Air bag arming device including steering shaft rotation sensor

The inflatable safety cushion for a vehicle comprises an electric arming device (19) which is capable of detecting a dangerous situation and of very rapidly actuating a pyrotechnic gas generator for triggering said safety cushion, and which is powered by a cell (12). It also comprises an arming device (20, 21) for detecting that the vehicle is being used in order automatically to arm the device (19) as soon as it is detected that the vehicle is being used, and a device (21a) for interrupting the armed state as soon as, for a predetermined length of time, no use of the vehicle has been detected.

The present invention relates to an inflatable safety cushion for a motor 
vehicle, commonly called an airbag, which may in particular be housed in 
the steering wheel for protecting the driver of said vehicle, comprising 
an electrical arming device capable of detecting a danger situation and of 
very rapidly actuating a pyrotechnic member for triggering said safety 
cushion. 
FR-A-2,582,269 discloses an appliance for triggering a member for 
tightening a seatbelt. This appliance comprises an electric arming device, 
capable of detecting a danger situation and of very rapidly actuating a 
pyrotechnic gas generator for triggering said tightening member. This 
device comprises a circuit powered by a cell and comprises, mounted in 
series, at least one sensor able to close said circuit when the danger 
situation is detected, and an electric ignitor capable of being fired in 
order to actuate the pyrotechnic gas generator under the action of the 
electrical current which flows in the circuit as soon as the sensor closes 
said circuit, the cell, the sensor and the ignitor being concentrated at 
the same place and being connected by a very short circuit. 
In order to save the energy stored by the cell, this device comprises a 
stop/start switch which is a magnetic strip-type relay capable of 
detecting the passage of a magnetic tape woven into the strap of the belt. 
This switch turns on the device when the seatbelt is unrolled by a user of 
the vehicle, and switches it off again when the belt is released by said 
user and is wound back up on the winder. 
Such a device cannot equip an airbag. 
The object of the invention is to propose an airbag of the aforementioned 
type equipped with an electric arming device powered by a cell, this 
device being entirely autonomously capable of being housed entirely inside 
the steering wheel of a vehicle, whilst being perfectly reliable, secure 
and capable of protecting the corresponding passenger of a vehicle 
effectively under all the conditions of use of said vehicle. 
The inflatable safety cushion or bag envisaged by the invention comprises 
an electric arming device of the aforementioned type. 
According to the invention, this safety cushion is one wherein it comprises 
arming means for detecting a use of the vehicle in order automatically to 
arm said device as soon as it is detected that said vehicle is being used, 
and means for interrupting the armed state as soon as, for a predetermined 
length of time, no use of the vehicle has been detected. 
The advantage of the Applicant Company is to have analyzed the conditions 
of use of a vehicle and to have discovered that it is sufficient, for an 
airbag of the abovementioned type, to associate, in combination, means for 
detecting a use of the vehicle, and time-delay means for keeping the 
arming device operational during a period of predetermined length after a 
last signal of use of the vehicle: this device thus remains operational in 
specific conditions under which it is difficult to detect the use of a 
vehicle whilst there still remains a risk of danger, for example while 
stopping in front of a traffic light showing the red stop light, in 
traffic jams, when driving along a dead-straight and perfectly flat road, 
etc. 
It is thus possible to produce an entirely autonomous airbag, capable of 
being installed on any type of vehicle, even in the case of a steering 
wheel which does not comprise a revolving contact for supplying the energy 
from the battery to inside the steering wheel. The airbag in accordance 
with the invention may operate even in the case of an incident of accident 
giving rise to the cables providing the distribution of electric current 
from the battery being cut. 
According to an advantageous version of the invention, the arming means 
comprise a detector of rotation of a revolving shaft. 
It is thus easy to switch the arming device on as soon as the steering 
wheel is turned, at least as far as the airbag intended to protect the 
driver of the vehicle is concerned. 
According to a preferred version of the invention, the arming means 
comprise at least one magnet and at least one magnetic relay capable of 
opening or closing by passing in front of the magnet in the case of 
relative displacement with respect to the magnet(s). 
Other features and advantages of the invention will emerge from the 
detailed description hereafter.

In the embodiment of FIG. 1, the assembly of a steering wheel, represented 
by the reference 1, is fastened to a steering column 2 which turns inside 
a sheath shown diagrammatically at 3. The assembly of the steering wheel 1 
comprises the actual ring 4 which is fitted with an upper lining 5 and a 
lower lining 6 respectively covering a folded protection airbag 7 ready 
for use, a pyrotechnic gas generator 8 allowing the airbag 7 to be 
inflated very rapidly, an electric ignitor 9, an impact sensor 10. All 
this equipment, of any type, is known per se and it is not necessary to 
describe it in detail here. The assembly 1 also contains other elements 
which will be described later. 
