METHOD FOR MANUFACTURING AN AIRBAG COVER WITH A DEFINED BREAK LINE AND A DEFINED TEAR RESISTANCE

Description

[0001] The invention relates to a method for producing a predetermined breaking line in an airbag cover, which is particularly advantageous when the airbag cover is made of a non-transparent material.

[0002] To ensure that an airbag located behind an airbag cover can reliably deploy in an emergency, it is known to provide the airbag cover with a predetermined breaking line that tears open when the airbag deploys, allowing the airbag to pass through the resulting opening. When manufacturing such a predetermined breaking line, also called a weak line or tear line, it is crucial that the line exhibits a defined tear resistance along its entire length. This resistance can be constant along its length or subject to a defined variation, for example, being lower locally where the tearing is to be initiated. The tear resistance must not be too high, so that the predetermined breaking line tears open reliably and completely when the airbag deploys.At the same time, it must not be too small, in order to rule out the possibility that the predetermined breaking point will break due to unintentional mechanical impacts from vehicle occupants.

[0003] Most methods for creating a predetermined breaking point in an airbag cover involve subtractive processes in which material is removed in the form of a groove or a series of blind holes, or slots that do not completely penetrate the material.

[0004] For an airbag cover made of a homogeneous material with a homogeneous density and a constant thickness, it is sufficient to remove or cut the material to a constant depth to achieve a constant tear resistance. In this case, a constant removal depth is equivalent to a constant residual wall thickness (thickness of the remaining material below the resulting blind holes or grooves).

[0005] If the thickness of the airbag cover is not constant along the desired predetermined breaking line, then the material removal must be carried out in such a way that a constant residual wall thickness remains. In this case, the depth is not constant.

[0006] According to the state of the art, purely mechanical cutting or milling tools, ultrasound-assisted cutting or milling tools, and lasers are used to create such predetermined breaking lines. Various measuring methods are employed, which are more or less suitable for determining the depth of material removal or the remaining wall thickness depending on the properties of the airbag cover and the tool used.

[0007] EP 1 459 943 B1 discloses a device and a method in which an ultrasonic sonotrode designed as a cutting blade is moved along a line, cutting a predetermined breaking line into the material of an airbag cover. During cutting, the distance between a distance sensor carried with the cutting blade and a counter-cutting plate holding the material to be cut is measured. The remaining wall thickness is calculated as the difference between the distance between the distance sensor and the counter-cutting plate, and the distance between the distance sensor and the base surface. The penetration depth of the cutting blade is adjusted so that the remaining wall thickness is constant along the length of the predetermined breaking line. The measuring method used here is dependent on the use of a mechanical tool whose penetration depth determines the depth of the cut into the material.

[0008] From DE 10 2005 040 017 A1, a method for processing a component, in particular an automotive interior trim component, is known, in which the component is irradiated with laser radiation of a first frequency, wherein the material of the component absorbs the laser radiation and a layer is present in the component which, upon striking the laser radiation, re-emits at a second frequency different from the first frequency. If the re-emitted radiation is measured, then the material has been removed down to this layer.

[0009] This method can be advantageously applied to the processing of a component that has a layer essentially opaque to laser radiation of both frequencies. The measuring sensor is positioned on the same side of the component as the laser. The accuracy of the cutting depth is achieved here depending on the precise depth of a material layer within the component.

[0010] US Patent 5,883,356 A discloses a device and a method for precisely scoring a workpiece, in which the workpiece thickness is first determined at predetermined points along the desired predetermined breaking line, and then a weakening line with a varying depth is created using a laser, corresponding to the previously measured thickness. The thickness is determined by scanning the opposing surfaces of the workpiece support and the workpiece itself. The laser power and the trajectory of the laser beam are controlled as a function of the determined thickness along the predetermined breaking line. This method is used for the precise pre-weakening of airbag covers. At least one process parameter is controlled as a function of the workpiece thickness along the desired breaking line to achieve a consistent material removal depth.

