train signal PT whose 
frequency depends upon the rotational speed of the propeller shaft. The 
pulse train signal PT is applied to a speed calculating unit 7, in which 
the period of the pulse train signal PT is measured and the vehicle 
running speed is calculated on the basis of the resulting period. A 
running speed signal S indicating the calculated vehicle running speed is 
output from the speed calculating unit 7 and supplied to a discrimination 
unit 8 to which the detected speed signal VS is also supplied. The 
discrimination unit 8 is composed of a reference signal generator 9 and a 
comparing unit 10, and serves to discriminate whether or not collision has 
occurred in the vehicle in response to the detected speed signal VS and 
the running speed signal S. 
As illustrated in FIG. 3, the reference signal generator 9 comprises a map 
calculation unit 11 in which the map calculation is carried out in 
response to the running speed signal S in accordance with map data 
corresponding to characteristics showing the relationship between the 
vehicle running speed and correction values, and the correction value 
corresponding to the vehicle running speed indicated by the running speed 
signal S at each instant is determined by the map calculation in the map 
calculation unit 11. Then, a correction signal HD representing the 
calculated correction value is output from the map calculation unit 11. 
The reference signal generator 9 further comprises a standard signal 
generator 12 which produces a standard signal RD representing a standard 
level, and the correction signal HD is added to the standard signal RD by 
an adding unit 13 in the polarity shown in FIG. 3 to produce a reference 
signal AO. The reference signal AO is applied to the comparing unit 10 for 
discriminating whether or not a collision has occurred in the vehicle on 
the basis of the level of the detected speed signal VS. 
Returning to FIG. 1, the detected speed signal VS is applied to the 
comparing unit 10 to compare the level of the detected speed signal VS 
with that of the reference signal AO. As explained in the foregoing, the 
reference signal AO is indicative of the reference level which is used for 
determining whether or not a collision has occurred in the vehicle on the 
basis of the detected speed signal VS, and the reference level described 
above is usually determined as follows. 
In order to complete the necessary operation of the air-bag 2 before the 
displacement of the head of the occupant(s) due to a collision has reached 
a predetermined allowable maximum distance, that is, 10 to 15 centimeters, 
required for ensuring their protection, assuming that the delay time from 
the time of the production of an air-bag triggering signal to the complete 
inflation of the air-bag 2 is t.sub.d and the head of the occupant(s) is 
displaced by the predetermined allowable maximum distance at t.sub.a, that 
is, after the passage of time t.sub.s from the occurrence of the 
collision, the level of the detected vehicle speed at the passage of time 
t.sub.a -t.sub.d from the time of the collision is determined at t.sub.0 
as the standard level -L.sub.a (see FIG. 2). 
Consequently, in the case where the vehicle running speed is lower than Va, 
the reference level indicated by the reference signal AO is equal to 
-L.sub.a because the correction value is zero. In this case, the comparing 
unit 10 is operated to produce a trigger signal TS when the level of the 
detected vehicle speed indicated by the detected speed signal VS becomes 
less than -L.sub.a. As will be understood from the graph shown in FIG. 4, 
the correction value varies in proportion to the vehicle running speed 
when the running speed is between V.sub.a and V.sub.b. As a result, the 
level of the reference signal AO becomes greater as the vehicle running 
speed increases, so that early determination of the collision is made. If 
the vehicle running speed is greater than V, the correction value is fixed 
at a predetermined constant valve which is not equal to zero. The 
characteristic curve shown in FIG. 4 is only one example, and, of course, 
any characteristic curves different from that shown in FIG. 4 can be 
alternatively selected. 
In any case, when the level of the detected vehicle speed as indicated by 
the detected speed signal VS becomes smaller than the reference level 
indicated by the reference signal AO, the air-bag triggering signal TS is 
generated from the discrimination unit 8 and applied to a driving circuit 
14. The driving circuit 14 is responsive to the air-bag triggering signal 
TS and provides a firing current to the electrically fired actuator 3 to 
operate the air-bag 2. 
With this constitution, the detected speed signal VS produced by the signal 
processing unit 5 is compared with the reference signal AO produced by the 
reference signal generator 9. When the level of the detected vehicle speed 
decreases due to a vehicle collision and has become less than the 
reference level, the air-bag 2 is actuated. In the control system 
according to the present invention, since the reference level is varied in 
accordance with the vehicle running speed as described above, a larger 
change in the magnitude of the detected vehicle speed is required to 
trigger the air-bag 2 during the relatively low speed running of the 
vehicle, while a smaller change in the magnitude of the detected vehicle 
speed suffices as the vehicle running speed becomes higher. 
Accordingly, a larger impact is needed to trigger the air-bag 2 in the case 
where the vehicle is running at a relatively low speed, and the impact 
necessary for triggering the air-bag 2 becomes less as the vehicle running 
speed increases. This means that, in the case where an impact is applied 
to the vehicle, the triggering signal TS is produced earlier during high 
speed running of the vehicle than during the low speed running of the 
vehicle. As a result, the control system 1 is able to effectively avoid 
triggering the air-bag 2 erroneously when the vehicle is running at a 
relatively low speed, while the control system 1 is capable of early 
collision detection when the vehicle is running at a relatively high 
speed, whereby ideal control operation can be realized. 
The arrangement of the discrimination unit 8 shown by FIGS. 1 and 3 is one 
embodiment and the discrimination unit 8 can be alternatively arranged, 
for example, as shown in FIG. 5. In FIG. 5, the comparing unit 10, the map 
calculation unit 11 and the standard signal generator 12 are the same as 
those shown in FIGS. 1 and 3. The arrangement of FIG. 5 is different from 
that shown by FIGS. 1 and 3 in that the level of the detected speed signal 
VS is corrected in accordance with the correction signal HD by an adding 
unit 15 to produce corrected speed signal M, and the discrimination of the 
occurrence of the vehicle collision is made by comparing the corrected 
speed signal M with the standard signal RD. However, the operations 
performed are substantially the identical in the two cases and the same 
effect will be obtained in either case.