In a known fashion, the aforementioned equipment constitutes, in 
combination with other equipment, an electric arming device 11 capable of 
detecting a danger situation and of very rapidly actuating the pyrotechnic 
gas generator 8 for inflating the bag 7. This device further comprises, in 
a known fashion (see FIGS. 2 and 3): 
means forming a continuous current source 12, 
a circuit 14 powered by the source 12 and comprising, mounted in series, 
the sensor 10 capable of closing said circuit 14 when the danger situation 
is detected, and the electric ignitor 9 capable of being fired in order to 
actuate the pyrotechnic gas generator 8 under the action of the electrical 
current which flows in the circuit 14 as soon as the sensor 10 closes said 
circuit. 
In the embodiment shown diagrammatically in FIG. 2: 
the source 12 consists of at least one long life cell 12 with low losses 
capable of supplying a charge of several amp-hours (Ah) under a voltage of 
a few volts which remains substantially constant during almost the whole 
life of the cell; 
the sensor 10 is a passive impact sensor which does not consume electricity 
when it is not activated; 
a capacitor 13 consists of at least one capacitor with very low impedance 
and very low leakage current; 
the electric ignitor 9 is an ignitor with very low ohmic resistance and 
very low ignition energy. 
The cell 12, the capacitor 13, the sensor 10 and the ignitor 9 are 
concentrated at the same place in the steering wheel 1 and are connected 
by a very short circuit of low resistance. 
The voltage of the cell 12, the capacitance of the capacitor 13 and the 
ohmic resistance of the circuit 14 are chosen so that discharging the 
capacitor 13 causes a current to flow in the circuit 14 for a 
predetermined time and with a predetermined strength, necessary for firing 
the ignitor 9, in the event of said circuit being closed by the sensor 10. 
The cell 12 is for example a lithium/thionyl chloride cell delivering a 
voltage of approximately 3.6 volts and capable of supplying energy of 
approximately 10 Ah. The cell is capable of resisting the temperatures 
likely to prevail inside the steering wheel, and its voltage remains 
substantially constant during almost all its life. 
The capacitor 13 has an impedance which is advantageously lower than 1 ohm, 
preferably lower than 0.2 ohm, in the aforementioned temperature range, 
and a leakage current advantageously less than 5 .mu.A, preferably less 
than 2 .mu.A, in the same temperature range. 
The electric ignitor 9 is, for example, capable of being fired under the 
action of a current whose strength is less than approximately 0.8 A with a 
minimum duration-of approximately 2 ms and preferably under the action of 
a current of 0.6 A. It requires, for example, to be made to burn, a firing 
energy lower than approximately 2.2 mJ supplied for approximately 2 ms, 
and preferably an energy of approximately 1.7 or 1.8 mJ. 
The device 11 may be limited to the circuit 14 consisting of the four 
aforementioned components namely cell 12, capacitor 13, sensor 10 and 
ignitor 9, connected as shown in FIG. 2, the current being returned, of 
course, via earth. 
Preferably, and as represented diagrammatically in FIG. 2, the device 11 
also comprises means for monitoring the voltage of the cell 12, and 
warning means which are actuated by the monitoring means when the voltage 
of the cell becomes less than a minimum predetermined threshold. 
In a known fashion, the monitoring means comprise a voltage comparator 15 
which receives the voltage of the cell 12 and a reference voltage 16. If 
the voltage of the cell 12 becomes less than the minimum threshold defined 
by the reference 16, the comparator 15 powers a timebase 17 which causes a 
warning light-emitting diode (LED) 18 to flash. 
In the embodiment shown diagrammatically in FIG. 3, the monitoring device 
19 in accordance with the invention comprises all the elements which 
constitute the device 11 of FIG. 2, and which retain the same reference 
number. 
The device 19 further comprises arming means 20 for detecting that the 
vehicle is being used in order automatically to arm said device as soon as 
it is detected that said vehicle is being used, and means for interrupting 
the armed state as soon as, for a predetermined length of time, no use of 
the vehicle has been detected. 