[0011] In all the aforementioned methods, the measured variable is the depth of removal or the remaining wall thickness.

[0012] DE 10 2013 202 081 B3 discloses a device for making cuts in a planar material web and a method for controlling a cutting device. The device includes a cutting device for making cuts in the material web, a support surface for the planar support of the material web, a measuring device for determining a remaining residual wall thickness along the cutting path, and a control and regulating device that, based on the determined measured values ​​and depending on predetermined setpoints, provides corresponding control signals that are transmitted to a drive unit, which, based on the control signals, causes an adjustment of the relative position of the cutting device to the support surface.Determining the remaining wall thickness is made possible here by using a cutting device and a support surface made of an electrically conductive material, and by connecting a voltage source to both. The measuring device measures the electric current flowing between the cutting device and the support surface during cutting, which depends on the specific resistance of the material web located between them. Unlike the previously mentioned methods, the measurement result is then a value that does not change solely depending on the remaining wall thickness if the material is inhomogeneous.

[0013] However, a disadvantage is that this measuring method can only be implemented with a mechanical tool and the material to be processed must be conductive.

[0014] EP 0 827 802 B1 describes a method for producing a predetermined breaking line, in which a predetermined breaking line is created in a flat material by selectively ablating the material on one side using controlled pulsed laser radiation. This involves a relative movement between the laser radiation and the flat material in the direction of the predetermined breaking line to be created. Blind holes are created, arranged in a linear sequence and separated from each other by a remaining rib. During the process, the laser radiation transmitted through the bottom of each blind hole is detected, and an integral value is calculated over the signals detected during the creation of each blind hole. When a threshold value for the integral value is reached, which correlates with the desired remaining wall thickness, the ablating laser radiation is switched off.After a relative movement over a predetermined distance, the laser radiation is switched on again, and another blind hole is created. The measurement method used here is contingent upon the use of laser radiation as a tool and the creation of a predetermined breaking line in the form of a perforation line in a material with at least some transparency. Upon reaching the threshold value for each blind hole, a consistent remaining wall thickness can only be inferred if the flat material is homogeneous. If the flat material exhibits density variations along the predetermined breaking line, the threshold value in a denser region will only be reached at a reduced remaining wall thickness, which, however, will have a comparable tear resistance to a greater remaining wall thickness in a lower-density region. This method is therefore also advantageously applicable when the flat material exhibits density variations.

[0015] German patent DE 102 54 377 B3 describes a method for introducing an integrated predetermined breaking line into an extensively developed structure, particularly one made of an inhomogeneous material such as a fabric. To produce a predetermined breaking line with the most homogeneous tear resistance possible, it is proposed to increase the laser power from zero to a target value for each blind hole along the predetermined breaking line. The laser is then immediately switched off if the detector registers a signal exceeding a predefined threshold before the target value is reached. This prevents further weakening or complete removal of the material where it is already weaker than the target weakening point.

[0016] The latter two methods are limited to laser ablation processes and require that the material be at least slightly transparent to the laser radiation if the material is not to be completely penetrated.

[0017] WO 2008 / 104230 discloses a method for pre-weakening an airbag cover using laser radiation, wherein the component has an indicator layer. Sound is generated at this indicator layer, which is then detected by an acoustic sensor, thereby controlling the processing laser. A disadvantage of this method is, in particular, the necessary presence of an indicator layer, which makes it impossible to take into account inhomogeneities in the remaining wall thickness or the material when controlling the processing tool.

[0018] US Patent 4,504,727 A describes a device for drilling a workpiece, particularly a multilayer printed circuit board, which is controlled using photoacoustic signals. The device includes at least one adjustable, emitting laser source. The different layers have different photoacoustic properties and thus each a characteristic acoustic signature that is photoacoustically activated by the laser source shining onto the substrate. The triggered acoustic signal is detected and forwarded to a control system, which evaluates the detected acoustic signal and adjusts the laser parameters accordingly. During the evaluation, a sudden signal change is detected, which is caused by sound generated at the layer interface.

