Ignition capsule, which can be inductively activated, for occupant restraint systems, and a test circuit for said ignition capsule

Abstract

The invention relates to a firing capsule for the gas generator of a passenger restraint system in a motor vehicle, especially for the gas generator of an inflatable impact protection cushion (airbag), in which the electrical energy necessary for detonating the igniting agent is inductively coupled in by means of alternating magnetic fields. The invention moreover relates to a test circuit with which the serviceability of the inductively activatable firing capsule can be monitored.

Description

[0001] The invention relates to a firing capsule for the gas generator of a passenger restraint system in a motor vehicle with an igniting agent which is caused to explode by a resistance wire, the electric power necessary for detonation being inductively coupled into the firing capsule. The invention moreover relates to a test circuit by which it can be ascertained whether the firing capsule is serviceable.

[0002] Among the most frequently used passenger restraint systems in motor vehicles are inflatable impact protection cushions (airbags) and belt tighteners. Both restraint systems require a gas generator with a propellant charge which is caused to explode by a firing capsule in case of a motor vehicle collision. The amount of gas released by this inflates the airbag or causes the turbine wheel of the belt tightener to rotate.

[0003] A gas generator for an airbag is disclosed for example in U.S. Pat. No. 5,803,494 A. The known gas generator has several electrically activatable firing capsules, as are described for example in DE 197 33 353 C1. One such firing capsule has a pot-shaped housing which is filled at least partially with an igniting agent and which is sealed by an insulating body. Two contact pins project through the insulating body into the interior of the firing capsule. The two contact pins are electrically connected conductively to one another by a resistance wire which penetrates the igniting agent. In a motor vehicle collision an impact sensor is activated and sends a signal to the trigger circuit. The trigger circuit thereupon delivers a current pulse to the contact pins of the firing capsule so that the resistance wire is caused to glow or melt. The thermal energy released in doing so causes the igniting agent to explode. The firing capsule splits at scored points intended for this purpose and thus allows the hot explosion gas to flow into the interior of the gas generator. As a result the propellant charge located there is caused to detonate.

[0004] One serious disadvantage of electrically activatable firing capsules is that their resistance wire can corrode and become brittle over time so that by means of special test circuits, compare in this regard for example DE 198 14 589 A1, it must be tested at regular time intervals whether the firing capsule is still serviceable. The test pulses delivered by one such test circuit could however lead to misfiring with fatal consequences under unfavorable conditions, if for example at high speed the driver is blinded for seconds by the suddenly inflating airbag. But misfiring can also occur due to electrostatic charges in the motor vehicle and by leakage currents if they are routed via the contact pins into the interior of the firing capsule.

[0005] DE 38 38 896 A1 discloses a firing capsule by which the aforementioned problems can be prevented in that the igniting agent is not caused to explode via the heat generation of an electrically heated resistance wire, but by incident laser light radiation. For this purpose the firing capsule is sealed with a window which is transparent to laser light. A layer which is in thermal contact with the igniting agent and which is highly absorptive at the wavelength of the incident laser light is applied to the inside of this window. Thus the layer is heated up when light is incident and the igniting agent is detonated. With respect to better light coupling, in the known firing capsules it is provided that the transparent window be made as a lens. The disadvantage of the optically activatable firing capsule is that considerable light intensities must be made available if reliable ignition is to be ensured.

[0006] The object of the invention is to devise a firing capsule for the gas generator of a passenger restraint system which essentially does not age, in which misfires as a result of electrostatic charges and leakage currents are precluded, and which can be detonated extremely reliably. This firing capsule is designed moreover to replace existing electrically or optically activatable firing capsules and accordingly to be composed such that conventional gas generators can be easily retrofitted with it. The object of the invention is furthermore to devise a test circuit by which the serviceability of the firing capsule can be monitored and which cannot trigger misfires.

[0007] The aspects of these two objects are achieved by a firing capsule as claimed in claim 1 and a test circuit as claimed in claim 13; advantageous developments and embodiments are given in the dependent claims.