In the example represented, the arming means comprise a detector 20 of 
rotation of a revolving shaft, in this case the steering column 2. The 
function of the detector 20 is to deliver a signal, for example a change 
of state of a contact signal, or a presence or absence of voltage signal, 
and the detector is associated with a conventional electronic circuit 21 
which it is not necessary to describe here, capable of controlling the 
arming or, to the contrary, of interrupting the arming of the device 19, 
by switching said device on or off. The electronic circuit 21 particularly 
comprises a time-delay device represented diagrammatically at 21a which 
keeps the device 19 in the armed state for a predetermined time, for 
example 1 hour, after the last signal of use of the vehicle was received 
from the detector 20. 
The device 19 further comprises, preferably, means for actuating the 
warning means when the arming means 20, 21 put said device into the armed 
state: the circuit 21 is thus, for example, connected to the voltage 
comparator 15 so that the latter actuates the time base 17 and makes the 
light-emitting diode 18 flash when the device 19 is switched on. Thus, the 
motorist who is seated in his car and turns his steering wheel 1 is 
informed, by the flashing of the diode 18, that the device 19 is switched 
on and in the armed state. 
When the circuit 14 is thus switched on, the cell 12 lets out a small 
current in order to supplement the charge of the capacitor 13 and 
compensate for losses which have arisen in the latter since the last time 
the vehicle was used. This light flow from the cell has the great 
advantage of at least partially eliminating the passivation of the 
electrodes of the cell 12, so that the latter is permanently in optimum 
operating conditions. 
The arming means may comprise at least one magnet and at least one magnetic 
relay (Reed bulb) capable of opening or closing by passing in front of the 
magnet in the case of relative displacement with respect to the magnet(s). 
In the embodiment represented diagrammatically in FIG. 4, the rotation 
detector 22 comprises a flexible strip switch 23 contained in a bulb 24 of 
cylindrical shape. Around the bulb 24 there is placed a magnet 25 of 
annular shape which moves freely axially inside an annular housing 26 
placed around the bulb 24 and coaxially with respect to the latter. 
If the detector 22 is fixed inside the steering wheel 1, for example so 
that its axis is horizontal when the steering wheel is in its mean 
position, a rotation in the clockwise direction of the steering wheel 1 
will lead to a displacement of the magnet 25 to the right of FIG. 4 with 
respect to the bulb 24; this displacement of the magnet 25 will take place 
towards the left of the figure if the steering wheel is turned in the 
counter-clockwise direction. Each time the magnet 25 passes in line with 
the flexible strip switch 23, the latter is closed and delivers a change 
of state signal processed by the electronic circuit 21. 
In the example represented, and in order to avoid parasitic signals in the 
case of rebound of the magnet at the two ends of the housing 26, means are 
provided for braking the annular magnet 25 at the end of its travel. These 
means may be of any known type: thus, at the right-hand end of FIG. 4 
springs 27 have been represented and at the left-hand end of-the figure 
elastic masses 28 and flexible braking strips 29 have been represented 
capable of braking the magnet 25 without blocking it. A U-shaped magnet 30 
has also been represented surrounding the annular housing 26 in the 
longitudinal direction and having its branch 31 constituting its S pole 
facing the face 32 of the magnet 25 constituting the S pole of the latter. 
Likewise, the branch 33 of the magnet 30 constituting its N pole is facing 
the face 34 of the magnet 25 constituting the N pole of the latter. 
Thus, the magnetic field created by the magnet 30 tends to brake the magnet 
25 when the latter approaches either one of the branches 31 or 33, with a 
force which increases as the magnet 25 gets closer to one of these 
branches. 
Of course only one of these braking means may be used, or several of them 
could be used in combination, as needs be. 
In the example represented in FIGS. 5A and 5B, the rotation detector 35 
comprises a flexible strip switch 36 fastened to the steering wheel 1, and 
an element forming a pendulum 37, suspended from a spindle 38 itself fixed 
to the steering wheel, so as to turn freely about this spindle 38. The 
pendulum 37 carries at least one magnet 39 which passes in front of the 
flexible strip switch 36 when the steering wheel 1 turns with respect to 
its mean position, in which the spindle 38 is substantially horizontal. 
In the example of FIG. 5B, the pendulum carries four magnets 39a, 39b, 39c, 
39d which, each in turn, cause the switch 36 to close by passing in front 
of the latter when the steering wheel turns, which allows a finer analysis 
of this rotation. 
A mass 40 of ferromagnetic metal is further placed on the other side of the 
flexible strip switch 36 with respect to the magnets 39, 39a, 39b, 39c, 
39d so as to concentrate the lines of the magnetic field and enhance the 
precision of the signal given by the switch 36. 