[0019] It is known to experts that acoustic emission measurements can be used to test materials such as metals, ceramics, glasses, rocks, concrete, brittle polymer materials, wood and composite materials, detecting damage to components such as cracking, crack propagation, delamination, dislocation movement,

[0020] Phase transformations and corrosion processes can be detected. The respective damage leads to a significant change in the sound waves emitted by the component.

[0021] Conventional acoustic detectors (microphones) use a diaphragm, piezoelectric crystals, or similar electrical or mechanical devices to measure the sound waves generated during material processing. These methods always involve moving or deflectable parts. The accuracy of such measurement methods is limited because mechanical systems are subject to vibrations and their deflection is mechanically restricted. Air currents or sound waves generated within the measurement structure also interfere with the measurement. In electrical measurement methods, stray electromagnetic fields interfere with the measurement. This makes it impossible to reliably measure large pressure changes across a wide frequency range (e.g., 10 Hz - 1 MHz).

[0022] US Patent 8,301,029 B2 discloses an acousto-optic converter comprising a laser, an optical detector, and a beam splitter. The beam splitter divides the laser beam into a first and a second beam. The first beam travels a path length exposed to an acoustic field. The second beam travels an acoustically isolated path length. The propagation velocity of the first beam changes due to pressure changes induced by the acoustic field, and the optical detector generates an electrical signal dependent on these changes in the propagation velocity of the first beam.

[0023] In an advantageous embodiment, the first and second beams are each reflected several times by two plane-parallel mirrors, the space between one pair of mirrors being exposed to an acoustic field, while the space between another pair of mirrors is acoustically isolated.

[0024] From US patent 2015 / 0139451 A1, a device is known which includes a measuring interferometer and at least one reference interferometer. The signal-to-noise ratio can be improved by using multiple reference interferometers. Light from the same source is coupled into the interferometers. The measuring interferometer is affected by changes in a physical parameter, while the at least one reference interferometer is isolated from these influences. Changes in one or more physical parameters alter the intensity of the light coupled out of the measuring interferometer. These changes are measured by a photodetector, thus detecting the changes in the physical parameter(s) by measuring the change in the intensity of the light emitted by the interferometer.The device also includes a signal processor which generates a differential signal from the respective detected signals of the measuring and reference interferometers.

[0025] The aforementioned US patent 2015 / 0139451 A1 also discloses an implementation of the device as an optical microphone. For this purpose, the measuring interferometer is acoustically coupled to the environment, and the physical parameter is atmospheric pressure. Local changes in atmospheric pressure or density, caused by a sound field, result in local changes in the refractive index of the interferometer. This modulates the amplitude of the laser radiation coupled out of the interferometer. The intensity of the coupled-out laser radiation depends on the incoming sound waves.

[0026] This optical microphone enables the measurement of sound waves with a wide bandwidth (up to 1 MHz in air, up to 25 MHz in liquids). It can also measure relatively high frequencies (>100 kHz). Conventional methods are typically limited to a bandwidth of approximately 100 kHz. The measurement method is contactless, and the device is diaphragm-free and very compact. Therefore, it is not susceptible to interference from mechanical vibrations. Another advantage over optical measurement methods is that electromagnetic radiation from the environment does not affect the detected signal.

[0027] The article "Acoustic Monitoring for Laser Material Processing," published in the journal "Laser," issue 2 / 2019, explains that capturing airborne sound emissions in the frequency range of 10 Hz to 1 MHz, now possible with an "optical microphone," opens up new avenues for monitoring machine processes. The application examples for process monitoring cited here—namely, the detection of irregularities or defects on surfaces, the monitoring of surface conditions (such as zinc coatings) during welding or ablation, the real-time detection of crack formation in laser welding processes, or the detection of spatter formation during welding processes—all involve the detection of abrupt changes in state.