[0008] The invention proceeds from the idea of inductively coupling the heat output necessary to detonate the igniting agent into the firing capsule. As a result no electrical supply lines are necessary which penetrate the housing of the firing capsule so that electrostatic charges or leakage currents which can trigger a misfire cannot be routed into the interior of the firing capsule. The housing of the firing capsule can be hermetically sealed because these feed lines are absent. This ensures that moisture cannot reach the resistance wire and the igniting agent. The resistance wire is thus protected against corrosion and the igniting agent remains dry and thus ignitable even under the unfavorable climatic conditions to which the motor vehicle is exposed. Moreover, the resistance wire is largely protected against temperature fluctuations which otherwise are relayed via electrical feed lines into the housing of the firing capsule. Accordingly it is guaranteed that the resistance wire essentially does not age or become brittle.

[0009] The firing capsule as claimed in the invention moreover enables the use of a test circuit which inductively couples a very weak alternating current signal into the firing capsule. In this way it is possible with the most simple circuity means to monitor the serviceability of the firing capsule.

[0010] The principle of inductive coupling of electrical heat output into a consumer is known. Thus, for example U.S. Pat. No. 5,905,343 A describes an electrical filament lamp with a filament wire which is made as a closed conductor loop. In the vicinity of the lamp bulb there are magnet cores via which the alternating magnetic fields generated by the primary windings are routed to the filament wire. These fields penetrate the secondary winding formed by the filament wire and induce in it the heating current necessary for illumination.

[0011] Furthermore, the prior art discloses firing capsules which can be inductively activated or tested. Thus DE 24 33 555 A1 discloses proximity firing pulse transmission for airbags in which the electrical energy needed to trigger the airbag is transmitted inductively from stationary to rotary motor vehicle parts. The transformer used here has a primary winding which is permanently connected to the steering column, and a secondary winding which is attached to the steering shaft or the steering wheel and which turns coaxially in the primary winding.

[0012] DE 195 30 586 A1 discloses an arrangement for monitoring the resistance of a load which is connected to a transformer, in which an additional resistance is connected parallel to the load and in which there are means which measure the primary-side input resistance of the transformer and signal a fault of the load when the input resistance deviates from a given value. The known arrangement is used preferably for monitoring the serviceability of the firing pellets of airbag systems.

[0013] The invention is detailed below using embodiments.

[0014] FIG. 1 schematically shows a section through the firing capsule as claimed in the invention, and in the form of a block diagram the pertinent trigger circuit and the test circuit as claimed in the invention. The firing capsule 1 has a pot-shaped housing 2 which is filled at least partially with an igniting agent 4. The top of the firing capsule 1 facing the interior of the gas generator which is not shown here is made as a bursting disk 3 or has scoring so that it is ensured that in the case of a motor vehicle collision the hot explosion gases travel quickly and reliably into the interior of the gas generator, where they cause the propellant charge to detonate. The housing 2 on its bottom is hermetically sealed by a sealing body 5. In this embodiment the sealing body 5 is penetrated by a magnet core 11. In one preferred embodiment this magnet core 11 can also be made in two parts so that an opening in the housing 2 is not necessary. Within the housing 2 is a suitably held resistance wire 8 which penetrates the igniting agent 4 and which together with the secondary winding 7 forms a secondary circuit 6. What is important here is that the electrical supply lines which are routed from the outside into the firing capsule and thus which can be the cause of misfiring are absent. Outside the housing 2 the magnet core 11 is surrounded by a primary winding 10. The ratio of the number of turns of the primary winding 10 to the secondary winding 7 is such that in the secondary circuit 6 a relatively strong current is induced which reliably causes the resistance wire 8 to glow or melt and thus causes the explosion of the igniting agent 4.

[0015] The primary winding 10 together with a trigger circuit 12 forms a primary circuit 9. The trigger circuit 12 is connected to an impact sensor 13. In the case of motor vehicle collision the impact sensor 13 releases a signal to the trigger circuit 12, as a result of which it generates a relatively large AC voltage. The AC voltage can be for example sinusoidal or rectangular; what is decisive for the operation of the firing capsule is simply that as a result of this AC voltage on the secondary side an alternating current is induced which heats the resistance wire 8 enough.