In the embodiment of FIG. 6, the steering wheel 41 comprises a device 42 
which comprises a first flange 43 fixed to the sheath 3 perpendicularly to 
the steering column 2, and a second flange 44 fixed to the steering wheel 
41 parallel to the first flange 43. 
The first and second flanges 43, 44 carry conjugate respective means such 
that the means carried by the second flange 44 deliver at least one change 
of state signal by passing in front of the means carried by the first 
flange 43 during a rotation of the steering wheel 41. 
In the example represented, the first flange 43 is a disk fixed to the end 
of the sheath 3 by any means, for example by a pin 45 which prevents the 
disk 43 from turning. The flange 44 is another disk fixed to the lower end 
of the steering wheel 41, and which carries two elastic pawls 46 capable 
of clipping into the disk 43 in order to prevent a recoil of the steering 
wheel 41 without hindering the latter's rotation. 
In the example-represented in FIG. 7, in solid line, and in FIG. 8A, the 
first disk 43a is made of metal and comprises openings 47 evenly 
distributed over one and the same circumference, and the second disk 44a 
comprises a first feeler 48 under voltage which permanently rubs against 
the first disk 43a and applies a voltage to it, as well as a second feeler 
49 which is isolated with respect to the disk 43a and which comes into 
contact sometimes with the openings 47 sometimes with a metal part 50 
lying between two adjacent openings 47. The feeler 49 therefore detects, 
in the event of the steering wheel 41 rotating, either the absence of 
voltage if it is in front of an opening 47, or the presence of voltage if 
it is in front of a metal part 50. 
In the embodiment represented in dotted lines in FIG. 7 and in solid lines 
in FIG. 8B, the first disk 43b carries several magnets 51 distributed 
evenly over one and the same circumference, and the second disk 44b 
carries at least one flexible strip switch 52 which delivers a change of 
state signal each time it passes in front of one of the magnets 51. 
Various embodiments of the device of the invention have thus been 
described, all of which are capable of being installed in a steering wheel 
and therefore make it possible to fit a novel steering wheel equipped with 
a protection airbag to any vehicle, even when the initial steering wheel 
of said vehicle does not comprise a revolving contact making it possible 
to supply current from the battery to the steering wheel. 
According to the results of tests carried out by the Applicant Company, the 
device of the invention may have, under the conditions prevailing in a 
motor vehicle, a durability of at least five years, or even longer if the 
device comprises arming means. Such durability, which up until now was 
thought impossible to achieve, allows the device to be mounted in a place 
of the steering wheel which is difficult to access in order to respect the 
safety requirements, since the customer is guaranteed that it will not be 
necessary to dismantle the steering wheel of his vehicle for many years in 
order to change the cell of his arming device. 
Of course, the invention is not limited to the embodiments which have just 
been described, and numerous modifications and changes may be made thereto 
without departing from the scope of the invention. 
It is thus possible to provide several cells in series and/or in parallel 
as well as several capacitors in parallel, in order to power the electric 
ignitor. 
It is possible to use any type of impact sensor, and any ignitor whatever 
meeting to the aforementioned requirements. 
Of course any type of rotation detector whatever may be used or this 
detector may be replaced by another detector for detecting a use of the 
vehicle, for example an inertia detector of low inertia sensitive to the 
accelerations and decelerations of the vehicle. 
Of course all the elements described hereinabove may be combined with one 
another or with other equivalent means. 
Obviously, the invention is not limited to only airbag systems housed in 
the steering wheel of a motor vehicle, and the device of the invention may 
be used for any safety appliance triggered by a pyrotechnic member itself 
actuated by an electric ignitor, whether this be in the automobile field, 
for example for a seatbelt pretensioner, or in any other field, it being 
possible for the vehicle to be any machine. This device may thus be used 
for airbag systems housed, for example, in the doors of a vehicle, for 
protection against lateral impacts, or in the backrests of front seats, 
for protecting the back seat passengers, in order to avoid having to 
install bundles of conductor wires reaching to these places. 
Likewise, in that which concerns the airbag capable of protecting the front 
passenger, it is easy to provide, on the electric arming circuit, a 
magnetic relay located close to the steering wheel and capable of being 
actuated by a magnet integral with the steering wheel: even if it 
comprises two electrical conductors going from the sheath of the steering 
wheel to the airbag, this electrical arming circuit, as for the one for 
the airbag housed in the steering wheel, remains perfectly automatic, 
reliable and secure.