[0028] The aforementioned article, "Acoustic Monitoring for Laser Material Processing," also states that the "optical microphone" can be used in a variety of ways in laser structuring. It can be used to determine the focal point as well as to perform real-time parameter checks of the laser tool. From this, the laser penetration time or the ablation volume can be derived.

[0029] The object of the invention is to find a method for manufacturing an airbag cover with a predetermined breaking line and a defined tear resistance, in which the setting of the tear resistance can be controlled independently of the transparency of the airbag cover and independently of the tool used.

[0030] The task is fulfilled for a process for manufacturing an airbag cover with a predetermined tear line, in which a tool is guided along a line relative to the airbag cover while removing material at machining points along the line. The material removal is controlled by a control signal that is compared to a reference signal to which a predefined tear resistance is assigned. The material removal at the respective machining point is stopped when the control signal equals the reference signal within a predefined tolerance, where the control signal is an optical signal, namely an interference signal with an acoustic signature.

[0031] According to the invention, the interference signal is generated from a measuring laser beam, at least a portion of which is guided along a path through a medium that is influenced by sound waves caused by the tool at the respective processing location, so that a phase shift required to generate the interference signal is induced.

[0032] It is advantageous that the track is arranged in a fixed relative position to the respective processing location.

[0033] It is particularly advantageous if the path is arranged along a side of the airbag cover facing away from the tool.

[0034] The tool can be a mechanical tool, but it is advantageous to use a processing laser beam with laser parameters different from the measuring laser beam.

[0035] The measuring laser beam is advantageously a beam component coupled out from the processing laser beam.

[0036] The instantaneous values ​​of the control signal assigned to the processing locations are advantageously stored when the processing is completed at each processing location, and a quality pass for the processed airbag cover is created from the stored instantaneous values.

[0037] The method according to the invention is explained in more detail below using various exemplary embodiments with reference to drawings.

[0038] This shows: Fig. 1 shows a device setup for a first embodiment of the method with a processing beam as a tool, Fig. 2 shows a device setup for a second embodiment of the method with a processing beam as a tool, Fig. 3 shows a device setup for a third embodiment of the method with a processing beam as a tool and a measuring laser beam coupled from it, and Fig. 4 shows a device setup for a fourth embodiment of the method with a milling cutter as a tool.

[0039] In all embodiments of the method, a tool is guided along an imaginary line L relative to an airbag cover 2. Material removal at machining points along line L creates a predetermined tear line 3 with a remaining residual wall in the airbag cover 2. The material removal is controlled such that the remaining residual wall along the predetermined tear line 3 exhibits a substantially defined, preferably constant, tear resistance. During the material removal process, the tear resistance is a variable value that differs over time and space across the machining points and depends on both the remaining residual wall thickness and the density and chemical composition of the material being machined. For an airbag cover 2 made of a material with a homogeneous density distribution, a constant tear resistance is achieved after material removal when the residual wall thickness is constant at all machining points.If the density distribution of the material varies along line L, then a residual wall thickness dependent on the density distribution is generated in order to create an essentially equal tear resistance at the machining locations along the predetermined breaking line 3.

[0040] During the production of the predetermined breaking line 3, an optical control signal is generated, in accordance with the state of the art, and compared with a reference signal to control the material removal. The control signal can be understood as a sequence of instantaneous values, each assigned to a specific machining location. An instantaneous value, as described here, represents a segment of the control signal that is assigned to a particular machining location over the time assigned to the dwell time S of the tool at that location. The machining locations can be directly adjacent to one another, resulting in a slot or, with interruptions, several slots. Alternatively, they can be separated by regular intervals, creating blind holes.

[0041] The essential aspect of the invention is that the optical control signal is an interference signal which is influenced by an acoustic signal (sound waves or a sound field) which is generated at the processing location by the action of the tool on the airbag cover 2, which is why it is referred to as an interference signal with a sound signature.