[0016] A test circuit 14 is connected parallel to the primary winding 10. At regular time intervals, for example at an interval of a few seconds, or continuously, this circuit produces a weak AC voltage with a frequency which can be clearly different from the AC voltage which is made available for detonation, therefore for example can be much higher. As long as the resistance wire 8 together with the secondary winding 7 forms a closed secondary circuit 6, this circuit is therefore in other words intact, the test circuit 14, as soon as it delivers its test signals to the primary winding 10, is loaded with respect to power in an exactly defined manner. If at this point for example the resistance wire 8 breaks, which can be the result of continuous vehicle vibrations, the secondary circuit 6 is suddenly open. The load on the test circuit 14 changes as a result. To implement the test circuit 14 as claimed in the invention, besides an AC voltage generator for generating the test voltage, therefore only one circuit is necessary which detects a change in the load by the secondary circuit and thereupon delivers a warning signal, which can be for example acoustic or optical. A circuit in which a test signal is inductively coupled into the current feed line of a conventional firing capsule of an airbag is known from publication DE 38 12 633 C2. In contrast to this prior art, the test signal in the test circuit as claimed in the invention is coupled into a secondary circuit which is completely electrically insulated from the environment and which contains the resistance wire 8. One special advantage of this arrangement is that the test signal, if it causes only minimum heating of the resistance wire, can also be sent quite without interruption.

[0017] In the embodiment shown in FIG. 2, the secondary circuit 6 consists only of one closed ring of resistance wire 8, therefore of a secondary winding with exactly one turn. On either side of the firing capsule there are a first primary winding 10a and a second primary winding 10b with one magnet core 11a, 11b each. The two primary windings 10a, 10b are supplied with alternating current in case of a motor vehicle collision by two trigger circuit 12a, 12b in the above explained manner. The magnetic fields produced in doing so penetrate the closed ring of resistance wire in the same direction, as symbolized in FIG. 2 by an arrow, and induce the heating current necessary for detonating the igniting agent 4.

[0018] This special configuration of the firing capsule as claimed in the invention enables very effective protection against misfiring caused by faults in the trigger circuit. If the alternating currents flowing in the two primary circuits 9a, 9b are dimensioned such that first the simultaneous interaction of the two heats up the resistance wire 8 to the required detonation temperature, then one of the primary circuits can be actuated as a result of a technical fault, without misfiring with the above described fatal consequences occurring. Of course the firing capsule as claimed in the invention can also be equipped with more than two interacting primary circuits. In the preferred embodiment there are 4 primary windings around the firing capsule, with an inductive power dimensioned such that only the interaction of 3 primary circuits leads to detonation. Therefore the system also remains fully serviceable even if one of the primary circuits has failed. At the same time misfires of the aforementioned type are reliably prevented. Of course at least one of the primary circuits can again be equipped with a test circuit as claimed in the invention.

[0019] FIG. 3a shows in a perspective how the resistance wire which is made as a closed conductor loop can be safely accommodated in the firing capsule 1 as claimed in the invention. For this purpose the resistance wire 8 is fixed on a carrier plate 15 of insulating material. Here the arrow also illustrates how the secondary circuit is penetrated by the alternating magnetic field of the primary circuit for purposes of inductive coupling. It is advantageous if the wire, as is shown in FIG. 3b in a section, is inserted into a groove 16 of the carrier plate 15. Reference number 4 in turn labels the igniting agent. FIG. 3c shows in a section another embodiment in which two annular resistance wires 8a, 8b are mounted electrically insulated from one another on either side of the carrier plate 15. If one of the resistance wires fails for example due to rupture or corrosion, the firing capsule still remains fully serviceable. It goes without saying that several arrangements of the type shown in FIG. 3c can be housed stacked on top of one another or next to one another in a firing capsule as claimed in the invention. In this case it is especially advantageous if the firing capsule 1 is surrounded by at least one primary winding 10; 10a, 10b and for example is used as the winding body for the primary winding. An additional magnet core for the primary winding is not necessary in this embodiment.

[0020] The object of the invention is furthermore to be able to produce the secondary circuit which is responsible for detonation in a manner as simple and economical as possible.

[0021] This further aspect of the object is achieved by an inductively activatable firing capsule with the features listed in claim 16.

[0022] The basic idea is to form at least one secondary circuit which is located within the firing capsule housing by a closed resistance element. This resistance element can be produced in diverse ways, for example by coating an annular core of insulating or poorly conductive material with a suitably dimensioned metal layer. Alternatively a carrier plate can be used which has an annular groove which is filled with metal. Likewise the metal can be applied by conventional coating processes such as for example by vapor deposition directly to the carrier plate.

[0023] The invention is detailed below using some embodiments shown in the drawings.