[0042] To generate the interference signal, a measuring laser beam ML is split into interfering partial beams, which are either influenced by the sound waves over different distances, or only one portion of the measuring beam, i.e., one of the two partial beams, is influenced by the sound field, while the other partial beam is acoustically isolated. Crucially, the distance S over which at least one of the partial beams is influenced by the sound field must have the same relative position and length with respect to the processing locations, ensuring that the influence occurs under identical conditions regardless of the processing location.A part of the device used to carry out the method, responsible for the measurement and comprising a measuring laser beam source 4 and an optical detector 5, must be moved synchronously depending on whether the tool or the airbag cover 2 performs the movement required for processing. The path of influence can be located either on the side of the airbag cover 2 facing the tool or, preferably, on the side facing away from the tool.

[0043] A first embodiment of a method according to the invention is described below with reference to Fig. 1The diagram describes a suitable device setup, illustrated in a schematic diagram. A processing laser beam source 1 is shown, which guides a processing laser beam BL along a line L, perpendicular to the plane of the drawing, relative to an airbag cover 2, thereby cutting a predetermined breaking line 3. Upon impact of the processing laser beam BL, sound waves SW are generated at the respective processing point. The properties of these sound waves are characteristic of the parameters of the remaining wall below the predetermined breaking line 3 and propagate, forming a sound field. The sound field extends in all directions from its source, the point of impact of the tool on the airbag cover. The propagation of the sound waves is indicated in the figures within a region relevant to its effect.

[0044] To obtain an optical signal influenced by the generated sound waves, the part of the device setup relevant for the measurement comprises a measuring laser beam source 4, an optical detector 5, and four mirrors 6.1-6.4. A measuring laser beam ML emitted by the measuring laser beam source 4 is split into a first partial beam T1 and a second partial beam T2 by a partially transparent first mirror 6.1. During its propagation over a distance S, i.e., a path of defined length, the first partial beam T1 is exposed to the sound waves SW generated at the respective processing location. These sound waves cause pressure changes in the medium along the distance S, over which the first partial beam T1 propagates in a fixed position relative to the processing location. These pressure changes lead to a change in the refractive index and thus to a change in the propagation speed of the first partial beam T1.The phase of the first partial beam T1 is thus shifted. The second partial beam T2 is guided through a vacuum, completely unaffected by the sound waves SW, before being deflected via two mirrors 6.2 and 6.3 and then via a partially reflective mirror 6.4, where it is brought into interference with the first partial beam T1. The resulting interference signal is detected by the optical detector 5. Assuming that the temperature and wavelength of the measuring laser beam are constant, the phase of the first partial beam T1 shifts depending on the pressure changes caused by the sound waves, and the detected interference signal allows conclusions to be drawn about the pressure changes and thus the characteristics of the sound waves. The refractive index of the medium located along the path S is modulated by the sound waves.The generation of the detection signal by interference of a first partial beam T1, influenced by the sound waves, with an uninfluenced second partial beam T2 has the advantage that fluctuations in the parameters of the measuring laser beam ML have no influence on the measurement result. For an objective and reproducible measurement, it is important that the geometric path traversed by the first partial beam T1, the distance S, has a constant length and the same relative position to the respective processing location throughout the entire process.

[0045] A second embodiment of a method according to the invention is described using the example in Fig. 2 The schematically depicted device setup is explained.

[0046] In this embodiment, unlike the previously described embodiment, an interference signal is generated by coupling a measuring laser beam ML into a resonator 7. The medium between the resonator mirrors 7.1, 7.2 is exposed to the sound waves generated during the ablation process, resulting in a phase shift in the waves circulating in the resonator. This phase shift causes the signal coupled out of the resonator 7 to be modulated, due to the interference of the measuring laser beam ML with itself within the resonator, depending on the pressure differences generated by the sound waves SW.

[0047] To eliminate fluctuations in the laser parameters from the measurement result, it is advantageous to decouple a small part of the processing laser beam and detect it through a monitoring detector 8, which serves to monitor the laser parameters during the processing process.