[0024] FIG. 4 shows a section through the firing capsule as claimed in the invention which is labeled 1 overall. The firing capsule 1 has a firing capsule housing 2 which is hermetically sealed by a sealing body 5. In contrast to conventional firing capsules, in this embodiment current supply wires which penetrate the sealing body 5 are not necessary in this version. The firing capsule 1 is dimensioned and configured overall such that conventional passenger restraint systems can be easily equipped with it. Within the firing capsule housing 2 is a high- explosive igniting agent 4 which in the case of a motor vehicle collision is caused to explode by the heat generation of a resistance element. For this reason, in the sealing body 5 there is a closed groove 16 into which at least one metal layer 17 is placed. The heat output required for detonation is inductively coupled into the firing capsule. For this reason, outside of the firing capsule there are two primary windings 10a, 10b with a host of turns which are wound onto a common core 11, preferably of magnetizable material, for better routing of the magnetic flux. In the case of a motor vehicle collision an impact sensor 13 delivers a signal which is illustrated by an arrow to two trigger circuits 9a, 9b, of which each then sends a trigger current into the primary winding 10a and 10b assigned to it.

[0025] The entire arrangement is configured such that the magnetic fields produced in the two primary windings 10a, 10b are rectified at each instant, therefore overall yield a magnetic field which, as the vertical arrow is designed to show, penetrates the secondary circuit free of loss as much as possible. The two primary circuits, as have already been discussed in detail, are dimensioned for reasons of safety such that the heat output which can be inductively coupled into the secondary circuit by each individual one of them, is not enough to cause the igniting agent 4 to explode.

[0026] Misfires are thus essentially precluded. Only when the two primary circuits are activated by the signal of the impact sensor 13 does detonation occur. It can be especially advantageous to trigger each of the primary circuits via a separate impact sensor 13. It furthermore goes without saying that more than two primary circuits can also be used; in a simpler version the novel firing capsule of course makes do with a single primary circuit.

[0027] In the embodiment shown in FIG. 5, the firing capsule as claimed in the invention is not sealed by a separate sealing body 5, but has an already closed firing capsule housing 2 in which the igniting agent 4 is located. Within the firing capsule housing 2 there is a carrier plate 15 which is surrounded by the igniting agent 4 and which on each side has an annular groove 16 in which at least one metal layer 17 is placed. In this embodiment there are thus two secondary circuits which are independent of one another. Here the vertical arrow also indicates the magnetic flux which is produced by at least one primary winding which is no longer shown here, in the case of a motor vehicle collision. In the two resistance elements this magnetic flux induces currents by which the two are heated up and thus cause the igniting agent 4 to explode. The firing capsule as claimed in the invention then also remains serviceable even if one of the two secondary circuits for example should fail due to a break in the resistance element.

[0028] FIGS. 6a to 6e show as a section five possibilities for implementing the resistance elements for the inductively activatable firing capsule as claimed in the invention.

[0029] In the embodiment as shown in FIG. 6a a closed, annular groove which is filled with at least one metal layer 17 is machined into a carrier plate 15. This corresponds essentially to the arrangement as shown in FIG. 4, but with the difference that there is only one single resistance element. In the embodiment as shown in FIG. 6b at least one metal layer 17 surrounds the carrier plate 15 on its outside edge like a tire. FIG. 6c shows an alternative in which at least one metal layer 17 is applied directly to the surface of the carrier plate 15. Various processes are possible for this production step, for example vapor deposition using the corresponding masks or large-area coating with subsequent etching out of the desired conductor structure. In the embodiment as shown in FIG. 6d the carrier plate 15 was first completely coated with at least one metal layer 17 and then provided with a hole so that only the annular, closed resistance element remains. Finally, FIG. 6e shows one especially preferred version in which an annular core 18 of insulating or poorly conductive material has been coated with at least one metal layer.

Claims

1. Inductively activatable firing capsule for the gas generator of a passenger restraint system in a motor vehicle, especially for a gas generator of an inflatable impact protection cushion, with a pot-shaped housing (2) which is filled at least partially with an igniting agent (4), outside the housing (2) there being at least one primary circuit (9; 9a, 9b) with a primary winding (10; 10a, 10b) which in the case of a vehicle collision is supplied with a trigger current, and within the housing (2) there being at least one secondary circuit (6) which contains a secondary winding (7) and connected in series thereto a resistance wire (8), the resistance wire (8) penetrating the igniting agent (4) or being located in the immediate vicinity of the igniting agent (4), and at least one secondary winding (7) being inductively coupled to at least one primary winding (10; 10a, 10b).

2. Firing capsule as claimed in claim 1, wherein the housing (2) is hermetically sealed by a sealing body (5).

3. Firing capsule as claimed in claim 1, wherein the housing (2) consists of an insulating material.

4. Firing capsule as claimed in claim 2, wherein the sealing body (5) consists of an insulating material.

5. Firing capsule as claimed in claim 1, wherein at least one primary winding (10; 10a, 10b) and at least one secondary winding (7) each surround a magnet core (11; 11a, 11b).

6. Firing capsule as claimed in claim 5, wherein there is a common magnet core (11) which penetrates the sealing body (5).

7. Firing capsule as claimed in claim 5, wherein at least one primary winding (10; 10a, 10b) and at least one secondary winding (7) each have its own magnet core (11; 11a, 11b).

8. Firing capsule as claimed in claim 1, wherein at least one secondary circuit (6) consists of an annularly closed resistance wire (8) which is located in the housing (2) of the firing capsule (1) such that it is inductively coupled to at least one primary winding (10; 10a, 10b) outside of the housing (2).

9. Firing capsule as claimed in claim 8, wherein there is at least one annularly closed resistance wire (8) on the carrier plate (15).

10. Firing capsule as claimed in claim 9, wherein two annularly closed resistance wires (8; 8a, 8b) at a time are located electrically insulated from one another on opposite sides of each carrier plate (15).

11. Firing capsule as claimed in claim 1, wherein there are at least two primary circuits (9; 9a, 9b) and one secondary circuit (6), the electrical power which is coupled into the secondary circuit (6) by one of the primary circuits (9; 9a, 9b) at a time being dimensioned such that the igniting agent (4) is not caused to explode.

12. Firing capsule as claimed in claim 1, wherein the trigger current is a sinusoidal or rectangular alternating current.

13. Test circuit for an inductively activatable firing capsule as claimed in one of claims 1 to 12, wherein it is connected in parallel to at least one primary winding (10; 10a, 10b) and delivers a weak AC voltage signal to at least one primary winding (10; 10a, 10b) and wherein it contains a device which detects the change of the alternating current flowing in at least one primary winding (10; 10a, 10b).

14. Test circuit as claimed in claim 13, wherein it produces an alarm signal when the alternating current flowing in at least one primary winding (10; 10a, 10b) changes.

15. Test circuit as claimed in claim 13, wherein the weak AC voltage signal is delivered at regular time intervals or continuously.

16. Inductively activatable firing capsule as claimed in one of the preceding claims, wherein at least one secondary circuit located within the firing capsule housing (2) is formed by a closed resistance element.

17. Inductively activatable firing capsule as claimed in claim 16, wherein the closed resistance element has a rectangular, square or round cross section.

18. Inductively activatable firing capsule as claimed in claim 16, wherein the closed resistance element has an annular configuration.

19. Inductively activatable firing capsule as claimed in one of claims 16 to 18, wherein the closed resistance element has fewer than 10, preferably exactly one turn.

20. Inductively activatable firing capsule as claimed in one of claims 16 to 19, wherein the closed resistance element is formed by an annular core (18) of insulating or poorly conductive material which is coated by at least one metal layer (17).

21. Inductively activatable firing capsule as claimed in one of claims 16 to 19, wherein the closed resistance element consists of at least one metal layer (17) which is applied to a carrier plate (15) of insulating or poorly conductive material.

22. Inductively activatable firing capsule as claimed in one of claims 16 to 19, wherein the closed resistance element consists of at least one metal layer (17) which is embedded in a groove (16) of a carrier plate (15) of insulating or poorly conductive material.

23. Inductively activatable firing capsule as claimed in claim 21 or 22, wherein the carrier plate (15) is formed by the sealing body (5) of the firing capsule housing (2).

24. Inductively activatable firing capsule as claimed in claim 20, wherein the annular core (18) consists of glass, ceramic or plastic.

25. Inductively activatable firing capsule as claimed in one of claims 21 to 23, wherein the carrier plate (15) consists of glass, ceramic or plastic.

26. Inductively activatable firing capsule as claimed in one of the preceding claims, wherein at least one metal layer (17) consists of metals with a high specific resistance.

27. Inductively activatable firing capsule as claimed in one of the preceding claims, wherein there are at least two primary circuits which are triggered by a common impact sensor (13).

28. Inductively activatable firing capsule as claimed in one of the preceding claims, wherein there are at least two primary circuits, of which each is triggered by a common impact sensor (13).