[0048] A third embodiment of a method according to the invention differs in that the measuring laser beam ML is a beam component coupled out of the processing laser beam BL. For this purpose, a device setup is used, as shown in Fig. 3 , recourse was made to the part relevant for measurement which is constructed according to the second embodiment.

[0049] According to a fourth embodiment, in which the measuring laser beam is also coupled out of the processing laser beam, the phase shift required for interference is realized according to the second embodiment.

[0050] Other embodiments differ from those mentioned above by using a different tool. Instead of a processing laser beam, as in Fig. 4As shown, a mechanical tool 9 is used. This could be, for example, a peeling knife or a milling cutter. The control signal is acquired analogously to the previously described embodiments.

[0051] The route S is according to the Figs. 1 and 2 on the side facing away from the tool and according to the Figs. 3 and 4 arranged on the side facing the tool. The arrangement of the line segment S is interchangeable for the exemplary embodiments and thus for the figures.

[0052] When the section S is positioned on the side of the airbag cover facing away from the tool, it is advantageous that the tool does not influence the sound waves, and the generated sound waves only pass through the remaining wall. This means that the material adjacent to the remaining wall, which defines the predetermined breaking line 3, has at least a lesser influence on the propagation of the sound waves and thus their parameters. The parameters of the sound waves are more clearly influenced by the parameters of the remaining wall than the sound waves on the side facing the tool. Positioning the section S on the side facing away from the tool is particularly advantageous if the airbag cover consists of a layered material and the remaining wall is formed by only a portion of one layer.

[0053] In all embodiments, the detected interference signal is compared with a reference signal, which is characteristic of the defined tear resistance of the airbag cover 2 along the predetermined breaking line 3, to control the material removal at the respective processing locations.

[0054] If the detected interference signal at the respective processing location matches the reference signal at the same processing location within a predefined tolerance, the material removal at that location is stopped. The reference signal was previously obtained by processing an airbag cover in the same manner as described in the procedure, and subsequently confirming, particularly through destructive testing, that the resulting predetermined breaking point exhibits the expected tear resistance. The instantaneous values ​​of the control signal that triggered the termination of processing at each processing location are then stored as a reference signal and assigned to a specific time. Reference symbol list

[0055] 1. Processing laser beam source 2. Airbag cover 3. Break line 4. Measuring laser beam source 5. Optical detector 7.1-6.4 6.1-6.4 7. Mirror 7. Resonator 7.2 8. Monitoring detector 9. Mechanical tool L line SW sound waves S section ML measuring laser beam T1 first partial beam T2 second partial beam BL processing laser beam

Claims

1. A method for producing an airbag cover (2) having a predetermined breaking line (3), in which a tool is guided along a line (L) relative to the airbag cover (2), while the tool removes material at machining locations along the line (L), wherein the material removal is controlled by a control signal which is compared with a reference signal to which a predetermined tear resistance is assigned, and the material removal at the respective machining location is terminated when the control signal equals the reference signal up to a predetermined tolerance, wherein the control signal is an optical signal, characterized in that the optical signal is an interference signal with an acoustic signature, wherein the interference signal is generated from a measuring laser beam (ML), at least a portion of which is guided along a path (S) through a medium which is influenced by the sound waves (SW) caused by the tool at the respective machining location, so that a phase shift required to generate the interference signal is induced.

2. The method according to claim 1, characterized in that the path (S) is arranged in a fixed position relative to the respective machining location.

3. The method according to claim 1, characterized in that the path (S) is arranged along a side of the airbag cover (2) facing away from the tool.

4. The method according to claim 1, characterized in that the tool is a machining laser beam (BL) with laser parameters differing from those of the measuring laser beam (ML).

5. The method according to claim 1, characterized in that that the tool is a machining laser beam (BL) and the measuring laser beam (ML) is a beam portion coupled out of the machining laser beam (BL).

6. The method according to any one of the preceding claims, characterized in that instantaneous values of the control signal assigned to the machining locations are stored when machining is completed at each of the machining locations, and a quality certificate for the machined airbag cover (2) is created from the stored instantaneous values.